Palladium (Pd0) Loading-Controlled Catalytic Activity and Selectivity for Chlorophenol Hydrodechlorination and Hydrosaturation

Reductive catalysis by zero-valent palladium nanoparticles (Pd⁰NPs) has emerged as an efficient strategy for promoting the detoxification of chlorophenols (CPs) via hydrogenation. Most studies achieved hydrodechlorination of CP to phenol for detoxification, but it requires considerably high energy input and harsh conditions to further hydrosaturate phenol to cyclohexanone (CHN) as the most desired product for resource recovery. This study documented 4-CP hydrodechlorination and hydrosaturation catalyzed by Pd⁰NPs deposited on H₂-transfer membranes in the H₂-based membrane catalyst-film reactor, which yielded up to 99% CHN selectivity under ambient conditions. It was further discovered that the Pd⁰ morphology and size, both determined by Pd⁰ loading, were the key factors controlling the catalytic activity and selectivity: while sub-nano Pd particles catalyzed only 4-CP hydrodechlorination, Pd⁰NPs were able to catalyze the subsequent hydrosaturation that requires more Pd⁰ reactive sites than hydrodechlorination. In addition, better dispersion of Pd⁰, caused by lower Pd⁰ loading, yielded higher activity for hydrodechlorination but lower activity for hydrosaturation. During the 15 day continuous tests, the substantial product from 4-CP hydrogenation was constantly phenol (>98%) for 0.2 g-Pd/m² and CHN (>92%) for 1.0 g-Pd/m². This study opens the door for selectively manipulating desired products from Pd⁰-catalyzed CP hydrogenation under ambient conditions.