Ruthenium carbenes supported on mesoporous silicas as highly active and selective hybrid catalysts for olefin metathesis reactions under continuous flow

Article abstract of DOI:10.1002/chem.201203893

In the search for a highly active and selective heterogenized metathesis catalyst, we systematically varied the pore geometry and size of various silica-based mesoporous (i.e., MCM-41, MCM-48, and SBA-15) and microporous (ZSM-5 and MWW) versus macroporous materials (D11-10 and Aerosil 200), besides other process parameters (temperature, dilution, and mean residence time). The activity and, especially, selectivity of such "linker-free" supports for ruthenium metathesis catalysts were evaluated in the cyclodimerization of cis-cyclooctene to form 1,9-cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry. The optimized material showed not only exceptionally high selectivity to the valuable product, but also turned out to be a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex. In the ring-opening/ring-closing metathesis (RO-RCM; see picture) of cis-cyclooctene, mesoporous silicas (i.e., MCM-41, MCM-48, SBA-15) are preferred supports for homogeneous ruthenium carbenes (see picture). The optimized "linker-free" support showed not only exceptionally high selectivity toward 1,9-cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry, but is also a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex. Copyright

Full text of DOI:10.1002/chem.201203893

FULL PAPER
DOI: 10.1002/chem.201203893
Ruthenium Carbenes Supported on Mesoporous Silicas as Highly Active
and Selective Hybrid Catalysts for Olefin Metathesis Reactions
under Continuous Flow
Miriam Bru,[a] Richard Dehn,[b] J. Henrique Teles,[b] Stephan Deuerlein,[b]
Manuel Danz,[b] Imke B. Mꢀller,[c] and Michael Limbach*[a, b]
Abstract: In the search for a highly
active and selective heterogenized
metathesis catalyst, we systematically
varied the pore geometry and size of
various silica-based mesoporous (i.e.,
MCM-41, MCM-48, and SBA-15) and
microporous (ZSM-5 and MWW)
versus macroporous materials (D11-10
and Aerosil 200), besides other process
parameters (temperature, dilution, and
mean residence time). The activity and,
especially, selectivity of such “linker-
free” supports for ruthenium metathe-
sis catalysts were evaluated in the cy-
clodimerization of cis-cyclooctene to
form 1,9-cyclohexadecadiene, a valua-
ble intermediate in the flavor and fra-
grance industry. The optimized materi-
al showed not only exceptionally high
selectivity to the valuable product, but
also turned out to be a truly heteroge-
neous catalyst with superior activity
relative to the unsupported homogene-
ous complex.
Keywords: cyclooligomerization
·
macrocycles · mesoporous materi-
als · metathesis · ruthenium · sup-
ported catalysts
Introduction
als are mostly based on the late-transition-metal ruthenium,
albeit there are examples with early-transition-metal com-
plexes.[11] The disadvantageous leaching of precious metal
from the support[12] can partially be overcome by a covalent
link between the surface and, for example, phosphine li-
gands,[13] N-heterocyclic carbenes (NHCs),[14] alkylidene
groups,[15] or ligands that serve as surrogates for anionic li-
gands.[16] Thus, besides the ligands, the obligatory solid sup-
ports, that is, various polymers,[13b] silicas,[17,18c] aluminas,[18]
or polymer/silica hybrids,[19] require appropriate and often
laborious functionalization.
In turn, truly heterogeneous hybrid metathesis catalysts
based on a weak physical interaction (i.e., in the dimension
of a hydrogen-bond bridge)[20] between an unmodified orga-
nometallic ruthenium carbene species and an inorganic sur-
face are scarce and were first reported by Sels and co-work-
ers[20] and later others.[21] We have developed tools to identi-
fy suitable pairs of organometallic complex and inorganic
support based on such a weak physical interaction,[22,23] and
indeed a Grubbs/Hoveyda 2 complex supported on the com-
mercially available macroporous silica D11-10 enabled the
cross-metathesis of even polar substrates (i.e., methyl
oleate) and the ring-opening/ring-closing metathesis (RO-
RCM) of cycloalkenes with high catalytic turnover.[24]
The catalyst and reactor concepts of the bulk-chemical in-
dustry, that is, continuous processing with heterogeneous
catalysts, are gaining slowly ground in the fine-chemical and
pharmaceutical industry.[1] This development is also true in
the field of olefin metathesis,[2] in which classical heteroge-
neous catalysts have been of limited applicability in pharma-
ceutical and fine-chemical process development[2–4] due to
the either harsh reaction conditions (e.g., WO3, >3008C)[5]
or the coactivators (e.g., tetraalkylstannanes for Re2O7)[6] re-
quired for the high catalytic activity of those systems. On
the other hand, highly active homogeneous early-[7] or late-
transition-metal carbenes[8] lack the processability and over-
all lifetime of classical heterogeneous catalysts,[9] and the
separation of the catalyst or catalyst-deactivation products
from the valuable product remains a challenge. Consequent-
ly, immobilization concepts have been developed to com-
bine the advantages of homogeneous catalysts with those of
heterogeneous metathesis catalysts.[10] Those hybrid materi-
[a] Dr. M. Bru, Dr. M. Limbach
CaRLa–Catalysis Research Laboratory
Im Neuenheimer Feld 584, 69120 Heidelberg (Germany)
Fax : (+49)621-60-6648957
RO-RCM allows the one-step assembly of macrocyclic
rings, which are attractive intermediates in the flavor and
fragrance industry;[3,24–26] for example, 1,9-cyclohexadeca-
diene would be easily available from cis-cyclooctene (COE;
Scheme 1, i=1), if the intrinsic selectivity issue between pol-
ymerization and selective cyclization could be overcome in
favor of a maximal selectivity in the cyclic dimer (2 mer).
