Methyltrioxorhenium heterogenized on commercially available supporting materials as cyclooctene metathesis catalyst

Article abstract of DOI:10.1016/j.jorganchem.2005.07.051

Methyltrioxorhenium (MTO) is immobilized on various commercially available supporting materials by sublimation under vacuum. Up to 15 wt% MTO (10 wt% Re) can be immobilized on the different supporting materials. Metathesis experiments with cis-cyclooctene as substrate are performed aiming at a high yield of low molecular weight metathesis products (dimers, trimers, tetramers), which are interesting for aroma and flavour industries. Up to ca. 60% of these desired products can be obtained. A high rhenium loading leads usually to good catalytic activities of the materials in the cyclooctene metathesis. Prolonged reaction times lead to higher yields of the high molecular weight metathesis products. When the same catalyst material is used repeatedly in several catalytic runs, leaching of the grafted Re compounds plays an increasing role, resulting in a reduction of the catalyst performance. None of the examined commercially available carrier materials, however, presents an ideal means for the heterogenization of MTO to obtain particularly high amounts of the desired low molecular weight metathesis products of cyclooctene.

Full text of DOI:10.1016/j.jorganchem.2005.07.051

Journal of Organometallic Chemistry 690 (2005) 4712–4718
www.elsevier.com/locate/jorganchem
Methyltrioxorhenium heterogenized on commercially
available supporting materials as cyclooctene metathesis catalyst
Alexandra M.J. Rost, Horst Schneider, Jochen P. Zoller,
*
*
Wolfgang A. Herrmann , Fritz E. K u¨ hn
Lehrstuhl f u¨ r Anorganische Chemie der Technischen Universit a¨ t M u¨ nchen, Lichtenbergstrasse 4, D-85747 Garching bei M u¨ nchen, Germany
Received 11 May 2005; received in revised form 11 July 2005; accepted 11 July 2005
Available online 22 August 2005
Abstract
Methyltrioxorhenium (MTO) is immobilized on various commercially available supporting materials by sublimation under
vacuum. Up to 15 wt% MTO (10 wt% Re) can be immobilized on the different supporting materials. Metathesis experiments with
cis-cyclooctene as substrate are performed aiming at a high yield of low molecular weight metathesis products (dimers, trimers,
tetramers), which are interesting for aroma and flavour industries. Up to ca. 60% of these desired products can be obtained. A high
rhenium loading leads usually to good catalytic activities of the materials in the cyclooctene metathesis. Prolonged reaction times
lead to higher yields of the high molecular weight metathesis products. When the same catalyst material is used repeatedly in several
catalytic runs, leaching of the grafted Re compounds plays an increasing role, resulting in a reduction of the catalyst performance.
None of the examined commercially available carrier materials, however, presents an ideal means for the heterogenization of MTO
to obtain particularly high amounts of the desired low molecular weight metathesis products of cyclooctene.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Cyclooctene; Heterogeneous catalysis; Metathesis; Methyltrioxorhenium
1
. Introduction
of methyltrioxorhenium (MTO) [24]. Since tin contain-
ing co-catalysts are essential for the metathesis of
functionalized olefins [25,26], it was soon discovered
that MTO supported on acidic metal oxides forms
metathesis catalysts that are active without additives,
even for functionalized olefins [27]. Standard supports
are Al O -SiO , or Nb O and the activity was reported
to be related to the surface acidity [19,28–30]. A high
metathesis activity was observed when MTO is chemi-
sorbed on the surface. Evidence for a surface carbene
species was found to be somewhat controversial, but
there appeared to be a correlation between the catalytic
activity and the presence of alkyl fragments on the
surface [27–29,31]. It has been shown later, however,
Since the 1980s several research groups [1–17] deve-
loped homogeneous systems usable as highly active
W-, Mo- and Ru-based catalysts for metathesis. Some
of these catalysts were successfully immobilized on poly-
styrene [18]. The most active heterogeneous metathesis
catalysts known to date are based on the elements
rhenium, molybdenum and tungsten [19].
2
3
2
2
5
The system Re O /Al O is an effective heteroge-
2
7
2
3
neous catalyst for carrying out olefin metathesis under
mild conditions and its activity can be further increased
by the addition of tetraalkyl tin compounds [20–23].
