ADIABATIC PROCESSES

TECHNICAL FIELD

[0001] The present disclosure relates to catalyst compositions for oxidative coupling of methane (OCM), more specifically catalyst compositions based on oxides of alkaline earth metals, and rare earth elements for OCM, and methods of making and using same.

BACKGROUND

[0002] Hydrocarbons, and specifically olefins such as ethylene, are typically building blocks used to produce a wide range of products, for example, break-resistant containers and packaging materials. Currently, for industrial scale applications, ethylene is produced by heating natural gas condensates and petroleum distillates, which include ethane and higher hydrocarbons, and the produced ethylene is separated from a product mixture by using gas separation processes.

[0003] Oxidative coupling of the methane (OCM) has been the target of intense scientific and commercial interest for more than thirty years due to the tremendous potential of such technology to reduce costs, energy, and environmental emissions in the production of ethylene (C ₂ H ₄ ). As an overall reaction, in the OCM, methane (CH ₄ ) and oxygen (0 ₂ ) react exothermically over a catalyst to form C ₂ H ₄ , water (H ₂ 0) and heat.

[0004] Ethylene can be produced by OCM as represented by Equations (I) and (II):

2CH ₄ + 0 ₂ → C ₂ H ₄ + 2H ₂ 0 ΔΗ = - 67 kcal/mol (I)

2CH ₄ + l/20 ₂ → C ₂ H ₆ + H ₂ 0 ΔΗ = - 42 kcal/mol (II)

[0005] Oxidative conversion of methane to ethylene is exothermic. Excess heat produced from these reactions (Equations (I) and (II)) can push conversion of methane to carbon monoxide and carbon dioxide rather than the desired C ₂ hydrocarbon product (e.g., ethylene):

CH ₄ + 1.50 ₂ → CO + 2H ₂ 0 ΔΗ = - 124 kcal/mol (III)

CH ₄ + 20 ₂ → C0 ₂ + 2H ₂ 0 ΔΗ = - 192 kcal/mol (IV)

The excess heat from the reactions in Equations (III) and (IV) further exasperate this situation, thereby substantially reducing the selectivity of ethylene production when compared with carbon monoxide and carbon dioxide production.

[0006] Additionally, while the overall OCM is exothermic, catalysts are used to overcome the endothermic nature of the C-H bond breakage. The endothermic nature of the bond breakage is due to the chemical stability of methane, which is a chemically stable molecule due to the presence of its four strong tetrahedral C-H bonds (435 kJ/mol). When catalysts are used in the OCM, the exothermic reaction can lead to a large increase in catalyst bed temperature and uncontrolled heat excursions that can lead to catalyst deactivation and a further decrease in ethylene selectivity. Furthermore, the produced ethylene is highly reactive and can form unwanted and thermodynamically favored deep oxidation products.

[0007] Generally, in the OCM, CH ₄ is first oxidatively converted into ethane (C ₂ H ₆ ), and then into C ₂ H ₄ . CH ₄ is activated heterogeneously on a catalyst surface, forming methyl radicals (e.g., CH ₃ *), which then couple in a gas phase to form C ₂ H ₆ . C ₂ H ₆ subsequently undergoes dehydrogenation to form An overall yield of desired C ₂ hydrocarbons is reduced by non-selective reactions of methyl radicals with oxygen on the catalyst surface and/or in the gas phase, which produce (undesirable) carbon monoxide and carbon dioxide. Some of the best reported OCM outcomes encompass a -20% conversion of methane and -80% selectivity to desired C ₂ hydrocarbons.

[0008] There are many catalyst systems developed for OCM processes, but such catalyst systems have many shortcomings. For example, conventional catalysts systems for OCM display catalyst performance problems, stemming from a need for high reaction temperatures to achieve desired conversions and selectivities, while displaying unstable performance across wide temperature ranges. Thus, there is an ongoing need for the development of catalyst compositions for OCM processes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a detailed description of the preferred aspects of the disclosed methods, reference will now be made to the accompanying drawing in which:

[0010] Figure 1 displays a graph of oxygen conversion as a function of temperature for an oxidative coupling of the methane (OCM) reaction for different catalysts in Example 2;

[0011] Figure 2 displays a graph of C ₂₊ selectivities as a function of temperature for an OCM reaction for different catalysts in Example 2;

[0012] Figure 3 displays a graph of oxygen conversion as a function of temperature for an OCM reaction for different catalysts in Example 3; and

[0013] Figure 4 displays a graph of C ₂₊ selectivities as a function of temperature for an OCM reaction for different catalysts in Example 3.

DETAILED DESCRIPTION

[0014] Disclosed herein are oxidative coupling of methane (OCM) catalyst compositions and methods of making and using same. In an aspect, an OCM catalyst composition can be characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states. [0015] A method of making an OCM catalyst composition can generally comprise the steps of (a) forming an OCM catalyst precursor mixture; wherein the OCM catalyst precursor mixture comprises one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or a third rare earth element cation; wherein the first rare earth element cation, the second rare earth element cation, and the third rare earth element cation, when present, are not the same; wherein the OCM catalyst precursor mixture is characterized by a molar ratio of first rare earth element to alkaline earth metal of b: l, wherein b is from about 0.3 to about 10.0; wherein the OCM catalyst precursor mixture is characterized by a molar ratio of second rare earth element to alkaline earth metal of c: 1, wherein c is from about 0 to about 10.0; and wherein the OCM catalyst precursor mixture is characterized by a molar ratio of redox agent or third rare earth element to alkaline earth metal of d: 1 , wherein d is from about 0 to about 10.0; and (b) calcining at least a portion of the OCM catalyst precursor mixture to form the OCM catalyst composition.

[0016] In some aspects, the OCM catalyst composition disclosed herein can be employed in single stage OCM processes. In other aspects, the OCM catalyst composition disclosed herein can be employed in multi-stage OCM processes.

[0017] Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, and the like, used in the specification and claims are to be understood as modified in all instances by the term "about." Various numerical ranges are disclosed herein. Because these ranges are continuous, they include every value between the minimum and maximum values. The endpoints of all ranges reciting the same characteristic or component are independently combinable and inclusive of the recited endpoint. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations. The endpoints of all ranges directed to the same component or property are inclusive of the endpoint and independently combinable. The term "from more than 0 to an amount" means that the named component is present in some amount more than 0, and up to and including the higher named amount.

[0018] The terms "a," "an," and "the" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. As used herein the singular forms "a," "an," and "the" include plural referents.

[0019] As used herein, "combinations thereof is inclusive of one or more of the recited elements, optionally together with a like element not recited, e.g., inclusive of a combination of one or more of the named components, optionally with one or more other components not specifically named that have essentially the same function. As used herein, the term "combination" is inclusive of blends, mixtures, alloys, reaction products, and the like. [0020] Reference throughout the specification to "an aspect," "another aspect," "other aspects," "some aspects," and so forth, means that a particular element (e.g., feature, structure, property, and/or characteristic) described in connection with the aspect is included in at least an aspect described herein, and may or may not be present in other aspects. In addition, it is to be understood that the described element(s) can be combined in any suitable manner in the various aspects.

[0021] As used herein, the terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, include any measurable decrease or complete inhibition to achieve a desired result.

[0022] As used herein, the term "effective," means adequate to accomplish a desired, expected, or intended result.

[0023] As used herein, the terms "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "include" and "includes") or "containing" (and any form of containing, such as "contain" and "contains") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0024] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art.

[0025] Compounds are described herein 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 the carbon of the carbonyl group.

[0026] In some aspects, a method for producing olefins as disclosed herein can comprise multiple stages (e.g., as part of a multi-stage process), wherein each individual stage can comprise an oxidative coupling of methane (OCM) reactor or reaction zone, and wherein each individual stage can be repeated as necessary to achieve a target methane conversion for the overall multi-stage process. For purposes of the disclosure herein, a stage of a process can be defined as a single pass conversion through a given catalyst bed. Any suitable physical configuration and arrangement of components of a catalyst bed (e.g., catalyst particles, inert media, spacers, support structures, screens, etc.) within the given catalyst bed may be employed. A multi-stage process generally comprises a plurality of individual stages (e.g., a plurality of reaction zones), wherein each individual stage (e.g., reaction zone) comprises a single pass conversion through a given catalyst bed. While the current disclosure will be discussed in detail in the context of a single stage comprising a single reactor comprising a given catalyst bed, it should be understood that any suitable stage/reactor/catalyst bed configurations can be used. For example, two or more stages of a multistage process can be housed in one or more reactors. As will be appreciated by one of skill in the art, and with the help of this disclosure, multiple stages can be housed within a single reaction vessel, for example a vessel comprising two or more catalyst beds in series. Further, as will be appreciated by one of skill in the art, and with the help of this disclosure, multiple vessels can be part of a single stage, for example two or more vessels in parallel, wherein a reactant mixture is distributed between and introduced to the two or more vessels in parallel. While the current disclosure will be discussed in detail in the context of a multi-stage process comprising 2 stages, it should be understood that any suitable number of stages can be used, such as for example, 2 stages, 3 stages, 4 stages, 5 stages, 6 stages, 7 stages, 8 stages, 9 stages, 10 stages, or more stages. Such multi-stage processes may be implemented via a corresponding plurality of reactors in series, as is described herein.

[0027] In an aspect, a method for producing olefins can comprise a first stage and a second stage, wherein the first stage comprises a first reactor (e.g., first adiabatic reactor, first OCM reactor), and wherein the second stage comprises a second reactor (e.g., second adiabatic reactor, second OCM reactor), and wherein the first reactor and the second reactor are in series, with the second reactor downstream of the first reactor.

[0028] In an aspect, a method for producing olefins as disclosed herein can comprise (i) introducing a first reactant mixture (e.g., OCM reactant mixture) to a first adiabatic reactor comprising a first OCM catalyst composition, wherein the first reactant mixture comprises methane (CH ₄ ) and oxygen (0 ₂ ), wherein the first reactant mixture is characterized by a first inlet temperature, and wherein the first OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; and (ii) allowing at least a portion of the first reactant mixture to contact at least a portion of the first OCM catalyst composition and react via an OCM reaction to form a first product mixture comprising unreacted methane and olefins.

[0029] The OCM reactant mixture (e.g., first reactant mixture, second reactant mixture) can be a gaseous mixture. The OCM reactant mixture can comprise a hydrocarbon or mixtures of hydrocarbons, and oxygen. In some aspects, the hydrocarbon or mixtures of hydrocarbons can comprise natural gas (e.g., CH ₄ ), liquefied petroleum gas comprising C ₂ -C ₅ hydrocarbons, heavy hydrocarbons (e.g., C ₆ to C ₂ hydrocarbons such as diesel fuel, jet fuel, gasoline, tars, kerosene, etc.), oxygenated hydrocarbons, biodiesel, alcohols, dimethyl ether, and the like, or combinations thereof. In an aspect, the OCM reactant mixture can comprise CH ₄ and 0 ₂ . As will be appreciated by one of skill in the art, and with the help of this disclosure, methane (or a hydrocarbon or mixtures of hydrocarbons) is introduced into a multi-stage process in the first stage into the OCM reactor (e.g., a first reactor); the OCM reactant mixture for subsequent stages (e.g., a second stage) will utilize the unreacted methane and any other hydrocarbons present that were recovered from the first stage (after passing through any other processes that are part of the first stage). In some aspects, some methane (or a hydrocarbon or mixtures of hydrocarbons) could be optionally added to reactant mixtures in stages other than the first stage (e.g., fresh hydrocarbon feed at one or more stages subsequent to a first stage), to supplement a recovered unreacted methane, if necessary.

[0030] The 0 ₂ used in the OCM reactant mixture (e.g., first reactant mixture, second reactant mixture) can be oxygen gas (which may be obtained via a membrane separation process), technical oxygen (which may contain some air), air, oxygen enriched air, and the like, or combinations thereof. 0 ₂ can be introduced in the first reactor, as well as in any subsequent stages in any OCM reactor (e.g., a second reactor).

[0031] The OCM reactant mixture (e.g., first reactant mixture, second reactant mixture) can further comprise a diluent. A diluent can be introduced in the first reactor, as well as in any subsequent stages in any OCM reactor (e.g., a second reactor). The diluent is inert with respect to the OCM reaction, e.g., the diluent does not participate in the OCM reaction. In an aspect, the diluent can comprise water, nitrogen, inert gases, and the like, or combinations thereof. In an aspect, the diluent can be present in the OCM reactant mixture in an amount of from about 0.5% to about 80%, alternatively from about 5% to about 50%, or alternatively from about 10% to about 30%, based on the total volume of the OCM reactant mixture.

[0032] In an aspect, the diluent comprises steam. Steam can be present in the first reactant mixture in an amount of from about 5% to about 70%, alternatively from about 10% to about 60%, or alternatively from about 15% to about 50%, based on the total volume of the first reactant mixture.

[0033] In an aspect, the OCM reactant mixture (e.g., first reactant mixture, second reactant mixture) can be characterized by an inlet temperature (e.g., can be introduced to the OCM reactor at an inlet temperature) of from about 300°C to about 900°C, alternatively from about 550°C to about 800°C, alternatively from about 575°C to about 750°C, or alternatively from about 600°C to about 700°C. As will be appreciated by one of skill in the art, and with the help of this disclosure, while the OCM reaction is exothermic, heat input is necessary for promoting the formation of methyl radicals from CH ₄ , as the C-H bonds of CH ₄ are very stable, and the formation of methyl radicals from CH ₄ is endothermic. In an aspect, the OCM reactant mixture can be introduced to the OCM reactor at a temperature effective to promote an OCM reaction.