Ways to prevent the formation of polymers by ring-opening
E-mail: michael.limbach@basf.com
[b] Dr. R. Dehn, Dr. J. H. Teles, Dr. S. Deuerlein, Dipl.-Ing. M. Danz,
Dr. M. Limbach
Process Research & Chemical Engineering
BASF SE, Carl-Bosch-Strasse 38, 67056 Ludwigshafen (Germany)
[c] Dr. I. B. Mꢀller
Quantum Chemistry, BASF SE
Carl-Bosch-Strasse 38, 67056 Ludwigshafen (Germany)
Chem. Eur. J. 2013, 19, 11661 – 11671
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ated in suspension in the
RO-RCM of cis-cyclooctene
(Scheme 2,
Table 1).
The
course of the reaction was
monitored by GC analysis, and
the heterogeneous nature of
the catalyst was ensured by a
split test.[31] As obvious from
previous studies,[3] determina-
tion of the selectivity of the
catalyst solely based on GC
area (%) is misleading, as all
higher oligomers and polymers
Scheme 1. Ring-chain equilibria and schematic reaction network in the RO-ROMP of cis-cyclooctene.
metathesis polymerization (ROMP; Scheme 1)[27] are 1) a ki-
netic-reaction regime as opposed to a thermodynamically
driven reaction, 2) high dilution, 3) low temperature, and
4) minimal conversion, which are all detrimental conditions
from a process point of view.
that are untraceable by GC analysis remain undetected, thus
leading to high, but biased selectivity in favor of oligomers
smaller than the pentamer. Thus, we developed a GC meth-
odology that allowed not only the reliable analysis of the
The positive effect of meso-
Table 1. Characterization of the materials employed in the current study.
porous molecular sieves on the
activity of supported catalysts
has already been demonstrated
in the metathesis reaction[28,29]
and beyond.[30] This effect is
why we were interested in a
systematic survey on the influ-
ence of a “linker-free” hybrid
catalytic material on the selec-
tivity in the RO-RCM of cis-
cyclooctene to 1,9-cyclohexa-
Material
Ø
V
A
A
N2 BET
[m2 gÀ1
[Ru]
[%]
Loading
[%]
Split
[a]
[a]
[ꢂ][a]
[mLgÀ1
]
[m2gÀ1
]
U
]
MCM-41[b,e]
MCM-41[b,e]
MCM-41[b,e]
MCM-41[b,e]
MCM-41[e,f]
MWW[c,f]
90
0.77
0.89
1.70
1.30
3.10
4.72
4.72
–
0.56
9.51
2.22
2.58
1.68
0.88
2.84
364
39
409
668
313
218
325
293
0.099
0.079
0.083
0.10
0.20
–
0.16
0.17
0.16
0.22
0.17
0.20
0.22
0.17
0.21
96
90
100
100
95
–
73
81
76
100
81
95
100
81
100
+
+
+
+
+
–
140
160
180
180
4
232
172
235
132
132
–
MWW[c,f]
4
631[g]
–
+
Alumina[f]
ZSM-5[e]
macro
180
60
70
95
macro
370
60
+
[h]
67
750
954
357
343
–
–
SBA-15[e,f]
Aerosil[f,i]
408
367
415
262
106
314
+
ACHTUNGTRENNUNGdecadiene. Besides various
+
Aerosil[f,j]
D11-10[f,i]
+
[h]
process parameters (tempera-
ture, dilution, and mean resi-
dence time (MRT)), we sys-
tematically varied the pore
size and geometry of various
silica-based mesoporous (i.e.,
MCM-41, MCM-48, and SBA-
15), microporous (ZSM-5 and
MWW) versus macroporous
supports (D11-10 and Aerosil 200).
