Langmuir 1997, 13, 2373-2379
2373
Cumene Photo-oxidation over Powder TiO2 Catalyst R. Wittenberg,† M. A. Pradera,‡ and J. A. Navio*,† Instituto de Ciencia de Materiales de Sevilla, Centro Mixto, CSIC-Universidad de Sevilla, Dpto. de Quı´mica Inorga´ nica, and Dpto. de Quı´mica Orga´ nica, Facultad de Quı´mica, Universidad de Sevilla, 41012 Sevilla, Spain Received October 31, 1996. In Final Form: January 20, 1997X The heterogeneous photoassisted oxidation of cumene in liquid acetonitrile-oxygenated dispersions containing TiO2 catalyst has been studied in a photochemical reactor radiating predominantly at 365-366 nm. Acetophenone and CO2 have been detected as sole products. Particular attention is focused on the extent of cumene adsorption on the catalyst particle and its relation with the mechanistic features of cumene oxidation. A mechanism invoking methyl instead of phenyl migration, as observed in the thermal catalytic reaction, is proposed.
Introduction Heterogeneous photocatalysis is the ambient-temperature process in which the surface of a solid brings into contact the reactants with electrons and/or holes which are generated within the solid by photons of enegy higher than the band gap (Eg) of the solid. These semiconductor compounds usually have a moderate-sized band gap (Eg ) 1-3.7 eV) between their valence and conduction bands. Under illumination with photons of band gap or greater energy, the valence band electrons are photoexcited into the conduction band, creating highly reactive electronhole pairs, which, after migration to the particle surface, may participate in charge transfer reactions with adsorbates (molecules or ions) and cause the reduction or oxidation of such species, assuming that the adsorbate possesses a redox potential appropriate for a thermodynamically permitted reaction.1-5 The applicability of semiconductor-mediated photocatalysis for functional group transformations of organic compounds has been widely reviewed.2-4 In short, partial oxidations by heterogeneous photocatalysis are worth consideration in organic synthesis, although only in a limited number of cases in the present state of knowledge.3,4 Titanium dioxide, TiO2, has been employed extensively in studies of heterogeneous photocatalysis and is accepted as one of the best photocatalysts.1-5 The aromatic ring is very resistant to oxidation, as is shown by the absence of reactivity of gaseous6 or liquid7 benzene. The oxidation of gaseous alkyltoluene, RC6H4CH3 (R ) C2H5, (CH3)2CH, (CH3)3C), has been studied in a differential-flow photoreactor with a fixed bed of TiO2.6 In all cases, the selectivity for RC6H4CHO was high. For R ) (CH3)3C, no CO2 from mineralization processes was * Corresponding author. † Instituto de Ciencia de Materiales de Sevilla, Centro Mixto, CSIC-Universidad de Sevilla, and Dpto. de Quı´mica Inorga´nica, Facultad de Quı´mica, Universidad de Sevilla, 41012 Sevilla. ‡ Dpto. de Quı´mica Orga ´ nica, Facultad de Quı´mica, Universidad de Sevilla. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Serpone, N.; Pelizzetti, E. PhotocatalysissFundamentals and Applications; Wiley Interscience: New York, 1989. (2) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (3) Herrmann, J. M.; Guillard, C.; Pichat, P. Catal. Today 1993, 17, 7. (4) Pichat, P. Catal. Today, 1994, 19, 313. (5) Navio, J. A.; Cerrillos, C.; Colo´n, G. Trends in Photochemistry and Photobiology; Research Trends Vol. 3; Council of Scientific Research Integration: India, 1994, p 445. (6) Mozzanega, M. N.; Herrmann, J. M.; Pichat, P. Tetrahedron Lett. 1977, 34, 2965. (7) Fujihira, M.; Satoh, Y.; Osa. Electroanal. Chem. 1981, 126, 177.
