| dc.description.abstract | Nanosized NiCoB alloy catalyst has been reported to be a good catalyst for the liquid phase hydrogenation reactions due to their excellent activity and selectivity. In this research, Two investigations were carried out on the NiCoB catalyst.
First, the effects of gas-holdup, external mass transfer, intra-particle mass transfer were studied based on the theoretical calculations. The results demonstrated that the hydrogenation reaction under the experimental conditions we adopted was reaction-controlled.
Second, W was used as the additive to modify the properties of NiCoB catalyst. A series of W–doped NiCoB catalysts with various W/Ni molar ratios were synthesized by chemical reduction of nickel acetate, cobalt acetate and ammonium metatungstate. Sodium borohydride in 50 vol. % methanol/water mixed solvent was used as the reducing agent. The preparation was carried out at room temperature under vigorous stirring and nitrogen stream was used as the curtain gas to remove dissolved oxygen in solvent. The atomic ratio of Ni/Co was fixed at 10; and W/Ni ratios were 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0, respectively. In order to fully reduce Ni and Co cations, excess amount of NaBH4 was added (B/Ni atomic ratio was 3). These catalysts were characterized by X–ray diffraction, transmission electron microscopy, high resolution transmission electron microscopy, energy dispersive spectra, X-ray photoelectron spectroscopy and differential scanning calorimetry. The catalytic properties of W-NiCoB catalysts were studied on hydrogenation of p–chloronitrobenzene (p-CNB). The conditions for hydrogenation reaction were 1.2 MPa H2 pressure, 353 K reaction temperature and 500 rpm stirring speed. Methanol was used as the solvent, the concentration of p-CNB was 0.2 M (2.54g p–CNB in 80 ml methanol) and the amount of W-NiCoB catalyst was 0.002 mol. The results showed that the increase of W content made the particle size of NiCoB smaller. W was mainly in the form of hydroxide and acted as a spacer, which could keep NiCoB in amorphous state, and would suppress sintering in reaction. The sample with W/Ni atomic ratio of 0.7 had the highest activity for p–CNB hydrogenation. The selectivities of p-chloroaniline (p-CAN) for W-NiCoB catalysts were also very high. It demonstrated that W-doped NiCoB catalysts were suitable for liquid phase hydrogenation catalyst. Then, the deactivation tests to compare the catalytic life of NiCoB, W-NiCoB and Mo-NiCoB (the best result done in our lab) were also studied. The results showed that only Mo additive could extend the life of NiCoB. After the hydrogenation reaction, the NiCoB catalyst sintered to numerous large grains, whereas, W-NiCoB turned into the form of milkly-white powder. However, Mo-NiCoB retained the original black nano-clusters, the catalytic activities were maintained at high level after 3 batch runs.
Additionally, gold catalyst for p-CNB hydrogenation with high selectivity and the monolith catalyst on this kind of reaction were also elementary studied.
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