Remarkable activity enhancement by trimethylsilylation in oxidation of alkenes and alkanes with H2O2 catalyzed by titanium-containing mesoporous molecular sieves

Article abstract of DOI:10.1039/a706175g

Titanium-containing mesoporous materials, Ti-MCM-41 and Ti-MCM-48, have been successfully modified by trimethylsilylation to exhibit enhanced catalytic activity in the oxidation of alkenes and alkanes with H₂O₂.

Full text of DOI:10.1039/a706175g

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Remarkable activity enhancement by trimethylsilylation in oxidation of alkenes
and alkanes with H2O2 catalyzed by titanium-containing mesoporous
molecular sieves
Takashi Tatsumi,* Keiko A. Koyano and Naoko Igarashi
Engineering Research Institute, School of Engineering, The University of Tokyo, Yayoi, Tokyo 113, Japan
Titanium-containing mesoporous materials, Ti-MCM-41
and Ti-MCM-48, have been successfully modified by
trimethylsilylation to exhibit enhanced catalytic activity in
the oxidation of alkenes and alkanes with H2O2.
as TS-1 and Ti-b,10,11 has significantly decreased upon
trimethylsilylation. Similar phenomena were also observed with
Ti-MCM-48. The band was also detected in the spectra of pure
silica MCM-41 or -48 (non-sil) and completely disappeared
upon trimethylsilylation. Moreover, the intensity of a broad
peak in the range 3700–3500 cm21 corresponding to silanol
groups and adsorbed water in the spectra of both Ti-MCM-41
and Ti-MCM-48 decreased significantly upon trimethylsilyla-
tion. To determine the concentration of surface trimethylsilyl
groups 29Si MAS NMR measurements were carried out. Before
trimethylsilylation, the 29Si MAS NMR spectrum of Ti-MCM-
41 (non-sil) exhibited two broad resonances, at d 2108.7
(73.7%) for the Q4 silicate unit and at d 2100.5 (26.3%) for the
Q3 environment. After trimethylsilylation, a new peak ascribed
to the silicon atoms of TMS appeared at d 14.3 and the ratio of
Q3 silicate was significantly decreased. Deconvolution of the
spectrum into three peaks (TMS, Q3, Q4) was made; the molar
ratio of TMS:Q3 :Q4 was 17.2:11.7:71.1, indicating that the
content of silanol groups after trimethylsilylation was 0.208
mol (mol Si)21 [mol Si for Ti-MCM-41 (non-sil)], while that of
silanol groups before trimethylsilylation was 0.263 mol (mol
Si)21. Thus a part of original silanol groups could not be
trimethylsilylated and furthermore, the increase in Q4 species
was moderate compared to the expected increase resulting from
the substitution of trimethylsilyl groups for silanol H atoms,
suggesting that some silanol groups were newly formed during
the trimethylsilylation. The incomplete trimethylsilylation is
considered to be due to the bulkiness of the trimethylsilyl group
(ca. 0.6 nm). Despite the incomplete modification of silanol
groups, Ti-MCM-41 (sil) was highly hydrophobic; the amount
of adsorbed water was measured by TG after exposure to
moisture over saturated aqueous solution of NH4Cl at room
temperature for 24 h; Ti-MCM-41 (sil) adsorbed only 0.29
mass% H2O, whereas Ti-MCM-41 (non-sil) adsorbed 55
mass% H2O.
The recent discovery of a new family of mesoporous molecular
sieves has received much attention.1,2 Ti-substituted MCM-413
and MCM-484 and Ti-substituted hexagonal mesoporous silica
(Ti-HMS5) have been synthesized, and pioneered the potential
to oxidize bulky molecules which cannot enter into the
