CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority’ to and the benefit of U.S. Provisional Application No. 63/332,470 having a filing date of April 19, 2022, the disclosure of which is incorporated herein by reference in its entirety.

FIELD

[0002] The present disclosure relates to oxidation of C8-r aromatic hydrocarbons and, more particularly, oxidation of p-xylene in preference to other C8+ aromatic hydrocarbons and separation thereof.

BACKGROUND

[0003] C8+ aromatic hydrocarbons, including o-, rn- and p-xylene isomers and mixtures thereof may be produced through various processes, such as via alkylation of lower aromatic hydrocarbons (e.g., benzene and/or toluene), transalkylation, toluene disproportionation, catalytic reforming, isomerization, cracking, and the like. While vanous processes may be utilized for alkylating lower aromatic hydrocarbons, alkylation with methanol and/or dimethyl ether under zeolite catalyst promotion may be particularly effective and advantageous. Illustrative processes for alkylating lower aromatic hydrocarbons with methanol and/or dimethyl ether are described in, for example, U.S. Patent Application Publication 20200308085 and International Patent Application Publication WO/2020/197888, each of which is incorporated herein by reference. Such alkylation processes may produce p-xylene (1,4- dimethylbenzene) at considerably higher selectivity compared to o-xylene and m-xylene, in combination with small amounts of C9+ aromatic hydrocarbons and byproducts such as styrene and phenol, or other product components resulting therefrom. Isomerization and other processes for producing C8+ aromatic hydrocarbons, in contrast, may form an equilibrium mixture of xylene isomers containing a much smaller proportion of p-xylene, as well as significant quantities of byproducts, such as ethylbenzene, that may complicate further processes for oxidizing p-xylene into other compounds of value.

[0004] Worldwide production capacity’ of p-xylene from various industrial sources is about 40 million tons per year. p-Xylene is a valuable chemical feedstock that may be obtained from C8+ aromatic hydrocarbons, primarily for conversion into 1,4-benzenedicarboxylic acid (terephthalic acid), which may be used in synthetic textiles, bottles, and plastic materials among other industrial applications. While mixed xylenes may be converted into blends of benzenedicarboxylic acid isomers (i.e., phthalic acid, isophthalic acid, and terephthalic acid), with the terephthalic acid being separated subsequently, the markets for phthalic acid and isophthalic acid are much smaller than that of terephthalic acid, thereby raising an oversupply issue for the other benzenedicarboxylic acid isomers, not to mention the non-selective oxidation representing an inefficient use of feedstock. As further complications, even small quantities of o-xylene can inhibit the oxidation process for p-xylene, and small quantities of ethylbenzene in a xylene source may lead to processing issues resulting from concurrent form anon of benzoic acid during oxidation. Although at least partial separation of p-xylene from other xylene isomers and byproducts, such as ethylbenzene, is feasible prior to oxidation, this approach may too be undesirable from an energy efficiency and process complexity standpoint.

SUMMARY

[0005] In some aspects, the present disclosure provides processes for selectively oxidizing p-xylene. The processes comprise: providing a C8-f- aromatic hydrocarbon feed comprising at least p-xylene; optionally, separating ethylbenzene, if any, from the C8+ aromatic hydrocarbon feed; exposing at least a portion of the C8+ aromatic hydrocarbon feed to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxygencontaining C8 aromatic compounds, wherein the selectivated oxidation catalyst is ineffective to oxidize m-xylene and o-xylene, if present, and the one or more oxygen-containing C8 aromatic compounds comprise a compound selected from the group consisting of p- methylbenzyl alcohol, p-methylbenzaldehyde, p-methyl benzoic acid, and any combination thereof) and forming a product stream comprising the one or more oxygen-containing C8 aromatic compounds from the C8+ aromatic hydrocarbon feed under the oxidation conditions. [G006 ] In some or other aspects, the present disclosure provides processes for oxidizing p- xylene into terephthalic acid. The processes comprise: providing a C8+ aromatic hydrocarbon feed comprising o-xylene, m-xylene, at least about 50 wt% p-xylene, and about 1000 ppm or less ethylbenzene, based on total mass of the C8+ aromatic hydrocarbon feed; separating an o- xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene from the C8+ aromatic hydrocarbon feed; exposing the o-xylene-lean stream to oxidation conditions and an oxidation catalyst effective to convert the o-xylene-lean stream into a product stream comprising terephthalic acid or a mixture of terephthalic, acid and isophthalic acid; and separating the product stream from the o-xylene-lean stream.

[0007] In still other aspects, the present disclosure provides processes for selectively oxidizing p-xylene within a C8+ aromatic hydrocarbon feed. The processes comprise: providing a C8+ aromatic hydrocarbon feed comprising o-xylene, m-xylene, at least about 50 wt% p-xylene, and about 1000 ppm or less ethylbenzene, based on total mass of the C8+ aromatic hydrocarbon feed; separating an o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene from the C8+ aromatic hydrocarbon feed; exposing the o- xylene-lean stream to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxygen-containing C8 aromatic compounds; wherein the selectivated oxidation catalyst is ineffective to oxidize m-xylene, if present, and the one or more oxy gen-containing C8 aromatic compounds comprise a compound selected from the group consisting of p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, and any combination thereof; forming a product stream comprising the one or more oxygencontaining C8 aromatic compounds from the o-xylene-lean stream under the oxidation conditions; and separating the product stream from the o-xylene-lean stream.

[0008] These and other features and attributes of the disclosed methods and compositions of the present disclosure and their advantageous applications and/or uses will be apparent from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents m form and function, as will occur to one having ordinary skill m the art and having the benefit of this disclosure.

[0010] To assist one of ordinary skill in the relevant art in making and using the subject matter hereof, reference is made to the appended drawings, wherein :

[0011] FIG. 1 is a block diagram schematically illustrating systems and processes for producing xylene isomers by alkylation of lower aromatic hydrocarbons with methanol and/or dimethyl ether and subsequently oxidizing p-xylene into a product stream comprising terephthalic acid or a terephthalic acid precursor according to the present disclosure.

[0012] FIG. 2 is a block diagram schematically illustrating the systems and processes of FIG. 1 coupled to a reforming process and a xylene isomerization process.

[0013] FIG. 3 is a block diagram schematically illustrating systems and processes for producing xylene isomers by disproportionation of toluene and subsequently oxidizing p- xylene into a product stream comprising terephthalic acid or a terephthalic acid precursor according to the present disclosure. DETAILED DESCRIPTION

[0014] The present disclosure relates to oxidation of C8+ aromatic hydrocarbons and, more particularly, oxidation of p-xylene in preference to other C8+ aromatic hydrocarbons and separation thereof.

Definitions

[0015] Various specific embodiments, versions and examples of the invention will now be described, including preferred embodiments and definitions that are adopted herein for purposes of understanding the claimed invention. While the following detailed description gives specific, preferred embodiments, those skilled in the art will appreciate that these embodiments are exemplary only, and that the invention may be practiced in other ways. For purposes of determining infringement, the scope of the invention will refer to any one or more of the appended claims, including their equivalents, and elements or limitations that are equivalent to those that are recited. Any reference to the ‘'invention” may refer to one or more, but not necessarily all, of the inventions defined by the claims.

[0016] In this disclosure, a process may be described as comprising at least one “step.” It should be understood that each step is an action or operation that may be earned out once or multiple times in the process, in a continuous or discontinuous fashion. Unless specified to the contrary’ or the context clearly indicates otherwise, multiple steps in a process may be conducted sequentially in the order as they are listed, with or without overlapping with one or more other step, or in any other order, as the case may be. In addition, one or more or even all steps may be conducted simultaneously with regard to the same or different batch of material. For example, in a continuous process, while a first step in a process is being conducted with respect to a raw material just fed into the beginning of the process, a second step may be carried out simultaneously with respect to an intermediate material resulting from treating the raw materials fed into the process at an earlier time in the first step. Preferably, the steps are conducted in the order described.

[0017] Unless otherwise indicated, all numbers indicating quantities in this disclosure are to be understood as being modified by the term “about” in all instances. It should also be understood that the precise numerical values used in the specification and claims constitute specific embodiments. Efforts have been made to ensure the accuracy of the data in the examples. However, it should be understood that any measured data inherently contain a certain level of error due to the limitation of the technique and equipment used for making the measurement. [0018] As used herein, the indefinite articles '‘a” or “an'’ shall mean “at least one'’ unless specified to the contrary or the context clearly indicates otherwise. Thus, for example, embodiments using “a fractionation column” include embodiments where one, two or more fractionation columns are used, unless specified to the contrary or the context clearly indicates that only one fractionation column is used.

[0019] As used herein, the term “consisting essentially of” means a composition, feed, stream or effluent that includes a given component or group of components at a concentration of at least about 60 wt%, preferably at least about 70 wt%, more preferably at least about 80 wt%, more preferably at least about 90 wt%, or still more preferably at least about 95 wt%, based on the total weight of the composition, feed, stream or effluent.

[0020] The following abbreviations may be used herein for the sake of brevity: RT is room temperature (and is 23°C unless otherwise indicated), kPag is kilopascal gauge, psig is poundforce per square inch gauge, psia is pounds-force per square inch absolute, and WHSV is weight hourly space velocity.

[0021] As used herein. “wt%” means percentage by weight, “vol%” means percentage by volume, “mol%” means percentage by mole, “ppm” means parts per million, and “ppm wt” and “wppm” are used interchangeably to mean parts per million on a weight basis. All concentrations herein are expressed on the basis of the total amount of the composition in question. All ranges expressed herein should include both end points as two specific embodiments unless specified or indicated to the contrary'.

[0022] Nomenclature of elements and groups thereof used herein are pursuant to the Periodic Table used by the International Union of Pure and Applied Chemistry' after 1988. An example of the Periodic Table is shown in the inner page of the front cover of Advanced Inorganic Chemistry’, 6 ^t!i Edition, by F. Albert Cotton et al. (John Wiley & Sons, Inc., 1999).

[0023] As used herein, the term “hydrocarbon” means (i) any compound consisting of hydrogen and carbon atoms or (ii) any mixture of two or more such compounds in (i). The term “Cn hydrocarbon,” where n is a positive integer, means (i) any hydrocarbon compound comprising carbon atom(s) in its molecule at the total number of n, or (ii) any mixture of two or more such hydrocarbon compounds in (i). Thus, a C2 hydrocarbon can be ethane, ethylene, acetylene, or mixtures of at least two of such at any proportion. A “Cm to Cn hydrocarbon” or “Cm-Cn hydrocarbon,” where m and n are positive integers and m < n, means any of Cm, Cm-i-1, Cm+2, ... , Cn-1, Cn hydrocarbons, or any mixtures of two or more thereof. Thus, a “C2 to C3 hydrocarbon” or “C2-C3 hydrocarbon” can be any of ethane, ethylene, acetylene, propane, propene, propyne, propadiene, cyclopropane, and any mixtures of two or more thereof at any proportion between and among the components. A “saturated C2-C3 hydrocarbon” can be ethane, propane, cyclopropane, or any mixture thereof of two or more thereof at any proportion. A “CrH- hydrocarbon” means (i) any hydrocarbon compound comprising carbon atom(s) in its molecule at the total number of at least n, or (ii) any mixture of two or more such hydrocarbon compounds in (i). A “Cn- hydrocarbon” means (i) any hydrocarbon compound comprising carbon atoms in its molecule at the total number of at most n, or (ii) any mixture of two or more such hydrocarbon compounds in (i). A “Cm hydrocarbon stream” means a hydrocarbon stream consisting essentially of Cm hydrocarbon^). A “Cm-Cn hydrocarbon stream” means a hydrocarbon stream consisting essentially of Cm-Cn hydrocarbon(s).

[002.4] As used herein, an “aromatic hydrocarbon” is a hydrocarbon comprising an aromatic ring in the molecular structure thereof. An aromatic compound may have a cyclic cloud of pi electrons meeting the Huckel rule. A “non-aromatic hydrocarbon” means a hydrocarbon other than an aromatic hydrocarbon.

[0025] As used herein, the term “lower aromatic hydrocarbons” refers to benzene, toluene, or a mixture of benzene and toluene

[0026] An “effluent” or a “feed” is sometimes also called a “stream” in this disclosure. Where two or more streams are shown to form ajoint stream and then supplied into a vessel, it should be interpreted to include alternatives where the streams are supplied separately to the vessel where appropriate. Likewise, where two or more streams are supplied separately to a vessel, it should be interpreted to include alternatives where the streams are combined before entering into the vessel as joint stream(s) where appropriate.

[0027] The term “selectivity” refers to the degree to which a particular reaction forms a specific product, rather than another product. For example, for the methylation of tol uene, 50% selectivity for p-xylene means that 50% of the products fonned are p-xylene, and 100% selectivity for p-xylene means that 100% of the product formed is p-xylene. The selectivity is based on the product formed, regardless of the conversion of the particular reaction. The selectivity for a given product produced from a given reactant can be defined as -weight percent (wt%) of that product relative to the total weight of the products formed from the given reactant in the reaction. The term “selectivated” refers to a process that has been modified to occur with a specified selectivity or a composition that has been modified to provide a specified selectivity. A “selectivated” catalyst, for example, may promote a catalytic reaction with a specified selectivity for forming a given product.

[0028] As used herein, the term “liquid-phase” means reaction conditions in which aromatic hydrocarbons present in a reactor are substantially in a liquid state. “Substantially in liquid phase” means > about 90 wt%, preferably > about 95 wt%, preferably > about 99 wt%, and preferably the entirety of the aromatic hydrocarbons, is in liquid phase.

[0029] As used herein, the term ‘‘vapor-phase” means reaction conditions in which aromatic hydrocarbons present in a reactor are substantially in a vapor state. “Substantially in vapor phase” means > about 90 wt%„ preferably > about 95 wt%, preferably > about 99 wt%, and preferably the entirety' of the aromatic hydrocarbons, is in vapor phase.

[0030] As used herein, the term “alkylation” means a chemical reaction in which an alkyl group is transferred to an aromatic ring as a substitute group thereon from an alkyl group source compound. “Methylation” means alkylation in which the transferred alkyl group is a methyl group. Thus, methylation of benzene can produce toluene, xy lenes, trimethylbenzenes, and the like; and methylation of toluene can produce xylenes, trimethylbenzenes, and the like.

[0031] As used herein, the term “methylated aromatic hydrocarbon” means an aromatic hydrocarbon comprising at least one methyl group and only methyl group(s) attached to the aromatic rmg(s) therein. Examples of methylated aromatic hydrocarbons include toluene, xylenes, trimethyl benzenes, tetramethylbenzenes, pentamethylbenzene, hexamethylbenzene, methylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes, tetramethylnaphthalenes, and the like.

[0032] As used herein, the terms “oxidation,” “oxidizing,” and variants thereof refer to a process in which an atom or group of atoms loses electrons and attains a higher effective charge. The “oxidation state” of an atom or group of atoms is representative of the effective charge. Oxidation may produce an oxygen-containing compound. An “oxygen-containing” compound contains a functional group containing at least one oxygen atom.

[0033] In this disclosure, o-xylene means 1,2-dimethylbenzene, m-xylene means 1,3- dimethylbenzene, and p-xylene means 1,4-dimethylbenzene. Herein, the generic term “xylene(s),” either in singular or plural form, collectively means one of or any mixture of two or three of p-xylene, m-xylene, and o-xylene at any proportion thereof.

[0034] hi this disclosure, phthalic acid means 1,2-benzenedicarboxylic acid, isophthalic acid means 1,3-benzenedicarboxylic acid, and terephthalic acid means 1,4- benzenedicarboxylic acid. The generic term “phthalic acid(s)” means one or any mixture of two or three of phthalic acid, isophthalic acid, and terephthalic acid at any proportion thereof. [0035] As used herein, the term “rich” or “enriched,” when describing a component m a stream or feed, means that the stream or feed comprises the component at a concentration higher than a source material from which the stream is derived. As used herein, the term “depleted” or “lean,” when describing a component in a stream or feed, means that the stream or feed comprises the component at a concentration lower than a source material from which the stream or feed is derived. A stream or feed that is lean in a given component need not necessarily lack that given component entirely.

[0036] Unless otherwise specified herein, any stream or feed herein that is “rich” in a particular component may “consist of’ or “consist essentially of’ that component.

[0037] Unless otherwise specified herein, any stream or feed that is “lean” in a particular component may be “free of” or “substantially free of” that component. “Essentially free of” and “substantially free of,” as interchangeably used herein, mean that a composition, feed, stream or effluent comprises a given component at a concentration of at most about 10 wt%, preferably at most about 8 wt%, more preferably at most about 5 wt%, more preferably at most about 3 wt%, and still more preferably at most about 1 wt%, based on the total mass of the composition, feed, stream or effluent in question.

[0038] A stream or feed that is lean in one component may be rich in another component. For example, an o-xylene-lean stream may be rich in m-xylene and/or p-xylene.

[0039] As used herein, the term “molecular sieve” means a crystalline or semi -crystalline substance, such as a zeolite, with pores of molecular dimensions that permit the passage of molecules below' a certain threshold size.

[0040] As used herein, the term “aviation gasoline” or “AvGas” interchangeably means a fuel composition suitable for the internal combustion engines of airborne vehicles. Specifications for AvGas are provided in, for example, ASTM D910 and various government regulations such as those from the Federal Aviation Administration of the United States. In this disclosure, “motor octane number” is determined by ASTM D2700. Mien used alone herein, “octane” and “octane number” mean motor octane number. Motor octane number is sometimes abbreviated as “MON” herein. A “high octane number” means a MON > about 95, preferably > about 96, more preferably > about 97, more preferably > about 98, more preferably > about 99, and still more preferably > about 100. Pure p-xylene, o-xylene, m-xylene, ethylbenzene, and 1,3,5-trimethylbenzene have MONs of about 105, 85-94, 105, 90-102, and 120, respectively. As such, p-xylene, m-xylene, and trimethylbenzenes are more preferable than are o-xylene and ethylbenzene as fuel components from the perspective of octane number of the fuel composition formulated from the fuel component, especially if a high octane number of > about 98 is desired for the fuel composition.

C8+ Aromatic Compound Oxidation Processes

[0041 ] As discussed above, p-xylene is a valuable feedstock for producing terephthalic acid. Although p-xylene may be readily oxidized to terephthalic acid, there are often difficulties in doing so. p-Xylene is usually produced concurrently wi th other xylene isomers, as well as with other components that may inhibit the oxidation chemistry to produce terephthalic acid and/or introduce impurities into the terephthalic acid. For instance, o-xylene may inhibit the oxidation process, and ethylbenzene (another C8+ aromatic compound) may be oxidized to benzoic acid, which reduces the terephthalic acid purity. As such, these components may need to be at least partially separated from an aromatic hydrocarbon feed being converted to terephthalic acid, even though doing so may decrease the process efficiency and lessen process economics.

