Preparation of Fischer-Tropsch Catalysts

The present invention is concerned with the preparation of Fischer-Tropsch catalysts, and the catalysts so produced.

The Fischer-Tropsch (FT) reaction for conversion of synthesis gas, a mixture of CO and hydrogen, possibly also containing essentially inert components like CO ₂ , nitrogen and methane, is commercially operated over catalysts containing the active metals iron (Fe) or cobalt (Co). However, the iron catalysts exhibit a significant shift reaction, producing more hydrogen in addition to CO ₂ from CO and steam. Therefore the iron catalyst will be most suited for synthesis gas with low H ₂ /CO ratios (< 1.2), e.g. from coal or other heavy hydrocarbon feedstock, where the ratio is considerably lower than the consumption ratio of the FT reaction (2.0-2.1).

In systems where the H ₂ /CO ratio in the synthesis gas is higher, e.g. where the hydrocarbon feedstock is methane, cobalt is the preferred catalyst. The present invention is particularly concerned with Co based catalysts.

Normally, the active FT metal is dispersed on a solid, porous support. In this way, a large portion of the Co is exposed as surface atoms where the reaction can take place. The support can be alumina, titania or silica, but in fact, other oxides such as zirconia, magnesia, zeolites have been used, as has carbon.

To enhance the catalyst performance, e.g. by facilitating the reduction of cobalt oxide to cobalt metal prior to the FT synthesis, it is common to add different promoters. In this regard, rhenium, ruthenium, platinum, iridium and other transition metals can all be beneficial.

A number of different impregnation procedures are known in the art which use various solvents and chemicals, and which are suitable. However, in the present specification, the examples involve melt impregnation or incipient wetness impregnation of cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ .6H ₂ O) onto the support. It is well known that the impregnation method may influence the dispersion of the active metal (cobalt) and hence the catalytic activity. Generally, after impregnation, the impregnated catalyst is dried, typically at 80- 120 ⁰ C, to remove water from the catalyst pores, and then calcined typically at 200-450 ⁰ C, e.g. at 300 ⁰ C for 2-16 h.

The conditions during calcination may also have a significant effect on the final catalyst properties. According to EP-A-O 421 502, the activity may be improved, if during calcination, the catalyst is exposed to an atmosphere containing large amounts of nitrogen oxides. In contrast, however, van de Loosdrecht et at. (Topics in Catalysis, Vol. 26, 2003, pp. 121-127) recorded high cobalt metal surface areas and high catalytic activities when the concentration of nitrogen oxides and water was kept low during calcination.

It is an object of the present invention to provide an improved F-T catalyst.

According to the invention, there is provided a method of producing a catalyst for use in a Fischer-Tropsch synthesis reaction, comprising the steps of: impregnating a catalyst support material with an active metal-based catalyst component to form a catalyst precursor; and calcining the catalyst precursor in an atmosphere of a dry calcining gas.

Preferably, the water content of the calcining gas is reduced to produce a vapour pressure <14.5mm Hg. Preferably, the water content of the gas used in the calcination step has been reduced by one or more steps of condensing,

absorption, membrane separation, dilution and pressure reduction. Preferably, the calcining step is carried out by passing the dry calcining gas through or over a bed of catalyst precursor particles and preferably, the active metal-based catalyst component is cobalt. The dry calcining gas may be an inert gas, such as nitrogen, or an oxygen containing gas such as air. Preferably, the impregnation step includes impregnation with a catalyst promoter component, preferably a rhenium component and the catalyst , after calcination and reduction, contains an amount of rhenium in the range 0.1 to 2 wt% as a promoter, assuming complete reduction of rhenium.

Preferably, the precursor is subjected to a drying step, prior to the calcining step. Preferably, the drying step is carried out at a temperature in the range 80 to 16O ⁰ C, preferably 110 to 15O ⁰ C, for a period of 0.2 to 10 hours, preferably 0.5 to 4 hours. Preferably, the catalyst is subjected to a reduction step after the calcining step. Preferably, the catalyst has a metal surface area that is increased by at least 10% as compared to a catalyst prepared by calcinations in air without water vapour pressure reduction.

