Reduction Catalyst for Carbon Monoxide, Process for Preparing the Catalyst and Process for Producing Hydrocarbon [Field of the Invention] The present invention relates to catalyst for reducing monoxide, processes for preparing such catalysts, and processes for producing hydrocarbons by reducing carbon monoxide. (Background of the Invention] In recent years, regulations concerning the sulfur content of liquid fuels such as gasoline and gas oils have been enforced more rapidly than before. Therefore, it is now absolutely necessary to produce environment friendly clean liquid fuels that are less in sulfur and aromatic hydrocarbon contents. As an example of processes for producing such clean fuels, there is the Fischer-Tropsch (FT)synthesis. The FT synthesis can simultaneously produce a clean liquid fuel base stock that is rich in paraffin content and free of sulfur and wax (FT wax) . The FT wax can be converted to a middle fraction (clean fuel base stock for kerosene or gas oil) through hydrocracking. The Fischer-Tropsch synthesis is carried out -1using catalysts comprising an active metal such as iron or cobalt supported on a support such as silica or alumina (for example, see Patent Document No. 1 below). It has been reported that these catalysts are improved in catalyst performances by the use of secondary metals in combination with such an active metal (for example, see Patent Document No. 2 below). Examples of the secondary metals include alkali or alkaline earth metals such as sodium, lithium and magnesium, zirconium, and hafnium, which are properly used depending on purposes such as improving the carbon monoxide conversion rate and increasing the chain growth probability ( a ) which will be an index of the wax production. In order to produce efficiently a fuel base stock which is a middle fraction, high carbon monoxide conversion rate and high chain growth probability (a are demanded for the FT synthetic catalyst. Although the aforesaid secondary metals take an important role to improve the catalyst performances, the effects of the metals are not maximized under the present situation. Generally, an impregnation method such as Incipient Wetness method is employed to support the secondary metal highly dispersedly. However, the FT synthesis is an extreme exothermic reaction and thus -2- -3 it is assumed that the reaction is likely to occur in the vicinity of the outer surface of the catalyst. Therefore, it is presumed that the secondary metal selectively supported in the vicinity of the catalyst outer surface functions more advantageously to improve the catalyst performances. However, there is no example wherein the secondary metal is supported in the vicinity of the catalyst outer surface, and thus an improvement in the catalyst performances has been hindered. When carbon monoxide is reduced using a catalyst, the optimum reaction conditions for producing hydrocarbons efficiently vary depending on the type of the support and active metal of the catalyst and the method of supporting the secondary metal. (1) Patent Document No. 1: Japanese Patent Laid-Open Publication No. 4-227847 (2) Patent Document No. 2: Japanese Patent Laid-Open Publication No. 59-102440 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present -3A invention as it existed before the priority date of each claim of this application. [Disclosure of the Invention] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. As the result of extensive research and development carried out by the present inventors, they have accomplished the present invention on the basis of the finding that a catalyst produced by supporting zirconia which is the secondary metal selectively in the vicinity of the outer surface of a metal oxide and then supporting ruthenium and/or cobalt was capable of producing hydrocarbons efficiently through a reduction reaction (FT synthesis) of carbon monoxide. That is, the present invention relates to a process for producing hydrocarbons by reducing monoxide using a catalyst comprising a support comprising a metal oxide and zirconium in the form of oxide supported selectively in the vicinity of the outer surface of the metal oxide, and one or more type of metal selected from cobalt and ruthenium supported on the support. Alternatively, the present invention relates to the foregoing process for producing hydrocarbons, using a catalyst comprises a support produced by supporting and then calcining zirconium on the metal oxide having been pre-treated with an aqueous solution with a pH of 7 or lower and one or more type of metal selected from cobalt and ruthenium supported on the support. The present invention also relates to a process for preparing a reduction catalyst for carbon monoxide, comprising producing a support by pre-treating a metal oxide with an aqueous solution with a pH of 7 or lower -4- -5 and supporting