Non-oxidative coupling reaction of methane to ethane and hydrogen catalyzed by the silica-supported tantalum hydride: (≡SiO)2Ta-H

Article abstract of DOI:10.1021/ja800863x

Silica-supported tantalum hydride, (□ SiO)₂Ta-H (1), proves to be the first single-site catalyst for the direct non-oxidative coupling transformation of methane into ethane and hydrogen at moderate temperatures, with a high selectivity (>98%). The reaction likely involves the tantalum-methyl-methylidene species as a key intermediate, where the methyl ligand can migrate onto the tantalum-methylidene affording the tantalum-ethyl. Copyright

Full text of DOI:10.1021/ja800863x

Published on Web 03/26/2008
Non-Oxidative Coupling Reaction of Methane to Ethane and
Hydrogen Catalyzed by the Silica-Supported Tantalum
Hydride: (tSiO)2Ta-H
Daravong Soulivong,† Se´bastien Norsic,† Mostafa Taoufik,† Christophe Cope´ret,†
Jean Thivolle-Cazat,† Sudhakar Chakka,‡ and Jean-Marie Basset*,†
Laboratoire de Chimie Organome´tallique de Surface, UMR CNRS, ESCPE UCB 5265, 43 blVd
du 11 NoVembre 1918, F-69616 Villeurbanne Cedex, France, and BP Products North America
Inc., 150 West WarrenVille Road, NaperVille, Illinois 60563
Received February 3, 2008; E-mail: basset@cpe.fr
Although the proven and potential deposits of natural gas
are enormous, a majority of these exist in remote areas of the
world. Methane, which is the main component of natural gas,
represents an important reserve of fossil carbon but contributes
much more than CO2 to the global warming on earth. Trans-
formation of remote natural gas into easily transportable higher
hydrocarbons remains an important economical target and a
huge scientific challenge.1,2 Conversion of methane to syngas3
followed by its transformation to heavier hydrocarbons by
Fischer-Tropsch synthesis4 is rather well-established but
requires high temperatures for the first step leading to limited
overall yield,5 in addition requiring very high capital costs. The
direct non-oxidative coupling of methane (NOCM) into ethane
and hydrogen shows a large positive Gibbs free energy,
affording very limited equilibrium conversions (Figure S1).6
Various attempts to accomplish the NOCM reaction used
classical and ill-defined heterogeneous catalysts, either via a
multistep process, (i) methane dissociation on Ru particles by
chemisorption (and stepwise surface carbon-carbon bond
formation) and (ii) liberation of products by hydrogenation, or
via a single-step process with a Pt/sulfated zirconia catalyst.6-8
Another approach, called the oxidative coupling of methane
(OCM), consists of using O2 to increase the yield of the coupling
reaction of methane into C2 products. However, the global yield
remained limited to <25% despite extensive efforts over two
decades due to the gas phase free radical character of this
reaction, hence the formation of large amounts of carbon
oxides.9-11
Figure 1. Non-oxidative coupling reaction of methane catalyzed by the silica-
supported tantalum hydride (tSiO)2Ta-H, 1, at various temperatures: (a)
conversion; (b) turnover (TON); O 250 °C; 4 300 °C; ) 375 °C; 0 475 °C.
Figure 2. Selectivity obtained at 300 °C in the non-oxidative coupling
reaction of methane catalyzed by the silica-supported tantalum hydride
(tSiO)2Ta-H, 1: 0 hydrogen; ( ethane.
(tSiO)xTa(dCHtBu)(CH2tBu)3-x, 2a and 2b (a: x ) 1 or b: x
) 2),16 resulting from the grafting of Ta(dCHtBu)(CH2tBu)3
3
17 on silica (Degussa, 200 m2/g) previously dehydroxylated at
500 °C; at that temperature of reduction, about 30% of
(tSiO)2Ta-H was already transformed into (tSiO)3Ta and
(tSi-H).18 When methane was contacted with 1 (5.35 wt %
Silica-supported tantalum hydride (tSiO)2Ta-H, 112 (in
equilibrium under hydrogen with (tSiO)2Ta(H)3),13 is known
to catalyze the total hydrogenolysis of light alkanes, including
the hydrogenolysis of ethane14 into methane, in contrast to the
silica-supported zirconium hydrides which do not cleave
ethane.14 In an attempt to upgrade methane into ethane or higher
hydrocarbons, it was interesting to consider the possibility of
exploiting the microscopic reversibility of ethane hydrogenolysis
reaction,14 that is, the coupling of methane into ethane and
hydrogen. Despite the severe thermodynamic limitations (vide
supra), we report here the first example of the selective catalytic
coupling of methane into ethane and hydrogen, which occurs
on the single-site catalyst (tSiO)2Ta-H, 1 (eq 1):
Ta) in a continuous flow reactor (PCH ) 50 bar, T ) 250 to
4
475 °C, at a weight hourly space velocity WHSV ) 0.44 h-1),
throughout the temperature range of 250 to 375 °C, ethane was
formed selectively (>98% among hydrocarbons) along with an
equimolar amount of H2. At the beginning, the conversion rate
went through a maximum up to the thermodynamic ceiling,
which is temperature-dependent (Figure S1), before reaching a
pseudo steady state (Figure 1a), affording a cumulative turnover