S0743-7463(96)01055-4 CCC: $14.00
detected. Under the same conditions, toluene yields only traces of benzaldehyde. These results confirm the stability of the aromatic ring and of the methyl group directly attached to it when no other alkyl group is present. The heterogeneous oxidation of toluene, in the liquid phase, using illuminated semiconductor materials, has been studied by Fujihira et al.7-9 and recently by Navio et al.10 using various experimental conditions. The main photocatalytic products found were benzaldehyde, benzyl alcohol, and benzoic acid. A detailed study concerning the selectivity of cumene oxidation over transition metal oxides has been reported.11-13 For the catalysts studied the main oxidation products were cumene hydroperoxide, dimethylphenylcarbinol, and acetophenone. In addition, a mechanistic study of liquid phase low-temperature cumene oxidation using heterogeneous catalysts has been reported.14 Although there are previous works describing the thermal heterogeneous catalytic oxidation of cumene, little attention has been paid to oxidation by the heterogeneous photocatalytic method. In this paper we report results of the heterogeneous photoassisted oxidation of cumene in liquid organic oxygenated dispersions containing pure TiO2 photocatalyst. Experimental Details Materials. We used TiO2 (Degussa, P-25) without further treatment as a photocatalyst from the same batch that we used in our previous studies;10 the BET surface area of this sample was around 49 m2 g-1. Electron micrographs showed that it consisted of polyhedral-shaped discrete particles from 10 to 50 nm, most of them falling within the 20-30 nm range. The anatase-to-rutile (a/r) weight ratio, calculated according to the method described elsewhere,15 was estimated as 0.773. All reactants such as cumene (GC, 99%), cumene hydroperoxide (tech, 80%), and acetonitrile (HPLC grade, 99.9+%) were supplied by Aldrich-Chemie GmbH & Co. KG and were used without purification. For IR spectra recording, reagent grade sodium (8) Fujihira, M.; Satoh, Y.; Osa. Nature 1981, 293, 205. (9) Fujihira, M.; Satoh, Y.; Osa. Chem. Lett. 1981, 81, 1053. (10) Navio, J. A.; Garcia-Go´mez, M.; Pradera Adrian, M. A.; Fuentes Mota, J. J. Mol. Catal. A: Chem. 1996, 104, 329. (11) Melville, H. W.; Richards, S. J. Chem. Soc. 1954, 3, 944. (12) Hock, H.; Kropf, K. Z. Prakt. Chem., 1959, 9, 173. (13) Emanuel, N. M., Ed. Teoria i Practica Gidrophaznago Okyslenia; Nanska: Moskva, 1974; p 330. (14) Maksimov, Y.; Suzdalev, I. P.; Tsodikov, M. V.; Kugel, V. Y.; Bukhtenko, O. V.; Slivinsky, E. V.; Navio, J. A. J. Mol. Catal. A: Chem. 1996, 105, 167. (15) Navio, J. A.; Macias, M.; Gonzalez-Catalan, M.; Justo, A. J. Mater. Sci. 1992, 27, 3036.
