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 anhydride has been investigated in the liquid phase (chlorobenzene as solvent) over the H-forms of various zeolites. H-Y and H-Beta have been shown to be efficient 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 generates the electrophilic acylium ion, which then reacts with veratrole in the liquid phase. The competitive adsorption of both reactants and products has 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 catalyst 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 the reaction. (C) 2000 Elsevier Science B.V. All rights reserved.