MESOPOROUS SILICA CARRIER FOR THE OXIDATIVE COUPLING OF METHANE AND ITS PREPARATION

Field of the Invention

The present invention relates to a mesoporous silica-supported catalyst, a method of preparing such a catalyst, and a process for the oxidative coupling of methane .

Background of the Invention

Methane is a valuable resource which is used not only as a fuel, but is also used in the synthesis of chemical compounds such as higher hydrocarbons .

The conversion of methane to other chemical compounds can take place via indirect conversion wherein methane is reformed to synthesis gas (hydrogen and carbon monoxide) , followed by reaction of the synthesis gas in a Fischer-Tropsch process. However, such indirect conversion is costly and consumes a lot of energy.

Consequently, it is desirable for industry to be able to convert methane directly to other chemical compounds without requiring the formation of intermediates such as synthesis gas. To this end, there has been increasing focus in recent years on the development of processes for the oxidative coupling of methane (OCM) .

The oxidative coupling of methane converts methane into saturated and unsaturated, non-aromatic hydrocarbons having 2 or more carbon atoms, including ethylene. In this process, a gas stream comprising methane is contacted with an OCM catalyst and with an oxidant, such as oxygen or air. In such a process, two methane molecules are first coupled into one ethane molecule, which is then dehydrogenated into ethylene. Said ethane and ethylene ("C2 compounds") may further react into saturated and unsaturated hydrocarbons having 3 or more carbon atoms (C3+ ) , including propane, propylene, butane, butene, etc. Besides said C2+ compounds, the gas stream leaving an OCM process may contain a mixture of water, hydrogen, carbon monoxide, carbon dioxide and saturated and unsaturated hydrocarbons having 5 or more carbon atoms.

In general, the conversion to saturated and unsaturated hydrocarbons having 2 or more carbon atoms ("C2+") compounds that can be achieved in an OCM process is relatively low. Besides, at a higher conversion, the selectivity decreases so that it is generally desired to keep the conversion relatively low. As a result, a large amount of unconverted methane leaves the OCM process. The proportion of unconverted methane in the OCM product gas stream may be as high as 50 to 60 mol% based on the total molar amount of the gas stream. This unconverted methane has to be recovered from the desired products, such as ethylene and other saturated and unsaturated hydrocarbons having 2 or more carbon atoms, thus

requiring additional separation steps .

A further difficulty with OCM processes is that a competing reaction that takes place is the oxidation of methane to carbon monoxide, carbon dioxide and water, as well as further oxidation of ethane and ethylene to carbon monoxide, carbon dioxide.

In view of the afore-mentioned issues, there has been a great deal of attention focused on developing catalysts for use in OCM processes which are capable of increasing selectivity to C2+ hydrocarbons at lower reaction temperatures .

The currently best-performing catalysts in the OCM field typically comprise manganese, tungsten and sodium impregnated onto an amorphous silica (Si0 ₂ ) carrier (Mn- Na ₂ W0 ₄ /Si0 ₂ ) .

Chua et al . studied the oxidative coupling of methane for the production of ethylene over sodium- tungsten-manganese-supported silica catalyst (Na-W- Mn/Si0 ₂ ) in Applied Catalysis A: General 343 (2008) 142- 148. The performance of Mn-Na ₂ W0 ₄ /Si0 ₂ catalyst was further reviewed by Arndt et al . in Applied Catalysis A: General 425-426 (2012) 53-61 and Lee et al . in Fuel 106 (2013) 851-857.

Work in the OCM field has focused on further improving the performance of catalyst compositions in the oxidative coupling of methane, for example, by changing the dopants and carrier supports therein and modifying the way in which the catalyst compositions are prepared.

In ChemCatChem 2011, 3, 1935-1947, Zavyalova et al . conducted statistical analysis of past catalytic data on oxidative methane coupling for new insights into the composition of high-performance catalysts.

US 2013/0023709A describes the high throughput screening of catalyst libraries for the oxidative coupling of methane and tests on various catalysts including catalysts comprising sodium, manganese and tungsten on silica and zirconia carriers .

US 2014/0080699 Al describes a solution combustion synthesis method for the preparation of catalysts such as Mn-Na ₂ W0 ₄ /Si0 ₂ catalyst material. Yildiz et al . , Chem. Commun . , 2014, 50, 14440, reports a Mn _x O _y - a ₂ W04/Si02 catalyst prepared by wet impregnation on mesoporous ordered silica (SBA-15), wherein collapse of the ordered mesoporous structure of SBA-15 occurred following calcination.

It is highly desirable in the OCM field to develop further catalysts for the oxidative coupling of methane that show high C2+ selectivity and yield as well as high catalyst stability.

Summary of the Invention

It was found that a mesoporous silica-supported catalyst composition comprising manganese and one or more alkali metals prepared in situ by a template- assisted one-pot synthesis method exhibits improved performance and stability relative to analogous

catalysts prepared by post-impregnation on amorphous or mesoporous silica supports .

Accordingly, in a first aspect of the present invention there is provided a catalyst composition comprising manganese and one or more alkali metals supported on an ordered mesoporous silica carrier.

In another aspect there is provided a catalyst composition comprising manganese, one or more alkali metals, optionally another transition metal and an ordered mesoporous silica carrier material.

In a further aspect there is provided a method of preparing a catalyst composition comprising manganese and one or more alkali metals and an ordered mesoporous silica carrier material, wherein said method comprises - providing a solution comprising (i) template material, (ii) silica precursor, (iii) manganese precursor, alkali metal precursor, and optionally other metal precursors, and (iv) solvent;

evaporating the solvent to obtain a solid material; optionally comminuting the solid material;

- calcining the solid material.

In a further aspect of the present invention, there is provided a process for the oxidative coupling of methane comprising converting methane to one or more C2+ hydrocarbons, wherein said process comprises contacting a reactor feed comprising methane and oxygen with the afore-mentioned catalyst composition.

In contrast to prior art silica-supported

catalysts which lose their mesoporosity after active metal loading on the mesoporous silica support by impregnation followed by the necessary calcination step, the in situ template-assisted mesoporous silica- supported catalyst according to the present invention was shown by low angle XRD to exhibit ordered

mesoporosity even when the catalyst was calcined at 800 °C for several hours. Without wishing to be bound by theory, it is believed that the template-assisted in situ preparation method as described herein is

advantageous for uniform dispersion of the active metals throughout the support and aids in maintaining the ordered mesoporosity of the support. It is further believed that the intact, non-collapsed ordered

mesoporous structure of the catalyst composition as defined herein is better capable of retaining the active metal inside the mesoporous matrix, thus preventing leaching out of active metals from the catalyst support. Brief Description of the Drawings

Figure 1 shows a low-angle XRD graph of a

mesoporous silica-supported catalyst composition according to the invention.

