International patent publication no. WO 01/89689 discloses an iron-based Fischer-Tropsch catalyst composition wherein the iron phase is the ferrihydrite. The catalyst composition includes natural promoters which may be selected from manganese or chromium or a mixture thereof and chemical promoters selected from magnesium zinc, copper and alkaline or alkali earth metals. The catalyst is best bound to a refractory oxide support such as silica. According to the specification, the catalyst composition produces significant yield of higher paraffins, olefins and alcohols.

It is an object of this invention to provide an improved iron-based catalyst with increased activity and selectivity towards alcohols and olefins.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided an iron-based Fischer-Tropsch catalyst composition wherein the main iron phase is ferrihydrite and wherein the catalyst composition includes alumina as a structural promoter.

By"iron-based"is meant that Fe makes up at least 30% (by mass) of the composition. The term"the main iron phase is ferrihydrite"means that at least 75% of the iron phase is ferrihydrite, as determined by X-ray diffraction using Co K alpha radiation. The preferred catalyst compositions exhibit hyperfine interaction parameters similar to those of ferrihydrite, as determined by Mössbauer absorption spectroscopy (MAS).

A"structural promoter"is a chemical species/element that helps to stabilise the ferrihydrite phase of the catalyst.

A"chemical promoter"is a chemical species/element that alters the product selectivity and activity of a catalyst.

Advantageously, the iron-based catalyst composition also includes Mn.

Preferably, the iron-based catalyst composition also includes a chemical promoter or promoters selected from Zn, Mg, Cu, Ru, Pd, Rh and/or an alkali or alkaline earth metal such as K, Na or La.

Advantageously, the alumina comprises 5% to 20%, by mass, of the catalyst composition.

Typically, the catalyst composition comprises, by mass, 35% to 60% Fe, 1% to 25% Mn, 1% to 15% Zn, 1% to 25% Cu, and 0. 1% to 3% K2O.

The catalyst does not have to be bound with a binder and has a surface area of 150-300m2/g and a pore volume of 0.1-0. 5 cm3/g.

According to a second aspect of the invention there is provided a process for preparing an iron-based catalyst pre-cursor wherein the main iron phase is ferrihydrite, and wherein the catalyst composition includes alumina, wherein the alumina is included by co-precipitation with the iron phase.

Typically the process for preparing an iron-based catalyst pre-cursor wherein the main iron phase is ferrihydrite, and wherein the catalyst composition includes alumina, includes the following steps: 1. preparing a solution in a polar solvent, the solution containing Fe and Al ions; 2. adding a precipitation agent, typically a basic solution, to the solution to form a catalyst precipitate wherein the main iron phase is ferrihydrite; 3. washing the precipitate; 4. drying, typically spray-drying the washed precipitate; and 5. calcining the dried precipitate.

Alternatively, the Al ions may be included in step 2 with the precipitation agent.

The first solution may be formed by dissolving a ferric salt, such as iron nitrate, and an alumina salt such as aluminium nitrate, in the polar solvent.

Advantageously, the ions of structural promoters such as Mn, Cu, Zn, Cd, Ni, Co and chemical promoters such as Zn, Mg, Cu, Cr, Ru, Pd, Rh or and alkaline or alkali earth metals such as K, Na and La are included in the first solution.

Typically, the first solution includes iron nitrate, manganese nitrate, aluminium nitrate, copper nitrate and zinc nitrate.

The preferred base is KOH, however NaOH and Na2 (CO) 3 and K2 (CO) 3 can also be used.

In another embodiment of the invention the process for preparing the iron- based catalyst pre-cursor wherein the main iron phase is ferrihydrite, and wherein the catalyst composition includes alumina, includes the following steps: 1. preparing a solution in a polar solvent, the solution containing Fe and Al ions; 2. adding a precipitation agent, typically a basic solution, to the solution to form a catalyst precipitate wherein the main iron phase is ferrihydrite; 3. washing the precipitate; 4. drying, typically spray-drying the washed precipitate; 5. calcining the dried precipitate; and 6. further impregnating the calcined precipitate with an alkaline or alkaline earth metal.

Alternatively, the Al ions may be included in step 2 with the precipitation agent.

The calcined precipitate is further impregnated with a desired level of the alkaline or alkaline earth metal, such as K, Na and La.

According to a third aspect of the invention there is provided a process for producing higher paraffins, alcohols and olefins selectively, by reacting hydrogen with carbon monoxide in the presence of a catalyst substantially as described herein above. In a preferred embodiment of the invention there is provided a process for producing linear paraffins, alcohols and olefins selectively.

DESCRIPTION OF EMBODIMENTS In broad terms this invention relates to a catalyst composition for, and method of, selectively converting synthesis gas under Fischer-Tropsch conditions (at pressures of 20 to 100 bar (2 to 10 MPa) and low temperatures of 200 to 310°C) to paraffins, olefins and, more especially, to linear alcohols in significant yields, up to and including detergent alcohols.

Catalyst compositions according to preferred embodiments of the invention are iron-based and the main iron phase is ferrihydrite and include alumina as a structural promoter.