and calcining zirconium on the metal oxide, and supporting one or more type of metal selected from cobalt and ruthenium on the support. The present invention also relates to a reduction catalyst for carbon monoxide prepared by the foregoing process. In one aspect, the invention provides a process for preparing a catalyst for Fischer-Tropsch synthesis comprising steps of (a) to (d): (a) pre-treating by soaking spherical alumina or silica in an aqueous solution with a pH of 7 or lower; (b) supporting zirconium on the pre-treated alumina or silica; (c) calcining the alumina or silica supporting zirconium, to produce a support comprising zirconium oxide which is supported selectively in the vicinity of the outer surface of the support; and (d) supporting one or more metals selected from the group consisting of cobalt and ruthenium on the support in an amount of from 3 to 50 percent by mass on the basis of the catalyst, wherein 75 percent by mass or more of the total amount of the zirconium oxide are supported in 1/5 or less of the radial area ranging from the outer surface of the catalyst to the center thereof in the catalyst.
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In another aspect, the invention provides a catalyst for Fischer-Tropsch synthesis, wherein 75 percent by mass or more of the total amount of the zirconium oxide are supported in 1/5 or less of the radial area ranging from the outer surface of the catalyst to the center thereof in the catalyst, prepared by a process comprising steps (a) to (d): (a) pre-treating by soaking spherical alumina or silica in an aqueous solution with a pH of 7 or lower; (b) supporting zirconium on the pre-treated alumina or silica; (c) calcining the alumina or silica supporting zirconium, to produce a support comprising zirconium oxide which is supported selectively in the vicinity of the outer surface of the support; and (d) supporting one or more metals selected from the group consisting of cobalt and ruthenium on the support in an amount of from 3 to 50 percent by mass on the basis of the catalyst. [Effects of the Invention] The process of preparing a reduction catalyst for carbon monoxide of the present invention enables an oxide of zirconium to be supported selectively in the vicinity of the outer surface of a metal oxide and thus -5B is capable of producing a catalyst with excellent properties as required for the FT synthetic catalyst. Further, the process of the present invention using a catalyst for reducing carbon monoxide comprising a support produced by supporting selectively and then calcining an oxide of zirconium in the vicinity of the outer surface of a metal oxide and one or more type of metal selected from cobalt and ruthenium supported on the support is capable of producing efficiently hydrocarbons, i.e., fuel base stocks at a higher carbon monoxide conversion rate and a higher chain growth probability (a).
[Best Mode for Carrying out the Invention] The present invention will be described in more detail below. The catalyst used in the present invention is a catalyst which comprises a support comprising a metal oxide and zirconium in the form of oxide supported selectively in the vicinity of the outer surface of the metal oxide, and one or more type of metal selected from cobalt and ruthenium. There is no particular restriction on the metal oxide of the catalyst used in the present invention. However, examples of the metal oxide include silica, titania, alumina, and magnesia, and preferable examples include silica and alumina. There is no particular restriction on the properties of the metal oxide. However, the specific surface area of the metal oxide measured by a nitrogen adsorption method is preferably from 50 to 800 m 2 /g, more preferably from 150 to 500 m 2 /g. The average pore diameter of the metal oxide is preferably from 6 to 40 nm, more preferably from 10 to 20 nm. An average pore diameter of smaller than 6 nm is not preferable because the time for supporting zirconium would tend to be longer. An average pore diameter of larger than 40 nm is not preferable because -6zirconium would likely penetrate to the interior of the metal oxide. There is no particular restriction on the shape of the metal oxide. In view of practicability, the shape is preferably spherical, cylindrical or trefoil shape which has been employed in actual petroleum refinery or petrochemical units. There is also no particular restriction on the particle diameter. However, in view of practicability, the particle diameter is preferably from 10 pm to 10 mm. In the present invention, the process for preparing a support comprising a metal oxide and zirconium in the form of oxide supported selectively in the vicinity of the outer surface of the metal oxide is specifically carried out by pre-treating a metal oxide with an aqueous solution with a pH of 7 or