Inorganic hydroxide fluorides as solid catalysts for acylation of 2-methylfuran by acetic anhydride

Abstract

Several inorganic hydroxide fluorides MFn−x(OH)x with M being either aluminum, iron, or magnesium were synthesized using a sol–gel method. These mesoporous fluoride materials exhibited very high specific surface areas (between 60 and 370m2g−1) depending on the synthesis parameters used (nature of the metal, HF/Al ratio, and thermal treatment). It is shown that their acidic properties can also be adjusted by tuning these parameters. Indeed, in the case of aluminum hydroxide fluorides, bi-acidic catalysts containing both Brønsted and Lewis acid sites were obtained, the amount of acid sites and the Lewis/Bronsted ratio (in the range of 1.9–7.6) being very dependent on the synthesis parameters. For example, the increase of the calcination temperature of aluminum hydroxide fluorides led to a decrease of both Lewis and Brønsted acid site, but also an increase of the Lewis/Bronsted ratio. The most acid fluoride exhibited 542μmolg−1 of Lewis acid sites and 291μmolg−1 of Brønsted acid sites.These inorganic hydroxide fluorides were successively used as catalysts for the solvent-free acylation of 2-methylfuran by acetic anhydride under mild conditions (50°C, atmospheric pressure) yielding selectively 2-acetyl-5-methylfuran. The activity of fluoride can be related to the amount of both Lewis and Brønsted acid sites. In general the higher these values, the greater the activity in acylation. A reaction mechanism was proposed involving a bi-acidic site which could be a Lewis acid site (unsaturated aluminum) in the vicinity of a Brønsted acid site.