WO2006010936A1

WO2006010936A1 - Cobald and rhenium containing fischer-tropsch catalyst

Info

Publication number: WO2006010936A1
Application number: PCT/GB2005/002966
Authority: WO
Inventors: Erling Rytter; Sigrid Eri
Original assignee: Statoil Asa; Petroleum Oil & Gas Corporation Of South Africa (Pty) Limited; Copsey, Timothy, Graham
Priority date: 2004-07-30
Filing date: 2005-07-27
Publication date: 2006-02-02
Also published as: GB2416715A; GB0417080D0

Abstract

A Fischer-Tropsch catalyst containing at least 8 wt % cobalt, assuming full reduction of cobalt, on a porous catalyst carrier having a pore volume of at last 0.2 ml/g characterised by a total content of alkali metal in the catalyst of less than 2000 ppm by weight.

Description

Fischer-Tropsch catalyst

The Fischer-Tropsch (FT) reaction for conversion of synthesis gas, a mixture of CO and hydrogen, possibly also containing essentially inert components like CO₂, 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 CO₂ from CO and steam. Therefore the iron catalyst will be most suited for synthesis gas with low H₂/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). Only Co based catalysts will be considered in the following.

A variety of products can be made by the FT-reaction, but from supported cobalt the main primary product is long-chain hydrocarbons that can be further upgraded to products like diesel fuel and petrochemical naphtha. The selectivity of the reaction towards longer chains often is described by the so- called SFA or S-F (Shultz-Flory- Anderson) alpha-value, or alternatively by the fraction of C₅₊ products. By-products can include olefins and oxygenates.

To achieve sufficient activity, it is customary to disperse the Co on a catalyst carrier, also often called the catalyst support. Thereby a large portion of the Co is exposed as surface atoms where the reaction can take place. The carrier often used is alumina, silica or titania, but generally speaking, other oxides like zirconia, magnesia, zeolites as well as mixed-oxides and carbon can and have been used. Some of these carriers can exhibit a number of crystalline phases with a variety of properties, like gamma, theta, alpha and other transition or precursor aluminas, as well as the anatase and ratile types of titania. To enhance the catalyst performance, e.g. by facilitating reduction of cobalt oxide to cobalt metal, it is common to add different promoters, and rhenium, ruthenium, platinum, iridium and other transition metals can all be beneficial.

It can further be useful to add a second type of promoters to enhance selectivity to desired products. This second promoter can be lanthanum oxide or a mixture of oxides of the lanthanides or other difficult reducible compounds. An example of using lanthanum or barium as a stabilising agent for gamma- alumina is found in WO 01/70394. Here 2-3 wt% La or Ba was impregnated/doped onto the Catapal or Pural/Puralox families of commercially available aluminas. It was demonstrated that high temperature calcination stabilised the surface area of the alumina, and no detrimental effect on the catalyst activity was detected. Therefore, lanthanum or barium, an alkali earth element, does seem to have a neutral effect on activity in these systems.

The second promoter can alternatively be metals from the alkali group in the periodic table of elements. It is described in US 4,880,763 (Statoil) how alkali metals may improve the selectivity towards long-chain paraffins, i.e. to improve the C₅₊ selectivity of the FT-reaction. In this patent it is stated that the addition of an alkali to the catalyst serves to increase the average molecular weight of the product, as evidenced by an increase in the S-F alpha value. However, the activity may decrease as the alkali content increases (for potassium).

Therefore, it is suggested that for any particular situation, there is an optimum alkali level that balances desired average product molecular weight and catalyst activity, potassium being more effective than lithium. In other words, it is taught that alkali may have a positive or negative overall effect, but no particular concentration limits are claimed. Only one example or mentioning of sodium has been given, with a concentration in the finished catalyst of 2400ppm by weight. Further, US 4,880,763 teaches that Li has no effect on activity at the 500pρm level, and that K has no or little effect at 1000 or 2000ppm, the lowest concentrations investigated for these elements. The gamma-alumina supports used, Harshaw al 4100P and Ketjen CK300, may themselves possibly contain alkali metal impurities, but no chemical analysis of these supports is provided.

The effects of potassium or lanthanum additives on the catalytic properties of alumina-supported cobalt catalysts have been examined through carbon monoxide hydrogenation reactions by Jeon et. al. (Korean J. Chem. Eng. VoI 21 (2), 2004, pp. 365-369). With the addition of potassium, the overall carbon monoxide conversion decreased, while the selectivity to higher hydrocarbon and olefin increased. By adding 1000, 5000 and 10000 ppm potassium, the overall carbon monoxide conversion decreased from 6.1% to 5.0, 4.0 and 2.0 % respectively. The effect of lanthanum on activity and selectivity in carbon monoxide hydrogenation was less significant than the effect of potassium, with only a small effect for addition of 10000 ppm lanthanum.

