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Tetralkylammonium and Sodium Decatungstate Heterogenized on Silica: Effects of the Nature of Cations on the Photocatalytic Oxidation of Organic Substrates A. Molinari,† R. Amadelli,† A. Mazzacani,‡ G. Sartori,‡ and A. Maldotti*,† Dipartimento di Chimica, Centro di Studio su Fotoreattivita` e Catalisi, Universita` di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy, and Dipartimento di Chimica Organica e Industriale, Universita` di Parma, Viale delle Scienze, 43100 Parma, Italy Received July 5, 2001 Tetralkylammonium and sodium decatungstates have been heterogenized on silica. The so-obtained materials have been characterized by UV-vis and Fourier transform infrared spectroscopies, N2 adsorption experiments, and measurements of surface polarity. Both the anion W10O324- and the countercations are anchored on the surface of the support without undergoing any appreciable modification. The size of the cations plays a key role in determining the degree of coverage and, consequently, the surface polarity of silica. The heterogenized systems are characterized by different and tunable photoreactivities depending on the nature of the countercation. In particular, tetralkylammonium cations enhance the efficiency of cyclohexane photooxidation both to radical species and to the stable oxygenation products cyclohexanol and cyclohexanone. The cation makes the environment where the photoactive anion is localized more or less hydrophobic; this influences the possibility of the approach of the substrate and, consequently, the photocatalytic efficiency of the process. The stability of these photocatalysts is demonstrated by the fact that they can be used at least three times without any loss of activity. Some alcohols with increasing dielectric constants, like cyclohexanol, ethanol, and methanol, have been used in an electron paramagnetic resonance spin trapping investigation in order to gain more information about the action of the different cations. With bulky and organic cations, alcohols were oxidized in the order from the less polar to the more polar, in terms of dielectric constants. On the contrary, the order is reversed with inorganic cations, such as Na+.
Introduction
Scheme 1
A number of authors are investigating the noticeable photocatalytic activity of polyoxotungstates in the oxidation of organic compounds.1 These photocatalysts are stable and, then, can be employed without loss of activity in several catalytic cycles. Some of our recent contributions in this research area have dealt with the use of (nBu4N)4W10O32 for the oxygenation of cyclohexane with molecular oxygen.2 We demonstrated that suitable experimental conditions can be found in organic solvents in order to avoid mineralization of this substrate. This is a major advantage when accumulation of valuable oxidation intermediates is sought and total photodegradation is an unwanted competing reaction. Cyclohexane oxygenation is an important commercial reaction, as the resultant products, alcohol and ketone, are precursors in the syntheses of adipic acid, which is in turn an intermediate in the production of nylon.3 The proposed photochemical pathway of the polyoxotungstate action involves the mechanism schematized by eqs 1-8 (Scheme 1).4 The photoexcited decatungstate can initiate the oxidation of cyclohexane through hydrogen † ‡
Universita` di Ferrara. Universita` di Parma.
(1) Hill, C. L.; Prosser McCartha, C. M. Photosensitization and Photocatalysis using inorganic and organometallic Compounds; Kluwer Academic Publishers: Dordrecht, 1993. (2) (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. (c) Maldotti, A.; Amadelli, R.; Carassiti, V.; Molinari, A. Inorg. Chim. Acta 1997, 256, 309. (d) Molinari, A.; Maldotti, A.; Amadelli, R.; Sgobino, A.; Carassiti, V. Inorg. Chim. Acta 1998, 272, 197. (3) Weissermel, K.; Arpe, H. J. In Industrial Organic Chemistry; VCH: Weinheim, Germany, 1997; p 239.
abstraction (eqs 1 and 2). Formation of oxidation products (cyclohexanol, cyclohexanone, and cyclohexyl-hydroper(4) (a) Renneke, R. F.; Pasquali, M.; Hill, C. L. J. Am. Chem. Soc. 1990, 112, 6585. (b) Ermolenko, L. P.; Giannotti, C. J. Chem. Soc., Perkin Trans. 1996, 1205. (c) Chambers, R. C.; Hill, C. L. Inorg. Chem. 1989, 28, 2509. (d) Hiskia, A.; Papaconstantinou, E. Inorg. Chem. 1992, 31, 163. (e) Yamase, T.; Takabayashi, N.; Kaji, M. J. Chem. Soc., Dalton Trans. 1984, 793.
