Active sites and active oxygen species for photocatalytic epoxidation of propene by molecular oxygen over TiO2-SiO2 binary oxides

Article abstract of DOI:10.1021/jp0277006

TiO₂-SiO₂ binary oxides of low Ti content promoted photocatalytic epoxidation of propene by molecular oxygen. By catalytic runs and UV spectra, it was revealed that the isolated tetrahedral Ti species on the TiO₂-SiO₂ samples are active for the photoepoxidation of propene, while the aggregated titanium oxide species are active mainly for the side reactions. By ESR and stoichiometric reaction tests of radical species, the following mechanism was proposed. Over the isolated tetrahedral Ti species, a [Ti³⁺-O_L^-]* radical pair is formed by UV irradiation. The Ti³⁺ moiety reacts with O₂ to form O₂^-, which could not activate propene by itself. The O_L^- moiety, a hole center on lattice oxygen, reacts with O₂ to form O₃^-, and the O₃^- reacts with propene to yield PO. It is first suggested that the O₃^- is the electrophilic oxygen species effective for the epoxidation of propene. When the O_L^- moiety reacts with propene, acrolein or ethanal is produced through H abstraction or CC bond fission. The lower selectivity of the aggregated titanium oxide species was attributed to the lower stability of the O₃^- and high activity for the consecutive reaction of PO.

Full text of DOI:10.1021/jp0277006

4364
J. Phys. Chem. B 2003, 107, 4364-4373
Active Sites and Active Oxygen Species for Photocatalytic Epoxidation of Propene by
Molecular Oxygen over TiO2-SiO2 Binary Oxides
Chizu Murata, Hisao Yoshida,* Jun Kumagai, and Tadashi Hattori
Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity,
Nagoya 464-8603, Japan
ReceiVed: December 12, 2002; In Final Form: March 3, 2003
TiO2-SiO2 binary oxides of low Ti content promoted photocatalytic epoxidation of propene by molecular
oxygen. By catalytic runs and UV spectra, it was revealed that the isolated tetrahedral Ti species on the
TiO2-SiO2 samples are active for the photoepoxidation of propene, while the aggregated titanium oxide
species are active mainly for the side reactions. By ESR and stoichiometric reaction tests of radical species,
the following mechanism was proposed. Over the isolated tetrahedral Ti species, a [Ti3+-OL-]* radical pair
-
is formed by UV irradiation. The Ti3+ moiety reacts with O2 to form O2 , which could not activate propene
-
-
-
by itself. The OL moiety, a hole center on lattice oxygen, reacts with O2 to form O3 , and the O3 reacts
with propene to yield PO. It is first suggested that the O3- is the electrophilic oxygen species effective for the
epoxidation of propene. When the OL moiety reacts with propene, acrolein or ethanal is produced through
-
H abstraction or CC bond fission. The lower selectivity of the aggregated titanium oxide species was attributed
-
to the lower stability of the O3 and high activity for the consecutive reaction of PO.
Introduction
supported metal oxides for propene photoepoxidation, TiO2/
SiO2 showed the highest PO yield.29 In addition, we confirmed
Propene oxide (PO) is an intermediate for the chemical
industry with a worldwide production capacity of nearly 5 ×
106 tons per year.1 Despite its importance, there is still no direct
oxidation process by molecular oxygen for the preparation of
PO.1 Currently, industrial production of PO is performed by
the chlorohydrin process or the hydroperoxide process. However,
these processes suffer from byproducts and coproducts. Recently
developed was propene epoxidation using H2O2 as oxidant with
TS-1 catalyst.2,3 This process is very selective to produce PO,
and the only byproduct is H2O, which is environmentally
friendly. However, this process has not been commercialized
widely because of the high cost of H2O2.
Many researchers have attempted the direct epoxidation of
propene using molecular oxygen.1 Although some workers have
reported a new approach for propene epoxidation using O2 and
reducing agents such as H24-6 and Zn powder,7 it is certain that
the process using only molecular oxygen and propene is the
best for PO production. Several systems have been reported for
the direct epoxidation of propene by using only molecular
oxygen, for example, heterogeneous catalytic systems such as
modified Ag catalysts,1,8,9 metal nitrate-modified Ti-MCM
catalysts,10 zeolite-supported TiO2 catalysts,11 NaCl-modified
VCe1-xCux oxide catalysts,12 and noncatalytic radical reaction
systems.13,14
that the photoepoxidation of propene over TiO2/SiO2 proceeded
catalytically, and the TiO2-SiO2 sample prepared by the sol-
gel method showed higher PO selectivity than the sample
prepared by the impregnation method.30 In the present study
we prepared a series of TiO2-SiO2 binary oxide samples
containing different amounts of Ti to examine the active sites
and the active oxygen species for propene photoepoxidation.
Experimental Section
TiO2-SiO2 binary oxide samples were prepared by the sol-
gel method consisting of a two-stage hydrolysis procedure.31
Tetraethoxyorthosilicate (Kishida), ethanol (Kishida), distilled
water, and nitric acid (Kishida) were mixed in the amounts 0.5,
1.9, 0.5, and 0.043 mol, respectively. The mixture was stirred
at 353 K for 3 h to hydrolyze tetraethoxyorthosilicate partially,
and the obtained sol was cooled to room temperature. A
2-propanol (Kishida, 20 mL) solution of titanium isopropoxide
(Kishida, 0.0005-0.05 mol) was added to the sol and stirred
for 2 h. Then, an aqueous nitric acid solution (H2O and HNO3
were 0.5 and 0.043 mol, respectively) was added to the sol and
stirred until the gelation was completed (about 3-14 days). The
gel was heated to 338 K at the heating rate 0.2 K min-1 and
dried for 5 h. After additional drying for 5 h at 373 K, it was
calcined at 773 K in flowing air for 8 h. Ti content was
determined by inductively coupled plasma (ICP) measurement.
