Photocatalytic Oxygenation of Cyclohexane on Titanium Dioxide

Visible-Light-Induced Partial Oxidation of Cyclohexane by Cr/Ti/Si Ternary Mixed ... Photocatalytic Oxidation of Cyclohexane over TiO2: Evidence for a...
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Photocatalytic Oxygenation of Cyclohexane on Titanium Dioxide Suspensions: Effect of the Solvent and of Oxygen P. Boarini, V. Carassiti, A. Maldotti, and R. Amadelli* Centro di Studio su Fotoreattivita` e Catalisi (C.N.R.) and Dipartimento di Chimica dell’Universita` di Ferrara, via L. Borsari, 46, 44100 Ferrara, Italy Received April 15, 1997. In Final Form: December 2, 1997 The photoassisted monooxygenation of C6H12 to C6H10O and C6H11OH by molecular oxygen has been studied on TiO2 powder catalyst dispersed in neat C6H12 and in C6H12/CH2Cl2 mixtures. The composition of the mixed solvent has a strong influence on the selectivity of the process: an increase in the content of CH2Cl2 brings about both an enhancement in the rate of formation of mono-oxygenated products and a decrease in the production of CO2. At the same time, the alcohol to ketone ratio increases in the mixed solvent. An explanation of this behavior is proposed which is based on the observed decrease in the adsorption strength of intermediates (C6H11OH and radicals) as the solvent composition is varied from pure C6H12 to mixtures of it with increasing amounts of CH2Cl2. The results of experiments with different O2 partial pressures are reported. The process is unaffected for O2 partial pressures > 200 Torr. For lower values the formation of dicyclohexyl becomes significant and reaches a maximum at a pO2 of 60 Torr. In O2-free media containing C(NO2)4 as the electron scavenger, the formation of C6H10O decreases markedly while that of C6H11OH is essentially the same as that in oxygenated media. In the mechanism proposed, the reaction of cyclohexyl radicals with O2 and/or activated oxygen species is the main route leading to the ketone.

Introduction The oxo functionalization of hydrocarbons with cleavage of a C-H bond and formation of a C-O bond is an interesting reaction from the applied point of view. Recent research, in the framework of new sustainable technologies, has focused on oxidations at low temperatures and ambient pressure. In particular, oxidations by molecular oxygen are attracting considerable interest. The advantages of using oxygen for the oxidation of organic compounds have been outlined in recent publications.1 On the basis of pure thermodynamic considerations, most of organic compounds are not stable with respect to oxidation by O2. There are, however, kinetic limitations imposed by the high bond energy which characterizes the oxygen molecule, and thus its activation or that of the hydrocarbon is needed. Photocatalysis on semiconductor suspensions can achieve the activation of both reactants. The excitation of TiO2 with light of λ < 400 nm leads to charge separation: electrons are promoted to the conduction band, and holes are left in the valence band. The separated charges will recombine unless either electrons or holes or both are efficiently scavenged by oxidants or reductants, respectively. In the case under examination, the holes are captured by the hydrocarbon (or by surface OH groups) while oxygen can react with conduction band electrons. It is generally accepted that the role of O2 is not just that of scavenging the photogenerated electrons; its reduction products (O2- and H2O2) constitute the so-called “selective oxygen species”2 which take part effectively in the process of oxo functionalization of hydrocarbons.3,4 (1) (a) Roty, A. K.; Kingsley, J. P. Chemtech 1996, 39. (b) Bielanski, A.; Haber, J. Oxygen in Catalysis; Marcel Dekker: New York, 1991. (c) Active Oxygen in Chemistry; Foote, C. S., Ed.; Chapman & Hall: New York, 1995. (d) Golodets, G. I. Heterogeneous Catalytic Reactions Involving Molecular Oxygen; Elsevier: Amsterdam, The Netherlands, 1983. (2) Akimoto, M.; Echigoya, E. J. Chem. Soc., Faraday Trans. 1 1977, 73, 193.

