Carbon dioxide conversion to dimethyl carbonate: The effect of silica as support for SnO2 and ZrO2 catalysts

Article abstract of DOI:10.1016/j.crci.2010.08.003

Abundant in nature, CO₂ poses few health hazards and consequently is a promising alternative to phosgene feedstock according with the principles of Green Chemistry and Engineering. The synthesis organic carbonates from CO₂ instead of phosgene is highly challenging as CO₂ is much less reactive. As part of our ongoing research on the investigation of catalysts for dimethyl carbonate (DMC) synthesis from methanol and CO ₂, we herein report results aimed at comparing the catalytic behavior of new SnO₂-based catalysts with that of ZrO₂. Silica-supported SnO₂ and ZrO₂ exhibit turnover numbers which are an order of magnitude higher than those of the unsupported oxides. Tin-based catalysts also promote methanol dehydration which makes them less selective than the zirconium analogues. Last but not least, comparison with soluble Bu₂Sn(OCH₃)₂ highlights the superiority of the organometallic precursor for achieving 100% selectivity to DMC but it deactivates by intermolecular rearrangement into polynuclear tin species.

Full text of DOI:10.1016/j.crci.2010.08.003

C. R. Chimie 14 (2011) 780–785
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Full paper/M e´ moire
Carbon dioxide conversion to dimethyl carbonate: The effect of silica
as support for SnO and ZrO catalysts
2
2
a,
b,d
b,c
Danielle Ballivet-Tkatchenko *, Jo a˜ o H.Z. dos Santos , Karine Philippot
Sivakumar Vasireddy
,
a
a
Institut de chimie mol e´ culaire, CNRS, universit e´ de Bourgogne, 9, avenue Alain-Savary, 21000 Dijon, France
b
Laboratoire de chimie de coordination, CNRS, 205, route de Narbonne, 31077 Toulouse, France
c
LCC, INPT, UPS, universit e´ de Toulouse, 31077 Toulouse, France
d
Instituto de Qu ı´ mica, Universidade Federal do Rio Grande do Sul, avenida Bento Gon c¸ alves 9500, 91501-970 Porto Alegre, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history:
Abundant in nature, CO2 poses few health hazards and consequently is a promising
alternative to phosgene feedstock according with the principles of Green Chemistry and
Received 1 April 2010
Accepted after revision 24 August 2010
Available online 16 October 2010
Engineering. The synthesis organic carbonates from CO
challenging as CO is much less reactive. As part of our ongoing research on the
investigation of catalysts for dimethyl carbonate (DMC) synthesis from methanol and CO
we herein report results aimed at comparing the catalytic behavior of new SnO -based
2
instead of phosgene is highly
2
2
,
Keywords:
2
Carbon dioxide fixation
Sustainable chemistry
Tin
2 2 2
catalysts with that of ZrO . Silica-supported SnO and ZrO exhibit turnover numbers
which are an order of magnitude higher than those of the unsupported oxides. Tin-based
catalysts also promote methanol dehydration which makes them less selective than the
Zirconium
2 3 2
zirconium analogues. Last but not least, comparison with soluble Bu Sn(OCH ) highlights
Supported catalysts
the superiority of the organometallic precursor for achieving 100% selectivity to DMC but
it deactivates by intermolecular rearrangement into polynuclear tin species.
