The Applicant is aware of known processes for preparing cobalt based catalyst precursors and which involve slurry phase impregnation of a catalyst support with a cobalt salt, drying of the impregnated catalyst support, and calcination of the dried impregnated catalyst support, to achieve a desired cobalt loading of the support. The resultant precursors are then activated by reduction thereof, to obtain cobalt based Fischer- Tropsch catalysts. These catalysts can display good intrinsic activities when used for Fischer-Tropsch synthesis; however, catalysts having enhanced or superior intrinsic activities cannot readily be obtained using the known processes. It is thus an object of the present invention to provide a cobalt based Fischer-Tropsch catalyst having enhanced initial and/or stabilized intrinsic Fischer-Tropsch synthesis activity, as well as a process for preparing such a catalyst.

According to a first aspect of the invention, there is provided a process for preparing a cobalt based catalyst precursor, which process includes

in a first support impregnation/drying/calcination stage, impregnating a particulate porous catalyst support with a cobalt salt, partially drying the impregnated support, and calcining the partially dried impregnated support to obtain a calcined material ; partially reducing the calcined material ; and in a second support impregnation/drying/calcination stage, impregnating the partially reduced material with a cobalt salt, partially drying the impregnated material, and calcining the partially dried impregnated material, to obtain the cobalt based catalyst precursor.

The resultant cobalt catalyst precursor can then be reduced to obtain a cobalt based Fischer-Tropsch catalyst. It was surprisingly found that this catalyst has enhanced or superior initial as well as stabilized intrinsic Fischer-Tropsch synthesis activity.

Thus, according to a second aspect of the invention, there is provided a process for preparing a cobalt based Fischer-Tropsch catalyst, which process includes in a first support impregnation/drying/calcination stage, impregnating a particulate porous catalyst support with a cobalt salt, partially drying the impregnated support, and calcining the partially dried impregnated support to obtain a calcined material ; partially reducing the calcined material ; in a second support impregnation/drying/calcination stage, impregnating the partially reduced material with a cobalt salt, partially drying the impregnated material, and calcining the partially dried impregnated material, to obtain a cobalt based catalyst precursor; and reducing the cobalt based catalyst precursor, to obtain the cobalt based Fischer-Tropsch catalyst.

In this specification, unless explicitly otherwise stated, where reference is made to catalyst mass, the mass given pertains to the calcined catalyst mass, ie the catalyst mass before any reduction of the catalyst is effective.

In the partial reduction of the calcined material, cobalt oxide present in the calcined material, ie a starting cobalt oxide phase, is partially reduced to a cobalt oxide phase containing less oxygen than the starting cobalt oxide phase. Without wishing to be bound thereby, it is believed that this phase containing the lower level of oxygen than the starting cobalt oxide phase, is a CoO like phase. Within the ambit of this specification, when reference is made to a CoO phase, it is thus intended that this means the partially reduced cobalt oxide phase.

A successful partial reduction of the calcined material may be assessed by performing X-ray diffraction analyses or by carrying out Temperature Programmed Reduction experiments on the calcined material.

The partial reduction of the calcined material may be performed at a temperature between a minimum temperature which is determined by the lowest temperature at which the partial reduction of the starting cobalt oxide phase to the CoO phase starts, and at a maximum temperature which is determined by the lowest temperature at which reduction of the CoO phase to a metallic cobalt phase starts. The minimum temperature may thus be 100°C, preferably 130°C, while the maximum temperature may be 300°C, preferably 250°C.

The partial reduction of the calcined material may be effected by contacting the calcined material with a hydrogen and/or carbon monoxide containing gas as a reducing gas, with an outlet gas being produced.

After completion of the partial reduction, which thus takes place in a partial reduction stage or vessel, the CoO phase may be unloaded into an inert atmosphere at the partial reduction temperature. Instead, however, it may be unloaded in air after first cooling down, or being cooled down, from the partial reduction temperature to room temperature. The CoO phase is stable in air at room temperature.

The partial reduction stage may be provided by any suitable contacting configuration of the calcined material with the reducing gas, such as a fluidized bed of the calcined material particles, with the reducing gas acting as the fluidizing medium; a fixed bed of the calcined material particles through which the reducing gas passes; or the like.

