WO2021050208A1

WO2021050208A1 - Highly selective mixed oxide catalyst for oxidative coupling of methane

Info

Publication number: WO2021050208A1
Application number: PCT/US2020/046639
Authority: WO
Inventors: Wugeng Liang; Vidya Sagar Reddy SARSANI; David West; Hector PEREZ; Robin WOODBURY
Original assignee: Sabic Global Technologies, B.V.
Priority date: 2019-09-10
Filing date: 2020-08-17
Publication date: 2021-03-18

Abstract

A supported oxidative coupling of methane (OCM) catalyst composition characterized by the general formula Mn-Na2WO4/SiO2; wherein equal to or greater than about 50% of any 100 mm2 regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1.

Description

HIGHLY SELECTIVE MIXED OXIDE CATALYST FOR OXIDATIVE COUPLING OF METHANE

TECHNICAL FIELD

[0001] The present disclosure relates to catalyst compositions for oxidative coupling of methane (OCM), more specifically catalyst compositions based on Mn-Na₂W0₄ 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 (C2H4). As an overall reaction, in the OCM, methane (CH₄) and oxygen (O₂) react exothermically over a catalyst to form C₂H , water (H₂0) and heat.

[0004] Ethylene can be produced by OCM as represented by Equations (I) and (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 C₂ hydrocarbon product (e.g., ethylene):

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, CH₄ is first oxidatively converted into ethane (C₂H₆), and then into C₂H . CH is activated heterogeneously on a catalyst surface, forming methyl radicals (e.g., CH₃·), which then couple in a gas phase to form C₂H₆. C₂H₆ subsequently undergoes dehydrogenation to form C₂H₄. An overall yield of desired C₂ 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) deep oxidation products, such as carbon monoxide (CO) and/or carbon dioxide (CO₂). Some of the best reported OCM outcomes encompass a ~20% conversion of methane and ~80% selectivity to desired C₂ 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, such as low selectivity towards desired products (e.g., C₂₊ hydrocarbons, such as ethylene) owing to an increased selectivity towards undesired products (e.g. deep oxidation products, such as CO₂). 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 (O₂) conversion and C₂₊ selectivity as a function of temperature in an oxidative coupling of the methane (OCM) reaction for two different catalysts;

[0011] Figure 2 displays a graph of selectivity to carbon monoxide (CO) and carbon dioxide (CO₂) as a function of temperature in an OCM reaction for two different catalysts;

[0012] Figure 3 displays a graph of a ratio of CO₂ formation as a function of temperature in an OCM reaction between two different catalysts;

[0013] Figure 4 displays a scanning electron microscope (SEM) micrograph of a reference OCM catalyst;

[0014] Figure 5 displays another SEM micrograph of a reference OCM catalyst;

[0015] Figure 6 displays an SEM micrograph of a supported OCM catalyst;

[0016] Figure 7 displays a graph of methane (CH ) conversion and C₂₊ yield as a function of temperature in an OCM reaction for two different catalysts;

[0017] Figure 8 displays an SEM micrograph of a type of sodium tungstate (Na₂WO₄);

[0018] Figure 9 displays an SEM micrograph of another type of Na₂WO₄; and

[0019] Figure 10 displays an SEM micrograph of yet another type of Na₂WO₄. DETAILED DESCRIPTION

[0020] Disclosed herein are supported oxidative coupling of methane (OCM) catalyst compositions and methods of making and using same. In an aspect, a supported OCM catalyst composition can be characterized by the general formula Mn-Na₂WO₄/SiO₂; wherein equal to or greater than about 50% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of sodium (Na) to tungsten (W) (Na:W) of from about 1.0:1 to about 4:1. In an aspect, the supported OCM catalyst composition as disclosed herein can be prepared by using sodium tungstate (Na₂WO₄) in the substantial absence of water; wherein the resulting supported OCM catalyst composition is characterized by a significant reduction in deep oxidation products (CO_x) formation in an OCM reaction, thereby resulting in an increased C₂₊ selectivity.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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. [0025] 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. [0026] As used herein, the term “effective,” means adequate to accomplish a desired, expected, or intended result.

[0027] 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.

[0028] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art.

[0029] 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. [0030] As used herein, the terms “C_x hydrocarbons” and “C_xs” are interchangeable and refer to any hydrocarbon having x number of carbon atoms (C). For example, the terms “C hydrocarbons” and “C₄s” both refer to any hydrocarbons having exactly 4 carbon atoms, such as n-butane, iso-butane, cyclobutane, 1 - butene, 2-butene, isobutylene, butadiene, and the like, or combinations thereof.

[0031] As used herein, the term “C_x+ hydrocarbons” refers to any hydrocarbon having equal to or greater than x carbon atoms (C). For example, the term “C₂₊ hydrocarbons” refers to any hydrocarbons having 2 or more carbon atoms, such as ethane, ethylene, C s, C₄s, C₅s, etc.

[0032] In an aspect, a supported OCM catalyst composition as disclosed herein can be characterized by the general formula Mn-Na₂WO₄/SiO₂; wherein equal to or greater than about 50% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of sodium (Na) to tungsten (W) (Na:W) of from about 1.0:1 to about 4:1. For purposes of the disclosure herein, the term “surface molar ratio” refers to molar ratio of atoms (e.g., Na, W, etc.) present at an external surface of the catalyst (as opposed to in the bulk of the catalyst), wherein the external surface of the catalyst is exposed and available for contact with reactants (e.g., oxygen (O₂), methane (CH ), etc.). [0033] Without wishing to be limited by theory, the general formula of the catalyst (Mn- Na₂WO₄/SiO₂) gives a theoretical stoichiometric molar ratio ofNa:W of 2.0:1 over the entire catalyst (e.g., the bulk of the catalyst). In other words, and without wishing to be limited by theory, by using sodium tungstate (Na₂WO₄) as the source for both Na and W in the catalyst, the bulk catalyst should contain 2 moles of Na for every 1 mole of W. However, and as will be appreciated by one of skill in the art, and with the help of this disclosure, for certain catalysts (e.g., conventional Mn-Na₂WO₄/SiO₂ catalysts), the surface concentration of atoms (i.e., distribution of the atoms on an external or exposed surface of the catalyst) is not necessarily uniform (i.e., does not necessarily follow the bulk theoretical stoichiometric molar ratio); which can lead to catalyst performance issues, such as decreased selectivity to desired products.

[0034] 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. Without wishing to be limited by theory, the different metals (Na, Mn, and W) present in the supported OCM catalyst compositions as disclosed herein can 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 (Na, Mn, and W) present in the supported OCM catalyst compositions as disclosed herein can generate formation of surface oxygen vacancies (e.g., uncompensated oxygen vacancies), which can lead to further improvement of catalyst performance, for example in terms of conversion, selectivity, etc., as will be discussed in more detail later herein.

[0035] Further, 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.

[0036] Without wishing to be limited by theory, an OCM reaction can propagate by following a mechanism according to reactions (l)-(8):

wherein “s” denotes a species adsorbed onto the catalyst surface. As will be appreciated by one of skill in the art, and with the help of this disclosure, two or more of reactions ( 1 )-(8) can occur concurrently (as opposed to sequentially). According to reaction (1), the activation of methane occurs with the participation of active adsorbed oxygen sites [0]_s, leading to the formation of methyl radicals and adsorbed hydroxyl group [OH]_s. According to reaction (2), the coupling of methyl radicals to form the coupling product ethane (C₂H₆) occurs in gas phase; wherein reaction (2) has a low activation energy, and therefore, does not limit the overall reaction rate. According to reaction (3), methyl radicals can react with gas phase oxygen to form an oxygenate product CH O₂. According to reaction (4), methyl radicals can also re-adsorb onto the catalyst surface and react with surface oxygen (e.g., active adsorbed oxygen sites [0]_s) to form an oxygenate species [CH₃0]_s. The oxygenates formed according to reactions (3) and (4) can further form CO and CO₂, and as such the reaction steps according to reactions (3) and (4) are the main reactions controlling the selectivity of various OCM catalysts.

