WO2006090190A1

WO2006090190A1 - Catalysts

Info

Publication number: WO2006090190A1
Application number: PCT/GB2006/050036
Authority: WO
Inventors: Sharon Bale; Cornelis Martinus Lok
Original assignee: Johnson Matthey Plc
Priority date: 2005-02-25
Filing date: 2006-02-14
Publication date: 2006-08-31
Also published as: GB0503818D0

Abstract

A process is described for the preparation of a supported cobalt catalyst comprising the steps of; (i) mixing a support material with a solution of a thermally decomposable cobalt ammine carboxylate complex, e.g. cobalt ammine formate, to form a catalyst precursor, (ii) separating any excess solution from the catalyst precursor, (iii) drying the catalyst precursor, and (iv) heating the dried catalyst precursor under non-oxidising conditions so that at least part of the cobalt is converted to its elemental form. The catalyst may be used for the hydrogenation of unsaturated compounds or the Fischer-Tropsch synthesis of hydrocarbons.

Description

Catalysts

The application relates to supported cobalt catalysts, as well as to the preparation and use thereof.

Supported cobalt catalysts wherein the cobalt is in its elemental or reduced state are well known and find use in many reactions involving hydrogen such as hydrogenation reactions, and the Fischer-Tropsch synthesis of hydrocarbons. The activity of the catalysts is believed to be directly proportional to the cobalt surface area of the reduced catalysts (e.g. see lglesia et al, Advances in Catalysis, vol 39, (1993) pages 221-302), but in order to achieve high cobalt surface areas, the cobalt should be well dispersed on the support. Supports that have been used for cobalt catalysts include alumina, silica, titania, alumino silicates and carbon.

Preparation of supported cobalt catalysts suitable for hydrogenation reactions or the Fischer- Tropsch synthesis of hydrocarbons has heretofore typically been by impregnation of soluble cobalt compounds into 'pre-formed' support materials or by precipitation of cobalt compounds from solution in the presence of support powders or extrudates, followed by a heating step and then, prior to use, activation of the catalysts by reduction of the resulting cobalt compounds to elemental, or 'zero-valent' form typically using a hydrogen-containing gas stream. Alternatively supported cobalt catalysts may be prepared by simultaneous co-precipitation of soluble cobalt and support compounds by addition of a base, followed again by heating and reduction stages.

The catalyst reduction stage may be performed in-situ, for example in a hydrogenation or hydrocarbon synthesis reactor, or the catalyst may be pre-reduced by the catalyst manufacturer. However the reduction stage, whether performed in-situ or by the catalyst manufacturer, often requires considerable quantities of heated reducing gas and comprises a complex and expensive part of the overall catalyst activation process. Accordingly it is desirable to replace/minimise the catalyst reduction stage as a means to generate an active catalyst.

We have developed a process whereby the reduction process is simplified by utilising catalyst precursors comprising one or more cobalt carboxylates derived from cobalt ammine carboxylates.

The present invention provides a process for the preparation of a supported cobalt catalyst precursor comprising the steps of;

(i) mixing a support material with a solution of a thermally decomposable cobalt ammine carboxylate complex to form a catalyst precursor, (ii) separating any excess solution from the catalyst precursor, and (iii) drying the catalyst precursor. The invention further provides a process for the preparation of a supported cobalt catalyst comprising steps (i) to (iii) and further (iv) heating the dried catalyst precursor under non- oxidising conditions so that at least part of the cobalt is converted to its elemental form.

The invention further provides catalysts obtainable by the above process and the use of such catalysts for performing hydrogenation reactions and the Fischer-Tropsch synthesis of hydrocarbons.

We have found that the use of a thermally decomposable cobalt ammine carboxylate complex provides an improved route to alumina-supported cobalt catalysts. Any soluble cobalt ammine carboxylate complex that is able to decompose to form elemental cobalt may be used. By the term "carboxylate" we mean a complex of cobalt with a carboxylic acid. Preferably the cobalt ammine carboxylate is cobalt ammine formate, cobalt ammine acetate or cobalt ammine oxalate, more preferably cobalt ammine formate and cobalt ammine oxalate and most preferably cobalt ammine formate. One or more of the cobalt complexes may be used.

GB 439274 states that a cobalt catalyst suitable for the hydrogenation of aliphatic nitriles may be formed by the pyrogenic decomposition of cobalt formate supported on active carbon, silica gel, pumice stone or bleaching earths. However despite this known property of cobalt formate, commercial cobalt catalysts are not produced in this way. Cobalt formate has a low solubility in water (1.5% Co by weight at 2O⁰C) making it unsuitable for commercial scale impregnation of support materials because in order to achieve catalytically useful cobalt levels, a large number of separate impregnation steps are required. We have found surprisingly that cobalt ammine carboxylates have considerably improved solubility making them more suitable for catalyst preparation.

The cobalt ammine carboxylate may be conveniently formed by reacting the respective cobalt carboxylate, e.g. cobalt (II) formate, with ammonia. This is readily achieved by dissolving the cobalt carboxylate in aqueous ammonia, particularly 30% ammonia. In this way the ammonia is able to complex with the cobalt to form the soluble cobalt ammine carboxylate complex, rather than react with the carboxylic acid to form ammonium carboxylate salts.

The parent cobalt carboxylates such as cobalt oxalate, cobalt formate and cobalt acetate are available commercially from various sources. If necessary, a cobalt carboxylate can be prepared by reacting a carboxylic acid with cobalt metal or cobalt hydroxide. Alternatively, basic cobalt (II) carbonate may be reacted with the corresponding carboxylic acid (e.g. formic acid, acetic acid or oxalic acid) and then aqueous ammonia added to the resulting cobalt carboxylate. The molar ratio of carboxylic acid to cobalt carbonate should be about 2:1 carboxylic acid : Co for monofunctional carboxylic acids such as formic and acetic acid or may be about 1 :1 for difunctional carboxylic acids such as oxalic acid. Ammonia should preferably be present in excess.