[0034] In an aspect, the first reactant mixture can be characterized by a first inlet temperature (e.g., can be introduced to the first reactor at a first inlet temperature) of from about 300°C to about 900°C, alternatively from about 550°C to about 800°C, alternatively from about 575°C to about 750°C, or alternatively from about 600°C to about 700°C.

[0035] In an aspect, the first inlet temperature can be less than an inlet temperature for an otherwise similar adiabatic reactor comprising an OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1. For OCM reactions performed in adiabatic reactors, each reactor is characterized by an inlet temperature (e.g., the temperature of a reactant mixture entering the adiabatic reactor) and an outlet temperature (e.g., the temperature of a product mixture exiting the adiabatic reactor), which provides for a temperature rise or increase in the adiabatic reactor, wherein the temperature rise is the difference between the outlet temperature and the inlet temperature. For example, if the inlet temperature is 700°C and the outlet temperature is 900°C, the temperature rise is 200°C. As will be appreciated by one of skill in the art, and with the help of this disclosure, enhanced OCM catalyst performance is required over a broad temperature range (e.g., a temperature range of equal to or greater than about 150°C). Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, the OCM catalyst has to be stable and display high C ₂₊ selectivity (e.g., greater than 70% C ₂₊ selectivity) at both the inlet temperature and the outlet temperature, as well as the temperatures between the inlet temperature and the outlet temperature. Adiabatic processes require for OCM catalyst compositions to be active at the inlet temperature, to be stable at the outlet temperature, and to maintain high C ₂₊ selectivity over a broad temperature range (e.g., temperature rise). Conventional catalysts can generally display high C ₂₊ selectivity at elevated temperatures (e.g., over about 800°C), but the C ₂₊ selectivity at lower temperatures (e.g., less than about 700°C) is fairly low (e.g., less than about 60%), and thus conventional catalysts do not display a high C ₂₊ selectivity over a broad temperature range. As such, conventional OCM catalysts display an overall low C ₂₊ selectivity over a broad temperature range. The performance of conventional OCM catalysts over broad temperature ranges is discussed in more detail in D. Leyshon, in: A. Holman et al., (Eds), Natural Gas Conversion, Elsevier B.V., Amsterdam, 1991, pp. 497-507; which is incorporated by reference herein in its entirety. Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, some conventional OCM catalysts display poor stability at high temperatures (e.g., about 900°C or more), which makes such catalysts unsuitable for adiabatic operation.

[0036] Without wishing to be limited by theory, when the temperature (e.g., reactor temperature) reaches values of above about 900°C, thermal reactions (e.g., methane steam reforming) will start to make significant contribution to the overall reaction, which in turn will result in lower C ₂₊ selectivity. As will be appreciated by one of skill in the art, and with the help of this disclosure, since the upper limit of the operating temperature is dictated by a significant contribution of thermal reactions, a broad temperature rise in an adiabatic reactor performing an OCM reaction can be achieved by lowering the inlet temperature; which in turn requires for the OCM catalyst to display high C ₂₊ selectivity at low reactor temperatures (e.g., less than about 700°C). However, and without wishing to be limited by theory, at low reactor temperatures, the selectivity that can be achieved is limited by the gas-phase methyl radical deep oxidation reactions; which are more significant at lower reaction temperatures, as described in more detail in Sinev et al., "Kinetics of oxidative coupling of methane: Bridging the gap between comprehension and description", J. Natural Gas Chemistry, 2009, vol. 18, p. 273; which is incorporated by reference herein in its entirety.

[0037] In an aspect, the OCM reactor (e.g., the first reactor, the second reactor) can be an adiabatic reactor. The OCM reactors can be fixed bed reactors, such as axial flow reactors, or radial flow reactors. As will be appreciated by one of skill in the art, and with the help of this disclosure, certain fixed bed reactors, such as radial flow reactors, can decrease a reactor pressure drop, which may in turn increase a desired selectivity. In an aspect, the reactor can comprise a catalyst bed comprising the OCM catalyst composition disclosed herein.

[0038] The OCM reactor (e.g., the first reactor, the second reactor) can be characterized by a temperature of from about 400°C to about 1,000°C, alternatively from about 500°C to about 950°C, or alternatively from about 600°C to about 900°C.

[0039] The OCM reactor (e.g., the first reactor, the second reactor) can be characterized by a pressure of from about ambient pressure (e.g., atmospheric pressure) to about 500 psig, alternatively from about ambient pressure to about 200 psig, or alternatively from about ambient pressure to about 150 psig. In an aspect, the method for producing olefins as disclosed herein can be carried out at ambient pressure.

[0040] The OCM reactor (e.g., the first reactor, the second reactor) can be characterized by a gas hourly space velocity (GHSV) of from about 500 h ^"1 to about 10,000,000 h ^"1 , alternatively from about 500 h ^{" 1} to about 1,000,000 h ^"1 , alternatively from about 500 h ^"1 to about 500,000 h ^"1 , alternatively from about 1,000 h ^"1 to about 500,000 h ^"1 , alternatively from about 1,500 h ^"1 to about 500,000 h ^"1 , alternatively from about 2,000 h ^"1 to about 500,000 h ^"1 , alternatively from about 5,000 h ^"1 to about 500,000 h ^"1 , alternatively from about 10,000 h ^"1 to about 500,000 h ^"1 , or alternatively from about 50,000 h ^"1 to about 500,000 h ^"1 . Generally, the GHSV relates a reactant (e.g., reactant mixture) gas flow rate to a reactor volume. GHSV is usually measured at standard temperature and pressure.

[0041] The OCM reactor (e.g., the first reactor, the second reactor) can comprise an OCM catalyst composition (e.g., first OCM catalyst composition, second OCM catalyst composition) as disclosed herein characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; wherein c is from about 0 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; wherein d is from about 0 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; and wherein x balances the oxidation states. As will be appreciated by one of the skill in the art, and with the help of this disclosure, each of the A, Z, E and D can have multiple oxidation states within the OCM catalyst composition, and as such x can have any suitable value that allows for the oxygen anions to balance all the cations. Without wishing to be limited by theory, the different metals (A, Z, E, and D) present in the OCM catalyst compositions as disclosed herein display synergetic effects in terms of conversion and selectivity. Further, and without wishing to be limited by theory, different ion radii and valences of the multiple metals (A, Z, E, and D) present in the OCM catalyst compositions as disclosed herein can generate formation of uncompensated oxygen vacancies, which can lead to further improvement of catalyst performance, for example in terms of conversion, selectivity, stability, etc.

[0042] The OCM catalyst composition as disclosed herein can comprise an alkaline earth metal (A). The alkaline earth metal (A) can be selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and combinations thereof. In an aspect, the alkaline earth metal (A) is strontium (Sr).

[0043] The OCM catalyst composition as disclosed herein can comprise a first rare earth element (Z), wherein the first rare earth element (Z) can be selected from the group consisting of lanthanum (La), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), and combinations thereof. In an aspect, the first rare earth element (Z) is lanthanum (La). As will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the first rare earth element (Z) can comprise a single rare earth element, such as lanthanum (La). Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the first rare earth element (Z) can comprise two or more rare earth elements, such as lanthanum (La), and neodymium (Nd), for example; or lanthanum (La), neodymium (Nd), and promethium (Pm), as another example; etc.

[0044] The OCM catalyst composition as disclosed herein can comprise a second rare earth element (E) and/or a third rare earth element (D), wherein E and D are different. The second rare earth element (E) and the third rare earth element (D) can each independently be selected from the group consisting of scandium (Sc), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), yttrium (Y), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and combinations thereof.

[0045] As will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the second rare earth element (E) can comprise a single rare earth element, such as ytterbium (Yb). Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the second rare earth element (E) can comprise two or more rare earth elements, such as ytterbium (Yb), and neodymium (Nd), for example; or ytterbium (Yb), and thulium (Tm), as another example; or ytterbium (Yb), neodymium (Nd), and thulium (Tm), as yet another example; etc.

[0046] As will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the third rare earth element (D) can comprise a single rare earth element, such as ytterbium (Yb). Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the third rare earth element (D) can comprise two or more rare earth elements, such as ytterbium (Yb), and neodymium (Nd), for example; or ytterbium (Yb), neodymium (Nd), and promethium (Pm), as another example; etc.

[0047] The OCM catalyst composition as disclosed herein can comprise a redox agent (D). As will be appreciated by one of skill in the art, and with the help of this disclosure, D can be either a redox agent or a third rare earth element. The redox agent (D) can be selected from the group consisting of manganese (Mn), tungsten (W), bismuth (Bi), antimony (Sb), tin (Sn), cerium (Ce), praseodymium (Pr), and combinations thereof. A redox agent generally refers to a chemical species that possesses the ability to undergo both an oxidation reaction and a reduction reaction, and such ability usually resides in the chemical species having more than one stable oxidation state other than the oxidation state of zero (0). As will be appreciated by one of skill in the art, and with the help of this disclosure, some rare earth elements, such as Ce and Pr, can also be considered redox agents. Further, as will be appreciated by one of skill in the art, and with the help of this disclosure, when D is Ce and/or Pr, D can be considered either a redox agent or a third rare earth element.

[0048] In some aspects, the redox agent (D) is manganese (Mn). In other aspects, the redox agent (D) is tungsten (W).

[0049] As will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the redox agent (D) can comprise a single element, such as manganese (Mn). Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, in some aspects, the redox agent (D) can comprise two or more rare earth elements, such as manganese (Mn), and tungsten (W), for example; or manganese (Mn), tungsten (W), and praseodymium (Pr), as another example; etc.

[0050] In an aspect, the second rare earth element (E) and/or the third rare earth element (D) can be basic (e.g., can exhibit some degree of basicity; can have affinity for hydrogen; can exhibit some degree of affinity for hydrogen). Nonlimiting examples of rare earth elements that can be considered basic for purposes of the disclosure herein include scandium (Sc), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), yttrium (Y), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and combinations thereof. As will be appreciated by one of skill in the art, and with the help of this disclosure, and without wishing to be limited by theory, the OCM reaction is a multi-step reaction, wherein each step of the OCM reaction could benefit from specific OCM catalytic properties. For example, and without wishing to be limited by theory, an OCM catalyst should exhibit some degree of basicity to abstract a hydrogen from CH ₄ to form hydroxyl groups [OH] on the OCM catalyst surface, as well as methyl radicals (CH ₃ *). Further, and without wishing to be limited by theory, an OCM catalyst should exhibit oxidative properties for the OCM catalyst to convert the hydroxyl groups [OH] from the catalyst surface to water, which can allow for the OCM reaction to continue (e.g., propagate). Furthermore, as will be appreciated by one of skill in the art, and with the help of this disclosure, and without wishing to be limited by theory, an OCM catalyst could also benefit from properties like oxygen ion conductivity and proton conductivity, which properties can be critical for the OCM reaction to proceed at a very high rate (e.g., its highest possible rate). Furthermore, as will be appreciated by one of skill in the art, and with the help of this disclosure, and without wishing to be limited by theory, an OCM catalyst comprising a single metal might not provide all the necessary properties for an optimum OCM reaction (e.g., best OCM reaction outcome) at the best level, and as such conducting an optimum OCM reaction may require an OCM catalyst with tailored composition in terms of metals present, wherein the different metals can have optimum properties for various OCM reaction steps, and wherein the different metals can provide synergistically for achieving the best performance for the OCM catalyst in an OCM reaction.

[0051] In an aspect, the OCM catalyst composition as disclosed herein can comprise one or more oxides of A; one or more oxides of Z; one or more oxides of E; one or more oxides of D; or combinations thereof. The OCM catalyst composition can comprise one or more oxides of a metal, wherein the metal comprises A, Z, and optionally E and/or D. In some aspects, the OCM catalyst composition can comprise, consist of, or consist essentially of the one or more oxides.

[0052] In an aspect, the one or more oxides can be present in the OCM catalyst composition in an amount of from about 0.01 wt.% to about 100.0 wt.%, alternatively from about 0.1 wt.% to about 99.0 wt.%, alternatively from about 1.0 wt.% to about 95.0 wt.%, alternatively from about 10.0 wt.% to about 90.0 wt.%), or alternatively from about 30.0 wt.%> to about 70.0 wt.%>, based on the total weight of the OCM catalyst composition. As will be appreciated by one of skill in the art, and with the help of this disclosure, a portion of the one or more oxides, in the presence of water, such as atmospheric moisture, can convert to hydroxides, and it is possible that the OCM catalyst composition will comprise some hydroxides, due to exposing the OCM catalyst composition comprising the one or more oxides to water (e.g., atmospheric moisture). Further, as will be appreciated by one of skill in the art, and with the help of this disclosure, a portion of the one or more oxides, in the presence of carbon dioxide, such as atmospheric carbon dioxide, can convert to carbonates, and it is possible that the OCM catalyst composition will comprise some carbonates, due to exposing the OCM catalyst composition comprising the one or more oxides to carbon dioxide (e.g., atmospheric carbon dioxide).

