Bimetallic and trimetallic Pt-based catalysts for selective hydrogenation of p-chloronitrobenzene to p-chloroaniline

The selective hydrogenation of p-chloronitrobenzene (p-CNB) is a crucial step in the preparation of p-chloroaniline (p-CAN). However, the competitive reactions of NO2 hydrogenation and C-Cl hydrogenolysis make it challenging to inhibit the undesired dechlorination side reaction during this process. Herein, we developed a series of bimetallic PtM/γ-Al2O3 (M = Co, Cu, Ni, Fe, Zn, Ga, and Sn) and trimetallic PtCuCox/γ-Al2O3 catalysts to enhance the selectivity of p-CAN and suppress the formation of aniline (AN). Among the bimetallic catalysts, PtCo/γ-Al2O3 showed the highest hydrogenation activity due to its high electron-rich degree of Pt. Conversely, PtCu/γ-Al2O3 displayed the highest p-CAN selectivity and the lowest AN selectivity, primarily owing to the presence of electron-deficient Pt. Remarkably, the trimetallic PtCuCo/γ-Al2O3 catalyst exhibited superior hydrogenation performance, achieving 97.3 % p-CAN selectivity and 0.4 % AN selectivity at complete conversion of p-CNB, while maintaining good stability over five cycles. This enhancement was attributed to the synergistic effect among Pt, Cu and Co. To further investigate the effect of support, we prepared bimetallic PtM3 (M = Co, Cu, Ni and Zn) catalysts supported on reducible CeO2. Interestingly, the use of CeO2 as a support facilitated the condensation path from p-CNB to p-CAN via azobenzene compounds, distinct from the direct path observed with γ-Al2O3 support. Furthermore, the AN selectivity on all PtM3/CeO2 was less than 1 %, indicating effective inhibition of dechlorination reactions. This study highlights the importance of a multi-metal synergistic strategy and the effect of the support to achieve superior performance in chloronitrobenzene hydrogenation.