DE69010321T2 - Process for the preparation of dispersed cobalt-containing surface-impregnated catalysts and hydrocarbon synthesis using these catalysts. - Google Patents

Description

Field of the invention

This invention relates to a process for the preparation of cobalt catalysts in which the metal is impregnated and distributed as a thin layer (film) or coating on the peripheral or outer surface of a particulate carrier or support, in particular a titania carrier or support.

2. Background of the invention

Particulate catalysts are commonly used in chemical, petroleum and petrochemical processing to chemically alter various gaseous and liquid feedstocks or to convert various gaseous and liquid feedstocks into more desirable products. Such catalysts are formed by dispersing (distributing) catalytically active metal or metals or compounds thereof on particulate carriers or supports. According to conventional wisdom, the catalytically active metal or metals are generally distributed as evenly as possible throughout the particle, thereby providing uniformity of catalytically active sites from the center of a particle outward.

The Fischer-Tropsch synthesis, a process for producing hydrocarbons from carbon monoxide and hydrogen (synthesis gas), has been used commercially in some parts of the world. This process may gain wider acceptance, possibly in this country, if adequate improvements can be made in the existing technology. The earlier Fischer-Tropsch catalysts consisted largely of base metals dispersed throughout a porous inorganic oxide support. In Fischer-Tropsch reactions, The Group VIII base metals iron, cobalt and nickel have been widely used and these metals have been activated with various other metals and supported in various ways on various substrates, primarily alumina. However, most commercial experience has been with cobalt and iron catalysts. The first commercial Fischer-Tropsch operation used a cobalt catalyst, although later more active iron catalysts were also commercialized. The cobalt and iron catalysts were formed by thoroughly mixing the metal and an inorganic oxide support. However, the early cobalt catalysts generally had low activity, requiring the use of a multi-stage process and low syngas throughput. The iron catalysts, on the other hand, produce too much carbon dioxide from the syngas, with too little of the carbon monoxide being converted to hydrocarbons.

US-A-4,794,099 describes and claims a hydrocarbon synthesis catalyst comprising cobalt in a catalytically active amount associated with an inorganic refractory support consisting of a major proportion of titania to which a minor proportion of up to about 15% by weight of silica in the form of silica or a silica precursor has been added.

EP-A-068 192 (and its Canadian counterpart CA-A-1 182 439) describes and claims a process for the preparation of abrasion-resistant coated catalysts from a surface-roughened, inert support with a particle size of 0.5 to 6 mm and a shell of active catalyst material, the shell surrounding and being anchored in the support, by agitating a charge of the support and spraying onto it a suspension of the starting material for the shell, whereby dispersant is partially removed by a gas stream at 20 to 250°C, a substantially constant residual moisture the shell is maintained, dried and tempered, the carrier charge is set in a mixing movement by mechanical action and at the same time is loosened from below by forcing in a swirling gas stream which intensifies the mixing process, the suspension of a precursor of the catalytically active material being fed to the powder in a countercurrent direction to the gas stream in amounts which increase with increasing thickness of the shell, and the suspension contains a binder, the amounts of binder removed and binder sprayed on are kept in a substantially constant ratio which is determined by the combination of carrier and precursor used in each case, the thermal expansion coefficients of the carrier and the dried, powdered precursor are adjusted so that they differ from each other by a maximum of 15%, and the shell is solidified on completion of the spraying process by continuing the intensified mixing movement, whereupon the mechanical mixing movement is stopped, the material is dried in the gas which continues to flow and finally tempered.

3. Tasks

It is therefore the primary object of the present invention to provide further improvements in the formation of these highly productive Fischer-Tropsch cobalt catalysts using a method of impregnation by spraying a catalytically effective amount of cobalt or of cobalt and an additional metal or metals onto the peripheral or outer surface of a particulate inorganic oxide support, particularly a particulate titania or titanium-containing support.

A further object is to provide a spray coating process according to the main object stated above, in which the cobalt or the cobalt and an additional metal or additional metals are applied to the surface of the particulate carrier particles sprayed, impregnated and distributed, while the latter are kept in a heated and swirled state.