+
D11-10[e,f]
MCM-48[e,f]
–
1252
+
[h]
[h]
+
[a] Hg porosimetry; Ø=pore diameter. [b] Strained with Ludox and poly(ethyleneoxide) (PEO; 1–1.6 mm).
[c] Deboronated; particle size: 0.6–1 mm; trimodal macroporosity: 12000, 6200, and 1800 ꢂ. [d] Strained with
Ludox and PEO (0.6–1 mm). [e] Calcined at 5008C. [f] Powder. [g] Langmuir [m2 gÀ1]. [h] Isomerization.
[i] Uncalcined. [j] Calcined at 5508C.
starting material cis-cyclooctene, but also of all cyclic
oligomers up to the pentamer.[32] cis-Cyclooctene and the
dimer were calibrated from commercially available samples
Results and Discussion
of cis,cis-1,9-cyclohexadecadiene[33] and the other cyclic
oligomers of cyclooctene (3–5 mers) were assumed to have
the same response factor as the dimer. This approximation
is good because the ratios of C/H atoms and double bonds/
C atoms are identical for all the oligomers, and the response
factors of cyclooctene and the dimer are also almost identi-
cal.
Preparation of materials and characterization of supports:
First, we prepared the Grubbs/Hoveyda 2 complex on vari-
ous mesoporous and nonmesoporous supports. The activity
and selectivity of the thus-obtained materials was first evalu-
MCM-41,[34] MCM-48,[35] SBA-15,[34,36] ZSM-5,[37] and
MWW[38] were prepared according to reported procedures,
activated alumina (Wako Presep-C), D11-10, and Aero-
sil 200 were purchased from commercial sources.[39] As de-
scribed previously, the hybrid materials were prepared by
simple impregnation of a suspension of the inorganic sup-
port in a solution of the Grubbs/Hoveyda 2 complex 1 in
Scheme 2. RO-RCM of cis-cyclooctene to 1,9-cyclohexadecadiene
(amongst other oligomers and polymers; see Table 1 and Figure 1a for
details).
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Mesoporous-Silica-Supported Ru Catalysts for Olefin Metathesis Reactions
FULL PAPER
CH2Cl2 for 3 hours, removal of
the solvent, and drying of the
material under vacuum at a
maximum
of
408C.[20,22,24]
Some materials had been cal-
cined prior to loading with the
complex (Table 1). Although
the initial loading of 1 on all
the materials was high (0.079–
0.22 wt% ruthenium corre-
sponds to 73–100% of the ini-
tial complex; Table 1) and re-
sulted in green materials, the
alumina-based material 1@alu-
mina decomposed rapidly as
indicated by a color change of
green to gray overnight. This
intrinsic instability of 1 on alu-
mina might be a consequence
of the Lewis acidic properties
of the support and opens
routes to catalyst deactivation,
as already described by Sels
and co-workers[20] and us.[22,24]
In suspension, all the materials
worked as heterogeneous cata-
lysts in the RO-RCM of cis-cy-
clooctene, as indicated by a
split test, apart from 1@ZSM-
5, in which 1 leached off the
support.
Despite of their good activi-
ty in the conversion of cis-cy-
clooctene (53, 99, 92, and
100% conversion after 1 h;
Figure 1a),
1@MCM-48,
1@ZSM-5, and calcined mac-
roporous materials 1@D11-10
and 1@Aerosil 200 led to sig-
nificant isomerization of the
olefin double bonds. This side
reaction is absolutely detri-
mental because it leads,
through secondary metathesis,
to the unsymmetrical cleavage
of the so-far symmetrical
double bonds in 1,9-cyclohexa-
decadiene to, for example, cy-
cloheptene and cyclononene
instead of two equivalents of
cis-cyclooctene. Over time, this
led to an inseparable mixture
of homologous macrocycles
(Scheme 3).
Figure 1. a) Activity test in suspension. b) Selectivity toward small oligomers (2–5 mers), oligomers (6–
20 mers), and polymers (>20 mer) as analyzed by GC and GPC.
The silicate ZSM-5 is
Brønsted acidic, which might
be the reason for the double-
Scheme 3. Byproducts formed in the RO-RCM of cis-cyclooctene by double-bond isomerization and unsym-
metrical metathetical cleavage of C16H28.
