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-') aerogel pellets were obtained, consisting of ca. 5 nm boehmite crystals. Fluorination of such aerogels with CHF, resulted in high conversion to fluoride with the formation of mainly WAIF, and some P/y-AIF,. Substantial crystal growth was observed with uniform pellet shrinkage and a large decrease in surface area. In contrast to fluorinated materials prepared from commercial y-A1203 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 CF,CH,CI.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.

Interaction of trifluoromethane (CHF3) with alkali hydroxides and carbonates

“…These reactions, usually performed at 573-973 K, include catalytic hydrolyses [15,16] and fluorinations with CHF 3 [17][18][19][20][21][22]. It should be noted that these processes operate at temperatures that are considerably lower than those used in the non-catalytic incineration.…”

Reactivity of the β-AlF3(100) surface: defects, fluorine mobility and catalysis of the CCl2F2 dismutation reaction

“…The surfaces of α-AlF 3 are known to be less catalytically active than the surfaces of the β phase, which show moderate catalytic activity, and the amorphous HS materials, which show high catalytic activity 10 . Many experimental studies have been performed to investigate the structure and chemical properties of AlF 3 , including solid state NMR [11][12][13] , powder X-ray diffraction 11,12,[14][15][16][17][18] , infrared spectroscopy [17][18][19] , X-ray photoelectron spectroscopy 11,19,20 and temperature programmed desorption 21 . The majority of traditional surface science techniques for determining surface structure require large, pure, crystalline samples.…”

Aluminium(III) fluoride originating from decomposition of hydrazinium fluoroaluminate(III) under oxidative conditions: Syntheses, X-ray photoelectron spectroscopy and some catalytic reactions

“…Only the activated LHF-materials, F-Al 2 O 3 and HS-AlF 3 show high catalytic activity at 573 K with no loss of performance during several hours on-stream. Treatment, using an HFC or HCFC is known to result in partial fluorination of an alumina surface [11,43,44], and fluorination of aluminium alkoxide-derived fluoride gels with an HCFC or CFC is a route to HS-AlF 3 [1]. In view of the combined XPS and FTIR results described above, it is tempting to suggest that treatment with the CFCs, CCl 2 F 2 or CCl 2 FCClF 2 , is a way of decreasing the concentration of both, surface hydroxyl groups and [NH 4 ] + species, leading to catalytic activity, with an increase in the concentration of accessible Lewis acid sites on the surface.…”

“…Finally, the behaviour of the materials as possible catalysts in two reactions, isomerisation of 1,1,2-trichlorotrifluoroethane and the room temperature dehydrochlorination of tert-butyl chloride is examined. These reactions are among several that have been used as operational tests for Lewis acidity in HS-AlF 3 and related binary fluorides [3,9] [11][12][13]. It had a similar function in the present work and was prepared from commercial g-alumina pellets (Shell S 618) having nominal diameter 0.8 mm and a specific surface area of 240 m 2 g À1 .…”