[0042] Two complementary approaches for addressing the foregoing issues associated with p-xylene oxidation are provided herein. First, selectivated oxidation catalysts may be used to oxidize p-xylene in preference to other xylene isomers to afford one or more oxy gen-containing C8 aromatic compounds, in which only one of the methyl groups upon the p-substituted aromatic ring undergoes oxidation, referred to as partial oxidation herein. Any of p- methylbenzyl alcohol, p-methylbenzaldehyde (p-tolualdehyde), and p-methylbenzoic acid (p- toluic acid) may be produced when using the selectivated oxidation catalyst. Such partially oxidized organic compounds may be readily separated from p-xylene and other xylene isomers, as well as from problematic and byproducts, by virtue of their differing boiling points and chemical properties. Once formed and obtained in sufficiently pure form, the partially oxidized organic compounds may undergo subsequent oxidation to produce terephthalic acid. Any process providing p-xylene or a mixture of p-xylene in combination with other xylene isomers may be utilized as a p-xylene source when performing oxidation with a selectivated oxidation catalyst according to the present disclosure, provided o-xylene and/or ethylbenzene are not present in excessive quantities (preferably <2000 ppm, more preferably <1000 ppm each) or may be decreased to a level below excessive quantities. Illustrative sources from winch p- xylene may be obtained include, for example, a toluene alkylation process, a toluene disproportionation process, a xylenes isomerization process, a reformate process, a cracking process, or any combination thereof. If needed, other xylene isomers (e.g , o-xylene) and/or problematic, byproducts may be at least partially removed from any of these sources in order to render them suitable for providing p-xylene of sufficient quality for undergoing oxidation. Additional details regarding the selectivated oxidation catalysts is provided below.

[0043] Second, it was recognized that toluene alky lation with methanol and/or dimethyl ether as an alkylation agent affords p-xylene in considerably greater than equilibrium quantities relative to other xylene isomers, particularly o-xylene, as well as produces low concentrations of problematic byproducts such as ethylbenzene. As such, xylene feed streams originating from a toluene alkylation process with methanol and/or dimethyl ether may be advantaged compared to other types of xylene sources. At the very least, p-xylene obtained through toluene alkylation with methanol and/or dimethyl ether may be separated and reacted more readily and efficiently in comparison to that obtained from an equilibrium xylenes mixture or similar product stream producing xylene isomers non-selectively and/or forming problematic byproducts. Thus, toluene alkylation processes with methanol and/or dimethyl ether may afford p-xylene that may be readily isolated from other components and oxidized to terephthalic acid using conventional oxidation catalysts and technology, given the lower amounts of other xylene isomers formed, particularly o-xylene. At the very least, a greater percentage of the feed material may be converted into desired products than when starting with alternative sources of p-xylene. Alternately, axylene-containing product stream obtained from toluene alkylation with methanol and/or dimethyl ether but is not fully separated from m-xylene may be oxidized conventionally, thereby affording a product mixture comprising predominantly terephthalic acid and smaller amounts of isophthalic acid. This product mixture may be reacted together to form polyesters comprising' predominantly terephthalic acid monomers and a small amount of isophthalic acid monomers. As still another option, a mixture of p-xylene and m-xylene obtained from toluene alkylation with methanol and/or dimethyl ether may be contacted with a selectivated oxidation catalyst to selectively oxidize the p-xylene and form the partially oxidized products (oxy gen-containing C8 aromatic compounds) referenced above, which may then be separated and converted to terephthalic acid. Use of a selecti vated oxidation catalyst in this regard may be desirable if high-quality terephthalic acid is desired, and p-xylene cannot be adequately isolated from a given xylenes source to meet a given terephthalic acid purity' specification. Unreacted m-xylene may be withdrawn and used elsewhere (e.g, as an AvGas blending component) or isomerized to generate an equilibrium mixture of xylene isomers. In each of the foregoing instances, efficient utilization of the initial aromatic hydrocarbon feed may be realized.

[0044] Toluene alkylation processes with methanol and/or dimethyl ether may be an advantaged source of p-xylene due to their high production rates of this xylene isomer, particularly relative to the production rate of o-xylene. For instance, the production rate of p- xylene may be about 4-6 times that of m-xylene and about 8-18 times that of o-xylene. Thus, o-xylene separation from a toluene alkylation process to facilitate oxidation does not represent a crippling under-utilization of the initial aromatic hydrocarbon feed. Ethylbenzene (obtained via reduction of a styrene byproduct formed dining toluene alkylation with methanol and/or dimethyl ether) is also not produced at problematic levels (<2000 ppm by weight) during such toluene alkylation processes. Moreover, styrene and/or ethylbenzene may be separated or mitigated readily and concurrently with separating o-xylene from other xylene isomers obtained from toluene alkylation processes with methanol and-'or dimethyl ether. Thus, toluene alkylation processes may lessen the separation burdens associated with p-xylene oxidation processes considerably compared to other xylene sources.

[0045] Separation of o-xylene from other xylene isomers may be conducted through distillation, wherein o-xylene is obtained within a bottoms fraction and a mixture of m-xylene and p-xylene is obtained within an overhead fraction. Alternately, adsorption chromatography or membrane separation may be utilized to perform this separation. Once o-xylene has been initially separated from m-xylene and p-xylene in the foregoing manner, the remaining m- xylene and p-xylene become more readily separable from each other, such as through simulated moving bed chromatography or crystallization, as discussed in further detail below. Moreover, the resulting m-xylene and p-xylene mixture may be suitable for undergoing oxidation, either with a conventional, non-selectivated oxidation catalyst (if producing a mixture of isophthalic acid and predominantly terephthalic acid) or a selectivated oxidation catalyst (if producing a product stream comprising predominantly terephthalic acid or a precursor thereof). Any xylene isomers (z.e., o-xylene or m-xylene) not undergoing oxidation to form a benzenedicarboxylic acid product may be recovered and isomerized to an equilibrium xylenes mixture, if desired, thereby facilitating more complete conversion of the initial aromatic hydrocarbon feed into products.

[0046] In addition, the styrene byproduct arising from toluene alkylation with methanol and/or dimethyl ether, a potential source for forming ethylbenzene, tends to co-distill predominantly with o-xylene in the bottoms fraction when separating xyl ene isomers from one another in accordance with the disclosure above. Thus, toluene alkylation processes employing o-xylene separation may largely avoid problematic levels of ethylbenzene and/or styrene in combination with p-xylene in the overhead fraction, where the styrene might otherwise compromise the separation of p-xylene. If ethylbenzene and/or styrene is/are formed in excessive quantities before conducting an oxidation reaction, they may also be readily mitigated through processes discussed in further detail below.

[0047] Any o-xylene and/or m-xylene not undergoing oxidation in accordance with the foregoing may be recovered and incorporated in other value products, or these xylene isomers may undergo catalytic isomerization and/or transalkylation to promote further conversion of the initial aromatic hydrocarbon feed into p-xylene. For example, m-xylene recovered from simulated moving bed chromatography or another suitable separation technique may be of sufficient purity to serve as a precursor to isophthalic acid, or be utilized directly as a fuel component (e.g., within a high-octane motor gasoline blend or as an AvGas co-blend). If o- xylene and/or tn-xylene are recycled and undergo isomerization and/or transalkylation to increase the p-xylene output, optionally with further oxidation thereof, phenol and/or styrene therein may need to be addressed before performing the isomerization.

[0048] Before discussing more particular aspects of the foregoing in further detail, the oxidation processes of the present disclosure will be described with reference to the drawings. FIGS. 1 and 2 show system and process flow diagrams of an illustrative toluene alkylation process with methanol and/or dimethyl ether, in which various locations are indicated where oxidation of p-xylene may be conducted, either selectively or non-selectively. FIG. 3 shows a system and process flow diagram of an illustrative toluene disproportionation process, in which various locations are indicated where oxidation of p-xylene may be conducted, either selectively or non-selectively.

[0049] FIG. I is a block diagram schematically illustrating systems and processes 100 for producing xylene isomers by alkylation of lower aromatic hydrocarbons with methanol and/or dimethyl ether and subsequently oxidizing p-xylene into a product stream composing terephthalic acid or a terephthalic acid precursor according to the present disclosure. In system and process 100, lower aromatic hydrocarbon feed 102 is provided to alkylation reactor 110 containing an alkylation catalyst. Lower aromatic hydrocarbon feed 102 may comprise benzene, toluene, or a mixture of benzene and toluene. Alkylation feed 104 comprising an alkylating agent such as methanol and/or dimethyl ether is concurrently provided to alkylation reactor 110 and reacted with lower aromatic hydrocarbon feed 102 under alkylation conditions in the presence of an alkylati on catalyst. Suitable alkyl ation catalysts and alkylation condi tions are discussed further hereinbelow ⁷ . Alkylation stream 120a exits alkylation reactor 110 and comprises a mixture of xylene isomers (preferably, predominantly p-xylene), water, and unreacted lower aromatic hydrocarbons or lower aromatic hydrocarbons that have not been fully converted to C8+ aromatic hydrocarbons. Optionally, residual alkylating agent, such as methanol and/or dimethyl ether, may be present in alkylation stream 120a. If desired or needed, unreacted lower aromatic hydrocarbons and/or residual alkylating agent within alkylation stream 120a may be separated from the mixture of xylene isomers using separation sub-system 130 (which may utilize at least one column), such that lower aromatic hydrocarbon recycle stream 132 may be returned to alkylation reactor 110 in conjunction with lower aromatic hydrocarbon feed 102, and alkylating agent recycle stream 134 may be returned to alkylation reactor 110 together with alkylation feed 104. As such, lower aromatic hydrocarbon recycle stream 132 may be provided as at least a portion of lower aromatic hydrocarbon feed 102. Lower aromatic hydrocarbon recycle stream 132 may comprise a toluene-rich stream in some examples. If lower aromatic hydrocarbon feed 102 contains benzene, at least a portion of the benzene may be present in lower aromatic hydrocarbon recycle stream 132.

[0050] Alkylation stream 120b exits separation sub-system 130 and is provided to o-xylene separator 140. As discussed above, alkylation stream 120b comprises at least o-xylene, m- xylene, and p-xylene (preferably, predominantly p-xylene) and may further comprise phenol and styrene. o-Xylene separator 140 may include any column that is capable of effectively separating o-xylene from a mixture of m-xylene and p-xylene under suitable process conditions. o-Xylene separator 140 may be a column containing a plurality of trays, or the column may be a packed column. In addition, o-xylene separator 140 may be multiple columns, which may individually contain a plurality of trays, appropriate packing, or any combination thereof. When o-xylene separator 140 is a tray -based column containing a plurality of trays, a sufficient number of trays may be present to afford a desired degree of o- xylene separation based upon an amount of energy input thereto. In non-limiting examples, suitable tray-based columns may contain at least about 100 separation trays, or at least about 120 separation trays, or at least about 140 separation trays, or at least about 160 separation trays, or at least about 180 separation trays, or at least about 200 separation trays, up to a maximum number of trays that can physically be incorporated in the column.

[0051] Alternately or additionally, o-xylene separator 140 may perform separation of alkylation stream 120b (or alkylation stream I20a if separation sub-system 130 is omitted) through membrane separation, adsorption chromatography, or any combination thereof.

[0052] Alkylation stream 120b is a first exemplary location within a toluene alkylation process from which a stream comprising p-xylene may be withdrawn and converted to a terephthalic acid precursor using a selectivated oxidation catalyst, provided the o-xylene content is sufficiently low or o-xylene is removed therefrom prior to oxidation. As shown, at least a portion of alkylation stream 120b may be diverted from o-xylene separator 140 via line 180 and provided to oxidation reactor 190 having suitable oxidation conditions and containing a selectivated oxidation catalyst, as described in further detail herein. Optionally, styrene maybe removed (e.g. , through clay treatment, acid treatment, or mild hydrotreating) from alkylation stream 120b before being provided to oxidation reactor 190. Upon conducting oxidation m oxidation reactor 190, terephthalic acid precursor stream 191 and recycle stream 192 may be obtained. Recycle stream 192 is lean in p-xylene and enriched in m-xylene relative to alkylation stream 120b. Terephthalic acid precursor stream 191 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-nietliylbenzoic acid, or any combination thereof, which may then be oxidized to terephthalic acid in a subsequent oxidation step using a non-selectivated oxidation catalyst. Recycle stream 192 may be returned to o-xylene separator 140 or fed to a xylenes isomerization process, as described in further detail below,

[0053] Upon separating alkylation stream 120b (or alkylation stream 120a) in a column of o-xylene separator 140, optionally after removing styrene therefrom as described above, a mixture predominantly comprising o-xylene and C9+ aromatic hydrocarbons may be obtained as bottoms fraction 150, and a mixture comprising predominantly m-xylene and p-xylene may be obtained as overhead fraction 152. Overhead fraction 152 is subsequently provided to p- xylene separation unit 160. As such, bottoms fraction 150 may be enriched in o-xylene and lean in m-xylene and p-xylene relative to an aromatic hydrocarbon mixture within lower aromatic hydrocarbon feed 102 and within alkylation stream 120b (or alkylation stream 120a). Bottoms fraction 150 may further comprise styrene or a reacted form thereof (e.g, ethylbenzene and/or heavy aromatic hydrocarbons produced through hydrotreating or contact with a clay or an acid) and may be carried along as bottoms fraction 150 is further processed. At least a maj on ty of the styrene or a reacted form thereof in the aromatic hydrocarbon mixture may enter bottoms fraction 150, and preferably at least about 90% of the styrene or a reacted form thereof present in the aromatic hydrocarbon mixture may enter bottoms fraction 150. o- Xylene may be farther separated from bottoms fraction 150 to provide an o-xylene stream, if desired, or bottoms fraction 150 may be further processed in an alternative manner. For example, o-xylene may be separated from bottoms fraction 150 prior to conducting xylenes isomerization, as shown in FIG. 2.

[0054] Overhead fraction 152 is a second exemplary location within a toluene alkylation process from which a stream comprising p-xylene may be withdrawn and converted to a terephthalic acid precursor using a select! vated oxidation catalyst. As shown, at least a portion of overhead fraction 152 may be diverted from p-xylene separation unit 160 via line 181 and provided to oxidation reactor 193 having suitable oxidation conditions and containing a selectivated oxidation catalyst, as described in further detail herein. Upon conducting oxidation in oxidation reactor 193, terephthalic acid precursor stream 194 and recycle stream 195 may be obtained. Terephthalic acid precursor stream 194 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof, which may- then be oxidized to terephthalic, acid m a subsequent oxidation step using a non-selectivated oxidation catalyst. Optionally, a non-selectivated oxidation catalyst may be utilized in oxidation reactor 193 if an isophthalic acid/terephthalic acid mixture is the product being targeted. Recycle stream 195 is lean in p-xylene and enriched in m-xylene relative to overhead fraction 152. Recycle stream 195 may be returned to p-xylene separation unit 160 or fed to a xylenes isomerization process, as described in further detail below.

[0055] Overhead fraction 152 is provided to p-xylene separation unit 160, which may employ a simulated moving bed chromatography separation unit or a crystallization separation unit, in which m-xylene stream 162 and p-xylene stream 164 may be at least partially separated from one another. The relatively high content of p-xylene in overhead fraction 152 may significantly decrease the energy intensity and cost of subsequent separation burdens. Given the high percentage of p-xylene produced in alkylation reactor 110 when alkylating lower aromatic hydrocarbon feed 102 with methanol and/or dimethyl ether as alkylation feed 104, p- xylene stream 164 may represent a majority' stream (provided p-xylene has not been previously consumed in optional oxidation reaetor(s) 190 and/or 193) and m-xylene stream 162 may be a minority stream obtained from p-xylene separation unit 160. When p-xylene separation unit 160 comprises a simulated moving bed chromatography separation unit, m-xylene stream 162, which may be referred to as a raffinate stream, may comprise m-xylene al a high concentration such as, for example, > about 90%, > about 95%, or > about 98%, based on the total mass of m-xylene stream 162. Such a high-concentration m-xylene stream 162 can be advantageously sold as-is for use as an aviation gasoline blending component, for conversion into isophthalic acid (e.g., using a conventional oxidation catalyst), or the like. When p-xylene separation unit 160 comprises a crystallization separation unit, m-xylene stream 162, which may be referred to as a filtrate stream, may comprise m-xylene at various concentrations such as, for example, > about 60%, > about 70%, or > about 80%, based on total mass of m-xylene stream 162, in combination with some residual p-xylene. In this case, the filtrate stream may be recombined with overhead fraction 152 and subjected to separation again in p-xylene separation unit 160, or the filtrate stream may be utilized elsewhere or undergo alternative processing to obtain more p-xylene. For example, the filtrate stream may be further separated by simulated moving bed chromatography to isolate additional p-xylene, crystallized at a lower temperature to isolate additional p-xylene, or undergo isomerization to generate p-xylene in an amount above that currently present. In the case of m-xylene stream 162 comprising appreciable concentrations of both p-xylene arid m-xylene and a low concentration of o-xylene (preferably substantially free of o-xylene), m-xylene stream 162 can be advantageously used as an aviation gasoline blending stock, undergo oxidation to a blend of isophthalic acid and terephthalic, acid, or be recycled to a crystallization separation unit or a simulated moving bed chromatography separation unit to recover an additional quantity of p-xylene therefrom, which may then be further oxidized. Alternately, m-xylene stream 162 may be fed to a xylenes isomerization process, as described in further detail below.

[0056] p-Xylene stream 164 is a third exemplary location within a toluene alkylation process from which a stream comprising p-xylene may be withdrawn and converted to a stream comprising a terephthalic acid precursor (when using a selectivated oxidation catalyst) or terephthalic acid (when using a conventional oxidation catalyst). Since p-xylene stream 164 may contain minimal amounts of ethylbenzene and o-xylene, p-xylene stream 164 may be oxidized conventionally (non-selectively) to produce high-quality terephthalic acid directly. However, if desired, terephthalic acid oxidation may be conducted sequentially by first producing a terephthalic acid precursor using a selectivated oxidation catalyst and then oxidizing the terephthalic acid precursor to afford terephthalic acid. As shown in FIG, 1, at least a portion of p-xylene stream 164 may be withdrawn via line 182 and provided to oxidation reactor 196 having suitable oxidation conditions and containing a selectivated oxidation catalyst (if producing a terephthalic acid precursor) or a conventional oxidation catalyst (if producing terephthalic acid) to afford product stream 197. Terephthalic acid precursor within product stream 197 may comprise p-methylbenzyl alcohol, p-methyl benzaldehyde, p- methylbenzoic acid, or any combination thereof.

[0057] With reference to FIG. 1, it is to be understood that oxidation need not necessarily take place in each of oxidation reactors 190, 193 and 196, nor are each of these oxidation reactors necessarily present in a given embodiment of system and method 100. However, conventional oxidation or selective oxidation may take place in at least one of these locations to form a product stream comprising a terephthalic acid precursor and/or a terephthalic acidcontaining product (e.g. , terephthalic acid or a mixture of isophthalic acid and terephthalic acid) m accordance with particular production goals. In addition, a feed stream from any of the above locations may be conveyed to a common oxidation reactor, thereby providing flexibility to provide a variety of product stream compositions, rather than having a dedicated oxidation reactor for receiving a feed stream from a fixed feed location. For example, a common oxidation reactor may receive a p-xylene-containing feed from any one of the above locations, and at least one additional p-xylene-containing feed from an additional location (e.g., from a xylenes isomerization process). Moreover, depending on the amount of takeoff at each location downstream from separation sub-system 130, it is to be further recognized that o-xylene separator 140 and p-xylene separation unit 160 need not necessarily be present, for example, if the entirety of alkylation stream 120b or overhead fraction 152 is withdrawn. [0058] Lower aromatic hydrocarbon feed 102 may be produced in conjunction with a reforming process. For example, system and process 100 may be coupled to a reforming process and a xylene isomerization process, as discussed below in reference to FIG. 2. FIG. 2 is a block diagram schematically illustrating the system and process of FIG. 1 coupled to a reforming process and a xylene isomerization process. As shown in FIG. 2 for process 200, reformate feed stream 202 comprising C6+ aromatic hydrocarbons is first separated at reformate splitter 210, which may employ one or more columns, to provide C8+ aromatics stream 212 and lower aromatic hydrocarbon stream 214 as at least one of a benzene stream, a toluene stream, or a mixed benzene/toluene stream, in combination with non-aromatic byproducts. Lower aromatic hydrocarbon stream 214 may be referred to as a C7- aromatic hydrocarbon stream. Lower aromatic hydrocarbon stream 214 is then separated via extraction in unit 220 to produce stream 222 rich in non-aromatic byproducts and purified lower aromatic hydrocarbons, which may include stream 223 rich in benzene and stream 224 rich in toluene, or streams 223 or 224 may be combined and comprise a mixture rich in both toluene and benzene. Streams 223 and 224, or a combined stream rich in benzene and toluene, may then be provided as at least a portion of lower aromatic hydrocarbon feed 102 to alkylation reactor 110. Xylene isomers produced in alkylation reactor 1 10 may then be separated from one another and undergo further processing in a manner identical to that described above for system and process 100 in FIG. I.

[0059] Like in system and process 100, oxidation of p-xylene to produce a product stream comprising a terephthalic acid precursor or a terephthalic acid-containing product may occur in one or more of oxidation reactors 190, 193, or 196 (or in a common oxidation reactor fluidly connected to one or more of these locations) in system and process 200 in FIG. 2, as discussed in further detail above. Recycle streams 192 and 195 may similarly be processed in the manner described above in reference to FIG. I, but may alternately be further processed in xylenes loop 240, discussed subsequently, as may bottoms fraction 150 (containing o-xylene) and/or m- xylene stream 162 as well ,

[0060] C8+ aromatics stream 2.12 is introduced to xylenes loop 2.40 and separated via xylenes splitter 250, which may comprise at least one column, to produce overhead fraction 252 rich in C8 aromatic hydrocarbons (e.g, a mixture of o-, m- and p-xylene) and bottoms fraction 254 rich in C9+ aromatic hydrocarbons. Overhead fraction 252 may be provided to p- xylene separation unit 260, which may employ a simulated moving bed chromatography separation unit or a crystallization separation unit, similar to those described above in connection with FIG. I, to separate p-xylene stream 262 and mixed xylene stream 264 from one another. Optionally, o-xylene may bypass p-xylene separation unit 260. Mixed xylene stream 264 is then provided to isomerization unit 270, which produces an equilibrium xylene mixture under isomerization reaction conditions in the presence of a suitable isomerization catalyst. The equilibrium xylenes mixture may be provided as recycle stream 272 and returned to xylenes splitter 250 for further processing with C8+ aromatics stream 212. Small amounts of C9+ aromatic hydrocarbons produced in isomerization unit 270 are also returned to xylenes spliter 250 with recycle stream 272 and undergo separation therein into botoms fraction 254. Xylenes splitter 250, p-xylene separation unit 260, and isomerization unit 270 together define a mam portion of xylenes loop 240. If needed, sty rene and/or ethylbenzene may be at least partially removed from mixed xylene stream 264 before conducting isomerization in isomerization unit 270. Optionally, a transalkylation unit (not shown) containing a transalkylation catalyst may be present in xylenes loop 240 to convert benzene, toluene, and/or C9-i- aromatic hydrocarbons into additional xylenes.

[0061] Optionally, at least a portion of the o-xylene and/or m-xylene obtained from alkylation reactor 1 10 and separated as above (i.e. , within bottoms fraction 150, or within m- xylene stream 162, either as a raffinate stream or a crystallization stream) may be transferred to xylenes loop 240 and further processed therein, provided that phenol and/or styrene is/are at least partially removed therefrom prior to being introduced to xylenes loop 240. For example, styrene in bottoms fraction 150 may be treated with clay or an acidic catalyst to convert the styrene into heavy aromatic compounds (or hydrogenated to produce ethylbenzene if no oxidation to terephthalic acid or a precursor thereof is to be conducted in xylenes loop 240, or if ethylbenzene may otherwise be removed prior to oxidation), and phenol in m-xylene stream 162, if present, may be removed by passage through a fixed bed of sorbent, such as alumina, and/or treatment with caustic. Styrene or heavy aromatic compounds formed from styrene may be removed using an additional column prior to being introduced to xylenes loop 240 (additional column not shown), or removal may take place in botoms fraction 254. Botoms fraction 150 and/or m-xylene stream 162 may be introduced to xylenes loop 240 together or separately (separate introduction shown in FIG. 2). Moreover, although bottoms fraction 150 and/or m-xylene stream 162 are shown in FIG. 2 as being introduced to isomerization unit 270, it is to be appreciated that other points of introduction into xylenes loop 240 are possible and the depicted location should not be considered limiting. For instance, bottoms fraction 150 and/or m-xylene stream 162 may be introduced to xylenes splitter 250, by way of non-limiting example. [0062] Similarly, the xylenes within either of recycle streams 192. and 195 may be fed to xylenes loop 240, either together or separately with each other and/or together or separately with bottoms fraction 150 and/or m-xylene stream 162. Again, if needed, suitable removal of styrene (or ethylbenzene) and/or phenol may be conducted to protect the isomerization catalyst in isomerization unit 270. Processing of styrene within either of recycle streams 192 and 195 may be conducted in a manner to avoid formation of ethylbenzene (or ethylbenzene may be removed) if p-xylene being produced in xylenes loop 240 is targeted for oxidation in accordance with the disclosure herein.

[0063] Within xylenes loop 240, there are various locations at which oxidation of p-xylene may take place using a selectivated oxidation catalyst and/or a conventional oxidation catalyst. Again, it is to be recognized that oxidation need not necessarily take place at ev ery hereinafter- described location within xylenes loop 240 (or there may be a common oxidation reactor for processing a feed stream from one or more locations, including replacing one or more of oxidation reactors 190, 193, or 196), nor does oxidation necessarily need to take place at ah within xyl enes loop 240, For example, one or more mixed xylenes or a p-xylene product stream may be withdrawn from xylenes loop 240 and utilized elsewhere, such as incorporation as a gas-bl ending component, for instance. Moreover, if performed, oxidation may be performed selectively or non-selectively within xylenes loop 240 to form a terephthalic acid precursor and/or a terephthalic acid-containing product (e.g., terephthalic acid or a mixture of isophthalic acid and terephthalic acid) in accordance with particular production goals.

[0064] Overhead fraction 252 is one exemplary location from which a xylenes-containing feed may be suitably withdrawn for oxidation in accordance with the disclosure herein, provided at least a portion of the o-xylene is removed therefrom prior to oxidation. The o- xylene removed from overhead fraction 252 may be diverted directly to isomerization reactor 270, if desired. Since overhead fraction 252 contains mixed xylene isomers, oxidation may be conducted using a selectivated oxidation catalyst, as described further herein. As shown in FIG. 2, at least a portion of overhead fraction 252 may be diverted from p-xylene separation unit 260 and converted to a product stream comprising a terephthalic acid precursor using a selectivated oxidation catalyst. Namely, at least a portion of overhead fraction 252 may be diverted from p-xylene separation unit 260 via line 280 and provided to oxidation reactor 290 having suitable oxidation conditions and containing a selectivated oxidation catalyst, as described in further detail herein. Upon conducting oxidation in oxidation reactor 2.90, terephthalic acid precursor stream 291 and recycle stream 292 may be obtained. Recycle stream 292 is lean in p-xylene and enriched in m-xylene relative to overhead fraction 252. Terephthalic acid precursor stream 291 may comprise p-methy [benzyl alcohol, p- methylbenzaldehyde, p-methyl benzoic acid, or any combination thereof, which may then be oxidized to terephthalic acid in a subsequent oxidation step using a non-selectivated oxidation catalyst. Recycle stream 292 may be returned to xylenes loop 240 for re-isomerization of the m-xylene into an equilibrium xylenes mixture.

[0065] Similarly, the equilibrium xylenes mixture within recycle stream 272 is another exemplary location from which a xylenes-con taming feed may be suitably withdrawn for oxidation in accordance with the disclosure herein, provided at least a portion of the o-xylene is removed therefrom prior to oxidation. o-Xylene removed may be returned to isomerization reactor 270 for further conversion. Since recycle stream 272 contains mixed xylene isomers, oxidation may be conducted using a selectivated oxidation catalyst, as described further herein. As shown in FIG. 2, at least a portion of recycle stream 272 may be diverted from xylenes splitter 250 and converted to a product stream comprising a terephthalic acid precursor using a selectivated oxidation catalyst. Namely, at least a portion of recycle stream 272 may be diverted from xylenes splitter 250 via line 281 and provided to oxidation reactor 293 having suitable oxidation conditions and containing a selectivated oxidation catalyst, as described m further detail herein. Upon conducting oxidation in oxidation reactor 293, terephthalic acid precursor stream 294 and recycle stream 295 may be obtained. Recycle stream 295 is lean in p-xylene and enriched in m-xylene relative to recycle stream 272. Terephthalic acid precursor stream 294 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof, which may then be oxidized to terephthalic acid in a subsequent oxidation step using a non-selectivated oxidation catalyst. Recycle stream 295 may be relumed to xylenes loop 240 for re-isomerization of the m-xylene into an equilibrium xylenes mixture.

[0066] p-Xylene stream 262 is yet another exemplary location from which a xylenes- containing feed may be suitably withdrawn for oxidation in accordance with the disclosure herein. Since p-xylene stream 262 is enriched in p-xylene, oxidation may be conducted conventionally to form a product stream comprising high-quality terephthalic acid. However, if desired, terephthalic acid oxidation may be conducted sequentially by first producing a terephthalic acid precursor and then oxidizing the terephthalic acid precursor to afford terephthalic acid. As shown in FIG. 2, at least a portion of p-xylene stream 262. may be provided to oxidation reactor 296 having suitable oxidation conditions and containing a selectivated oxidation catalyst (if producing a terephthalic acid precursor) or a conventional oxidation catalyst (if producing terephthalic acid) to afford product stream 297. The terephthalic acid precursor within product stream 297 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof.

[0067] In still another example, a selectivated oxidation catalyst may be used in conjunction with oxidizing p-xylene or a mixture of xylene isomers obtained from a toluene disproportionation process. FIG. 3 is a block diagram schematically illustrating systems and processes for producing xylene isomers by disproportionation of toluene and subsequently oxidizing p-xylene into a product stream comprising terephthalic acid or a terephthalic acid precursor according to the present disclosure. As shown, system and process 300 provides toluene feed 302 to disproportionation reactor 310 containing a disproportionation catalyst and suitable disproportionation conditions. Upon undergoing disproportionation, toluene is converted into a mixture of xylene isomers, benzene and light gases. Benzene stream 312, containing the benzene and light gases, is separated from xylenes mixture 314, which is provided to xylene splitter 320 for further separation thereof. Recycle stream 316 is returned to disproportionation reactor 310 or toluene feed 302 entering disproportionation reactor 310. Xylenes mixture 314 may contain ethylbenzene prior to being provided to xylenes splitter 320, but the ethylbenzene may be removed or decreased to acceptable levels by membrane separation, simulated moving bed chromatography, or through selective conversion to benzene. Thus, xylenes mixture 314 may contain a low level of ethylbenzene and be suitable for processing according to the disclosure herein. Xylene splitter 320 may comprise at least one column to produce overhead fraction 322 rich in m- and p-xylene and bottoms fraction 324 rich in o-xylene. Additional separation details may be similar to those described above for xylene separator 140 (FIG. 1).

[0068] Overhead fraction 322 may be provided to p-xylene separation unit 330, which may employ a simulated moving bed chromatography separation unit or a crystallization separation unit similar to those described above in connection with FIG. 1, to separate p-xylene stream 332 and m-xylene stream 334 from one another. m-Xylene stream 334 may comprise at least some p-xylene in the event a crystallization separation unit is used. At least a portion of p- xylene stream 332. may be provided to oxidation reactor 340 having suitable oxidation conditions and containing a selectivated oxidation catalyst (if producing a terephthalic acid precursor) or a conventional oxidation catalyst (if producing terephthalic acid) to afford product stream 342. The terephthalic acid precursor wi thin product stream 342 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof. [0069] m-Xylene stream 334 may be provided to xylenes loop 350, such as to isomerization unit 352 within xylenes loop 350. o-Xylene within bottoms fraction 324 may also be provided to xylenes loop 350 as well. Isomerization unit 352 converts the o-xylene and m-xylene into an equilibrium xylene mixture under isomerization reaction conditions in the presence of a suitable isomerization catalyst, in a similar manner to that discussed above in connection with FIG. 2. Again, a transalkylation reactor containing a transalkylation catalyst may also be present.

[0070] The equilibrium xylenes mixture may be provided as recycle stream 354 to xylenes splitter 360, which may separate the xylenes as overhead fraction 362 from byproducts 361 formed during isomerization (e.g.. C9+ aromatics). Optionally, o-xylene may be excluded from overhead fraction 362 and provided directly to isomerization unit 352, such that the o- xylene bypasses p-xylene separation unit 370, Overhead fraction 362 may then be provided to p-xylene separation unit 370, which may employ a simulated moving bed chromatography separation unit or a crystallization separation unit similar to those described above in connection with FIGS. 1 and 2, to separate p-xylene stream 372 and m-xylene stream 374 from one another. m-Xylene stream 374 is then provided to isomerization unit 352 to continue with production of additional xylenes.

[0071] As with the toluene alkylation processes discussed above in connection with FIGS. 1 and 2, there are various locations at which oxidation of p-xylene may take place in system and process 300 using a selectivated oxidation catalyst and/or a conventional oxidation catalyst. Again, it is to be recognized that oxidation need not necessarily take place at every or any of the hereinafter-described locations, nor are all of the oxidation reactors necessarily present (or a common oxidation reactor may receive a feed from one or more various locations).

[0072] Oxidation of p-xylene stream 332 in oxidation reactor 340 was addressed previously, wherein a selectivated oxidation catalyst (if producing a terephthalic acid precursor) or a conventional oxidation catalyst (if producing terephthalic acid) may be used to afford product stream 342. The terephthalic acid precursor may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof.

[0073] In another example, selective oxidation may take place before separating p-xylene from overhead fraction 322, As shown, at least a portion of overhead fraction 322 may be provided to oxidation reactor 324 containing oxidation conditions and a suitable oxidation catalyst to form product stream 326 and recycle stream 328. Product stream 326 may comprise a terephthalic acid precursor if a selectivated oxidation catalyst is used. The terephthalic acid precursor within product stream 326 may comprise p-methylbenzyl alcohol, p- methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof, which may then be oxidized to terephthalic acid in a subsequent oxidation step using a non-selectivated oxidation catalyst. Recycle stream 328 is lean in p-xylene relative to overhead fraction 322 and may be returned to overhead fraction 322 for further processing as discussed herein. Alternately, recycle stream 328 may be provided directly (not shown) to xylenes loop 350.

[0074] In another example, selective oxidation may take place before providing m-xylene stream 334 to xylenes loop 350, provided that m-xylene stream 334 contains at least some p- xylene. As shown, at least a portion of m-xylene stream 334 may be provided to oxidation reactor 335 containing oxidation conditions and a suitable oxidation catalyst to form product stream 336 and recycle stream 337. Product stream 336 may comprise a terephthalic acid precursor if a select!vated oxidation catalyst is used. The terephthalic acid precursor within product stream 336 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p- methyibenzoic acid, or any combination thereof, which may then be oxidized to terephthalic acid in a subsequent oxidation step using a non-selectivated oxidation catalyst. Recycle stream 337 may be enriched in m-xylene relative to m-xylene stream 334 and may be provided to xylenes loop 350 in combination with m-xylene stream 334 for further processing m accordance with the disclosure herein.

[0075] Oxidation to a terephthalic acid precursor and/or terephthalic acid may also take place within xylenes loop 350. It is again to be recognized the oxidation need not necessarily take place at each depicted location in FIG. 3 (or a common oxidation reactor may receive a feed mixture from one or more locations), nor does oxidation necessarily take place at all in xylenes loop 350. Alternately, one or more mixed xylenes or p-xylene may be withdrawn from xylenes loop 350 and utilized elsewhere, for instance as a gas blending component.

[0076] Overhead fraction 362 is one exemplary location from which a xylenes-contaming feed may be suitably withdrawn for oxidation in accordance with the disclosure herein. Any o-xylene removed from overhead fraction 362 (removal not shown) may bypass p-xylene separator 370 and proceed directly to isomerization reactor 352. Since overhead fraction 362 contains mixed xylene isomers, oxidation may be conducted using a selectivated oxidation catalyst, as described further herein, provided at least a portion of the o-xylene is removed therefrom prior to oxidation. As shown in FIG. 3, at. least a portion of overhead fraction 362 may be diverted from p-xylene separation unit 370 and converted to a terephthalic acid precursor using a selectivated oxidation catalyst. Namely, at least a portion of overhead fraction 362 may be diverted from p-xylene separation unit 370 via line 380 and provided to oxidation reactor 382 having suitable oxidation conditions and containing a selectivated oxidation catalyst, as described in further detail herein. Upon conducting oxidation in oxidation reactor 382, terephthalic acid precursor stream 383 and recycle stream 384 may be obtained. Recycle stream 384 is lean in p-xylene and enriched in m-xylene relative to overhead fraction 362. Terephthalic acid precursor stream 383 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof, which may then be oxidized to terephthalic acid in a subsequent oxidation step using a non-selectivated oxidation catalyst. Recycle stream 384 may be returned to xylenes loop 350 for re-isomerization of the o-xylene and m-xylene into an equilibrium xylenes mixture.

[0077] Similarly, the equilibrium xylenes mixture within recycle stream 354 is another exemplary location from which a xylenes-containing feed may be suitably withdrawn for oxidation in accordance with the disclosure herein, provided at least a portion of the o-xylene is removed therefrom prior to oxidation. o-Xylene removed from recycle stream 354 may be returned to isomerization reactor 352 for re-isomerization. Since recycle stream 354 contains mixed xylene isomers, oxidation may be conducted using a selectivated oxidation catalyst, as described further herein. As shown in FIG. 3, at least a portion of recycle stream 354 may be diverted from xylenes splitter 360 and converted to a terephthalic acid precursor using a selectivated oxidation catalyst. Namely, at least a portion of recycle stream 354 may be diverted from xylenes splitter 360 via line 390 and provided to oxidation reactor 391 having suitable oxidation conditions and containing a selectivated oxidation catalyst, as described in further detail herein. Upon conducting oxidation in oxidation reactor 391, terephthalic acid precursor stream 392 and recycle stream 393 may be obtained. Recycle stream 393 is lean in p-xylene and enriched in m-xylene relative to recycle stream 354. Terephthalic acid precursor stream 392 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof, which may then be oxidized to terephthalic, acid in a subsequent oxidation step using a non-selectivated oxidation catalyst. Recycle stream 393 may be returned to xylenes loop 350 for re-isomerization of the m-xylene into an equilibrium xylenes mixture.