Titania-silica samples were denoted as T-S(x), where x (mol
% of Ti) ) NTi/(NTi + NSi) × 100. Amorphous silica was
prepared from tetraethoxyorthosilicate by another sol-gel
method followed by calcination in a flow of air at 773 K for 5
h.29 The employed TiO2 sample was a Japan Reference Catalyst
(JRC-TIO-4; equivalent to P-25). The BET surface area of the
samples was determined by N2 adsorption after the sample was
pretreated in a flow of He at 673 K for 0.5 h. The specific
surface area of each TiO2-SiO2 and SiO2 sample was similarly
On the other hand, photosystems have been proposed as
another potential approach for the epoxidation using only
molecular oxygen.15 The epoxidations of higher alkenes in the
liquid phase using photocatalyst were reported in both
heterogeneous16-18 and homogeneous19-21 systems. As for the
photoepoxidation of propene, several systems using TiO2,22
Ba-Y type zeolite,23-25 Nb2O5/SiO2,26 MgO/SiO2, and SiO2
27,28
have been reported. In the TiO2 system, the selectivity to PO
was low.22 In other systems, the activity for photoepoxidation
was still low and it was not clear whether the reaction proceeded
catalytically.23-28 In our previous screening study of silica-
10.1021/jp0277006 CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/11/2003

Epoxidation of Propene by Molecular Oxygen
TABLE 1: Results of the Photooxidation of Propene over the SiO2, TiO2, and TiO2-SiO2 Samplesa
selectivityc/%
Ti content/mol % surface area/m2 g-1 conversionb/% PO yield/% PO propanal acetone acrolein ethanal HC COx
J. Phys. Chem. B, Vol. 107, No. 18, 2003 4365
run
sample
1
2
3
4
5
TiO2-SiO2
0.08
0.34
1.0
4.1
8.3
0.34
0.34
387
423
535
483
416
423
423
558
34
4.4
9.2
12.5
21.0
32.1
0.2
0.9
0.7
14.1
2.6
5.3
6.3
4.5
60.2
57.5
50.5
21.5
5.7
5.5
0.9
2.9
2.7
6.2
19.1
24.5
1.6
9.4
3.5
0.0
10.4
5.8
8.3
11.4
13.6
2.9
0.0
25.8
1.3
1.1
1.2
1.7
2.4
3.1
0.0
0.0
15.2
0.0
17.7
21.1
22.1
24.1
26.4
3.0 3.9
5.1 6.6
3.3 7.4
3.4 14.4
2.9 19.1
1.8
6d TiO2-SiO2
trace
trace
0.2
70.0 20.0 0.0
16.4 73.3 0.0
18.1 10.0 5.1
7e
8
SiO2
22.3
0.0
9f TiO2
0.0
0.8
1.7 96.2
a Catalyst 0.2 g, C3H6 100 µmol, O2 200 µmol, reaction time 2 h. b Based on propene. c PO denotes propene oxide, HC denotes ethene + butenes,
and COx denotes CO + CO2. d In the dark at 330 K for 2 h. e In the absence of O2. f Irradiation time was 1 h.
high enough (400-550 m2 g-1), as shown in Table 1. Thus, in
this study the conversion and PO yield were shown without
normalization by surface area.
Prior to each reaction test and spectroscopic measurement,
the sample was treated with 100 Torr oxygen (1 Torr ) 133.3
N m-2) at 773 K for 1 h, followed by evacuation at 673 K for
1 h. The photooxidation of propene was performed with a
conventional closed system (123 cm3). The sample (200 mg)
was spread on the flat bottom (12.6 cm2) of the quartz vessel.
Propene (100 µmol, 15 Torr) and O2 (200 µmol, 30 Torr) were
introduced into the vessel, and the sample was irradiated by a
200 W Xe lamp. The temperature of the catalyst bed was
elevated by ∼20 K from room temperature due to the photoir-
radiation. After determination of the amount of CO and CO2 in
the gas phase by GC-TCD, the products and residual propene
in the gas phase were collected by a liquid nitrogen trap and
analyzed by GC-FID. After the nontrapped gas components (O2
and CO) were evacuated, the catalyst bed was heated at 573 K
in vacuo with a liquid nitrogen trap to collect the products
desorbed from the catalyst. The results presented here are the
sum of the product amounts collected at each step. Yield and
selectivity were calculated on the basis of propene.
The reactivity of PO over the TiO2-SiO2 samples was tested
Figure 1. Results of the photooxidation of propene over TiO2-SiO2
in two ways: (A) Thermal reaction of PO; PO (20 µmol) was
adsorbed by the sample in the dark at room temperature, and
then the sample was heated at 573 K to collect the products by
a liquid nitrogen trap. (B) Photooxidation of PO; PO (20 µmol)
and O2 (200 µmol) were introduced to the reactor, and then the
sample was irradiated for 1 h. The products were collected and
analyzed in the same manner as that for the reaction test of the
propene photooxidation.
samples. Conversion (a), PO yield (b), and selectivity to PO (c), ethanal
(d), propanal (e), and CO + CO2 (f).
dation of propene over the TiO2-SiO2, SiO2, and TiO2 samples.
Partial oxidation of propene took place over all the TiO2-SiO2
samples under the photoirradiation (runs 1-5). The major
products were PO, ethanal, propanal, CO, CO2, acetone, and
acrolein; and small amounts of 2-propanol, ethene, and butene
were also formed. It was confirmed that the epoxidation of
propene did not proceed on the T-S(0.34) sample in the dark
(run 6). In the absence of O2, propene conversion was very low,
and the main products were ethene, butane, and ethanal (run
7), which would be produced by the photometathesis of propene
and the reaction between the lattice oxygen and propene.28,32-37
This result indicates that molecular O2 is essential for the
photoepoxidation of propene over the TiO2-SiO2 samples. The
SiO2 sample showed low conversion and 22% PO selectivity
(run 8),29,30 while the TiO2 sample showed high activity for the
complete oxidation of propene to CO2 (run 9),30 as previously
reported.
