Method for preparing a catalyst for conversion of methane to higher hydrocarbons and for dehydrogenating hydrocarbons

Abstract: A method for preparing a catalyst for conversion of methane to higher hydrocarbons, discloses the steps of: forming a mixture of a manganese aqueous solution and an oxidative agent aqueous solution in an inorganic oxide support material; varying a pH value of the mixture between acidic and basic values to obtain a precipitate of manganese oxide; drying the manganese oxide precipitate; impregnating the dried manganese oxide precipitate with a promoter to obtain an impregnated solid; drying the impregnated solid; and calcining the impregnated solid until a surface atomic ratio of promoter to manganese is at least 5. The catalyst so acquired may be contacted with a methane gas to obtain stable conversion values over extended reaction times.

Description: The present invention relates to a process for converting methane gases to high hydrocarbons, a process for dehydrogenating hydrocarbons and a method for preparing a catalyst for use in such processes. Methods for conversion of methane to high hydrocarbons are typically carried out using metal-based catalysts. Among the known conversion processes, catalysts based on manganese, magnesium and lead using alkali metal promoters have given good results in synthesis of methane to obtain ethane/ethylene type products. Among these catalysts used in such processes, catalysts based upon oxides of manganese have been extensively studied and described. U. S. Patent No. 4,499,322 to Jones et al. discloses a process of methane conversion using an oxidative synthesizing agent containing a promoting amount of alkali metal and/or compounds thereof. U. S. Patent No. 4,547,608 discloses the use of a support material for a catalyst which comprises alkali earth metal oxides and silicon oxides, alumina and mixtures thereof. EP-A-0316075 discloses a process for conversion of methane to higher hydrocarbons using a catalyst comprising manganese oxides which incorporate at least one of the elements tin, titanium, tungsten, tantalum, silicon, germanium, lead, phosphorus, arsenic, antimony, boron, gallium, indium, a lanthanide or an actinide. The method for preparation of this catalyst is disclosed as comprising the steps of calcining the manganese dioxide at 1000°C for 16 hours to obtain Mn 3 O 4 , which is impregnated with a solution of sodium chloride and finally diluted with fines of silica. EP-A-0365181 discloses a process for preparation of a cogel catalyst for use in conversion of methane to higher hydrocarbons. The disclosed process involves the mixing of an aqueous solution of a soluble salt of an alkali or alkaline earth metal with a solution of a soluble compound of a metal thermally decomposable to a metal oxide. A solution in a hydroxylic solvent of a hydrolyzable silane is then added to this mixture, and this mixture is then maintained under such conditions that the mixture forms a cogel, while avoiding the formation of precipitates or particles. This cogel is then dried and calcined to obtain the desired catalyst. While the above-described catalysts obtain satisfactory initial conversion rates of methane to higher hydrocarbons, this initial activity decreases rapidly during reaction time. US-A-4517398 discloses a method for converting methane to higher hydrocarbons which comprises contacting a gas comprising methane with a contact agent comprising (a) at least one reducible oxide of at least one metal, (b) a support comprising a magnesia derived from magnesium hydroxide or from a magnesium-containing component contacted with hydroxyl-containing material, and wherein the support (b) has been calcined prior to the addition of the reducible metal oxide (a). US-A-4769508 discloses a method for converting methane to higher hydrocarbon products by contacting a gas comprising methane with a catalyst comprising (a) a reducible oxide of Mn, (b) at least one alkali metal or compounds thereof, and (c) at least one member of the group consisting of oxides of Ti, mixed oxides of Ti and Si, mixed oxides of Ti and at least one alkaline earth metal. In a preferred embodiment the ratio of alkali metal: Mn in the catalyst is within the range of about 0.01 to 10:1. Accordingly, it is the principal object of the present invention to provide a method for preparing a catalyst which possesses high conversion rates which do not rapidly decrease during reaction time. It is a further object of the present