lower, and then supporting and calcining the zirconium on the metal oxide. Description will be given of a method of pre-treating a metal oxide with an aqueous solution with a pH of 7 or lower. Example of the aqueous solution with a pH of 7 or lower include nitric acid aqueous solution, acetic acid aqueous solution, sulfuric acid aqueous solution, -7hydrochloric acid aqueous solution, ion-exchange water, and distilled water. The pH is preferably from 5 to 7, more preferably from 6 to 7. A pH of lower than 5 is not preferable in economical sense because it arises the necessity of increasing the concentration of zirconium to be supported after the pre-treatment. The pre-treatment may be carried out by pouring an aqueous solution with a pH of 7 or lower into a vessel containing a metal oxide. The time for soaking a metal oxide in an aqueous solution with a pH of 7 or lower is from 10 to 72 hours for the case where it is left to stand, from 1 to 12 hours for the case where it is vibrated, and from 1 to 30 minutes for the case it is exposed to ultrasonic wave. In either case, even if the metal oxide is soaked in the solution for the time longer than needed, it is not adversely affected. The aforesaid time is applied to the case where the temperature of the solution is room temperature. The soaking time can be shortened by heating the solution up to 50'C. When the temperature of the solution is higher than 500C, it is not preferable because the water would likely evaporate, resulting in change in pH. After the pre-treatment is carried out for a predetermined time, zirconium may be supported on the -8metal oxide by pouring a solution of excess zirconium into the vessel containing the pre-treated metal oxide. Thereupon, supernatant liquid of the solution after the pre-treatment is preferably removed because a smaller vessel may be used. The term "excess" used herein denotes a volumetric amount twice or more as much as the volume of the metal oxide. The zirconium source used herein may be preferably zirconyl sulfate, zirconyl acetate, ammonium zirconyl carbonate or zirconium trichloride and is preferably ammonium zirconyl carbonate or zirconyl acetate. The amount of zirconium to be supported is preferably 40 percent by mass or less, more preferably from 1 to 30 percent by mass, of the metal oxide. When the amount is more than 40 percent by mass, zirconium would not likely be supported selectively in the vicinity of the outer surface of the metal oxide. There is no particular restriction on the time for supporting zirconium since the time depends on the intended amount of zirconium to be supported. However, it is usually from 3 to 72 hours. After completion of supporting zirconium, the support (the metal oxide supporting zirconium)is separated from the solution and then dried. There is no particular restriction on the drying method. -9- Examples of the drying method include natural drying in the air and deaeration drying in vacuo. Drying is carried out at a temperature of100 to 200 0 C, preferably 110 to 130 0 C for 2 to 24 hours, preferably 5 to 12 hours. After the drying treatment, calcination is carried out to convert zirconium to oxide. There is no particular restriction on the calcination method. Calcination is usually carried out under air atmosphere at a temperature of 340 to 600 0 C, preferably 400 to 450 0 C for 1 to 5 hours. In this way, a support is produced which comprises a metal oxide and an oxide of zirconium supported selectively in the vicinity of the outer surface of the metal oxide. Thereafter, ruthenium and/or cobalt are supported on the support. Generally, examples of active metals for FT synthesis include ruthenium, cobalt and iron. However, active metals used in the present invention are limited to ruthenium, cobalt and combination thereof in order to make use of characteristics of zirconia. There is no particular restriction on precursor compounds containing ruthenium or cobalt. Therefore, salts or complexes of these metals may be used. Examples include nitrate, hydrochloride, formate, -10propionate and acetate. There is no particular restriction on the amount of ruthenium or cobalt on the basis of the support. However, ruthenium or cobalt may be supported in an amount of usually from 3 to 50 percent by mass, preferably from 10 to 30 percent by mass. If the amount is less than 3 percent by mass, activation would be insufficient. If the amount is in excess of 50 percent by mass, the active metal would likely aggregate and thus would be assumed to be reduced in utility value as a practical FT synthetic catalyst. There is no particular restriction on the method of supporting the active metal. There may be used an impregnation method such as Incipient Wetness method. After the active metal is supported, it is usually dried at a temperature of 100 to 2000C, preferably 110 to 1300C for 2 to 