Decomposition characteristics and reaction mechanisms of methylene chloride and carbon tetrachloride using metal-loaded zeolite catalysts

Abstract

The catalytic activities and selectivities of four zeolite-Y catalysts (HY, CoY, NaY and Co-z.sbnd;Y/CA) for the oxidation of methylene chloride and carbon tetrachloride were compared. Reactor experiments were carried out in a fixed bed reactor with temperatures ranging from 150 to 350°C and a space velocity of 2400 h −1, at atmospheric pressure. Other catalyst characteristics, including oxygen adsorption capacities, surface area, acidity and catalyst composition were measured and compared. The CoY catalyst showed excellent activity compared to the other catalysts with complete conversion of both feeds at temperatures as low as 200°C, and therefore was used as a model catalyst for reaction mechanistic investigations. Variable space velocity reactor runs (100 to 47000 h −1, were conducted at 350°C with this catalyst to distinguish series/parallel reaction mechanisms. It was found that CO was the predominant deep oxidation product (> 95% selectivity) and that the formations of CO and CO 2 occurred by parallel reactions in the oxidation of methylene chloride. Also, during the oxidation of carbon tetrachloride, phosgene was found to be a reaction intermediate. To elucidate the surface reaction mechanisms, in situ FTIR experiments were performed using a transmission reaction cell at 300°C. The results indicated that the CVOCs adsorbed on the Brønsted acid sites of the zeolite. The FTIR results also suggested the formation of an unstable intermediate (COHCI) during the oxidation of methylene chloride. The formation of phosgene as a reaction intermediate during the formation of CO 2 was observed for the oxidation of carbon tetrachloride, consistent with the variable space velocity experiments. Based on these results and the current literature, deep oxidation reaction mechanisms have been proposed for these two systems.