Facile fabrication of size-controlled Pd nanoclusters supported on Al2O3 as excellent catalyst for solvent-free aerobic oxidation of benzyl alcohol

Abstract

Selective oxidation of benzyl alcohol (BzOH) to benzaldehyde is one of the most important organic transformations. It is vital to explore a stable, economic and highly efficient catalyst. Here, Pd/R-Al2O3 (R = Al(NO3)3/urea molar ratio) was prepared by hydrothermal process followed by cation-assisted polyol method, and utilized for solvent-free aerobic oxidation of BzOH without any additives. Characterization results from TEM, NH3/CO2/O2-TPD and XPS showed that the size of Pd nanoclusters on Pd/R-Al2O3 was controlled by base number on R-Al2O3, and the order of oxygen mobility was Pd/1:6-Al2O3 > Pd/1:7-Al2O3 > Pd/1:8-Al2O3 > Pd/1:9-Al2O3. In the BzOH oxidation, R-Al2O3 as a basic additive could improve the benzaldehyde selectivity, and all surface atoms of Pd nanoclusters displayed the same catalytic activity. Excellent oxygen mobility indicated a high catalytic performance since it accelerated oxygen transfer from gaseous molecular oxygen to the surface of Pd nanoclusters to rapidly regenerate active Pd0 species. Among Pd/R-Al2O3 catalysts, Pd/1:6-Al2O3 exhibited a turnover frequency of 70381 h−1 due to the smallest Pd size and the best oxygen mobility, and good stability after five recycles owing to the strong interaction between Pd0 species and R-Al2O3. Therefore, Pd/R-Al2O3 may be great potential as a promising candidate for numerous catalytic reactions.