Characterization of the structure and catalytic behavior of AlF3-x(OH)(x) with aluminum successively replaced by chromium and magnesium

The calcination of alpha-AlF3 . 3H(2)O forms a beta-AlF3 phase which is catalytically active for dismutation of C-1-hydrocarbons such as dichlorodifluoromethane. The stepwise replacement of aluminum by chromium and magnesium leads to considerable alterations in structure and surface properties of the calcination products. This is accompanied by significant changes in the catalytic activity. The synthesis of the catalysts was carried out by coprecipitation of mixed metal fluoride trihydrates and subsequent calcination procedures. The stepwise replacement with chromium leads to a rebuilding of the lattice from the pseudo-hexagonal tungsten bronze (HTB) structure of beta-AlF3 into a cubic pyrochlore structure of CrF3-x(OH)(x). Hexagonal beta-CrF3 or mixed crystals beta-(Al,Cr)F-3 were not obtained via this route. The maximum catalytic activity for CCl2F2 dismutation was obtained for the 50% chromium sample, which is accompanied by a maximum BET surface area and a maximum number of Lewis acid sites. CrF3-x(OH)(x) exhibits a dramatic loss of catalytic activity as well as BET surface area. Possible explanations are given by comparing the pseudo-hexagonal tungsten bronze structure with the cubic pyrochlore structure with regard to accessibility of the Lewis acid metal cations. The presence of hydroxyl groups within the pyrochlore lattice enables the formation of hydrogen bridge bonds which is accompanied by a shielding of the metal cations. In the case of the replacement with magnesium the aim was to tune the strength of the Lewis acidity. The catalytic activity of the calcined samples passes through a maximum at the 10% magnesium sample. Due to the decrease of the strength of Lewis acid sites with further increasing Mg content, the catalytic activity is more and more diminished. At 50% Mg and higher the strength of the Lewis acid sites is no longer sufficient to catalyze the dismutation. (C) 1996 Academic Press, Inc.