Selective catalysts and their preparation for catalytic hydrocarbon synthesis

Abstract: Supported catalysts for Fischer-Tropsch processes prepared by contacting a suitable support with a catalyst in the form of a metal salt and depositing the catalyst on the support. Where rim type catalyst are preferred, a molten salt starting material is used. For non-rim type catalyst, such as powdered catalyst, catalytic metal salt-solution starting materials may be used. The catalyst is prepared without high temperature calcination of the catalyst before reduction by reducing the salt directly on the support to the metal. During the reduction step, the heating rate is a slow single steady heating rate.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to improved catalysts, their manufacture, and use in hydrocarbon synthesis, Fischer-Tropsch type reactions, to prepare heavy hydrocarbons, C₅+, from synthesis gas, carbon monoxide and hydrogen. The Prior Art Hydrocarbon synthesis, Fischer-Tropsch, reactions involving the reaction of carbon monoxide and hydrogen over a catalyst, to produce heavier hydrocarbons, C₅+, are diffusion limited. Hydrogen diffuses at a faster rate into the catalyst particle than carbon monoxide and the H₂/CO ratio at any point below the exterior profile of the catalyst will be greater than the hydrogen to carbon monoxide ratio in the bulk gas. As the reactants, hydrogen and carbon monoxide, proceed further into the catalyst particle towards its center, the ratio of hydrogen to carbon monoxide will continue to increase as long as the catalyst metal sites of the catalyst are available to consume H₂ and CO. The stoichiometric consumption ratio of hydrogen and carbon monoxide to produce heavy hydrocarbons is 2.1/1. Ratios above the stoichiometric ratio favor increasing formation of methane, CH₄. However, methane is not a desirable product of the hydrocarbon synthesis reaction, although the synthesis gas is often prepared from methane, for example, by partial oxidation or catalytic reforming. Various methods for overcoming the diffusion limitation phenomenon of hydrocarbon synthesis catalysts have been developed in the prior art, one of which is the coated or rim-type catalysts. Examples of coated or rim type catalysts are U.S. patent No. 2,512,608 and 4,599,481, and 4,637,993. The latter two patents disclose cobalt or promoted cobalt catalysts on supports which may be silica, alumina, or silica-alumina. Other examples of rim-type catalysts appear in Everson et al, Fischer-Tropsch Reaction Studies with Supported Ruthenium Catalysts, Journal of Catalysis, 53, 186-197 (1978) and Dixit and Tavlarides, Kinetics of the Fischer-Tropsch Synthesis, Ind., Eng. Chem. Proc. Des. Dev. 1983, 22, 1-9, where various metals such as cobalt, ruthenium, and iron are disclosed as hydrocarbon synthesis catalysts and a coated ruthenium on alumina catalyst is utilized in hydrocarbon synthesis rate studies. The catalyst has a rim of about 300 µm in thickness. The use of coated or rim type catalysts has been successful in limiting methane formation and increasing C₅+ yields in hydrocarbon synthesis reactions. However, the preparation of rim type catalysts is relatively difficult and the use of available catalyst metal atoms as sites for promoting the desired reactions has not necessarily been efficient. In addition, during the reduction of the catalyst metal presursor, which typically requires heating in the presence of a reducing agent, the catalyst precursor is not heated at a single steady heating rate. In the prior art, heating is carried out over a broad range of temperatures for a specified time to maximum temperature. For example, U.S. Patent 2,696,475 to Farrow et al teaches a method for preparing nickel, cobalt or copper catalyst using a hydrogen reducing agent. The catalyst precursor is heated from 300°C to 500°C for about 4 hours. In U.S. Patent 4,568,663 to Mauldin a rhenium promoted cobalt catalyst precursor is reduced at temperatures ranging from 250°C to 500°C for periods ranging from 0.5 to 24 hours. In neither case is a particular heating rate for the catalyst precursor material specified. The rate of heating during reduction later became more important after it was recognized that the reduction of the catalyst metal precursor to improve catalyst activity was temperature path dependent. However, these processes either employ a multiplicity of heating rates or calcine the catalyst at high temperatures before the reduction step. Reducing the catalyst metal precursor directly, without calcination, was then known to result in catalysts having inferior activity than that of catalysts produced by a process that employs a calcination step. For example, U.S. Patent 4,801,573 issued to Eri et al, shows a method for preparing a rhenium and cobalt catalyst on a support. The reduction is carried out at the single steady rate of 1°C/min. to the maximum temperature of 350°C. However, the process requires calcination of the catalyst precursor before reduction. U.S. Patent 4,605,676 issued to Koblinski et al, reveals a process for directly reducing cobalt catalyst precursors without calcining. The catalyst precursors are prepared from cobalt nitrate solutions and cobalt carbonyls on alumina based supports. During the reduction step, the heating rate is held steady at 1°C/min. to 350°C. For comparative purposes, the directly reduced catalyst (R350) was subjected to an additional oxidation and reduction step to produce a second catalyst (ROR). Koblinski teaches that the catalyst prepared by the ROR procedure has much higher activity than catalyst prepared by direct reduction (R350) at a heating rate of 1°C/min. In U.S. Patent 4,493,905 to Beuther et al, gas synthesis catalysts are prepared by directly reducing the catalyst from a catalyst metal salt. Although calcination was omitted, the reduction was carried out by a multiple of heating steps of different rates, each to a different maximum temperature. For instance, in one embodiment, the catalyst is initially heated at 1°C/min. up to 100°C and held at that temperature for about 1 hour and