This observation led to the methyl tin based synthesis
that matrix-isolated MTO tautomerizes to H C@Re-
2
(
OH)O under the influence of UV light; the carbene
*
2
Corresponding authors. Fax: +49 89 289 13473.
E-mail addresses: wolfgang.herrmann@ch.tum.de (W.A. Herr-
1
3
has been characterized in its normal, D- and C-enri-
ched isotopic forms by IR spectroscopy with results
mann), fritz.kuehn@ch.tum.de (F.E. K u¨ hn).
0
022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.051

A.M.J. Rost et al. / Journal of Organometallic Chemistry 690 (2005) 4712–4718
4713
in good agreement with quantum chemical calculations
32]. DFT calculations indicate that H C@Re(OH)O
lies nearly 90 kJ/mol higher in energy than MTO, and
[37–40]. Nevertheless, the application of MTO as hete-
rogeneous metathesis catalyst is of significant interest,
due to the very easy synthetic accessibility and the
usually high stability of this organometallic compound
[41,42].
[
2
2
hence is inaccessible under normal thermal conditions.
2
Using the complex CH ReO {(g -OSiH ) O} as a mod-
3
2
2 2
el, i.e., the condensation product of MTO with disilanol,
H Si(OH)} O, shows that the tautomeric H-atom
transfer occurs preferentially to an Re–O–Si bridging
C15–20-bodies of cyclic and unsaturated hydrocar-
bons are used in high quantities in aroma and flavour
industry. Such substances can be synthesized by meta-
thetic dimerization of cis-cyclooctene with Re O /
{
2
2
oxygen rather than to an Re@O unit. The resulting car-
2
7
2
bene species, H C@ Re{(g -OSiH )(O)(SiH OH)}, is
Al O catalysts pre-activated with tin compounds.
2 3
2
2
2
effectively stabilized by nearly 50 kJ/mol relative to
However, besides the lower weight metathesis products
considerable amounts of products, consisting of five
and more cyclooctene units are often obtained as
by-products (see Scheme 2) [43,44]. In this work, we
examine the application of supported MTO as catalyst
without pre-activation to compare the product yields
with the Re O /Al O catalyst system. We apply dif-
ferent commercially available carriers in order to find
out whether some of these systems could be utilized as
carrier materials for heterogeneous MTO without fur-
ther modifications and may be applied in the cyclooc-
tene metathesis.
2
CH ReO -{(g -OSiH ) O}. This alternative H-atom
3
2
2 2
transfer to the Re–O–Si bridge appears to be a more
plausible mechanism for carbene formation on the sup-
porting materials [32,33]. A surface compound of similar
composition is considered to be a plausible candidate for
the catalytic active species in heterogeneous MTO cata-
lyzed metathesis reactions by some researchers (see
Scheme 1) [32–36]. Isolated and fully characterized
Rhenium carbene complexes featuring Re in high oxida-
tion states and displaying also oxo ligands attached to
the rhenium carbene moiety are, however, still quite rare
2
7
2
3
O
O
OH
CH3
Si
CH3
Re
O
Si
O
-
H O
2
O
O
O
O
Re
O
O
+
O
O
O
O
Si
Si
O
OH
O
O
O
O
OH
H2C
CH3
Re
O
Si
Si
O
O
O
O
O
O
O
O
Re
O
Si
O
Si
O
O
O
O
2
Scheme 1. Reaction of methyltrioxorhenium (MTO) with a SiO -surface containing free –OH groups may lead to a Re-carbene species as suggested
by Morris et al. [33]. The presence of excess water (e.g., in a not sufficiently dried solvent) inverts the reaction, leading to the removal of MTO from
the surface.
C16
+
0.15-0.17 mol% [Re]
C24
+
C32
+
C40
+
C48
+
....
Scheme 2. Metathesis of cis-cyclooctene.

4714
A.M.J. Rost et al. / Journal of Organometallic Chemistry 690 (2005) 4712–4718
2
. Results and discussion
in most cases. This was already demonstrated previously
in quantitative experiments [27]. Only for samples 1 and
3 significant decomposition is observed. During the
immobilization of MTO on SiO /Al O the formation
2
.1. Supporting materials and catalyst loading
2
2
3
For our examinations, we tested primarily commer-
of methane, ethane and ethene resulting from the
decomposition of MTO was monitored by GC–MS.
