Chemistry: fischer-tropsch processes; or purification or recover – Group viii metal containing catalyst utilized for the...
Reexamination Certificate
2001-02-08
2002-10-15
Parsa, Jafar (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Group viii metal containing catalyst utilized for the...
C518S700000, C518S721000, C502S263000, C502S303000, C502S439000
Reexamination Certificate
active
06465530
ABSTRACT:
The present invention relates to a process for synthetizing hydrocarbons from a mixture comprising CO—(CO
2
)—H
2
(i.e., a mixture comprising carbon monoxide and hydrogen and possibly carbon dioxide, known as synthesis gas). This process comprises using a catalyst comprising at least one group VIII metal, preferably cobalt, supported on a silica-alumina, prepared by co-precipitating and calcining at a temperature in the range from about 500° C. to about 1200° C. for at least 6 hours, so as to have a specific surface area of less than 260 m
2
/g.
PRIOR ART
The skilled person is aware that synthesis gas can be converted to hydrocarbons in the presence of a catalyst containing transition metals. Such conversion, carried out at high pressures and under pressure, is known in the literature as the Fischer-Tropsch synthesis. Metals from group VIII of the periodic table such as iron, ruthenium, cobalt and nickel catalyse the transformation of CO—(CO
2
)—H
2
mixtures (i.e., a mixture of carbon monoxide, hydrogen and possibly carbon dioxide, known as synthesis gas) to liquid and/or gaseous hydrocarbons.
Different methods have been described and developed in the prior art that are intended to improve the preparation of Fischer-Tropsch catalysts based on cobalt supported on different supports. The most widely used supports are alumina, silica and titanium dioxide, occasionally modified by additional elements.
International patent WO-A-99/39825 describes the use of a support comprising a titanium dioxide base on which a binder constituted by silica and alumina has been incorporated. The mechanical properties of the catalyst obtained are improved, in particular for use in a three-phase reactor, generally known as a slurry reactor. Usually, such a reactor is of the slurry bubble column type.
WO-A-99/42214 describes adding a stabilising element to an Al
2
O
3
support used to prepare a catalyst that is active in the Fischer-Tropsch process.
The stabiliser can be selected from the group formed by: Si, Zr, Cu, Mn, Ba, Co, Ni and/or La. It can substantially reduce the solubility of the support in acid or neutral aqueous solutions.
Cobalt-based Fischer-Tropsch catalysts described in the invention cited above and used in a three-phase reactor can lead to excessive losses of catalyst in the paraffin waxes produced, by the formation of submicronic fines. The catalyst losses, expressed with respect to the cobalt, can reach 50 mg of cobalt per kilogram of wax.
U.S. Pat. Nos. 5 169 821 and 5 397 806 describe including silicon, zirconium or tantalum in a cobalt-based catalyst supported on TiO
2
in the form of anatase to stabilise it to high temperature regeneration.
WO-A-96/19289 describes the use of a catalyst to convert synthesis gas to hydrocarbons based on cobalt, ruthenium or iron supported on a mesoporous crystalline aluminosilicate with a particular pore structure.
U.S. Pat. No. 4 497 903 describes incorporating cobalt into the crystalline layers of an aluminosilicate. The catalyst obtained is active on converting synthesis gas into liquid hydrocarbons principally constituted by branched hydrocarbons with a high octane number.
U.S. Pat. No. 5 045 519 describes a process for preparing silica-alumina leading to a high purity product that is heat stable. It is prepared by hydrolysis of an aluminium alkoxide and simultaneous or successive addition of orthosilicic acid previously purified by ion exchange. The silica-aluminas obtained are used as a support for a desulphurisation catalyst, in DeNO
x
catalysis, for oxidation, in hydrocracking, in mild hydrocracking, in automobile exhaust catalysis, and in isomerisation.
The present invention concerns a process for synthetizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen, and possibly carbon monoxide, in the presence of a catalyst comprising at least one group VIII metal, preferably cobalt, supported on a particular silica-alumina which will be described below. The catalyst is preferably used in suspension in a liquid phase in a perfectly stirred autoclave type three-phase reactor or slurry bubble column. It is also suitable for use in a fixed bed.
The Applicant has discovered that using a silica-alumina support prepared by co-precipitating and calcining at a high temperature for a time sufficient to encourage interactions between the alumina and the silica (Al—O—Si bonds) can, after impregnation with at least one group VIII metal, preferably cobalt, produce a catalyst that is particularly active in a process for synthetizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen. Further, said catalyst has improved mechanical properties, in particular when, as is preferable, it is used in suspension in a liquid phase in a three-phase reactor, and it has better resistance to attrition phenomena.
The silica-alumina used in the process of the present invention is preferably a silica-alumina that is homogeneous on the micrometric scale and in which the quantity of anionic impurities (for example SO
4
2−
, Cl
−
) and cationic impurities (for example Na
+
) is preferably less than 0.1% by weight, more preferably less than 0.05% by weight.
The silica-alumina used in the process of the invention is prepared by co-precipitation. By way of example, the silica-alumina support used in the process of the invention can be prepared by true co-precipitation under controlled stationary operating conditions (average pH, concentration, temperature, mean residence time) by reacting a basic silicon-containing solution, for example in the form of sodium silicate, optionally aluminium, for example in the form of sodium aluminate, with an acidic solution containing at least one aluminium salt, for example aluminium sulphate.
After co-precipitation, the support is obtained by filtering and washing, optionally washing with an ammoniacal solution to extract the residual sodium by ion exchange, drying and forming, for example by spray drying then calcining, preferably in air in a rotary oven and at a high temperature, generally in the range about 500° C. to about 1200° C., for a time sufficient to encourage the formation of interactions between the alumina and the silica, generally at least 6 hours. These interactions lead to a better mechanical strength of the support and thus of the catalyst used in the process of the invention.
A further method for preparing the silica-alumina of the invention consists of preparing, from a water-soluble alkaline silicate, a solution of orthosilicic acid (H
4
SiO
4
, H
2
O) decationised by ion exchange then mixed with a cationic aluminium salt in solution, for example the nitrate, and finally co-precipitating the solution obtained with ammonia under controlled operating conditions. After filtering and washing, drying with forming and calcining between about 500° C. and about 1200° C. for at least 6 hours, a silica-alumina support that can be used in the process of the invention is obtained.
A preferred method, disclosed in U.S. Pat. No. 5 045 519, consists of preparing a decationised orthosilicic acid as above then simultaneously or consecutively adding an aluminium alkoxide such as aluminium trihexanoate to the hydrolysed product. After intense homogenisation of the suspension by vigorous stirring, optional adjustment of the dry material content by filtering then re-homogenisation, the product is dried and formed, then calcined between about 500° C. and about 1200° C. for at least 6 hours.
In all of the preparation methods described, during any step of the preparation it may be desirable to add a small proportion of a stabilising element selected from the group formed by lanthanum, praseodymium and neodymium. The stabilising element is preferably added in the form of a soluble salt, for example a nitrate.
Preferably, a soluble salt of at least one stabilising element is added to the aqueous cationic aluminium salt or, as disclosed in U.S. Pat. No. 5 045 519, simultaneously or consecutively with addition of the orthosilicic acid to the hydrolysed product of at least one aluminium alkoxide
Courty Philippe
Normand Laurent
Roy-Auberger Magalie
Zennaro Roberto
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Parsa Jafar
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