Inorg. Chem. 1996, 35, 3925-3930
3925
Photocatalytic Oxidation of Aromatic Hydrocarbons Yun Mao and Andreja Bakac* Ames Laboratory, Iowa State University, Ames, Iowa 50011 ReceiVed February 8, 1996X
In acidic aqueous solutions UO22+ serves as a photocatalyst (λirr g 425 nm) for the oxidation of benzene by H2O2. Under conditions where 50% of the excited state *UO22+ is quenched by H2O2 (k ) 5.4 × 106 M-1 s-1) and 50% by benzene (k ) 2.9 × 108 M-1 s-1), the quantum yield for the formation of phenol is 0.70. The yield does not change when benzene is replaced by benzene-d6, but decreases by a factor of ∼4 upon the change of solvent from H2O to D2O. Photocatalytic oxidation of toluene by UO22+/H2O2 produces PhCHO, PhCH2OH, and a mixture of cresols with a total quantum yield of 0.28 under conditions where 50% of *UO22+ is quenched by H2O2. The quenching of *UO22+ by benzene and substituted benzenes takes place with k > 108 M-1 s-1. The system UO22+/t-BuOOH/C6H6/hν does not result in the oxidation of benzene, but instead yields methane and ethane.
Introduction Catalytic oxidation of hydrocarbons has scientific and practical importance. Biological systems that utilize dioxygen or hydrogen peroxide as oxidants are catalyzed by oxygenases and peroxidases, respectively.1-3 Laboratory and industrial oxidation of hydrocarbons utilize transition metal complexes as catalysts. Many of the reactive species involved are analogous to those encountered in the biological systems and include superoxo, peroxo, and oxo metal complexes, as well as metalfree intermediates such as O2•-/HO2• and HO•.3-7 Hydrogen peroxide has been explored extensively as a hydroxylating agent. Catalysts are needed because the reactivity of H2O2 toward hydrocarbons is low. Transition metal complexes usually activate H2O2 by converting it into reactive metal peroxy and hydroperoxy species,3,8-10 or to hydroxyl radicals in Fenton-type reactions,11-14 eqs 1-2. Photochemical cleavage of H2O2 to hydroxyl radicals, eq 3, has also been explored.15
Mn + H2O2 f M(OO)n-2 + 2 H+ (or M(OOH)n-1 + H+) (1) Mn + H2O2 f Mn+1OH + HO• hν(UV)
H2O2 98 2HO•
(2) (3)
The reaction of HO• radicals with benzene produces cyclohexadienyl radicals, which undergo a number of reactions, X Abstract published in AdVance ACS Abstracts, June 1, 1996. (1) Karlin, K. D. Science 1993, 261, 701. (2) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry; University Science Books: Mill Valley, CA, 1994. (3) Simandi, L. I. Catalytic ActiVation of Dioxygen by Metal Complexes; Kluwer Academic Publishers: Dordrecht, The Netherlands/Boston, MA/London, 1992. (4) Shilov, A. E. ActiVation of Saturated Hydrocarbons by Transition Metal Complexes; D. Reidel Publishing Company: Dordrecht, The Netherlands/Boston, MA/Lancaster, England, 1984. (5) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; Wiley: New York, 1992; Chapter 10. (6) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic: New York, 1981. (7) (a) Ito, S.; Yamasaki, T.; Okada, H.; Okino, S.; Sasaki, K. J. Chem. Soc., Perkin Trans. 2 1988, 285. (b) Ito, S.; Kunai, A.; Okada, H.; Sasaki, K. J. Org. Chem. 1988, 53, 296. (c) Jintoku, T.; Taniguchi, H.; Fujiwara, Y. Chem. Lett. 1987, 1865. (d) Kimura, E.; Machida, R. J. Chem. Soc., Chem. Commun. 1984, 499.
S0020-1669(96)00142-5 CCC: $12.00
depending on the catalyst and reaction conditions. Both oxidation states of the catalyst usually react with cyclohexadienyl radicals, as shown in eqs 4-6 using Fe3+/Fe2+ as example. The yield of phenol therefore depends on the redox properties of the catalyst and on the relative concentrations of the oxidized and reduced forms of the catalyst.
