Visible-Light-Induced Aerobic Oxidation of Alcohols in a Coupled Photocatalytic System of Dye-Sensitized TiO2 and TEMPO

摘要

Selective catalytic oxidation of alcohols is a fundamental transformation for the synthesis of fine chemicals. Catalysts based on transition metals, such as palladium, platinium, and copper, have been used for efficient aerobic oxidation of alcohols. Selective transformation of alcohols by dioxygen can also be achieved in the presence of both transition-metal ions and nitroxyl radicals (such as 2,2,6,6-tetramethylpiper-idinyloxyl, TEMPO). However, the use of large amounts of transition-metal complexes could potentially lead to environmental pollution. Therefore, considerable efforts have been made to find a clean catalytic system without transition metals for the aerobic transformation of alcohols. Recently, a novel transition-metal-free TEMPO-based catalysis system was developed, in which Br2 and NaNO2 were used as cocatalysts, and the effective oxidation of alcohols by molecular oxygen was accomplished at 80°C. To facilitate the removal of the TEMPO catalyst after use, a heterogeneous variant in which TEMPO is supported on SBA-15 was prepared; this, together with NaNO2 as cocatalyst, was also used successfully for the selective oxidation.