Non-oxidative coupling of methane catalysed by supported tungsten hydride onto alumina and silica-alumina in classical and H2 permeable membrane fixed-bed reactors

Article abstract of DOI:10.1039/c0cc00007h

Non-oxidative coupling of methane with high selectivity into ethane (>99% among hydrocarbon) in a classical fixed-bed reactor catalysed by SiO₂-Al₂O₃ or γ-Al₂O ₃ supported tungsten hydride is presented. Continuous hydrogen separation, using a Pd-Ag membrane in a fixed-bed reactor, led to methane coupling far beyond the thermodynamic equilibrium conversion.

Full text of DOI:10.1039/c0cc00007h

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Non-oxidative coupling of methane catalysed by supported tungsten
hydride onto alumina and silica–alumina in classical and H2 permeable
membrane fixed-bed reactorsw
K. C. Szeto,a S. Norsic,a L. Hardou,a E. Le Roux,a S. Chakka,b J. Thivolle-Cazat,a
A. Baudouin,a C. Papaioannou,b J.-M. Basset*a and M. Taoufik*a
Received 17th February 2010, Accepted 29th March 2010
First published as an Advance Article on the web 20th April 2010
DOI: 10.1039/c0cc00007h
Non-oxidative coupling of methane with high selectivity into
ethane (499% among hydrocarbon) in a classical fixed-bed
reactor catalysed by SiO2–Al2O3 or c-Al2O3 supported tungsten
hydride is presented. Continuous hydrogen separation, using a
Pd–Ag membrane in a fixed-bed reactor, led to methane
coupling far beyond the thermodynamic equilibrium conversion.
simple fixed-bed reactor catalyzed by Ta-H supported on
silica.17 The proposed mechanism for this reaction involves
tantalum-hydride-methylidene species as key intermediate.17
On the other hand, an alternative system based on W–H on
alumina or silica–alumina, which was developed in our
laboratory, has shown improved activity in alkane metathesis
compared to Ta–H on silica.18,19 Moreover, this particular
metal site is also active in unexpected reactions like direct
conversion from ethene to propene and production of 2,3-
dimethylbutane from isobutane.20,21 Since metal-carbene-
hydride seems to be an important intermediate in all these
reactions,18–23 systems based on tungsten hydride supported
on SiO2–Al2O3 or g-Al2O3 (called W-H@support) have been
investigated for NOCM. However, coupling of methane to
hydrogen and ethane is thermodynamically unfavourable
(DG E 17 kJ molÀ1 in the range of 25–500 1C) and thereby
results in an extremely low equilibrium conversion
(Fig. S1, ESIw).11 To enhance the CH4 conversion, a hydrogen
selective membrane reactor can be used in order to shift the
equilibrium in eqn (1) to the right hand side by constant
removal of hydrogen.24 Considerable research efforts
have been devoted to the development of various membrane
reactors based on separating/removing the products out
of the reaction zone to improve the yield of desired product.
Successful examples have been reported using Pd-based
or ceramic proton conductor membranes to selectively
remove H2.25–29
The utilisation of natural gas to produce petrochemical
derivatives has become an important research field as a result
of oil depletion. One the most abundant components in
natural gas is methane. However, a large portion of methane
is presently flared,1 due to the lack of conventional methods to
convert it into beneficial products.2 Since gases emitted from
CH4 flaring are believed to contribute significantly to global
warming and are under discussion for further restrictions, its
conversion into valuable added products will most likely be an
industrial key step in the future.3 Currently, conversion of
CH4 usually involves several steps, in which at least one of
them requires high temperature and often elevated pressure,
that make the overall reaction highly energy consuming and
expensive.4–6 Thus, development of reaction pathways that
convert CH4 directly and selectively to added value products
under mild conditions is greatly desired. An ideal reaction is
the non-oxidative coupling of methane (NOCM) to form
ethane and hydrogen (eqn (1)).
2CH4 " C2H6 + H2
(1)
We herein report the intriguing catalytic performance of
tungsten hydride supported onto SiO2–Al2O3 or g-Al2O3 for
NOCM using two types of fixed-bed reactor i.e. classical and
equipped with a Pd–Ag membrane for H2 removal.
Hydrogen is regarded as a clean energy carrier, in particular
for vehicles,7 and is in high demand. Ethane can easily be
converted to heavy alkanes through metathesis reactions.8,9
Moreover, ethane dehydrogenation into ethylene is central
for polyethylene production.10 Accounts reporting heavy
hydrocarbon formation from CH4 are predominantly based
upon supported metal systems.11–13 For instance Re and Mo
supported on H-ZSM-5 convert CH4 mainly to aromatics,
with the selectivity to C2H6 being less than 7%.14–16
Recently, we reported exceptionally selective coupling of
methane to hydrogen and ethane at moderate temperature in a
a
´
Universite Lyon 1, Institut de Chimie Lyon, CPE Lyon, CNRS,
UMR 5265 C2P2, LCOMS, Baˆtiment 308 F, 43 Blvd du 11
Novembre 1918, F-69616 Villeurbanne Cedex, France.
E-mail: taoufik@cpe.fr, basset@cpe.fr; Fax: +33 (0)472431795;
Tel: +33 (0)472431798
b BP, 150 W Warrenville Road, IL 60563, Naperville, USA
