Is MCM-41 really advantageous over amorphous silica? The case of grafted titanium epoxidation catalysts

Article abstract of DOI:10.1039/b103057b

MCM-41 can be used in the same way as amorphous silica for the preparation of supported titanium catalysts; MCM-41 does not show clear advantages over silica in epoxidation reactions with either TBHP or H₂O₂; only in one case is a positive effect observed and this involves the recycled catalyst with H₂O₂.

Full text of DOI:10.1039/b103057b

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Is MCM-41 really advantageous over amorphous silica? The case of
grafted titanium epoxidation catalysts†
José M. Fraile,a José I. García,a José A. Mayoral,*a Eugenio Vispe,a D. R. Brownb and M. Naderib
a Departamento de Química Orgánica, Instituto de Ciencia de Materiales de Aragón, Facultad de
Ciencias, Universidad de Zaragoza-C.S.I.C., E-50009 Zaragoza, Spain.
E-mail: mayoral@posta.uniza.es
b Department of Chemical and Biological Sciences, University of Huddersfield, Huddersfield, UK
HD1 3DH
Received (in Cambridge, UK) 4th April 2001, Accepted 28th June 2001
First published as an Advance Article on the web 26th July 2001
MCM-41 can be used in the same way as amorphous silica
for the preparation of supported titanium catalysts; MCM-
41 does not show clear advantages over silica in epoxidation
reactions with either TBHP or H2O2; only in one case is a
positive effect observed and this involves the recycled
catalyst with H2O2.
density of hydroxy groups16 (ca. 2 OH nm22). In the case of
amorphous silica, this agreement (1.07 mmol g21, 475 m2 g21
,
3.7 hydrogen-bridged OH nm22) is also observed. Treatment of
the catalyst with tartaric acid was carried out as described for
silica.12 The Ti content of MCM-Ti(TA) was found to be very
similar to that found in the silica counterpart (Table 1).
Si-Ti(OPri) has previously been characterised by MAS-
NMR,7 EXAFS18 and IR.12 The similarity of the IR spectra of
the solids (ESI†), together with the agreement observed in the
titanium loading, seem to indicate that the surface species’ are
similar irrespective of the structure of the silica. The exclusion
of water in the preparation method prevents the formation of
titania (anatase) on the silica surface, as demonstrated by the
absence of a UV absorption above 300 nm.
Catalysts based on mesoporous crystalline silicas, especially
MCM-41, have been widely applied in recent years.1 Among
the advantages proposed for this type of solid are the presence
of a regular porous system, and the large pore size. In many
cases comparisons have only been made regarding the behav-
iour of zeolites and MCM-41, and comparisons with amorphous
silica are generally ignored.
Titanium catalysts are among the most important oxidation
catalysts2 and titanium centres can be incorporated in MCM-
413,4 or amorphous silica6–9 either within the framework, by
substitution of silicon atoms,3 or by grafting titanium species
onto the surface. Several of these catalysts have been used in the
epoxidation of alkenes with dilute hydrogen peroxide.10–12 In
the case of MCM-41, grafting has been described as being more
efficient than framework substitution.10,13,14 However, no
direct comparison has been made between amorphous and
crystalline mesoporous silicas. In this communication we report
the behaviour of MCM-41 as a support for Ti(OPri)4 using the
same methodology employed for amorphous silica.
All the catalysts were tested in the epoxidation of cyclohex-
ene with 30% hydrogen peroxide in tert-butyl alcohol at 353 K
(Scheme 1). The active hydroperoxotitanium species is able to
epoxidise directly cyclohexene with hydrogen peroxide and
also to produce free radicals, which give rise to an allylic
hydroperoxide. This compound can also be involved in
titanium-catalysed epoxidation of cyclohexene, leading to
cyclohex-2-en-1-ol as a by-product. Finally, the acidity of the
catalyst promotes the hydrolysis of cyclohexene oxide to the
corresponding diol. The results of this study are gathered in
Table 1.
As far as catalysts prepared with Ti(OPri)4 are concerned,
MCM-41 leads to a solid that is less active than the silica-based
one in terms of both hydrogen peroxide conversion and turnover
number to epoxidation products. Moreover, the epoxidation/
allylic oxidation selectivity is also slightly lower, indicating a
lower contribution of the direct epoxidation. The main ad-
vantage of MCM-41 comes from the reusability of the catalyst,
in fact the turnover number for epoxidation products is nearly
constant up to the third run. However, the epoxidation/allylic
oxidation selectivity is almost 50/50, which indicates that only
The synthesis of pure siliceous MCM-4115 and titanium
grafting7 were carried out as previously described. The titanium
loading onto MCM-41 (Table 1) is in good agreement with the
surface area of the support (802 m2 g21) and the reported
† Electronic supplementary information (ESI) available: experimental
details of preparation and characterisation of the catalysts, IR spectra and
results of the cyclohexene epoxidation with TBHP. See http://www.rsc.org/
suppdata/cc/b1/b103057b/
Table 1 Results obtained in the epoxidation of cyclohexene with dilute H2O2 in the presence of titanium catalystsa
Yieldd (24 h)
Ti
Conv.
H2O2
TON
(ep + diol)
(ep + diol)/
enol
Catalyst
Run contentb
H2O2/Ti
ep
diol
enol
chhp
ep/diol
c
Si-Ti(OPri)
MCM-Ti(OPri)
Si-Ti(TA)
1
1.07
0.74
1.25
1.01
0.99
0.56
1.01
0.94
58.4
86.7
50.0
61.9
63.0
111.6
61.9
66.5
72
75
56
67
92
108
37
38
9
2
9
3
9
19
5
10
21
2
12
7
15
32
10
17
18
13
1
17.2
3.6
11.4
9.3
25.3
52.8
14.3
10.6
71/29
36/64
64/36
44/56
73/27
57/43
66/34
56/44
29/71
50/50
37/63
20/80
22/78
40/60
22/78
59/41
3e
1
14
12
32
28
18
7
13
18
15
35
12
13
3e
1
3e
1
MCM-Ti(TA)
3e
4
a Reaction conditions: 200 mg catalyst, 250 mmol cyclohexene, 12.5 mmol H2O2 (30%), 25 mL tert-butyl alcohol 353 K, 24 h. b mmol g21 c % conversion
to cyclohexene oxidation products. d Determined by gas chromatography. ep = cyclohexene oxide; diol = trans-cyclohexane-1,2-diol; enol = cyclohex-
2-en-1-ol; chhp = cyclohexenyl hydroperoxide. e Ti content after the third run. Ratios are referred to this value.
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Chem. Commun., 2001, 1510–1511
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103057b