© 1997 American Chemical Society
2374 Langmuir, Vol. 13, No. 8, 1997 carbonate, phenol, and acetophenone supplied by Panreac Montplet & Esteban S. A., E. Merck (Darmstadt), and AldrichChemie GmbH & Co. KG were used. The photo-oxidation reactions have been carried out in acetonitrile, which is a very well-known redox-inert solvent.1-3 The photo-oxidation experiments were carried out in an Applied Photophysics Ltd. photochemical reactor equipped with a 400 W medium-pressure mercury arc lamp, radiating predominantly at 365-366 nm. This lamp produces more than 5 × 1019 photons s-1 within the reaction flask. It was contained in a double-glass immersion well, through which water was passed for cooling. A gas inlet reaction flask (400 mL) was used; a double surface condenser was fitted to the reaction flask to prevent ‘creep’ and loss of vapor. Methods. The powdered photocatalyst (3.5 g L-1) was independently suspended in 0.01 mol L-1 solutions (V ) 370 mL) of the reactants, and these suspensions were treated for 5 min with ultrasound to prevent agglomeration of the particles. Oxygen was bubbled through, and a positive pressure of the gas was maintained during the period of illumination. Three milliliter aliquots were taken from the photoreactor at set time intervals during illumination. The photocatalyst was removed by centrifugation, and the liquid phase was analyzed by GC-MS technique using a Kratos-MS 80 RFA instrument fitted to a Carlo Erba GC. Separations were achieved on a CP-SIL 5 CBWCOT (25 m × 0.32 mm) column whose temperature was programmed from 30 °C (10 min) to 250 °C (15 min, 10 °C/min). The method of external standard was used for semiquantitative determinations. The quantity of evolved CO2 was monitored as BaCO3 by gravimetric analysis, bubbling the generated CO2 through a saturated solution of Ba(OH)2 (0.01 mol L-1). The X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Leybold-Heraeus spectrometer, working at a constant pass energy of 50 eV; Mg KR radiation (hν ) 1253.6 eV) was used as excitation source. A final pressure of 10-9 Torr was attained before XPS recording. IR spectra were recorded on a Perkin-Elmer apparatus, model 883, using KBr disks. The samples were obtained by stirring an acetonitrile suspension containing TiO2 (3.5 g L-1) and the adsorbed substance (0.1 mol L-1), which was studied by IR spectroscopy. UV-vis spectra were performed on a Shimadzu UV2101PC apparatus using acetonitrile as reference.
Results and Discussion Adsorption Measurements. The degree of adsorption of cumene onto TiO2 was calculated by monitoring the decreasing concentration of solute (∆c) in acetonitrile solutions (10-3 to 10-2 mol L-1) containing powdered TiO2 (3.5 g L-1), stirred for 1.5 h in the dark at room temperature (see Figure 1a). The progress of the adsorption measurements was followed by monitoring the changes in the optical absorbance of the cumene solutions (free of TiO2) at 268 nm. Careful measurements of the absorbance spectra for cumene were performed at several concentrations in acetonitrile. From these measurements, typical plots were made of the absorbances at 268 nm vs concentration. An appreciable relationship (〈r2〉 ) 0.9916) between absorbance and concentration was found below a concentration of about 0.02 mol L-1. According to results reported in Figure 1a, the isotherm for adsorption of cumene onto hydroxylated TiO2 surfaces can be described by the two initial steps (I and II) of a Langmuir-type isotherm16 in the lower concentration region (Figure 1b). However, in the final step (III) an S-type isotherm is observed, indicating, in accordance with Giles et al.,17 a cooperative adsorption when the number of adsorbed molecules increases. The limits of solute adsorption (nmax) resulting from competitive adsorption(16) Langmuir, I. Trans. Faraday Soc. 1921, 17, 621. (17) Giles, G. H.; Macewan, T. H. J. Chem. Soc. 1960, Part IX, 3973.
Wittenberg et al.
Figure 1. (a) Adsorption isotherm of cumene in acetonitrile suspensions of TiO2 (3.5 g L-1) at room temperature. (b) Amplified detail of step I from the isotherm.
desorption equilibria between solute and solvent18 can be deduced and correspond to ca. 3 and 8.6 molecules/nm2 for adsorption steps I and II, respectively. A calculation based on the model plane 111 of the anatase19 indicated that each Ti4+ ion at the TiO2 surface must be totally coordinated by, on average, two or three hydroxyl groups, a water molecule, and two or three O2ions, the latter in the anionic layer under the hydrated/ hydroxylated surface layer. At the same time, a monolayer of OH- groups and/or H2O molecules on the surface of the hydrated/hydroxylated surface of TiO2 (anatase) corresponds to ca. 14-16 OH/ nm2.20 Thus, given the value of 8.6 cumene molecules/ nm2 obtained by adsorption measurements, we can correlate, tentatively, that about one cumene molecule seems to be adsorbed per each Ti4+ ion and per two hydroxyl groups. Surface Analysis by XPS and IR. X-ray photoelectron spectroscopy experiments have provided complementary information as well as further evidence of the degree of adsorption of cumene molecules onto TiO2 . Thus, the XPS characterization of cumene/TiO2 at the saturation step revealed the analytical surface structure composition indicated in Table 1. Several surface analyses by XPS (18) (a) Cunningham, J.; Al-Sayyed, G. J. Chem. Soc., Faraday Trans. 1 1990, 86, 3935. (b) Cunningham, J.; Al-Sayyed, G.; Srijaranai, S. In Aquatic and Surface Photochemistry; Helz, G. R., et al., Eds.; Lewis Publishers: London, 1994; Chapter 22, p 317. (19) Munuera, G.; Moreno, F.; Gonzalez, F. Proceedings of the 7th International Symposium on Reaction Solids; Chapman & Hall: London, 1972. (20) (a) Gonzalez-Elipe, A. R.; Munuera, G.; Soria, J. Chem. Phys. Lett. 1978, 57, 265. (b) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J. Chem. Soc., Faraday Trans. 1, 1979, 75, 736. (c) Gonzalez-Elipe, A. R.; Munuera, G.; Sanz, J.; Soria, J. J. Chem. Soc., Faraday Trans. 1 1979, 75, 743.