The cobalt concentration that may be achieved in water is suitably between 50 g/litre and 200g/litre. The pH of the solution is suitably between 8.0 and 11.0, preferably between 8.5 and

11. If desired, buffer salts of the carboxylic acid may be added (e.g. ammonium formate).

The support material may comprise alumina, silica, alumino-silicate, titania, zirconia, carbon or other suitable support. Preferably, the support material comprises silica, alumina, zirconia, titania or combinations of these in powder or shaped, e.g. extruded or pelleted, form. Preferably the powdered supports have been subjected to a step of spray drying.

Silica supports may be formed from natural sources, e.g. as kieselguhr, may be a pyrogenic or fumed silica or may be a synthetic, e.g. precipitated silica. Precipitated silicas are preferred. The particulate silica may be in the form of a powder or a shaped material, e.g. as extruded, pelleted or granulated silica pieces. Suitable powdered silicas typically have particles of surface weighted mean diameter D[3,2] in the range 3 to 100 μm. Shaped silicas may have a variety of shapes and particle sizes, depending upon the mould or die used in their manufacture. For example the particles may have a cross-sectional shape which is circular, lobed or other shape and a length from about 1 to > 10 mm.

The surface area of the powdered or granular silica is generally in the range 10 - 500 m²/g, preferably 100 - 400 m²g^"1. The pore volume is generally between about 0.1 and 4 ml/g, preferably 0.2 - 2 ml/g and the mean pore diameter is preferably in the range from <2 to about 30 nm.

If desired, the silica may be mixed with another metal oxide, such as titania or zirconia. The silica may alternatively be present as a coating on a shaped unit, which is preferably of alumina typically as a coating of 0.5 to 5 monolayers of silica upon the underlying support.

Titania supports may be formed from natural sources or may be a synthetic, e.g. precipitated titania. The titania may optionally comprise up to 20% by weight of another refractory oxide material, typically silica, alumina or zirconia. The titania may alternatively be present as a coating on a support which is preferably of silica or alumina, for example as a coating of 0.5 to 5 monolayers of titania upon the underlying support. Therefore when we refer to titania we include titania-coated supports. Conventional titania supports suitable for Fischer-Tropsch catalysts are based upon rutile forms of titania, which has superior attrition resistance compared with anatase forms. The BET surface area is generally in the range 10 - 500 m²/g, preferably 100 to 400 m²/g. The pore volume of the titania is generally between about 0.1 and 4 ml/g, preferably 0.2 to 2 ml/g and the mean pore diameter is preferably in the range from 2 to about 30 nm.

Similarly zirconia supports may be formed from natural sources or may be a synthetic, e.g. precipitated zirconia. The zirconia may optionally comprise up to 20% by weight of another refractory oxide material, typically silica, alumina or titania. The zirconia may alternatively be present as a coating on a support, which is preferably of silica or alumina, for example as a coating of 0.5 to 5 monolayers of zirconia upon the underlying support. Therefore when we refer to zirconia we include zirconia-coated supports.

The support material is preferably a transition alumina. Transition aluminas are defined in "Ullmans Encyklopaedie der technischen Chemie", 4., neubearbeitete und erweiterte Auflage, Band 7 (1974), pp.298-299. Suitable transition alumina may be of the gamma-alumina group, for example eta-alumina or chi-alumina. These materials may be formed by calcination of aluminium hydroxides at 400 to 750⁰C and generally have a BET surface area in the range 150 to 400 m²/g. Alternatively, the transition alumina may be of the delta-alumina group which includes the high temperature forms such as delta- and theta- aluminas which may be formed by heating a gamma group alumina to a temperature above about 800⁰C. The delta-group aluminas generally have a BET surface area in the range 50 to 150 m²/g. Alternatively, the transition alumina may be alpha-alumina. The transition aluminas contain less than 0.5 mole of water per mole Of AI₂O₃, the actual amount of water depending on the temperature to which they have been heated. Alternatively a spinel, such as a metal aluminate, may also be used as a support.

A suitable transition alumina powder generally has a surface-weighted mean diameter D[3,2] in the range 1 to 200 μm. In certain applications such as for catalysts intended for use in slurry reactions, it is advantageous to use very fine particles which are, on average, preferably less than 20 μm, e.g. 10 μm or less. For other applications e.g. as a catalyst for reactions carried out in a fluidised bed, it may be desirable to use larger particle sizes, preferably in the range 50 to 150 μm. The term surface-weighted mean diameter D[3,2], otherwise termed the Sauter mean diameter, is defined by M. Alderliesten in the paper "A Nomenclature for Mean Particle Diameters"; Anal. Proα, vol 21 , May 1984, pages 167-172, and is calculated from the particle size analysis which may conveniently be effected by laser diffraction for example using a Malvern Mastersizer.

It is preferred that the alumina powder has a relatively large average pore diameter as the use of such aluminas appears to give catalysts of particularly good selectivity. Preferred aluminas have an average pore diameter of at least 10 nm, particularly in the range 15 to 30 nm. [By the term average pore diameter we mean 4 times the pore volume as measured from the desorption branch of the nitrogen physisorption isotherm at 0.98 relative pressure divided by the BET surface area]. During the production of the compositions of the invention, cobalt compounds are deposited in the pores of the alumina, and so the average pore diameter of the composition will be less than that of the alumina employed, and decreases as the proportion of cobalt increases. It is preferred that the catalysts or precursors have an average pore diameter of at least 8 nm, preferably above 10 nm and particularly in the range 15 to 25 nm.

