Novel insights into the mechanism for protic solvent promoting Pd/C-catalyzed hydrodechlorination of chlorinated organic compounds

Solvent effects in liquid-phase hydrodechlorination (HDC) of chlorinated organic compounds (COCs) over Pd/C catalyst were systematically studied. For the HDC of 4-chlorobiphenyl (4-CBP) with triethylamine (Et3N), Pd/C exhibited much higher catalytic activity in protic solvents than that in aprotic polar and non-polar solvents, and a clear correlation between the HDC rate and polarity of protic solvents was observed. Moreover, water promoted Pd/C-catalyzed HDC of 4-CBP in homogeneous alcohol-water solvents and biphasic organic-water solvents with Et3N, and the HDC rate of 4-CBP increased with the increase of water content in the above solvent systems. Catalyst characterization (FT-IR and XRD) combined with solvent parameters and isotope labelling analysis were introduced to explain in-depth the solvent effects in the HDC of 4-CBP with Et3N. It was found that protic solvents would transfer active hydrogen atom (H*) via hydrogen bonding network or exchange their hydroxyl hydrogen with H*, which was beneficial to the stabilization of H* and interface HDC reaction. Thus, the higher HDC rate was achieved in solvent systems contained protic solvents with higher hydrogen-bond donor capability (alpha) values. On the other hand, non-polar and polar aprotic solvents had a negative effect on H* and mass transfer, and thus suppressed interface HDC reaction. Similar solvent effects were observed in Pd/C-catalyzed hydrogenation of unsaturated bonds-containing groups (nitryl group, carbonyl group, and carbon-carbon double bond). The results presented here would provide vital guidance for the rational selection of solvent systems for Pd/Ccatalyzed HDC of COCs and hydrogenation of unsaturated bonds-containing groups.