Cumene Photo-oxidation over Powder TiO2 Catalyst
Langmuir, Vol. 13, No. 8, 1997 2375
Table 1. XPS Analysis of Freshly Prepared Cumene/TiO2 Surfaces element
binding energy (eV)
atom %
C (1s) Ti (2p) O (1s)
284.6 458.5 529.8
14.28 23.70 62.00
always allowed values for carbon impurities close to 2%. By considering nine carbon atoms per cumene molecule, the results obtained could indicate a percent of cumene species of 1.5. On the other hand, using the SBET of TiO2, the percentage composition of Ti(2p) atoms (Table 1), and the data that there are approximately 7 Ti/nm2, it can be estimated that about one cumene molecule per surface Ti4+ is adsorbed. At the same time, taking into account the composition XPS analysis of Ti(2p) and O(1s), it can be estimated that about 14.6% of the oxygen atoms are in excess with respect to the stoichiometric amount expected from the Ti(2p) composition. Let us assume that the excess oxygen observed can be associated with the oxygen atoms from adsorbed hydroxyl groups and water molecules. This latter result indicates that the adsorption of cumene molecules onto TiO2 surfaces seems to occur without appreciable dehydration or dehydroxylation. Figure 2a shows the IR spectrum of a freshly prepared cumene/TiO2 sample equilibrated at the saturation step and the changes in this spectrum following UV illumination in air of the cumene/TiO2 specimen (Figure 2b-d). The IR spectra of freshly prepared specimens containing species which might possibly be formed during the photooxidation of cumene such as cumene hydroperoxyde, acetophenone, methanol, phenol, and carbonate ions adsorbed on TiO2 surfaces are also included for comparison (Figure 2e-i). For a better understanding of the results obtained, the observed bands and their assignments are given in Table 2. In the background spectrum for hydroxylated TiO2 (not shown), a broad band centered at about 3300 cm-1 is observed which can be assigned to the presence of different types of OH groups remaining on the surface.21,22 Also the appearance of the 1630 cm-1 band due to the H-O-H deformational vibration indicates the presence of molecular water. When cumene is adsorbed on this hydroxylated surface (Figure 2a), the bands at 3300 and 1630 cm-1 remain present, indicating that the adsorption of cumene molecules does not appear to produce appreciable surface dehydration/dehydroxylation, as indicated by the XPS results. The bands at 1600 and 1500 cm-1 are ascribed to the in-plane skeletal vibration of the aromatic ring.23,24 The participation of π-electrons in the interaction of benzene and toluene with TiO2 surfaces has been previously investigated by Suda.23 These authors concluded that these molecules are adsorbed by the formation of a Ti4+/π-electron type complex on the dehydroxylated surface and by the formation of an OH/p-electron type complex on the hydroxylated surface. This conclusion was raised by the appearance of a band at 1478-1480 cm-1. Thus, the appearance of a weak band at 1480 cm-1 in the cumene/ TiO2 specimen (Table 2) could suggest the formation of a weak π-complex between the cumene molecule and the TiO2 surface. Upon UV illumination in air of the cumene/TiO2 specimen, the preadsorbed cumene species is transformed, (21) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem., 1971, 75, 1221. (22) Munuera, G.; Moreno, F.; Prieto, J. A. Z. Phys. Chem. 1972, 78, 112. (23) (a) Nagao, M.; Suda, Y. Langmuir 1989, 5, 42. (b) Suda, Y. Langmuir 1988, 4, 147. (24) Busca, G. J. Chem. Soc., Faraday Trans. 1993, 89 (4), 753.