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11663

M. Limbach et al.
bond isomerization activity; indeed, Brønsted acidic surface
silanol groups might have also been present after calcina-
tion[40] on nonmesoporous materials 1@Aerosil 200 and
1@D11-10 at approximately 5008C prior to loading with
complex 1. Although their uncalcined counterparts and
1@MCM-41 did not show this undesired isomerization activ-
ity, we cannot yet exclude that it is a consequence of hy-
drides formed in the course of the reaction.[41]
In suspension, 1@MCM-41 turned out to be the most
active material, closely followed by 1@Aerosil 200 (uncal-
cined) and 1@SBA-15 (99, 100, and 97% conversion after
1 h; Figure 1a). All three ma-
indicated by the closed mass balance, we considered MCM-
41 to be the preferential support for further studies under
continuous-flow conditions to address the influence of pore
size of this selected support on selectivity. Four MCM-41-
type silicates with pore diameters of 90, 140, 160, and 180 ꢂ
were prepared by employing amine templates of various
chain lengths,[34] and the thus-derived powders were molded
with Ludox[42] and PEO and calcined in air (to remove the
PEO binder) to obtain, after splitting and sieving, shaped
catalyst bodies with a mean diameter of 1–1.6 mm. This
molding had no influence on the pore diameter, but pore
terials were at least as active
as the homogeneous complex
1 itself under otherwise identi-
cal conditions (91% conver-
sion after 1 h; [COE]=100 mm
in CH2Cl2 instead of cyclo-
hexane due to its better solu-
bility, [COE]/ACHTUNGTRENNUNG[cat.]= ~100:1 at
208C). Materials 1@MCM-48,
1@D11-10 (uncalcined), and
1@MWW (uncalcined) turned
out to be significantly less
active (53, 51, and 44% con-
version after 1 h).
There was a clear effect of
pore geometry on selectivity in
the RO-RCM of cis-cyclooc-
tene (Figure 1b). Not only did
1@D11-10 and 1@Aerosil 200
form a significant amount of
higher oligomers (6–20 mers),
they yielded >10 wt% of poly-
mers (>20 mer), which pre-
cipitated during the reaction
due to their low solubility.
SBA-15 formed up to 45% of
those polymers, despite having
a narrow pore size (60 vs. 70
and >300 ꢂ), which is similar
to the reaction catalyzed by
the homogeneous complex
itself. This outcome might be
an indication that the complex
did not reside in the pores of
the SBA-15, but was deposited
on the outside of the particles,
thus leading to the low selec-
tivity. Although 1@MCM-41
gave the lowest selectivity of
the valuable dimer, it was
most selective for the smaller
oligomers amongst all the ma-
terials (>60% for <20 mer).
As this MCM-41 does not lead
to the formation any polymer,
Figure 2. a) Activity over time under continuous-flow conditions. b) Conversion after 1 h at [COE]0 =50 mm.
Selectivity at 30% (for 1@MCM-41 with pore sizes of 90 and 140 ꢂ) and 70% conversion (for 1@MCM-41
with pore sizes of 160 and 180 ꢂ).
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Mesoporous-Silica-Supported Ru Catalysts for Olefin Metathesis Reactions
FULL PAPER
volume and BET surface of the molded catalyst bodies were
lower than those of the corresponding powders (Table 1, en-
tries 4 and 5).
initially the least active (27 and 53% conversion after 1 h),
but the first material kept its activity over 400 minutes at
much with a less-pronounced deactivation tendency relative
to materials 1@MCM-41 with pore sizes of 140, 160, or even
180 ꢂ.
We benchmarked the activity and selectivity obtained
with mesoporous materials 1@MCM-41 against strands of
the reference material 1@D11-10 (Figure 2a).[24] The initial
activity of all the mesoporous materials was measured at
[COE]0 =25 and 50 mm (i.e., 0.77 and 2.00 wt%) and turned
out to be significantly higher throughout the whole experi-
ment (400 min) than that of 1@D11-10, which started at ap-
proximately 25% conversion and deactivated fast. Never-
theless, the activity of the mesoporous materials correlated
well with their pore-size. Although the materials 1@MCM-
41 with pore sizes of 160 and 180 ꢂ started at similar initial
activities (i.e., 92 and 94% conversion after 1 h; [COE]=
50 mm in cyclohexane), the material with the larger pores
deactivated faster (77 and 45% conversion after 400 min).
Materials 1@MCM-41 with pore sizes of 90 and 140 ꢂ were
The same activity trend was observed for materials
1@MCM-41 with pore sizes of 160 and 180 ꢂ at [COE]=
25 mm in cyclohexane (52 and 77% conversion after 1 h).
Assuming that most of the catalytically active sites are locat-
ed at the inner surface of the materials, the reactivity is
most likely controlled by the diffusion rate of the substrate
into the pore, which is higher at larger pore diameters (Fig-
ure 1a). To our surprise, there was only a minor, but never-
theless clear effect on the selectivity toward the dimer and
all the other oligomers traceable by GC analysis up to the
pentamer: At a smaller pore size, the selectivity towards the
dimer and small oligomers (2–5 mers) was higher, for exam-
ple, 29 and 78% for 1@MCM-41 with a pore size of 90 ꢂ
Figure 3. a–c) Geometric dimensions of the three isomers of C16H28 as calculated by using DFT calculations. d) Schematic representation of a likely bind-
ing mode of the complex on the inorganic support.
