FISCHER-TROPSCH CATALYST

Abstract: A Fischer-Tropsch catalyst comprises cobalt or iron on a support which is carbon having a BET surface area of at least 100 m<2>/g, ratio of BET to basal plane surface area not greater than 4:1 and a ratio of basal plane surface area to edge surface area of at least 10:1.

Description: FISCHER-TROPSCH CATALYST " - The present invention relates to catalysts suitable for use in the Fischer-Tropsch process, and to a Fischer- ropsch process carried out using the catalysts. In the well-known Fischer-Tropsch reaction a mixture of carbon monoxide and hydrogen (usually referred to as "syngas") is reacted to give relatively low molecular weight hydrocarbons, a substantial proportion of which are liquid at normal temperature and pressure. The Fischer-Tropsch process is disclosed in numerous references for example US 4 088 671. This discloses that the catalysts usually used are supported Co and Fe catalysts. US 4088 671 is concerned with a Co catalyst modified by the addition of Ru. Various catalysts supports are mentioned including activated carbon, coke, and charcoal. Although carbon is sometimes included in lists of supports which can be used for Fischer-Tropsch catalysts, it is recognised by skilled persons that carbon is in general not a satisfactory support for Fischer-Tropsch catalysts. In practice although carbon may sometimes be mentioned as a support, the experiments are carried out with oxide supports, as in US 4 088 671. GB 1565 074 discloses a catalyst for ammonia synthesis on a graphitised carbon support of defined BET/basal plane and basal plane/edge surface area ratios. There is nothing to suggest that this support would offer any advantages in the different Fischer-Tropsch reaction. We have now found that by appropriate selection of the support a substantial increase In activity can be obtained by comparison with a Fischer-Tropsch catalyst on an oxide support. According to the present invention a Fischer-Tropsch catalyst comprising cobalt or iron on a support is characterised in that the support is a carbon having a BET surface area of at least 100 m2/g, a ratio of BET to basal plane surface area not greater than 4:1, and a ratio of basal plane surface area to edge surface area of at least 10:1. Carbons may be characterised by their BET, basal plane, and edge surface areas. The BET surface area is the surface area determined by nitrogen adsorption using the method of Brunauer Eπtmett and Teller J. Am Chem. Soc. 60,309 (1938). The basal plane surface area is the surface area determined from the heat of adsorption on the carbon of n-dotriacontane from n-heptane by the method described In Proc.Roy.Soc. A314 pages 473-498, with particular ref rence to page 489. The edge surface area is the surface area determined from the heat of adsorption on the carbon of n-butanol from n-heptane as disclosed in the Proc.Roy.Soc. article . mentioned above with particular reference to page 495. The carbon supports are known, for example from-GB 1 565 074. However the catalysts of the present invention behave in a completely different manner from the catalysts of GB 1565 074. Thus the catalysts of GB 1 565 074 have their activity increased by the presence of alkali metals, and the presence of chloride ion is stated to be undesirable. The catalysts of the present invention have their activity destroyed by the presence of excess alkali metals. In a preferred embodiment of the present invention the catalyst additionally comprises a platinum or palladium, preferably platinum itself. The platinum group metal is conveniently deposited by Impregnation with an aqueous solution of a compound of a platinum group which Is reducible to give the metal. The platinum group metal may for example be deposited on the carbon support before the cobalt or iron. It may be preferred to carry out a reduction step to leave the metal before the deposition of the cobalt or iron, particularly if the compound used to deposit the platinum group metal leaves components which interfere with reduction of the