24 hours, preferably 5 to 10 hours and then calcined under air atmosphere at a temperature of 340 to 6000C, preferably 400 to 4500C for 1 to 5 hours to convert the active metal to an oxide thereby preparing a reduction catalyst for carbon monoxide used in the present invention. The catalyst thus prepared is excellent in performances required for the FT synthetic catalyst. When the metal oxide of the catalyst of the present -11invention is in the form of sphere, 75 percent by mass or more, preferably 80 to 95 percent by mass of the total amount of the oxide of zirconium is supported in 1/5 or less of the radial area (outer surface side) ranging from the catalyst outer surface to the center. When carbon monoxide is reduced using a catalyst prepared by the above-described process, the reaction temperature is usually from 180 to 320*C, preferably from200 to 300'C. A reaction temperature of lower than 180'C is not preferable because carbon monoxide hardly reacts and thus the yield of hydrocarbon would be reduced. A reaction temperature of higher than 320'C is not also preferable because the production of gas such as methane would likely be increased. There is no particular restriction on the gas hourly space velocity. However, it is usually from 500 to 4000 h1, preferably from 1000 to 3000 h~1. When the gas hourly space velocity is less than 500 h-1, the productivity of liquid fuel would likely be reduced. When the gas hourly space velocity is in excess of 4000 h1, the production of gas would likely be increased, resulting from an increase in the reaction temperature. There is no particular restriction on the reaction pressure (partial pressure of a synthetic gas of carbon monoxide and hydrogen). However, the reaction may be -12carried out within the range of usually 1 to 7 MPa, preferably 2 to 4 MPa. When the reaction pressure is less than 1 MPa, the yield of liquid fuel would likely be reduced. When the reaction pressure is in excess of 7 MPa, the amount of facility investment would likely be increased. There is no particular restriction on the feedstock as long as it is composed of mainly carbon monoxide and hydrogen. However, the molar ratio of hydrogen/carbon monoxide is usually from 1.5 to 2.5, preferably from 1.8 to 2.2. [Examples] Hereinafter, the present invention will be described in more detail by way of the following examples and comparative examples, which should not be construed as limiting the scope of the invention. [Preparation of catalyst A] Into a 250 ml glass bottle were weighed 30 g of spherical silica (average pore diameter: 10 nm, average particle diameter: 1.8 mm), and 100 ml of an nitrate aqueous solution with a pH of 6.6 were added thereinto, followed by application of ultrasonic wave at a temperature of 401C for 10 minutes. Thereafter, about -13- 50 ml of the supernatant liquid were pumped out with a Pasteur pipette and then 150 ml of an aqueous solution of 0.2 mol/L of ammonium zirconyl carbonate was added. The mixture was left to stand at room temperature for 24 hours. Thereafter, the mixture was filtered with a filter paper and then dried in vacuo at a temperature of 1200C for 6 hours and calcined under air atmosphere at a temperature of 4300C for 3 hours. The resulting support was impregnated with an aqueous solution of cobalt nitrate in such an amount that 10 percent by mass of metal cobalt on the basis of the support were contained by Incipient Wetness method. Thereafter, the support was dried at a temperature of 1200C for 12 hours and then calcined at a temperature of 4200C for 3 hours thereby producing a catalyst A. The amount of the zirconium in this catalyst was quantified using a fluorescent X-ray. Further, the distribution and amount of zirconium in the radial direction of the catalyst were measured with an electron probe micro analyzer (EPMA). Table 1 sets forth the amount of the zirconium in the catalyst and the ratio of the amount of the zirconium present in 1/5 or less of the radial area ranging from the outer surface to the center (vicinity of the outer surface) to the -14total amount of the zirconium. [Preparation of catalyst B] Into a 250 ml glass bottle were weighed 30 g of cylindrical alumina (average pore diameter: 115 nm, diameter: 1/16 inch, length; about 3 mm), and 100 ml of ion-exchanged water (pH 7.0) were added thereinto, followed by application of ultrasonic wave at a temperature of 400C for 10 minutes. Thereafter, about 50 ml of the supernatant liquid were pumped out with a Pasteur pipette and then 150 ml of an aqueous solution of 0.2 mol/L of ammonium zirconyl carbonate was added. The mixture was left to stand at room temperature for 36 hours. Thereafter, the mixture was filtered with a filter paper and then dried in vacuo at a temperature of 1200C for 6 hours and calcined under air atmosphere at a temperature of 4300C for 3 hours. The resulting