Silica-alumina-supported, tungsten-based heterogeneous alkane metathesis catalyst: Is it closer to a silica- or an alumina-supported system?

Article abstract of DOI:10.1002/adsc.200600436

Grafting of [W(≡C-t-Bu)(CH₂-t-Bu)₃] (1) on silica-alumina partially dehydroxylated at 500°C generates a surface complex [(≡SiO)W(≡C-t-Bu)-(CH₂-t-Bu)₃] (2) as evidenced by mass balance analysis, IR, and NMR spectroscopy. Upon treatment of this species under H₂, a tungsten hydride derivative, [(≡SiO)(EO)W(H)_x] (3), (E=Si or Al), is formed. Both of these complexes are active as alkane metathesis catalysts. The activity of this hydride is similar to that observed for the tungsten hydride-supported on alumina and much greater than that obtained on silica. Noteworthy are the selectivities in higher alkane homologues, that is C_n+1>C _n+2, which are fully consistent with olefin metathesis being the key homologation process.

Full text of DOI:10.1002/adsc.200600436

FULL PAPERS
DOI: 10.1002/adsc.200600436
Silica-Alumina-Supported, Tungsten-Based Heterogeneous
Alkane Metathesis Catalyst: Is it Closer to a Silica- or an
Alumina-Supported System?
Erwan Le Roux,a Mostafa Taoufik,a,* Anne Baudouin,a Christophe CopØret,a
Jean Thivolle-Cazat,a Jean-Marie Basset,a,* Barry M. Maunders,b
and Glenn J. Sunleyc
a
Laboratory of Surface Organometallic Chemistry, UMR 9986, Ecole SupØrieure de Chimie Physique Electronique de
Lyon, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne Cedex, France
Fax : (+33)-(0)4-7243-1795; e-mail: taoufik@cpe.fr
BP Chemicals Ltd, Chertsey Road, Sunbury-On-Thames, Middlesex, TW167 LN, U.K.
BP Chemicals Ltd, Hull Research and Technology Centre, Saltend, Hull, HU128DS, U.K.
b
c
Received: August 25, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
ꢀ
Abstract: Grafting of [W
A
G
on silica-alumina partially dehydroxylated at 5008C on alumina and much greater than that obtained on
generates a surface complex [( SiO)W
ACHTREUNG
sis, IR, and NMR spectroscopy. Upon treatment of fully consistent with olefin metathesis being the key
this species under H2, a tungsten hydride derivative, homologation process.
ꢀ
[( SiO)(EO)W(H)x] (3), (E=Si or Al), is formed.
Both of these complexes are active as alkane meta- Keywords: alkanes; hydrides; metathesis; silica-alu-
thesis catalysts. The activity of this hydride is similar mina; tungsten
Introduction
cases reported so far, activity and stability of the sys-
tems are still a challenge.
Conversion of alkanes is still of major interest in the
We have recently developed W-based systems,
petrochemical industry, and the current oil prices and giving higher turnover numbers and having a greater
the expected future situation in terms of supply and stability than the corresponding Ta-based systems.
demand make the efficient use and transformation of Nonetheless, in the case of W, it is necessary to use an
this natural resource of major importance.[1,2] Since alumina support because the silica-supported systems
the early 1990s, we have developed a series of oxide- are either inactive or less active. In fact, it has not
supported metal hydrides[3–6] for the low temperature been possible to prepare a silica-supported W hydride
alkane transformation.[7–9] In contrast to hydrogenoly- so far; one reason being the sintering during the treat-
sis, alkane metathesis allows both lower and higher ment under hydrogen of W-supported species. We
homologues to be obtained, and we have shown re- report below the grafting and the characterization of
cently through a combination of structure-reactivity[10] [W
U
G
ꢀ
reactor based on dehydrogenation-hydrogenation cat-
alysts in the presence of olefin metathesis catalysts,[12]
and recently a homogeneous catalytic system based
on the same principle has been reported.[13] In all
Adv. Synth. Catal. 2007, 349, 231 – 237
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231

Erwan Le Roux et al.
