17898
J. Phys. Chem. B 2006, 110, 17898-17905
Ti-Containing Mesoporous Organosilica as a Photocatalyst for Selective Olefin Epoxidation Masatsugu Morishita, Yasuhiro Shiraishi,* and Takayuki Hirai Research Center for Solar Energy Chemistry, and DiVision of Chemical Engineering, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka 560-8531, Japan ReceiVed: April 17, 2006; In Final Form: July 14, 2006
We previously found that Ti-containing mesoporous silica (T-S) with isolated and tetrahedrally coordinated Ti-oxide species, when photoactivated in acetonitrile with molecular oxygen (O2), catalyzes highly selective epoxidation of olefins (Chem. Commun. 2005, 5977). The system showed the highest epoxide selectivity among the photocatalytic systems proposed so far, but showed insufficient olefin conversion. In the present work, we have employed Ti-containing mesoporous organosilicas (T-OS), synthesized by a surfactanttemplating method with an organosilane precursor, as the photocatalyst and have studied the effects on the olefin conversion and the epoxide selectivity. The T-OS catalysts demonstrate the same high epoxide selectivity as does T-S, but scarcely improve the olefin conversion. Photoluminescence measurement reveals that the T-OS catalysts with high surface hydrophobicity enhance the access of hydrophobic olefins to the photoexcited Ti-oxide species as expected, but destabilize the excited species themselves. ESR analysis demonstrates that the T-OS catalysts also destabilize the active oxygen radical (O3•-), a crucial oxidant for olefin epoxidation, formed on the excited Ti-oxide species. These destabilizations counteract the enhanced olefin access to the excited species, resulting in almost no improvement in olefin conversion. Through detailed analyses, we have summarized the changes in photocatalytic properties of the Ti-oxide species, associated with the organic modification of the catalyst.
1. Introduction There has been much interest in the selective epoxidation of olefins with molecular oxygen (O2) driven in a heterogeneous catalytic system.1 Photocatalytic olefin oxidation processes have been studied extensively,2 especially using a semiconductor, titanium dioxide (TiO2); however, the systems lack sufficient epoxide selectivity.3 In a previous work,4 we have found that a Ti-containing MCM-41 mesoporous silica (T-S) with isolated and tetrahedrally coordinated Ti-oxide species (Ti-O4), when photoactivated in acetonitrile (MeCN), catalyzes highly selective photocatalytic epoxidation of olefins with O2. The system demonstrates the highest epoxide selectivity among the photocatalytic systems proposed so far.2,3,5 The high epoxide selectivity of the T-S system is due to a “shield effect” driven by MeCN. The catalytic mechanism for the selective olefin epoxidation on T-S is summarized in Scheme 1. Photoexcitation of the Ti-O4 species (I) leads to LMCT (ligand-to-metal charge transfer) between Ti4+ and lattice oxygen (OL2-), resulting in a formation of excited state [Ti3+OL-]* species (II). Reaction of the species (II) with O2 produces two types of oxygen radicals, O2•- and O3•- (III and IV). The electrophilic O3•- formed on OL- (III) adds directly to olefin (route A), leading to a formation of the corresponding epoxide. The vacant OL- sites on species II and IV are also electrophilic and, hence, act as positive holes. Without MeCN, a proton transfers from olefin to OL- to produce the corresponding olefin radical, which reacts with O2•- and affords undesirable allylic oxidation products (routes B and C).5 In contrast, with MeCN, the proton transfer is suppressed because MeCN, a weak base, stabilizes the olefin by solvation via a hydrogen-bonding effect.6 * To whom correspondence should be addressed. E-mail: shiraish@ cheng.es.osaka-u.ac.jp. Fax: +81-6-6850-6273. Phone: +81-6-6850-6271.
SCHEME 1: MeCN-Assisted Selective Olefin Photoepoxidation on T-S Catalyst
In this case, MeCN does not deactivate the OL- site, suppress the reaction of OL- with O2, or destabilize the O3•- formed. The MeCN-assisted selective suppression of the olefin radical formation on the OL- site, so-labeled “shielding effect”, promotes route A preferentially, resulting in high epoxide selectivity.4 The system, however, shows very low olefin conversion (98%) as does T-S. This indicates that no improvement in the olefin conversions on the T-OS catalysts may be attributable to the properties of the photoexcited Ti-O4 species and/or of the oxygen radicals formed on the species. 2.4. Properties of Active Site and Oxygen Radicals. As shown in Scheme 1, reaction of O2 with the excited Ti-O4 species (formation of O3•- radical) is also an important step for olefin epoxidation. As shown by dotted lines in Figure 3AD, addition of O2, in place of CHE, to the T-S and T-OS catalysts also leads to a decrease in PL intensity of [Ti3+-OL-]* due to the formation of oxygen radicals (Scheme 1, II f III and II f IV). As summarized in Table 4, both T-S and T-OS catalysts show almost the same decrease in PL intensity (7177%). This suggests that the increased surface hydrophobicity does not affect the access of O2 to the [Ti3+-OL-]* species.
17902 J. Phys. Chem. B, Vol. 110, No. 36, 2006
Figure 5. ESR spectra (77 K) of (A) T-S and (B) T-OS(10) catalysts (i) obtained by photoirradiation with O2 (0.3 µmol). Spectrum ii is obtained by letting sample i stand at room temperature for 1.5 min. Spectrum iii is obtained by subtracting spectrum ii from spectrum i. Spectrum iv is the simulated spectrum obtained by a trial-and-error method according to eqs 1 and 2.
A notable fact of the PL spectra obtained in vacuo (Figure 3, bold lines) is the lower PL intensity of the T-OS catalysts compared to that of T-S. As summarized in Table 4, the PL intensity decreases with an increase in the BTESE ratio of the catalysts, meaning that the quantity of the photoformed [Ti3+OL-]* species decreases with the BTESE ratio. Anpo et al. have suggested that the PL intensity of the [Ti3+-OL-]* species decreases with an increase in surface hydrophobicity of the catalyst.20 The strong deactivation of the [Ti3+-OL-]* species on the T-OS catalysts may be because the highly polarized (charge-separated) [Ti3+-OL-]* species are unstable in a hydrophobic (less polar) environment. The obtained findings suggest that the [Ti3+-OL-]* species formed are deactivated strongly on the hydrophobic catalyst surface, and that the deactivation of the species may be the crucial factor for the low olefin conversions on the T-OS catalysts. For further confirmation, we determined the quantity and stability of the oxygen radicals formed on the Ti-O4 species by means of ESR measurement. Figure 5A.i shows the ESR spectrum of the T-S catalyst (25 mg containing 0.35 µmol of Ti-O4) obtained at 77 K with photoirradiation in the presence of O2 (0.32 µmol). The complicated spectrum is assigned to two types of oxygen radicals, such as O2•- (gxx ) 2.003, gyy ) 2.009, gzz ) 2.026) and O3•- (g|| ) 2.008, g⊥ ) 2.002).4,5,21,22 Figure 5B.i shows the ESR spectrum of T-OS(10) catalyst. It is noteworthy that the intensity of the T-OS(10) spectrum is much lower than that of the T-S spectrum. Total quantities of the oxygen radicals formed on the T-S and T-OS catalysts, determined by double integration of the total ESR signal,4,5 are 1 (T-S), 0.097 (T-OS(10)), 0.063 (T-OS(20)), and 0.085 (TOS(100)), respectively, where the quantities on the T-OS catalysts are 20% of that formed on T-S. This suggests that the very low quantities of oxygen radicals formed on the T-OS catalysts are attributable to the deactivation of radical species themselves on the T-OS catalysts, in addition to the destabilization of the [Ti3+-OL-]* species. The strong deactivation of the oxygen radical species on the T-OS catalysts
Morishita et al.
Figure 6. Relative quantity of (black) O3•- and (white) O2•- formed on the respective catalysts determined by ESR measurement. The numbers in the figure denote the quantity ratio O3•-/O2•-.
may be because these radicals are unstable in hydrophobic (less polar) environment.23 The crucial oxygen radical for selective epoxidation is O3•(Scheme 1).4,5 We then determined the quantity of O3•- formed on the T-OS catalysts. As reported5 and described previously,4,22 when the ESR sample of the T-S catalyst (spectrum i, Figure 5A) is left to stand at room temperature for 1.5 min, the O3•signal disappears and only the O2•- signal remains (spectrum ii). This is because O3•- is unstable at room temperature, while O2•- is stable.5 By subtracting spectrum ii from i, the lost O3•signal is derived (spectrum iii). The relative quantity of O3•and O2•- can therefore be determined by double integration of the ESR signals, ii and iii, respectively.4,22 The ratio of O3•-/ O2•- is therefore estimated to be 0.24/0.76 ()0.32) (total quantity ) 1) (Figure 6). To determine the respective quantities of O2•- and O3•- formed on T-OS(10) (total quantity ) 0.097), the pure O2•- and O3•- spectra of the T-S sample (spectra ii and iii, Figure 5A) were summed up with weighting factors, R and β, to produce a simulated spectrum, as below.
simulated spectrum )
spectrum (O2•- ) × 0.76 spectrum (O3•- ) × β (1) R+ 0.24 R + β ) 0.097
(2)
A trial-and-error method22 determined the values R (0.09) and β (0.007), which produced the nearest shaped spectrum (spectrum iv) to the observed spectrum (spectrum i). The ratio of O3•-/O2•- was therefore estimated to be 0.007/0.09 ()0.08). Figure 6 summarizes the relative quantities of the oxygen radicals formed on the respective catalysts. The quantities of O3•- are 0.24 (T-S), 0.007 (T-OS(10)), 0.005 (T-OS(20)), and 0.006 (T-OS(100)), indicating that the O3•- quantities on T-OS catalysts are much lower than that on T-S (