Preferably, the alumina material is a gamma alumina or a theta alumina, more preferably a theta alumina, having a BET surface area of 90 - 120 m²/g and a pore volume of 0.4 -0.8 cm³/g.

The alumina support material may be in the form of a spray dried powder or formed into shaped units such as spheres, pellets, cylinders, rings, or multi-holed pellets, which may be multi-lobed or fluted, e.g. of cloverleaf cross-section, or in the form of extrudates known to those skilled in the art. The alumina support may be advantageously chosen for high filterability and attrition resistance.

The support material is mixed with the cobalt ammine carboxylate solution to allow the solution to impregnate the pores of the support and form the catalyst precursor. In one embodiment, the support in the form of shaped units, e.g. extrudates, pellets or granules is immersed in a solution of the cobalt ammine carboxylate for a period of time to permit impregnation of the pores to occur. Once the shaped support material is saturated, the excess solution is separated e.g. by decanting or filtration, and the impregnated shaped supports dried.

In another embodiment, the support material is in the form of a powder and sufficient solution is used so that the resulting mixture is a mobile slurry. If desired, the slurry may be heated, e.g. to >60°C, to displace ammonia prior to separation of the catalyst precursor. By removing ammonia, the pH of the solution is lowered and the less soluble cobalt carboxylate precipitates from solution onto the support material.

Alternatively and preferably, an incipient wetness technique is used on powder or shaped units whereby only sufficient cobalt solution to fill up the pores of the support material is used. By using an incipient wetness method, also known as 'dry-impregnation', the amount of waste cobalt solution is minimised.

The impregnation conditions are in part dictated by the support, the temperature and the cobalt solution concentration. The temperature is preferably between 10 and 99⁰C. The cobalt concentration should preferably be as high as practical under the temperature condition. The impregnation of cobalt ammine carboxylate onto the support material should preferably be continued until the cobalt content of the dried catalyst precursor is ≥ 10% by weight, more preferably ≥ 15% by weight.

In the process of the present invention the catalyst precursor is separated from any excess cobalt ammine carboxylate solution and dried. Separation of excess cobalt solution may be by decantation, filtration or centrifugation or another method known to those skilled in the art. Any separated cobalt solution is preferably re-used.

The impregnation may be repeated on the catalyst precursor until the desired cobalt content is achieved. The upper limit is dictated by the cobalt content of the cobalt ammine carboxylate solution, number of impregnations it is practical to perform and the pore volume and surface area of the support material, but may be about 40% by weight. We have found that a cobalt content of the catalyst precursor ≥10% by weight is desirable in order to provide sufficient catalytically active cobalt metal upon thermal decomposition of the cobalt complex. The cobalt- support interaction that is known to occur with oxidic supports, results in the formation of cobalt species that do not form elemental zero valent cobalt upon heating, but require reduction under hydrogen at high temperatures. For example, cobalt aluminate and cobalt silicate species that form when cobalt compounds are deposited on alumina and silica respectively require heating under hydrogen at high temperature, e.g. >550°C, to form elemental cobalt. Hence preparation of the catalyst precursor under conditions in which cobalt-support interactions are minimised is preferred. The catalyst precursors of the present invention have at least a portion of the cobalt in thermally decomposable form, by which we mean in a form that provides elemental zero valent cobalt upon heating under non-oxidising conditions.

The drying step for cobalt complex impregnation may be performed at 20-120⁰C, preferably 95- 11O⁰C, in air or under an inert gas such as nitrogen, or in a vacuum oven. Care should be taken not to decompose the cobalt complex to oxidic form. Hence, in the present invention calcination in air at temperature >200°C is not performed. The absence of a calcination step is of direct benefit to the catalyst producer, and will potentially reduce the amount of cobalt spinel, e.g. cobalt aluminate that is present in the catalyst precursor. This can provide catalysts in which more of the cobalt is readily reduced and thereby give high cobalt surface areas. High cobalt surface areas are desirable as they lead to high activity catalysts.

The dried catalyst precursor may then be provided to the hydrogenation or hydrocarbon synthesis reactor and heated under non-oxidising conditions to generate active cobalt catalyst in-situ. Providing the dried catalyst precursor in this way overcomes the need for catalyst encapsulation. This is of direct benefit to the catalyst producer. Alternatively, the dried catalyst precursor may be provided to the Fischer-Tropsch or Hydrogenation plant, placed in a holding vessel where it is heated under non-oxidising conditions to effect partial or full activation before passing the partially or fully activated catalyst to the Fischer-Tropsch or hydrogenation reaction vessel. Where partial activation is performed, the subsequent activation may be performed under the same or different conditions.

Alternatively, the catalyst may be provided in 'pre-reduced' form, where the dried catalyst precursor has been subjected to a heating step under non-oxidising conditions in order to decompose at least a portion of the cobalt complex so that at least part of the cobalt is transformed into the elemental 'zero-valent' state. The pre-reduced catalyst is preferably provided to the reactor in either encapsulated or passivated form to prevent re-oxidation of the cobalt metal.

