Acetylation of dimethoxybenzenes with acetic anhydride in the presence of acidic zeolites

The acetylation of 1,2-, 1,3- and 1,4-dimethoxybenzenes with acetic anhydri de has been investigated in the liquid phase (chlorobenzene as solvent) ove r the H-forms of various zeolites. H-Y and H-Beta have been shown to be eff icient catalysts in such a reaction, and led to the selective formation of the corresponding dimethoxyacetophenones. 1,2-Dimethoxybenzene (veratrole) has been chosen as a model substrate for the kinetic study of the reaction over H-Y (Si/Al = 15) as catalyst. The reaction proceeds through a modified Eley-Rideal type mechanism, wherein the chemisorbed acetic anhydride gener ates the electrophilic acylium ion, which then reacts with veratrole in the liquid phase. The competitive adsorption of both reactants and products ha s been evidenced. It has been especially shown that the deactivation of the catalyst was due to a partial adsorption on the active sites of the cataly st of either the reaction product (3',4'-dimethoxyacetophenone in the case of 1,2-dimethoxybenzene as the substrate) or/and of acetic acid formed in t he reaction. (C) 2000 Elsevier Science B.V. All rights reserved.