A particular object of this invention is to provide a process for the preparation of cobalt catalysts and activated cobalt catalysts useful in Fischer-Tropsch synthesis, which employs a process for impregnating by spray coating a compound comprising cobalt or compounds comprising cobalt and an activator metal or metals onto the peripheral or external surface of a particulate inorganic oxide support, particularly a particulate titania or titanium-containing support, while maintaining the latter in a heated and fluidized condition.

Another, more specific object is to provide a new process for spray coating and impregnating cobalt or cobalt and an additional cobalt modifying or activating metal or cobalt modifying and activating metals onto a heated, fluidized, particulate titania or titania-containing support, particularly one in which the titania or titania-containing support, and especially one in which the titania or titania component has a high rutile:anatase ratio .

4. Description of the invention

These and other objects are achieved by this invention which comprises a process for the preparation of catalysts comprising dispersing a catalytically effective amount of cobalt as a layer on the peripheral outer surface of a particulate, porous, inorganic oxide support to form a catalyst useful for the conversion of synthesis gas to hydrocarbons, and spraying the support particles into containing a decomposable compound of the metal or metals, the process

- maintaining a bed of the carrier particles in a fluidized state at a temperature in the range of about 50ºC to about 100ºC (e.g. in the range of about 70 to about 90ºC) by contact with a gas at a temperature in the range of about 50ºC to about 100ºC,

- spraying the bed of heated support particles with a liquid having dispersed therein a compound or compounds of cobalt at a flow rate sufficient to provide a ratio of liquid flow rate:fluid gas flow rate of less than about 0.6 g liquid/foot³ (0.6 g/28.32 L) of fluidizing gas to form a surface layer of the metal on the particles having an average thickness in the range of about 20 microns (20 µm) to about 250 microns (250 µm), the loading of the metal calculated as metallic metal per packed bulk volume of the catalyst being in the range of about 0.01 g/cm³ to about 0.15 g/cm³.

Preferred inorganic oxide supports are alumina, silica, silica/alumina and titania, and of these, a titania or titania-containing support is most preferred, particularly a rutile-titania support or a support in which the titania or titania component has a rutile:anatase weight ratio of at least about 3:2. The catalytically active surface layer or film has an average thickness in the range of about 20 microns (20 µm) to about 250 microns (250 µm), preferably about 40 microns (40 µm) to about 150 microns (150 µm), with the cobalt loading expressed as the weight of metallic cobalt per packed Bulk volume of the catalyst is in the range of about 0.01 g/cm³ to about 0.15 g/cm³, preferably about 0.03 g/cm³ to about 0.09 g/cm³.

Metals such as rhenium, zirconium, hafnium, cerium, thorium and uranium or compounds thereof may be added to the cobalt to increase the activity and regenerability of the catalyst. Therefore, the thin, catalytically active layers or films formed on the surface of the support particles, particularly the titanium dioxide or titanium dioxide-containing support particles, may contain, in addition to a catalytically active amount of cobalt, one or more of rhenium, zirconium, hafnium, cerium, thorium and uranium or mixtures of these with each other or with other metals or compounds thereof. Preferably thin, catalytically active layers or films which are applied to a carrier, in particular a titanium dioxide or a titanium dioxide-containing carrier, therefore contain cobalt-rhenium, cobalt-zirconium, cobalt-hafnium, cobalt-cerium, cobalt-thorium and cobalt-uranium, with or without the additional presence of other metals or compounds thereof.

