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Metathesis of Alkanes Catalyzed by Silica-Supported Transition Metal Hydrides

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Science  04 Apr 1997:
Vol. 276, Issue 5309, pp. 99-102
DOI: 10.1126/science.276.5309.99

Abstract

The silica-supported transition metal hydrides (≡Si-O-Si≡)(≡Si-O-)2Ta-H and (≡Si-O-)xM-H (M, chromium or tungsten) catalyze the metathesis reaction of linear or branched alkanes into the next higher and lower alkanes at moderate temperature (25° to 200°C). With (≡Si-O-Si≡)(≡Si-O-)2Ta-H, ethane was transformed at room temperature into an equimolar mixture of propane and methane. Higher and lower homologs were obtained from propane, butane, and pentane as well as from branched alkanes such as isobutane and isopentane. The mechanism of the step leading to carbon-carbon bond cleavage and formation likely involves a four-centered transition state between a tantalum-alkyl intermediate and a carbon-carbon -bond of a second molecule of alkane.

Paraffins, particularly methane and light alkanes, constitute an abundant yet low-value fossil feedstock. Light alkanes would be very valuable if they could be transformed into higher molecular weight hydrocarbons (1); this represents a continuing scientific challenge (2). Here, we report observations of a catalytic reaction that we designate "metathesis of alkanes" and which, to our knowledge, has not previously been reported (3). The metathesis reaction proceeds by both the cleavage and the formation of the C-C bonds of acyclic alkanes, which are transformed into a mixture of higher and lower homologs. It was observed in the presence of various silica-supported metal-hydride catalysts, particularly tantalum hydride (4), all prepared by the surface organometallic chemistry route (5, 6). Metathesis reactions of alkenes and alkynes, discovered a few decades ago, are now well documented and understood and are used in several industrial chemical processes. In contrast to the metathesis of alkenes (7) or alkynes (8), for which the cleavage of the molecule occurs selectively at the C=C or C≡C bond, the metathesis of acyclic alkanes seems to involve, at least to some extent, the reaction of all C-C bonds. Thus, the reaction, even if it is selective in terms of the formation and cleavage of C-C bonds, is not restricted to the formation of the first higher and lower homologs but also can yield the next several higher and lower ones. The metathesis of acyclic olefins is hindered by thermodynamic limitations; such a reaction is close to thermoneutrality for most alkanes (9).

We recently reported that the reaction of Ta(-CH2CMe3)3(=CHCMe3) (Me, methyl) (10) with the surface hydroxyl groups of a dehydroxylated silica (11) leads to the formation of a mixture of two species: (≡Si-O-) Ta(-CH2CMe3)2(=CHCMe3) (~65%) and (≡Si-O-)2Ta(-CH2CMe3)(=CHCMe3) (~35%) (12). Treatment of these two surface complexes under hydrogen at 150°C overnight yields mainly a surface tantalum (III) monohydride, (≡Si-O-Si≡)(≡Si-O-)2 Ta-H ([Ta]s-H), which has been fully characterized by infrared spectroscopy, extended x-ray absorption fine structure (EXAFS) analysis, microanalysis, and quantitative chemical reactions (4).

When [Ta]s-H was contacted at room temperature with a cyclic alkane (4, 13), no catalytic reaction occurred. Only a C-H bond activation was observed, which led to the stoichiometric formation of a tantalum (III)-cycloalkyl species with the evolution of 1 mol of hydrogen. Tantalum is extremely electrophilic, and thus we assume that this activation occurs by a σ-bond metathesis Embedded Image(1) (where R = cyclopentyl, cyclohexyl, cycloheptyl, or cyclooctyl), rather than by an oxidative addition-reductive elimination pathway (13). When [Ta]s-H was contacted with an acyclic alkane, a catalytic metathesis reaction occurred at moderate temperature (25° to 200°C), leading to the formation of the higher and lower homologs (Table 1). The simplest case is the metathesis of ethane, which has only one C-C bond and consequently does not present any problem of selectivity. This alkane yielded propane and methane in comparable amounts, as well as trace amounts of n-butane and isobutane (Fig. 1). In this example, it is clear that a methyl fragment is transferred from one molecule of ethane to a second one. A labeling experiment carried out with 13C-monolabeled ethane yielded unlabeled, monolabeled, dilabeled, and trilabeled propane (5:44:43:8), which proved the cleavage of the 13C-12C bond of ethane and the redistribution of the 13C atoms into the propane molecules.

