Process for steam reforming of hydrocarbon

Abstract: The present invention provides a catalyst for steam reforming of hydrocarbons which comprises ruthenium supported on a zirconia carrier; and a catalyst for steam reforming of hydrocarbons which comprises (A) at least one element selected from the group consisting of rhodium and ruthenium as an element imparting mainly reforming activity and (B) at least one element selected from the group consisting of nickel, lanthanum, praseodymium, neodymium, samarium, thorium, uranium, chromium, magnesium, calcium, and yttrium as an element for imparting co-catalyst function which are supported on a zirconia carrier; and furthermore a catalyst for steam reforming of hydrocarbons which comprises at least one element selected from the group consisting of rhodium and ruthenium which is supported on a partially stabilized zirconia carrier.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a process for steam reforming of hydrocarbons and more particularly to a process for steam reforming of hydrocarbon in the presence of a catalyst which has superior properties such as high catalyst activity, high heat resistance and mechanical strength and long life and which can be suitably utilized, for example, in a hydrogen producing plant for fuel cells and a small size hydrogen producing plant. (2) Description of the Related Art Hitherto, as catalysts for acceleration of steam reforming reaction comprising reacting a hydrocarbon with steam to form hydrogen, carbon monoxide, methane and carbon dioxide, there have been catalyst systems as disclosed in (1) Japanese Patent Kokoku No.12917/78, (2) Japanese Patent Kokai No.91844/81 and (3) Japanese Patent Kokai No.4232/82. The catalyst system disclosed in Japanese Patent Kokoku No.12917/78 comprises an alumina carrier which supports metallic nickel as a catalyst active component and, as co-catalysts, metallic silver in an amount of at least 2 mg atom per 100 g of catalyst and at least one of oxides of yttrium lanthanum, cerium, praseodymium, neodymium and samarium in an amount of at least 2 mg atom per 100 g of catalyst and at a ratio in number of atom to silver of 10 or less. The catalyst system disclosed in the above Japanese Patent Kokai No.91844/81 comprises rhodium and zirconium oxide. The catalyst system disclosed in the above Japanese Patent Kokai No. 4232/82 comprises an active alumina containing 0.5-10% by weight of silica and 1% by weight or less, in terms of oxide, of an alkali metal or an alkaline earth metal and 0.05-20% by weight of ruthenium supported on the active alumina. However, the alumina-supported catalyst systems disclosed in the first and the third patent publications suffer from the problems that they have activity at temperatures of higher than about 500°C and if the temperature is raised in order to further increase the activity carbon is deposited on the catalyst and that when the firing temperture is raised, the surface area is conspicuously reduced resulting in a reduction of catalyst activity or the mechanical strength decreases due to shrinking or the pressure loss of the catalyst layer increases due to powdering to make the operation difficult. With reference to the catalyst system disclosed in the second patent publication, the reaction temperature may be lower than for the above alumina supported catalyst systems, for example, 500°C, but the catalyst activity is not sufficient and besides the steam/carbon ratio is high, namely, 24mol/mol and thus such catalyst system is not necessarily advantageous for an industrial process for reforming hydrocarbons with steam. The inventors have already found that a catalyst system comprising rhodium metal supported on a zirconium carrier is a catalyst effective for reforming hydrocarbons with steam, which has high activity and can be controlled in the amount of formation of carbon on the catalyst. [cf. Igarashi et al; The 58th Catalyst Forum (A), 4 Ren B12, Nagoya]. This catalyst system is still not enough to be an industrial catalyst owing to its short life and insufficient activity. GB-A-1 029 235 discloses a catalytic composition containing a platinum group metal, such as rhodium, and a refractory inorganic oxidic