The cobalt salt may, in particular, be cobalt nitrate, Co (NO3) 26H20.

The particulate porous modified catalyst support may be any commercial available porous oxidic catalyst support, such as alumina (Al203), silica (SiO2), a silica-alumina (Si04. A1203), titania (TiO2) and magnesia (MgO).

The support may be a protected modified catalyst support containing, for example, silicon as a modifying component, as described in WO 99/42214 and which is hence incorporated herein by reference.

The support impregnation as well as the impregnation of the partially reduced material may, in principle, be effected by any suitable known impregnation method, eg incipient wetness impregnation, or slurry phase impregnation. Similarly, the calcination of the impregnated support as well as the calcination of the partially reduced material that was impregnated may be performed in any suitable known calcination unit, eg

in a fluidized bed, in a fixed bed, in a furnace, in a rotary kiln, and/or in a torbed calciner, preferably at temperatures between 150°C and 300°C.

In particular, the calcination may be in accordance with that described in PCT/IBOO/01745, and which is thus incorporated herein by reference.

The calcination may thus involve fluidized bed calcination as described in PCT/IBOO/01745.

The support and partially reduced material impregnation, drying and calcination may, in particular, be in accordance with the process described in our copending WO 00/20116, which is thus incorporated herein by reference. The precursor preparation may thus involve a 2-step slurry phase impregnation, partial drying and calcination process as described in WO 00/20116, which is dependant on a desired active component (cobalt) loading requirement and the pore volume of the porous oxidic catalyst support.

The support impregnation and drying may typically be effected in a conical vacuum drier with a rotating screw or in a tumbling vacuum drier.

The catalyst precursor may contain between 5gCo/100g support and 70g Co/100g support, preferably between 20gCo/100g support and 50g Co/100g support.

During either or both of the impregnation stages, a water soluble precursor salt of palladium (Pd) or platinum (Pt) or a mixture of such salts may be added, as a dopant capable of enhancing the reducibility of the cobalt.

Preferably, the dopant is added in a mass proportion of the palladium metal, the platinum metal or the mixture of palladium and platinum metals to the cobalt metal of between 0,01: 100 to 0,3: 100.

The invention extends also to a cobalt based Fischer-Tropsch catalyst, when produced by the process of the second aspect of the invention, and to a cobalt based catalyst precursor, when produced by the process of the first aspect of the invention.

According to a third aspect of the invention, there is provided a process for producing hydrocarbons, which includes contacting synthesis gas comprising hydrogen (H2) and carbon monoxide (CO) at an elevated temperature between 180°C and 250°C and an elevated pressure between 1 and 40 bar with a cobalt catalyst according to the invention, in a slurry phase Fischer-Tropsch reaction of the hydrogen with the carbon monoxide, to obtain hydrocarbons.

The invention extends also to hydrocarbons when produced by the process as hereinbefore described.

The invention will now be described in more detail with reference to the following non-limiting examples and with reference to the drawings.

In the drawings, FIGURE 1 shows the temperature programmed reduction (TPR) pattern of catalyst G, after the 1St impregnation/drying and calcination stage; FIGURE 2 shows the TPR pattern of catalyst G, after the partial reduction stage;

FIGURE 3 shows the TPR pattern of catalyst G, after the second and final impregnation/drying and calcination stage.

EXAMPLE 1 Catalyst B (30q Co/100a A1203) (not in accordance with the invention) Preparation A Pt promoted catalyst was prepared on SASOL Germany GmbH's trademark product: Puralox SCCa 5/150, as a selected pre-shaped Al203 support, in accordance with the method of aqueous slurry phase impregnation and vacuum drying, followed by direct fluidized bed calcination, in accordance with Catalyst Example 1 of WO 00/20116, or one of Catalysts D, E, G or H of PCT/IB00/01745.

