Direct transformation of ethylene into propylene catalyzed by a tungsten hydride supported on alumina: Trifunctional single-site catalysis

Article abstract of DOI:10.1002/anie.200701199

A trifunctional single-site catalyst: Ethylene is selectively transformed into propylene in a continuous-flow reactor in the presence of the supported tungsten hydride W(H)₃/Al₂O₃. Since the catalyst is also active for olefin metathesis, the reaction is likely to proceed at a "trifunctional catalytic site" by ethylene dimerization, butene isomerization, and cross-metathesis of ethylene with 2-butenes (see picture). (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200701199

Communications
DOI: 10.1002/anie.200701199
Ethylene to Propylene
Direct Transformation of Ethylene into Propylene
Catalyzed by a Tungsten Hydride Supported on
Alumina: Trifunctional Single-Site Catalysis**
Mostafa Taoufik,* Erwan Le Roux, Jean Thivolle-Cazat, and Jean-Marie Basset*
Dedicated to Yves Chauvin and
to Süd-Chemie on the occasion
of its 150th anniversary
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There is an increasing worldwide demand for propylene due
to a higher demand for polypropylene than polyethylene.
Propylene is usually obtained as a co-product of the
petroleum industry, either from naphtha steam crackers
(about 69%) or from gasoline-making fluidized catalytic
crackers.[1,2] Only 2–3% of global propylene production
comes from dehydrogenation of propane or Fischer–Tropsch
synthesis. The cross-metathesis reaction between ethylene
and 2-butenes[3–5] is an alternative route to propylene that is
currently undergoing significant industrial development,[2]
although it first requires the dimerization of ethylene into
2-butenes. Herein we report a new and efficient catalytic
reaction that transforms ethylene directly into propylene with
a selectivity higher than 95% [Eq. (1)].
This reaction is catalyzed by a recently discovered
tungsten hydride supported on g-alumina, namely W(H)3/
Al2O3 (3),[6] which functions as a “trifunctional single-site”
catalyst that catalyzes ethylene dimerization into 1-butene,
isomerization of 1-butene into 2-butenes, and cross-meta-
thesis between 2-butenes and ethylene to finally form
propylene. The tungsten hydride 3, which has also proved to
be an excellent catalyst for alkane metathesis,[7] was obtained
&
^
Figure 1. a) Conversion of ethylene ( ) and TON ( ). b) Selectivities
during the reaction of ethylene to propylene catalyzed by 3
(3.86 wt% W) in a continuous-flow reactor (1508C, PC H =1 bar,
2
4
4 mLminÀ1, VHSV=260 hÀ1).
=
ethylidene species {W}(CH2CH3)( CHCH3) by the elemen-
tary steps displayed in Equation (2).
À
ꢀ
by heating the complex [(Al O)W( CtBu)(CH2tBu)2] (2),
[8]
ꢀ
which results from grafting [W( CtBu)(CH2tBu)3] (1) onto
g-alumina(500) (Johnson Matthey; 200 m2 gÀ1), at 1508C under
H2.
Propylene was obtained selectively (95%) by passing
ethylene over 3 in a continuous-flow reactor (PC H = 1 bar,
Butene isomers then begin to appear in the gas phase. The
initial 1-butene/trans-2-butene/cis-2-butene/isobutene iso-
meric distribution of 5.2:1:2:3.1 (t = 100 min) changed over
time, tending to the ratio 1.7:3.4:1:1.56 (see the Supporting
Information). This ratio reveals a deficiency of 2-butenes and
isobutene compared to the thermodynamic equilibrium
values (1:8:3.84:29.6).[9] In fact, the initial dimerization of
ethylene to 1-butene is followed by a fast isomerization of 1-
butene to cis- and trans-2-butenes (see the Supporting
Information): the newly formed catalyst {W}(CH2CH3)-
2
4
T= 1508C, flow rate 4 mLminÀ1 or volume hourly space
velocity (VHSV) 260 hÀ1). The reaction presents an initial
steep maximal conversion rate of 0.68 molC H molWÀ1 minÀ1
2
4
before reaching a pseudo-plateau of 0.1 molC H molWÀ1 minÀ1,
2
4
with an overall turnover number (TON) of 1120 after 120 h
(Figure 1a). The selectivity for propene increases rapidly up
to a plateau at 95%, while that of butenes remains below
4.5%. Higher olefins are present in only trace amounts (less
than 0.5%; Figure 1b).
