Amphiphilic Mesoporous Sandwich-Structured Catalysts for Selective Hydrogenation of 4-Nitrostyrene in Water.

Selective catalytic hydrogenation of substituted nitro compounds (NCs) of hydrophobic nature in aqueous solution using transition-metal-based catalysts is highly desirable yet fairly challenging. Herein, we propose the idea of amphiphilic mesoporous catalysts for selective hydrogenation of hydrophobic NCs in aqueous solution. The amphiphilic catalyst Co@Co-N-C@SBA-15 with a sandwich-like structure is constructed by a one-step solvent-free melting coating method. The catalyst has an external hydrophilic silica support that facilitates catalyst dispersion in water. It has unique Co-N-C catalytic layers uniformly coated in the inner mesopore surfaces of the silica support, which enhance the selective adsorption and activation of hydrophobic NCs. It has a high surface area (448.2 m/g) and a uniform mesopore size (∼7.0 nm) for fast mass transportation. It possesses ultrafine metallic Co nanoparticles uniformly anchored within the N-doped carbon (N-C) layers for easy magnetic separation. These features make the catalyst excellent for the selective hydrogenation of 4-nitrostyrene to form 4-aminostyrene, with a high conversion of 98.0% in 1.0 h, a superior selectivity of 98.8%, and a good stability under mild conditions. A comprehensive study confirms the excellence of the amphiphilic mesoporous catalysts compared with other control catalysts. The Co-N sites are the intrinsic active sites. They can selectively adsorb and activate the nitro groups other than the vinyl groups, leading to superior selectivity. Water as the solvent results in the best performance compared with typical organic solvents probably because of an enhanced water-mediated hydrogen spillover and transfer.