Epoxidation of styrene on a novel titanium-Silica catalyst prepared by ion beam implantation

Article abstract of DOI:10.1006/jcat.1999.2406

A Ti-SO₂ catalyst with highly isolated titanium ions was prepared by ion beam implantation. Thecatalyst wastested for theepoxidation of styrene using dilute H₂O₂ (30%) as the oxidizing agent. Two kinds of product were form

Full text of DOI:10.1006/jcat.1999.2406

Journal of Catalysis 183, 128–130 (1999)
Article ID jcat.1999.2406, available online at http://www.idealibrary.com on
Epoxidation of Styrene on a Novel Titanium–Silica Catalyst
Prepared by Ion Beam Implantation
1
�
Qihua Yang, Can Li, Shandong Yuan, Jian Li, Pinliang Ying, Qin Xin, and Weidong Shi
�
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,China; and State Key
Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116023, China
Received September 23, 1998; revised December 8, 1998; accepted January 4, 1999
In this paper we report for the first time a novel titanium–
A Ti–SiO2 catalyst with highly isolated titanium ions was pre- silica catalyst prepared by a physical method, ion beam im-
pared by ion beam implantation. The catalyst was tested for the epo- plantation, and the epoxidation of styrene as a test reaction
xidation of styrene using dilute H2O2 (30%) as the oxidizing agent.
Two kinds of product were formed: epoxide and benzaldehyde. The
selectivity for epoxide approaches 100% at 298 K and decreases at
for this new catalyst. It is found that the Ti–SiO2 catalyst
prepared by ion beam implantation shows quite good ac-
tivity and selectivity in the epoxidation of styrene.
Ion beam implantation has been widely used to modify
elevated reaction temperatures. A turnover frequency for styrene up
�
1
to 11.7 h was observed. The activity and selectivity for epoxide
decreased when the catalyst was calcined at higher temperatures.
It is proposed that there are at least two types of active sites on the
materials (10, 11), but no work has been reported on its use
in preparing catalysts for selective oxidation. Principally,
ion beam implantation is an effective method for preparing
isolated atoms on a solid surface. In this work, the titanium
atom is ionized first, then the ionized titanium beam is ac-
celerated under a high voltage and the titanium ion beam is
implanted at high speeds in a support (silica in this work).
The titanium ions implanted in the support are highly iso-
lated because the ion density of the beam is very small and
the ions are repelled from each other in the beam. These
highly isolated titanium sites may resemble the titanium
sites in TS-1 zeolites.
3
+
4+
catalyst: Ti(�) (isolated Ti and Ti ) for the selective formation of
epoxide and Ti(�) (TiO2 clusters) for the formation of benzaldehyde.
�
c 1999 Academic Press
Epoxides are important intermediates in organic synthe-
sis of fine chemicals and pharmaceuticals. The direct epox-
idation of alkenes has been the main process for preparing
the epoxides. Epoxidation has traditionally been carried
out using peracids, and the procedures are very costly and
usually produce huge amounts of pollutants. It is highly
desirable to replace the conventional process with an envi-
ronmentally benign procedure. One possibility is titanium
silicalite-1 (TS-1) which shows unique catalytic activity and
selectivity in the oxidation of a large number of organic sub-
strates, such as alkenes, alcohols, aromatics, phenols, and
alkanes (1–5), using H2O2 as oxidant under mild reaction
conditions. The high activity and selectivity of TS-1 catalyst
are attributed to its framework titanium sites which are iso-
lated monoatomic sites. But the small pore size, 0.5–0.6 nm,
of this zeolite limits its applications to the epoxidation of
small molecules (6). Many efforts have been made to find
alternative catalysts with highly isolated titanium sites. The
preparation of such catalysts has focused mainly on using
chemical methods, such as the impregnation of silica with
titanium-containing precursors (7), a sol–gel procedure fol-
lowed by supercritical drying (8), and different approaches
to zeolite synthesis (9).
