Modification mechanism of Sn 4+ for hydrogenation of p-chloronitrobenzene over PVP-Pd/γ-Al 2O 3

Abstract

PVP-Pd (1.5 wt.%)/γ-Al 2O 3 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 Sn 4+ 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 H 2/mol Pd s on PVP-Pd/γ-Al 2O 3 catalyst to 1.96 mol H 2/mol Pd s on the PVP-Pd-Sn 4+/γ-Al 2O 3 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 Sn 4+ to Pd was up to 5. The great promotion role of Sn 4+ could be owing to the interaction between Sn 4+ and −NO 2 group of the substrate. The combination of Sn 4+ with oxygen in −NO 2 increased the polarity of N O bond. The increase of the polarity of N O benefited the activated dihydrogen to attack the N O bond, and the hydrogenation was accelerated. At the same time, the increase of the polarity of N O bond caused the more lone pair electron of p orbital on chlorine atom to dislocate to phenyl ring, so C Cl bond was strengthened and the polarity of C Cl was weakened. Furthermore, these were unfavorable for the activated dihydrogen to attack C Cl bond and the hydrogenation selectivity was greatly improved.