The decomposition of MTO observed during the immo-
bilization process was <5% in all examined cases [27],
with the exception of 1 and 3, where it was significantly
higher. MTO without any co-catalyst in homogeneous
phase, however, is also not an active metathesis catalyst
with cyclooctene being the substrate.
cially available supporting materials with well-defined
surface areas (BET) and pore volumes (see Table 1).
MTO, being a highly volatile, easily sublimable solid
can be brought on the supporting material by sublima-
tion under vacuum. Excess MTO, which does not under-
go a reaction with the surface –OH groups is removed
from the surface under vacuum. The supporting material
has to be thermally pre-treated in order to reduce the
decomposition of the immobilized and easy hydrolysable
rhenium species [35].
2.2. Results of the metathesis experiments
In the immobilization experiments an excess of MTO
2.2.1. Batch experiments
(
15 wt% with respect to the supporting material) has
In order to test the activity of the prepared heteroge-
neous catalyst systems, metathesis experiments with
0.15–0.17 mol% Re, with respect to the substrate
cyclooctene, at 50 °C were performed with n-hexane
being the solvent (Scheme 1). The results of the catalysis
experiments are summarized in Table 2. None of the
supporting materials catalyzed the cyclooctene metathe-
sis under the applied conditions when no MTO was
grafted on it prior to application.
Aiming at a high yield of lower homologues, which
are used in aroma and flavour industry no highly active
metathesis catalyst is needed. Such catalysts produce
rapidly a quite large amount of higher homologues
due to their fast conversion of the starting material. A
catalyst with lower activity and conversion gains a high-
er yield of lower homologues.
been used to reach a maximum Re loading of the respec-
tive supporting materials. The rhenium content of the
product materials was determined by elementary analysis
(
EA). A high quantity of molybdenum in the supporting
material 9 prevents the rhenium content determination
by elementary analysis. The rhenium content is in this
case calculated from the amount of resublimated MTO
after immobilization. This method has proven to be a
reliable method to determine the Re content also in cases
where no molybdenum is present and the Re content of
the layer can be independently determined by EA. The
extremely low Re loading in the cases of 9, 10 and 13
originates from the low specific surface area (BET) <
5
for reaction ( Si CP-MAS-NMR evidence).
2
m /g and the few surface hydroxyl groups available
2
9
The resulting Re/C ratio being approximately 1:1 in
most examined cases shows that MTO decomposition
during the immobilization process is not pronounced
Material 2 having a higher MTO loading, a higher
specific surface area and a higher pore volume than
material 1 is a good metathesis catalyst, particularly
Table 1
Prepared catalysts
a
b
2
3
No.
Supporting material
Ratio (wt%)
Re (wt%)
Re:C
BET (m /g)
Pore volume (cm /g)
1
2
3
4
5
6
7
8
9
SiO
SiO
2
/Al
/Al
2
O
O
3
84/16
87/13
82.5/3.5/14
100
100
100
86/14
96.5/3.5
100
2.0
8.0
8.1
8.1
9.0
10.0
8.7
9.8
0.6
1.4
1.0:2.6
1.0:1.0
1.0:1.8
1.0:1.0
1.0:1.1
1.0:1.0
1.0:1.4
1.0:0.9
–
170
264
0.4
0.7
0.3
0.3
0.3
0.3
0.3
0.3
–
2
2
3
Al
2
Al
2
Al
2
Al
2
Al
2
Al
2
O
O
O
O
O
O
3
3
3
3
3
3
/CoO/MoO
3
<200
<200
<200
<200
<200
<200
<5
c
d
e
/MoO
/CoO
3
MoO
CoO
3
1
1
1
1
0
1
2
3
a
100
–
–
–
c
e
Al
Al
2
O
O
3
/CoO/MoO
/CoO/MoO
3
82.5/3.5/14
82.5/3.5/14
100
7.5
8.7
<0.1
1.0:1.1
1.0:1.1
–
<200
<200
<5
0.3
0.3
–
2
3
3
Nb
2
O
5
Relative error: ± 2%.
For Re relative error: ± 2%, for C absolute error: ± 0.3%.
b
c
Neutral.
Alkaline.