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Results and Discussion
which would have been replaced by a siloxy ligand of
the surface. Moreover, treatment under H2 of this
ꢀ
Monitoring the reaction of [W
N
A
(1) with SiO2-Al2O3À(500) by IR spectroscopy shows through hydrogenolysis of the perhydrocarbyl ligands,
the following features: a pale yellow colouring of the and their amount shows that the solid contained an
disk, a disappearance of the band associated with the average of 14–15 carbons per grafted W. Overall, the
isolated silanol [n(SiOH) =3747 cmÀ1, Figure 1 a and b), data are consistent with the loss of approximately one
the appearance of (i) a broad band at 3700 cmÀ1 con- equivalent of 2,2-dimethylpropane during the grafting
sistent with the presence of remaining silanols in in- process, leaving on average 3 “neopentyl-like” ligands
teractions with the grafted organometallic moiet- around the metal centre as expected for the formation
ies[14–16] and (ii) two groups of bands in the 3000– of 2, [( SiO)W
( C-t-Bu)(CH2-t-Bu)2]SA (Scheme 1).
A
ꢀ
ꢀ
2700 cmÀ1 and 1470–1330 cmÀ1 regions assigned to
1
n(CH) and d(CH) vibrations of perhydrocarbyl ligands.
ꢀ
ꢀ
ꢀ
When grafting of 1, [W
A
(CH2-t-Bu)3], was those observed for [( SiO)W
A
A
[17]
performed on larger quantities of SiO2-Al2O3–(500) in Bu)2]SiO , prepared by grafting of 1 on SiO2–(700)
.
In
2
1
the absence of solvent at 668C, a pale yellow solid the H NMR, the broad signal centred at 1.1 ppm is
was obtained after washing the excess of 1 with anhy- assigned to the methyl protons of t-Bu fragments, and
drous pentane and drying under vacuum. The per- one shoulder is also observed at 2.0 ppm, which can
centage of grafted W typically ranges between 7.5 and be attributed to the methylene protons of the neopen-
8.3%, that is 0.40–0.45 mmolgÀ1of solid, which shows tyl ligand according to 2D HETCOR solid state
that 50% of the initial OH of silica-alumina NMR spectroscopy (see Supporting Information, Fig-
(0.88 mmol OH/g) have been consumed in agreement ure S1)[18,19] and by analogy with the corresponding
with what has been observed by IR spectroscopy. silica-supported complex (Figure 2 a).[17] Similarly, the
13
During this process, an average of 0.9 equivs. of 2,2- 13C NMR spectrum of 10% C enriched [W
A
ꢀ
dimethylpropane (t-BuCH3) evolved per grafted W, in Bu)(C*H2-t-Bu)3] grafted on SiO2-Al2O3–(500) (Figure 2
agreement with the loss of one neopentyl ligand, b) displays four signals at 318, 95, 52 and 32 ppm, as-
Figure 1. Monitoring the formation of tungsten hydride supported on SiO2-Al2O3–(500) (3) by IR spectroscopy: a) Silica-alumi-
ꢀ
ꢀ
na dehydroxylated at 5008C. b) [( SiO)W
U
G
ꢀ
Scheme 1. Grafting of [W
G
(CH2-t-Bu)3] (1) onto SiO2-Al2O3–(500)
.
232
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Silica-Alumina-Supported, Tungsten-Based Alkane Metathesis Catalyst
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Bu)2]SA is heated under H2 for 15 h at 1508C, the pale
yellow solid darkens to brown, and the IR spectrum
shows the following (Figure 1 c and d) : i) the disap-
pearance of 95% of the n(C and d(C bands, ii) the
À
H)
À
H)
re-appearance of a band at 3747 cmÀ1 associated with
isolated silanols (20%), iii) the appearance of bands
at 2270 cmÀ1 and in the 1980–1800 cmÀ1 region.
Treatment of this solid with D2 at 808C for 15 min
leads to the disappearance of most of the bands in the
1980–1800 cmÀ1 region (90%) (Supporting Informa-
tion, Figure S2), which are regenerated (95%) upon
treatment with H2 at 808C. Therefore, these bands
can be assigned to n(W
vibrations. Moreover, one
À
H)
broad signal of weak intensity at 1910 cmÀ1 remains
unchanged upon treatment under D2: it is assigned to
a n(Al
vibration by comparison with what is ob-
À
H)
served for the W hydride supported on alumina.[6]
Similarly, the band at 2270 cmÀ1 is not affected by this
[3–5]
treatment, and can be assigned to a n(Si
.
Addi-
À
H)
tionally, the band at 2961 cmÀ1 associated with the re-
sidual alkyl ligands (5%, Figure 1 c) are modified,
while a weak band at 2214 cmÀ1, consistent with n(C
À
D)
À
vibrations, appears: these C H bonds are associated
to remaining alkyl fragment probably bonded to W.