Figure 2. IR spectra from the surfaces of the following samples: (a) cumene adsorbed at the TiO2 surface, cumene/ TiO2; cumene/TiO2 after UV illumination in air for periods of time of (b) 3 h, (c) 5 h, and (d) 10 h, (e) cumene hydroperoxide/ TiO2; (f) acetophenone/TiO2; (g) methanol/TiO2; (h) phenol/TiO2; (i) sodium carbonate/TiO2.
giving a set of new overlapping bands. Most of them can be attributed to the presence of adsorbed acetophenone species, which result from the comparison of the bands that appeared from spectra a-c with those of spectrum f and the literature data.21-25 On the other hand, our IR analyses lead to the observation that at short illumination times (Figure 2b and c) bands attributed to cumene hydroperoxide species are also detected whereas at prolonged illumination times bands assigned to carbonate species seem to be present. This last observation suggests that the heterogeneous photoassisted oxidation of cumene by TiO2 could occur to give acetophenone, possibly via cumene hydroperoxide as intermediate; also the appearance of carbonate species upon prolonged illumination in air of the cumene/TiO2 surface seems to be qualitative proof that a mineralization process accompanies this photoprocess. In order to explore the latter aspects in (25) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 2nd ed.
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Wittenberg et al.
Table 2. Assignmenta of the IR Bands Found for the Irradiated Species Adsorbed at the TiO2 Surface surface cumene(ads)/TiO2
acetophenone(ads)/TiO2
phenol(ads)/TiO2 methanol(ads)/TiO2
CO32-(ads)/TiO2
a
ν (cm-1)
assignment
3300 1630 1600 1500 1480 1460 1380-1370 1300 1050-1020 1000-900 (shoulders) 3300 1685 1630-1600 1450 1360 1265 3440 broad band (1690, 1630, 1590) peaks at 1380, 1100-1000, 880 3420 1630 1520-1510 1390-1380 1000-900 3480 1450 1410 (shoulder) 1200
OsH stretching vibration HOH deformational vibration in-plane skeletal vibration of aromatic ring OH-π-interaction δa CH(CH3)2 δs CH(CH3)2 ν(ArsCH) δ(CH) ν(CsC) OsH stretching vibration ν(CdO)
OsH stretching vibration OsH stretching vibration HOH deformational vibration δa(CH3) δs(CH3) ν(CsC) OsH stretching vibration CsO stretching vibration CsO stretching vibration
According to refs 21-25.