10.1021/la0110141 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/19/2002
Decatungstates Heterogenized on Silica
oxide) occurs mainly according to a photocatalytic cycle (eqs 3-7), which involves both the oxidized state and the photoreduced form of the decatungstate. The final alcohol to ketone concentration ratio depends also on the possibility that further oxidation of the alcohol to cyclohexanone occurs (eq 8) in competition with that of cyclohexane. Finally, we have to consider that radical intermediates may be involved in autoxidation chain processes yielding a mixture of alcohol and ketone. In our previous work, we established that the efficiency of the photocatalytic process assisted by the decatungstate and the relative distribution of the hydrocarbon oxidation products depend on (i) the polarity of the reaction medium,5 (ii) the concentration of dissolved oxygen,2a (iii) the presence of cocatalysts such as metalloporphyrins,2b,2d and (iv) the heterogenization on solid supports such as silica.5a,6 Concerning this last point, it is noteworthy that heterogenization of photocatalysts on solid matrixes has a positive effect on photocatalytic efficiency.5a Generally speaking, another main advantage in the use of heterogeneous systems is that the solid support makes the catalysts more easily handled and recycled than in homogeneous solution.7 The influence of alkali metal cations on the redox properties of polyoxotungstates has been previously investigated.8 In particular, it has been demonstrated that an increase in the inorganic cation size results in the formation of more intimate ion pairs, that possess larger electron affinity. In this work, we present more insights into the photocatalytic behavior of silica-supported W10O324- through the investigation of the effect of the countercation on its photooxidizing activity. In particular, the effect of Na+ on the photooxidative properties of W10O324- is compared with that of some tetralkylammonium cations, which are characterized by a minimum pairing with the decatungstate.8b Attention is focused on cations with different sizes and hydrophobicities such as (nBu4N)+, (Et4N) +, (Et3NH) +, (Me4N) +, (NH4) +, and Na+. The substrates investigated are cyclohexane and the following alcohols with different dielectric constants: cyclohexanol, ethanol, and methanol. The supported photocatalysts are characterized by UV-vis and Fourier transform infrared (FT-IR) spectroscopies, N2 adsorption experiments, and measurements of surface polarity. Comparison of their photocatalytic properties is carried out on the basis of the yields of final stable products and of primary radical species, which are detected by an electron paramagnetic resonance (EPR) spin trapping investigation. Experimental Section Materials. The syntheses of the decatungstates (nBu4N)4W10O32, (Et4N)4W10O32, (Et3NH)4W10O32, (Me4N)4W10O32, (NH4)4W10O32, and Na4W10O32 were performed following literature procedures.9 Their subsequent heterogenization on silica, to give (nBu4N)4W10O32/SiO2 (1), (Et4N)4W10O32/SiO2 (2), (Et3NH)4W10O32/ SiO2 (3), (Me4N)4W10O32/SiO2 (4), (NH4)4W10O32/SiO2 (5), and (5) (a) Molinari, A.; Amadelli, R.; Andreotti, L.; Maldotti, A. Dalton Commun. 1999, 1203. (b) Boarini, P.; Carassiti, V.; Maldotti, A.; Amadelli, R. Langmuir 1998, 14, 2080. (6) Molinari, A.; Amadelli, R.; Carassiti, V.; Maldotti, A. Eur. J. Inorg. Chem. 2000, 91. (7) (a) Mizuno, N.; Misono, M. Chem. Rev. 1998, 98, 199. (b) Neumann, R.; Miller, H. J. Chem. Soc., Chem. Commun. 1995, 2277. (8) (a) Grigoriev, V. A.; Hill, C. L.; Weinstock, I. A. J. Am. Chem. Soc. 2000, 122, 3544. (b) Grigoriev, V. A.; Cheng, D.; Hill, C. L.; Weinstock, I. A. J. Am. Chem. Soc. 2001, 123, 5292. (c) Metelski, P. D.; Swaddle, T. W. Inorg. Chem. 1999, 38, 301. (9) (a) Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shun, W. Inorg. Chem. 1979, 18, 93. (b) Renneke, R. F.; Pasquali, M.; Hill, C. L. J. Am. Chem. Soc. 1990, 112, 6585.
