Titania-silica catalysts prepared by sol-gel method for photoepoxidation of propene with molecular oxygen

Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

845

Titania-silica catalysts prepared by sol-gel method for photoepoxidation of propene with molecular oxygen Chizu Murata, Hisao Yoshida, and Tadashi Hattori Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Photooxidation of propene to propene oxide (PO) by molecular oxygen was performed over TiO2-SiO2 binary oxides prepared by sol-gel method (TS(s)) and TiO2/SiO2 supported oxides prepared by impregnation method (TS(i)). On both the TS(s) and TS(i) systems, the selectivity to PO increased with decreasing Ti content. Diffuse reflectance UV spectra showed that the samples of low Ti content consisted of the isolated tetrahedral Ti species predominantly, while the [TiOz]n clusters were also formed and became larger with increasing Ti content. Thus, it was concluded that the isolated tetrahedral Ti species were the active sites for PO production. The TS(s) sample of low Ti content exhibited much higher selectivity to PO (60 %) than the corresponding TS(i) sample (40 %) although the dominant Ti species seemed to be the same in both samples. It was confirmed that the consecutive reaction of PO was much suppressed on the sample prepared by the sol-gel method. 1. INTRODUCTION The direct gas phase epoxidation of propene by molecular oxygen has been desired, and has been attempted by many researchers [1-4]. As a new approach, 'photoepoxidation' of propene using only Oz has been investigated over several systems such as TiO2, Ba-Y type zeolite, Nb205/SiO2, MgO/SiO2 and SiO2 [5-10]. But these activities were still low and it was not clear whether the reaction proceeded catalytically or not. In our previous screening study of silica-supported metal oxides for propene photoepoxidation, TiO2/SiO2 showed the highest PO yield [11]. In the present study, we prepared titania-silica catalysts by two kinds of preparation method, the two-stage sol-gel method [12] and the conventional impregnation method, and compared their propene photooxidation activity. 2. EXPERIMENTAL TiOa-SiOa mixed oxide samples were prepared by the sol-gel method consisting of two-stage hydrolysis procedure, by which Ti species are expected to be highly dispersed in silica since the aggregation of Ti is suppressed during the hydrolysis process [12]. A mixture of Si(OCaH5)4, CaH5OH, HaO and HNO3 (0.5, 1.9, 0.5 and 0.043 mol, respectively) was stirred at 353 K for 3 h to hydrolyze Si(OC2H5)4 partially, and the obtained sol was cooled down to room temperature. A 2-propanol (20 ml) solution of

846 titanium isopropoxide (0.0005-0.05 mol) was added to the sol and stirred for 2 h. Then, an aqueous HNO3 solution (H20 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 up to 338 K at 0.2 K m-in-1 and dried for 5 h. After additional drying for 5 h at 373 K, it was calcined at 773 K in a flowing air for 8 h. Ti content was determined by inductively coupled plasma (ICP) measurement. Titania-silica samples thus prepared by the sol-gel method were denoted as xTS(s), where x (mol% of Ti) = NTi/(NTi + Nsi)xl00. TiOz/SiO2 supported oxide samples were prepared by the conventional impregnation method; amorphous silica (1.0 g) was impregnated with an aqueous solution (50 ml) of (NH4)2[TiO(C204)2]" 2H20, then dried at 383 K for 12 h and calcined at 773 K in flowing air for 5 h. Amorphous silica was prepared from Si(OC2H5)4 by another sol-gel method followed by calcination in a flow of air at 773 K for 5 h [11]. Titania-silica samples prepared by the impregnation method in this way were denoted as xTS(i) similarly to above. The TiO2 sample employed was a Japan Reference Catalyst (JRC-TIO-4; equivalent P-25). The BET surface area of the samples was determined by N2 adsorption. Prior to each reaction test and spectroscopic measurement, the sample was treated with 100 Tort oxygen (1 Torr = 133.3 N m -2) at 673 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 cm 3). The sample (200 mg) was spread on the flat bottom (12.6 cm 2) of the quartz vessel. Propene (100 l.tmol, 15 Torr) and oxygen (200/Ltmol, 30 Torr) were introduced into the vessel, and the sample was irradiated by a 200 W Xe lamp. After collecting the products in gas phase, the catalyst bed was heated at 573 K in vacuo to collect the products adsorbed on the catalyst by a liquid nitrogen trap. These products were separately analyzed by GC. The results presented here are the sum of each product yield. The reactivity of PO over titania-silica catalysts was tested in two ways. (a)Thermal isomerization of PO; PO (20 [.tmol) was introduced to the reactor in the dark, then the sample was heated at 573 K to collect the products. (b)Photooxidation of PO; PO (20 ptmol) and O2 (200 ~mol) were introduced to the reactor, then the sample was irradiated for 1 h. After collecting products in gas phase, the sample was heated at 573 K to collect the adsorbed products. Diffuse reflectance UV spectra of the samples in vacuo were recorded on a JASCO V-570 spectrophotometer at room temperature. The crossing-point between the base line and the tangential line of the inflection point was employed as the wavelength of absorption edge. Temperature-programmed desorption of NH3 (NH3-TPD) was examined to estimate the amount of surface acid sites. The pretreated sample (200 mg) was exposed to NH3 at 373 K for 0.5 h, and evacuated at 373 K to remove weakly adsorbed NH3. The temperature of the sample increased linearly 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. 3. RESULTS

