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GAS PHASE SELECTIVE PHOTOXIDATION OF ALCOHOLS USING LIGHT-ACTIVATED TITANIUM DIOXIDE AND MOLECULAR OXYGEN
SahleDemessie*, E AND U R. Pillai**. GAS PHASE SELECTIVE PHOTOXIDATION OF ALCOHOLS USING LIGHT-ACTIVATED TITANIUM DIOXIDE AND MOLECULAR OXYGEN. Presented at American Chemical Society, Orlando, FL, 04/7-11/02.
Gas Phase Selective Oxidation of Alcohols Using Light-Activated Titanium Dioxide and Molecular Oxygen
Gas phase selective oxidations of various primary and secondary alcohols are studied in an indigenously built stainless steel up-flow photochemical reactor using ultraviolet light and immobilized titanium dioxide film, prepared by dip-coating technique, as catalyst. The goal of Green Chemistry and Engineering is accomplished by forming the desired carbonyl compounds in a selective manner and by minimizing by-products and pollutants when compared to the conventional techniques. The system is found to be specifically suitable for the selective oxidation of primary and secondary aliphatic alcohols to their corresponding carbonyl compounds. Aromatic alcohols, however, form mainly secondary reaction products due to their particular orientation on the catalyst surface, which facilitates a stronger interaction of the surface active sites with the substrate molecule. The effects of different reaction parameters like oxygen/substrate ratio, water vapor, UV light and contact time on the conversion and product distribution are described. Presence of oxygen is found to be critical for the photo oxidation but its concentration seems to be not very important. Water vapor in the feed is also found to be helpful in the reaction although it is not as critical as in the case of hydrocarbon oxidations where it is necessary for hydroxylating the catalyst surface and sustaining the catalyst life. In the case of alcohol oxidation, surface hydroxylation can be partially provided by the hydroxyl groups of the alcohol itself. One disadvantage with the system is the catalyst deactivation, which is attributed to the surface accumulation of reaction intermediates and products. However, the catalyst regains its original activity after regeneration by calcining in air for 3h. Conversion and product distribution obtained are explained on the basis of different oxidation mechanisms involving a variety of nucleophilic and electrophilic reaction pathways.