BACKGROUND

[0001] Aromatic hydrocarbons, for example, benzene, toluene, ethylbenzene, xylenes, and polyaromatic hydrocarbons such as naphthalene, are important commodity chemicals in the petrochemical industry.

[0002] A method of preparing aromatic hydrocarbons is by the dehydroaromatization (DHA) of methane, as methane is one of the most abundant organic compounds on earth. For example, methane is the major constituent of natural gas; large amounts of methane are trapped in marine sediments as hydrates and in coal shale as coal bed methane; and it can also be derived from a biomass as a biogas.

[0003] The dehydroaromatization of methane is becoming increasingly important.

Methane dehydroaromatization over catalysts is a promising process for the production of valuable aromatic compounds and hydrogen from natural gas.

[0004] Catalyst deactivation due to coke formation is one of the main drawbacks of the dehydroaromatization of methane. Coke formation can decrease the life of the catalyst. The equilibrium of the dehydroaromatization of methane is also low due to coke formation on the catalyst. Coke formation is largely favored at high temperatures, particularly at 700°C and above. Coke formation can be of two types, hard coke and soft coke. Hard coke is mainly known as graphitic type coke. Soft coke can be polyaromatic deposits.

[0005] This invention addresses the problem of reduced catalyst activity in a methane dehydroaromatization reaction due to coke formation.

BRIEF DESCRIPTION

[0006] Disclosed herein are processes for improving catalyst regeneration and stability of catalysts in a methane dehydroaromatization process. The processes relate to the regeneration of a dehydroaromatization catalyst and to a method for dehydroaromatization of methane

[0007] A method of regenerating a dehydroaromatization catalyst used in the

dehydroaromatization of methane can comprise: contacting the deactivated

dehydroaromatization catalyst with a first regeneration gas stream comprising hydrogen at a temperature of 700°C to 850°C, and contacting the deactivated dehydroaromatization catalyst with a second regeneration gas stream comprising 0.1 to 10 percentage by volume (vol.%) oxygen at a temperature of 400°C to 600°C. [0008] A method for dehydroaromatization of methane can comprise: passing a feedstock comprising methane gas to an aromatization reactor; converting a portion of the methane gas in the feedstock to aromatic hydrocarbons with a dehydroaromatization catalyst at a reaction temperature of 700°C to 850°C; and regenerating at least a portion of the

dehydroaromatization catalyst by contacting the dehydroaromatization catalyst with a first regeneration gas stream comprising hydrogen at a temperature of 700°C to 850°C, preferably 800°C to 850°C, contacting the dehydroaromatization catalyst with a second regeneration gas stream comprising 0.1 to 20 vol.% oxygen at a temperature of 400°C to 600°C.

[0009] The above described and other features are exemplified by the following figures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following figures are exemplary embodiments wherein the like elements are numbered alike.

[0011] FIG. 1 shows methane conversion on a Mo/ZSM-5 catalyst.

[0012] FIG. 2 shows Benzene selectivity in mol% on a Mo/ZSM-5 catalyst.

[0013] FIG. 3 shows methane conversion over time on Mo/HZSM-5 at 1,050 GHSV and 725°C using only oxygen regeneration at 550°C at 2500 GHSV under 5% 0 ₂ and 95% N _2.

[0014] FIG. 4 shows methane conversion over time on Mo/HZSM-5 at 1050 GHSV and 725°C using only hydrogen based regeneration at 800°C and 1,050 GHSV under 100% H _2.

[0015] FIG. 5 shows methane conversion over time on Mo/HZSM-5 at 1,050 GHSV and 725°C using both hydrogen based regeneration at 800°C and 1,050 GHSV under 100% H ₂ for 30 minutes each followed by oxygen regeneration at 550°C at 2,500 GHSV under 5% 0 ₂ and 95% N ₂ .

