Low-temperature hydrodechlorination mechanism of chlorobenzenes over platinum-supported and palladium-supported alumina catalysts

Abstract

Hydrodechlorination of monochlorobenzene was carried out on Pt-supported and Pd-supported alumina, silica, titania, and silica-alumina catalysts at 298 K using a gas-phase fixed bed flow reactor. The Pt and Pd catalysts supported on alumina with Lewis acid sites indicated higher activity than those supported on silica-alumina and titania that have Brønsted acid sites. The activities of the Pt-supported and Pd-supported silica lay between those of the alumina and the silica-alumina catalysts. On each carrier, the Pd catalysts resulted in higher activity than the Pt ones. The pure Pt and Pd powder catalysts indicated less activities than the metal-supported alumina catalysts. Reaction products were only benzene and cyclohexane; on the Pd catalysts, cyclohexane was a minor product. No cleavage or decomposition of aromatic structure occurred. All the catalysts showed gradual activity decrease with process time, which originated from the accumulation of chlorines or chloride ions on the catalyst surfaces. The accumulation took place on both the metal and carrier surfaces, but the contribution of the latter was much larger than that of the former. In the reactions of 1,4-dichlorobenzene and 1,3,5-trichlorobenzene, similar results were also obtained. Furthermore, the conversion of 1,4-dichlorobenzene increased with increasing the amount of supported Pd at a constant amount of alumina carrier and with increasing the amount of alumina at a constant amount of supported Pd. Based on the above experimental results, the mechanism of the hydrodechlorination on the Pt-supported and Pd-supported alumina catalysts was discussed.