Modification mechanism of Sn4+ for hydrogenation of p-chloronitrobenzene over PVP-Pd/γ-Al2O3

PVP-Pd (1.5 wt.%)/γ-Al2O3 was prepared and used as a catalyst for the hydrogenation of p-chloronitrobenzene (p-CNB) to form p-chloroaniline (p-CAN), so that a serious dehalogenation reaction was happened. However, the catalytic property of this catalyst was remarkably affected by some metal cationic additives. Especially, when Sn4+ was introduced into the reaction system, the activity of the catalyst was not only promoted, but the dehalogenation reaction was also greatly suppressed. The average rate of hydrogenation increased from 1.28 mol H2/mol Pd s on PVP-Pd/γ-Al2O3 catalyst to 1.96 mol H2/mol Pd s on the PVP-Pd-Sn4+/γ-Al2O3 catalyst (molar ratio of Pd to Sn = 1:1), and the selectivity for p-CAN increased from 66.8 to 96.6%. The dehalogenation reaction was completely restrained as the molar ratio of Sn4+ to Pd was up to 5. The great promotion role of Sn4+ could be owing to the interaction between Sn4+ and −NO2 group of the substrate. The combination of Sn4+ with oxygen in −NO2 increased the polarity of NO bond. The increase of the polarity of NO benefited the activated dihydrogen to attack the NO bond, and the hydrogenation was accelerated. At the same time, the increase of the polarity of NO bond caused the more lone pair electron of p orbital on chlorine atom to dislocate to phenyl ring, so CCl bond was strengthened and the polarity of CCl was weakened. Furthermore, these were unfavorable for the activated dihydrogen to attack CCl bond and the hydrogenation selectivity was greatly improved.

Graphical abstractThe hydrogenation activity of p-CNB and the selectivity to form p-CAN over PVP-Pd/γ-Al2O3 was obviously promoted by the addition of Sn4+. Hydrogenolysis reaction of CCl was completely blocked when the molar ratio of Sn4+ to Pd was up to 5. The modification mechanism of Sn4+ could be the interaction between Sn4+ and –NO2 group of the substrate increased the polarity of NO bond and accelerated the hydrogenation of nitro group. Because of the same reason, the polarity of CCl bond was decreased and the strength of CCl bond was increased, so that the dehalogenation reaction was greatly inhibited. Figure optionsDownload full-size imageDownload as PowerPoint slide