Langmuir, Vol. 18, No. 14, 2002 5401 Na4W10O32/SiO2 (6), was carried out in agreement with the previously described impregnation procedure.6 Decatungstate (0.1 g) was dissolved in a CH3CN/H2O mixture (4/1) containing 1 g of colloidal silica (0.012 µm, Strem Chemicals). The suspensions were shaken at room temperature for an hour, and then the excess of solvent was slowly evaporated. The obtained sample was dried at 100 °C for 24 h. All the solvents purchased from Fluka were spectroscopic grade and used as received. R-Phenyl N-tert-butyl nitrone (pbn) (Aldrich) was employed without any further purification. Apparatus. UV-vis diffuse reflectance spectra of the heterogenized decatungstate were recorded with a Lambda 6 spectrophotometer from Perkin-Elmer, equipped with an integrating sphere. Infrared spectra were obtained with a Nicolet 510P FTIR instrument in KBr, fitted with a Spectra-Tech collector diffuse reflectance accessory (range from 4000 to 200 cm-1). Surface areas were obtained by the Brunauer-Emmett-Teller (BET) method using a C. Micromeritics Pulsechemisorb 2705. Continuous irradiations were carried out with a medium-pressure Hanau Q-400 mercury lamp (15 mW cm-2) using a glass cutoff filter (λ > 300 nm). Gas chromatographic analyses were performed using two different gas chromatographs: a Dani 8521-a, equipped with a flame ionization detector (FID) and with a packed column (Carbowax 20M, 5% on Chromosorb W-AW); a HP 6890 equipped with a FID and with either a HP-WAX (cross-linked poly(ethylene glycol), 30 m, 0.32 mm × 0.5 µm film thickness) column or a HP-5 (cross-linked 5% PH ME siloxane, 30 m, 0.32 mm × 0.25 µm film thickness) column. Quantitative data have been obtained on the basis of calibration curves with authentic samples of the products. Each photocatalytic experiment has been repeated four times in order to evaluate errors which never exceeded (10%. EPR spectra were recorded with an X-band Bruker 220 SE spectrometer. Procedures. In the continuous irradiation experiments, the heterogeneous photocatalyst was suspended by magnetic stirring in 3 mL of neat cyclohexane (15 g dm-3). Irradiation was carried out inside a Pyrex cell with only one optical face, at λ > 300 nm, under 1 atm of O2. The chosen amount of polyoxoanion was such that the maximum absorption of incident light was obtained. An analogous procedure has been followed suspending the heterogeneous photocatalysts in CH3CN solutions of cyclohexanol (2 × 10-3 mol dm-3). After irradiation, the sample was centrifuged and the gas chromatographic analyses were performed from the liquid. Then, the irradiated powders were resuspended at least three times in CH3CN in order to extract all the oxidation products for analysis. Gas chromatographic analyses were carried out with columns of different polarity and with different temperature programs. The determination of hydroperoxides was performed by a spectrophotometric standard method reported in the literature.10 Control experiments showed that no detectable oxidation product was observed when blank experiments were run in the absence of light or irradiating the pristine SiO2. Solutions (1 × 10-5 mol dm-3) of (nBu4N)4W10O32 for homogeneous phase experiments were prepared from more concentrated CH3CN solutions of decatungstate 1 × 10-3 mol dm-3. EPR spin trapping experiments were carried out by irradiating the heterogeneous photocatalyst dispersed in the required medium containing the spin trap pbn (1 × 10-1 mol dm-3). Photochemical excitation was carried out with light of wavelength higher than 300 nm directly inside the EPR cavity, using a flat quartz cell as the reaction vessel. The quantity of pbn was established by performing experiments where the signal intensities of the paramagnetic adducts between the photogenerated radicals and pbn were followed as a function of the amount of pbn in solution. The amount of spin trap used corresponds to the plateau region. Signals were not observed in the dark and in the absence of supported photocatalysts. The paramagnetic radical 2,2,6,6-tetramethylpiperidin-1-oxyl was used as a standard substance for the measurement of spin concentration. In this calculation, the intensities of the spectra were compared after double integration of the signals. For the polarity measurements, the investigated systems were treated with a solution of the pyridinium N-phenolate betaine, known as Reichardt’s dye, in CH2Cl2 (ca. 1 × 10-4 mol dm-3) at (10) Mair, R. D.; Graupner, A. J. Anal. Chem. 1964, 36, 194.