Figure 2 shows comparative catalytic performance data .

Detailed Description of the Invention

To facilitate an understanding of the present invention, it is useful to define certain terms relating to the oxidative coupling of methane and the associated catalyst performance.

As used herein, "methane (CH ₄ ) conversion" means the mole fraction of methane converted to product (s) .

"C _x selectivity" refers to the percentage of converted reactants that went to product (s) having carbon number x and "C _x+ selectivity" refers to the percentage of converted reactants that went to the specified product (s) having a carbon number x and higher. Thus, "C ₂ selectivity" refers to the percentage of converted methane that formed ethane and ethylene. Similarly, "C ₂₊ selectivity" means the percentage of converted methane that formed compounds having carbon numbers of 2 and higher.

"C _x yield" is used to define the percentage of products obtained with carbon number x relative to the theoretical maximum product obtainable. The C _x yield is calculated by dividing the amount of obtained product having carbon number x in moles by the theoretical yield in moles and multiplying the result by 100. "C ₂ yield" refers to the total combined yield of ethane and ethylene. The C _x yield may be calculated by multiplying the methane conversion by the C _x selectivity.

As used herein, the term "reactor temperature" or "reaction temperature" should be interpreted to refer to the temperature as measured at the entrance of the catalyst bed, i.e. the temperature of the reactor feed gas just before entering the catalyst bed. In practice, due to the presence of heating elements in the region upstream of the catalyst bed and/or radiative heat transfer from the catalyst bed due to reaction

exothermicity, the temperature of the reactor feed gas just before entering the catalyst bed is not necessarily the same as the temperature of the feed gas at the inlet of the reactor, i.e., it may for example be somewhat or substantially higher.

As used herein, in the context of the silica support material, the term "mesoporous" refers to a material containing pores, wherein the pores have a diameter from 2 to 50 nm. The porous structure provides for a large internal surface area with adsorptive capacity for molecular or ionic species, which are capable of entering therein. Typically, the mesoporous silica support material of the catalyst composition according to the present invention has pores with a diameter ranging from 2 to 50 nm, more preferably in the range of from 3 to 15 nm, more preferably in the range of from 6 to 12 nm. Typically, at least 90 % by volume of the mesoporous silica support material has a pore diameter larger than 7 nm. Typically, less than 10 % by volume of the mesoporous silica support material has a pore diameter smaller than 5 nm. The term "ordered mesoporous", used herein in the context of the silica support material, is well known in the art and generally refers to a mesoporous material comprising extended regions having a narrow pore-size distribution, as described in detail in, for example, F. Liebau, Microporous and Mesoporous Materials 58 (2003) pp. 15-72. Typically, the ordered mesoporous silica support material of the catalyst compositions of the present invention are characterized by comprising at least 50 wt%, and/or at least 10 vol% of regions in which the pores have a very narrow size distribution (standard deviation < 1%) and a high long-range (> 7 nm) order .

The pore size and pore size distribution can be determined by a variety of nanoscale characterization techniques, including transmission electron microscopy

(TEM) , small-angle X-ray scattering (SAXS), low-angle X- ray diffraction (XRD) , gas adsorption-desorption and mercury intrusion porosimetry, combined with analysis techniques including Density Functional Theory (DFT) , Horvath-Kawazoe (HK) modeling and Barrett-Joyner-Halenda

(BJH) modeling. ISO standard nr. 15901 describes methods for measuring pore size distribution and porosity of solid materials by mercury porosimetry and gas

adsorption .

The term "supported on" used herein in the context of the ordered mesoporous silica carrier refers to the common way of distributing the catalytically active material over the high surface area carrier ("support") material. Contrary to the impregnation method known in the art for affixing catalytically active material to the catalyst carrier material (wherein an existing solid - amorphous or ordered - carrier material in dry or suspension form is treated with a solution of one or more metal precursors until sufficient metal is affixed to the carrier, followed by calcination) , in the present invention all catalyst constituents, i.e. catalyst carrier and metals, are combined in their precursor state in the presence of a suitable template material, followed by drying, optional comminution steps and calcination in order to provide a carrier material with interspersed active metal.

As used herein in the context of catalyst

constituents, "weight percent" (wt%) refers to the ratio of the total weight of the carrier, the metal-containing component or the metal in the dopant to the total weight of the catalyst composition. Said percentages are determined with respect to the weight of the total dry catalyst composition. Suitably, the weight of the total dry catalyst composition may be measured following drying for at least four hours at temperatures of at least 120, preferably at least 150 °C, more preferably at least 300 °C.

Percentages of metals from the metal-containing constituents in the catalyst composition may be

determined by X-ray fluorescence (XRF) , or by

dissolution followed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) or Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) , all techniques being known in the art. The metals content of the catalyst composition may also be inferred or controlled via its synthesis.

The components of the catalyst composition are to be selected in an overall amount adding up to 100 wt . %. As used herein, the term "compound" refers to the combination of a particular element with one or more different elements by surface and/or chemical bonding, such as ionic and/or covalent and/or coordinate bonding.

The term "ion" or "ionic" refers to an

electrically chemical charged moiety; "cation" or

"cationic" being positive, "anion" or "anionic" being negative, and "oxyanion" or "oxyanionic" being a negatively charged moiety containing at least one oxygen atom in combination with another element (i.e., an oxygen-containing anion) . It is understood that ions do not exist in vacuo, but are found in combination with charge-balancing counter ions when added.

The term "oxidic" refers to a charged or neutral species wherein an element in question is bound to oxygen and possibly one or more different elements by surface and/or chemical bonding, such as ionic and/or covalent and/or coordinate bonding. Thus, an oxidic compound is an oxygen-containing compound which also may be a mixed, double or complex surface oxide.

Illustrative oxidic compounds include, but are not limited to, oxides (containing only oxygen as the second element), hydroxides, nitrates, sulfates, carboxylates , carbonates, bicarbonates, oxyhalides, etc. as well as surface species wherein the element in question is bound directly or indirectly to an oxygen either in the substrate or the surface.

