Superfine CoNi Alloy Embedded on Al2O3 Nanosheets for Efficient Tandem Catalytic Reduction of Nitroaromatic Compounds by Ammonia Borane
Aromatic amino compounds are one type of important and universally used chemical intermediate in a wide range of industrial fields. To produce them with high efficiency and selectivity under ambient conditions is expected and demanded in modern industry. Herein, a series of superfine CoNi alloy nanoparticles embedded on Al2O3 nanosheets (CoxNi1-x/Al2O3, x represents the contents of Co in precursor) catalysts have been fabricated from CoNiAl-LDH, and used to catalyze the tandem dehydrogenation of ammonia borane (AB) and hydrogenation of nitroaromatics to the corresponding amines. Systematic experiments indicate that the composition, size, morphology and catalytic performance of these CoxNi1-x/Al2O3 catalysts can be easily controlled by changing the contents of Ni in CoNiAl-LDH precursors. Particularly, the Co0.67Ni0.33/Al2O3 exhibits the best tandem catalytic performance among the six different samples. This as-prepared catalyst shows not only moderate turn-over-frequency value (TOF: 34.5 〖"mol" 〗_("H" _"2" ) 〖"mol" 〗_(〖"Co" 〗_"0.67" 〖"Ni" 〗_"0.33" )-1 min-1 at 298 K without bases or additives) and relatively low activation energy (32.4 kJ/mol) for the dehydrogenation of AB, but also superior catalytic activity (conversion yields reaching up to 100%) and selectivity (> 99%) for the tandem reductive transformation of excess sixteen kinds of nitroaromatics to aromatic amines. Density functional theory (DFT) calculations suggest that the construction of CoNi alloy optimized the electronic structure respect to the pure component, promoting its more active for AB hydrolysis and nitroaromatics hydrogenation. Finally, the catalyst could be easily recycled by magnets due to the magnetic properties of Co0.67Ni0.33 alloy.