[0053] The one or more oxides can comprise a single metal oxide, mixtures of single metal oxides, a mixed metal oxide, mixtures of mixed metal oxides, mixtures of single metal oxides and mixed metal oxides, or combinations thereof. [0054] The single metal oxide comprises one metal selected from the group consisting of A, Z, E, and D. A single metal oxide can be characterized by the general formula M _m O _y ; wherein M is the metal selected from the group consisting of A, Z, E, and D; and wherein m and y are integers from 1 to 7, alternatively from 1 to 5, or alternatively from 1 to 3. A single metal oxide contains one and only one metal cation. Nonlimiting examples of single metal oxides suitable for use in the OCM catalyst compositions of the present disclosure include CaO, MgO, SrO, BaO, La ₂ 0 ₃ , Sc ₂ 0 ₃ , Y ₂ 0 ₃ , Ce0 ₂ , Ce ₂ 0 ₃ , Pr ₂ 0 ₃ , Pr0 ₂ , Nd ₂ 0 ₃ , Pm ₂ 0 ₃ , Sm ₂ 0 ₃ , Eu ₂ 0 ₃ , Gd ₂ 0 ₃ , Tb ₂ 0 ₃ , Dy ₂ 0 ₃ , Ho ₂ 0 ₃ , Er ₂ 0 ₃ , Lu ₂ 0 ₃ , Yb ₂ 0 ₃ , Tm ₂ 0 ₃ , W0 ₃ , Mn0 ₂ , W ₂ 0 ₃ , Sn0 ₂ , and the like, or combinations thereof.

[0055] In an aspect, mixtures of single metal oxides can comprise two or more different single metal oxides, wherein the two or more different single metal oxides have been mixed together to form the mixture of single metal oxides. Mixtures of single metal oxides can comprise two or more different single metal oxides, wherein each single metal oxide can be selected from the group consisting of CaO, MgO, SrO, BaO, La ₂ 0 ₃ , Sc ₂ 0 ₃ , Y ₂ 0 ₃ , Ce0 ₂ , Ce ₂ 0 ₃ , Pr ₂ 0 ₃ , Pr0 ₂ , Nd ₂ 0 ₃ , Pm ₂ 0 ₃ , Sm ₂ 0 ₃ , Eu ₂ 0 ₃ , Gd ₂ 0 ₃ , Tb ₂ 0 ₃ , Dy ₂ 0 ₃ , Ho ₂ 0 , Er ₂ 0 , Lu ₂ 0 , Yb ₂ 0 , Tm ₂ 0 , W0 , Mn0 ₂ , W ₂ 0 , and Sn0 ₂ . Nonlimiting examples of mixtures of single metal oxides suitable for use in the OCM catalyst compositions of the present disclosure include SrO- La ₂ 0 ₃ , SrO-MgO-La ₂ 0 ₃ , SrO-Yb ₂ 0 ₃ -La ₂ 0 ₃ , SrO-Er ₂ 0 ₃ -La ₂ 0 _3> SrO-Ce0 ₂ -La ₂ 0 ₃ , SrO-Mn0 ₂ -La ₂ 0 ₃ , SrO- W0 ₃ -W ₂ 0 ₃ -La ₂ 0 ₃ , SrO-W0 ₃ -Tm ₂ 0 ₃ -La ₂ 0 ₃ , SrO-W0 ₃ -Tm ₂ 0 ₃ -La ₂ 0 ₃ , SrO-BaO-Ce0 ₂ -Er ₂ 0 ₃ -La ₂ 0 ₃ , SrO- Ce0 ₂ -Ce ₂ 0 ₃ -Er ₂ 0 ₃ -La ₂ 0 ₃ , SrO-BaO-W0 ₃ -W ₂ 0 ₃ -La ₂ 0 ₃ , SrO-BaO-Sm ₂ 0 ₃ -W0 ₃ -W ₂ 0 ₃ -La ₂ 0 ₃ , SrO-MgO- Ce0 ₂ -Ce ₂ 0 ₃ -W0 ₃ -W ₂ 0 ₃ -La ₂ 0 ₃ , SrO-CaO-Pr0 ₂ -Pr ₂ 0 ₃ -MnO-Mn ₂ 0 ₃ -La ₂ 0 ₃ , and the like, or combinations thereof.

[0056] The mixed metal oxide comprises two or more different metals, wherein each metal can be independently selected from the group consisting of A, Z, E, and D. A mixed metal oxide can be characterized by the general formula M ¹ _m iM ² _m2 O _y ; wherein M ¹ and M ² are metals; wherein each of the M ¹ and M ² can be independently selected from the group consisting of A, Z, E, and D; and wherein ml, m2 and y are integers from 1 to 15, alternatively from 1 to 10, or alternatively from 1 to 7. In some aspects, M ¹ and M ² can be metal cations of different chemical elements, for example M ¹ can be a lanthanum cation and M ² can be a strontium cation. In other embodiments, M ¹ and M ² can be different cations of the same chemical element, wherein M ¹ and M ² can have different oxidation states. For example, the mixed metal oxide can comprise Mn ₃ 0 ₄ , wherein M ¹ can be a Mn (II) cation and M ² can be a Mn (III) cation. Nonlimiting examples of mixed metal oxides suitable for use in the OCM catalyst compositions of the present disclosure include La/SrO; LaYb0 ₃ ; SrYb ₂ 0 ₄ ; Sr ₂ Ce0 ₄ ; Mn ₃ 0 ₄ ; La/MgO; Sm ₂ Ce ₂ 0 ₇ ; Er ₂ Ce ₂ 0 ₇ ; CaTm ₂ 0 ₄ ; MgYb ₂ 0 ₄ ; SrCe ₍₁ . _y) Yb _y 0 , wherein y can be from about 0.01 to about 0.99; and the like; or combinations thereof.

[0057] In an aspect, mixtures of mixed metal oxides can comprise two or more different mixed metal oxides, wherein the two or more different mixed metal oxides have been mixed together to form the mixture of mixed metal oxides. Mixtures of mixed metal oxides can comprise two or more different mixed metal oxides, such as La/SrO; LaYb0 ₃ ; SrYb ₂ 0 ₄ ; Sr ₂ Ce0 ₄ ; Mn ₃ 0 ₄ ; La/MgO; Sm ₂ Ce ₂ 0 ₇ ; Er ₂ Ce ₂ 0 ₇ ; CaTm ₂ 0 ₄ ; MgYb ₂ 0 ; SrCe ₍ i. _y) Yb _y 0 ₃ , wherein y can be from about 0.01 to about 0.99; and the like; or combinations thereof.

[0058] In an aspect, mixtures of single metal oxides and mixed metal oxides can comprise at least one single metal oxide and at least one mixed metal oxide, wherein the at least one single metal oxide and the at least one mixed metal oxide have been mixed together to form the mixture of single metal oxides and mixed metal oxides.

[0059] The OCM catalyst compositions suitable for use in the present disclosure can be supported OCM catalyst compositions and/or unsupported OCM catalyst compositions. In some aspects, the supported OCM catalyst compositions can comprise a support, wherein the support can be catalytically active (e.g., the support can catalyze an OCM reaction, such as MgO). In other aspects, the supported OCM catalyst compositions can comprise a support, wherein the support can be catalytically inactive (e.g., the support cannot catalyze an OCM reaction, such as Si0 ₂ ). In yet other aspects, the supported OCM catalyst compositions can comprise a catalytically active support and a catalytically inactive support. Nonlimiting examples of a support suitable for use in the present disclosure include MgO, A1 ₂ 0 ₃ , Si0 ₂ , Zr0 ₂ , Ti0 ₂ , and the like, or combinations thereof. As will be appreciated by one of skill in the art, and with the help of this disclosure, the support can be purchased or can be prepared by using any suitable methodology, such as for example precipitation/co-precipitation, sol-gel techniques, templates/surface derivatized metal oxides synthesis, solid-state synthesis of mixed metal oxides, microemulsion techniques, solvothermal techniques, sonochemical techniques, combustion synthesis, etc.

[0060] In an aspect, the OCM catalyst composition can further comprise a support, wherein at least a portion of the OCM catalyst composition contacts, coats, is embedded in, is supported by, and/or is distributed throughout at least a portion of the support. In such aspect, the support can be in the form of powders, particles, pellets, monoliths, foams, honeycombs, and the like, or combinations thereof. Nonlimiting examples of support particle shapes include cylindrical, discoidal, spherical, tabular, ellipsoidal, equant, irregular, cubic, acicular, and the like, or combinations thereof.

[0061] In an aspect, the OCM catalyst composition can further comprise a porous support. As will be appreciated by one of skill in the art, and with the help of this disclosure, a porous material (e.g., support) can provide for an enhanced surface area of contact between the OCM catalyst composition and the reactant mixture, which in turn would result in a higher C¾ conversion to

[0062] The OCM catalyst composition as disclosed herein can be made by using any suitable methodology. In an aspect, a method of making an OCM catalyst composition can comprise a step of forming an OCM catalyst precursor mixture; wherein the OCM catalyst precursor mixture comprises one or more compounds comprising an alkaline earth metal (A) cation, one or more compounds comprising a first rare earth element (Z) cation, one or more compounds comprising a second rare earth element (E) cation, and one or more compounds comprising a redox agent or a third rare earth element (D) cation; and wherein the first rare earth element cation, the second rare earth element cation, and the third rare earth element cation, when present, are not the same (i.e., are different). The OCM catalyst precursor mixture is characterized by a molar ratio of first rare earth element to alkaline earth metal of b: l, wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5. The OCM catalyst precursor mixture can be characterized by a molar ratio of second rare earth element to alkaline earth metal of c: l, wherein c is from about 0 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5. The OCM catalyst precursor mixture can be characterized by a molar ratio of redox agent or third rare earth element to alkaline earth metal of d: l, wherein d is from about 0 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5.

[0063] The one or more compounds comprising an alkaline earth metal cation can comprise an alkaline earth metal nitrate, an alkaline earth metal oxide, an alkaline earth metal hydroxide, an alkaline earth metal chloride, an alkaline earth metal acetate, an alkaline earth metal carbonate, and the like, or combinations thereof. The one or more compounds comprising a first rare earth element cation can comprise a first rare earth element nitrate, a first rare earth element oxide, a first rare earth element hydroxide, a first rare earth element chloride, a first rare earth element acetate, a first rare earth element carbonate, and the like, or combinations thereof. The one or more compounds comprising a second rare earth element cation can comprise a second rare earth element nitrate, a second rare earth element oxide, a second rare earth element hydroxide, a second rare earth element chloride, a second rare earth element acetate, a second rare earth element carbonate, and the like, or combinations thereof. The one or more compounds comprising a redox agent cation can comprise a redox agent nitrate, a redox agent oxide, a redox agent hydroxide, a redox agent chloride, a redox agent acetate, a redox agent carbonate, and the like, or combinations thereof. The one or more compounds comprising a third rare earth element cation can comprise a third rare earth element nitrate, a third rare earth element oxide, a third rare earth element hydroxide, a third rare earth element chloride, a third rare earth element acetate, a third rare earth element carbonate, and the like, or combinations thereof.

[0064] In some aspects, the OCM catalyst precursor mixture can be formed in the presence of water, for example by contacting water or any suitable aqueous medium with one or more compounds comprising an alkaline earth metal (A) cation, one or more compounds comprising a first rare earth element (Z) cation, and optionally one or more compounds comprising a second rare earth element (E) cation and/or one or more compounds comprising a redox agent or a third rare earth element (D) cation. In such aspects, the OCM catalyst precursor mixture comprises water. [0065] In other aspects, the OCM catalyst precursor mixture can be formed in the absence of water (e.g., substantial absence of water; without adding water, etc.), for example by contacting the one or more compounds comprising an alkaline earth metal (A) cation, one or more compounds comprising a first rare earth element (Z) cation, and optionally one or more compounds comprising a second rare earth element (E) cation and/or one or more compounds comprising a redox agent or a third rare earth element (D) cation with each other. In such aspects, the one or more compounds comprising an alkaline earth metal (A) cation, one or more compounds comprising a first rare earth element (Z) cation, and optionally one or more compounds comprising a second rare earth element (E) cation and/or one or more compounds comprising a redox agent or a third rare earth element (D) cation can be mixed together, for example by grinding, dry blending, or otherwise intimately mixing to obtain a homogeneous mixture (e.g., OCM catalyst precursor mixture). As will be appreciated by one of skill in the art, and with the help of this disclosure, while the one or more compounds comprising an alkaline earth metal (A) cation, one or more compounds comprising a first rare earth element (Z) cation, and optionally one or more compounds comprising a second rare earth element (E) cation and/or one or more compounds comprising a redox agent or a third rare earth element (D) cation can be mixed without adding water, in some instances, a small amount of water can be added to promote or enable an uniform mixing of the compounds, for example by forming a paste. Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, whether water is used or not for forming the OCM catalyst precursor mixture, the OCM catalyst precursor mixture can be further subjected to a step of drying and/or calcining as disclosed herein.

[0066] Without wishing to be limited by theory, some of the one or more compounds comprising an alkaline earth metal (A) cation, one or more compounds comprising a first rare earth element (Z) cation, one or more compounds comprising a second rare earth element (E) cation, one or more compounds comprising a redox agent or a third rare earth element (D) cation, or combinations thereof can be insoluble in water, or only partially soluble in water (e.g., lanthanum oxide, ytterbium oxide, strontium carbonate, neodymium oxide, etc.); and in such instances, these compounds cannot be solubilized in water, but rather mixed as dry materials, or with little water as to (e.g., an amount of water effective to) form a paste (e.g., homogeneous mixture).

[0067] Further, without wishing to be limited by theory, and as will be appreciated by one of skill in the art, and with the help of this disclosure, even when the OCM catalyst precursor mixture is formed without water addition, the OCM catalyst precursor mixture can contain a small amount of water, for example water from atmospheric moisture.

[0068] In an aspect, the step of forming the OCM catalyst precursor mixture can comprise solubilizing the one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or a third rare earth element cation in an aqueous medium to form an OCM catalyst precursor aqueous solution. The aqueous medium can be water, or an aqueous solution. The OCM catalyst precursor aqueous solution can be formed by dissolving the one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, one or more compounds comprising a redox agent cation or a third rare earth element cation, or combinations thereof, in water or any suitable aqueous medium. As will be appreciated by one of skill in the art, and with the help of this disclosure, the one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or a third rare earth element cation can be dissolved in an aqueous medium in any suitable order. In some aspects, the one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or a third rare earth element cation can be first mixed together and then dissolved in an aqueous medium.