DETAILED DESCRIPTION

[0016] Hard coke can only be removed under dilute oxygen at lower temperatures (400 to 550°C), while soft coke can be removed by pure hydrogen at higher temperatures (700 to 850°C). As described herein, the term "coke" is used to mean carbon containing solid materials, which are essentially non-volatile solids at the reaction conditions. Because cooling is required for regeneration of the catalyst using dilute oxygen, hydrogen is considered to be more effective for catalyst regeneration. However, hydrogen based regeneration alone does not give very long term catalyst stability. Therefore, periodic regeneration of the catalyst by decoking with dilute oxygen is described. This needs to be done at a lower temperature as regeneration with dilute oxygen at high temperatures can damage the catalyst.

[0017] Disclosed herein is the usage of both hydrogen and oxygen (e.g., dilute oxygen) to de-coke the catalyst and regain catalyst activity. It has been discovered herein that regeneration of a coked catalyst with hydrogen containing stream (e.g., comprising at least 90% by volume (vol%) hydrogen) and periodic regeneration with oxygen (e.g., dilute oxygen containing stream comprising less than or equal to 10 vol% oxygen, for example dilute oxygen 0.1 to 5 vol%) will improve the overall process and feasibility of catalyst regeneration.

[0018] The regeneration of a catalyst for the dehydroaromatization of methane with a stream of hydrogen containing gas is done for several cycles, followed by periodic regeneration with oxygen at 400 to 600°C. This combination of regeneration protocols remarkably improves catalyst stability and regeneration. Moreover, it was found that catalyst performance is even further improved when the dehydroaromatization of methane is carried out for 10 to 30 minutes (min), preferably 10 to 20 minutes, e.g., for 15 min.

[0019] The methane dehydroaromatization reaction disclosed herein converts methane to an aromatic compound by contacting the catalyst with a methane feed to produce an aromatic compound. A methane gas feed used for converting to an aromatic compound can be supplied from, for example, at least one of a natural gas, methane hydrates, coal bed methane, synthetic gas, and biogas. The aromatic compound can comprise at least one of benzene, toluene, naphthalene, and xylene. The aromatic compound can comprise at least one of benzene and toluene. The aromatic compound can further comprises traces of C ₂ H ₄ , C ₂ H ₆ , or C ₃ H ₆ , or a combination thereof. Further, the aromatic compound can be substantially benzene (e.g., greater than or equal to 80 vol.% of the aromatic compound can be benzene).

[0020] The aromatic compound can be produced under reaction conditions at a temperature range from 700°C to 850°C. The temperature range for the production of the aromatic compound can be derived from any two exemplary temperatures. For example, the temperature can range from 710°C to 825°C, preferably 720°C to 800°C, or 720°C to 750°C.

[0021] The aromatic compound can be produced under reaction conditions at a gas hourly space velocity (GHSV) of 300 milliliters/grams/hour (ml/g/h) to 2,000 ml/g/h. The GHSV can be in a range derived from any two exemplary values. 1000 to 3000 ml/g/hr.

[0022] A zeolite catalyst can be used in a methane dehydroaromatization reaction, for example a Mo/ZSM-5, Mo/ZSM-11 and Mo/MCM22 catalyst can be used. For example, a ZSM-5 catalyst can be used in a methane dehydroaromatization reaction, e.g., a molybdenum oxide ZSM-5 (Mo-oxide/ZSM-5) catalyst. A Mo-oxide/ZSM-5 catalyst provides good catalytic performance, a high aromatic compound selectivity and productivity (for example greater than 85%, and even as high as 90% selectivity), a high hydrocarbon conversion (for example conversion greater than or equal to 6%). Coke build up on the Mo-oxide/ZSM-5 catalyst can reduce catalyst performance and catalyst lifetime. The method disclosed herein reduces coke build up on a Mo-oxide/ZSM-5 catalyst, thereby increasing the productivity of the methane dehydroaromatization reaction and the lifetime of the catalyst maintained a conversion of greater than 8% for at least 10 hours (hrs), for example, for 20 hrs. For example, the Mo-oxide/ZSM-5 catalyst can be reduced with hydrogen to form methane for several cycles and periodically cooled and exposed to oxygen (e.g., dilute oxygen) to form CO/C0 ₂ , as shown in Figures 3, 4, and 5.