[0037] Further, and without wishing to be limited by theory, easy removal of methyl radicals from oxygen centers will result in increasing C₂₊ selectivity; while oxygen centers that display strong bonding will promote the oxidation of methyl radicals via [CH₃0]_s, thereby leading to the formation of deep oxidation products (CO_x). The direction of reaction (3) depends on temperature, while the direction of reaction (4) depends both on temperature and catalyst. O-containing compounds (e.g., O-containing compounds formed according to reactions (3)-(4)) are precursors of deep oxidation products, such as CO and CO₂, and thus the conversion of methyl radicals via reactions (3)-(4) will lead to C₂₊ selectivity loss. [0038] Furthermore, and without wishing to be limited by theory, as described in reactions (l)-(5), an OCM reaction starts with methyl radical formation, coupling of which leads to the formation of ethane; wherein ethane can be further converted to ethylene through parallel reactions of thermal dehydrogenation and catalytic oxidative dehydrogenation, according to reaction (6). Furthermore, according to reaction (7), ethylene dehydrogenation can produce acetylene. In addition to the oxygenates formed according to reactions (3) and (4), a portion of the C₂₊ products formed (e.g., C₂H ) can also undergo deep oxidation to form CO and CO₂. For example, according to reaction (8), ethylene can undergo deep oxidation to CO and CO₂. The mechanism of OCM reaction is described in more detail in Lomonosov, V.I. and Sinev, M.Y., Kinetics and Catalysis, 2016, vol. 57, pp. 647-676; which is incorporated by reference herein in its entirety. [0039] Furthermore, and without wishing to be limited by theory, in order to increase the C₂₊ selectivity, catalyst activity for reactions (3) and (4), and for C₂₊ deep oxidation in reaction (8) need to be reduced. The supported OCM catalyst composition as disclosed herein can reduce the re-adsorption of methyl radicals and re-adsorption of the formed products, thereby reducing deep oxidation and improving C₂₊ selectivity.

[0040] In aspects where Na and W are spatially separated (e.g., are relatively far apart) on the surface of the catalyst, for example in the case of conventional catalysts prepared by using aqueous solutions of Na₂WO₄, some external catalyst surface regions can display an increased Na content (i.e., increased Na surface concentration), while other external catalyst surface regions can display an increased W content (i.e., increased W surface concentration). Without wishing to be limited by theory, when Na₂WO₄ is dissolved in water, the Na₂WO₄ aqueous solution comprises Na⁺ cations and WO₄ ^2- anions randomly distributed in the solution, which provides the opportunity for the Na⁺ cations to generate regions on the catalyst that are concentrated in Na (e.g., enriched in Na), as well as for the WO₄ ^2- anions to generate regions on the catalyst that are concentrated in W (e.g., enriched in W), respectively. Further, and without wishing to be limited by theory, catalyst surface regions displaying an enriched W surface concentration (i.e., increased W surface concentration) will display formation of large amounts of W0 . Generally, W0₃ is characterized by a lower basicity and displays strong redox and deep oxidation properties. When methyl radicals re-adsorb onto W0 catalyst surface sites, they can form the oxygenate species [CH 0]_s as shown in reaction (4), and can also produce CO_x as the final products, thereby resulting in a lower selectivity towards desired products (e.g., C₂₊ selectivity).

[0041] In aspects where Na and W are spatially neighboring each other (e.g., are relatively close to each other) on the surface of the catalyst, for example in the case of the supported OCM catalyst compositions as disclosed herein (which are prepared with Na₂WO₄ under substantially anhydrous conditions, as will be discussed in more detail later herein), the external catalyst surface can display a substantially uniform distribution of Na and W in the bulk of the catalyst, as well as on the external surface of the catalyst. For purposes of the disclosure herein, the terms “substantially anhydrous,” “substantially free of water,” and “substantially water-free” refer to a medium (e.g., surrounding medium), a composition (e.g., an OCM catalyst precursor), conditions (e.g., contacting conditions, mixing conditions), and the like, or combinations thereof; wherein the medium and/or composition comprise less than about 15 wt.%, alternatively less than about 10 wt.%, alternatively less than about 5 wt.%, alternatively less than about 4 wt.%, alternatively less than about 3 wt.%, alternatively less than about 2 wt.%, alternatively less than about 1 wt.%, alternatively less than about 0.5 wt.%, alternatively less than about 0.1 wt.%, alternatively less than about 0.01 wt.%, alternatively less than about 0.001 wt.% water (H₂0), based on the total weight of the medium and/or composition, respectively. Further, for purposes of the disclosure herein, the term “in the substantial absence of water” refers to the use of a substantially anhydrous, substantially free of water, or substantially water-free medium (e.g., surrounding medium), composition (e.g., an OCM catalyst precursor), conditions (e.g., contacting conditions, mixing conditions), and the like, or combinations thereof. As will be appreciated by one of skill in the art, and with the help of this disclosure, a substantially anhydrous, substantially free of water, and/or substantially water-free medium (e.g., surrounding medium), composition (e.g., an OCM catalyst precursor), conditions (e.g., contacting conditions, mixing conditions), and the like, or combinations thereof can be exposed to and/or comprise environmental water, such as atmospheric moisture. Further, as will be appreciated by one of skill in the art, and with the help of this disclosure, a substantially anhydrous, substantially free of water, and/or substantially water-free medium (e.g., surrounding medium), composition (e.g., an OCM catalyst precursor), conditions (e.g., contacting conditions, mixing conditions), and the like, or combinations thereof can be exposed to and/or comprise hydration water from crystal hydrates, such as water from sodium tungstate dihydrate (Na₂WO₄·2H₂0). [0042] Without wishing to be limited by theory, when Na₂WO₄ is used in the substantial absence of water (e.g., Na₂WO₄ is used under substantially anhydrous conditions, Na₂WO₄ is used in solid form or state), the Na₂WO₄ comprises Na⁺ cations and WO₄ ^2- anions evenly distributed in a crystal lattice (e.g., rhombic crystal lattice, orthorhombic crystal lattice, etc.), which provides for the Na⁺ cations and for the WO₄ ^2- anions being in a relatively fixed position with respect to each other in the bulk of the catalyst, as well as on the external surface of the catalyst. When Na₂WO₄ is used in the substantial absence of water, Na and W are substantially uniformly distributed on the external catalyst surface, as well as in the bulk of the catalyst (e.g., supported OCM catalyst composition). For purposes of the disclosure herein, the terms “substantially uniform distribution” and “substantially uniformly concentration” refer to a composition (e.g., supported OCM catalyst composition, OCM catalyst precursor, etc.) having less than about +25%, alternatively less than about +20%, alternatively less than about +15%, alternatively less than about +10%, alternatively less than about +9%, alternatively less than about +8%, alternatively less than about 7+%, alternatively less than about +6%, alternatively less than about +5%, alternatively less than about +4%, alternatively less than about +3%, alternatively less than about +2%, or alternatively less than about +1% variation in the bulk concentration and/or surface concentration of a chemical species (e.g., Na, W, MnO₂, SiO₂, etc.) in the bulk of the composition (e.g., catalyst, catalyst precursor) and/or on the external surface of the composition (e.g., catalyst, catalyst precursor), respectively.

[0043] The external surface of the OCM catalyst composition can be characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1, alternatively from about 1.1:1 to about 3.5:1, alternatively from about 1.2:1 to about 3:1, or alternatively from about 1.25:1 to about 2.0:1.

[0044] In an aspect, equal to or greater than about 50%, alternatively equal to or greater than about 60%, alternatively equal to or greater than about 70%, or alternatively equal to or greater than about 75% of any 100 mm² regions, alternatively any 75 mm² regions, alternatively any 50 mm² regions, alternatively any 25 mm² regions, or alternatively any 10 mm² regions of the external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1, alternatively from about 1.1:1 to about 3.5:1, alternatively from about 1.2:1 to about 3:1, or alternatively from about 1.25:1 to about 2.0:1. As will be appreciated by one of skill in the art, and with the help of this disclosure, some regions of the external surface of the catalyst (e.g., external surface of the OCM catalyst composition) may contain Na and/or W, while other regions of the external surface of the catalyst may contain exposed support (e.g., exposed silica), without catalytic material (e.g., Na and/or W). In other words, and for example, from 100 random 100 mm² regions of the external surface of the OCM catalyst composition (wherein such regions have Na and/or W), at least 50 of the 100 random 100 mm² regions of the external surface of the OCM catalyst composition are characterized by a surface molar ratio of Na to W (Na: W) of from about 1.0:1 to about 4: 1.

