J. Chem. SOC., Faraday Trans. 1, 1482, 78, 2121-2128 Photoinduced Metathesis Reaction of C,H, on Supported MOO, Catalyst BY MASAKAZU ANPO,* ICHIRO TANAHASHI AND YUTAKA KUBOKAWA* Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 59 1 , Japan Received 11 th August, 198 1 Ultraviolet irradiation of MOO, supported on porous Vycor glass (PVG) in the presence of C,H, has been found to induce the formation of approximately equimolar amounts of C,H, and 2-C4H, as well as a small amount of CH,CHO, suggesting that the metathesis reaction of C,H, occurs. The dependence of the yields upon the excitation wavelength is in good agreement with the excitation band of the phosphorescence of MoO,/PVG, which arises from the charge-transfer transition hv hvr (Mo6+=O2-) (Mo5+-O- ).* The quenching of the phosphorescence of MoO,/PVG and the decrease in the yield of C,H, metathesis on adding 0, and CO suggest that the photoinduced metathesis reaction is closely associated with the charge-transfer excited triplet state of MoO,/PVG.From these results, together with those of e.s.r. experiments reported previously, the mechanism of the photoinduced C,H, metathesis, especially the primary process of metalkarbene formation, is discussed. We have recently investigated the photoluminescence of metal oxides supported on porous Vycor glass (PVG) containing 1 wt % metal, as well as its correlation with the photoreactions on these oxides.' The charge-transfer excited states (Mo5+-O- ) * Play a significant role in the photoreactions, the trapped holes (0-) as well as electrons (Mo5+) being involved in the reactions; e.g.the presence of electrons in the neighbourhood of holes is necessary for the fission of the C=C bond in alkenes.2 It has been proposed by a number of workers that photolysis of liquid water on TiO,, SrTiO,, ZnO etc. proceeds via a photoelectrochemical mechanism whereby reduction and oxidation of the species take place on different surface^.^ On the other hand, Somorjai et aL4 have proposed that a photocatalytic mechanism is applicable to the photolysis of water on SrTiO, without added metal; i.e. the surface sites active for oxidation and reduction exist close together. Such a situation is similar to that with the excited complex 0-)*, with the close existence of electrons and holes.In the present work, as an extension of previous studies, the photoreaction of C,H, on MoO,/PVG has been investigated. As has been shown by Kazansky et a/.,5 studies of e.s.r. and optical spectra of supported metal oxides with low metal contents can give valuable information on the structure and reactivity of low-coordinated metal ions on the support. Although the importance of ' surface coordinative unsaturation ' in heterogeneous catalysis appears to be established, the role of surface ions in low coordination in photocatalytic reactions on oxides is still unclear. Studies of supported metal oxides with an extremely low metal content are expected to provide information on the role of surface coordinative unsaturation in photocatalysis.69 2121 FAR 12122 REACTION OF C,H, ON SUPPORTED MOO, EXPERIMENTAL The gases used were of 99.5% purity (Takachiho Kogyo Co.) and were used after purification by low-temperature fractional distillation. MOO, supported on PVG (MoO,/PVG) (0.007 wt %) was prepared by impregnation of PVG (Corning no. 746685-7930, 160 m2 g-', 9.0 x 3.0 x 1.0 mm3) with an aqueous solution of (NH,),Mo,O,,. The catalysts were dried at 350 K and heated in 0, at 773 K followed by evacuation at 613 K. The metal content was determined by atomic absorption and/or calorimetry. The photoluminescence spectra were measured with a Shimazu RF-501 spectrofluorophotometer with filters to eliminate scattered light between 77 and 300 K. E.s.r. measurements were carried out at 77 K using a JES-ME-X (X-band) spectrometer. Mn2+ in MgO powder was used for g-value and sweep calibrations.U.V. irradiation was carried out at 290 K using a 75 W high-pressure mercury lamp with a colour filter (A < 280 nm) and a water filter. The reaction products were separated by low-temperature fractional distillation and