In preparation for laboratory scale slurry phase continuous stirred tank reactor ('CSTR') Fischer-Tropsch synthesis runs, this calcined material was reduced and wax coated in accordance with the following procedure: 27,5g of the catalyst was reduced at 1 bar pure H2 (space velocity > 200m &num n H2/g catalyst/h) whilst the temperature was increased from 25°C to 380°C-425°C at a rate of 1 °C/min whereafter the temperature was kept constant at this temperature of 380°C-425°C for 16 hours.

The reduced catalyst was allowed to cool down to room temperature at which stage the hydrogen was replaced by argon, and the catalyst unloaded in molten Fischer-Tropsch wax under the protection of an argon blanket. This wax coated catalyst was then transferred to the slurry reactor.

CSTR Fischer-Tropsch synthesis run An extended slurry phase CSTR Fischer-Tropsch synthesis run (number 106F) was performed on catalyst B. This run lasted about ('ca') 90 days, during which the following synthesis conditions were maintained: Reactor temperature : 220, 5°C Reactor pressure : 20, 3 bar Catalyst inventory : 20,8g (H2+CO) space velocity : 2169m #n/(g catalyst. h) APG space velocity : 2452mtn/(g catalyst. h), where'APG'is an acronym for Arge Pure Gas, ie the commercial synthesis gas produced at Schumann- Sasol (Pty) Limited in Sasolburg, South Africa, according to the method of coal gasification, followed by Rectisol (trademark) purification.

Feed gas composition: H2 49, 1 vol% CO : 25, 9 vol% CH4 : 9,3 vol% CO2 : 0,5 vol% Ar : 15,2 vol% The observed synthesis performance data of this run (ie 106F) is presented in Table 1.

Relative (Fischer-Tropsch) Intrinsic Activity Factor ('R. I. A. F.') is defined as follows :

Consider an arbitrary slurry phase cobalt Fischer-Tropsch catalyst, displaying the following observed synthesis performance in a CSTR: rFT Z moles CO converted to Fischer-Tropsch products per gram catalyst per second, observed at T=y Ke) vin, at the following set of reactor partial pressures: PH2 v bar Pco z bar then the definition of R. I. A. F. is as follows : R. I. A. F = [Z (1 + 1.82 #)2] / [49480.9e(-11113.4/y) ##] Initial intrinsic Fischer-Tropsch activity (ai) of a slurry phase cobalt based catalyst is defined as follows : ai = the R. I. A. F. after 15 hours on stream (ie ti = time initial) of continuous exposure to the following set of gradientless slurry phase synthesis conditions: 220°C, 20 bar, % (H2 + CO) conversion in excess of 50%, obtained with a feed gas of composition: ca 50 vol% H2 and ca 25 vol% CO, the balance consisting of Ar, N2, CH4 and/or CO2.

EXAMPLE 2 EXAMPLE OF A1203 SUPPORTED COBALT SLURRY PHASE CATALYST IN ACCORDANCE WITH THE INVENTION (CATALYST G) THAT DISPLAYED ENHANCED INITIAL INTRINSIC FISCHER-TROPSCH ACTIVITIES Catalyst G (30gCo/0, 075gPt/100gAl2O3) A Pt promoted cobalt catalyst was prepared on SASOL Germany GmbH's trademark product: Puralox SCCa 5/150 as the selected pre-shaped

support material, in accordance with the method of aqueous slurry phase impregnation and vacuum drying, followed by direct fluidized bed calcination disclosed in US 5733839, WO 99/42214 and WO 00/20116.

In particular, the catalyst was prepared as follows : 44.6g Co (NO3) 2. 6H20 was dissolved in 40m1 distilled water and 0.0248g (NH3) 4Pt (NO3) 2 was dissolved in 10m1 distilled water. These two solutions were mixed together in a 500mi round ball flask in a rotavapor at 60°C and atmospheric pressure, and 50g of the Puralox SCCa 5/150 support material was added. Aqueous slurry phase impregnation and vacuum drying was effected via the following procedure: Temperature of oil bath Rotavapor pressure (°C) (mbar) Time (minutes) 60 Atmospheric 10 60 240 30 70 244 90 85 242 60 85 50 240 This vacuum dried intermediate product was directly subjected to a fluidized bed calcination step according to the following procedure: -Continuous air flow of 1. 7 dm3n/min -Temperature program: 25°C 1 c/miny 250°C 6hoursw 250°C

This calcined intermediate product of the 1 st cobalt/platinum impregnation and calcination step was then subjected to a partial reduction step prior to the 2nd (and last) cobalt/platinum impregnation step. For this purpose this intermediate material was reduced at 1 bar pure hydrogen (space velocity of 2000 min/g intermediate. h) whilst the temperature was increased from 25°C to 230°C at a rate of 1 °C/min whereafter the temperature was kept constant at 230°C for 2 hours.