It has now, surprisingly, been found that the use of alumina as a structural promoter in an iron-based catalyst composition wherein the main iron phase is ferrihydrite increases the activity and selectivity of the catalyst (compared to the catalyst described in WO 01/89689) by 1.5 to 3 times.

The catalyst composition also preferably includes manganese as a structural promoter, and chemical promoters selected from magnesium and zinc. A preferred catalyst composition according to the invention includes alumina, manganese, zinc, copper and potassium.

A typical composition of this invention comprises, by mass, 35% to 65% Fe, 1% to 25% Mn, 1% to 15% Zn, 1% to 25% Cu, 0. 1% to 3% K20, and 5% to 20% Al203.

The catalyst composition of the invention may be produced by making a first acidic solution containing Fe nitrate (Fe (N03) 3-9H20), Mn nitrate (Mn (NO3) 3-4H20), Zn nitrate Zn (NO3) 3-6H2O), Cu nitrate Cu (NO3) 3- 3H2O), K nitrate (KN03) and Al nitrate (Al (N03) 3-9H20), and heating the solution to 75°C. A second basic solution, containing 25%, by mass, KOH and at a temperature of 45°C is then added to the solution. The rate at which the second solution is added to the first solution is adjusted so that the pH is maintained at a range of approximately 8 and the temperature at approximately 70°C. The addition of the second solution to the first solution causes the formation of a precipitate, which is the catalyst composition of the invention. The precipitate is then filtered, and washed and the filter cake then reslurried and spray-dried at an inlet temperature of 260°C and an outlet temperature of 120°C. Thereafter, on-spec catalyst is calcined at 450°C for 16 hours and sieved to a particle size of 38-150, um.

Although the above described method of preparation is a preferred method, the alumina can be included by co-precipitation by adding Al nitrate to the second basic solution, or it could be added to the washed precipitate by mixing a solution containing Al nitrate or Al hydroxide with the slurried precipitate and spray-drying and calcining the resulting mixture. The K can also be added in other ways, for example by impregnation after calcination of the composition. Descriptions of these alternative methods are provided in the Examples.

The catalyst composition according to the invention so-formed is iron- based, the main iron phase is ferrihydrite, and the catalyst has a surface area of 150-300m2/g and a pore volume of 0.1-0. 5, typically 0.3 m3/g.

Although it is possible to bind the abovementioned catalyst to a refractory metal oxide, it is not necessary to do so. Thus, the preparation stage is simplified as the binder addition stage may be eliminated.

A Fischer-Tropsch synthesis process according to an embodiment of the invention is carried out with an iron-based catalyst composition according to the invention as described above in a slurry bed reactor containing a crude synthetic paraffin or wax liquid with a carbon chain length varying from C10 to C120, such as the wax obtained from a slurry bed reactor process, using either Fe or Co based catalysts. An iron-based catalyst composition as described above is then suspended in the slurry medium, the catalyst loading ranging between 10 and 40 % by weight of the slurry. The slurry is stirred and conditioned by causing pure H2, CO or a hydrogen rich H2/CO mixture to flow continuously through the medium for approximately 20 hours. Alternatively, catalyst conditioning (that is reduction and carbiding) may be carried out at atmospheric pressure using H2, CO or H2/CO.

Thereafter, synthesis gas is caused to flow continuously through the conditioned slurry. The composition of the synthesis gas feed generally comprises H2 and CO in an H2 : CO molar ratio in the range of about 5: 1 to about 1: 5, preferably in the range of about 1: 1 to 2: 1. The feed synthesis gas may also comprise about 1 to 25 volume percent CO2, N2, and/or CH4.

Throughout the conditioning process and synthesis process, the reactor is operated at a temperature between 200 and 310°C ; preferably between 220 and 250°C, most preferably at about 240 °C ; and pressure between 10 and 100 bar (1 and 10 MPa).

Compared to the catalysts previously bound by Si02 such as those described in WO 01/89689, Fischer-Tropsch activity is increased 1.5-3 times. Productivity towards olefins and alcohols is increased 1.5-3 times and selectivities increased 1.5-2 times. A further advantage of the catalyst of the invention is enhancement of the Ce+ alcohol and olefin fractions in the order of 1.5-2 times. Preparation of the catalyst composition of the invention according to the preferred method also has economic and environmental advantages as co-precipitation of aluminium together with all other elements reduces the loss of metals during the preparation process.

Example 1 A solution containing 1787g of Fe (N03) 3. 9H2O ; 400g of Mn (N03) 3. 4H20 ; 345g of AI (N03) 3. 9H20 ; 69g of Cu (N03) 3. 3H20 and 143g of Zn (NO3) 3. 6H20 in 4200 ml of water was heated to 75°C and mixed with a second solution containing 25% (mass) KOH kept at 45°C. The rate at which these solutions were fed was adjusted such that the precipitation pH was-8 and the temperature was- 70°C.

The precipitate was filtered, washed thoroughly with water until the conductivity was 2. 0mSv. The filter cake was then reslurried and spray dried at an inlet temperature of 260°C and outlet of 120°C.

Thereafter the on-spec catalyst was calcined at 450°C for 16 hours and finally sieved between 38 and 150 microns before being characterised and tested. The composition of the first catalyst of the invention is represented in Table 1.