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Table 1. Surface Areas and Polarity Measurementsa of Unmodified SiO2 and of (nBu4N)4W10O32/SiO2 (1), (Et3NH)4W10O32/SiO2 (3), and Na4W10O32/SiO2 (6) material
surface area (m2/g)
λmax (nm)
ETN
SiO2 1 3 6
95 ( 2 54 ( 2 83 ( 2 83 ( 2
470 606 505 496
0.93 0.51 0.80 0.83
a
SiO2 and 1, 3, and 6 were separately put in contact with a solution of Reichardt’s dye in CH2Cl2 (1 × 10-4 mol dm-3) at room temperature. The very fast adsorption of the dye caused different coloring of the solids. After the removal of the solvent, diffuse reflectance UV-vis spectra were recorded and the maxima of the lowest energy charge-transfer transitions (λmax) were used to calculate ETN values from eqs 9 and 10.
room temperature. The solids adsorbed the dye from the solution within a few seconds. Then the solvent was removed on a rotary evaporator and the remaining traces were eliminated under vacuum. The diffuse reflectance UV-vis spectra were recorded, and the maxima of the lowest energy charge-transfer transitions were measured.
Results and Discussion Characterization of the Photocatalysts. The UVvis spectra of the heterogeneous systems 1-6, prepared as reported in the Experimental Section, revealed that the W10O324- clusters are present on the surface of silica without any appreciable modification (typical absorption at 325 nm). The preservation of the decatungstate structure was confirmed by infrared spectra, which showed the characteristic bands of W10O324- (ν ) 958, 890, and 802 cm-1). Infrared spectra also gave evidence that the alkylammonium counterions are adsorbed on the surface of silica showing their typical C-H stretching at about 2870 cm-1. Anchoring of the decatungstate on silica is accompanied by a decrease of surface area, as shown by the N2 adsorption data reported in the first column of Table 1. The photocatalysts 1, 3, and 6 have been chosen as representative of the different nature of the countercations employed: organic-bulky, intermediate, and inorganicsmall, respectively. The most significant effect is observed with (nBu4N)4W10O32, so indicating that the size of the counterions of the decatungstate plays a key role in determining the degree of surface coverage. Polarity effects are of fundamental importance in heterogeneous catalysis, where reactions occur either at the catalyst surface, inside pores, or in a thin liquid film at the surface. In mechanistic terms, surface polarity can govern the adsorption of substrates and desorption of products, which are often rate-limiting steps. The use of solvatochromic indicators is widespread in the determination of solvent parameters such as polarity; one of the most used is the pyridinium N-phenolate betaine dye known as Reichardt’s dye.11 Clark and co-workers have recently demonstrated that this probe can be successfully employed as a surface polarity indicator for chemically and thermally treated silicas.12 To verify the possible effect of different cations on the intrinsic surface polarity of silica, pristine SiO2, 1, 3, and 6 were separately put in contact with a solution of Reichardt’s dye in CH2Cl2 (ca. 1 × 10-4 mol dm-3) at room (11) Reichardt, C. Chem. Rev. 1994, 94, 2319. (12) (a) Tavener, S. J.; Clark, J. H.; Gray, G. W.; Heath, P. A.; MacQuarrie, D. J. Chem. Commun. 1997, 1147. (b) MacQuarrie, D. J.; Tavener, S. J.; Gray, G. W.; Heath, P. A.; Rafelt, J. S.; Saulzet, S. I.; Hardy, J. J. E.; Clark, J. H.; Sutra, P.; Brunel, D.; di Renzo, F.; Fajula, F. New J. Chem. 1999, 23, 725. (c) Adolph, S.; Spange, S.; Zimmermann, Y. J. Phys. Chem. 2000, 104, 6429.