invention to provide a process for the conversion of methane to higher hydrocarbons which utilizes a catalyst prepared according to the present invention, to yield enhanced results in the conversion process. Other aspects, objectives and advantages of the present invention will be apparent to those skilled in the art after a consideration of the accompanying detailed description of the invention and figures which are referred to therein. Summary of the invention Subject matter of the present invention is a method for preparing a catalyst for conversion of methane to higher hydrocarbons according to Claim 1. Preferred embodiments thereof are subject matter of Claims 2 to 15. Subject matter of the present invention is also a process for converting methane to higher hydrocarbons according to Claim 16. Preferred embodiments thereof are subject matter of Claims 17 and 18. A further subject matter of the invention is a process for dehydrogenating hydrocarbons according to Claim 19; preferred embodiment of this process is subject matter of Claim 20. The method for preparation of a catalyst for the conversion of methane to higher hydrocarbons comprises the steps of: forming a mixture of a manganese aqueous solution and an oxidative agent aqueous solution in an inorganic oxide support material; varying a pH value of the mixture between acidic and basic values to obtain a precipitate of manganese oxide; drying the manganese oxide precipitate; impregnating the dried manganese oxide precipitate with an alkali metal promoter to obtain an impregnated solid; drying the impregnated solid; and calcining the impregnated solid until a surface atomic ratio of promoter to manganese is at least 5. The catalytic agent is preferably supplied in the form of an aqueous solution of manganese chloride. The oxidative agent is preferably a chlorate, hypochlorite, thiosulfate, or bromate of an alkali metal. The inorganic oxide support material is preferably selected from oxides of silicon, zirconium and/or magnesium. The promoter which is impregnated into the dried manganese oxide precipitate is preferably any alkali metal halide, and most preferably sodium chloride. A process is also disclosed, according to the present invention, whereby the catalyst of the present invention is employed in the conversion of methane to higher hydrocarbons. This process comprises the steps of providing a methane gas; providing a catalyst based on manganese oxide and impregnated with an alkali metal promoter, wherein a surface atomic ratio of promoter to manganese oxide is at least 5; and contacting the methane gas with the catalyst. The improved stability of the catalyst prepared according to the disclosed method is believed to result from the high surface ratio of promoter to manganese which is more clearly set forth in the detailed description and examples to follow. BRIEF DESCRIPTION OF THE DRAWINGS A detailed description of the invention follows, with reference to the accompanying drawings, wherein: Figure 1 is a graph showing the selectivity of catalysts prepared according to the present invention, as compared to a prior art catalyst; Figure 2 is a graph showing the selectivity, conversion, yield and ratio C 2 =/C 2 of a catalyst prepared according to the present invention, as disclosed in Example 1 hereafter; Figure 3 is a graph showing the selectivity, conversion, yield and ratio C 2 =/C 2 of a catalyst prepared according to the present invention, as disclosed in Example 2 hereafter; and Figure 4 is a graph showing the selectivity, conversion, yield and ratio C 2 =/C 2 of a catalyst prepared according to the present invention, as disclosed in Example 3 hereafter. DETAILED DESCRIPTION The disclosed invention pertains to a method for preparing catalyst for use in conversion of methane to higher hydrocarbons. In accordance with the invention, a mixture is made of a manganese aqueous solution and an oxidative agent aqueous solution in an inorganic oxide support material. The catalytic agent aqueous solution is preferably an aqueous solution of manganese chloride. The oxidative agent is preferably a chlorate, hypochlorite, thiosulfate, or bromate of an alkali metal. Most preferably, potassium chlorate is used as the oxidative agent contained in the oxidative agent aqueous solution. The inorganic oxide support material is preferably based on oxides of silicon, zirconium and/or magnesium. In an alternate embodiment of the invention, and when silicon oxide is used as the support material, the