C6H6(OH)• + Fe3+ f C6H5OH + Fe2+ + H+
(5)
C6H6(OH)• + Fe2+ + H+ f C6H6 + Fe3+ + H2O (6) Much less is known about the mechanism of oxidation of C6H6 by peroxo-metal species, but of those studied so far, peroxo-vanadium complexes appear to be the most efficient in hydroxylating aromatic hydrocarbons.8-10 All the mechanistic studies agree that these processes involve radicals. Some workers proposed an intramolecular electron transfer resulting in a VIV(O2•-) intermediate, which then inserts into a C-H bond.10 Others8 suggested the involvement of a one-electron reduced peroxovanadium complex. We have recently used UO22+ as a photocatalyst for the oxidation of aliphatic hydrocarbons and toluene by molecular oxygen.16,17 All of the reactions studied in that work involve a hydrogen atom abstraction by *UO22+ from the substrate, as demonstrated by the kinetic isotope effects and the nature of the products. As expected, benzene was unreactive, owing to the strength of the C-H bonds and the ease with which benzene quenches *UO22+ in a chemically unproductive reaction. (8) Bonchio, M.; Conte, V.; Di Furia, F.; Modena, G. J. Org. Chem. 1989, 54, 4368. (9) Butler, A.; Clague, M. J.; Meister, G. E. Chem. ReV. 1994, 94, 625. (10) Mimoun, H.; Saussine, L.; Daire, E.; Postel, M.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1983, 105, 3101. (11) Walling, C. Acc. Chem. Res. 1975, 8, 125. (12) Goldstein, S.; Meyerstein, D.; Czapski, G. Free Radical Biol. Med. 1993, 15, 435. (13) Czapski, G.; Ilan, Y. A. Photochem. Photobiol. 1978, 28, 651. (14) Oturan, M. A.; Pinson, J. J. Phys. Chem. 1995, 99, 13948. (15) Lunak, S.; Sedlak, P. J. Photochem. Photobiol. A 1992, 68, 1. (16) Wang, W.-D.; Bakac, A.; Espenson, J. H. Inorg. Chem. 1995, 34, 6034. (17) Mao, Y.; Bakac, A. J. Phys. Chem. 1996, 100, 4219.
© 1996 American Chemical Society
3926 Inorganic Chemistry, Vol. 35, No. 13, 1996
Mao and Bakac
We have now explored the use of H2O2 as oxidant in the photochemical UO22+/benzene system. Two fundamentally different mechanisms might be expected to operate. U(VI) is known to form complexes with H2O2,18-21 and chemistry similar to that reported for the peroxovanadium complexes is possible if photoexcitation is provided. (In the absence of light, the peroxouranium complexes are ineffective as oxidants.)22 On the other hand, H2O2 may reduce the photoexcited UO22+ to UO2+, which would then engage in Fenton-type chemistry. The results of a kinetic and mechanistic study of the oxidation of benzene and several other aromatic hydrocarbons by UO22+/ H2O2/hν are reported herein. A brief study of the oxidation of UO2+ by H2O2 has also been conducted. Experimental Section Chemicals. Benzene, toluene and p-xylene (Aldrich) were distilled prior to use. Phenol, benzaldehyde, p-benzoquinone, p-tolualdehyde, 4-methylbenzyl alcohol, phthalic dicarboxaldehyde, p-toluic acid, 1,3,5mesitylene (all Aldrich), and 3,5-dimethylbenzaldehyde (Lancaster) were used without further purification. Stock solutions of uranyl perchlorate were prepared by dissolving uranium trioxide (Strem Chemicals, 99.8%) in aqueous perchloric acid. Hydrogen peroxide (Fisher) and tert-butyl hydroperoxide (Aldrich) were used as received. Instrumentation. Product analyses were carried out by use of a Waters high performance liquid chromatograph, equipped with a C18 column and a photodiode array detector (Waters 996), which simultaneously records the chromatogram and the absorption spectrum. The eluent was usually the 40% aqueous acetonitrile. In some experiments the proportion of acetonitrile was changed for better separation of products. The GC-MS spectrometer (Magnum, Finnigan MAT) was equipped with a capillary column (DB5, 0.25 mm i.d. and 0.25 µm film), EI source, and an ion trap assembly and operated by use of ITS40 software package. Methane and ethane were detected by use of a gas chromatograph (Hewlett-Packard, Model 5790) equipped with a flame ionization detector and a VZ-10 column. 