then heated at 1°C/min. up to 200°C and held at that temperature for about 2 hours and finally, heated at 10°C/min. up to 360°C and held at that temperature for 16 hours. We have now discovered a method for preparing coated or rim type catalysts having a relatively uniform rim of predictable thickness and wherein the efficiency of the catalyst metal atoms and promotion of desired reactions is greatly enhanced. Furthermore, we have discovered a process whereby catalyst dispersion and activity can be increased by directly reducing a catalyst metal precursor, in the form of a salt, in the presence of a reducing agent where heating during the reduction is a single steady heating rate up to a maximum temperature. US-A-4,605,680 describes a process for converting synthesis gas to straight chain paraffins using a catalyst of cobalt, preferably promoted, on a support of gamma-alumina, eta-alumina or mixtures thereof. The support is calcined and impregnated with a non-aqueous solution of cobalt and thoria salts, e.g. by an "incipient wetness" technique. The catalyst is slowly reduced by heating in two steps in the presence of hydrogen. In the first step it is heated at a rate of 0.5 to 5 C/min to a hold temperature of 180 to 220 C where it is held for 6 to 24 h. Thereafter it is heated at a rate of 5 to 20 C to a final hold temperature of 250 to 400 C where it is held for 6 to 65 h. No rim type catalysts are described. According to the present invention there is provided a method of preparing a supported cobalt rim-type catalyst particle which comprises: (a) contacting a support particle with a molten salt of a catalyst metal comprising or consisting of cobalt at a temperature between its melting point and its decomposition temperature for a period sufficient to deposit the catalyst metal salt on the support; (b) removing any liquid-phase component from the particle produced in step (a), and; (c) reducing catalyst metal in the particle produced in step (b) to metallic form by heating the particle in the presence of H₂, wherein the heating rate is a single steady rate of less than 1°C/min. to a maximum temperature in the range of from 200°C to 500°C, to produce a supported metal catalyst particle. The novel process disclosed herein for preparing hydrocarbon synthesis catalyst provides many advantages such as: (a) the capability of being carried out in fewer process steps than previous processes, (b) eliminating the need for high temperature calcination of the catalyst metal precursor before reduction, (c) eliminating the need to expose the catalyst to further high temperature oxidation or reduction after the initial reduction and (d) avoids the need to reduce the catalyst precursor more than once. The catalyst may be prepared by melt impregnation techniques. The catalyst metal is directly reduced to the metal from a salt starting material. As such, the normal calcining step between drying and the initial reduction is eliminated and any subsequent oxidation or reduction steps are not needed to achieve a catalyst of high dispersion and activity. Furthermore, because site density is a function of catalyst metal dispersion, high temperature calcination of the dried, impregnated catalyst which produces agglomeration is therefore avoided. Description of the Figures Figures 1 through 3 are photomicrographs of a cross section of a Co/SiO₂ catalyst prepared in accordance with the preferred process of this invention by the procedure outlined below under Experimental Procedures B. Cobalt Distributions. Figures 4 through 6 are similarly obtained photomicrographs of Co/Zr/SiO₂ catalyst prepared by the method disclosed in U.S. Patent 4,599,481. Figures 7 and 8 are similarly obtained photomicrographs of catalyst E and catalyst F described below and prepared by the method of this invention. Figure 7 has a scale that shows not only that the rim is very clearly defined by thickness but that the rim is about 150 µm in thickness. Detailed Description The hydrocarbon synthesis, Fischer-Tropsch, reaction is well known and the operating conditions for it have been well described in many publications. In general, temperatures may range from about 160 to 300°C, preferably about 190°C to 260°C while pressures may vary widely, e.g., above about 5.52 bar (80 psig), preferably 5.52 bar (80 psig) to 41.37 bar (600 psig), more preferably 9.65 bar (140 psig) to 27.58 bar (400 psig). The gas hourly space velocity can range from 100 v/hr/v to 5000 v/hr/v, preferably 300 to 1500 v/hr/v. The hydrogen to carbon monoxide ratio may range widely, for example, at least 0.5/1 to as high as 10/1, preferably 0.5/1 to 4/1 and more preferably 1/1 to 2.5/1. With the catalyst of this invention, lower hydrogen to carbon monoxide ratios, that is, less than about 1.7/1 or 1.5/1 to 1.7/1 have been found particularly effective. Catalyst site density, as defined herein, is the number of catalyst surface atoms [sites] per catalyst volume. Site density may be qualitatively defined as the number of sites per unit volume in the catalyst promoting the reaction between hydrogen and carbon monoxide to from C₅+ hydrocarbons. Quantitatively, site density is defined herein as A B x D x g wherein A is the number of grams of catalyst metal per gram of catalyst, g Co/gm cat; B is the molecular weight of the catalyst metal, g/g-atom; D is dispersion, g-atom surface catalyst metal/g-atom total catalyst metal, as determined by hydrogen chemisorption; and g is the specific gravity of the catalyst (support + catalyst metal), g/liter. Catalytic metals for Fischer-Tropsch reactions have been widely reported as cobalt, ruthenium, iron and nickel. However, when making rim-type catalyst cobalt is the preferred metal and is employed in amounts of 5 to 50 wt.% of the catalyst, and preferably in amounts of 7 to 30 wt.%. Promoters may be employed such as zirconium and, if employed, used in amounts ranging from 0.1 to 10 wt.