material, such as zirconia. This known catalyst does not contain an element for imparting co-catalyst function. US-A-4 077 913 discloses a catalyst for use in oxidation or reduction reactions comprising an inert unitary structural support, such as zirconia, having deposited thereon a mixture or alloy comprising 5 to 75 weight% ruthenium, from a trace to 30 weight% base metal and balance platinum in an amount of not less than 5 weight%. Summary of the Invention It is an object of the present invention to provide a process for steam reforming of hydrocarbons to obtain gases rich in hydrogen by using a long life catalyst which is free from the above-mentioned problems and which has high activity even at a reaction temperature lower than 500°C, is excellent in heat resistance and mechanical strength and improved in stability and reformability and which can perform steam reforming with a low steam/carbon ratio. According to the present invention, this object is attained by a process for steam reforming of hydrocarbons comprising reacting hydrocarbons with steam in the presence of a catalyst which is characterized by using a catalyst which comprises a zirconia carrier and supported thereon, (A) at least one element selected from the group consisting of rhodium and ruthenium as an element imparting mainly reforming activity and (B) at least one element selected from the group consisting of nickel, lanthanum, praseodymium, neodymium, samarium, thorium, uranium, chromium, magnesium, calcium and yttrium as an element for imparting co-catalyst function, provided that platinum is excluded from the elements of groups (A) and (B); and performing the steam reforming reaction at a steam/carbon ratio of 3-12 and at a reaction temperature of 400-850°C, thereby obtaining mainly hydrogen. DESCRIPTION OF THE PREFERRED EMBODIMENTS Zirconia carrier: It is especially important that zirconia is selected as a carrier in the present invention. This is because this zirconia carrier is especially high in reactivity with water, performs improvement of the ability to reform hydrocarbons with steam and restrains deposition of carbon on the catalyst. Such zirconia carrier includes zirconium oxide and a substance capable of being converted to zirconium oxide at the time of the preparation of catalyst or at steam reforming. The above zirconium oxide may be commercially available one. As the substance capable of being converted to zirconium oxide at the time of the preparation of catalyst or at steam reforming, mention may be made of, for example, zirconium hydroxide, zirconium halide, zirconium nitrate, zirconyl nitrate, zirconium acetate, zirconium oxalate, zirconium alkoxide, zirconium oxychloride and organozirconium compound. Sparingly soluble salts may be solubilized and used by adding, for example, an acid. The above various zirconium compounds may be used alone or in combination of two or more. Among them, zirconyl nitrate is especially preferred. When zirconyl nitrate is used, zirconium oxide can be prepared by subjecting zirconyl nitrate to precipitation with ammonia water and thermally decomposing the precipitate. This may also be prepared by hydrolysis of an alkoxide compound of zirconium. The zirconium oxide used as a carrier may be anhydrous or may contain water of crystallization. Most preferred is zirconium oxide. In the present invention, other carriers may also be used together with the aforementioned zirconium compounds as zirconia carriers as far as attainment of the object of the present invention is not obstructed. Other carriers include silica, alumina and zeolite. The above-mentioned zirconium compounds and other carriers can be used as mixtures, or compositions such as composite oxides or composites comprising said zirconium compounds supported or coated on other carriers. The shape of the zirconia carrier is not critical and may have optional shapes such as fine powders, beads, pellets, plates, films and monolith. Elements of group (A): As ruthenium sources for supporting ruthenium, mention may be made of, for example, ruthenium halides such as ruthenium iodide and ruthenium chloride, haloruthenates such as ammonium chlororuthenate, haloruthenic acids such as chlororuthenic acid, ruthenium