Two Photoexcitation Steps for Photometathesis of Propene over FSM-16

Oct 14, 2000 - evacuation of FSM-16 at high temperature such as 1073 K react with propene ... Mesoporous silica materials such as FSM-161,2 and MCM-...
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10304

J. Phys. Chem. B 2000, 104, 10304-10309

Two Photoexcitation Steps for Photometathesis of Propene over FSM-16 Yoshitaka Inaki, Hisao Yoshida,* and Tadashi Hattori Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity, Nagoya 464-8603, Japan ReceiVed: May 31, 2000; In Final Form: August 31, 2000

Photometathesis of propene over siliceous mesoporous silica, FSM-16, was revealed to be a novel type metathesis system consisting of two photoexcitation steps. For both steps, photoirradiation is essential, but the effective wavelength for each step is different: UV light is necessary for the first step, while visible light as well as UV light is effective for the second step. In the first step, photoabsorption sites generated by evacuation of FSM-16 at high temperature such as 1073 K react with propene under UV light irradiation to form surface intermediates. While, in the second step, the intermediates thus formed react with propene under the light including the visible region to form metathesis products.

Introduction FSM-161,2

and MCMMesoporous silica materials such as 413,4 were recently synthesized and attract a great deal of attention as a new class of catalyst and adsorbent. Most of the applications to catalysis were performed through the introduction of heteroatoms5,6 to generate catalytic functions. This trend seems to be based on a common sense that the silica materials are catalytically inert, but silica actually catalyzes a few reactions.7-13 Recently, it was found that unmodified mesoporous silica has some advantages to amorphous silica, which has mild acidity14-16 and radical-like function17,18 to catalyze some reactions. Moreover, photocatalytic activities were reported on amorphous silica19-24 and mesoporous silica.25-27 We reported recently that unmodified mesoporous silica, FSM-16, exhibits the photocatalytic activity for metathesis of propene.25,26 Although it had been reported that amorphous silica exhibited an activity for photometathesis of propene previously,19,20 the activity of FSM-16 was much higher than amorphous silica.25,26 Characteristic thin wall structure of FSM-16 would effectively contribute to the formation of photometathesis active sites, which were suggested to be the strained siloxane bridges or the related sites generated by dehydroxylation of isolated hydroxyl groups under evacuation at high temperature.25 Most catalysts for metathesis contain transition metals such as Mo, W, and Re, and it has been believed that these are essential elements as active species for metathesis.28 The reaction mechanism for metathesis on transition metal oxide catalyst has been well understood, and it is believed that the reaction consists of the catalytic cycle via two kinds of intermediates: metal carbene and metallacyclobutane.29 One might suspect that such species are hardly formed on silica surface. However, the presence of the metallacyclobutane intermediates was suggested in photometathesis over amorphous silica.19 It is considered that the photoirradiation enables the metathesis over nontransition metal oxide such as silica materials. But the details for the photoactivation mechanism of photometathesis over silica materials have not been investigated. * Author to whom correspondence should be addressed. Phone: +8152-789-4609. Fax: +81-52-789-3193. E-mail: [email protected].

