Reactivity of fluorinated γ-alumina and β-aluminium(iii) fluoride surfaces towards hydrogen halides and tert-butyl chloride

Abstract

The Lewis acids β-aluminium() fluoride and γ-alumina, fluorinated at room temperature with sulfur tetrafluoride, both interact with hydrogen fluoride and chloride, as demonstrated by radiotracer measurements using [ 18 F] and [ 36 Cl]. The different behaviour of HCl towards the two surfaces is rationalised by considering plausible surface sites and, in the case of β-AlF 3 , the role of residual water. Both materials promote dehydrochlorination of tert-butyl chloride. β-Aluminium() fluoride also has some catalytic activity in Friedel-Crafts alkylation whereas oligomerisation of Bu t Cl dominates on fluorinated γ-alumina. The different behaviour appears to be due to the presence of both Lewis and Brønsted surface acidity on γ-alumina that has been fluorinated under static conditions. A description for this surface is proposed.

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

“…Secondly, and this is more important, the HF behaves as Lewis base, reacting with the strong Lewis acid sites, which become thereby blocked. A similar situation has been demonstrated previously [9] for HF on b-AlF 3 , a modification that has the hexagonal tungsten bronze structure [16]. Retention of small quantities of HF after the second stage fluorination using this reagent is therefore a probability.…”

“…We have shown previously that anhydrous HCl is strongly adsorbed at hydrated surface sites of b-AlF 3 [9]. If hydrated or hydroxylated sites were to be present at HS-AlF 3 they would contribute to the HCl adsorption observed.…”

“…This could have been a reflection of the far smaller BET area (72 vs. 205 m 2 g À1 ) of the sample, although, in other studies using this Geiger-Müller monitoring method, positive results have been obtained for materials that have even smaller BET areas (e.g. [9]). Samples CFC3 and HF3 (Table 1) were used in a more extensive test of the possibility that the adsorption behaviour of H 36 Cl, which is formed via dehydrochlorination over HS-AlF 3 , can depend on the fluorination reagent used, CCl 2 F 2 /N 2 or aHF/ N 2 .…”

“…The application of this method and procedure to b-AlF 3 under identical conditions results in a far milder reaction, which leads to the slow formation of HCl in the vapour over a period of several hours [9]. Nanoscopic AlF 3 , HS-AlF 3 , therefore is far more reactive than the b-modification with respect to room temperature dehydrochlorination.…”

“…This was ascribed to the high activity of ZSM-5 in competing reactions, for example oligomerisation, cracking and aromatisation [6]. The probe molecule, tert-butyl chloride is particularly interesting, since, in the presence of acidic solids, it may behave as an alkylation agent or may undergo dehydrochlorination, depending on the conditions used [8,9].…”