The carrier particles are fluidized in a bed and sprayed with a suspension or solution containing a cobalt compound or a compound or compounds of cobalt and an additional metal or metals which are catalytically active in a Fischer-Tropsch synthesis reaction. The temperature of the bed during spraying should be in the range of about 50°C to about 100°C and more preferably in the range of about 70°C to about 90°C. Suitably the carrier particles are fluidized in a gas, preferably air, and the fluidized bed of carrier particles is sprayed with the suspension or solution via one or a plurality of nozzles. A novel and key feature of the process is that a very high particle drying capacity is created during spraying by maintaining a temperature within a specified range of Conditions of temperature and flow rate of air, solution concentration of the metal or metals, compound or compounds and flow rate of suspension or solution from the nozzle or nozzles into the bed. In forming a film or coating of metal or metals of the required uniform thickness on the particulate support particles, e.g., as in coating a cobalt metal film on a particulate titanium dioxide support, the temperature of the bed during spraying or the spray temperature must be maintained in the range of about 50°C to about 10,000°C, and more preferably about 7,000°C to about 9,000°C, and the ratio of the solvent feed rate to the fluidizing gas rate or air rate must be maintained below about 0.6 g solution/ft³ (0.6 g/28.32 L) of air, preferably about 0.1 g/ft³ (28.32 L) to about 0.6 g/ft³ (28.32 L), and more preferably about 0.3 g/ft³ (28.32 L) to about 0.5 g/ft³ (28.32 L). These parameters combined provide high metal recovery, control of drying capacity, and produce catalysts having a uniform film or coating of metal or metals having an average thickness of about 20 microns (20 µm) to about 250 microns (250 µm), preferably about 40 microns (µm) to about 150 microns (150 µm), with the loading of metal or metals expressed as weight of metallic metal per packed bulk volume of catalyst ranging from about 0.01 g/cc to about 0.15 g/cc, preferably about 0.03 g/cc to about 0.09 g/cc of catalyst. When the temperature of the fluidizing gas is on the higher side, solution:gas ratios on the higher side of the specified range are preferred, and conversely, when the temperature of the fluidizing gas is on the lower side, solution:gas ratios on the lower side of the specified range are preferred. The extent of loading of a metal or metals in the solution and the porosity of the support also affect to some extent the thickness of the film or coating of the metal or metals at a given Set of spray conditions. For cobalt, eg when operating in the preferred range of conditions indicated, the thickness of the cobalt film will be in the range of about 100 microns (100 µm) to about 250 microns (250 µm) at a volumetric cobalt loading of about 6 g/100 cm3.

Generally, the concentration of the metal compound or compounds in the suspension or solution should be greater than about 5% by weight, preferably 10% by weight. Preferably, a solution is used and the solution may be saturated or unsaturated with the metal compound or compounds. The optimum concentration may be best determined depending on the specific compound or compounds used.

Low concentrations require longer spray times and, conversely, high concentrations shorten the spray time. A balance is necessary between the specific solution concentration and the time required for spraying in order to obtain the required loading of metal or metals to film thickness.

Various types of fluidized bed devices are described in the literature, but the specific type and size of a fluidized bed device does not appear to have any effect on the depth of impregnation of the metal or metals below the peripheral surface of the particulate support. Known devices include, for example, fluidized bed granulators/dryers equipped with nozzles entering from either above or below the bed, rotary granulators, and Wurster columns. Such devices are available from Glatt Air Technigues, Inc., Ramsey, New Jersey, namely a one liter head sprayer (Versa-Glatt model) and a 20 liter head sprayer, model GPCG-5, and from Coating Place, Inc., Verona, Wisconsin, namely a 20 liter bottom sprayer. A lesser degree of particle breakage appears to occur with head-entering nozzles. Typically, the air flow rate in operating these units is adjusted to that required to properly fluidize the carrier particles, and the rate of metal solution addition is then adjusted in relation to the air flow rate.

A cobalt-titania catalyst is particularly preferred. The cobalt-titania catalyst is one in which the cobalt or the cobalt and an activator is distributed as a thin, catalytically active film on titania or a titania-containing carrier or support, wherein the titania has a rutile:anatase weight ratio of at least about 3:2 as determined according to ASTM D3720-78: Standard Test Method for Anatase:Rutile Ratio in Titanium Dioxide Pigments Using X-Ray Diffraction. Generally, the catalyst is one in which the titania has a rutile:anatase ratio in the range of at least about 3:2 to about 100:1 or greater, more preferably from about 4:1 to 100:1 or greater. When one or more of the metals rhenium, zirconium, hafnium, cerium, thorium or uranium is added to the cobalt as an activator to form the thin, catalytically active film, the metal is added to the cobalt in a concentration sufficient to provide a cobalt:metal activator weight ratio in the range of about 30:1 to about 2:1, preferably about 20:1 to about 5:1. Rhenium and hafnium are the preferred activator metals, with rhenium being more effective in promoting improved activity maintenance on an absolute basis and hafnium being more effective on a cost-effectiveness basis. These catalyst compositions have been found to provide a product containing predominantly linear C10+ paraffins and olefins and very few oxygenated (oxygenated) products, with high productivity and low methane selectivity. These catalysts also provide high activity, high selectivity and high activity maintenance at the conversion of carbon monoxide and hydrogen to distillate fuels.