[0069] The OCM catalyst precursor aqueous solution can be dried to form the OCM catalyst precursor mixture. In an aspect, at least a portion of the OCM catalyst precursor aqueous solution can be dried at a temperature of equal to or greater than about 75°C, alternatively of equal to or greater than about 100°C, or alternatively of equal to or greater than about 125°C, to yield the OCM catalyst precursor mixture. The OCM catalyst precursor aqueous solution can be dried for a time period of equal to or greater than about 4 hours, alternatively equal to or greater than about 8 hours, or alternatively equal to or greater than about 12 hours.

[0070] In an aspect, a method of making an OCM catalyst composition can comprise a step of calcining at least a portion of the OCM catalyst precursor mixture to form the OCM catalyst composition, wherein the OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states. The OCM catalyst precursor mixture can be calcined at a temperature of equal to or greater than about 750°C, alternatively equal to or greater than about 800°C, or alternatively equal to or greater than about 900°C, to yield the OCM catalyst composition. The OCM catalyst precursor mixture can be calcined for a time period of equal to or greater than about 2 hours, alternatively equal to or greater than about 4 hours, or alternatively equal to or greater than about 6 hours. [0071] In some aspects, at least a portion of the OCM catalyst precursor mixture can be calcined in an oxidizing atmosphere (e.g., in an atmosphere comprising oxygen, for example in air) to form the OCM catalyst composition. Without wishing to be limited by theory, the oxygen in the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x can originate in the oxidizing atmosphere used for calcining the OCM catalyst precursor mixture. Further, without wishing to be limited by theory, the oxygen in the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x can originate in the one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or a third rare earth element cation, provided that at least one of these compounds comprises oxygen in its formula, as is the case with nitrates, oxides, hydroxides, acetates, carbonates, etc.

[0072] In some aspects, the method of making an OCM catalyst composition can further comprise contacting the OCM catalyst composition with a support to yield a supported catalyst (e.g., an OCM supported catalyst, an OCM supported catalyst composition, etc.).

[0073] In other aspects, the method of making an OCM catalyst composition can comprise forming the OCM catalyst composition in the presence of the support, such that the resulting OCM catalyst composition (after the calcining step) comprises the support. For example, at least a portion of the OCM catalyst precursor aqueous solution can be contacted with a support to yield a supported OCM catalyst precursor. In an aspect, at least a portion of the supported OCM catalyst precursor can be further dried (e.g., at a temperature of equal to or greater than about 75°C) and calcined (e.g., at a temperature of equal to or greater than about 750°C) to form the OCM catalyst composition.

[0074] In an aspect, a method for producing olefins as disclosed can comprise allowing at least a portion of the reactant mixture (e.g., first reactant mixture, second reactant mixture) to contact at least a portion of the OCM catalyst composition (e.g., first OCM catalyst composition, second OCM catalyst composition) and react via an OCM reaction to form a product mixture (e.g., first product mixture, second product mixture) comprising unreacted methane and olefins.

[0075] In an aspect, a method for producing olefins as disclosed herein can comprise allowing at least a portion of the first reactant mixture to contact at least a portion of the first OCM catalyst composition and react via an OCM reaction to form a first product mixture comprising unreacted methane and olefins.

[0076] In an aspect, the method for producing olefins as disclosed herein can comprise recovering at least a portion of the first product mixture from the first adiabatic reactor, wherein the first product mixture is characterized by a first outlet temperature, and wherein the first outlet temperature is greater than the first inlet temperature. The first product mixture can comprise olefins, water, CO, C0 ₂ , and unreacted methane. [0077] The first outlet temperature can be from about 700°C to about 950°C, alternatively from about 750°C to about 925°C, or alternatively from about 800°C to about 900°C. A first temperature rise or increase (i.e., the difference between the first outlet temperature and the first inlet temperature, wherein the first inlet temperature can be from about 300°C to about 900°C) can be equal to or greater than about 150°C, alternatively equal to or greater than about 175°C, alternatively equal to or greater than about 200°C, alternatively from about 150°C to about 300°C, alternatively from about 150°C to about 275°C, or alternatively from about 150°C to about 250°C. For purposes of the disclosure herein, the first temperature rise can be referred to as a "broad temperature rise," or a "broad temperature increase." Further, for purposes of the disclosure herein, the term "broad temperature rise" refers to a temperature rise of equal to or greater than about 150°C.

[0078] In an aspect, a method for producing olefins as disclosed herein can comprise cooling the first product mixture from the first outlet temperature to a second inlet temperature. In some aspects, the second inlet temperature and the first inlet temperature can be the same or different. The second inlet temperature can be from about 300°C to about 900°C, alternatively from about 550°C to about 800°C, alternatively from about 575°C to about 750°C, or alternatively from about 600°C to about 700°C.

[0079] A step of cooling the first product mixture can occur in a first heat exchanger. In some aspects, the first heat exchanger can heat a methane feed stream to the first inlet temperature, wherein the first reactant mixture comprises at least a portion of a heated methane feed stream. In other aspects, the first heat exchanger can heat the first reactant mixture. Any suitable process stream can be used in the first heat exchanger to cool the first product mixture, including cooling water, steam generation (e.g., electricity cogeneration), etc. That is, the first heat exchanger can heat any suitable stream, whether in an OCM process or in a process other than an OCM process.

[0080] In an aspect, a method for producing olefins as disclosed herein can comprise a second stage, wherein the second stage comprises a second adiabatic reactor (e.g., OCM reactor, second OCM reactor) in series with and downstream from the first adiabatic reactor. In such aspect, a method for producing olefins can comprise (i) introducing a second reactant mixture to a second adiabatic reactor comprising a second OCM catalyst composition, wherein the second reactant mixture comprises at least a portion of the first product mixture and 0 ₂ , wherein the second reactant mixture is characterized by the second inlet temperature, and wherein the first inlet temperature and the second inlet temperature are the same or different; (ii) allowing at least a portion of the second reactant mixture to contact at least a portion of the second OCM catalyst composition (e.g., OCM catalyst composition as disclosed herein) and react via an OCM reaction to form a second product mixture comprising unreacted methane and olefins, wherein an amount of unreacted methane in the second product mixture is less than an amount of unreacted methane in the first product mixture, with the proviso that no fresh or supplemental methane is added to the second stage to desirably produce an increase in a methane concentration, and wherein an amount of olefins in the second product mixture is greater than an amount of olefins in the first product mixture, with the proviso that no olefins are separated or recovered from the first product mixture to desirably produce a decrease in an olefin concentration; (iii) recovering at least a portion of the second product mixture from the second adiabatic reactor, wherein the second product mixture is characterized by a second outlet temperature, wherein the second outlet temperature is greater than the second inlet temperature, and wherein the first outlet temperature and the second outlet temperature can be the same or different; (iv) optionally cooling the second product mixture to form a cooled second product mixture (e.g., via a second heat exchanger), wherein the cooled second product mixture can be characterized by a temperature that is about the same as the first inlet temperature and/or the second inlet temperature; and (v) recovering at least a portion of the olefins from the second product mixture and/or the cooled second product mixture. For purposes of the disclosure herein, all descriptions related to the first stage (such as descriptions of first adiabatic reactor (e.g., OCM reactor), first OCM catalyst composition, first reactant mixture (e.g., OCM reactant mixture), first product mixture (e.g., OCM product mixture), first heat exchanger, first inlet temperature, first outlet temperature, first temperature rise, etc.) can be applied to the corresponding components of the second stage (such as descriptions of second adiabatic reactor (e.g., OCM reactor), second OCM catalyst composition, second reactant mixture (e.g., OCM reactant mixture), second product mixture (e.g., OCM product mixture), second heat exchanger, second inlet temperature, second outlet temperature, second temperature rise, etc., respectively), unless otherwise specified herein.

[0081] As will be appreciated by one of skill in the art, and with the help of this disclosure, in some instances, the methane reacting in the second stage in the second adiabatic reactor is primarily methane that was introduced to the first adiabatic reactor, that did not react in the first adiabatic reactor, and that was subsequently recovered as unreacted methane (as part of the first product mixture), with the proviso that no fresh or supplemental methane was added to the second stage to desirably produce an increase in a methane concentration. Further, as will be appreciated by one of skill in the art, and with the help of this disclosure, when fresh methane is introduced to the second stage, an amount of unreacted methane recovered from the second stage (as part of the second product mixture) minus the amount of fresh methane introduced to the second stage is less than the amount of unreacted methane that was recovered from the first stage (as part of the first product mixture) and was subsequently introduced to the second stage. In some aspects, a method for producing olefins can further comprise introducing additional CH ₄ to the second reactor.

[0082] In an aspect, a method for producing olefins as disclosed herein can be a multi-stage process, wherein the multi-stage process further comprises one or more additional stages downstream of the first stage and/or the second stage (with each successive downstream stage having a corresponding OCM reactor in series with and downstream of an immediately preceding stage/reactor), as necessary to achieve a target methane conversion and/or a target C ₂₊ selectivity for the overall multi-stage process. Each additional stage can comprise (i) introducing a reactant mixture to a reactor comprising an OCM catalyst composition as disclosed herein, wherein the reactant mixture comprises CH ₄ and 0 ₂ , and wherein the reactant mixture is characterized by an inlet temperature; (ii) allowing at least a portion of the reactant mixture to contact at least a portion of the OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins; (iii) recovering at least a portion of the product mixture from the reactor, wherein the product mixture is characterized by an outlet temperature, and wherein the outlet temperature is greater than the inlet temperature; and (iv) optionally cooling the product mixture (e.g., via a heat exchanger). In some aspects, the reactant mixture can comprise at least a portion of an upstream product mixture recovered from an upstream reactor. In other aspects, the reactant mixture can further comprise at least a portion of a downstream product mixture recovered from a downstream reactor. For purposes of the disclosure herein, all descriptions related to the first stage (such as descriptions of first adiabatic reactor (e.g., OCM reactor), first OCM catalyst composition, first reactant mixture (e.g., OCM reactant mixture), first product mixture (e.g., OCM product mixture), first heat exchanger, first inlet temperature, first outlet temperature, first temperature rise, etc.) can be applied to the corresponding components of any subsequent stage (such as descriptions of reactor (e.g., OCM reactor), OCM catalyst composition, reactant mixture (e.g., OCM reactant mixture), product mixture (e.g., OCM product mixture), heat exchanger, inlet temperature, outlet temperature, temperature rise, etc., respectively), unless otherwise specified herein.

[0083] In an aspect, the multi-stage process can have from 2 to 4 stages, alternatively from 2 to 3 stages, alternatively 2 stages, alternatively 3 stages, or alternatively 4 stages. In an aspect, the multi-stage process as disclosed herein can have less stages than an otherwise similar multi-stage process employing an OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1. As will be appreciated by one of skill in the art, and with the help of this disclosure, for adiabatic operation in an OCM process, the total temperature rise (e.g., overall temperature rise) necessary to achieve a target methane conversion and/or a target C ₂₊ selectivity is known under a given set of operation conditions. For example, if a total temperature rise necessary to achieve a target methane conversion and/or a target C ₂₊ selectivity is about 600°C, and if the OCM catalyst composition as disclosed herein can provide for a 200°C temperature rise while maintaining a high C ₂₊ selectivity (e.g., greater than 70% C ₂₊ selectivity) over the entire temperature rise of 200°C in a particular stage, then only 3 stages are necessary to achieve the overall necessary temperature rise of 600°C. However, when conventional catalysts are used, such conventional catalysts can maintain a high C ₂₊ selectivity (e.g., greater than 70% C ₂₊ selectivity) over a temperature rise of only about 50°C, for example, in a particular stage, resulting in 12 stages being necessary to achieve the overall necessary temperature rise of 600°C. In an aspect, the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can advantageously maintain a high C ₂₊ selectivity (e.g., greater than 70% C ₂₊ selectivity) over a broad temperature rise (e.g., equal to or greater than about 150°C), thereby providing for a multi-stage process as disclosed herein that has less stages and thus is more cost effective when compared to a multi-stage process employing conventional catalysts (more stages).

[0084] In an aspect, a method for producing olefins as disclosed herein can comprise recovering at least a portion of the product mixture (e.g., first product mixture, cooled first product mixture, second product mixture, cooled second product mixture) from the reactor (e.g., OCM reactor, first adiabatic reactor, second adiabatic reactor). In an aspect, a method for producing olefins as disclosed herein can comprise recovering at least a portion of the C ₂₊ hydrocarbons from the product mixture. The product mixture can comprise C ₂₊ hydrocarbons (including olefins), unreacted methane, and optionally a diluent. The C ₂₊ hydrocarbons can comprise C ₂ hydrocarbons and C ₃ hydrocarbons. In an aspect, the C ₂₊ hydrocarbons can further comprise C ₄ hydrocarbons (C ₄ s), such as for example butane, iso-butane, n-butane, butylene, etc. The C ₂ hydrocarbons can comprise ethylene (C ₂ H ₄ ) and ethane (C ₂ H ₆ ). The C ₂ hydrocarbons can further comprise acetylene (C ₂ H ₂ ). The C ₃ hydrocarbons can comprise propylene (C ₃ H ₆ ) and propane (C ₃ H ₈ ).

[0085] The water produced from the OCM reaction and the water used as a diluent (if water diluent is used) can be separated from the product mixture prior to separating any of the other product mixture components. For example, by cooling down the product mixture to a temperature where the water condenses (e.g., below 100°C at ambient pressure), the water can be removed from the product mixture, by using a flash chamber for example.