The ion implantation was carried out in a Mevva 80-10
18
ion implantation system at 573 K with a dose of 3 � 10 ions
�
2
cm at 40 keV. The vacuum system during the implanta-
�
3
� 2
tion was maintained at 1.07 � 10 N m . Silica powder of
2
� 1
4
0–60 mesh and with a BET surface area of ca. 300 m g
was used as the support. The silica powder was kept rotat-
ing in a vessel during the implantation to ensure a uniform
distribution of the titanium ions on the silica surface. Tita-
nium metal with a purity of 99.9% was used as the titanium
source. The catalyst prepared by the ion beam implanta-
tion is referred to as Ti–SiO2(298). The titanium content of
the Ti–SiO2(298) catalyst was measured to be 0.28 wt% by
ICP-AES. The catalyst was calcined at 573, 673, and 773 K
in air for 2 h before use. The samples calcined at these
different temperatures are referred to as Ti–SiO2(573), Ti–
SiO2(673), and Ti–SiO2(773), respectively.
Table 1 gives the results of the epoxidation of styrene at
3
33 K on the implanted Ti–SiO2 catalyst calcined at differ-
ent temperatures. Two types of products are formed: the
epoxide and benzaldehyde. For the uncalcined catalyst, Ti–
SiO2(298), the turnover frequency (TOF) is estimated to
1
To whom correspondence should be addressed. Fax: 86–0411–
4
694447. E-mail: canli@ms.dicp.ac.cn.
128
0021-9517/99 $30.00
Copyright �c 1999 by Academic Press
All rights of reproduction in any form reserved.

EPOXIDATION OF STYRENE ON Ti–SiO2
129
TABLE 1
Epoxidation of Styrene on Ti–SiO2 Catalyst Calcined at Different Temperatures
Selectivity (mol% )
a
b
TOF
(h
Convstyrene
(mol% )
EffH2
O
2
(% )
�
1
Catalyst
)
Epoxide
Benzaldehyde
Phenylacetaldehyde
Others
Ti–SiO2(298)
Ti–SiO2(573)
Ti–SiO2(673)
Ti–SiO2(773)
TS-1c
12.3
11.7
4.4
3.1
5.9
4.8
4.5
1.7
1.2
35.4
37.8
36.3
11.5
8.5
55.5
52.5
53.1
0
44.5
47.5
46.9
100
0
0
0
0
74.2
0
0
0
0
2.0
22.8
1.0
Note. Catalyst, 0.4 g; styrene, 18 mmol; H2O2, 18 mmol; solvent CH3CN, 4 ml; reaction temperature, 338 K; reaction time, 3 h.
The products were separated from the catalyst and analyzed using gas chromatography (50 m � 0.2 mm i.d. capillary column).
a
TOF = moles of styrene converted per mole of Ti in the catalyst per hour. All titaniums ions are assumed to be on the surface
�
5
of the catalyst. One gram of Ti–SiO2 contains 5.8 � 10 mol of Ti.
b
Peroxide efficiency is defined as millimoles of substrate converted per millimole of hydrogen peroxide consumed.
From Ref. (12); the reaction temperature was 333 K.
c
be at least two times larger than that of TS-1 (12). The se- seems that benzaldehyde is formed mainly via the further
lectivity for epoxide is more than 50% for the implanted oxidation of the epoxide, and the higher reaction tempera-
Ti–SiO2 while TS-1 produced mainly phenylacetaldehyde, ture favors the oxidation of epoxide.
the isomeric product of the epoxide. This indicates that the
isolated titanium ions in the implanted catalyst possess very of styrene on the Ti–SiO2 catalyst can be explained in terms
high activity and selectivity in the epoxidation of styrene. of the unique active sites derived by ion beam implantation.
