High-temperature hydrodechlorination of ozone-depleting chlorodifluoromethane (HCFC-22) on supported Pd and Ni catalysts

Abstract

The hydrodechlorination of chlorodifluoromethane (HCFC-22) was performed by a catalytic reaction and noncatalytic thermal decomposition at high temperatures of 400-800 degrees C. After 47 h of time-on-stream on a supported palladium (Pd) catalyst, the gas-phase composition of difluoromethane (HFC-32) is 41.0%, with 4.9% of the HCFC-22 remaining, indicating the conversion of up to 95.1% of HCFC-22. The supported nickel catalyst's deactivation is significant as it exhibits the low conversion of HCFC-22 under the same reaction conditions. The deactivation of the catalyst is caused by the polymerization of adsorbed methyl radicals, which competes with the formation of HFC-32. With concentrated reactants at high reaction temperatures, there was an increase in the catalytic activity; however, unwanted tar, methane, and trifluoromethane (HFC-23) by-products are also produced. The use of catalyst suppresses the formation of these by-products. Considering the compositions of the products of the catalytic and noncatalytic reactions, we demonstrate that the use of the supported-metal catalysts and hydrogen flow suppresses tar formation and lowers the required reaction temperature.