[0086] In an aspect, at least a portion of the C ₂₊ hydrocarbons can be separated (e.g., recovered) from the product mixture to yield recovered C ₂₊ hydrocarbons. The C ₂₊ hydrocarbons can be separated from the product mixture by using any suitable separation technique. In an aspect, at least a portion of the C ₂₊ hydrocarbons can be separated from the product mixture by distillation (e.g., cryogenic distillation).

[0087] In an aspect, at least a portion of the recovered C ₂₊ hydrocarbons can be used for ethylene production. In some aspects, at least a portion of ethylene can be separated from the product mixture (e.g., from the C ₂₊ hydrocarbons, from the recovered C ₂₊ hydrocarbons) to yield recovered ethylene and recovered hydrocarbons, by using any suitable separation technique (e.g., distillation). In other aspects, at least a portion of the recovered hydrocarbons (e.g., recovered C ₂₊ hydrocarbons after olefin separation, such as separation of C ₂ H ₄ and C ₃ H ₆ ) can be converted to ethylene, for example by a conventional steam cracking process.

[0088] A method for producing olefins as disclosed herein can comprise recovering at least a portion of the olefins from the product mixture. In an aspect, at least a portion of the olefins can be separated from the product mixture by distillation (e.g., cryogenic distillation). As will be appreciated by one of skill in the art, and with the help of this disclosure, the olefins are generally individually separated from their paraffin counterparts by distillation (e.g., cryogenic distillation). For example, ethylene can be separated from ethane by distillation (e.g., cryogenic distillation). As another example, propylene can be separated from propane by distillation (e.g., cryogenic distillation).

[0089] In an aspect, at least a portion of the unreacted methane can be separated from the product mixture to yield recovered methane. Methane can be separated from the product mixture by using any suitable separation technique, such as for example distillation (e.g., cryogenic distillation). At least a portion of the recovered methane can be recycled to the reactant mixture.

[0090] In an aspect, the 0 ₂ conversion in each stage of the OCM as disclosed herein can be equal to or greater than about 90%, alternatively equal to or greater than about 95%, alternatively equal to or greater than about 99%, alternatively equal to or greater than about 99.9%, or alternatively about 100%. Generally, a conversion of a reagent or reactant refers to the percentage (usually mol%) of reagent that reacted to both undesired and desired products, based on the total amount (e.g., moles) of reagent present before any reaction took place. For purposes of the disclosure herein, the conversion of a reagent is a % conversion based on moles converted. As will be appreciated by one of skill in the art, and with the help of this disclosure, the reactant mixture in OCM reactions is generally characterized by a methane to oxygen molar ratio of greater than 1 : 1, and as such the 0 ₂ conversion is fairly high in OCM processes, most often approaching 90%- 100%. Without wishing to be limited by theory, oxygen is usually a limiting reagent in OCM processes. The oxygen conversion can be calculated b using equation (1):

2 = number of moles of 0 ₂ that entered the adiabatic reactor as part of the reactant mixture; and ^2 ^¾ ^ ⁼ number of moles of 0 ₂ that was recovered from the adiabatic reactor as part of the product mixture.

[0091] In an aspect, the OCM catalyst composition as disclosed herein can be characterized by a reactor temperature effective for achieving an 0 ₂ conversion of equal to or greater than about 90% that is decreased by equal to or greater than about 25°C, alternatively by equal to or greater than about 50°C, alternatively by equal to or greater than about 75°C, or alternatively by equal to or greater than about 100°C, when compared to a reactor temperature effective for achieving an 0 ₂ conversion of equal to or greater than about 90% of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1.

[0092] In some aspects, the OCM catalyst composition as disclosed herein can be characterized by a reactor temperature effective for achieving an 0 ₂ conversion of equal to or greater than about 90% of less than about 700°C, alternatively less than about 600°C, or alternatively less than about 500°C. As will be appreciated by one of skill in the art, and with the help of this disclosure, the reactor temperature effective for achieving an 0 ₂ conversion of equal to or greater than about 90% is dependent upon specific reactor conditions, such as for example methane to oxygen molar ratio, type and size of reactor, GHSV, etc.

[0093] In an aspect, the OCM catalyst composition as disclosed herein can be characterized by a C ₂₊ selectivity of equal to or greater than about 70%, alternatively equal to or greater than about 75%, alternatively equal to or greater than about 80%, or alternatively equal to or greater than about 85% over a temperature rise in the adiabatic reactor (e.g., in a single stage), wherein the temperature rise is a difference between the outlet temperature and the inlet temperature, and wherein the temperature rise can be equal to or greater than about 150°C (e.g., broad temperature rise).

[0094] Generally, a selectivity to a desired product or products refers to how much desired product was formed divided by the total products formed, both desired and undesired. For purposes of the disclosure herein, the selectivity to a desired product is a % selectivity based on moles converted into the desired product. Further, for purposes of the disclosure herein, a C _x selectivity (e.g., C ₂ selectivity, C ₂₊ selectivity, etc.) can be calculated by dividing a number of moles of carbon (C) from CH ₄ that were converted into the desired product (e.g., C _C2H4 , C _C2H6 , etc.) by the total number of moles of C from CH ₄ that were converted (e.g., C _C2H 4, C _C2H 6, C _C2H 2, Cc3H6, C _C3 H8, C _c4s , C _C0 2, C _c0 , etc.). C _C2H 4 = number of moles of C from CH ₄ that were converted into C ₂ FL _t ; C _C2H 6 ⁼ number of moles of C from CH ₄ that were converted into C ₂ H ₆ ; C _C2H 2 ⁼ number of moles of C from CH that were converted into C ₂ H ₂ ; C _C3H6 ⁼ number of moles of C from CH that were converted into C ₃ H ₆ ; C _C 3 _HS ⁼ number of moles of C from CH that were converted into C ₃ H ₈ ; C _C4s = number of moles of C from CH that were converted into C hydrocarbons (C s); C _C o2 ⁼ number of moles of C from CH that were converted into C0 ₂ ; C _C o ⁼ number of moles of C from CFL _t that were converted into CO; etc.

[0095] A C ₂₊ selectivity (e.g., selectivity to C ₂₊ hydrocarbons) refers to how much C ₂ H ₄ , C ₃ H ₆ , C ₂ H ₂ , C ₂ H ₆ , C ₃ H ₈ , and C s were formed divided by the total products formed, including C ₂ FL _t , C ₃ H ₆ , C ₂ H ₂ , C ₂ H ₆ , C ₃ H ₈ , C s, C0 ₂ and CO. For example, the C ₂₊ selectivity can be calculated by using equation (2):

2 + 2 ¹ + 2 ¹ + ¹ + ¹ + ¹ m ^ ₂ H ₄ ^ C ₂ H ₆ ^ C ₂ H ₂ ^ C ₃ H ₆ ^ ₃ ¾ ^ C _4s x l00%

2 Γ^ C ₂ H ₄ + 2 s ¹ C ₂ H _b + 2 s ¹ C ₂ H ₂ + 3 ^ ¹ C ₃ H ₆ + 3 ^ ¹ ₃ ¾ + 4 ^ ¹ _{4 s} + Γ ^ C0 ₂ + Γ ^ CO °

As will be appreciated by one of skill in the art, and with the help of this disclosure, if a specific product and/or hydrocarbon product is not produced in a certain OCM reaction/process, then the corresponding C _Cx is 0, and the term is simply removed from selectivity calculations.

[0096] Without wishing to be limited by theory, when the selectivity (e.g., C ₂₊ selectivity) of an OCM process increases, less methane is converted to undesirable products, such as deep oxidation products (e.g., CO, C0 ₂ ), which in turn means that more oxygen (which is often the limiting reagent in OCM processes) is available for the conversion of methane to desirable products (e.g., C ₂ products, C ₂ H ₄ , C ₂₊ products, etc.), thus enabling an increased yield of desired C ₂₊ products. As will be appreciated by one of skill in the art, and with the help of this disclosure, the higher the temperature in the reactor, the more deep oxidation products will be produced, and as such lower temperatures for achieving an 0 ₂ conversion of equal to or greater than about 90% will lead to a lower amount of deep oxidation products produced, thus resulting in the increased methane conversion to the desirable products.

[0097] In an aspect, the method for producing olefins as disclosed herein can further comprise minimizing deep oxidation of methane to CO _x products, such as carbon monoxide (CO) and/or carbon dioxide (C0 ₂ ). In some aspects, the product mixture can comprise less than about 15 mol%, alternatively less than about 10 mol%, or alternatively less than about 5 mol% carbon monoxide (CO) and/or carbon dioxide (C0 ₂ ).

[0098] In an aspect, the OCM catalyst composition as disclosed herein can be characterized by a C ₂₊ selectivity that is increased by equal to or greater than about 5%, alternatively equal to or greater than about 10%, alternatively equal to or greater than about 15%, or alternatively equal to or greater than about 20% over a temperature rise of equal to or greater than about 150°C (e.g., in a single stage), when compared to a C ₂₊ selectivity of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1.

[0099] In an aspect, the OCM catalyst composition as disclosed herein can be characterized by a S ₇₀₊ temperature rise that is increased by equal to or greater than about 50%, alternatively equal to or greater than about 60%), or alternatively equal to or greater than about 75%> when compared to a S ₇₀₊ temperature rise of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1; wherein a S ₇₀₊ temperature rise of a catalyst is a temperature rise wherein the catalyst is characterized by a C ₂₊ selectivity of equal to or greater than about 70%.

[00100] In an aspect, the OCM catalyst composition as disclosed herein can be characterized by a S ₇₀₊ temperature rise that is increased by equal to or greater than about 25°C, alternatively by equal to or greater than about 50°C, alternatively by equal to or greater than about 75 °C, or alternatively by equal to or greater than about 100°C, when compared to a S ₇₀₊ temperature rise of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1.

[00101] In an aspect, the OCM catalyst composition as disclosed herein can be characterized by the general formula A _a La _b O _x ; wherein A is an alkaline earth metal; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; and wherein x balances the oxidation states. As will be appreciated by one of the skill in the art, and with the help of this disclosure, at least some of the A and La can have multiple oxidation states within the OCM catalyst composition, and as such x can have any suitable value that allows for the oxygen anions to balance all the cations.

[00102] In an aspect of the OCM catalyst composition characterized by the general formula A _a La _b O _x , A is Sr. In such aspect, the OCM catalyst composition can be characterized by the general formula Sr _a La _b O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; and wherein x balances the oxidation states.

[00103] In another aspect, the OCM catalyst composition as disclosed herein can be characterized by the general formula A _a La _b E _c O _x ; wherein A is an alkaline earth metal; wherein E is a second rare earth element; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; wherein c is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; and wherein x balances the oxidation states. As will be appreciated by one of the skill in the art, and with the help of this disclosure, at least some of the A, La, and E can have multiple oxidation states within the OCM catalyst composition, and as such x can have any suitable value that allows for the oxygen anions to balance all the cations.

[00104] In an aspect of the OCM catalyst composition characterized by the general formula A _a La _b E _c O _x , A is Sr, and E is Yb. In such aspect, the OCM catalyst composition can be characterized by the general formula Sr _a La _b Yb _c O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; wherein c is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; and wherein x balances the oxidation states.

[00105] In yet another aspect, the OCM catalyst composition as disclosed herein can be characterized by the general formula Sr _a La _b E _c D _d O _x ; wherein E is a second rare earth element; wherein D is a third rare earth element; wherein the second rare earth element and the third rare earth element are different; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; wherein c is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; wherein d is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; and wherein x balances the oxidation states. As will be appreciated by one of the skill in the art, and with the help of this disclosure, at least some of the Sr, La, E, and D can have multiple oxidation states within the OCM catalyst composition, and as such x can have any suitable value that allows for the oxygen anions to balance all the cations.

[00106] In an aspect of the OCM catalyst composition characterized by the general formula Sr _a La _b E _c D _d O _x , E is Yb, and D is Tm. In such aspect, the OCM catalyst composition can be characterized by the general formula Sr _a La _b Yb _c Tm _d O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; wherein c is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; wherein d is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; and wherein x balances the oxidation states.

[00107] In an aspect of the OCM catalyst composition characterized by the general formula Sr _a La _b E _c D _d O _x , E is Nd, and D is Yb. In such aspect, the OCM catalyst composition can be characterized by the general formula Sr _a La _b Nd _c Yb _d O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0, alternatively from about 0.5 to about 8, or alternatively from about 1 to about 5; wherein c is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; wherein d is from about 0.01 to about 10.0, alternatively from about 0.1 to about 8, or alternatively from about 0.5 to about 5; and wherein x balances the oxidation states.

[00108] In an aspect, a method of making an OCM catalyst composition can comprise the steps of (a) forming an OCM catalyst precursor aqueous solution comprising an alkaline earth metal nitrate, a La nitrate, a second rare earth element nitrate, and a redox agent nitrate or a third rare earth element nitrate; wherein the La nitrate, the second rare earth element nitrate, and the third rare earth element nitrate, when present, are not the same; wherein the OCM catalyst precursor aqueous solution is characterized by a molar ratio of La to alkaline earth metal of b: l, wherein b is from about 0.3 to about 10.0; wherein the OCM catalyst precursor aqueous solution is characterized by a molar ratio of second rare earth element to alkaline earth metal of c: l, wherein c is from about 0 to about 10.0; and wherein the OCM catalyst precursor aqueous solution is characterized by a molar ratio of redox agent or third rare earth element to alkaline earth metal of d: l, wherein d is from about 0 to about 10.0; (b) drying at least a portion of the OCM catalyst precursor aqueous solution at a temperature of about 125°C for about 12-18 h to form an OCM catalyst precursor mixture; and (c) calcining at least a portion of the OCM catalyst precursor mixture at a temperature of about 900°C for about 4-8 h, for example in an oxidizing atmosphere, to form the OCM catalyst composition, wherein the OCM catalyst composition is characterized by the general formula A _a La _b E _c D _d O _x ; wherein A is the alkaline earth metal; wherein E is the second rare earth element; wherein D is the redox agent or third rare earth element; wherein La, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states.

