Cross-metathesis of propane and methane: A catalytic reaction of C-C bond cleavage of a higher alkane by methane

Article abstract of DOI:10.1002/anie.200460982

Methane, a building block for basic chemicals through its incorporation into alkanes: when a methane/propane mixture is passed over a tantalum hydride catalyst at 250°C, propane is transformed into two ethane molecules through the incorporation of one methane unit. This reaction corresponds to a cross-metathesis of propane and methane (see scheme; Alk = alkyl).

Full text of DOI:10.1002/anie.200460982

Communications
CꢀC Bond Formation
Cross-Metathesis of Propane and Methane: A
Catalytic Reaction of CꢀC Bond Cleavage of a
Higher Alkane by Methane**
Daravong Soulivong, Christophe CopØret,*
Jean Thivolle-Cazat, Jean-Marie Basset,*
Barry M. Maunders, Richard B. A. Pardy, and
Glenn J. Sunley
In memory of Ian P. Rothwell
Natural gas is the most abundant hydrocarbon resource:
reserves for 60 years of normal consumption have already
been proven, and for about 200 years or greater are expected
to be discovered, which makes it the probable future source
[
*] Dr. D. Soulivong, Dr. C. CopØret, Dr. J. Thivolle-Cazat, Dr. J.-M. Basset
Laboratoryof Surface Organometallic Chemistr y, UMR 9986
Ecole SupØrieure de Chimie Physique Electronique de Lyon
43 Bd du 11 Novembre 1918, 69616 Villeurbanne Cedex (France)
Fax: (+33)4-7243-1795
E-mail: coperet@cpe.fr
basset@cpe.fr
Dr. B. M. Maunders
BP Chemicals Ltd
ChertseyRoad, Sunbur y- On-Thames, Middlesex, TW16 7LN (UK)
Dr. R. B. A. Pardy, Dr. G. J. Sunley
BP Chemicals Ltd
Hull Research and TechnologyCenter
Saltend, Hull, HU12 8DS (UK)
[
**] We are grateful to BP Chemicals, the CNRS and the ESCPE Lyon for
financial supports. We thank Dr. O. Mauryfor helpful discussions.
We would like to dedicate this paper to the memoryof Ian P.
Rothwell, a keyactor in tantalum chemistry.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460982
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1
3
not only for energy but also for basic chemicals. However, its
main component, methane, is one of the most inert hydro-
carbons. Apart from using methane for power generation the
only other current methods commercially used to valorize
methane are “steam reforming”, “partial oxidation” (leading
to “syngas”, a mixture of carbon monoxide and hydrogen
which is converted into fuels and chemicals, such as gasoline,
methanol, and dimethyl ether), and the oxidative pyrolysis to
acetylene, which also produces a syngas byproduct. The
syngas processes are indirect, since they require first the
chemical transformation of CH to CO and H , which are the
A mixture of C-labeled methane (500 equiv; 64 kPa) and
ethane (10 equiv; 1.25 kPa) was heated at 1658C over 1
(53 mg, 5wt% in Ta (see Method of preparation in Exper-
imental Section)) in a glass reactor (0.28 L), and the
1
3
distribution of the C isotopomers of ethane (unlabeled,
mono-, and dilabeled) was monitored over time by GC/MS.
Incorporation of C labels in ethane occurred over 200 h;
hydrogenolysis of ethane into methane was also detected as a
1
3
competitive parallel reaction, since H had been produced
2
during the first step of CꢀH bond activation (formation of
[
15,16]
TaꢀR from TaꢀH (1), R=Me, Et).
Note that the initial
4
2
building blocks for further chemistry. All these processes are
formation of monolabeled ethane followed by the dilabeled
isotopomer is in agreement with a stepwise incorporation of
the label (Figure 1). Additionally, propane was also produced
limited by selectivity, with formation of CO as a by-product,
2
and require high temperatures which make them relatively
unattractive in terms of energy and environmental consid-
[
1]
erations. Therefore, a tremendous effort has been directed
at finding direct catalytic transformations of methane into
more valuable and useful carbon-containing products. Tran-
sition metals have been shown to readily activate the CꢀH
[
2–6]
bond of methane to form metal–methyl complexes
and, in
some instances, enable the reforming of this CꢀH bond; these
reactions are, however, either stoichiometric or degenerate
(
these types of exchange reaction can only be detected by
isotope labeling!). The transformation of methane into
methanol by platinum salts in sulfuric acid is also noteworthy
Figure 1. Evolution of the isotopomeric distribution of a) ethane {unla-
[
7]
despite highly acidic conditions. Methane has also been
reported to react with olefins to give higher alkanes in the
beled (CH
and b) propane {unlabeled (C
(C **), and trilabeled (C
and C-labeled methane.
CH
3
3
), monolabeled (CH
), monolabeled (C
***)} in the cross-metathesis of ethane
3
CH
3
*), and dilabeled (CH
3 3
*CH *)}
3
H
8
3 8
H
*), dilabeled
[
8–10]
3
H
8
H
3 8
presence of superacids or organometallic catalysts
more recently, its coupling reaction with silanes has been
and,
1
3
[
11]
disclosed. The difficulty in achieving selective transforma-
tion of methane into valuable chemical products resides in the
low reactivity of methane compared to co-reactants (e.g.
olefins) and/or the products. However, this problem would be
overcome by the reaction of methane with alkanes to produce
other alkanes, which all have similar reactivity to the starting
alkanes. Herein we report the successful accomplishment of
this reaction through a low-temperature incorporation of
methane into another alkane, propane.
by the known self-metathesis of ethane and was also
converted into its isotopomers. The incorporation of the
label into ethane clearly shows that cross-metathesis occurs
and that methane can participate in the carbon–carbon
formation of higher alkanes. In the case of ethane, this
reaction yields ethane, a degenerate process, which can only
be detected by labeling. The reaction conditions (batch
reactor, methane/ethane reaction) were designed to detect
this possible phenomenon, but were not optimized to study a
productive reaction.
Recently, we have shown that a highly electrophilic
tantalum hydride supported on silica [(ꢁSiO) Ta-H] (1),
2
[
12]
prepared by surface organometallic chemistry, can catalyti-
cally transform a given alkane into its higher and lower
To observe a productive cross-metathesis of methane with
an alkane, the reaction with propane was investigated in a
continuous-flow reactor, and the reaction conditions tuned to
work at full conversion of propane. This reaction has a
[
13,14]
homologues.
This reaction, called “alkane metathesis”,
involves a successive cleavage and formation of carbon–
carbon bonds: hence two ethane molecules give one methane
and one propane molecule. The free energy of the reaction is
ꢀ
1
positive free energy (8.2 kJmol at 1508C for a 1:1 methane/
propane ratio), but its conversion can be, in principle,
thermodynamically driven by a high methane to propane
ratio. For example for a methane/propane ratio of 1250:1 the
conversion of propane can be up to 98% at 2508C; the
temperature and the contact time are also important param-
eters (kinetic). Note that the production of ethane does not
guarantee that such a reaction takes place since 1 is also
known to readily catalyze the hydrogenolysis of propane into
ꢀ
1
slightly negative (ꢀ8.2 kJmol at 1508C, driven by the
formation of methane), and the equilibrium conversion is
around 87% at 1508C. This raises the question: is it possible
to drive this reaction in the reverse direction, that is, to react
methane with another alkane to give a mixture of alkanes
with incorporation of methane? This reaction would corre-
spond to the cross-metathesis of methane with a higher
alkane.
[
17]
a mixture of methane/ethane
or the self-metathesis of
To test this concept, we studied the degenerate reaction of
methane with ethane [Eq.(1)] which requires C labeling to
detect the phenomenon.
propane (production of ethane and butane). Nonetheless if
100% cross-metathesis takes place, two ethane molecules are
produced per propane molecule consumed [Eq. (2)], while
only one ethane is produced per propane molecule in the case
of hydrogenolysis [Eq. (3)], and half an ethane (and half a
1
3
o
ðTÞ
*
CH þ CH ꢀCH ! CH þ* CH ꢀCH DG ¼ 0
4
3
3
4
3
3
ð1Þ
Angew. Chem. Int. Ed. 2004, 43, 5366 –5369
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5367