[0045] As described previously herein, in the case of conventional catalysts prepared by using aqueous solutions of Na₂WO₄, some external catalyst surface regions can display an increased Na content (i.e., increased Na surface concentration), while other external catalyst surface regions can display an increased W content (i.e., increased W surface concentration). In aspects where conventional catalysts are prepared by using aqueous solutions of Na₂WO₄, less than about 40%, alternatively less than about 30%, alternatively less than about 25%, or alternatively less than about 20% of any 100 mm² regions of the external surface of such conventional catalysts having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1, alternatively from about 1.1:1 to about 3.5:1, alternatively from about 1.2:1 to about 3:1, or alternatively from about 1.25:1 to about 2.0:1. In other words, and for example, from 100 random 100 mm² regions of the external surface of a conventional catalyst prepared by using aqueous solutions of Na₂WO₄ composition (wherein such regions have Na and/or W), less than 40 of the 100 random 100 mm² regions of the external surface of the OCM catalyst composition are characterized by a surface molar ratio of Na to W (Na: W) of from about 1.0:1 to about 4: 1.

[0046] Without wishing to be limited by theory, when Na₂WO₄ is used in the substantial absence of water (e.g., Na₂WO₄ is used under substantially anhydrous conditions, Na₂WO₄ is used in solid form or state), and Na and W are relatively close to each other as well as uniformly distributed on the external surface of the OCM catalyst composition (as is the case with the supported OCM catalyst composition as disclosed herein), the catalytically active sites on the catalyst surface will display increased basicity as compared to catalytically active sites enriched in W0₃ (which are present in conventional catalysts characterized by the general formula Mn-Na₂WO₄). Consequently, in the case of the supported OCM catalyst composition as disclosed herein, and without wishing to be limited by theory, the chance of methyl radical re-adsorption is reduced (compared to conventional catalysts), and the chance for deep oxidation of adsorbed species is also reduced (compared to conventional catalysts); thereby leading to an increase in C₂₊ selectivity. Further, and without wishing to be limited by theory, in the case of the supported OCM catalyst composition as disclosed herein, the chance of adsorbing reaction products such as ethylene (C₂H₄) and ethane (C₂H₆) onto the catalyst surface is also reduced (compared to conventional catalysts) and the chance for deep oxidation of adsorbed products is also reduced (compared to conventional catalysts); thereby leading to an increase in C₂₊ selectivity.

[0047] In an aspect, the supported OCM catalyst composition characterized by the general formula Mn-Na₂WO₄/SiO₂ as disclosed herein can comprise Mn-Na₂WO₄, Na/Mn/O, Na₂WO₄, Mn₂0₃-Na₂WO₄, Mn₃O₄-Na₂WO₄, MnWO₄-Na₂WO₄, MnWO₄-Na₂WO₄, Mn-WO₄, and the like, or combinations thereof. In an aspect, the OCM catalyst composition characterized by the general formula Mn-Na₂WO₄/SiO₂ as disclosed herein can comprise Mn-Na₂WO₄. In an aspect, the OCM catalyst composition characterized by the general formula Mn-Na₂WO₄/SiO₂ as disclosed herein can comprise an element with redox properties, such as manganese (Mn) and/or tungsten (W). For purposes of the disclosure herein, a chemical species that has redox properties, can also be referred to as a “redox agent.” 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, Mn and/or W can be redox agents. Further, and as will be appreciated by one of skill in the art, and with the help of this disclosure, some metals of the OCM catalyst composition characterized by the general formula Mn-Na₂WO₄/SiO₂ as disclosed herein are redox agents (e.g., Mn and/or W) and other metals of OCM catalyst composition characterized by the general formula Mn-Na₂WO₄/SiO₂ as disclosed herein are not redox agents (e.g., Na).

[0048] In an aspect, the supported OCM catalyst composition as disclosed herein can comprise

Na₂WO₄ in an amount of from about 0.1 wt.% to about 15 wt.%, alternatively from about 1 wt.% to about

12.5 wt.%, or alternatively from about 2.5 wt.% to about 10 wt.%, based on the total weight of the supported OCM catalyst composition.

[0049] In an aspect, the supported OCM catalyst composition as disclosed herein can comprise manganese (Mn) in an amount of from about 0.1 wt.% to about 10 wt.%, alternatively from about 0.5 wt.% to about 7.5 wt.%, or alternatively from about 1 wt.% to about 5 wt.%, based on the total weight of the supported OCM catalyst composition. The supported OCM catalyst composition as disclosed herein can be characterized by a weight ratio of Mn to W of from about 0.1 to about 5.0, alternatively from about 0.25 to about 4.5, or alternatively from about 0.5 to about 4.0.

[0050] In an aspect, the supported OCM catalyst composition as disclosed herein can comprise manganese (Mn) as manganese oxide (MnO₂). In an aspect, the MnO₂ in the supported OCM catalyst composition as disclosed herein can have any suitable desired shape and/or size specifications, for example as required by a specific application. In some aspects, the MnO₂ can comprise nanostructures, wherein a nanostructure is defined as a three-dimensional object characterized by at least one external dimension of less than about 1,000 nm. 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, three-dimensional objects are characterized by three external dimensions. For example, any three-dimensional object can be placed in a three-dimensional Cartesian coordinate system (i.e., a Cartesian coordinate system for a three-dimensional space) having axes x, y, and z, wherein the three-dimensional object is characterized by a first external dimension along x, a second external dimension along y, and a third external dimension along z. In some aspects, the MnO₂ of the supported OCM catalyst can comprise nanoparticles, nanofibers, nanoplates, or combinations thereof; wherein nanoparticles, nanofibers, and nanoplates are three-dimensional objects defined in accordance with ISO/TS 80004-2:2015.

[0051] In an aspect, the supported OCM catalyst composition as disclosed herein comprises a silica (SiO₂) support, wherein at least a portion of the OCM catalyst composition (e.g., characterized by the general formula Mn-Na₂WO₄) contacts, coats, is embedded in, is supported by, and/or is distributed throughout at least a portion of the support. As will be appreciated by one of skill in the art, and with the help of this disclosure, the support (i.e., SiO₂) is catalytically inactive or non-selective (e.g., the support cannot catalyze an OCM reaction or cannot give high selectivity). Further, as will be appreciated by one of skill in the art, and with the help of this disclosure, the silica 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 metal oxides, microemulsion techniques, solvothermal techniques, sonochemical techniques, combustion synthesis, etc.

[0052] In an aspect, the support can be 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 supported OCM catalyst composition and a reactant mixture, which in turn would result in a higher CH₄ conversion to CH₃·.

[0053] In an aspect, the supported OCM catalyst composition as disclosed herein can comprise SiO₂ in an amount of from about 5 wt.% to about 98 wt.%, alternatively from about 25 wt.% to about 96 wt.%, or alternatively from about 35 wt.% to about 95 wt.%, based on the total weight of the supported OCM catalyst composition. As will be appreciated by one of skill in the art, and with the help of this disclosure, the amount of catalytically active material composition (e.g., characterized by the general formula Mn- Na₂WO₄) on the support, and consequently the amount of support in the catalyst composition, depends on the catalytic activity of the catalytically active material.

[0054] In an aspect, the supported OCM catalyst composition as disclosed herein can be in the form of powders, particles, pellets, monoliths, foams, honeycombs, and the like, or combinations thereof. Nonlimiting examples of supported OCM catalyst composition particle shapes include cylindrical, discoidal, spherical, tabular, ellipsoidal, equant, irregular, cubic, acicular, and the like, or combinations thereof. [0055] The supported OCM catalyst can have any suitable desired particle specifications, for example as required by a specific application. For example, the supported OCM catalyst can be characterized by a size suitable for use in a particular reactor (e.g., OCM reactor). As will be appreciated by one of skill in the art, and with the help of this disclosure, the catalyst size can be determined for a particular application to achieve the best performance for the OCM reaction (e.g., desired conversion, desired selectivity, etc.).