oxides such as ruthenium hydroxide, ruthenium dioxide and ruthenium tetraoxide, ruthenates such as potassium ruthenate and organic ruthenium compounds such as ruthenium carbonyl. Such ruthenium sources may be used alone or in combination of two or more. The amount of ruthenium supported is generally 0.1-5% by weight, preferably 0.3-3% by weight of zirconia carrier. In this case, if the amount of ruthenium is too much, the carrier is covered with the supported metal and effects corresponding to the supporting amount cannot be obtained. Preparation of catalyst: The process for the preparation of the catalyst has no special limitation and there may be optionally employed, for example, an impregnation process, dipping process, wet adsorption process, dry adsorption process, CVD process, solvent evaporation process, dry mixing process, wet mixing process, spray coating process and combination of them. The operation method for supporting also includes any of a stationary method, stirring method, solution passing method or solvent refluxing method. An example of a suitable preparation of the catalyst is explained below. That is, zirconium hydroxide is fired, for example, at 500-800°C for 1 hour to several hours and the resulting fired product is dipped in a solution or colloidal dispersion containing compounds of elements selected from group (A), such as the above mentioned ruthenium compound, and from group (B) normally for 0.1-10 hours, preferably 0.5-5 hours, followed by mixing or kneading and evaporating to dryness. The fired product of zirconium oxide also may be dipped in a solution or a colloidal dispersion of compounds of elements of group (A) and then in a solution or a colloidal dispersion of compounds of elements of group (B) or vice versa. Then, the product is dried at 100-200°C over 0.1-24 hours and fired at 500-800 °C for 0.1-20 hours in air or nitrogen stream. If necessary,the fired product is molded into desired shape and subjected to reducing treatment to obtain a catalyst for steam reforming as used in the present invention. Solvents used for solution or colloidal dispersion of the compounds of groups (A) and (B) are not limitative as far as they can dissolve the respective compounds or can keep stably as a uniform colloid solution. Aqueous solvents, non-aqueous solvents and mixed solvents thereof can be used. In carrying out the dipping, if necessary, various acids, bases, salts, oxidizing agents, reducing agents, pH modifiers, buffers and resolution accelerators can be added to the solution or colloidal dispersion of the respective compounds. Preferably, the reducing treatment mentioned above is performed by contacting the catalyst precursor packed in a reaction tube with hydrogen under heating. As rhodium source for supporting rhodium metal, mention may be made of, for example, rhodium halides such as rhodium chloride, halorhodates such as sodium chlororhodate and ammonium chlororhodate, halorhodic acids such as chlororhodic acid, rhodium (III) hydroxide, rhodium (IV) hydroxide, rhodium nitrate, rhodium oxide and organorhodium compounds such as rhodium carbonyl. Such rhodium sources may be used alone or in combination of two or more. Preferred are rhodium halides and especially preferred is rhodium trichloride. In the catalyst used in the invention, elements of group (A) are supported on a zirconia carrier. In this case, the amount of the metal supported is normally 0.1-5% by weight, preferably 0.3-3% by weight of zirconia carrier and there is no limitation in the ratio of two elements when two elements of group (A) are supported. Elements used as those of group (B): As nickel sources for supporting nickel, mention may be made of, for example, nickel chloride (hexahydrate), nickel chloride (anhydride), nickel bromide (hexahydrate), nickel bromide (anhydride), nickel iodide (hexahydrate), nickel iodide (anhydride), nickel nitrate, nickel sulfate, nickel acetate, nickel formate, nickel oxalate, nickel hydroxide, nickel oxide, nickel carbonate, nickel acetylacetonate and nickel carbonyl. Lanthanum sources, praseodymium sources, neodymium sources, samarium sources, chromium sources, uranium sources, thorium sources, magnesium sources, calcium sources, or