Insight into the selectivity of nano-catalytic nitroarenes reduction over other active groups by exploring hydrogen sources and metal components

Abstract

Catalytic reduction of nitroarenes to the corresponding anilines is a crucial reaction in the fine and bulk chemical industry, however, the control of chemoselectivity remains enormous challenges when additional reducible groups exist in the same molecule of nitroarenes. Herein, we summarize the breakthroughs over the recent 10 years in this review, especially the recent 5 years, which focus on the selectivity control of catalytic reduction of nitroarenes that bear other reducible functional groups, including nitrostyrene, halogenated nitrobenzene, nitrobenzaldehyde, nitroacetophenone, nitrobenzonitrile, and nitrophenylacetylene. We mainly discuss the origin of the catalytic selectivity, which are classified as natural selectivity, intrinsic selectivity, and preferential adsorption. We attempt to discover how the hydrogen sources and metal catalysts affect the catalytic selectivity, and how to design and fabricate efficient catalysts for the selective reduction of -NO2 group. The selectivity caused by hydrogen sources is assigned as natural selectivity, and it is interesting that the hydrogen source contains protonic active hydrogen has natural selectivity to nitro group. Through analyzing various reported experimental results, we have rendered such a blueprint about the selectivity sequence for diverse hydrogen sources: in some cases, the more protonic active hydrogen in the molecule, the better its natural selectivity towards nitro reduction (N2H4·H2O > HCOOH > NaBH4, NH3·BH3 > H2). Moreover, the catalytic selectivity of metal catalysts for reducing nitroarenes containing other reducible groups depends on two central factors: intrinsic selectivity and preferential adsorption. Various approaches to improve the selectivity of metal catalysts have been discussed, including the adjustment of the metal-support interactions, confinement of metals in the pores of the supports, modification of metals with non-metallic elements (N, S, P, etc.), construction of bimetallic alloys or intermetallics, formation of single-atom catalysts, and so on. This review will further strengthen the understanding of the selectivity for the reduction of substituted nitroarenes and provide guidance for the design and preparation of highly efficient and selective metal catalysts.