Enhancing β-hydroxy ketone selectivity in the aldol condensation of furfural and acetone over N–B–O sites in calcined boron nitride

Abstract

Controlling aldol condensation while suppressing dehydration to afford β-hydroxy ketones is a potential and promising synthetic method for obtaining chiral alcohols. However, this strategy only works well in homogeneous processes, and further dehydration of the β-hydroxy ketone occurs irrepressibly in most heterogeneous systems. In this work, intrinsically inert BN calcined at a high temperature in air was found to be able to catalyze the aldol condensation of biomass-derived furfural and acetone to obtain hydrated 4-(furan-2-yl)-4-hydroxybutan-2-one (FAc-OH) with a high selectivity. Two types of oxygen-substituted nitrogen defect sites over air-calcined BN were convincingly identified and successfully correlated with the corresponding catalytic performance for aldol condensation with and without dehydration. A B-centered Lewis acid-catalyzed mechanism was proposed, where the N–B–O sites can selectively produce the β-hydroxy ketone for aldol condensation and the β-hydroxy ketone is more likely to further dehydrate at the O–B–O sites. The controlled synthesis of the two distinct active sites offers an efficient way to switch the distribution of dehydrated and hydrated products produced by aldol condensation.