[0023] The regeneration of the coked catalyst with hydrogen can be carried out at the temperature (700°C to 850°C or above). For example, the hydrogen regeneration can be at a temperature of 750°C to 850°C, e.g., 770°C to 825°C, or 800°C to 850°C. The hydrogen stream can be pure hydrogen (for example 100 vol% hydrogen), or 90 vol% hydrogen or greater. The hydrogen stream can be added directly to the methane dehydroaromatization reactor for the first regeneration of the Mo-oxide/ZSM-5 catalyst. The hydrogen flow (GHSV) could be, for example, 1,000 to 3,000 ml/g/hr for 30 to 60 min for every 15 to 30 min reaction cycle. In other words, after each 15 to 30 min reaction cycle for the production of the aromatic compound, the catalyst can be regenerated with a hydrogen flow of 1,000 to 3,000 ml/g/hr for 30 to 60 min.

[0024] Optionally, the hydrogen gas can be supplied from the product stream of the methane dehydroaromatization reaction. Particularly, the product stream of the methane dehydroaromatization reaction can be processed further to separate pure hydrogen from the remainder of the products. The hydrogen separated from the product stream can be greater than 90 vol% hydrogen. The hydrogen separated from the product stream with no more than 0.1 to 3% residual methane of the methane dehydroaromatization reaction can be used as the hydrogen stream for catalyst regeneration to save cost of supplying an additional hydrogen stream.

[0025] Coke removal by oxidation can be carried out with oxygen (e.g., dilute oxygen) at 400°C to 600°C, preferably at 500°C to 550°C. Following the methane dehydroaromatization reaction at 700°C to 850°C, the catalyst bed can be cooled to below 600°C (e.g., to 500 to 550°C) for regeneration with oxygen. Regeneration with oxygen at higher temperatures can damage the Mo-oxide/ZSM-5 catalyst. Once the catalyst bed cools to the desired temperature range, the oxygen can be introduced, e.g., at a GHSV of 1,000 to 3,000 ml/g/hr for about 30 to 60 min to complete the regeneration. The oxygen can be consumed by oxidizing the coke to C0 ₂ or CO. After the regeneration step is complete, the catalyst bed can be heated back to reaction temperature of 700°C to 850°C. Desirably, an inert gas is flowing (e.g., a gas that does not react with the catalyst, such as nitrogen gas) during the heating of the catalyst to sweep away any unreacted oxygen.

[0026] Optionally, the oxygen regeneration can be based upon changes in the catalyst activity. For example, an initial catalyst activity (AO can be determined (e.g., of the fresh catalyst). Then, the catalyst activity (e.g., reaction catalyst activity (AO) can be monitored. The reaction catalyst activity can be compared to the initial catalyst activity such that when the reaction catalyst activity is at least 30% (preferably at least 25%) less than the initial catalyst activity, the catalyst can be regenerated with oxygen. In other words, the following equation can be used to determine when to regenerate the catalyst with oxygen:

(Ai - O.3A0 > A _r .

Preferably the catalyst is regenerated with oxygen when the following equation is met:

(Ai - 0.25 AO > A _r .

[0027] The percent conversion using the catalyst with both hydrogen and oxygen based regeneration can be maintained at 6% and 12% for 1,000 minutes, and even for 1,440 minutes. When using both hydrogen and oxygen based regeneration the conversion to the aromatic compound can be maintained at greater than or equal to 7% for 500 min., for example, for 800 min, and even for 1,000 min. When using both hydrogen and oxygen based regeneration the conversion to the aromatic compound can be maintained at greater than or equal to 8% for 800 min., and even for 1,000 min. When using both hydrogen and oxygen based regeneration the conversion to the aromatic compound can be maintained at greater than or equal to 9% for 800 min.