Acid.
d
e

A.M.J. Rost et al. / Journal of Organometallic Chemistry 690 (2005) 4712–4718
4715
Table 2
amounts, however, were in most cases quite similar to
those listed in Table 2 (50 °C reaction temperature).
Results of the metathesis experiments with cis-cyclooctene as substrate
at 50 °C (reaction time: 24 h; 0.15–0.17 mol% Re:cyclooctene)
a
b
No.
Cyclooctene
Lower
Higher
2.2.2. Time-dependent experiments
Time-dependent metathesis experiments were perfor-
c
c
c
(%)
homologues (%)
homologues (%)
1
2
3
4
5
6
7
8
9
100
–
–
1
1
–
–
med in a tubular reactor in order to evaluate the long
time effect on selectivity and catalytic activity. In the
tubular reactor unit, the reaction medium was circulated
(flow rate 5 mL/min. The results are shown in Table 4).
The product distribution is similar to the batch expe-
riments. The conversion is completed after 7 h. If the
reaction is continued, the dimer concentration decreases
and the concentration of the higher homologs increases
slowly. The higher viscous components of the reaction
mixture, however, cause technical problems. After 2
days the reaction medium is so viscous that the flow rate
has to be increased. In consequence the contact time
changes and the steady state model can not be applied
any more.
57
46
60
59
58
53
61
–
43
54
39
40
38
47
39
–
4
–
–
100
100
4
1
1
1
1
0
1
2
3
a
–
–
58
61
–
38
39
–
–
100
Sum of dimer, trimer and tetramer.
Sum of pentamer and higher homologues.
Relative error: ± 5%.
b
c
2
.3. Leaching-experiments
with respect to forming the lower molecular weight
products. Materials 1, 9, 10 and 13 display no metathesis
activity.
An important parameter in heterogeneous reactions
is the stability of the heterogeneous catalyst. It can be
determined by leaching tests. Catalyst 3 was refluxed
with the reaction mixture at 50 °C for 24 h. Then the
solution was filtered off, the catalyst was washed and
new substrate was added. Two further cycles were
performed. After three cycles 4 wt% Re were detected
by EA to remain on the supporting material. This
amount is approximated half of original Re content.
The product yield decreases from nearly quantitative
The comparatively low Re loading of these materials
seems not to be sufficient to obtain metathesis activity.
A larger content of heavier metathesis products is
observed with material 3. Materials 2, 4–8, 11, and 12,
having all a Re loading between 7.5% and 10% display
a comparable metathesis activity. In all these cases the
amount of the desired lower weight metathesis products
is somewhat higher than the amount of the heavier
metathesis products.
(first cycle) to 63% (second cycle) to 10% (third cycle).
Some of the catalysts leading to the favoured reaction
products were tested at a lower reaction temperature
No free MTO was detectable by NMR in the con-
centrated washing solution. A deactivation process,
removing the Re compound from the surface leads to
perrhenates (EA evidence) and the formation of meth-
ane, ethane, etc.
(
20 °C), in order to see whether this would additionally
contribute to a higher amount of lower molecular
weight metathesis products (see Table 3). The observed
Another deactivation process of the heterogeneous
catalyst occurs if wet solvent is present. Water promotes
the catalyst deactivation by leading to the formation of
free MTO (inactive in olefin metathesis), which can be
detected by NMR and IR spectroscopy and perrhenic
Table 3
Results of the metathesis experiments with cis-cyclooctene as substrate
at room temperature (reaction time: 24 h; 0.15-0.17 mol% Re:cycloc-
tene; 20°C)
a
b
No.
Cyclooctene
Lower
Higher
c
c
c
(%)
homologues (%)
homologues (%)
d
2
–
2
4
–
7
–
–
–
56
49
45
58
48
59
49
55
44
49
51
42
45
41
51
45
Table 4
3
4
5
6
7
8
Results of the continuous metathesis experiments with cis-cyclooctene
with catalyst 2 (Re:substrate: 1.3 mol%; 20 °C)
a
b
Time (h)
Cyclooctene
Lower
Higher
c
c
c
(
%)
homologues (%)
homologues (%)
7
24
48
a
–
–
–
55
50
47
45
50
53
1
1
a
b
c
Sum of dimer, trimer and tetramer.
Sum of pentamer and higher homologues.