After spraying is complete, the impregnated metal compounds must be decomposed to the corresponding metal oxides. Preferably, the catalyst is contacted with oxygen, air or other oxygen-containing gas at a temperature sufficient to oxidize the metal or metal component, e.g. convert cobalt to CO₃O₄. Temperatures in the range of above about 150°C and preferably above about 200°C are sufficient to convert the metals, particularly cobalt, to the oxide. Temperatures above about 500°C are to be avoided. Suitably, the decomposition of the metal compound or compounds is achieved at temperatures ranging from about 150°C to about 300°C. The decomposition may be carried out in the fluidized bed apparatus or in an external furnace or calciner.

Cobalt catalysts prepared by the process of the invention have been found to be particularly useful for the production of liquid hydrocarbons from synthesis gas at high productivities and low methane formation. They contain substantially all of the active metal, particularly cobalt, on the outer peripheral surface of the support particles, with the metal substantially excluded from the inner surface of the particles. The high loading of metal or metals in a thin rim or shell on the outer surface of the particles maximizes the conversion of hydrogen and carbon monoxide at the surface of the catalytic particles. This avoids the normal diffusion-related limitation of most previously known catalysts, with the catalyst behaving more ideally, approaching the behavior of a powdered catalyst which is not diffusion-limited. Unlike when using powdered catalysts, the flow of reactants through the However, the catalyst bed remains almost unhindered due to the larger particle size of the catalyst. The large-scale production of this type of catalyst is easily achieved by the process of the invention, whereas it is very difficult, if at all possible, by known techniques.

In conducting synthesis gas reactions, the total gauge pressure on the CO and H2 reaction mixture is generally maintained above about 80 psig (0.552 MPa) and preferably above about 140 psig (0.965 MPa). It is generally desirable to use carbon monoxide and hydrogen in a molar ratio of H2:CO above about 0.5:1 and preferably equal to or above about 1.7:1 to increase the concentration of C10+ hydrocarbons in the product. Suitably, the H2:CO molar ratio is in the range of about 0.5:1 to about 4:1 and preferably, carbon monoxide and hydrogen are used in a molar ratio of H2:CO above about 0.5:1 and preferably equal to or above about 1.7:1. :CO in the range of about 1.7:1 to about 2.5:1. Generally, the reaction is carried out at hourly gas volume flow rates in the range of about 100 V/h/V to about 5000 V/h/V, preferably about 300 V/h/V to about 1500 V/h/V, measured as standard volume of the gas mixture of carbon monoxide and hydrogen (0°C, 1 atm) per hour per volume of catalyst. The reaction is carried out at temperatures in the range of about 160°C to about 290°C, preferably about 190°C to about 260°C. The gauge pressures preferably range from about 80 psig (0.552 MPa) to about 600 psig (4.137 MPa), more preferably from about 140 psig (0.965 MPa) to about 400 psig (2.758 MPa). The product generally contains and preferably contains 60% or more and more preferably 75% or more of liquid C₁₀+ hydrocarbons boiling above 160°C (320°F).

The invention will be more clearly understood by reference to the following examples and demonstrations which show comparative data illustrating its salient features. All parts are in units of weight, unless otherwise stated.

EXAMPLES 1 - 10

A range of supports for use in the preparation of catalysts were obtained from reliable commercial sources or prepared from particulate titania-alumina (96.5% TiO2/3.5% Al2O3), silica and alumina in both extrudate and spherical form. The identity of the supports, their physical size and shape and certain physical properties, namely surface area (SA) in square metres per gram (BET), pore volume (PV) measured by mercury intrusion and porosity are given in Table 1 15. Table 1 Carrier number Composition Shape Porosity Extrudates Spheres Extrudates

The catalysts prepared from these supports are described in Table 2, where the catalyst used in the catalyst preparation The carrier used is identified by the carrier number given in Table 1.

