Angewandte
Chemie
whether the rhodium complex would exhibit typical behavior
associated with its homogeneous analogue.
Keywords: heterogeneous catalysis · hydrogenation ·
mesoporous materials · rhodium · silicates
.
The catalytic activity of several hybrid materials for
hydrogenation was evaluated by using substrates of varying
steric encumbrance and compared to the activity of a
homogeneous catalyst, [RhCl(PPh ) ]. Among the materials
[
1] a) S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, J.
Am. Chem. Soc. 1999, 121, 9611; b) T. Asefa, M. J. MacLachlan,
N. Coombs, G. A. Ozin, Nature 1999, 402, 867; c) C. Yoshima-
Ishii, T. Asefa, N. Coombs, M. J. MacLachlan, G. A. Ozin, Chem.
Commun. 1999, 2539.
3
3
tested were two different lots of a model system prepared by
postsynthetic grafting of precursor 1 onto calcined SBA-15
silica 3. Results obtained for the catalytic tests are given in
Table 1.
[2] S. Inagaki, S. Guan, T. Ohsuna, O. Terasaki, Nature 2002, 416,
304.
[
[
[
3] The transition-metal precursor, [RhCl{PPh (CH ) Si-
2 2 2
(
(
OCH CH ) } ] (1), was prepared by treating [{RhCl-
2 3 3 3
cod)}2] (cod = 1,5-cyclooctadiene) with PPh (CH ) Si-
[
a]
Table 1: Hydrogenation of alkenes by different rhodium-based catalysts.
2
2 2
[
b]
À1
Substrate
styrene
Catalyst
%wt Rh
Substrate/Rh Conv [%] TOF [h
]
(OCH CH ) according to the procedure reported: O.
2 3 3
Krꢁcher, R. A. Kꢁppel, M. Frꢁba, A. Baiker, J. Catal. 1998,
78, 284.
3
2
3
2
3
2
3
2
0.8
0.7
0.8
0.7
0.8
0.7
1.1
1.7
7600
8500
7550
8560
7500
8890
5500
3400
6600
85
93
100
100
4
2150
2650
2590
2850
97
1
4] a) J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz,
C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson,
E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L.
Schlenker, J. Am. Chem. Soc. 1992, 114, 10834; b) C. T.
Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S.
Beck, Nature 1992, 359, 710.
5] Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng,
T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schꢂth,
G. D. Stucky, Chem. Mater. 1994, 6, 1176.
cyclohexene
acrolein
4
104
[
c]
[c]
crotonaldehyde
styrene
ꢀ7
ꢀ5
20
[
c]
[c]
9
À4
[d]
[d]
[RhCl(PPh ) ] 3.5ꢂ10
m
75
3300
3
3
solution
[
a] Conditions: P(H )=20 bar, 708C, 3 h in benzene. TOF=turnover frequency, [6] In a typical procedure, CTAB was first dissolved in water,
2
conv=conversion. [b] From elemental analysis. [c] 19 h. [d] Homogeneous catal-
ysis, 1.5 h.
HCl, and half of the amount of acetonitrile used in the gel
composition. TEOS was then prehydrolyzed over 15 min
at room temperature prior to the addition of the organ-
ometallic precursor that had been dissolved in the
remaining portion of acetonitrile. The resulting solution
was stirred for 3 h at room temperature. The solid product was
collected by filtration, washed with water, and dried under
vacuum overnight at 258C.
In all cases, the modified SBA-3 materials, including 2,
exhibited similar activity to that of model system 3; thus, it is
clear that the rhodium centers are available for catalysis.
[
7] Optimized molar composition: TEOS 1; H O 120; CH CN 4.3;
2
3
Indeed, for styrene hydrogenation, the activity is in the same
HCl 9.2; CTAB 0.12. When a siloxane-containing transition-
metal complex is used, the calculation of the molar composition
is based on the total number of condensable silicon centers, from
TEOS and the complex.
range as that observed for the homogeneous complex.
Furthermore, catalytic activity was observed even for the
more hindered double bonds of cyclohexene and crotonalde-
hyde (see Table 1. The chemoselectivity (100%) of the
catalyst for the carbon–carbon double bond in a,b-unsatu-
rated aldehydes is the same as that reported for the
[8] The template was removed from the as-made material by batch
extraction with dry ethanol at 508C for 2 h. Three extraction
cycles were necessary to complete the process.
[
15]
[9] The postsilylation reaction was performed in dry toluene at 508C
for 1 h using either (CH ) SiCl or (CH ) SiCl as the silylating
homogeneous catalyst. Limited attempts at recycling the
catalyst have shown that catalytic activity is undiminished
over several cycles, but more rigorous testing is needed.
In summary, we have developed a new synthetic protocol
that allows the incorporation of phosphine-ligated transition-
metal complexes into SBA-3 type silicas without significant
loss of the mesoscopic order of the silica framework or the
coordination environment and catalytic activity of the tran-
sition-metal complex. Key novel aspects of the protocol are
the use of acetonitrile as a cosolvent, the acidic conditions, the
relatively low temperatures, and silylation of the material
prior to extraction of the SDA. The method has been
successfully extended to incorporate other siloxy organo-
phosphanyl transition-metal complexes into hybrid materials,
notably those of platinum and palladium. The evaluation of
these materials, in terms of activity and stability, for a variety
of catalytic applications is ongoing.
3
3
3
2
2
agents.
[
10] The expansion of the structure upon silylation and extraction,
indicated by the increase of the d(10) spacing is likely to be
related to the decreased degree of hydrogen-bonding interac-
tions between the surface hydroxy groups, which leads to a
partial release of the internal surface tension.
[
[
[
11] Elemental analysis: 1.5%wt P and 1.7%wt Rh.
12] D. W. Sindorf, G. E. Maciel, J. Am. Chem. Soc. 1983, 105, 3767.
13] Thermogravimetric analyses were conducted from 25 to 10008C
À1
in air at a heating rate of 58Cmin
.
[14] We have demonstrated for an SBA-15 material that the
postsynthetic grafting of the rhodium complex onto the surface
of unmodified calcined silica dramatically reduces the specific
area and the median pore-size diameter and that the effect varies
inversely with rhodium loading (see the Supporting Informa-
tion).
15] a) P. N. Rylander, Hydrogenation Methods, Academic Press,
London, 1985; b) J. M. Grosselin, C. Mercier, G. Allmang, F.
Grass, Organometallics 1991, 10, 2126; c) P. Clause, Top. Catal.
[
1998, 5, 51.
Received: February 6, 2005
Published online: April 28, 2005
Angew. Chem. Int. Ed. 2005, 44, 3475 –3477
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3477