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

A series of chromium oxofluorides was prepared by thermal reaction between (NH4)3CrF6 and Cr2O3. No CrF3-phases were detected in the reaction products either by XRD or by XANES. The shift in the peak intensities of the powder diffraction pattern shows that the structure of Cr2O3 (Eskolaite) was modified with increasing fluorine content. Fluorine was found to replace oxygen in the structure of Cr2O3, resulting in an increasing distortion of the Cr2O3-lattice. Acidic sites of the samples were determined by analyzing characteristic IR-bands of pyridine adsorbed on the solid surface using FTIR-photoacoustic spectroscopy. The halogen exchange activity of the samples was studied in (i) the dismutation of CCl2F2 and (ii) the fluorination of CH2ClCF3 with HF. All chromium oxofluorides exhibited a significantly higher catalytic activity than both, pure CrF3 and Cr2O3. The reasons for this enhanced catalytic activity of the oxofluoride phases are discussed.