It is a fact that selectivity is a key issue in the catalysis of fine chemicals’ production. Having this goal in mind, the use of TiO2 has a drawback in that, due to the high positive energy of the photogenerated holes, most organic compounds can undergo complete oxidation. This makes it an interesting photocatalyst for the detoxification of water, as witnessed by the large amount of literature available on this subject.5 Clearly, the exploitation of the stabililty and practical use of TiO2 for photosynthetic purposes requires some strategy in the control of its high oxidation power. The present work is a continuation of previous research by us and other authors6 on the photoassisted monooxygenation of cyclohexane on TiO2 at room temperature and 760 Torr. This is an important commercial reaction, as the resultant products (alcohol and ketone) are precursors in the synthesis of adipic acid, in turn, an intermediate in the production of nylon.6c In our recent work on the photoassisted functionalization of cyclohexane,6b we reported that the surface modification of TiO2 with bonded iron porphyrins brings about an increase in the monooxygenation products with respect to CO2. As the latter, deriving from complete photodegradation, must be counted (3) (a) Linsebigler, A.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (b) Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 7626. (c) Wong, J. C. S. Linsebigler, A.; Lu, G.; Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 335. (4) (a) Gerischer, H.; Heller, A. J. Phys. Chem. 1991, 95, 5261. (b) Gerischer, H. Electrochim. Acta 1993, 38, 3. (c) Gerischer, H.; Heller, A. J. Electrochem. Soc. 1992, 139, 113. (5) (a) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69 and references therein. (b) Pichat, P. Catal. Today 1994, 19, 313. (c) Bahnemann, D. W.; Cunningham, J.; Fox, M. A.; Pichat, P.; Pelizzetti, E.; Serpone, N. In Aquatic and Surface Photochemistry; Helz, G. R., Crosby, D. G., Zepp, R. G., Eds.; CRC: Boca Raton, FL, 1994; p 261. (6) (a) Mu, W.; Herrmann, J. M.; Pichat, P. Catal. Lett. 1989, 3, 73. (b) Polo, E.; Amadelli, R.; Carassiti, V.; Maldotti, A. In Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Perot, G., Montassier, C., Eds.; Elsevier: The Netherlands, 1993; p 409. (c) Weissermel, K.; Arpe, H. J. Industrial Organic Chemistry; VCH: Weinheim, Germany, 1997; pp 239-241.

S0743-7463(97)00384-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/14/1998

Photocatalytic Oxygenation of Cyclohexane

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among the reaction products, we could conclude that the overall selectivity of the process improved. We examine the effect of solvent composition on the photooxidation of cyclohexane by TiO2. Solvent effects in heterogeneous catalysis have been reported for different processes7 but not, to our knowledge, for the process of interest here. We compare the data in pure cyclohexane with those obtained in mixed cyclohexane/dichloromethane media. We show that, in the latter case, the rate of conversion of cyclohexane to alcohol and ketone is higher while the formation of CO2 decreases. We can therefore claim an increase of the selectivity of the process for this system where CO2 is an undesired reaction product. The effect of the oxygen partial pressure on the nature and rate of formation of the products is also discussed as part of an effort directed at understanding the reaction mechanism. Experimental Section Materials. The titanium dioxide employed was the wellcharacterized Degussa P25; phenyl-N-tert-butylnitrone (PBN) and tetranitromethane were obtained from Aldrich while all other chemicals were Fluka products. The purity of the cyclohexane, cyclohexanol, and cyclohexanone was checked by gas chomatography. When needed, cyclohexane and dichloromethane were dried respectively over metallic sodium and CaH2 or CaCl2. Apparatus and Methods. TiO2 suspensions in the chosen medium (4 g/dm3) were irradiated at λ > 360 nm using a Hanau Q 400 medium-pressure mercury lamp (6.3 × 10-2 mW/cm2). Products’ analysis was carried out by gas chromatography using a DANI 8521-a gas chromatograph equipped with with a flame ionization detector, using columns packed with Carbowax 20 M 5% on Chromosorb W-AW. The reaction products were determined by comparison of their retention times with those of authentic samples. In some cases (detection of bicyclohexyl), a capillary column was used (Fisons Instruments, 9000 Series, SE52 column and a EL980 F). Gas-mass analysis was carried out using the following instrumentation: GC, HPGC5890 Series II PLUS (5 × MS capillary column He carrier, flux of 1 mL/min); MS, HP 5989B quadrupole and HP software. ESR spin-trapping experiments were performed using a Bruker 220 SE spectrometer. Irradiation was carried out inside the ESR cavity using a flat quartz cell containing the suspension and PBN.