[0056] The supported OCM catalyst composition as disclosed herein can be made by using any suitable methodology, provided that Na₂WO₄ is used under substantially anhydrous conditions.

[0057] In an aspect, a method of making a supported OCM catalyst composition can comprise a step of contacting sodium tungstate (Na₂WO₄), in the substantial absence of water, with silica (SiO₂) and manganese oxide (MnO₂) to form a supported OCM catalyst precursor mixture. The supported OCM catalyst precursor mixture can be characterized by a weight ratio of Mn to W of from about 0.1 to about 5.0, alternatively from about 0.25 to about 4.5, or alternatively from about 0.5 to about 4.0.

[0058] MnO₂, Na₂WO₄, and SiO₂ can be contacted with each other in any suitable order to form the supported OCM catalyst precursor mixture; provided that Na₂WO₄ is not contacted with water. For example, MnO₂ can be contacted with SiO₂ and water or any suitable aqueous medium to form an OCM catalyst precursor aqueous slurry, wherein the OCM catalyst precursor aqueous slurry comprises MnO₂, SiO₂, and water. The aqueous medium can be water, or an aqueous solution.

[0059] MnO₂ and SiO₂ can be contacted with water or any suitable aqueous medium in any suitable order to form the OCM catalyst precursor aqueous slurry. 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, MnO₂, and SiO₂ are insoluble in water, thus forming slurries when contacted with water (as opposed to solutions, as is the case when contacting compounds that are water soluble with water).

[0060] In some aspects, the MnO₂ used for preparing the supported OCM catalyst composition as disclosed herein can comprise MnO₂ nanostructures, wherein a nanostructure is defined as a three- dimensional object characterized by at least one external dimension of less than about 1,000 nm.

[0061] In some aspects, MnO₂ and SiO₂ can be first mixed together and then with water or any suitable aqueous medium to form the OCM catalyst precursor aqueous slurry. For example, MnO₂ and SiO₂ can be contacted with each other in the substantial absence of water (e.g., without adding water); for example by dry mixing, for example by blending, grinding, milling, crushing, chopping, and the like, or combinations thereof; or otherwise intimately mixing to obtain a substantially homogeneous mixture; wherein such substantially homogeneous mixture can be further contacted with water or any suitable aqueous medium to form the OCM catalyst precursor aqueous slurry. As will be appreciated by one of skill in the art, and with the help of this disclosure, while MnO₂ and SiO₂ 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; wherein such paste can be further contacted with water or any suitable aqueous medium to form the OCM catalyst precursor aqueous slurry.

[0062] MnO₂ can be contacted with water or any suitable aqueous medium to form a MnO₂ paste (very little water is used) or slurry (an amount of water sufficient to suspend MnO₂ particles is used). SiO₂ can be contacted with water or any suitable aqueous medium to form a SiO₂ paste (very little water is used) or slurry (an amount of water sufficient to suspend SiO₂ particles is used).

[0063] In an aspect, SiO₂, SiO₂ paste, SiO₂ slurry, or combinations thereof can be contacted with MnO₂, MnO₂ paste, MnO₂ slurry, or combinations thereof to form the OCM catalyst precursor aqueous slurry. For example, SiO₂ or SiO₂ paste can be contacted with a MnO₂ slurry to form the OCM catalyst precursor aqueous slurry. As another example, MnO₂ or MnO₂ paste can be contacted with a SiO₂ slurry to form the OCM catalyst precursor aqueous slurry.

[0064] In an aspect, the step of contacting MnO₂ with SiO₂ and water to form an OCM catalyst precursor aqueous slurry further comprises agitating the OCM catalyst precursor aqueous slurry; wherein agitating comprises stirring, shaking, blending, mixing, sonicating, and the like, or combinations thereof. Agitating the OCM catalyst precursor aqueous slurry can provide for a substantially homogeneous slurry, e.g., a slurry having a substantially uniform concentration of MnO₂ and SiO₂ throughout the bulk of the slurry.

[0065] In some aspects, the OCM catalyst precursor aqueous slurry can be heated at a temperature of from about 40°C to about 200°C, alternatively from about 60°C to about 150°C, or alternatively from about 80°C to about 120°C. The OCM catalyst precursor aqueous slurry can be heated under agitation.

[0066] The OCM catalyst precursor aqueous slurry can be heated and/or agitated for a time period of equal to or greater than about 5 minutes, alternatively equal to or greater than about 15 minutes, alternatively equal to or greater than about 30 minutes, alternatively equal to or greater than about 1 hour, or alternatively equal to or greater than about 2 hours.

[0067] In an aspect, at least a portion of the OCM catalyst precursor aqueous slurry can be dried to form an OCM catalyst precursor mixture, wherein the OCM catalyst precursor mixture comprises MnO₂ and SiO₂. The OCM catalyst precursor mixture is substantially free of water. In an aspect, at least a portion of the OCM catalyst precursor aqueous slurry can be dried at a temperature of equal to or greater than about 75°C, alternatively of equal to or greater than about 90°C, alternatively of equal to or greater than about 100°C, alternatively of equal to or greater than about 110°C, or alternatively of equal to or greater than about 125°C, to yield the OCM catalyst precursor mixture. The OCM catalyst precursor aqueous slurry can be dried for a time period of equal to or greater than about 2 hours, alternatively 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. [0068] In an aspect, Na₂WO₄ can be contacted under substantially anhydrous conditions with at least a portion of the OCM catalyst precursor mixture to form the supported OCM catalyst precursor mixture, wherein the supported OCM catalyst precursor mixture comprises Na₂WO₄, MnO₂, and SiO₂. The supported OCM catalyst precursor mixture is substantially free of water. For example, Na₂WO₄ and the OCM catalyst precursor mixture can be contacted with each other in the substantial absence of water (e.g., without adding water); for example by dry mixing, for example by blending, grinding, milling, crushing, chopping, and the like, or combinations thereof; or otherwise intimately mixing to obtain a substantially homogeneous mixture (i.e., supported OCM catalyst precursor mixture), e.g., a mixture having a substantially uniform concentration of Na₂WO₄, MnO₂, and SiO₂ throughout the bulk of the mixture.

[0069] The Na₂WO₄ can be used in hydrated form (Na₂WO₄·2H₂0) which displays an orthorhombic crystal structure, and/or in anhydrous form (Na₂WO₄) which displays a rhombic crystal structure.

[0070] In an aspect, a method of making a supported OCM catalyst composition as disclosed herein can comprise a step of calcining at least a portion of the supported OCM catalyst precursor mixture to form the supported OCM catalyst composition, wherein the supported OCM catalyst composition is characterized by the general formula Mn-Na₂WO₄/SiO₂; and wherein equal to or greater than about 50% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1. The supported OCM catalyst precursor mixture can be calcined at a temperature of equal to or greater than about 700°C, alternatively equal to or greater than about 750°C, alternatively equal to or greater than about 800°C, alternatively equal to or greater than about 850°C, or alternatively equal to or greater than about 900°C, to yield the supported OCM catalyst composition. The supported 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. In some aspects, at least a portion of the supported 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 supported OCM catalyst composition.

[0071] In an aspect, a method of making a supported OCM catalyst composition as disclosed herein can comprise a step of sizing the supported OCM catalyst composition to form the supported OCM catalyst composition into desired particle specifications (e.g., required particle specifications). The supported OCM catalyst composition can be sized by using any suitable methodology. In an aspect, the supported OCM catalyst composition can be subjected to grinding, crushing, milling, chopping, and the like, or combinations thereof to form the supported OCM catalyst composition into desired particle specifications (e.g., required particle specifications). As previously described herein, the supported OCM catalyst composition can have any suitable desired particle specifications, for example as required by a specific application. [0072] In an aspect, a method for producing olefins as disclosed herein can comprise (A) introducing a reactant mixture (e.g., OCM reactant mixture) to an OCM reactor comprising the supported OCM catalyst composition as disclosed herein, wherein the reactant mixture comprises methane (CH ) and oxygen (O₂); and (B) allowing at least a portion of the reactant mixture to contact at least a portion of the supported OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins.