ß 2010 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
R E´ S U M E´
Mots cl e´ s :
Le dioxyde de carbone est naturellement abondant sans toxicit e´ particuli e` re ce qui en fait
un substitut au phosg e` ne int e´ ressant dans le contexte des principes de ‘‘Green Chemistry
and Engineering’’. Cependant, remplacer le phosg e` ne par le dioxyde de carbone pose le
probl e` me de sa plus faible r e´ activit e´ . Dans le cadre de nos travaux sur la synth e` se
Fixation du dioxyde de carbone
Chimie durable
E´ tain
Zirconium
catalytique du carbonate de dim e´ thyle (DMC) a` partir du m e´ thanol et du CO
2
, nous
Catalyseurs support e´ s
d e´ crivons ici les r e´ sultats obtenus avec de nouveaux catalyseurs a` base d’oxyde d’ e´ tain et
de zirconium. Les oxydes support e´ s sur silice poss e` de des nombres de rotation qui sont dix
fois sup e´ rieurs a` ceux des oxydes non support e´ s. Les catalyseurs a` base d’ e´ tain sont aussi
actifs en r e´ action de d e´ shydratation du m e´ thanol ce qui les rend moins s e´ lectifs que ceux a`
base de zirconium. Enfin, la comparaison avec les pr e´ curseurs catalytiques solubles du
2 3 2
type Bu Sn(OCH )
montre que ceux-ci sont totalement s e´ lectifs en DMC mais ils se
d e´ sactivent dans les conditions r e´ actionnelles.
ß 2010 Acad e´ mie des sciences. Publi e´ par Elsevier Masson SAS. Tous droits r e´ serv e´ s.
1. Introduction
*
Corresponding author.
Carbon dioxide utilization as chemical feedstock could
E-mail addresses: ballivet@u-bourgogne.fr (D. Ballivet-Tkatchenko),
˜
o H.Z. dos Santos), Karine.Philippot@lcc-toulouse.fr
have currently a significant role in addressing the
emission excess of CO in the atmosphere, but more
jhzds@iq.ufrgs.br (Joa
(
K. Philippot), svasir1@lsu.edu (S. Vasireddy).
2
1
631-0748/$ – see front matter ß 2010 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.08.003

D. Ballivet-Tkatchenko et al. / C. R. Chimie 14 (2011) 780–785
781
importantly transforming an end-product into a valuable
feedstock [1]. Carbon dioxide has been employed for
decades as a raw material for the production of urea,
salicylic acid, and ethylene (propylene) carbonates, as
transesterification of ethylene carbonate which co-pro-
duces ethylene glycol leading to a decrease of the AE value
to 59 wt%. The methanolysis of urea is more favorable (AE =
72 wt%). Moreover, the co-formed ammonia may be
recycled for urea synthesis. It is worthy of note that entries
well as for methanol synthesis from a blend of CO and CO
Recent interest in the use of CO as feedstock is motivated
by the development of new reactions from so-called
renewable carbon. However, CO displays a limited
2
.
2
2
3 and 4 correspond to an indirect use of CO since ethylene
carbonate and urea are thereof industrially produced.
Entry 5 highlights that the direct synthesis of DMC from
2
reactivity when compared to other C1 molecules such
as carbon monoxide and phosgene. Nevertheless, the need
for friendly reactants, products, and safer processes
according to the principles of Green Chemistry and
Engineering [2] offers the opportunity to investigate
CO
of 83 wt%. Interestingly, in the context of non-fossil carbon
resources for making organics coupling methanol with CO
presents the unique advantage of utilizing CO as carbon
feedstock, provided that methanol is thereof obtained by
hydrogenation [11].
2
and methanol leads, as entry 2, to the highest AE factor
2
2
2
new reaction pathways based on CO .
The alternative to phosgene is a challenging option that
may open new perspectives for carbonate and carbamate
synthesis including polymers [3]. The phosgene technolo-
gy has still a leading role in a range of industries, including
production of pharmaceuticals, agrochemicals, and others
that raises obvious technical and economical barriers.
Phosgene is widely recognized as one of the most acutely
toxic substances used in commerce today. It is currently
produced from brine and CO in a two-step process:
2
The key role played by DMC and CO in the practice of
green chemistry has stimulated fundamental research
for developing innovative catalytic methodologies.