In WO 02/07883 (Sasol) it is claimed how sodium and potassium, among others may be used as a modifying agent/component to form a protected modified catalyst support which is less soluble in the aqueous acid and/or the neutral aqueous impregnation solution during preparation of the catalyst. However, no effect on the FT-reaction has been mentioned for these modifications, in fact, alkali metals are not among the examples given at all.

In spite of the previous art being inconclusive as to the effect of alkali metals, it now has been discovered that certain amounts of alkali metals, and specifically sodium (Na), has a detrimental effect on the activity level of the catalyst. In addition, it has also been discovered that impurities in the carrier or in the chemicals used can give the same detrimental lowering of the activity level. In the present invention, different levels of Na have been added to the FT-catalyst by different methods to establish the limiting level of this element.

The challenge of trace elements in the preparation of FT-catalysts has been addressed by Front Friede et al. (Topics in Catalysis, Vol. 26, 2003, pp 3-12). In their description of the development of the BP Fischer-Tropsch catalyst, they selected one vendor for scale-up and state that key aspects proved to be sourcing the correct raw materials, validating and guaranteeing their quality, as well as the quality of the water used. It took a year of "painstaking analysis" to confirm which of the trace impurities introduced in the commercial manufacturing route were detrimental to catalyst performance and to eliminate these from the production method. The actual nature of the trace compounds or elements is not revealed.

Catalyst carriers may contain a certain level of alkali, in particular sodium, depending on the particular preparation method. Some references may be found in the alumina product catalogue of Almatis (previously part of Alcoa). For example, a low-density pseudo-boemite alumina, G250, gives a high porosity, high surface area gamma-alumina upon heat treatment at 500⁰C. The sodium level in terms of Na₂O is specified as being below 800ppm. For CSS alumina materials, Catalyst Substrate Spheres, a typical Na₂O level is given as 3500 ppm. On the other hand, alumina prepared by the alkoxide route contains a very low level of sodium. These materials are prepared by Sasol (previously Condea) under the trade name Puralox. Similar aluminas are offered by Almatis as HiQ materials with a Na2O level of only ca. 20ppm.

Catalyst supports Amorphous catalyst support materials typically have specific surface areas between 50 and 500m²/g, more typically between 100 and 300m²/g. The starting alumina materials used for the most part in the present invention are all, at least predominantly, of the γ-alumina type, preferably with specific surface areas between 150 and 200m /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- 120μm. After spray-drying, the material is calcined at a high temperature to give the appropriate crystal size and pore structure.

It is also important that the total pore volume is sufficiently high, above 0.2cm³/g or better, above 0.4 cm³/g, or even above 0.6 cm³/g. The pore volume is often measured by the BET method applying nitrogen as the adsorption gas. 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 absorbtivity, 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.

Upon high temperature treatment, the γ-aluminas or the different alumina hydrates will be converted to transition phase aluminas, denoted δ, θ, η, χ or K - aluminas, that all finally will be converted to α-alumina, with a gradual decrease in specific surface areas. These aluminas may also be suitable as support materials for cobalt for the Fischer-Tropsch synthesis, even though they may have specific surface areas in the range 10-50m²/g. The specific surface areas and pore volumes must be balanced towards the requirements for sufficiently high cobalt metal loading and dispersion. However, it is also possible to increase the high temperature surface stability of aluminas by adding certain stabilising agents like lanthanum (lanthanum oxide). In this way, the γ-phase can be retained, even above 1000°C. Other stabilising agents have been used, such as magnesia and ceria.

The catalyst supports used in the present investigation are the following for the different catalysts investigated (surface area, SA; pore volume, PV); all supports are based on gamma-alumina (before possible high-temperature treatment as indicated):

A and A': Sasol, Puralox SCCa, SA = 191m²/g, PV = 0.76ml/g.

Bl: Grace sample 2, SA = 162m²/g, PV = 0.25ml/g.

B2: Grace SMR, SA = 176 m²/g, PV = 0.52ml/g.

C: Saint-Gobain CRR/RDA, SA = 238m²/g, PV = 0.69ml/g.

D : Alcoa Ga-200L, G317, SA = 235m²/g, PV = 0.68 ml/g.

Contains 3 wt% La.

El: Akzo 1, SA = 194 m²/g, PV = 0.72 ml/g.

E2: Akzo 2, SA = 167 m²/g, PV = 0.57 ml/g.

R, S, T and U: Sasol, Puralox SCCa, SA = 170m²/g, PV = 0.73ml/g.

W: As A, then calcined at 1130⁰C for 16h, SA = 18 m²/g. X: As R, then calcined at 1140⁰C for 16h, SA = 10.3m²/g.