The results in Table 1 show that the calcination temper- Since the Ticontent ofTi–SiO2 isonly0.28% and the surface
The excellent activity and selectivity in the epoxidation
2
� 1
ature has a large effect on activity and selectivity. The ac- area ofSiO2 ismore than 200m g , the possibilityofaggre-
tivity and selectivity decrease when the catalyst is calcined gated titanium ions is small. Therefore most of the titanium
at higher temperatures. When the catalyst was calcined at ions are in a well-isolated form on the silica. XPS failed to
7
73 K, no epoxide product was detected. This implies that determine the valence state of the titanium ions in Ti–SiO2
the nature of the active sites in the catalyst was significantly because the titanium content is far below the detection
changed with the calcination treatment. limit. A sharp EPR signal at a g value of 2.003 was observed
The effect of reaction temperature on styrene conversion for Ti–SiO2(298). This EPR signal is tentatively assigned to
3+
and product selectivity is summarized in Table 2. As ex- the isolated Ti species (13) and/or a trapped single elec-
pected, by increasing the reaction temperature from 298 to tron, implying that some Ti3 may be stabilized in the SiO2
+
3
58 K, the conversion of styrene increased from 1.8 to 6.7% . substrate. The EPR signal gradually weakens when the cal-
The selectivity for epoxide is 100% at 298 K, but decreases cination temperature is increased, and disappears when the
dramatically at higher temperatures, while the selectivity sample is calcined at 773 K in air. The disappearance of the
for benzaldehyde increased at the cost of the epoxide. It EPR signal is possibly due to the oxidation of Ti3 to Ti
+
4+
because this signal is maintained for the calcination in N2.
The titanium sites on the Ti–SiO2 catalyst can be grouped
TABLE 2
into at least two kinds of species: isolated Ti3 and Ti
+
4+
Dependence of Catalytic Activity and Selectivity on Reaction
Temperature for Ti–SiO2(298) Catalyst
ions, referred to as Ti(�); and small oxide clusters, (TiO2)n
n > 1), referred to as Ti(�). It is suggested that the surface
(
3+
4+
titanium ions, either Ti or Ti on the SiO2 support, are
Selectivity (mol% )
3+
a
TOF
(h
Convstyrene
(mol% )
stabilized in the form of oxide species, i.e., TiO1.5(Ti ) and
�
1
4+ + 2+
Benzaldehyde TiO (Ti ). Other Ti and Ti ions are quite unstable and
2
T (K)
)
Epoxide
3+
4+
easily oxidized into Ti or Ti and their concentrations
in the Ti–SiO2 catalyst will be extremely small, especially
when the sample is calcined in air at high temperature.
2
3
3
98
38
58
4.7
12.3
17.5
1.8
4.8
6.7
100
55.5
16.0
0
44.5
84.0
3+
4+
Therefore the TiO1.5(Ti ) and TiO2(Ti ) species should
predominate on the catalyst surface. In addition, some ag-
gregated oxide clusters (TiO2)n (n > 1) may be formed in
air during calcination at high temperature. According to
the correlation between the catalytic data and calcination
Note. Catalyst, 0.4 g; styrene, 18 mmol; H2O2, 18 mmol; solvent CH3CN,
4
ml; reaction time, 3 h.
a
TOF = moles of styrene converted per mole of Ti in the catalyst per
hour. All titanium ions are assumed to be on the surface of the catalyst.
One gram of Ti–SiO2 contains 5.8 � 10 mol of Ti.
�
5

1
30
YANG ET AL.
temperature, we assume that Ti(�) sites contribute to the
high activity and selectivity for the epoxide, while Ti(�)
sites contribute mainly to the formation of benzaldehyde.
The lower activity of the catalyst calcined at higher temper-
atures is due to the decrease in Ti(�) sites.
TABLE 3
Reusability of Ti–SiO2(573) in Epoxidation of Styrene
Selectivity (mol% )
a
No. of times
used
TOF
(h
Convstyrene
(mol% )
�
1
)
Epoxide
Benzaldehyde
To confirm the stability of Ti3 sites under calcination at
high temperature, Ti–SiO2 was heated in N2 instead of air
at 773 K for 2 h and cooled down to room temperature in
N2. This catalyst shows a conversion of styrene of 7.2% and
a selectivity to epoxide of 66.8% which is even higher than
that of Ti–SiO2(298). The catalyst also shows an EPR signal
+
1
2
3
4
5
11.7
11.7
11.9
11.2
11.4
4.5
4.5
4.6
4.3
4.4
52.5
50.9
52.8
50.7
49.6
47.5
49.1
47.2
49.3
50.4
3+
Note. Catalyst, 0.4g;styrene, 18mmol;H2O2, 18mmol;solvent CH3CN,
4 ml; reaction temperature, 338 K; reaction time, 3 h.
at g = 2.003. This strongly suggests that the Ti sites are
quite stable and can be oxidized to Ti4+ only in air at eleva-
a
TOF = moles of styrene converted per mole of Ti in the catalyst per
ted temperatures. The higher conversion and epoxide selec-
hour. All titanium ions are assumed to be on the surface of the catalyst.