temperature. The solids adsorbed the dye from the solution within a few seconds. After the removal of the solvent, diffuse reflectance UV-vis spectra were recorded. Following a known procedure,11,12 it has been possible to calculate the molar energy of the electronic transition of the dye (ET), measured in kilocalories per mole at 25 °C and 1 bar according to eq 9.
ET (kcal mol-1) ) hcνmaxNA ) 28591/λmax (nm)
(9)
In this equation, λmax is the wavelength corresponding to the maximum absorption of the longest wavelength π-π* band of betaine. ETN represent the dimensionless normalized values of ET. They are defined according to eq 10,
ETN )
ET(solvent) - ET(TMS) ET(water)
)
ET(solvent) - 30.7 32.4 (10)
using water and tetramethylsilane (TMS) as extreme polar and nonpolar references, respectively. Therefore, the ETN scale ranges from 0 for TMS to 1 for water. The measured λmax and the calculated ETN values are reported in Table 1. The ETN value obtained for the unmodified SiO2 was in agreement with that reported in the literature.12a Catalysts 3 and 6 were quite similar to silica, with ETN values of 0.80 and 0.83, respectively. For 1, we observed a remarkable decrease of surface polarity (ETN ) 0.51), due to the significant decrease of free surface following impregnation with (nBu4N)4W10O32. Generally speaking, ionic adsorption on silica occurs because of the amphoteric properties of surface silanol groups, which can be present in their protonated (SiOH2+) or deprotonated (SiO-) forms. In any case, these surface groups are able to attract ions of opposite charge. On the basis of previous work,13 anchoring of the decatungstate in an acidic environment (pH ) 1-2) is expected to occur through an ion-mediated pathway, which may be schematized by the sequence SiOH2+/W10O324-/counterion+. On the other hand, it has been reported that at a pH close to neutrality, organic quaternary ammonium cations may be adsorbed on the negatively charged surface of silica (SiO-) through electrostatic interactions.14 The positive ions should then act as a bridge between the negative surface and the decatungstate according to an ionmediated adsorption pathway of the type SiO-/counterion+/ W10O324-. This kind of interaction has already been proposed to be responsible for the anchoring of some polyoxomolybdates on silica.15 The second pathway appears to be more likely in this work, since immobilization of the decatungstates is carried out from CH3CN/H2O mixtures at a pH close to neutrality. Establishing the precise stoichiometry of the association complexes counterion+/W10O324- on silica is not an easy assignment. Previous works in homogeneous solution8 report that solutions of polyoxometalates having alkali metal ions as countercations contain equilibrium (13) (a) Piquemal, J.-Y.; Manoli, J.-M.; Beaunier, P.; Ensuque, A.; Tougne, P.; Legrand, A-P.; Bre´geault, J.-M. Microporous Mesoporous Mater. 1999, 29, 291. (b) Bre´geault, J.-M.; Piquemal, J.-Y.; Manoli, B. E.; Duprey, E.; Launay, F.; Salles, L.; Vennat, M.; Legrand, A.-P. Microporous Mesoporous Mater. 1999, 29, 291. (14) Ogawa, M. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 1999, 94, 209. (15) (a) Rocchiccioli-Deltcheff, C.; Amirouche, M.; Che, M.; Tatibout, J. M.; Fournier, M. J. Catal. 1990, 125, 292. (b) Fournier, M.; Thouvenot, R.; Rocchiccioli-Deltcheff, C. J. Chem. Soc., Faraday Trans. 1991, 87, 349.