Figure 1. Formation of monooxygenation products from the photooxidation of cyclohexane in TiO2 suspensions in the neat hydrocarbon (A, cyclohexanone, B, cyclohexanol) and in a 1:1 C6H12/CH2Cl2 mixture (A′, cyclohexanone; B′, cyclohexanol). λ > 360 nm, room temperature.

Results and Discussion Effect of the Solvent. Our earlier work on the oxo functionalization of cyclohexane on illuminated TiO2 was carried out using suspensions of the semiconductor in the pure hydrocarbon.6b Under these conditions, the formation of the ketone largely predominates over that of the alcohol. On the other hand, in the homogeneous phase we achieved a selective monooxygenation of cyclohexane to cyclohexanone by irradiation of an iron porphyrin bearing an OH axial ligand in the pure hydrocarbon.8 Significantly, here, as in the case of TiO2, one observes the lightinduced formation of OH• radicals, which are likely initiators of the hydrocarbon oxidation. We were intrigued by the possibility of additional similarities between the homogeneous and the heterogeneous photocatalytic systems. In particular, the ob(7) (a) Cerveny, L.; Ruzicka, V. Adv. Catal. 1981, 30, 335. (b) Gilbert, L.; Mercier, C. In Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Perot, G., Montassier, C., Eds.; Elsevier: The Netherlands, 1993, p 51. (c) Sackett, D. A.; Fox, M. A. J. Phys. Org. Chem. 1979, 83, 3146. (d) Fox, M. A.; Ogawa, H.; Pichat, P. J. Org. Chem. 1989, 54, 3847. (e) Bickley, R. I.; Munuera, G.; Stone, F. S. J. Catal. 1973, 31, 398. (8) Amadelli, R.; Bregola, M.; Polo, E.; Carassiti, V.; Maldotti, A. J. Chem. Soc., Chem. Commun. 1992, 1355.

Figure 2. Variation of the cyclohexanol/cyclohexanone ratio as a function of irradiation time of TiO2 suspensions in C6H12/ CH2Cl2 mixtures containing the indicated amount of CH2Cl2.

servation that the polarity of the solvent has a remarkable effect on the selectivity of the photooxidation process in the homogeneous process9 stimulated our ongoing investigations with TiO2 suspensions along this direction. The results reported in Figure 1 show the formation of cyclohexanone and cyclohexanol in illuminated suspensions of TiO2 in pure cyclohexane and in a 1:1 cyclohexane/ CH2Cl2 mixed solvent. It is seen that the overall rate of formation of cyclohexanol and cyclohexanone increases in the mixed solvent. In particular, the rate of formation of cyclohexanol is significantly higher than that in neat cyclohexane. This is clearly seen in Figure 2, where the ratio of cyclohexanol to cyclohexanone is plotted as a function of the amount of CH2Cl2 in the mixed solvent and of the irradiation time. Carbon dioxide is always a product of the complete oxidation of organic substrates on illuminated TiO2, on account of the high oxidizing power of this semiconductor. While this is an advantage in the (9) Maldotti, A.; Bartocci, C.; Varani, G.; Molinari, A.; Battioni, P.; Mansuy, D. Inorg. Chem. 1996, 35, 1126.

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RH2 + h+ f RH• + H+

(1)

Ti-OH + h+ f Ti-OH•+

(2)

RH2 + (OH•)ads f RH• + H2O

(3)

RH• + (OH•)ad f (HROH)ads

(4)

(HROH)ads + 2OH• f RO + 2H2O

(5)

Table 1. Effect of the Reaction Medium on the Photoassisted Oxidation of Cyclohexane on Titanium Dioxide Suspensions

sample TiO2 in neat C6H12 TiO2 in C6H12/CH2Cl2 80/20 TiO2 in C6H12/CH2Cl2 50/50

[C6H10 O + [CO2] CO2 % C6H11OH] (×104 of the (×103 mol/dm3) mol/dm3) products 7.15 20.3 27.2

3.7 5.04 2.69

5.2 2.48 1.24

Figure 3. Adsorption of cyclohexanol onto TiO2 in (A) C6H12, (B) C6H12/CH2Cl2 (80/20), and (C) neat CH2Cl2.