iron or cobalt. The quantity of platinum group metal on the catalyst may for example be in the range 0.2 to 10% by weight of total catalyst, preferably 0.5 to 5% by weight. Clearly if the objective is to make a Fischer-Tropsch catalyst materials which destroy the activity of the catalyst (eg alkali metals as indicated above) must be excluded. Whether given materials adversely effect the activity may be determined by simple comparative tests. The quantity of iron or cobalt deposited on the catalyst may be In the range 5 to 100% wt based on weight of carbon. The catalyst may be prepared by Impregnation with an aqueous solution of a salt of cobalt or Iron. Examples of suitable salts are nitrates. The preferred carbons for use in the present invention have a BET surface area of at least 200 m /g,most preferable at least 300 ι_r/g. The BET surface area is-preferably not greater than 1000 πr/g, more preferably not greater than 750 m^/g. The ratio of BET to basal plane surface area is preferably not greater than 2.5:1. It is particularly preferred to use carbons with ratios of BET to basal plane surface area of not greater than 1.5:1. It is preferred to use carbons with ratios of basal plane surface area to edge surface area of at least 100:1. It is not believed that there is an upper limit on the ratio, although in practice it will not usually exceed 200:1. The preferred carbon support may be prepared by heat treating a carbon-containing starting material. The starting material may be an oleophilic graphite e.g. prepared as disclosed in GB 1 168 785 or may be a carbon black. However oleophilic graphites contain carbon in the form of very fine particles in flake form and are therefore not very suitable materials for use as catalyst supports. We prefer to avoid their use. Similar considerations apply to carbon blacks which also have a very fiae particle size. The preferred materials are activated carbons derived from vegetable materials e.g. coconut charcoal, or from peat or coal or from carbonizable polymers. The materials subjected to the heat treatment preferably have particle sizes not less than these indicated above as being preferred for the carbon support. The preferred starting materials have the following characteristics: BET surface area of at least 100, more preferably at least 500 m 2 /g. The preferred heat treatment procedure for preparing carbon supports having the defined characteristics, comprise successively (1) heating the carbon in an inert atmosphere at a temperature of from 900°C to 3300°C, (2) oxidizing the carbon at a temperature between 300°C and 1200°C, (3) heating in an inert atmosphere at a temperature .of between 900°C and 3000°C. The oxidation step is preferably carried out at temperatures between 300° and 600°C when oxygen (eg as air) is used as the oxidising agent. The duration of the heating in inert gas is not critical. The time needed to heat the carbon to the required maximum temperature is sufficient to produce the required changes in the carbon. The oxidation step must clearly not be carried out under conditions such that the carbon combusts completely. It is preferably carried out using a gaseous oxidizing agent fed at a controlled rate to avoid over oxidation. Examples of gaseous oxidising agents are steam, carbon dioxide, and gases containing molecular oxygen eg air. The oxidation is preferably carried out to give a carbon weight loss of at least 10% wt based on weight of carbon subjected to the oxidation step, more preferably at least 15% wt. The weight loss is preferably not greater than 40 % wt of the carbon subjected to the oxidation step, more preferably not greater than 25 % wt of the carbon. The rate of supply of oxidizing agent is preferably such that the desired weight loss takes place over at least 2 hours, more preferably at least 4 hours. Where an inert atmosphere is required it may be supplied by nitrogen or an Inert (Group 0) gas. The catalysts of the present invention may be used for the 5 Fischer-Tropsch conversion of synthesis gas to hydrocarbons which are liquid at normal temperatures and pressures. The temperature may for example be In the range 150 to 300°C, .preferably 200 to 250°C. The pressure may for example be in the range 1 to 50 bar absolute (0.1 to 5 MPa). The molar ratio of 10 hydrogen to carbon monoxide may for example be in the range 3 :1 to 1:1, more preferably about 2:1. The gas hourly space velocity may be 100 to 10,000, preferably 500 to 3000. T e invention will now be illustrated by reference to the following Examples. 