WO2017001579A1

WO2017001579A1 - Catalyst and process for the oxidative coupling of methane

Info

Publication number: WO2017001579A1
Application number: PCT/EP2016/065340
Authority: WO
Inventors: Hendrik Dathe; Henri Preising; Andrew David Horton; Yvon Maria KRUITHOF
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2015-07-01
Filing date: 2016-06-30
Publication date: 2017-01-05

Abstract

The invention relates to a catalyst composition comprising manganese, one or more alkali metals and tungsten on a titanium-containing carrier, wherein the carrier has a B.E.T. surface area of greater than 90 m²/g; and a process for the oxidative coupling of methane using said catalyst composition.

Description

CATALYST AND PROCESS FOR THE OXIDATIVE COUPLING OF

METHANE

Field of the Invention

The present invention relates to 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 may further react into saturated and unsaturated hydrocarbons having 3 or more carbon atoms, including propane, propylene, butane, butene, etc.

Therefore,—usually, the gas stream leaving an OCM process contains a mixture of water, hydrogen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, propane, propylene, butane, butene and saturated and unsaturated hydrocarbons having 5 or more carbon atoms .

In general, the conversion 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 low. As a result, a relatively 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, which are also present in such gas streams.

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.

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

One of the best-performing catalysts that has been found to date in the OCM field comprises manganese, tungsten and sodium on a silica carrier (Mn-Na₂W0₄/Si0₂ ) .

Chua et al . studied the oxidative coupling of methane for the production of ethylene over sodium- tungsten-manganese-supported silica catalyst (Na-W- Mn/Si0₂) in Applied Catalysis A: General 343 (2008) 142- 148.

The performance of Mn-Na₂W0₄/Si02 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.

US 6596912 Bl describes a two-stage process for the conversion of methane to C4+ hydrocarbons, wherein in the first stage, methane and oxygen are reacted in an oxidative coupling reactor over a Mn-Na₂W0₄/Si0₂ catalyst at 800 °C to convert the methane to ethylene and in the second stage the ethylene product is oligomerised to higher hydrocarbons.

Work in the OCM field has focussed 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 . conduct statistical analysis of past catalytic data on oxidative methane coupling for new insights into the composition of high-performance catalysts.

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

US 2014/0080699 Al describes a specific method for the preparation of catalysts such as Mn-Na₂W0₄/Si02 catalyst which is said to provide an improved catalyst material .

Various manganese and titanium-containing catalysts for the oxidative coupling of methane are researched in the literature and are disclosed in various patent publications including Gong et al . Catalysis Today 24 (1995), 259-261, Gong et al . Catalysis Today 24 (1995), 263-264, Jeon et al . Applied Catalysis A: General 464-465 (2013) 68-77, US 4769508 A and US 2013/0178680 Al .

US 2013/017868 Al for example describes a catalyst for oxidative coupling of methane, comprising a titanium- containing carrier and sodium tungstate and manganese oxide (Na₂W0₄ and Mn₂0₃) , and having maximum C₂

hydrocarbon selectivity and yield of about 46% and 17%, respectively.

US 5053578 and US 5130286 describe catalytic compositions comprising a Group IA metal, a Group IIA metal and a third component, with a preferred surface areas of less than about 25 m²/gram.

It is highly desirable in the OCM field to develop further catalysts for the oxidative coupling of methane which not only exhibit the selectivity benefits of catalysts comprising manganese, tungsten and sodium on a silica carrier, but which also show improvement in the space time yield, thereby allowing the development of smaller reactor sizes for fixed catalyst beds.

Summary of the Invention

The present invention has surprisingly found that catalyst compositions comprising manganese, tungsten and one or more alkali metals on a specific titanium- containing carrier not only have advantageous C2+ selectivity, but that said catalysts also demonstrate beneficial space time yields during the oxidative coupling of methane.

Accordingly, in a first aspect of the present invention there is provided a catalyst composition comprising manganese, one or more alkali metals and tungsten on a titanium-containing carrier, wherein the carrier has a B.E.T. surface area of greater than 90 m²/g .

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.

Brief Description of the Drawings

Figure 1 is a schematic diagram showing a typical reactor set-up for oxidative coupling of methane.

Figure 2 shows the space time yield results obtained for the various catalysts tested at 750 °C.

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 (CH₄) conversion" means the mole fraction of methane converted to product (s) .

"Cx selectivity" refers to the percentage of converted reactants that went to product (s) having carbon number x and "Cx+ selectivity" refers to the percentage of converted reactants that went to the specified product (s) having a carbon number x or more. Thus, "C2 selectivity" refers to the percentage of converted methane that formed ethane and ethylene. Similarly, "C2+ selectivity" means the percentage of converted methane that formed compounds having carbon numbers of 2 or more.

"Cx yield" is used to define the percentage of products obtained with carbon number x relative to the theoretical maximum product obtainable. The Cx 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. "C2 yield" refers to the total combined yield of ethane and

ethylene. The Cx yield may be calculated by multiplying the methane conversion by the Cx selectivity.

"Space time yield Cx" refers to the volume of products having carbon number x formed per volume of the reactor and time. As used herein in the context of catalyst dopants, "weight percent" refers to the ratio of the total weight of the carrier, the metal-containing dopant or the metal in the dopant to the total weight of the catalyst composition the catalyst. 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 120 to 150 °C.

Percentages of metals from the metal-containing dopants in the catalyst composition may be determined by XRF as is known in the art. The metals content of 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 not to exceed 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.

The titanium-containing carrier used in the catalyst composition of the present invention may be selected from titanium oxides and mixed titanium oxides, such as

TiSi0₄.

In a preferred embodiment of the present invention, the titanium-containing carrier is a titanium oxide.

Preferably, said titanium-containing carrier comprises titanium (IV) oxide (titania) (Ti0₂) . In a more preferred embodiment, the titanium-containing carrier is titanium (IV) oxide. Titanium (IV) oxide may be present in the carrier as rutile and/or anatase.

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

titanium-containing carrier prior to doping with

manganese, tungsten and one or more alkali metals.

The titanium-containing carrier used in the catalyst composition of the present invention has a B.E.T. surface area of greater than 90 m²/g, preferably in the range of from 90 to 350 m²/g, more preferably in the range of from 100 to 300 m²/g and most preferably in the range of from 200 to 300 m²/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 material is measured by drying the catalyst, measuring the weight and adding water until all pores are filled. After centrifuging the excess water off the remaining amount is measured by weight and the total pore volume determined.

The total pore volume of the titanium-containing carrier used in the catalyst composition of the present invention is not narrowly critical and may be, for example, at least 0.9 cm³/g. Preferably, said carrier has a pore volume greater than 1.1 cm³/g, more preferably at least 1.2 cm³/g. The pore volume is typically at most 1.8 cm³/g, or, preferably, at most 1.42 cm³/g.

Generally, as the total pore volume of a carrier increases, the ability to deposit catalytic material on the carrier increases. However, at higher total pore volumes, the titanium-containing carrier, or a catalyst composition comprising the carrier, may have lower crush strength or attrition resistance.

The titanium-containing carrier 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, relative to the total weight of the catalyst composition.

Optionally, the catalyst composition of the present invention may further comprise one or more additional carriers therein selected from silica, alumina and zirconia . 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 a preferred embodiment 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_xO_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 Mn0₂, Mn₂0₃, Mn₃0₄ and mixtures thereof.

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

Particularly preferred alkali metals are lithium and sodium .

The one or more alkali metals are preferably each present in an amount of in the range of from 0.1 to 1.5 % by weight, more preferably in the range of from 0.4 to 1.2 % by weight, and most preferably in the range of from 0.4 to 0.9 % by weight, relative to the total weight of the catalyst composition.

The catalyst composition of the present invention further comprises 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 the preparation of the catalyst composition of the present invention, the one or more alkali metals and tungsten may be doped as separate metals and/or metal- containing compounds into said composition. However, in a preferred embodiment of the present invention, 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, tungsten 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 Na₂W0₄,

Na₂W₂0₇, Mn₂0₃ and MnW0₄. Preferably, the catalyst

composition of the present invention comprises one or more of Na₂W0₄, Na₂W₂0₇, Mn₂0₃ and MnW0₄.

During the preparation of the catalyst composition of the present invention, the specific form in which the manganese-containing dopant, the alkali metal-containing dopants, the tungsten-containing dopant and any optional co-promoters and/or additional metal-containing dopants are provided is not limited, and may include any of the wide variety of forms known.

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

Generally, suitable compounds are those which can be solubilized in an appropriate solvent, such as a water- containing solvent .

As will be appreciated by persons skilled in the art, while specific forms of the afore-mentioned metal- containing dopants 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-containing dopant, the alkali metal-containing dopants, the tungsten-containing dopant and/or any optional co-promoters and/or additional metal-containing dopants that may ultimately exist on the catalyst composition during use.

Additionally, it should be understood that while a particular compound may be used during catalyst

preparation (e.g., in an impregnation solution), it is possible that the counter ion added during catalyst preparation may not be present in the finished catalyst composition .

As previously discussed, the specific form in which the one or more alkali metals is provided is generally not limited, and may include any of the wide variety of forms known. For example, the one or more alkali metal- containing dopants may be provided as ions (e.g., cation), or as alkali metal compounds.

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 alkali metal-containing dopant 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 catalyst composition of the present invention may in principle be prepared by any suitable technique known in the art for similar catalyst compositions.

Thus, methods such as precipitation, co- precipitation, impregnation, granulation, spray-drying or dry-mixing can be used.

The manganese-, tungsten- and alkali metal- containing dopants can be composited or associated with the titanium-containing carrier by any of the methods associated with the preparation of supported catalyst compositions known in the art.

Such "supported" compositions may be prepared by methods such as adsorption, impregnation, precipitation, co-precipitation, granulation, spray drying, or dry mixing .

One suitable method of preparation is to impregnate the titanium-containing carrier with solutions of compounds of the manganese, alkali metals and tungsten.

After impregnation, the impregnated carrier is dried to remove solvent and the dried solid is then calcined, preferably in air.

Calcination may take place at a temperature in the range of from 700 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.

The process of the present invention comprises utilising the catalyst composition as hereinbefore described in a reactor suitable for the oxidative coupling of methane.

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 one embodiment 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 in 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, C0₂, H₂ and H₂0) . 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 a preferred embodiment, the methane and oxygen added to the reactor as mixed feed, optionally comprising further components therein, at the same reactor inlet. However, in another embodiment 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 a preferred embodiment, 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.

Figure 1 is a schematic representation showing a typical reactor and product separation set-up for the oxidative coupling of methane.

Feed gas comprising methane and oxygen (or air) is introduced into the OCM reactor 101, via lines 107 and 108, respectively. The methane may consist of fresh feed and recycled methane (derived from the separation stage of the process) . The product mixture exiting the OCM reactor is passed to condensation vessel 102, where the majority of the water by-product of OCM is removed. The product from 102 is then sent to the separation section 103, wherein the desired C2+ hydrocarbons are separated

(stream 104 ) , either as a mixed hydrocarbon stream or as separated streams of ethylene, ethane, propylene and other hydrocarbons . Unreacted methane separated from the OCM product mixture in 103 may optionally be recycled, as stream 106, which is combined with fresh feed stream 107, before entering the reactor. Undesired products of OCM, such as CO and C0₂, as well as N₂ in the case of OCM with air feed, are also separated from the product mixture in 103 and leave the process as stream 105. The separation section may also include a section for conversion of alkanes to olefins (e.g. ethane cracker) .

The processes of the present invention is not limited to any particular reactor or flow configurations, and those depicted in Figure 1 are merely exemplary.

Furthermore, the sequence in which various feed

components are introduced into the process and their respective points of introduction, as well as the flow connections, may be varied from that depicted in Figure

1.

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 C2+ hydrocarbons at a reactor temperature 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 900 °C, more preferably in the range of from 650 to 850 °C, even more preferably in the range of from 650 to 800 °C and most preferably in the range of from 700 to 775 °C.

In a preferred embodiment of the present invention, the conversion of methane to one or more C2+ 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 of the reactor feed divided by the catalyst bed volume at standard conditions. Preferably, said gas hourly space velocity is in the range of from 10000 to 300000 h^-1, and most preferably in the range of from 20000 to 70000 h^"1.

In a preferred embodiment, the process of the present invention has a C2+ hydrocarbon selectivity of greater than 40 %, and most preferably greater than 60 %.

In a preferred embodiment, the process of the present invention has results in an ethane : ethene weight ratio of less than 1.6 , and most preferably less than 0.6.

Preferably, the afore-mentioned C2+ hydrocarbon selectivity, C2 hydrocarbon selectivity and ethane : ethene ratio values are determined at a temperature of less than 850 °C, and most preferably at a temperature in the range of from 750 to 800 °C.

The invention is further illustrated by the

following Examples .

Examples and Comparative Example

Catalyst Preparation Procedure

A catalyst composition comprising 2 wt . % Mn, 5.0 wt . % Na₂W0₄ on a titanium carrier having a B.E.T.

surface area of 33 m²/g was prepared by the following method : -

(NH₄) 2 (C₂0₄) . H₂0 (37.3 mmol, 5.31 g) and anhydrous citric acid (11.1 mmol, 2.14 g) were dissolved in 20 ml H₂0 at 75°C. To 4 ml of this solution, Na₂W0₄.2H₂0 (1.85 mmol, 0.61 g) was added. To this solution Mn (N0₃) ₂.4H₂0 (4.05 mmol, 1.017 g) was added.

Under vigorous stirring, 2.0 ml of HN0₃ 65 vol. /vol. % (48 mmol, 3.02 g) was added to the suspension.

From this metal stock-solution, 3.22 ml was added on top of 5.37 g pre-dried Ti0₂ (lh at 300 °C) .

Rolling: 23h at a slow rotation speed on a roller bench . Calcination took place with the following

temperature regime: 6h 120 °C (ramp 120 °C/h) , 6h 500 °C (ramp 253 °C/h) , 8h 850 °C (ramp 175 °C/h) .

The material was then cooled to room temperature.

The obtained material was than pressed and sieved in 30 - 60 mesh to be tested.

Further catalyst samples for testing were prepared in an analogous fashion using different carrier samples as detailed in Table 1 below.

Carrier samples used in Table 1 were all

commercially available carriers .

Table 1

General Testing Procedure

Each of the afore-mentioned catalyst compositions in Table 1 were tested in accordance with the following general testing procedure.

The active test was carried out in a quartz fixed- bed microreactor with an isothermal zone of 4 cm and internal diameter (i.d.) of 2 mm.

Typically, 50 g of the catalyst composition to be tested was loaded in the reactor filled with a solid quartz tube in rest space of the reactor to minimise the contribution from any gas-phase reactions . The reagents of CH₄ (>99.9 %) and 0₂ (99.9 %) were used without further purification. The reactor feed comprised methane and oxygen in a mole ratio of 4:1, with 5 mol . % nitrogen as inert gas.

The catalyst composition was evaluated at 700 °C, 725 °C, 750 °C and 800 °C, and 3.5 barg (350 kPa) pressure with a flow of 4.8 Nl/h.

The reaction products were then analyzed with an on^¬ line GC device equipped with a 2 TCD and 2 FID using different columns for the separation of CH₄, C0₂, C₂H_X, C₃H_X, C₄H_X, CsH_x, O₂, ₂, CO.

The conversion, selectivities , yields and space time yields were all calculated for the products identified. Results

Results of catalyst testing are tabulated in Tables 2-5 below.

In addition, Figure 2 also plots the results obtained at 750 °C.

Testing at 700 °C

Table 2

(II) Testing at 725 °C

Table 3

(Ill) Testing at 750 °C

Table 4

(IV) Testing at 800 °C

Table 5

Discussion

In industrial OCM processes, it is highly desirable to utilise catalysts which not only exhibit similar selectivity benefits to known OCM catalysts comprising manganese, tungsten and sodium on a silica carrier, but which also show attractive space time yields, thereby allowing the development of smaller reactor sizes.

As is summarized in Table 2-5, the catalysts comprising manganese, one or more alkali metals and tungsten formulated on a titanium-containing carrier show not only advantageous C2+ selectivity, but also a surprising increase in CH₄ conversion with increasing carrier surface area.

Furthermore, surprisingly, high C2+ space time yields can be achieved with the catalysts according to the present invention. Indeed, when compared to Catalyst E, Catalyst D demonstrates similar or better C2+ space time yields. At temperatures of 750 and 800 °C, Catalyst C also displays advantageous C2+ space time yields as compared to Catalyst E.

At 700 °C, it is apparent that the Catalysts C and D according to the present invention demonstrate greater methane conversion than comparative Catalysts A and B, whilst also displaying similar selectivities to the reference Catalyst E.

Similarly, at 725 °C, Catalysts C and D according to the present invention demonstrate greater methane conversion than comparative Catalysts A and B.

It is apparent that at temperatures of 750°C and higher, the catalysts of the present invention offer a significant advantages over the comparative Catalysts A and B demonstrating higher methane conversions and also good C2+ selectivities and C2+ space time yields. The performance of Catalysts C and D at temperatures of 750 °C and higher is in the direction of the

performance of Catalyst E under similar conditions.

The present invention surprisingly allows for the application of catalyst compositions comprising

manganese, one or more alkali metals and tungsten on a titanium-containing carrier with increased overall product yields .

Claims

C L A I M S

1. A catalyst composition comprising manganese, one or more alkali metals and tungsten on a titanium-containing carrier, wherein the carrier has a B.E.T. surface area of greater than 90 m²/g.

2. Catalyst composition according to Claim 1, wherein the titanium-containing carrier is selected from titania and titanium silicate.

3. Catalyst composition according to Claim 1 or 2, wherein the carrier has a B.E.T. surface area in the range of from 90 to 350 m²/g.

4. Catalyst composition according to Claims 1 to 3, wherein the carrier has a B.E.T. surface area in the range of from 100 to 300 m²/g.

5. Catalyst composition according to Claims 1 to 4, comprising one or more of Na₂W0₄, Na₂W₂0₇, Mn₂0₃ and MnW0₄.

6. Catalyst composition according to Claims 1 to 5, wherein the carrier is selected from rutile and anatase.

7. 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 a catalyst composition according to Claims 1 to 6.

8. Process according to Claim 7, wherein the conversion is carried out at a reactor temperature in the range of from 700 to 800 °C.

9. Process according to Claim 7 or 8, wherein the conversion is carried out at a reactor temperature in the range of from 750 to 800 °C.