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Steady-State Photocatalytic Epoxidation of Propene by O2 over V2O5/SiO2 Photocatalysts Fumiaki Amano, Tsunehiro Tanaka,* and Takuzo Funabiki Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Received October 26, 2003. In Final Form: February 10, 2004 A silica-supported, lowly loaded vanadium oxide (V2O5/SiO2) photocatalyst promotes the photocatalytic epoxidation of propene with O2 at steady state in a flow reactor system. Very little deep oxidation of propene into CO2 takes place over V2O5/SiO2, in contrast to the results obtained over a TiO2 photocatalyst in which total oxidation is the main path. With each loading, the sums of the selectivities into propene oxide (PO) and propanal (PA) at steady state were almost the same. The monomeric VO4 tetrahedral species dispersed on SiO2 yield PO under UV irradiation. The less dispersed vanadium oxide species on SiO2 promote the isomerization of PO into PA. We utilized a flow reactor system in which the short contact time reduced the isomerization and resultant decomposition of PO over the catalyst surface.
Introduction Propene oxide (PO) is an important material in the chemical industry. A titanium silicalite (TS-1) catalyst promotes the epoxidation of propene with H2O2;1 however, because H2O2 is fairly expensive, molecular O2 is a preferred oxidant. Haruta et al. have reported that Au supported on mesoporous titanosilicates (e.g., Au/TiMCM-41) promotes the epoxidation of propene with O2 and H2 at 423 K.2,3 The authors propose that, in this system H2O2-like species form on Au nanoparticles. The problem is that the efficiency of H2 utilization is very low. Other investigators report that Ti-containing catalysts (e.g., Ti/ silicalite and Ti-Al-HMS) promote epoxidation of propene with O2 at 523-573 K;4,5 although the conversion rate of propene is very high, the PO selectivity is still low. The combination of photochemical and photocatalytic oxidation of propene with O2 at room temperature under irradiation is also attractive.6 It has been demonstrated that small alkenes adsorbed in Ba-Y-type zeolites can be selectively oxidized with O2 by visible light irradiation.7-10 The propene in the Ba-Y zeolite was photooxidized to allyl hydroperoxide at a low temperature, near 173 K. At room temperature, the allyl hydroperoxide converts to acrylaldehyde (acrolein, AL) or reacts with an unreacted propene molecule to yield PO.8,10 Using the photocatalytic system, we have found that Nb2O5/SiO2 catalyzed the photooxidation of propene into PO with O2 under UV irradiation.11 Recently, Yoshida et al. have reported that * To whom correspondence should be addressed. Fax: +81-75383-2561. E-mail:
[email protected]. (1) Danciu, T.; Beckman, E. J.; Hancu, D.; Cochran, R. N.; Grey, R.; Hajnik, D. M.; Jewson, J. Angew. Chem., Int. Ed. 2003, 42, 1140-1142. (2) Sinha, A. K.; Seelan, S.; Akita, T.; Tsubota, S.; Haruta, M. Appl. Catal., A 2003, 240, 243-252. (3) Sinha, A. K.; Seelan, S.; Akita, T.; Tsubota, S.; Haruta, M. Catal. Lett. 2003, 85, 223-228. (4) Murata, K.; Kiyozumi, Y. Chem. Commun. 2001, 1356-1357. (5) Liu, Y. Y.; Murata, K.; Inaba, M.; Mimura, N. Catal. Lett. 2003, 89, 49-53. (6) Maldotti, A.; Molinari, A.; Amadelli, R. Chem. Rev. 2002, 102, 3811-3836. (7) Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 1812-1820. (8) Blatter, F.; Sun, H.; Frei, H. Catal. Lett. 1995, 35, 1-12. (9) Blatter, F.; Sun, H.; Vasenkov, S.; Frei, H. Catal. Today 1998, 41, 297-309. (10) Xiang, Y.; Larsen, S. C.; Grassian, V. H. J. Am. Chem. Soc. 1999, 121, 5063-5072.
a series of highly dispersed transition metals such as Ti4+, Cr6+, and Zn2+, in a SiO2 matrix, exhibited photocatalytic activity for PO formation.12-17 These SiO2-based photocatalysts were different from bulk semiconductor metal oxides such as TiO2 in terms of their mechanism of photoexcitation and their photocatalytic ability to bring about selective mild oxidation.6 In the case of silicasupported vanadium oxide (V2O5/SiO2), a highly dispersed VO4 species supported on SiO2 was found to be photoactive for partial oxidation of hydrocarbons,18 although the bulk V2O5 was photoinactive.19 We have previously examined the photooxidation of propene over V2O5/SiO2 with O2 under UV irradiation in a closed circulation reactor.20,21 The products of the partial oxidations were ethanal (acetaldehyde, AA), acrylaldehyde (AL), and propanal (propionaldehyde, PA). However, PO was not detected as a product, although we predicted the formation of PO over V2O5/SiO2 as a precursor to PA.21 If the decomposition of the unstable PO intermediate is suppressed, the epoxidation of propene with O2 will be established over V2O5/SiO2. In the present study, we adopted a flow reactor system instead of a closed reactor system to obtain a short contact time between the substrates and the catalysts; as a result, we were able to observe the formation of PO over V2O5/SiO2 for the first time. (11) Tanaka, T.; Nojima, H.; Yoshida, H.; Nakagawa, H.; Funabiki, T.; Yoshida, S. Catal. Today 1993, 16, 297-307. (12) Yoshida, H.; Murata, C.; Hattori, T. Chem. Commun. 1999, 15511552. (13) Yoshida, H.; Murata, C.; Hattori, T. Chem. Lett. 1999, 901-902. (14) Yoshida, H.; Murata, C.; Hattori, T. J. Catal. 2000, 194, 364372. (15) Murata, C.; Yoshida, H.; Hattori, T. Chem. Commun. 2001, 24122413. (16) Murata, C.; Yoshida, H.; Kumagai, J.; Hattori, T. J. Phys. Chem. B 2003, 107, 4364-4373. (17) Yoshida, H.; Shimizu, T.; Murata, C.; Hattori, T. J. Catal. 2003, 220, 226-232. (18) Yoshida, S.; Magatani, Y.; Noda, S.; Funabiki, T. Chem. Commun. 1981, 601-602. (19) Pichat, P.; Herrmann, J. M.; Disdler, J.; Mozzanega, M. N. J. Phys. Chem. 1979, 83, 3122-3126. (20) Yoshida, S.; Tanaka, T.; Okada, M.; Funabiki, T. J. Chem. Soc., Faraday Trans. 1984, 80, 119-128. (21) Tanaka, T.; Ooe, M.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1986, 82, 35-43.
10.1021/la0359981 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/30/2004
Photocatalytic Epoxidation of Propene over V2O5/SiO2
Figure 1. Time course of photooxidation of propene by O2 over 0.5 wt % V2O5/SiO2 under irradiation: 0.3 g of catalyst; total flow rate 100 mL min-1; reactants 20% C3H6, 10% O2, and 70% He; conversion rate of C3H6 (a) and selectivity into AA (b), AL (c), PO (d), PA (e), and CO2 (f).
Experimental Section Amorphous SiO2 was synthesized by the hydrolysis of distilled tetraethyl orthosilicate (TEOS) in a water-ethanol mixture at the boiling point, followed by calcination in dry air at 773 K for 5 h. The specific surface area was about 550 m2 g-1, as determined by N2 adsorption at 77 K. The silica-supported vanadium oxide catalysts (V2O5/SiO2) were prepared by an incipient wetness impregnation of SiO2 with an aqueous solution of NH4VO3 at 353 K. The TiO2 (JRC-TIO-4) was provided by the Catalysis Society of Japan (rutile/anatase ) 3/7, specific surface area 48 m2 g-1). The samples pressed into disks were calcined at 773 K for 5 h in dry air and crushed to 26-50 mesh in a mortar. We prepared catalysts of various V5+ loadings, 0.18, 0.5, 2.5, and 10.0 wt %, as V2O5, to reveal the relationship between the degree of V2O5 dispersion and photocatalytic activity. The photoreactions of propene were carried out in a fixed bed flow reactor system at atmospheric pressure.22 Either 300 mg of V2O5/SiO2 or 600 mg of TiO2 was mounted on a reaction cell (cell volume 0.75 mL, irradiated area 7.5 cm2) made of quartz glass. Before each reaction test, the catalysts were heated at 673 K under a stream of a 20% O2/80% He mixture flowing at 50 mL min-1 for 2 h. The reaction was carried out under a total flow rate of 100 mL min-1 of a gas mixture of 20% C3H6/10% O2/70% He (GHSV 8000 h-1, contact time 0.45 s). For the photoirradiation, we used a 300 W Xe lamp (Ushio) without a UV cutoff filter at room temperature. During the irradiation, the temperatures of the reaction cell reached about 323 K. The products were analyzed by on-line TCD and FID gas chromatographs. The photoluminescence measurements were performed with an F-3010 spectrometer (Hitachi) at a liquid nitrogen temperature. The emission and excitation spectra were recorded in the machine’s phosphorescence mode (delayed time 1 ms). The UVvis-near-IR diffuse reflectance spectra were obtained using a Lambda-19 spectrometer (Perkin-Elmer) equipped with a reflectance spectroscopy accessory, RSA-PE-19 (Labsphere), at room temperature. BaSO4 was used as a standard reflection sample. Prior to each measurement, the samples were dehydrated by an oxidation treatment at 673 K in a static atmosphere quartz cell, the process including an evacuation for 30 min, oxidation with 80 Torr of O2 for 1 h, and evacuation for 30 min. The samples were measured without exposure to air after the oxidation treatments.
Results and Discussion Photoepoxidation of Propene over V2O5/SiO2 Photocatalysts. Figure 1 shows the result of photooxidation of propene with O2 over 0.5 wt % V2O5/SiO2. After 2 h on stream, no deactivation was observed, and the conversion rate of propene at steady state was about 0.7 µmol min-1 (42 µmol h-1). The amount of surface vanadium ions was (22) Tanaka, T.; Ito, T.; Takenaka, S.; Funabiki, T.; Yoshida, S. Catal. Today 2000, 61, 109-115.
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Figure 2. C3H6 conversion rate of photooxidation of propene over V2O5/SiO2: 0.18 wt % (a), 0.5 wt % (b), 2.5 wt % (c), 10.0 wt % (d), TiO2 (e), and SiO2 (f) under irradiation; catalysts TiO2 (0.6 g), V2O5/SiO2 (0.3 g), and SiO2 (0.4 g); total flow rate 100 mL min-1; reactants 20% C3H6, 10% O2, and 70% He.
15.2 µmol in 300 mg of 0.5 wt % V2O5/SiO2, and the apparent turnover frequency (TOF) was 2.76 h-1. The main products were AA and PO; the selectivities were 43 and 30 C%, respectively. The initial yield of AL, which had been a main product in a closed circulation system, was high, but the yield decreased drastically. The remaining photogenerated AL could have poisoned the active sites of AL formation under reaction, because the rate of photodesorption of AL from the catalyst surface is lower than that of thermal desorption.20 We have previously proposed reaction routes of the photooxidation of propene: the oxidative fission of the olefinic double bond to yield AA and formation of a π-allyl intermediate which is converted to AL and an epoxide intermediate which is isomerized to PA.20,21 The PA yield was very low, suggesting that the decomposition of PO into PA was suppressed by the flow reactor system. For the analogous Nb catalyst, a high dispersion of NbO4 species on SiO2 is essential for the photoepoxidation of propene; PO is selectively produced over monomeric NbO4 tetrahedra, whereas PA is selectively generated over polymeric NbO4 tetrahedra.11 Therefore, it is necessary to examine the effect of vanadate dispersion on the SiO2 surface. The characterization of V2O5/SiO2 with different loadings by means of UV-vis-near-IR is reported in the next section. Figure 2 shows the propene conversion rate over V2O5/SiO2 with different vanadate loadings. Each V2O5/SiO2 catalyst showed a very similar steady-state activity after 5 h on stream. The nonloaded amorphous SiO2 exhibited hardly any photocatalytic activity. It is obvious that the surface vanadium oxide species are the photocatalytic active sites. Although the conversion rate of propene over TiO2 is relatively high, the main product is not a partial oxidation product but the total oxidation product CO2.19 Figure 3 shows the selectivity of propene into PO and PA over V2O5/SiO2 with various vanadia loadings. The PO selectivity increased with a decrease in the loading amount. On the other hand, the PA selectivity decreased with a decrease in the loading amount. The sums of the selectivities into PO and PA at steady state were almost the same, about 35-40 C%, for V2O5/SiO2 with each loading. This result indicates that PA is formed as a secondary product via PO, and that highly loaded V2O5/ SiO2 promotes the isomerization of PO into PA. The isomerization of PO over vanadium oxide species on SiO2 has been confirmed under photoirradiation and thermal treatment in a closed circulation reactor.21 Table 1 shows the results of the photooxidation of propene after 5 h on stream. The selectivity into CO2 over TiO2 was about 80 C%.19 The TiO2 semiconductor photo-
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Figure 3. Selectivity into PO and PA of photooxidation of propene over V2O5/SiO2: 0.18 wt % (a), 0.5 wt % (b), 2.5 wt % (c), and 10.0 wt % (d) under irradiation; catalyst V2O5/SiO2 (0.3 g); total flow rate 100 mL min-1; reactants 20% C3H6, 10% O2, and 70% He. Table 1. Results of Photooxidation of Propene over Various Catalysts under Irradiationa selectivityb (C%)
catalyst
conversion rate (µmol min-1)
PO
PA
AC
AL
AA
CO2
TiO2 SiO2 0.18 VS 0.18 VSc 0.5 VS 2.5 VS 10.0 VS
1.11 0.01 0.71 0.00 0.72 0.65 0.65
0 100 43 0 30 23 9
0 0 2 0 5 14 34
6 0 7 0 5 2 5
0 0 6 0 8 20 10
13 0 33 0 43 33 36
80 0 7 0 9 7 5
a The data were obtained at 5 h on stream. Conditions: TiO (0.6 2 g), x VS ) x wt % V2O5/SiO2 (0.3 g), SiO2 (0.4 g); total flow rate 100 mL min-1; reactants 20% C3H6, 10% O2, and 70% He. b PO ) propene oxide, PA ) propanal, AC ) acetone, AL ) acrylaldehyde, and AA ) ethanal. c Reaction in the dark at 373 K.
catalyst is effective for the degradation and mineralization of toxic organic compounds in aqueous and gaseous conditions.23 TiO2 promotes the deep oxidation of hydrocarbons under UV irradiation, so that PO is not formed over TiO2. On the other hand, partial oxidation compounds such as PO and AA were mainly obtained over V2O5/SiO2. Although photoepoxidation occurs even over amorphous silica, the conversion level was quite low,12-14 so that the determination of PO selectivity contained considerable error. Among the V2O5/SiO2 catalysts, the highest PO selectivity (43 C%) was observed over an 0.18 wt % loaded catalyst. In the dark at 373 K, propene oxidation did not proceed at all over the 0.18 wt % V2O5/SiO2. Evidently, V2O5/SiO2 is not thermally activated but photoactivated. It is also worth mentioning that a small amount of PO was formed over highly loaded V2O5/SiO2 even at 10 wt % in a flow reactor. To some extent, the short contact time between the substrates and the catalysts afforded by the flow reactor system must prevent the PO from decomposing into aldehydes over a highly loaded catalyst, because, in the case of a closed circulation reactor, PO was not detected over 5.0 wt % V2O5/SiO2.20 We examined the effective wavelength for photocatalytic epoxidation over V2O5/SiO2 by the use of a 300 W Xe lamp with and without a UV cutoff filter. The UV cutoff filter greatly diminished the UV light intensity at wavelengths less than 380 nm. The propene conversion rate over 0.18 wt % V2O5/SiO2 decreased remarkably under irradiation with the UV cutoff filter. Because of the incomplete removal of the UV radiation, the photooxidation of the propene still proceeded to a small extent using the visible light; we know this because the product selectivity hardly changed whether we used the UV cutoff filter. Therefore, it is evident that the photoepoxidation of propene over 0.18 wt % V2O5/SiO2 needs the irradiation of UV light (λ < 380 nm). (23) Herrmann, J. M. Catal. Today 1999, 53, 115-129.
Figure 4. Near-IR diffuse reflectance spectra of dehydrated SiO2 (a) and V2O5/SiO2, 0.18 wt % (b), 0.5 wt % (c), 2.5 wt % (d), and 10.0 wt % (e).
Figure 5. UV-vis diffuse reflectance spectra of dehydrated SiO2 (a) and V2O5/SiO2, 0.18 wt % (b), 0.5 wt % (c), 2.5 wt % (d), and 10.0 wt % (e).
Characterization of V2O5/SiO2 Photocatalysts. The surface vanadium oxide species at low loading over SiO2 are known to be isolated VO4 tetrahedra in the dehydrated state; the local structure is changed by the adsorption of water molecules.24 Figure 4 shows the near-IR diffuse reflectance spectra of the dehydrated V2O5/SiO2. The intensity of the peak at around 7310 cm-1, indicating the presence of isolated SiOH groups over silica, decreased with an increase in the vanadia loading.25 This means that surface vanadium oxide species are interacting with isolated SiOH groups. Apparently, the surface hydroxyl groups of SiO2 are replaced with VO4 tetrahedra such as (SiO)3sVdO. Figure 5 shows the UV-vis diffuse reflectance spectra of dehydrated V2O5/SiO2. No absorption band was observed on amorphous SiO2. In the case of the 0.18 wt % V2O5/SiO2, a ligand-metal charge-transfer (LMCT) (24) Yoshida, S.; Tanaka, T.; Nishimura, Y.; Mizutani, H.; Funabiki, T. Proc. 9th Int. Congr. Catal. 1988, 3, 1473-1480. (25) Gao, X. T.; Bare, S. R.; Weckhuysen, B. M.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 10842-10852.
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Table 2. Band Gap Energies for the Allowed Transitions of V2O5/SiO2 and Reference Compounds sample
Eg/eV
sample
Eg/eV
0.18 wt % V2O5/SO2 0.5 wt % V2O5/SiO2 2.5 wt % V2O5/SiO2 K3VO4a
3.85 3.73 3.62 3.51
10.0 wt % V2O5/SiO2 NH4VO3b V2O5c
3.39 3.22 2.36
a
Isolated VO4. b Polymerized VO4. c Polymerized VO5/VO6.
band from O2- to V5+ emerged around 200-350 nm and was assigned to a tetrahedral VO4 monomer.25 However, with an increase in the vanadia loading, the peak position of the absorption band shifted to a longer wavelength and the spectral shape broadened. Wachs et al. reported a correlation between the bandgap energy (Eg) and the number of the covalent V-O-V bonds around the central V5+ species.26 Therefore, it is possible to estimate the local structure of the vanadium oxide species in the samples by comparing the Eg value with that of reference compounds. Table 2 shows the Eg for the allowed transitions of V2O5/SiO2 and the reference compounds by finding the intercept of a straight line in the low-energy edge of an [F(R)hν]2 against hν plot, where F(R) is the Kubelka-Munk function and hν is the incident photon energy.26,27 The values of Eg for the 0.18-2.5 wt % V2O5/SiO2 samples are still higher than that of the reference compound K3VO4, which consists of a VO4 monomer. The addition of alkali-metal ions into the V2O5/ SiO2 changed neither the coordination number nor the isolated state of the vanadium species, but caused a red shift of the edge of the UV spectra.28 Therefore, the electronic state of the reference compound K3VO4 would also be different from that of the VO4 monomer on the SiO2. However, it is reasonable to conclude that monomeric VO4 tetrahedra are the main surface species in the case of the dehydrated V2O5/SiO2 under low loadings. The gradual decrease of the Eg value with an increase up to 2.5 wt % in the vanadia loading might be due to slight changes in the electronic state derived from a local distortion of the monomeric VO4 tetrahedra. The Eg of the 10 wt % V2O5/SiO2 catalyst was intermediate between those of the K3VO4 and the NH4VO3 consisting of polymeric VO4 units (metavanadate chain).25 Evidently, there are a number of metavanadate chains in addition to the VO4 monomers on highly loaded V2O5/ SiO2. Moreover, in the case of the 10 wt % V2O5/SiO2, the UV-vis diffuse reflectance spectra exhibit a weak broad band in the 400-550 nm region. This is indicative of the presence of V2O5-like VO5/VO6 polymers on the SiO2 surface. The bulk V2O5 has a layered structure of VO5 sheets, which can also be regarded as distorted VO6 polymers.25 Figure 6 shows photoluminescence spectra of the dehydrated 0.18 wt % V2O5/SiO2 at liquid nitrogen temperature. The lifetime of the emission was measured by a decay curve as 13 ms. The VO4 monomers supported on the SiO2 display a phosphorescence emission with a vibrational fine structure assigned to the stretching of the terminal vanadyl VdO bonds.29-31 On the other hand, (26) Gao, X. T.; Wachs, I. E. J. Phys. Chem. B 2000, 104, 1261-1268. (27) Gao, X. T.; Bare, S. R.; Fierro, J. L. G.; Wachs, I. E. J. Phys. Chem. B 1999, 103, 618-629. (28) Takenaka, S.; Tanaka, T.; Yamazaki, T.; Funabiki, T.; Yoshida, S. J. Phys. Chem. B 1997, 101, 9035-9040. (29) Gritscov, A. M.; Shvets, V. A.; Kazansky, V. B. Chem. Phys. lett. 1975, 35, 511-512. (30) Anpo, M.; Tanahashi, I.; Kubokawa, Y. J. Phys. Chem. 1980, 84, 3440-3443. (31) Iwamoto, M.; Furukawa, H.; Matsukami, K.; Takenaka, T.; Kagawa, S. J. Am. Chem. Soc. 1983, 105, 3719-3720.
Figure 6. Photoluminescence spectra of dehydrated 0.18 wt % V2O5/SiO2 catalyst at 77 K. The phosphorescence excitation spectrum (a) was monitored at 510 nm, and the phosphorescence emission spectrum (b) was excited at 300 nm.
the separation between the 0-0 and the 0-1 transitions of the vibrational progression is usually observed to be a lower frequency than that of the VdO stretch, so that Scott et al. proposed that the progression should be attributed to a stretching of the V-O-Si ligands.32 However, the assignment to the basal V-O stretch in the VO4 tetrahedra is open to argument now, because it is known that the terminal VdO bond length in the excited triplet state is longer than that in the ground state.33,34 In the present study, a discussion of the vibrational emission structure is not necessary, but the wavelength of the phosphorescence excitation is more important. It is generally accepted that the triplet emission state involves an intersystem crossing from the singlet excited state localized on an oxovanadium group.35 The promotion of an electron into the antibonding orbital of the VdO bond (“π f π*” transition) destabilizes and activates the lattice terminal oxygen that is responsible for the photooxidation of the hydrocarbons. The phosphorescence excitation spectrum showed a distinct band at around 320 nm, a result that was in good agreement with the wavelength which was measured for the photoepoxidation over the 0.18 wt % V2O5/SiO2 (λ < 380 nm). The shape and the maximum of the phosphorescence excitation that is dominated by the π f π*-type VdO transition were not consistent with those of the diffuse reflectance UV spectrum for the 0.18 wt % V2O5/SiO2. This discrepancy indicates that the higher energy LMCT from the basal oxygens to the vanadium center contributes less to the phosphorescence emission. The basal V-O bonds would be strongly deactivated because they are covalently bound to the silica networks.35 We concluded that the photoexcited (Vδ-dOδ+)* species of the VO4 monomers supported on the SiO2 are the photocatalytically active sites for the oxofunctionalization of propene into PO under UV irradiation. Figure 7 illustrates the mechanism of the photooxidation of propene with O2 over V2O5/SiO2 under irradiation. The isolated VO4 tetrahedra dispersed on the SiO2 are the photocatalytic active sites for the formation of PO. However, the highly loaded V2O5/SiO2, containing less dispersed V5+ species, promote the isomerization of PO into PA. The oxygen incorporated into the products do not come from gaseous oxygen, but rather from the lattice oxygen in the V2O5/SiO2.20 (32) Tran, K.; Hanninglee, M. A.; Biswas, A.; Stiegman, A. E.; Scott, G. W. J. Am. Chem. Soc. 1995, 117, 2618-2626. (33) Kobayashi, H.; Yamaguchi, M.; Tanaka, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1985, 81, 1513-1525. (34) Krauss, M. J. Mol. Struct.: THEOCHEM 1999, 458, 73-79. (35) Tran, K.; Stiegman, A. E.; Scott, G. W. Inorg. Chim. Acta 1996, 243, 185-191.
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Figure 7. Mechanism of photooxidation of propene with O2 over V2O5/SiO2 under irradiation.
Conclusions In a previous study, we examined the photooxidation of propene over V2O5/SiO2 with O2 under UV irradiation in a closed circulation reactor. Although PO was not detected in the products, we predicted the formation of a PO intermediate prior to the production of aldehydes. If the contact time between the substrate and the catalysts is reduced, the decomposition of the unstable PO would be suppressed. In the present study, we adopted a flow reactor system in the photooxidation of propene over V2O5/ SiO2. The lowly loaded V2O5/SiO2 promotes the photocatalytic epoxidation of propene with O2 at steady state in a flow reactor system. Very little deep oxidation of propene took
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place, and the main products were PO and AA over V2O5/ SiO2. On the other hand, because the main product over the TiO2 was CO2, it is a suitable photocatalyst for degradation and mineralization. The sums of selectivities into PO and PA at steady state were almost the same for all of the V2O5/SiO2 samples with various loadings. A highly loaded V2O5/SiO2 promotes the isomerization of the PO intermediate into PA. By means of UV-vis diffuse reflectance spectra, we found that the VO4 monomers dispersed on SiO2 are photocatalytically active sites for the formation of PO, and the polymerized/aggregated vanadium oxide species on SiO2 are responsible for the isomerization of the PO into PA. The flow reactor system shortened the contact time between the substrates and the catalysts, so that, to some extent, the PO’s decomposition into PA over less dispersed vanadium oxide species was inhibited. The photoexcited (VdO)* species of the VO4 monomers supported on the SiO2 are active sites for the photocatalytic epoxidation of propene with O2 under UV irradiation. Supporting Information Available: Output spectra of a 300 W Xe lamp with a UV cutoff filter and the result of photooxidation under irradiation with a UV cutoff filter. This material is available free of charge via the Internet at http://pubs.acs.org. LA0359981