[0028] The selectivity and productivity of benzene, and an activity preservation ratio (APR) of the catalyst composition can be measured at time-on-stream (TOS) in the range from 1 hour to 40 hours. For example, the TOS can range from 3 hours to 38 hours, or from 5 hours to 30 hours, or from 8 hours to 25 hours.

[0029] The benzene yield of the disclosed catalyst composition including regeneration with both hydrogen and oxygen (e.g., 0.1 to 5 vol% oxygen) can be greater than or equal to 7%, even greater than or equal to 7.2%, when measured at a temperature between 670°C and 720°C.

[0030] The APR of the catalyst composition with regeneration with both pure hydrogen and dilute oxygen in the conversion of the methane to an aromatic compound, at 24 hours, can be 70% to 97%, for example 75% to 85%, or 75% to 80%. The APR of the catalyst composition with regeneration with both pure hydrogen and dilute oxygen in the conversion of the methane to an aromatic compound can be at least 95.6 % or greater at 24 hours. For example, the APR of the catalyst composition with regeneration with both pure hydrogen and dilute oxygen in the conversion of the methane to an aromatic compound can be 97% or greater at 24 hours.

EXAMPLES

[0031] This disclosure is further illustrated by the following examples, which are non- limiting.

[0032] A methane to benzene reaction was carried out in a fixed bed reactor using a Mo zeolite catalyst, charged into a quartz tubular down-flow reactor. The performance of the Mo zeolite catalyst was studied at a temperature 700 °C to 800 °C at a gas hourly space velocity of 525 to 3,000 ml/g/hr. The catalyst was first pre-carburized and then the reaction was started. Reaction was carried out under 100% methane for 15 minutes followed by 30 minutes of hydrogen regeneration at a temperature 800 to 850°C. The major products of the methane to benzene reaction are benzene, toluene, naphthalene and coke.

[0033] Coke formation on the surface of the catalyst blocks the pores which results in the deactivation of the catalyst. The coke was removed from the surface of the catalyst using hydrogen regeneration and periodic oxygen regeneration. Oxygen regeneration was carried out at 450 to 600°C for 15 to 60 min. Temperatures were raised again to 700 °C to 800 °C in presence of methane after the dilute oxygen regeneration.

[0034] During the oxidative regeneration the carbon deposited on the catalyst is converted to CO and C0 ₂ . The reaction was carried out at 700 to 800°C for 72 cycles. Each cycle had 15 minutes of reactions under CH4 followed by 30 min of regeneration under hydrogen. Methane conversion on a Mo/ZSM-5 catalyst at 525 to 3,000 ml/g/hr space velocity is shown in Figure 1.

[0035] Oxygen regenerations were done at 450 to 600°C when the catalyst activity fell below 25% of the initial catalyst activity. The benzene yields during these 72 cycles were stable which shows that the catalyst was active during all these cycles. Benzene selectivity during all these 75 cycles was found to be between 85 to 100 mol% as shown in Figure 2.

[0036] The methods and processes disclosed herein include(s) at least the following embodiments:

[0037] Embodiment 1: A method for dehydroaromatization of methane, comprising: passing a feedstock comprising methane gas to an aromatization reactor; converting a portion of the methane gas in the feedstock to aromatic hydrocarbons with a dehydroaromatization catalyst at a reaction temperature of 700°C to 850°C; and regenerating at least a portion of the dehydroaromatization catalyst by contacting the dehydroaromatization catalyst with a first regeneration gas stream comprising hydrogen at a temperature of 700°C to 850°C, preferably 800°C to 850°C, contacting the dehydroaromatization catalyst with a second regeneration gas stream comprising 0.1 to 20 vol.% oxygen at a temperature of 400°C to 600°C.