Temperature-programmed desorption of NH3 (NH3-TPD) was
examined to estimate the amount of surface acid sites. The
pretreated sample (300 mg) was exposed to NH3 at 373 K for
0.5 h and evacuated at 373 K to remove weakly adsorbed NH3
for 0.5 h. The temperature of the sample was linearly elevated
to 1073 K by 5 K min-1 in a He carrier stream. The amount of
desorbed NH3 was monitored with the mass spectrometer at
m/e ) 16.
Diffuse reflectance UV spectra were recorded on a JASCO
V-570 spectrophotometer at room temperature. ESR spectra
were measured at 77 K by a X-band JEOL JES-TE200
spectrometer at a microwave power level (1.0 mW) at which
microwave power-saturation of the signals did not occur. The
sample was irradiated by a 500 W ultrahigh-pressure Hg lamp
at 77 K. The magnetic field was calibrated with a JEOL NMR
FIELD METER ES-FC5.
Figure 1 depicts the results of photooxidation of propene over
the TiO2-SiO2 samples from Table 1. The conversion of
propene increased with an increase of Ti content, while the yield
of PO increased with Ti content up to 1 mol % Ti and then
decreased. The T-S(0.08) and T-S(0.34) samples showed high
selectivity to PO, such as 60%. With increasing Ti content, the
selectivity to PO decreased, while the selectivity to CO2 and
Results
Photooxidation of Propene by Molecular Oxygen over the
TiO2-SiO2 Samples. Table 1 shows the results of photooxi-

4366 J. Phys. Chem. B, Vol. 107, No. 18, 2003
Murata et al.
Figure 2. Time course of the photooxidation of propene over the T-S(0.34) (A) and T-S(4.1) (B) samples. Catalyst 0.2 g, C3H6 100 µmol, O2
200 µmol. Conversion (a), PO yield (b), and selectivity to PO (c), ethanal (d), propanal (e), and CO + CO2 (f).
propanal increased. Selectivity to ethanal was not so affected
by Ti content.
Figure 2 shows time courses of propene photooxidation over
the T-S(0.34) and T-S(4.1) samples. Over both samples, the
conversion increased with an increase of irradiation time. With
increasing conversion, the PO selectivity slightly decreased, and
the selectivity to ethanal and propanal much decreased in the
initial 2 h, while the selectivity to COx increased. This indicates
that ethanal and propanal were consecutively oxidized to COx
more easily than PO was. Comparing the selectivity at similar
conversion, the difference between the T-S(0.34) and T-S(4.1)
samples was evident. From the results in Figures 1 and 2, it
Figure 3. Diffuse reflectance UV spectra of SiO2, TiO2-SiO2, and
was clearly shown that the TiO2-SiO2 samples with less Ti
TiO2 samples. SiO2 (a), T-S(0.08) (b), T-S(0.34) (c), T-S(1.0) (d),
content are more effective for the photoepoxidation of propene.
T-S(4.1) (e), T-S(8.3) (f), and TiO2 (g).
Over the T-S(0.34) sample the PO yield reached 15.7% at 10
h (Figure 2A). Even if all the Ti atoms were assumed to be the
LMCT from O to Ti of aggregated titanium oxide species
active sites, the turnover number, TON ) (the amount of
produced PO)/(the amount of active sites), was 1.4. This means
(aggregated TiOx units, x ) 4-6).38-40 The absorption edge
was shifted to longer wavelength with increasing Ti content
that the photoepoxidation of propene over the TiO2-SiO2
and became close to that of the TiO2 sample. It is generally
known that the absorption edge is shifted to longer wavelength
samples proceeds catalytically in a similar manner as the
previously reported result over the TiO2-SiO2 sample.30
as the size of the aggregated species becomes larger and as the
Active Sites for Photoepoxidation of Propene on TiO2-
coordination number of Ti increases.38-40 In the SiO2-supported
SiO2 Samples. Diffuse reflectance UV spectroscopy was
TiO2 system, the following were suggested on the basis of UV-
vis-NIR, Raman, and XANES spectroscopies: When the Ti
employed to determine the structure of Ti species on the TiO2-
SiO2 samples (Figure 3). The TiO2 sample showed a large
content is low, the isolated TiO4 units exist, and with increasing
absorption band below 400 nm. The SiO2 sample scarcely
Ti content, one-dimensional polymerized TiO4 units and two-
showed absorption. The T-S(0.08) and T-S(0.34) samples
exhibited a narrow absorption band below 250 nm centered at
dimensional polymerized TiO5 units would be formed, followed
by the formation of small particles comprised of TiO6 units,
around 205 nm, which was assigned to the LMCT (ligand-
and then the particle size becomes larger.39 This spectroscopic
metal charge transfer) from O to Ti (eq 1) of the isolated
aspect could be applied to the present TiO2-SiO2 binary oxide
system; that is, the coordination number and the size of titanium
oxide species should increase with increasing Ti content in the
TiO2-SiO2 samples.
tetrahedral Ti species (isolated TiO4 unit).38-40
[Ti4+-OL2-] {hν
\
} [Ti3+-OL-]*
(1)
When compared with the results of reaction tests, it was
demonstrated that the isolated tetrahedral Ti species on SiO2
are responsible for propene photoepoxidation, while the ag-
The samples containing more than 1 mol % Ti showed additional
absorption bands above 250 nm, which are assigned to the

Epoxidation of Propene by Molecular Oxygen
J. Phys. Chem. B, Vol. 107, No. 18, 2003 4367
-
respectively. It was clearly shown that the O3 is more stable
over the T-S(0.34) sample than over the T-S(4.1) sample.
