GB2416715A

GB2416715A - Fischer-Tropsch catalyst

Info

Publication number: GB2416715A
Application number: GB0417080A
Authority: GB
Inventors: Erling Rytter; Sigrid Eri
Original assignee: Statoil ASA; Petro SA
Current assignee: Equinor ASA; Petro SA
Priority date: 2004-07-30
Filing date: 2004-07-30
Publication date: 2006-02-08
Also published as: WO2006010936A1; GB0417080D0

Abstract

A Fischer-Tropsch catalyst composition comprising cobalt on a porous alumina carrier, which cobalt composition contains alkali metal in the amount of less than 2000ppm by weight.

Description

Fischer-Tropsch catalyst The Fscher-Tropsch (FT) rcacton 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 Hz/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-reacton (2.0-2.1). Only Co based catalysts Will be considered In the following.

A variety of products can be made by the FT-rcaction, 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 SEA or S-F (Shultz-Flory-Anderson) al "'a-value, or altenatvely by the fraction of Cal products. By-products can include olefins and oxygenates.

To achieve sufficient activity, it is customary to disperse the Co on a 2() 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 ttana, but generally speaking, other oxides like zirconia, magnesia, zeoLtcs as weld 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, 1kc gamma, theta, alpha and other transition or precursor alumnus, as well as the anatase and futile types of ttania. To enhance the catalyst performance, e.g. by facihtatng 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 sclectvty to desired products. This second promoter can be lanthanum oxdc or a l mixture of oxides of the lanthandes or other difficult reducible compounds. An example of using lanthanum or barium as a stablsng agcut for gamma-alumina is found in WO ()1/70394. Ilcre 2-3 wt% La or Ba was impregnated/doped onto the Catapal or Pural/Puralox famhcs of commercially avalablc aluminas. It was demonstrated that high temperature calcinaton stablsed the surface area of the alumina, and no detrimental ef't'ect on the catalyst activity was detected. Therefore, lanthanum or barium, an alkali earth element, does seem to have a neutral effect on acOvty In these systems.

The second promoter can altcrnatvcly 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-chan paraffins, uc. to improve the C5.r selectivity of the F'l'-reacton. 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 m the S-F alpha value. llowever, 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 bcng more effective than}hum. In other words, it is taught that alkah may have a positive or ncgatve overall effect, but no particular concentrahon limits arc claimed. Only one example or mcntonng of sodium has been given, with a concentration in the finished catalyst of 2400ppm by weight. Further, US 4,880,763 teaches that L' has no effect on activity at the 500ppm}evel, and that K has no or little cf'f ct at 1000 or 20()0ppm, 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.

In WO 02/07883 (Sasol) it is clamcd 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 m the aqueous acid and/or the neutral 3() aqueous nnpregnaton solution during preparation of the catalyst. Ilowever, no ef'f'ect on the L'T-reacton has been mentioned for these modifications, In fact, alkah metals are not among the examples given at all.

In spite of the previous art bemg mconclusve 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 mpuntcs in the carrier or in the chemicals used can gvc the same detrimental lowering of the activity level. In the present invention, different levels of Na have been added to the FT-catalyst by dl't'erent methods to establish the lmtng level of this element.

I'he challenge of trace elements in the preparation of FT-catalysts has been addressed by Front Fncde et al. (Topics In Catalysis, Vol. 26, 2003, pp 3-12). In their description of the development of the BP FscherTropsch 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 quahty, 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 Almats (previously part of Alcoa). For example, a low-densty pseudo-bocmte alumina, G250, gives a high porosity, high surface area gamma-alumina upon heat treatment at 50() C. The sodium level in terms of Na2O is specified as biting 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 alkoxde route contains a very low level of sodium. These matcrals arc prepared by Sasol (previously Condea) under the trade name Puralox. Similar alumnas are offered by Almatis as IliQ materials with a Na20 level of only cat 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 alumma materials used for the most part in the present invention are all, at least predominantly, of the y-alumina type, preferably with specific surface areas between 150 and 200m2/g. These supports can be prepared by spray-drying techniques of an appropriate solution m order to obtain essentially spherical particles of appropriate size, e.g. 80% m the range between 30-120m. After spray-dryng, the material is calcned 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 1() 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 ntrogcn as the adsorption gas. This method does not take into account large pores where a mercury porosmetcr is more relevant. A less accurate, but more practical parameter is the measured water absorbtvty, which can be drcctly correlated with the amount of cobalt that can be impregnated on the catalyst by the incipient wetness procedure. A high pore volume will gvc a light material suitable for operation m a slurry environment and ease the impregnation by mnimismg the number of impregnation steps required.