Progress in this field has been slow, however, mainly
2
due to the low reactivity of CO . The current main issue
for the reaction reported in entry 5 of Table 1 is to
enhance the yield and rate for DMC formation. One
option consists in adding water traps to the catalytic
systems for preventing catalyst deactivation by water as
well as to shifting the equilibrium [12]. Nevertheless,
there is a need for exploring new catalysts for rate
enhancement in the absence of water trap additives. In
recent years, a variety of heterogeneous catalysts has
been screened. The reaction is commonly run under high
electrolysis of brine for forming Cl
which is further reacted with CO on activated charcoal
catalyst leading to COCl [4]. In the case study of organic
2
(chlor-alkali process)
2
carbonate synthesis, the reaction co-produces HCl. Thus,
-
-
the chlorine cycle Cl ! Cl
2
! Cl is, overall, energy-
intensive. Phosgene-free production of DMC is mainly
based on the oxidative carbonylation of methanol which
limits scale-up due to inherent hazards [5]. Nevertheless,
much attention is being focused on extending DMC uses as
intermediate, e.g. in organic synthesis [6], phosgene-free
synthesis of polymers [7], and gasoline blending [8]. As a
matter of fact, DMC fulfils green chemistry criteria [2]. It
has a low toxicity, an absence of any irritant or mutagenic
effects and low atmospheric loss, and is highly biodegrad-
able [7,9]. Therefore, there is a real need for friendly
synthetic routes to DMC.
pressure, i.e. 100–300 bar, at 130–180 8C over ZrO
2
-
based catalysts [13], CeO [14], Cu-Ni/graphite [15], V-
2
doped Cu–Ni/AC catalysts [16], and polyoxometalates
[17]. Zirconia-based catalysts are those exhibiting the
highest selectivity to DMC (ꢀ100%). Interestingly,
organometallic complexes such as diorganotin (IV)
compounds are also active precursors under the same
reaction conditions. Importantly enough, they are
superior to heterogeneous catalysts in terms of atom
economy since they are totally selective [12(b),18].
However, organometallic tin compounds in general
suffer from environmental pressure due to issues around
tributyltin derivatives.
Table 1 reports the various alternative feedstocks
of current interest to get DMC. The atom economy factor
(
AE) is also given as a helpful indicator of the amount
As part of our ongoing research on tin compounds for
of co-produced waste to assess the greenness of a
reaction [10].
the synthesis of DMC from methanol and CO
report preliminary results on the catalytic behavior of
supported SnO catalysts for comparison with unsupport-
ed SnO and diorganotin (IV) compounds. The synthesis of
unsupported SnO was achieved by a free-chloride sol–gel
route from tin tetra-tert-butoxide precursor providing
SnO nanoparticles with an average diameter of 4 nm [19].
Supported SnO was prepared from the same tin precursor
2
, we herein
The conventional phosgene route (entry 1) gives an AE
factor of 55 wt% which shows that nearly half the weight of
the reactants is transformed into hydrogen chloride. A
much more favorable factor (83 wt%) is obtained from the
on-stream oxidative carbonylation of methanol with the
co-formation of water (entry 2). Entry 3 corresponds to the
2
2
2
2
2
with the objective to avoid handling nanoparticles. A
preliminary screening with alumina, silica-alumina, and
silica as supports showed that silica was the best candidate
due to its inertness towards methanol dehydration and
Table 1
The different feedstocks to DMC, and the corresponding atom economy
factor (AE).
2
DMC decomposition. ZrO catalysts were also prepared
Entry
Feedstock
COCl + 2 CH
Product
AE (wt%)
following identical methods to get a reference under our
reaction conditions since they are known to be the most
selective to DMC among the heterogeneous catalysts
reported so far [13]. The synthesis of the silica-supported
tin and zirconium oxides is also reported as there is no
previous report.