[0038] Embodiment 2: A method of regenerating a dehydroaromatization catalyst used to convert methane to an aromatic compound, the method comprising: contacting the deactivated dehydroaromatization catalyst with a first regeneration gas stream comprising hydrogen at a temperature of 700°C to 850°C; and contacting the deactivated dehydroaromatization catalyst with a second regeneration gas stream comprising 0.1 to 10 vol.% oxygen at a temperature of 400°C to 600°C.

[0039] Embodiment 3: The method of any of the preceding Embodiments, wherein the second regeneration gas stream comprises 3 vol.% to 8 vol.% oxygen.

[0040] Embodiment 4: The method of any of Embodiments 1 - 2, wherein the second regeneration gas stream comprises 0.1 vol.% to 5 vol.% oxygen.

[0041] Embodiment 5: The method of any of any of Embodiments 1 - 2, wherein the second regeneration gas stream comprises air.

[0042] Embodiment 6: The method of any of the preceding Embodiments, wherein the deactivated dehydroaromatization catalyst is contacted with the first regeneration gas for at least 15 minutes, at a rate of at least 800 ml/g/h at STP conditions.

[0043] Embodiment 7: The method of any of the preceding Embodiments, wherein the deactivated dehydroaromatization catalyst is contacted with the second regeneration gas for at least 15 minutes, at a rate of at least 800 ml/g/h at STP conditions.

[0044] Embodiment 8: The method of any of the preceding Embodiments, wherein the first regeneration gas stream further comprises an inert gas.

[0045] Embodiment 9: The method of any of the preceding Embodiments, wherein the dehydroaromatization catalyst includes a zeolite-based catalyst.

[0046] Embodiment 10: The method of any of the preceding claims, further comprising determining an initial catalyst activity of the dehydroaromatization catalyst; monitoring a reaction catalyst activity of the dehydroaromatization catalyst; comparing the reaction catalyst activity to the initial catalyst activity; and when the reaction catalyst activity is at least 25% less than the initial catalyst activity, contacting the dehydroaromatization catalyst with the second regeneration gas stream.

[0047] Embodiment 11: The method of Embodiment 10, wherein the comparing is performed subsequent to contacting the dehydroaromatization catalyst with the first regeneration gas stream. [0048] Embodiment 12: The method of any of the preceding embodiments, further comprising, subsequent to contacting the dehydroaromatization catalyst with the second regeneration gas stream, heating the dehydroaromatization catalyst back to the reaction temperature while flowing an inert gas over the dehydroaromatization catalyst.

[0049] Embodiment 13: The method of Embodiment 11, wherein the inert gas is nitrogen gas.

[0050] Embodiment 14: The method of any of the preceding embodiments, wherein the dehydroaromatization conditions include a time period of 1-30 mins, and a gas hourly space velocity of 1,000 -ml/g/h to 20,000 ml/g/h at STP conditions.

[0051] All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other (e.g., ranges of "up to 25 wt.%, or, more specifically, 5 wt.% to 20 wt.%", is inclusive of the endpoints and all intermediate values of the ranges of "5 wt.% to 25 wt.%," etc.). "Combinations" is inclusive of blends, mixtures, alloys, reaction products, and the like. The terms "first," "second," and the like, do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms "a" and "an" and "the" do not denote a limitation of quantity, and are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. "Or" means "and/or" unless clearly stated otherwise. Reference throughout the specification to "some embodiments", "an embodiment", and so forth, means that a particular element described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments.

[0052] Unless specified to the contrary herein, all test standards are the most recent standard in effect as of the filing date of this application, or, if priority is claimed, the filing date of the earliest priority application in which the test standard appears.

[0053] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this application belongs. All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.

[0054] Compounds are described using standard nomenclature. For example, any position not substituted by any indicated group is understood to have its valency filled by a bond as indicated, or a hydrogen atom. A dash ("-") that is not between two letters or symbols is used to indicate a point of attachment for a substituent. For example, -CHO is attached through carbon of the carbonyl group.

[0055] While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.