support was impregnated with an aqueous solution of cobalt nitrate in such an amount that 10 percent by mass of metal cobalt on the basis of the support were contained by Incipient Wetness method. Thereafter, the support was dried at a temperature of 1200C for 12 hours and then calcined at a temperature of 4200C for 3 hours thereby producing a catalyst B. -15- The amount of the zirconium in this catalyst was quantified using a fluorescent X-ray. Further, the distribution and amount of the zirconium in the radial direction of the catalyst were measured with an electron probe micro analyzer (EMPA). Table 1 sets forth the amount of the zirconium in the catalyst and the ratio of the amount of the zirconium present in 1/5 or less of the radialarea ranging from the outer surface to the center (vicinity of the outer surface) to the total amount of the zirconium. [Preparation of catalyst C] The same preparation and analysis as those for the catalyst A were carried out except that 30 g of silica used for preparation of the catalyst A were impregnated with a nitric acid aqueous solution containing 1.2 g of zirconium by Incipient Wetness method. The results are set forth in Table 1. [Preparation of catalyst D] The same preparation and analysis as those for the catalyst B were carried out except that 30 g of alumina used for preparation of the catalyst B were impregnated with a nitric acid aqueous solution containing 1.2 g of zirconium by Incipient Wetness method. The results -16are set forth in Table 1. [Preparation of catalyst E) The same preparation and analysis as those for the catalyst B were carried out except that an ammonium aqueous solution (pH: 8.5) was used in place of the ion-exchanged water (pH: 7.0). The results are set forth in Table 1. [Example 1] Into a fixed-bed circulation type reactor were charged 30 g of the catalyst A. The catalyst A was reduced under a hydrogen gas stream at a temperature of 400*C for 2 hours before a reaction was initiated. Thereafter, a feedstock mixed gas containing hydrogen and carbon monoxide at a molar ratio of 2/1 was supplied at a gas hourly space velocity of 2000 h-1, and the reaction was carried out at a temperature of 215 0 C and a pressure of 2. 5 MPa. The gas composition and oil thus produced at the outlet of the reactor were analyzed using a gas chromatography to calculate the carbon monoxide conversion rate and chain growth probability (a) in accordance with a conventional method. The results are set forth in Table 2. -17- [Example 2) A reaction was carried out under the same conditions as and using the same catalyst as those of Example 1 except that the reaction temperature was changed to 2250C. The results are set forth in Table 2. [Example 3] A reaction was carried out under the same conditions as those of Example 1 except that 30 g of the catalyst B were used in place of the catalyst A. The results are set forth in Table 2. [Comparative Example 1] A reaction was carried out under the same conditions as those of Example 1 except that 30 g of the catalyst C were used in place of the catalyst A. The results are set forth in Table 2. [Comparative Example 2] A reaction was carried out under the same conditions as those of Example 1 except that 30 g of the catalyst D were used in place of the catalyst A. The results are set forth in Table 2. -18- [Comparative Example 3] A reaction was carried out under the same conditions as those of Example 1 except that 30 g of the catalyst E were used in place of the catalyst A. The results are set forth in Table 2. Table 1 Amount of Total amount of zirconium in the zirconium vicinity of the outer surface in (mass %) the total amount of zirconium (%) Catalyst A 3.8 91 Catalyst B 3.6 83 Catalyst C 3.6 42 Catalyst D 3.7 39 Catalyst E 6.2 39 Table 2 Carbon monoxide Chain growth conversion rate probability (a) mol % Example 1 Catalyst A 58 0.93 Example 2 Catalyst A 82 0.91 Example 3 Catalyst B 56 0.92 Comparative Catalyst C 42 0.89 Example 1 Comparative Catalyst D 41 0.87 Example 2 Comparative Catalyst E 50 0.87 Example 3 -19- As set forth in Table 1, the preparation process of the present invention is capable of producing a reduction catalyst for carbon monoxide, supporting zirconia selectively in the vicinity of the outer surface. Further, as set forth in Table 2, the catalysts supporting zirconia selectively in the vicinity of the outer surface (1/5 or less of the radial area ranging from the outer surface to the center of the catalyst) are high in carbon monoxide conversion rate and chain growth probability ( a ) and the use of the catalysts makes it possible to produce a fuel base stock efficiently. [Applicability in the Industry] The process of the present invention is capable of producing hydrocarbon that is fuel base stock, at a high carbon monoxide conversion rate and a high chain growth probability (a ) -20-