(256e) Elucidating the Mechanism of p-Chloronitrobenzene Hydrogenation to Chloroaniline and Factors That Affect Rates and Selectivities over Pd Catalyst

Active pharmaceutical ingredients like haloanilines are produced by the selective catalytic hydrogenation of the corresponding halo-substituted nitroarenes (e.g., para-chloronitrobenzene (PCNB)). However, PCNB can undergo C-Cl bond scission forming undesired products such as aniline. The reaction network is complex, and it is not well understood as to why certain adsorption configurations are preferred for the adsorbed species as well as which adsorbates are more likely to undergo C-Cl scission. In this study, we investigate the hydrogenation of p-chloronitrobenzene to p-chloroaniline on Pd using theory and experiments to understand the factors that lower selectivity and elucidate the reaction mechanism.

Low-coverage density functional theory (DFT) calculations show that PCNB adsorbs on Pd(111) with its phenyl ring parallel to the slab at different coverages which contradicts experiments that show under reaction conditions, nitrobenzene adsorbs in a vertical configuration on Pd nanoparticles. Entropic stabilizations and interaction energy between co-adsorbates make the vertical configuration more stable at higher coverages. Intermediates formed due to C-Cl bond cleavage were thermodynamically more stable than those formed due to hydrogenation. However, activation barriers for elementary steps show that, in the early stages of the reaction, the hydrogenation pathway has a lower barrier as compared to the C-Cl bond cleavage of the corresponding intermediate. As the extent of reaction increases, the hydrogenation barrier rises; yet barriers for C-Cl scission are unaffected and are independent of the type of hydrogenated reaction intermediate. These results indicate that intermediates become more susceptible to C-Cl cleavage with the extent of hydrogenation. Under batch reaction conditions using Pd nanoparticles, aniline has the highest selectivity amongst the dechlorinated side product providing validity to our conclusions from calculations. These results help in understanding which intermediates are more likely to undergo C-Cl bond scission and can give insight about the structure-function relationship that governs the hydrogenation of PCNB.