greater depth, a series of IR experiments were carried out. Thus, Figure 3 shows the IR spectrum of a freshly prepared cumene hydroperoxide/TiO2 surface equilibrated at the saturation step (Figure 3a) and the changes in this spectrum following UV illumination in air of the cumene hydroperoxide/TiO2 sample (Figure 3b and c). The IR spectra of freshly prepared acetophenone/TiO2 and CO32-/ TiO2 specimens are also included for comparison (Figure 3d and e). As can be seen, the UV illumination in air of the cumene hydroperoxide/TiO2 surface leads to the IR spectra in Figure 3b and c, which are similar to those obtained after UV illumination of cumene/TiO2 surfaces in air (Figure 2b and c). On the other hand, it should be noted that the spectrum of Figure 3c could be interpreted as a mixture of the spectra from Figure 3d and e, because the spectrum of Figure 3c shows most of the bands which are present in the latter two. All these results readily suggest that the partial photoinduced oxidation of cumene at the TiO2 surface occurs via cumene hydroperoxide as intermediate to give acetophenone and CO2 as partial and total oxidation products, respectively. Photocatalytic Reaction. The oxidation of cumene has been previously studied under two different experimental conditions: (i) under UV illumination in the absence of TiO2 with bubbling O2 and (ii) under UV illumination in the presence of TiO2 and of bubbling O2. Without TiO2 under UV illumination no product, other than cumene, was detected even after 5 h of illumination and bubbling oxygen. This result is expected according to the UV-vis spectrum of cumene (Figure 4) and the characteristic emission lines of the mercury lamp radiating predominantly at 365-366 nm. When a TiO2 suspension is irradiated with UV light, the concentration of cumene markedly decreases with reaction time (Figure 5); at the same time the simultaneous appearance of acetophenone and CO2 as sole oxidation products was detected. The progressive evolution of CO2 during the heterogeneous photoassisted oxidation of cumene is shown in Figure 6. It should be noted that the concentration of acetophenone
Figure 3. IR spectra from the surfaces of the following samples: (a) cumene hydroperoxide adsorbed at the TiO2 surface; cumene hydroperoxide/TiO2 after illumination in air for periods of time of (b) 1 h and (c) 5 h; (d) acetophenone/TiO2; (e) sodium carbonate/TiO2.
seems to decrease at prolonged illumination times (Figure 5) whereas the amount of CO2 (Figure 6) increases
Cumene Photo-oxidation over Powder TiO2 Catalyst
Figure 4. UV-vis spectrum of cumene (10-2 M) in acetonitrile.
Langmuir, Vol. 13, No. 8, 1997 2377
Figure 7. Kinetic plot, according to pseudo-first-order kinetics, of the TiO2-assisted phototransformation of cumene (data from Figure 5).
TiO2 occurs via cumene hydroperoxide as intermediate as shown by the IR results, a certain amount of the partial oxidation products is lost through total mineralization to CO2. The overall chemical equation of this photo-oxidation reaction can be schematized as follows:
Figure 5. Variation of the concentration of cumene on an illuminated acetonitrile suspension of TiO2 as a function of illumination time. The progressive evolution of acetophenone in the medium is also indicated. Experimental conditions: cumene (10-2 M), TiO2 (3.5 g L-1), bubbling O2.
Figure 6. Progressive evolution of CO2 during the heterogeneous photocatalytic experiment reported in Figure 5.
progressively over time. These results could indicate that the mineralization process takes place also with the acetophenone molecule. According to the results reported in Figure 6, it can be estimated that, after 5 h of illumination, the amount of CO2 produced corresponds to a molar percentage of CO2 of about 17% of the theoretical amount of CO2 which would be generated if the total original cumene were mineralized. However, the amount of CO2 produced by TiO2-assisted phototransformation of cumene hydroperoxide under the same experimental conditions as for cumene was evaluated after 5 h of illumination as about 30% of the theoretical amount of CO2 which could be generated from the original. This result indicates that if cumene photo-oxidation over
The TiO2-photoassisted transformation of cumene proceeds via reasonably good pseudo-first-order kinetics (Figure 7). The value of the pseudo-first-order rate constant for the cumene TiO2-assisted phototransformation has been estimated as 19.6 × 10-2 h-1 (〈r2〉 ) 0.992). The turnover number (T.N.), defined as the number of the reacting molecules or product molecules formed per surface active site in heterogeneous (photo) catalysis, will be estimated to establish whether this process is truly catalytic (T.N. . 1) or is stoichiometric (T.N. ) 1).26 In order to estimate the number of surface active sites, we propose the assumption of Somorjai27 that about 10% or less of the surface sites are active in any given reaction. Assuming the above considerations and a surface density of OH- groups of 1014 per cm2 for TiO2, the specific T.N. was determined as ∼350. This value, considered as a conservative estimate of the real turnover, clearly indicates that the TiO2-assisted phototransformation of cumene is catalytic. The apparent quantum yield (qy) for the TiO2-photoassisted oxidation of cumene molecules (from results reported in Figure 5) has been calculated as 2 × 10-3. This qy value has been estimated by assuming that all photons supplied by the lamp are absorbed by the catalyst grains. This last assumption is not strictly accurate because, for heterogeneous systems as in the case of the present study, photons below 360 nm entering the reactor during a given time are, in part, scattered; in part, absorbed by the solid particles, which may or may not produce a photocatalytic reaction; in part, transmitted through dispersion; and, in part, absorbed by the substrate, which may or may not produce a photoreaction. On the (26) Serpone, N.; Pelizzetti, E.; Hidaka, H. In Photochemical and Photoelectrochemical Conversion and Storage of Solar Energy; Tian, Z. W., Cao, Y., Eds.; International Academic Publishers: Beijing, 1993; p 33. (27) Somorjai, G. A.; In Photocatalysis-Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interscience: New York, 1989; Chapter 9.