% of catalyst. The hydrocarbon synthesis reaction may be carried out in either fixed-bed, trickle-bed, slurry or fluid bed systems. In a fixed-bed system, catalyst pellets of 0.5-3 mm effective diameter as spheres or cylinders are loaded into a tube and synthesis gas is passed into the tube for conversion to heavy hydrocarbons as liquids. The hydrocarbons produced are C₅+, preferably C₁₀ to C₄₀ and higher, preferably substantially paraffinic. Large pellets are needed in fixed-bed reactors in order to minimize pressure drop restrictions. The cobalt metal is supported on a carrier and, generally, inorganic refractory oxides are employed as supports. Preferred supports are silica, magnesia, alumina, silica-alumina, and titania and of these, supports having an increasing surface area are preferred relative to supports of lower surface area because the higher surface area supports stabilize higher Co dispersions. The particular preparation method of this invention permits high catalyst loadings with high dispersion levels and greater site density. Generally, higher surface area materials will permit higher catalyst loadings and dispersions. Frequently, they also provide higher pellet porosity, thus enhancing reactant diffusion rates. Preferred surface areas range from 50-500m²/g of Al₂O₃, SiO₂, carbon, MgAl₂O₃, MgCr₂O₄, TiO₂ or substituted titanias. A key aspect in the preparation of rim type catalyst is the use of a cobalt source of sufficient viscosity and at loading conditions, to prevent capillary action within the support material from pushing the cobalt salt into the interior regions of the support. By virtue of the invention, a catalyst can be prepared that has a well defined rim, that is, where the cobalt is virtually completely located in a narrow band on the outer surface of the particle. In contrast to catalysts prepared by other methods, e.g., dipping from a solution, virtually none of the cobalt migrates to the interior of the catalyst particle. See, for example, Figures 4, 5, 6, and 8 which are examples of a prior art dipping technique and show that cobalt is deposited well inside the catalyst particle and thereby negates to a substantial degree the rim concept for eliminating diffusion limitation problems in Fischer-Tropsch reactions. Generally, the cobalt is concentrated in the outer portion of the support as a rim. The term coating while used herein is inappropriate because it suggests a catalyst particle where the cobalt forms a coating on the support material as opposed to a catalyst particle wherein the cobalt is deposited within the internal pore structure of the support material and located near the external surface of the pellet. A better term is a rim catalyst and a coated catalyst for purposes of this invention is deemed synonymous to a rim catalyst. The thickness of the rim is important because of diffusion limitation criteria. When hydrogen diffuses through the liquid-filled pore system into the particle more quickly than carbon monoxide, the catalytic metal - cobalt - must be located close enough to the exterior surface of the particle so that it will promote the hydrocarbon synthesis reaction before the hydrogen/carbon monoxide ratio becomes so great as to affect the reaction product and lead to increased methane production. The rim thickness should be no greater than about 300 µm and rim thickness above this limit (at usual temperature, pressure, and catalyst productivities) the H₂/CO ratio will be too high for maximizing the desired C₅+ products. Preferably, the coating or rim is lens than about 200µm in thickness and more preferably 50-150µm, wherein the site density is relatively high, for example, greater than about 0.04 g-atom surface/liter catalyst. The new rim catalyst is prepared by a novel technique that employs a catalyst metal, preferably cobalt, in liquid form, as a molten salt or molten hydrated salt, of sufficient viscosity to prevent capillary forces from driving the liquid into the interior of the support. The composition for a rim type catalyst produced by following this invention has at least 95%, preferably at least 98% of the cobalt located on the rim. Assuming, for example, a spherical catalyst particle of 2.2 mm diameter, all of the cobalt located in a rim of 200 µm will be located in a rim having a volume of 45% of the total catalyst particle volume; for 150 µm rim the volume will be 35.5% and for a 100 µm rim the volume will be 24.8%. The method of preparation using a cobalt containing liquid of appropriate viscosity allows the preparation of rim catalysts of the proper rim thickness. This may result from a limited intrinsic metal ion transport rate in viscous media or may be due to the rapid crystallization and pore blocking by the cooling molten salt as it contacts the colder particles. The desired preparation is achieved by using the molten form of a water soluble cobalt salt, for example cobalt nitrate Co(NO₃)₂. Cobalt nitrate may also occur as a hydrate, Co(NO₃)₂·6H₂O, and is a solid at room temperature. In making the rim catalyst, the salt is melted at a temperature slightly above the melting point of the salt (65°C) but not so high as to change substantially the viscosity of the resulting liquid, e.g., 5 to 25°C higher than the melting point. The molten salt is preferably employed in the substantial absence of water or any other low viscosity polar or a polar protic solvent, although up to about 10 wt.% water may be tolerated, preferably less than about 5 wt.