[0073] The OCM 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., CH₄), liquefied petroleum gas comprising C₂-C₅ hydrocarbons, C₆₊ heavy hydrocarbons (e.g., C₆ to C₂₄ 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 CH₄ and O₂.

[0074] The O₂ used in the OCM 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.

[0075] The OCM reactant mixture can further comprise a diluent. 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 (e.g., steam), 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.

[0076] The OCM reactor can comprise an adiabatic reactor, an autothermal reactor, an isothermal reactor, a tubular reactor, a cooled tubular reactor, a continuous flow reactor, a fixed bed reactor, a fluidized bed reactor, a moving bed reactor, and the like, or combinations thereof. In an aspect, the OCM reactor can comprise a catalyst bed comprising the supported OCM catalyst composition.

[0077] In an aspect, the OCM reactor can be characterized by any suitable OCM reactor operational parameters, such as temperature (e.g., feed preheat temperature, reactor effluent temperature, etc.), pressure, flow rate (e.g., space velocity), and the like, or combinations thereof.

[0078] The OCM reaction mixture can be introduced to the OCM reactor at a temperature (e.g., feed preheat temperature) of from about 150°C to about 1,000°C, alternatively from about 225°C to about 900°C, or alternatively from about 250°C to about 800°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 , as the C-H bonds of CH are very stable, and the formation of methyl radicals from CH is endothermic. In an aspect, the OCM reaction mixture can be introduced to the OCM reactor at a temperature effective to promote an OCM reaction.

[0079] The OCM reactor can be characterized by a reactor effluent temperature of from about 400°C to about 1,200°C, alternatively from about 500°C to about 1,100°C, or alternatively from about 600°C to about 1,000°C.

[0080] The OCM 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.

[0081] The OCM 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 100,000 h^-1, alternatively from about 500 h^-1 to about 50,000 h^-1, alternatively from about 1,000 h^-1 to about 40,000 h^-1, or alternatively from about 1,500 h^-1 to about 25,000 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.

[0082] In an aspect, the method for producing olefins as disclosed herein can comprise recovering at least a portion of the product mixture from the OCM reactor, wherein the product mixture can comprise olefins, water, CO, CO₂, and unreacted methane. In an aspect, a method for producing olefins as disclosed herein can comprise recovering at least a portion of the olefins from the product mixture. The product mixture can comprise C₂₊ hydrocarbons (including olefins), unreacted methane, and optionally a diluent. The C₂₊ hydrocarbons can comprise C₂ hydrocarbons and C hydrocarbons. In an aspect, the C₂₊ hydrocarbons can further comprise C₄ hydrocarbons (C₄s), such as for example butane, iso-butane, n-butane, butylene, etc. The C₂ hydrocarbons can comprise ethylene (C₂H ) and ethane (C₂H₆). The C₂ hydrocarbons can further comprise acetylene (C₂H₂). The C₃ hydrocarbons can comprise propylene (C₃H₆) and propane (C₃H₈).

[0083] 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.

[0084] 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).

[0085] 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.

[0086] In an aspect, the O₂ conversion of the OCM reaction 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 O₂ 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 by using equation (9):

wherein = number of moles of O₂ that entered the OCM reactor as part of the reactant mixture;

and = number of moles of O₂ that was recovered from the OCM reactor as part of the product

mixture.

[0087] In an aspect, the supported OCM catalyst composition as disclosed herein can be characterized by a methane conversion that is increased when compared to a methane conversion at the same reaction temperature of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water. In an aspect, the supported OCM catalyst composition as disclosed herein can be characterized by a methane conversion that is increased by equal to or greater than about 5%, alternatively by equal to or greater than about 10%, or alternatively by equal to or greater than about 15% when compared to a methane conversion at the same reaction temperature of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water. The methane conversion can be calculated by using equation (10):

wherein = number of moles of C from CH that entered the adiabatic reactor as part of the

reactant mixture; and = number of moles of C from CH that was recovered from the adiabatic

reactor as part of the product mixture.

[0088] In an aspect, the supported OCM catalyst composition as disclosed herein can be characterized by a C₂₊ selectivity that is increased when compared to a C₂₊ selectivity of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water. In an aspect, the supported OCM catalyst composition as disclosed herein can be characterized by a C₂₊ selectivity that is increased by equal to or greater than about 1%, alternatively equal to or greater than about 2.5%, alternatively equal to or greater than about 5%, alternatively equal to or greater than about 7.5%, or alternatively equal to or greater than about 10% when compared to a C₂₊ selectivity of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.

[0089] 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., C₂ selectivity, C₂₊ selectivity, etc.) can be calculated by dividing a number of moles of carbon (C) from CH that were converted into the desired product (e.g.,

etc.) by the total number of moles of C from CH₄ that were converted (e.g.,

number of moles of C from CH₄ that were converted into C₂H₄; C_C2H6 ⁼ number of moles of C from CH₄ that were converted into C₂H₆; C_C2H2 ⁼ number of moles of C from CH₄ that were converted into C₂H₂; C_C3H6 ⁼ number of moles of C from CH₄ that were converted into C H₆; C_C3H8 ⁼ number of moles of C from CH₄ that were converted into C H₈; C_C4s = number of moles of C from CH₄ that were converted into C₄ hydrocarbons (C₄s); C_CO2 ⁼ number of moles of C from CH₄ that were converted into CO₂; C_CO ⁼ number of moles of C from CH₄ that were converted into CO; etc. [0090] A C₂₊ selectivity (e.g., selectivity to C₂₊ hydrocarbons) refers to how much C₂H₄, C H₆, C₂H₂, C₂H₆, C₃H₈, and C s were formed divided by the total products formed, including C₂H , C₃H₆, C₂H₂, C₂H₆, C₃H₈, C S, CO₂ and CO. For example, the C₂₊ selectivity can be calculated by using equation (11):

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.

[0091] 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 (CO₂), as previously described herein. Without wishing to be limited by theory, when the selectivity to desired products (e.g., C₂₊ selectivity) of an OCM process increases, less methane is converted to undesirable products, such as deep oxidation products (e.g., CO, CO₂), 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., C₂ products, C₂H , C₂₊ products, etc.), thus enabling an increased yield of desired C₂₊ products.

[0092] In an aspect, the supported OCM catalyst composition as disclosed herein can be characterized by a CO₂ selectivity that is decreased when compared to a CO₂ selectivity of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water. In an aspect, the supported OCM catalyst composition as disclosed herein can be characterized by a CO₂ selectivity that is decreased by equal to or greater than about 5%, alternatively equal to or greater than about 10%, alternatively equal to or greater than about 15%, alternatively equal to or greater than about 20%, or alternatively equal to or greater than about 25% when compared to a CO₂ selectivity of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.

[0093] In an aspect, the supported OCM catalyst composition as disclosed herein can be characterized by the general formula Mn-Na₂WO₄/SiO₂; wherein the supported OCM catalyst composition is characterized by a substantially uniform external surface concentration of Na and/or W. For example, wherein equal to or greater than about 50% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1. [0094] In an aspect, a method for producing olefins as disclosed herein can comprise the steps of (a) contacting manganese oxide (MnO₂) with silica (SiO₂) and water to form an OCM catalyst precursor aqueous slurry; (b) drying at least a portion of the OCM catalyst precursor aqueous slurry at a temperature of equal to or greater than about 100°C to form an OCM catalyst precursor mixture, wherein the OCM catalyst precursor mixture comprises MnO₂ and SiO₂, and wherein the OCM catalyst precursor mixture is substantially free of water; (c) contacting sodium tungstate (Na₂WO₄), in the substantial absence of water, with at least a portion of the OCM catalyst precursor mixture to form a supported OCM catalyst precursor mixture; (d) calcining at least a portion of the supported OCM catalyst precursor mixture at a temperature of equal to or greater than about 800°C to form a supported OCM catalyst composition; wherein equal to or greater than about 75% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.25:1 to about 2:1; (e) placing at least a portion of the supported OCM catalyst composition in an OCM reactor; (f) introducing a reactant mixture to the OCM reactor comprising the supported OCM catalyst composition, wherein the reactant mixture comprises methane (CH₄) and oxygen (O₂); (f) allowing at least a portion of the reactant mixture to contact at least a portion of the supported OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins; (g) recovering at least a portion of the product mixture from the OCM reactor; and (h) recovering at least a portion of the olefins from the product mixture. The supported OCM catalyst composition can be characterized by an increased methane conversion, an increased C₂₊ selectivity, and a decreased CO₂ selectivity when compared to methane conversion, a C₂₊ selectivity, and a CO₂ selectivity, respectively of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.