These observations are already in sharp contrast to
what has been observed upon treatment of
ꢀ
ꢀ
[( SiO)W( C-t-Bu)(CH2-t-Bu)2]SiO , for which silanols
A
2
1
Figure 2. a) Solid-state H MAS NMR spectrum of 2. The
are regenerated and no or only weak bands around
1900 cmÀ1 , assigned to W H, appear. Moreover, in
spectrum was recorded with 8 scans and a relaxation delay
of 1 s. No line broadening was applied before Fourier trans-
form. b) 13C CP/MAS solid-state NMR spectrum of 2. The
spectrum was recorded with 7000 scans, a relaxation delay
of 1 s and a CP contact time of 1 ms. An exponential line
broadening of 80 Hzwas applied before Fourier transform.
À
contrast to the tungsten hydride supported on alumi-
na,[6] the IR spectrum of this supported tungsten hy-
À
dride on silica-alumina, [W H/SiO2-Al2O3]SA (3) show
several tungsten hydride bands, residual surface alkyl
species as well as both Si H and Al H species. These
latter hydride species are probably formed by the re-
À
À
ꢀ
signed respectively to the carbynic (W C), the meth- action of tungsten hydride intermediates with adja-
[3–5]
À
ꢀ À À ꢀ
ꢀ À À
ylene (W CH2-t-Bu), the quaternary carbon of the cent ( Si O Si ) and ( Si O Als) bridges,
thus
ꢀ
ꢀ
neopentylidyne ligand (W CCMe3) and the methyl providing bisgrafted species, [( SiO)2WHn] and
ꢀ
ꢀ À
À
groups of the t-Bu [C
(CH3)3], respectively.[17]
[( SiO)(AlsO)WHn] along with ( Si H) and (Als H).
G
ꢀ
In summary, the reaction of [W
N
N
Bu)3] (1) with SiO2-Al2O3–(500) leads to the formation spectrum)[4] suggests the absence of formation of tris-
of a supported organotungsten species whose general grafted species.
ꢀ
ꢀ
structure can be formulated as [( SiO)W
A
Hydrolysis of these hydrides by reaction with H2O
ACHTREUNG
ꢀ
À
ed to the surface via one covalent bond ( SiO W) as gives 1.9Æ0.2 equivs. of H2/W and 0.3Æ0.1 equivs. of
evidenced by mass balance analysis, IR and NMR t-BuCH3/W. Moreover, the reaction with 50 equivs. of
spectroscopy. Moreover, the structure of [1/SiO2- CH3I at room temperature gives 0.9Æ0.1 equivs. of
Al2O3–(500)] was reported earlier to be an alkylidene W CH4 and 0.9Æ0.2 equivs. and H2 per W. Hydrolysis of
complex because of its high performances in olefin the resulting powder gives 0.01 H2/W, 0.03 CH4/W and
metathesis.[20] Here, we show that it is in fact a well- 0.02 t-BuCH3/W, which shows that the reaction of the
defined alkylidyne complex, and the necessary propa- metal hydrides with CH3I is complete, and that CH3I
gating alkylidene ligand[21] is probably generated in does not react to give a (hydrolysable) W
ACHTREUNG
oxide supported group
plexes.[17,18,22,23]
6
alkylidyne com- number of H atoms/W is about 2.7 (0.9 CH4/W+1.8
H/W), some of which can be readily released as H2
The preparation of the corresponding hydride was and could be associated to W
(h2-H2) species. More-
N
ꢀ
ꢀ
then investigated. When [( SiO)W
N
N
Adv. Synth. Catal. 2007, 349, 231 – 237
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233

Erwan Le Roux et al.
FULL PAPERS
ꢀ
ꢀ
CH3-t-Bu/W, point out that there are on average 3.0 even if the spectroscopic features of 2, [( SiO)W
(H/R) ligands/W. Hydrolysis data (1.9 equivs. of H2/ t-Bu)
W) are also consistent with this hypothesis if one con- SiO)W
siders that half of the measured H2 (0.95 H2) has been ent from these of [(AlsO)W
released by de-coordination of H2 from W
ACHTREUNG
ꢀ
(CH2-t-Bu)2]SA, are identical to these of [(
ꢀ
U
ACHTREUNG
2
ꢀ
( C-t-Bu)
(CH2-t-Bu)2]Al O
.