35. Og of this partially reduced intermediate material was transferred to the aqueous impregnation solution of the 2nd cobalt/platinum impregnation step under the protection of an argon blanket, an impregnation solution that was prepared as follows : 22.7g Co (N03) 2. 6H20 was dissolved in 28ml distilled water and 0.0401g (NH3) 4Pt (NO3) 2 was dissolved in 7ml distilled water. These two solutions were mixed together in a 500ml round ball flask in a rotavapor at 60°C and atmospheric pressure.

Aqueous slurry phase impregnation and vacuum drying was effected via the following procedure: Temperature of oil bath Rotavapor pressure (°C) (mbar) Time (minutes) 60 Atmospheric 10 60 300 30 70 307 90 85 301 60 85 50 240

This vacuum dried intermediate product was directly subjected to a fluidized bed calcination step, according to the following procedure: -Continuous air flow of 1.7 dm3n/min -Temperature program: 25°C i°c/min 250°C 250°C Temperature programmed reduction experiments were performed on the catalyst intermediates after l't impregnation and calcination step, after the partial reduction as well as after the 2nd impregnation and calcination step. It can be seen in Figure 1 that the TPR pattern of the catalyst intermediate after l't impregnation and calcination shows several reduction steps. The first two reduction peaks are probably the reduction to CoO and the last broad peak the reduction of CoO to metallic cobalt. The TPR pattern of the catalyst sample after the partial reduction step (ie Figure 2) shows only the broad reduction peak of the reduction of CoO to metallic cobalt. The partial reduction thus caused the reduction of cobalt oxides to CoO. The TPR pattern of the catalyst intermediate after 2nd impregnation and calcination (ie Figure 3) again shows several reduction peaks, similar to the TPR pattern of the sample after l't impregnation and calcination.

In preparation for laboratory scale slurry phase CSTR Fischer-Tropsch synthesis runs, the finally calcined catalyst precursor was reduced and wax coated in accordance with the procedure described for the preparation of catalyst B.

Catalyst G was tested for its Fischer-Tropsch performance in a similar manner as described in example 1. The observed results are presented in Table 1.

Table 1 Catalyst B G Catalystcharacteristics: 30gCo/0,075g 30gCo/0,075gPt/ Composition Pt/1 OOgAl203 1 OOg Synthesis performance data: Run analysis number 106F 46£1 Time on stream (hours) 15 15 % (H2 + CO) conversion 73 65 Reactor partial pressures: H2 (bar) 3,7 5,0 CO (bar) 2,4 2,9 H20 (bar) 5,0 4,4 CO2 (bar) 0,3 0,3 Initial Relative Intrinsic (Fischer-2,7 4,7 Tropsch) Activity Factor (i. e.: a, = R. I. A. F. at t,)

The following conclusion can be drawn from Table 1: Catalyst G, ie 30gCo/0. 075gPt/100gAl2O3 prepared by using a partial reduction step as an additional preparation step, displayed an initial RIAF of 4,7, which is about 70% higher than the initial RIAF of 2,8 of catalyst B, ie 30gCo/0. 075gPt/100gAl2O3 prepared without a partial reduction step.

In this invention it was thus surprisingly found that cobalt based Fischer- Tropsch synthesis catalysts displayed an increased intrinsic catalytic performance when these catalysts were prepared using a partial reduction step as an additional preparation step prior to the final

impregnation/drying and calcination step, eg the partial reduction step was performed in between the 1st and 2nd impregnation/drying/ calcination steps.