analysed by gas chromatography or by a Shimazu quadrupole mass-spectrometer. Details of the experimental procedures have been described previously.', 2* RESULTS AND DISCUSSION QUENCHING OF THE MoO,/PVG PHOSPHORESCENCE WITH C,H, Fig. 1 shows the photoluminescence of MoO,/PVG and its spectral change caused by added C,H, together with the corresponding excitation spectrum. As described previously,'? the excitation spectrum of the photoemission having A, = 295 nm can be attributed to a charge-transfer process; i.e.hv (M06+=02-) = (M05+-0-)* hV' where Mo5+ is tetrahedrally coordinated.? The emissions at 440 and 340 nm are assigned to phosphorescence from the charge-transfer excited triplet states and to 320 400 500 600 0 wavelength/ n m FIG. l.-(a) Photoluminescence of MoO,/PVG at 298 K (excitation, 290k7.5 nm): (i) O,, 0.03 Torr; (ii) 0,, 0.13 Torr; (iii) C,H,, 0.42 Torr. (6) Excitation spectrum of 500 nm emission (slit width for excitation, 5.0 nm; slit width for emission, 7.0 nm). t According to our recent studies, PVG outgassed at higher temperatures shows emission at 400 nm with an excitation band at 255 nm, arising from the presence of specific sites of low coordination. Little or no emission of PVG is observed with light in the range > 280 nm.Since the measurements in the present work have been carried out with light in this region, it is concluded that PVG makes no contribution to the observed emission (e.g. fig. 1).M. ANPO. I. TANAHASHI A N D Y. KUBOKAWA 2123 fluorescence from the excited singlet states, respectively, since on decreasing the temperature from 300 to 77 K, the former increased while the latter scarcely changed in intensity. As seen in fig. 1, only the phosphorescence is quenched by added C3H6, suggesting that C3H6 molecules interact with the excited triplet states. As described previously,6a the possibility that collisional quenching, whereby gaseous molecules interact with the excited states, takes place may be excluded, since the quenching became irreversible at lower temperatures.Accordingly, quenching in the present system is attributable to the formation of an adsorption complex between added gases and the oxide surface. As shown in fig. 1, 0, quenches the phosphorescence more efficiently than C3H6, i.e. the interaction of MoO,/PVG with 0, is stronger than with C,H6. As regards such a discrepancy, there appears to be some difference in the nature of adsorption complexes for 0, and C3H6. As will be shown later, 0; anion radicals are formed under U.V. irradiation of MoO,/PVG in the presence of 0,. Formation of anion radicals would not be expected for C3H6. PHOTOINDUCED METATHESIS REACTION OF C3H6 U.V. irradiation (3, > 280 nm) of Mo03/PVG in the presence of C3H6 was found to induce the formation of approximately equimolar amounts of C,H, and 2-C4H,. This suggests that the metathesis reaction of C3H6 occurs, its yield increasing with pressure of C3H6 (fig.2). As shown in fig. 3, on U.V. irradiation of MoO,/PVG in the presence of C3H6 the metathesis of C3H6 takes place immediately, its yields increasing with irradiation time. As soon as U.V. irradiation ceases, the reaction stops. As expected from this lack of metathesis Of C3H6 in the dark, no interaction O f C3H6 with MoO,/PVG occurred at room temperature in the dark, since the C3H6 introduced was completely recovered by desorption up to 323 K. 0.5 1.0 1.5 2.0 2.5 initial C, H, pressure/Torr FIG. 2.-Effect of pressure of C,H, upon the yields of photoinduced metathesis reaction at 290 K. Fig. 4 shows the dependence of the yield of photoinduced metathesis upon the excitation wavelength.From a comparison with the results shown in fig. 1, it is seen that the dependence of the yields upon excitation wavelength is in good agreement with the excitation band of the photoemission of MoO,/PVG. This suggests that the photoinduced metathesis reaction is closely associated with the charge-transfer excited states of MoO,/PVG. After the photoreaction, the temperature of the Mo03/PVG was raised stepwise, and the desorption products were analysed. As shown in fig. 5, in addition to C,H, 69-22124 REACTION OF C,H, ON SUPPORTED MOO, v, v, .