Green oxidation of alcohols to carbonyl compounds by heterogeneous photocatalysis.

The selective conversion of alcohols to carbonyl compounds is essential for the preparation of fragrances, food additives, and many organic intermediates. This transformation is carried out by catalytic processes that are of fundamental importance in synthetic chemistry. Catalytic oxidation has long been carried out in environmentally harmful organic solvents (often chlorinated ones) at high temperatures and pressures by employing stoichiometric amounts of various inorganic oxidants, for example chromate and permanganate species, that play the role of oxygen donors. The latter oxidants are not only expensive and toxic but also produce large amounts of dangerous wastes, so that investigations have been concerned with substituting them for safer systems. In recent years, significant progress has been made in the development of effective catalytic processes for the aerobic oxidation of alcohols with environmentally benign and inexpensive oxidants such as oxygen or air. Typically, the aerobic oxidation of alcohols involves the use of catalysts based on platinum-group metals and transition metal compounds. The latter are (1) transition metal complexes, which comprise a central metal atom and surrounding organic ligand(s) ; and (2) inorganic catalysts, mainly polyoxometalates (POMs), typically oxides of molybdenum, tungsten, and vanadium. POMs are well-defined early transition metal–oxygen clusters with unique structural characteristics and catalytic properties. 7] Interestingly, many POMs share photochemical characteristics very similar to those of semiconductor photocatalysts, so that POMs represent the analogues of semiconductor metal oxides. According to thermodynamics, an alcohol molecule with singlet electronic configuration cannot directly react with an unactivated dioxygen molecule, which has a triplet electronic configuration. The working mechanism for all metal oxidation catalysts is that the metal atom mediates the electron transfer, and thereby induces the formation of singlet oxygen. In this respect, a wide range of homogeneous metal catalysts has been found capable to use molecular oxygen as the only oxidant. For example, a water-soluble palladium(II) batho-phenanthroline complex has been used as catalyst at a pressure of 30 bar and a temperature of 373.16 K for the aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic water/alcohol system. The alcohol conversion was almost complete and the yield of carbonyl compounds was in the range 79–90%, depending on the substrate. The use of homogeneous catalysts, however, offers obvious disadvantages over heterogeneous systems with respect to ease of handling and catalyst recycling. A wide range of supported platinum and palladium catalysts has long been reported to exhibit high catalytic performance in the oxidation of alcohols. Au/Pd-TiO2 catalysts showing high turnover frequencies (up to 270000 turnovers per hour) have been also proposed. These catalysts, used for carrying out the oxidation of benzyl alcohol to benzaldehyde, showed an alcohol conversion of 74.5% and an aldehyde selectivity of 91.6%, benzyl benzoate being the only detected byproduct. Even if the catalytic systems have proven rather effective, from a green chemistry perspective their application is often limited by environmental risks. Consequently the search for economic and safe reactions to afford selective oxidation of alcohols using molecular oxygen but avoiding high pH values and the presence of metal complexes catalysts has attracted great attention. In this context the search for an oxidation catalyst capable of directly activating dioxygen is an interesting as well as challenging task. Heterogeneous photocatalysis (HP) by polycrystalline semiconductor oxides is an unconventional technology that has been mainly applied in the field of environmental remediation to degrade organic and inorganic pollutants in air and wastewaters. TiO2 is the most-often-used photocatalyst, owing to its reliability, low cost, and (photo)stability under irradiation, although different semiconductor and insulator solids have also been used as photocatalysts. Some of them, for example ZnO, give rise to anodic photocorrosion in water and consequently they can not be used safely. In addition to decontamination, HP has been applied for carrying out many selective photocatalytic reactions in the absence of solvent or in nonaqueous solvents such as acetonitrile or water/acetonitrile mixtures. Heteropolyoxometalate catalysts of the type [S2M18O62] 4+ (M=W, Mo) have been used to carry out the photo-oxidation of aromatic alcohols under sunlight and UV/Vis light in acetonitrile. Mechanistic investigations of photocatalysis by these nontoxic compounds indicate that near-UV/Vis irradiation of a POM solution results in an O-to-M charge-transfer excited state that has strong oxidation ability and is responsible for the oxidation of organic substrates, photoexcited POMs [a] Prof. V. Augugliaro, Prof. L. Palmisano “Schiavello-Grillone” Photocatalysis Group Dipartimento di Ingegneria Chimica dei Processi e dei Materiali Universit di Palermo Viale delle Scienze, 90128 Palermo (Italy) Fax: (+39)091-7025020 E-mail : [email protected]