Surface area is determined in accordance with the well-known B.E.T. (Brunauer-Emmett-Teller ) nitrogen adsorption technique, often simply termed the "B.E.T. method". Herein, the general procedure and guidance of ASTM D4365-95 is followed in the application of the "B.E.T. method" to the materials. "B.E.T. surface area" as used herein refers to the surface area of the

silicon-containing carrier prior to and after doping with manganese and one or more alkali metals, and optionally other metals.

Typically, the mesoporous silica-supported

catalyst composition of the present invention has a B.E.T. surface area of greater than 1 m ² /g, preferably in the range of from 1 to 50 m ² /g, more preferably in the range of from 2 to 20 m ² /g and most preferably in the range of from 2 to 10 m ² /g, according to ASTM D4365-95.

The total pore volume may be measured by a

conventional water pore volume method where the total amount of water to fill the pores of the material is measured by drying the catalyst, measuring the weight and adding water until the material gets wet, signifying that all pores are filled. After centrifuging the excess water off the remaining amount is measured by weight and the total pore volume determined.

The mesoporous silica support material may be conveniently present in the catalyst composition in an amount in the range of from 75 to 96 % by weight, preferably in the range of from 85 to 92 % by weight, relative to the total weight of the catalyst

composition.

Typically, the catalyst composition of the present invention comprises manganese in an amount of in the range of from 1 to 10 % by weight, preferably in the range of from 1 to 5 % by weight, more preferably in the range of from 1.3 to 3 % by weight and most preferably in the range of from 1.7 to 2.5 % by weight, relative to the total weight of the catalyst composition. In some embodiments of the present invention, the manganese is present in the catalyst composition in the form of one or more manganese-containing dopants such as one or more manganese-containing oxides. Said manganese- containing oxides may be reducible oxides of manganese and/or reduced oxides of manganese. However, in the active state, the catalyst composition comprises at least one reducible oxide of manganese. Such reducible oxides include compounds of the general formula Mn _x O _y wherein x and y designate the relative atomic

proportions of manganese and oxygen in the composition and one or more oxygen-containing Mn compounds which contain manganese, oxygen and additional elements.

Particularly preferred reducible oxides of manganese include Mn0 ₂ , Mn ₂ 0 ₃ , Mn ₃ 0 ₄ and mixtures thereof.

The catalyst composition of the present invention comprises one or more (Group 1) alkali metals. Said alkali metals are typically one or more of lithium, sodium, potassium, rubidium and cesium. In some

embodiments the alkali metals are sodium and/or lithium.

The one or more alkali metals are typically each present in an amount of in the range of from 0.1 to 2.0 % by weight, more preferably in the range of from 0.4 to 1.5 % by weight, and most preferably in the range of from 0.4 to 1.2 % by weight, relative to the total weight of the catalyst composition.

The catalyst composition of the present invention may further comprise tungsten. Said tungsten may be present in a preferred amount of in the range of from 1 to 4.5 % by weight, more preferably in the range of from 1.5 to 3.5 % by weight, relative to the total weight of the catalyst composition. In one embodiment, the catalyst composition comprises manganese, tungsten and one or more alkali metals. In a preferred embodiment, the catalyst

composition comprises manganese, tungsten and sodium, generally denoted in the field as Mn/Na ₂ W0 ₄ . Thus, in a preferred embodiment, the alkali metal is sodium and the other transition metal is tungsten.

In another embodiment, the catalyst composition comprises a ternary oxide compound having the

composition M ¹ M ² Mn0 ₃ , wherein M ¹ and M ² are the same or different and are alkali metals selected from lithium, sodium and potassium. In one embodiment of the present disclosure, the ternary oxide compound has the

composition M ₂ Mn0 ₃ , wherein M is an alkali metal selected from lithium, sodium and potassium. Preferably, M is lithium. In one embodiment, the catalyst composition comprises a ternary oxide compound having the

composition Li ₂ Mn0 ₃ .

In the preparation of the catalyst composition of the present invention, the one or more alkali metals and optionally tungsten may be doped as separate metals and/or metal-containing compounds into said composition. If the catalyst composition also comprises tungsten, in some embodiments, the one or more alkali metals and tungsten may be doped into the catalyst composition in the form of one or more compounds comprising both alkali metal (s) and tungsten therein. Suitable examples of such compounds include sodium tungstate and lithium

tungstate .

During the oxidative coupling of methane, the specific form of the manganese, one or more alkali metals, optionally other transition metals, and any optional co-promoters and/or additional metal-containing dopants in the catalyst composition may be unknown.

Thus, when sodium, tungsten and manganese are present in combination in the catalyst composition, they may be preferably present as one or more of Na ₂ W0 ₄ , Na ₂ W ₂ 0 ₇ , Mn ₂ 0 ₃ and MnW0 ₄ . Preferably, the catalyst composition of the present invention comprises one or more of Na ₂ W0 ₄ , Na ₂ W ₂ 0 ₇ , Mn ₂ 0 ₃ and MnW0 ₄ .

In another aspect, the invention relates to a process for preparing a catalyst composition comprising manganese and one or more alkali metals, wherein said method comprises

- providing a solution comprising (i) template material, (ii) silica precursor, (iii) manganese precursor, alkali metal precursor, and optionally other metal precursors, and (iv) solvent;

evaporating the solvent to obtain a solid

material ;

optionally comminuting the solid material;

- calcining the solid material.

The mesoporous silica-supported catalyst

composition of the present invention is prepared in situ by a template-assisted solution-gelation (sol-gel) inorganic polymerization method in a solution also comprising manganese precursor, alkali metal precursor, and optionally other metal precursors. Template-assisted sol-gel and hydrothermal preparation methods for order mesoporous compounds, including silica support

materials, have been known in the art for several decades; see for example US 3,556,725; WO 1991/011390; and Huo et al . , Chem. Mater. 1994, 6, 1176-1191. In such methods, the organic template material serves as a structure-directing agent for the formation of ordered mesoporous silica via self-organization of the organic template material (e.g. into micelles) and silica precursor compound (s) into three-dimensional periodic phases, followed by removal of the organic matter by high-temperature calcination to provide a three- dimensional, porous structure with long-range order. Examples of commercially available ordered mesoporous silica compounds are the hexagonal ordered mesoporous molecular sieve known as MCM-41 having a 2-3 nm pore size and SBA-15 comprising ordered hexagonal mesoporous silica particles with a tunable, uniform pore size.

