Hydrogenation of p-Chloronitrobenzene on Tungsten-Modified NiCoB Catalyst

Abstract

NiCoB has been reported to be a good catalyst for the hydrogenation of p-chloronitrobenzene (p-CNB). However, it aggregates easily. In this study, a series of tungsten-modified NiCoB catalysts with various tungsten contents were synthesized by the chemical reduction method using NaBH4 as the reducing agent. The products were characterized by X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The catalysts were tested for the liquid-phase hydrogenation of p-CNB. Ni, Co, W, and B were present in both elemental and oxidized states. The addition of tungsten could influence the surface composition of the catalyst. The electronic structures of Ni, Co, W, and B were also changed with changing content of tungsten. XPS showed that the elemental states of Ni, Co, W, and B formed an amorphous nanoalloy. The interactions of these species affected their electron densities and further influenced their catalytic activities. The tungsten oxide located among the NiCoB particles could act as a spacer that separates the NiCoB particles from their neighbors and inhibits aggregation. Compared to the unmodified NiCoB catalyst, the catalytic activity of tungsten-modified NiCoB exhibited an obvious increase, along with a slight decrease in the selectivity of p-chloroaniline (p-CAN). The differences in the atom radius and electronegativity of tungsten from those of the other elements in the alloy resulted in the formation of active sites for the catalytic reaction. The concentration of boron on the catalyst surface decreased with increasing tungsten concentration, which was responsible for the slight decrease in the selectivity of p-CAN. Considering the electronic effects, the structural effects, and the amount of B3+ (Lewis acid), good catalytic performance in the hydrogenation of p-CNB was achieved by modifying the NiCoB catalysts with the proper amount of tungsten. Tungsten species not only acted as a spacer to prevent NiCoB particles from aggregating, but also donated partial electrons to Ni and Co. An overdose of tungsten would cover the surface of Ni and result in low activity.