In the present study, we focused our attention on the photoactivation process of photometathesis over mesoporous silica, FSM-16, and proposed the reaction scheme. Experimental Section Mesoporous silica, FSM-16, was prepared by the same manner as described in ref 1, except that hexadecyltrimethylammonium bromide [C16H33(CH3)3N+Br-] was used as a template. Synthesized precursor of FSM-16 was calcined at 523 K in a flow of N2 for 1 h and subsequently at 873 K in a flow of air for 5 h to remove the organic fraction. Powder X-ray diffraction (XRD) pattern was recorded on a Rigaku diffractometer RINT 1200 using a radiation of Ni-filtered Cu KR (40 kV, 20 mA). N2 adsorption isotherm was recorded on Coulter Omnisorp Series 100 CX at 77 K. Before measurement of the isotherm, the catalyst was evacuated for 3 h at 673 K. The purity of FSM-16 calcined was measured by ICP emission spectrometry, and only a small amount of Al was contained as a contaminant (Si/Al ) 372), which would be originated from water glass. Before catalytic reaction, a pretreatment of catalyst was performed at 1073 K in the presence of 100 Torr O2 for 1 h, followed by evacuation for 1 h at the same temperature. Photocatalytic reaction of propene was carried out under photoirradiation using a 250 W ultrahigh-pressure Hg lamp in a closed circulation system (295 cm3; propene 250 µmol) at ambient temperature. The powdered catalyst (200 mg) was spread on the flat bottom (12 cm2) of a quartz reactor, and photoirradiated from outside of the reactor. The temperature of the catalyst bed was elevated by ca. 10 K from room temperature by the photoirradiation. The wavelength of photoirradiation light was limited by using TOSHIBA UV-cut glass filters: UV-29, UV-31, UV-33, UV-35, UV-37, and Y-43, which allow the transmission of the light with wavelength λ > 290, 310, 330, 350, 370, and 430 nm, respectively. After each photoirradiation period, products were analyzed by gas chromatography. In the metathesis, one ethene molecule and one 2-butene molecule were obtained from two propene molecules (eq 1). Since ideal metathesis has occurred (ethene/ butene ) 1.0) in the photometathesis over FSM-16 and amorphous silica with a little amount of byproduct, 1-butene,19,20,26

10.1021/jp001962v CCC: $19.00 © 2000 American Chemical Society Published on Web 10/14/2000

Photometathesis of Propene over FSM-16

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Figure 1. XRD patterns of synthesized FSM-16 after removing the template by calcination at 873 K (a), and after pretreatment at 1073 K (b).

Figure 2. (A) N2 adsorption isotherms of FSM-16 after pretreatment for 3 h at 673 K (a), and at 1073 K. (B) Pore size distributions of FSM-16 pretreated at 673 K (a), and at 1073 K (b), calculated from N2 adsorption isotherm.

the conversion of photometathesis was defined as a sum of the yields of ethene and butene.

2C3H6 f C2H4 + C4H8

(1)

Diffuse reflectance UV-vis spectra were recorded on a JASCO V-570 equipped with integrating sphere covered with BaSO4. The spectra were recorded at room temperature by using an in-situ closed quartz cell consisting of pretreatment part and recording part. Pretreatment of catalyst was carried out in the same way as that for the reaction test mentioned above in the pretreatment part of in-situ cell, then the catalyst was moved to the recording part to record spectra without exposure to air. Results Characterization of FSM-16. As shown in Figure 1a, the XRD pattern of synthesized FSM-16 sample after calcination at 873 K exhibited four diffraction lines at low angle region as reported,1 indicating that this material has hexagonal regularity. The d spacing of (100) was 3.74 nm. Figure 2A(a) shows the N2 adsorption isotherm of the FSM-16 sample. The adsorption isotherm was type IV of IUPAC classification, indicating the presence of mesopores. The pore size distribution calculated by using the BJH method (adsorption blanch) was very narrow at 3.1 nm diameter (Figure 2B(b)). The BET surface area was 924 m2 g-1. These results confirmed that the FSM-16 sample was prepared precisely as reported.1 Figure 1b shows the XRD pattern of FSM-16 pretreated at 1073 K. The position of the hexagonal lines was slightly shifted to a higher angle, but the pattern was almost the same as the sample before pretreatment (Figure 1a). Figure 2 shows the N2 adsorption isotherm (Figure 2A(b)) and the pore size distribution (Figure 2B(b)) of FSM-16 pretreated at 1073 K. Pore size distribution was shifted to a shorter region, but was still sharp. BET surface area was 998 m2 g-1, indicating that there is no change within error level. Thus, it was confirmed that the structure of FSM-16 was maintained after the pretreatment at 1073 K. Time Course of the Reaction. Figure 3 shows the response of the propene conversion to photoirradiation without the UV-