1
2
3
4
5
2
3
OH
DMC + 2 HCl
55
83
59
72
83
CO + 1/2 O + 2 CH
(CH O) CO + 2 CH OH
(NH CO + 2 CH OH
CO + 2 CH OH
2
3
OH
DMC + H
DMC + HO(CH
DMC + 2 NH
DMC + H
2
O
2
2
3
2 2
) OH
)
3 2
3
3
2
3
2
O

7
82
D. Ballivet-Tkatchenko et al. / C. R. Chimie 14 (2011) 780–785
2
. Results and discussion
2
.1. Preparation of silica-supported tin and zirconium oxides
t
t
4 4
from Sn(O Bu) and Zr(O Bu)
Supported catalysts may exhibit different structure-
activity relationship depending on the dispersion of the
active phase, metal loading, and interaction with the
support. In most cases, with catalysts prepared by a
conventional impregnation method from inorganic salts,
dispersion of the active phase is difficult [20]. Using
metalorganic compounds as precursors allows a better
control of this dispersion [21]. Accordingly, monomeric
metal alkoxides were chosen in this work as convenient
precursors for reacting with the hydroxyl groups of the
support, thus allowing chemical grafting. Tin and zirconium
2 2
tetra-tert-alkoxide dissolved in CH Cl were contacted,
respectively, with silica (Aerosil Degussa 200, SSA = 196
2
ꢁ1
m g ) pretreated at 450 8C. The initial M/SiO
ratio was varied from 10 to 20 wt%. Afterfiltration, washings
with CH Cl and drying at 40 8C under vacuum, the solids
2
(M = Sn, Zr)
2
2
contained 4.9 and 4.4 ꢂ 0.3 wt% of tin and zirconium,
respectively. These metal contents remain unchanged when
the reaction is performed either at 25 or 60 8C. Further
washings with tetrahydrofuran or methanol did not promote
metal leaching. These results show that chemical grafting of
the complexes did occur at the saturation value of 1.3 ꢂ 0.1 Sn
2
(
or Zr) per nm . As the OH content of silica pretreated at 450 8C
2
isabout2 OHper nm [22], they may havereacted through two
of the four tert-butoxy ligands. However, using other mono-
t
mers such as nBu
3
SnOCH
3
and nBu
2
Sn(O Bu)
2
has no noticable
changes in the loading value. As it is known that only the
methoxy ligand of nBu SnOCH undergoes protolysis [23], it
-l
Fig. 2. IR spectra in the 4000-2600 cm region: (a) SiO
2
pretreated at
t
450 8C, (b) adsorption of Sn(O Bu)
4
(5 Sn wt%) at 25 8C followed by
3
3
vacuum for 4 h, (c) heat-treatment at 110 8C for 4 h followed by vacuum
for 4 h.
can, therefore, be inferred that the saturation loading under
these experimental conditions is mainly controlled by steric
effects.
The thermal stability of the anchored tert-butoxy tin
species was studied and first evaluated by the tempera-
ture-programmed desorption technique (TPD) using a
thermal conductivity detector (TCD). As shown in Fig. 1,
the onset of decomposition occurs at 100 8C over a narrow
range of temperature with a major peak at 180 8C. Analysis
of the gas evolved by gas chromatography (GC) revealed
the main presence of isobutene.
Investigation of the grafting mode was performed by
FTIR transmission spectroscopy (Fig. 2). The IR spectrum of
pure silica pretreated at 450 8C shows the presence of a
ꢁ
1
sharp n(OH) band at 3747 cm characteristic of isolated
ꢁ1
silanol groups and a weak shoulder at ꢀ3675 cm due to
the presence of H-bonded silanols (Fig. 2a) in agreement
with the literature [24].
t
Adsorption of Sn(O Bu)
4
at room temperature led to the
consumption of isolated silanol groups, leaving a broad
ꢁ
1
band at ca. 3620 cm (Fig. 2b). After heating to 110 8C
followed by evacuation, the (C–H) bands of the tert-butyl
groups have almost disappeared while the (O–H) band of
n
n
the isolated silanols is partially restored (Fig. 2c). Although
it was not our objective to study the reaction mechanism
involved, the restoration of Si–OH groups and the
formation of isobutene analyzed as analyzed by TPD
experiments in the same temperature range are likely due
to a dehydration process. Such a reaction is induced by
protolysis of tin tert-butoxide by the isolated OH groups.
