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Abstract

Heterogeneous reactions of trifluoromethane CHF3 with solid xerogel oxides, alkali
hydroxides and carbonates were investigated under static and dynamic conditions and at
moderate temperatures. In all cases, main reaction is fluorination characterised by a
complete decomposition (mineralisation) of the fluorinating agent, CHF3, and the
formation of the solid metal fluorides. Extent of fluorination strongly depends on
temperature and is dictated by the nature of the solid precursors. Alkali hydroxides, KOH,
NaOH, or LiOH, are the most reactive solids that start to react with CHF3 at relatively
low temperatures, 100–210 °C. Onset temperatures for fluorination of corresponding
carbonates are 100–150 °C higher. In the case of alumina-gallia xerogels the starting
temperature of fluorination depends on the gallia content and varies from 125 to 180 °C.
Reactivity of the solids towards CHF3 is directly related to their basicity. The reactions
are rationalised on the basis of the acid-base interactions between CHF3 as a weak C–H
acid, and basic oxygen species on the surface of the solids. These interactions apparently
play a key role during the initial binding of CHF3 molecule to the solid reactant, in this
way enabling or facilitating other decomposition steps that finally lead to a complete
destruction of the CHF3 molecule. Total decomposition of CHF3 with KOH or NaOH that
is achievable at relatively low temperatures, could in some specific cases represent a
possible alternative to the energy-demanding CHF3 decomposition by incineration. On the
other side, interaction of single γ-Al2O3 and γ-Ga2O3, and mixed γ-Al2O3/γ-Ga2O3
xerogels with CHF3 at intermediate temperatures (from room temperature to 450 ºC)
resulted in partial fluorination. Fluorinated oxides remain amorphous and retain a
considerable part of the initial surface area; for the fluorinated Al-based materials surface
areas in all cases exceed 100 m2 g–1. Lewis acidity of mixed oxides, either before or after
fluorination, is strongly influenced by the presence of surface Ga3+ ions, mainly due to
their strong preference to replace highly acidic Al3+ ions in tetrahedral positions. Ion
replacement leads to the formation of acidic sites with lower strengths what is confirmed
by the model catalytic reaction, isomerisation of CCl2FCClF2. Activity of γ-Al2O3 and γ-
Ga2O3, and mixed γ-Al2O3/γ-Ga2O3 can therefore be described in terms of coordination
preferences of surface Al3+ and Ga3+ ions and the clear difference in the strength of
related acid sites. XPS investigations indicate that fluorination of mixed oxides is
accompanied by substantial surface reconstructions and preferential formation of Al–F
based phases with Ga remaining mainly in O environments. Further segregation
processes, such as slow crystallisation of Al(F,OH)3·nH2O phases, are probably promoted
by water adsorption.