Suitable template materials are anionic

surfactants, non-ionic surfactants and cationic

surfactants, and combinations thereof. Suitable non- ionic surfactants for use as structure-directing template material include nonionic triblock copolymers composed of a central hydrophobic chain of

poly (propylene glycol) (poly (propylene oxide)) flanked by two hydrophilic chains of poly (ethylene glycol) (poly (ethylene oxide)) ["PEG-PPG-PEG" or "PEO-PPO-PEO" ] , also known as "poloxamers" . Examples of such compounds are those sold under the trade mark "Pluronic" such as PLURONIC P-123, which is a PEG-PPG-PEG triblock

copolymer having a molecular weight of about 5,800 Da; and PLURONIC F-127, which is a PEG-PPG-PEG triblock copolymer having a molecular weight of approximately 12,500 Da.

Suitable cationic surfactants include hexadecyl amine (HDA) , and quaternary ammonium salts such as cetyltrimethyl ammonium chloride (CTAC) , cetyltrimethyl ammonium bromide (CTAB) , dodecyltrimethyl ammonium chloride (DTAC) , cetylpyridinium chloride (CPC) ,

benzalkonium chloride (BAC) , benzethonium chloride

(BZT) , dimethyl dioctadecylammonium chloride and

dioctadecyl dimethylammonium bromide (DODAB) . A

preferred cationic surfactant is cetyltrimethyl ammonium bromide (CTAB) .

Suitable anionic surfactants include alkyl

sulfates, alkylbenzene sulfonates, alkyl carboxylates and di-alkyl sulfosuccinates, such as sodium dodecyl sulfate (SDS) , stearic acid and dioctyl sodium

sulfosuccinate (also known as Aerosol-OT) . Preferred anionic surfactants are sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate and stearic acid.

The silica precursor may be any conventionally known silica or silicate, such as fumed silica,

colloidal silica, sodium silicate, and organic silicates or a combination thereof. In a preferred embodiment, an organic silicate, such as quaternary ammonium silicate, is employed. Examples of such quaternary ammonium silicates are tetramethylammonium silicate (TMAS) , tetramethyl orthosilicate (TMOS) and tetraethyl

orthosilicate (TEOS) . Preferably tetraethyl

orthosilicate (TEOS) is used as the silica precursor.

Optionally, the catalyst composition of the present invention may further comprise one or more additional carriers therein selected from titania (Ti0 ₂ ) , alumina (A1 ₂ 0 ₃ ) and zirconia (Zr0 ₂ ) in amounts of from 2 to 50 wt % based on total weight of the catalyst

composition, i.e. support, and manganese, alkali metal compounds, optionally tungsten, and optionally other compounds . Typically, the catalyst synthesis process involves providing a solution of the template material, or of a combination of template materials, and mixing with one or more silica precursor compounds to create a support material solution. Typically, the weight ratio of template compound to silica precursor used in the synthesis process is in the range of 0.1:1 to 5:1, preferably 0.2:1 to 2:1, more preferably 0.3:1 to 1:1. The catalyst synthesis process typically further involves providing one or more solutions of the

manganese precursor, the alkali metal precursor, and optionally other metal precursors, such as tungsten precursor, either as separate solutions or as solutions comprising two or more of the required metal and alkali metal precursors. Wherever reference is made herein to

"solvent", this should be understood as a single solvent compound or a mixture of solvent compounds. The skilled person will be capable of selecting suitable solvents that are capable of dissolving the template material and the metal and alkali metal, which can subsequently be removed by (thermal) evaporation. Examples of suitable solvents include water, ethanol, acetone, isopropanol and methanol, and any combination thereof.

In a preferred embodiment, the alkali metal precursor is a sodium precursor and the optional other transition metal is tungsten.

Preferably, an organic or inorganic acid is added to catalyze hydrolysis of the silica precursor and subsequent polymerization (condensation) into a three- dimensional silica network. Suitable examples of such acids include citric acid, acetic acid, nitric acid and hydrochloric acid, and the skilled person will be capable of selecting a preferred combination of silica precursor and hydrolysis catalyst in the appropriate amounts. Typically, acid is added to the silica

precursor in a molar ratio of acid to silica precursor in the range of from 0.1:1 to 2:1, preferably in the range of from 0.2:1 to 1:1, more preferably in the range of from 0.3:1 to 0.8:1.

Suitably, a chelating agent may be added, such as, for example, ammonium oxalate or citric acid. Typically, agitation of the slurry of metal and alkali metal solution (s) and (polymerized) silica is conducted over a period of several hours, for example between 1-20 hours, preferably between 2-15 hours, more preferably between 4-14 hours .

Typically the template-containing support material solution is combined with the metal and alkali metal solution (s), and optionally other components, under agitation to form a slurry. Subsequently, the one or more solvents are removed by evaporation, typically at moderately elevated temperatures in the range of 40-140 °C, preferably 50-100 °C, more preferably 60-95 °C, to provide a solid material. The thermal solvent removal step further contributes to the aging (e.g., formation of cross-links, strengthening, stiffening, shrinkage) of the polymerized silica network into the desired solid mesoporous structure.

If preferred, the particle size of the resulting solid material may be reduced (comminuted) by any technique known in the art for reducing the size of catalyst particles, including grinding, milling, crushing, cutting, sieving, etc., and any combination thereof . Calcination of the, optionally comminuted, solid material yields a catalyst composition comprising manganese, one or more alkali metals and optionally other metals supported on an ordered mesoporous silica carrier. Calcination may take place at a temperature in the range of from 600 to 1000 °C, preferably in the range of from 700 to 900 °C, and most preferably in the range of from 800 to 850 °C. While above this

temperature suitable mesoporous structures are still formed, to a minor extent collapse of the mesoporous structure may occur. Calcination is typically conducted over a period of several hours, for example 2-10, preferably 4-8, more preferably 5-7 hours. In some embodiments, calcination is carried out by a step-wise increase of the temperature and intermediate dwelling, for example 0.5 hr dwelling at 600 °C, 2 °C/min until 800 °C and 4 hrs dwelling at 800 °C .

During the preparation of the catalyst composition of the present invention, the specific form in which the precursors of the manganese, the alkali metal,

optionally other transition metals (such as tungsten) and any optional co-promoters and/or additional metal- containing dopants are provided is not limited, provided that they can be solubilized in an appropriate solvent, such as a water- and/or alcohol-containing solvent.

For example, precursors of the manganese, an alkali metal, optional other transition metal (such as tungsten) and an optional co-promoter and/or additional metal-containing dopant may suitably be provided as dissolved ions (e.g., cation, anion, oxyanion, etc.), or as dissolved compounds (e.g., alkali metal salts, salts of a further co-promoter, etc.) .

