Aluminum Chloride as a Solid Is Not a Strong Lewis Acid

Abstract

Aluminum chloride is used extensively as Lewis acid catalyst in a variety of industrial processes, including Friedel-Crafts and Cl/F exchange reactions. There is a common misconception that pure AlCl3 is itself a Lewis acid. In the current study, we use experimental and computational methods to investigate the surface structure and catalytic properties of solid AlCl3. The catalytic activity of AlCl3 for two halide isomerization reactions is studied and compared with different AlF3 phases. It is shown that pure solid AlCl3 does not catalyze these reactions. The (001) surface of crystalline AlCl(3) is the natural cleavage plane and its structure is predicted via first principles calculations. The chlorine ions in the outermost layer of the material mask the Al3+ ions from the external gas phase. Hence, the experimentally found catalytic properties of pure solid AlCl3 are supported by the predicted surface structure of AlCl3.

The very strong solid Lewis acids aluminium chlorofluoride (ACF) and bromofluoride (ABF)—Synthesis, structure, and Lewis acidity

“…AlCl 3 is a strong Lewis acid in the molecular state, but the solid itself possesses no or only very low acidity [82] (see also discussion in Section 4.3). From the point of view today many older results are doubtable.…”

“…It has been shown recently by experimental and theoretical methods, that the (0 0 1) surface of solid AlCl 3 has no Lewis acidic centres [82], and is very similar to the catalytically inactive a-AlF 3 . All aluminium ions at the surface are coordinated by six chloride ions (Fig.…”

Non-aqueous sol–gel synthesis of nano-structured metal fluorides

“…In contrast, HS-AlF 3 was immediately active both in liquid as in gas phase reactions. Computational investigations showed that in the regularly ordered AlCl 3 all Al atoms next to the surface are completely covered and efficiently blocked by Cl atoms, in contrast to the highly disordered HS-AlF 3 [37].…”

“…(8)) [25] and isomerisation of CCl 2 FCClF 2 (Eq. (9)) [37] and of CBrF 2 CBrFCF 3 (Eq. (10)) [21] used.…”

“…Reaction (9), isomerisation of CCl 2 FCClF 2 , which is of technical interest, was studied in more detail [37]. It could be shown that AlCl 3 , which is known to be useful in batch reactions with liquid CCl 2 FCClF 2 only, is not able to catalyse the isomerisation unless it had become converted by the fluorinating action of CCl 2 FCClF 2 in the much more active aluminium chlorofluoride (ACF).…”