Oxidation of chlorinated hydrocarbons over Pt zeolite catalysts 1-mechanism of dichloromethane transformation over PtNaY catalysts

摘要

The catalytic oxidation of dichloromethane (DCM, 1000 ppm) in wet air was carried out in a fixed bed reactor over NaY, pure, exchanged with 0.5 wt% Pt or added with 0.4 wt% PtSiO2. At low temperatures (less than or equal to 250 degreesC) DCM is selectively hydrolyzed into 2/1 molar mixture of HCl and formaldehyde; at higher temperatures with Pt catalysts, DCM hydrolysis is followed by formaldehyde oxidation into CO2 and water, which suggests a bifunctional scheme. DCM transformation was investigated over pure NaY, the zeolite samples being characterized after reaction by adsorption of nitrogen, of pyridine followed by IR spectroscopy, and of DCM followed by microcalorimetry. During the first 15 min of reaction, a significant decrease in conversion could be observed as well as large changes in the NaY properties: disappearance of the strongest sites for DCM adsorption, creation of acidic bridging OH groups, etc., suggesting a stoichiometric reaction of DCM with NaY. Afterward, the process became catalytic with no more change in DCM conversion and in the physicochemical properties of the zeolite. A mechanism of DCM hydrolysis is proposed involving as intermediates chloromethoxy, hydroxymethoxy, and NaCl species as well as the OH groups created on the NaY zeolite. (C) 2003 Elsevier Science (USA). All rights reserved. [References: 55]