At the same time the support, and the final catalyst, should have sul'tcent 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 y-alumnas or the different alumina hydrates will be converted to transition phase alumnas, denoted &, 8, '1, X or K - alumnas, that all finally Will be converted to a-alumina, with a gradual decrease In specific surface areas. These alummas may also be suitable as support materials for cobalt for the Fschcr-Tropsch synthesis, even though they may have specific surface areas m the range 10-50m2/g. The specific surface areas and pore volumes must be balanced towards the rcqurcments for sufficiently high cobalt metal loading and dispersion. However, it Is also possible to increase the high temperature surface stability of alumnus by adding certain stabhsng agents like lanthanum (lanthanum oxide). In this way, the y-phasc can be retaped, even above 1000 C. Other stabilsmg agents have been used, such as magnesia and cereal The catalyst supports used In the present mvestgaton are the following for the different catalysts investigated (surface area, SA; pore volume, PV); all supports are based on gamma-alumna (before possible hgh-tcmperature treatment as educated): A and A': Sasol, Puralox SCCa, SA = 191 m2/g, PV = 0.7Gml/g.

B 1: Grace sample 2, SA = 162m2/g, PV = 0.25ml/g.

B2: Grace sample 3, SA - 170m2/g, SA =) 0.21ml/g.

B3: Grace SMR, SA = 176 m2/g, PV = 0.52ml/g.

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

D: Alcoa Ga-200L, G317, SA = 235m2/g, PV = 0.68 Contains 3 wt% La.

R. S. 'I' end U: Sasol, Puralox SCCa, SA = 170m2/g, PV = 0.73ml/g.

] 5 W: As A, then calcmed at 1130 C for 16h.

X: As R. then calcined at 1140 C for Ah, SA = 10.3m2/g.

Y: As R. then impregnated with 10 wt% Zn from Zn(NO3)2, dried, calcified at 350 C then at 1140 C for 16h, SA = 1 1.9m2/g.

The sodium levels of some supports arc included in Table 1. A separate high purity sample from Alcoa, HQ7213CC 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 loadmg 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 3() cobalt can vary between 18 and 22 wt% of the total reduced catalyst weight.

Bet'ore Impregnation, the catalyst support Is somctmes precalcned at approximately 500 C. Impregnation is usually In one step, but multiple steps can also be employed, fiom a mixed aqueous solution of appropriate metal salts, generally of cobalt mtrate, perrhenic acid and other water soluble solutions of desired promoters, preferably nitrate solutions. The TTnpregnatTon technique TS by the pore Filing, or "mcTpent wetness", method that imphcs 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 givmg an impregnated catalyst that has a completely dry appearance to sticky snow-take. In no Instance there are 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 mtrate (Co(NO3)2)*6H2O) and perrhenTc acid (HReO4). Alternatives Include using cobalt acetate(s), cobalt halide(s), cobalt carbonyl(s), cobalt oxalate(s) , cobalt phosphate(s), organic cobalt compounds, ammonium perrhenate, rhenium halde(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, nnpregnaton 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 bclTeved to be non-structure sensitive, the dispersion should not influence the selectivity.

The impregnated catalyst is dried, typically at 8()-120 C, to remove water from the catalyst pores, and then calcmed at typically 2()0-450 C, e.g. at 300 C for 2-16 hours.

The prepared catalysts are summarsed in Table 1. There are three series of catalysts depending on the support, all of the alumina type. The first series Illustrates variation in gamTna-alumTna from different supphers, the second different sodium levels on a hTgh-purTty, high porosity gammaalumna, and the third series have supports which essentially consists of alf:a-alumTna or a mixture of alfa alumma and a spTncl alummatc. When sodium Is added, it Is either mpregnated as a NaNO3 solution, then dried and calcned, or added to the Co/Re metals soluton. The Re precursor s perrhenc acd, except for samples R-U where ammonum perrhenate s used.

The gven amounts of cobalt n the catalyst compositons are assumed fully reduced.

Table 1. Catalysts

C' I Metals mpregnaton Sodum C'atalyst perfrn-nance code C'o Re Added * Analysed Rel at ve Relat ve _. _ (wt2/o) _,(wt\),, (ppmwt) (ppmwt) actvty (:'5+sclcct Sertcs I Gamma- alumna supports, ms ccllaneous sup] lcrs A 20 _ _ j _. . _. 1.69 0,t33() A' 12 0.5 0 () 80 0 946 --31--1 _ 12-.- 0.5 o 1,_X, ,_ 0,31 0(307 B2 12 1 0.5 0 _. _. 0.35 () 930 113 20 0.5 0 284*** 1 15 0.935 _ C.__ _20,, 05 1 _._0 _ _X,,_,0 -- 1 _ _ _. 20 _ _ _. _. _. , ().78 0.'301 Seres 2Gamma-alumna supports, se nes Puralox-S( ('a R I 200.5 0 1.51 1 ().914 _. _ _. _ _ _ _. _ _ _ _ _ S - -o- - l 0.5 100 1.45 1 0.923 l' 20 0. 5 200 X 1.38 0.927 tJ_ I 20 o 5 1 400. _ 0.91 1 0.935 Scrcs 311gh temperature treated gamma-alum ma supports, s' me modfie d _._ _ _. . . ._. _ 1.05 0.987 X 12 1 0.5 40() 0 14 0.984 Y 12 0 25 200 ** 1 X 0,33 0,973 * Impregnated on support and calcl 1ed ** Added wth metals *** Support Catalyst testing One cntical step before testn.g s the actvation of the catalyst that nvolves 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 hke nitrogen, noble gases or steam and suitable temperatures and pressures should be applied. If a fludsed bed reactor Is applied for activation, it might be convcnent to use recycle of (part of) the rcductve 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 Fscher- Tropsch reactor pressure. Selcchon 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 Lmt with four parallel fixed-bed reactors. Approximately lg of catalyst particles in a size fraction between 53 and 90 microns was mend with 10-20 times the volume of inert SAC.

] 5 Reduction was performed nil site with hydrogen before a mixture of hydrogen and CO at ratio 2:1 was carefully added. After 20h on stream at 21 0 C and 20 bar total pressure, the space velocity was adjusted to give an estimated conversion level of CO after 90h of 45 +/- 3 %. It Is very important to perform selectivity, as well as activity, comparisons at the same Icvel of conversion, as the level of steam generated In the reaction has a profound Influence on the catalyst performance.

The catalysts A and A' in Table I are reference catalyst for 20 and 12 wt% Co, respectively, using a high purity gamma-alumna catalyst carrier. The activity level will vary somewhat with the Rc loading, but is reduced only by ca.10 % for half the starting loading (see US 2,880,763). in the table, relatvc activity = I corresponds to a rate of cat I. I g.yr,car,,, s / gulch and relative C5+ selectivity = I corresponds to 78 % after 90h tme-on-stream m the fixed-ecd 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 astonshng as the ltcrature teaches that even sgmtcantly higher levels do not affect the catalytic activity. Scrips I illustrates that a similar effect is found when the sodium Is an mhercnt 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 alfa-alumna 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. Sample Y illustrates that the adverse effect of sodium also is present when Na follows the nnpregnaton solution.

Claims

Claims 1. A Fscher-Tropsch catalyst composition comprising cobalt on a

porous alumina carrier, which cobalt composition contains alkali metal m the amount of less than 200()ppm by weight.
2. A Fscher-Tropsch catalyst composition according to claim I In which the amount of cobalt is at least 8 wt% cobalt (assuming full reduction of cobalt).
3. A Fscher-Tropsch catalyst composition according to clang 1 or 2 in which the porous alumina carrier has a pore volume of at least 0.2ml/g.
4. A Fscher-Tropsch catalyst composition according to any of the claims 13 m which the amount of alkah metal Is less than 800ppm by weight.
5. A Fscher-Tropsch catalyst composition according to any of the clanks 14 In which the amount of alkah metal is less than 400ppm by weight.
6. A 11'scher-Tropsch catalyst composition according to any of the claims 1-5 In which the amount of alkali metal Is less than 200ppm by weight.
7. A Fscher-Tropsch catalyst composition according to any of the claims 16 in which the amount of alkali metal Is less than 1 O()ppm by weight.
8. A Fscher-Tropsch catalyst composition according to any of the claims 17 In which the amount of alkah metal Is less than 50ppm by weight.
9. A F'scher-Tropsch catalyst composition according to any of the 3() claims 1-8 nl which the alkali metal Is sodium. 1()
10. A Fischer-Tropsch catalyst composition according to any of the claims 1-8 m which the alkali metal is potassium.
11. A Fscher-Tropsch catalyst composition according to any of the clanns 1-10 In which the carder comprises substantial amounts of an alumina.
1 2. A Fscher-Tropsch catalyst composition according claim 11 In which the alumina Is gamma-alumna or alpha-alumna.
13. A F'scher-Tropsch catalyst composition according to any of the claims 1-1() in which the carrier comprises substantial amounts of a spiel aluminate.
14. A Fscher-Tropsch catalyst composition according to claim 13 In which the spinet aluminate Is nickel or zinc aluminate.
15. A Fschcr-Tropsch catalyst composition according to any of the claims 1-10 in which the carrier Is comprised by substantial amounts of titana.
16. A Fscher-Tropsch catalyst composition according to any of the preceding claims In which the catalyst carrot has a pore volume of at least 0.41/g.
17. A Fscher-Tropsch catalyst composition according to any of the prcccdng clamps in which the catalyst carrier has a pore volume of at least 0.6ml/g.
18. A F,scher-Tropsch catalyst composition according to any of the preceding clamps In which the catalyst contains at least 12 wt% cobalt.
19. A Fscher-Tropsch catalyst conposton according to any of the preceding claims In which the catalyst m addition contains rtemum as a promoter.
20. A Fscher-Tropsch catalyst composition according to any of the preccdng claims in which the catalyst contains less than 800ppm sodium and less than 1 wt% lanthanum.
21. A Ftscher-Tropsch catalyst composition according to any of the preceding claims m which the alkalms part of the catalyst carrier.
22. A Fischer-Tropsch catalyst composition according to any of the prccedng clamps In which the alkah has been Impregnated onto the catalyst earner.
23. A Fischer-Tropsch catalyst composition according to any of the preccdng claims In which the alkali stems from the metal precursors used in the catalyst preparation.