Decatungstates Heterogenized on Silica
Langmuir, Vol. 18, No. 14, 2002 5403 Scheme 2
Figure 1. Cyclohexanol and cyclohexanone concentrations obtained upon 4 h irradiation (λ > 300 nm) of suspensions of catalysts 1-6 (15 g dm-3) in neat cyclohexane, in the presence of 760 Torr of O2.
distributions of free anions and ion-paired structures of variable stoichiometry. In contrast, the tetralkylammonium salts here employed are characterized by a minimum pairing between ions, which should give unstable association complexes of unknown stoichiometry. Control experiments revealed that the decatungstate is attached on silica strongly enough as to be not released in a detectable amount when the heterogeneous systems 1-6 were dispersed in CH3CN. Therefore, this solvent has been chosen as a dispersing medium in addition to cyclohexane for the photocatalytic investigations described in the following paragraph. Photocatalytic Oxidation of Cyclohexane. In a typical experiment, the individual catalysts 1-6 were dispersed in neat cyclohexane (15 g dm-3) and irradiated at λ > 300 nm in the presence of 760 Torr of O2. Gas chromatographic analysis of the photoproducts, carried out after their exhaustive extraction from the heterogeneous phase, revealed that (i) irradiation induces the oxidation of cyclohexane to cyclohexanol and cyclohexanone as main products, whose concentration grows linearly as a function of time, and (ii) the concentration of other minor products was evaluated to be at least 20 times lower than that of cyclohexanol and cyclohexanone. Iodometric analysis gave evidence that hydroperoxides, which are the primary products during the oxygenation of alkanes by illuminated W10O324- (eqs 1-3), were accumulated only in negligible amounts (about 20 times less than that of cyclohexanol and cyclohexanone). The possibility that catalytic processes may occur to some extent in the solution phase due to desorption phenomena is a very crucial point in heterogeneous catalysis. In fact, catalytic reactions in the homogeneous phase may be 1-4 orders of magnitude more efficient than in the heterogenized systems. Two main results indicate that the decatungstate is not desorbed in significant amounts during the irradiation of the photocatalysts 1-6 in neat cyclohexane: (i) all these systems could be employed at least three times without suffering appreciable loss of their photocatalytic activity; (ii) control experiments indicated that the solubility of all the employed decatungstates in this solvent is always lower than 1 × 10-6 mol dm-3. Additional experiments carried out in a mixed cyclohexane/CH3CN solvent containing (nBu4N)4W10O32 up to 1 × 10-5 mol dm-3 showed that no appreciable photooxidation of cyclohexane occurred. This finding is not unexpected considering that in these conditions only a minor amount of the excitation light is absorbed by the decatungstate (less than 5%). The concentrations of cyclohexanol and cyclohexanone after 4 h irradiation are reported in Figure 1 as a function