number, TON, of up to 40 after 147 h at 475 °C (Figure 1b).
Kinetics is increased because high pressure of methane inhibits
the reverse reaction of hydrogen with ethane. At 475 °C, besides
trace amounts of propane, olefins (mainly ethylene) were
observed, and their amount increased at lower contact time.
2CH4 h C2H6 + H2
(1)
There was an initiation during which hydrogen was the main
product (100%), and then the selectivities of hydrogen and
ethane reached 50% each (Figures 2 and S2). At higher
temperatures (e.g., 475 °C), the selectivity of hydrogen was
higher than that of ethane due to dehydrogenation processes
into olefins (mainly ethylene) (Figure S3).
The silica-supported tantalum hydride 1 was prepared by
treatment under H2 at 250 °C of the surface complexes
† Laboratoire de Chimie Organome´tallique de Surface.
‡ BP Products North America Inc.
9
5044 J. AM. CHEM. SOC. 2008, 130, 5044–5045
10.1021/ja800863x CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism for the Non-oxidative Coupling
Reaction of Methane Catalyzed by the Silica-Supported Tantalum
Hydride (tSiO)2Ta-H, 1
The cumulative amount of H2 evolved during the initiation
period corresponded to 1.45 and 1.33 equiv per Ta at 250 and
375 °C, respectively. The hydrogen evolution arose from (i) its
liberation from some remaining trishydride species,13 (ii) the
C-Hbondactivationofmethane,leadingtosurfacetantalum-methyl
species, and (iii) partial dehydrogenation processes into tanta-
lum(V) carbene hydride and tantalum(V) carbyne13 (eq 2):
catalyzed by a “single-site” catalyst, the silica-supported
tantalum hydride (tSiO)2Ta-H, 1. In the continuous flow
reactor, at each temperature between 250 and 375 °C, the
conversion at thermodynamic limitation was reached with a high
selectivity (>98%) into ethane among hydrocarbon products.
Thereactionislikelytoinvolvethetantalum-methyl-methylidene
species as a key intermediate where the methyl ligand can
migrateontothetantalum-methylidene,affordingthetantalum-ethyl.
Such elementary steps are those frequently proposed in
Fischer-Tropsch synthesis. The only difference between the
two processes is that the CO dissociation step does not occur
when starting from methane whereas it does with carbon
monoxide.
Indeed, such a process was evidenced by NMR spectroscopy
of 1 on MCM-41 supports treated with 13CH4 at 250 °C. In
particular, peaks around 180 and 300 ppm, respectively, assigned
to carbene and carbyne species, were observed (Figure S4).13
Moreover, it was also found that Ta-Me could transfer the
methyl to silicon to produce ca. 30% of (tSiO)3Ta(III) and
tSi-Me.
This process of stepwise C-H bond activation and dehydro-
genation of Ta-Me to Ta(H)(dCH2) and TatC-H was
confirmed by the temperature-programmed reaction (TPRn) of
methane (1 bar) carried out on (tSiO)2Ta-H between 25 and
375 °C (Figure S5). Two peaks of H2 evolution (respectively,
0.66 equiv per Ta at 180 °C and 2.31 equiv per Ta at 375 °C)
were consistent with the successive steps of C-H bond
activation of methane (# 0.7 equiv of Ta), the progressive
transformation of Ta-CH3 to TatCH and the catalytic forma-
tion of gaseous ethane by NOCM. TPRn experiment, carried
out under 50 bar of methane (Figure S6), presented the same
profile but with a shift of the two peaks by about 80 °C,
consistent with an inhibiting effect of methane pressure on the
carbyne species formation.
The reaction is then postulated to proceed first via a C-H
bond activation of methane, leading to a surface tantalum-methyl
species plus molecular hydrogen (eq 2). This tantalum-methyl
undergoes dehydrogenation to a carbene-hydride and a carbyne
(plus hydrogen) species. Both species are able to activate another
molecule of methane by a σ-bond metathesis process,19 affording
a methyl-methylidene key intermediate. Then, as shown for
Cp*2Ta(dCH2)CH3,20 migratory insertion of the methyl group
onto the carbene ligand would yield a tantalum-ethyl. Finally,
the ethyl ligand can be displaced by methane in large excess
via σ-bond metathesis, liberating ethane and leading back to a
tantalum-methyl species. This mechanism is still speculative
since the NMR spectra have been obtained at atmospheric
pressure on a tantalum hydride supported on MCM-41, whereas
the catalytic reaction has been performed on silica at 50 atm.
Acknowledgment. This work was supported by BP Chemi-
cals, the CNRS, and the ESCPE Lyon. The authors wish to thank
B. Maunders and G. Sunley for fruitful discussions.
Supporting Information Available: Experimental details, Fig-
ures S1-S6. This material is available free of charge via the Internet
at http://pubs.acs.org.
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In summary, we have shown the first example of the non-
oxidative coupling reaction of methane into ethane and H2,
JA800863X
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