2
3
(h2-H2) spe- Secondly, for [W-H/SiO2-Al2O3]SA (3) the initial rate
À
cies and the other half from hydrolysis of W H spe- after 1 h and the number of turnovers after 120 h are
cies (0.95 H2). Note that a small amount of H2 proba- 8.5 mol P/mol W/h and 123, respectively. Again, the
bly comes from the hydrolysis of Al H species (small silica-alumina-supported tungsten hydride (3) displays
À
amount according to IR spectroscopy, see Supporting catalytic performances (activity/selectivity) similar to
Information Figure S2). Since bisgrafted surface com- those of the alumina supported W hydrides (Support-
plexes are probably the major species (absence of ing Information, Figure S3).[6] In contrast, the corre-
SiH2 species), it is therefore possible to propose an sponding silica-supported tungsten hydride system is
idealized view of surface species as a 1:1 mixture of a not very active (10 TON/Wtotal), but in this case, the
dihydrido and a tetrahydrido species, some of which presence of only a low amount of active sites, due to
(ca. 30%) having an alkyl ligand in place of a hydride sintering during the preparation, might be the main
with some W atoms attached to a bissiloxy system origin of this difference. In addition, as observed pre-
and others to one siloxy and one aluminoxy viously, linear alkanes are favoured over branched
(Scheme 2). ones independently of the catalysts used (2 or 3), and
The reactivity of these species is also noteworthy. the selectivities of higher homologues are as follows:
ꢀ
ꢀ
Firstly, [( SiO)W
G
(CH2-t-Bu)2]SA (2) is an
Cn+1 >Cn+2 @Cn+3. The observed selectivities are very
active catalyst precursor for the metathesis of pro- close to those observed for alumina supported W-
pane under standard reaction conditions (Table 1, see based systems and are fully consistent with the model
Supporting Information Table S1 for experimental de- proposed earlier (Scheme 3),[10,11] which is based on
tails). The initial rate is 0.7 mol propane (P)/mol W/h, olefin metathesis intermediates and which predicts
and 30 TON have been reached after 120 h. These that the favoured pathway for the formation of higher
catalytic performances (activity/selectivity) are similar homologues involves a [1,3]-disubstituted metallacy-
to those of the corresponding alumina-supported clobutane rather than
a [1,2]-disubstituted one
[6]
while (Scheme 3).[24] The higher selectivity of linear versus
ꢀ
complex [(AlsO)W
( C-t-Bu)
E
,
3
the corresponding silica-supported system 2is inactive branched compounds is also consistent with this
Scheme 2. Proposed structures for tungsten hydrides supported on silica-alumina.
234
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Adv. Synth. Catal. 2007, 349, 231 – 237

Silica-Alumina-Supported, Tungsten-Based Alkane Metathesis Catalyst
FULL PAPERS
model since the latter would involve the less reactive
disubstituted carbenes. Note also the low amount of
methane, which shows that W-based catalysts are
more selective than the corresponding Ta ones, proba-
bly because they do not catalyse the competitive reac-
tion of alkane hydrogenolysis.[9]
These results show that attaching W to a siloxy
group does not prevent to obtain highly active cata-
lysts, but that alumina, whether as a bulk or in a
silica-alumina, plays a crucial role. One possibility is
that alumina prevents the mobility of surface species
(sintering), and could stabilize surface intermediates
in alkane metathesis.
Conclusions
In conclusion, we have shown that 1 reacts selectively
on the silanol of a silica alumina partially dehydroxy-
lated at 5008C to provide a well-defined surface spe-
ꢀ
ꢀ
cies 2 [( SiO)W
G
N
by IR and NMR spectroscopies. Upon treatment with
H2, this surface species further reacts with the oxide
surface to generate probably bisgrafted species ac-
cording to reactivity studies with H2O and CH3I. The
major surface species are best described as a 1:1 mix-
ture of a bis- and a tetrahydride W surface species.
Both species are alkane metathesis catalyst precur-
sors, with activities and selectivities similar to those
observed for the corresponding alumina supported
species, which shows that alumina has a critical role
in the W based systems and could possibly prevent
the mobility of reactive surface species. Further stud-
ies along this line are currently under investigation.
Experimental Section
General Procedures
All experiments were carried out under controlled atmos-
phere, using Schlenk and glove-box techniques for the or-
ganometallic synthesis. For the synthesis and treatments of
the surface species, reactions were carried out using high-
vacuum lines (1.34 Pa) and glove-box techniques. Argon, hy-
drogen and propane were purified over molecular sieve and
deoxo catalysts. Pentane was distilled over Na/K amalgam.