2378 Langmuir, Vol. 13, No. 8, 1997
Wittenberg et al. Scheme 1
TiO2 + hν
TiO2*(e––h+)
TiO2*(e––h+) e–+
h+
e–+ h+ heat, Ω, etc.
H2O + h+
OH• + H+
OH– + h+
OH•
O2 + e– RH +
h+
R• + HO2• R• + O2 •
RO2 + RH R• + OH•
+
(1)
exciton dissociation
(2)
exciton recombination
(3)
hydroxyl radical formation by hole trapping
(4) (5)
H+
O2•– R•
exciton formation
HO2•
formation of superoxide O2•– and HO2• radicals
H+
(6) (7)
ROOH RO2•
hydroperoxide formation
(8)
mineralization
(9)
ROOH + R• ROH
[O]
CO2 + H2O
other hand, if the intermediates are adsorbed at the TiO2 surface, they will be further transformed by the photocatalytic mode. This last effect must be considered when the efficiency of the procedure is considered. Because the term quantum yield seems to be ambiguous and inapplicable in heterogeneous photocatalysis, as has been emphasized by Serpone et al.,26 we have estimated the relative photonic efficiency by the method protocol suggested earlier,28 which relates the initial rate of substrate disappearance with the rate of incident photons reaching inside the front window of the reactor. According to this method a value of ca. 0.01 is given for the photonic efficiency of the cumene TiO2-assisted phototransformation. Mechanistic Features. The study of the kinetics and mechanism of low-temperature cumene oxidation over the surface of simple metal oxides has been previously carried out and discussed in refs 11 and 13. The catalyst surface S was suggested as participating in the steps of heterogeneous chain initiation according to the scheme
RH + O2 + S f [RH]ads + [O2]ads f R• + RO2• f products while the succeeding reactions (chain propagation, branching, and termination) proceed homogenously. Recently, Maksimov et al.14 have reported the results of a kinetic study of low-temperature liquid phase cumene oxidation either in the presence of a homogeneous initiator or over complex oxides, including titania catalysts. The study led to the conclusion that the active surface of complex oxides participates in chain initiation, most probably via R-H bond rupture. The products detected were dimethylphenylcarbinol, acetophenone, and cumene hydroperoxide. These heterogeneous thermal catalytic results show a marked contrast with the industrial synthesis of phenol from the homogeneous catalytic oxidation of cumene. In both cases, thermal heterogeneous and homogeneous catalytic oxidation of cumene, the cumene hydroperoxide species is considered as an intermediate. Illumination of TiO2 powder with photons of energy equal to or greater than its band gap energy (3.0 eV) results in the formation of an electron (e-)-hole (h+) pair (Scheme 1, eq 1). These charge carriers can recombine or the holes can be scavenged by oxidizable species (e.g. H2O, OH-, or organic molecules) and the electrons by reducible species (e.g. O2 or H+) (eqs 3-6). (28) Serpone, N.; Sauve´, G.; Koch, R.; Tahiri, H.; Pichat, P.: Piccinini, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol., A: Chem. 1996, 94, 191.
