A comparative study of surface acidity in the amorphous, high surface area solids, aluminium fluoride, magnesium fluoride and magnesium fluoride containing iron(III) or aluminium(III) fluorides

The ethylene glycol-mediated sol-gel synthesis of nano AlF3: structural and acidic properties after different post treatments

In this work, the structure and surface acidity of nano AlF₃ prepared by the ethylene glycol-mediated sol-gel process, followed by different post treatments including post-fluorination and calcination, were systematically investigated. FT-IR, elemental analysis, XPS and TG-DTA-MS results indicate the ethylene glycol strongly interacts with the as-prepared AlF₃ precursor, thus stabilizing the formed nano particles. MAS NMR spectroscopy combined with in situ FT-IR and HRTEM techniques reveal that AlF₆, AlO_6-xF_x and AlO₆ species are present in the resulting X-ray amorphous nano AlF₃. The fraction of AlF₆ species formed after post-fluorination significantly increases, whereas more AlO_6-xF_x species are formed just after calcination. After a comparable post-fluorination treatment, the CHClF₂ dismutation activity at room temperature indicates that nano AlF₃ prepared according the ethylene glycol-mediated route does not possess the same super-strong acidity as HS-AlF₃ prepared by the fluorolytic sol-gel method, although NH₃-TPD and N₂-sorption results indicate larger BET surface areas and high concentration of acid sites for the former as compared to the latter. This might be rationalized based on the absence of terminal fluorine species and the presence of a significant number of AlO_6-xF_x and AlO₆ species in the resulting nano AlF₃ as revealed by ²⁷Al, ¹⁹F MAS NMR and HRTEM. An interesting consequence is that these oxygen-containing species stabilize the microstructure of AlF₃ formed, resulting in improved thermal stability of these phases as compared to "classically" prepared HS-AlF₃.

A Computational Study of AlF3 and ACF Surfaces

By applying first principles density functional theory (DFT) methods, different metal fluorides and their surfaces have been characterized. One of the most investigated metal fluorides is AlF3 in different polymorphs. Its chloride-doped analogon AlClxF3−x (ACF) has recently attracted much attention due to its application in catalysis. After presenting a summary of different first-principle studies on the bulk and surface properties of different main group fluorides, we will revisit the problem of the stability of different α -AlF3 surfaces and extend the investigation to chloride-doped counterparts to simulate the surface properties of amorphous ACF. For each material, we have considered ten different surface cuts with their respective terminations. We found that terminations of ( 01 1 ¯ 0 ) and ( 11 2 ¯ 0 ) yield the most stable surfaces for α -AlF3 and for the chlorine substituted surfaces. A potential equilibrium shape of the crystal for both α -AlF3 and ACF is visualized by a Wulff construction.

Metal Fluorides, Metal Chlorides and Halogenated Metal Oxides as Lewis Acidic Heterogeneous Catalysts. Providing Some Context for Nanostructured Metal Fluorides

Aspects of the chemistry of selected metal fluorides, which are pertinent to their real or potential use as Lewis acidic, heterogeneous catalysts, are reviewed. Particular attention is paid to β-aluminum trifluoride, aluminum chlorofluoride and aluminas γ and η, whose surfaces become partially fluorinated or chlorinated, through pre-treatment with halogenating reagents or during a catalytic reaction. In these cases, direct comparisons with nanostructured metal fluorides are possible. In the second part of the review, attention is directed to iron(III) and copper(II) metal chlorides, whose Lewis acidity and potential redox function have had important catalytic implications in large-scale chlorohydrocarbons chemistry. Recent work, which highlights the complexity of reactions that can occur in the presence of supported copper(II) chloride as an oxychlorination catalyst, is featured. Although direct comparisons with nanostructured fluorides are not currently possible, the work could be relevant to possible future catalytic developments in nanostructured materials.