abatement of pollutants, it is a drawback in the present case, where the aim is the accumulation of valuable reaction intermediates. The concentrations of monooxygenation products and of CO2 after 90 min of irradiation are given in Table 1 for pure C6H12 and its mixtures with CH2Cl2. The production of CO2 is seen to decrease markedly as the amount of CH2Cl2 in the reaction medium increases. This indicates that we are not just observing an increase in the overall rate of the process but rather the selective formation of monooxygenation products. There are previous reports in the literature about solvent effects in heterogeneous catalysis, especially on hydrogenation reactions.7 According to Gilbert and Mercier7b the main factors affecting selectivity are (i) reagent solubility, (ii) polarity, (iii) reactivity or acidobasicity of solvents, and (iv) competitive chemisorption of products and solvents. The last factor is certainly a major one in that the nature of the solvent can control surface reactions through a modification of adsorption-desorption equilibria of reagents and intermediates. An interesting article by Fox et al.7d is particularly relevant to the present work in that it shows that the regioselectivity in the oxidation of 1-4 propanediol is controlled by solventdependent adsorption phenomena with little selective adsorption occurring in CH2Cl2. While cyclohexane interaction with hydroxylated surfaces has been shown to be very weak and nonspecific,10 the interaction of polar compounds such as cyclohexanol and cyclohexanone is expected to be stronger. Indeed, Figure 3 shows that the adsorption of cyclohexanol TiO2 is strong enough for the decrease of the alcohol in solution to be easily followed by gas chromatography, after an established equilibration time in the dark. In comparison, cyclohexanone must be much less adsorbed, since we could not detect any variation in its concentration with the same procedure. The reaction mechanism at the interface includes the following steps not involving O2 or its reduction products:4 (10) Whalen, J. W. J. Phys. Chem. 1962, 66, 511.

We confirmed experimentally that cyclohexanol is oxidized to cyclohexanone6a and that the rate of disappearance of the alcohol is 4 times faster in cyclohexane than in dichloromethane, in agreement with adsorption data. Reactions that lead to the formation of the ketone, other than eq 5, will be discussed later in this paper. We draw attention to the fact that species that are strongly adsorbed can capture the photogenerated holes or the OH radicals more efficiently. It is clear then that the observed decrease in cyclohexanol adsorption and oxidation, as the composition of the medium is changed from pure cyclohexane to pure dichloromethane through mixtures of the two, can lead to accumulation of cyclohexanol in the liquid bulk; this can possibly account for the observed increase of the alcohol to ketone ratio (Figure 2). In the next sections we will comment further on adsorption phenomena as a key factor in controlling the reactivity of the system. ESR and Gas-Mass Spectrometry Measurements. It is evident that radicals have a fundamental role in the mechanism of the processes of interest here; most radicals are, however, too unstable to be revealed at room temperature. In this respect, the ESR-spin-trapping technique represents an excellent tool for the detection of shortlived radicals11 and has been successfully employed in studies of the mechanism of reactions in illuminated semiconductor suspensions.12 The ESR spectra shown in Figure 4 were recorded on irradiation of TiO2 suspensions in C6H12, CH2Cl2 and a 1:1 mixture of the two, using phenyl-tert-butylnitrone (PBN) as the spin trap. They reveal the formation of radical adducts according to

H O|

|

H O• |

|

C6H5sCdNsC(CH)3 + R• f C6H5sCsNsC(CH)3 +

|

R

(6)

The spectra obtained consist of triplets of doublets which are better resolved in the C6H6/CH2Cl2 mixed solvent (Figure 4c) than in pure cyclohexane (Figure 4a). The coupling constants derived from Figure 4a are aN ) 13.7 G and aH ) 2 G. The same spectra were observed when deuterated cyclohexane was used. In no case was a significant ESR signal observed with TiO2 in the dark or under illumination without TiO2. (11) (a) Janzen, E. G. Acc. Chem. Res. 1971, 4, 31. (b) Maldotti, A.; Bartocci, C.; Varani, G.; Molinari, A.; Battioni, P.; Mansuy, D. Inorg. Chem. 1996, 35, 1126. (c) Maldotti, A.; Amadelli, R.; Varani, G.; Tollari, S.; Porta, F. Inorg. Chem. 1994, 33, 2968. (12) (a) Howe, R. F. Adv. Colloid Interface Sci. 1982, 18, 1. (b) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (c) Maldotti, A.; Amadelli, R.; Carassiti, V. Can. J. Chem. 1988, 66, 76. (d) Amadelli, R.; Maldotti, A.; Bartocci, C.; Carassiti, V. J. Phys. Chem. 1989, 93, 6448. (e) Maldotti, A.; Amadelli, R.; Bartocci, C.; Carassiti, V. J. Photochem. Photobiol., A: Chem. 1990, 53, 263. (e) Brezova, V.; Stasko, A. J. Catal. 1994, 147, 156. (f) Brezova, V.; Stasko, A.; Biskupic, S.; Blazkova, A.; Havlinova, B. J. Phys. Chem. 1994, 98, 8977.