inorganic oxide is calcined prior to its addition to the mixture of manganese and oxidative agent. The mixture formed of the above components is then treated to obtain a manganese oxide precipitate. This precipitate is preferably obtained by varying the pH value of the mixture between acidic and basic values until the desired manganese oxide precipitate is obtained. The proper manipulation of the pH of the mixture can be obtained through the addition of, for example, ammonium hydroxide to obtain a basic value pH, and by the addition of, for example, nitric acid to obtain the desired acidic values. During the additions of these pH manipulating agents, the mixture should be continuously stirred. Once a desired amount of solid manganese oxide precipitate is obtained, the solid is evaporated to eliminate water surplus, and then dried in a furnace, preferably at a temperature of 100°C for a period of 4 hours. This solid is then impregnated with an alkali metal promoter to improve the catalytic activity of the obtained material. Promoters which are particularly suitable to the present invention include those selected from a group of alkali metal halides, and most preferably, sodium chloride. This promoter is impregnated into the solid in a solution, using techniques which are known in the art. The impregnated solid is then dried, preferably at a temperature in the range of 100-140°C, and the dried solid is then calcined at a temperature in the range of 300-1000°C, and more preferably in the range of 600-800°C. For reasons that are not known, the resulting catalyst, when subjected to suitable testing, exhibits a surface atomic ratio of promoter to manganese which is higher than those of catalysts prepared according to methods of the prior art. Further, catalysts obtained according to the method of the present invention exhibit a greatly enhanced stability of conversion over periods of reaction time. According to an alternate embodiment of the invention, in the method wherein silica is used as the support material, the precipitated manganese oxide may be calcined prior to its impregnation at a temperature in the range of 900-1200°C, and most preferably at a temperature of 1000°C. The aforementioned surface ratio of promoter to catalytic agent is preferably measured by using X-ray photoelectron spectroscopy (XPS). Signals generated by such spectroscopy allow the determination and estimation of the amount of materials which are present at the surface of the catalyst, and which are, accordingly, responsible for catalytic activities possessed by the catalyst. XPS results in excitement of the atoms of the material being measured. The energy spectrum of the electrons emitted by photo emission are then measured. The kinetic energy with which the electrons are liberated is such that only those that are near the surface of the catalyst (within a distance of less than 100 angstroms) escape from the material to be detected. Thus, this method gives an accurate measure of the composition of materials at the surface of the catalyst. As will be more clearly demonstrated in the following examples, catalysts prepared according to the prior art processes in general possess a relatively low ratio of promoter to manganese at the surface, as measured by XPS techniques, and the ratio of the prior art catalysts decreases with use. Also as clearly shown in the following examples, however, catalysts prepared according to the present invention exhibit ratios of promoter to manganese which are in the range of at least 5, and these ratios either increase, or remain relatively constant, over periods of use. Accordingly, a method is described whereby catalysts can be produced having high ratios of promoter to catalytic agent located at the surface of the catalyst, which catalysts exhibit great stability in conversion rates over extended periods of use. The catalyst produced according to the invention may preferably be used in a process of conversion of methane to higher hydrocarbons, wherein the methane to be converted may be either pure methane or a methane-containing gas. Thus, the methane may be mixed with a molecular oxygen containing oxidant gas such as oxygen, air or an air/oxygen mixture. In this instance, it is preferable to employ methane and oxygen in a ratio by volume in the range of 2-10:1 of methane to oxygen. Inert diluents may also be present in the methane to be treated, such as argon, helium and nitrogen. The treatment process comprises the steps of providing the