3.1. Photooxidation of Propene Table 1 shows BET surface area of the SiO2, TiO2, titania-silica samples and the results of photooxidation of propene over them. The specific surface area of each

847 titania-silica and silica samples were similarly high (400-550 m2g'l). The SiO2 sample showed a low conversion as previously reported [10], while the TiO2 sample showed high activity for the complete oxidation of propene to CO2. Partial oxidation of propene took place over all the titania-silica samples, both the TS(i) and TS(s) samples, under photoirradiation. The major products were propene oxide(PO), ethanal, propanal, CO, CO2, acetone and acrolein; and small amount of2-propanol, ethene and butene were also formed. It was confirmed that propene was not converted on the titania-silica in the dark, as typically shown over the 0.34TS(s) sample. Over both the TS(i) and TS(s) systems, the conversion of propene increased with an increase of Ti content, while the yield of PO increased with increasing Ti content up to about 1 mol % and then decreased. Fig. 1 depicts the propene conversion and the selectivity in the photooxidation of propene over the TS(i) and TS(s) systems from Table 1. In the TS(i) system (Fig. l a), the samples containing small amount of Ti (0.01-0.1 mol %) showed high selectivity to PO such as 40 %. Increasing Ti content, the selectivity to PO decreased and the selectivity to COx and propanal increased. Also in the TS(s) system (Fig. lb), the distribution of products changed according to Ti content similarly to that over the TS(i) system. However, comparing at the same content of Ti, the TS(s) system always showed higher PO selectivity than the TS(i) system. Especially, the TS(s) sample containing small amount of Ti (0.08 mol %) showed the best selectivity to PO such as 60 % among all the samples tested in the present study and the previously reported photoepoxidation systems [5, 8-11 ]. Each TS(s) sample showed lower selectivity to ethanal and acrolein than TS(i) sample (Table 1). Table 1. Results of the photooxidation of propene over the SiO2, TiO2 and titania-silica samples. SA a Conv. b PO ~ Selectivity / C% Sample /mEg q / C % /C% PO d propanal acetone acrolein ethanal HC e CO t. SiO2 558 0.7 0.2 22.3 3.5 25.8 15.2 18.1 10.0 5.1 TiO2 34 59.5 0.0 0.0 0.4 0.2 0.0 0.1 0.4 98.8 0.0ITS(i) 544 1.9 0.7 37.0 3.0 15.1 7.6 26.8 2.7 4.9 0.1TS(i) 478 9.1 3.7 40.8 3.3 11.3 4.2 24.5 2.3 10.1 0.5TS(i) 495 17.8 5.4 30.1 11.0 12.4 4.2 20.0 1.8 16.4 1.5TS(i) 473 24.4 4.7 19.2 17.6 9.3 8.7 21.5 2.4 14.1 5.0TS(i) 453 23.8 1.1 4.8 10.2 12.2 2.6 26.4 3.7 37.0 0.08TS(s) 387 4.4 2.6 60.2 2.9 10.4 1.1 17.7 3.0 3.9 0.34TS(s) 423 9.2 5.3 57.5 2.7 5.8 1.2 21.1 5.1 6.6 1.0TS(s) 535 12.5 6.3 50.5 6.2 8.3 1.7 22.1 3.3 7.4 4.1TS(s) 483 21.0 4.5 21.5 19.1 11.4 2.4 24.1 3.4 14.4 8.3TS(s) 416 32.1 1.8 5.7 24.5 13.6 3.1 26.4 2.9 19.1 0.34TS(s) g 423 0.5 trace 5.5 1.6 2.9 0.0 70.0 20.0 0.0 Catalyst 0.2 g, propene 100 tool, 02200 moi, reaction time 2 h. aBET surface area. b Conversion based on propene, c PO yield, d Propene oxide, e Ethene and butenes, f CO and CO2. g In the dark for 2 h at 323 K.