Photocatalytic Oxygenation of Cyclohexane

Figure 4. ESR spectra obtained upon irradiation of TiO2 suspensions in (a) C6H12 (or C6D12), (b) CH2Cl2, and (c) a 1:1 C6H12 (or C6D12)/CH2Cl2 mixture, using PBN as a spin trap.

Comparison with literature data13 permits to rule out OH• or radicals derived from the reduction of O2 as the trapped species. On the other hand, the reported values for the adduct between a cyclohexyl radical and PBN are aN ) 14. 4 G and aH ) 2.2 G, i.e., rather higher13 than those observed here. Values of aN below 14 G are more characteristic of trapped alkoxy radicals than of alkyl radicals.13 The fact that experiments carried out under the same conditions as in Figure 4c but with added cyclohexanol gave the same ESR spectra suggests that the trapped species are cyclohexanoxy radicals. We wish to emphasize that the formation of cyclohexyl radicals as a first step in the oxidation process cannot be ruled out, since the radical trapping process is kinetically controlled. The higher intensity of the signals and the better resolution observed in the mixed solvent compared to pure cyclohexane indicate that the radical trapping reaction is easier in the first case. This is in agreement with a stronger adsorption of the radical species on TiO2 in pure cyclohexane, thus confirming the conclusions of the previous section. The ESR spectrum shown in Figure 4b results from the irradiation of a TiO2 suspension in pure CH2Cl2 containing PBN. This spectrum, due to the superimposition of signals of different spin adducts, was observed also in our work on irradiated CH2Cl2 solutions containing PBN and iron porphyrins.9 The signal shown in the figure has reached its maximum intensity. Despite its weakness, the signal might nevertheless indicate some decomposition of dichloromethane. Since we wished to gain more insight into a possible interference of CH2Cl2 in the cyclohexane oxidation in the mixed solvent (1:1), we carried out a gas-mass chromatographic analysis of the products obtained after 90 min of irradiation. The results showed that, neglecting CO2, cyclohexanone and cyclohexanol together represent ∼68% of the total products. Surprisingly, dicyclohexyl is the one main secondary product even though the reaction medium is saturated with O2; its amount is ∼1.5% of that of the total products. Radical coupling is also observed in the oxidation of phenols on TiO2.14 The only clearly identified chlorinated compounds of cyclohexane are 2-chlorocyclohexanol, chlorocyclohexane, and 2-chlorocyclohexanone, which represent 3, 1.3, and 0.5% of the total products, respectively. A plethora of (13) Rehorek, D. Z. Chem. 1980, 20, 325. (14) Minero, C.; Pilizzetti, E.; Pichat, P.; Sega, M.; Vincenti, M. Environ. Sci. Technol. 1995, 29, 2226.

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Figure 5. Effect of the O2 partial pressure on the formation of cyclohexanone and cyclohexanol from the photooxidation of cyclohexane in suspensions of TiO2 in a 1:1 C6H12/CH2Cl2 medium.