Communications
higher hydrocarbon detected (> 99% of selectivity; see
Supporting Information) along with residual methane. 1.9 ꢂ
moles of ethane are produced per mole of propane consumed
and 95% of the propane is converted by cross-metathesis, that
is through its reaction with methane to produce two ethane
molecules.
The reaction of methane with higher alkanes was further
1
3
supported by running an identical reaction, in which C-
labeled methane was used in place of unlabeled methane. In
this case the ethane produced at steady-state conditions is
>
85% dilabeled according to GC/MS, which confirms that
ethane is produced by the reaction with methane (see
Supporting Information). The multiple labeling is due to
running the reaction at full conversion of propane, which
implies a long contact time and secondary reactions, such as
[
14]
the successive degenerate reaction of monolabeled ethane
with labeled methane, as described above, leading finally to
dilabeled ethane as the main isotopomer. Ethane produced
initially (t < 200 min) is unlabeled, clearly showing that it
comes from the hydrogenolysis of propane.
Figure 2. a) Conversion of propane as a function of time on
stream. b) Number of equivalents of ethane formed bypropane
consumed y in% as a function of time on stream.
butane) per propane in the case of propane
“
self-metathesis” [Eq. (4)]. Therefore, to make
a clear cut distinction between cross-metathe-
sis and side reactions (hydrogenolysis and self-
metathesis) it is critical to establish the mass
balance of the overall reaction.
CH þ C H ! 2 C H
ð2Þ
ð3Þ
ð4Þ
4
3
8
2
6
H þ C H ! CH þ C H
2
3
8
4
2
6
1
1
C
3
H
8
! =
2
C
2
H
6
þ =
2 4 8
C H
To make this distinction, the reaction is
ꢀ1
carried out by passing a 1.5 mLmin flow of a
1
1
250:1 methane/propane mixture at 50 ”
0 kPa over 300 mg of 1 at 2508C. The
2
alkane mixture was first contacted at room
ꢀ
1
temperature and then a ramp of 2508Ch was
applied up to 2508C (Figure 2). During this
period a large amount of hydrogen is produced
by reaction of the alkane mixture (methane
and propane) with 1 to form the propagating
species, surface alkyl species, which are formed
by CꢀH activation (See Supporting Informa-
[
15,16]
tion).
Propane is partially consumed and
converted into a methane/ethane mixture by
[
17]
hydrogenolysis.
After the evolution of
hydrogen (produced in the activation step)
has ceased, the amount of ethane in the stream
increases until it reaches a plateau after
Scheme 1. Possible mechanisms via carbene intermediates: a) on two sites and b) on
1
000 min (Figure 2b); ethane is the major
one site.
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Angew. Chem. Int. Ed. 2004, 43, 5366 –5369