[0095] In an aspect, the supported OCM catalyst compositions characterized by the general formula Mn-Na₂WO₄/SiO₂; wherein equal to or greater than about 50% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1; 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 supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.

[0096] The supported OCM catalyst compositions as disclosed herein can advantageously display improved conversion, C₂₊ selectivity, activity and stability when compared to the conversion, C₂₊ selectivity, activity and stability, respectively, of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.

[0097] The supported OCM catalyst compositions as disclosed herein can advantageously display decreased selectivity to CO₂, when compared to the CO₂ selectivity of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water. Additional advantages of the supported OCM catalyst compositions as disclosed herein; and methods of making and using same, can be apparent to one of skill in the art viewing this disclosure.

EXAMPLES

[0098] 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

[0099] Oxidative coupling of methane (OCM) catalyst compositions were prepared as follows.

[00100] A reference catalyst (Mn-Na₂WO₄/SiO₂) was prepared by using the following procedure, wherein Na₂WO₄ was contacted with water.

[00101] Mn(N0₃)₂·4H₂0 (5.20 g) was dissolved in deionized water (30 mL) to form a manganese nitrate solution. Na₂WO₄·2H₂0 (3.37 g) was dissolved in deionized water (30 mL) to form a sodium tungstate solution. The manganese nitrate solution and the sodium tungstate solution were then added into 164.73 g of silica sol with a silica content of 34% (Nicol 1034A). The resulting mixture was agitated at 85°C for 2 hours, and then was dried overnight at 125°C, followed by calcination at 800°C for 6 hours under airflow to obtain the Mn-Na₂WO₄/SiO₂ reference catalyst (1.9%Mn-5.0%Na₂WO₄/SiO₂).

[00102] Different Mn-Na₂WO₄/SiO₂ catalysts were prepared as follows without contacting Na₂WO₄ with water, and were compared with the reference catalyst.

[00103] Catalyst #1 (Mn-Na₂WO₄/SiO₂) was prepared by using the following method. Nano MnO₂ (0.95 g wet cake, with MnO₂ content 10%, purchased from Novarials Corporation) was dispersed in deionized water (4.0 mL) to form a MnO₂ slurry. Then the MnO₂ slurry was added into 13.72 g of silica sol with silica content of 34% (Nicol 1034A) to form a MnO₂/SiO₂ slurry. The MnO₂/SiO₂ slurry was agitated at 85°C for 2 hours, and then was dried overnight at 125°C to yield a dry material. 0.25 g of solid Na₂WO₄ (anhydrous) with sheet structure was mixed with the dry material to yield a mixture, wherein the mixture was further mixed to achieve a uniform mixture in dry powder form. The dry powder was then calcined at 800°C for 6 hours under airflow to obtain the Mn-Na₂WO₄/SiO₂ Catalyst #1 (1.5%Mn-5.0%Na₂WO /SiO₂). When comparing Catalyst #1 to the reference catalyst, the Mn content in Catalyst #1 is slightly lower; however, experimental data indicated that such small difference in Mn content has no impact on catalyst performance.

[00104] Catalyst #2 (Mn-Na₂WO₄/SiO₂) was prepared by using the following method. Nano MnO₂ (0.95 g wet cake, with MnO₂ content 10%, purchased from Novarials Corporation) was dispersed in deionized water (4.0 mL) to form a MnO₂ slurry. Then the MnO₂ slurry was added into 13.72 g of silica sol with silica content of 34% (Nicol 1034A) to form a MnO₂/SiO₂ slurry. The MnO₂/SiO₂ slurry was agitated at 85°C for 2 hours, and then was dried overnight at 125°C to yield a dry material. 0.25 g of solid Na₂WO₄ (anhydrous) with a sheet structure different from the sheet structure of the Na₂WO₄ used to prepare Catalyst #1 was mixed with the dry material to yield a mixture, wherein the mixture was further mixed to achieve a uniform mixture in dry powder form. The dry powder was then calcined at 800°C for 6 hours under airflow to obtain the Mn-Na₂WO₄/SiO₂ Catalyst #2 (1.5%Mn-5.0%Na₂WO₄/SiO₂).

[00105] Catalyst #3 (Mn-Na₂WO₄/SiO₂) was prepared by using the following method. Nano MnO₂ (0.95 g wet cake, with MnO₂ content 10%, purchased from Novarials Corporation) was dispersed in deionized water (4.0 mL) to form a MnO₂ slurry. Then the MnO₂ slurry was added into 13.72 g of silica sol with silica content of 34% (Nicol 1034A) to form a MnO₂/SiO₂ slurry. The MnO₂/SiO₂ slurry was agitated at 85°C for 2 hours, and then was dried overnight at 125°C to yield a dry material. 0.28 g of solid Na₂WO₄-2H₂0 (the same Na₂WO₄-2H₂0 used for preparing the reference catalyst) was mixed with the dry material to yield a mixture, wherein the mixture was further mixed to achieve a uniform mixture in dry powder form. The dry powder was then calcined at 800°C for 6 hours under airflow to obtain the Mn-Na₂WO₄/SiO₂ Catalyst #3 (1.5%Mn-5.0%Na₂WO₄/SiO₂). During the preparation of the Catalyst #3, Na₂WO₄-2H₂0 was not contacted with water (as opposed to the preparation method of the reference catalyst, where the Na₂WO₄-2H₂0 was dissolved in water).

[00106] Catalyst #4 (Mn-Na₂WO₄/SiO₂) was prepared by using the following method. Nano MnO₂ (0.95 g wet cake, with MnO₂ content 10%, purchased from Novarials Corporation) was dispersed in deionized water (10.0 mL) to form a MnO₂ slurry. Then the MnO₂ slurry was added into 13.57 g of silica sol with silica content of 34% (Nicol 1034A) to form a MnO₂/SiO₂ slurry. The MnO₂/SiO₂ slurry was agitated at 85°C for 2 hours, and then was dried overnight at 125°C to yield a dry material. 0.51 g of solid Na₂WO₄ (anhydrous) with sheet structure (the same Na₂WO₄ used to prepare Catalyst #1) was mixed with the dry material to yield a mixture, wherein the mixture was further mixed to achieve a uniform mixture in dry powder form. The dry powder was then calcined at 800°C for 6 hours under airflow to obtain the Mn-Na₂WO₄/SiO₂ Catalyst #4 (2.8%Mn-9.7%Na₂WO₄/SiO₂).

EXAMPLE 2

[00107] The performance of the supported 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.

[00108] Performance test. The catalysts obtained as described in Example 1 were performance tested in a 4.0 mm ID quartz tube reactor. The reactor was loaded with 100 mg of catalyst. A mixture of methane and oxygen at a fixed CH :O₂ ratio of 7.4 was fed to the reactor at a total flow rate of 33.3 seem. Products obtained were analyzed by using online GC with TCD and FID detectors.

[00109] Mn-Na₂WO₄/SiO₂ Catalyst #1. The performance obtained with

1.5%Mn-5.0%Na₂WO₄/SiO₂ Catalyst #1 is shown in Figure 1, compared to the reference catalyst.