The decomposition temperature at which the cobalt carboxylate is converted to elemental cobalt is preferably greater than 200⁰C but the precise temperature will depend upon the cobalt complex or complexes chosen and whether a substance is present that promotes decomposition of the cobalt complex or complexes. The rate of temperature increase during the heating step is preferably 0.5 to 20°C/minute. If this rate is exceeded there is a risk of reducing the cobalt dispersion. A slow rate of increase is preferable as this may itself lead to lower decomposition temperatures. The preferred temperature range for thermal decomposition is 200 - 55O⁰C, more preferably 200 - 45O⁰C. The catalyst precursors may be subjected to temperatures in this range for between 0.05 to 24 hours, preferably 0.1 - 10 hours. The optimum conditions, i.e. the conditions of temperature and time that result in catalysts with the highest cobalt dispersion (i.e. surface area/g cobalt), are dependant upon the cobalt complex used, whether a decomposition promoter is present and the non-oxidising conditions employed.

By "non-oxidising conditions" we mean conditions under which the bulk of the cobalt in the elemental 'zero-valent' state is not substantially oxidised to inactive cobalt oxides. In a first embodiment, the heating stage is performed under an inert gas such as nitrogen, helium or argon. In a second embodiment, the heating stage is performed under vacuum. In a third embodiment, the heating stage is performed in the presence of a hydrogen-containing gas, preferably pure hydrogen or hydrogen mixed with one or more inert or reducing gases. The use of a hydrogen-containing gas to augment the thermal decomposition has been found to lead to high cobalt surface areas. Thus the heating step may be performed by passing a hydrogen-containing gas such as hydrogen, synthesis gas, e.g. a hydrogen/carbon monoxide gas mixture or a mixture of hydrogen with nitrogen, methane or other inert gas over the dried catalyst precursor at elevated temperature, for example by passing the hydrogen-containing gas over the composition at temperatures in the range 200-550⁰C for between 1 and 24 hours, preferably 300-500⁰C at atmospheric or higher pressures up to about 25 bar. However, the presence of a hydrogen-containing gas is not essential in the present invention in order to form active cobalt catalysts. Thus in the present invention, active catalysts containing cobalt in the elemental zero valent state may be prepared by heating the catalyst precursor under non- oxidising conditions in the absence of hydrogen.

Because the cobalt carboxylate complex decomposes on heating to give elemental cobalt, hydrogen reduction may be omitted or the amount of hydrogen or other reducing gas used reduced compared to current catalyst types. This offers a considerable saving to the producer/user. Furthermore upon decomposition, the gases generated (carbon monoxide/carbon dioxide and hydrogen) are non-corrosive and generally compatible with the typical end uses in hydrogenation and Fischer-Tropsch synthesis. Thus the present invention overcomes the difficulties associated with reduction of cobalt-nitrate derived catalysts that can generate potentially corrosive nitrogen oxides.

The cobalt content of the dried catalyst precursor is preferably ≥ 10% by weight, more preferably ≥15% by weight, most preferably 15-35% by weight. In these regions, catalysts with very high dispersions, expressed as cobalt surface area per gram cobalt may be obtained by the process of the present invention. Preferably the cobalt surface area of the catalysts is in the range 25->100m²/g cobalt (in the active catalysts).

Cobalt surface areas may conveniently be determined by hydrogen chemisorption. The preferred methods are as follows;

1 ) Cobalt surface area of catalyst prepared by thermal decomposition in absence of hydrogen: An accurately known weight of sample (approx 0.5g) is charged to a chemisorption flow- through tube that is attached to a Micromeritics 2010 Chemisorption Analyser. The sample is heated to >200°C, e.g. 35O⁰C, in flowing helium (3.4-6.4 litres/h) at 3°C/min. The sample is maintained in the flowing gas at this temperature for 60 min and then evacuated to <10μmHg whilst still at this temperature and held under vacuum for 120 min. The sample is then cooled under vacuum to 15O⁰C. Once 15O⁰C has been reached the sample is held under vacuum for a further 30 mins. The chemisorption analysis is carried out at 15O⁰C using pure (100%) hydrogen gas. An automatic analysis program is used to measure a full isotherm over the range 100 mmHg up to 760 mmHg pressure of hydrogen. The analysis is carried out twice; the first measures the "total" hydrogen uptake (i.e. includes chemisorbed hydrogen and physisorbed hydrogen) and immediately following the first analysis the sample is put under vacuum (< 5mm Hg) for 30 min. The analysis is then repeated to measure the physisorbed uptake. A linear regression may then be applied to the "total" uptake data with extrapolation back to zero pressure to calculate the volume of gas chemisorbed (V). 2) Cobalt surface area of catalyst prepared by thermal decomposition in presence of hydrogen: An accurately known weight of sample material (approx 0.2-0.5g) is firstly degassed and dried by heating to 140⁰C at 10°C/min in flowing helium and holding it at 140⁰C for 60 min. The degassed and dried sample is then heated from 140⁰C to >200°C, e.g. 425⁰C at a rate of 3°C/min under a 50 ml/min flow of hydrogen and then holding it under the same hydrogen flow, at this temperature for 6 hours. Following heating under hydrogen, the sample is placed under vacuum and heated to 45O⁰C at 10°C/min and held under these conditions for 2 hours. The sample is then cooled to 150⁰C and held for a further 30 minutes under vacuum. The chemisorption at 15O⁰C is then performed as described above.

Cobalt surface areas were calculated in all cases using the following equation; Co surface area = (6.023 x 10²³ x V x SF x A ) / 22414, where V = uptake of H₂ in ml/g,

SF = Stoichiometry factor (assumed 2 for H₂ chemisorption on Co) A = area occupied by one atom of cobalt (assumed 0.0662 nm²).

This equation is described in the Operators Manual for the Micromeretics ASAP 2010 Chemi System V 2.01 , Appendix C, Part No. 201-42808-01 , October 1996.

