In situ X-ray diffraction and solid-state NMR study of the fluorination ofgamma-Al2O3 with HCF2Cl

In situ X-ray diffraction (XRD) and NMR methods were used to follow the str uctural changes that occur during the dismutation reaction of hydrochlorofl uorocarbon-22 (CHClF2) over gamma -alumina. Use of a flow cell allowed diff raction patterns to be recorded, while the reaction products were simultane ously monitored downstream of the catalyst bed, by gas chromatography. No v isible structural changes of gamma -Al2O3 were observed at 300 degreesC, th e temperature at which this material becomes active for catalysis. A new ph ase began to form at 360 degreesC, which by 500 degreesC completely dominat ed the XRD powder pattern. F-19/Al-27 cross-polarization (CP) experiments o f gamma -Al2O3 activated at 300 degreesC showed that AlF3 had already begun to form at this temperature. By 400 degreesC, resonances from a phase that resembles alpha -AlF3 dominate both the F-19 and Al-27 NMR spectra of the used catalyst. In situ XRD experiments of the catalytically inactive alpha -AlF3 phase were performed to investigate the structural changes of this ma terial, associated with the extent of tilting of the AlF6 octahedra in this ReO3-related structure, as a function of temperature. Structural refinemen ts of this sample, and the catalytically active phase that grows over gamma -Al2O3, demonstrate that the catalyst is structurally similar to the rhomb ohedral form of alpha -AlF3. Differences between the two phases are ascribe d to defects in the catalyst, which limit the flexibility of the structure; these may also be responsible for the differences in the catalytic behavio r of the two materials.