[00110] It can be seen from Figure 1 that using MnO₂ nanooxide and adding Na₂WO₄ in solid form into the catalyst composition (without contacting Na₂WO₄ with water), the obtained catalyst (1.5%Mn-5.0%Na₂WO /SiO₂ Catalyst #1) shows almost the same activity (in terms of oxygen conversion) as the reference catalyst. However, the selectivity (C₂₊ selectivity) of Catalyst #1 has improved clearly over the reference catalyst. At relatively low reactor temperatures (from 650°C to 725°C), about 10% increase in selectivity was observed for Catalyst #1 over the reference catalyst; and at relative high reactor temperatures (from 750°C to 850°C), about 2% to 5% increase in selectivity was observed for Catalyst #1 over the reference catalyst.

[00111] The CO and CO₂ formation for Catalyst #1 and for the reference catalyst are compared in Figure 2. It can be seen that the CO formation with these two catalysts are very close, but clearly lower CO₂ formation is observed for Catalyst #1 over the reference catalyst. The CO₂ formation for Catalyst #1 is reduced by 20-30% when compared to the reference catalyst as shown in Figure 3. Figure 3 displays a ratio of CO₂ formation of Catalyst #1 to the reference catalyst.

EXAMPLE 3

[00112] The external surface composition of the supported OCM catalyst compositions prepared as described in Example 1 was investigated. Specifically, the local composition analysis of Catalyst #1 was compared to the reference catalyst.

[00113] A scanning electron microscope (SEM) micrograph of the reference catalyst is shown in Figure 4. It is observed that there are dark areas in Figure 4. Energy dispersive X-ray spectroscopy (EDS or EDX) composition analysis of 100 mm² regions (areas) are shown in Table 1.

Table 1 - Composition analysis by EDS of 100 mm² regions (areas)

Note: Some spectra mentioned in Table 1 are not displayec in the Figures.

[00114] Very high concentration ofNa and very low concentration of W are seen in the dark area in Figure 4, with a Na:W molar ratio = 302.50, indicating that this area is significantly enriched with Na. With the same reference catalyst, in another area as shown in Figure 5, the composition analysis shows higher W content, with a Na:W molar ratio = 0.11 (W:Na molar ratio = 9.38), which is also displayed in Table 1. For the reference catalyst used for collecting the data in Table 1 (i.e., a conventional catalyst prepared by using aqueous solution of Na₂WO₄), 8 of the 9 investigated 100 mm² regions of the external surface of the reference catalyst had Na and/or W; and out of the 8 x 100 mm² regions of the external surface of the reference catalyst having Na and/or W, only 1 region was characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1 (i.e., spectrum 1, wherein the molar ratio of Na to W (Na:W) is 1.77), accounting for 12.5% of the 100 mm² sampled regions having Na and/or W. Therefore, with the method of using Na₂WO₄ in aqueous solution (reference catalyst), the Na and W are spatially separated in the catalyst; wherein some portions of the reference catalyst display an enriched Na content, and wherein other portions of the reference catalyst display an enriched W content. For the portions having enriched W content, W0₃ will be formed in the catalyst. Without wishing to be limited by theory, and as previously disclosed herein, W0₃ has relatively lower basicity and also displays strong redox and deep oxidation properties. Further, and without wishing to be limited by theory, when a methyl radical re-adsorbs onto W0₃ sites, it will form [CH₃0]_s and produce CO_x as the final products, according to reaction (4), thereby resulting in a lower selectivity towards desired products (i.e., lower C₂₊ selectivity).

[00115] With the catalyst made of solid Na₂WO₄ in the substantial absence of water (e.g., Catalyst #1), its SEM is displayed in Figure 6. Composition analysis of the marked area shows a Na:W molar ratio = 1.59, as given in Table 1. For Catalyst #1 used for collecting the data in Table 1 (i.e., a supported OCM catalyst composition as disclosed herein prepared by using anhydrous Na₂WO₄), all of the 9 investigated 100 mm² regions of the external surface of Catalyst #1 had Na and/or W; and 6 out of the 9 regions were characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0:1 to about 4:1 (i.e., spectra 13, 14, 17, 19, 20, and 21), accounting for 66.7% of the 100 mm² sampled regions having Na and/or W. Therefore, when the catalyst was prepared using solid Na₂WO₄ in the substantial absence of water with a solid mixing method, much less separation between Na and W was achieved in the final catalyst composition as compared to the reference catalyst. Without wishing to be limited by theory, when Na is connected to W in a crystal lattice of Na₂WO₄, as in the case of Catalyst #1, the active sites will have higher basicity when compared to W0₃. Further, and without wishing to be limited by theory, the chance for methyl radical re-adsorption is reduced, and the chance for deep oxidation is also reduced. In addition, and without wishing to be limited by theory, with the feature of the less spatial separation between Na and W, the desired products formed, such as C₂H₄ and/or C₂H₆, will also have less chance to be further adsorbed on the catalyst surface and be oxidized, thereby resulting in a higher selectivity towards desired products (i.e., higher C₂₊ selectivity). Consequently, keeping Na and W bonded in a crystal lattice format can be critical for a catalyst more selective towards desired products (i.e., C₂₊ hydrocarbons).

[00116] Under the testing condition with CH to O₂ ratio of 7.4 (as described in Example 2), and without wishing to be limited by theory, oxygen is the limiting agent. With more CO₂ formed in the products (given that CO₂ formation uses more O₂ than the formation of other products that do not contain oxygen, such as hydrocarbons), there will be less oxygen available for CH₄ conversion, thereby resulting in lower CH₄ conversion. Consequently, a lower CH₄ conversion was observed with the reference catalyst when compared to Catalyst #1, as displayed in Figure 7. The C₂₊ yields obtained with the reference catalyst as well as with Catalyst #1 are also shown in Figure 7, and it can be seen that better yields are observed for Catalyst #1 over the reference catalyst.

EXAMPLE 4

[00117] The performance of the supported OCM catalyst compositions prepared as described in Example 1 was investigated. Specifically, the performance of Catalysts #1, #2, #3, and #4 was compared to the performance of the reference catalyst. OCM reactions were conducted by using catalysts prepared as described in Example 1 , by using the testing procedure described in Example 2, and the resulting data are displayed in Table 2.

Table 2. Performance comparison at 800°C reactor temperature

[00118] It can be seen from Table 2 that compared to the reference catalyst, all catalysts prepared using solid Na₂WO₄ (in the substantial absence of water) show better C₂₊ selectivity (Catalysts #1, #2, #3, and #4). It can also be seen that the increase in C₂₊ hydrocarbons mainly comes from the reduction of CO₂ formation in the products, indicating the reduction of deep oxidation activity with the new preparation method (using solid Na₂WO₄ under substantially anhydrous conditions).

[00119] Comparing catalysts made with solid Na₂WO₄ but with different structures (e.g., sheet structure, absence of sheet structure, etc.), the differences in performance are small, although their surface morphologies are quite different from each other, as shown in Figures 8, 9, and 10. Figure 8 displays an SEM micrograph of the solid Na₂WO₄ used for preparing Catalysts #1 and #4. Figure 9 displays an SEM micrograph of the solid Na₂WO₄ used for preparing Catalyst #2. Figure 10 displays an SEM micrograph of the solid Na₂WO₄ used for preparing Catalyst #4.

[00120] Catalyst #4 was prepared with the same materials as Catalyst #1, but Catalyst #4 has a higher active sites concentration when compared with Catalyst #1, and it can be seen that almost the same selectivity was obtained with both Catalysts #1 and #4. With the conventional catalyst preparation method, for example such as the preparation method used to make the reference catalyst, there is an optimal catalyst composition range. The conventional catalyst preparation method is described in more detail in Ji, et al., Applied Catalysis A: General 225 (2002) 271; which is incorporated by reference herein in its entirety. For W, when its content is higher than 3.0% (corresponding to a Na₂WO₄ content higher than 5.0%), CO_x formation will be increased significantly. Without wishing to be limited by theory, this increase in CO_x formation is due to the higher W0 content in the catalyst because of the Na and W separation as described in more detail previously herein. However, in the case of Catalyst #4, with a Na₂WO₄ content close to 10%, no higher CO_x formation was observed, this confirms that with the catalyst preparation method using solid Na₂WO₄ in the substantial absence of water, with no significant spatial separation of Na and W, a higher Na₂WO₄ content can be used in the catalyst. The ability to increase the Na and/or W content in the OCM catalysts could result in a more stable catalyst. As will be appreciated by one of skill in the art, and with the help of this disclosure, one cause of OCM catalyst deactivation is loss of Na during reaction. Na is generally introduced in the OCM catalyst composition with Na₂WO₄. With the OCM catalyst preparation method using solid Na₂WO₄ in the substantial absence of water, a higher amount of Na₂WO₄ could be used, such that there would be a higher amount of Na available in the catalyst, and as a result, catalyst life would be extended.