The catalyst may in addition to cobalt, further comprise one or more suitable additives and promoters useful in hydrogenation reactions and/or Fischer-Tropsch catalysis. For example, the catalysts may comprise one or more additives that alter the physical properties and/or promoters that effect the reducibility or activity or selectivity of the catalysts. Suitable additives are selected from compounds of metals selected from molybdenum (Mo), nickel (Ni), copper (Cu), iron (Fe), manganese (Mn), titanium (Ti), zirconium (Zr), lanthanum (La), cerium (Ce), chromium (Cr), magnesium (Mg) or zinc (Zn). Suitable promoters include rhodium (Rh), iridium (Ir), ruthenium (Ru), rhenium (Re), platinum (Pt) and palladium (Pd). Additives and/or promoters may be incorporated into the catalysts by use of suitable compounds such as acids, e.g. perrhenic acid, metal salts, e.g. metal nitrates or metal acetates, or suitable metal-organic compounds, such as metal alkoxides or metal acetylacetonates. Typical amounts of promoters are 0.1 - 10% metal by weight on cobalt.

If desired, the compounds of additives and/or promoters may be added in suitable amounts to the solution of the cobalt ammine carboxylate. Alternatively, they may be combined with the catalyst precursor before or after drying.

Preferably one or more promoters selected from Pt, Pd, Ir, Re or Ru are included in the catalyst. Palladium is a particularly preferred promoter as it is believed to reduce the temperature at which the cobalt complex on the support decomposes to form elemental cobalt. The palladium is preferably added to the cobalt ammine carboxylate solution, e.g. as palladium acetate, in sufficient amount such that the palladium content of the catalyst precursor is greater than 10ppm, preferably about 250-1500 ppm (on catalyst precursor). Thus in a preferred embodiment the cobalt catalyst comprises Co and Pd and optionally one or more of Ru, Re, Ir, and R.

Cobalt catalysts having cobalt in the elemental or zero-valent state can be difficult to handle as they can react spontaneously with oxygen in air, which can lead to undesirable self-heating and loss of activity. Consequently active cobalt catalysts suitable for hydrogenation reactions may be passivated following the final heating step with carefully controlled small amounts of an oxygen-containing gas, often air or oxygen in carbon dioxide and/or nitrogen. Passivation provides a thin protective layer sufficient to prevent undesirable reaction with air, but which is readily removed once the catalyst has been installed in a hydrogenation process by treatment with a hydrogen-containing gas. Considerably lower amounts of hydrogen are required to activate such passivated catalysts compared to the un-reduced catalysts. For catalysts suitable for Fischer-Tropsch processes, passivation is not preferred and the active cobalt catalyst is preferably protected by encapsulation of the catalyst particles with a suitable barrier coating. In the case of a Fischer-Tropsch catalyst, this may suitably be a FT-hydrocarbon wax.

The catalysts may be used for hydrogenation reactions and for the Fischer-Tropsch synthesis of hydrocarbons.

Typical hydrogenation reactions include the hydrogenation of aldehydes and nitriles to alcohols and amines respectively, and the hydrogenation of cyclic aromatic compounds or unsaturated hydrocarbons. The catalysts of the present invention are particularly suitable for the hydrogenation of unsaturated organic compounds particularly oils, fats, fatty acids and fatty acid derivatives like nitriles. Such hydrogenation reactions are typically performed in a continuous or batch-wise manner by treating the compound to be hydrogenated with a hydrogen-containing gas under pressure in an autoclave at ambient or elevated temperature in the presence of the cobalt-catalyst, for example the hydrogenation may be carried out with hydrogen at 80-250⁰C and a pressure in the range 0.1- 5.0 x 10⁶ Pa.

The Fischer-Tropsch synthesis of hydrocarbons is well established. The Fischer-Tropsch synthesis converts a mixture of carbon monoxide and hydrogen to hydrocarbons. The mixture of carbon monoxide and hydrogen is typically a synthesis gas having a hydrogen: carbon monoxide ratio in the range 1.7-2.5:1. The reaction may be performed in a continuous or batch process using one or more stirred slurry-phase reactors, bubble-column reactors, loop reactors or fluidised bed reactors. The process may be operated at pressures in the range 0.1-10Mpa and temperatures in the range 150-350⁰C. The gas-hourly-space velocity (GHSV) for continuous operation is in the range 100-25000hr^"1. The catalysts of the present invention are of particular utility because of their high cobalt surface areas/g catalyst. The invention will now be further described by reference to the following examples. Throughout, the aqueous ammonia used was ammonia (30%) from BDH. The solutions prepared in Examples 1 (a) and (b) were prepared using a commercial cobalt formate

(Co(O₂CH)₂^H₂O) available from City Chemical, 139 Allings Crossing Rd., West Haven, CT 06516, U.S.A (Co content 31.5% wt). For Example 1 (c) cobalt formate was prepared in the laboratory according to the following method. 200ml methanol (BDH) was added to 5Og basic cobalt carbonate (General Grade), and the resultant mixture stirred for a few minutes to form a slurry. Then, a formic acid solution containing 10Og of 90% formic acid (BDH) and 100ml methanol was added to the slurry at room temperature, and the mixture allowed to react at 65°C for 30 min. Immediately after the reaction, the reaction mixture was filtered while hot. The resultant filter cake was washed with 50ml methanol three times and dried under vacuum or in a conventional oven at 80⁰C for 2 hours. Yield 6Og (Co contents 37.1 % wt (vacuum dried) and 36.8% wt (oven dried)). Ammonium formate was used as received from BDH. The gamma alumina used was Puralox HP14/150 available from Sasol. The silica used was ES70X synthetic amorphous silica powder available from lneos Silica Limited. The palladium acetate was used as received from Alfa Aesar. Cobalt and palladium contents in solutions or dried precursors were measured using ICP atomic absorption spectroscopy (ICP-AAS) and particle sizes by diffraction using a Malvern Mastersizer.

