Green Oxidation of Alcohols to Carbonyl Compounds by Heterogeneous Photocatalysis

Abstract

The selective conversion of alcohols to carbonyl compounds is essential for the preparation of fragrances, food additives, and many organic intermediates.[1] This transformation is carried out by catalytic processes that are of fundamental importance in synthetic chemistry.[2] 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 [3] 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 [4,5] (POMs), typically oxides of molybdenum, tungsten, and vanadium. POMs are well-defined early transition metal-oxygen clusters with unique structural characteristics and catalytic properties. [6,7] Interestingly, many POMs share photochemical characteristics very similar to those of semiconductor photocatalysts, so that POMs represent the analogues of semiconductor metal oxides.[8] According to thermodynamics, an alcohol molecule with singlet electronic configuration cannot directly react with an unactivated dioxygen molecule, which has a triplet electronic configuration.[9] 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. [10][11][12][13] 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.[14] 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.[15-17] Au/Pd-TiO 2 catalysts showing high turnover frequencies (up to 270 000 turnovers per hour) have been also proposed.[18] These catalysts, u...

Homogeneous, Heterogeneous and Nanocatalysis

“…163 Nevertheless, photocatalysis has recently been used for the oxidation reactions of both aliphatic and aromatic alcohols and a review on the transformation of alcohols by heterogeneous photocatalysts was published fairly recently. 164 Pristine and doped titania have been mainly used as photocatalysts, owing to their reliability, stability under irradiation and low cost, although different semiconductor and insulator solids have also been used. Recently, Augugliaro and co-workers 165 investigated the UV photo-oxidation of piperonyl alcohol in water using a TiO 2 suspension.…”

Highly selective phenol production from benzene on a platinum-loaded tungsten oxide photocatalyst with water and molecular oxygen: selective oxidation of water by holes for generating hydroxyl radical as the predominant source of the hydroxyl group

“…The Pt/WO 3 photocatalysts exhibited significantly greater selectivity for phenol production than did the Pt/TiO 2 system, which showed high activity for oxidative degradation of benzene and other intermediates including phenols. A possible reason for the difference in reactivity between the two systems could be a 45 thermodynamically, even through the photoexcited electrons generated in the conduction band of WO 3 . The small amount of Pt loading (0.1 wt.%) also was effective for enhancing the rate of hydroxylation and oxidation of benzene on Pt/WO 3 -K photocatalysts, strongly suggesting the occurrence of multi-70 electron reduction of O 2 on Pt cocatalyst in the present system.…”

“…Chemical syntheses using semiconductor photocatalysts can be environmentally benign processes that possess great potential for reducing energy consumption in industrial production of useful chemicals by using light energy. [1][2][3] The high reduction and oxidation potentials of photoexcited electrons and holes, 30 respectively, generated on semiconductor particles such as TiO 2 can promote various chemical reactions, even endothermic reactions. Another advantage of photocatalytic organic syntheses is the availability of molecular oxygen (O 2 ) or water (H 2 O) as abundant and harmless reductants or oxidants, instead of 35 conventional, expensive reductants and oxidants that generally generate non-recyclable wastes requiring separation from the products.…”

Solar photocatalytic oxidation of NO by electronspun TiO2/ZnO composite nanofiber mat for enhancing indoor air quality

“…An advanced oxidation process (AOP) such as photocatalysis is a promising technology for air purification. Among the AOPs, photocatalytic oxidation (PCO) oxidizes pollutants to harmless H 2 O and CO 2 without the need for additional carrier gas . The process uses radicals generated from the catalyst in reaction with water vapor in air at ambient temperature and pressure.…”

“…The process uses radicals generated from the catalyst in reaction with water vapor in air at ambient temperature and pressure. A relatively inexpensive photocatalyst, titanium dioxide (TiO 2 ), has been demonstrated to give high removal rate for many pollutants such as hydrocarbons, chlorinated hydrocarbons, carbonyls, sulfur dioxide, nitrogen oxide and odor compounds …”