Hydrogenation of p-chloronitrobenzene on Ni–B Nanometal Catalysts

Abstract

A series of Ni–B catalysts were prepared by mixing nickel acetate in 50% ethanol/water or methanol/water solution. The solution of sodium borohydride (1 M) in excess amount to nickel was then added dropwise into the mixture to ensure full reduction of nickel cations. The mol ratio of boron to nickel in mother solution was 3 to 1. The effects of preparation conditions such as temperature, stirring speed, and sheltering gas on the particle size, surface compositions, electronic states of surface atoms and catalytic activities of the Ni–B catalysts were studied. Ranel nickel catalyst was included for comparison. These catalysts were characterized by N2 sorption, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The catalysts were tested for liquid phase hydrogenation of p-chloronitrobenzene. All of the catalysts prepared in this study had nanosized particles. The preparation condition has significant influence on the particle size and surface compositions of the catalyst. The Ni–B catalyst was passivated by boron; therefore it was more stable than Raney nickel and did not catch fire after exposure to air. The catalysts prepared under N2 flow could suppress the oxidation of Ni by the dissolved oxygen in water and had metallic state of nickel. The catalyst prepared with vigorous stirring at 25°C under N2 stream yielded the smallest particles and resulted in the highest activity. It was much more active than the Raney nickel catalyst. The reaction condition also has pronounced effect on the hydrogenation activity. Using methanol as the reaction solvent increased p-chloronitrobenzene conversion to a large extent, compared to that using ethanol as the reaction medium. The selectivity of main product (p-chloroaniline) was greater than 99% on all of the Ni–B catalysts.