[00109] In an aspect, a method for producing ethylene can comprise a multi-stage process, wherein each stage comprises (i) introducing a reactant mixture to an adiabatic reactor comprising an OCM catalyst composition, wherein the reactant mixture comprises CH ₄ and 0 ₂ , wherein the reactant mixture is characterized by an inlet temperature, and wherein the OCM catalyst composition is characterized by the general formula A _a La _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein E is a second rare earth element; wherein D is a redox agent or third rare earth element; wherein La, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; (ii) allowing at least a portion of the reactant mixture to contact at least a portion of the OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins; (iii) recovering at least a portion of the product mixture from the reactor, wherein the product mixture is characterized by an outlet temperature, and wherein the outlet temperature is greater than the inlet temperature; and (iv) optionally cooling the product mixture; wherein each stage is characterized by a broad temperature rise, wherein the broad temperature rise in each stage is a difference between the outlet temperature for that particular stage and the inlet temperature for that particular stage, and wherein the broad temperature rise is equal to or greater than about 200°C. In such aspect, the OCM catalyst composition in each stage is characterized by a C ₂₊ selectivity of equal to or greater than about 80% over the broad temperature rise for that particular stage.

[00110] In an aspect, the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; and methods of making and using same, as disclosed herein can advantageously display improvements in one or more composition characteristics when compared to conventional OCM catalysts, e.g., an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1.

[00111] The OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can display improved conversion and selectivity, when compared to the conversion and selectivity, respectively, of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1. As will be appreciated by one of skill in the art, and with the help of this disclosure, high selectivity over a broad temperature range of a catalyst for methane conversion to C ₂₊ products via OCM is critical for commercialization of the OCM process.

[00112] In an aspect, the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can advantageously be used in adiabatic reactors. In such aspect, the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can advantageously allow for the use of a lower inlet temperature, when compared to an inlet temperature that can be used with an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3 : 1 to about 10.0: 1. As will be appreciated by one of skill in the art, and with the help of this disclosure, some conventional OCM catalysts, such as Mn-Na ₂ W0 ₄ /Si0 ₂ and Na- Mn0 ₂ /MgO, require for the feed to the reactor to be pre-heated to about 800°C, which translates in a high energy cost, as well as expensive specialized materials for a heat exchanger used for pre-heating the reactor feed; in addition to providing a very narrow temperature range with a high C ₂₊ selectivity (e.g., equal to or greater than about 70%).

[00113] The OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can advantageously allow for the use of a lower outlet temperature in an adiabatic reactor, when compared to an inlet temperature that can be used with an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3 : 1 to about 10.0: 1. As will be appreciated by one of skill in the art, and with the help of this disclosure, some conventional OCM catalysts, such as Li/MgO and Mn-Na ₂ W0 ₄ /Si0 ₂ , are unstable at elevated temperatures (e.g., about 900°C), which translates into a narrow temperature range available for adiabatic operation.

[00114] The OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can advantageously allow for the use of a broad temperature rise in an adiabatic reactor, while maintaining high C ₂₊ selectivity (e.g., equal to or greater than about 70%). The OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can advantageously display stable performance at elevated temperatures (e.g., up to about 950°C, or greater).

[00115] In an aspect, the composition of OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein, can be advantageously adjusted as necessary, based on the needs of the OCM reaction, to meet target criteria, such as a target selectivity and/or a target conversion, owing to a broad range of A, Z, E and D content; and as such the OCM catalyst compositions as disclosed herein can display better performance when compared to otherwise similar OCM catalyst compositions (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1. Additional advantages of the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x , as disclosed herein; and methods of making and using same, can be apparent to one of skill in the art viewing this disclosure.

EXAMPLES

[00116] The subject matter having been generally described, the following examples are given as particular embodiments of the disclosure and to demonstrate the practice and advantages thereof. It is understood that the examples are given by way of illustration and are not intended to limit the specification of the claims to follow in any manner.

EXAMPLE 1

[00117] Oxidative coupling of methane (OCM) catalyst compositions were prepared as follows. A reference catalyst composition following the general formula Sri ₀ Ce _0.9 Ybo _. iO _x , wherein x balances the oxidation states, was prepared as follows. In an attempt to produce 10 g of Sri _. oCe _0.9 Yb ₀ iO _x , 4.23 g of Sr(N0 ₃ ) ₂ , 7.82 g of Ce(N0 ₃ ) ₃ x 6H ₂ 0 and 0.90 g of Yb(N0 ₃ ) ₃ x 5H ₂ 0 were added into 25 ml deionized (DI) water to provide a mixture, which mixture was further agitated until all solids were dissolved and a clear solution was obtained. The obtained clear solution was dried at 125°C overnight to produce a dried OCM catalyst precursor mixture. The dried OCM catalyst precursor mixture was calcined under air flow at 900°C for 6 hours to produce the reference catalyst #1 (e.g., Sri ₀ Ce _0.9 Yb _0.1 O catalyst).

[00118] Other catalysts with different compositions were prepared by using the same method used above for the reference catalyst, but with different required amounts of raw materials; and their performance was compared to the performance of the reference catalyst (e.g., Sri _. oCe _0.9 Yb ₀ iO _x catalyst). As examples, the preparation method of catalysts #1 and #2 is described below.

[00119] In an attempt to produce 10 g of catalyst #1 (S^ ₀ La _{0 9} O _x catalyst), 4.02 g of Sr(N0 ₃ ) ₂ , and 7.41 g of La(N0 ₃ ) ₃ x 6H ₂ 0 were mixed and dissolved into 30 ml deionized (DI) water to provide a mixture, which mixture was further agitated until all solids were dissolved and a clear solution was obtained. The obtained clear solution was dried at 125°C overnight to produce a dried OCM catalyst precursor mixture. The dried OCM catalyst precursor mixture was calcined under air flow at 900°C for 6 hours to produce catalyst #1 (e.g., Sri ₀ La ₀ 9O _x catalyst).

[00120] In an attempt to produce 10 g of catalyst #2 (Sr _t oLa _{0 9} Ybo _Λ Ο _χ catalyst), 4.02 g of Sr(N0 ₃ ) ₂ , 7.40 g of La(N0 ₃ ) ₃ x 6H ₂ 0 and 0.85 g of Yb(N0 ₃ ) ₃ x 5H ₂ 0 were added into 25 ml deionized (DI) water to provide a mixture, which mixture was further agitated until all solids were dissolved and a clear solution was obtained. The obtained clear solution was dried at 125°C overnight to produce a dried OCM catalyst precursor mixture. The dried OCM catalyst precursor mixture was calcined under air flow at 900°C for 6 hours to produce catalyst #2 (e.g., Sr _{! 0} La _{0 9} Yb ₀ iO _x catalyst).

[00121] Other catalysts (e.g., catalysts #3, #4, #5, #6, #7, and #8) were prepared in a similar fashion to the reference catalyst, and catalysts #1 and #2, but with using different amounts of various nitrates, as necessary. Catalyst #3 is characterized by the general formula Sr _{! 0} La ₀ gYb _{0 3} O _x . Catalyst #4 is characterized by the general formula Sr _{! 0} La ₀ gYb ₀ iTm _{0 2} O _x . Catalyst #5 is characterized by the general formula Sr _{! 0} La _{! 2} Yb ₀ iO _x . Catalyst #6 is characterized by the general formula Sr _{! 0} La _{! 5} Yb _{0 3} O _x . Catalyst #7 is characterized by the general formula Sr _{! 0} La _{0 3} Yb ₀ iO _x . Catalyst #8 is characterized by the general formula Sr _{! 0} La ₀ gNd _{0 7} Yb ₀ iO _x .

EXAMPLE 2

[00122] The performance of the OCM catalyst compositions prepared as described in Example 1 was investigated. Specifically, the performance of catalyst #1 was compared to the performance of the reference catalyst. OCM reactions were conducted by using catalysts prepared as described in Example 1 as follows. A mixture of methane and oxygen along with an internal standard, an inert gas (neon) were fed to a quartz reactor with an internal diameter (I.D.) of 2.3 mm heated by traditional clamshell furnace. A catalyst (e.g., catalyst bed; given catalyst bed) loading was 20 mg, and a total flow rate of reactants was 40 standard cubic centimeters per minute (seem). The reactor was first heated to a desired temperature (measured with the use of a thermocouple) under an inert gas flow and then a desired gas mixture was fed to the reactor. All OCM reactions were conducted at a methane to oxygen (CH ₄ :0 ₂ ) molar ratio of 7.4. The products obtained from the OCM reaction were analyzed by using an online Agilent 7890 gas chromatograph (GC) with a thermal conductivity detector (TCD) and a flame ionization detector (FID).

[00123] The performance of catalyst #1 was compared to the performance of the reference catalyst, and the data are displayed in Figures 1 and 2. From Figure 1 it can be seen that catalyst #1 shows much higher activity than the reference catalyst: the 0 ₂ conversion is higher at the same reactor temperature for catalyst #1 as compared to the reference catalyst. As will be appreciated by one of skill in the art, and with the help of this disclosure, in the case of catalyst #1 having higher activity (e.g., higher 0 ₂ conversion), the inlet temperature for adiabatic operation can be lowered. Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, the feed for OCM processes needs to be heated to the inlet temperature to carry out the OCM reaction; therefore, the lower the inlet temperature, the less external heat is needed to heat up the feed, making the operation more cost effective.

[00124] Figure 2 displays a comparison of the C ₂₊ selectivity for catalyst #1 and the reference catalyst, and it can be seen that the C ₂₊ selectivity is higher at the same reactor temperature for catalyst #1 as compared to the reference catalyst. Figure 2 shows that the lowest temperature required for catalyst #1 to achieve 75% C ₂₊ selectivity was about 700°C; and higher than 75% C ₂₊ selectivity was obtained across a broad temperature range of from 700°C to 850°C. Additionally, in the temperature range of 700°C to 800°C, higher than 80% C ₂₊ selectivity was obtained for catalyst #1. For the reference catalyst, the lowest temperature required to achieve 75% C ₂₊ selectivity was about 775°C, and the temperature range that enabled 75% C ₂₊ selectivity was 775°C to 825°C. The highest C ₂₊ selectivity obtained with the reference catalyst was 75.5%, which value is much lower than that obtained with catalyst #1. At high temperatures, such as 850°C or greater, catalyst #1 can still produce products with 75% or higher C ₂₊ selectivity. While the catalysts were not tested at temperatures higher than 850°C, the data can be extrapolated, as indicated by the dashed line in Figure 2. As such, it can be predicted that catalyst #1 will have a C ₂₊ selectivity higher than 75% at 900°C, as shown by the dashed line. For the reference catalyst, the highest temperature where the reference catalyst can still achieve 75% C ₂₊ selectivity is 825°C.

[00125] Based on the data in Figure 2, for catalyst #1, the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 700°C to 900°C (200°C temperature range); and for the reference catalyst, the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 775°C to 825°C (50°C temperature range). Catalyst #1 clearly displays a broader temperature range that allows for equal to or greater than 75% C ₂₊ selectivity, when compared to the reference catalyst.

[00126] If the catalysts were employed in an adiabatic reactor, to achieve equal to or greater than 75% C ₂₊ selectivity, catalyst # 1 could be employed in an adiabatic reactor operated with an inlet temperature of 700°C and an outlet temperature of 900°C, and a temperature rise of 200°C. To achieve equal to or greater than 75% C ₂₊ selectivity, the reference catalyst could be employed in an adiabatic reactor operated with an inlet temperature of 775°C and an outlet temperature of 825°C, and a temperature rise of 50°C. The data in Figure 2 clearly indicate that the C ₂₊ selectivity would drop significantly if the adiabatic reactor would be operated outside of 700°C to 900°C for catalyst #1, and 775°C to 825°C for the reference catalyst.

[00127] As will be appreciated by one of skill in the art, and with the help of this disclosure, for adiabatic operation, the total temperature rise is known under a given set of operation conditions. For example, a total temperature rise of 600°C could be necessary to achieve a target methane conversion and selectivity. When catalyst #1 would be used, given a temperature rise in each stage of 200°C, 3 stages would be needed. When the reference catalyst would be used, given a temperature rise in each stage of 50°C, 12 stages would be needed, which would make the process impractical for commercial applications. The data clearly demonstrates that catalyst #1 could be used for adiabatic reactor operation in a cost effective manner. When catalyst #1 would be used in an adiabatic reactor with a 200°C temperature rise (e.g., 700°C - 900°C), the selectivity obtained would be the average of the selectivity from 700°C to 900°C as shown in Figure2, which is clearly higher than the average selectivity obtained by the reference catalyst across a 50°C temperature rise. Therefore, catalyst #1 would have higher C ₂₊ selectivity in a reactor system that additionally would have less stages compared to a reactor system employing the reference catalyst.

[00128] Table 1 displays a summary of the comparison between the performances of catalyst #1 and the reference catalyst.

Table 1

predicted

[00129] S ₇₅ o/ ₀₊ temperature range of a catalyst is a temperature range wherein the catalyst is characterized by a C ₂₊ selectivity of equal to or greater than about 75%. S _80%+ temperature range of a catalyst is a temperature range wherein the catalyst is characterized by a C ₂₊ selectivity of equal to or greater than about 80%). The reference catalyst displays no temperature that allows for equal to or greater than 80%> C ₂₊ selectivity. However, for catalyst #1, the temperature range that allows for equal to or greater than 80%> C ₂₊ selectivity is 700°C to 800°C (100°C temperature range).

