Adsorption and transformation of halogenated hydrocarbons over zeolites

The chloroflurocarbons (CFCs) are weakly adsorbed in the internal channels of the zeolite resulting in small wavenumber shifts on zeolites possessing no Bronsted acid centers, but hydrogen bonded adsorbed species are formed on Bronsted acidic samples. Some decomposition of CFCs was found upon adsorption at room temperature (RT) for HCFC 22, but CFCs having no hydrogen proved to be more stable against fragmentation. At higher temperature, fundamental interactions take place between the reactant and the zeolite, which can be followed easily by multinuclear (Si-29, Al-27, Na-23) solid state MAS NMR spectroscopy. The surface intermediate for the destruction of hydrogen containing reactant, i.e. HCOCl, is unstable and decomposes to CO and HCl as final products of the reaction. For CFCs known as more stable derivatives of the CFC family, phosgene was the surface intermediate. For this reaction, CO2 and HCl or NaCl were the final products, depending on which (H+ or Na+) is the charge compensating cation. The complex feature of the destruction reaction of CFCs over zeolites is reflected by the appearance of different CFCs as products of dismutation reactions. A general observation was that AlCl3 fall-out appeared in the cold part of the reactor or spectroscopic cell as a product of the dealumination of zeolites at higher temperature. The destruction of zeolites proved to be less pronounced in the presence of oxygen and/or in the case of a zeolite which has a higher Si to Al ratio than Y-FAU zeolites like mordenite or ZSM-5 zeolites. (C) 1999 Elsevier Science B.V. All rights reserved.