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M. Limbach et al.
versus 26 and 69% for 1@MCM-41 with a pore size of
140 ꢂ (both at 30% conversion), respectively, or 25 and
75% for 1@MCM-41 with a pore size of 160 ꢂ versus 25
and 73% for 1@MCM-41 with a pore size of 180 ꢂ (both at
70% conversion; Figure 2b), respectively.
Analysis by gel-permeation chromatography (GPC) of the
crude reaction mixture obtained with 1@D11-10 revealed
that greater than 7% of the starting material was converted
into oligomers from >13 mer up to a 48 mer, which repre-
sented the upper detection limit of our GPC setup. This
reason might explain the fast deactivation of the supported
catalyst, whereas the activity
SBA-15[20,23,44] because the P ligand would be most likely
protonated off.
Optimization of the reaction parameters for MCM-41:
There was a clear positive correlation between reaction tem-
perature and the activity and selectivity obtained in the RO-
RCM reaction under continuous-flow conditions (Fig-
ure 4a). Whereas the activity of the catalyst increased with
temperature (e.g., 48% conversion at 408C versus 76% con-
version at 608C after 100 min), the selectivity for all thre
small oligomers up to the pentamer reached nearly 100% at
and selectivity of 1@MCM-41
even increased over time and
no oligomers greater than a
nonamer formed (analyzed by
GPC).
This minor influence of even
small pores on the selectivity
toward small oligomers (2–
5 mers) becomes understanda-
ble if one considers the actual
size of the three isomers of the
cyclic dimer, that is, t,t-C16H28,
c,t-C16H28, and c,c-C16H28 (t,t-
C16H28/c,t-C16H28/c,c-C16H28 =
29:58:13 at 258C).[43] We have
determined the geometry of
these conformers in the gas
phase with single-point DFT
calculations
(cp2k
MD,
12008C, small basis set,
COSMO). A hull was created
and the minimal/maximal di-
ameter of each individual con-
former was determined (Fig-
ure 3a–c). Even the biggest
isomer t,t-C16H28 has only a
width and length of approxi-
mately 7ꢃ13 ꢂ, which is much
too small to be influenced by a
pore size of 90–180 ꢂ in diam-
eter.
The nature of the catalyti-
cally active species on the sup-
port remains elusive. Most
likely, we expect it to be the
unmodified
or
isomerized
Grubbs/Hoveyda 2 complex[19b]
hydrogen bonded through one
or two anionic chloride ligands
to the surface silanol groups of
the support (Figure 3d). This
supposition is in line with ob-
servations that complexes con-
taining P ligands are not stable
on silica materials such as
Figure 4. a) Influence of temperature and b) [COE]0 on the activity and selectivity of 1@MCM-41 with a pore
diameter of 180 ꢂ under continuous-flow conditions.
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Mesoporous-Silica-Supported Ru Catalysts for Olefin Metathesis Reactions
FULL PAPER
608C, which means that virtually all cis-cyclooctene is con-
verted into small oligomers, which stay in solution. In turn,
the deactivation rate of 1@MCM-41 was higher at 60 than at
408C (i.e., 45% conversion at 408C versus 60% conversion
at 608C after 400 min), and this material deactivated within
minutes at 808C. To clarify whether catalyst deactivation
was a consequence of a slow but steady coating of the cata-
lyst surface with oligomers and/or polymers, the reaction
was stopped after 480 minutes, and the catalyst was washed
with pure cyclohexane under the reaction conditions for ap-
proximately 17 hours. After this time, the initial solution of
cis-cyclooctene in cyclohexane was fed again (Figure 5).
Although we expected desorption of oligomers from the
coated catalyst surface during the washing in case of poly-
mer poisoning, less than 6% of small oligomers up to the
pentamer were eluted. By GPC analysis, there were less
than 15% of other medium-size polymers present in the col-
lected crude-product fractions before washing, and approxi-
mately 8% afterward washing. Elemental analysis of the
spent support after the reaction revealed that besides virtu-
ally no loss of ruthenium (0.100 of fresh material vs.
