Zeolite Catalysts as Solid Solvents in Fine Chemicals Synthesis: 1. Catalyst Deactivation in the Friedel–Crafts Acetylation of Anisole

Abstract

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 catalyst was carried out in a batch reactor at 90°C, without and with addition 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 equal amounts during the initial stages of the reaction but acetic acid is consumed at long reaction time and high conversion. Partial zeolite dealumination of the used catalyst was evidenced by 27Al MAS NMR spectroscopy and the regenerated catalyst showed a lower activity agreeing with its reduced Al content, i.e., acidity. Without added p-MOAP, acetylation occurs rapidly at low conversion but deactivation becomes important as conversion increases. The reaction rate is largely decreased when p-MOAP is added to the reaction mixture, indicating inhibition of the reaction by p-MOAP. A detailed kinetic analysis using a Langmuir–Hinshelwood model was performed to quantify the nature and extent of the reaction inhibition by p-MOAP. It shows that the adsorption equilibrium constant for p-MOAP exceeds by a factor of at least 6 the adsorption equilibrium constant for any of the reactants and that the occupancy of the intracrystalline volume of the zeolite by p-MOAP increases rapidly with conversion, thereby reducing the access of the reactants to the catalytic sites. Comparison of our results with literature data enabled 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 and 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 and products in the zeolite intracrystalline volume which can be rationalized 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 fine chemicals synthesis would be better performed using catalytic reactor configurations minimizing the role played by competitive adsorption effects.