Zeolite Catalysts as Solid Solvents in Fine Chemicals Synthesis: 2. Competitive Adsorption of the Reactants and Products in the Friedel–Crafts Acetylations of Anisole and Toluene

Abstract

The liquid-phase acetylations of anisole by acetic anhydride and isopropenyl acetate and that of toluene by acetic anhydride, using zeolites HBEA, HMFI, and HFAU as catalysts, were carried out under batch conditions using various molar ratios of the reactants without added solvent. Evidence has been obtained for competitive adsorption effects of the reactants resulting from differences in their adsorption equilibrium constants (AEC). Estimated values of the AEC show that preferential adsorption is in the order toluene<acetic anhydride (AA)<anisole<isopropenyl acetate (IPA). In all cases, the acetylation products are adsorbed more strongly than the reactants, but the competitive adsorption of acetic acid and acetone appear to be negligible when AA and IPA are used as acetylating agents, respectively. The effect of dealumination was investigated for the acetylation of toluene by acetic anhydride, using HBEA as catalyst. Both the initial and the quasi-stationary-state reaction rates are proportional to Al content, i.e., the number of Brønsted sites at low Al content. The rates decrease at high aluminium content, indicating that competitive adsorption effects are enhanced due to the increased zeolite polarity and polarisability. Initial rate constants for the acetylation of anisole, derived by assuming a Langmuir–Hinshelwood model, show that the rate-determining step is most likely to be the reaction of anisole with an acyl cation equivalent formed by reaction of either acetic anhydride or isopropenyl acetate with the zeolite (HBEA). A possible mechanism is proposed in which the formation of an acyl cation–zeolite complex is the initial step. This proposal is based in part on the colour change of the zeolite when it is contacted with the reactant mixture and during reaction, the colour being attributed to the formation of the zeolite–acyl cation complex. The present work confirms that competitive adsorption effects, involving both reactants and products, need to be controlled in order to achieve maximum catalytic performance when zeolites are used as catalysts in the liquid phase.