[0078] p-Xylene stream 372 is yet another exemplary location from which a xylenes- containing feed may be suitably withdrawn for oxidation in accordance with the disclosure herein. Since p-xylene stream 372 is enriched in p-xylene, oxidation may be conducted conventionally to produce high-quality terephthalic acid. However, if desired, oxidation to terephthalic acid may be conducted sequentially by first producing a terephthalic acid precursor and then oxidizing the terephthalic acid precursor to afford terephthalic acid. As shown in FIG. 3, at least a portion of p-xylene stream 372 may be provided to oxidation reactor 374 having suitable oxidation conditions and containing a selectivated oxidation catalyst (if producing a terephthalic acid precursor) or a conventional oxidation catalyst (if producing terephthalic acid) to afford product stream 376. The terephthalic acid precursor (if formed) in product stream 376 may comprise p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof.

[0079] Both conventional (non-selectivated) and selectivated oxidation catalysts suitable for use m the disclosure herein may feature similar inorganic frameworks incorporating at least one oxidizing metal element. The at least one oxidizing metal element may be present as a framework atom in the inorganic framework or be incorporated within one or more pores defined within the inorganic framework. The non-selectivated and selectivated oxidation catalysts may differ from one another in that at least a portion of an outer surface of the selectivated oxidation catalysts may be coated with at least one passivating agent or otherwise deactivated with an agent that promotes passivation (e.g, through steaming). The at least one passivating agent may temper reactivity of the oxidation catalyst and allow p-xylene oxidation to occur selectively to form p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, or any combination thereof.

[0080] Tire inorganic frameworks of the oxidation catalysts may comprise one or more zeolite frameworks. Any zeolite framework capable of incorporating an oxidizing metal element may be suitable for use as an oxidation catalyst in the disclosure herein. The oxidizing metal element may be its zero-valent state or in a salt form. In particular examples, the zeolite may have an intermediate pore size of about 5-7 A, such as zeolites having AEL, AFI, MWW, MFI, MEL, MFS, MEI, MTW, EUO, MTT, HEU, PER, or TON inorganic frameworks. Examples of suitable intermediate pore size zeolites include ZSM-5, ZSM-1 1, ZSM-12, ZSM- 22, ZSM-23, ZSM-34, ZSM-35, ZSM-38, ZSM-48, ZSM-50, ZSM-57, MCM-22, MCM-49, MCM-56, and S APO-5, SAPO-1 1, silicalite 1, and silicalite 2. Titanosilicates, gallosilicates, aluminosilicates, and gallium-containing aluminosilicate zeolites having a MFI structure may be utilized in any of the selectivated or non-selectivated oxidation catalysts disclosed herein.

[0081] The oxidation catalysts of either type may be formulated with a binder, or the oxidation catalyst may be an unbound free powder. The binder may comprise a binder material resistant to the temperature and other conditions employed during the oxidation reaction conducted therewith. Examples of suitable binder materials include clays, alumina, silica, silica-alumina, silica-magnesia, silica-zirconia, silica-thona, silica- beryllia, and silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica- alumina-magnesia and silica-magnesia-zirconia The zeolite bearing the at least one oxidizing metal element may also be blended with a second zeolite as a binder material, thereby forming a zeolite-bound zeolite, as described in U.S. Patent Nos. 5,993,642 and 5,994,603 and each incorporated herein by reference. The relative proportions of zeolite and binder material may range from about 1 :99 to about 99: 1 on a mass basis. In illustrative examples, the zeolite bearing the at least one oxidizing metal element may be present in an amount of 10% to about 70% by mass of the zeolite-bound zeolite, or about 26% to about 50% by mass of the zeolitebound zeolite.

[0082] Oxidizing metal el ements may include any metal or metal ion that may interact with a methyl group of one or more xylene isomers to convert the methyl group into an oxygenated functional group (/%., an alcohol, an aldehyde, and/or a carboxylic acid). In non-limiting examples, suitable oxidizing metal elements may include, transition metals and certain rare earth metals. Example oxidizing metal elements that may be present in the oxidation catalysts include, but are not limited to, Ti, Co, Mn, Cu, Fe, Ru, Rh, Ce, or any combination thereof. The oxidizing metal element may be present in the oxidation catalysts in an amount ranging from about 0. 1 wt% to about 4 wt%, for example.

[0083] Selectivated oxidation catalysts may be prepared by treating any of the foregoing non-selectivated oxidation catalysts with a passivating agent that forms a surface coating upon at least a portion of an outer surface of the oxidation catalysts or a bound form thereof (i.e., a catalyst extrudate) or an agent that otherwise modifies outer surface of the oxidation catalysts (e.g., steaming). In more particular examples, the selectivated oxidation catalyst may have a framework comprising one or more of a silicalite (e.g. , titanium silicalite), a 10-membered ring zeolite containing an oxidizing metal element in a pore thereof, or any combination thereof, and in which at least a portion of an outer surface of the framework is passivated with a passivating agent, including steaming in some examples. In at least one example, the 10- membered ring zeolite may have an MFI framework, such as ZSM-5.

[0084] Any passivating agent capable of tempering the activity of the oxidation catalyst to confer selectivity toward p-xylene oxidation may be suitably used in the disclosure herein. Illustrative examples of suitable passivating agents may include at least one substance selected from coke, a phosphorus compound, a silicon compound, a magnesium compound (e.g. , MgO), a metal oxide, or any combination thereof For example, select! vati on may be conducted with a silicon compound in combination with a magnesium compound (or other alkali metal compound) and/or a phosphorus compound. Steaming may also serve in a passivating agent capacity. [0085] Coke selectivation may be conducted by exposing the oxidation catalyst a thermally decomposable organic compound (e.g. , benzene, toluene, or the like) at a temperature in excess of the decomposition temperature of the organic compound. In illustrative embodiments, the decomposition temperature at which coke deposition occurs may range from about 400°C to about 650°C or about 425 °C to about 550°C. Coke passivation may take place at a WHSV of about 0. 1 to about 20 pounds of feed per pound of oxidation catalyst per hour, at a pressure of about 1 to about 100 atmospheres, and in the presence of 0 to about 2 moles of hydrogen per mole of decomposable organic compound, such as about 0. 1 to about 1 moles of hydrogen per mole of organic compound. Optionally, 0 to about 10 moles of nitrogen or another inert gas per mole of decomposable organic compound may be present during coke passivation. This process may be conducted for a period of time until a suffici ent quantity of coke has deposited on the catalyst surface, which may be at least about 2 wt% and, more preferably, about 8 wt% to about 40 wt% coke.

[0086] A silicon compound may also be used to selectivate the catalyst. The silicon compound may be converted through calcination to a silica coating upon the catalyst after deposition thereon. The silicon compound may comprise a silicone compound (a poly siloxane), a siloxane compound, silane compound such as a disilane or alkyoxysilane, or any combination thereof Suitable polysiloxanes may have a formula of (SiRl R2O)n, wherein R1 and R2 are the same or different and are selected independently from hydrogen, fluorine, hydroxy, alkyl, aryl, aralkyl, alkaryl, or fluoroalkyl. Hydrocarbyl groups may contain from 1 to about 10 carbon atoms and preferably are methyl or ethyl groups. Variable n is an integer of at least 2 and generally in the range of 2 to about 1000, wherein the molecular weight of the silicone may range from about 80 to about 20,000 or about 150 to about 10,000. The silicone may be linear or cyclic. Representative silicones and polymerized or oligomerized forms thereof may include, for example, dimethylsilicone, di ethylsilicone, phenylmethylsilicone, methylhydrogensilicone, ethylhydrogen silicone, phenylhydrogensilicone, fluoropropylsilicone, ethyltrifluoropropylsilicone, tetrachlorophenylmethylsilicone, methylethylsilicone, phenylethylsilicone, diphenylsilicone, niethyltrisilicone, tetrachldrophenylethyl silicone, methylvinylsilicone, ethylvinylsilicone, or the like. Representative cyclic silicones may include, but are not limited to, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, hexaphenylcyclotrisiloxane, methyltrisilicone, and octaphenylcyclotetrasiloxane. Other representative siloxanes and polymerized or oligomerized variants thereof include, for example, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, hexamethyldisiloxane, octamethytrisiloxane, decamethyltetrasiloxane, hexaethylcyclotrisiloxane, octaethylcyclotetrasiloxane, hexaphenylcyclotrisiloxane and octaphenylcyclo-tetrasiloxane.

[0087] Suitable silanes and disilanes may include, but are not limited to, dimethylphenylsilane, phenytrimethylsilane, triethylsilane, and hexamethyldisilane. Suitable alkoxysilanes may include those with at least one silicon-hydrogen bond.

[0088] Suitable metal oxides that may be utilized to selectivate the outer surface of the oxidation catalyst include, but are not limited to, alkaline earth metal oxides (e.g., calcium oxide, magnesium oxide, and the like), rare earth metal oxides (e.g. , lanthanum oxide and other +3 rare earth metal oxides), and other metal oxides such as titania, magnesia, boron oxide, antimony oxide, and the like. The metal oxide may be formed from a precursor that is deposited upon the outer surface of the oxidation catalyst and subsequently calcined.

[0089] Loading of the passivating agent upon the oxidation catalyst may range from about 0. 1 wt% to about 40 wt%, or 0. 1 wt% to about 25 or about 1 wt% to about 25 wt%, or about 1 wt% to about 15 wt%. A given select! vation technique may be conducted one time or multiple times to deposit a coating having a desired weight upon the oxidation catalyst. Any of the various types of passivating agents may be calcined after being deposited upon the outer surface of the oxidation catalysts.

[0090] It is to be appreciated that passivation of the oxidation catalyst is not limited to the foregoing types of passivation. Described further below are passivating agents for conducting passivation upon other types of catalysts. Any of these alternative passivating agents may be utilized for producing a selectivated oxidation catalyst suitable for use in the disclosure herein. Furthermore, the passivating agents described above may be utilized to passivate other types of catalysts as well.

[0091] Suitable oxidation conditions employed when utilizing a selectivated oxidation catalyst or a non-selectivated oxidation catalyst may be similar. In non-limiting examples, suitable oxidation conditions may include exposure to an oxidizing agent (in addition to the at. least one oxidizing metal element in the oxidation catalyst) at a temperature ranging from about room temperature up to about 200°C, Oxidizing agents that may be utilized in this regard may include, but are not limited to, air, hydrogen peroxide, organic peroxides, organic hydroperoxides, hypohalite compounds and salts thereof (HOX, X C i . Br, or I), halic acid compounds and salts thereof (HX03, X=C1, Br, or I) perhalic acid compounds and salts thereof (HX04, X—Cl, Br, or I), or any combination thereof. In addition, the oxidation conditions may be such that the oxidation reaction takes place substantially in a liquid phase. [0092] In some embodiments, oxidation of C8 aromatic compounds using a selectivated oxidation catalyst and isomerization of any C8 aromatic compounds not undergoing oxidation (e.g., o-xylene and/or m-xylene) may take place in a single reactor. For example, a single reactor may contain both a selectivated oxidation catalyst and an isomerization catalyst, wherein the conditions in the reactor are suitable for conducting both oxidation and isomerization. As p-xylene is converted into oxygen-containing C8 aromatic compounds in the reactor, the oxygen-containing C8 aromatic compounds may be rendered less susceptible toward isomerization. Unreacted o-xylene and/or m-xylene may then undergo isomerization to afford an equilibrium xylenes mixture, from which additional p-xylene may be consumed to produce additional oxygen-containing C8 aromatic compounds. Coupling selective oxidation and isomerization together in this manner may drive the combined reaction process toward desired products derived from p-xylene.

[0093] Accordingly, the present disclosure provides processes for oxidizing C8+ aromatic compounds selectively to afford one or more oxygen-containing C8 aromatic compounds. Such processes for oxidizing C8+ aromatic compounds may comprise: providing a C8+ aromatic hydrocarbon feed comprising at least p-xylene; optionally, separating ethylbenzene, if any, from the C8+ aromatic hydrocarbon feed; exposing at least a portion of the C8+ aromatic hydrocarbon feed to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxygen-containing C8 aromatic compounds; wherein the selectivated oxidation catalyst is ineffective to oxidize m-xylene and o-xylene, if present, and the one or more oxygen-containing C8 aromatic compounds comprise a compound selected from the group consisting of p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, and any combination thereof; and forming the one or more oxygen-containing C8 aromatic compounds from the C8+ aromatic hydrocarbon feed under the oxidation conditions. The processes may further comprise separating the product stream to obtain a stream rich in the one or more oxygen-containing C8 aromatic compounds and a stream lean in the one or more oxygen-containing C8 aromatic compounds. The product stream may be further separated from the C8+ aromatic hydrocarbon feed. Separation of the one or more oxygen- containmg C8 aromatic compounds may take place by one or more of distillation, crystallization, acid-base extraction, complex formation, or any combination thereof.

[0094] The C8+ aromatic hydrocarbon feed may be derived from any source containing p- xylene or from which p-xylene may be fonned, including those containing impurities that may interfere with the oxidation chemistry, provided that any interfering impurities may be removed prior to conducting an oxidation reaction. For example, an otherwise unsuitable C8+ aromatic hydrocarbon feed containing ethylbenzene may be rendered suitable by removing at least a portion of the ethylbenzene, such as about 2000 ppm or less by mass, preferably below about 1000 ppm by mass. In non-limiting embodiments, the C8+ aromatic compounds may be present in or produced from a reformate process, a xylenes isomerization process, a toluene disproportionation process, a toluene alky lation process (including toluene alky lation processes with methanol and/or dimethyl ether), a transalkylation process, a cracking process, or any combination thereof In some embodiments, a fraction at least partially enriched in p-xylene may be produced from any of the foregoing, which may offer a processing advantage of having to expose a lower volume of C8+ aromatic hydrocarbon feed to the selectivated oxidation catalyst to afford a specified amount of oxidation product. The fraction enriched in p-xylene may be lean in o-xylene or substantially free of o-xylene, according to some process configurations.

[0095] According to some embodiments, processes of the present disclosure may comprise contacting toluene with a disproportionation catalyst under disproportionation conditions to produce a disproportionation mixture comprising o-xylene, m-xylene, p-xylene, benzene, and C9+ aromatic hydrocarbons; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the disproportionation mixture.

[0096] According to some embodiments, processes of the present disclosure may comprise forming an isomerized stream comprising at least p-xylene under catalytic isomerization conditions from a p-xylene-lean stream; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from die isomerized stream.

[0097] According to some embodiments, the C8+ aromatic hydrocarbon feed may comprise mixed xylenes comprising o-xylene, m-xylene, and p-xylene. Such feeds may be preferably oxidized with a selectivated oxidation catalyst, described further herein, to avoid unwanted conversion of o-xylene and m-xylene to benzenedicarboxylic acid compounds isomeric with terephthalic acid. More preferably, at least a portion of the o-xylene may be removed prior to exposing the p-xylene to oxidation conditions. When the C8+ aromatic hydrocarbon feed comprises predominantly or solely p-xylene as the only xylene isomer, oxidation may be conducted conventionally with a non-selectivated oxidation catalyst, or, if desired, with a selectivated oxidation catalyst. The oxidation of p-xylene may be conducted in two steps with a selectivated oxidation catalyst and a non-selectivated oxidation catalyst if purification may be conducted more effectively before terephthalic acid has been formed. That is, processes of the present disclosure may comprise further oxidizing the one or more oxygen- containing C8 aromatic compounds to a second product stream comprising terephthalic acid, such as terephthalic acid alone or a mixture of isophthalic acid and terephthalic acid.

[0098] According to some embodiments, the C8+ aromatic hydrocarbon feed may comprise a mixture of p-xylene and m-xylene in an o-xylene-lean stream Such C8+ aromatic hydrocarbon feeds may be obtained from any suitable source, but may preferably be obtained from a toluene and/or benzene alkylation process with methanol and/or dimethyl ether, given the relatively low amounts of o-xylene and other byproducts typically produced in such processes. The low amount of o-xylene may facilitate a decrease in the size of separation columns and plates needed in a xylene splitter to provide a mixture of p-xylene and m-xylene suitable for undergoing oxidation, optionally after further separation thereof. Accordingly, processes of the present disclosure may comprise: separating an o-xylene-lean stream an d an o-xylene-rich stream from the C8+ aromatic hydrocarbon feed, the o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene; wherein the o-xylene-lean stream is exposed to the oxidation conditions and the selectivated oxidation catalyst, optionally after further separation thereof.

[0099] Separation of the aromatic hydrocarbon mixture may take place via any suitable technique sufficient to separate o-xylene from a mixture of m-xylene and p-xylene. In illustrative embodiments, separating the aromatic hydrocarbon mixture may take place in a distillation column, such that o-xylene is obtained within a bottoms fraction and a mixture of m-xylene and p-xylene is obtained as an overhead fraction. In illustrative embodiments, the distillation column for separating o-xylene from the mixture of m-xylene and p-xylene may be a tray-based distillation column comprising at least about 140 separation trays.

[0100] Other suitable techniques for separating o-xylene from a mixture of m-xylene and p-xylene may include, for example, absorption chromatography, membrane separation, or the like.

[0101] The C8+ aromatic hydrocarbon feed may comprise ethylbenzene at a concentration that does not interfere with the oxidation reaction to produce the one or more oxygencontaining C8 aromatic compounds. Preferably, the C8+ aromatic hydrocarbon feed comprises ethylbenzene at a concentration no greater than about 1000 ppm, based on total mass of C8 aromatic hydrocarbons in the C8+ aromatic hydrocarbon feed, when exposed to the oxidation conditions and the selectivated oxidation catalyst. If the C8+ aromatic hydrocarbon feed does not already contain a concentration of ethylbenzene below this level, the processes described herein may further comprise separating at least a portion of the ethylbenzene from the C8+ aromatic, hydrocarbons to a suitably low level, preferably below about 1000 ppm, more preferably below about 500 ppm or below about 100 ppm.

[0102] In some embodiments, at least a portion of the C8+ aromatic hydrocarbons may be obtained from a toluene and/or benzene alkylation process with methanol and/or dimethyl ether. Accordingly, such processes may further comprise: contacting toluene and/or benzene with an alkylating agent comprising methanol and/or dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions to produce an alkylation mixture comprising at least about 50 wt% p-xylene, based on total mass of the alkylation mixture; and obtaining at least a portion of the C8-i- aromatic hydrocarbon feed from the alkylation mixture.