15 Example 1 This Example shows the preparation of a cobalt Fischer-Tropsch catalyst- supported on a graphitised carbon. • ' The carbon used as support was prepared from " a commercially available extrudate activated carbon sold by Degussa under the 20 designation Katepon BKIV. The carbon was in the form of extrudates of 4mm diameter and had typical BET, basal plane, and edge surface areas of 939,182 and 32 rn^/g respectively. The activated carbon was heat treated as follows. The carbon was heated from room temperature in a stream of helium to 1700°C over a period of about 1 25 hour. When the temperature reached 1700°C the carbon was allowed to cool in the stream of helium to 25°C- The carbon was then heated in air in a muffle furnace at approximately 520°C for a time known from experience to give a weight loss of 20 %wt. The carbon was then heated in helium to between 1800°C and 1850°C as in the helium 30 heating step mentioned above. The carbon was allowed to cool to room temperature in a helium atmosphere. Typical values found for carbon treated in this manner were: BET surface area 710-749 m 2 /g Basal plane surface area 416-666 2/g 35 Edge surface area 3.6-3.8 m 2 /g T£_ § support was ground and sieved to 16—30 mesh (0.5-1.0 mm) and washed by refluxing in (1) dilute (10% w/w) hydrochloric acid and (2) distilled water to remove sulphur and impurity transition metals before use. 5 The carbon was then impregnated with an aqueous solution of cobalt nitrate (analytical reagent grade). The quantity of cobalt nitrate used was such that lOg of carbon support were treated with 2g of cobalt (giving a nominal metal loading of 16.7% w/w based on weight of catalyst). The quantity of water used was the minimum 10 used necessary to ensure even wetting of the carbon support (a few drops of methanol may be added to assist in wetting of the support). The impregnation of the carbon was conducted in a rotary evaporator at 50-80°C under a vacuum of 200-800 bar (20 kPa-80 kPa). The impregnated carbon was dried overnight id a 15 vacuum oven at 120°C, 200-300 m bar (20 kPa-80 kPa) . The catalyst was tested (using a 2.2 ml sample) in a once-through microreactor equipped with an on-line gas chromatography apparatus to analyse for CO, CO2, and CJ-C^Q organic products. The catalyst was reduced before use by treating 20 for 2-8 hours with a stream of hydrogen (25-100 ml/min) at 400-450°C and 8 bar (0.8 MPa) gauge. Synthesis gas at a molar ratio of H2 C0 of 2:1 was then admitted. The conditions used and the results obtained are given in Table 1. Definition of Terms used in the Results 25 The CO conversion is defined as the percentage of the carbon monoxide fed to the reactor that was converted to analysed products. It may be represented as: - 100-r molar % CO in product xl00j% 30 l_SUM(molar % carbon-containing material x carbon number) _1 The selectivity to organic products is defined as the percentage of the total carbon monoxide converted to C]_-C^Q organic products rather than to CO2. It may be represented as: SUM(molar % organic products x carbon number) x 100% 35 SUM(molar % organic products x carbon number) + molar % CO2 ___ The alpha factor represents the relative quantities of the organic " *products and is defined by P. Biloen and W.M.H. Sachtler in "Advances in Catalysis", Volume 30, pp 165 et seq, published by the Academic Press Inc 1981. In the present application the factor is calculated for products with carbon numbers from C3-G1 Q . The gas hourly space velocity (GHSV) is defined as the ml/hour of CO/H2 feedstock (at