Figure 3. Response of the metathesis conversion over FSM-16 to the photoirradiation without the UV-cut filter.

cut filter. When the light was turned on, the conversion increased. Under photoirradiation, the ideal metathesis occurred (ethene/butene ) 1.0). When the light was turned off, the conversion did not increase. The result that the reaction took place only under the photoirradiation indicates that the photoirradiation is essential for the reaction. Another important result is that the increment of the conversion increased each time the light was turned on, or, in other words, the photometathesis is suggested to start with the induction period, meaning that the reaction proceeded consecutively. Figure 4 shows the time course of the reaction without using the UV-cut filter. The reaction did not proceed without photoirradiation (Figure 4a). Under photoirradiation, the conversion increased with photoirradiation time (Figure 4b). At the initial of this irradiation period, the induction period was observed. After the conversion reached a certain extent, products and unreacted propene in gas phase were evacuated from the reactor, and fresh propene was reintroduced into the reactor (A). When the propene was reintroduced, small amounts of desorbed products (the ratio of products deviated from ideal metathesis,

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Inaki et al.

Figure 4. Time course of the metathesis reaction over FSM-16 in the dark (a, c), and under photoirradiation without the UV-cut filter (b, d). Before the period (c), the gaseous molecules were removed followed by reintroduction of fresh propene (A).

Figure 6. Time course of the metathesis reaction over FSM-16 under photoirradiation with the UV-cut filters: Y-43 (a), UV-37 (b), UV-35 (c), UV-33 (d), UV-31 (e), and UV-29 (f), respectively. In the box, the vertical axis was enlarged (15 times).

Figure 5. Time course of the metathesis reaction over FSM-16 under photoirradiation with the UV-cut filter Y-43 (a, d), UV-33 (b), and in the dark (c), Before the period (c), the gaseous molecules were removed followed by reintroduction of fresh propene (A).

i.e., ethene/butene ) 0.5) were observed, but their amount did not increase in the dark (Figure 4c). After that, the reaction proceeded again under photoirradiation (Figure 4d). It is noteworthy that no induction period was observed in this case, and the reaction rate was higher than that in the first photoirradiation period shown in Figure 4b. In a separated run, photoirradiation was carried out without reintroduction of propene at the point (A) in Figure 4. In this case, the amount of photodesorbed products were very small (ca. 0.1%). Photoirradiation after reintroduction of propene resulted in the formation of metathesis products at the yield similar to that of region (d) in Figure 4. Dependence of the Reaction upon the Wavelength of Light. Figure 5 shows the time course of the reaction with using the UV-cut filter. When only visible light was irradiated through the filter Y-43, metathesis products were not observed (Figure 5a). When the light including the UV region was irradiated through the filter UV-33, the conversion increased and the remarkable induction period was observed (Figure 5b). These results indicate that the UV light is essential to start the metathesis over FSM-16. After the evacuation of gaseous molecules followed by reintroduction of propene (A), the reaction did not proceed in the dark (Figure 5c). However, the reaction proceeded even under visible light with the filter Y-43 without the induction period (Figure 5d). This result indicates that the visible light is also effective after the reaction proceeded to a certain extent. In Figure 6, the effect of the wavelength on the reaction was investigated by using a series of UV-cut filters. When only visible light was irradiated through the filter Y-43, the metathesis products were not observed (Figure 6a). When the wavelength region of irradiation light was extended to shorter region

Figure 7. Time course of the metathesis reaction over FSM-16 in the dark (a), under photoirradiation without the UV-cut filter (b, g), and with the UV-cut filters: Y-43 (c), UV-37 (d), UV-35 (e), and UV-29 (f), respectively.