% H₂O, based on weight of solid salt. The essential feature of the use of a molten salt or hydrated salt is the use of a salt, normally solid at room temperature, that melts before it decomposes. Thus, the useable temperature range of any salt in the practice of this invention is that temperature range between the melting point and the decomposition temperature. If a hydrated salt is used or if added water is present, the useful temperature range is preferably in the lower portion of the usable temperature range and minimal experimentation can further define the limits. Use of molten salts provides the appropriate viscosity to allow preparation of rim-type catalysts. We have illustrated the invention with molten cobalt nitrate, whether or not hydrated but other cobalt molten salts that meet the criteria of melting before decomposing may be used, for example, CoBr₂·6H₂O (Tm = 47°C), CoCl₂·6H₂O (Tm = 86°C), Co(C₁₆H₃₁O₂)₂ (palmitate, Tm = 70.5°C), Co(OOCC₆H₅)₂ (benzoate, Tm = 115°C) or Co(C₅H₇O₂)₃ (pentanedionate, Tm = 216°C). While not wishing to be bound by an particular theory, the use of the substantially anhydrous molten salt for impregnation may create a situation where the active species, e.g., the cobalt nitrate molten salt, is strongly adsorbed by the support particle and diffusion of this active species is limited. The presence of water reduces the adsorptivity of the active species while decreasing solution viscosity and allows capillary inhibition to be the predominant transport process, thereby creating a particle throughout which the cobalt is evenly distributed. However, if capillary forces are small compared to the inertial (viscous) forces, the particle will have the cobalt concentrated on its exterior surface with very little, if any, cobalt on the interior surface. The use of additives to change capillary transport characteristics and limiting the time of contact are well known procedures for concentrating a catalytic metal at any portion or portions of the catalyst particle. (Reference: S. Kulkarni et al, J. Catal. 69 , 445 (1981), and references therein. After depositing the cobalt onto the support, such as, silica, the catalytic precursor is dried, for example, at about 120°C for about 30 minutes to several hours. The reduction follows the drying step directly so that a separate high temperature calcination step (wherein the catalyst is heated above 250-500°C in an oxygen containing stream to convert the catalytic metal salt to the oxide) is omitted. The reduction takes place in the presence of a reducing agent, preferably hydrogen gas, at a steady heating rate of less than about 1°C/min. and preferably less than about 0.4°C/min. and more preferably less than about 0.2°C/min. to a maximum temperature ranging from 200-500°C for a period of time sufficient to substantially reduce the valence state of the cobalt to zero. The phrase "steady heating rate" as used herein, is defined as a rate where the ratio of temperature change in degrees centigrade (°C) as a function of time (°C/min.) is constant. The phrase "drying" or "dried" as used herein is defined as a procedure wherein the raw catalyst sample is heated to substantially remove liquid phase components, such as water or other solvents, from the catalyst particle. This is generally achieved by heating the catalyst to temperatures up to 120°C for from 30 minutes to several hours. Table A, below, shows the effect of high temperature calcination on a cobalt/silica catalyst and on site density. As can be seen, greater site density is achieved in the absence of the usual calcination step. During high temperature calcination, generally conducted at temperatures in excess of about 200°C, e.g, 300 to 400°C, cobalt tends to agglomerate, as the oxide, into larger particles which remain after the subsequent reduction step and result in lower dispersion. As noted above, by eliminating the high temperature calcination step and directly reducing the catalyst to the metal, agglomeration is avoided and dispersion is enhanced. EFFECT OF CALCINATION ON COBALT DISPERSION (Co/SiO₂, Catalyst B) Pretreatment Site Density g-atom Co surface Dispersion (%) liter catalyst Calcination/450°C 0.032 2.9 Reduction/450°C Direct Reduction/450°C 0.064 5.7 We have also found that slower single steady heating rates during the reduction step of less than about 1°C/min. avoid catalyst metal sintering and produce a catalyst having increased dispersion. One possible theory is that the slower heating rate causes a decreased evolution rate of cobalt salt solution reduction products, such as, NO₂, H₂O, NOx, and potential decomposition products of the anion of the salt (e.g., HBr, HOAc, etc. for nitrate free salts). This in turn is believed to cause dissolution and sintering of the cobalt particles. In addition, the relatively slow heating rate allows ample time at lower temperatures for the precursor salt to decompose. This precludes the need to expose the catalyst to excessively high temperatures which would lead to thermal sintering, i.e., the growth of large, relatively low surface area cobalt crystallites. The method of this invention allows the synthesis of highly dispersed cobalt, and reduces the need for other structural or electronic promoters such as thoria, ruthenium and platinum, which in the prior art are taught to increase catalyst activity. Preparation of catalysts in accordance with this invention leads to catalysts of enhanced dispersion, wherein the cobalt particles are smaller than expected, i.e., below about 17 nm (170 angstroms), and more of the cobalt atoms are available to catalyze the desired reaction between carbon monoxide and hydrogen to form heavy hydrocarbons such as C₅+. As the cobalt site density is increased, the use of the rim catalyst in hydrocarbon synthesis reactions also increases the C₅+ productivity (defined as the volume of CO converted to C₅+ per volume of catalyst per hour). The invention will be further illustrated by the following examples. The examples are not to be taken as limiting the scope of the invention in any way. Experimental Procedures A. Chemisorption Physisorption and chemisorption techniques in the characterization of supported catalysts was first demonstrated in the elegant work of Emmett, P.H., Brunauer, S., J. Am. Chem. Soc., 56 , 35 (1934). Measurements of total surface area using dinitrogen physisorption (BET) were followed by the use of selective dinitrogen chemisorption to determine the number of Fe surface atoms on promoted ammonia synthesis catalysts. We have used molecular hydrogen as a probe of surface cobalt atoms in hydrocarbon synthesis catalysts. We measured dihydrogen uptakes in an all-glass static chemisorption unit, pumped by diffusion and roughing pumps isolated from the system by liquid nitrogen traps, and capable of a dynamic vacuum of 10⁻⁷ Torr. Prereduced and passivated samples were rereduced in flowing dihydrogen (200 