[0103] Preferably, the toluene and/or benzene provided to a toluene alkylation process may be obtained from a reformate stream comprising C6+ aromatic hydrocarbons, and the reformate stream is further separated into a raffinate stream comprising C8+ aromatic hydrocarbons, the C8+ aromatic hydrocarbons comprising at least a mixture of o-xylene, m-xylene, and p-xylene. Processes of the present disclosure may further comprise separating an intermediate stream comprising at least a portion of the p-xylene from the raffmate stream. Further, processes of the present disclosure may comprise optionally, separating ethylbenzene, if any, from the raffinate stream or the intermediate stream; exposing the raffinate stream or the intermediate stream to the oxidation conditions and the selectivated oxidation catalyst (after moving o- xylene therefrom, if needed), the oxidation conditions and the selectivated catalyst to which the raffmate stream or the intermediate stream is exposed being the same as or different than the oxidation conditions and the selectivated catalyst to which the C8+ aromatic hydrocarbon feed is exposed; and forming at least some of the one or more oxy gen-containing C8 aromatic compounds from the raffinate stream or the intermediate stream under the oxidation conditions. Still further, processes of the present disclosure may comprise: separating the one or more oxy gen-con raining C8 aromatic compounds from the raffinate stream or the intermediate stream; obtaining a p-xylene-lean stream after exposing the raffinate stream or the intermediate stream to the oxidation conditions and the selectivated oxidation catalyst; exposing the p- xylene-lean stream to catalytic isomerization conditions to obtain an isomerized stream comprising p-xylene at a higher concentration than in the p-xylene-lean stream; and recycling the isomerized stream to the oxidation conditions and the selectivated oxidation catalyst m combination with the raffinate stream or the intermediate stream. At least a portion of the C8+ aromatic hydrocarbon feed may also be obtained from the isomerized stream. [0104] The C8+ aromatic hydrocarbon feed may comprise o-xylene at various concentrations ranging from cl wt% to c2 wt%, based on the total mass of the aromatic hydrocarbon mixture, where cl and c2 can be, independently, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 80, as long as cl < c2. Desirably, at least a majority, preferably > about 55%, preferably > about 60%, preferably > about 65%, preferably > about 70%, preferably > about 75%, preferably > about 80%, preferably > about 85%, preferably > about 90%, preferably > about 91 %, preferably > about 92%, preferably > about 93%, preferably > about 94%, preferably > about 95%, preferably > about 96%, preferably > about 97%, preferably > about 98%, preferably > about 99%, of the o-xylene present in the C8+ aromatic hydrocarbon feed may be obtained in a bottoms fraction when separating o-xylene from m-xylene and p-xylene prior to oxidation.

[0105] When present in the C8+ aromatic hydrocarbon feed (e.g., in an aromatic hydrocarbon feed resulting from toluene alkylation with methanol and/or dimethyl ether), at least a majority of the styrene and/or C9+ aromatic hydrocarbons may enter the bottoms fraction with o-xylene when performing a separation by distillation. In illustrative embodiments, the C8+ aromatic hydrocarbon feed obtained from toluene alkylation may comprise up to about 5000 ppm styrene by weight, such as about 10 ppm to about 5000 ppm styrene by weight or about 1000 ppm to about 2000 ppm styrene by weight, based on total mass of the C8+ aromatic hydrocarbon feed Desirably, at least a majority, preferably > about 55%, preferably > about 60%, preferably > about 65%, preferably > about 70%, preferably > about 75%, preferably > about 80%, preferably > about 85%, preferably > about 90%, preferably > about 91%, preferably > about 92%, preferably > about 93%, preferably > about 94%, preferably > about 95%, preferably > about 96%, preferably > about 97%, preferably > about 98%, preferably > about 99%, of the styrene present in the C8+ aromatic hydrocarbon feed may enter the bottoms fraction with o-xylene when performing a separation by distillation. Preferably, the bottoms fraction may comprise about 50 ppm styrene by weight or less, preferably < about 40 ppm, preferably < about 30 ppm, preferably < about 20 ppm, preferably < about 10 ppm, or preferably ri about 5 ppm, based on total mass of the bottoms fraction.

[0106] If desired, o-xylene may be separated from the bottoms fraction as a value material . In particular, the bottoms fraction obtained from distillation, may be separated into a stream enriched in o-xylene and lean in C9+ aromatic hydrocarbons and styrene relative to the bottoms fraction. Separation into a stream enriched in o-xylene may be conducted via distillation, adsorption chromatography, or the like, in non-limiting examples. When separated according to the foregoing, the stream enriched in o-xylene may comprise o-xylene in an amount of at least about 80 wt% based on total mass, or at least about 85 wt%, or at least about 90 wt%, or at least about 95 wt%, or at least about 96 wt%, or at least about 97 wt%, or at least about 98 wt%, or at least about 99 wt%. As still another option, discussed further below, the stream enriched in o-xylene may be isomerized into an equilibrium mixture of xylene isomers. C9+ aromatic hydrocarbons separated from o-xylene may be supplied to a transalkylation unit together with a lower aromatic hydrocarbon stream, from which an additional quantity of xylenes may be produced.

[0W7] A ratio of the total amount of o-xylene in the bottoms fraction to the total amount of o-xylene in the overhead fraction may range from rl to r2, where rl and r2 can be, independently, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, or 50, as long as rl < r2. Preferably rl is greater than or equal to about 1, More preferably rl is greater than or equal to about 2. Still more preferably rl is greater than or equal to about 5. Still more preferably rl is greater than or equal to about 10.

[0108] Separation of the C8t- aromatic hydrocarbons may take place using any suitable technique capable of achieving sufficient resolution of m-xylene and p-xylene from o-xylene or problematic byproducts for conducting an oxidation reaction. In some embodiments, separation may take place using one or more distillation columns by virtue of the separation in boiling points of the xylene isomers (o-xylene, m-xylene, and p-xylene have normal boiling points of 144' ^: 'C, 139°C, and 138°C, respectively). As such, the higher boiling o-xylene may be obtained within the bottoms fraction through use of a sufficiently efficient distillation column and distillation process. In addition or alternately, separation of the C8+ aromatic hydrocarbons may utilize separation techniques such, as membrane separation or adsorption chromatographic separation.

[0109] lire overhead fraction enriched in m-xylene and p-xylene and lean in o-xylene may be further processed to at least partially separate m-xylene and p-xylene. Preferably, the overhead stream may comprise o-xylene in an amount of about 1000 ppm or less by mass. The m-xylene and p-xylene in the overhead fraction may be at least partially separated from one another, if desired, or be maintained together when conducting an oxidation reaction with a selectrvated oxidation catalyst or a non-selectivated oxidation catalyst In non-limiting examples, the overhead fraction may be separated into a stream enriched in p-xylene and lean in m-xylene and a stream enriched in m-xylene and lean in p-xylene. The stream enriched m p-xylene may optionally consist essentially of p-xylene. The stream enriched in m-xylene may optionally consist essentially of m-xylene or consist essentially of a mixture of m-xylene and p-xylene. In non-limiting examples, separation of the overhead stream may take place by simulated moving bed chromatography, crystallization, absorption chromatography, or any combination thereof. In at least the case of crystallization, the stream enriched in m-xylene may comprise a mixture of m-xylene and p-xylene, in which m-xylene is present in a greater amount than is p-xylene.

[0110] Any of the foregoing streams or fractions obtained from a toluene alkylation process with methanol and/or dimethyl ether may be oxidized with a selectivated or non-selectivated oxidation catalyst according to the disclosure herein. The overhead stream may be oxidized directly to afford a product stream comprising terephthalic acid as a majority component in combination with isophthalic acid, if using a non-selectivated oxidation catalyst. Alternately, a selecti vated oxidation catalyst may be utilized to oxidize the overhead stream, if formation of m-substituted compounds in the product stream is not desired. If oxidizing the stream enriched in p-xylene obtained from the overhead fraction, oxidation may preferably take place using a non-selectivated oxidation catalyst, although a selectivated oxidation catalyst may be al ternately used if i t is desired to obtain the terephthalic acid in two steps.

[0111] In some embodiments, separation of the overhead fraction into the stream enriched in p-xylene may take place using simulated moving bed chromatography in a simulated moving bed chromatography separation unit, details of which will be familiar to one having ordinary skill in the art. Commercially available simulated moving bed chromatography processes are available from Axens, a French corporation, as ELUXYL® technology .

[0112] When produced by a simulated moving bed chromatography separation unit, the stream enriched in p-xylene may comprise p-xylene at a concentration of > about 95%, > about 97%, > about 98%, > about 99%, or even > about 99.5%, based on the total mass.

[0113] When produced by a simulated moving bed chromatography separation unit, the stream enriched in m-xylene may comprise m-xylene at a concentration of > about 90%, > about 92%, > about 94%, > about 95%, > about 97%, or > about 99%, based on the total mass. [0114] In other embodiments, the overhead fraction may be separated using crystallization in a crystallization separation unit. Crystallization techniques may take advantage of the much higher melting point of p-xylene (13°C) compared to o-xylene (-25°C), m-xylene (-48°C), and ethylbenzene (-95°C). By cooling the overhead stream to a temperature lower than the p- xylene crystallization temperature (melting point), p-xylene may preferentially crystallize from a mixture of p-xylene and m-xylene, optionally containing other impurities. The p-xylene may then be separated from the filtrate by filtration, centrifugation, decantation, and like techniques. The p-xylene may optionally be further purified by melting and recrystallization, if desired, to produce a high purity p-xylene product. Description of crystallization-based p-xylene purification processes can be found in, for example, Handbook of Petrochemicals Production Processes, Second Edition, by Robert A. Meyers, Ph.D., Chapter 1.5, the relevant contents of which are incorporated herein by reference.

[0115] When produced by a crystallization unit, the stream enriched in p-xylene may comprise p-xylene at a concentration of > about 95%, > about 97%, > about 98%, > about 99%, or even > about 99.5%, based on the total mass.

[0116] When produced by a crystallization unit, the stream enriched in m-xylene may comprise p-xylene at a concentration from c(pX)5 wt% to c(pX)6 wt%, and m-xylene at a concentration from c(mX)5 wt% to c(mX)6 wt%, based on the total mass, wherein c(pX)5 and c(pX)6 can be, independently, for example, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 20, 22, 24, 25, 26, 28, 30, 32, 34, as long as c(pX)5 < c(pX )6; and c(mX)5 and c(mX)6 can be, independently, for example, 60, 65, 70, 75, 80, 85, 90, 91 , 92, as long as c(raX)5 < c(mX)6. Optionally, the m-xylene and/or p-xylene may be isolated from this stream and further processed according to the disclosure herein.

[0117] The processes disclosed herein may be conducted after producing or obtaining any aromatic hydrocarbon mixture containing xylenes. The xylenes present in the aromatic hydrocarbon mixture may represent an equilibrium mixture of xylenes or at least one xylene isomer may be produced in abundance compared to an equilibrium mixture. For instance, the aromatic hydrocarbon mixture may comprise p-xylene in a higher amount than an equilibrium mixture, preferably wherein p-xylene is a majority xylene isomer of the aromatic hydrocarbon mixture, and more preferably wherein p-xylene is a majority component of the aromatic hydrocarbon mixture. Illustrative aromatic hydrocarbon mixtures may comprise at least about 50 wt% xylenes, or at least about 60 wt% xylenes, or at least about 70 wt% xylenes, or at least about 80 wt% xylenes, based on total mass of the aromatic hydrocarbon mixture.

[0118] In the processes disclosed herein, one or more unreacted xylene isomers may be provided to catalytic isomerization conditions to form an equilibrium mixture of xylene isomers. p-Xylene may be at least partially separated from the equilibrium mixture of xylene isomers and undergo selectivated or non-selectivated oxidation in accordance with the disclosure herein. For example, when utilizing a selectivated oxidation catalyst, at least one of unreacted o-xylene or m-xylene obtained prior to oxidation and/or after exposing the C8+ aromatic hydrocarbon feed to the oxidation conditions and the selectivated oxidation catalyst may be exposed to catalytic isomerization conditions downstream from the oxidation conditions and the selectivated oxidation catalyst. [0119] The aromatic, hydrocarbon mixture may be produced or obtained from any suitable source of aromatic hydrocarbons. In non-limiting examples, the aromatic hydrocarbon mixture may be produced by performing catalytic alkylation on benzene and/or toluene using methanol and/or dimethyl ether as an alkylating agent. Such catalytic alkylation processes may be advantageous by forming p-xylene with high selectivity. The benzene and/or toluene may, in turn, be obtained from a reforming process producing a reformate stream. As such, the separation processes disclosed herein may be coupled to a reforming process in at least this respect. Moreover, as discussed further hereinafter, a C8+ aromatic hydrocarbon feed obtained from reforming may be processed in parallel to xylene isomerization, as disclosed herein, to realize various advantages in the present disclosure. By virtue of the low ethylbenzene production and relatively high fraction of p-xylene produced in such benzene and/or toluene alkylation processes, an overhead fraction obtained following separation of o-xylene from p- xylene and m-xylene may be conducted using a selectivated or non-selectivated oxidation catalyst. Such processes may afford a mixture of terephthalic acid and isophthalic acid if conducted with a non-selectivated oxidation catalyst, wherein the ratio of terephthalic acid to isophthalic acid may be determined by the ratio of p-xylene to m-xylene in the overhead stream. Alternately, a selectivated oxidation catalyst may be used to provide one or more oxidized organic compounds that may be subsequently oxidized to terephthalic acid, and leave behind unreacted m-xylene, which may be processed subsequently

[0120] More specifically, C8+ aromatic hydrocarbon oxidation processes conducted non- selectively in accordance with the foregoing may comprise: providing a C8+ aromatic hydrocarbon feed comprising o-xylene, m-xylene, at least about 50 wt% p-xylene, and about 1000 ppm or less ethylbenzene, based on total mass of the C8+ aromatic hydrocarbon feed; separating an o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene from the C8+ aromatic hydrocarbon feed; exposing the o-xylene-lean stream to oxidation conditions and an oxidation catalyst effective to convert the o-xylene-lean stream into a product stream comprising terephthalic acid or a mixture of terephthalic acid and isophthalic acid; and separating the product stream from the o-xylene-lean stream. In non-limiting examples, separation of the product stream may take place by ciystallization, acid-base extraction, solvent extraction, adsorption chromatography, or any combination thereof.

[0121] Likewise, C8+ aromatic hydrocarbon oxidation processes conducted in accordance with the foregoing using a selectivated oxidation catalyst may comprise: providing a C8+ aromatic hydrocarbon feed comprising o-xylene, m-xylene, at least about 50 wt% p-xylene, and about 1000 ppm or fess ethylbenzene, based on total mass of the C8+ aromatic hydrocarbon feed; separating an o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m- xylene from the C8+ aromatic hydrocarbon feed; exposing the o-xylene-lean stream to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxygen-con taming C8 aromatic compounds; wherein the selectivated oxidation catalyst is ineffective to oxidize m-xylene and o-xylene, if present, and the one or more oxygen- containing C8 aromatic compounds comprise a compound selected from the group consisting of p-methylbenzyl alcohol, p-methylbenzaldehyde, p-methylbenzoic acid, and any combination thereof; forming a product stream comprising the one or more oxygen-containing C8 aromatic, compounds from the C8+ aromatic hydrocarbon feed under the oxidation conditions; and separating the product stream from the o-xylene-lean stream. In non-limiting examples, separation of the product stream may take place by crystallization, acid-base extraction, solvent extraction, adsorption chromatography, or any combination thereof.

[0122] Processes for providing the C8+ aromatic hydrocarbon feed may take place through alkylation of toluene and/or benzene using methanol and'' or dimethyl ether. Such processes may comprise: contacting toluene and/or benzene with an alkylating agent comprising methanol and/or dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions to produce an alkylation mixture comprising at least about 50 wt% p-xylene, based on total mass of the alkylation mixture; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the alkylation mixture.

[0123] Reforming processes may produce a reformate stream comprising C6+ hydrocarbons, such as a mixture of non-aromatic hydrocarbons, lower aromatic hydrocarbons (/.e., benzene and/or toluene), ethylbenzene, an equilibrium mixture of xylenes (C8 aromatic hydrocarbons), and C9+ aromatic hydrocarbons. Such reforming processes may provide the benzene and/or toluene for alkylation in accordance with the disclosure above. A reformate stream suitable for further processing according to the disclosure herein may be obtained by providing a feed stream comprising paraffins and/or naphthenes into a reformer under conditions suitable to convert at least a portion of the paraffins and/or naphthenes into one or more aromatic hydrocarbons ('. e. , one or more of benzene, toluene, ethylbenzene, xylenes, and C9-r- aromatic hydrocarbons). Optionally, at least a portion of the ethylbenzene produced from reforming may be removed prior to providing benzene and/or toluene to an alkylation process and/or xylenes for re-isomerization and further oxidation using a selective oxidation catalyst according to the disclosure herein. Reforming may be conducted in the presence of a reforming catalyst under suitable reforming conditions. Illustrative reforming conditions may include, for example, a temperature ranging from about 427 °C to about 565 °C (about 800°F to about 1050°F), a pressure ranging from about 241 kilopascal (gauge) to about 3447 kilopascal (gauge) (from about 35 psig to about 500 psig), and a liquid hourly space velocity (“LHSV”) ranging from about 0.3 hr" ^! to about 3.0 hr ^-1 , to produce a reformate stream comprising one or more of the foregoing aromatic hydrocarbons. Preferably, reforming is performed under high seventy' reforming conditions including a temperature of about 52.7°C to about 543°C (about 980°F to about 1010°F), which may result in a low concentration of linear paraffins and a high octane number of the reformate stream. Illustrative processes and catalysts useful for reforming linear/branched paraffins and naphthenes to produce aromatic hydrocarbons and high octane number liquid products can be found in, for example. Catalytic Reforming, by Donald M Little, Penn Well Publishing Company (1985), the relevant contents of which are incorporated herein by reference in its entirety. The feed stream provided for reforming may be derived from, for example, a crude distillation column, a crude cracker effluent, a stream cracker effluent, a fluid catalytic cracker (“FCC”) effluent, the like, and any combination thereof

[0124] A reformate stream may be separated, such as through distillation and/or extraction, into a C8+ aromatics stream and a lower aromatic hydrocarbon stream, such as at least one of a benzene stream, a toluene stream, or a mixed benzene/toluene stream. The benzene and/or toluene in the lower aromatic hydrocarbon stream may be further reacted, as discussed further above, to produce the aromatic hydrocarbon mixture comprising xylenes, which may be further separated according to the disclosure herein and/or oxidized as further described above. Nonaromatic compounds may be present in the lower aromatic hydrocarbon stream and may be separated therefrom prior to conversion of the benzene and/or toluene into xylenes, as discussed further herein. Preferably, the lower aromatic hydrocarbon stream is substantially free of non-aromatic compounds before being converted into xylenes. To ensure that the lower aromatic hydrocarbon stream is substantially free of non- aromatic compounds, solvent extraction may be performed after separating the lower aromatic hydrocarbon stream and the C8+ aromatics stream from each other. As discussed further hereinbelow, xylenes in the C8+ aromatics stream may be processed in parallel with xylenes obtained from the aromatic hydrocarbon mixture and further oxidized to achieve an overall high conversion of feed stream to xylene-derived products, preferably p-xylene-derived products.

[0125] ^r flie lower aromatic hydrocarbon stream obtained from a reformate stream may be further processed to benzene and/or toluene suitable for being converted into an aromatic hydrocarbon mixture comprising xylenes, which may be subsequently separated from one another and oxidized in accordance with the disclosure herein. Further processing of the lower aromatic hydrocarbon stream is addressed hereinafter.