848 40

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Fig. 1 Results of photooxidation of propene over the TS(i) (a) and TS(s) (b) samples. Conversion (C)), and selectivity to PO (O), propanal (A), ethanal (Y) and COx (•

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4 6 8 Irradiation time / h Fig. 2 shows time courses of propene photooxidation over the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples. Over all these samples, the conversion increased with an increase of irradiation time. With increasing conversion, the PO selectivity decreased slightly, and the selectivity to ethanal and propanal much decreased, while the selectivity to COx increased. This indicated that ethanal and propanal were consecutively oxidized to COx more easily than PO was. Over the 0.34TS(s) sample the PO yield reached 15.7 % at 10 h, while over the 0.1TS(i) sample it reached 9.2 % at 8 h (Fig. 2). Even if all the Ti atoms were assumed to 0

2

-

Fig. 2. Time course of photooxidation of propeneover the 0.34TS(s) (a), 4.1TS(s) (b) and 0.1TS(i)(c) samples. Conversion (o), PO yield ( , ) , and selectivity to PO ("), propanal (zx),

849 be the active sites, the turnover number, TON = (the amount of produced PO) / (the amount of active sites), was 1.4 and 2.8, respectively. This means that the photoepoxidation of propene over titania-silica proceeds catalytically [13]. In all the runs mentioned above, most of products except for CO2 were collected by heating at 573 K in Vacuo. Thus, we examined the desorption temperature of each product over the 0.34TS(s) and 4.1TS(s) samples by heating the catalysts stepwise at 323 K, 373 K, 473 K and 573 K. Over both of the samples, most of PO and ethanal were collected by heating up to 373 K. These results suggest that PO and ethanal weakly adsorbed on the catalysts. On the contrary, propanal was mainly collected by 573 K heating. Propanal adsorbed on the catalyst more strongly than PO and ethanal. 3.2. The reactivity of propene oxide (PO) To know the possibility of the conversion of produced PO, the thermal isomerization of PO and the photooxidation of PO were examined over the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples (Table 2). By the thermal isomerization of PO (Table 3 (a)), PO was converted mainly to propanal without irradiation. A very small amount of PO was converted over the 0.34TS(s) and 0.1TS(i) samples, while 18.8 % of PO was converted over the 4.1TS(s) sample. By the photooxidation of PO with molecular oxygen (Table 3 (b)), ethanal, acrolein and COx were obtained in addition to propanal over all the samples. These results suggested that propanal was produced by the thermal isomerization of PO in the dark predominantly, and others such as ethanal and CO,, were mainly produced by the photooxidation of PO. The amount of PO converted by the photooxidation, which would correspond to the difference between (b) and (a), was 2.1%, 20.3 % and 11.7 % over the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples, respectively. Table 2. Reaction of propene oxide (PO) over the representative titania-silica catalysts. Yield/C% b Method a Sample