NTP) per ml of catalyst. Example 2 A catalyst was prepared as in Example 1 but containing 5g of Co per lOg of support (nominal metal loading 33.3% w/w). This catalyst was tested at two different space velocities and the results obtained are given in Tables 1 and 2. Comparative Test A This is a comparative test not according to the invention. A sample of oxide-supported Fischer-Tropsch catalyst (the - conventional' type) was tested using the apparatus used in Example 1. The catalyst contained cobalt, magnesium oxide, and zirconium oxide. It was prepared on a 50-100 mesh silica support according to the general method described by J R Anderson in "Structure of Metallic Catalysts", page 455, published by Academic Press, 1975. It contains 22.7% w/w CO, 1.2% w/w MgO and 2.16% w/w Zrθ2« It was reduced before use as in Example 1. The activity and selectivity of this catalyst under the conditions used for the previous examples are given in Tables 3 and 4. Comparison of the results with Tables 1 and 2 (a catalyst according to the invention) shows that the oxide-based catalyst possessed an activity (expressed as CO conversion) comparable with the catalyst of Example 1 and considerably poorer than the catalyst of Example 2 at comparable temperatures. When the relative densities of the catalyst as loaded into the reactor are taken into consideration (0.40g/ml for the catalyst of the Example 1, 0.53g/ml for the catalyst of Example 2 and 0.77g/mm for the catalyst of Comparative Test A), it is apparent that the catalysts of the invention possess considerable higher activity per unit mass of metal present in the reactor. Table 1 (a) GHSV:- 500 h -1 H 2 :C0;- 2:1 Pressure:- 8 Bar Gauge (0.9 MPa absolute) Table 2 Catalyst (% w/w Co) Example 2 - 33.3% w/w Co Temperature (°C) 205 228 CO Conversion (%) 31 99 Selectivity to Organic Products (%) 98 75 " Carbon Dioxide (%) 2 25 Alpha Factor 0.81 0.46 Table 3 Comparative Tests Temperature (°C) 222 241 CO Conversion (%) 60 99 Selectivity to organic products (%) 88 84 " carbon dioxide (%) 12 . 16 Alpha Factor 0.87 0.72 (a) GHSV:- 500 h~l H2:C0:- 2:1 Pressure:- 8 Bar gauge (0.9 MPa absolute) Table 4 Comparative Tests Temperature (°C) 195 256 270 CO Conversion (%) 0 59 81 Selectivity to organic products (%) - 90 50 " carbon dioxide (%) - 10 50 Alpha Factor — 0.77 0.57 (b) GHSV:- 1000 h " 1 H 2 : C0 :- 2 : 1 • Pressure:- 8 Bar gauge (0.9 MPa absolute) Example 3 A sample of graphitised carbon support (as used in Example 1) was impregnated with chloroplatlnic acid from aqueous solution. The technique of impregnation and drying was similar to that described for cobalt Impregnation in Example 1. The quantity of chloroplatinic acid used was such that lOg of carbon support were treated with 0.2g of platinum (giving a nominal metal loading of 1.9% w/w). The dried catalyst was reduced in- a stream of flowing hydrogen at atmospheric pressure, 200°C for 5 hours. It was then purged with nitrogen and re-impregnated with cobalt nitrate as described in Example 1. The quantities used were such that lOg of the platinum/carbon catalyst were treated with 2g of cobalt (nominal metal loading of the finished catalyst 16.7% w/w cobalt and 1.7% w/w platinum). The catalyst was then reduced and tested as in Example 1, the results ' being shown in Table 5. Table 5 GHSV:- 500 h~l H 2 :C0:- 2:1 Pressure:- 6 Bar Gauge (0.7 MPa absolute) The benefits of the platinum addition to the cobalt catalyst are clearly demonstrated by comparison with Table 6. This shows typical performance data obtained under similar test conditions from a 16.7% w/w cobalt catalyst prepared by the technique used in Example 1. Table 6 GHSV:- 500 h~l H2 C0:- 2:1 Pressure:- 6 Bar Gauge (0.7 MPa absolute) It may be seen by comparison of Tables 5 and 6 that the effect of platinum addition to the cobalt catalyst is to increase the CO conversion achieved at a particular temperature, with other aspects of catalyst performance largely unchanged. 