Zeolite catalysts as solid solvents in fine chemicals synthesis - 1. Catalyst deactivation in the Friedel-Crafts acetylation of anisole

The liquid phase acetylation of anisole by acetic anhydride (anisole:acetic anhydride in a molar ratio of 2:1) using zeolite HBEA (Si/Al = 11) as cata lyst was carried out in a batch reactor at 90 degrees C, without and with a ddition of the product, p-methoxyacetophenone (p-MOAP) (molar ratio anisole /p-MOAP = 3 and 6). As expected, acetic acid and p-MOAP are produced in equ al amounts during the initial stages of the reaction but acetic acid is con sumed at long reaction time and high conversion. Partial zeolite dealuminat ion of the used catalyst was evidenced by Al-27 MAS NMR spectroscopy and th e regenerated catalyst showed a lower activity agreeing with its reduced Al content, i.e., acidity. Without added p-MOAP, acetylation occurs rapidly a t low conversion but deactivation becomes important as conversion increases . The reaction rate is largely decreased when p-MOAP is added to the reacti on mixture, indicating inhibition of the reaction by p-MOAP. A detailed kin etic analysis using a Langmuir-Hinshelwood model was performed to quantify the nature and extent of the reaction inhibition by p-MOAP. It shows that t he adsorption equilibrium constant for p-MOAP exceeds by a factor of at lea st 6 the adsorption equilibrium constant for any of the reactants and that the occupancy of the intracrystalline volume of the zeolite by p-MOAP incre ases rapidly with conversion, thereby reducing the access of the reactants to the catalytic sites. Comparison of our results with literature data enab led us to derive an approximate activation energy for this reaction, i.e., ca. 11 kcal mol(-1). A good agreement is observed between the calculated an d experimental reaction rates as a function of conversion. It is concluded that the deactivation of the catalyst as conversion increases is mainly due to product inhibition, i.e., the competitive adsorption of the reactants a nd products in the zeolite intracrystalline volume which can be rationalize d in terms of the zeolite acting as a solid solvent. Our work suggests that the application of zeolites and other microporous solids as catalysts to f ine chemicals synthesis would be better performed using catalytic reactor c onfigurations minimizing the role played by competitive adsorption effects. (C) 1999 Academic Press.