Preferably, the impregnating step is achieved by means of melt impregnation or incipient wetness impregnation techniques. Other suitable impregnation techniques include: precipitation from solution; precipitation/deposition; chemical vapour deposition; and vacuum impregnation. Preferably, the support material is impregnated with cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ .6H ₂ O). Preferably, the support material is impregnated with an aqueous solution of cobalt nitrate hexahydrate, containing an amount of cobalt to give a catalyst with a cobalt loading in the range 10 to 50 wt%, preferably 10 to 25 wt%, assuming complete reduction of cobalt. Preferably, the aqueous solution also contains an amount of rhenium to give a catalyst with a rhenium loading in the range 0.1 to 2 wt%, preferably 0.2 to 1 wt%, as a promoter, assuming complete

reduction of rhenium.

Preferably, the support material is alumina, titania, silica, zirconia, magnesia or zeolite. The most preferred material is γ-alumina, though any other form of alumina alone or in combination with a spinel-type aluminate, e.g. α-alumina together with nickel aluminate, may be used.

Preferably, the support material contains at least 10% by weight of a spinel compound composed of a divalent metal and aluminium. Preferably, the divalent metal is nickel or zinc. Preferably, the support material, prior to impregnation, has a specific surface area in the range 20 to 500 m ² /g, preferably 30 to 200 m /g. Preferably, the support material,, prior to impregnation, has a pore volume greater than 0.1 cm ³ /g, preferably greater than 0.2 cm ³ /g.

We have found, surprisingly, that the humidity of the gas to which the material is exposed during calcination can have an effect of the F-T activity of the resulting catalyst. Hence, we have found that exposure of the catalyst precursor to dry gases, i.e. gases in which the water content has been reduced, during the calcination step, offers improved Fisher-Tropsch catalysts. The gas used during calcination may be an oxygen-containing gas, e.g. air, or an inert gas such as nitrogen, or mixtures of these. Air is preferred.

Preferably, the calcining step is conducted at a temperature in the range 150 to 45O ⁰ C, more preferably 200 to 350 ⁰ C, for a period of 1 to 16 hours, more preferably 1 to 10 hours. In the present invention the water content of the gas used in the calcination step may be been reduced by one or more steps of condensing, absorption, membrane separation, dilution or pressure reduction, to have a water vapour pressure preferably <14.5mm Hg. More preferably, the

gas passed over the catalyst precursor during the calcining step has a humidity defined by a water vapour pressure of < 5 mm Hg, most preferably < 1 mm Hg, providing a dew point of < 1.3°C, more preferably < -17.3°C respectively.

The term "passed over the catalyst precursor" means that the gas is passed over the individual particles of the precursor. Conveniently, this is achieved by passing the gas through a bed of the precursor particles. The gas may be preheated prior to being passed through the precursor particles. In a preferred embodiment, the calcination is performed on a fluidised bed of catalyst precursor in fluidised bed calciner.

Preferably, the dry calcining gas contains less than 500 vppm of water. Thus, the calcining gas may contain, in the range of 20 to 1000 vppm of water, preferably between 50 and 500 vppm of water. The dry calcining gas may be obtained by cooling the gas thereby condensing water or by using a sorption material to remove water. The reduced water vapour pressure in the calcining gas may be obtained by mixing an un-dried gas with a stream of dried gas, by compressing the gas to condense water and separating the condensed water, by passing the gas through a membrane that physically separates the water from the gas, or by performing the calcination under vacuum, preferably at a pressure of less than 0.2 atm.

Preferably, the gas is passed through a bed of catalyst precursor particles. Preferably, the gas a flow rate giving a gas hourly space velocity (GHSV) >400hr ^"1 , preferably > 1000 hr ^"1 .

Preferably, the surface area of the cobalt in the catalyst, after calcination and reduction is in the range 5 to 30% m ² /g catalyst, preferably 10 to 25 m ² /g.

The invention also extends to a catalyst produced by the above method. Preferably, the cobalt content of the catalyst is from 10 to 40% by weight, preferably from 10 to 25% by weight.

The invention also extends to the use of such a catalyst in an F-T synthesis reaction. Preferably, the reaction is carried out in a slurry bubble column reactor. Preferably, H ₂ and CO are supplied to a slurry in the reactor, the slurry comprising the catalyst in suspension in a liquid including the reaction products of the H ₂ and CO, the catalyst being maintained in suspension in the slurry at least partly by the motion of the gas supplied to the slurry.

The invention also extends to a process for the production of hydrocarbons which comprise subjecting H ₂ and CO gases to a Fischer-Tropsch synthesis reaction in a reactor in the presence of the catalyst.