desired methane or methane gas mixture, preparing a catalyst having a surface ratio of promoter to catalytic agent of at least 5, according to the aforesaid method, and contacting the methane gas with the catalyst at a temperature in the range of 500-1000°C, and more preferably in the range of 600-900°C. The gas is preferably supplied to the catalyst at a space velocity measured at standard temperature and pressure in the range of 100-100,000 h -1 , and more preferably at a space velocity of 300-6,000 h -1 . This process is also preferably carried out in a continuous manner. The conversion rates obtained by this process are demonstrated more clearly in the examples which follow. Example No. 1 A starting solution of MnCl 3 4H 2 O (14.43 g in 20 ml of H 2 O) and KClO 3 (4 g in 40 ml of H 2 O) was added to 19 g of silica which was previously calcined (1000°C, 16 h) and suspended in 40 ml of H 2 O, at environmental temperature and maintaining continuous stirring. NH 4 OH was introduced into the mixture to obtain a basic solution. HNO 3 was then added, during continuous stirring, to obtain an acidic solution. As a result of the manipulation of the pH of the solution, a solid manganese oxide precipitate was obtained. This solid was evaporated to eliminate water surplus, and then dried in a furnace at 100°C for a period of 4 hours. The dried solid was then calcined at a temperature of 1000°C for a period of 16 h. This calcined solid was then impregnated with a solution of sodium chloride (18 g in 80 ml of H 2 O) during a period of 2 hours of constant stirring. The impregnated solid was then dried in a furnace at 100°C for 4 hours, and calcined at 700°C for 16 hours. Example No. 2 A catalyst was prepared following the identical method as described in Example 1, but omitting the step of calcining the dried solid prior to the impregnation step. Example No. 3 A catalyst was prepared in an identical procedure as that described in Example 1, except that the silicon oxide had not previously been calcined, and the dried solid was also not calcined prior to its impregnation. Example No. 4 A catalyst was prepared using a procedure identical to that described in Example No. 3, but the support material was zirconium oxide. Example No. 5 A catalyst was prepared following a procedure identical to that described in Example No. 3, except that the support material was magnesium oxide. Table I contains the results obtained from evaluation of the catalysts prepared according to Examples 1-5 above, regarding their conversion of methane to ethylene/ethane products. Example T(°C) % onversion C % Selec. C 2 % Yield C 2 C = 2 /C 2 CH 4 O 2 1 720 25 92 53 13 3.2 740 32 100 59 19 5.6 2 720 35 100 59 21 4.0 740 33 100 61 20 5.5 3 720 34 100 62 21 5.7 740 35 100 60 21 5.8 4 740 25 100 57 15 2.1 5 740 21 100 38 8 1.2 Operating conditions: 600 h -1 , CH 4 /O 2 = 4, atmospheric pressure, 1cc of catalyst. T(°C) temperature of catalytic bed. Comparative Example A catalyst was prepared according to the method disclosed in afore-described European Patent Application No. 0316075-A1. MnO 2 was calcined at 1000°C for 16 hours in order to obtain Mn 3 O 4 , which was then impregnated with a solution of NaCl. This substance was then diluted with a finely divided silica. Table II below contains the results obtained for evaluation of this catalyst for the conversion of methane. Catalyst EC T(°C) % Conversion CH 4 % Selec. C 2 % Yield C 2 C = 2 /C 2 NaCl/Mn 3 O 4 /SiO 2 720 29 59 17 3.4 Operating conditions: 1200 h -1 , CH 4 /O 2 = 4, atmospheric pressure, 5 cc of catalyst. T(°C) = temperature of catalytic bed. The catalyst prepared according to the teachings of the prior art, as well as the catalysts prepared in accordance with Examples 1-5 above, were then tested with X-ray photoelectron spectroscopy (XPS) in order to evaluate the ratio of promoter to catalytic agent at the surface of these materials. The studies were carried out using a Leybold LHS-10 surface analysis apparatus using a radiation energy of 1253.6 eV, provided through a magnesium cathode with a power of 300 watts. The X-ray source was operated through a constant and sufficiently high power to assure high intensity of the signal without saturation of the detector (less than 120,000 cps). The accumulated data were obtained by using a pulse counting system which is controlled by a computer, with intervals of energy of 0.2 eV. Energy sweeps were conducted until an adequate signal/noise ratio was reached. The procedure was