Relative error: ± 5%.
Sum of dimer, trimer and tetramer.
Sum of pentamer and higher homologues.
Relative error: ± 5%.
b
c
d
In chloroform soluble components.

4716
A.M.J. Rost et al. / Journal of Organometallic Chemistry 690 (2005) 4712–4718
higher homologues of
cyclooctene
H C
2
O
HO
Si
O
O
O
Si
H2C
Re
O O
HO
Si
O
O
Si
CH
HC
Re
O
O
CH
Re
O
O
HO
Si
O
HO
Si
O
O
Re
O
O
O
Si
Si
HO
Si
O
O
Si
Scheme 3. Hypothetical mechanism of the metathesis of cis-cyclooctene with a heterogeneous MTO catalyst, based on a ‘‘Scherer-type’’ catalytic
active species.
1
7
acid as decomposition product. If O-labelled water is
added, the labelled oxygen is included both into the
formed MTO and into the perrhenic acid. This reaction
may be explained by an inversion of the catalyst forma-
tion reaction (see Scheme 1).
4. Experimental
4.1. General procedures
All experiments were carried out under argon atmo-
sphere using glove box and Schlenk technique. All
solvents were dried and distilled under argon with the
standard methods. Nitrogen gas was deoxygenated by
a BTS-catalyst and dehydrated by molecular sieves
3
. Conclusions
˚
Methyltrioxorhenium (MTO) can be heterogenized
(4 A) prior to use.
by simple sublimation on several commercially available
supporting materials. A quite high Re loading of ca.
NMR spectra were measured on a Bruker Avance
1
DPX-400. H NMR spectra were obtained at 400.13
1
3
29
1
0 wt% can be reached with some of these materials.
MHz, C NMR spectra at 100.85 MHz. Si-CP-
MAS-NMR spectra are recorded at 79.49 MHz, with a
All materials containing a reasonable Re loading main-
tain cyclooctene metathesis without any additives. The
amount of lower weight metathesis products (consisting
of less than four cyclooctene units), which are interesting
for the aroma and flavour industry are obtained at room
temperature and 50 °C in ca. 50–60% yield after 24 h
reaction time. Longer reaction times lead to a slow
increase of the content of higher homologues. During
several runs the catalyst materials loose Re due to
catalyst decomposition. This leads to a considerable
reduction of the metathesis activity. A hypothetical reac-
tion mechanism, based on previously published examin-
ations is shown in Scheme 3. This mechanism might help
to explain the observed water sensitivity of the heteroge-
neous systems, however, further work will be necessary
to elucidate the real mechanism of such reactions.
Further work in this direction is currently under way
in our laboratory.
1
(9.4 T) Bruker MSL 400P spectrometer, with 5.5 ls H
90° pulses, 8 ms contact time, a spinning rate of
4.5 kHz and 4 s recycle delays. The molar mass distribu-
tion of the metathesis products was obtained by GPC.
The solvent in the samples of the reaction mixture were
distilled off and replaced by CHCl for the measure-
3
ments. The determination was performed at a Hewlett-
Packard HP series 110 gel permeation chromatograph
3
5
with two polystyrene cross-linked columns (10 and 10
˚
A), refraction index detectors and UV-detectors. The
support morphology was obtained by the BET-method
on an AS AP 2010 nitrogen sorptionporosimeter (Micro-
meritics).
4.2. Synthesis procedures
4.2.1. Preparation of the supporting material
The obtained results show that despite of some diffe-
rences in the obtained yields of the desired lower mole-
cular weight products, none of the yet commercially
available carrier materials presents an ideal supporting
system for MTO to be applied in the catalytic cyclooc-
tene metathesis.
The following commercial available products were
used as supporting materials. Materials 7, 8, 11 and 12
are mixed substances consisting of the corresponding
pure metal oxides (see Table 5).
For dehydration of the supporting materials under
vacuum and nitrogen flow a tube furnace (Heraeus,

A.M.J. Rost et al. / Journal of Organometallic Chemistry 690 (2005) 4712–4718
4717
Table 5
Supporting material
and cooled with liquid nitrogen to avoid the loss of
MTO during the evacuation. MTO was sublimated on
the support under vacuum. To get evenly distributed
sublimation the flask was regularly shaken. After 24 h
reaction time a pre-weighed and liquid nitrogen cooled
cold trap was installed between the flask and the vacuum
line and the excess of MTO was removed within 5 h. The
immobilized amount of MTO was calculated from the
amount of removed MTO and determined by Re
elemental analysis. The resulting solid with immobilized
rhenium was used as metathesis catalyst.