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epoxidation with alkyl hydroperoxides or to a more rapid
deactivation of the silica-based catalysts. In order to clarify this
point, the four catalysts were compared in the epoxidation of
cyclohexene with tert-butyl hydroperoxide (TBHP). The results
(ESI†) show that the catalysts with isopropoxy groups have
nearly the same catalytic activity and selectivity. Treatment
with tartaric acid noticeably reduces the activity, irrespective of
the type of support, but Si-Ti(TA) is clearly more active than
MCM-Ti(TA). Thus, the hypothesis regarding the higher
activity of MCM-catalysts is not confirmed.
In conclusion, the three factors studied (support, titanium
environment and nature of the oxidant) have a significant
influence on both the results of the reaction and the stability of
the catalyst. Furthermore, the three factors are not completely
independent and a careful selection of each parameter is
necessary to optimise the behaviour of this kind of catalyst.
Treatment of MCM-41 with tartaric acid has a detrimental
effect with both oxidants used (H2O2 and TBHP) in such a way
that silica-grafted systems become far more active. On using
amorphous silica, treatment with tartaric acid leads to a catalyst
that is more active in the epoxidation with dilute hydrogen
peroxide but is less active with TBHP. Ti(OPri) species
immobilised on amorphous silica are only slightly more active
than those grafted onto MCM-41. However, the use of the
crystalline MCM-41 leads to more stable catalysts.
It is clear that none of the supports are particularly
advantageous over the others and that the choice of one or other
is influenced by other factors concerning the titanium environ-
ment and the reaction conditions.
This work was made possible by the generous financial
support of the CICYT (project MAT99-1176).
Scheme 1
the radical mechanism takes place with the recovered catalyst.
The presence of different titanium species, with either two or
three bonds to the surface, or the higher titanium dispersion due
to the larger surface area of MCM-41 could be responsible for
these differences in behaviour.
Treatment with tartaric acid improves the activity of the
silica-based catalyst without causing a modification in the
epoxidation/allylic oxidation selectivity. This higher activity is
reflected in the higher hydrogen peroxide conversion and the
higher turnover number for epoxidation products. It has been
shown that some of the titanium species generated in the
treatment with tartaric acid are able to pass into solution.12
However, some other more active species remain on the solid,
as shown by the higher turnover number attained with the
recovered catalyst. Even with this recovered catalyst the
epoxidation/allylic oxidation selectivity is > 50/50, signifying a
contribution of the direct epoxidation with hydrogen peroxide.
Similar treatment with tartaric acid does not cause the same
beneficial effect in the case of the MCM-41 catalyst. This solid
is much less active in terms of the productive conversion of
hydrogen peroxide and turnover numbers and, in addition, the
selectivities are very similar to those obtained with the parent
MCM-Ti(OPri) catalyst. In this case the species are more
strongly bonded to the surface, as demonstrated by the lower
degree of titanium leaching.
A particularly interesting point concerns the stability of these
catalysts in comparison to other similar systems described in the
literature. One of the few studies regarding the stability of
grafted titanium species on MCM-41 describes a titanium loss
of 50–61% after two reactions, with a total H2O2/Ti ratio in the
range 228–686.10 However, this leaching does not seem to be
proportional to either the H2O2/Ti ratio or the catalytic activity.
In our case, titanium loss is 7–19% for MCM catalysts after
three runs, with a total H2O2/Ti ratio in the range 150–200.
These values indicate that the solids described here have a
higher stability. The solids described here also show a higher
stability in comparison with related silica-based catalysts.8 This
stability is demonstrated by the the fact that catalysts prepared
with TiF4 or tetraneopentyltitanium cannot be used with dilute
hydrogen peroxide.
Notes and references
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The IR spectra of the recovered catalysts (ESI†) show that
some by-products remain adsorbed on the surface.
Another interesting feature of these systmes is the lower
content of cyclohexenyl hydroperoxide (chhp) in the final
reaction mixture when MCM-41-based catalysts are used. This
may be due to a higher activity of these catalysts in the
Chem. Commun., 2001, 1510–1511
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