conducted using a gap opening of 1 millimeter with a constant absolute resolution employing an energy step of 200 eV. The ratio of promoter to catalytic agent on the surface of the catalyst was determined by measuring the corrected signal area of the promoter to the catalytic agent. Sensitivity factors were applied to this data to enable the surface atomic ratio to be evaluated. The results obtained by the XPS testing of the catalysts prepared according to the invention in Examples 1-5 and of the prior art catalyst are shown in Table III hereinbelow, wherein % of Na and Mn are shown at the surface of the catalyst, as well as the ratio of Na to Mn. CATALYST % Na % Mn Na/Mn Previous art before reaction 2.3 0.6 3.8 Previos art after reaction 3.0 1.1 2.7 Example 1 before reaction 10.6 1.9 5.6 Example 1 after reaction 15.6 1.3 12.0 Example 2 before reaction 6.6 1.3 5.1 Example 3 after reaction 14.2 2.4 5.9 Example 3 before reaction 7.3 1.1 6.6 Example 3 after reaction 6.1 1.1 5.5 Example 4 before reaction 12.7 2.4 5.3 Example 4 after reaction 14.0 2.5 5.6 Example 5 before reaction 12.4 2.3 5.4 Example 5 after reaction 12.2 2.4 5.1 The percentage of Na located at the surface of these catalysts was also evaluated per the following table and equation: As can be seen, the disclosed method also results in a larger percentage of total Na being deposited on the surface of the catalyst than in the prior art. The catalysts prepared according to the procedure of Examples 1, 2 and 3 above were tested for their conversion of methane to higher hydrocarbons. 1 cc of each catalyst was introduced in a quartz reactor of 20 cm length and 5 mm diameter. The reactor was brought to a temperature in the range of 720-740°C. A methane/oxygen mixture at a ratio of methane to oxygen of 4:1 was contacted with the catalyst at a space velocity of 600 h -1 and at atmospheric pressure. These operating conditions were maintained constant for all tests. The results for the catalyst according to Example 1 are indicated below in Table IV. Example No. 1 t 1 (min) % Conversion % Sel. C 2 % Yield C 2 C = 2 /C 2 CH 4 O 2 90 34 99 60 20 5.4 126 34 99 59 20 5.3 153 33 99 59 19 5.2 172 33 99 57 19 5.2 1200 32 99 57 18 4.7 1229 32 99 57 18 4.7 1263 32 99 57 18 4.7 1291 32 99 57 18 4.6 1359 31 99 57 18 4.6 1499 31 99 57 18 4.5 1579 31 99 57 18 4.5 2573 29 90 57 16 4.0 2611 28 90 57 16 4.0 2635 28 89 57 16 3.9 The results for the catalyst of Example 2 are incorporated below in Table V. Example No. 2 t 1 (min) % Conversion % Sel. C 2 % Yield C 2 C = 2 /C 2 CH 4 O 2 45 35 100 59 21 4.0 75 35 100 59 21 3.9 100 35 100 51 21 3.9 135 35 100 59 21 4.0 1200 30 77 60 18 2.8 1223 30 79 60 18 2.9 1246 30 78 60 18 2.9 The results of the testing of the catalyst according to Example 3 are contained in Table VI. Example No. 3 t 1 (min) % Conversion % Sel. C 2 % Yield C 2 C = 2 /C 2 CH 4 O 2 100 31 100 59 18 4.7 145 31 100 59 18 4.7 200 31 100 58 18 4.8 240 30 96 57 17 4.7 270 31 95 56 18 4.5 1170 31 92 52 16 3.4 1270 29 92 52 16 3.4 1480 29 92 54 16 3.4 1560 28 91 54 15 3.3 1610 28 92 55 15 3.3 With reference to Figure 1, the ethylene/ethane selectivity of the prior art catalyst and the catalysts prepared according to Examples 1-3 are graphically illustrated. As can be observed, catalysts prepared according to the present invention reach their maximum selectivity at the beginning of the reaction and then, unlike the prior art catalyst, maintain a substantially stable activity during the entire reaction time. It is also observed that the highest apparent stability is that exhibited by the catalyst of Example 1, which also exhibited the highest ratio of promoter to catalytic agent. A correlation is therefore indicated between the ratio and catalytic stability. The catalyst of the present invention is also useful in Oxidative Dehydrogenation Processes. The process comprises contacting oxygen or oxygen containing gas which also contains one or more dehydrogenatable hydrocarbons such as ethane, propane, cyclo alkanes or other alkanes, with a catalyst according to the present invention. The dehydrogenated product obtained by such a process depends upon the feedstock selected, for example alkanes may be treated to obtain olefins, diolefins, alkynes. Operating temperatures for such dehydrogenation procedures are generally in a range of 500-1000°C, preferably 600-900°C. Space velocity of the process at standard temperature and pressure is generally in a range of from 100-100,000 -1 , and preferably about 300-5000h -1 .

