Fluorine modified chromium oxide and its impact on heterogeneously catalyzed fluorination reactions

A series of chromium oxofluorides was prepared by thermal reaction between (NH4)(3)CrF6 and Cr2O3. No CrF3-phases were detected in the reaction produc ts either by XRD or by XANES. The shift in the peak intensities of the powd er diffraction pattern shows that the structure of Cr2O3 (Eskolaite) was mo dified with increasing fluorine content. Fluorine was found to replace oxyg en in the structure of Cr2O3, resulting in an increasing distortion of the C2O3-lattice. Acidic sites of the samples were determined by analyzing char acteristic IR-bands of pyridine adsorbed on the solid surface using FTIR-ph otoacoustic spectroscopy. The halogen exchange activity of the samples was studied in (i) the dismutation of CCl2F2 and (ii) the fluorination of CH2Cl CF3 with HF. All chromium oxofluorides exhibited a significantly higher cat alytic activity than both, pure CrF3 and Cr2O3. The reasons for this enhanc ed catalytic activity of the oxofluoride phases are discussed. (C) 2000 Els evier Science S.A. All rights reserved.