Preferably, the reaction is a three-phase reaction in which the reactants are gaseous, the product is at least partially liquid and the catalyst is solid. Preferably, the reaction is carried out in a slurry bubble column reactor. Preferably, the H ₂ and CO are supplied to a slurry in the reactor, the slurry comprising the catalyst in suspension in a liquid including the reaction products of the H ₂ and CO, the catalyst being maintained in suspension in the slurry at least partly by the motion of the gas supplied to the slurry.

Preferably the reaction temperature is in the range 190 to 26O ⁰ C, preferably 210 to 24O ⁰ C, and the pressure is in the range 10 to 60 bar, preferably 15 to 35 bar. Preferably, the ratio H ₂ /CO of the gases supplied to the Fischer-Tropsch synthesis reactor is in the range 1.1 to 2.2, preferably 1.5 to 1.95, and the superficial gas velocity in the reactor is in the range 5 to 60 cm/s, preferably 20 to 40 cm/s.

Preferably, the product of the Fischer-Tropsch synthesis reaction is subsequently subjected to post-processing. The post-processing may include de-waxing, hydro-isomerisation, hydro-cracking and combinations of these.

The invention also extends to an apparatus for carrying out the method of the invention, comprising a calcination vessel through which the catalyst precursor and calciner gas may pass having a catalyst precursor inlet, a calcined catalyst precursor outlet, a calciner gas inlet and a calciner gas outlet, and in which the calciner gas inlet is operatively connected to gas drying apparatus. Preferably, the gas drying means comprises condensing means that cool the gas to below the dew point of the water contained therein to condense and optionally to freeze the water and thereby separate the condensed or frozen water from the air fed to the calcination. Alternatively, the gas drying apparatus comprises a vessel having a wet gas inlet and a dry gas outlet and an absorption material, preferably a particulate zeolite, disposed between the wet gas inlet and the dry gas outlet.

Porous catalyst support materials typically have specific surface areas between 30 and 500 m /g. These supports can be prepared by spray drying techniques of an appropriate solution in order to obtain essentially spherical particles of appropriate size, e.g. 80% in the range between 30-150 μm. After spray-drying, the material is calcined at a high temperature to give the appropriate support crystal size and pore structure.

It is important that the total pore volume of the support is sufficiently high, above 0.1 cm /g or better, above 0.2 cm /g, or even better above 0.6 cm /g. The pore volume is often measured by nitrogen adsorption/desorption. This method does not take into account large pores where a mercury porosimeter is more

relevant. A less accurate, but more practical parameter is the measured water absorptivity, which can be directly correlated with the amount of cobalt that can be impregnated on the catalyst by the incipient wetness procedure. A high pore volume will give a light material suitable for operation in a slurry environment and ease the impregnation by minimising the number of impregnation steps required. At the same time the support, and the final catalyst, should have sufficient strength for extended operation of months and years with minimal attrition of the materials. This can be tested in a slurry environment or by the ASTM method applicable for testing FCC (fluid catalytic cracking) catalysts.

The invention may be carried into practice in various ways and some embodiments will now be specifically described in the following non-limiting examples.

The starting alumina material used for all catalysts in the present invention was Puralox SCCa γ-Al ₂ O ₃ from Sasol.

EXAMPLE 1 - Effect of Air Circulation

A catalyst precursor was prepared by one-step incipient wetness impregnation of γ-Al ₂ O ₃ (BET surface area = 187 m ² /g, Pore volume = 0.73 cm ³ /g) with an aqueous solution of cobalt nitrate hexahydrate. The sample contained a nominal amount of 20 wt% cobalt and 1 wt% rhenium, as calculated assuming reduced catalysts with complete reduction of cobalt. The actual metal loading as determined by XRF (x-ray fluorescent spectroscopy) or ICP (inductively coupled plasm spectrometry) may vary up 10%, e.g. for a catalyst with nominal loading of 20 wt%, the actual amount cobalt can vary between 18 and 22 wt% of the total reduced catalyst weight. The effect of air humidity for the catalytic

properties was tested by varying the calcination arrangement. One (sample 1) was calcined in a crucible located inside an oven. The air flow was not passed directly through the sample. The other sample (sample 2) was calcined as a bed of catalyst particles inside a glass reactor located inside the same oven and subjected to a continuous flow of air. In this case, the air flowed directly through the sample. The calcination temperature and time were 300 ⁰ C and 10 h, respectively. For Sample 1, ambient air surrounded the crucible before heating, whereas dry instrument air at room temperature was passed via a tube through the wall of the oven for Sample 2. In the latter case, the water content of the air was 500 vppm.

