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From Olefin to Alkane Metathesis A Historical Point of View.

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Highlights
DOI: 10.1002/anie.200602155
Alkane Metathesis
From Olefin to Alkane Metathesis: A Historical Point of
View**
Jean-Marie Basset, Christophe Copret,* Daravong Soulivong, Mostafa Taoufik,
and Jean Thivolle-Cazat
Keywords:
alkane metathesis · dehydrogenation · heterogeneous
catalysis · homogeneous catalysis · olefin metathesis
Alkane conversion has been a major
focus of petrochemical research for the
past century,[1–3] and in fact the discovery
of olefin metathesis on supported MoO3
or WO3 systems by Banks and Bailey
was the result of the investigation of the
reactivity of alkane and olefin mixtures
on such types of catalysts.[4, 5] Under
these conditions, only olefins were transformed, which led to the development of
very important industrial processes such
as the Lummus ABB process, which
converts ethylene into propylene
through cross-metathesis with 2-butenes
on W-based catalysts.[6] This discovery
was rapidly turned into an alkane conversion process first by Banks (Philipps)
and then by Hughes (Chevron)[7, 8] by
combining heterogeneous dehydrogenation/hydrogenation and olefin metathesis catalysts, which allows a given
alkane to be converted into its lower and
higher homologues. This reaction
[*] Dr. J.-M. Basset, Dr. C. Cop0ret,
Dr. D. Soulivong, Dr. M. Taoufik,
Dr. J. Thivolle-Cazat
LCOMS UMR 9986
CPE Lyon
43 Bd du 11 Novembre 1918, 69616
Villeurbanne Cedex (France)
Fax: (+ 33) 4-7243-1795
E-mail: coperet@cpe.fr
[**] We are indebted to the CNRS, the MinistBre d0l0gu0 C l’Enseignement sup0rieur et
C la Recherche, the R0gion RhEnes-Alpes,
and CPE Lyon for financial support. We
would also like to thank our industrial
partners INEOS and BP for continuous
support, and more specifically S. Chakka,
B. M. Maunders, S. Spitzmesser, and G. J.
Sunley. Finally, we thank the graduate
students and postdoctoral fellows who
carry out research in the LCOMS.
6082
[Eq. (2)],
like
olefin
metathesis
[Eq. (1)], is thermoneutral (Figure 1),
but requires the intermediate formation
of olefins from alkanes [Eq. (3)], which
Figure 1. Thermodynamic data (conversion as
a function of temperature) of the chemical
processes olefin metathesis [Eq. (1)], alkane
metathesis [Eq. (2)], and alkane dehydrogenation [Eq. (3)].
is highly disfavored. Therefore, high
temperatures are required for high concentration of olefins (Figure 1). For
example, in the Chevron process, which
relies on a mixture of Pt/Al2O3 (dehydrogenation/hydrogenation) and WO3/
SiO2 (metathesis), high temperatures
and high pressures are necessary, typically 400 8C and 60 bar (Figure 2).[9]
In 1997, within a research program
targeted at the activation of C H bonds
by supported transition-metal catalysts,
J.-M. Basset et al. reported the conver-
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
sion of alkanes at low temperatures and
pressures, typically 150 8C and 1 bar,
into their lower and higher homologues.
The catalyst was a silica-supported tantalum hydride,[10] and the reaction was
named alkane metathesis by analogy
with olefin metathesis (Scheme 1 a). An
important research effort in collaboration with BP Chemicals and BP has
yielded numerous catalytic systems
based on Group 5 and Group 6 hydride
and alkyl systems,[11–13] and currently the
best system is a W hydride supported on
alumina (turnover number (TON) = 120
after 120 h under ca. 0.8 bar of propane;
Scheme 1 b).[13] Notably, the catalytic
systems based on hydrides give better
results under pressure and can be regenerated by treatment under H2.
Recent kinetic and mechanistic studies in our laboratories[12, 14] have shown
that the key carbon–carbon bond-breaking and bond-making steps correspond
to those of olefin metathesis, that is, a
[2+2] cycloreversion and a cycloaddition step.[15, 16] The transformation of the
alkane into an olefin and carbenic
propagating species involves additional
key elementary steps. It has been proposed that an alkyl species is formed by
C H activation of alkanes, which, by a
subsequent b-H or a-H transfer step,
yields the olefin or a hydridocarbene
intermediate, the necessary propagating
center
for
olefin
metathesis
(Scheme 1 c). The corresponding reverse step yields the alkane metathesis
product. Notably, and in contrast to the
Chevron process, alkane metathesis is
carried out on a dual catalyst based on a
single metal having all the necessary
properties (C H activation, dehydroAngew. Chem. Int. Ed. 2006, 45, 6082 – 6085
Angewandte
Chemie
Figure 2. Schematic view of the homologation processes taking
place within a fixed-bed reactor.
genation, metathesis, and hydrogenation), and moreover, these catalytic
systems can be used to incorporate
methane into higher homologues.[17] Finally, these systems are selective towards Cn 1 and Cn+1 homologues, because the major pathway involves the
cross-metathesis of the terminal olefin
(R’ = Cn 2, Scheme 1 c) and the alkylidene species resulting from the activation of the terminal C H bond of the
alkane (R = Cn 1, Scheme 1 c)
After nearly 30 years without external reports, Chevron has published new
patents on their system,[18, 19] and just
recently a collaborative effort of the
groups of Brookhart, Goldman, and
Chevron has provided a homogeneous
catalytic system related to the original
Chevron process,[20] but based on a
combination of homogeneous catalysts,
the Kaska Ir complexes 1[21–23] as dehydrogenation/hydrogenation
catalysts,
and the Mo imido olefin metathesis
catalyst 2 of Schrock[24, 25] (Figure 3 a).