yttrium sources for supporting lanthanum, praseodymium, neodymium samarium, chromium, uranium, thorium, magnesium, calcium or yttrium include nitrates, sulfates, carbonates, acetates, hydroxides, oxides, basic salts, alkoxides and organic compounds of these metals. These sources may be used alone or in combination of two or more. The amount of element of group (B) obtained from the above metal sources can vary depending on the kind of metal and cannot be indiscriminately specified, but normally 0.1-10% by weight, preferably 0.3-5% by weight of metals are supported. Steam reforming of hydrocarbon: The above-mentioned catalyst for steam reforming is used for accelerating steam reforming of hydrocarbons. The hydrocarbons to be reformed are not limitative and include, for example, straight chain or branched chain saturated aliphatic hydrocarbons such as methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane and cycloaliphatic saturated hydrocarbons such as cyclohexane, methylcyclohexane and cyclooctane. These hydrocarbons are used alone or in combination of two or more or may be various purified petroleum fractions. Steam to be reacted with hydrocarbons is not limitative. The above hydrocarbons are considered to react with steam in accordance with the following reaction formulas. C n H m + nH₂O → nCO + (n+m/2)H₂ (I) C n H m + 2nH₂O → nCO₂ + (2n+m/2)H₂ (II) [n in the formulas (I) and (II) represents a real number of 1 or more and m represents a real number of 2 or more.] In addition to the above formulas, reaction (III) of formation of CH₄ by hydrogenolysis of hydrocarbon and the following equilibrium reactions can be considered to occur jointly. CH₄ + H₂O ⇄ CO + 3H₂ (IV) CO + H₂O ⇄ CO₂ + H₂ (V) Therefore, theoretically, the amounts of hydrocarbon and steam used can be determined in stoichiometric amounts which satisfy the above reaction formulas (I)-(V), but when the catalyst of the present invention is used, the amounts of hydrocarbon and steam are determined so that the steam/carbon ratio is 3-12, preferably 3-8. By employing such a steam/carbon ratio, gases rich in hydrogen can be efficiently and stably obtained. The reaction temperature is 400-850°C. It is worth notice that the catalyst used in the present invention has sufficiently high catalytic activity even at a reaction temperature of 500°C or lower. The reaction pressure is normally 0-49 bar·G (0-50 kg/cm²G), preferably 0-19.6 bar·G (0-20 kg/cm²G). The reaction method may be any of the continuous flowing method or batch method, but the continuous flowing method is suitable. When the continuous flowing method is employed, the gas space velocity (GHSV) of the mixed gas of hydrocarbon and steam is usually 1,000-40,000 h⁻¹ , preferably 2,000-20,000 h⁻¹. What is worth notice when the above described catalyst is used in the present invention is that continuous operation is possible even at such high gas space velocity. The reaction type is not limitative and may be any of the fixed bed type, moving bed type, or fluidized bed type. The type of reaction apparatus is also not limitative and, for example, a tube type reactor may be employed. In this way, when hydrocarbon and steam are allowed to react in the presence of the above described catalyst, the reaction proceeds normally and mainly in accordance with the above reaction formula (I), but since the reaction of the reaction formula (II), the equilibrium reaction (V) where produced carbon monoxide and water react to produce carbon dioxide and hydrogen and the equilibrium reaction (IV) where carbon monoxide and hydrogen react to produce methane and water also take place simultaneously, as a result there is obtained a mixture of hydrogen, methane, carbon monoxide and carbon dioxide, although the main product is hydrogen. The resulting mixed gas can be used as it is or can be separated into respective gas components, which are used for respective purposes. The present invention is explained by the following examples. Reference Example 1 A catalyst for steam reforming (Ru/ZrO₂) was prepared by supporting 0.5% by weight of RuCl₃ in terms of Ru on a ZrO₂ carrier obtained by firing ZrO₂·xH₂O and subjecting this catalyst to a reducing treatment with hydrogen in a