Engineering an atomically well defined active site for the catalytic epoxidation of alkenes

Article abstract of DOI:10.1039/a706387c

The improvement effected by replacing one of the three O-Si≡ groups to which a Ti^IV ion is anchored (to a mesoporous silica support) by a O-Ge≡ group is described, as is the detailed structure retrieved by in situ X-ray absorption spectroscopy of the created active site.

Full text of DOI:10.1039/a706387c

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Engineering an atomically well defined active site for the catalytic epoxidation
of alkenes
Richard D. Oldroyd, John Meurig Thomas* and Gopinathan Sankar
Davy–Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albermarle Street, London, UK W1X 4BS
The improvement effected by replacing one of the three
O–Si· groups to which a TiIV ion is anchored (to a
mesoporous silica support) by a O–Ge· group is described, as
is the detailed structure retrieved by in situ X-ray absorption
spectroscopy of the created active site.
scanning transmission electron microscopy), show that the
exposed active sties in the Ti—MCM41 catalyst
(Si:Ti = 44.7:1) are TiIV centres tripodally attached to the
support via three Ti–O–Si· linkages (Fig. 1). We have grafted
GeIV centres onto the MCM41 surface using a procedure similar
to that of Ne´dez et al.11 to which the catalytically active TiIV
centres have been subsequently attached. The synthesis entails
exposing the dehydrated mesoporous silica (surface
area = 1020 ± 10 m2 g21) to tetrabutylgermanium at 180 °C and
then, after washing, calcining and dehydrating the resulting
sample, introducing the TiCp2Cl2 in an analogous fashion to the
procedure used for unmodified MCM41. The resulting atomic
ratios are Si:Ge:Ti = 44.9:0.07:1 and 44.7:0:1 for the
germanium-modified and Ti—MCM41 catalysts, respectively.
Full preparative and analytical details are to be given else-
where.12
The immediate atomic environment of a catalytically active
metallic ion may be determined by X-ray absorption spectro-
scopy.1–3 Using this technique we find that, in the case of TiIV
ions anchored onto the walls of a high-area, crystalline
mesoporous silica4 (Ti—MCM41) by condensing titanocene
dichloride and pendant ·Si–OH groups in a manner previously
described,5,6 the coordination numbers and spatially averaged
bond distances for the epoxidation catalyst (see Fig. 1) are as
shown in Table 1. It has hitherto been established6,7 that this
epoxidation catalyst functions in a radical-free fashion, and that
it epoxidises cyclohexene with a selectivity as high as 95% and
turnover numbers of ca. 35 h21 (under the conditions used in the
cited study) with alkyl hydroperoxide oxidants. This perform-
ance is a good deal superior to that of the catalyst produced by
incorporation of TiIV ions into the interior of the walls during
the synthesis of the MCM41 mesoporous silica from the parent
gel (Ti?MCM41).8–10
Titanium and germanium K-edge extended X-ray absorption
fine structure (EXAFS) spectroscopy, supplemented by FTIR
spectroscopy and spatially resolved elemental analysis (using
Fig. 2 Representation of the derived structures of calcined Ge—MCM41 (a)
and Ti—Ge—MCM41 (b). The insets show the respective FTIR spectra.
Most of the surface silanol groups have been omitted for clarity. (Note that
the ratio of the different active sites shown here are not the same as in the
real catalyst, but are purely illustrative.)
Fig. 1 Illustration of the Ti environment in Ti—MCM41 before (a) and after
(b) removal of the Cp moiety by calcination. (Ti = cyan, C = green,
O = red, H = white). For clarity, the SiO2 background of the mesoporous
support is not shown in atomic detail.
Table 1 Bond distances and angles derived from analysis of Ti and Ge K-edge EXAFS data
Material
Edge
Ti
Atom type
Occupancy
Distance/Å
2s2/Å2
Angle/°
Ti—MCM41 (calcined)
O
4.1
2.3
0.8
3.9
2.7
3.8
1.7
1.1
4.1
0.8
3.3
1.82
3.19
3.37
1.73
3.04
1.81
3.19
3.34
1.72
2.80
3.10
0.005
0.006
0.006
0.004
0.005
0.002
0.004
0.004
0.004
0.003
0.003
Ti–O–Si(A):138
Ti–O–Si(B):160
Si(A)a
Si(B)
O
Ge—MCM41 (calcined)
Ge
Ti
Ge–O–Si:132
Si
O
Si(A)a
Si(B)
O
Ti—Ge—MCM41(calcined)
Ti–O–Si(A):139
Ti–O–Si(B):158
Ge
Ge–O–Si:136
Ge–O–Ti:106
Ti
Si
a Self-consistent results pertaining to coordination numbers and bond lengths are best achieved by recognising two distinct Si shells.
Chem. Commun., 1997
2025

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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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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