Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By alkyl transfer – e.g. – disproportionation – etc.
Reexamination Certificate
1999-10-05
2001-05-22
Knode, Marian C. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By alkyl transfer, e.g., disproportionation, etc.
C585S644000, C585S646000
Reexamination Certificate
active
06235958
ABSTRACT:
The invention relates to a process for metathesis of olefinic C
5
cuts (i.e., containing pentenes) with a catalyst comprising delta alumina, rhenium and cesium.
Preferably, the present invention provides a process for converting a C
5
hydrocarbon cut as defined above, after selective hydrogenation of diolefins and acetylenic compounds and hydroisomerisation of monoolefins, by metathesis with ethylene or propylene in the presence of a catalyst based on rhenium improved by the incorporation of cesium 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
5
hydrocarbon cut which principally contains C
5
diolefins, methylbutenes, n-pentenes and pentane, 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 or propylene of an olefinic C
5
cut, which has already undergone selective hydrogenation of diolefins and acetylenic compounds and hydroisomerisation of monoolefins, can produce isobutene, a compound which is in great demand for synthesising methyl tertio-butyl ether for use as a fuel additive or for polymerisation.
When such a reaction is carried out using the usual rhenium-based metathesis catalysts, olefins such as the isobutene produced and the 2-methyl-2-butene contained in the C
5
cut polymerise on contact with the catalyst and that secondary reaction causes a large reduction in duration of the cycle for the catalyst between two regeneration operations.
Incorporating alkali metals into a rhenium-based metathesis catalyst to carry out metathesis of C
5
olefins has been described in European patent EP-A-0 691 318, although no mention is made of the importance of using any particular alkali metal over any others.
Incorporating cesium 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, cesium 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 cesium 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 cesium-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 cesium by impregnation, calcining at 500° C., then activating in nitrogen (T. Kawai et al., Journal of Molecular Catalysis, vol. 76, pp. 249-261, 1992).
SUMMARY OF THE INVENTION
We have now discovered that the use of a rhenium- and cesium-based metathesis catalyst which has previously been activated at a temperature of more than 750° C. leads to a substantial reduction in the deactivation rate, without substantially affecting the activity for the metathesis reaction, 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 cesium.
More precisely, the process forming the subject matter of the invention concerns a process for converting an olefinic C
5
cut by metathesis, either with ethylene to produce an effluent containing principally propylene, isobutene and n-butenes, or with propylene to produce an effluent containing principally isobutene and n-butenes. Preferably, said C
5
cut has previously undergone selective hydrogenation of the polyunsaturated compounds and hydroisomerisation of the monoolefins, advantageously obtained simultaneously with hydrogenation. Metathesis is carried out in the presence of a catalyst comprising (and preferably constituted by) delta alumina, rhenium and cesium. Preferably, the catalyst comprises at least one rhenium compound deposited on a support composed principally by alumina which has been treated at a temperature of more than 750° C., so as to transform a portion of the alumina to delta alumina, and modified by at least one cesium compound. Metathesis takes place at a temperature in the range 20° C. to 1 50° C., and at a pressure at least equal to the vapour tension of the reaction mixture at the reaction temperature, and is followed by separation of the isobutene produced.
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 cesium.
In this catalyst preparation 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 cesium. In a preferred implementation, the precursor containing rhenium but not cesium, which has been heat treated at more than 750° C., is impregnated with a cesium compound, dried than activated.
The porous starting support is based on gamma 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% to 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 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
Commereuc Dominique
Hugues Francois
Saussine Lucien
Dang Thuan D.
Institut Francais du Pe'trole
Knode Marian C.
Millen White Zelano & Branigan P.C.
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