Chemistry of hydrocarbon compounds – Saturated compound synthesis – By alkyl transfer – e.g. – disproportionation – etc.
Reexamination Certificate
1999-03-12
2001-05-08
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Saturated compound synthesis
By alkyl transfer, e.g., disproportionation, etc.
Reexamination Certificate
active
06229060
ABSTRACT:
The present invention relates to a process for the metathesis of alkanes.
It is well known that alkanes commonly known as paraffins are molecules which are difficult to convert because of their chemical inertia.
It is known to convert alkanes by hydrogenolysis reactions of carbon—carbon bonds. It has been possible, over certain metal catalysts, simultaneously to observe homologation reactions of alkanes but the latter always remain very minor reactions with respect to the hydrogenolysis reactions. This is because these reactions are always carried out in the presence of hydrogen, at temperatures varying from 250 to 350° C., in contact with catalysts based on transition metals in massive form or in the form of films or else in the form of metal particles supported on oxides. The best results seem to have been obtained by Donohoe, Clarke and Rooney with linear C
5
to C
7
alkanes over tungsten film (J. Chem. Soc. Farad. Trans. I, 1980, 76, 345; J. Chem. Soc. Chem. Commun., 1979, 648); the reaction proves to be slower by an order of magnitude with branched alkanes and does not take place with cyclic alkanes. Sarkany has also studied the homologation of butane and various C
4
or C
5
alkanes over catalysts of nickel black, Ni/SiO
2
, platinum black, Pt/SiO
2
or Pd/Al
2
O
3
type (J. Chem. Soc. Farad. Trans. I, 1986, 82, 103; J. Catal., 1984, 89, 14); this reaction is also favoured in the case of linear alkanes with respect to branched alkanes but always remains a minor process beside isomerization reactions (for example of butane to isobutane over Pt/SiO
2
) or hydrogenolysis reactions (over Ni/SiO
2
). The homologation-aromatization of pentane or cyclopentane to benzene has been reported by Peter and Clarke over rhodium film alloyed with copper, tin, gold or silver (J. Chem. Soc. Farad. Trans. I, 1976, 72, 1201) and by Sarkany over Ni/SiO
2
(J. Chem. Soc. Chem. Commun., 1980, 525); only alloyed rhodium films give a suitable selectivity for benzene while hydrogenolysis remains predominant with nickel catalysts, even for reduced conversions. Thus, the performances of these known homologation reactions of alkanes remain very modest or linked to an aromatization process.
Nevertheless, if it were known how to convert alkanes into their higher homologues, this would constitute a means for enhancing the value of certain petroleum fractions, such as in particular the C
4
or C
5
fractions. It would be possible to envisage numerous applications in the field of oils, fuels, polymers or organic chemical synthesis, making it possible to lengthen side chains or to obtain higher branched hydrocarbons by other routes than acidic or superacidic catalysis. This is because it is known that high octane numbers are particularly desired in the field of fuels. It is also well known that low molecular weight alkanes cannot be enhanced in value to any significant extent, whereas heavier alkanes are of greater commercial interest.
The object of the present invention is thus to provide a process which makes it possible to convert alkanes into their higher and lower homologues with high selectivity.
Another object of the invention is to provide such a process which has a wide field of application which can cover, simultaneously, the fields of oils, fuels, polymers or organic chemical synthesis, the elongation of the side chains of compounds comprising them or the production of higher branched hydrocarbons. Such an object is thus in particular the enhancement in value of light alkanes to heavier alkanes of greater advantage industrially.
Another object of the invention is to provide such a process which can be employed under moderate operating conditions, namely in particular at relatively low temperature and relatively low pressure.
Yet another object of the invention is to provide novel catalysts of use in the context of the process for the metathesis of alkanes.
It has been discovered, which is the subject-matter of the present invention, that it is possible, by using the catalysts of the invention, to carry out the metathesis of alkanes to higher and lower homologues with high selectivity and even at low temperature, in particular at temperatures of less than 200° C.
A subject-matter of the invention is therefore a process for the metathesis of linear or branched starting alkanes, in which process the starting alkane or alkanes is/are reacted over a solid catalyst comprising a metal hydride grafted to and dispersed over a solid oxide. It seems probable that this catalyst acts as catalytic precursor. When it is brought into the presence of alkanes, this catalyst has a tendency to form an alkylmetal complex which would be the catalytically active species.
The reaction according to the invention provides for the metathesis of sigma C—C bonds and the conversion of an alkane into its higher and lower homologues. Thus, starting with an alkane, for example ethane, it is possible to obtain, directly and successively, all the higher alkanes and in particular branched alkanes.
The reaction can be written according to the equation:
C
n
H
2n+2
→C
n−i
H
2(n−i)+2
+C
n+i
H
2(n+i)+2
(I)
i=1, 2, 3 . . . n−1
The invention thus makes possible the direct conversion of light alkanes into heavier alkanes and thus an enhancement in value of these light alkanes, in particular C
4
-C
5
petroleum fractions. It also makes possible the elongation of side hydrocarbon-comprising chains on certain cyclic compounds.
In an entirely preferred way, the catalysts according to the invention are obtained from an organometallic complex of following formula (II):
MR
a
(II)
where
M is a transition metal selected from those of groups 5 and 6 of the Periodic Classification of the Elements, preferably from tantalum, tungsten and chromium;
the R groups are identical or different, saturated or unsaturated, preferably C
1
, to C
10
, hydrocarbon-comprising ligands bonded to the M by one or more carbons (it being possible for the metal-carbon bonds to be simple, double or triple bonds);
a is less than or equal to the valency of M, which is 5 or 6.
Mention may in particular be made, among the appropriate ligands, of methyl, neopentyl, neopentylidene, neopentylidyne, benzyl or their mixtures, for example neopentyl-neopentylidene or neopentyl-neopentylidine.
The neopentyl-neopentylidene and neopentyl-neopentylidine mixtures are particularly advantageous when complexed with tantalum, respectively tungsten.
According to an embodiment which is particularly appropriate to the use of the solid catalyst, the dispersing and the grafting of the organometallic compound are carried out over and to a very anhydrous solid oxide. The solid oxide, for example silica, is subjected to an exhaustive heat treatment (with the intention of providing for dehydration and dehydroxylation), in particular between 200 and 1100° C. for several hours (for example, from 10 to 20 hours). Of course, a person skilled in the art will take care not to exceed the degradation temperature or stability limit temperature of the solid oxide which he has chosen to use. For silica, the dehydration is carried out between 200 and 500° C., preferably in the vicinity of 500° C., for a simple dehydration reaction or at a temperature greater than 500° C., if it is desired additionally to obtain the formation of surface siloxane bridges.
The transfer of the complex onto the solid oxide can be carried out in particular by sublimation or in solution.
In the case of sublimation, the organometallic complex in the solid state is heated under vacuum and under temperature conditions which provide for its sublimation and its migration in the vapour state onto the solid oxide, which itself is preferably in the pulverulent state or in the form of pellets or the like. The sublimation is carried out in particular between 50 and 150° C., preferably in the vicinity of 80° C. The deposition can be monitored, for example by infrared spectroscopy.
The grafting takes place by reaction of the complex with the functional groups o
Basset Jean-Marie
Theolier Albert
Thivolle-Cazat Jean
Vidal Véronique
Centre National de la Recherche Scientifique (C.N.R.S.)
Dang Thuan D.
Griffin Walter D.
Jacobson Price Holman & Stern PLLC
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