0.097 wt% Ru after the run), there was only a minimal
amount of 0.5 wt% of carbon on the support. Upon restart-
ing of the reaction after the purge, there was a significant
drop in catalyst activity (30 vs. 60% conversion), thus point-
ing to other intrinsic deactivation pathways rather than de-
activation through polymer deposition.
dimer, which was approximately two times higher for the di-
luted feeds (e.g., 20% selectivity in dimer at 90% conver-
sion for [COE]0 =0.77 wt% vs. 10% selectivity at 90% con-
version for [COE]0 =2.00 wt%). This effect was even more
pronounced for the small oligomers (2–5 mers; e.g., 65% se-
lectivity to dimer at 90% conversion for [COE]0 =0.77 wt%
vs. 40% selectivity at 90% conversion for [COE]0 =
2.00 wt%) because high dilution favors backbiting of the
growing oligomer chain to the ruthenium carbene
(Scheme 1).
Finally, to maximize the selectivity toward the dimer and
small oligomers, the MRT in the reactor was varied (Fig-
ure 6a). Its effect on catalyst activity was obvious because
the catalyst had longer contact with the feed at the slowest
flow rate (4 mLminÀ1, MRT=20 min) relative to higher
flow rates (8 and 16 mLminÀ1, MRT=10 and 5 min, respec-
tively), in which the catalyst had less time to react to result
in decreasing conversion at shorter MRTs (45, 67, and 93%
conversion after 4 h at MRT=5, 10, and 20 min, respective-
ly). The selectivity toward the dimer at approximately 45%
conversion (the only point at which all three experiments
were comparable at the same conversion) decreased with in-
creasing MRT (10, 18, and 29% selectivity at MRT=20, 10,
and 5 min, respectively). This observation implies that the
small oligomers are kinetic products and are not formed
through backbiting from a larger oligomer or polymer chain
(Scheme 1).[25] The same trend applied for the shorter
oligomers (2–5 mers: 30, 30, and 76% selectivity at MRT=
20, 10 and 5 min, respectively).
The outcome of the reaction was strongly dependent on
the initial concentration of cis-cyclooctene in the feed (Fig-
ure 4b). The catalyst deactivated faster at constant MRT at
2.00 wt% cis-cyclooctene in cyclohexane relative to a feed
of lower concentration (0.77 wt%). Again, the lower con-
centration had a major influence on the selectivity to the
A small selection of precomplexes 1–5 with chelating li-
gands (Figures 6b and 7), that is, the Hoveyda paragon 1,[45]
the Grela nitro catalyst 2,[15d] O-chelated lactate 3,[46] S-che-
late 4,[47] and N-chelate 5[48] were chosen to determine the
effect of the organometallic
species on the outcome of the
reaction. As anticipated, the
initial activity of 2 was highest
due to its fast initiation (71%
conversion after 1 h). Due to
its fast deactivation after ap-
proximately 300 minutes, cata-
lyst 2 reached the break-even
point to 1 at approximately
30% conversion. Lactate
3
had an initial activity similar
to 2 (45 vs. 52% conversion
after 1 h, respectively) but also
deactivated more quickly. In
our heterogeneous reaction
setup, the Umicore catalys 5
and the Evonik catalyst
4
showed virtually no activity
(<5%) at 40, 60, or 808C.
Amongst complexes 1–3, there
was no effect of the chelate
structure on selectivity because
all the complexes gave virtual-
Figure 5. Conversion and selectivity over time with
a solvent flush run overnight. Conditions: 608C,
16 mLminÀ1, 0.35 wt% cis-cyclooctene, 1@MCM-41(pore size: 180 ꢂ).
Chem. Eur. J. 2013, 19, 11661 – 11671
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11667

M. Limbach et al.
substrate concentration, tem-
perature, and finally mean resi-
dence time, we obtained
a
maximal selectivity of 35%
toward a cyclic dimer, which is
a remarkable value in view of
the wealth of potentially possi-
ble products. Although the
pore diameter had little effect
on the selectivity, the compet-
ing ROMP reaction was virtu-
ally suppressed. The nature of
the initial ruthenium carbene
species had no effect on the se-
lectivity, but had
a strong
impact on the activity of the
catalytic material. This differ-
ence in reactivity might be a
consequence of the individual
sensitivity of the complexes to
acid surface silanols, which
were responsible for double-
bond isomerization as a major
side reaction and is a future
field of development of this
valuable technology.
Experimental Section
General: All the reactions and ma-
nipulations were performed in an
argon atmosphere by using standard
Schlenk techniques. Cyclohexane, di-
ethyl ether, THF, and dichlorome-
thane were dried by means of
a
MBraun solvent-purification system.
All the solvents were degassed with
argon prior to use. GC analyses were
carried out on a Agilent 6890N mod-
ular GC base equipped with a split-
mode capillary-injection system and
flame-ionization detector by using a
standard HP-5 capillary column
(30 mꢃ0.32 mmꢃ0.25 mm; He flow:
2.0 mLminÀ1; program: 508C (1 min),
Figure 6. a) Influence of MRT on the activity and selectivity of 1@MCM-41 with a pore diameter of 180 ꢂ
under continuous-flow conditions. b) Evaluation of precomplexes 1–5 adsorbed on MCM-41 in the RO-RCM
of cis-cyclooctene under continuous-flow conditions.