Example 1 : Preparation of cobalt ammine carboxylate solutions

a) A saturated solution of cobalt ammine formate/ammonium formate in aqueous NH₃ was prepared by initially dissolving 15.Og cobalt formate in 52.5ml aqueous NH₃ followed by the addition of 6.6g ammonium formate. The solution was filtered to remove residual solids. Concentration = 8.7% Co. Solution pH = 10.1.

b) A saturated solution of cobalt ammine formate in aqueous NH₃ was prepared by initially dissolving 35.Og cobalt formate in 96ml aqueous NH₃. The solution was filtered to remove residual solids. Concentration = 13.1% Co. Solution pH = 8.6.

c) A saturated solution of cobalt ammine formate in aqueous NH₃ with Pd promoter was prepared by dissolving 5Og cobalt formate in 100ml aqueous NH₃. The solution was filtered to remove residual solids. Following this palladium was added as palladium acetate. Concentration Cobalt = 15.0%. Concentration Palladium = 0.055% (550 ppm). Solution pH = 10.8. Example 2: Impregnation Of AI₇Q₃ support by incipient wetness method

a) Using the solution as described above in Example 1 (b). Approximately 25g gamma AI₂O₃ powder was placed in a 500ml round bottom flask. The AI₂O₃ was stirred whilst adding the cobalt ammine formate solution dropwise through a dropping funnel. The addition was terminated at what was deemed as incipient wetness (when the solid just began to stick together). The solid was dried at 95°C for 16 hours. The dried powder was sieved to < 1.Omm particle size. Four further impregnations were carried out using the same method. Impregnation details, cobalt loadings (measured by ICP-AAS), particle size analysis (measured using a Malvern Mastersizer) and nitrogen adsorption analysis results for BET surface area, average pore volume and average pore diameter are given below.

Thermogravimetric mass spectroscopic (TGMS) analysis on the dried catalyst precursors heated under helium from room temperature at a rate of 10°C/minute revealed that the thermal decomposition to yield cobalt in elemental form occurred around 300-320⁰C.

b) The method of Example 2(a) was repeated using the solution of Example 1 (c).

Thermogravimetric mass spectroscopic (TGMS) analysis on the dried catalyst precursor containing Pd obtained after 5 impregnations and heated from room temperature at 5°C/minute under helium revealed that the thermal decomposition to yield cobalt in elemental form occurred around 235⁰C. Thus the presence of Pd desirably lowers the of decomposition temperature of the Co complex.

Example 3 : Preparation of Catalysts

a) Without Hydrogen Augmentation:

The precursors prepared in example 2(a) were heated in a stream of helium at 35O⁰C for 1 hour and then under vacuum for 2 hours and the Co surface area determined by hydrogen chemisorption at 15O⁰C according to the method described above. The weight loss on heating (WLOH) was also measured. The results are as follows;

The results demonstrate that when the Co content of the dried precursor is >10% wt, particularly >15% wt, heating at 35O⁰C leads to catalysts with high cobalt surface areas. It has been found that heating these precursors under helium at temperatures greater than 35O⁰C upto 425⁰C reduces the resulting cobalt surface areas.

The dried catalyst precursors prepared in example 2(b), containing Pd, were heated in a stream of helium at 400⁰C for 1 hour and then under vacuum for 2 hours and the Co surface area determined by hydrogen chemisorption at 15O⁰C according to the method described above. The weight loss on heating (WLOH) was also measured. The results are as follows;

Comparing the cobalt surface areas with those where palladium is absent, these results suggest that the presence of palladium is having a beneficial effect on cobalt dispersion in the catalysts.

(b) With Hydrogen-Augmentation:

The catalyst precursors prepared in Example 2(a) were subjected to thermal decomposition under hydrogen at a temperature of 425⁰C for 6 hours followed by heating to 45O⁰C under vacuum for 2 hours and cobalt surface areas determined by hydrogen chemisorption at 15O⁰C according to the method as described above.

It can be seen that heating under hydrogen is increasing the elemental cobalt content of the catalysts and as a consequence the cobalt surface areas are higher.

The precursor prepared in example 2(b), containing Pd were subjected to thermal decomposition under hydrogen at a temperature of 425⁰C for 6 hours followed by heating to 45O⁰C under vacuum for 2 hours cobalt surface areas determined by hydrogen chemisorption at 15O⁰C according to the method as described above.

It appears that again the presence of Pd has had a beneficial effect on the resulting cobalt surface area. Example 4: Impregnation of AbO₃ support by slurry method

A series of catalysts were prepared as follows;

(a) 16.1g laboratory prepared cobalt formate (Co content 37.1% wt) was dissolved in 96ml aqueous ammonia and 94ml demineralised water to form the cobalt formate ammine complex solution. pH solution = 10.4. 3g AI₂O₃ (Puralox HP14/150) was placed in a 3-litre round bottom flask along with the cobalt ammine complex solution. The contents of the flask were continuously agitated whilst heating to 100⁰C. Ammonia/water was distilled off until pH = 7.2. At this stage the solution was very pink. The flask contents were filtered whilst hot. The filtrate was pink, the solid was black. The filter cake was oven dried at 105⁰C for approximately 65 hours. Approximately 5.9g material was produced. Co content of dried precursor = 37.4% wt by ICP-AAS.