As will be appreciated by those skilled in the art, while specific forms of the afore-mentioned compounds may be provided during catalyst preparation, it is possible that during the conditions of preparation of the catalyst composition and/or during use in oxidative coupling of methane, the particular forms initially present may be converted to other forms.

Furthermore, in many instances, analytical techniques may not be sufficient to precisely identify the forms that are present. Accordingly, the present disclosure is not intended to be limited by the exact form of the manganese, the one or more alkali metals, the optional other transition metals, and/or any optional co- promoters and/or additional metal-containing dopants that may ultimately exist on the catalyst composition during use .

Examples of suitable alkali metal compounds include, but are not limited to, alkali metal salts and oxidic compounds of the alkali metals, such as the nitrates, nitrites, carbonates, bicarbonates , oxalates, carboxylic acid salts, hydroxides, halides, oxyhalides, borates, sulfates, sulfites, bisulfates, acetates, tartrates, lactates, oxides, peroxides, and iso- propoxides, etc.

As previously mentioned, the one or more alkali metals may comprise a combination of two or more alkali metal dopants. Non-limiting examples include

combinations of lithium and sodium, lithium and

potassium, lithium and rubidium, lithium and cesium, sodium and potassium, sodium and rubidium, sodium and cesium, potassium and rubidium, potassium and cesium and rubidium and cesium.

Optionally, the catalyst compositions of the present invention may further comprise one or more co- promoters and/or additional metal-containing dopants.

Examples of co-promoters and metal-containing dopants that may be conveniently used therein include lanthanum, cerium, niobium and tin.

The catalyst composition of the present invention may comprise said optional co-promoters and/or metal- containing dopants in a total amount of in the range of from 0.1 to 5 % by weight, and most preferably in the range of from 0.5 to 2 % by weight, relative to the total weight of the catalyst composition.

The process of the present invention further comprises utilizing the catalyst composition as

hereinbefore described in a reactor suitable for the oxidative coupling of methane. Accordingly, in one aspect the invention relates to process for the

oxidative coupling of methane comprising converting methane to one or more C2+ hydrocarbons, wherein said process comprises contacting a reactor feed comprising methane and oxygen with a catalyst composition as defined herein.

The reactor may be any suitable reactor, such as a fixed bed reactor with axial or radial flow and with inter-stage cooling or a fluidized bed reactor equipped with internal and external heat exchangers.

In some embodiments of the present invention, the catalyst composition may be packed along with an inert packing material, such as quartz, into a fixed bed reactor having an appropriate inner diameter and length. Optionally, the catalyst composition may be pretreated at high temperature to remove moisture and impurities therefrom. Said pretreatment may take place, for example, at a temperature in the range of from 100- 300 °C for about one hour in the presence of an inert gas such as helium.

Various processes and reactor set-ups are

described in the OCM field and the process of the present invention is not limited in that regard. The person skilled in the art may conveniently employ any of such processes in conjunction with the catalyst

composition as hereinbefore described.

Suitable processes include those described in EP 0206042 Al, US 4443649 A, CA 2016675 A, US 6596912 Bl, US 2013/0023709 Al, WO 2008/134484 A2 and WO 2013/106771 A2.

As used herein, the term "reactor feed" is understood to refer to the totality of the gaseous stream at the inlet of the reactor. Thus, as will be appreciated by one skilled in the art, the reactor feed is often comprised of a combination of one or more gaseous stream(s), such as a methane stream, an oxygen stream, a recycle gas stream, a diluent stream, etc.

During the oxidative coupling of methane, a reactor feed comprising methane and oxygen is introduced into the reactor. Optionally, the reactor feed may further comprise one or more of a diluent gas, together with minor components of the methane feed (ethane, propane etc.) or the methane recycle stream (e.g.

ethane, ethylene, propane, propylene, CO, C0 ₂ , H ₂ and

H ₂ 0) . The diluent represents the balance of the feed gas and is an inert gas. Examples of suitable inert gases are nitrogen, argon and helium.

In some embodiments, the methane and oxygen are added to the reactor as mixed feed, optionally

comprising further components therein, at the same reactor inlet. However, in some embodiments of the present invention, the methane and oxygen may be added in separate feeds, optionally comprising further components therein, to the reactor at separate inlets.

Methane may be present in the reactor feed in a concentration of at least 35 mole-%, and most preferably at least 40 mole-%, relative to the total reactor feed. Similarly, methane may be present in the reactor feed in a concentration of at most 90 mole-%, and most

preferably at most 85 mole-%, relative to the total reactor feed.

In some embodiments of the present invention, methane may be present in the reactor feed in a

concentration in the range of from 35 to 90 mole-%, and most preferably in the range of from 40 to 85 mole-%, relative to the total reactor feed.

In addition to methane, the reactor feed further comprises oxygen, which may be provided either as pure oxygen or air. In an oxygen-based process, high-purity (at least 95 mole-%) oxygen or very high purity (at least 99.5 mole-%) oxygen is employed.

In general, the oxygen concentration in the reactor feed should be less than the concentration of oxygen that would form a flammable mixture at either the reactor inlet or the reactor outlet at the prevailing operating conditions. Often, in practice, the oxygen concentration in the reactor feed may be no greater than a pre-defined percentage (e.g., 95%, 90%, etc.) of oxygen that would form a flammable mixture at either the reactor inlet or the reactor outlet at the prevailing operating conditions.

Although the oxygen concentration in the reactor feed may vary over a wide range, the oxygen

concentration in the reactor feed is typically at least 7 mole-%, or at least 10 mole-%, relative to the total reactor feed. Similarly, the oxygen concentration of the reactor feed is typically at most 25 mole-%, or at most 20 mole-%, relative to the total reactor feed.

In some embodiments, oxygen may be present in the reactor feed in a concentration in the range of from 7 to 25 mole-%, and most preferably in the range of from 10 to 20 mole-%, relative to the total reactor feed.

It is within the ability of one skilled in the art to determine a suitable concentration of oxygen to be included in the reactor feed, taking into consideration, for example, the overall composition of the reactor feed, along with the other operating conditions, such as pressure and temperature.

However, in some embodiments, the methane : oxygen volume ratio in the process of the present invention is preferably in the range of from 2/1 to 10/1, more preferably in the range of from 3/1 to 6/1.

The reactor feed optionally may further comprise a diluent gas, such as helium, argon, nitrogen, or a combination thereof.