cm³(STP)/g-cat-min) for 1-2 hours at 200-800°C, and then evacuated to less than 10⁻⁶ Torr for 0.5-1 hour at a temperature sufficient to remove all chemisorbed hydrogen (>250°C). The samples were then cooled to the adsorption temperature (-77 to 150°C) and isotherms were measured at 3 to 5 hydrogen pressures between 100 and 700 Torr. A backsorption isotherm was sometimes measured by evacuating the sample to 10-6 Torr at the adsorption temperature for 0.5h, and then measuring the hydrogen uptakes again between 100 and 500 Torr. Adsorption and backsorption isotherms were extrapolated to zero pressure to obtain the total and weak chemisorption uptakes, respectively. Dispersions were calculated from the total hydrogen uptakes at 100°C and from the cobalt content of the samples, assuming a 1:1 stoichiometry of hydrogen to surface cobalt atoms. Dispersion is defined as the fraction or percentage of the total number of cobalt atoms in the catalyst that reside at the surface of cobalt crystallites and provides a convenient measure of the number of surface sites that are available for catalysis. B. Cobalt Distributions Cobalt distributions within support pellets were obtained by X-ray line profiles using a JEOL-35C (JEOL, Tokyo, Japan) SEM (Scanning Electron Microscope) equipped with an annular, backscattered electron detector, a four crystal X-ray wavelength spectrometer, and a PGT System 4 X-ray energy dispersive spectrometer (Princeton Gamma Tech., Princeton, New Jersey, USA). Wavelength dispersive scans collect element-specific Co K-α radiation, at an accelerating voltage of 25 Kev to measure the cobalt distribution across the pellet. C. Support Surface Area/Pore Volume Distributions Support surface areas and pore volume distributions were measured by dinitrogen physisorption at 77K, as described in Smith, J.M., Chemical Engineering Kinetics, McGraw-Hill, pp. 291-318, 2nd Edition, 1970. EXAMPLE 1 Preparation of 13% Co/SiO₂ Rim Catalyst (Catalyst B) by Melt-Impregnation Technique Silica spheres of nominal 2.2 mm diameter are calcined for 16 hours at 600°C. Surface area measures 80 m²/g with a pore volume of 1.0 ml/g. 12.5 grams of the spheres are weighed out. 50 grams of Co(NO₃)₂·6H₂O are weighed out into a flask and heated to melt the cobalt nitrate salt. Temperature is kept between 85 and 95°C. No additional water or other solvent is added to the melt. The silica spheres are added to a vacuum filter that contains a 5 to 10 mm layer of 6 mm hollow glass beads. The molten cobalt nitrate solution is poured over the silica spheres with vacuum applied so that the residence time of the molten liquid is 2-4 sec. The spheres are then dried at 120°C. Measurement of the penetration depth of the cobalt is 0.05-0.15 mm (Fig. 1). The cobalt loading is 13%. This indicates that the local loading in the rim is 40 to 50%. The sample is reduced at 0.2 deg. C/min. from room temperature to 350 deg. C. The dispersion of cobalt is 5.5% corresponding to 17 nm (170Å) diameter crystallites assuming hemispherical particles. A comparative sample is prepared by dissolving the 50 grams of cobalt nitrate in 75cc of water. This solution is added to 12.5 grams of the silica spheres in a vacuum funnel with a layer of porous glass beads. The sample is dried at 120°C. Cobalt now has penetrated into the entire sphere. The cobalt distribution is very similar to that of Catalyst A (see Example 3). EXAMPLE 2 Preparation of 20.9% Co/SiO₂ Rim Catalyst (Catalyst E) by Melt-Impregnation Technique Silica spheres of nominal 2.2 diameter are calcined for 16 hours at 600°C. Surface area measures 350m²/g with a pore volume of 1.3 ml/g. 12.5 grams of the spheres are weighed out. 50 grams of Co(NO₃)₂·6H₂O are placed in a flask and heated to melt the cobalt nitrate salt. Temperature is kept at between 75 and 85°C. No additional water or other solvent is added to the melt. The silica spheres are added to a vacuum filter that contains a 5 to 10 mm layer of 6 mm hollow glass beads. The molten cobalt nitrate solution is poured over the silica spheres with vacuum applied so that the residence time of the molten liquid is 2-4 sec. The spheres are then dried at 120C. The sample is reduced at 0.2 deg. C/min from room temperature at 350 deg. C. Melt impregnation and reduction procedures are repeated on prereduced sample. Cobalt dispersion is 7.3% corresponding to 13 nm (130Å) diameter crystallites. The higher surface area support leads to better Co dispersion than in Catalyst B, in spite of the much higher cobalt loading in this catalyst. EXAMPLE 3 Preparation of Evenly Impregnated 13% Co/SiO₂ Spheres (Catalyst A) Co(NO₃)₂·6H₂O (17.4g) was dissolved in enough water to make a solution of 30 cc total volume. This solution was contacted (incipient wetness) with 20 grams of the silica spheres used in Catalyst B. The spheres were dried at 120°C. Examination indicated that the cobalt had penetrated into the entire sphere. The cobalt loading is 13% wt. (Catalyst A). EXAMPLE 4 Preparation of 13% Co/SiO₂ Powder by Grinding of Catalyst B (Catalyst C) Catalyst B is ground and passed through 80/140 mesh screens to isolate catalysts particles sized between 100 and 150 microns. EXAMPLE 5 Preparation of 14.5% Co/SiO₂ Powder by Incipient Wetness of Powder with Cobalt Nitrate Solution (Catalyst D) SiO₂ Powder (Davison 62, 100-180 um pellet size, 280m²g⁻¹) was impregnated to incipient wetness with cobalt nitrate solution to give a uniform cobalt loading of 14.5% wt. The catalyst was dried overnight at 120°C, and reduced in flowing hydrogen at 450°C. EXAMPLE 6 Preparation of 11% Co/0.6% Zr/SiO₂ Catalyst as Described in U.S. Patent No. 4,599,481 (Catalyst F) This preparation was carried out following the procedure for catalyst 4 outlined in the patent, 138g of Co(NO₃)₂·6H₂O was dissolved in 200 cc of 99.9% ethanol (15 wt% Co). 15g of silica spheres 350m²/g, 2.2 mm diameter were immersed in the above solution at room temperature for 5 seconds. The spheres were then dried for 2 hours at 100°C and then calcined at 500°C for 1 hour. The immersion/dry/calcine sequence was repeated one more time to complete the cobalt loading step, as described in the aforementioned patent. The zirconium was loaded using incipient wetness. 