of the nature of the countercation employed. The photocatalytic efficiency of 1-4 in terms of overall oxidation products was similar, while a significant decrease is observed with both the ammonium and the sodium salts (catalysts 5 and 6). Most likely, tetralkylammonium cations create a less polar and more hydrophobic environment around the photoactive species on the surface than do Na+ and NH4+, so favoring the approach of the cycloalkane and its subsequent oxidation to cyclohexanol and cyclohexanone according to eqs 1-8. Moreover, we observed that the ratio of cyclohexanone to cyclohexanol is about 2-fold higher for the case of the smaller countercations. This modest but reproducible difference in reactivity can possibly suggest prospective surface modifications toward higher chemoselectivity. EPR Spin Trapping Investigation. The EPR spin trapping technique has been successfully employed in studies on the mechanism of cyclohexane oxidation by photoexcited polyoxotungstates in homogeneous solutions.2a,2c This technique is particularly relevant when insights are sought into the reaction mechanisms because it allows determination of both the nature and the formation rate of short-lived radical species, which often are the primary products of a photocatalytic process.16 In previous papers, we demonstrated that the rate of formation of the adduct between the primary radical species and the spin trap can be considered proportional to the rate of oxidation of the substrate.17 In fact, the presence of a large excess of pbn warrants a constant percentage of trapping, although the true radical concentration may be higher than the spin concentration derived from the intensity of the signal. Typical experiments were carried out irradiating (λ > 300 nm) 1, 3, or 6 dispersed in the desired medium containing the spin trap pbn (1 × 10-1 mol dm-3). The formation of radical species (R•) could be monitored by detection of the more stable paramagnetic nitroxides, which are formed according to eq 11 (Scheme 2). The nature of the trapped radicals could be identified by the hyperfine coupling constants of the so-obtained spectra.18 All the radical species described in the following were formed with initial rates that did not depend on the presence of O2, so indicating that they originate from primary photoredox reactions between the substrate and the photoexcited decatungstate and not during subsequent O2-mediated autoxidation processes. Cyclohexane Oxidation. Irradiation of 1 suspended in neat cyclohexane yielded the EPR spectrum reported in Figure 2a, consisting of a broad triplet with nitrogen hyperfine coupling constant aN ) 14.2 G. This result indicates, in line with the results obtained in homogeneous solutions,2c that photoexcitation of the heterogenized (n-Bu4N)4W10O32 induces the oxidation of cyclohexane to the cyclohexyl radical according to eq 2. The spectrum obtained here indicates that there are strong interactions between the spin adduct and the surface of silica. In fact, comparison with the spectrum (16) (a) Janzen, E. G. Acc. Chem. Res. 1971, 4, 31. (b) Maldotti, A.; Amadelli, R.; Varani, G.; Tollari, S.; Porta, F. Inorg. Chem. 1994, 33, 2968. (17) Amadelli, R.; Maldotti, A.; Bartocci, C.; Carassiti, V. J. Phys. Chem. 1989, 93, 6448. (18) Buettner, G. R. Free Radical Biol. Med. 1987, 3, 259.
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Figure 4. Spin concentrations of the adducts [RO-pbn]• obtained after 220 s irradiation of suspensions of 1, 3, or 6 in CH3CN containing pbn (1 × 10-1 mol dm-3) and one of the desired alcohols (1 × 10-2 mol dm-3).
Figure 2. EPR spin trapping spectra of the adduct [C6H11pbn]• obtained after 220 s irradiation of (a) photocatalyst 1 in neat cyclohexane and (b) (nBu4N)4W10O32 (2 × 10-4 mol dm-3) in a CH2Cl2/CH3CN/C6H12 (6/3/1) solvent mixture. pbn concentration: 1 × 10-1 mol dm-3.
Figure 3. EPR signal intensity of the adduct [C6H11-pbn]• at a fixed field position during irradiation of 1 (squares) or 6 (circles) in neat cyclohexane containing pbn (1 × 10-1 mol dm-3).