The order and manner in which the components of the reactor feed are combined prior to contacting the catalyst composition is not limited, and they may be combined simultaneously or sequentially. However, as will be recognized by one skilled in the art, it may be desirable to combine certain components of the inlet feed gas in a specified order for safety reasons. For example, oxygen may be added to the inlet feed gas after the addition of a dilution gas for safety reasons.

Similarly, as will be understood by one of skill in the art, the concentration of various feed components present in the inlet feed gas may be adjusted throughout the process, for example, to maintain a desired

productivity, optimize the process, etc. Accordingly, the above-defined concentration ranges were selected to cover the widest possible variations in the composition of the reactor feed during normal operation.

It is an advantage of the catalyst composition as described herein that it shows oxygen (0 ₂ ) conversion rates close to 100 %, thus significantly reducing downstream separation costs.

It is another advantage that, whereas in prior art catalyst compositions prepared by impregnation on a pre- formed mesoporous silica support the mesoporous

structure was found to collapse after calcination, in the catalyst composition as defined herein the ordered mesoporosity of the silica substrate is maintained. This may prevent leaching out of active metals from the silica support matrix, thus improving long-term

stability of the catalyst composition.

Figure 1 shows a low-angle X-ray diffraction (XRD) diffractogram of a 2 wt% Mn/2 wt% Na ₂ W0 ₄ /mesoporous Si0 ₂ catalyst composition prepared using Pluronic P-123 as template and having been calcined at 800 °C for 4 hours. As evidenced by the characteristic high-intensity peak at 2Θ-0.8, the in situ template-assisted prepared mesoporous silica-supported catalyst composition as disclosed herein exhibits ordered mesoporosity even if the composition was calcined at 800 °C for 4 h. This is in contrast to prior art mesoporous silica-supported catalyst compositions, which lose their mesoporosity after loading active metal on the carrier by

impregnation method following the necessary calcination step (see, e.g. Yildiz et al . , Chem. Commun . , 2014, 50, 14440) .

In the process of the present invention, the reactor feed comprising methane and oxygen is contacted with a catalyst composition as hereinbefore described in order to effect the conversion of methane to one or more C ₂₊ hydrocarbons. Typically, the reactor temperature is in the range of from 500 to 1000 °C. Preferably, said conversion is effected at a reactor temperature in the range of from 650 to 1000 °C, more preferably in the range of from 700 to 950 °C, even more preferably in the range of from 750 to 900 °C and most preferably in the range of from 800 to 850 °C.

In some embodiments of the present invention, the conversion of methane to one or more C ₂₊ hydrocarbons is effected at a reactor pressure in the range of from 1 to 25 MPa. More preferably, said reactor pressure is in the range of from 2 to 10 MPa.

The gas hourly space velocity (GHSV) in the process of the present invention is the entering volumetric flow rate (m ³ /s) of the reactor feed (at standard conditions) divided by the catalyst bed volume. Preferably, said gas hourly space velocity is in the range of from 3,000 to 1, 000, 000 h ^-1 . It should be noted that suitable and favorable space velocities differ markedly between laboratory test reactors and industrial reactors. For the latter, the GHSV is typically in the range of 10,000 to 300,000 h ^-1 , preferably in the range of from 20,000 to 150, 000 T ¹ . Said GHSV is measured at standard

temperature and pressure, namely 0 °C and 1 bara (100 kPa) .

In some embodiments, the process of the present invention has a C ₂₊ hydrocarbon selectivity of greater than 40 %, and most preferably greater than 60 %.

In some embodiments, the process of the present invention results in an ethane : ethene ( C ₂ /C ₂ = ) ratio of less than 1.6 , and most preferably less than 0.6.

The invention is further illustrated by the following Examples.

EXAMPLES

Example 1: catalyst samples A-J

In-situ template-assisted synthesis of mesoporous Si0 ₂ supported Mn _x O _Y -Na ₂ W0 ₄ catalysts (samples B, E, F, G, H,

A mesoporous silica catalyst containing Mn _x O _y -Na ₂ W0 ₄ was synthesized using a template-assisted method. The total amount of catalyst synthesized, including the silica support, was approximately 20 grams. About 30 grams of template material (samples B, F, G, I: Pluronic P-123 ex Sigma-Aldrich; sample E: cetyltrimethylammonium bromide (CTAB) ex Sigma-Aldrich; sample H: stearic acid ex Sigma-Aldrich) was dissolved in 150 ml ethanol at room temperature, followed by dropwise addition of 69 grams of tetraethyl orthosilicate (TEOS; ex Sigma-Aldrich) to the solution under continuous stirring to provide a support material solution. A metal and alkali metal precursor solution was prepared separately by dissolving 0.4-3 grams of sodium tungstate dihydrate and 1.5-5 gram of manganese nitrate in the desired metal/alkali metal ratios together in approximately 20 ml of ethanol, followed by addition of 12 ml of concentrated nitric acid (HN0 ₃ ) . The metal/alkali metal precursor solution was added to the support material solution under

vigorous stirring. The mixture was stirred overnight to form a slurry, and subsequently heated in an oven for 7 hours at 90 °C. The temperature was reduced to 50 °C and any remaining ethanol was permitted to evaporate

overnight. The obtained solid sample was ground to a fine powder and calcined at 600 °C for 4 h then at 800 °C for 4 hours under static air (ramp in both cases 2 °C /min) .

Synthesis of MnxOy-Na ₂ W0 ₄ catalysts impregnated on amorphous Si0 ₂ support (samples A and C; comparative) A kilogram scale batch of Mn/Na ₂ W0 ₄ /amorphous silica catalyst was prepared by a two-step impregnation

procedure. 90 grams of Mn (N0 ₃ ) ₂ ·4Η ₂ 0 was dissolved in 1.6 L demineralized water. After adding 15.6 mL of

concentrated nitric acid the total volume was adjusted to 1.8 L by adding demineralized water. The solution was sprayed onto 910-940 g of amorphous Si0 ₂ (PQ Corp., BET surface area 300 m ² /g, water pore volume = 1.8 mL/g) in a rotating drum attached to an ERWEKA driving unit. The drum was allowed to rotate for an additional 30 minutes, while supported by a Leister fan for indirect dynamic drying until water removal of >95%. The batch was dried overnight at 120°C. 22-55 g of Na ₂ W0 ₄ ^» 2H ₂ 0 and citric acid in a 1:2 molar ratio were dissolved in 1.6 L

demineralized water and the solution was topped up with demineralized water to a total volume of 1.8 L. This solution was sprayed onto the Mn-impregnated silica and dried dynamically as before. After further overnight drying, the resulting powder was calcined with a heating rate of 3°C/min until 850°C for 5 hours dwell time.