including UV light by changing the UV-cut filter to UV-37, the conversion increased slightly (Figure 6b). When the filter was changed to UV-35, the reaction rate became higher (Figure 6c). Each time the wavelength region was extended (Figure 6df), the rate increased. These results indicate that the effective wavelength for the start of the reaction is in the UV region at least below 370 nm, and that the shorter UV light is more effective. In Figure 7, after the reaction proceeded to a certain extent under photoirradiation without the UV-cut filter (Figure 7b), the effect of the wavelength on the reaction was examined in a similar way to Figure 6. In this case, photoirradiation of visible light was effective for the progress of the reaction (Figure 7c) in contrast to Figure 6a, indicating that the visible light is also effective for the reaction after the progress of the reaction to a

Photometathesis of Propene over FSM-16

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Figure 8. Time course of the metathesis reaction over FSM-16 under photoirradiation without the UV-cut filter (a, e), with the UV-cut filter Y-43 (c, d), and in the dark (b). At the two points (A, B), the gaseous molecules were removed followed by reintroduction of fresh propene.

certain extent. The reaction rate decreased by further irradiation of visible light (Figure 7c). Irradiating through the filters UV37 (Figure 7d) and UV-35 (Figure 7e), the rate did not change so much. Irradiation through the filter UV-29 (Figure 7f) increased the rate slightly, and the irradiation without the filter (Figure 7g) resulted in further increase of the rate. Figure 8 shows the time course of the reaction including a long period in the dark and under the visible light. The reaction proceeded to a certain extent under photoirradiation without the UV-cut filter (Figure 8a). After gaseous molecules were evacuated and fresh propene was reintroduced (A), the catalyst was kept for a long period in the presence of propene in the dark (Figure 8b). A small amount of desorbed products was observed only at the initial period, but the amount did not increase further in the dark. It is noteworthy that product ratio (ethene/butene ) 0.1) deviated from the ideal value () 1.0) in this period (Figure 8b), similarly to Figure 4c. Because of the significant deviation from ideal value, these results (Figures 4c and 8b) are not considered to be related to the main path of metathesis. When the visible light was irradiated with the filter Y-43 (Figure 8c), the reaction proceeded. Moreover, product ratio gradually changed into the ideal value () 1.0) with the increase of the conversion. However, further irradiation of visible light stopped the reaction (Figure 8c). After the evacuation of gaseous molecules followed by reintroduction of propene (B), no desorbed products in the dark, and no products were formed under the visible light (Figure 8d). When the UV light was irradiated, the reaction proceeded and the induction period was observed again (Figure 8e), suggesting that the catalyst was activated again by UV light. UV-Vis Spectra. Figure 9A shows the UV-vis spectra of FSM-16. The sample calcined at 1073 K in air exhibited only broad and small absorption band (Figure 9Aa). After the evacuation at 1073 K, the intensity increased and the line shape changed (Figure 9A(b)), indicating that the new absorption sites were generated by the evacuation at 1073 K. These new sites absorbed the UV light mainly. After the introduction of propene on the evacuated FSM-16, intensity of absorption band increased slightly (Figure 9A(c)), although the line shape of the spectrum was almost the same as that before introducing propene (Figure 9A(b)). After photoirradiation for 15 min without using UVcut filter in the presence of propene, however, the absorption band around λ ) 250 nm decreased drastically (Figure 9A(d)). Further irradiation for 60 min (Figure 9A(e)) did not bring about a significant change of UV spectrum. A difference spectrum depicted in Figure 9B was obtained by subtracting spectrum-d from spectrum-c in Figure 9. This difference spectrum was below ca. 390 nm and had a maximum at around 250 nm.

Figure 9. (A) Diffuse reflectance UV-vis spectra of FSM-16: after calcination at 1073 K in an atmosphere (a), after treatment in O2 at 1073 K followed by evacuation at 1073 K (b), after subsequent introduction of propene at room temperature (c), after subsequent photoirradiation in the presence of propene without the UV-cut filter for 15 min (d), and for 60 min (e). (B) Difference spectrum given by subtracting spectrum (d) from (c).

