MIL-53 (Al) derived single-atom Rh catalyst for the selective hydrogenation of m-chloronitrobenzene into m-chloroaniline

The catalytic hydrogenation of halonitroarenes to haloanilines is a green and sustainable process for the production of key nitrogen-containing intermediates in fine chemical industry. Chemoselective hydrogenation poses a significant challenge, which requires the rational design of the catalysts with proper hydrogenation ability for nitro group and simultaneously preventing dehalogenation of halogen group. Herein, a highly effective Rh@Al 2 O 3 @C single-atom catalyst (SAC) was developed for the hydrogenation of m -chloronitrobenzene ( m -CNB) to m -chloroaniline ( m -CAN), through an in-situ grafting of metal during the assembly of MIL-53 (Al), followed by confined pyrolysis . Extensive characterizations reveal an exquisite structure of the Rh@Al 2 O 3 @C, containing atomically dispersed Rh sites onto Al 2 O 3 confined by the amorphous carbon. The five-coordinated aluminum (Al V ) species are essential for achieving the atomic dispersion of Rh atoms, providing the unsaturated coordinative sites for metal. Compared to the benchmark Rh/γ-Al 2 O 3 and Rh/C nanocatalysts, the Rh@Al 2 O 3 @C SAC affords an excellent turnover frequency of 2317 mol m -CNB ·mol Rh –1 ·h –1 , the highest value to date in heterogeneous catalyst systems for the hydrogenation of m -CNB at 313 K and 20 bar H 2 , together with a sustained selectivity to m -CAN (~98%) during five consecutive runs. The superior catalytic performance of the Rh@Al 2 O 3 @C is attributed to a proper modulation of electronic structure of hydrogenation metal by forming SAC, together with an enhanced accessibility of acid function sites.