Aqueous solutions of cobalt nitrate (11 to 13 wt.% Co) and perrhenic acid (1 to 1.3 wt.% Re) were used in the preparation of a series of 15 catalysts. These catalysts were individually prepared in fluid bed sprayers, the 1 liter and 20 liter top sprayers supplied by Glatt Air Techniques, Inc. and the 20 liter bottom sprayer supplied by Coating Place, Inc., respectively. The catalysts were calcined as indicated in Table 2. Table 2 also gives the weight of the fluidized bed sprayer charge, the solution rate in g/minute, the fluidized air rate in cubic feet (38.32 L)/minute (CFM), the solution:air ratio in g/cubic feet (28.32 L), the average bed temperature during spraying, the calcination method, the weight percent Co-Re on the finished catalyst, and the average depth from the outer surface or thickness of the Co-Re on the outer surface of the support particles in microns (µm), namely the RIM thickness, as measured by electron probe analyzer (manufactured by JEOL Company, Model No. JXA-50A). The thickness of the Co-Re surface layer or RIM is a measure of the success or failure of a given catalyst preparation. It was found that the metal distribution throughout this depth was not uniform and the concentration often decreased with increasing distance from the peripheral surface of the particles. However, the depth within a particle and measurements taken from particle to particle generally showed good uniformity, particularly with respect to catalyst preparations Nos. 1 to 10 inclusive, which were successful in accordance with the invention. In addition to average RIM thickness, metal recovery is also an important criterion for success, with metal recoveries being above 90 wt.% except for Example 12. Table 2 Catalyst Preparation No. Support Solution Rate Fluidize Air Rate Solution-Air Ratio Average Bed Temp Calcination Method RIM Thickness Microns Calcination Method A = 220ºC in the sprayer B = 300ºC in the tube unit C = 250ºC in the furnace only 60% of the cobalt deposited on the catalyst Catalyst No. 6 was prepared by depositing additional metal on top of Catalyst No. 5.

The following conclusions can be drawn from the preparations described in Table 2, namely:

Preparations 1-4a illustrate the strong relationship of spray temperature to RIM thickness, with other conditions being relatively constant. The higher the temperature in the range of 50 to 100ºC, the thinner the RIM. Note that Preparations 1-3 were sprayed at low solution:air ratios.

Preparations 4a and 4b show that RIM thickness depends to some extent on the final calcination. The molten nitrate salt also migrates into the support during heating before decomposing to the oxide. Migration of the metals to the center of the particles can be minimized by heating rapidly, and the sprayer provides the best means of doing this. An alternative approach is to use low solution:air ratios during spraying. The extra drying helps minimize the tendency of the RIM to migrate later during calcination. The solution:air ratio of 0.56 is at the upper end of the acceptable range. The RIM increased significantly when a small portion was withdrawn from the sprayer immediately after spraying and then calcined in a tubular unit.

Preparation #5, compared to Preparation #2, shows that RIM thickness is a function of cobalt loading. The less cobalt sprayed onto the substrate, the thinner the RIM is at constant spray conditions.

Preparation #6 shows that exceptionally thin RIMs can be made at high metal loadings. In this case, Preparation #5 was simply returned to the sprayer after calcination for another impregnation. Compared to a single impregnation, Preparation #2, a very thin RIM was obtained.

Preparations 7 and 8 show that RIM thickness is a function of the support porosity. The RIM is thinner (Preparation #7 vs. Preparation #4a) when the support has a larger pore volume. The RIM is much thicker (Preparation #8 vs. Preparation #4a, 7) when a small pore volume is present.

Preparations 9 and 10 show the success with silica and alumina supports.