Synthesis of MnxOy-Na2W04 catalysts impregnated on pre- synthesized mesoporous Si0 ₂ support (sample D;

comparative)

A mesoporous silica support was synthesized using a template-assisted method. The total amount of support synthesized was approximately 20 grams. 30 grams of template material was dissolved in 150 ml ethanol at room temperature, followed by dropwise addition of 69 grams of tetraethyl orthosilicate (TEOS) to the solution under continuous stirring to provide a support material solution .

The mixture was stirred overnight to form a slurry, and subsequently heated in an oven for 7 hours at 90 °C. The temperature was reduced to 50 °C and any remaining ethanol was permitted to evaporate overnight . The obtained solid sample was ground to a fine powder and calcined at 600 °C for 4 h then at 800 °C for 4 hours under static air (ramp 2 °C /min) to produce a solid powderous mesoporous silica support. A metal and alkali metal precursor solution was prepared separately by dissolving 0.4-3 grams of sodium tungstate dihydrate and 1.5-5 gram of manganese nitrate in the desired

metal/alkali metal ratios together in approximately 20 ml of ethanol . The mesoporous silica support was

impregnated by adding the metal and alkali metal

precursor solution dropwise under continuous stirring to the dry powderous mesoporous silica support until the support material wetted. The mixture was further stirred until the support went dry, after which the procedure was repeated until the alkali metal and metal precursor solution was completely consumed by the support. The entire mass was dried overnight at 90 °C followed by calcination at 800 °C for 4 h (2 °C /min ramp) .

Table 1 provides pore size and pore volume data of the catalyst samples A-I determined using nitrogen

adsorption-desorption analysis carried out at 77 K on a Micromeritics Tristar II 3020 static volumetric

analyzer. The adsorption isotherms were analyzed

applying Density Functional Theory using built-in automated analysis software.

Catalytic Performance Testing Procedure

Each of the catalyst compositions A-J prepared according the procedures described above was tested for oxidative methane coupling performance in accordance with the following general testing procedure. The catalysts (250-400 μπι sieve fraction) were tested in a 16-barrel nanoflow unit (Flowrence, Avantium) . The methane (CH ₄ ; >99.9 %) and oxygen (0 ₂ ; 99.9 %) reagents were used without further purification. The reactor feed comprised methane and oxygen in a mole ratio of 4:1, with 5 mol . % of nitrogen as inert gas. The gases were fed each via a mass flow controller and then passed through a manifold. The mixed feed was split by a glass chip with 16 channels and sent individually to each of the 16 reactors. The nanoflow unit is divided in 4 blocks, each block containing 4 reactors. The reactions were carried out in quartz tubes (2 mm inner diameter, 3 mm outer diameter) where the catalyst bed (10 mm bed length) was laid on top of a quartz insert. Downstream, each reactor effluent was diluted with helium (He) , which flowed along the outer side of the reactors walls. The pressure was indirectly monitored by means of pressure indicators (4 per block) allocated in the entrance of the He side diluent lines. The pressure was controlled by the parallel pressure block: the effluent flow is passed through one side of a membrane that is pushed by N ₂ (controlled by a pressure controller valve, does NOT mix with the effluent) at the other side of the membrane. In this way, the force equilibrium between the flows at the membrane surface serves to control the pressure. GHSV: 20,000 h ^_1 ; Pressure: 1.3 barg. The effluent gas leaving the membrane was analyzed on-line by gas chromatography (GC) , where N ₂ was used as

internal standard, and by mass spectrometry (MS) .

Conversion of methane and oxygen and product composition was, after condensation of the water vapour in a separator, measured with an on-line GC

(Thermoscientific GC, Breda) equipped with two TCD detectors and two FID detectors for quantitative analyses of oxygen, nitrogen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, C3, C4 and C5 hydrocarbons . The total off-gas flow of the nanoflow unit was determined by the amount and concentration of nitrogen (in Nl/hr) in the reactor feed and in the off gas

(determined from the results of the on-line GC

analyses) . From this total off-gas flow, the individual component flows were calculated in Nl/hr. From these individual component flows, the total carbon balance was calculated, which in most experiments was between 98 and 102 %C . Besides the carbon balance, oxygen and methane conversions as well as C ₂₊ selectivity and yields were calculated.

The conversion, selectivities and yields are summarized in Table 2.

Example 2: catalyst samples I-III

Three catalyst compositions comprising 2 wt% Mn/5 wt% Na ₂ W0 ₄ on a silica support, were prepared by (I)

impregnation on amorphous Si0 ₂ ; (II) impregnation on pre- synthesized mesoporous Si0 ₂ , followed by calcination; (III) in situ sol-gel synthesis of mesoporous Si0 ₂ in the presence manganese precursor, alkali metal precursor, and tungsten precursor and stearic acid template, followed by calcination, using the preparations methods as described in detail in Example 1.

Catalytic oxidative methane coupling (OCM)

performance testing was conducted as described in detail in Example 1, under the following conditions: 2 mm ID quartz reactor; Catalyst bed length: 19 mm; Feed ratio: CH ₄ /0 ₂ =4:1; GHSV: 25, 000 h ^"1 ; Pressure: 1.3 barg;

Temperature: 750-800 °C (< 20 hr) ; 850 °C (> 20 hr) . The results are presented in Figure 2. Shown herein are the C ₂₊ selectivity (closed circles), C ₂₊ yield (open circles), oxygen conversion (triangles) and methane conversion (squares) as a function of reaction runtime for each of catalyst compositions I to III.

Discus sion

Table 2 summarizes the results of the catalytic

performance testing of Example 1 of catalyst preparation samples B, E, F, G, H and J (according to the invention) and catalyst preparations A, C and D (comparative) .

Figure 2 presents the results of the catalytic

performance testing of Example 2 of catalyst preparation samples I, II (both comparative) and III (according to the invention) .

As can be seen from these results, the catalyst preparations according to the present invention, using various template materials, display similar or improved C ₂₊ yields (Y _C 2 ₊ ) and selectivity (S _C 2 ₊ ) behavior over time compared to both analogous catalyst compositions on amorphous (samples A and C; I) or pre-synthesized silica carriers (samples D; II) . The results further show improved conversion of oxygen (X ₀₂ ) and reduced

selectivity to carbon monoxide (S _C o) for the catalyst preparations of the present invention as compared to both analogous catalyst compositions on amorphous or pre-synthesized mesoporous silica carriers.

