Polyhedron Vol. 7, No. 6, pp. 477481, 1988 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Printed in Great Britam 0277-5387/M $3.00+.00 0 1988 Pergamon Press plc zyxwvutsr SELECTIVE PHOTOCATALYTIC OXIDATION OF ALCOHOLS BY HETEROPOLYTUNGSTATES A. HISKIA and E. PAPACONSTANTINOU” Chemistry Department, N.R.C. Demokritos, Athens, Greece zyxwvutsrqponmlkjihgfedc (Received 29 M ay , 1987 ; accepted after revision 13 October 1987) Abstract-A series of primary, secondary, saturated and unsaturated alcohols have been selectively oxidized photocatalytically to the corresponding aldehydes and ketones by a series of heteropolytungstates using near visible and UV light. Stoichiometrically, one photon adds two electrons onto the heteropolytungstates and produces one molecule of aldehyde or ketone. Using oxygen-saturated solutions, the photocatalytic cycle is obtained at the early stage of the photoredox reaction, with optimization of the quantum yield for the oxidized organic product being N 0.1. After at least 20 photocatalytic cycles, when the production of the organic oxidation product was 20 times the amount of catalyst, there was no sign of decomposition of the catalyst. In the process of converting solar energy into chemical fuel homogeneously, several systems have been used that are based on a photosensitizer (S) for the absorption of light, and a relay (R) which carries the electrons of the photoredox reactions and delivers them for the production of fuel, for instance reduction of H+ into HZ, in the presence of a Pt catalyst, according to the scheme S+R%S++R- R-+H+%R+1/2H2 S+ + EDTA -+ S + EDTA(ox.). The process involves, as is well known, an organic compound (mainly EDTA and/or TEA) which is consumed (oxidized) during the process (sacrificial reagent) to revert the sensitizer to its original form.2 The system that has produced the most sat- isfactory results and has been the focus of many prominent laboratories is based on Ru(bpy):+ as the sensitizer and MV2+ as the relay, using the above mentioned sacrificial reagents. Polyoxometalates especially of the keggin type, i.e. [XW1204,J- (XW;;), (in this paper X = P, Si, Fe, H2)3, and Dawson structures i.e. [P2W i806J6 *Author to whom correspondence should be addressed. (P2Wf<),” have been shown to be effective in an analogous process for H2 production5 and conver- sion of sun light into electricity (photogalvanic cells).6 As has been mentioned elsewhere, they pres- ent the advantage that, unlike other systems, the polyoxometal acts as a sensitizer, relay and cata- lyst, with the main drawback being the limited effective absorption of the visible light (< 400 nm). In the process, they also use organic species that are oxidized during the process. Another ad- vantage is that the organic species are not sacrifi- cial reagents, in the sense that the oxidized pro- ducts are useful reagents, so that the process of hydrogen production can be converted to an effective photocatalytic oxidation of certain organic compounds. A previous communication has outlined the basic principles of the process.7 During the course of this investigation other works have been published on the subject. Generally their results, whenever there is an overlap in the investigation, are in good agree- ment with ours.8 EXPERIMENTAL Polyoxometalates were prepared according to well established methods.’ Aqueous solutions of polyoxometalates in the presence of organic com- pounds were photolyzed with a high pressure Hg arc and a 150 W Xe lamp using filters, in the pres- 477