Catalytic dechlorination and deep hydrogenation of 4-chlorophenol in a hydrogen-based internal circulation reactor for efficient wastewater detoxification

Abstract

Catalytic dechlorination is an attractive technique for chlorophenol degradation but was restricted to the low reaction kinetics and high toxicity of the intermediate products. This work aimed to develop a hydrogen-based internal circulation reactor (H2-ICR) for the efficient detoxification of 4-chlorophenol (4-CP). The structure and catalytic performance of commercial Pt/C, Pd/C, and Ru/C catalysts were systematically investigated. Compared to Pd and Ru, Pt-based catalysts exhibited better performance for the detoxification of chlorophenols. Over 99 % 4-CP was removed within 6 min by employing Pt/C as the catalyst for an initial 4-CP concentration of 2 mM (equivalent to 257 mg L−1). The reaction kinetic constant reaches 0.68 min−1, with the performance almost unchanged at different solution pH (3–11). Mechanistic analysis indicates that phenol hydrogenation is the rate-limiting step on Pt/C. The main product of Pt/C is cyclohexanol, suggesting the simultaneous occurrence of dechlorination and hydrosaturation reactions. In contrast, traditional Pd/C catalysts can hardly catalyze the hydrosaturation reaction, leaving toxic phenol as the main product. Biotoxicity assessment both in silico and by experiment demonstrates that the Pt catalyst can realize efficient detoxification of 4-CP wastewater. Both electron spin resonance (ESR) and radical quenching experiments confirmed the pivotal role of atomic hydrogen (H*). This study provides useful guidance for the design of hydrogenation catalysts and reactors with high safety, which may lead to a paradigm shift for chlorophenol wastewater remediation.