Table 1: Pore size and pore volume of the catalyst samples as measured using nitrogen adsorption-desorption analysis.

Table 2: Comparative catalyst performance data for oxidative coupling of methane. Reaction conditions: Catalyst bed length: 10 mm; Feed ratio: CH ₄ /0 ₂ =4:1; GHSV: 20,000 h ^-1 ; Pressure: 1.3 barg .

Sample Composition Support Temp . Time o2 XcH Sc2+ S _co Yc2+ c ₂ /c ₂₌

(total (°C) ( ) (%) (%) (%) (%) (%) ratio catalyst

weight incl .

support)

750 8.7 41. 7 6. 9 28. .8 21. .4 2. 0 4.6

2wt% Mn/2 wt%

A Amorphous Si0 ₂ 800 27.1 67. 7 19 .0 66. .1 14. .0 12 .6 1.0

Na ₂ W0 ₄ (18.0

(comp) 850 96.8 100 .0 30 .1 71. .5 14. .4 21 .5 0.5

mg) 850 146.3 98. 9 28 .3 73. .3 18. .1 20 .7 0.5

850 229.8 97. 0 28 .3 68. .4 18. .4 19 .4 0.5

750 7.6 43. 5 6. 4 15. .6 18. .2 1. 0 5.9

2 wt% Mn/2 In situ rap-

800 25.9 82. 1 17 .7 49. .0 11. .9 8. 7 1.5

B wt% Na ₂ W0 ₄ Si0 ₂ ; Pluronic 850 39.2 100 .0 27 .2 67. .2 10. .0 18 .3 0.7

(32.3 mg) P-123 template 850 145.0 99. 2 28 .5 73. .8 15. .6 21 .0 0.5

850 228.7 98. 0 28 .5 68. .1 16. .7 19 .4 0.6

Sample Sample Support Temp . Time o2 XcH Sc2+ S _co Yc2+ c ₂ /c ₂₌

(°C) ( )

(total (%) (%) (%) (%) (%) ratio catalyst

weight)

750 10.0 14. 0 3. 3 58. .6 16 .7 2. 0 6.0

2 wt% Mn/5

C Amorphous Si0 ₂ 800 28.4 39. 4 13 .1 78. .5 10 .9 10 .3 1.4 wt% Na ₂ W0 ₄ 850 41.6 97. 3 29 .8 74. .7 13 .2 22 .3 0.5

(comp)

(15.2 mg) 850 147.7 90. 7 28 .6 74. .8 17 .2 21 .4 0.5

850 228.6 89. 0 28 .3 73. .2 18 .8 20 .7 0.5

Pre- 750 14.1 13. 4 2. 7 45. .9 14 .4 1. 3 7.9

2 wt% Mn/5

D synthesized 800 27.4 30. 1 9. 4 71. .1 8. 2 6. 7 2.3 wt% Na ₂ W0 ₄ 850 40.6 85. 9 27 .3 73. .9 9. 6 20 .2 0.6

(comp) Si0 ₂ ; Pluronic

(27.3 mg) 850 146.7 84. 0 26 .3 74. .7 13 .5 19 .6 0.5

P-123 template

850 227.6 88. 9 28 .4 74. .7 13 .6 21 .2 0.5

750 7.9 26. 7 4. 7 30. .2 17 .8 1. 4 5.8

2 wt% Mn/5 In situ mp-

800 23.7 54. 2 14 .5 66. .8 9. 7 9. 7 1.5

E wt% Na ₂ W0 ₄ Si0 ₂ ; CTAB 850 39.5 100 .0 29 .1 73. .0 9. 5 21 .2 0.5

(21.8 mg) template 850 145.4 91. 8 27 .3 75. .1 14 .8 20 .5 0.5

850 229.0 86. 8 27 .9 74. .2 15 .9 20 .7 0.5

750 10.3 22. 2 4. 2 39. .3 14 .4 1. 6 6.0

2 wt% Mn/5 In situ mp- 800 23.5 46. 1 13 .8 67. .6 8. 9 9. 3 1.7

F wt% Na ₂ W0 ₄ Si0 ₂ ; Pluronic 850 39.3 98. 9 30 .0 72. .9 9. 0 21 .9 0.5

(23.8 mg) P-123 template 850 145.2 91. 1 27 .5 74. .9 13 .9 20 .6 0.5

850 228.9 90. 9 29 .3 73. .0 14 .2 21 .4 0.5

Sample Sample Support Temp . Time o2 XcH Sc2+ S _co Yc2+ c ₂ /c ₂₌

(°C) ( )

(total (%) (%) (%) (%) (%) ratio catalyst

weight)

750 11.4 32. 4 5. 3 29. .3 15 .2 1. 6 5.4

5 wt% Mn/5 In situ rap- 800 27.2 66. 6 16 .3 63. .1 9. 2 10 .3 1.4

G wt% Na ₂ W0 ₄ Si0 ₂ ; Pluronic 850 40.5 100 .0 28 .3 71. .6 8. 9 20 .2 0.6

(25.9 mg) P-123 template 850 146.5 97. 5 27 .8 74. .8 12 .3 20 .8 0.5

850 222.3 97. 2 29 .7 74. .4 13 .1 22 .1 0.5

750 9.2 26. 2 5. 6 51. .2 12 .1 2. 9 4.1

2 wt% Mn/10 In situ rap- 800 30.1 56. 2 16 .2 76. .0 7. 7 12 .3 1.2

H wt% Na ₂ W0 ₄ Si0 ₂ ; stearic 850 97.3 97. 1 28 .9 75. .7 11 .1 21 .9 0.5

(30.4 mg) acid template 850 146.8 97. 1 27 .5 76. .0 12 .5 20 .9 0.5

850 227.7 96. 5 29 .0 74. .9 13 .5 21 .7 0.5

750 10.6 18. 8 4. 0 57. .7 10 .0 2. 3 5.6

2 wt% Mn/10 In situ rap- 800 29.0 40. 1 13 .1 77. .1 8. 0 10 .1 1.5

J wt% Na ₂ W0 ₄ Si0 ₂ ; Pluronic 850 39.7 100 .0 29 .3 74. .4 10 .5 21 .8 0.5

(27.8 mg) P-123 template 850 145.5 95. 0 28 .6 71. .9 13 .1 20 .5 0.5

850 229.2 94. 5 28 .6 70. .0 15 .4 20 .1 0.5