Angewandte
Chemie
In summary, we have shown that 1 can catalyze the
reaction of methane with a higher hydrocarbon, namely
propane, and thereby incorporate methane into a higher
alkane by cross-metathesis. While we earlier proposed
mechanism based on cleaving the CꢀC bond system by
[16] V. Vidal, A. Theolier, J. Thivolle-Cazat, J.-M. Basset, J. Corker,
J. Am. Chem. Soc. 1996, 118, 4595.
[
17] M. Chabanas, V. Vidal, C. CopØret, J. Thivolle-Cazat, J.-M.
Basset, Angew. Chem. 2000, 112, 2038; Angew. Chem. Int. Ed.
2000, 39, 1962.
[
18] C. CopØret, O. Maury, J. Thivolle-Cazat, J.-M. Basset, Angew.
Chem. 2001, 113, 2393; Angew. Chem. Int. Ed. 2001, 40, 2331.
[
13]
either s-bond metathesis or oxidative addition,
we now
propose that it proceeds via carbene intermediates and olefin
[19] E. Le Roux, M. Chabanas, A. Baudouin, A. de Mallmann, C.
CopØret, E. A. Quadrelli, J. Thivole-Cazat, J.-M. Basset. A.
Losage, L. Emsley, G. J. Sunley, J. Am. Chem. Soc., in press.
[20] V. Dufaud, J.-M. Basset, Angew. Chem. 1998, 110, 848; Angew.
Chem. Int. Ed. 1998, 37, 806.
[
18] [19]
metathesis as suggested by previous studies.
,
Since the
alkane-metathesis active site can be generated from either
III
V
Ta or Ta intermediates, and since olefin metathesis would
require the formation of olefins, one and two-site mechanisms
can be proposed (Scheme 1), which are under investigation.
The overall reaction provides a route for the direct
transformation of methane into more valuable hydrocarbon
materials. This reaction makes possible new processes, such as
replacing expensive hydrogen by low-cost methane in the
[
20]
recycling of polyolefins.
Experimental Section
Method of preparation of catalyst 1: Silica (1 g), previously treated at
5
2
0
008C under vacuum, in suspension in pentane (15 mL) was treated at
58C with a pentane solution of [Ta(=CHtBu)(CH tBu) ] (0.155 g,
2
3
.33 mol) under an inert atmosphere. The reaction mixture was
stirred for 2 h at 258C and filtered. The remaining orange solid was
then washed three times with pentane and dried under vacuo. This
solid was then contacted with dry and deoxygenated H (200 equiv)
2
for 15 h at 2508C, and the amounts of methane was monitored by GC.
Typically after 3 cycles of treatment under H , less than 1% of carbon
2
(as methane) is present in the gas phase.
Received: June 16, 2004
Keywords: alkanes · cross-metathesis · heterogeneous catalysis ·
.
methane homologation · tantalum
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