Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By alkyl transfer – e.g. – disproportionation – etc.
Reexamination Certificate
2001-07-03
2003-09-23
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By alkyl transfer, e.g., disproportionation, etc.
C585S646000
Reexamination Certificate
active
06624338
ABSTRACT:
The invention relates to a catalyst comprising (and preferably constituted by) delta alumina, rhenium and caesium, and to a process for preparing the catalyst. The invention also relates to a metathesis process using this catalyst, and more particularly to a process for metathesis of an olefinic C
4
cut (i e., containing butenes).
In one particular aspect, the present invention relates to a process for converting a C
4
hydrocarbon cut containing isobutene, after selective hydrogenation of butadiene and acetylenic compounds and hydroisomerisation of 1-butene to 2-butene, by metathesis with ethylene in the presence of a catalyst based on rhenium improved by the incorporation of caesium and by the presence of delta alumina.
Cracking light paraffins produces ethylene and propylene required for petrochemistry. It also produces a certain number of other products including a C
4
hydrocarbon cut which principally contains butadiene, isobutene, n-butenes and butanes, also traces of acetylenic hydrocarbons.
The metathesis reaction converts such cuts, which are often inadequately used, to compounds with a higher intrinsic value. As an example, metathesis with ethylene of an olefinic C
4
cut, which has already undergone selective hydrogenation of butadiene and acetylenic compounds and hydroisomerisation of 1-butene to 2-butene, can produce propylene.
When such a reaction is carried out using the usual rhenium-based metathesis catalysts, with a C
4
cut containing isobutene, this partially polymerises on contact with the catalyst and that secondary reaction causes a large reduction in the duration of the cycle for the catalyst between two regeneration operations.
Incorporating caesium into a rhenium-based metathesis catalyst has been described in French patent FR-A-2 373 504, for the preparation of branched olefins. In U.S. Pat. No. 5,057,644, caesium has been cited as a possible additional alkaline element in a catalyst activated at 300-600° C. brought into the presence of organic borane as a promoter. FR-A-1 572 314 indicates that the selectivity of a metathesis catalyst can be increased by adding caesium in particular, and that activation occurs at 300-750° C.
The alumina used as a support in the prior art is generally a gamma alumina and the catalyst is heat activated in a temperature range which usually does not exceed 750° C., although U.S. Pat. No. 3,594,440 indicates that it is possible to go up to 900° C., with no mention of a particular advantage. In a more recent publication, it is indicated that there is no interest in activating a caesium-rhenium-alumina catalyst at a temperature of more than 300° C., the catalyst being prepared by impregnating rhenium into alumina, calcining at 550° C., introducing caesium by impregnation, calcining at 500° C., then activating in nitrogen (T. Kawai et al., Journal of Molecular Catalysis, vol. 76, pp. 249-261, 1992).
We have now discovered that the use of a rhenium- and caesium-based catalyst which prior to the reaction has been treated at a temperature of more than 750° C., generally at most 1000° C., leads to a substantial reduction in the deactivation rate, without substantially affecting the activity for the metathesis reaction, for example, which substantially increases the duration of cycles between two regeneration steps.
Without wishing to be bound by a particular interpretation, heat activation of a gamma alumina at temperatures beyond 750° C., and generally of at most 1000° C., is known to partially transform it into delta alumina. The beneficial effect on performance could be considered to be attributed to the presence of this type of alumina combined with the presence of caesium.
The catalyst used in the process of the invention thus comprises (and is preferably constituted by) at least three components:
a porous alumina-based support; more generally, the support is principally composed by alumina, and advantageously it contains at least 75% by weight of alumina; preferably it is constituted by alumina, with at least a portion of the alumina being delta alumina (at least 0.5% by weight and preferably at least 1% by weight, or more preferably, 5% by weight and preferably 5-50% by weight)
0.01% to 20% by weight of rhenium;
and 0.01% to 5% by weight of caesium.
The present invention also concerns a process for preparing said catalyst. In this process, a catalyst precursor based on gamma alumina and rhenium is formed and said precursor undergoes heat treatment at more than 750° C. in a non reducing gas atmosphere. In one implementation, the precursor also contains caesium. In a preferred implementation, the precursor containing rhenium but not caesium, which has been heat treated at more than 750° C., is impregnated with a caesium compound, dried than activated.
The porous starting support is based on gamma alumina, and preferably contains at least 75% by weight of alumina, and advantageously has an appreciable surface area, for example at least 10 m
2
/g, and preferably at least 50 m
2
/g, and a sufficient pore volume, for example at least 0.1 ml/g, preferably 0.3-1 ml/g.
The rhenium compound can be introduced into the support, for example by vapour phase sublimation or by impregnation in solution. In general, the dry impregnation method is preferably used, where the rhenium compound is dissolved in water or in an organic solvent, for example a hydrocarbon, an alcohol or an ether. The quantity of rhenium on the support is adjusted by selecting the concentration of the impregnating solution. When the quantity of rhenium which is to impregnated is higher than that which a solution at its saturation limit will allow, the operation must be carried out several times with intermediate drying steps to eliminate the impregnation solvent, at a temperature of 90° C. to 250° C., for example, preferably 100° C. to 180° C. This enables 0.01% 20%, preferably 0.1% to 15%, more advantageously 0.5% to 8% by weight of metallic rhenium, to be introduced. Preferred rhenium compounds are rhenium heptoxide, ammonium perrhenate and perrhenic acid.
After the rhenium impregnation step, a catalyst precursor is obtained, then drying is carried out at a temperature of 90° C. to 250° C., for example, preferably 100° C. to 180° C., followed by calcining at a temperature of more than 750° C. and advantageously at most 1000° C., preferably more than 800° C. and advantageously at most 900° C., in a non reducing gas atmosphere, for example oxygen, nitrogen or argon, oxygen diluted with nitrogen, preferably in air, under static or dynamic conditions, a slow gaseous stream being preferable, however. The amount of moisture in the gaseous stream is preferably kept below 200 ppm (parts per million). However, it is possible to heat in an atmosphere constituted by methane combustion gases or a natural gas in the presence of an excess of air. The duration of this activation treatment is, for example, from 10 minutes to 5 hours or more, after which the precursor obtained is cooled in an atmosphere which is preferably anhydrous. During this calcining treatment, a portion of the gamma alumina is transformed into delta alumina, for example at least 0.5% by weight, preferably 5% to 50% by weight of delta alumina with respect to the starting alumina.
The caesium compound can be introduced into the support using any of the usual methods used in heterogeneous catalysis, for example by solution impregnation. In general, it is preferable to use the dry impregnation method, described above. The caesium compound is dissolved in water. The volume of the solution is less than or at a maximum equal to the volume of the pores of the support. The quantity of caesium on the support is adjusted by selecting the concentration of the impregnation solution. When the quantity which is to be impregnated is higher than that which can be introduced by a solution at its saturation limit, the operation must be carried out several times, with intermediate drying steps to eliminate the impregnation solvent, at a temperature of 90° C. to 250° C., for example, preferably 100° C. to 180° C. This enables
Commereuc Dominique
Hugues François
Saussine Lucien
Dang Thuan D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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