Construction of intimate Lewis acid and basic sites on an Al2O3–NC composite catalyst with enhanced performance in transfer hydrogenation of cinnamaldehyde

Catalytic transfer hydrogenation is an effective means of producing allylic alcohols from α,β-unsaturated carbonyl compounds with extremely high selectivity. A composite catalyst bearing Lewis acid and base sites was prepared from Al(NO3)3 and dopamine hydrochloride in a one-pot synthesis followed by thermal treatment in nitrogen. The resultant catalyst exhibited remarkably high performance in transfer hydrogenation of cinnamaldehyde (CAL) to selectively produce cinnamyl alcohol (COL) using isopropanol as both hydrogen donor and solvent. A CAL conversion of 99.0% with COL selectivity of 98.9% was achieved under optimal conditions. This indicates that the nitrogen functional groups on N-doped carbon (NC), particularly pyrrolic-N and graphitic-N, significantly enhanced the catalytic efficiency of Al2O3. The enhancement of pyrrolic-N and graphitic-N in the vicinity of Al2O3 might be attributed to their concerted function in helping to deprotonate isopropanol. Based on the catalyst poisoning results, a catalytic mechanism with Lewis acid–base pairs was tentatively proposed. In addition, the composite catalyst exhibited high performances in the transfer hydrogenation of other α,β-unsaturated carbonyl compounds, including furfural, benzaldehyde, cyclohexanone, and syringaldehyde, to selectively produce the corresponding allylic alcohols.