EXAMPLE 3

[00130] The performance of the OCM catalyst compositions prepared as described in Example 1 was investigated. Specifically, the performance of catalysts #1, #2, and #3 was compared, in order to investigate the effect of Yb on the performance of the catalyst. OCM reactions were conducted as described in Example 2. Catalysts #2 and #3, are similar to catalyst #1, except for the presence of various amounts of Yb in catalysts #2 and #3.

[00131] Table 2 displays a summary of the comparison between the performances of catalysts #1, #2 and #3. Table 2

[00132] Figure 3 displays the 0 ₂ conversion for catalysts #1, #2 and #3. From Figure 3 it can be seen that catalyst #3, which has the highest Yb content, shows the highest activity; followed by catalyst #2, which has a lower Yb content than catalyst #3; and followed by catalyst #1, which contains no Yb. The data in Figure 3 indicate that Yb increases catalyst activity, and as a result, high 0 ₂ conversion can be obtained at lower reactor temperatures.

[00133] Figure 4 displays a comparison of the C ₂₊ selectivity for catalysts #1, #2 and #3, and it can be seen that at temperatures below about 700°C, the C ₂₊ selectivity is higher at the same reactor temperature for higher Yb content, with the highest selectivity displayed by catalyst #3 (highest Yb content), followed by catalyst #2 (low Yb content), and followed by catalyst #1 (no Yb). The catalysts containing Yb (catalyst #2 and #3) provide a broader temperature range having high C ₂₊ selectivity.

[00134] Based on the data in Figure 4, for catalyst #1 (no Yb), the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 700°C to 850°C (150°C temperature range - in the absence of data extrapolation); for catalyst #2 (low Yb content), the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 700°C to 850°C (150°C temperature range); and for catalyst #3 (high Yb content), the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 675°C to 850°C (175°C temperature range).

[00135] Based on the data in Figure 4, for catalyst #1 (no Yb), the temperature range that allows for equal to or greater than 80% C ₂₊ selectivity is 700°C to 800°C (100°C temperature range); for catalyst #2 (low Yb content), the temperature range that allows for equal to or greater than 80% C ₂₊ selectivity is 750°C to 825°C (75°C temperature range); and for catalyst #3 (high Yb content), the temperature range that allows for equal to or greater than 80% C ₂₊ selectivity is 700°C to 825°C (125°C temperature range).

[00136] The catalyst with the highest Yb content (catalyst #3) displays the broadest temperature ranges for achieving a high C ₂₊ selectivity (e.g., equal to or greater than 75%, equal to or greater than 80%).

EXAMPLE 4

[00137] The performance of the OCM catalyst compositions prepared as described in Example 1 was investigated. Specifically, the performance of catalysts #2, and #4 was compared, in order to investigate the effect of Tm on the performance of the catalyst. OCM reactions were conducted as described in Example 2. Catalyst #4 is similar to catalyst #2, except for the presence of Tm in catalyst #4. [00138] Table 3 displays a summary of the comparison between the performances of catalysts #2, and

#4.

Table 3

[00139] The data in Table 3 indicate that the presence of Tm (e.g., Tm promotion) increases (e.g., enhances) catalyst activity and, at the same time, broadens the temperature range that allows for achieving 80% or higher C ₂₊ selectivity. For catalyst #2 (no Tm), the temperature range that allows for equal to or greater than 80% C ₂₊ selectivity is 750°C to 825°C (75°C temperature range); while for catalyst #4 (with Tm), the temperature range that allows for equal to or greater than 80% C ₂₊ selectivity is 725°C to 850°C (125°C temperature range).

EXAMPLE 5

[00140] The performance of the OCM catalyst compositions prepared as described in Example 1 was investigated. Specifically, the performance of catalysts #2, #5, #6, and #7 was compared, in order to investigate the effect of La content on the performance of the catalyst. OCM reactions were conducted as described in Example 2. Catalysts #5 and #7 are similar to catalyst #2, except for a different La content in catalysts #5 (higher La content) and #7 (lower La content). Catalyst #6 is similar to catalyst #2, except for different La and Yb content in catalyst #6 (higher La content and higher Yb content).

[00141] Table 4 displays a summary of the comparison between the performances of catalysts #2, #5, #6, and #7.

Table 4

[00142] The data in Table 4 indicate that increasing the content of La increases (e.g., enhances) catalyst activity. For catalyst #2, the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 700°C to 850°C (150°C temperature range), while for catalyst #5, the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 650°C to 850°C (200°C temperature range); although the temperature range that allows for equal to or greater than 80% C ₂₊ selectivity is narrower for catalyst #5 as compared to catalyst #2. When the La content is low, such as in catalyst #7, the catalyst activity is lower, so that a much higher temperature is needed to achieve equal to or greater than 90% 0 ₂ conversion: 775°C for catalyst #7.

EXAMPLE 6

[00143] The performance of the OCM catalyst compositions prepared as described in Example 1 was investigated. Specifically, the performance of a catalyst containing Nd (catalyst #8) was investigated. OCM reactions were conducted as described in Example 2. Catalyst #8 is similar to catalyst #2, except for the presence of Nd in catalyst #8.

[00144] Table 5 displays a summary of catalyst performance for catalyst #8.

Table 5

[00145] The data in Table 5 indicate that the presence of Nd in the catalyst composition increases catalytic activity, such that equal to or greater than 90% 0 ₂ conversion can be achieved at 650°C. Further, the temperature range that allows for equal to or greater than 75% C ₂₊ selectivity is 650°C to 850°C (200°C temperature range), which is broader than the similar temperature range for catalyst #2 (150°C temperature range).

[00146] Overall, the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x as disclosed herein display enhanced (higher) activity that allows for the OCM reaction to proceed at a lower inlet temperature; and enhanced (higher) selectivity, as well as enhanced (higher) selectivity over a broader temperature range, as compared to an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1. The OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x as disclosed herein display a lower deep oxidative property, which enables the higher selectivity. The lower deep oxidative property is especially important in limiting the CO _x formation at high temperatures. Further, the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x as disclosed herein display enhanced methyl radical formation and coupling rates, which in turn results in high catalytic activity, such that the OCM catalysts can be used at a low reactor temperature. Furthermore, the OCM catalyst compositions characterized by the general formula A _a Z _b E _c D _d O _x as disclosed herein display a significantly increased selectivity at lower temperatures. [00147] For the purpose of any U.S. national stage filing from this application, all publications and patents mentioned in this disclosure are incorporated herein by reference in their entireties, for the purpose of describing and disclosing the constructs and methodologies described in those publications, which might be used in connection with the methods of this disclosure. Any publications and patents discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention.

[00148] In any application before the United States Patent and Trademark Office, the Abstract of this application is provided for the purpose of satisfying the requirements of 37 C.F.R. § 1.72 and the purpose stated in 37 C.F.R. § 1.72(b) "to enable the United States Patent and Trademark Office and the public generally to determine quickly from a cursory inspection the nature and gist of the technical disclosure." Therefore, the Abstract of this application is not intended to be used to construe the scope of the claims or to limit the scope of the subject matter that is disclosed herein. Moreover, any headings that can be employed herein are also not intended to be used to construe the scope of the claims or to limit the scope of the subject matter that is disclosed herein. Any use of the past tense to describe an example otherwise indicated as constructive or prophetic is not intended to reflect that the constructive or prophetic example has actually been carried out.

[00149] The present disclosure is further illustrated by the following examples, which are not to be construed in any way as imposing limitations upon the scope thereof. On the contrary, it is to be clearly understood that resort can be had to various other aspects, embodiments, modifications, and equivalents thereof which, after reading the description herein, can be suggest to one of ordinary skill in the art without departing from the spirit of the present invention or the scope of the appended claims.

ADDITIONAL DISCLOSURE

[00150] A first aspect, which is an oxidative coupling of methane (OCM) catalyst composition characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states.

[00151] A second aspect, which is the composition of the first aspect, wherein the alkaline earth metal is selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and combinations thereof. [00152] A third aspect, which is the composition of any one of the first and the second aspects, wherein the first rare earth element is selected from the group consisting of lanthanum (La), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), and combinations thereof.

[00153] A fourth aspect, which is the composition of any one of the first through the third aspects, wherein the second rare earth element and the third rare earth element can each independently be selected from the group consisting of scandium (Sc), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), yttrium (Y), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and combinations thereof.

[00154] A fifth aspect, which is the composition of any one of the first through the fourth aspects, wherein the redox agent is selected from the group consisting of manganese (Mn), tungsten (W), bismuth (Bi), antimony (Sb), tin (Sn), cerium (Ce), praseodymium (Pr), and combinations thereof.

[00155] A sixth aspect, which is the composition of any one of the first through the fifth aspects comprising one or more oxides of A; one or more oxides of Z; one or more oxides of E; one or more oxides of D; or combinations thereof.

[00156] A seventh aspect, which is the composition of any one of the first through the sixth aspects having the general formula A _a La _b O _x ; wherein A is an alkaline earth metal; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; and wherein x balances the oxidation states.

[00157] An eighth aspect, which is the composition of the seventh aspect having the general formula Sr _a La _b O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; and wherein x balances the oxidation states.

[00158] A ninth aspect, which is the composition of any one of the first through the sixth aspects, having the general formula A _a La _b E _c O _x ; wherein A is an alkaline earth metal; wherein E is a second rare earth element; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0.01 to about 10.0; and wherein x balances the oxidation states.

[00159] A tenth aspect, which is the composition of the ninth aspect having the general formula Sr _a La _b Yb _c O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0.01 to about 10.0; and wherein x balances the oxidation states.

[00160] An eleventh aspect, which is the composition of any one of the first through the sixth aspects having the general formula Sr _a La _b E _c D _d O _x ; wherein E is a second rare earth element; wherein D is a third rare earth element; wherein the second rare earth element and the third rare earth element are different; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0.01 to about 10.0; wherein d is from about 0.01 to about 10.0; and wherein x balances the oxidation states. [00161] A twelfth aspect, which is the composition of the eleventh aspect having the general formula Sr _a La _b Yb _c Tm _d O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0.01 to about 10.0; wherein d is from about 0.01 to about 10.0; and wherein x balances the oxidation states.

[00162] A thirteenth aspect, which is the composition of any one of the eleventh and the twelfth aspects having the general formula Sr _a La _b Nd _c Yb _d O _x ; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0.01 to about 10.0; wherein d is from about 0.01 to about 10.0; and wherein x balances the oxidation states.

[00163] A fourteenth aspect, which is the composition of any one of the first through the thirteenth aspects further comprising a support, wherein at least a portion of the OCM catalyst composition contacts, coats, is embedded in, is supported by, and/or is distributed throughout at least a portion of the support; wherein the support comprises MgO, A1 ₂ 0 ₃ , Si0 ₂ , Zr0 ₂ , Ti0 ₂ , or combinations thereof; and wherein the support is in the form of a powder, a particle, a pellet, a monolith, a foam, a honeycomb, or combinations thereof.

[00164] A fifteenth aspect, which is the composition of any one of the first through the fourteenth aspects, wherein the OCM catalyst composition is characterized by a C ₂₊ selectivity of equal to or greater than about 70% over a temperature rise of equal to or greater than about 150°C.

[00165] A sixteenth aspect, which is the composition of any one of the first through the fifteenth aspects, wherein the OCM catalyst composition is characterized by a C ₂₊ selectivity that is increased by equal to or greater than about 5% over a temperature rise of equal to or greater than about 150°C, when compared to a C ₂₊ selectivity over the same temperature rise of equal to or greater than about 150°C of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3 : 1 to about 10.0: 1.

[00166] A seventeenth aspect, which is the composition of any one of the first through the sixteenth aspects, wherein the OCM catalyst composition is characterized by a S ₇₀₊ temperature rise that is increased by equal to or greater than about 50% when compared to a S ₇₀₊ temperature rise of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3 : 1 to about 10.0: 1 ; wherein a S ₇₀₊ temperature rise of a catalyst is a temperature rise wherein the catalyst is characterized by a C ₂₊ selectivity of equal to or greater than about 70%.

[00167] An eighteenth aspect, which is the composition of any one of the first through the seventeenth aspects, wherein the OCM catalyst composition is characterized by a S ₇₀₊ temperature rise that is increased by equal to or greater than about 25°C when compared to a S ₇₀₊ temperature rise of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3 : 1 to about 10.0: 1 ; wherein a S ₇₀₊ temperature rise of a catalyst is a temperature rise wherein the catalyst is characterized by a C ₂₊ selectivity of equal to or greater than about 70%.

[00168] A nineteenth aspect, which is the composition of any one of the first through the eighteenth aspects, wherein the OCM catalyst composition is characterized by a reactor temperature effective for achieving an 0 ₂ conversion of equal to or greater than about 90% that is decreased by equal to or greater than about 25°C, when compared to a reactor temperature effective for achieving an 0 ₂ conversion of equal to or greater than about 90% of an otherwise similar OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3: 1 to about 10.0: 1.