1 wt% Zr was prepared by dissolving 0.25g of ZrO(NO₃)₂ xH₂O (H₂O = 3%) in 13cc H₂O. This solution was added dropwise of 10g of the above silica spheres. The solution was dried at 100°C for 2 hours and calcined at 500°C for 1 hour. The sample was then reduced at 250°C for 24 hours with an 0.2°C/min heat-up rate in flowing hydrogen. EXAMPLE 7 Comparison of Evenly Distributed Catalyst (Catalyst A) and Rim Catalyst (Catalyst B); Cobalt on 2.2 mm SiO₂ Spheres The hydrocarbon synthesis activity and selectivity of evenly impregnated (A) and radially impregnated (B) Co on 2.2 mm SiO₂ spheres were measured in a fixed bed plug flow reactor at 200°C, 2100 KPa, H₂/CO=2, at a space velocity required for 60-65% CO conversion. All reported data ware obtained after at least 75h on stream. Placing the active Co sites within 100-150 microns of the SiO₂ sphere surface increased the volumetric activity from 90 to 150 h⁻¹, in spite of a higher local loading (40 vs 13%) and a concomitant lower dispersion 5.5 vs 6.3% in the rim catalyst. (Table 1) Also CH₄ selectivity decreases (12.0 to 8.3%) and C₅+ selectivity increases (82 to 87%) when cobalt is non-uniformly impregnated. The activity and selectivity on the rim cobalt catalyst resemble those on an evenly impregnated 150 µm Co/SiO₂ powder (Table 1). CH₄ selectivity is slightly higher in the rim catalyst (8.3 vs 7.5%) but because of a much lower C₂-C₄ selectivity (4.4 vs 9.2%), C₅+ selectivity is actually higher in the rim catalyst (87 vs 83%). The results suggest that diffusional limitations are almost completely eliminated by concentrating cobalt in the outer surface and that the volumetric activity and selectivity of conventional powders can be achieved with commercial-sized pellets, by using radially impregnated (or rim) catalysts prepared by the method of this invention. EXAMPLE 8 Crushing of Co/SiO₂ Rim An additional activity and C₅+ selectivity increase was obtained when the rim catalyst was crushed into smaller particles, suggesting slight diffusional limitations were still operative in 100-150 µm rims (Catalyst B). Volumetric rates increased from 150 to 210 h⁻¹ and C₅+ selectivity from 87 to 90.5 %. CH₄ selectivity decreased from 8.3 to 4.7% but C₂-C₄ selectivity was not affected. Comparison of Even and Rim Co on SiO₂ Spheres (13% wt. Co) Catalyst A Even (2.2 mm Spheres) Catalyst B Rim (100-150 µm on 2.2 mm spheres) Catalyst D 13% Co/SiO₂ (100-150 µm powder) Site Density (g-atom surface Co/liter cat) 0.069 0.064 0.065 Space Velocity (cm³ H₂ + CO/cm³ cat·h) 420 650 920 CO Conversion (%) 64.5 66.0 48.0 Volumetric Rate (cm³ CO converted/cm³ cat·h) 90 150 150 Selectivity (%) CH₄ 12.0 8.3 7.5 C₂-C₄ 5.1 4.4 9.2 C₅+ 82.0 87.0 83.2 CO₂ 0.9 0.4 0.2 Cobalt-Time Yield (mole CO/g-atom Co·h) 3.3 5.7 5.3 Cobalt dispersion (%) 6.3 5.5 6.0 Site-Time-Yield (mole-CO/g-atom Co surface·h) 50 100 90 Run 135-149 134-268 128-827 Time on stream (h) 193 318 76 Conditions: 200°C, 2100 kPa, H₂/CO=2 Comparison of Crushed and Uncrushed Rim Co on SiO₂ (13% wt. Co) Catalyst B Rim (2.2 mm SiO₂ Spheres) Crushed Catalyst B (0.1-0.2 mm granules) Site Density (g-atom surface Co/liter cat) 0.064 0.064 Space Velocity (cm³ H₂ + CO/cm³ cat·h) 650 940 CO Conversion (%) 66.0 48.0 Volumetric Rate (cm³ CO converted/cm³ cat·h) 150 210 Selectivity (%) CH₄ 8.3 4.7 C₂-C₄ 4.4 4.7 C₅+ 87.0 90.5 CO₂ 0.4 0.1 Cobalt-Time Yield (mole CO/g-atom Co·h) 5.7 5.5 Cobalt dispersion (%) 5.5 5.5 Site-Time-Yield (mole-CO/g-atom Co surface·h) 100 150 Run 134-268 137-538 Time on stream (h) 318 175 Conditions: 200°C, 2100 kPa, H₂/CO=2 EXAMPLE 9 Comparison of High Loading/High Surface Area (Catalyst E) and Low Loading/Low Surface Area (Catalyst B) Rim Catalysts Prepared by Melt Impregnation Increasing support surface area from 80 to 350 m²g⁻¹ allowed the use sequential melt impregnation procedures to give higher cobalt loading (20.9 vs 13 wt.%) with an actual increase in cobalt dispersion. As a result, volumetric rates increased from 150 to 342 h⁻¹ at 2100 kPa, 200°C, and 2.1 H₂/CO ratio (Table 3). A the same conditions, C₅+ productivity increased from 130 to 278 h⁻¹. The required catalyst volume for a given productivity per reactor can be decreased by a factor of two. C₅+ selectivity decreases slightly because of increased CO diffusional limitations for the higher activity 20.9% Co/SiO₂ catalyst, but it is more than compensated by the higher total volumetric productivity of Catalyst E. Comparison of High Surface Area/High Loading and Low Surface Area/Low Loading Co/SiO₂ Rims (200°C) Site Density (g-atom surface Co/liter cat) 0.064 0.129 Rim Thickness (µm) 80 ± 30 120 ± 30 Catalyst B E (support area/m² g⁻¹) 80 350 % Co Loading 13 20.9 Space Velocity (cm³ H₂ + CO/Cm³ cat·h) 650 1750 CO Conversion (%) 66 64 Volumetric Rate (cm³ CO converted/cm³ cat·h) 150 342 Selectivity (%) CH₄ 8.3 12.7 C₂-C₄ 4.4 5.3 C₅+ 87.0 81.3 CO₂ 0.4 0.7 C₂+ productivity (cm³ CO to C₂+/cm³ cat·h) 137 296 C₅+ productivity (cm³ CO to C₅+/cm³ cat·h) 130 278 Cobalt-Time Yield (mole CO/g-atom Co·h) 5.7 9.5 Cobalt Dispersion (surface CO/total Co x 100%) 5.5 7.3 Site-Time-Yield (mole-CO/g-atom Co surface·h) 100 130 Run 134-268 154-657 Time on stream (h) 318 315 Pressure (kPa) 2100 2100 H₂/CO ratio 2.1 2.1 EXAMPLE 10 Comparison of 20.9% Co/SiO₂ Rim (Catalyst E) at 2.1 and 1.65 H₂/CO Ratios CH₄ selectivity can be decreased, and C₅+ selectivity increased, with little change in total volumetric productivity by operating the catalyst at a sub-stoichiometric H₂/CO ratio (1.65 to 2.1) (Table 4). CH₄ selectivity decreases markedly from 12.7 to 7.7%, while C₅+ selectivity increases from 81.3 to 85.9%. C₅+ productivity decreases by about 10%, from 278 h⁻¹ to 241 h⁻¹. This mode of operation allows more effective utilization of the synthesis gas, while maintaining the high volumetric productivity observed at the stoichiometric consumption ratio (2.1/1) on these Co/SiO₂ rim catalysts. Effect of Decreasing H₂/CO Ratio From 2.10 to 1.65 (20.9% Co/SiO₂ Rim) Space Velocity (cm³ H₂ + CO/cm³ cat·h) 1750 1250 CO Conversion (%) 64 62.5 Volumetric Rate (cm³ CO converted/ cm³ cat·h) 342 280 Selectivity (%) CH₄ 12.7 7.7 C₂-C₄ 5.3 5.6 C₅+ 81.3 85.9 CO₂ 0.70 0.80 C₂+ productivity (cm³ CO to C₂+/cm³ cat·h) 296 258 C₅+ productivity (cm³ CO to C₅+/cm³ cat·h) 278 241 Cobalt-Time Yield (mole CO/g-atom Co·h) 9.5 7.7 Cobalt Dispersion (surface Co/total Co x 100%) 7.3 7.3 Site-Time-Yield (mole-CO/g-atom Co surface·h) 130 107 Run 154-657 154-681 Time on stream (h) 315 385 Pressure (kPa) 2100 2100 H₂/CO ratio 2.1 1.65 EXAMPLE 11 Comparison of 20.9% Co/SiO₂ Rim (Catalyst E) at 630 and 2100 kPa (H₂/CO - 2.1) Methane Selectivity can be decreased, and C₅+ selectivity and productivity increased by operating the catalyst at elevated pressure. Increasing H₂/CO pressure from 630 to 2100 kPa at 2.1/1 H₂/CO ratio decreases methane selectivity from 20.6 to 13.5%, while C₅+ selectivity and productivity increase from 67 to 80.9% and 94.5% to 302 h⁻¹, respectively (Table 5). This elevated pressure mode of operation not only decreases catalyst volume for a given reactor productivity, but also allows more efficient utilization of the synthesis gas feed, by minimizing the fraction of it converted to undesired products. Pressure Effect on 20.9% Co/SiO₂ Rim (Catalyst E) (H₂O/CO - 2.1) Space Velocity (cm³ H₂ + CO/cm³ cat·h) 750 1915 CO Conversion (%) 61 63.5 Volumetric Rate (cm³ CO converted/ cm³ cat·h) 141 373 Selectivity (%) CH₄ 20.6 13.5 C₂-C₄ 10.8 5.1 C₅+ 67.0 80.9 CO₂ 1.6 0.5 C₂+ productivity (cm³ CO to C₂+/cm³ cat·h) 110 321 C₅+ productivity (cm³ CO to C₅+/cm³ cat·h) 94.5 302 Cobalt-Time Yield (mole CO/g-atom Co·h) 3.9 10.3 Cobalt Dispersion (surface Co/total Co x 100%) 7.3 7.3 Site-Time-Yield (mole-CO/g-atom Co surface·h) 50 140 Run 154-628 154-635 Time on stream (h) 145 170 Pressure (kPa) 630 2100 H₂/CO ratio 2.1 2.1 EXAMPLE 12 Comparison of 20.9% Co/SiO₂ Rim (Catalyst E) at 2100 and 4000 kPa (H₂/CO - 1.65) Methane Selectivity can be decreased, and C₅+ productivity increased by operating the catalyst at elevated pressure. Increasing H₂/CO pressure from 2100 to 4000 kPa at a 1.65 H₂/CO ratio decreases methane selectivity from 7.5 to 7.2%, while C₅+ productivity increases from 221 to 264 h⁻¹, (Table 6). This elevated pressure mode of operation not only decreases catalyst volume for a given reactor productivity, but also allow more efficient utilization of the synthesis gas feed, by minimizing the fraction of it converted to undesired products. Effect of Increasing Pressure from 200 kPa to 4000 kPa (H₂/CO - 1.65, Catalyst E) 20.9 Co/SiO₂ Rim Catalyst Space Velocity (cm³ H₂ + CO/cm³ cat·h) 1170 1500 CO Conversion (%) 61.5 58 Volumetric Rate (cm³ CO converted/ cm³ cat·h) 255 310 Selectivity (%) CH₄ 7.5 7.2 C₂-C₄ 4.9 6.9 C₅+ 86.7 85.2 CO₂ 0.85 0.80 C₂+ productivity (cm³ CO to C₂+/cm³ cat·h) 234 285 C₅+ productivity (cm³ CO to C₅+/cm³ cat·h) 221 264 Cobalt-Time Yield (mole CO/g-atom Co·h) 7.07 8.50 Cobalt Dispersion (surface Co/total Co x 100%) 7.3 7.3 Site-Time-Yield (mole-CO/g-atom Co surface·h) 98 118 Run 154-688 154-720 Time on stream (h) 508 530 Pressure (kPa) 2100 4000 H₂/CO ratio 1.65 1.65 EXAMPLE 13 Comparison of Rim Catalyst from U.S. 4,599, 481 (Co/Zr/SiO₂, Catalyst F) With Melt Impregnated Catalysts of This Invention (Catalyst B and E) Catalysts B and E show 2.0 to 4.6 times the C₂+ productivity of Catalyst F (Table 7). Total volumetric rates are 2.1 to 4.9 times higher on the catalysts of this invention. These improvements are the result of the thinner Co-coated region and of the sharper cobalt profiles obtained with the melt impregnation technique. The absence of Zr, and our pretreatment conditions (no calcination/slow reduction) also lead to a larger fraction of the loaded cobalt being available to reactants during catalysis (higher cobalt dispersion). Finally, when compared at equal CH₄ and C₂+ selectivities, rather than at constant H₂/CO ratio, Catalyst E still give 4.3 times higher C₂ productivity than Catalyst F (Table 8). EXAMPLE 14 A Variation on the Method for Preparing Rim Type Cobalt on Silica Catalysts (Catalyst G) The preparation is a vacuum-assisted molten cobalt nitrate (Co/NO₃)₂·6H₂O) loading method. Silica spheres were placed in a filter funnel with a vacuum flask underneath. The molten cobalt nitrate was poured over and filtered through the evacuating silica sphere bed. Specifically, using a 1.5 inch diameter glass filter funnel with a 12 mesh 304 SS screen inserted in the bottom. A charge of 12.5 grams of silica spheres (1.5 mm diameter) was placed in the filter funnel creating a 1 inch bed height. The filter funnel is placed on top of a filter flask and a vacuum was pulled on the bed. 30 grams of cobalt nitrate (ACS Grade) were placed in a beaker on a hot plate and heated to 80 degrees centigrade. The cobalt nitrate liquid was removed from the hot plate and allowed to cool to 70 degrees centigrade. The molten cobalt nitrate was quickly poured over the evacuating silica spheres and the excess liquid was pulled through the bed and into the vacuum flask. The silica spheres were immediately stirred with a glass stirring rod to minimize sticking and poured out into a porcelain bowl. The cobalt nitrate coated spheres were oven dried at 100 degrees centigrade for two hours and then reduced in flowing hydrogen at 320 degrees centigrade. The entire procedure was repeated to increase the cobalt loading to 27% wt. This method results in a catalyst of well defined metal penetration, and high site density. Table 9 shows that this preparation procedure leads to a catalyst of high activity and site density, having with decreased CH₄ selectivity (5.1 vs. 7.7%) and increased C₅+ selectivity (90.1 vs 85.9%). Comparison of Catalyst E (Example 2) and G (Example 14) Catalyst G E Space Velocity (cm³ H₂ + CO/Cm³ cat·h) 1100 1250 CO Conversion (%) 64 62.5 Volumetric Rate (cm³ CO converted/cm³ cat·h) 250 280 Selectivity (%) CH₄ 5.1 7.7 C₂-C₄ 3.9 5.6 C₅+ 90.1 85.9 CO₂ 0.9 0.80 C₂+ productivity (cm³ CO/ to C₂+/cm³ cat·h) 235 258 C₅+ productivity (cm³ CO to C₅+/cm³ cat·h) 226 241 Cobalt-Time Yield (mole CO/g-atom Co·h) 4.9 7.7 Cobalt dispersion (surface