PROCESS FOR CONVERTING SYNTHESIS GAS IN THE PRESENCE OF A...

Details PROCESS FOR CONVERTING SYNTHESIS GAS IN THE PRESENCE OF A... PROCESS FOR CONVERTING SYNTHESIS GAS IN THE PRESENCE OF A...

2001-10-24

2003-02-04

Parsa, Jafar (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Group viii metal containing catalyst utilized for the...

C518S700000, C518S717000, C518S721000, C502S164000

Reexamination Certificate

active

06515035

ABSTRACT:

The present invention relates to a process for synthesising hydrocarbons from a mixture comprising CO—(CO
2
)—H
2
(i.e., a mixture comprising CO—H
2
and possibly CO
2
, known as synthesis gas). This process comprises using a catalyst comprising at least one group VIII metal, preferably cobalt, supported on a support based on alumina modified by silica.
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 temperatures 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 CO—H
2
and possibly CO
2
, 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 application WO-A-99/39825 describes the use of a support comprising a titanium dioxide base onto 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 slurry reactor.
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 aim is to avoid dissolving the alumina in the wake of an acid or aqueous attack. The stabiliser can be Si, Zr, Cu, Mn, Ba, Co, Ni and/or La. It can substantially reduce the solubility of the support in acidic or neutral aqueous solutions. A preferred method for introducing the stabiliser, in particular silica, is to graft on organic compounds such as TEOS (triethoxysilane) or TMOS (trimethoxysilane) onto a pre-formed alumina. The quantities of silica that are deposited are preferably in the range of of 0.06 at. Si
m
2
(atoms of silicon per square nanometer of specific surface area of the support) to 2.8 at. Si
m
2
, and optionally up to 14.8 at. Si
m
2
for a Puralox alumina with a specific surface area of 150 m
2
/g.
French patent application 00/01168 claims a process for synthesising hydrocarbons from a mixture comprising carbon monoxide and hydrogen in the presence of a catalyst comprising at least one group VIII element supported on a silica-alumina prepared by co-precipitation and calcined at a temperature in the range of of about 500° C. to about 1200° C. to obtain a silica-alumina with a specific surface area of less than 260 m
2
/g.
European patent application EP-A-0 184 506 describes a process for producing alumina-based catalyst supports with improved durability. The production process is characterized in that in a first step, the support is impregnated with an aqueous solution of quaternary ammonium silicate then dried, and the impregnated support is then calcined.
SUMMARY OF THE INVENTION
The present invention concerns a process for synthesising hydrocarbons from a mixture comprising carbon monoxide and hydrogen CO—H
2
, and possibly carbon monoxide CO
2
, in the presence of a catalyst comprising at least one group VIII metal, supported on a support comprising alumina modified by impregnating with an aqueous solution of quaternary ammonium silicate. The catalyst is preferably used in suspension in a liquid phase in a perfectly stirred autoclave type three-phase reactor (slurry bubble column). It is also suitable for use in a fixed bed.
DETAILED DESCRIPTION OF THE INVENTION
The Applicant has surprisingly discovered that a support comprising alumina modified by impregnation with an aqueous quaternary ammonium silicate solution such that the quantity of silica introduced into the support is in the range of about 3% to about 10% by weight with respect to the finished support, after impregnating with at least one element from group VIII, can produce a catalyst that is particularly active in a process for synthesising 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, without the need to introduce silica via a precursor, i.e., primarily chemically bonded to the support during the step for adding said silica precursor. Simple dry impregnation is suitable for introducing said precursor.
The support used in the process of the invention can, for example, preferably be prepared using the operating procedure described in EP-B-0 184 586, i.e., as follows:
In a first step, the support comprising the alumina (i.e., usually at least 55% by weight, preferably at least 65% by weight, more preferably at least 80% by weight and still more preferably at least 90% by weight of alumina) is impregnated with an aqueous solution of quaternary ammonium silicate solution, preferably selected from the group formed by: tetramethylammonium silicate, tetraethylamrnonium silicate, and tetrahydroxyammonium silicate. In a preferred implementation of the invention, the alumina used is a gamma alumina. In a preferred implementation of this preparation step of the invention, the support is dry impregnated, i.e., the total volume of the solution used is approximately equal to the total pore volume of the support;
In a second step, the impregnated support is dried at a temperature in the range of 80° C. to 500° C. to transform the compounds used into precursors of the desired oxides in the dispersed state on the alumina support.
In a third step, the support dried during the second step is calcined at a temperature in the range of 500° C. to 1000° C., preferably in the range of 600° C. to 900° C. for a period in the range of 1 to 24 hours.
The supports, also termed “silicated aluminas” used in the present invention comprise in the range of about 3% by weight to about 9.5% by weight of SiO
2
, preferably in the range of about 4% by weight to about 9.5% by weight, more preferably in the range of about 4.5% by weight to about 9% by weight of silica, and still more preferably in the range of 5% by weight to 8% by weight of silica with respect to a support comprising alumina and silica. These amounts correspond to quantities of atoms of Si
m
2
in the range of about 2 to about 6 atoms of Si
m
2
, preferably in the range of about 2.5 to about 6 atoms of Si
m
2
, more preferably in the range of about 3.0 to about 5.7 atoms of Si
m
2
and still more preferably in the range of about 3.3 to about 5.1 atoms of Si
m
2
for a support with a specific surface area of 160 m
2
/g. In this application, the modifying element of the support, such as silica, is chemically bonded to the support.
In a variation of the catalyst preparation process of the invention, its is possible to introduce into the support before, after or simultaneously with the quaternary ammonium silicate, at least one precursor of an oxide of an element selected from the group formed by: rare earths, alkaline-earths and zirconium, which result in the corresponding oxides by thermal decomposition, i.e., to oxides of the rare earth or alkaline-earth and/or zirconium, dispersed over the support. The overall quantity of said oxides in the catalyst of the invention is preferably less than 50% by weight, more preferably in the range of 1% by weight to 30% by weight, and more preferably in the range of 3% by weight to 15% by weight.
Preferably, the quaternary ammonium silicate and optional rare earth, alkaline-earth and zirconium precursors are impregnated onto the formed support.
Preferably, the support used before impregnating the aqueous quaternary ammonium silicate solution is a gamma alumina and is in the form of a fine calibrated powder obtained, for example, by spray drying and with a gra

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