(b) 32.1g lab prepared cobalt formate (Co content 37.1% wt) was dissolved in 192mls aqueous ammonia and 188ml demineralised water to form the cobalt formate ammine complex solution. pH solution = 11.0. 6.1g AI₂O₃ (Puralox H P14/150) was placed in a 3-litre round bottom flask along with the cobalt ammine complex solution. The contents of the flask were continuously agitated whilst heating to 100⁰C. Ammonia/water was distilled off until pH = 7.6. At this stage the solution was very pink. The flask contents were filtered whilst hot. The filtrate was purple, the solid was black. The filter cake was oven dried at 105⁰C for approximately 16 hours. Approximately 14.8g material was produced. Co content of dried precursor = 39.2% wt.

Example 5 : Preparation of Catalysts

a) Without Hydrogen Augmentation: The precursors prepared in examples 4(a-b) were heated without hydrogen augmentation in a stream of helium at 400⁰C and then under vacuum for 2 hours and the Co surface area determined by hydrogen chemisorption at 15O⁰C according to the method described above. The weight loss on heating (WLOH) was also measured. The results are as follows;

b) With Hydrogen Augmentation: The precursors prepared in examples 4(a-b) were heated under hydrogen at 425⁰C followed by heating to 45O⁰C under vacuum for 2 hours and the Co surface area determined by hydrogen chemisorption at 15O⁰C according to the method described above. The weight loss on heating (WLOH) was also measured. The results are as follows;

Example 5: Cobalt ammine formate on theta alumina

Gamma alumina was fired at 1050⁰C in air for 4hours to produce theta phase AI₂O3. A cobalt ammine formate solution was prepared by dissolving 75g commercial cobalt formate in 180ml aqueous ammonia in a round bottom flask over 16 hours. After 16 hours the solution was filtered to remove residual solids.

Approximately 5Og theta AI₂O₃ powder was placed in a Pascall mixing barrel and placed onto the Pascall Conical Lab Mixer. The AI₂O₃ was stirred whilst adding the cobalt ammine formate solution dropwise onto the support. The addition was terminated at what was deemed as incipient wetness. Once incipient wetness had been reached the cover was placed on the barrel and the barrel rotated at full speed for a further 10 minutes. The solid was dried at 105°C/16h. Three further impregnations were carried out using the same method. The details are given below.

a) Results without H₂ Augmentation:

The catalyst precursor was heated in a stream of helium at 350⁰C for 1 hour and then under vacuum for 2 hours and the Co surface area determined by hydrogen chemisorption at 150⁰C. The weight loss on heating (WLOH) was also measured. TGA/MS analysis shows thermal decomposition of the cobalt formate to elemental cobalt at 300 to 320⁰C. The results are as follows:

b) Results with H? Augmentation:

The catalyst precursor was subjected to thermal decomposition under hydrogen at a temperature of 425°C for 6 hours followed by heating to 450⁰C under vacuum for 2 hours and the Co surface area determined by hydrogen chemisorption at 150⁰C. The weight loss on reduction (WLOR) was also measured. The results are as follows:

Cobalt surface areas of cobalt formate catalysts made with theta AI₂O₃ using the hydrogen- augmented reduction method appear to be better than those made with gamma AI₂O₃. However, when using the heating in helium method, the catalysts prepared with gamma AI₂O₃ appear to be superior to those prepared with theta AI₂O₃.

Example 6: Cobalt ammine formate on silica

A cobalt ammine formate solution was prepared by dissolving 20Og commercial cobalt formate in 500ml aqueous NH₃ in a 1 litre round bottom flask over 8 hours. The solution was filtered to remove residual solids. pH solution = 11.2.

Approximately 10Og SiO₂ powder (as received) was placed in a Pascall mixing barrel and place onto the Pascall Conical Lab Mixer. The SiO₂ was stirred whilst adding the cobalt ammine formate solution dropwise onto the support. The addition was terminated at what was deemed as incipient wetness. Once incipient wetness had been reached the cover was placed on the barrel and the barrel rotated at full speed for a further 10 minutes. The solid was dried at 105°C/16h. Further impregnations were carried out using the same method.

Impregnation details, cobalt loadings (measured by ICP-AAS), particle size analysis (measured using a Malvern Mastersizer) and nitrogen adsorption analysis results for BET surface area, average pore volume and average pore diameter are given below.

a) Results without H₂ Augmentation:

The catalyst precursor was heated in a stream of helium at 250 and 300 ⁰C for 1 hour and then under vacuum for 2 hours and the Co surface area determined by hydrogen chemisorption at 150⁰C. The weight loss on heating (WLOH) was also measured. TGA/MS analysis shows thermal decomposition to form elemental cobalt at 300 to 320⁰C. The results are as follows:

He 250°C

He 300°C

b) Results with H?Augmentation:

Example 7 Catalysts Testing

a) in-situ reduction with hydrogen.

Catalysts were used for the Fischer-Tropsch synthesis of hydrocarbons in a laboratory-scale reactor. About 0.2 g of unreduced catalyst in a diluted bed (ca. 4 mm ID by 50 mm depth) was first reduced at 430⁰C for 420min in a hydrogen flow of 9851 litres/hr/kg of unreduced catalyst. Then hydrogen and carbon monoxide at a 2:1 ratio were passed through the bed at 200⁰C or 210°C at 20 barg. The space velocity was adjusted after 30 hrs to obtain as close as possible 50% CO conversion. The activity and selectivity of the catalyst to CH₄, C2-C4 and C5+ hydrocarbons were measured using known Gas Chromatography (GC) techniques.

For comparison, a 20%wt Co catalyst promoted with 1 % wt Re, prepared by impregnation of Co nitrate and perrhenic acid on the gamma-alumina and calcined in air was also reduced and tested under the same conditions (Std 1 ). The activity of the catalysts of the present invention is given relative to this standard catalyst. The results are as follows;

Example 7a was the Co-formate/Pd/alumina catalyst precursor of Example 2(b) prepared by 4 impregnations.