other peaks is observed in the chromatogram whose integrated areas are, however, smaller than those of the identified compounds just mentioned. In the mixed solvent, the contribution of Cl• radicals, originating from the decomposition of CH2Cl2, in extracting hydrogen from cyclohexane cannot be ruled out a priori. However, the fact that dicyclohexyl is the main side product in the mixed solvent too is an indication that the amount of cyclohexyl radicals is relatively high, and if the concentration of Cl• was high too, one would reasonably observe higher amounts of chlorinated derivatives than actually observed. We think, rather, that the low amount of chlorinated compounds is a good indication that cross reactions involving hydrocarbon radicals and radicals deriving from a possible oxidation of CH2Cl2 are not significant. Role of O2. The involvement of O2 and of its reduction products in the formation of hydrocarbon oxidation products is a topic of considerable interest and relevance to the work discussed here. Figure 5 shows how the formation of monoxygenation products depends on the O2 partial pressure. A mixed C6H12/CH2Cl2 solvent was chosen, since in this case it is possible to follow the formation of both cyclohexanol and cyclohexanone. It is seen that the concentration of the monooxygenation products increases rapidly in the partial pressure range from 10 to 100 Torr and that above 200 Torr the dependence on pO2 becomes negligible. This is in accord with previous studies of the oxidation of cyclohexane with molecular oxygen.1 Figure 5 also shows that the ratio alcohol/ketone depends on pO2 for pressures below 100 Torr. The interesting point to note is that in the lower partial pressure range cyclohexanol is the main product. It has already been mentioned above that dicyclohexyl is always a reaction product, indicating that the formation of cyclohexyl radicals is an efficient process. As a matter of fact, we noticed that some oxidation of cyclohexane still occurs even in deaerated suspensions. Quantitative experiments, with controlled values of the oxygen partial pressure, showed that the concentration of dicyclohexyl reached a maximum (5 × 10-5 mol dm-3) around 60 Torr. Evidently, at the higher pO2 values cyclohexyl radicals are scavenged by oxygen species, while at the lower values electron-hole recombination finally prevails It is now well-established that in the photocatalytic oxidation of organic compounds either in the homogeneous

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phase using polyoxometalate or iron porphyrins15 or on dispersed semiconductors3-5 the role of O2 is not merely that of acting as an electron acceptor. Typical reactions that involve molecular oxygen and hydrocarbon radicals are the following:

HR• + O2 f HRO2•

(7)

HRO2• + RH2 f RH• + HROOH

(8)

HRO2• + e-(TiIII) f RO + TiIVOH

(9)

where the species are involved in adsorption-desorption equilibria. In addition, the alkyl radicals can react with the products of O2 reduction, i.e., O2- and HO2•, according to

HR• + O2- f RO + OH•

•

(10)

HR + HO2 f RO + H2O

(11)

HRO2• + HO2• f HROOH + O2

(12)

Figure 6. Formation of cyclohexanone and cyclohexanol upon irradiation of a O2 free suspension of TiO2 in pure cyclohexane containing 0.01 mol dm-3 C(NO2)4.

Reaction 16 leads to alcohol formation, as do reactions 14 and 15 through reaction 4. Several aspects of the reaction mechanism on semiconductor oxides are currently the object of debate. One of the arguments concerns oxidations mediated by OH• radicals versus direct capture of the photogenerated hole

by reducing species; the latter is considered more likely in the absence of water. The state of the art of these studies is well-summarized in recent review articles.3,5 The source of OH• radicals is also a discussed matter even in aqueous solutions. Thus, for example, Matthews20 reports that OH radicals are formed mainly upon capture of holes by surface OH groups or adsorbed water while, according to Okamoto et al.,21 the decomposition of H2O2 (reactions 13 and 14) has a major role. We thought that changing the electron scavenger might provide some indirect support of the involvement of activated oxygen species in the oxidation mechanism. We were aware of the fact that the choice of an alternative electron acceptor instead of oxygen is not straightforward; this is because, first, the use of pure cyclohexane restricts the choice drastically and, second, changing the nature of the compound inevitably introduces new problems. We decided to use tetranitromethane because it is known to be a good electron and radical scavenger22 and has been employed earlier in studies of charge transfer at colloidal TiO2.23 The reported reaction rate of this compound with free electrons is 6.2 × 1010 mol dm3 s-1,22 while, for comparison, those for O2 and H2O2 are 1.88 × 1010 and 1.23 × 1010 mol dm3 s-1, respectively.24 Figure 6 shows the formation of cyclohexanol and cyclohexanone, as a function of irradiation time, in deaerated suspensions of TiO2 in pure cyclohexane containing C(NO2)4. The salient point here is that the production of cyclohexanol is substantially higher than that when O2 is employed (cf. Figure 1) and that its concentration grows with time of irradiation. This is also in agreement with the data of Figure 5 at very low O2 partial pressures. In addition, contrary to the behavior observed in the oxygenated system, the concentration of cyclohexanone rapidly stabilizes to a value which depends on the C(NO2)4 concentration. In any case, this value is always lower than that in O2, even if the concentration of C(NO2)4 is made 0.01 mol dm-3 (Figure 6), i.e., approximately that of dissolved O2 in these media under

