Preparation and properties of highly fluorinated alumina prepared from boehmite aerogels or commercial γ-Al2O3

Abstract

Pellets of alumina-based aerogels, doped with chromium or chromium and titanium, have been prepared according to a proposed preparation procedure from preformed and partly dried aqueous gels precipitated from metal chloride solutions. After supercritical drying, low density and high surface area (>300 m² g^–1) aerogel pellets were obtained, consisting of ca. 5 nm boehmite crystals. Fluorination of such aerogels with CHF₃ resulted in high conversion to fluoride with the formation of mainly α-AIF₃ and some β/γ-AIF₃. Substantial crystal growth was observed with uniform pellet shrinkage and a large decrease in surface area. In contrast to fluorinated materials prepared from commercial γ-Al₂O₃ by fluorination with HF, fluorinated aerogels have a much more expanded and heterogeneous structure with a broad distribution of size/shape of particles and pores. Fluorinated aerogels show catalytic activity in trichloroethene hydrofluorination to CF₃CH₂Cl. By the fluorination of alumina-based aerogels, very porous, high surface area and catalytically active fluorides can be prepared. Such highly dispersed materials are believed to be very useful in studying heterogeneous catalytic reactions of technically important fluoro-derivatives of lower halogenated hydrocarbons.