Insight into the Effective Aerobic Oxidative Cross-Esterification of Alcohols over Au/Porous Boron Nitride Catalyst

摘要

Boron nitride (BN) has attracted great attention with an unexpected ability in aerobic catalysis. Still, its related probe reactions are relatively rare, and the effect of the BN-supported metal catalyst on O-2 activation is still ambiguous, and opinions are varied. In this work, the porous BN (pBN)-supported Au catalyst with a porous structure and exposed edges exhibits high activity in the oxidative cross-esterification reactions between the aromatic and C-1-C-3 aliphatic alcohols at ambient temperature. The turnover frequency value for methyl benzoate is 118 h(-1) at 30 degrees C, and the calculated apparent activation energy (Ea, 58 kJ/mol) is comparable to that of AuPd/TiO2, Ru/Al2O3, and PdBiTe catalysts. Combined with temperature-programmed desorption (TPD) results, the loading of Au enhances the desorption of O-2 and the interaction with alcohols; thus, a synergistic effect between the O-rich pBN and Au is considered. The free-radical scavenger can dramatically suppress the conversion (similar to 6%), suggesting that the reaction proceeds via the O-2* radicals. According to the vibration of nu(O-O), delta(OO-H), and nu(B-O-O-B) detected by attenuated total reflectance-infrared spectroscopy (ATR-IR), we are prone to consider the oxygen activation route by the edge B atoms. Then, a possible L-H reaction mechanism was proposed: benzyl alcohol and O-2 adsorb on the Au/pBN initially, then O-2 is converted to O-2*, and the alpha-H elimination proceeds; as the semi-acetal formed, another alpha-H elimination proceeds and methyl benzoate is finally formed.