Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2002-11-27
2004-12-28
Nguyen, Cam N. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S243000, C502S252000, C502S260000, C502S326000, C502S327000, C502S330000, C502S332000, C502S333000, C502S334000, C502S339000
Reexamination Certificate
active
06835690
ABSTRACT:
FIELD OF THE INVENTION
THIS INVENTION relates to cobalt catalysts. In particular, the invention relates to a cobalt based Fischer-Tropsch catalyst, to a precursor of such a cobalt catalyst, to a process for preparing a precursor of such a cobalt catalyst, to a process for preparing such a cobalt catalyst, and to a process for producing hydrocarbons using such a cobalt catalyst.
BACKGROUND OF THE INVENTION
The Applicant is aware of known processes for preparing cobalt based catalyst precursors and which involve slurry phase impregnation of a catalyst support with a cobalt salt, drying of the impregnated catalyst support, and calcination of the dried impregnated catalyst support, to achieve a desired cobalt loading of the support. The resultant precursors are then activated by reduction thereof, to obtain cobalt based Fischer-Tropsch catalysts. These catalysts can display good intrinsic activities when used for Fischer-Tropsch synthesis; however, catalysts having enhanced or superior intrinsic activities cannot readily be obtained using the known processes. It is thus an object of the present invention to provide a cobalt based Fischer-Tropsch catalyst having enhanced initial and/or stabilized intrinsic Fischer-Tropsch synthesis activity, as well as a process for preparing such a catalyst.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a cobalt based Fischer-Tropsch catalyst, the catalyst including a porous catalyst support and metallic cobalt crystallites within the support, and the catalyst having
i. a proportion of its cobalt in reducible form, with this proportion being expressible as &OHgr; mass %, based on the total pre-reduced catalyst mass;
ii. when freshly reduced, a mono-modal Gaussian metallic cobalt crystallite size distribution;
iii. when freshly reduced, a metallic cobalt surface area, in m
2
per gram of catalyst, of from 0.14 &OHgr; to 1.03 &OHgr;; and
iv. when freshly reduced, a catalyst geometry that ensures a stabilized Fischer-Tropsch synthesis effectiveness factor of 0.9 or greater.
It was surprisingly found that the cobalt based catalyst of the first aspect of the invention displayed enhanced initial and stabilized intrinsic Fischer-Tropsch activity.
By ‘stabilized Fischer-Tropsch synthesis effectiveness factor’ is meant the effectiveness factor of the stabilized catalyst. A stabilized cobalt based Fischer-Tropsch catalyst is defined as a catalyst that has been conditioned completely during slurry phase Fischer-Tropsch synthesis at realistic Fischer-Tropsch synthesis conditions using an ultra pure synthesis gas, ie a synthesis gas that contains no contaminant compounds other than H
2
and CO that could affect catalytic deactivation.
By ‘realistic Fischer-Tropsch synthesis conditions’ is meant reaction conditions of 225±5° C. and 20 bar, and % (H
2
+CO) conversion of 60±10% using a feed gas comprising about 50 vol % H
2
, about 25 vol % CO, and with the balance being Ar, N
2
, CH
4
and/or CO
2
.
By ‘freshly reduced’ is meant a catalyst that has been activated without subjecting such a catalyst to Fischer-Tropsch synthesis.
In this specification, unless explicitly otherwise stated, where reference is made to catalyst mass, the mass given pertains to the calcined catalyst mass or pre-reduced catalyst mass, ie the catalyst mass before any reduction of the catalyst is effected.
Preferably, the metallic cobalt surface area of the catalyst, when freshly reduced, may be, in m
2
per gram of catalyst, from 0.28 &OHgr; to 0.89 &OHgr;.
The sizes of a majority of the cobalt crystallites may be greater than 8 nm.
It was surprisingly found that the supported cobalt catalysts of the present invention with their large cobalt crystallites, ie with the majority of metallic cobalt crystallite sizes greater than 8 nm and thus relatively low cobalt metal areas, were not severely affected by oxidation. With these catalysts, while their time zero (ie at the start of a run) intrinsic activities, when used for Fischer-Tropsch synthesis, may be lower than those of supported cobalt catalysts having smaller cobalt crystallites (i.e. having the majority of cobalt crystallite sizes smaller than 8 nm), superior or enhanced initial and stabilized intrinsic activities were surprisingly obtained since there was much less deactivation with the novel catalysts than with supported cobalt catalysts having the majority metallic cobalt crystallite sizes smaller than, or equal to, 8 nm.
The porous catalyst support may be a calcined support. Thus, the supported cobalt catalyst of the first aspect of the invention may be that obtained by impregnating a porous catalyst support with cobalt or a cobalt precursor, particularly cobalt nitrate, calcining the impregnated support to obtain a cobalt catalyst precursor; and reducing the catalyst precursor to obtain the supported cobalt catalyst.
The catalyst support may be a modified catalyst support. The modified catalyst support may comprise catalyst support particles coated with a modifying agent, which may be carbon or one or more metals of Group IA and/or Group IIA of the Periodic Table of Elements, ie one of more metals selected from Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, St, Ba, Ra and mixtures thereof. It was found that the catalyst had a surprisingly large increase in initial intrinsic activity when used for Fischer-Tropsch synthesis, and hence a surprisingly high stabilized intrinsic activity, when a modified catalyst support was used, particularly when a carbon or barium coated support was used.
When barium is used to coat the support, the maximum amount of barium that can be used is determined by the influence of the barium on the pore volume of the support as well as on the solubility of the barium precursor. The pore volume of the support should be large enough to accommodate the required amount of cobalt nitrate to be able to obtain a supported cobalt catalyst with the required cobalt loading. The solubility of the barium precursor should be large enough to be able to add the barium precursor in one impregnation step to the support. In practice the maximum amount of barium may be 10% by mass, based on the catalyst mass. The minimum amount of barium is determined by the minimum amount of barium that is effective in increasing the stabilized intrinsic Fischer-Tropsch activity of the cobalt catalysts, and may be 0.2% by mass.
When carbon is used to coat the support, the maximum amount of carbon that can be used as an effective coating is determined by the influence of the carbon coating on the pore volume of the original catalyst support, as the pore volume of the catalyst support determines how much cobalt can be impregnated into the catalyst support. This is particularly important when a catalyst with a relatively high cobalt loading is required. Similarly, the minimum amount of carbon that can be used as an effective coating is determined by the minimum level of carbon that still provides the required positive effect on the stabilized intrinsic Fischer-Tropsch synthesis performance of the cobalt catalyst. Thus, the maximum level of carbon may be 40 g C/100 g support, preferably 20 g C/100 g support, and more preferably 10 g C/100 g support, while the minimum level of carbon may be 0.1 g C/100 g support, preferably 0.5 g C/100 g support, and more preferably 1.2 g C/100 g support.
In principle, the coating of the catalyst support particles can be effected by any suitable method. For example, the carbon coated catalyst support may be prepared by coating pre-shaped spherical porous catalyst support particles with a uniform carbon based layer in accordance with the method as described in EP 0681868, which is hence incorporated herein by reference.
According to a second aspect of the invention, there is provided a cobalt based catalyst precursor, which includes a porous catalyst support and cobalt oxide crystallites within the support, the precursor having a proportion of its cobalt in reducible form, with this proportion being expressible as &OHgr; mass %, based on the total precursor mass,
Loosdrecht Jan Van De
Van Berge Peter Jacobus
Visagie Jacobus Lucas
Ladas & Parry
Nguyen Cam N.
Sasol Technology Limited
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