50–2508C
(10 min). GC-MS analyses were car-
ried out on Agilent 19091S-433
modular GC base equipped with a
(208CminÀ1),
2508C
a
ly the same selectivity of approximately 35% toward the
dimer at 50% conversion.[49]
split-mode capillary-injection system and flame-ionization detector by
using a standard HP-5 capillary column (30 mꢃ0.25 mmꢃ0.25 mm; He
flow: 0.6 mLminÀ1
;
program: 508C (1 min), 50–2508C (208CminÀ1),
2508C (10 min)). GPC was measured on a 55er system with refraction
index (RI) detection equipped with four columns of 7.5ꢃ300 mm; PLgel:
5 mm, pore width: 5 nm, eluent: THF, flow: 1 mLminÀ1, temperature:
358C. Polystyrene (266–5000 Da) was used as a reference material for
the assignment of molecular weight. All the other starting materials were
purchased in reagent-grade purity from Acros, Aldrich, Fluka, or Strem
and were used without further purification.
Conclusion
We have found that the longevity and the selectivity of a
heterogenized Grubbs/Hoveyda complex in a RO-RCM re-
action was significantly increased by running the reaction
under continuous-flow conditions on a mesoporous support.
By tuning the pore diameter of the mesoporous support,
Procedure for the heterogenization of complexes on solid support: The
solid support (1.5 g) was suspended under argon in CH2Cl2 (10 mL). A
11668
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Chem. Eur. J. 2013, 19, 11661 – 11671

Mesoporous-Silica-Supported Ru Catalysts for Olefin Metathesis Reactions
FULL PAPER
Aerosil 200 (uncalcined): cis-Cyclooc-
tene (162.6 mg, 1.424 mmol), material
1@Aerosil 200 (742.9 mg, [Ru]=
0.17%).
Aerosil 200 (calcined at 2008C): cis-
Cyclooctene (149.7 mg, 1.311 mmol),
material 1@Aerosil 200 (725.8 mg,
[Ru]=0.16%).
Aerosil 200 (calcined at 6008C; SJ 3-
41):
1.391 mmol),
(738.6 mg, [Ru]=0.20%).
cis-Cyclooctene
(158.8 mg,
SJ3–39
material
MCM-41 (180 ꢁ): cis-Cyclooctene
(145.6 mg, 1.275 mmol), material
1@MCM-41
(764.9 mg,
[Ru]=
0.20%).
MCM-41 (180 ꢁ, strands, 0.6–1 mm):
cis-Cyclooctene (135.4 mg,
1.186 mmol), material 1@MCM-41
(678.0 mg, [Ru]=0.25%).
Figure 7. Ruthenium carbenes 1–5 with various chelating ligands employed in the flow study (see Figure 6b
for details).
solution of Grubbs/Hoveyda 2 complex (20.5 mg, 32.7 mg) in CH2Cl2
(10 mL) was added to this suspension. The green suspension was shaken
for 4 h (200 min) at room temperature under argon. The solvent was re-
moved under vacuum at a maximum of 408C to give the loaded materials
as green solids. The content of ruthenium was measured by means of in-
ductively coupled plasma/mass spectrometry (ICP-MS).
MCM-41 (180 ꢁ, strands, 1–1.6 mm): cis-Cyclooctene (135.3 mg,
1.154 mmol), material 1@MCM-41 (668.4 mg, [Ru]=0.22%).
SBA-15: cis-Cyclooctene (149.3 mg, 1.620 mmol), material 1@SBA-15
(743.3 mg, [Ru]=0.22%).
Procedure for the preparation and testing of various precomplexes on
MCM-41 under continuous-flow conditions: Shaped and split MCM-41
(1–1.6 mm, 15.12–15.59 g) was suspended under argon in CH2Cl2
MWW: Ru calcd (%): 0.22; found: 0.16 (73% of theory); alumina-acti-
vated (Wako 45 mm; 300 mesh): Ru calcd (%): 0.22; found: 0.17 (81% of
theory); D11-10 (calcined): Ru calcd (%): 0.22; found: 0.17 (81% of
theory); D11-10 (uncalcined): Ru calcd (%): 0.22; found: 0.22 (100% of
theory); SBA-15: CH2Cl2 (30 mL); Ru calcd (%): 0.22; found: 0.22
(100% of theory); MCM-41: Ru calcd (%): 0.21; found: 0.20 (95% of
theory); ZSM-5: Ru calcd (%): 0.21; found: 0.16 (76% of theory);
MCM-48: Ru calcd (%): 0.21; found: 0.21 (100% of theory); Aerosil 200
(uncalcined): Ru calcd (%): 0.21; found: 0.17 (81% of theory); Aero-
sil 200 (calcined): Ru calcd (%): 0.21; found: 0.20 (95% of theory).