Scheme 2
If we assume this mechanism, the formation of cumene hydroperoxide species may be found in steps 7 and 8 via two different pathways. Once the cumene hydroperoxide is formed, a mechanism invoking methyl group migration could be involved to give acetophenone and methanol, as illustrated in Scheme 2. It is interesting to note at this point that in the well established mechanism for cumene oxidation to phenol, a transposition of cumene hydroperoxide is invoked, thus implying phenyl group migration. On the basis of our results, the only way to explain the methyl group migration, despite the phenyl group, is to postulate that, because the phenyl rings of cumene and cumene hydroperoxide seem to be linked to the TiO2 surface via a π-OH interaction, an inhibition effect may be produced, preventing phenyl group migration; the transposition of cumene hydroperoxide via methyl group migration could then be favored, thus giving acetophenone and methanol. The fact that methanol is not detected during our photoreaction could be explained by the strong adsorption capability of this species at the TiO2 surface and its consequent rapid mineralization to CO2. Finally, mechanisms involving hydroxyl radical attack, which have been proposed as primary steps in the photocatalytic degradation of organic compounds to CO2,29 must be invoked to explain the photogeneration of CO2 as indicated in step 9 of Scheme 1. Actually, this reaction is in conformity with the pioneer work of Teichner et al.,30,31 who always found a C-C bond breaking in the mild photocatalytic oxidation of methyl(29) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (30) Djeghri, N.; Formenti, M.; Juillet, F.; Teichner, S. J. J. Chem. Soc., Faraday Trans. 1974, 58, 185. (31) Djeghri, N.; Teichner, S. J. J. Catal. 1980, 62, 99.
Cumene Photo-oxidation over Powder TiO2 Catalyst
branched alkanes. Presently, our photoreaction is performed in a nonaqueous medium which protects the substrate from the direct oxidative degradation and which, in our oppinion, represents experimental conditions compatible with those of Teichner et al.30,31 Regarding the overall chemical equation of the heterogeneous photocatalytic oxidation of cumene, it should be noted that the photogeneration of acetophenone and CO2 is in agreement with results reported by Teichner et al.,30,31 who always detected one carbon loss per molecule when there was a methyl branch in the hydrocarbon to be oxidized. Conclusions Although the catalytic oxidation of cumene has been previously studied by several authors, particularly the homogeneous and heterogeneous thermal catalytic processes, little attention, however, has been devoted to investigating the heterogeneous photocatalytic one. Our investigation attempted to correlate the role of the adsorption effects on the mechanism through which the photo-oxidation of cumene proceeds. From our results we can draw the following conclusions: Cumene is adsorbed Langmuir-like, forming several layers on highly hydroxylated TiO2 surfaces. The limits of solute adsorption are calculated as 3 cumene molecules per nm2 for the first layer and 8.6 cumene molecules per
Langmuir, Vol. 13, No. 8, 1997 2379
nm2 for the remainder. From this, a number of about 1 cumene molecule per Ti4+ can be deduced. Cumene seems to be adsorbed on TiO2 surfaces involving weak OH/π-interactions, according to the band at 1480 cm-1 obtained in the IR spectrum. This interaction leads to a change in the established mechanism of cumene oxidation, invoking a methyl instead of a phenyl migration. Acetophenone and probably methanol are formed. At prolonged illumination times, the concentration of acetophenone decreased, probably through a mineralization process. The fact that methanol is not detected under our experimental conditions could be due to its strong adsorption on TiO2 surfaces and its rapid mineralization. The photoassisted transformation of cumene, in the presence of TiO2, follows pseudo-first-order kinetics. Due to a turnover number of ca. 350, the studied reaction is catalytic with respect to TiO2. The relative photonic efficiency for the cumene phototransformation has been estimated to be ca. 0.01 under our experimental conditions. Acknowledgment. We thank the European Commission (ERASMUS-scholarship to R.W.) and the ‘Direccio´n General de Investigacio´n Cientifica y Tecnica’ (DGICYT, Project PB93-0917) for partial financial support. The help of XPS service at Seville University is also gratefully acknowledged. LA961055A