Table 1. ELEMENT Catalyst 1 Fe [%] 42.9 Cu [/100gFe] 8. 3 Mn [/100gFe] 32.6 Zn [/100gFe] 12.6 K2O [/100gFe] 0.64 Al2O3[/100gFe] 15.7 Example 2 A second catalyst of the invention was prepared by the same procedure outlined in Example 1 but with the aim of removing all the K20 during washing. The K20 level after calcination was 0. 16g/100g Fe. The catalyst was impregnated to the desired K20 level of 1. 00g/100g Fe using the slurry impregnation method: 0.2612g of KNO3 was dissolved in 10moi of distilled water and mixed with 60ml of methanol. The mixture was then added to a flask containing 30g of catalyst and heated in a rotavapor at 65°C from 800mmHg to 50mmHg.

The dried catalyst was calcined at 450°C for 16 hours. The composition of the impregnated second catalyst of the invention is presented in Table 2.

Table 2 ELEMENT Catalyst 2 Fe [%] 38.8 Cu [/100gFe] 6. 8 Mn [/100gFe] 27. 5 Zn [/100gFe] 11.5 K2O [/100gFe] 1.04 Al203 [/100gFe] 29.7 Example 3 Catalyst 3 was prepared by the same procedure outlined in Example 1. The K20 level after calcination was 5. 3g/100g Fe. The excess K2O was removed by washing the catalyst with dilute HN03 as follows : 100ml of distilled water was added to 30g of catalyst and mixed thoroughly.

Dilute HN03 (1: 3 dilution of 55.5 M) was added drop-wise until the pH was - 5.5. The solvent was decanted and the wet catalyst was dried using methanol in a rotavapor. The composition of the third catalyst of the invention is presented in Table 3.

Table 3 ELEMENT Catalyst 3 Fe [%] 34.5 Cu [/100gFe] 6. 8 Mn [/100gFe] 36.1 Zn [/100gFe] 10.7 K20 [/100gFe] 0.59 Al203 [/100gFe] 30.7 Examples 4-6 Catalysts 4-6 are prepared via a continuous preparation method. A metal solution (Fe, Zn, Mn and Cu nitrates) was heated up to 70 °C and the KOH solution temperature kept at-45 °C. The rate at which these solution were fed was adjusted such that the precipitation pH was-8 and the temperature was- 70 °C. After precipitation, the precipitate was filtered and washed until the conductivity was 2.0 mSv. The precipitate was then separated into different batches and Al additions investigated as described in the Examples 4-6.

Example 4 In a first procedure Al nitrate was mixed with the slurry prepared as described in section 2. This step is referred to as the binder addition stage.

After mixing AI nitrate with the iron slurry, the slurry was spray dried and calcined as in Example 1. The final composition of the catalyst is presented in Table 4.

Table 4 ELEMENT Catalyst 4 Fe [%] 52. 1 Cu [/100gFe] 7. 5 Mn [/100gFe] 11.2 Zn [/100gFe] 10.9 K20 [/100gFe] 1.19 Al2O3[/100gFe] 12.3 Example 5 Al hydroxide was mixed with the washed iron slurry to obtain desired alumina levels. This procedure is followed by spray drying and calcination.

The final composition of the fifth catalyst is present in Table 5.

Table 5 ELEMENT Catalyst 5 Fe [%] 51.4 Cu [/100gFe] 8. 5 Mn [/100gFe] 12.2 Zn [/100gFe] 11.9 K2O [/100gFe] 1.68 Al203 [/100gFe] 13.4 Table 6 Shows the performance of the catalysts 1 and 4 of the invention described above in comparison to a catalyst made according to WO 01/89689. Comparative Catalyst 1 Catalyst 4 catalyst Partial H2 20.68 20. 80 21.18 Pressures in Reactor /(bar) CO 6.02 6.82 5.59 H2O 4.06 2.64 1.83 CO2 3.43 3.80 5.23 GHSV ml (n) /gr caShr 2468. 68 4978. 68 7038. 80 Activity mmolCO 16. 66 28. 13 43. 22 converted to hydrocarbons/gr cat/h Alcohol mmol/gcat/hr 0.88 2.33 1.98 Production Alcohol C% 13.66 19.61 16.32 Selectivity C6+ Alcohol C% 4.02 4.46 7.42 Selectivity Olefin mmol/gcat/hr 0.80 2.57 3.67 Production Olefin C% 17.93 27.57 32.16 Selectivity C6+ Olefin C% 4.96 4.35 11.93 Selectivity Example 7 Catalysts 7 and 8 were prepared with co-precipitating Al (NO3) 3.9H2O together with the other metal nitrates whereas in catalysts 9 and 10, AI (NO3) 3. 9H20 was mixed into the catalyst slurry before spray drying.

Table 7 This Table shows that co-precipitating Al nitrate reduces the loss of the other metal promoters. % Promoter losses Cu Mn Zn Catalyst Catalyst 7 6 Catalyst 8 - 4.5 4.2 Catalyst 9 < 1 < 1 < 1 Catalyst 10 < 1 < 1 1