Y1-Y5: As R, then impregnated with 10 wt% Zn from Zn(NO₃)₂ or

5 wt% Ni from Ni(NO₃)₂, dried, and calcined at 1140⁰C for 16h, SA = 12 m²/g.

Zl, Z2: TiO₂ (Degussa P25) calcined at 700⁰C for 10 hours, SA = 34 m²/g.

The sodium levels of some supports are included in Table 1. A separate high purity sample from Alcoa, HiQ7213CC made by the alkoxy route, contained 38ppm sodium.

Catalyst preparation and catalyst modification

The catalysts contain a nominal amount of 12 or 20 wt% Co and 0.25, 0.5 or 1.0 wt% Re, as calculated assuming reduced catalysts with complete reduction of cobalt. The actual metal loading as determined by XRF or ICP may vary up to 10%, e.g. for a catalyst with nominal loading of 20 wt% Co, the actual amount of cobalt can vary between 18 and 22 wt% of the total reduced catalyst weight.

Before impregnation, the catalyst support is sometimes precalcined at approximately 500⁰C. Impregnation is usually in one step, but multiple steps can also be employed, from a mixed aqueous solution of appropriate metal salts, generally of cobalt nitrate, perrhenic acid and other water soluble solutions of desired promoters, preferably nitrate solutions. The impregnation technique is by the pore filling, or "incipient wetness", method that implies that the solution is mixed with the dry support until the pores are filled. The definition of the end point of this method may vary somewhat from laboratory to laboratory giving an impregnated catalyst that has a completely dry appearance to sticky snow-like. In no instance is there any free flowing liquid present.

A number of alternative impregnation procedures have been described in the literature, e.g. using alternative solvents and chemical. Our standard procedure involves aqueous incipient wetness with solutions of cobalt nitrate (Co(NO₃)₂)*6H₂O) and perrhenic acid (HReO₄). Alternatives include using cobalt acetate(s), cobalt halide(s), cobalt carbonyl(s), cobalt oxalate(s), cobalt ρhosphate(s), organic cobalt compounds, ammonium perrhenate, rhenium halide(s), rhenium carbonyl(s), industrial metal salt solutions, organic solvents, etc. Further, the impregnation technique may encompass all available methods beside incipient wetness, like precipitation, impregnation from slurry with surplus liquid, chemical vapour deposition etc. It is well known that the impregnation method may influence the dispersion of the active metal (cobalt) and hence the catalytic activity, but the FT-reaction is believed to be non- structure sensitive, the dispersion should not influence the selectivity.

The impregnated catalyst is dried, typically at 80- 12O⁰C, to remove water from the catalyst pores, and then calcined at typically 200-450⁰C, e.g. at 300⁰C for 2-16 hours.

The prepared catalysts are summarised in Table 1. There are four series of catalysts depending on the support, three of them being of the alumina type. The first series illustrates variation in gamma-alumina from different suppliers, the second different sodium levels on a high-purity, high porosity gamma- alumina, and the third series have supports which essentially consists of alpha- alumina or a mixture of alpha alumina and a spinel aluminate. The fourth series is on titania. When sodium is added, it is either impregnated as a NaNO₃ solution, then dried and calcined, or added to the Co/Re metals solution. The Re precursor is perrhenic acid, except for samples R-U where ammonium perrhenate is used.

The given amounts of cobalt in the catalyst compositions are assumed fully reduced. Table 1. Catalysts

* Impregnated on support and calcined ** Added with metals Catalyst testing

One critical step before testing is the activation of the catalyst that involves reduction of cobalt oxide(s) to cobalt metal. Passing a suitable reducing gas over the catalyst particles can perform this reduction. Particularly suitable are hydrogen or carbon monoxide or mixtures thereof. The reducing gas can be mixed with inerts like nitrogen, noble gases or steam and suitable temperatures and pressures should be applied. If a fluidised bed reactor is applied for activation, it might be convenient to use recycle of (part of) the reductive gas and a slight atmospheric total overpressure just to secure a suitable gas flow. It is also possible to use elevated total pressures, let us say up to 8 bars or higher, or even the Fischer-Tropsch reactor pressure. Selection of the reduction temperature strongly depends on the actual catalyst formulation, in particular on the presence and nature of promoters.

The fixed bed testing was performed in a laboratory unit with four parallel fixed-bed reactors. Approximately Ig of catalyst particles in a size fraction between 53 and 90 microns was mixed with 10-20 times the volume of inert SiC. Reduction was performed in situ with hydrogen before a mixture of hydrogen and CO at ratio 2:1 was carefully added. After 2Oh on stream at 21O⁰C and 20 bar total pressure, the space velocity was adjusted to give an estimated conversion level of CO after 9Oh of 45 +/- 3 %. It is very important to perform selectivity, as well as activity, comparisons at the same level of conversion, as the level of steam generated in the reaction has a profound influence on the catalyst performance.