[00121] The preparation method disclosed herein using solid Na₂WO₄ in the substantial absence of water, without wishing to be limited by theory, due to less spatial segregation of Na and W, the methyl radicals and products formed will have less chance to be re-adsorbed, such that that CO_x formation is reduced and C₂₊ selectivity is improved (i.e., increased).

[00122] For the purpose of any U.S. national stage fding 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.

[00123] 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.

[00124] 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. [00125] 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.

Claims

CLAIMS What is claimed is:

1. A supported oxidative coupling of methane (OCM) catalyst composition characterized by the general formula Mn-Na₂WO₄/SiO₂; wherein equal to or greater than about 50% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio of Na to W (Na:W) of from about 1.0: 1 to about 4: 1.

2. The supported OCM catalyst composition of claim 1, wherein equal to or greater than about 75% of any 100 mm² regions of an external surface of the OCM catalyst composition having Na and/or W are characterized by a surface molar ratio ofNa:W of from about 1.25:1 to about 2.0:1.

3. The supported OCM catalyst composition of claim 1, wherein the supported OCM catalyst composition comprises manganese (Mn) in an amount of from about 0.1 wt.% to about 10 wt.%, based on the total weight of the supported OCM catalyst composition.

4. The supported OCM catalyst composition of claim 1, wherein the supported OCM catalyst composition comprises Na₂WO₄ in an amount of from about 0.1 wt.% to about 15 wt.%, based on the total weight of the supported OCM catalyst composition.

5. The supported OCM catalyst composition of claim 1 , wherein at least a portion of the Mn is present in the OCM catalyst composition as MnO₂ nanostructures, wherein a nanostructure is defined as a three- dimensional object characterized by at least one external dimension of less than about 1,000 nm.

6. A method of making a supported oxidative coupling of methane (OCM) catalyst composition comprising:

(a) contacting sodium tungstate (Na₂WO₄), in the substantial absence of water, with silica (SiO₂) and manganese oxide (MnO₂) to form a supported OCM catalyst precursor mixture; and

(b) calcining at least a portion of the supported OCM catalyst precursor mixture to form the supported OCM catalyst composition of claim 1.

7. The method of claim 6, wherein the step (a) of contacting Na₂WO₄, in the substantial absence of water, with SiO₂ and MnO₂ to form a supported OCM catalyst precursor mixture comprises (i) contacting MnO₂ with SiO₂ and water to form an OCM catalyst precursor aqueous slurry; (ii) drying at least a portion of the OCM catalyst precursor aqueous slurry to form an OCM catalyst precursor mixture, wherein the OCM catalyst precursor mixture comprises MnO₂ and SiO₂; and (iii) contacting Na₂WO₄ with at least a portion of the OCM catalyst precursor mixture to form the supported OCM catalyst precursor mixture.

8. The method of claim 7, wherein the step (i) of contacting MnO₂ with SiO₂ and water to form an OCM catalyst precursor aqueous slurry further comprises agitating the OCM catalyst precursor aqueous slurry; wherein agitating comprises stirring, shaking, blending, mixing, sonicating, or combinations thereof.

9. The method of claim 7, wherein the OCM catalyst precursor aqueous slurry is dried at a temperature of equal to or greater than about 75°C.

10. The method of claim 7, wherein the step (iii) of contacting Na₂WO₄ with at least a portion of the OCM catalyst precursor mixture to form the supported OCM catalyst precursor mixture further comprises dry mixing the Na₂WO₄ with the OCM catalyst precursor mixture to form the supported OCM catalyst precursor mixture; and wherein dry mixing comprises blending, grinding, milling, crushing, chopping, or combinations thereof.

11. The method of claim 6, wherein the supported OCM catalyst precursor mixture is calcined at a temperature of equal to or greater than about 700°C.

12. A method of making a supported oxidative coupling of methane (OCM) catalyst composition comprising:

(a) contacting manganese oxide (MnO₂) with silica (SiO₂) and water to form an OCM catalyst precursor aqueous slurry;

(b) drying at least a portion of the OCM catalyst precursor aqueous slurry at a temperature of equal to or greater than about 90°C to form an OCM catalyst precursor mixture, wherein the OCM catalyst precursor mixture comprises MnO₂ and SiO₂, and wherein the OCM catalyst precursor mixture is substantially free of water;

(c) contacting sodium tungstate (Na₂WO₄), in the substantial absence of water, with at least a portion of the OCM catalyst precursor mixture to form a supported OCM catalyst precursor mixture; and

(d) calcining at least a portion of the supported OCM catalyst precursor mixture at a temperature of equal to or greater than about 750°C to form the supported OCM catalyst composition of claim 1.

13. The method of claim 12, wherein the step (a) of contacting MnO₂ with SiO₂ and water to form an OCM catalyst precursor aqueous slurry further comprises agitating the OCM catalyst precursor aqueous slurry to form a substantially homogeneous slurry; and wherein agitating comprises stirring, shaking, blending, mixing, sonicating, or combinations thereof.

14. The method of claim 13 further comprising heating the OCM catalyst precursor aqueous slurry at a temperature of from about 40°C to about 200°C while agitating the OCM catalyst precursor aqueous slurry.

15. The method of claim 12, wherein the step (c) of contacting Na₂WO₄, in the substantial absence of water, with at least a portion of the OCM catalyst precursor mixture to form a supported OCM catalyst precursor mixture further comprises dry mixing the Na₂WO₄ with the OCM catalyst precursor mixture to form a substantially homogeneous mixture; and wherein dry mixing comprises blending, grinding, milling, crushing, chopping, or combinations thereof.

16. A method for producing olefins comprising:

(a) introducing a reactant mixture to an oxidative coupling of methane (OCM) reactor comprising the supported OCM catalyst composition of claim 1 , wherein the reactant mixture comprises methane (CH₄) and oxygen (O₂);

(b) allowing at least a portion of the reactant mixture to contact at least a portion of the supported 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 OCM reactor; and

(d) recovering at least a portion of the olefins from the product mixture.

17. A method for producing olefins comprising:

(a) contacting sodium tungstate (Na₂WO₄), in the substantial absence of water, with silica (SiO₂) and manganese oxide (MnO₂) to form a supported oxidative coupling of methane (OCM) catalyst precursor mixture;

(b) calcining at least a portion of the supported OCM catalyst precursor mixture at a temperature of equal to or greater than about 700°C to form the supported OCM catalyst composition of claim 1;

(c) placing at least a portion of the supported OCM catalyst composition in an OCM reactor;

(d) introducing a reactant mixture to the OCM reactor comprising the supported OCM catalyst composition, wherein the reactant mixture comprises methane (CH ) and oxygen (O₂);

(e) allowing at least a portion of the reactant mixture to contact at least a portion of the supported OCM catalyst composition and react via an OCM reaction to form a product mixture comprising unreacted methane and olefins;

(f) recovering at least a portion of the product mixture from the OCM reactor; and

(g) recovering at least a portion of the olefins from the product mixture.

18. The method of claim 17, wherein the supported OCM catalyst composition is characterized by a C₂₊ selectivity that is increased when compared to a C₂₊ selectivity of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.

19. The method of claim 17, wherein the supported OCM catalyst composition is characterized by a CO₂ selectivity that is decreased when compared to a CO₂ selectivity of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.

20. The method of claim 17, wherein the supported OCM catalyst composition is characterized by a CH conversion that is increased when compared to a CH conversion at the same reaction temperature of an otherwise similar supported OCM catalyst composition that was prepared by contacting Na₂WO₄ with water.