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

Controlling aldol condensation while suppressing dehydration to afford beta-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 beta-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 beta-hydroxy ketone for aldol condensation and the beta-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.