(75 mL).
A solution of the precomplex (80–144 mg, 127–128 mg) in
CH2Cl2 (10 mL) was added to this suspension. The green suspension was
shaken for 2 h (200 min) at room temperature under argon. The solvent
was removed under vacuum at a maximum of 408C to give the loaded
materials as green solids. The content of ruthenium was measured by
means of ICP-MS.
A steel reactor (diameter: 14 mm, length: 45 cm) was charged with the
catalyst. COE in cyclohexane solution (0.35% by weight, 25 mmolLÀ1
)
was pumped through the reactor at 608C with a flow rate of 16 mLminÀ1
and a pressure of 2 bar. The reactions were monitored by sampling the
resulting solution with a GC-MS probe.
Procedure for the catalytic testing of the loaded materials and the split
test to determine the heterogeneity of the catalyst: In the glove box, do-
decane (1.008 g, internal standard) and cis-cyclooctene (131.9 mg,
1.15 mmol, 96.5%) were added to a suspension of the supported material
(720 mg) in a vial (50 mL) in hexane (16 mL), and the suspension was
shaken at room temperature at 200 rpm. Samples were taken in the given
time intervals and the samples were quenched with a drop of ethyl vinyl
ether. GC analyses were performed on a DB-5 column (30 m, ID:
0.32 mm, film: 0.25 mm; temperature program: 608C (5 min), 60–3008C
(108CminÀ1), 3008C (15 min), 300–3208C (108CminÀ1), 3208C (18 min).
1: 80 mg (128 mmol), 15.12 g MCM-41; [Ru]=0.082%. 2: 85 mg
(127 mmol), 15.58 g MCM-41; [Ru]=0.075%. 3: 82 mg (127 mmol),
15.52 g MCM-41; [Ru]=0.077%. 4: 94 mg (128 mmol), 15.58 g MCM-41;
[Ru]=0.060%. 5: 144 mg (128 mmol), 15.59 g MCM-41; [Ru]=0.075%.
After a conversion of approximately 50%, two samples were taken from
the reaction mixture, one was quenched with a drop of ethyl vinyl ether
and the other one remained unquenched. The conversion in the un-
quenched sample was compared after 4 h to the conversion of the
quenched sample. If the conversion of the samples remained equal, the
reaction was heterogeneous. Consecutive conversion indicated that there
was a species catalytically active in the liquid phase.
Acknowledgements
M.B. and M.L. work at CaRLa of Heidelberg University and are cofi-
nanced by Heidelberg University, the state of Baden-Wꢀrttemberg, and
BASF SE. Support of these institutions is gratefully acknowledged.
MWW: 720 mg material (0.16 wt% Ru).
MWW (strands, 0.6–1 mm): cis-Cyclooctene (156.1 mg, 1.367 mmol), ma-
terial 1@MWW (735.1 mg, [Ru]=0.18%).
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Chem. Eur. J. 2013, 19, 11661 – 11671

Mesoporous-Silica-Supported Ru Catalysts for Olefin Metathesis Reactions
FULL PAPER
tene): 5.5 min, tR (C16H28, 2 mer): 20.1 min, tR (C24H42, 3 mer):
28.2 min, tR (C32H56, 4 mer): 37.1 min, tR (C40H70, 5 mer): 55.1 min.
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[49] The broader applicability of the methodology has successfully been
demonstrated in the RCM of 5-hexenyl undec-10-enoate (referen-
ce [2a]) to give a mixture of (E/Z)-exaltolide in 80% yield (E/Z=
2.46:1); experimental conditions: 5-hexenyl undec-10-enoate (3.39 g,
12.7 mmol), 1@MCM-41 (9.75 g), 0.17 wt% olefin in cyclohexane,
608C, 25 h; GC-conditions: HP-5 capillary column (30 mꢃ0.32 mmꢃ
0.25 mm; He flow: 2.0 mLminÀ1; program: 608C (5 min), 60–3008C
(108CminÀ1), 3008C (11 min), 300–3208C (108CminÀ1), 3208C
(18 min).
[39] D11-10 is a precipitated silica, which is available from BASF SE on
request; Aerosil 200 is a hydrophilic fumed silica with a specific sur-
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a commercial product of Helm AG, and can be purchased from
Sigma-Aldrich.
Received: October 31, 2012
Revised: May 28, 2013
Published online: July 12, 2013
Chem. Eur. J. 2013, 19, 11661 – 11671
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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