These results show that the extrusion of the tert-butoxy
ligands is occurring under mild conditions restoring
partially the hydroxyl groups of silica. Further work is
t
Fig. 1. TPD profile under helium of Sn(O Bu)
4
adsorbed on SiO
2
(4.9 Sn
wt%).

D. Ballivet-Tkatchenko et al. / C. R. Chimie 14 (2011) 780–785
783
presently under study to check whether silica-supported
zirconium tert-butoxide behaves in a similar manner. After
a vacuum treatment at 130 8C, the BET surface area of silica
observed (Fig. 3b). As a matter of fact, the reaction was
found to be non-selective in DMC. Almost equal amounts
of DME were analyzed. Therefore, the additional accumu-
lation of water in the reactor likely contributes to DMC
decomposition with prolonged reaction time. Indeed, the
activity was restored after ex-situ reactivation of the
catalysts at 130 8C for 2 h under vacuum. After four
2
slightly decreased with supported tin from 196 to 181 m
ꢁ
1
2
ꢁ1
g
, and with supported zirconium to 186 m
g .
2.2. Dimethyl carbonate synthesis from methanol and CO
2
on
unsupported ZrO
2
and SnO and silica-supported catalysts
2
recyclings of ZrO
The same observation stands for SnO
2
, the DMC yield remained identical.
with which
2
The direct synthesis of dimethyl carbonate from
methanol and CO is usually conducted in the temperature
range 120–180 8C. The reaction takes place under CO
pressure due to the reversibility of carbon dioxide
insertion into M–OCH moieties leading to the methyl
carbonato species, M–OCO CH , a key species in the
reaction mechanism [18(b),25]. The catalytic experiments
herein described were performed at 150 8C under 200 bar
in a batch autoclave. Under these conditions, the reactants
are forming a single dense phase [26].
extensive recyclings were performed after reaction times
ranging from 12 to 60 h. With cumulative reaction
2
2
2
duration of 17 days with SnO , the SSA was slightly
decreased by 14%. Thus, it can be concluded that the
catalysts exhibit excellent stability at the lab scale.
Our next objective was to compare under the same
experimental conditions the activity of silica-supported
3
2
3
2 2
SnO and ZrO prepared from tin- and zirconium tetra-
butoxide, thus avoiding the issue of handling nanoparti-
cles. The metal loading was fixed at the saturation limit as
determined by the aforementioned grafting study, but the
impregnation method of the metal tetrabutoxide was
applied. As heating to 130 8C under vacuum induces the
elimination of the butoxy ligands (see precedent section),
the supported samples were pretreated under this condi-
tion giving a final tin and zirconium loading of 4.9 and 3.1
wt%, respectively. The corresponding catalysts were tested
for DMC synthesis.
Fig. 4 reports the DMC yield expressed as the DMC/M
(M= Zr, Sn) molar ratio with time for comparison with the
unsupported catalysts. As observed in Fig. 3 for the
unsupported oxides, the DMC yield increases steadily
with reaction time over several hours.
2 2
Our previous study with unsupported SnO and ZrO
catalysts showed that the DMC yield linearly increased
with the specific surface area (SSA) of the oxides [19]. High
2
ꢁ1
SSA up to 170 m
g
could be obtained by a sol–gel
procedure from tin- and zirconium tetra-tert-butoxide
thanks to the formation of oxide grains in the nanometric
range (4 nm in average). Fig. 3 reports the activity for DMC
synthesis expressed as the DMC/M (M= Zr, Sn) molar ratio
2
ꢁ1
for ZrO
2
and SnO
2
samples with SSA of 172 and 170 m g
(Fig. 3a), the DMC yield
,
respectively. In the case of ZrO
2
increases steadily for several hours of reaction before
reaching a plateau. The reaction was quite selective in
DMC. During the first 6 h of a run, no other organics could
be detected. However, dimethyl ether (DME) was found in
trace amounts at longer reaction times. Its formation is
likely due to methanol dehydration. The co-produced
water may inhibit DMC formation under batch conditions
as it accumulates in the autoclave which might explain the
DMC plateau after 80 h run.