- 2 30- (3 c-’ \D 2 r, : 20- v, al h 22 .- 5 10- - 5 I 1 reaction time/min FIG. 3.-Time course of photoinduced metathesis reaction of propene on MoOJPVG.C,H,, 0.92 Torr; excitation, 3 10 f 9 nm; temperature, 290+ 2 K. FIG. excitation wavelength/nm -Effect of excitation wavelength upon the yield of photoinduced metat..esis reaction of propene on MoOJPVG. C,H,, 1.2 Torr; temperature, 298 f 2 K ; slit width 9 nm. and 2-C4H,, CH,, 1-C,H, and CH,CHO were desorbed, the sum of the three minor products being ca. 3% of the total desorption products. As regards the desorption of CH,CHO, essentially the same thermal desorption pattern was obtained with the desorption after adsorption of CH,CHO on MoO,/PVG at 300 K. This suggests that CH,CHO is formed from the photoreaction at 290 K and not by thermal reaction due to the temperature rise of MoO,/PVG. EFFECT OF THE ADDITION OF 0, AND co UPON THE PHOTOINDUCED METATHESIS REACTION Fig.6 shows the effect of added 0, upon the yield of C,H, metathesis reaction under U.V. irradiation at 290 K. Both C,H4 and 2-C4H, yields decrease with increasingM. ANPO, I. TANAHASHI AND Y. KUBOKAWA 2125 h % 5 5.0- 4 ”, c - 3 *? 1 0 e . r/: + 0 e c 2 . 5 - L 0 3 c2 H4 desorption temperature/ K FIG. 5. pressure of oxygen/Torr FIG. 6. FIG. 5.-Photoformed products and their desorption pattern. C,H,, 0.66 Torr. FIG. 6.-Effect of the addition of oxygen upon the yield of photoinduced metathesis of propene on MoO,/PVG at 290 K. C,H,, 0.66 Torr. pressure of 0,. In addition to CH,CHO, new oxygen-containing compounds such as C,H,CHO were produced in the presence of 0,. E.s.r. studies shows that U.V. irradiation of MoO,/PVG at 77 K in the presence of 0, leads to the formation of (o;),,, anion radicals.Such 0; formation has been reported for various oxides by several workers.l9 Formation of new oxygen-containing compounds described above appears to be caused by the presence of (o;),,, species. Details will be reported in the near future. The decrease in the yield of C,H, metathesis on adding 0, is attributed to the quenching of the excited triplet states of MoOJPVG by 0,, since 0, quenches the excited triplet states more efficiently than C,H,, as shown in fig. 1. The effect of added CO upon the C,H, metathesis has been similarly investigated. As shown in fig. 7, a similar decrease in metathesis yield occurred, its extent being smaller than that of 0,. This less efficient quenching with CO could result from the fact that the interaction of CO with the excited states of MoO,/PVG is weaker than that of 0,.Fig. 1 shows that in the metathesis of C,H, more C,H, is formed than 2-C4H,, the extent being more significant in fig. 6 and 7. Although the true nature of such features is unclear, the adsorption of C,H,, which includes its dimerization, was found to be induced by U.V. irradiation while the 2-C4H, adsorption was not. Similar results have been reported by Morikawa and coworkers.8 U.V. irradiation of MoO,/PVG in the presence of C,H, is expected to cause the photoreduction of MOO,, i.e. formation of Mo5+ and/or Mo4+ ions which are known to be the active sites for alkene metathe~is.~ If the photoreduced Mo ions were responsible for the photoinduced metathesis, such an immediate response for the intermittence of U.V.irradiation (fig. 3) would be unexpected, since the photoreduced Mo ions should exist even in the dark. Furthermore, the decrease in the yield of C,H,2126 REACTION OF C3H6 ON SUPPORTED MOO, t 0 1.0 2.0 3.0 4.0 pressure of CO/Torr FIG. 7.-Effect of the addition of carbon monoxide upon the yield of photoinduced metathesis of propene on MoOJPVG at 290 K. C,H,, 0.66 Torr. metathesis on the addition of CO (fig. 7), which is expected to accelerate the photoreduction of MOO,, would be unexpected on such a basis. This again confirms that the excited state (Mo5+-O-)* is responsible for the photoinduced metathesis reaction. MECHANISM OF THE PHOTOINDUCED METATHESIS OF C3H6 It has been generally accepted that with homogeneous catalytic reactions alkene metathesis proceeds via metal-carbene and metallocyclobutane intermediates.1° A number of workers have proposed that a similar mechanism is applicable to heterogeneous catalyst^.