Before use, silica-alumina (HA-S-HPV from Akzo-Nobel:
Si/Al=3 and 390 m2g)À1 was calcined under N2/O2 and parti-
ally dehydroxylated under vacuum at 5008C (SiO2-
ꢀ
Al2O3–(500)). [W
U
G
The preparation of 1* was performed according to the
same procedure as that used for 1, but starting with a 90:10
mixture of non-labeled t-BuCH2Cl and (99% atom 13C)
monolabeled t-BuC*H2Cl to form t-BuCH2MgCl (10% 13C
labelled) as alkylating agents (yield 80–85%); the 99% 13C
monolabeled t-BuC*H2Cl was prepared as previously de-
scribed.[19]
Adv. Synth. Catal. 2007, 349, 231 – 237
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235

Erwan Le Roux et al.
FULL PAPERS
Scheme 3.
Analyses by gas chromatography were performed using a
KCl/Al2O3 column (50 m, 0.32 mm) equiped with an FID
detector.
volume around 5 L) in order to quantify 2,2-dimethylpro-
pane evolved during grafting. Analysis by gas chromatogra-
phy indicated the formation of 1.45Æ0.05 mmol of 2,2-dime-
thylpropane (0.9Æ0.1 CH3-t-Bu/W). Pentane (10 mL) was
introduced into the reactor by distillation, and the solid was
washed three times. After evaporation of the solvent, the re-
sulting brown powder was dried under vacuum to yield 3.6 g
of [1/SiO2-Al2O3–(500)] (2). Elemental analysis: W 8.3%wt.
Infrared spectra were recorded on a Nicolet Magna 550
FT spectrometer equipped with a cell designed for in situ re-
actions under controlled atmosphere. Elemental analyses
were performed at the Service Central d’Analyses of CNRS
in Solaize.
1H MAS and 13C CP-MAS solid state NMR spectra were
recorded on a Bruker Avance-500 spectrometer with a con-
ventional double resonance 4 mm CP-MAS probe. The sam-
ples were introduced under Ar in a zirconia rotor, which
was then tightly closed. In all experiments, the rotation fre-
quency was set to 10 kHz. Chemical shifts were given with
Preparation of the W-H/SiO2-Al2O3 (3)
The solid 2 [1/SiO2-Al2O3–(500)] (0.50 g) was heated at 1508C
in the presence of a large excess of anhydrous H2 (77
330 Pa) in a glass reactor of 500 mL. After 15 h, the gaseous
product was quantified by GC. After evacuation of the gas
phase, a second treatment under H2 is performed under the
same experimental conditions.
1
respect to TMS as external references for H and 13C nuclei.
ꢀ
Monitoringthe Graftingof [W
on Silica-Alumina and the Formation of the Hydride
by IR Spectroscopy
G
(CH2-t-Bu)3]
Representative Procedure for Propane Metathesis
In a glove-box, the solid 2 (45 mg, 20.1 mmole) was intro-
duced in a batch reactor of 500 mL. After evacuation of Ar,
dry propane was added (600 Torr; sub/cat=876), and the re-
action mixture was heated at 1508C. To monitor the reac-
tion, aliquots were expanded in a small volume, brought to
atmospheric pressure with air and analysed by GC. Similar
procedures were used with solid 3 and other catalysts men-
tioned in Table 1 (see Supporting Information Table S1 for
experimental details).
Silica-alumina (20–50 mg) was pressed into an 17 mm self-
supporting disk, adjusted in the sample holder, and put into
a glass reactor (300 mL) equipped with CaF2 windows. The
support was calcined under N2/O2 and partially dehydroxy-
lated under vacuum at 5008C, SiO2-Al2O3–(500). The com-
pound 1 was then sublimed under dynamic vacuum at 70–
808C onto the oxide disk, which typically turned orange-
yellow. The solid was then heated at 808C for 2 h, and the
excess of 1 was removed by reverse sublimation at 70–808C
and condensed into a lateral tube cooled in liquid N2, which
was then sealed off using a torch. After evacuation under
vacuum for 30 min,
a large excess of anhydrous H2
Acknowledgements
(77,330 Pa) was introduced, and the solid was heated at
1508C for 15 h. The gaseous products were analysed by GC,
and evacuated. An IR spectrum was recorded after each
step.
This work was supported by BP Chemicals,the CNRS,and
the ESCPE Lyon.
Representative Procedure for Graftingof 1 on SiO -
Al2O3À(500) on a Larger Scale [1/SiO2-Al2O3–(500)] (2)
2
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compounds were condensed into another reactor (of known
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Silica-Alumina-Supported, Tungsten-Based Alkane Metathesis Catalyst
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