Table 1.

Metathesis reaction of acyclic alkanes catalyzed by the [Ta]s-H complex at 150°C.

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Fig. 1.

Metathesis reaction of ethane catalyzed by the silica-supported [Ta]s-H complex at 150°C under 1 atm (C2H6/Ta ratio, ~800).

When higher linear alkanes were used, several higher and lower homologs were produced, because various C-H and C-C bonds can be involved. For example, metathesis of propane yielded mainly n-butane and isobutane together with ethane, and n-butane yielded n-pentane and isopentane as well as propane; besides these main products, some higher and lower alkanes were also produced (Fig. 2). This suggests, as observed for ethane, that the main reaction involves the transfer of a methyl fragment from one molecule of the initial alkane to a second one; the wider product distribution observed with higher alkanes (for example, butane and pentane) (Fig. 2) also suggests the probable occurrence of secondary reactions, the possible transfer of higher alkyl fragments such as ethyl, or both. The occurrence of the two processes can hardly be distinguished; secondary reactions can take place as the reaction proceeds and the conversion increases. (The two related processes may possibly be distinguished by a kinetic study in a dynamic reactor with varying contact time.) All the alkanes present in the medium can be presumed to compete for the adsorption. In the case of ethane, secondary reactions seem to occur very soon with the appearance of butane and especially isobutane, the formation of which necessarily arises from a secondary reaction of propane. Branched alkanes such as isobutane or methyl butane also underwent the metathesis reaction; however, neopentane only led to the stoichiometric formation of a surface tantalum-neopentyl species and did not react further, probably for steric reasons.

Fig. 2.

Distribution of the various hydrocarbons produced during the metathesis reaction of the first lower linear alkanes (at 3% conversion), catalyzed by the silica-supported [Ta]s-H complex at 150°C (ethane, propane, or butane/Ta ratio, ~800, P = 1 atm; pentane/Ta ratio, ~400, P = 400 torr). The starting alkane in excess is not represented.

During the metathesis reaction, infrared spectroscopy showed that the tantalum hydride disappeared and was transformed into a tantalum-alkyl intermediate. At 150°C, aging of the catalyst was observed, preventing higher conversions. In a few exploratory experiments, it was observed that similar silica-supported hydrides of tungsten or chromium (14) were also able to catalyze the alkane metathesis reaction (Table 1), whereas silica-supported hydrides of zirconium, hafnium, or aluminum (14) did not show any activity. To our knowledge, this alkane metathesis reaction is an unprecedented catalytic reaction; it can be described by the general equation Embedded Image(2) with i = 1, 2, …, n - 1, but where i = 1 is generally favored.

Regarding the mechanism of the reaction (15) (Scheme 1), we assume that in a first step, a C-H bond of acyclic alkane (for example, ethane) is cleaved by a σ-bond metathesis, producing hydrogen and a tantalum-alkyl (for example, ethyl) species; this elementary step is indeed observed during the C-H bond activation of cycloalkanes on the [Ta]s-H species, yielding the corresponding tantalum-cycloalkyl surface complexes (Eq. 1), and occurs without modification of the oxidation state of the metal (4). The next step involves the transfer of a methyl group and necessarily implies the cleavage and the formation of C-C bonds. Among the known elementary steps, two possibilities can be considered: (i) The oxidative addition of the ethane C-C bond (16) would yield the dimethyl(ethyl)tantalum(V) surface complex; reductive elimination of propane from this complex could follow, with the concomitant release of a tantalum-methyl species. (ii) A σ-bond metathesis mechanism, which we prefer because of the highly electrophilic character of the silica-supported tantalum-alkyl species, would involve a four-centered transition state with the presence of an sp3 carbon in the middle of the metallocycle (Scheme 2).

Scheme 1.
Scheme 2.

Related but different four-centered intermediates, in which a hydrogen atom replaces the central carbon, have been invoked in recent studies concerning the C-H bond activation of alkanes (17); other four-centered intermediates have been proposed in the dehydropolymerization of silane compounds (18). In the case of ethane, this step of C-C bond formation and cleavage enables the liberation of propane and the formation of a tantalum-methyl surface complex. The last step in the catalytic cycle must liberate methane and regenerate the active species, the tantalum-ethyl surface complex; a new four-centered transition state is assumed in which a hydrogen is transferred from an ethane molecule to the methyl ligand (Scheme 1).