-
It is known that O2 is generated by the reaction of Ti3+
with O2 in the following equation:48-51
Ti3+ + O2 f Ti4+-O2
(2)
-
On the other hand, it is generally accepted that O3- is generated
in the following scheme:50
O- + O2 f O3
(3)
-
In eq 3, two types of O- have been proposed. One is generated
- 52-54
by the dissociation of O2
lattice oxygen, OL
.
Another is the hole trapping
.
- 51,55-57 In the present system, the OL- was
generated by the photoirradiation in the absence of O2 (Figure
4). In addition, both O3- and O2- were immediately generated
-
-
by UV irradiation at 77 K, and the intensity of O3 and O2
Figure 4. ESR spectra at 77 K of the T-S(0.34) and T-S(4.1) samples
signals increased with increasing irradiation time over both the
T-S(0.34) and T-S(4.1) samples with constant ratio. These
results suggest that O3- is formed by the reaction of OL- with
O2 in the following equation:
and cell after the photoirradiation in vacuo at 77 K.
gregated titanium oxide species are responsible for propanal,
ethanal, and COx production.
Photoformed Radical Species on the TiO2-SiO2 Samples.
The T-S(0.34) and T-S(4.1) samples showed no ESR signals
in the dark at 77 K. UV irradiation of the T-S(0.34) and
T-S(4.1) samples in vacuo at 77 K gave rise to the ESR signals
shown in Figure 4. Both samples showed ESR signals with an
axial symmetric g tensor assigned to Ti3+ (g| ) 1.971 and g
-
OL- + O2 f O3
(4)
-
The introduction of propene into the samples containing O2
and O3 at 77 K in the dark did not change the O2 and O3
signals and produced no signals attributed to propene. The
subsequent UV irradiation did not affect the signals. When the
samples were warmed to room temperature, O3 disappeared
-
-
-
) 1.907),41-46 and a signal due to OL (g ) 2.001), a hole
-
-
center on the lattice oxygen.46,47 This hole center has also been
observed over some irradiated systems, for example, UV-
irradiated TiO241-45 and γ-irradiated silica-based materials.46,47
It was verified that UV irradiation results in the LMCT from O
to Ti (eq 1). The signals attributed to Ti3+ and OL- were stable
at 77 K after the photoirradiation was stopped. When the sample
was warmed to room temperature, these signals disappeared.
When the T-S(0.34) sample was irradiated at 77 K in the
presence of O2 (1.0 Torr), the signals shown in Figure 5a (solid
line) were observed. In the range from 0.1 to 30 Torr O2, the
intensity and shape of ESR spectra were analogous to each other
for the T-S(0.34) sample. The heights of these signals were
about 10 times larger than those of Ti3+ and OL-. Without UV
irradiation, no signals appeared. These photoformed signals were
stable at 77 K even after the photoirradiation was turned off.
The total spectrum was attributed to the superimposition of the
-
and O2 remained even in the presence of propene. It is
-
generally known that O2 is less reactive to hydrocarbon
- 50
molecules than other oxygen species such as O- and O3
.
-
These results suggest that the O2 is not active to propene by
-
itself. On the other hand, since the O3 disappeared at room
temperature in both the presence and absence of propene, the
reactivity of the O3- to propene could not be confirmed by ESR,
and it will be discussed later.
When the T-S(0.34) and T-S(4.1) samples were irradiated
in the presence of propene (1.0 Torr) at 77 K without molecular
oxygen, a complicated ESR signal was overlapped on the Ti3+
signal (Figure 6). The intensity of the Ti3+ signal in the presence
of propene was larger than that in vacuo. In Figure 7, a sum of
simulated spectra for CH2C˙ HCH2 (π-allyl radical, g ) 2.0023,
hyperfine coupling constant (hfcc), 0.406 mT for an R proton,
1.393 mT and 1.483 mT for two â protons, respectively),58 •CH3
(methyl radical, g ) 2.0023, hfcc, 2.3 mT for three R protons),58
and H3C(CH2)nC˙ HCH3 (alkyl radical, g ) 2.0023, hfcc, 2.3 mT
for three methyl â protons, 2.2 and 4.4 mT for methylene â
protons of secondary radicals, the anisotropic hfcc values of
the R proton were 1.17, 1.96, and 3.52 mT)59 is compared with
an experimental one. The simulated spectrum coincides with
the experimental one very well except for the part of the Ti3+
signal at g ) 1.982. From these results, it was suggested that
O2 (gxx ) 2.003, gyy ) 2.009, gzz ) 2.025)48-50 and T-type
-
O3- (g| ) 2.008, g ) 2.002)49-51 signals. The shape of T-type
-
O3 is shown in the following model:
When the sample was warmed to room temperature for several
-
-
minutes, the O3 signal disappeared and only the O2 signal
-
OL reacted with propene to form the allyl radical or methyl
-
remained (Figure 5b (broken line)). This O2 is very stable,
radical and that the consumption of the hole on OL would
-
and the intensity of the O2 signal very slowly decreased at
stabilize Ti3+ as shown in eqs 5 and 6.
room temperature in several days while the O3- on the TiO2-
SiO2 samples was unstable at room temperature.
Ti3+sOL- + H2CdCHsCH3 f
Ti3+sOLH + H2CC˙ HCH2 (5)
These oxygen radicals are also observed over the T-S(4.1)
sample, similarly to the case of the T-S(0.34) sample (Figure
5). However, the ratio of the two radicals was obviously dif-
ferent. The relative amount of O3- and O2- was determined by
Ti3+sOL- + H2CdCHsCH3 f
Ti3+sOLsCHdCH2 + •CH3 (6)
-
the double integration of ESR signals. The ratios of O3-/O2
were 0.58 and 0.16 over the T-S(0.34) and T-S(4.1) samples,

4368 J. Phys. Chem. B, Vol. 107, No. 18, 2003
Murata et al.
