Solvent-free aerobic oxidation of benzyl alcohol over Pd monometallic and Au-Pd bimetallic catalysts supported on SBA-16 mesoporous molecular sieves

Pd monometallic and Au-Pd bimetallic catalysts supported on surface-functionalized SBA-16 were prepared by a conventional adsorption method and were examined using X-ray diffraction, nitrogen physisorption, UV-vis spectroscopy, and high-resolution transmission microscopy. SBA-16 with the unique "super-cage" structure effectively controlled the formation of dispersed noble metal nanoparticles in the mesoporous channels. These confined nanoparticles with a narrow particle size distribution exhibited excellent catalytic activity in the solvent-free benzyl alcohol selective oxidation with molecular oxygen. Amine-functionalization remarkably improved the selectivity towards benzaldehyde. Au-Pd bimetallic catalysts showed enhanced catalytic performance compared to the Au and Pd monometallic catalysts. The highest turnover frequency of 8667 h^-1 was achieved over a bimetallic catalyst with Au:Pd molar ratio of 1:5; this good catalytic activity can be maintained after five recycling runs. The characterization results of scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed that the bimetallic catalyst was constructed of uniformly alloyed nanoparticles with Pd clusteron-Au cluster structure. The synergetic effect between Au and Pd nanocluster was suggested to account for the better catalytic activity of bimetallic catalysts because the size-dependent effect can be ruled out due to the effective confinement of noble metal nanoparticles by SBA-16 mesostructure.