dc.description.abstract | Amorphous NiB alloy catalysts are beneficial for the hydrogenation of p-chloronitrobenzene (p-CNB). They possess better catalytic activity and selectivity in the hydrogenation reaction. In this study, we used sodium borohydride to reduce the precursor salts of nickel acetate, cobalt acetate and additives (Mo, La, Fe and W), and prepared a series of modified Ni-B and NiCoB catalysts by chemical reduction method. The molar ratio of Ni/Co/B and Ni/additive was relatively 1:0.1:3 and 1:0.1, respectively, and an overdose of B was used to completely reduce the precursor salts. Ni catalysts were prepared in 50 vol.% methanolic solution at 298 K under N2 curtain gas with vigorous stirring of 500 rpm. In order to prevent the catalysts from oxidation and deactivation during the preparation, N2 curtain gas was used. The high activity of fresh catalysts was tested for the hydrogenation of p-CNB. In order to analyze the physical, chemical and surface properties, the as-prepared catalysts were characterized by nitrogen sorption (BET), X–ray diffraction (XRD), transmission electron microscopy (TEM) and X–ray photoelectron spectroscopy (XPS). The activity and selectivity of catalysts were tested for the liquid-phase hydrogenation reaction of p-CNB at 353 K and 1.2MPa H2 pressure under vigorous stirring of 500 rpm in a batch reactor (Parr Reactor Model 4842). The reaction medium was 80 ml methanol. During the run, the samples were withdrawn periodically (10 min) and analyzed by a gas chromatograph. The results revealed that adding the additives into the catalysts would enhance the thermal stability, dispersing effect of active sites, activity and selectivity. The selectivity of main product, p-chloroaniline (p-CAN) was more than 90%.
The results of the reaction showed that the performance of the modified NiFeB (1:0.1) was the best one between a series of Fe-doped NiB catalysts and the selectivity also increased significantly. For modified NiCoB catalysts, Mo- and W-doped ones were better than the other catalysts, and more obvious electron transfer contributed to the best activity and selectivity. In order to study the influence of Mo/Ni molar ratio on the catalytic activity of Mo-doped NiCoB catalysts, the molar ratio of Mo/Ni was modified and found that the Mo-NiCoB (0.6:1:0.1) catalyst had excellent hydrogenation activity. It achieved 100% conversion within 10 min with a selectivity of 98%.
In order to discuss the characterization of modified catalysts, the XRD patterns confirmed the formation of alloy. All the metals would form alloy and hence the major focus was on the phenomenon of electron transfer. Furthermore, the relationship between the particle size and additives was correlated. The particle size would decrease and the surface area would increase with the introduction of the additives. Therefore, the catalysts were in nano-scale range as confirmed from the TEM images and BET measurement. | en_US |