Figure 5. ESR spectra at 77 K of the T-S(0.34) and T-S(4.1) samples after the photoirradiation in the presence of O2 (1.0 Torr) at 77 K (a),
followed by warming the samples to room temperature for several minutes (b). The spectra in part c were obtained by subtracting spectra b from
spectra a.
Figure 7. A sum of simulated ESR spectra (broken line) and observed
ESR spectra (solid line) of the T-S(0.34) sample photoirradiated in
Figure 6. ESR spectra at 77 K of the T-S(0.34) and T-S(4.1) samples
after the photoirradiation in the presence of propene (1 Torr) in the
absence of O2 at 77 K.
the presence of propene (1.0 Torr) at 77 K. The following parameters
are employed for the simulation. CH2C˙ HCH2 (π-allyl radical, g )
2.0023, hyperfine coupling constant (hfcc), 0.406 mT for an R proton,
1.393 mT and 1.483 mT for two â protons, respectively),54 •CH3 (methyl
radical, g ) 2.0023, hfcc, 2.3 mT for three R protons),54 and H3C-
(CH2)nC˙ HCH3 (alkyl radical, g ) 2.0023, hfcc, 2.3 mT for three methyl
â protons, 2.2 and 4.4 mT for methylene â protons of secondary
radicals, the anisotropic hfcc values of the R proton were 1.17, 1.96,
and 3.52 mT).55 The relative ratio of CH2C˙ HCH2/•CH3/H3C(CH2)nC˙ HCH3
is 1.0/0.1/4.4.
These allyl and methyl radicals would react with propene to
produce the alkyl radical, H3C(CH2)nC˙ HCH3, in the following
schemes:
H2CdC˙ HdCH2 + H2CdCHsCH3 f
H2CdCHsCH2sC˙ HsCH3 (7)
reacts with propene. When these samples warmed to room
temperature, the radical species derived from propene disap-
peared, and O2 remained. O2 was a stable species even in
the presence of a propene molecule.
•CH3 + H2CdCHsCH3 f H3CsCH2sC˙ HsCH3 (8)
-
-
When O2 (1 Torr) was subsequently added to the samples at 77
K, the Ti3+ signal decreased and the O2- signal appeared over
both samples in addition to the signals derived from propene
(Figure 8). This result indicates that the photoformed Ti3+ reacts
with O2 to produce O2- in the presence of O2 and propene (eq
The Ti3+ signal of the T-S(0.34) sample almost disappeared
by O2 introduction, while that of the T-S(4.1) sample partly
remained (Figure 8). This agrees well with the result that the
T-S(4.1) sample contains the aggregated titanium oxide species
(Figure 3). The T-S(0.34) sample would have Ti3+ species on
-
2), while the prodution of O3- (eq 4) does not occur after OL

Epoxidation of Propene by Molecular Oxygen
J. Phys. Chem. B, Vol. 107, No. 18, 2003 4369
In method B, propene was introduced into the T-S(0.34)
sample, and the reactor was photoirradiated at 77 K, followed
by O2 introduction at 77 K in the dark. This procedure will
-
bring about allyl radial, methyl radical, alkyl radical, and O2
radical species on the T-S(0.34) sample according to eqs 1, 2,
and 5-8. After the sample was warmed to room temperature,
ethanal, acrolein, and propanal were mainly obtained (run 2).
In method C, after the T-S(0.34) sample was photoirradiated
at 77 K in the presence of O2 and warmed to room temperature,
-
only the O2 species was confirmed by the separate ESR
measurement (eqs 1 and 2). When propene was introduced to
this sample, ethanal, propanal, and acrolein were mainly
generated (run 3). This product distribution is very similar to
that by method B (run 2).
In method D, the T-S(0.34) sample was reduced by H2 at
773 K and evacuated at 773 K, followed by O2 introduction at
room temperature. This reduction procedure generates Ti3+ on
the sample in the following scheme:48,50
Ti4+ H2 reduction8 Ti3+
(9)
Figure 8. ESR spectra at 77 K of the T-S(0.34) and T-S(4.1) samples
after the photoirradiation in the presence of propene (1.0 Torr) at 77
K, followed by the introduction of O2 (1.0 Torr) in the dark at 77 K.
The Ti3+ immediately reacts with O2 to produce O2- according
to eq 2.48,50 The existence of O2- was confirmed by ESR. When
-
the surface predominantly, while the T-S(4.1) sample would
have Ti3+ species not only on the surface but also in the bulk
lattice in which the Ti3+ cannot contact with molecular oxygen.
Reactivity of the Oxygen Radical Species to Propene. ESR
spectra showed that O3- and O2- were generated over the TiO2-
SiO2 samples irradiated at 77 K in the presence of oxygen, while
only O2- was observed at room temperature. On the other hand,
this O2 species was contacted with propene at room temper-
ature, no products were detected (run 4). This confirmed that
the O2 on the TiO2-SiO2 samples is not active to propene at
room temperature, as described above.
-
From the results of runs 1 and 4 in Table 2 (methods A and
-
D), it was revealed that O3 reacts with propene to produce
-
PO; in other words, O3 is the active oxygen species for the
-
it was confirmed that OL reacted with propene to form allyl
photoepoxidation of propene over the isolated tetrahedral Ti
species. On the other hand, the result of run 3 in Table 2 (method
radical, methyl radical, and alkyl radical species by photoirra-
-
diation at 77 K. Therefore, it is expected that we can control
B) demonstrated that the OL reacts with propene to produce
-
the selective preparation of the oxygen radical species, O3
,
ethanal and acrolein.
O2-, and OL- and can examine their reactivity to propene. Since
the amount of products collected by 573 K heating after ESR
measurement was too small to be detected by GC-FID, we
carried out separate experiments with larger amounts of samples
(2.0 g) in the same procedures as ESR measurements in order
to determine the products generated through the reaction of each
oxygen species with propene.
Table 2 lists the procedures for obtaining each expected
radical species and the products experimentally obtained by the
stoichiometric reaction of radical species with propene or O2.
First, we will show the results over the T-S(0.34) sample, which
contain the isolated tetrahedral Ti species predominantly. When
propene and O2 were introduced on the T-S(0.34) sample at
77 K in the dark and it was warmed to room temperature, no
products were detected (method E, run 5). It was confirmed
that the propene epoxidation does not proceed without UV
irradiation.
Runs 6-8 in Table 2 show the results over the T-S(4.1)
sample, which also contains the aggregated titanium oxide
species. In methods A-C, PO was not generated at all, while
ethanal, acrolein, and propanal were mainly obtained (runs 6-8).
This product distribution is quite different from that of the actual
reaction test (Table 1). Details will be discussed below.
Consecutive Reaction of Propene Oxide over the TiO2-
SiO2 Samples. To know the possibility of the consecutive
conversion of produced PO, the thermal reaction and the
photooxidation of PO were examined over the T-S(0.34) and
T-S(4.1) samples (Table 3). By the thermal reaction, PO was
isomerized mainly to propanal. The amount of converted PO
over the T-S(0.34) sample (run 1) was very small in compari-
son with that over the T-S(4.1) sample (run 2).
It is generally known that PO is isomerized into propanal on
acid sites and into acetone on basic sites.60 The amount of
surface acid sites was estimated from NH3-TPD and shown in
Table 3. The T-S(0.34) sample showed a very small amount
of acid sites (3 µmol g-1), which is a comparable level to that
of the SiO2 sample (<6 µmol g-1). The T-S(4.1) sample was
found to have 55 µmol g-1 of acid sites. This means that the Ti
species on the T-S(0.34) sample do not act as acid sites, while
those on the T-S(4.1) sample result in acid sites. It is generally
considered that Ti-O-Si bridges adjacent to TiO5 units and
TiO6 units cause the charge imbalance to exhibit acidity, while
TiO4 units bring about no acid sites.38 This agrees well with
the results mentioned above. The acid sites derived from the
aggregated titanium oxide species on the T-S(4.1) sample
would convert PO into propanal through the thermal reaction.
The photooxidation of PO with molecular oxygen followed
by thermal desorption was also examined. Ethanal and COx were
In method A, O2 was introduced into the T-S(0.34) sample,
and the reactor was cooled to 77 K, followed by the photoir-
-
radiation at 77 K. This procedure should bring about O3 and
O2- radical species on the T-S(0.34) sample according to eqs
1, 2, and 4. Subsequently, propene was introduced to the sample
at 77 K and the sample was warmed to room temperature in
the dark. By these procedures, it is expected that propene comes
-
-
in contact with O3 and O2 on the T-S(0.34) sample. As a
result, almost equimolecular amounts of PO and acetone were
detected as products (run 1). In the actual reaction test (Table
1), the selectivity to acetone was less than 10%. The quite
different reaction conditions, such as temperature and products’
yield, between these experiments would result in the discrepancy
of acetone selectivity.

4370 J. Phys. Chem. B, Vol. 107, No. 18, 2003
Murata et al.
TABLE 2: Results of Stoichiometric Reaction of Oxygen Radical Species with Propenea
run
sample
methodb
expected radical speciesc
productsd/%
-
1
2
3
4f
5
6
7
8
T-S(0.34)
A
B
C
D
E
A
B
C
O3-, O2
PO (52), acetone (48)
O2-, methyl radical, allyl radical, alkyl radical ethanal (49), acrolein (24), propanal (15), PO (4), acetone (8)
(OL-),e O2
ethanal (49), acrolein (25), propanal (19), PO (3), acetone (4)
none
none
-
-
O2
none
-
T-S(4.1)
O3-, O2
ethanal (52), acrolein (33), propanal (15)
O2-, methyl radical, allyl radical, alkyl radical ethanal (52), acrolein (9), propanal (8), acetone (20), C2H4 (11)
(OL-),e O2
ethanal (53), acrolein (33), propanal (14)
-
a Catalyst 2 g, O2 5 µmol, C3H6 20 µmol, irradiation time 0.5 h. b The methods are composed of the following sequential procedures. [A] (1) The
sample was irradiated at 77 K in the presence of O2. (2) C3H6 was introduced to the sample at 77 K in the dark. (3) The sample was warmed to
room temperature in the dark. [B] (1) The sample was irradiated at 77 K in the presence of C3H6. (2) O2 was introduced to the sample at 77 K in
the dark. (3) The sample was warmed to room temperature in the dark. [C] (1) The sample was irradiated at 77 K in the presence of O2. (2) The
sample was warmed to room temperature in the dark. (3) C3H6 was introduced to the sample at room temperature in the dark. [D] (1) The sample
was reduced by H2 at 723 K for 1 h. (2) The sample was evacuated at 723 K for 1 h. (3) O2 was introduced to the sample at room temperature in
the dark. (4) C3H6 was introduced to the sample at room temperature in the dark. [E] (1) O2 and C3H6 were introduced to the sample at 77 K in
the dark. (2) The sample was warmed to room temperature in the dark. c The expected radical species. The existence of a radical species was
confirmed by separate ESR measurements. d The products were collected by heating at 573 K in vacuo with a liquid nitrogen trap after the procedures
shown in methods A-E. The percentage of each product is shown in parentheses. e It could not be confirmed by separate ESR measurement.
f Catalyst 10 g.
TABLE 3: Results of the Reaction of Propene Oxide over the TiO2-SiO2 Samplesa
collected compounds/µmol
run
sample
Ti atomsb/(µmol g-1
)
acid sitesc/(µmol g-1
)
propanal acetone acrolein ethanal alcohols HC COxd sum/µmol
thermal reactione
1
2
T-S(0.34)
T-S(4.1)
55
675
3
55
0.3
2.9
trace
0.1
trace
0.0
trace
trace
0.0
0.0
trace trace
0.2 0.6
0.4
3.8
3
4
T-S(0.34)
T-S(4.1)
55
675
3
55
0.0
0.5
0.3
1.2
0.0
0.7
trace 0.3
0.3 1.0
1.0
7.9
a Catalyst 0.2 g. b The number of Ti atoms contained in 1 g of the sample. c The amount of acid sites calculated from the NH3-TPD profile.
d COx denotes CO + CO2. e The thermal reaction of propene oxide (20 µmol) by 573 K heating in vacuo. f The photooxidation of propene oxide
(20 µmol) for 1 h with O2 (200 µmol), followed by the thermal desorption of products by heating at 573 K.
acrolein and H2O.25 In the present study, the reaction of OL
with propene resulted in methyl radical or allyl radical through
CC bond fission or H abstraction (Figure 6). These radical
species would react with O2- to form ethanal and formaldehyde
or acrolein and H2O. However, as described in the previous
section, the O2- remained after the hydrocarbon radical species
disappeared at room temperature. The possibility of direct
reaction with molecular oxygen could not be excluded.
-
mainly obtained in addition to propanal over both samples (runs
3 and 4). In the photooxidation, the T-S(4.1) sample converted
more PO than the T-S(0.34) sample did. This agrees with the
high activity for complete photooxidation of the bulk TiO2
sample (Table 1, run 9). It is demonstrated that the aggregated
titanium oxide species promotes not only the thermal isomer-
ization of PO to propanal on the acid sites but also the
consecutive photooxidation of PO to COx.
The products distribution obtained in method C was very
similar to that obtained in method B. This result implied that
OL also existed on the samples at room temperature after the
Discussion
-
Proposed Mechanism for the Photooxidation of Propene
over the Isolated Tetrahedral Ti Species. As described above,
the O3- reacts with propene to produce PO. Kanai et al. reported
TiO2-SiO2 containing 5 mol % Ti produced PO in the
photooxidation of propene although the selectivity to PO was
low.61 Without direct experimental evidence, they speculated
that PO was produced through a titanaoxacyclobutane interme-
diate that was produced by the reaction of propene with OL
From the results in the present study, the reaction between
propene and OL provided other products. Therefore, their
photoirradiation at 77 K in the presence of O2 although the signal
-
of OL was not observed by ESR measurement (Figure 5b).
-
-
The OL would be formed through the decomposition of O3
at room temperature (the reverse reaction of eq 4). The spin-
lattice relaxation time of the OL- might become shorter because
-
the symmetry is changed by the presence of the O2
.
-
From these results, the tentative mechanism for the photo-
oxidation of propene at room temperature over the isolated
.
tetrahedral Ti species is proposed in Scheme 1. When the [Ti4+
-
-
OL2-] of the isolated tetrahedral Ti species (species I) absorbs
UV light (λ < 250 nm), LMCT from O to Ti occurs, and the
charge-transfer exited state [Ti3+-OL-]* is achieved (species
speculation seems to be excluded.
The OL- reacted with propene to produce methyl radical and
allyl radical. When the radical species contacted with molecular
oxygen, ethanal, acrolein, and propanal were mainly obtained
(method B, run 2). Yoshida et al. suggested that the photoexcited
VdO bonds on V2O5/SiO2 catalyst (5 wt % V2O5) activate
propene and O2 to produce a π-allyl intermediate and O2-, and
then ethanal or acrolein is generated.62 Xiang et al. speculated
that propene cation radical C3H6+ and O2- are generated through
an intermolecular C3H6-O2 charge transfer in the cage of Ba-
exchange zeolites and lead to ethanal and formaldehyde or
II). When the photoformed Ti3+ and OL react with O2, O2
-
-
and O3- are produced, respectively (species III). The O3- reacts
with propene (species IV) to produce PO (route A); otherwise,
-
it decomposes to OL and O2 (species V).
-
It is not clear how the O2 and the residual oxygen atom in
-
O3 are consumed in route A after producing PO. However,
since the selectivity to PO over the TiO2-SiO2 samples in the
-
reaction test reached 60%, the residual oxygen atom in O3

Epoxidation of Propene by Molecular Oxygen
J. Phys. Chem. B, Vol. 107, No. 18, 2003 4371
SCHEME 1: Proposed Mechanism for the Formation of PO, Ethanal, and Acrolein over the Isolated Tetrahedral Ti
Speciesa
a Route A is the cycle for PO production through species I-II-III-IV-I. Route B is the cycle for acrolein or ethanal production through
species I-II-(V)-VI or VII-I. L denotes either H or Si.
should not be responsible for side reactions and might produce
one more PO. If two oxygen atoms in O3 are employed for
chrome P-450, a monooxygenase, generates the electrophilic
oxygen species from O2 involving a reducing agent to catalyze
the epoxidation of alkenes and hydroxylation of alkanes and
aromatics.68 Recently, many workers examined the epoxidation
of olefin in the presence of O2 and reducing agents in order to
produce the electrophilic oxygen species.4-7,63,64
-
the PO production, TON (produced PO/Ti atoms) should exceed
2 per one catalytic cycle. Although confirmed TON was 1.4
over the T-S(0.34) sample in the present study, confirmed TON
reached 2.8 over the TiO2/SiO2 0.1 mol % sample, as previously
reported.30 This indicates that route A in Scheme 1 proceeds
catalytically in any case.
In the present study, O3- is claimed to be the active oxygen
-
species for the epoxidation of propene. The O3 is generated
by the reaction of the photoformed OL- (hole center on lattice
When OL- reacts with propene, allyl radical (species VI) or
methyl radical (species VII) is formed through H abstraction
-
oxygen) with O2. Therefore, the O3 should be the electron
-
deficient state and should have an electrophilic nature. So far,
or CC bond fission. These radical species reacted with O2 or
-
O3 was observed over various photoirradiated metal oxides
O2 to produce acrolein and H2O or ethanal and formaldehyde
(route B). O2 and propene would competitively react with the
and was suggested to be the active oxygen species for the 16O-
18O exchange reaction and CO oxidation.50,55,56,69-71 However,
there are no reports that O3- on metal oxide reacts with alkene
to produce epoxide. Takita et al. reported that the stoichiometric
reaction between propene and O3- on MgO gave CO2 as a main
-
-
OL to form the O3 and the hydrocarbon radical species,
respectively, resulting in the different products from each other.
It is not clear whether both O3- and O2- coexist on the same
[Ti-O] site (species III). However, since no Ti3+ signal was
observed in the presence of O2 under the photoirradiation, it
may be more possible that the O3- and O2- coexist on the same
product.72 The shape of the O3 on MgO is shown in the
-
following model:
-
[Ti-O] site. The existence of O2 on Ti3+ might result in the
-
separation of electric charge to stabilize the existence of O3
.
-
-
Thus, we tentatively draw O3 and O2 on the same [Ti-O]
site in Scheme 1.
Electrophilic Active Oxygen Species for the Photoepoxi-
dation of Propene. It is generally considered that the electro-
philic oxygen species are effective for the epoxidation of
alkene.63-67 In the Ag/Al2O3 catalytic system for epoxidation
of ethylene using molecular oxygen, it is suggested that the
electrophilic atomic oxygen on the Ag surface attacks the CdC
bonds in ethylene.65 In the epoxidation system using peroxide
reagents in the presence of homogeneous (Mo, W, V, Ti, and
Zr) or heterogeneous (TiO2/SiO2, TS-1) catalysts, the peroxo
species, generated from metal species and peroxide reagent, are
suggested to be the active oxygen species.66,67 It is considered
that the metal ion increases the electrophilicity of the peroxo
moiety by withdrawing electrons. It is well-known that cyto-
This is different from that on the TiO2-SiO2 samples. Therefore,
the nature of the O3- would also be different. Kubokawa et al.
-
reported that O3 was the active oxygen species for the
photooxidation of alkenes to ketones, aldehydes, and dienes over
porous Vycor glass (PVG).73 The O3 on PVG is T-type O3
,
-
-
similar to that on the TiO2-SiO2 samples; however, the
reactivity to alkene is different. Since it is known that the nature
of O- strongly depends on the adjacent metal atoms,50 that of
O3- is also expected to vary from oxide to oxide. Other factors
than the oxygen species would also be concerned. PVG is a
porous silica material containing some impurities such as B2O3
(2.95 wt %), Na2O (0.04 wt %), Al2O3, and ZrO2 (0.72 wt %),

4372 J. Phys. Chem. B, Vol. 107, No. 18, 2003
Murata et al.
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Al2O3/SiO2 sample showed low selectivity to PO because of
its acidity.29 The acidity on PVG might cause the absence of
epoxide yield. The existence of T-type O3- would be required
for the photoepoxidation; however, the other factor, such as
acidity, will also be concerned with the photoepoxidation
activity.
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showed lower selectivity to PO (21.5%) than the T-S(0.34)
sample (57.5%). The lower selectivity over the T-S(4.1) sample
-
related to a lower ratio of O3-/O2 (Figure 5), which should
correspond to species III/(species III + species V) in Scheme
1. Thus, route B is mainly promoted, or route A is less promoted
over the aggregated titanium oxide species because of the
-
smaller amount of O3
.
In the stoichiometric reactions over the T-S(4.1) sample,
-
when propene was contacted with O3 and O2-, PO was not
obtained, but ethanal, acrolein, and propanal were formed (Table
2, run 6). The products distribution is essentially identical to
that obtained in method C (run 8). The O3- over the T-S(4.1)
-
sample in method A (run 6) would decompose to OL and O2
before it reacts with propene during warming the sample from
-
77 K to room temperature. This result indicates that the O3
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-
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Conclusion
The isolated tetrahedral Ti species on the TiO2-SiO2 samples
catalyze the photoepoxidation of propene with molecular
oxygen. [Ti4+-OL2-] is excited by UV light to form a [Ti3+
-
OL-]* radical pair. The Ti3+ moiety reacts with O2 to form O2
.
-
-
The OL moiety, a hole center on lattice oxygen, reacts with
O2 to form O3-, which reacts with propene to yield PO. The
O3- would be the electrophilic oxygen species effective for the
-
epoxidation of propene. When the OL moiety reacts with
propene, acrolein or ethanal is produced through H abstraction
or CC bond fission.
The isolated tetrahedral Ti species exhibit the higher stability
-
of the O3 and lower activity for the consecutive reactions of
PO in comparison with the case of the aggregated titanium oxide
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tetrahedral Ti species for PO production.
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Acknowledgment. We thank Prof. Y. Okamoto, Dr. Y. Isobe
(Department of Applied Chemistry, Graduate School of Engi-
neering, Nagoya University), Mr. Y. Nagara, and Mr. M.
Kawahara (Japan Chemical Innovation Institute, Nagoya) for
their aid in ESR measurement. This work was supported by a
grant-in-aid from the Japanese Ministry of Education, Science,
Art, Sports and Culture, and by Nippon Sheet Glass Foundation
for Materials Science and Engineering.
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Epoxidation of Propene by Molecular Oxygen
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