micropores of zeolites such as TS-1, TS-2 and Ti-b. However,
it has been reported that Ti-MCM-41 samples show a lower
intrinsic activity and lower selectivity toward the use of H2O2
for alkene oxidation than either TS-1 or Ti-b owing to their high
hydrophilicity.6 It has been recently proposed that the hydro-
philic/hydrophobic property of Ti zeolites plays an important
role in their activity for liquid phase oxidations.7 We have
conducted trimethylsilylation of Ti-MCM-41 and Ti-MCM-48
in order to enhance activity in oxidation with dilute H2O2
through increasing their hydrophobicity. Here, we report that
trimethylsilylation of Ti-containing mesoporous molecular
sieves, Ti-MCM-41 and Ti-MCM-48, resulted in a remarkable
enhancement of catalytic activity in oxidation of alkenes and
alkanes with H2O2.
Ti-MCM-41 was synthesized as previously described.8
Tetraethylorthosilicate, tetrabutylorthotitanate and cetyl-
trimethylammonium chloride (CTMACl) were used, and the
molar composition of the gels subjected to hydrothermal
synthesis was SiO2 :0.01 TiO2 :0.6 CTMA:0.3 NMe4OH:60
H2O. Ti-MCM-48 was similarly synthesized by using
CTMACl/OH (Cl/OH = 70/30) as a template.9 The molar
composition of the gels was: SiO2 :0.02 TiO2 :CTMA:46.5
H2O. Trimethylsilylation was conducted according to literat-
ure;2 0.5 g Ti-MCM-41 or -48, 10 g Me3SiCl and 15 g
(Me3Si)2O were refluxed for 16 h under N2. The HCl produced
was stripped off and the dry powder obtained was thoroughly
washed with dry acetone with centrifuging, and dried at 393 K
for 3 h. The samples before and after trimethylsilylation are
denoted Ti-MCM-41 or -48 (non-sil) and Ti-MCM-41 or -48
(sil), respectively. A typical oxidation run used 50 mg of a
catalyst in 25 mmol of substrate in a round-bottom flask, to
which was added 5 mmol of H2O2 (30% aqueous solution). The
resulting mixture was stirred at various temperatures.
We performed oxidation of cyclohexene, hexane and pent-
2-en-1-ol with H2O2 on Ti-MCM-41 and Ti-MCM-48 samples.
The results are summarized in Table 2. Neither Ti-MCM-41
(non-sil) nor Ti-MCM-48 (non-sil) exhibited significant activ-
ity in the oxidation of cyclohexene. However, trimethylsilyla-
tion of these Ti-containing mesoporous solids resulted in a ca.
20-fold increase in the activity in the oxidation of cyclohexene.
Furthermore, non-productive decomposition of H2O2 was
inhibited by the trimethylsilylation. The turnover number
The X-ray diffraction patterns of Ti-MCM-41 and Ti-MCM-
48 materials closely matched those of the pure silica iso-
morph.1,2 Although the X-ray diffraction patterns and the d
spacings were virtually unchanged after trimethylsilylation, the
pore diameter determined by the Dolimore–Heal method, the
BET surface area, and the pore volume decreased upon
trimethylsilylation (Table 1). These observations could be
accounted for by assuming that the silanol groups inside the
pore were trimethylsilylated.
Table 1 Properties of Ti-containing mesoporous molecular sieves before
and after trimethylsilylation
Pore
Pore
BET surface
Si/Ti area/m2 g21
diameter/
nm
volume/
ml g21
Catalyst
Ti-MCM-41 (non-sil)
Ti-MCM-48 (non-sil)
Ti-MCM-41 (sil)
123
47
139
51
1015
1048
879
2.32
2.32
1.90
1.90
0.88
0.91
0.82
0.71
In the IR spectra of Ti-MCM-41 samples the band at 960
cm21, which is used as evidence for Ti incorporation into the
framework in Ti-containing microporous molecular sieves such
Ti-MCM-48 (sil)
839
Chem. Commun., 1998
325

