Photocatalytic epoxidation of propene by molecular oxygen over highly dispersed titanium oxide species on silica

Article abstract of DOI:10.1039/a904886c

Highly dispersed titanium oxide species on silica prepared by the sol-gel method catalyse selective epoxidation of propene by molecular oxygen upon photoirradiation.

Full text of DOI:10.1039/a904886c

Photocatalytic epoxidation of propene by molecular oxygen over highly
dispersed titanium oxide species on silica
Hisao Yoshida,* Chizu Murata and Tadashi Hattori
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan.
E-mail: yoshidah@apchem.nagoya-u.ac.jp
Received (in Oxford, UK) 17th June 1999, Accepted 2nd July 1999
Highly dispersed titanium oxide species on silica prepared
by the sol-gel method catalyse selective epoxidation of
propene by molecular oxygen upon photoirradiation.
the vessel. A 200 W Xe lamp was used as the light source.
Products in the gas phase and products desorbed by photo-
irradiation for 1 h were analyzed separately by gas chromatog-
raphy, followed by analysis of the desorbed products by heating
at 573 K. The results presented here are the sum of each product
yield. The products were propene oxide (PO), propanal (PA),
acetone (AC), acrolein (AL), acetaldehyde (AA), alcohols
Propene epoxidation is of industrial importance. The mixed
oxide of Ti and Si has been recognized as a catalyst for the
epoxidation of olefins with hydroperoxides and is employed
1
i
industrially to produce propene oxide (PO) from propene. TS-
(MeOH, EtOH and Pr OH), hydrocarbons (ethene and butenes;
1
, titanium-containing crystalline silica, is known to catalyse
2 x
HC) and CO and CO (CO ).
Fig. 1 shows diffuse reflectance UV-VIS spectra of the
samples. Bare silica showed no large absorption band [Fig.
2
epoxidation of propene using H
oxygen as oxidant would be the most economical way of
carrying out the oxidation. The discovery of catalysts for the
O
2 2
.
However, employing
1
2
1(a)], while bulk TiO showed a large absorption band below
epoxidation of propene by molecular oxygen is highly desir-
able. One of the most promising systems for this reaction should
the band gap [Fig. 1(d)]. Both T/S(0.1) and T-S(0.34) exhibited
a narrow band centred at ca. 210 nm [Fig. 1(b),(c)] which is
assigned to the ligand-to-metal charge transfer band of highly
dispersed tetrahedral titanium species.7 This means that
T/S(0.1) and T-S(0.34) consist of predominantly highly dis-
persed tetrahedral titanium species on silica.
be photocatalysis on silica-supported systems, since PO was
3
obtained as a product over Nb/SiO
2
, Mg/SiO
2
and even over
bare amorphous silica4 with significant selectivity. However,
the PO yield in these systems was still low, and it is not clear
whether the reaction proceeds catalytically or not.
,5
Table 1 shows the results of photooxidation of propene by
molecular oxygen on the samples. Bare silica (entry 1) exhibited
almost the same conversion (1.53%) and propene oxide
selectivity (22.3%) as previously reported.4 The conversion to
In the present study, we prepared highly dispersed titanium
oxide species on silica by two methods, impregnation and sol-
gel methods, and examined their photocatalytic activity for
propene epoxidation using molecular oxygen.
,5
CO
x
was not significant. Other major by-products are AC, AL
(entry 2) showed high photooxidation
Amorphous silica was prepared from Si(OEt)
4
by the sol-gel
and AA. Bulk TiO
2
method followed by calcination in a flow of air at 773 K for 5 h.6
The bulk titanium oxide employed was a Japan Reference
Catalyst (JRC-TIO-4; equivalent to P-25). A silica-supported
titanium oxide sample containing 0.1 mol% Ti (Ti/Ti + Si),
referred to as T/S(0.1), was prepared by the impregnation
activity; the conversion of propene was 14.1% when irradiated
for 1 h (half of the standard irradiation time). However, the
2 2
method.† A TiO –SiO mixed oxide sample containing 0.34
mol% Ti, T-S(0.34), was prepared by the sol-gel method.‡ The
Ti content was determined by inductively coupled plasma (ICP)
measurements. A diffuse reflectance UV-VIS absorption spec-
trum of the sample in vacuo was recorded at room temperature
with a JASCO V-570 spectrophotometer after the same
pretreatment as for the photoreaction test.
Before the photoreaction, the samples were heated in air up to
6
73 K, and then evacuated. Subsequently the samples were
2
2
treated with 100 Torr oxygen (1 Torr = 133.3 N m ) at 673 K
for 1 h, followed by evacuation at 673 K for 1 h.§ The
photooxidation of propene was carried out in a closed reaction
3
vessel made of quartz (123.6 cm ) for 2 h. The temperature of
the catalyst bed was elevated by ca. 20 K from room
temperature by the photoirradiation. The reactants were propene
(
100 mmol, 15 Torr) and oxygen (200 mmol, 30 Torr). The
Fig. 1 Diffuse reflectance UV-VIS spectra of (a) silica, (b) T/S(0.1),
(c) T-S(0.34) and (d) TiO .
2
2
catalyst (200 mg) was spread on the flat bottom (12.6 cm ) of
Table 1 Results of the photooxidation of propene
BET
surface
Selectivity (%)
area/
m g
Irradiation
time/h
Conversion of
propene (%)
PO yield
(%)
2
21
Entry Sample
PO
PA
AC
AL
AA
Alcohol HC
CO
x
1
2
3
4
SiO
TiO
T/S (0.1)
2
558
34
477
2
1
2
2
0.7
14.1
9.1
0.2
0.0
3.7
5.3
27.1
0.0
40.8
57.5
1.4
0.0
3.3
2.7
26.7
1.3
11.3
5.8
8.9
0.0
4.2
1.2
16.8
0.8
24.5
21.1
6.0
0.0
3.6
0.0
8.8
1.7
2.3
5.1
4.3
96.2
10.0
6.6
2
T-S (0.34) 423
9.2
Chem. Commun., 1999, 1551–1552
1551