1H NMR and UV absorption spectra were recorded by use of Varian 300 NMR and Shimadzu 3101 PC spectrometers, respectively. Molecular oxygen was quantitated by use of a YSI biological oxygen monitor (Model 5300) with a DAQdata acquisition software package. Time-resolved experiments were performed with use of a flash-lamp pumped dye-laser photolysis system described earlier.16 The dyes used were LD 423 and LD 490. Most of the reactions were monitored by observing the luminescence of *UO22+ at 515 nm. In some experiments the concentration of *UO22+ was obtained from the absorbance at 580 nm ( ) 4500 M-1 cm-1).16,23 Steady-state irradiations used a 250-W quartz tungsten halogen lamp (Oriel Corporation), equipped with a beam turning assembly. The irradiation wavelength was adjusted to >425 nm by use of a Corning 3-67 filter. Sample Preparation. Aqueous solutions of UO22+ at the desired pH (adjusted with H3PO4) were placed in a 1 cm quartz cell and sealed with a gastight septum. The appropriate gas (argon, oxygen, or air) was bubbled through the solution for 20 min, followed by injection and dissolution of the substrate. After photolysis, the reaction solution was introduced directly into the HPLC chromatograph. For GC-MS and 1H NMR spectra, several samples were combined and concentrated by extraction with diethyl ether. All experiments were carried out at room temperature. Solutions of UO2+ were prepared by the reduction24 of UO22+ (0.25 mM) by substoichiometric amounts of Cr(H2O)62+ (0.1 mM) in 5 mM (18) Thompson, M. E.; Nash, K. L.; Sullivan, J. C. Isr. J. Chem. 1985, 25, 155. (19) Sullivan, J. C.; Gordon, S.; Cohen, D.; Mulac, W.; Schmidt, K. H. J. Phys. Chem. 1976, 80, 1684. (20) Djogic, R.; Raspor, B.; Branica, M. Croat. Chim. Acta 1993, 66, 363. (21) Djogic, R.; Branica, M. Electroanalysis 1992, 4, 151. (22) Westland, A. D.; Tarafder, M. T. H. Inorg. Chem. 1981, 20, 3992. (23) Burrows, H. D. Inorg. Chem. 1990, 29, 1549. (24) Ekstrom, A. Inorg. Chem. 1973, 12, 2455.
Figure 1. Effect of the initial concentration of C6H6 on the yield of phenol. Conditions: [UO22+] ) 0.25 mM, [H2O2] ) 0.2 M, [H3PO4] ) 0.1 M, and irradiation time 60 min. HClO4. The kinetics of the UO2+/H2O2 reaction were monitored at the 255 nm maximum of UO2+.25
Results Identification of the Photochemical Product. A 60-min steady-state photolysis of a solution containing 0.25 mM UO22+, 0.2 M H2O2, and 3.7 mM C6H6 produced a single new peak in the HPLC chromatogram. The intensity of the peak increased linearly with irradiation time. The retention time and the UV spectrum of the product coincided with those of phenol. The GC-MS and 1H NMR spectra confirmed this assignment. The new component in the GC-MS chromatogram yielded a mass spectrum identical to that of phenol. The 1H NMR spectrum exhibits multiplets at δ 6.85 and 7.25 ppm (phenol), in addition to a singlet at 7.26 ppm corresponding to unreacted benzene. The quantum yield of phenol (ΦPhOH) was determined indirectly by comparison with the known quantum yield of benzaldehyde (ΦPhCHO ) 0.01)17 produced under identical experimental conditions in the system toluene/UO22+/O2. The experiments were conducted under “standard” reactions conditions, see later, such that 50% of *UO22+ was quenched by H2O2, and the rest by benzene. Similar experiments in the toluene/UO22+/H2O2 system yielded ΦPhCHO ) 0.11, approximately 10 times greater than that obtained with O2 as oxidant. Most of the product analyses were conducted after 1-2 h of irradiation at