propanal acetone acrolein ethanal alcohols HCCCOx d

Total yield / C%

(a) 0.34TS(s) 1.7 0.1 0.1 0.1 0.0 0.2 0.1 2.3 (a) 4.1TS(s) 14.3 0.7 0.0 0.0 0.0 1.1 2.7 18.8 (a) 0.1TS(i) 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.2 (b) 0.34T S (s) 1.3 0.2 0.0 1.4 0.0 0.2 1.3 4.4 (b) 4.1TS(s) 18.4 2.6 2.4 6.1 3.5 1.3 4.8 39.1 (b) 0.1TS(i) 1.5 0.6 1.5 2.8 0.7 0.5 4.3 11.9 Catalyst 0.2 g, propene oxide 20 lamol, a (a) The thermal isomerization of PO and (b) the photooxidation of PO. See text. bBased on PO. c Propene, ethene and butenes, d CO and COz.

850

3.3 Characterization

of the titania-silica

samples

Fig. 3 shows diffuse reflectance UV spectra of the samples. The TiO2 sample showed a large absorption band below 380 nm. The SiO2 -,,, \ sample scarcely showed absorption. The titania-silica samples of low Ti content less than 0.4 mol %, in both \ ,, \,, ,.,,,, \ the TS(i) and TS(s) systems, \ ',\', ,\, exhibited a narrow absorption band ~ 2. ~,, below 250 nm, which was assigned to the LMCT (ligand-metal charge transfer) from O to Ti of isolated tetrahedral Ti species [14, 15]. The samples containing more than 0.4 ' 315 ' I ' ~1 mol % of Ti in both the TS(i) and 2q)0 250 waveleng~... / nm 350 400 TS(s) systems showed the additional Fig. 3 Diffuse reflectance UV spectra of SiO2 (---), absorption in the region above 250 TS(i)(----), T S ( s ) ( ~ ) and TiO2(. . . . ). The samples nm. The absorption edge was shifted were evacuated at 673 K. SiO2 (a), 0.0ITS(i) (b), to longer wavelength with increasing 0.08TS(s) (c), 0.1TS(i) (d), 0.34TS(s) (e), 1.0TS(s) ~, Ti content and became close to that 0.5TS(i) (g), 4.1TS(s) (h), 1.5WS(i)(i), 8.3TS(s) ~,'), of the TiO2 sample. The absorption in 5.0TS(i) (k) and TiO2 (/). the 250 - 330 nm region are assigned to the [TiOz]n clusters, and it is generally known that the absorption edge is shifted to longer wavelength as the size of the [TiOa]n clusters become larger [14, 15]. From these results, it was indicated that both the TS(s) and TS(i) samples containing less than 0.4 mol o% of Ti consisted of the isolated tetrahedral Ti species predominantly. In the titania-silica samples containing more than 0.4 mol % of Ti, the [TiOz]n clusters were also formed, and the size of the [TiOz]n clusters become larger with increasing Ti content. Fig. 4 shows the relationship between the Ti content and the absorption edge in UV spectra. In the region of more than 0.4 mol % of Ti content, the TS(s) system showed the absorption edge of shorter wavelength than the TS(i) system. This indicates that the TS(s) system have more dispersed Ti species than the TS(i) system when they were compared at the same Ti content. Fig. 5 shows the NH3-TPD profiles of the 0.34TS(s), 4.1TS(s) and 0.1TS(i) samples. The 4.1TS(s) sample showed 54.7 ~tmol g-1 of desorbed NH3, which corresponds to about 8 % of Ti atoms in this sample. On the other hand, the 0.34TS(s) and 0.1TS(i) samples showed very small amount of desorbed NH3, which means that both of them have almost no acid site.

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500 600 700 Temperature / K Fig. 5. NH3-TPD profiles of 0.34TS(s) (a), 4.1TS(s) (b) and 0.1TS(i) (c).