The amount of alkali specified in the catalyst (and reported in table 1 under the heading of sodium) is the total analysed amount of alkali in the catalyst and not just based on the amount in the alkali added. It therefore includes any alkali present in the support or in the chemicals used in the preparation of the catalysts. The catalysts were analysed as follows to determine the alkali level.

A Ig catalyst sample was dissolved in a mixture of 5ml concentrated HCl, 5ml concentrated HNO₃ and 90ml distilled water. The mixture was heated to the boiling point and filtered down into a volumetric flask and aliquots of the liquid in the flask were used in order to determine the amounts of alkali (Na, K, Li, Rb and Cs) in the catalyst samples by means of atomic absorption photometry (on a Varian model 400 AAS instrument).

The catalysts A and A' in Table 1 are reference catalyst for 20 and 12 wt% Co, respectively, using a high purity gamma-alumina catalyst carrier. The activity level will vary somewhat with the Re loading, but is reduced only by ca.lO % for half the starting loading (see US4880763). In the table, relative activity = 1 corresponds to a rate of ca. 1.1 ghydrocarbons / gcat*h and relative C₅₊ selectivity = 1 corresponds to 87 % after 9Oh time-on-stream in the fixed-bed reactor.

Series 2 shows clearly that adding sodium to the support reduces the activity significantly, by as much as 40% at the 400ppm Na level. This is astonishing as the literature teaches that even significantly higher levels do not affect or only slightly affect the catalytic activity. Series 1 illustrates that a similar effect is found when the sodium is an inherent component of the catalyst support as prepared in the production process. In series 3, the activity is less than 20% of the reference sample at the 400ppm Na level for an alpha-alumina support, samples W and X. The strong effect is possibly related to the low surface area of these catalyst carriers and a consequently higher sodium surface coverage. Samples Y illustrate that the adverse effect of sodium also is present when Na follows the impregnation solution. Samples Z show that the same negative effect of Na in the catalyst is found when it is supported on titania. This means that the detrimental effect of alkali in the catalyst seems to be independent of the support.

Claims

1. A Fischer-Tropsch catalyst containing at least 8 wt% cobalt, assuming full reduction of cobalt, on a porous catalyst carrier having a pore volume of at least 0.2 ml/g characterised by a total content of alkali metal in the catalyst of less than 2000ppm by weight.

2. A Fischer-Tropsch catalyst as claimed in claim 1, in which the amount of alkali metal is less than 800ppm by weight.

3. A Fischer-Tropsch catalyst as claimed in claim 1 or claim 2, in which the amount of alkali metal is less than 400ppm by weight.

4. A Fischer-Tropsch catalyst as claimed in any one of claims 1-3, in which the amount of alkali metal is less than 200ppm by weight.

5. A Fischer-Tropsch catalyst as claimed in any one of claims 1-4, in which the amount of alkali metal is less than lOOppm by weight.

6. A Fischer-Tropsch catalyst as claimed in any one of claims 1-5, in which the amount of alkali metal is less than 50ppm by weight.

7. A Fischer-Tropsch catalyst as claimed in any one of claims 1-6, in which the alkali metal is sodium.

8. A Fischer-Tropsch catalyst as claimed in any one of claims 1-6, in which the alkali metal is potassium.

9. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the catalyst carrier is alumina.

10. A Fischer-Tropsch catalyst as claimed in claim 9, in which the alumina is substantially gamma-alumina or alpha-alumina.

11. A Fischer-Tropsch catalyst as claimed in any one of claims 1-8, in which the carrier comprises substantial amounts of a spinel aluminate.

12. A Fischer-Tropsch catalyst as claimed in claim 11, in which the spinel aluminate is nickel or zinc aluminate.

13. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the carrier comprises alumina and a spinel aluminate.

14. A Fischer-Tropsch catalyst as claimed in any one of claims 1-8, in which the carrier is comprised by substantial amounts of titania.

15. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the catalyst carrier has a pore volume of at least 0.4ml/g.

16. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the catalyst carrier has a pore volume of at least 0.6ml/g.

17. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the catalyst contains at least 12 wt% cobalt.

18. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the catalyst in addition contains rhenium as a promoter.

19. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the catalyst contains less than 800ppm sodium and less than 1 wt% lanthanum.

20. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the alkali is part of the catalyst carrier.

21. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the alkali has been impregnated onto the catalyst carrier.

22. A Fischer-Tropsch catalyst as claimed in any preceding claim, in which the alkali stems from the metal precursors used in the catalyst preparation.