Conversion of CCl_2F_2(CFC-12) in the presence and absence of H_2 on sol-gel derived Pd/Al_2O_3 catalysts

摘要

Conversion of CCl_2F_2 in the presence (hydrogenolysis) and absence of hydrogen was investigated on Al_2O_3, AlF_3 and Pd/Al_2O_3 xerogel and aerogel catalysts. CCl_2F_2 was found to form CClF_3 and CCl_3F on Al_2O_3 and AlF_3 in the presence and absence of hydrogen as well as on the Pd/Al_2O_3 catalysts in the absence of hydrogen. Overall activity increased during the hydrogenolysis reactions at 230 deg C as a function of time which was paralleled by a significant increase in the yield of CClF_3 formed through a Cl/F-exchange reaction. X-ray diffraction patterns of the spent catalyst recovered after 3 h of hydrogenolysis confirmed the presence of Pd(C) (Pd-carbon solid solution) and AlF_3 phases on Pd/Al_2O_3 catalysts indicated that the carbon incorporation into the Pd lattice and the transformation of Al_2O_3 to AlF_3 starts at the initial stage of the reaction. It was concluded that AlF_3 is responsible for the Cl/F-exchange reactions. CH_4, a complete hydrogenation product, is formed during hydrogenolysis. Another route for its formation is the reaction between hydrogen in the gas phase and the interstitial carbon.