4. DISCUSSION 4.1. The structure of Ti species and the acidity over titania-silica samples When Ti content was very low (< 0.4 mol %), the titania-silica samples consisted of the isolated tetrahedral Ti species predominantly, regardless of the preparation method (Fig. 3). In the titania-silica samples containing more than 0.4 tool % of Ti, the [TiO2]n clusters were also formed, and the clusters became larger with increasing Ti content (Fig. 3). The TS(s) samples contained more dispersed Ti species than the TS(i) samples when they were compared at the same Ti content (Fig. 4). By the sol-gel method employed in the present study, the aggregation of Ti atom would be reduced in comparison with the conventional impregnation method, as expected. The 0.34TS(s) and 0.1TS(i) samples predominantly having the isolated tetrahedral Ti species possessed almost no acid site, while the 4.1TS(s) sample containing the [TiOz]n clusters 60~oCk(a had acid sites (Fig. 5). It is known that Ti-O-Si ~" ) bridges where the Ti atoms reside in ~ 40 pentahedral or octahedral sites cause the charge i~ A"~ I imbalance and generate Bronsted acid sites [14] 9 ~5 Thus, the acid property of the 4.1TS(s) sample ~ 20 would be due to the presence of [TiO2]n clusters, which contained the octahedral or pentahedral 0 I I - ~ Ti moieties9 200 250 300 ~ 3 5 0 400 absorption edge / n m 4.2. The effect of Ti content on the Fig. 6. Relationship between the PO photocatalytic activity selectivity and the absorption edge in UV On both the TS(s)and TS (i) systems, the spectra of the TS(s) (a) and TS(i) (b) PO selectivity increased and the selectivity to samples. propanal and COx decreased with decreasing Ti

852 content (Fig. 1). As mentioned above, the dispersion of Ti species varied with Ti content. Fig. 6 shows the relationship between the PO selectivity and the absorption edge of UV spectrum of each sample. The selectivity to PO increased when the absorption edge was shifted to shorter wavelength. This means that more dispersed Ti species show higher PO selectivity. The 4.1TS(s) sample showed higher selectivity to propanal, ethanal and COx than the 0.34TS(s) sample at similar conversion (Fig. 2). This result would be explained by the result of PO conversion shown in Table 2. PO was not so much converted over the 0.34TS(s) sample, while a large amount of PO was converted into propanal over the 4.1TS(s) sample in the dark (Table 2(a)). Generally, it is known that PO is isomerized into propanal on acid sites, and into acetone on basic sites [16]. Thus, it was suggested that a part of PO produced on the 4.1TS(s) sample in the photooxidation of propene cannot be desorbed as PO, and was converted into propanal due to the acid sites arising from the [TiOz]n clusters. In addition, much larger amount of PO converted to ethanal and COx over the 4.1TS(s) sample than over the 0.34TS(s) sample under photoirradiation in the presence of molecular oxygen (Table 2(b)). The [TiOz]n clusters promoted not only the thermal isomerization of PO to propanal, but also the consecutive photooxidation of PO during the propene photooxidation. As a conclusion of this section, it is found that the isolated tetrahedral Ti species on SiO2 are effective for propene photoepoxidation, while the [TiOz]n clusters for other products such as propanal, ethanal and COx production. 4.3. The effect of the preparation method on the photocatalytic activity The TS(s) system showed higher selectivity to PO than the TS(i) system at any Ti content (Fig. 1). In Fig. 6, the plots of the TS(s) and TS(i) samples seem to be on the common curve in the region from 270 to 380 nm of the absorption edge. This means that PO selectivity would be determined by the dispersion of Ti species regardless of the preparation methods in this region. On the other hand, in the region less than 250 nm of the absorption edge, although the samples in the both systems consisted of the isolated tetrahedral Ti species predominantly, the plots of the TS(s) system showed higher PO selectivity than the TS(i) system (Fig. 6). The 0.1TS(i) sample showed higher selectivity to ethanal, acrolein and COx than the 0.34TS(s) sample (Table 1). This result agrees with the result of photooxidation of PO; more PO converted to ethanal, acrolein and COx over the 0.1TS(i) sample than over the 0.34TS(s) sample (Table 2(b)). The yield of propanal converted from PO was similar over both samples (Table 2), and the 0.1TS(i) sample showed almost no acid sites similarly to the 0.34TS(s) sample (Fig. 5). Therefore, the conversion of PO over the 0.1TS(i) sample should not be attributed to the acidity arising from [TiOz]n clusters. Although no evidences were obtained to clarify the structural differences between the TS(s) and TS(i) samples containing a small amount of Ti, some possibility can be considered. One is that the local structure of the isolated tetrahedral Ti species may be different. For example, Lamberti et al. [17] suggested the existence of two kinds of the isolated tetrahedral Ti species in TS-1; [Ti(OH)(OSi)3] and [Ti(OSi)4]. As a conclusion of this section, it should be noted that the sol-gel method gave more selective catalysts for propene photoepoxidation than the impregnation method.