In the presence of 1 a (L = C2H4) and 2,
hexane is converted into a mixture of
Angew. Chem. Int. Ed. 2006, 45, 6082 – 6085
Scheme 1. a) Analogy between olefin and alkane metathesis.
b) Examples of alkane metathesis catalysts developed by J.-M.
Basset et al. (TON data for experiments performed under the same
reaction conditions, p = 0.8 bar, T = 150 8C. The subscripts x and y
symbolize the number of O and H atoms (x = 1 for E = Si, 1 < x < 3
for E = Al; 1 < y < 3). c) Proposed reaction pathway with supported
hydride catalysts.
C2–C14 alkane products (0.75–2.05 m,
Figure 3 b), which corresponds to a
TON of about 49–128 based on 2.
It is noteworthy that the reaction
takes place at such a low temperature
because the amount of olefin must be
Figure 3. a) Homogeneous catalytic precursors; in 2: Ar = 2,6-iPr2C6H3, R = CMe2Ph, R’ =
C(CH3)(CF3)2. b) Product distribution y (expressed in mmol L 1) for hexane metathesis (7.6 m)
with 1 a (L = C2H4 ; 10 mm) and 2 (16 mm) after 4 days of reaction at 125 8C (data extracted from
reference [20]).
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6083
Highlights
very low, which shows that both catalysts
must be quite efficient. The Ir catalyst
precursors can be 1 a, 1 b, or 1 c, but in
each case, it is necessary to add an
activator,
3,3-dimethyl-1-butene
(2 equiv), a well-known H2 scavenger,[26, 27] to the dihydrogen derivative.
The catalytic system slowly deactivates
with time, and this is mainly a result of
the slow decomposition of 2. In fact,
adding an extra amount of 2 allows the
alkane mixture to be further converted.
Notably, the Grubbs-type Ru catalysts[28] are not efficient for this system
because they are not compatible with
complexes 1 (decomposition).
The product distribution (all linear
alkanes from C2–C14) clearly indicates
that the reaction corresponds to successive steps of dehydrogenation, olefin
metathesis, and hydrogenation. Notably,
the products do not arise from the
selective metathesis of 1-hexene, the
kinetic product of the Ir-based system,[29] which would give only the C2
and C10 homologues, but from the
cross-metathesis of all hexene isomers.
Moreover, the formation of alkanes of
carbon number greater than 10 in small
amounts is evidence for successive reactions such as the cross-metathesis of
hexane (as hexenes) with higher homologues. It is also noteworthy that the
distribution of the products is consistent
with their formation through nearly a
statistical metathesis of an equilibrated
mixture of hexene isomers (internal
olefins in greater amount than terminal
ones leading to homologues of shorter
chain lengths compared to those derived
from the metathesis of terminal olefins).
This is a major difference compared
with the systems based on catalysts
prepared through surface organometallic chemistry which give mainly the Cn 1
and Cn+1 homologues (see above). Finally, the absence of formation of methane with the homogenous catalytic system shows that the methylidene propagating species (and probably other
alkylidene species) does not react with
H2 through hydrogenolysis, and that
alkanes are solely formed by hydrogenation of olefins at the Ir center.
Moreover, one initiation product, 2methyl-2-phenylbutane, which probably
results from the cross-metathesis of the
neophylidene Mo complex 2 and 1-hexene and subsequent hydrogenation, has
6084
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been detected. More information on 2
through spectroscopy studies have so far
been hindered by the presence of many
propagating species and deactivation.
On the other hand, it was possible to
observe the resting states of the Ir systems by monitoring the reaction mixture
by NMR: depending on whether 1 a or
1 b is used, it is either the olefin or the
dihydrogen complex, respectively. This
type of study will probably allow further
improvement of the catalytic system
based on a structure–reactivity relationship.
Finally, owing to the instability of 2,
the combination of the Ir homogeneous
catalyst with a heterogeneous olefin
metathesis catalyst, Re2O7 on alumina,
has also been investigated. Starting from
decane, a distribution of alkane products is also obtained: a TON of 180/Ir
has been achieved after three hours,
after nine days of reaction this value
increases to about 400/Ir. It is noteworthy that the authors have chosen Rebased catalysts because they are usually
more susceptible to deactivation at higher temperatures than the analogous Moor W-based systems. The latter systems,
however, require higher temperatures,
which could be detrimental to the Ir
catalysts.
The groups of Brookhart and Goldman have reported the first approach to
a homogeneous alkane metathesis catalytic system based on the Chevron
technology that is the combination of
two catalysts: a dehydrogenation/hydrogenation catalyst and an olefin metathesis catalyst. A first attempt to combine the Ir dehydrogenation catalyst
with a more robust heterogeneous olefin
metathesis catalyst, Re2O7/Al2O3, has
yielded promising results, and the use
of spectroscopic studies has already
given some insight about the fate of
the homogeneous catalysts, which
should lead to the development of better
catalytic systems. Thus, new rapid advances in the field of alkane conversions
are expected, and significant developments can be anticipated.[30] There are
still several challenges ahead such as
finding stable—easily regenerated—catalysts and achieving a selective transformation of alkanes into a minimum
number of homologues.
Published online: August 29, 2006
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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