With ZrO
2
/SiO
2
, a plateau is again observed but for
, i.e. 120 vs
higher reaction time than for unsupported ZrO
2
70 h (Fig. 4a). Supported SnO
compared to supported ZrO
2
exhibits a similar activity
during the first 80 h, a
2
reaction time also higher than for unsupported SnO
2
(Fig. 4b). Then, the rate of DMC formation is somewhat
decreased. DME was detected but not analyzed quantita-
tively. This will be done in the near future for assessing the
selectivity. However, it is likely that the formation of water
during DME synthesis hinders the reaction to DMC as for
2
Interestingly, the SnO sample exhibits a similar
activity to DMC at the beginning of the run. Nevertheless,
a noticeable decrease in DMC yield beyond the 100 h run is
Fig. 3. DMC/M molar ratio (M = Sn or Zr) as a function of reaction time at
Fig. 4. DMC/M molar ratio (M = Sn or Zr) as a function of reaction time at
2
ꢁ1
2
ꢁ1
1
50 8C and 200 bar: (a) ZrO
2
(172 m
g
) and (b) SnO
2
(170 m
g ).
2 2 2 2
150 8C and 200 bar: (a) ZrO /SiO (3.1 Zr wt%), (b) SnO /SiO (4.9 Sn wt%).

7
84
D. Ballivet-Tkatchenko et al. / C. R. Chimie 14 (2011) 780–785
unsupported SnO
2
. Indeed, the activity was restored after
4.1. BET surface area
ex-situ reactivation of the catalysts at 130 8C for 2 h under
vacuum.
Nitrogen adsorption data were obtained at –196 8C with
a Micromeritics ASAP 2020 apparatus. The specific surface
areas (SSA) were calculated from the BET method.
At the beginning of the runs, the rate of DMC formation is
nearly identical on tin- and zirconium-supported catalysts.
Importantly enough, there is a drastic enhancement in
comparison with unsupported catalysts. The observed
difference is of an order of magnitude which demonstrates
the superiority of the supported catalysts. Although it is
premature to draw any correlation between structure and
activity to rationalize these high turnover numbers, the
physicochemical properties of the supported active species
at low coverage may be influenced by the support
interaction. Further work is in progress for characterization
and optimization of the activity with metal loading. The
results of DMC formation on silica-supported tin oxide can
be compared with those we previously obtained with the
4.2. Transmission FTIR
The spectra were recorded at room temperature on a FT
-
1
1725X Perkin-Elmer spectrometer (2 cm
resolution)
equipped with IRDM software. A self-supported disk of
silica (ꢀ15 mg) was placed in a quartz cell equipped with
2
CaF windows, then evacuated to 450 8C for 12 h prior to
t
t
4 2 2 4
Sn(O Bu) adsorption. A CH Cl solution of Sn(O Bu) was
deposited dropwise onto the silica disk under argon in a
glove box, in order to get around 5 wt% of metal loading.
Then, the solvent was evacuated under vacuum at room
temperature for 4 h.
2 3 2 2 3 2
homogeneous nBu Sn(OCH ) and tertBu Sn(OCH ) pre-
cursors under the same experimental conditions [26,27].