=
The initiation and propagation steps were elucidated by
identifying the products formed while heating to the reaction
temperature of 1508C (see the Supporting Information).
Interestingly, one equivalent of ethane is released per
tungsten center after initial contact of 3 with ethylene. This
finding suggests the formation of a surface tungsten–ethyl–
( CHCH3) can insert ethylene into the ethyl moiety to form
an n-butyl species, which affords 1-butene and then cis- and
trans-2-butenes after reinsertion of 1-butene and b-H elimi-
nation from the sec-butyl species [Eq. (3)].
[*] Dr. M. Taoufik, Dr. E. Le Roux, Dr. J. Thivolle-Cazat, Dr. J.-M. Basset
Laboratoire de Chimie OrganomØtallique de Surface
UMR-CNRS-ESCPE-UCB 5265
43 Blvd du 11 Novembre 1918, 69616 Villeurbanne Cedex (France)
Fax: (+33)4-7243-1795
E-mail: taoufik@cpe.fr
The high production of all butene isomers during the
initial stages of the reaction, combined with the relative
deficiency of 2-butenes, suggests that they are primary
reaction products. Propene is formed subsequently, which
suggests the occurrence of a simple metathesis process
involving the newly formed carbene-type complex {W}-
basset@cpe.fr
[**] We wish to thank E. A. Quadrelli and C. CopØret for fruitful
discussions.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
=
(CH2CH3)( CHCH3) [Eq. (4)].
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Communications
step of the reaction [Eq. (1)] is ethylene dimerization to
1-butene (the alumina support was found not to catalyze this
reaction) followed by isomerization to 2-butenes. Propylene is
formed in a follow-up cross-metathesis of the resultant
ethylene/2-butenes mixture, in agreement with the ability of
3 to catalyze olefin metathesis (Scheme 1).
The formation of isobutene, which also appears in the
early stages, requires another process involving free propene.
Thus, 2-methyltungstacyclobutane can isomerize into
3-methyl-tungstacyclobutane and then afford isobutenyl-
tungsten hydride by b-H abstraction from the b-C atom of
the metallacyclobutane;[10] isobutene then forms by ethylene
À
insertion into the W H bond and a-hydrogen transfer
[Eq. (5)].
Scheme 1. Proposed mechanism for the direct conversion of ethylene
into propylene.
Mechanistic information regarding the propagation step
was obtained by performing catalytic experiments under
steady-state conditions with different ethylene flow rates (2–
20 mLminÀ1 (VHSV= 123–1300 hÀ1)) while maintaining
chemical regime conditions, as confirmed by the observed
linear dependence of the conversion on the inverse space
velocity (proportional to contact time; see the Supporting
Information). Thus, while the selectivity for propene
decreases with decreasing inverse space velocity (or increas-
ing flow rate), the reverse is true for butenes. This relationship
indicates that butenes are primary products, since the
selectivity intercept with the y-axis extrapolates to a nonzero
value, and that propene is formed in a reaction that consumes
butenes, since their curves are mirror images at any flow rate
(Figure 2). Furthermore, the formation of 1-butene also
According to numerous examples,[12] olefin cross-meta-
thesis is expected to be catalyzed by a tungsten–ethyl–
ethylidene species following a classical Chauvin mechanism
(Scheme 1a).[13] The same tungsten species is also involved in
the observed ethylene dimerization reaction[14] according to
the classical Cosse–Arlman mechanism,[15,16] which proceeds
À
by a double ethylene insertion into the W H bond of the
tungsten ethylidene hydride (Scheme 1b); this latter species
also catalyzes the isomerization of 1-butene into 2-butenes
(Scheme 1c).
