ABSTRACT
In situ X-ray diffraction (XRD) and NMR methods were used to follow the structural changes that occur during the dismutation reaction of hydrochlorofluorocarbon-22 (CHClF(2)) over gamma-alumina. Use of a flow cell allowed diffraction patterns to be recorded, while the reaction products were simultaneously monitored downstream of the catalyst bed, by gas chromatography. No visible structural changes of gamma-Al(2)O(3) were observed at 300 degrees C, the temperature at which this material becomes active for catalysis. A new phase began to form at 360 degrees C, which by 500 degrees C completely dominated the XRD powder pattern. (19)F/(27)Al cross-polarization (CP) experiments of gamma-Al(2)O(3) activated at 300 degrees C showed that AlF(3) had already begun to form at this temperature. By 400 degrees C, resonances from a phase that resembles alpha-AlF(3) dominate both the (19)F and (27)Al NMR spectra of the used catalyst. In situ XRD experiments of the catalytically inactive alpha-AlF(3) phase were performed to investigate the structural changes of this material, associated with the extent of tilting of the AlF(6) octahedra in this ReO(3)-related structure, as a function of temperature. Structural refinements of this sample, and the catalytically active phase that grows over gamma-Al(2)O(3), demonstrate that the catalyst is structurally similar to the rhombohedral form of alpha-AlF(3). Differences between the two phases are ascribed to defects in the catalyst, which limit the flexibility of the structure; these may also be responsible for the differences in the catalytic behavior of the two materials.