TheorganometallicsaretotallyselectivetoDMCwhereastin
oxide also produces DME. The initial rate of DMC formation
during the first 6 h of the run is about twice with the
organometallics. However, they fastly desactivate due to
intermolecular rearrangement that leads to polynuclear tin
4.3. Temperature programmed desorption (TPD)
TPD experiments were conducted under a helium flow
3
ꢁ1
rate of 10 cm min
in a microreactor with 76 mg of
ꢁ1
sample at a heating rate of 6 8C min using a TCD detector
of a Shimadzu GC-14A. At the outlet of the TCD, a six-port
valve sampling allowed us to analyze the gas evolved by GC
2 2 2 2
species. The SnO /SiO and ZrO /SiO systems are much
more stable providing an almost constant rate for DMC over
6
0 h. The consequence is much higher turnover numbers.
using a FID detector. The sample was prepared from a
t
CH
2
Cl
2
solution of Sn(O Bu)
4
which was contacted with
2
ꢁ1
3
. Conclusion
pretreated silica (Aerosil Degussa 200, SSA = 196 m g ) at
room temperature for 2 h under stirring (Sn/SiO = 10 Sn
wt%). The obtained slurry was filtered, the solid washed
with aliquots of CH Cl , then dried under vacuum at 40 8C
2
We have shown that tin and zirconium oxides catalyze
the reaction of CO
2
with methanol to afford DMC. Using
2
2
silica-supported catalysts, it was possible to get a stable
activity over several hours at 150 8C and 200 bar pressure
as well as to achieve many recyclings without loss of
activity. The observed selectivity for DMC was much higher
with unsupported zirconium oxide and will be assessed
quantitatively on silica-supported catalysts for compari-
son. However, there is almost an order of magnitude in the
turnover numbers to DMC with the supported catalysts.
The turnover numbers are also much higher than with the
organotins we studied but selectivity and initial rate are
lower. Silica support likely contributes to promote and
stabilize the active species. Better dispersion and/or
specific interaction may be at the origin of this effect.
Overall, this work highlights for the first time that
supported tin- and zirconium oxides may compete with
soluble organotin precursors with a definite advantage for
the greenness of the catalysts.
for 3 h giving a final tin loading of 4.9 ꢂ 0.3 Sn wt%.
4.4. Catalytic experiments
Caution: since high pressures are involved, appropriate
safety precautions must be taken.
The catalysts (0.100–0.550 g) were pretreated under
vacuum at 130 8C for 2 h, then 20 ml of dried methanol was
added. The suspension was transferred under argon into a
120 ml batch stainless steel autoclave, followed by admis-
2
sion of 40 g of CO with an ISCO 260D pump to getting a
working pressure of 200 bar. The autoclave was heated to
150 8C (controlled by an internal K-thermocouple). DMC
yield was analyzed by GC (Fisons 8000, FID detector) as a
function of time by sampling with a high-pressure valve. At
the end of the experiments, degassing the autoclave at 0 8C
followed by GC analysis of the liquid phase allowed to check
consistency between the two procedures; the fit was better
than 3% (relative error). For recycling experiments, the
catalysts were recovered by centrifugation, pretreated at
130 8C for 2 h under vacuum, then reloaded into the
autoclave following the aforementioned procedure.
4
. Experimental
All the syntheses were performed under argon atmo-
sphere using the Schlenk tube technique. The solvents
purum grade) were deoxygenated, dried using standard
(
techniques, then stored under argon. Tin tetra-tert-
butoxide was prepared according to ref. [28] and zirco-
nium tetra-tert-butoxide was purchased from Aldrich.
Acknowledgements
Financial support from CNRS-INC and the CNRS
interdisciplinary program CPDD/RdR3 is greatly acknowl-
edged. S.V. thanks the Conseil r e´ gional de Bourgogne for a
postdoctoral grant.
2
ꢁ1
Silica (Degussa Aerosil 200, SSA = 196 m
g
) was
pretreated under vacuum at 450 8C for 12 h prior the
reaction with the metal alkoxides.

D. Ballivet-Tkatchenko et al. / C. R. Chimie 14 (2011) 780–785
785
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