(15) Maldotti, A.; Amadelli, R.; Carassiti, V.; Molinari, A. Inorg. Chim. Acta 1997, 256, 309. (16) Harima, Y.; Morrison, R. S. J. Phys. Chem., 1988, 92, 5716. (17) Phelps, J.; Santhanam, K. S. V.; Bard, A. J. J. Am. Chem. Soc. 1967, 89, 1752. (18) Lewis, I. C.; Singer, L. S. J. Chem. Phys. 1965, 43, 2712. (19) (a) Tafalla, D.; Salvador, P. J. Electroanal. Chem. 1987, 237, 225. (b) Salvador, P.; Decker, F. J. Phys. Chem. 1984, 88, 6116.

(20) Matthews, R. W. J. Chem. Soc., Faraday Trans. 1 1984, 80, 457 (21) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M.; Itaya, A. Bull. Chem. Soc. Jpn. 1985, 58, 2015. (22) Asmus, K. D.; Henglein, H. Ber. Bunsen-Ges. Phys. Chem. 1968, 68, 348. (23) Henglein, H. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 241. (24) Gordon, S.; Hart, E. J.; Matheson, M. S.; Rabani, J.; Thomas, J. K. Discuss. Faraday Soc. 1963, 36, 193.

An additional possible reaction pathway involves a “current doubling” phenomenon

(HR•)ads + (O2-)ads f (HRO2•)ads + ecb

(13)

In this case, the cyclohexyl radical injects an electron into the conduction band to give HR•+, which subsequently reacts with the superoxide ion. This possibility is supported by the work of Harima and Morrison,16 who observed current doubling for the oxidation of several alkanes, including cyclohexane, on TiO2 photoelectrodes. The occurrence of reaction 13 may in part explain the observed increase in the yield of monooxygenation products in C6H12/CH2Cl2. In fact, it has been reported that dichloromethane can stabilize organic cation radicals.17,18 This was actually assessed for several aromatic compounds; saturated hydrocarbons are not mentioned in the cited works, but this may be simply connected to the high ionization potentials that make their oxidation by the methods chosen by the authors unsuitable.18 While the above reactions constitute the main routes for cyclohexanone formation, one should also take into account that the reduction of peroxides is a source of radicals that sustain the oxidation process:19

H2O2 + O2- f OH• + OH- + O2

(14)

H2O2 + e-(TiIII) f OH• + TiIV-OH

(15)

HROOH + e-(TiIII) f HRO• + TiIV-OH

(16)

Photocatalytic Oxygenation of Cyclohexane

Langmuir, Vol. 14, No. 8, 1998 2085

760 Torr of the pure gas.25 In the present case, the formation of cyclohexanol can still occurr as shown above (reactions 3 and 4) but also through reaction 3 followed by23

HR• + C(NO2)4 + H2O f HROH + H+ + C(NO2)3- + NO2 (17) Likewise, the ketone is formed via reaction 5 or by the reaction of alkoxy radicals with C(NO2)4, according to

HRO• + C(NO2)4 + H2O f RO + H+ + C(NO2)3- + NO2 (18) The stable anion22 formed in this reaction is very likely to be strongly adsorbed on the oxide and undergo decomposition as its concentration reaches a critical value.23 The consequence will be an inhibition of both reactions 5 and 18 leading to cyclohexanone while formation of cyclohexanol from the oxidation of cyclohexane apparently still goes on by reason of the large excess and low adsorption of the hydrocarbon. Moreover, if this oxidation involves OH radicals, conduction band processes cannot be their source as in the case with O2 (reactions 14 and 15), and our conclusions are in better agreement with those of Matthews20 than with those of Okamoto et al.21 Contributions by Solution Phase Radical Reactions. As mentioned above, the radicals that are formed upon capture of photogenerated holes or electrons are involved in adsorption-desorption equilibria at the TiO2 particles. The desorbed species are also involved in homogeneous phase reactions of autoxidation that lead to the formation of both the alcohol and the ketone:26