Figure 10 shows the UV-vis spectra of FSM-16 after evacuation at 1073 K followed by photoirradiation through UVcut filter in the presence of propene. Each time the wavelength region of the irradiation light extended to shorter region, the absorption band decreased step by step (Figure 10b-g). When only visible light was irradiated through the filter Y-43, the spectrum was changed very slightly (Figure 10b). Irradiation through the filter UV-37 resulted in remarkable decrease of the absorption band (Figure 10c). Irradiating the light including shorter UV region, absorption band decreased further (Figure 10d-g). These results indicate that the effective wavelength for the decrease of absorption band was in UV region at least below 370 nm. Discussion Two Photoreaction Steps and Effective Wavelength. The photometathesis of propene over FSM-16 started with the induction period (Figures 3, 4b, 5b, 7b, and 8a), and after the reintroduction of fresh propene, the reaction was restarted with higher initial reaction rate (Figure 4d), but with no induction period (Figures 4d, 5d, and 8c). These results indicate that the photometathesis over FSM-16 proceeds consecutively via certain surface intermediates. In addition, the intermediates remained

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Inaki et al. SCHEME 1: Tentatively Proposed Reaction Scheme for Photometathesis of Propene over FSM-16

Figure 10. Diffuse reflectance UV-vis spectra of FSM-16: after treatment in O2 at 1073 K followed by evacuation at 1073 K and subsequent introduction of propene at room temperature (a), after successive photoirradiation with UV-cut filters: Y-43 (b), UV-37 (c), UV-35 (d), UV-33 (e), and UV-31 (f) for 15 min each, and subsequent photoirradiation for 15 min without the UV-cut filter (g).

as stable species in the dark, and under evacuation (Figures 3-5, 8), in contrast with reported photoinduced metathesis over MoO3/PVG,30 in which the carbene intermediates were unstable in the dark. Therefore, the reaction could be divided into at least two consecutive steps; in the first step, the surface intermediates are formed from propene, and in the second step, the metathesis products are formed from the intermediates. Photoirradiation was required for the formation of intermediates in the first step (Figures 3, 4a, 7a). On the other hand, even in the presence of intermediates, photoirradiation was required for the formation of products (Figures 3, 4c, 5c, and 8b). Thus, the photoirradiation is necessary for both reaction steps. Two photoreaction steps are clearly distinguishable by difference in the effective wavelength for each step. For the first step, the visible light was not effective (Figure 5a, 6a) and UV light below ca. 370 nm is essential (Figure 6b). Moreover, the light in the shorter UV region is more effective for the reaction (Figure 6). On the other hand, for the second step, the visible light as well as UV light is effective (Figures 5d, 7c, and 8c). The difference in the effective wavelength for two photoreaction steps would arise from the difference of photoactive species. The effective wavelength is only in UV region in the first step, which roughly agrees with the absorption band of FSM-16 evacuated at 1073 K (Figure 9A(b)). This suggests that the first step proceeds accompanying the absorption of UV light by the sites on evacuated FSM-16 as discussed later. On the other hand, in the second step, the visible light should be absorbed by the intermediates, but not by FSM-16 itself. Although the absorption band was not observed in the visible region after the progress of the reaction to a certain extent (Figure 9A(e)), it would be due to very small absorption coefficient of intermediates and/or very small concentration of intermediates. Photoexcitation Sites on FSM-16. The new absorption band in the UV region was generated on FSM-16 by evacuation at 1073 K (Figure 9A(b)). A part of the absorption band weakened