=
Thus, {W}(CH2CH3)( CHCH3), which bears both an alkyl
and an alkylidene fragment, is able to catalyze the dimeriza-
tion, isomerization, and metathesis of olefins at the same site.
In other words, it behaves as a trifunctional single-site
catalyst.
A few examples of the direct transformation of ethylene
into propylene have been reported, although none are
particularly selective or highly productive. For example, a
deficiency of ethylene formation compared to butenes has
been observed during the metathesis of propylene catalyzed
by [Mo(CO)6]/Al2O3 and attributed to the direct transforma-
tion of ethylene into propylene.[17] We have also observed this
reaction, with a short lifetime, during Fischer–Tropsch syn-
thesis in the presence of small iron nanoclusters formed by the
thermal decomposition of [HFe3(CO)11]À/MgO.[18] This reac-
tion has also been reported to proceed with catalysts based on
europium or ytterbium deposited on coal in combination with
titanium compounds (TiCl4, Ti(OiPr)4)[19] as well as with MOx/
Figure 2. Direct conversion of ethylene into propylene in a continuous-
flow reactor (1508C, PC H =1 bar, 520 mg of 3 (3.86 wt% W)). Selectiv-
ity versus inverse space velocity expressed in [(min)(volume of
2
4
~
catalyst)/(volume of ethylene)]; &: propylene, : butenes.
SiO2 or Ru/SiO2 catalysts.[21] However, the catalytic per-
[20]
formances of all these systems were either not quantified[17] or
proved to be very low[19–21] compared to the present W(H)3/
Al2O3 system.
In summary, the W(H)3/Al2O3 system is currently the best
single-site catalyst precursor for the direct transformation of
ethylene into propylene in terms of both activity and
selectivity. It operates as a trifunctional single-site catalyst
that catalyzes ethylene dimerization, 1-butene isomerization,
and ethylene/2-butenes cross-metathesis. We are currently
studying further improvements to this system.
appears to precede that of other butenes since it is the only
one for which the selectivity increases with decreasing inverse
space velocity (see the Supporting Information).
Complex 3 was independently shown to be an active
catalyst for propene metathesis (batch reactor, [C3H6]/[W] =
1470). Thus, propene was converted at 1508C within
50 minutes to a mixture of ethylene and n-butenes with a
conversion of 38% (thermodynamic equilibrium).[11] All the
data thus strongly support the proposal that the first catalytic
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All experiments were carried out using Schlenk and glovebox
techniques for the preparation and handling of organometallic
compounds. The alumina-supported tungsten hydride was prepared
as reported.[7]
[6] A recent theoretical simulation based on the IR frequencies of
the tungsten hydride fits with a tungsten tris(hydride) formula.
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A stainless-steel half-inch cylindrical reactor that can be isolated
from the atmosphere was charged with 3 (520 mg, 3.86 wt% W) in a
glovebox. After connection to the gas lines and purging of the tubes, a
controlled flow of ethylene was passed over the catalyst bed at 1508C
with flow rates varying from 2 to 20 mLminÀ1 (VHSV= 123–
1300 hÀ1). Hydrocarbon products and hydrogen were analyzed
online by GC (HP 8890 chromatograph fitted with an Al2O3/KCl
50 m ” 0.32 mm capillary column, FID detector for hydrocarbons with
a 3-m molecular sieve column and a catharometer for hydrogen).
See the Supporting Information for further experimental details
and Figures S1–S4.
[10] R. R. Schrock, M. Duval-Lungulescu, W. C. P. Tsang, A. H.
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[14] P. W. Jolly, G. Wilke, The Organic Chemistry of Nickel, Vol. 2,
Academic Press, New York, 1975.
Received: March 19, 2007
Published online: July 12, 2007
Keywords: dimerization · hydrides · metathesis ·
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