Low-temperature catalytic destruction of CCl4, CHCl3 and CH2Cl2 over basic oxides

Abstract

The catalytic destruction of CCl₄, CHCl₃ and CH₂Cl₂ in the presence of steam has been compared over a series of unsupported and alumina-supported lanthanide and alkaline earth oxides. It was found that (1) the destruction rate over basic oxides decreases with decreasing chlorine content of the CHC compound (CCl₄ > CHCl₃ > CH₂Cl₂); (2) the catalyst activity is always higher for lanthanide oxides than for alkaline earth oxides; (3) supported basic oxides are more active than their unsupported counterparts and (4) a stable destruction activity of more than 10 days can be maintained as long as an excess of steam is present in the gas stream. The reaction products are also dependent on the type of chlorinated hydrocarbon. CO₂ and HCl are the products for the destruction of CCl₄ and both compounds are formed from the reaction intermediate Cl₂CO. In the case of CHCl₃ a mixture of CO and HCl is produced, partially formed via the hydrolysis of the reaction intermediate HClCO. Finally, the reaction products for the destruction of CH₂Cl₂ are HCl, CO and H₂; the latter two are formed from H₂CO decomposition. In the case of supported basic oxides significant amounts of CH₃Cl are produced in the catalytic destruction of CH₂Cl₂, which is catalyzed by the partially uncovered Al₂O₃ support phase. In situ Raman and infrared spectroscopy were used to monitor the physicochemical changes taking place in the catalytic solid as well in the gas phase above the catalyst material. Based on these findings a plausible reaction mechanism is proposed. Lanthanide oxides and lanthanide oxide chlorides are both active phases in this catalytic process.