Simultaneous production of aromatic aldehydes and dihydrogen by photocatalytic dehydrogenation of liquid alcohols over metal-loaded titanium(IV) oxide under oxidant- and solvent-free conditions

Photocatalytic conversion of aromatic alcohols in acetonitrile (CH3CN) suspensions of metal-loaded titanium(IV) oxide under various conditions was examined. Benzaldehyde (PhCHO) and hydrogen (H2) were simultaneously produced with a molar ratio of 1:1 from benzyl alcohol (BnOH) in CH3CN suspensions of platinum-loaded titanium(IV) oxide (Pt–TiO2) under deaerated conditions and BnOH was converted quantitatively with high apparent quantum efficiency of 38% at 366 nm. The yield of PhCHO in dehydrogenation under deaerated conditions (>99%) was much higher than that (53%) obtained by dehydrogenation of BnOH in the presence of oxygen accompanying the formation of water instead of H2. When a small amount of water was intentionally added to the CH3CN suspension of Pt–TiO2, reaction rates of hydrogenation drastically decreased. Control experiments carried out under air and in the presence of water indicated that an oxygen-free condition was important for quantitative conversion of BnOH to PhCHO in two points, i.e., the reaction under oxygen-free conditions yields no water and active oxygen, that caused drastic decrease in the reaction rate and undesired oxidation of BnOH and PhCHO, respectively. Photocatalytic dehydrogenation of BnOH to PhCHO occurred even in a solvent-free condition in which Pt–TiO2 particles were suspended in BnOH. Photocatalytic hydrogenation under deaerated conditions was applied for conversions of various aromatic alcohols to corresponding aldehydes.

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► Photocatalytic conversion of aromatic alcohols in CH3CN suspensions of Pt–TiO2 was examined.
► Benzyl alcohol (BnOH) was quantitatively converted to benzaldehyde (PhCHO) and H2.
► Apparent quantum efficiency of the conversion reached 38% at 366 nm.
► An oxygen-free condition was important for quantitative conversion of BnOH to PhCHO.
► Photocatalytic dehydrogenation of BnOH occurred even in a solvent-free condition.