reaction tube. Steam reforming was carried out using a normal-pressure fixed-bed flow type reaction apparatus, n-butane as a starting material and the following conditions; reaction temperature: 450°C, reaction time factor: 622.7 (g-cat·min/n-butane mol) and a steam/carbon ratio (feeding ratio of steam and n-butane calculated from molar ratio): 12. Continuous analyses by a gas chromatography gave a total conversion rate of 60.6% as a catalyst activity after 10 hours from initiation of the reaction and the following composition of the resulting gas: carbon monoxide: 1.1 mol%, methane: 20.7 mol%, carbon dioxide: 18.5 mol% and hydrogen: 59.7 mol%. Examples 2-13 The procedure of Example 1 was repeated except that catalysts having the metals as shown in Table 1 supported were used. The results are shown in Table 1 together with those of Reference Example 1. Comparative Example 1 The procedure of Reference Example 1 was repeated except that a catalyst comprising zirconia carrier and rhodium supported thereon was used. The results are also shown in Table 1. Catalyst system Total conversion rate (%) Dry gas composition (mol%) CO CH₄ CO₂ H₂ Reference Example 1 Ru/ZrO₂ 60.6 1.1 20.7 18.5 59.7 Example 2 Rh-Ni/ZrO₂ 70.9 1.8 21.8 19.1 57.4 Example 3 Rh-Cr/ZrO₂ 61.3 1.9 17.2 20.0 60.8 Example 4 Rh-La/ZrO₂ 61.0 2.0 17.5 19.2 61.3 - Example 5 Rh-Pr/ZrO₂ 57.6 2.2 15.1 20.2 62.5 Example 6 Rh-Nd/ZrO₂ 68.3 2.5 16.1 19.7 61.8 Example 7 Rh-Sm/ZrO₂ 84.9 2.8 19.6 19.7 58.0 Example 8 Rh-Th/ZrO₂ 68.2 2.1 13.9 20.1 63.8 Example 9 Rh-U/ZrO₂ 75.1 1.9 20.5 20.0 57.8 Example 10 Rh-Mg/ZrO₂ 69.9 1.9 17.5 20.4 60.1 Example 11 Rh-Ca/ZrO₂ 59.1 1.9 13.7 20.0 64.4 Example 12 Rh-Y/ZrO₂ 71.2 2.0 17.4 20.4 60.1 Example 13 Rh-Th/ZrO₂ 73.0 1.9 22.1 18.8 57.2 Comp. Ex.1 Rh/ZrO₂ 50.6 2.1 12.4 19.2 66.4 Examples 14-15 The life of catalyst systems shown in Table 2 was evaluated using a normal-pressure fixed bed flow type reaction apparatus and n-butane as a starting material. The reaction conditions were the follwoing; reaction temperature: 450 °C, reaction time factor: 622.7 (g-Cat·min./n-butane mol) and a steam/carbon ratio: 12. The results are shown in Table 2. Comparative Example 2 The procedure of Examples 14-15 was repeated except that Rh/ZrO₂ was used as a catalyst. Catalyst system Total conversion rate (%) Initial After 1,000 hr Example 14 Rh-Th/ZrO₂ 68.2 68.1 Example 15 Rh-La/ZrO₂ 61.0 60.5 Comparative Example 2 Rh/ZrO₂ 50.7 47.0 The catalyst used in the present invention has good efficiency and is suitable, for example, for small sized plants for the preparation of hydrogen and for the production of hydrogen for fuel cells.

Claims: A process for steam reforming of hydrocarbons comprising reacting hydrocarbons with steam in the presence of a catalyst characterized by using a catalyst which comprises a zirconia carrier and supported thereon. (A) at least one element selected from the group consisting of rhodium and ruthenium as an element imparting mainly reforming activity and (B) at least one element selected from the group consisting of nickel, lanthanum, praseodymium, neodymium, samarium, thorium, uranium, chromium, magnesium, calcium and yttrium as an element for imparting co-catalyst function, provided that platinum is excluded from the elements of groups (A) and (B); and performing the steam reforming reaction at a steam/carbon ratio of 3-12 and at a reaction temperature of 400-850°C, thereby obtaining mainly hydrogen. The process according to claim 1 characterized by using a catalyst wherein the amount of at least one element selected from the group consisting of rhodium and ruthenium is 0.1-5% by weight of the zirconia carrier. The process according to claim 1 characterized by using a catalyst wherein the amount of at least one element selected from the group consisting of nickel, lanthanum praseodymium, neodymium, samarium, thorium, uranium. chromium, magnesium, calcium and yttrium is 0.1-10% by weight of the metals supported on the zirconia carrier.

Inventors: IGARASHI AKIRA C O KOHGAKUIN U, IGARASHI, AKIRA C/O KOHGAKUIN UNIVERSITY
Applicants: IGARASHI AKIRA C O KOHGAKUIN U, IGARASHI, AKIRA C/O KOHGAKUIN UNIVERSITY
Application year: 1989
Main classification: C01B3
IPC codes: B01J23, B01J21, C01B3