w Electronic supplementary information (ESI) available: Experimental
details, Fig. S1–S8 and Scheme S1. See DOI: 10.1039/c0cc00007h
Scheme 1 Proposed mechanism for the NOCM reaction catalysed by
the W–H supported onto SiO2–Al2O3 and g-Al2O3.
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Fig. 2 13C CP MAS NMR spectrum of W-H@SiO2–Al2O3 obtained
after 13CH4 activation at 150 1C.
Fig. 1 Catalytic performances at 350 1C, 50 bar of (a) W-H@SiO2–
Al2O3 and (b) W-H@g-Al2O3 for the NOCM in a classical fixed-bed
reactor.
during the initiation period corresponds to 2 H2 per W
(Fig. S6, ESI).w The hydrogen evolution can arise from two
sources: (i) formation of tungsten monohydride by liberation
of one H2 per W from the presumed tungsten trihydride
species, (ii) C–H bond activation of methane, releasing one
H2 per W and formation of tungsten methyl surface species
(Scheme S1, ESI).w These results are consistent with the
spectroscopic data and corroborate the postulated mechanism
for NOCM postulated for Ta-H@SiO2 surface species
(Scheme 1).17
W–H supported onto SiO2–Al2O3 or g-Al2O3 were
prepared from the grafted W(RCt-Bu)(CH2t-Bu)3 precursor
under H2 treatment at moderate temperatures as described
elsewhere.18,19
When CH4 is contacted with W–H@SiO2–Al2O3 in a
classical fixed-bed reactor, the conversion curve with respect
to time on stream increases regularly to a stable plateau at
0.12% (Fig. 1a). Crucially, the activity remains constant after
the initial state, giving the TON of 6 after 5800 min (about
4 days). Ethane is formed selectively (499% among hydro-
carbons) along with an equimolar amount of H2 (Fig. S2a,
ESI).w Importantly, no detectable traces of alkenes, alkynes,
or aromatic compounds are found. When the maximum
conversion is reached, a stable steady state is observed,
showing an essential improvement compared to the Ta-H@SiO2
which suffers from deactivation.17
W-H@g-Al2O3 was also tested in NOCM in a classical
fixed-bed reactor to study the effect of the support on the
activity under same conditions used for W-H@SiO2–Al2O3.
The conversion curve with respect to time on stream is described
by a continuously increase to a stable plateau at 0.2%
(Fig. 1b), which is notably higher than W-H@SiO2–Al2O3
(Fig. 1a), and corresponds to the thermodynamic equilibrium.
Importantly, the conversion remains constant under the
equilibrium conditions, giving the TON of 11 after 5800 min,
which is twofold as high as W-H@SiO2–Al2O3. Ethane and H2
are formed selectively in the same quantities, as shown in
Fig. S2b (ESI),w followed by trace of propane (about 0.3%),
originated from ethane metathesis.33 No decrease in conver-
sion or change in selectivity are observed after exposing the
catalyst under the same conditions after 10 days (TON = 35,
Fig. S7, ESI)w and thereby makes the W–H based system more
active than the previously reported Ta–H system in the long
run.17
Methane activation was monitored by IR spectroscopy and
solid-state NMR in order to get an insight in the mechanism.17
IR study shows that n(W–H) bands decrease strongly after 5 h
of reaction at 150 1C (E80% consumption) and considered to
be total after 24 h (Fig. S3, ESI).w After reaction of 13CH4
onto W-H@SiO2–Al2O3 at 150 1C for 24 h, the 13C CP MAS
NMR spectrum shows four distinct peaks at À4, 48, 237, and
395 ppm (Fig. 2). These peaks have chemical shifts compatible
with tungsten methyl (48 ppm), methylidene (237 ppm), and
methylidyne (395 ppm) ligands, respectively. These observa-
tions indicate that the methyl species formed after activation
of CH4 onto W-H@SiO2–Al2O3, leads to a partial evolution
Although the former approach offers a selective conversion
of the usually inert CH4 to added value products, thermo-
dynamic limitations result in low yield. A known method to
shift the equilibrium is to use a membrane reactor to remove
selectively the formed product.34 Conversion of CH4 into
C2H6 was then carried out exactly as in the classical reactor,
apart from that a Pd–Ag-based H2-permeable membrane
reactor with a dynamic sweep-gas was used. Fig. 3a displays
the conversion of the methane coupling reaction in a
hydrogen selective membrane fixed-bed reactor loaded with
W-H@g-Al2O3. The conversion curve is completely different
from what is observed in the classical fixed-bed reactor
(Fig. 3b) and exhibits a higher initial activity (conversion =
0.6%) and maintains at a high level after 2500 min.
of
a mixture of unsaturated surface species, such as
[W = CH2] and [WRCH], probably by subsequent a-H
abstraction and a-H elimination, respectively (Scheme S1,
1
ESIw).17,30,31 The H NMR spectrum is provided in the ESI
(Fig. S4).w The signal at À4 ppm is attributed to methyl
carbon attached to silicon ((Si–Me) or (Si(H)Me)) originated
from the transfer of W–Me.32 2D 1H-13C HETCOR solid-state
NMR spectra (Fig. S5, ESI)w confirms these attributions of
the [W–Me] and [W = CH2]. Similar observations have been
also observed for the previously reported system based onto
Ta-H@SiO2.17
There is long initiation stage (300 min) where hydrogen is
the only product. The cumulative amount of H2 evolved
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Chem. Commun., 2010, 46, 3985–3987 | 3987