X. Shen et al. / Applied Catalysis A: General 401 (2011) 37–45
39
Table 2
100
1
00
A
Catalytic results of Ti-YNU-1-227 in the epoxidation of cycloheptene at different
H2O2/c-C7H12 ratios .
a
80
80
60
40
20
0
Samples
H2O2/c-C7H12
Conv. (%)
Selectivity (%)
H2O2 eff. (%)b
Epoxide Others
6
4
2
0
0
0
0
Ti-YNU-1-227
Ti-YNU-1-227 1.5
Ti-YNU-1-227
1
71.2
74.6
75.8
92.6
91.8
91.5
7.4
7.2
8.5
79.7
66.8
65.0
2
a
◦
Reaction conditions: 60 C, 4 h, 0.1 g catalyst, 5 mmol substrate, 5–10 mmol H2O2
31% in aqueous solution), 5 mL MeCN.
(
b
H2O2 efficiency.
consumption of the oxidant would prevent the progress of the
reaction.
0
20
40
60
80
100
120
Liquid/solid ratio/(ml/g)
3
.2.3. Effect of acid treatment
It is known that the acid treatment of Ti-MWW before calci-
9
7
5
3
1
000
200
400
600
B
nation is necessary for obtaining high activity and selectivity in
the epoxidation of alkenes. Particularly, this process is indispens-
able for the formation of Ti-YNU-1 [15,16]. Thus, it is reasonable
to deduce that the acid-treatment conditions would have a strong
effect on the catalytic performance of Ti-YNU-1. Indeed, it was
found that the optimum liquid/solid (L/S) ratio should be between
L/S = 100 (mL/g), Si/Ti = 205
L/S = 70 (mL/g), Si/Ti = 165
4
0 and 50 mL/g in terms of cycloheptene conversion if the sam-
ple was treated with 2 M HNO3 for 20 h under reflux conditions
Fig. 2A). When the L/S ratio was too low, i.e. 30, the prepared sam-
L/S = 50 (mL/g), Si/Ti =142
L/S = 40 (mL/g), Si/Ti = 136
(
ple contained less Ti-YNU-1 (Fig. 2B). However, when Ti-MWW
precursor was subjected to heavy treatment with acid (the L/S
ratio > 70), a large amount of Ti species were washed away, thus
decreasing the active sites and lowering the conversion.
800
0
L/S = 30 (mL/g), Si/Ti = 119
3.2.4. Effect of reaction temperature
0
5
10
15
20
25
30
The epoxidation of cycloheptene over Ti-YNU-1-267 is greatly
2
Theta/degree
dependent on the reaction temperature and comparable with
that over Ti-Beta (Fig. 3). Cycloheptene conversion almost lin-
early increased with the temperature for both materials, while the
reverse trend was observed for epoxide selectivity. Nevertheless,
Ti-YNU-1-267 gave a very high selectivity, which was maintained
above 90% regardless of the reaction temperature, whereas the
epoxide selectivity on Ti-Beta-44 declined to 78% when the tem-
Fig. 2. Catalytic results (A) and XRD patterns (B) of the Ti-YNU-1 prepared by treat-
ing the postsynthesized Ti-MWW lamellar precursor with different amounts of 2 M
◦
HNO3 aqueous solution for 20 h before calcination (reaction conditions: 60 C, 2.5 h,
0.1 g catalyst, 5 mL MeCN, 5 mmol substrate, 7.5 mmol H2O2 (31% in aqueous solu-
tion)).
◦
perature was increased to 75 C. In addition, Fig. 3 shows that
higher conversion than Ti-Beta-44 and the difference became more
marked with increasing reaction time. The gradual reduction of
reaction rate is due to the consumption of the substrate, resulting
in a decrease in the substrate content, and the deposition of large-
molecule organic compounds in the channels [19]. In addition, it
can be seen that cycloheptene conversion over the Ti-YNU-1-155
could reach as high as 41% at the reaction time of 10 min, implying
the very fast initial reaction rate. The high selectivity (>90%) of Ti-
YNU-1-155 indicates that the ring opening of cycloheptene oxide is
not severe, but this side reaction occurs much more seriously over
the Ti-Beta-44 owing to the presence of weak acid sites [11,17].
Although H2O2 efficiency decreased as the reaction time increased
for both materials, it was higher on the Ti-YNU-1-155 than on the
Ti-Beta-44.
the reaction temperature has a significant influence also on the
H O efficiency. The highest H O efficiency for both materials was
2
2
2
2
◦
obtained at the same temperature of 40 C. This can be explained
as follows: H O2 was consumed via two routes. One was for
2
the oxidation of substrates; the other was consumed by thermal
decomposition. Generally, high conversion gives high H O2 effi-
2
ciency [17,18]. Therefore, it is reasonable of the observance that
H O efficiency increases with increasing reaction temperature
2
2
◦
below 40 C because of the marked increase in cycloheptene con-
version. However, at high reaction temperature, decomposition of
H O becomes appreciable, and the degree increased with increas-
2
2
ing reaction temperature as a result of higher activation energy for
decomposition of H O2 than for formation of epoxide through the
2
attack of active oxygen species by the ꢀ electrons of alkenes. There-
fore, the H O efficiency conversely decreased when the reaction
temperature was higher than 40 C.
2
2
3.2.6. Effect of catalyst amount
◦
The above studies show that Ti-YNU-1 is a potential, envi-
ronmentally benign heterogeneous catalyst for the epoxidation of
cycloheptene. With regard to the activity, epoxide selectivity and
H O efficiency, Ti-YNU-1 is highly superior to Ti-Beta. In particu-
3.2.5. Effect of reaction time
Fig. 4 shows the catalytic results of Ti-YNU-1-155 and Ti-Beta-44
2
2
for the oxidation of cycloheptene as a function of reaction time. It is
clear that cycloheptene conversion quickly increased to about 71%
and 59% within 1.5 h over the Ti-YNU-1-155 and Ti-Beta-44, respec-
tively. Then, a plateau was observed, especially while the reaction
time was longer than 2.5 h. Nevertheless, Ti-YNU-1-155 exhibited
lar, the catalytic stability of the former is also much better than that
of the latter [16]. After 8 repeated runs, the structure of Ti-YNU-
1 and the tetrahedral coordination state of Ti species were kept,
while the irreversible change of the Ti state from the tetrahedral to
octahedral coordination occurred to Ti-Beta [16]. Nevertheless, the