N-Monoalkylation of Amines with Alcohols by Tandem Photocatalytic and Catalytic Reactions on TiO2 Loaded with Pd Nanoparticles

TiO2 loaded with Pd particles (Pd/TiO2), when photoirradiated at lambda > 300 nm in alcohol containing primary amine, efficiently promotes N-monoalkylation of amine with alcohol, producing the corresponding secondary amine with almost quantitative yields. This occurs via tandem photocatalytic and catalytic reactions: (i) Pd-assisted alcohol oxidation on the photoactivated TiO2, (ii) condensation of the formed aldehyde with amine on the TiO2 surface, and (iii) hydrogenation of the formed imine by the surface H atoms on the Pd particles. The rate-determining step is the imine hydrogenation, and the reaction depends strongly on the size of the Pd particles. The catalyst with 0.3 wt % Pd, containing 2-2.5 nm Pd particles, shows the highest activity for imine hydrogenation, and smaller or larger Pd particles are inefficient. Calculations of the number of surface Pd atoms based on the cuboctahedron particle model revealed that the Pd atoms on the triangle site of Pd particles are the active site for hydrogenation. Larger Pd, particles contain a larger number of these Pd atoms and are effective for imine hydrogenation. Alcohols, however, are strongly adsorbed onto the larger triangle site and suppress imine hydrogenation. As a result of this, the catalyst with 2-2.5 nm Pd particles, which contains a relatively larger number of Pd atoms on the triangle site and does not promote strong alcohol adsorption, shows the highest activity for imine hydrogenation and promotes efficient N-monoalkylation of amine with alcohol under photoirradiation.