Co/total Co x 100%) 5.8 7.3 Site-Time-Yield (mole-CO/g-atom Co surface·h) 88 107 Run 51/848-842 154-681 Time on stream (h) 220 385 Pressure (kPa) 2100 2100 H₂/CO 1.65 1.65 Site Density (g-atom surface Co/liter cat) 0.126 0.129 % Co 27.0 20.9 EXAMPLE 15 Preparation of 10% Co/SiO₂ Powder Catalyst by Incipient Wetness Technique and Steady Heating Rates A cobalt solution, containing an equal volume cobalt solution to the pore volume of the support was prepared by dissolving 14.85 g of the hexahydrate of cobalt nitrite in acetone so that the total volume of solution equaled 54 cc. The solution was added drop-wise to 27 g of precalcined Davison 62 silica to impregnate by incipient wetness the cobalt on the silica support. For incipient wetness impregnation to occur, 2 cc of acetone per gram of silica was required. The resulting catalyst was dried at 120°C and reduced during various runs of different steady heating rates in the presence of H₂ to the temperature indicated. In some cases, the catalyst was subjected to the sequence of pretreatment steps as shown in Table 10. The results show improved dispersion for the slower single steady heating rates. The data also confirms that high dispersion, ergo enhanced catalyst activity, can be achieved by direct reduction of the catalyst from the impregnated cobalt nitrate support, rather than by first cooling the precursor. Most preferably, a combination of slow heating rate of the impregnated cobalt nitrate support (with no oxidation step) and a high gas space velocity produces the highest dispersion and most active catalysts. The data also shows that once formed by the initial reduction, subsequent oxidation/reduction cycles do not markedly affect the dispersion. Run Pretreatment Sequence Heating Rate and Reduction Temp. Temp. Chemisorption (°C) Percent Dispersion (°C/Min.) (°C) A H₂ 0.1 300 300 8.6 400 9.1 500 8.2 600 7.3 B H₂/O₂/H₂ 0.1 300 250 7.4 350 8.5 500 7.7 C H₂ 0.1 300 350 8.8 450 10.2 D H₂ 4.0 500 450 5.2 E O₂/H₂ 4.0 500 350 3.5 F H₂/O₂/H₂ 4.0 500 300 6.8 500 5.6 G H₂ 0.2 350 350 5.5 Notes : Runs A through F - Involved powdered catalysts prepared by incipient wetness impregnation. Run C - The normal gas space velocity SHSV of 3500 cm³/g cat/hr. was increased to 5300 cm³/g cat/hr. Run G - The catalyst was a rim type catalyst prepared by a melt impregnation technique (see Example 1).

Claims: A method of preparing a supported cobalt rim-type catalyst particle which comprises: (a) contacting a support particle with a molten salt of a catalyst metal comprising or consisting of cobalt at a temperature between its melting point and its decomposition temperature for a period sufficient to deposit the catalyst metal salt on the support; (b) removing any liquid-phase component from the particle produced in step (a), and; (c) reducing catalyst metal in the particle produced in step (b) to metallic form by heating the particle in the presence of H₂, wherein the heating rate is a single steady rate of less than 1°C/min. to a maximum temperature in the range of from 200°C to 500°C, to produce a supported metal catalyst particle. The method of claim 1 wherein the heating rate is a single steady rate of less than 0.4°C/min. (e.g., 0.2°C/min. or less). The method of claim 1 or claim 2 wherein the contacting occurs in the substantial absence of water other than any water of hydration. The method of any one of claims 1 to 3 wherein the catalyst metal salt is cobalt nitrate or hydrated cobalt nitrate. The method of any one of claims 1 to 4 wherein the metal salt is contacted with the support for less than about 10 seconds. The method of any one of claims 1 to 5 wherein the support is selected from the group consisting of silica, magnesia, alumina, silica-alumina, titania, and mixtures thereof. A method according to any one of claims 1 to 6 of preparing a supported cobalt rim-type catalyst particle which comprises: (a) contacting an inorganic refractory oxide support particle with a molten cobalt salt, in the substantial absence of water other than any water of hydration, at a temperature from 5 to 25 C higher than the melting point so as to provide sufficient viscosity and loading conditions to prevent capillary action within the support material from allowing the transport of cobalt salt into the interior regions of the support, for a period of time sufficient to deposit at least 95 wt.% of the cobalt in a rim of less than about 200 µm in thickness from the external surface of the particle; (b) removing any liquid-phase material from the particle produced in step (a); and (c) reducing cobalt component in the particle from step (b) to metallic cobalt by heating the particle ion the presence of H₂, wherein the heating rate is a single steady rate of less than about 1°C/min. to a maximum temperature in the range of from 200°C to 500°C and wherein the molten cobalt salt is at a temperature between its melting point and its decomposition temperature to produce a supported cobalt rim-type catalyst particle. The method of claim 7 wherein the catalyst particle has a cobalt dispersion of greater than about 4% and a cobalt site density of at least about 0.05 g-atom surface Co/liter. The method of any one of claims 1 to 8 wherein the support has a surface area in the range of from 50-500m²/gm. A cobalt catalyst particle prepared by the method of any one of claims 1 to 9 comprising cobalt on a support (e.g., silica) wherein substantially all of the cobalt is located in a rim of less than about 200 µm thickness from the external surface of the particle, the degree of dispersion of the cobalt is greater than about 4% and the cobalt site density is at least about 0.05 g-atom surface Co/liter of particles. The particle of claim 14 wherein the surface area of the support is at least about 50 m²/gm.

Inventors: BAUMGARTNER JOSEPH, FIATO ROCCO ANTHONY, IGLESIA ENRIQUE, SOLED STUART LEON, VROMAN HILDA, BAUMGARTNER, JOSEPH, FIATO, ROCCO ANTHONY, IGLESIA, ENRIQUE, SOLED, STUART LEON, VROMAN, HILDA
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1990
Main classification: B01J23
IPC codes: B01J23, B01J37, C07C1