11 Example 4 A catalyst containing 16.7% w/w iron and 0.8% w/w platinum was prepared from ferric nitrate using the technique described for Example 3. The performance data for this catalyst is presented in Table 7. Table 7 GHSV:- 500 h _1 H 2 :C0:- 2:1 Pressure:- 6 Bar Gauge (0.7 MPa absolute) The benefit of the platinum addition may be seen by comparison with a similar catalyst in which the platinum was omitted. Table 8 GHSV:- ' 500 h-! H2:C0:- . 2:1 Pressure:- 6 Bar Gauge (0.7 MPa absolute) Comparison of Tables 7 and 8 shows that, as in the case of Example . 3, the effect of platinum addition is primarily to increase the catalyst activity (CO conversion achieved at a particular temperature). Comparative Test B This is a comparative experiment not according to the invention. 1 2 A catalyst was prepared as in Example 3 containing 16.7% wt/wt cobalt and 1.7% wt/wt platinum. However the carbon used as support was not a carbon having the surface area characteristics required for the present invention. It was instead a commercial available activated carbon supplied by BDH Chemicals Ltd, Poole, England, and had the following surface area characteristics. Basal plane surface area: 128 2 g~^ BET surface area: 1020 m 2g-l Edge surface area: 30 mZg- 1 BET/based plane: 8 Basel plane/edge: 4.3 The catalyst was tested as in Example 3 at a gas hourly space velocity of 500 hr " "^, an H2ϊC0 molar ratio of 2:1 a pressure of 6 bar gauge (0.7 MPa absolute), and at various temperatures. The results are shown in Table 9. Table 9 GHSV:- 500 ft -1 H2:C0:- 2:1 Pressure:- 6 Bar Gauge (0.7 MPa absolute) A Comparison of Table 9 shows the very much greater conversion and higher alpha factor obtained using the catalysts according to the invention. A comparison of Table 9 with Tables 3 and 4 shows that the performance of the catalyst supported on active carbon is very much inferior to that obtained using a conventional oxide support. No skilled person studying the results obtained with activated carbon would believe that carbon was a satisfactory support for Fischer-Tropsch catalysts.

Claims: Claims: 1. A Fischer-Tropsch catalyst comprising cobalt or iron on a support characterised in that the support is a carbon having a BET surface area of at least 100 m 2 /g, a ratio of BET to basal plane surface area not greater than 4:1, and a ratio of basal plane surface area to edge surface area of at least 10:1. 2. A process according to claim 1 wherein the carbon has a BET surface area of at least 200 m 2 /g. 3. A process according to either one of claims 1 or 2 wherein the carbon has a ratio of basal plane to surface area to edge surface area of at least 100:1. 4. A process according to any one of the preceding claims wherein the ratio of BET to basal plane surface area is not greater than 2.5:1. 5. A process according to claim 4 wherein the ratio is not greater than 1.5:1. 6. A process according to any one of the preceding claims wherein the catalyst also contains platinum. 7. A process according to any one of the preceding claims wherein the quantity of platinum is in the range 0.2% to 10% by weight of the total catalyst. 8. A process according to any one of the preceding claims wherein the catalyst has a content of cobalt or iron in the range 5 to 100% wt, based on weight of carbon. 9. A process for the production of hydrocarbons which comprises bringing a mixture of carbon monoxide and hydrogen into contact with a catalyst according to any one of the preceding claims at a temperature in the range 150 to 300°C and a pressure in the range 0.1 to 5 MPa. 10. A process according to claim wherein the molar ratio of hydrogen to carbon monoxide is in the range 3:1 to 1:1.

Inventors: FOSTER ALAN IVOR, JOYNER RICHARD WILLIAM, MCCARROLL JOHN JAMES, PARTINGTON STEPHEN ROY, SPIERS RANALD GORDON LYON, TENNISON STEPHEN ROBERT, FOSTER, ALAN, IVOR, JOYNER, RICHARD, WILLIAM, MCCARROLL, JOHN, JAMES, PARTINGTON, STEPHEN, ROY, SPIERS, RANALD, GORDON, LYON, TENNISON, STEPHEN, ROBERT
Applicants: BRITISH PETROLEUM CO PLC, THE BRITISH PETROLEUM COMPANY P.L.C.
Application year: 1985
Main classification: C07C1
IPC codes: B01J21, B01J23, B01J35, C07C1