^ However, the mechanism of carbene formation is still unclear.As has been reported recently, U.V. irradiation of MoOJPVG at 77 K in the presence of C2H4 brings about the formation of C3H6, a bridged-type x-complex (C,H,=qH2)- \ I \\ ' \d being formed simultaneously.2 From these results it has been concluded that the interaction of C,H, with photoformed 0- hole centres leads to fission of the C=C bond in C,H,, resulting in the formation of methylene and HCHO. The C=C bond fission has been supposed to proceed as follows. The photoinduced increase in electron density in the Mo ions which constitute the excited states (Mo5+-O-)* enhances back-donation of electrons to the n* orbital of the bridged-type n-complex with consequent easier rupture of the C=C bond.A similar mechanism appears to be applicable to the reaction of C3H6 on U.V. irradiation of MoO,/PVG. However, no formation of the bridged-type ;n-complex occurred. This might be attributed to the low stability of the complex formed from C3H6, which results in a higher efficiency of formation of methylene as compared to the case of C2H,. Thus, it may be concluded that carbene formation in theM. ANPO, I. TANAHASHI AND Y. KUBOKAWA 2127 photoinduced metathesis of C,H, proceeds via an oxometallocyclobutane intermediate following the bridged-type n-complex: (Mo5+-0-)* + C,H, C,H2-FH-CH, excited complex Mo5+-O- n-complex \ I '\ J Tl H2 H2 H II I I Mo MO~+-O- -kC3H, I I I metathesis -C + O=CH-CH, +- C-C-CH, methylene ethanal.Simultaneous formation of CH,CHO and methylene is supported by the difference in the pressure dependence of the yields between C2H4 (or 2-C4H,) and CH,CHO. In contrast with the increase in yield of C2H4 (or 2-C4H,) with increasing C,H, pressure, as shown in fig. 2, the yield of CH,CHO was essentially independent of C,H, pressure above 0.7 Torr.* As has been discussed by a number of worker^,^ alkene metathesis is a chain reaction, with carbenes as the chain carriers reacting with alkenes to generate new carbenes and alkenes via intermediate metallocyclobutanes. From the ratio of C2H4 to CH,CHO in the C,H, metathesis, it is concluded that the photoformed methylenes recycle ca.500 times at 2.5 Torr of C,H,. An immediate response to the intermittence of U.V. irradiation (fig. 3) suggests that the photoformed carbenes are unstable, their concentration at steady state being negligibly small after interruption of U.V. irradiation. Although it is difficult to exclude the possibility that U.V. irradiation is essential for the propagation step, e.g. photoexcitation of the alkene-metallocarbene complex occurring, it seems unlikely that such a photoexcitation step plays a significant role in the metathesis reaction, since the dependence of its yield upon the excitation wavelength is in good agreement with the excitation band of the photoemission of MoO,/PVG.Formation of the oxometallocyclobutane intermediate shown above has already been proposed for carbene formation by Rooney and StewartlOa as follows: 0 CHR' 0-CHR1 O=CHR' M CHR2 M-CHR2 M=CHR2. II+II * I I * I I However, they consider that the strength of the Mo=O bond compared to that of the Mo=C bond is probably too large for carbene formation to proceed appreciably. In the charge-transfer excited states of MOO, formed under U.V. irradiation, the bond length of Mo=O becomes longer, i.e. its bond strength becomes weaker compared to that in the ground state.' Accordingly, direct involvement of the Mo=O bond in carbene formation described above is expected to occur more easily under U.V. irradiation. Thus, formation of the oxometallocyclobutane intermediate is also supported by the concept proposed by Rooney and Stewart.* The reduction of the MOO, proceeds with increasing amount of CH,CHO. However, its extent is negligibly small compared to the amount of MOO,, having little or no effect upon its catalytic activity. 1 Torr z 133.322 Pa.2128 to the Asahi Glass Foundation for financial support. 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