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Table 2 Effect of trimethylsilylation on catalytic activity of Ti-containing mesoporous molecular sieves
Conversion
(mol% of max.)
TON/
mol (mol Ti)21
H2O2 decomposition
(%)
Catalyst
Selectivity (%)
Cyclohexene oxidation
Alcohol
Ketone
Epoxide
Diol
Ti-MCM-41 (non-sil)a
Ti-MCM-48 (non-sil)b
Ti-MCM-41 (sil)a
0.72
2.1
13.3
38.5
5.4
6.1
112.1
120.9
30.0
26.7
14.4
21.3
15.2
32.8
21.0
17.0
0
54.7
35.7
50.7
59.4
57.6
61.9
0
4.7
13.9
2.2
Ti-MCM-48 (sil)a
0
Hexane oxidation
2-ol
3-ol
2-one
3-one
Ti-MCM-41 (non-sil)b
Ti-MCM-41 (sil)b
Ti-MCM-41 (sil)c
Ti-MCM-48 (non-sil)b
Ti-MCM-48 (sil)b
0
0
—
—
—
—
74.7
0.0
97.3
75.0
20.2
0.06
0.20
0
0.50
6.8
0
40.5
22.0
—
59.5
22.9
—
0.0
31.0
—
0.0
24.1
—
0.17
0.52
45.4
54.6
0.0
0.0
Pent-2-en-1-ol oxidation
Pent-2-enal
Epoxide
Ti-MCM-41 (non-sil)b
Ti-MCM-48 (non-sil)b
Ti-MCM-41 (sil)b
32.4
32.0
14.7
20.7
242
92.9
124
59.3
81.0
78.9
81.6
73.4
19.0
21.1
18.4
26.5
0
3.0
0
Ti-MCM-48 (sil)b
0
a Catalyst 50 mg, substrate 25 mmol, H2O2 5 mmol, 323 K, 3 h. b Catalyst 50 mg, substrate 25 mmol, H2O2 5 mmol, 323 K, 2 h. c Catalyst 50 mg, substrate
100 mmol, H2O2 20 mmol, 353 K, 16 h.
(TON) per hour was even higher than that observed with Ti-b at
333 K.12 The selectivity for epoxide/diol was increased at the
expense of allylic oxidation. In the oxidation of hexane, no
products were obtained with the non-trimethylsilylated sam-
ples, where H2O2 was mostly decomposed. On the other hand,
hexane was oxidized on the trimethylsilylated samples with
retarded decomposition of H2O2. The turnover number in-
creased to 6.8 mol (mol Ti)21 with increasing reaction
temperature, while the rate of H2O2 decomposition increased.
The difference in the activity for cyclohexene and hexane
oxidation between non-trimethylsilylated and trimethylsilylated
samples may be explained in terms of hydrophobicity/hydro-
philicity; excess water should prevent non-polar cyclohexene or
hexane from adsorbing into the pores of the non-trimethylsily-
lated catalysts and/or approaching active sites inside the pores.
It has been observed that trimethylsilylated catalysts exist in the
organic phase because of their hydrophobicity, resulting in
mitigation of the inhibition of oxidation caused by water. No
leaching of Ti was observed during the oxidation.
As shown in Table 2, pent-2-en-1-ol was oxidized with H2O2
on both Ti-MCM-41 and -48 (non-sil) at a much higher rate than
cyclohexene. The enhanced reactivity of this OH-containing
alkenes may be due to the possible formation of a ring structure
where alcoholic OH is bound to Ti as previously proposed.13,14
It should be noted that the difference in the reactivity between
alkenes and allylic alcohols on TS-1 was not so marked
compared to this case.15 Thus it is conceivable that, because of
the presence of a considerable amount of silanol groups, pent-
2-en-1-ol containing a polar hydroxyl group more easily
migrates into the pores of non-trimethylsilylated catalysts than
simple alkenes in competition with water, showing a much
higher reactivity than cyclohexene. Trimethylsilylated samples
Ti-MCM-41 (sil) and Ti-MCM-48 (sil) exhibited lower activity
in the oxidation of pent-2-en-1-ol than the corresponding non-
trimethylsilylated samples. Partial removal of silanol groups
would reduce the capability to absorb the allylic alcohols
through hydrogen bonding. The decreased activity might be
also due to the difficulty in access to the active site caused by the
introduction of rather bulky trimethylsilyl groups. Studies on
the steric environment of the Ti site are in progress using a
variety of reactants as probe molecules.
to moisture than that of pure silica samples.8 For example, the
d100 peak height of Ti-MCM-41 (non-sil) decreased by 80%
upon exposure to moisture over saturated aqueous solution of
NH4Cl for 3 days. In contrast the ordered structure of both Ti-
MCM-41 (sil) and Ti-MCM-48 (sil) was found to be intact upon
exposure to moisture for 30 days.
The authors are grateful to Dr S. Nakata and Mr Y. Tanaka
(Chiyoda Corp.) for measuring 29Si MAS NMR spectra. This
work was supported in part by Grant-in-aid for Scientific
Research on Priority Areas (No. 0742104) from the Ministry of
Education, Science and Culture, Japan.
Notes and References
* E-mail: tatsumi@catal.t.u-tokyo.ac.jp
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Trimethylsilylation leads to a further advantage: the meso-
porous structure of Ti-containing samples is much less resistant
Received in Cambridge, UK, 26th August, 1997; 7/06175G
326
Chem. Commun., 1998