2(HRO2•) f HROH + RO + O2

(19)

HROOH f HRO• + OH•

(20)

HRO• + RH2 f HROH + HR•

(21)

Above we described how a radical scavenger such as PBN can be used in ESR experiments to obtain information on the nature of radicals formed in the initial reaction stages, as soon as light is switched on. We expected that, in principle, the presence of PBN in experiments under continuous illumination should lead to the inhibition of reaction routes involving relatively free radicals, such as the above reactions (19-21) and/or reactions 7-12. In effect, the data of Figure 7 show a large effect of PBN on the relative distribution of the monooxygenation products. Specifically, it is interesting to note that the formation of cyclohexanol is barely affected by PBN while that of cyclohexanone decreases by over 50%. The experiments of Figure 7 were carried out in a mixed C6H12/ CH2Cl2 medium, but we observed a similar phenomenon in neat C6H12. In the latter case, however, PBN causes a less pronounced decrease (20-25%) in cyclohexanone formation. This is not surprising in view of the stronger adsorption of the intermediates in this medium, including radicals (see Figure 4). It seems possible to conclude that interphase reactions of the hydrocarbon radicals with oxygen (or activated (25) Murov, S. L. Handbook of Photochemistry; Marcel Dekker Inc.: New York, 1973. (26) (a) Maldotti, A.; Molinari, A.; Bergamini, P.; Amadelli, R.; Battioni, P.; Mansuy, D. J. Mol. Catal. 1996, 113, 147. (b) Maldotti, A.; Molinari, A.; Argazzi, R.; Amadelli, R.; Battioni, P.; Mansuy, D. J. Mol. Catal. 1996, 114, 141.

Figure 7. Effect of the addition of a radical trap (PBN) on the formation of cyclohexanone and cyclohexanol from the photooxidation of cyclohexane on TiO2 suspensions in 1:1 C6H12/CH2Cl2 mixtures: A, cyclohexanone (no PBN), B, cyclohexanol (no PBN), C, cyclohexanone (PBN); D, cyclohexanol (PBN).

oxygen species) have a paramount role in the formation of cyclohexanone. Conclusions The photocatalytic monooxygenation of cyclohexane by O2, using TiO2 as catalyst, yields fine chemicals such as cyclohexanone and cyclohexanol. Complete degradation of the hydrocarbon also occurs, giving CO2 as an undesired product. The results of our research, which focused on (i) the role of the solvent and (ii) the role of oxygen and activated oxygen species, show the following: (1) The formation of monooxygenated products (C6H11OH + C6H10O) is favored, and that of CO2 is inhibited, when the oxidation of C6H12 is carried out in C6H12/CH2Cl2 mixtures instead of the neat hydrocarbon. The nature of the solvent medium is then an important parameter in the control of the process selectivity toward fine chemicals’ production. The experimental data show that the nature of the solvent also controls the alcohol to ketone ratio by affecting the adsorption of intermediates. (2) The function of O2 is not just that of acting as an acceptor of the photogenerated electrons. The results presented here indicate that reaction of the hydrocarbon radicals with activated O2 species or O2 itself represents an important pathway that leads to the formation of cyclohexanone. This is shown experimentally by the fact that upon substitution of O2 with C(NO2)4, which is known to be a good electron acceptor, only the formation of the ketone is inhibited. This phenomenon occurs also in the presence of a radical trap (PBN), and it is more efficient in the mixed solvent than in the pure hydrocarbon substrate. This is rationalized in terms of a decrease in the adsorption strength of the radicals in the former case, as shown by ESR data, which facilitates their trapping by PBN. In the proposed reaction pathway, the formation of cyclohexanol is a valence band process. The oxidation of the alcohol to the ketone or further is more efficient in pure cyclohexane than in the more polar mixed solvent, due to stronger adsorption in the former medium. However, cyclohexanone is formed mainly by a parallel route which involves the reaction of cyclohexyl radicals with O2 and/or its reduction products. Acknowledgment. This research was supported by the National Council of Research (C.N.R.). LA970384F