[0126] Prior to being converted into an aromatic hydrocarbon mixture comprising xylenes, the lower aromatic hydrocarbon stream may be processed to remove non-aromatic compounds therefrom. Removal of non-aromatic compounds from benzene and/or toluene may take place via liquid-liquid extraction, extractive distillation, or any combination thereof. Non-aromatic compounds removed from the lower aromatic hydrocarbon stream may be utilized in other hydrocarbon processing operations (e.g , olefin oligomerization, alkylation, cracking, or the like) or as fuel for these or other processes. Following extraction or extractive distillation, the remaining benzene anchor toluene feed may be subsequently converted into an aromatic hydrocarbon mixture comprising xylenes for further separation and oxidation according to the disclosure herein.

[0127] Toluene and/or benzene in the lower aromatic hydrocarbon stream may be further reacted under alkylation conditions to form an aromatic hydrocarbon mixture comprising mixed xylenes, preferably a non-equilibrium xylenes mixture in which p-xylene is the predominant xylene isomer, which may be further separated and oxidized according to the disclosure herein. More specifically, processes of the present disclosure may comprise contacting toluene and/or benzene with an alkylating agent comprising methanol and/or dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions to produce an alkylation stream comprising o-xylene, m-xylene, p-xylene, styrene, C9-1- aromatic hydrocarbons, and toluene, and obtaining at least a portion of the aromatic hydrocarbon mixture from the alkylation stream.

[0128] Benzene, if present in the lower aromatic hydrocarbon stream, may be converted to toluene under the alkylation conditions and obtained in the alkylation stream. Toluene present in the alkylation stream may be separated and recycled to the alkylation reactor to convert the toluene into a further quantity of o-xylene, m-xylene, p-xylene, styrene, and C9+ aromatic hydrocarbons. As such, obtaining at least a portion of the aromatic hydrocarbon mixture from the alkylation stream may further comprise separating the alkylation stream into a toluene-rich stream and a C8+ aromatic hydrocarbon stream; recycling the toluene-rich stream to the alkylation reactor; and providing the C8+ aromatic hydrocarbon stream as at least a portion of the aromatic hydrocarbon mixture from which xylenes are then separated according to the disclosure herein.

[0129] The alkylation reactor producing the C8+ aromatic hydrocarbons from the lower aromatic hydrocarbon stream may be a fluidized bed reactor, which may produce styrene as a byproduct under the alkylation reaction conditions. In a fluidized bed reactor under suitable alkylation conditions, the C8+ aromatic hydrocarbon stream may comprise styrene at a concentration up to about 5000 ppm by weight, based on total mass of the C8+ aromatic hydrocarbon stream. Alternately, fixed bed or moving bed processes may be used.

[0130] Lower aromatic hydrocarbons may be alkylated (e.g. , methylated) by contacting a lower aromatic hydrocarbon with an alkylating agent comprising methanol anchor dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions. Alkylation of benzene may produce toluene. Toluene may be alkylated to produce xylenes, which may over-alkylate to produce C9+ aromatic hydrocarbons. The alkylation catalyst and alkylation conditions may be chosen such that the alkylation stream (aromatic hydrocarbon mixture) obtained from the alkylation reactor comprises a non-equilibrium mixture of xylenes. For example, the aromatic hydrocarbon mixture may comprise p-xylene at a high concentration, and o-xylene, m-xylene, and styrene at relatively low concentrations. Description of exemplary methylation catalysts, methylation agents, and methylation conditions can be found in, for example, U.S. Patent Nos. 6,423,879; 6,504,072; 6,642,426, and 9,440,893, the relevant contents of which are incorporated herein by reference. Due to their high p-xylene concentration, aromatic hydrocarbon mixtures formed under reaction conditions producing a non-equilibrium xylenes mixture may be advantageous for further xylenes separation according to the disclosure herein. For example, such aromatic hydrocarbon mixtures may facilitate separation of p-xylene and m-xylene from one another in satisfactory’ purity levels and without requiring m-xylene recycling via isomerization to increase overall p- xylene yields.

[0131] In non-limiting examples, an aromatic hydrocarbon mixture obtained from methylation of lower aromatic hydrocarbons with methanol and/or dimethyl ether may feature: an o-xylene concentration from c(oX)l wt% to c(oX)2 wt%, based on the total mass of the aromatic hy drocarbon mixture, wherein c(oX) 1 and c-(oX)2 can be, independently, for example, 0, 0.02. 0.04, 0.01, 0.06, 0 08, 0.1 , 0.2, 0.4, 0.5, 0.6, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 2.0, 22, 24, 25, 26, as long as c(oX)l < c(oX)2; an m-xylene concentration from c(mX)l wt% to c(mX)2 wt%, based on the total mass of the aromatic hydrocarbon mixture, wherein c(mX)l and c(mX)2 can be, independently, for example, 0, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 22, 24, 25, 26, as long as c(mX)l < c(mX)2; a p-xylene concentration from c(pX)I wt% to c(pX)2 wt%, based on the total mass of the aromatic hydrocarbon mixture, wherein c(pX)l and c(pX)2 can be, independently, for example, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 91, 92, 93, 94, 95, as long as c(pX)l < c(pX)2; an m-xylene/o-xylene ratio from r(m/o)l to r(m/o)2, where r(m/o)l and r(m/o)2 can be, independently, for example, 2.1, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 7.5, 8.0, 9.0, 10, 11, 12, 12.5, 13, 14, 15, 16, 17, 17.5, 18, 19, 20, as long as r(m/o)l < r(m/o)2; and an ethylbenzene concentration from c(EB)l wt% to c(EB)2 wt%, based on the total mass of the aromatic hydrocarbon mixture, wherein c(EB)l and c(EB)2 can be, independently, for example, 0, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 2, 3, 4, 5, as long as c(EB)l < c(EB)2. The alkylation reaction can be advantageously conducted under alkylation conditions employing relatively low reactor temperatures, such as for example, less than or equal to about 500°C, less than or equal to about 475°C, less than or equal to about 450°C, less than or equal to about 425°C, or less than or equal to about 400°C. Alkylation may be conducted at temperatures of greater than or equal to about 200°C, greater than or equal to about 250°C, or greater than or equal to about 300°C. In illustrative embodiments, suitable temperature ranges may include temperatures ranging from about 200 ^c 'C to about 500°C, about 275°C to about 475°C, about 300°C to about 450°C, or about 250°C to about 400°C. Such low-temperature reactions can be particularly utilized when a MWW framework type zeolite is present as the alkylation catalyst.

[0132] Operating pressures in the alkylation reactor can vaiy in a broad range, such as for example, from > 100 kPa, > 1000 kPa, > 1500 kPa, > 2000 kPa, > 3000 kPa, or > 3500 kPa, to < 8500 kPa, such as < 7000 kPa, or < 6000 kPa. In some examples, operating pressures may range from 700 kPa to 7000 kPa, such as from 1000 kPa to 6000 kPa, or from 2000 kPa to 5000 kPa.

[0133] WHSV values based on total aromatic hydrocarbon feed and alkylation agent feed may be in a range from, for example, 0.5 hr ^-1 to 50 hr ^-1 , such as from 5 hr ^-1 to 15 hr ^-1 , from I hr ^-1 to 10 hr ^-1 , or from 5 hr ^-1 to 10 hr ^-1 , or from 6.7 hr ^-1 to 10 hr’ ¹ . In some embodiments, at least part of the lower aromatic hydrocarbons, the alkylating agent and/or the aromatic hydrocarbon mixture may be present in the alkylation reactor in a liquid phase.

[0134] The alkylation reactor may comprise any suitable reactor system comprising, but not limited to, a fixed bed reactor, a moving bed reactor, a fluidized bed reactor, anchor a reactive distillation unit. In addition, the reactor may include a single alkylation reaction zone or multiple alkylation reaction zones therein. Injection of the alkylating agent can be effected at a single point in the alkylation reactor or at multiple points spaced along the alkylation reactor. The lower aromatic hydrocarbons and the alkylating agent may be premixed before entering the alkylation reactor or be introduced separately. [0135] Any suitable catalyst capable of converting toluene (or benzene) to xylenes with an alkylating agent can be used for alkylation in this disclosure. Examples of such catalysts are crystalline microporous materials including zeolite-based, as well as non-zeolite-based, molecular sieves and can be of the large, medium, or small pore type. Molecular sieves can have 3-dimensional, four-connected framework structure of comer-sharing [TO4] tetraliedra, where T can be a tetrahedrally coordinated atom. These molecular sieves are often described in terms of the size of the ring that defines a pore, where the size is based on the number of T atoms in the ring. Other framework- type characteristics include the arrangement of rings that form a cage, and, when present, the dimension of channels, and the spaces between the cages. See van Bekkum, et al, Introduction to Zeolite Science and Practice, Second Completely Revised and Expanded Edition, Volume 137, pages 1-67, Elsevier Science, B.V., Amsterdam, Netherlands (2001). Another convenient measure of the extent to which a molecular sieve provides control of molecules of vaiying sizes to its internal structure is the Constraint Index. The method by which Constraint Index is determined is described fully in U.S. Patent No. 4,016,218, which is incorporated herein by reference for details of the method.

[0136] Non-limiting examples of molecular sieves include small pore molecular sieves (e.g.. AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO, KPI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, THO, and substituted forms thereof), medium pore molecular sieves (e.g, AFO, AEL, EUO, HEU, FER, MEL, MFI, MTW, MTT, TON, and substituted forms thereof), large pore molecular sieves (e.g., EMT, FAU, and substituted forms thereof), intergrowths thereof, and combinations thereof. Other molecular sieves include, but are not limited to, ANA, BEA, CFI, CLO, DON, GIS, LTL, MER, MOR, MWW, SOD, intergrowths thereof, and combinations thereof. In some embodiments, the molecular sieve has an MWW framework type (morphology).

[0137] The small, medium, and large pore molecular sieves have from a 4-ring to a 12-ring or greater framework-type. In some embodiments, the zeolitic molecular sieves have 6-, 8-, 10- , or 12-ring structures and an average pore size in the range from about 3 A to 15 A. In other embodiments, the molecular sieves are aluminosilicate molecular sieves and have a 6-ring or an 8-ring structure and an average pore size of about 5 A or less, such as in the range from 3 A to about 5 A, for example from 3 A to about 4.5 A or from 3.5 A to about 4.2 A.

[0138] Other non-limiting examples of zeolitic and non-zeolitic molecular sieves include one or a combination of the following: Beta (U.S. Patent No. 3,308,069 and Reissue No. 28,341 ), ZSM-3 (U.S. Patent No 3,415,736), ZSM-4 (U.S. Patent No 4,021,947), ZSM-5 (U.S. Patent Nos. 3,702,886, 4,797,267 and 5,783,321 ), ZSM-11 (U.S. Patent No. 3,709,979), ZSM-12 (U.S. Patent No. 3,832,449), ZSM-12 andZSM-38 (U.S. Patent No. 3,948,758), ZSM- 14 (U.S. Patent No. 3,923,636), ZSM-I8 (U.S. Patent. No. 3,950,496), ZSM-20 (U.S. Patent No. 3,972,983), ZSM-22 (U.S. Patent No. 5,336,478), ZSM-23 (U.S. Patent No. 4,076,842), ZSM-34 (U.S. Patent No. 4,086,186), ZSM-35 (U.S. Patent No. 4,016,245), ZSM-38, ZSM-48 (U.S. Patent No. 4,397,827), ZSM-50, ZSM-58 (U.S. Patent No. 4,698,217), MCM-1 (U.S. Patent No. 4,639,358), MCM-2 (U.S. Patent No. 4,673,559), MCM-3 (U.S. Patent No. 4,632,811). MCM-4 (U.S. Patent No. 4,664,897), MCM-5 (U.S. Patent No. 4,639,357), MCM- 9 (U.S. Patent No. 4,880,611), MCM-10 (U.S. Patent No. 4,623,527), MCM-14 (U S. Patent No. 4,619,818), MCM-22 (U.S. Patent No. 4,954,325), MCM-41 (U.S. Patent No. 5,098,684), M-41S (U.S. Patent No. 5,102,643), MCM-48 (U.S. Patent No. 5,198,203), MCM-49 (U.S. Patent No. 5,236,575), MCM-56 (U.S. Patent No. 5,362,697), ALPO-11 (U.S. Patent No. 4,310,440), ultrastable Y zeolite (USY) (U.S. Patent Nos. 3,293,192 and 3,449,070), Dealuminized Y zeolite (Deal Y) (U.S. Patent No. 3,442,795), mordenite (naturally occurring and synthetic) (for synthetic mordenite U. S. Patent Nos. 3,766,093 and 3,894,104), SSZ-13, titanium aluminosilicates (TASOs) such as TASO-45 (European Patent No. EP-A-0 229 295), boron silicates (U.S. Patent No. 4,254,297), titanium aluminophosphates (TAPOs) (U.S. Patent No. 4,500,651), mixtures of ZSM-5 and ZSM-11 (U.S. Patent No. 4,229,424), ECR-18 (U.S. Patent No. 5,278,345), SAPO-34 bound ALPO-5 (U.S. Patent No. 5,972,203), those disclosed in International Publication No. WO 98/57743 published Dec. 23, 1988 (molecular sieve and Fischer-Tropsch), those disclosed in U.S. Patent No. 6,300,535 (MFI-bound zeolites), mesoporous molecular sieves (U.S. Patent Nos. 6,284,696, 5,098,684, 5,102,643 and 5,108,725). and the like, and intergrowths and/or combinations thereof.

[0139] In an embodiment, the methylation catalyst comprises an aluminosilicate catalyst composition. Aluminosilicates, as used herein, can include those having a molar relationship of X ₂ O ₃ :(n)YO ₂ (wherein X is a tnvalent element, e.g, Al; and Y is a tetravalent element, e.g.. Si), in which n < 500, such as < 250, < 100, such as from 30 to 100

[0140] Non-limiting examples of trivalent element X can include aluminum, boron, iron, indium, gallium, and combinations thereof. For example, X can be aluminum. Non-limiting examples of tetravalent element Y can include silicon, tin, titanium, germanium, and combinations thereof. For example, Y can be silicon.

[0141] Other non-limiting examples of aluminosilicate catalysts and compositions can be found, for instance, in U.S. Patent Application Publication No. 2003/0176751 and U.S. Patent No. 7,094,389. [0142] One class of molecular sieve suitable for use in a process of this disclosure lias a Constraint Index < 5, and is crystalline microporous material of the MWW framework type. In at least one embodiment, the crystalline microporous material is a zeolite. As used herein, the term “crystalline microporous material of the MWW framework type” includes one or more of: a) molecular sieves made from a common first degree crystalline building block unit cell, which unit cell has the MWW framework topology. (A unit cell is a spatial arrangement of atoms which if tiled in three-dimensional space describes the crystal structure. Such crystal structures are discussed in the “Atlas of Zeolite Framework Types”, Fifth edition, 2001, incorporated herein by reference); b) molecular sieves made from a second degree building block, being a 2-dimensional tiling of such MWW framework topology unit cells, forming a monolayer of one unit cell thickness, in an embodiment, one c-unit cell thickness; c) molecular sieves made from common second degree building blocks, being layers of one or more than one unit cell thickness, where the layer of more than one unit cell thickness is made from stacking, packing, or binding at least two monolayers of MWW framework topology unit cells. The stacking of such second degree building blocks can be in a regular fashion, an irregular fashion, a random fashion, or any combination thereof; and d) molecular sieves made by any regular or random 2-dimensional or 3-dimensional combination of unit cells having the MWW framework topology.

[0143] Crystalline microporous materials of the MWW framework type include those molecular sieves having an X-ray diffraction pattern comprising d-spacing maxima at 12.4^0.25, 6.9±0.15, 3.57±0,07 and 3.42±0.07 Angstrom. The X-ray diffraction data used to characterize the material are obtained by standard techniques using the K-alpha doublet of copper as incident radiation and a diffractometer equipped with a scintillation counter and associated computer as the collection system.

[0144] Examples of crystalline microporous materials of the MW W framework type include MCM-22 (U.S. Patent No. 4,954,325), PSH-3 (U.S. Patent No. 4,439,409), SSZ-25 (U.S. Patent No. 4,826,667), ERB-1 (European Patent No. 0293032), ITQ-1 (U.S. Patent No. 6,077,498), ITQ-2 (International Publication No. WO97/17290), MCM-36 (U.S. Patent No. 5,250,277), MCM-49 (U.S. Patent No. 5,236,575), MCM-56 (U.S. Patent No. 5,362,697), UZM-8 (U.S. Patent No. 6,756,030), UZM-8HS (U.S. Patent No. 7,713,513), UZM-37 (U.S. Patent No. 7,982,084), EMM-10 (U.S. Patent No. 7,842,277), EMM-12 (U.S. Patent No. 8,704,025), EMM-13 (U.S. Patent No. 8,704,023), UCB-3 (U.S. Patent No. 9,790, 143B2) and mixtures thereof.

[0145] In some embodiments, the crystalline microporous material of the MWW framework type may be contaminated with other crystalline materials, such as ferrierite or quartz. These contaminants may be present m quantities of < 10 wt%, such as < 5 wt%.

[0146] In some embodiments, the molecular sieves are not subjected to pre-treatments, such as high temperature steaming, to modify their diffusion properties. In other embodiments, the molecular sieves may be selectivated, either before introduction into the aromatization reactor or in-situ in the reactor, by contacting the catalyst with a selectivating agent, such as silicon, steam, coke, or a combination thereof. In one embodiment, the catalyst is silica- selectivated by contacting the catalyst with at least one oiganosilicon in a liquid carrier and subsequently calcining the silicon-containing catalyst in an oxy gen-containing atmosphere, e.g, air, at a temperature of 350°C to 550°C. A suitable silica-selectivation procedure is described in U.S. Patent No. 5,476,823. In another embodiment, the catalyst is selectivated by contacting the catalyst with steam. Steaming of the zeolite is effected at a temperature of > 950°C, such as from 950°C to 1075°C, or from 1000°C to 1050°C, for 10 minutes to 10 hours, such as from 30 minutes to 5 hours. The selectivation procedure, which may be repeated multiple times, alters the diffusion characteristics of the molecular sieve and may increase the xylene yield.

[0147] In addition to, or in place of, silica or steam selectivation, the catalyst may be subjected to coke selectivation. Tins optional coke selectivation typically involves contacting the catalyst with a thermally decomposable organic compound at an elevated temperature m excess of the decomposi tion temperature of said compound but below the temperature at which the crystallinity of the molecular sieve is adversely affected. Further details regarding coke selectivation techniques are provided in the U.S. Patent No. 4,1 17,026. In some embodiments, a combination of silica selectivation and coke selectivation may be employed.