recorded in the homogeneous phase (Figure 2b) shows a significant line broadening and the lack of any observable hydrogen interaction. This is an expected result since the formation and trapping of cyclohexyl radicals occurs within the diffusion layer. Double integration of the EPR signals allows us to calculate a spin concentration of 7 × 10-6 mol dm-3 after 220 s irradiation. This value accounts for at least 30% of the overall conversion of cyclohexane to oxygenated products after 4 h photoexcitation. Figure 3 reports the intensities of the EPR signal of the adduct between pbn and the cyclohexyl radical [C6H11pbn]• at a fixed field position during irradiation of 1 (squares) or 6 (circles). The growth of this signal is
significantly faster when the countercation is (nBu4N)+ than when it is Na+. In line with the results described in the previous paragraph, this result indicates that the organic countercation creates an environment around W10O324- which is favorable to the approach of cyclohexane. Oxidation of Alcohols. On the basis of the above results, we decided to extend the EPR spin trapping investigation to other substrates that may be easily oxidized by photoexcited W10O324- and, at the same time, are characterized by different dielectric constants. For this purpose, we chose the following alcohols: cyclohexanol, ethanol, and methanol. Alcohols are known to give strong interactions with polyoxotungstates, leading to the formation of association complexes.19 This complexation precedes photooxidation of these substrates, which takes place according to eq 8.4e Typical EPR experiments were carried out by irradiating 1, 3, or 6 suspended in CH3CN containing pbn (1 × 10-1 mol dm-3) and one of the mentioned alcohols (1 × 10-2 mol dm-3). In all cases, irradiation leads to the oxidation of alcohol to alkoxy radicals that were trapped by pbn to give broad triplets with aN values ranging from 14 to 14.5 G. Figure 4 shows the spin concentrations of the paramagnetic adducts between pbn and C6H11O•, C2H5O•, and CH3O• after 220 s irradiation of suspensions of 1, 3, or 6. When the counterion is nBu4N+, the photoreactivity decreases from the less polar alcohol to the more polar one in terms of dielectric constants. It is also evident that the amount of radicals obtained by cyclohexanol is significantly higher than those obtained by photooxidation of ethanol and methanol. That order is perfectly reversed with Na+ ions. This behavior may be ascribed to the fact that the nBu4N+ countercation makes the environment where the decatungstate is confined quite hydrophobic, and although methanol and ethanol should be easily oxidized on a thermodynamic basis,20 their approach to the surface-confined decatungstate is difficult. On the other hand, the interaction with the less polar and more organic cyclohexanol is largely favored. In the case of 6, the photoreactivity increases from the less polar alcohol to the more polar one. In fact, the inorganic ion Na+ is small and hydrophilic thus favoring the approach of methanol with respect to cyclohexanol. Once again, ethanol represents an intermediate situation between cyclohex(19) (a) Fox, M. A.; Cardona, R.; Gaillard, E. J. Am. Chem. Soc. 1987, 109, 6347. (b) Kothe, T.; Martschke, R.; Fischer, H. J. Chem. Soc., Perkin Trans. 1998, 2, 503. (20) Amadelli, R.; de Battisti, A.; Maldotti, A.; Bartocci, C.; Carassiti, V. Mater. Chem. Phys. 1987, 18, 57.
Decatungstates Heterogenized on Silica
anol and methanol, as its polarity is intermediate. The maximum spin concentrations of the adducts relative to methanol and ethanol are achieved with 3, which has counterions with intermediate characteristics that can help the approach of the alcohols to the decatungstate and simultaneously make the surface more polar. Conclusions This work is part of an effort whose broader purpose is the tuning of polyoxometalate-based catalysts for oxidation and electron-transfer processes. Evidence of anchoring of the decatungstates 1-6 on silica has been provided by spectroscopic and N2 adsorption experiments without any appreciable modification of the W10O324- anion. The size of the countercations of the decatungstate determines the degree of surface coverage and, consequently, the surface polarity of the support. W10O324- decatungstates heterogenized on amorphous silica provide stable photocatalytic systems, with different and tunable characteristics depending on the nature of the counterion. In particular, the described results show that the nature of the countercation plays a key role in
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determining the efficiency of the oxidation of cyclohexane to both radical species and stable oxygenation products. The cation makes the environment where the photoactive anion is localized more or less hydrophobic; this influences the possibility of the approach of the substrate and, consequently, the photocatalytic efficiency of the process. Some alcohols with increasing dielectric constants, like cyclohexanol, ethanol, and methanol, have been photooxidized in order to gain more information about the action of the different cations. It was found that with bulky and organic cations able to cover the surface at a high degree, alcohols were oxidized in the order from the less polar to the more polar, in terms of dielectric constants, while with inorganic cations, such as Na+, the order was reversed. Also, for the first time, we have direct evidence of the formation of alkoxy radicals as a consequence of alcohol oxidation by a photoexcited polyoxotungstate. Acknowledgment. This research was supported by MURST (Programmi di ricerca scientifica di rilevante interesse nazionale) and by CNR. LA0110141
