Anisole acetylation, protonic zeolites

Figure 12.6 Mechanism of anisole acetylation over protonic zeolites.

The acetylation over protonic zeolites of aromatic substrates with acetic anhydride was widely investigated. Essentially HFAU, HBEA, and HMFI were used as catalysts, most of the reactions being carried out in batch reactors, often in the presence of solvent. Owing to the deactivation effect of the acetyl group, acetylation is limited to monoacetylated products. As could be expected in electrophilic substitution, the reactivity of the aromatic substrates is strongly influenced by the substituents, for example, anisole > m-xylene > toluene > fluorobenzene. Moreover, with the poorly activated substrates (m-xylene, toluene, and fluoroben-zene) there is a quasi-immediate inhibition of the reaction. It is not the case with activated substrates such as anisole and more generally aromatic ethers. It is why we have chosen the acetylation of anisole and 2-methoxynaphtalene as an example. 

It is generally admitted that over zeolites, acetylation of aromatic substrates with acetic anhydride (AA) is catalyzed by protonic acid sites. The direct participation of Lewis sites was excluded by using two BEA samples with similar protonic acidities, but with very different Lewis acidities indeed, these samples were shown to have quasi-similar activities. The currently accepted mechanism is shown in Figure 12.6 for the anisole acetylation example. The limiting step of the process is the attack of anisole molecules by acylium ions. 

However, whereas this analysis of competition effects in anisole acetylation is very attractive, the following oversimplification which was made also has to be considered indeed, the competitive adsorption of the very polar acetic acid product, which inhibits arene acetylation[52,53] and the adsorption equilibrium constants were related to adsorption within the zeolite micropores only, without taking into consideration chemisorption over the protonic active sites. 

The liquid phase acetylation with AA of benzenic ethers, especially of anisole and veratrole [Reactions (3.1) and (3.2)], was investigated over various protonic zeolites with 12- (large pores) or 10- (medium pores) membered ring openings (Table 3.1). 

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