Cascade hydrogenation of nitrobenzene to dicyclohexylamine with Pd/γ-Al2O3: The role of acid sites

Abstract

Direct hydrogenation of nitrobenzene (NB) to dicyclohexylamine (DCHA) is a green but challenging process due to the competition of cross-coupling reaction (to DCHA) and hydrogenation reaction (to byproduct). Herein, we report that the cross-coupling reaction to desired DCHA is greatly enhanced by tuning the acidity of supported Pd catalysts. It was found that Pd NPs loading on commercial γ-Al2O3 (γ-Al2O3-Com) with moderate acidity were more selective than those on neutral SBA-15 or strongly acidic γ-Al2O3 nanorods. The presence of acid site could promote the coupling reaction. By analyzing the correlation between hydrogenation activity and DCHA selectivity, it was found that the matched rate of hydrogenation and cross-coupling is the key factor to achieve high selectivity. This study may open a new avenue for the hydrogenation of nitroarenes to DCHA by regulating the acidity of supports and the reaction conditions.