[00169] A twentieth aspect, which is a method of making an oxidative coupling of methane (OCM) catalyst composition comprising (a) forming an OCM catalyst precursor mixture; wherein the OCM catalyst precursor mixture comprises one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or a third rare earth element cation; wherein the first rare earth element cation, the second rare earth element cation, and the third rare earth element cation, when present, are not the same; wherein the OCM catalyst precursor mixture is characterized by a molar ratio of first rare earth element to alkaline earth metal of b: l, wherein b is from about 0.3 to about 10.0; wherein the OCM catalyst precursor mixture is characterized by a molar ratio of second rare earth element to alkaline earth metal of c: l, wherein c is from about 0 to about 10.0; and wherein the OCM catalyst precursor mixture is characterized by a molar ratio of redox agent or third rare earth element to alkaline earth metal of d: l, wherein d is from about 0 to about 10.0; and (b) calcining at least a portion of the OCM catalyst precursor mixture to form the OCM catalyst composition.

[00170] A twenty-first aspect, which is the method of the twentieth aspect, wherein the OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states.

[00171] A twenty-second aspect, which is the method of any one of the twentieth and the twenty-first aspects, wherein the step (a) of forming an OCM catalyst precursor mixture further comprises (i) solubilizing the one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or a third rare earth element cation in an aqueous medium to form an OCM catalyst precursor aqueous solution; and (ii) drying at least a portion of the OCM catalyst precursor aqueous solution to form the OCM catalyst precursor mixture.

[00172] A twenty-third aspect, which is the method of the twenty-second aspect, wherein the OCM catalyst precursor aqueous solution is dried at a temperature of equal to or greater than about 75°C.

[00173] A twenty-fourth aspect, which is the method of any one of the twentieth through the twenty- third aspects, wherein at least a portion of the OCM catalyst precursor aqueous solution is contacted with a support to yield a supported OCM catalyst precursor.

[00174] A twenty-fifth aspect, which is the method of the twenty-fourth aspect, wherein at least a portion of the supported OCM catalyst precursor is further dried and calcined to form the OCM catalyst composition.

[00175] A twenty-sixth aspect, which is the method of any one of the twentieth through the twenty- fifth aspects, wherein the OCM catalyst precursor mixture is calcined at a temperature of equal to or greater than about 750°C.

[00176] A twenty-seventh aspect, which is the method of any one of the twentieth through the twenty- sixth aspects, wherein the one or more compounds comprising an alkaline earth metal cation comprises an alkaline earth metal nitrate, an alkaline earth metal oxide, an alkaline earth metal hydroxide, an alkaline earth metal chloride, an alkaline earth metal acetate, an alkaline earth metal carbonate, or combinations thereof; wherein the one or more compounds comprising a first rare earth element cation comprises a first rare earth element nitrate, a first rare earth element oxide, a first rare earth element hydroxide, a first rare earth element chloride, a first rare earth element acetate, a first rare earth element carbonate, or combinations thereof; wherein the one or more compounds comprising a second rare earth element cation comprises a second rare earth element nitrate, a second rare earth element oxide, a second rare earth element hydroxide, a second rare earth element chloride, a second rare earth element acetate, a second rare earth element carbonate, or combinations thereof; wherein the one or more compounds comprising a redox agent cation comprises a redox agent nitrate, a redox agent oxide, a redox agent hydroxide, a redox agent chloride, a redox agent acetate, a redox agent carbonate, or combinations thereof; and wherein the one or more compounds comprising a third rare earth element cation comprises a third rare earth element nitrate, a third rare earth element oxide, a third rare earth element hydroxide, a third rare earth element chloride, a third rare earth element acetate, a third rare earth element carbonate, or combinations thereof. [00177] A twenty-eighth aspect, which is an OCM catalyst produced by the method of any one of the twentieth through the twenty-seventh aspects.

[00178] A twenty-ninth aspect, which is an oxidative coupling of methane (OCM) catalyst composition produced by (a) solubilizing one or more compounds comprising an alkaline earth metal cation, one or more compounds comprising a first rare earth element cation, one or more compounds comprising a second rare earth element cation, and one or more compounds comprising a redox agent cation or third rare earth element cation in an aqueous medium to form an OCM catalyst precursor aqueous solution; wherein the first rare earth element cation, the second rare earth element cation, and the third rare earth element cation, when present, are not the same; wherein the OCM catalyst precursor aqueous solution is characterized by a molar ratio of first rare earth element to alkaline earth metal of b: l, wherein b is from about 0.3 to about 10.0; wherein the OCM catalyst precursor aqueous solution is characterized by a molar ratio of second rare earth element to alkaline earth metal of c: l, wherein c is from about 0 to about 10.0; and wherein the OCM catalyst precursor aqueous solution is characterized by a molar ratio of redox agent or third rare earth element to alkaline earth metal of d: l, wherein d is from about 0 to about 10.0; (b) drying at least a portion of the OCM catalyst precursor aqueous solution at a temperature of equal to or greater than about 75°C to form an OCM catalyst precursor mixture; and (c) calcining at least a portion of the OCM catalyst precursor mixture at a temperature of equal to or greater than about 750°C to form the OCM catalyst composition.

[00179] A thirtieth aspect, which is the composition of the twenty-ninth aspect, wherein the OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states.

[00180] A thirty-first aspect, which is a method for producing olefins comprising (a) introducing a reactant mixture to an adiabatic reactor comprising an oxidative coupling of methane (OCM) catalyst composition, wherein the reactant mixture comprises methane (CH ₄ ) and oxygen (0 ₂ ), wherein the reactant mixture is characterized by an inlet temperature, and wherein the OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; (b) allowing at least a portion of the reactant mixture to contact at least a portion of the OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins; (c) recovering at least a portion of the product mixture from the adiabatic reactor, wherein the product mixture is characterized by an outlet temperature, and wherein the outlet temperature is greater than the inlet temperature; and (d) recovering at least a portion of the olefins from the product mixture.

[00181] A thirty-second aspect, which is the method of the thirty-first aspect, wherein the OCM catalyst composition is characterized by a C ₂₊ selectivity of equal to or greater than about 70% over a temperature rise in the adiabatic reactor, wherein the temperature rise is a difference between the outlet temperature and the inlet temperature.

[00182] A thirty-third aspect, which is the method of the thirty-second aspect, wherein the temperature rise is equal to or greater than about 150°C.

[00183] A thirty-fourth aspect, which is the method of any one of the thirty-first through the thirty- third aspects, wherein the inlet temperature is from about 550°C to about 800°C.

[00184] A thirty-fifth aspect, which is the method of any one of the thirty-first through the thirty- fourth aspects, wherein the outlet temperature is from about 700°C to about 950°C.

[00185] A thirty-sixth aspect, which is the method of any one of the thirty-first through the thirty-fifth aspects further comprising minimizing deep oxidation of methane to carbon dioxide (C0 ₂ ).

[00186] A thirty-seventh aspect, which is the method of any one of the thirty-first through the thirty- sixth aspects, wherein the OCM reaction in the adiabatic reactor is characterized by an oxygen conversion of equal to or greater than about 90%.

[00187] A thirty-eighth aspect, which is the method of any one of the thirty-first through the thirty- seventh aspects, wherein the inlet temperature is less than an inlet temperature for an otherwise similar adiabatic reactor comprising an OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3 : 1 to about 10.0: 1.

[00188] A thirty-ninth aspect, which is a method for producing olefins comprising (a) introducing a first reactant mixture to a first adiabatic reactor comprising a first oxidative coupling of methane (OCM) catalyst composition, wherein the first reactant mixture comprises methane (CH ₄ ) and oxygen (0 ₂ ), wherein the first reactant mixture is characterized by a first inlet temperature, and wherein the first OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; (b) allowing at least a portion of the first reactant mixture to contact at least a portion of the first OCM catalyst composition and react via an OCM reaction to form a first product mixture comprising unreacted methane and olefins; (c) recovering at least a portion of the first product mixture from the first adiabatic reactor, wherein the first product mixture is characterized by a first outlet temperature, and wherein the first outlet temperature is greater than the first inlet temperature; (d) cooling the first product mixture from the first outlet temperature to a second inlet temperature; (e) introducing a second reactant mixture to a second adiabatic reactor comprising a second OCM catalyst composition, wherein the second reactant mixture comprises at least a portion of the first product mixture and 0 ₂ , wherein the second reactant mixture is characterized by the second inlet temperature, wherein the first inlet temperature and the second inlet temperature are the same or different; wherein the second OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; and wherein the first OCM catalyst composition and the second OCM catalyst composition are the same or different; (f) allowing at least a portion of the second reactant mixture to contact at least a portion of the second OCM catalyst composition and react via an OCM reaction to form a second product mixture comprising unreacted methane and olefins, wherein an amount of unreacted methane in the second product mixture is less than an amount of unreacted methane in the first product mixture, and wherein an amount of olefins in the second product mixture is greater than an amount of olefins in the first product mixture; (g) recovering at least a portion of the second product mixture from the second adiabatic reactor, wherein the second product mixture is characterized by a second outlet temperature, wherein the second outlet temperature is greater than the second inlet temperature, and wherein the first outlet temperature and the second outlet temperature are the same or different; (h) optionally cooling the second product mixture to form a cooled second product mixture; and (i) recovering at least a portion of the olefins from the second product mixture and/or the cooled second product mixture.

[00189] A fortieth aspect, which is the method of the thirty-ninth aspect, wherein the first OCM catalyst composition is characterized by a C ₂₊ selectivity of equal to or greater than about 70% over a first temperature rise in the first adiabatic reactor, wherein the first temperature rise is a difference between the first outlet temperature and the first inlet temperature; and wherein the first temperature rise is equal to or greater than about 150°C. [00190] A forty-first aspect, which is the method of any one of the thirty-ninth and the fortieth aspects, wherein the second OCM catalyst composition is characterized by a C ₂₊ selectivity of equal to or greater than about 70% over a second temperature rise in the second adiabatic reactor, wherein the second temperature rise is a difference between the second outlet temperature and the second inlet temperature; and wherein the second temperature rise is equal to or greater than about 150°C.

[00191] A forty-second aspect, which is the method of any one of the thirty-ninth through the forty- first aspects, wherein producing olefins is a multi-stage process, wherein a first stage comprises steps (a) through (d), wherein a second stage comprises steps (e) through (h), and wherein the multi-stage process further comprises one or more additional stages downstream of the first stage and/or the second stage, as necessary to achieve a target methane conversion and/or a target C ₂₊ selectivity for the overall multi-stage process.

[00192] A forty-third aspect, which is the method of the forty-second aspect, wherein each additional stage comprises (i) introducing a reactant mixture to a reactor comprising an OCM catalyst composition, wherein the reactant mixture comprises CH ₄ and 0 ₂ , wherein the reactant mixture is characterized by an inlet temperature, and wherein the OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; (ii) allowing at least a portion of the reactant mixture to contact at least a portion of the OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins; (iii) recovering at least a portion of the product mixture from the reactor, wherein the product mixture is characterized by an outlet temperature, and wherein the outlet temperature is greater than the inlet temperature; and (iv) optionally cooling the product mixture.

[00193] A forty-fourth aspect, which is the method of the forty-third aspect, wherein the reactant mixture comprises at least a portion of an upstream product mixture recovered from an upstream reactor.

[00194] A forty-fifth aspect, which is the method of any one of the thirty-ninth through the forty- fourth aspects, wherein the reactant mixture comprises at least a portion of a downstream product mixture recovered from a downstream reactor.

[00195] A forty-sixth aspect, which is the method of any one of the thirty-ninth through the forty-fifth aspects, wherein the OCM reaction in the first adiabatic reactor and/or the second adiabatic reactor is characterized by an oxygen conversion of equal to or greater than about 90%. [00196] A forty-seventh aspect, which is the method of the forty-second aspect, wherein the multistage process has from 2 to 4 stages.

[00197] A forty-eighth aspect, which is the method of any one of the thirty-ninth through the forty- seventh aspects, wherein the multi-stage process has less stages than an otherwise similar multi-stage process employing an OCM catalyst composition (i) without the first rare earth element, or (ii) comprising an alkaline earth metal and the first rare earth element in a first rare earth element to alkaline earth metal molar ratio other than from about 0.3 : 1 to about 10.0: 1.

[00198] A forty-ninth aspect, which is a multi-stage method for producing olefins, wherein each stage comprises (i) introducing a reactant mixture to an adiabatic reactor comprising an oxidative coupling of methane (OCM) catalyst composition, wherein the reactant mixture comprises CH ₄ and 0 ₂ , wherein the reactant mixture is characterized by an inlet temperature, and wherein the OCM catalyst composition is characterized by the general formula A _a Z _b E _c D _d O _x ; wherein A is an alkaline earth metal; wherein Z is a first rare earth element; wherein E is a second rare earth element; wherein D is a redox agent or a third rare earth element; wherein the first rare earth element, the second rare earth element, and the third rare earth element, when present, are not the same; wherein a is 1.0; wherein b is from about 0.3 to about 10.0; wherein c is from about 0 to about 10.0; wherein d is from about 0 to about 10.0; and wherein x balances the oxidation states; (ii) allowing at least a portion of the reactant mixture to contact at least a portion of the OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins; (iii) recovering at least a portion of the product mixture from the reactor, wherein the product mixture is characterized by an outlet temperature, and wherein the outlet temperature is greater than the inlet temperature; and (iv) optionally cooling the product mixture, wherein each stage is characterized by a broad temperature rise, wherein the broad temperature rise in each stage is a difference between the outlet temperature for that particular stage and the inlet temperature for that particular stage, and wherein the broad temperature rise is equal to or greater than about 200°C.

[00199] A fiftieth aspect, which is the method of the forty-ninth aspect, wherein the OCM catalyst composition in each stage is characterized by a C ₂₊ selectivity of equal to or greater than about 70% over the broad temperature rise for that particular stage.

[00200] While embodiments of the disclosure have been shown and described, modifications thereof can be made without departing from the spirit and teachings of the invention. The embodiments and examples described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention.

[00201] Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are a further description and are an addition to the detailed description of the present invention. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference.