Catalytic combustion of chlorinated hydrocarbons over H-BETA and PdO/H-BETA zeolite catalysts

摘要

The catalytic combustion of selected chlorinated volatile organic compounds,namely dichloromethane and trichloroethylene,over H-BETA and PdO/H-BETA has been examined under dry and humid conditions at a space velocity of 15,000 h~(-1) in order to evaluate the influence of the addition of a noble metal to the zeolitic material on catalytic conversion and product distribution.The PdO/H-BETA zeolite catalyst showed an enhanced catalytic performance with respect to the pure protonic zeolite.Hence,the palladium-based system produced 50% conversion at 300 and 315 deg C in trichloroethylene and dichloromethane oxidation,respectively.In contrast,H-BETA required a temperature increase to 450 and 380 deg C,respectively,for similar conversion level.Palladium oxide resulted a more active (and stable) phase in comparison with metallic palladium under the working conditions.The oxidation of chlorinated hydrocarbons with zeolite-based catalysts gave rise to carbon monoxide,carbon dioxide,hydrogen chloride,and molecular chlorine as primary products.In addition,substantial quantities of methyl chloride and tetrachloroethylene were detected in the combustion of dichloromethane and trichloroethylene,respectively.The incorporation of PdO species promoted the selectivity towards carbon dioxide but also led to the generation of notable amounts of molecular chlorine.A slight improvement in conversion was observed over PdO/H-BETA catalyst under humid conditions since water facilitated the removal of chlorine species present on the catalyst surface.Likewise,the addition of water as a co-feed inhibited the formation of carbon monoxide,and enhanced the conversion of molecular chlorine to hydrogen chloride.