3+
tivity are partly due to the fact that the isolated Ti and
� 5
One gram of Ti–SiO2 contains 5.8 � 10 mol of Ti. The solid catalyst was
4+
Ti sites in the bulk of the silica move to the surface when
the catalyst is calcined.
filtered after reaction, washed with water, and dried at 393 K for 6 h and
finally calcined in air at 573 K for 2 h.
The following epoxidation routes of styrene are pro-
posed:
(
30% ) as oxidizing agent. The high activity and selectivity
are attributed to the highly isolated titanium sites on the
implanted silica surface. It is proposed that there are two
3+
types of active sites, on the catalyst, Ti(�) (isolated Ti
4+
and Ti ) and Ti(�) (TiO2 clusters), and the formation of
epoxide and benzaldehyde is catalyzed by the two kinds of
active sites, respectively.
It seems that the successive reactions are catalyzed respec-
tively by two types of active sites, Ti(�) and Ti(�). The epox-
idation may take place mainly on Ti(�) sites while the cleav-
age of the carbon–carbon bond takes place mainly on Ti(�)
sites because the selectivity to benzaldehyde is increased
for the Ti–SiO2 catalyst calcined at high temperature where
more Ti(�) species are produced. When the reaction tem-
perature is increased, the epoxide produced is further oxi-
dized on Ti(�) sites. Accordingly, the selectivity of epoxide
depends on both reaction temperature and concentration
of surface Ti(�) sites.
The reusability of the catalyst is reported in Table 3. No
obvious loss of activity and selectivity is observed after the
catalyst is reused five times. This result confirms that the
leaching of the titanium ions from the Ti–SiO2 catalyst is
negligible, at least the first several times. This may be due
to the chemical bonding of TiOx and (TiO2)n with SiO2.
In summary, a titanium–silica catalyst prepared by ion
beam implantation shows high catalytic activity and se-
REFERENCES
1
2
. Epsosito, A., Taramasso, M., and Buonomo, F., U.S. Patent 2,116,974
(1985).
. Neri, C., Esposito, A., Anfossi, B., and Buonomo, F., European Patent
Appl. 100,119 (1984).
. Esposito, A., Neri, C., and Buonomo, F., U.S. Patent 4,480,135 (1984).
. Tatsumi, T., Nakamura, M., Negishi, S., and Tominage, H., J. Chem.
Soc. Chem. Commun., 476 (1990).
5. Huybrechts, D. R. C., De Bruycker, L., and Jacobs, P. A., Nature 345,
40 (1990).
. Kochkar, H., and Figueras, F., J. Catal. 171, 420 (1997).
. Neumann, R., and Levin-Elad, M., J. Catal. 166, 206 (1997).
. Fraile, J. M., Garcia, J. I., Mayoral, J. A., der Menorval, L. C., and
Rachdi, F., J. Chem. Soc. Chem. Commun., 539 (1995).
9. Van der Waal, J. C., Rigutto, M. S., and van Bekkum, H., Appl. Catal.
A 167, 331 (1998).
0. Leon Shohet, J., IEEE Trans. Plasma Sci. 19, 725 (1991).
1. Coates, D. M., and Kaplan, S. L., MRS Bull., 43 (1996).
2. Kumar, S. B., Mirajkar, S. P., Pais, Godwin, C. G., Kumar, P., and Kumar,
R., J. Catal. 156, 163 (1995).
3
4
2
6
7
8
1
1
1
lectivity for the epoxidation of styrene using dilute H2O2 13. Brintzinger, H. H., J. Am. Chem. Soc. 89, 6871 (1967).