[0148] It may be desirable to combine the molecular sieve, prior to selectivating, with at least one oxide modifier, such as at least one oxide selected from elements of Groups 2 to 4 and 13 to 16 of the Periodic Table. In some embodiments, the oxide modifier is selected from oxides of boron, magnesium, calcium, lanthanum, and phosphorus. In some cases, the molecular sieve may be combined with more than one oxide modifier, for example a combination of oxides of phosphorus with calcium and/or magnesium, since in this way it maybe possible to reduce the steaming severity needed to achieve a target diffusivity value. In some embodiments, the total amount of oxide modifier present in the catalyst, as measured on an elemental basis, may be from 0.05 wt% and 20 wt%, such as from 0.1 wt% to 10 wt%, based on the weight of the final catalyst. Where the modifier comprises phosphorus, incorporation of modifier into the catalyst is conveniently achieved by the methods described in U.S. Patent Nos. 4,356,338; 5,1 10,776; 5,231,064; and 5,348,643

[0149] The molecular sieves may be used as the methylation catalyst without any binder or matrix, in a self-bound form. Alternatively, the molecular sieves may be composited with another material which is resistant to the temperatures and other conditions employed in the methylation reaction Such binder or matrix materials include active and inactive materials and synthetic or naturally occurring zeolites as well as inorganic materials such as clays and/or oxides such as alumina, silica, silica-alumina, zirconia, titania, magnesia or mixtures of these and other oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels comprising mixtures of silica and metal oxides. Clays may also be included with the oxide-type binders to modify the mechanical properties of the catalyst or to assist in its manufacture. Use of a material in conjunction with the molecular sieve whether combined therewith or present during its synthesis, which itself is catalytically active may change the conversion and/or selectivity' of the catalyst. Inactive materials suitably serve as diluents to control the amount of conversion so that products may be obtained economically and orderly without employing other means for controlling the rate of reaction. These materials may be incorporated into naturally occurring clays, for example, bentonite and kaolin, to improve the crash strength of the catalyst under commercial operating conditions and function as binders or matrices for the catalyst. Tire relative proportions of molecular sieve and inorganic oxide matrix vary widely, with the sieve content ranging from 1 wt% to 90 wi%, and in some embodiments the composi te is prepared in the form of beads, in the range of 2 wt% to 80 wt% of the composite.

[0150] As indicated above, a C8t- aromatics stream obtained from a reforming process may comprise xylene isomers and may be desirably processed in a xylenes loop (see FIG. 2 above) in parallel with the separation process for an aromatic hydrocarbon mixture described herein to increase the overall extent of xylene recovery'. The xylenes loop may comprise a xylenes splitter producing a C8 aromatics stream, a p-xylene separation unit producing a p-xylene stream and a raffinate stream depleted in p-xylene (a raffinate stream or a filtrate stream, depending on how the stream is produced), and a xylenes isomerization unit that may isomerize the raffinate stream depleted in p-xylene to produce an isomerized stream comprising an additional amount of p-xylene, namely p-xylene at a higher concentration than in the raffinate stream. The isomerized stream may be at least partly recycled to the xylenes splitter or the p- xylene separation unit. The isomerization unit may comprise an isomerization reactor operated under vapor phase conditions and/or under liquid phase conditions.

[0151] In non-limiting examples, the C8+ aromatics stream obtained from the reformate splitter may comprise ethylbenzene at a concentration of about c(EB)l wt% to c(EB)2 wt%, based on the total mass of the raffinate stream, where c(EB)l and c(EB)2 can be, independently, for example, 0, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 15, 16, 18, 20, 22, 24, 25, 26, 28, 30, as long as c(EB)l < c(EB)2. When the C8+ aromatics stream also comprises ethylbenzene at a sufficiently high concentration, such as greater than or equal to about 5 wt%, or greater than or equal to about 8 wt%, or greater than or equal to about 10 wt%, it may be desirable to remove at least a portion of the ethylbenzene prior to processing in the p-xylene separation unit. Exemplary' ethylbenzene conversion catalysts and processes therefor can be found, for example, in U.S. Patent No. 5,977,420, the relevant contents of which are incorporated herein by reference. Optionally, operations for removing at least a portion of the ethylbenzene may comprise a solvent extraction step. The resulting processed C8+ aromatics stream may have an ethylbenzene concentration that is suitably small for further processing to isolate xylenes therefrom according to the disclosure herein.

[0152] As indicated above, the C8+ aromatics stream obtained from a reformate stream via a reformate splitter may be processed separately from the aromatic hydrocarbon mixture produced by alkylating lower aromatic hydrocarbons Prior to separating the xylene isomers from each other, the C8+ aromatics stream may be further separated, such as in a xylenes splitter, to obtain a C8 aromatics stream comprising at least o-xylene, m-xylene and p-xylene. The C8 aromatics stream obtained from the xylenes splitter may comprise an equilibrium xylenes mixture in particular examples. The C8 aromatics stream can be then fed into the p- xylene separation unit comprising a simulated bed chromatography separation unit and/or a crystallization separation unit described above to produce a p-xylene stream and a stream depleted in p-xylene (either a raffinate or a filtrate stream, depending on how produced).

[0153] The stream depleted in p-xylene may be exposed to catalytic isomerization conditions in an isomerization unit to obtain a recycle stream comprising, for example, an equilibrium mixture of o-xylene, m-xylene, and p-xylene. The recycle stream may be at least partly returned to the xylenes splitter and/or to the p-xylene separation unit. The xylenes loop thus permits production of p-xylene from m-xylene and o-xylene that may be present in the C8+ aromatics stream obtained from the reformate splitter.

[0154] The isomerization conditions used to convert the stream depleted in p-xylene into an equilibrium xylenes mixture may include a temperature and a pressure such that a majority of the xylenes are in a vapor phase (“vapor-phase isomerization” or “VPI”). Alternatively, the isomerization conditions may include a temperature and a pressure such that a majority of the xylenes are in liquid phase (“liquid-phase isomerization” or “LPI”). LPI requires a lower temperature than VPI, and can be earned out without co-feeding a molecular hydrogen stream. As such, LPI may be preferred over VPI, especially when ethylbenzene is present at a low concentration. VPI may be favored when ethylbenzene is present at a high concentration, for example, greater than or equal to about 10 wt%, based on the total mass, because VPI can be more effective than LPI in converting ethylbenzene. Description of exemplary' VPI processes and catalysts can be found in, for example, U.S. Patent Application Publications 2011/03196881; 2012/0108867; 2012/0108868; 2014/0023563; 2015/0051430; and 2017/0081259, the relevant contents of which are incorporated herein by reference. Description of exemplary' LPI processes and catalysts can be found in, for example, U.S. Patent Application Publications 2011/0319688; 2012/0108867; 2013/0274532; 2014/0023563; and 2015/0051430, the relevant contents of which are incorporated herein by reference.

[0155] Optionally, o-xylene and/or m-xylene produced from alkylation of lower aromatic hydrocarbons and separated according to the disclosure herein may be further exposed to the isomerization conditions in order to at least partially convert these components to p-xylene as well. That is, processes of the present disclosure may further comprise exposing at least a portion of the o-xylene in the bottoms fraction and/or at least a portion of the stream rich in m- xylene to the catalytic isomerization conditions suitable for converting o-xylene and m-xylene into a mixture of xylene isomers. If so performed, phenol and/or styrene may be removed to protect the isomerization catalyst. Suitable techniques for mitigating phenol and/or styrene include those mentioned above.

[0156] Non-limiting embodiments disclosed herein can include:

[0157] A. Oxidation processes for C8+ aromatic compounds. The processes comprise: providing a C8+ aromatic hydrocarbon feed comprising at least p-xylene; optionally, separating ethylbenzene, if any, from the C8+ aromatic hydrocarbon feed; exposing at least a portion of the C8+ aromatic hydrocarbon feed to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxygen-containing C8 aromatic compounds; wherein the selectivated oxidation catalyst is ineffective to oxidize m- xylene and o-xylene, if present, and the one or more oxy gen-containing C8 aromatic compounds comprise a compound selected from the group consisting of p-methylbenzyl alcohol, p-methylberizaldehyde, p-methylbenzoic acid, and any combination thereof; and forming a product stream comprising the one or more oxygen-containing C8 aromatic compounds from the C8+ aromatic hydrocarbon feed under the oxidation conditions.

[0158] B. Oxidation processes for C8+ aromatic compounds containing primarily p- xylene. The processes comprise: providing a C8+ aromatic hydrocarbon feed comprising o- xylene, m-xy lene, at least about 50 wt% p-xylene, and about 1000 ppm or less ethylbenzene, based on total mass of the C8+ aromatic hydrocarbon feed; separating an o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene from the C8+ aromatic hydrocarbon feed; exposing the o-xylene-lean stream to oxidation conditions and an oxidation catalyst effective to convert the o-xylene-lean stream into a product stream comprising terephthalic acid or a mixture of terephthalic acid and isophthalic acid; and separating the product stream from the o-xylene-lean stream.

[0159] C. Selecti ve oxidation processes for C8+ aromati c compounds containing primarily p-xylene. The processes comprise: providing a C8+ aromatic hydrocarbon feed comprising o-xylene, m-xylene, at least about 50 wt% p-xylene, and about 1000 ppm or less ethylbenzene, based on total mass of the C8+ aromatic hydrocarbon feed; separating an o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene from the C8+ aromatic hydrocarbon feed; exposing the o-xylene-lean stream to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxygen-containing C8 aromatic compounds; wherein the selectivated oxidation catalyst is ineffective to oxidize m- xylene, if present, and the one or more oxygen-containing C8 aromatic compounds comprise a compound selected from the group consisting of p-methylbenzyl alcohol, p- methylbenzaldehyde, p-methylbenzoic acid, and any combination thereof; forming a product stream comprising the one or more oxygen-containing C8 aromatic compounds from the o- xylene-lean stream under the oxidation conditions; and separating the product stream from the o-xylene-lean stream.

[0160] Embodiments A-C may have one or more of the following additional elements in any combination:

[0161] Element 1 : wherein the process further comprises separating the product stream to obtain a stream rich in the one or more oxygen-containing C8 aromatic compounds and a stream lean in the one or more oxygen-containing C8 aromatic compounds.

[0162] Element 2: wherein the process further comprises further oxidizing the one or more oxy gen-containing C8 aromatic, compounds to form a second product stream comprising or consisting essentially of terephthalic acid. [0163] Element 3: wherein the C8+ aromatic hydrocarbon feed comprises, in addition to p-xylene, at least one of o-xylene and m-xylene.

[0164] Element 4: separating an o-xylene-lean stream and an o-xylene-rich stream from the C8+ aromatic hydrocarbon feed, the o-xylene-lean stream compri sing p-xylene or a mixture of p-xylene and m-xylene; wherein the o-xylene-lean stream is exposed to the oxidation conditions and the selectivated oxidation catalyst.

[0165] Element 5: wherein the C8+ aromatic hydrocarbon feed comprises ethylbenzene at a concentration no greater than about 1000 ppm, based on total mass of C8 aromatic hydrocarbons in the C8r- aromatic hydrocarbon feed, when exposed to the oxidation conditions and the selectivated oxidation catalyst.

[0166] Element 6: wherein the process further comprises contacting toluene and/or benzene with an alkylating agent comprising methanol and/or dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions to produce an alkylation mixture comprising at least about 50 wt% p-xylene, based on total mass of the alkylation mixture; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the alkylation mixture.

[0167] Element 7 : wherein the toluene and/or benzene is obtained from a reformate stream comprising C6+ aromatic hydrocarbons, and the reformate stream is further separated into a raffinate stream comprising C8+ aromatic hydrocarbons, the C8+ aromatic hydrocarbons comprising at least a mixture of o-xylene, m-xylene, and p-xylene.

[0168] Element 8: wherein the process further comprises separating an intermediate stream comprising at least a portion of the p-xylene from the raffinate stream.

[0169] Element 9: wherein the process further comprises optionally, separating ethylbenzene and/or o-xylene, if any, from the raffinate stream or the intermediate stream; exposing the raffinate stream or the intermediate stream to the oxidation conditions and the selectivated oxidation catalyst, the oxidation conditions and the selectivated oxidation catalyst to which the raffmate stream or the intermediate stream is exposed being the same as or different than the oxidation conditions and the selectivated oxidation catalyst to which the C8+ aromatic hydrocarbon feed is exposed; and forming at least some of the one or more oxygencontaining C8 aromatic compounds from the raffinate stream or the intermediate stream under the oxidation conditions.

[0170] Element 10: wherein the process further comprises separating the one or more oxygen-containing C8 aromatic compounds from the raffinate stream or the intermediate stream and obtaining a p-xylene-lean stream therefrom; exposing the p-xylene-lean stream to catalytic isomerization conditions to obtain an isomerized stream comprising p-xylene at a higher concentration than in the p-xylene-lean stream; and recycling the isomerized stream to the oxidation conditions and the selectivated oxidation catalyst in combination with the raffinate stream or the intermediate stream,

[0171] Element 11: wherein the process further comprises forming an isomerized stream comprising p-xylene under catalytic isomerization conditions from a p-xylene-lean stream; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the isomerized stream. [0172] Element 12: wherein the process further comprises contacting toluene with a disproportionation catalyst under disproportionation conditions to produce a disproportionation mixture comprising o-xylene, m-xylene, p-xylene, benzene, and C9+ aromatic hydrocarbons; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the disproportionation mixture,

[0173] Element 13: wherein the process further comprises wherein unreacted m-xylene is obtained after exposing the C8+ aromatic hydrocarbon feed to the oxidation conditions and the selectivated oxidation catalyst, the unreacted m-xylene being exposed to catalytic isomerization conditions downstream from the oxidation conditions and the selectivated oxidation catalyst.

[0174] Element 14: wherein the selectivated oxidation catalyst has a framework comprising one or more of titanium silicalite, a 10-membered ring zeolite containing an oxidizing metal element in a pore thereof, or any combination thereof, and an outer surface of the framework is at least partially passivated with a passi vating agent or by steaming.

[0175] Element 15: wherein the oxidizing metal element is selected from the group consisting of Co, Mn, Cu, Fe, Ru, Rh, Ce, Ti, V, Re, and any combination thereof.

[0176] Element 16: wherein the passivating agent comprises at least one substance selected from the group consisting of coke, a phosphorus compound, a silicon compound, a magnesium compound, a metal oxide, and any combination thereof

[0177] Element 17: wherein the 10-membered ring zeolite has an MFI framework

[0178] Element 18: wherein the 10-membered ring zeolite comprises ZSM-5. By way of non-limiting example, exemplary combinations applicable to A include, but are not limited to: 1 and 2; 1 and 3; 1 and 4; 1, 3 and 4; 1 and 5; 1, 3 and 5; 1 and 6; 1, 5 and 6; 1, 6 and 7; 1 and 5-7; 1 and 6-8; 1 and 5-8; 1 and 6-9; 1 and 5-9; 1 and 6-10; 1 and 5-10; 1 and 11; 1 and 12; 1 and 13; 1 and 14; 1, 14 and 16; 1, 14 and 17; 1, 14 and 18; 2 and 3; 2 and 4; 2-4; 2 and 5; 2, 3 and 5; 2 and 6; 2, 5 and 6, 2, 6 and 7; 2 and 5-7; 2 and 6-8, 2 and 5-8; 2 and 6-9, 2 and 5-9; 2 and 6-10; 2 and 5-10; 2 and 1 1; 2 and 12; 2 and 13; 2 and 14; 2. 14 and 16; 2, 14 and 17; 2, 14 and 18; 3 and 4; 3 and 5; 3 and 6; 3, 5 and 6; 3, 6 and 7; 3 and 5-7; 3 and 6-8; 3 and 5-8; 3 and 6-9; 3 and 5-9; 3 and 6-10; 3 and 5-10; 3 and 11; 3 and 12; 3 and 13; 3 and 14; 3, 14 and 16; 3, 14 and 17; 3, 14 and 18; 4 and 5; 4 and 6; 4-6; 4, 6 and 7; 4-7; 4 and 6-8; 4-8; 4 and 6-9; 4-9; 4 and 6-10; 4-10; 4 and 11; 4 and 12; 4 and 13; 4 and 14; 4, 14 and 16; 4, 14 and 17; 4, 14 and 18; 5 and 6; 5-7; 5-8; 5-9; 5-10; 5 and 11; 5 and 12; 5 and 13; 5 and 14; 5, 14 and 16; 5, 14 and 17; 5, 14 and 18; 5-7 and 11; 5-7 and 12; 5-7 and 13; 5-7 and 14; 5-7, 14 and 16; 5-7, 14 and 17; 5-7, 14 and 18, 5-8 and 13; 5-8 and 14; 5-8, 14 and 16; 5-8, 14 and 17; 5-8, 14 and 18; 5-9 and 13; 5-9 and 14; 5-9, 14 and 16; 5-9, 14 and 17; 5-9, 14 and 18; 5-10 and 13; 5-10 and 14; 5-10, 14 and 16; 5-10, 14 and 17; 5-10, 14 and 18; 6-8 and 13; 6-8 and 14; 6-8, 14 and 16; 6- 8, 14 and 17; 6-8, 14 and 18; 6-9 and 13; 6-9 and 14; 6-9, 14 and 16; 6-9, 14 and 17; 6-9, 14 and 18; 6-10 and 13; 6-10 and 14; 6-10, 14 and 16; 6-10, 14 and 17; 6-10, 14 and 18; 1 1 and/ or 12, and 13; 11 and/ or 12, and 14; 11 and,'' or 12, 14 and 16; 11 and/ or 12, 14 and 17; and 11 and/ or 12, 14 and 18. Illustrative combinations applicable to B include, but are not limited to, 6 and 7; 6-8; 6-9; and 6-10. Illustrative combinations applicable to C include, but are not limited to, 1 and 2; 1 and 6; l, 6 and 7; 1 and 6-8; 1 and 6-9; 1 and 6-10; 1 and 11; 1 and ! 2; 1 and 13; 1 and 14; 1, 14 and 16; 1, 14 and 17; 1, 14 and 18; 2 and 6; 2, 6 and 7; 2 and 6-8; 2 and 6-9; 2 and 6-10; 2 and 11; 2 and 12; 2 and 13; 2 and 14; 2, 14 and 16; 2, 14 and 17; 2, 14 and 18; 6 and 7; 6-8; 6-9; 6-10; 6 and 11; 6 and 12; 6 and 13, 6 and 14; 6, 14 and 16, 6, 14 and 17; 6, 14 and 18; 6, 7 and 11 ; 6, 7 and 12; 6, 7 and 13; 6, 7 and 14; 6, 7. 14 and 16; 6, 7, 14 and 17; 6, 7, 14 and 18; 6-8 and 11; 6-8 and 12; 6-8 and 13; 6-8 and 14; 6-8, 14 and 16; 6-8, 14 and 17; 6-8, 14 and 18; 6-9 and 11; 6-9 and 12; 6-9 and 13; 6-9 and 14; 6-9, 14 and 16; 6-9, 14 and 17; 6-9, 14 and 18; 6-11; 6-10 and 12; 6-10 and 13; 6-10 and 14; 6-10, 14 and 16; 6-10, 14 and 17; 6-10, 14 and 18; 1 1 and/or 12, and 13; 11 and/or 12, and 14; 11 and/or 12, 14 and 16, 1 1 and/or 12, 14 and 17; and 11 and/or 12, 14 and 18.