Claims: A method for preparing a catalyst for conversion of methane to higher hydrocarbons, comprising the steps of: forming a mixture of a manganese aqueous solution and an oxidative agent aqueous solution in an inorganic oxide support material; obtaining a precipitate of manganese oxide on the support material; drying the manganese oxide precipitate; impregnating the dried manganese oxide precipitate with an alkali metal promoter to obtain an impregnated solid; drying the impregnated solid; calcining the impregnated solid until a surface atomic ratio of promoter to manganese is at least 5. A method according to claim 1, wherein precipitate of manganese oxide is obtained by varying a pH value of the mixture between acidic and basic values. A method according to claim 1 or 2, wherein the manganese aqueous solution is an aqueous solution of manganese chloride. A method according to any of claims 1 to 3, wherein the oxidative agent aqueous solution is an aqueous solution containing an oxidative agent selected from the group consisting of alkali metal chlorates, hypochlorites, thiosulfates, and bromates. A method according to claim 4, wherein the oxidative agent is potassium chlorate. A method according to any of claims 1 to 5, wherein the inorganic oxide support material is selected from a group consisting of oxides of silicon, zirconium and magnesium. A method according to any of claims 1 to 6, wherein the promoter is an alkali metal halide. A method according to claim 7, wherein the promoter is sodium chloride. A method according to any of claims 1 to 8, further comprising the step of calcining the dried catalytic agent oxide precipitate prior to impregnation. A method according to claim 9, wherein the dried catalytic agent oxide precipitate is calcined at a temperature of 900-1200°C. A method according to claim 10, wherein the dried catalytic agent oxide precipitate is calcined at a temperature of 1000°C. A method according to any of claims 1 to 11, wherein the impregnated solid is calcined at a temperature of 300-1000°C. A method according to claim 12, wherein the impregnated solid is calcined at a temperature of 600-800°C. A method according to any of claims 1 to 13, wherein the impregnated solid is dried at a temperature of 100-140°C. A method according to any of claims 1 to 14, wherein the inorganic oxide support material, when silicon oxide is used as support material, is calcined prior to its mixture with the catalytic agent aqueous solution and the oxidative agent aqueous solution. A process for converting methane to higher hydrocarbons, comprising the steps of: providing a methane gas; providing a catalyst as prepared according to any of claims 1 to 15 and contacting the methane gas with the catalyst. A process according to claim 16, wherein the contacting step is carried out at a temperature of 600-900°C. A process according to claim 17, wherein methane gas is provided in a mixture with oxygen gas, wherein a ratio by volume of methane to oxygen is in the range of 2-10:1. A process for dehydrogenating hydrocarbons, comprising the steps of: providing an oxygen and dehydrogenatable hydrocarbon feedstock; providing a catalyst as prepared according to any of claims 1 to 15; and contacting the feedstock with the catalyst. A process according to claim 19, wherein the contacting step is carried out at a temperature of 660-900°C.

Inventors: NORA URBANO, OVALLES CESAR, ROSA FRANCISCO, NORA, URBANO, OVALLES, CESAR, ROSA, FRANCISCO, Nora, Urbano, Av. PPAL. Lomas Prados Del Este.
Applicants: INTEVEP SA
Application year: 1992
Main classification: C07C2
IPC codes: C07C9, C07C11, C07C2, C07B61, C07C5, B01J23