No. Supporting material Ratio (wt%) Distributor
1
2
3
4
5
6
7
8
9
0
1
2
3
a
SiO
SiO
2
/Al
/Al
2
O
O
3
84/16
87/13
82.5/3.5/14
100
100
100
86/14
96.5/3.5
100
Degussa-H u¨ ls AG
STREM
STREM
Merck
Merck
2
2
3
Al
Al
Al
Al
Al
Al
MoO
CoO
Al
Al
2
2
2
2
2
2
O
3
O
3
O
3
O
3
O
3
O
3
/CoO/MoO
3
a
b
c
c
Merck
/MoO
/CoO
3
Merck/STREM
Merck/STREM
STREM
c
3
1
1
1
1
100
Aldrich
a
c
2
O
O
3
/CoO/MoO
/CoO/MoO
3
82.5/3.5/14
82.5/3.5/14
100
Merck/Aldrich/STREM
Merck/Aldrich/STREM
STREM
4.3. Metathesis experiments
2
3
3
2 5
Nb O
Twelve millilitre of a 0.1 M cis-cyclooctene/n-hexane
solution were added to 300 mg catalyst under argon
atmosphere. The mixture was stirred, as indicated, for
Neutral.
Alkaline.
Acid.
b
c
24 h at 50 °C or room temperature. After this samples
for analysis were taken. Depending on the Re content
of the catalyst the reactions were performed with a
0.15–0.17 mol% ratio of immobilisized rhenium:cyclo-
octene.
Time-dependent metathesis experiments were done in
a tubular reactor (Scheme 4) 500 mL 0.1 M cis-cyclooc-
tene/n-hexane solution were filled in a storage vessel
and catalyst 2 (9.6 g, 1.3 mol% [Re]) in the tubular
reactor (chromatographic column, Aldrich, L = 30 cm,
Typ ROK/A 4/30) was used. Support 1 was first heated
at 400 °C for 12 h in a nitrogen flow in order to obtain
dehydrated silica. All other supporting materials were
heated at 200 °C for 3 h in a nitrogen flow.
4
.2.2. Immobilization of MTO on the supporting material
After dehydration MTO was added to the supporting
material under argon atmosphere. The flask was closed
Scheme 4.

4718
A.M.J. Rost et al. / Journal of Organometallic Chemistry 690 (2005) 4712–4718
(c) C. Coperet, New J. Chem. 28 (2004) 1;
(
9
(
´
d = 1.5 cm) under argon atmosphere. The displacement
of the pump was 5 mL/min. Samples were taken after
d) M. Chabanas, C. Cop e´ ret, J.M. Basset, Chem. Eur. J. 9 (2003)
71;
e) C.B. Rodella, J.A.M. Cavalcante, R. Buffon, Appl. Catal. A
74 (2004) 213;
7
, 24 and 48 h.
2
4.4. Leaching-experiments with the immobilisized MTO
systems
(f) C.B. Rodella, R. Buffon, Appl. Cat. A 263 (2004) 203;
(g) T. Oikawa, T. Ookoshi, T. Tanaka, T. Yamamoto, M. Onaka,
Micropor. Mesopor. Mater. 74 (2004) 93.
[
[
20] G.C.N. van den Aardweg, R.H.A. Bosma, J.C. Mol, J. Chem.
Soc., Chem. Commun. (1983) 262.
21] E. Verkuijlen, F. Kapstein, J.C. Mol, C. Boelhouwer, J. Chem.
Soc., Chem. Commun. (1977) 198.
Three hundred milligram of catalyst 3 were added to
2 mL 0.1 M cis-cyclooctene/n-hexane solution under
1
argon atmosphere. The mixture was refluxed at 50 °C
for 3 days. After each 24 h a sample was taken, the cat-
alyst was filtered, washed three times with 10 mL hexane
and new substrate was added. Two further cycles were
performed. Then the catalyst was filtered, washed, dried
under vacuum and the Re content of the material was
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