[0179] The present disclosure can further include the following non-limiting aspects and/or embodiments:

[0180] 1-1. A process for oxidizing C8+ aromatic compounds, comprising: providing a C8+ aromatic hydrocarbon feed comprising at least p-xylene; optionally, separating ethylbenzene, if any, from the C8+ aromatic hydrocarbon feed; exposing at least a portion of the C8+ aromatic hydrocarbon feed to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxy gen-containing C8 aromatic compounds, wherein the selectivated oxidation catalyst is ineffective to oxidize m-xylene and o-xylene, if present, and the one or more oxy gen-containing C8 aromatic compounds comprise a compound selected from the group consisting of p-methylbenzyl alcohol, p- methylbenzaldehyde, p-me thy 1 benzoic acid, and any combination thereof; and forming a product stream comprising the one or more oxygen-containing C8 aromatic compounds from the C8+ aromatic hydrocarbon feed under the oxidation conditions. [0181] 1-2. The process of 1-1, further comprising: separating the product stream to obtain a stream rich in the one or more oxy gencontaining C8 aromatic compounds and a stream lean in the one or more oxygen-containing C8 aromatic compounds.

[0182] 1-3. The process of 1-1 or 1-2, further comprising: further oxidizing the one or more oxy gen-containing C8 aromatic compounds to form a second product stream comprising or consisting essentially of terephthalic acid.

[0183] 1-4. The process of any of 1-1 to 1-3, wherein the C8+ aromatic hy drocarbon feed comprises, in addition to p-xylene, at least one of o-xylene and m-xylene.

[0184] 1-5. The process of 1-4, further comprising: separating an o-xylene-lean stream and an o-xylene-rich stream from the C St- aromatic hydrocarbon feed, the o-xylene-lean stream comprising p-xylene or a mixture of p- xylene and m-xylene; wherein the o-xylene-lean stream is exposed to the oxidation conditions and the selectivated oxidation catalyst.

[0185] 1-6. The process of any of 1-1 to 1-5, wherein the C8+ aromatic hydrocarbon feed comprises ethylbenzene at a concentration no greater than about 1000 ppm, based on total mass of C8 aromatic hydrocarbons in the C8+ aromatic hydrocarbon feed, when exposed to the oxidation conditions and the selectivated oxidation catalyst.

[0186] 1-7. The process of any of 1-1 to 1-6, further comprising: contacting toluene and/or benzene with an alkylating agent comprising methanol and/or dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions to produce an alkylation mixture comprising at least about 50 wt% p- xylene, based on total mass of the alkylation mixture; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the alkylation mixture.

[0187] 1-8. The process of 1-7, wherein the toluene and/or benzene is obtained from a reformate stream comprising C6 + aromatic hydrocarbons, and the reformate stream is further separated into a raffinate stream comprising C8+ aromatic hydrocarbons, the C8+ aromatic hydrocarbons comprising at least a mixture of o-xylene, m-xylene, and p-xylene.

[0188] 1-9. The process of 1-8, further comprising: separating an intermediate stream comprising at least a portion of the p-xylene from the raffinate stream.

[0189] 1-10. The process of 1-9, further comprising: optionally, separating ethylbenzene and/or o-xylene, if any, from the raffinate stream or the intermediate stream; exposing the raffinate stream or the intermediate stream to the oxidation conditions and the selectivated oxidation catalyst, the oxidation conditions and the selectivated oxidation catalyst to which the raffinate stream or the intermediate stream is exposed being the same as or different than the oxidation conditions and the selectivated oxidation catalyst to which the C8-i- aromatic hydrocarbon feed is exposed; and forming at least some of the one or more oxy gen-containing C8 aromatic compounds from the raffinate stream or the intermediate stream under the oxidation conditions. [0190] 1-11. The process of any of 1-8 to 1-10, further comprising: separating the one or more oxygen-containing C8 aromatic compounds from the raffinate stream or the intermediate stream and obtaining a p-xylene-lean stream therefrom, exposing the p-xylene-lean stream to catalytic isomerization conditions to obtain an isomerized stream comprising p-xylene at a higher concentration than in the p-xylene-lean stream; and recycling the isomerized stream to the oxidation conditions and the selectivated oxidation catalyst in combination with the raffinate stream or the intermediate stream.

[0191] 1-12. The process of any of 1-1 to 1-6, further comprising: forming an isomerized stream comprising p-xylene under catalytic isomerization conditions from a p-xylene-lean stream; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the isomerized stream.

[0192] 1-13. The process of any of 1-1 to 1-6, further comprising: contacting toluene with a disproportionation catalyst under disproportionation conditions to produce a disproportionation mixture comprising o-xylene, m-xylene, p-xylene, benzene, and C9+ aromatic hydrocarbons; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the disproportionation mixture, [0193] 1-14. The process of any of 1-1 to 1-13, wherein unreacted m-xylene is obtained after exposing the C8+ aromatic hydrocarbon feed to the oxidation conditions and the selectivated oxidation catalyst, the unreacted m-xylene being exposed to catalytic isomerization conditions downstream from the oxidation conditions and the selectivated oxidation catalyst.

[0194] 1-15. The process of any of 1-1 to 1-14, wherein the selectivated oxidation catalyst has a framework comprising one or more of titanium silicalite, a 10-membered ring zeolite containing an oxidizing metal element in a pore thereof, or any combination thereof, and an outer surface of the framework is at least partially passivated with a passivating agent or by steaming.

[0195] 1-16. The process of 1-15, wherein the oxidizing metal element is selected from the group consisting of Co, Mn, Cu, Fe, Ru, Rb, Ce, Ti, V, Re, and any combination thereof.

[0196] 1-17. The process of 1-15 or 1-16, wherein the passivating agent comprises at least one substance selected from the group consisting of coke, a phosphorus compound, a silicon compound, a magnesium compound, a metal oxide, and any combination thereof.

[0197] 1-18. The process of any of 1-15 to 1-17, wherein the 10-membered ring zeolite has an MFI framework.

[0198] 1-19. The process of any of 1-15 to 1-18, wherein the 10-membered ring zeolite comprises ZSM-5.

[0199] II-l. A process for oxidizing C8+ aromatic compounds, comprising: providing a C8+ aromatic hydrocarbon feed comprising o-xylene, m-xylene, at least about 50 wt% p-xylene, and about 1000 ppm or less ethylbenzene, based on total mass of the C8-+- aromatic hydrocarbon feed; separating an o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene from the C8+ aromatic hydrocarbon feed; exposing the o-xylene-lean stream to oxidation conditions and an oxidation catalyst effective to convert the o-xylene-lean stream into a product stream comprising terephthalic acid or a mixture of terephthalic acid and isophthalic acid; and separating the product stream from the o-xylene-lean stream.

[0200] II -2. The process of II-l, further comprising: contacting toluene and/or benzene with an alkylating agent comprising methanol and/or dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions to produce an alkylation mixture comprising at least about 50 wt % p- xylene, based on total mass of the alkylation mixture; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the alkylation mixture.

[0201] III-l . A process for oxidizing C8+ aromatic compounds, comprising: providing a C8+ aromatic hydrocarbon feed comprising o-xylene, m-xylene, at least about 50 wt% p-xylene, and about 1000 ppm or less ethylbenzene, based on total mass of the C8+ aromatic hydrocarbon feed; separating an o-xylene-lean stream comprising p-xylene or a mixture of p-xylene and m-xylene from the C8+ aromatic hydrocarbon feed; exposing the o-xylene-lean stream to oxidation conditions and a selectivated oxidation catalyst effective to convert p-xylene into one or more oxygen-containing C8 aromatic compounds; wherein the selectivated oxidation catalyst is ineffective to oxidize m-xylene, if present, and the one or more oxygen-containing C8 aromatic compounds comprise a compound selected from the group consisting of p-methylbenzyl alcohol, p-methylbenzaldehyde, p- methylbenzoic acid, and any combination thereof; forming a product stream comprising the one or more oxygen-containing C8 aromatic compounds from the o-xylene-lean stream under the oxidation conditions; and separating the product stream from the o-xylene-lean stream.

[0202] III-2, The process of III-l, further comprising: contacting toluene and/or benzene with an alkylating agent comprising methanol and/or dimethyl ether in the presence of an alkylation catalyst in an alkylation reactor under alkylation conditions to produce an alkylation mixture comprising at least about 50 wt% p- xylene, based on total mass of the alkylation mixture; and obtaining at least a portion of the C8+ aromatic hydrocarbon feed from the alkylation mixture.

[0203] III-3. The process of III-l or III-2, further comprising: further oxidizing the one or more oxy gen-containing C8 aromatic compounds to a second product stream comprising or consisting essentially of terephthalic acid.

[0294] III-4. The process of any of III-l to III-3, wherein the selectivated oxidation catalyst has a framework comprising one or more of titanium silicalite, a 10-membered ring zeolite containing an oxidizing metal element in a pore thereof, or any combination thereof, and an outer surface of the framework is passivated with a passivating agent or by steaming.

[G205] III-5. The process of III-4, wherein the oxidizing metal element is selected from the group consisting of Co, Mn, Cu, Fe, Ru, Rh, Ce, Ti, V, Re, and any combination thereof. [0206] III-6. The process of 111-4 or 111-5, wherein the passivating agent comprises at least one substance selected from the group consisting of coke, a phosphorus compound, a silicon compound, a magnesium compound, a metal oxide, and any combination thereof

[0207] IIJ-7, The process of any of III-4 to I II-6, wherein the 10-membered ring zeolite has an MFI framework.

[0288] HI-8. Tire process of any of 111-4 to III-7, wherein the 10-membered ring zeolite comprises ZSM-5.

[0209] To facilitate a better understanding of the embodiments of the present disclosure, the following examples of preferred or representative embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the invention.

Examples

[0210] Oxidation Catalyst Syntheses. Oxidation catalysts were synthesized by direct incorporation of catalytic metal within a zeolite framework during synthesis of the zeolite, or by post-synthesis exchange of the catalytic metal into the zeolite framework. Zeolite catalysts were obtained as free powders or zeolite-bound extrudates. Tetraethyl orthosilicate (TEOS) and tetraethyl orthotitanate (TEOT) were obtained from Sigma-Aldrich. Tetrapropylammonium hydroxide (Sigma-Aldrich or Fluka, TPAOH) was used as a zeolite templatmg agent in a 20 wt % aqueous solution. Selectivation was carried out as further described below (Examples 3-15).

[0211] Example 1: Direct Synthesis of Non-Selectivated Oxidation Catalyst by Direct Incorporation of Oxidizing Metal (Catalyst A, unbound). In a typical experiment, 0.6 mL of TEOT was added dropwise to 40 niL of TEOS contained in a stirred PTFE autoclave liner kept under nitrogen. The clear yellow solution obtained was then mixed for a further 30-60 minutes before dropwise addition of 30 mL of TPAOH. If, upon addition of the first few drops of TPAOH, the clear solution turned cloudy, further addition of TPAOH was only continued after the solution again became clear again. When all the TPAOH had been added, the PTFE liner was transferred to a water bath maintained at 80°C, and the solution was stirred for 3 hours. After heating, the reaction mixture was diluted to its original volume with deionized water and stirred overnight. The reaction mixture in the liner was then transferred to a Pan' autoclave reactor and crystalli zed for 96 hours at 175°C under autogenous pressure. After cooling, the resulting white gelatinous mixture was then centrifuged at 20,000 rpm for 10 minutes. The supernatant liquid was discarded, deionized water was added, and the white residue was re-dispersed under sonication for 5 minutes. The process was repeated 4-5 times until the pH of the supernatant liquid was close to neutral. The product (ZSM-5 containing Ti) was dried at 80-90°C overnight and then calcmed in air at 55O°C for 3-6 hours to afford a white powder.

[0212] Example 2: Synthesis of Non-Selectivated Oxidation Catalysts by Exchange of Oxidizing Metal (Catalyst B, zeolite bound). Zeolites having a ZSM-5 framework were synthesized from initial reaction mixtures having a SiZAl(gel) molar ratio of 25-90 and a pH of 11, differing in sequence of mixing and using various sources of Si, Al, and Na among runs. A1C13-6H ₂ O, A1(NO3)3-9H2O, sodium silicate, 27% Si 02, 10% NaOH, TEOS, and TPAOH (Fluka, 20 wt% in water) were used for preparation of the synthesis mixtures. The Si/TPAOH molar ratio was maintained at 2.65 for all the mixtures.

[0213] In brief, an Al source compound dissolved in 10 mL of distilled water and a Si source (22.6 g of TEOS or 24.5 g of sodium silicate) in 5 mL of ethanol were mixed and stirred for 90 minutes. A total of 40.75 g of TPAOH solution in 40 mL water was stirred for 90 minutes, added to the Si-Al mixture, and then the combined mixture was stirred for an additional 90 minutes. Alternately, the sequence of mixing was altered by adding TPAOH to TEOS prior to mixing with AlCh. A portion of the resulting gel was dried under vacuum at room temperature, and traces of water were removed at 60°C. Crystallization from the gels was carried out in a Teflon-lined autoclave with agitation at 170°C for 5 to 10 days. The products were then calcined at 550°C over 10 hours. The zeolite products were exchanged with Na* ions (0.1 M NaCl), followed by an oxidation-promoting metal solution [Co(II), Co(III), Mn(II), Cu(II), Fe(II), Ru(lll), Rh(III), or Ce(lll)]. In the case of Co ³¹ , ion exchange was performed three times with 0.05 M Co(NO3)3 at room temperature. Chemical compositions of the products (Si/Al and Co) were analyzed by X-ray fluorescence.

[0214] Catalyst extrudates were prepared by first mixing ZSM-5 core ciystals, prepared as above, with amorphous silica containing a trace amount of alumina and then extruding the mixture into a silica-bound extrudate. Next, the silica binder of the extrudate was converted to a second zeolite by aging the extrudate at elevated temperatures of 100°C-250°C in an aqueous solution containing tetrapropylammonium bromide as a template and sufficient hydroxide ions (NaOH) to convert the silica to binder zeolite crystals. In one instance, the zeolite comprised 70 wt % H-ZSM-5 core crystals (average particle size of 3.5 microns) having a silica to alumina mole ratio of 75: 1 and 30 wt. % ZSM-5 binder ciystals having a silica to alumina mole ratio of approximately 900: 1.

[0215 [ Example 3: Catalyst B (Co Form) Selectivation (triple treatment). The catalyst extrudate prepared as in Example 2 was selectivated by contacting Co-ZSM-5/silica bound (65 wt% H-ZSM-5: 35 wt% silica, 0.4 Dm particle size, 26: 1 Si:Al ₂ ratio) with 7.8 wt% dimethylphenylmethyl polysiloxane (Dow-550) dissolved in decane and subsequently calcining at 1000°C. The catalyst was treated with the polysiloxane twice more using substantially the same procedure.

[0216] Example 4: Catalyst B (Co Form) Selectivation (quadrapie treatment).

Example 3 was repeated, except an additional polysiloxane treatment was conducted.

[0217] Example 5: Catalyst B (Co Form) Selectivation (triple treatment followed by steaming). Example 3 was repeated, except after the third polysiloxane treatment, the catalyst was steamed at atmospheric pressure for 24 hours at 1 OOCf’C.

[0218] Example 6: Catalyst B (Co Form) Selectivation (quadruple treatment followed by steaming). Example 3 was repeated, except an additional polysiloxane treatment was conducted. After the fourth polysiloxane treatment, the catalyst was steamed at atmospheric pressure for 24 hours at 1000°C.

[0219] Example 7 (Prophetic): Catalyst A Selectivation (triple treatment). Example 3 is repeated, except Catalyst A (Example 1) is substituted for Catalyst B.

[0220] Example 8 (Prophetic). Catalyst B (Ru Form) Selectivation (triple treatment). Example 3 is repeated, except Catalyst B exchanged with 5 wt% RuC13 was used, and an additional polysiloxane treatment was conducted.

[022] ] Example 9 (Prophetic). Non-Selective H ₂ O ₂ Oxidation with Catalyst A. Oxidation is carried out by loading catalyst powder from Example 1 (5 g) into a batch reactor with 1 mole of mixed xylenes (25% para, 25% ortho, 50% meta), and 0.25 mole of H ₂ O ₂ (35% in water). The reaction is heated at 110°C for 3 hours.

[0222] Example 10 (Prophetic). Selective H ₂ O ₂ Oxidation with Selectivated Catalyst A. Example 9 is repeated, except the selectivated catalyst of Example 7 is used.

[0223] Example 11 (Prophetic). Non-Selective Air Oxidation with Catalyst B (Co Form). Oxidation is carried out by loading extrudate from Example 2 (5 g, Co form) into a batch reactor with 1 mole of mixed xylenes (25% para, 25% ortho, 50% meta). The reactor is pressurized with air and run under air flow conditions with heating at 150°C for 3 hours.

[0224] Examples 12A-12D (Prophetic). Selective Air Oxidation with Selectivated Catalyst B (Co Form). Example 11 is repeated, except the catalyst extrudates of Examples 3- 6 are used (all Co forms).

[0225] Example 13 (Prophetic). Selective Air Oxidation with Selectivated Catalyst B (Ru Form). Example 11 is repeated, except the catalyst extrudate of Example 8 is used.

[0226 ] Example 14 (Prophetic). SelectivHe ₂ O ₂ Oxidation with Selectivated Catalyst B (Ru Form). Oxidation is carried out by loading catalyst extrudate from Example 8 (5 g) into a batch reactor with 1 mole of mixed xylenes (2.5% para. 25% ortho, 50% meta), and 0.25 mole ofH ₂ O ₂ (35% in water) is added dropwise at 70°C over 3 hours.

[0227] Example 15 (Prophetic). Selective HOG Oxidation with Selectivated Catalyst B (Ru Form). Example 14 is repeated, except a HOG solution (30% in water) is added dropwise at room temperature over 3 hours.

[0228] Many alterations, modifications, and variations will be apparent to one having ordinary skill in the art in light of the foregoing description without departing from the spirit or scope of the present disclosure and that when numerical limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated.

[0229] All documents described herein are incorporated by reference herein for purposes of all jurisdictions where such practice is allowed, including any priority documents and/or testing procedures to the extent they are not inconsistent with this text. As is apparent from the foregoing general description and the specific embodiments, while forms of the disclosure have been illustrated and described, various modifications can be made without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the disclosure be limited thereby. For example, the compositions described herein may be free of any component, or composition not expressly recited or disclosed herein. Any method may lack any step not recited or disclosed herein. Likewise, the term “comprising’ ⁷ is considered synonymous with the term “including.” Whenever a method, composition, element or group of elements is preceded with the transitional phrase “comprising,” it is understood that we also contemplate the same composition or group of elements with transitional phrases “consisting essentially of,” “consisting of,” “’selected from the group of consisting of” or “is” preceding the recitation of the composition, element, or elements and vice versa.

[0230] One or more illustrative incarnations incorporating one or more invention elements are presented herein. Not all features of a physical implementation are described or shown in this application for the sake of clarity It is understood that in the development of a physical embodiment incorporating one or more elements of the present invention, numerous implementation-specific decisions must be made to achieve the developer's goals, such as compliance with system-related, business-related, government-related and other constraints, which vary by implementation and from time to time. While a developer's efforts might be time-consuming, such efforts w'ould be, nevertheless, a routine undertaking for those of ordinary skill in the art and having benefit of this disclosure.

[0231] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the present specification and associated claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the embodiments of the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claim, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. [0232] Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed, including the lower limit and upper limit. In particular, every range of values (of the form, ‘‘from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces.

[0233] Therefore, the present disclosure is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present disclosure may be modified and practiced in different but equivalent manners apparent to one having ordinary skill in the art and having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present disclosure. The embodiments illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein.