Process for manufacturing alkanes

Chemistry of hydrocarbon compounds – Saturated compound synthesis – By alkyl transfer – e.g. – disproportionation – etc.

Reexamination Certificate

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C585S700000, C585S705000, C585S931000

Reexamination Certificate

active

06469225

ABSTRACT:

PROCESS FOR MANUFACTURING ALKANES
The present invention relates to a process for the manufacture of alkanes in the presence of a supported organometallic compound which employs in particular an alkane cross-metathesis reaction.
Alkanes are generally products which are difficult to convert because of their chemical inertia. Nevertheless, the conversion of alkanes into other alkanes is known. Hydrogenolysis reactions, which consist of cleavage or opening reactions of a carbon-carbon bond by hydrogen, are known, for example. Isomerization reactions, which convert an alkane into one of its isomers, for example n-butane into isobutane, are also known. All these reactions are generally carried out at relatively high temperatures and in the presence of catalysts based on metals, in particular on transition metals, in the bulk form or in the form of films or alternatively in the form of metal particles deposited on inorganic supports essentially based on metal oxide. Thus, for example, the catalyst can be of the following type: nickel black, Ni/SiO
2
, platinum black, Pt/SiO
2
, Pd/Al
2
O
3
, or tungsten or rhodium film, optionally mixed with copper, tin or silver. With some metal catalysts, it was possible simultaneously to observe alkane homologation reactions, which consist of reactions which convert alkanes into higher alkanes. However, alkane homologation reactions are generally very minor reactions in comparison with the hydrogenolysis or isomerization reactions and their results are very poor.
Nevertheless, it remains the case that a process for the conversion of alkanes into their higher homologues would constitute a means for enhancing in value certain petroleum fractions, in particular the lightest fractions, such as the C4 or C5 fractions, or liquefied petroleum gas, also known as LPG. It is known that, as a general rule, alkanes of low molecular weight cannot be exploited to any great extent in chemistry or petrochemistry, other than as fuels, whereas heavier alkanes are often of greater commercial interest, such as, for example, to increase the octane number of engine fuels or alternatively to involve these heavier alkanes in thermal or thermal catalytic cracking or steam cracking reactions in order to manufacture, for example, olefins or dienes.
In this sense, Patent Application PCT/FR 97/01266 discloses a process for the conventional metathesis of alkanes in which at least one alkane is reacted with itself or several alkanes with one another in the presence of a solid catalyst comprising a metal hydride grafted to and dispersed over a solid oxide. Thus, a metathesis reaction is carried out in the presence of this metal hydride by cleavage and recombination of the carbon-carbon bonds, converting an alkane simultaneously into its higher and lower homologues. The reaction can be written according to the following equation (1):
2C
n
H
2n+2
→C
n−i
H
2(n−i)+2
+C
n+i
H
2(n+i)+2
  (1)
where i=1,2,3, . . . n−1 and n can range from 2 to 30 and even beyond.
The catalyst based on metal hydride comprises a transition metal chosen from those from groups 5 and 6 of the Table of the Periodic Classification of the Elements (as defined by IUPAC in 1991 and illustrated in “Hawley's Condensed Chemical Dictionary”, 12
th
edition, by Richard J. Lewis, Sr., published by Van Nostrand Reinhold Company, New York, 1993), such as, in particular, tantalum, chromium or tungsten. The preparation of the catalyst comprises a stage of hydrogenation of an organometallic compound comprising a transition metal dispersed over and grafted to a solid oxide beforehand, so that the transition metal is reduced to an oxidation state lower than its maximum value, thus resulting in the metal hydride. However, like any conventional alkane metathesis reaction, in particular carried out in the presence of this metal hydride, higher and lower homologous alkanes are simultaneously manufactured, which limits the degree of freedom in the choice of alkane to be manufactured, since several types of alkanes are manufactured simultaneously.
A novel process for the manufacture of alkanes has now been found which makes use in particular of an alkane cross-metathesis reaction carried out in the presence of an organometallic compound. This novel process makes it possible to give an additional degree of freedom and a greater flexibility in the synthesis of alkanes. Thus, for the first time, it has been possible to demonstrate an alkane cross-metathesis reaction which consists essentially of cleavage and recombination reactions of carbon-carbon and carbon-metal bonds between at least one startng (initial) alkane and an organometallic compound comprising at least one hydrocarbon-comprising radical. The novel process can be employed under less severe conditions than the majority of processes known to date and is capable of easily and efficiently producing more desirable alkanes of economic value with various applications according to their molecular mass.
The subject-matter of the invention is therefore a process for the manufacture of alkanes, characterized in that it comprises, as main stage, a cross-metathesis reaction between at least one starting alkane (A) and one organometallic compound (B) fixed to an inorganic support and comprising a metal bonded to at least one hydrocarbon-comprising radical, which reaction results in the formation of at least one other alkane (C), which is a higher or lower homologue of the starting alkane (A), by cleavage of the hydrocarbon-comprising radical with the metal of the organometallic compound (B) and recombination of the said radical with at least one other radical originating from a cleavage of the starting alkane (A).
Thus, the process of the invention comprises an alkane cross-metathesis reaction which consists essentially of a stoichiometric reaction between the starting alkane (A) and the organometallic compound (B). The products resulting from the reaction include in particular at least the other alkane (C) and at least one other organometallic compound (D) having a different hydrocarbon-comprising radical from that of the compound (B).
By way of illustration, the alkane cross-metathesis reaction can be written according to at least one of the following two equations (2) and (3):
in which equations H—R
1
—R
2
represents the starting alkane (A), H represents a hydrogen atom, R
1
and R
2
represent hydrocarbon-comprising radicals, and R
3
—M represents the organometallic compound (B) with M representing the metal bonded to R
3
, which itself represents a hydrocarbon-comprising radical.
Side reactions can take place in parallel with the alkane cross-metathesis reaction, in particular carbon-hydrogen bond cleavage and recombination reactions as described in the following equation (4):
The invention thus makes it possible, by virtue of a cross-metathesis reaction, to convert the starting alkane (A) into at least one of its higher or lower homologues (C) by reactions of cleavage and of recombination with the hydrocarbon-comprising radical of the organometallic compound (B). More specifically, it makes use of cleavage reactions of carbon-carbon bonds of the starting alkane (A) and of cleavage reactions of the carbon-metal bond of the organometallic compound (B). The cleavage reactions are themselves followed by recombination reactions which form new carbon-carbon and carbon-metal bonds and thus result in at least one other alkane (C) and one other organometallic compound (D). This reaction is all the more surprising since only one conventional alkane metathesis reaction was known until now, converting, by catalytic reaction, an alkane simultaneously into one or more of its higher homologues and into one or more of its lower homologues according to, for example, the abovementioned equation (1). The alkane cross-metathesis reaction, in contrast to the conventional metathesis reaction, does not employ a catalyst, such as a supported metal hydride, but an organometallic compound which takes part as reactant in a stoic

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