853 5. CONCLUSION The titania-silica catalyst of low Ti content prepared by the sol-gel method exhibited the highest selectivity to PO in the photooxidation of propene by molecular oxygen. The selectivity to PO decreased with increasing Ti content. It was concluded that the isolated tetrahedral Ti species were active sites for propene photoepoxidation, while the [TiOz]n clusters for the formation of propanal and CO2. Comparing samples TS(s) and Ts(i) which exhibited similar UV absorption band attributed to the isolated tetrahedral Ti species, the TS(s) sample showed much higher selectivity to PO, up to 60 %, than the TS(i) sample. It was found that the sol-gel method could provide effective titania-silica catalysts for the photoepoxidation of propene by molecular oxygen. ACKNOWLEDGEMENT 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. REFERENCES 1. Y. Wang and K. Otuka, J. Catal., 157 (1995) 450. 2. T. Hayashi, K. Tanaka and M. Haruta, J.Catal., 178 (1998) 566. 3. G. Lu and X. Zuo, Catal. Lett., 58 (1999) 67. 4. K. Murata and Y. Kiyozumi, Chem. Commun., (2001) 1356. 5. E Pichat, J. Herrmann, J. Disdier and M. Mozzanega, J. Phys. Chem., 83 (1979) 3122. 6. E Blatter, H. Sun, S. Vasenkov and H. Frei, Catal. Today, 41 (1998) 297. 7. Y. Xiang, S. C. Larsen and V. H. Grassian, J. Am. Chem. Soc., 121 (1999) 5063. 8. T. Tanaka, H. Nojima, H. Yoshida, H. Nakagawa, T. Funabiki, and S. Yoshida, Catal. Today, 16 (1993) 297. 9. H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki and S. Yoshida, Chem. Commun., (1996) 2125. 10. H. Yoshida, T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki and S. Yoshida, J. Catal., 171 (1997) 351. 11. H. Yoshida, C. Murata and T. Hattori, J. Catal., 194 (2000) 364. 12. R. Lange, J. Hekkink, K. Keizer and A. Burggraaf, J. Noncryst. Solids, 191 (1995) 1. 13. H. Yoshida, C. Murata and T. Hattori, Chem. Commun., (1999) 1551. 14. X. Gao and I. E. Wachs, Catal. Today, 51 (1999) 233, and references therein. 15. S. Bordiga, S. Colucia, C. Lamberti, L. Marchese, A. Zecchina, E Boscherimi, E Buffa, E Genomi, G. Leofanti, G. Petrini, and G. Vlaic, J. Phys. Chem., 98 (1994) 4125. 16. Y. Okamoto, T. Imanaka and S. Teranishi, Bull. Chem. Soc. Jpn., 46 (1973) 4. 17.C. Lamberti, S. Bordiga, D. Arduino, A. Zecchina, E Geobaldo, G. Span6, E Genoni, G. Petrini, A. Carati, E Villain and G. Vlaic, J. Phys. Chem. B, 102 (1998) 6382.