remarkably by photoirradiation in the presence of propene (Figure 9A(d)). This result suggests that propene was photoadsorbed on the absorption sites to form surface intermediates, and that the absorption sites became photoinactive. Thus, the difference spectrum (Figure 9B) would correspond to the band of absorption sites for propene photoadsorption. The wavelength region of the difference spectrum (below ca. 390 nm) agrees well with the effective wavelength for the first photoreaction step (Figure 6) and that for the decrease of the absorption band (Figure 10). This agreement would lead to the conclusion that the absorption sites exhibiting the band in Figure 9B should be the active sites for photometathesis, and that propene was photoadsorbed on the active sites to form surface intermediates accompanying the photoabsorption of UV light below 390 nm by the active sites. The band gap energy of the crystal SiO2 (quartz) is 8.1 eV (153 nm) in a vacuum UV region, but FSM-16 and amorphous silica are in marked contrast to crystal SiO2 because they have high surface area and surface hydroxyl groups. We recently proposed that the active sites for photometathesis would be the strained siloxane bridges (tSi-O‚‚‚‚‚‚Sit) or the related sites, which were generated by dehydroxylation of isolated hydroxyl groups at high temperature.25 Moreover, we observed the strained siloxane bridges, and these exhibited the radical type function.18 It was reported that tSi-O‚ site absorbs the light in the UV region,31 and that this radical site would be probably derived from the cleavage of the strained siloxane bridges.32-34 Thus, the absorption band observed in this study may be assignable to the strained siloxane bridges (tSi-O‚‚‚‚‚‚Sit) or the related sites such as tSi-O‚ site. Proposed Scheme of Photometathesis. In the above discussion, the reaction was divided into two photoexcitation steps; in the first step, the intermediates are formed under the UV light, while in the second step, the products are formed from the intermediates under the light including the visible region. With respect to the structure of intermediates, a metallacyclobutane-like species was suggested in the photometathesis over amorphous silica.19 Moreover, Gusel′mikov reported that the pyrolysis of the silicon-cyclobutane species gives the siliconcarbene species accompanied by desorption of alkene.35 Thus, the reaction scheme of photometathesis of propene over FSM16, consisting of two photoexcitation process, was tentatively proposed (Scheme 1) with referring to the above information and the conventional metathesis mechanism.29 In the first step, the active sites generated by evacuation of FSM-16 at 1073 K absorb the UV light (Figure 9B) and interact with propene followed by formation of surface intermediates (step-1). Carbene like species were tentatively proposed as the

Photometathesis of Propene over FSM-16 intermediates formed in the first step with referring to the mechanism of other metathesis.36 But this does not exclude metallacyclobutane species as possible initial intermediates. The induction period (remarkably shown in Figure 5b) is necessary until enough amount of intermediates are formed, because this first step is slower than others. On the other hand, the second step is the photometathesis cycle. This cycle would be further divided into two photoreaction steps; metallacyclobutane species are formed from carbene species and propene under the UV light (step-2a), and carbene species are formed from metallacyclobutane with desorption of metathesis products under visible light or UV light (step-2b). In this scheme, the visible light is proposed to be only effective for the conversion of metallacyclobutane species into metathesis products and carbene species (step-2b), but not for the regeneration of metallacyclobutane (step-2a) from the following results. The formation of metathesis products under visible light (Figures 5d, 7c, and 8c) indicates that the visible light is effective in the step-2b. And the results that the reaction was stopped by further irradiation of visible light (Figure 8c) can be rationalized by assuming that step-2a can only proceed under the UV light and the metallacyclobutane species is consumed under visible light. After the irradiation of the visible light for a long time, only carbene species would remain as the surface intermediates. UV light irradiation is essential for the completion of photometathesis cycle (Figures 7 and 8e). Summary The photometathesis of propene over FSM-16 could be clearly divided into two photoreaction steps having different effective wavelength in each. In the first step, the active sites, generated by evacuation at high temperature, absorb only UV light and interact with propene to form surface intermediates. In the second step, photoirradiation is also necessary for the conversion of intermediates to form metathesis products. Visible light is effective for the formation of products to some extent, but the UV light is essential for the continuous progress of the reaction. The second step would consist of at least two steps to complete the metathesis cycle, and visible light would promote a part of this cycle to form metathesis products. Acknowledgment. This work was partly supported by Nippon Sheet Glass Foundation for Materials Science and Engineering. References and Notes (1) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (2) Inagaki, S.; Koiwai, A.; Suzuki, N.; Fukushima, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1996, 69, 1449.

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