Such a mechanism is also operating in the case of higher alkanes. For example, the formation of n-butane and isobutane from propane is determined in the first step of C-H bond activation, which yields two species, tantalum-n-propyl and tantalum-isopropyl surface complexes (Scheme 3). The subsequent transfer of a methyl group from a second propane molecule to these propyl intermediates enables the evolution of n-butane and isobutane as well as the formation of a tantalum-ethyl complex, which in turn will yield ethane.

Scheme 3.

The metathesis of isobutane is particularly interesting with respect to the product distribution. In agreement with the proposed mechanism, the C-H bond activation step can produce two tantalum-alkyl intermediates (Scheme 4). We can expect that the formation of the tantalum-tert-butyl species can be disfavored (relative to that of the other possible tantalum-isobutyl complex) for steric reasons; despite this, the production of neopentane resulting from the methyl transfer step on the tantalum-tert-butyl occurs in substantial amounts.

Scheme 4.

REFERENCES AND NOTES

  1. CNRS, French Patent No. 96 09033.
  2. The reaction enthalpy for the metathesis of ethane to propane and methane (2C2H6 ⇔ C3H8 + CH4) at 400 K is ∆H0 = -2.15 kcal mol-1, whereas the standard free energy variation is ∆G0 = -1.98 kcal mol-1. These values were calculated from data found in D. R. Stull, E. F. Westrum Jr., G. C. Sinke, The Chemical Thermodynamics of Organic Compounds (Krieger, Malabar, FL, 1987).
  3. Aerosil Degussa silica (200 m2/g) was dehydroxylated at 500°C under vacuum [(SiO2)500].
  4. To obtain silica-supported tungsten hydride, we allowed tris(neopentyl)neopentylidyne tungsten, W(-CH2-CMe3)3(=C-CMe3) [ R. R. Schrock, Inorg. Synth. 26, 4 (1989)], to react with the silanols of (SiO2)500. The resulting grafted tungsten complexes were then treated at 80°C under 1 atm of H2 for 16 hours. Typical bands of W-H were observed at 1942 cm-1. To obtain silica-supported chromium hydride, we allowed Cr(-CH2-SiMe3)4 [W. Mowatet al., J. Chem. Soc. Dalton Trans. 1972, 533 (1972)] to react with the OH groups of (SiO2)500. The resulting grafted chromium complex, (=SiO-)Cr(-CH2-SiMe3)3 [J. A. N. Ajjou and S. L. Scott, Organometallics 16, 86 (1997)], was then treated at 150°C under 1 atm of H2 for 16 hours. Typical bands of Cr-H were observed at 2115 cm-1. To obtain silica-supported zirconium or hafnium hydrides, we allowed Zr(-CH2-CMe3)4 or Hf(-CH2-CMe3)4 [P. J. Davidson, M. F. Lappert, R. Pearce, J. Organomet. Chem. 57, 269 (1973)] to react with the OH groups of (SiO2)500. The resulting grafted complexes, (=Si-O-)Zr(-CH2-CMe3)3 and presumably (=Si-O-)Hf(-CH2-CMe3)3, were then treated at 150°C under 1 atm of H2 for 16 hours. Typical bands of Zr-H and Hf-H were observed at 1635 and 1702 cm-1, respectively. To obtain silica-supported aluminum hydride, we allowed Al(-CH2-CHMe2)3 to react with the OH groups of (SiO2)500. The resulting grafted aluminum complex, presumably (=Si-O-)Al(-CH2-CHMe2)2, was then treated stepwise at 300°C under 1 atm of H2. Typical bands of Al-H were observed at 1940 cm-1.
  5. In the early stages of the discovery of the olefin metathesis reaction, the most simple mechanism envisaged was a pairwise mechanism in which both olefins would react on the metallic center to give a quasicyclobutane intermediate. This mechanism was later refuted. The simple fact that metathesis of ethane gives methane and propane indicates that the mechanism is nonpairwise, that is, the two alkanes do not play the same role in the mechanism.
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