Preparations 11 and 12 show the adverse effect of operating outside the preferred temperature range. Preparation No. 11 shows that spraying below 50°C gives no RIM despite using a low solution:air ratio and depositing only 2.8% cobalt. Preparation No. 12 shows that spraying at 110°C gives a thin RIM, but only 60% of the sprayed metal remains on the catalyst. The rest of the cobalt and expensive rhenium were discharged overhead as particulate matter. "Spray drying" of the solution and decomposition of the nitrate appears to have occurred before the solution reached the support.

Preparations 13 and 14 show that good RIMs are not obtainable when large solution:air ratios are used.

Furthermore, a series of Fischer-Tropsch runs were performed on the catalysts successfully prepared by the fluidized bed method to demonstrate the utility of these RIM catalysts. All were successful and the data obtained agreed well with the predicted behavior based on the cobalt loading and RIM thickness of each catalyst preparation. The results obtained with catalyst preparation 4a can be considered illustrative. Therefore, preparation No. 4a was used to convert synthesis gas to heavy hydrocarbons. A portion of the catalyst was reduced at 450°C for 1 hour and then at 200°C, 280 psig (1.931 MPa gauge), GHSV = 1000 with a feed containing 65% H₂/31% CO/4% Ne. CO- conversion was 79% after 20 hours of operation. Product selectivity expressed as moles of CO converted to product per mole of CO converted was 6.0% CH₄, 0.4% CO₂, and 93.6% C₂+.

Claims (10)

1. A process for the preparation of catalysts comprising dispersing a catalytically effective amount of cobalt as a layer on the peripheral outer surface of a particulate, porous inorganic oxide support to form a catalyst useful for converting synthesis gas to hydrocarbons, and contacting the support particles with a spray containing a decomposable compound of the metal or metals, the process comprising: - maintaining a bed of the carrier particles in a fluidized state at a temperature in the range from about 50ºC to about 100ºC by contact with a gas at a temperature in the range from about 50ºC to about 100ºC, - spraying the bed of heated support particles with a liquid having dispersed therein a compound or compounds of cobalt at a flow rate sufficient to provide a ratio of liquid flow rate: fluidizing gas flow rate of less than about 0.6 g liquid/foot³ (0.6 g/28.32 L) of fluidizing gas to form a surface layer of the metal on the particles having an average thickness in the range of about 20 microns (20 µm) to about 250 microns (250 µm), the loading of the metal calculated as metallic metal per packed bulk volume of the catalyst being in the range of about 0.01 g/cm³ to about 0.15 g/cm³. 2. The method of claim 1, wherein the ratio of the liquid flow rate: fluidizing gas flow rate is in the range of about 0.3 g/ft³ (0.3 g/28.32 L) to about 0.5 g/ft₃ (0.5 g/28.32 L). 3. A method according to claim 1 or claim 2, wherein the decomposable metal compound or compounds comprise a cobalt-containing compound and optionally at least one compound of a further metal selected from the group consisting of rhenium, hafnium, zirconium, cerium, thorium and uranium, the cobalt being distributed together with the further metal or metals as a surface layer on the carrier. 4. A process according to any one of claims 1 to 3, wherein the catalytically active surface layer of catalyst has an average thickness in the range of about 40 microns (40 µm) to about 150 microns (150 µm), the cobalt loading being in the range of about 0.03 g/cm³ to about 0.09 g/cm³. 5. A process according to any one of claims 1 to 4, wherein the support comprises alumina, silica, silica-alumina, titania or mixtures comprising alumina, silica or titania. 6. The method of claim 5, wherein the support comprises titanium dioxide and the titanium dioxide of the support has a rutile: anatase weight ratio of at least about 3:2, e.g. a rutile:anatase weight ratio in the range of at least about 3:2 to about 100:11, and higher. 7. A process according to any one of claims 1 to 6, wherein the liquid in which the compound or compounds of cobalt is/are dispersed is water. 8. A method according to any one of claims 1 to 7, wherein the fluidizing gas is air. 9. A method according to any one of claims 1 to 8, wherein the temperature of the fluidizing gas is in the range of about 70°C to about 90°C. 10. Process for the synthesis of hydrocarbons from a mixture containing CO and H₂, in which this mixture is brought into contact with a catalyst prepared by the process according to any one of claims 1 to 9 under hydrocarbon synthesis conditions.