Example 7b was the Co-formate/alumina catalyst precursor of Example 2(a) prepared by 4 impregnations.

Example 7c was the Co-formate/silica catalyst precursor of Example 6 prepared by 5 impregnations.

These results indicate that the catalysts of the present invention may be more active than standard nitrate-based catalysts, and in the case of 7a and 7b in particular, give similar selectivity.

b) in-situ reduction with synthesis gas

Example 7a was repeated using the same reaction conditions (210°C/20 barg) using the Cobalt-formate and Pd-promoted Cobalt-formate on alumina catalysts, but without the preceding hydrogen reduction step, i.e. the catalyst precursors were simply brought up to the reaction temperature in the presence of the synthesis gas. No 'extra' reduction time was used. Such simplified activation offers a considerable advantage to the catalyst user compared to current activation processes.

The catalysts of the present invention were compared against a standard cobalt-nitrate derived 20% cobalt/alumina catalyst (Std 2) prepared by impregnation and calcined in air. This catalysts was subjected to the same synthesis gas-only activation procedure. The results were as follows;

Example 7d was the Co-formate/alumina catalyst precursor of Example 2(a) prepared by 4 impregnations.

Example 7e was the Co-formate/Pd/alumina catalyst precursor of Example 2(b) prepared by 4 impregnations.

These results show that under these reduction conditions, the catalysts of the present invention are more active than the standard nitrate-based catalyst.

Comparative Example C1. Preparation of Cobalt formate solution and impregnation of alumina.

4.1g cobalt formate (laboratory prepared, Co content 36.8% wt) was dissolved in 170ml cold demineralised H₂O in a round bottom flask. The solution was left to dissolve overnight. Solution pH = 6.8. Cobalt in solution = 0.94wt%.

Approximately 2Og gamma AI₂O₃ was placed in a round bottom flask. The AI₂O₃ was stirred whilst adding the cobalt formate solution dropwise through a dropping funnel. The addition was terminated at what was deemed as incipient wetness (when the solid just began to stick together). The colour of the solid was pink. The solid was dried at 105⁰C for 64 hrs. After drying the colour of the solid was lilac. The dried powder was sieved to < 1.0mm particle size. Three further impregnations were carried out using the same method, although drying was overnight (16 hours) rather than 64 hours. Final Co content of the dried precursor was measured at 4.1 % wt.

The comparative catalyst precursor was then subjected to heating to 35O⁰C under helium or hydrogen-augmented heating at 425⁰C and the cobalt surface areas measured by hydrogen chemisorption at 15O⁰C as described above. The results were as follows;

The surface areas are lower than obtained for the present invention.

Claims

Claims.

1. A process for the preparation of a supported cobalt catalyst precursor comprising the steps of; i) mixing a support material with a solution of a thermally decomposable cobalt ammine carboxylate complex to form a catalyst precursor, ii) separating any excess solution from the catalyst precursor, and iii) drying the catalyst precursor,

2. A process according to claim 1 wherein the cobalt ammine carboxylate is selected from the list consisting of cobalt ammine formate, cobalt ammine acetate or cobalt ammine oxalate or mixtures thereof.

3. A process according to acclaim 1 or claim 2 wherein the cobalt ammine carboxylate is cobalt ammine formate.

4. A process according to any one of claims 1 to 3 wherein the support material is alumina, silica, zirconia or titania.

5. A process according to any one of claims 1 to 4 wherein the support material is a gamma alumina or a theta alumina.

6. A process according to any one of claims 1 to 5 wherein one or more compounds of metals selected from molybdenum, nickel, copper, iron, manganese, titanium, zirconium, lanthanum, cerium, chromium, magnesium, zinc, nickel, rhodium, iridium, ruthenium, rhenium, platinum and palladium are included in the catalyst precursor.

7. A process according to any one of claims 1 to 6 wherein the mixture of support material and thermally decomposable cobalt ammine carboxylate complex is heated to remove ammonia and water prior to separating excess solution from the resulting catalyst precursor.

8. A process according to any one of claims 1 to 7 wherein the steps (i) to (iii) are repeated until the cobalt content of the dried catalyst precursor is ≥ 10% by weight.

9. A process according to claim 6 wherein sufficient palladium compound is present in the solution of cobalt ammine carboxylate such that the palladium content of the dried catalyst precursor is greater than 10 ppm.

10. A process according to any one of claims 1 to 9 further comprising (iv) converting the catalyst precursor to active catalyst by heating the dried catalyst precursor under non- oxidising conditions so that at least part of the cobalt is converted to its elemental form.

11. A process according to claim 10 wherein the temperature range for conversion is 200 to 55O⁰C.

12. A process according to claim 10 or claim 11 wherein the heating is performed under an inert gas such as nitrogen, helium or argon.

13. A process according to claim 10 or claim 11 wherein the heating is performed under vacuum.

14. A process according to claim 10 or claim 11 wherein the heating is performed in the presence of a hydrogen-containing gas.

15. A process according to claim 14 wherein the heating stage is performed under at temperatures in the range 200-550⁰C for between 1 and 24 hours at atmospheric or higher pressures up to about 25 bar.

16. A process according to any one of claims 10 to 15 wherein the catalyst precursor is disposed in a Fischer-Tropsch reactor and the reduction is performed in-situ.

17. A catalyst precursor obtainable by the process of any one of claims 1 to 9.

18. A catalyst obtainable by the process of any one of claims 10 to 15.

19. The use of a catalyst according to claim 18 for hydrogenation reactions.

20. The use of a catalyst according claim 18 for the Fischer-Tropsch synthesis of hydrocarbons.