The resulting so-called Ti—Ge—MCM41 catalyst was
investigated by scanning transmission electron microscopy,
X-ray absorption spectroscopy and also by IR spectroscopy.
The salient results are given in Table 1, from which the
representation of the created active sites (Fig. 2) is derived. It is
evident that titanium attaches to the germanium centres (as well
as unmodified regions of the MCM41 surface) from the
disappearance of the n(GeO–H)13 band at 3680 cm21 in the IR
spectrum and the appearance of a Ge···Ti shell from the Ge
K-edge EXAFS spectrum.
The difference in catalytic activity between the two catalysts
is quite dramatic. Although the germanium loading on the
MCM41 surface is low (only ca. 0.15 mass%) there is a
substantial improvement in catalytic activity of as much as 18%.
It can be calculated that for those titanium centres that are
surrounded tripodally by two O–Si· and one O–Ge· there is a
140% increase in turnover frequency with tert-butyl hydro-
peroxide (TBHP) and some 80% increase with cumene
hydroperoxide (CuHP) as oxidants at 30 °C compared to
titanium centres bound to three O–Si· moieties. Turnover
frequencies towards the epoxide were 33.8 and 39.8 h21 for
Ti—MCM41 and Ti—Ge—MCM41 respectively in the pres-
ence of TBHP, and 31.2 and 34.9 h21 using CuHP. In each case
the selectivity towards cyclohexene oxide was > 75%, the
remaining products being cyclohexenol and cyclohexan-
1,2-diol.
We also acknowledge the efforts of Dr D. Lewis for preparing
the figures.
Footnote and References
* E-mail: richie@ri.ac.uk
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G. E. Derbyshire and G. N. Greaves, Nature, 1991, 354, 465.
2 J. M. Thomas and G. N. Greaves, Science, 1994, 265, 1675.
3 J. M. Thomas, G. N. Greaves, G. Sankar, P. A. Wright, J. S. Chen,
A. J. Dent and L. Marchese, Angew. Chem., Int. Ed. Engl., 1994, 33,
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4 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck,
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D. W. Snelgrove, K. U. Ingold and D. D. M. Wayner, Angew. Chem.,
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7 R. A. Sheldon, J. Mol. Catal., 1980, 7, 106.
8 H. C. L. Abbenhius, S. Krijnen and R. A. van Santen, Chem. Commun.,
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9 G. Sankar, F. Rey, J. M. Thomas, G. N. Greaves, A. Corma,
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Attempts to modify the active site with Sn (using SnBu4 in
place of GeBu4)14 prior to anchoring the titanium were not
successful since there was a strong tendency for Sn–O–Sn
linkages to form, and hence crystallites of extraframework
SnO2, which significantly inhibited catalysis. A detailed
account of this work will be submitted elsewhere.12
We are grateful to the EPSRC for a rolling grant (to J. M. T.)
and a ROPA award (to R. D. O.) and the staff at the Daresbury
Laboratory for provision of facilities for the XAS experiments.
¨
12 R. D. Oldroyd, G. Sankar, J. M. Thomas and D. Ozkaya, J. Phys. Chem.,
in press.
13 H. Kosslick, V. A. Tuan and R. Fricke, Ber. Bunsen-Ges. Phys. Chem.,
1992, 96, 1761.
14 C. Ne´dez, A. Theolier, F. Lefebvre, A. Choplin, J. M. Basset and
J. F. Joly, J. Am. Chem. Soc., 1993, 115, 722.
Received in Basel, Switzerland, 3rd April 1997; Revised manuscript
accepted 8th August 1997; 7/06387C
2026
Chem. Commun., 1997