The catalytic properties of sample 1 and 2 are given in Table 1. The high activity of sample 2 compared to sample 1 was related to the lower amounts of water and NO _x in the calcination atmosphere for the latter sample. By passing dry air directly through the sample, we believe that the calcination decomposition products were removed very efficiently. The calcination effect is probably related to different cobalt surface areas after calcination since it is generally believed that the FT-reaction is non-structure sensitive.

Table 1. Catalytic properties of catalysts

Catalyst Relative activity Relative selectivity

1 1.53 0.908

2 1.91 0.909

EXAMPLE 2 - Effect of Air Humidity

The effect of air humidity for the cobalt surface area was tested for another series of catalysts. A catalyst precursor was prepared by one- step incipient

wetness impregnation of a γ-Al ₂ O ₃ support (of the same kind as used in Example 1) with an aqueous solution of cobalt nitrate hexahydrate. The sample contained a nominal amount of 20 wt% cobalt. After impregnation, the catalyst precursor was dried at 110 ⁰ C for 3 h and subsequently split into samples of 1.2 g. These samples were calcined for 1O h under different conditions (air at 30 ml/min; air at 50 ml/min; 50% air/50% steam at 30 ml (min) and at different temperatures. The water content in the air was < 40 vppm.

The cobalt surface areas obtained for the catalysts of this series are given in Figure 1. Calcination in humid air clearly resulted in lower cobalt surface area than calcination in dry air. Increasing the air flow rate and therefore decreasing the water content during calcinations is clearly beneficial. The difference was largest for the lowest calcination temperatures. The reason for the big difference was probably different Co ₃ O ₄ crystallite sizes prior to in situ reduction. X-ray diffraction data gave a crystallite size of 13.2 and 6 nm, respectively.

EXAMPLE 3 - Effect of Air Humidity

The catalysts of this series contain a nominal amount of 20wt% Co and 0.5 wt% Re. Cobalt nitrate hexahydrate was heated above the melting point and mixed with ammonium perrhenate. The mixture was then added to the support. Before impregnation, the catalyst support (BET surface area = 175 m2/g, Pore volume = 0.71 cm3/g) was pre-calcined at 500 ^° C.

The freshly prepared catalysts were dried at a temperature of 110 ^° C. After impregnation and drying, calcination was carried out in a fluidised bed reactor. In all cases, the calcination temperature and time were 25O ⁰ C and 2 hours, respectively. The effect of humidity during calcination was studied by feeding

air flows of different humidity to the calcination reactor. Both dry instrument air (dew point < -30 ^° C) and air with higher levels of humidity were used as calcination agents. The water content (and air dew point) was controlled by saturating air with water in a heated bubbler and mixing with dry air. In comparison to the present invention illustrated by experiments A, B, C & D, experiments E, F & G were run with humidity's above 14.5 mmHg. The prepared catalysts are summarised in Table 2, which clearly shows that the air humidity level is important for the catalytic activity. Fisher-Tropsch activities and selectivity's were measured relative to experiment G. Calcination in air with a dew point of O ⁰ C or below resulted in relative activities in the range 1.10 to 1.14 in the Fisher-Tropsch synthesis. The activities after calcination in humid air were lower when the inlet air dew point was 17° C or higher.

Table 2. Experimental conditions and catalytic data.

Hydrogen chemisorption

Hydrogen adsorption isotherms were recorded on a Micromeritics ASAP 2010 unit at 313 K. The samples were reduced in situ in flowing hydrogen at 623 K for 16 h. The temperature was increased by 1 K/min from ambient to 623 K. After reduction, the samples were evacuated at 623 K and cooled down under vacuum to 313 K. An adsorption isotherm was recorded in the pressure interval 20 to 510 mmHg. The amount of chemisorbed hydrogen was calculated by extrapolating the linear part of the isotherm to zero pressure. In order to calculate the cobalt surface area, it was assumed that two cobalt sites were covered by one hydrogen molecule and that the area of one cobalt atom is 6.62 ^• 10 ^'22 m ² /atom.