on silica proceeded catalytically. This is the first report of the
catalytic photooxidation of propene to propene oxide by
molecular oxygen.
In conclusion, highly dispersed titanium oxide species were
found to catalyse the photooxidation of propene to propene
oxide by molecular oxygen at room temperature. The selectivity
to propene oxide on titanium oxide species on silica was
affected by the preparation method; the sol-gel method was
superior to conventional impregnation methods. Titania–silica
prepared by the sol-gel method in the present study showed
higher selectivity to propene oxide than those previously
reported in the literature. It is expected that fine-tuning of the
preparation method, the amount of titanium oxide and the
reaction conditions should further enhance their activities.
This work was supported by a Grant-in-aid from the Ministry
of Education, Science, Art, Sports and Culture, Japan.
Fig. 2 Time course of (a) propene conversion, (b) PO yield and (c) PO
selectivity in the photooxidation of propene by molecular oxygen on
T/S(0.1).
Notes and references
†
Calcined silica was impregnated by aqueous solutions of ammonium
major product (96.2%) was CO
observed.
x
; no propene oxide was
titanyl oxalate, dried at 383 K for 12 h and calcined at 773 K in a flow of air
for 5 h.
On the other hand, the silica-supported titania sample
T/S(0.1) (entry 3) exhibited higher selectivity to PO (40.8%)
‡ A mixture of Si(OEt) , EtOH, H O and HNO (1, 3.8, 1, and 0.085 mol,
4
2
3
respectively) was stirred at 353 K for 3 h and cooled down to room
i
than bare silica and bulk TiO
T/S(0.1) was much higher than on bare silica, while selectivity
to CO was considerably lower than bulk TiO . Therefore, it
2
. The conversion of propene on
temperature. An Pr OH solution of titanium isopropoxide was added
dropwise and stirred at 293 K for 2 h, followed by very slow addition of an
aqueous solution of HNO
acid solution was equivalent to that of Si(OEt)
3
[1 and 0.085 mol respectively, the amount of this
used]. Two weeks later, the
x
2
4
2
was found that dispersed titanium oxide on silica has a moderate
ability to produce PO from propene and molecular oxygen upon
photoirradiation.
1
obtained gel was heated at a rate of 0.2 K min up to 338 K and dried for
h. After drying for an additional 5 h at 373 K, the TiO –SiO mixed oxide
was obtained by calcination at 773 K in a flow of air for 8 h.
This treatment temperature was lower than that of the previous reports
5
2
2
T-S(0.34) prepared by the sol-gel method showed almost the
same conversion as T/S(0.1); therefore, an accurate comparison
of product selectivity was allowed. Obviously the PO selectivity
on T-S(0.34) was higher than that on T/S(0.1); it was also higher
than the selectivities previously reported in the literature.3 The
§
(refs. 4,5) because low temperature (673 K, even room temperature) was
found to be sufficient to activate the sample.
¶ Turnover number is defined as (the amount of produced PO)/(the amount
of catalytic active sites).
–5
x
conversion to CO was also suppressed more than that of
T/S(0.1). It is thus indicated that T-S(0.34) has excellent
properties for this photoepoxidation; sol-gel methods should be
advantageous for producing active sites for selective photo-
epoxidation of propene using molecular oxygen.
1
2
3
B. Notari, Adv. Catal., 1996, 41, 253.
M. G. Clerich, G. Bellussi and U. Romano, J. Catal., 1991, 129, 159.
T. Tanaka, H. Nojima, H. Yoshida, H. Nakagawa, T. Funabiki and S.
Yoshida, Catal.Today, 1993, 16, 297.
Fig. 2 presents the time course of propene conversion, PO
yield and PO selectivity in the photooxidation on T/S(0.1). The
conversion of propene and yield of PO increased with
increasing irradiation time. The selectivity was not affected
significantly by the conversion at least until a conversion of
4
H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki and S. Yoshida,
Chem. Commun., 1996, 2125.
5 H. Yoshida, T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki and S.
Yoshida, J. Catal., 1997, 171, 351.
S. Yoshida, T. Matsuzaki, T. Kashiwazaki, K. Mori and K. Tarama, Bull.
Chem. Soc. Jpn., 1974, 47, 1564.
S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F.
Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini and G. Vlaic,
J. Phys. Chem., 1994, 98, 4125.
6
7
2
9
4.1%. After 8 h photoirradiation, the yield of PO achieved was
.2% of the initial amount of propene. The amount of PO
produced was 9.2 mmol, and the calculated amount of titanium
ions in the reactor was 3.33 mmol. Even if all the titanium ions
were active sites, the turnover number¶ would be 2.8, indicating
that the photoepoxidation of propene on titanium oxide species
Communication 9/04886C
1552
Chem. Commun., 1999, 1551–1552