In Situ X-ray Diffraction and Solid-State NMR Study of the Fluorination of γ-Al2O3 with HCF2Cl

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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 (CHClF2) over γ-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 γ-Al2O3 were observed at 300 °C, the temperature at which this material becomes active for catalysis. A new phase began to form at 360 °C, which by 500 °C completely dominated the XRD powder pattern. 19F/27Al cross-polarization (CP) experiments of γ-Al2O3 activated at 300 °C showed that AlF3 had already begun to form at this temperature. By 400 °C, resonances from a phase that resembles α-AlF3 dominate both the 19F and 27Al NMR spectra of the used catalyst. In situ XRD experiments of the catalytically inactive α-AlF3 phase were performed to investigate the structural changes of this material, associated with the extent of tilting of the AlF6 octahedra in this ReO3-related structure, as a function of temperature. Structural refinements of this sample, and the catalytically active phase that grows over γ-Al2O3, demonstrate that the catalyst is structurally similar to the rhombohedral form of α-AlF3. 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.

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