HBEA zeolites aromatics, acylation

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

Acylation of aromatics over a HBEA Zeolite. Effect of solvent and of acylating agent. 

Under mild conditions (liquid phase, 160°C) HBEA zeolites can catalyse the acylation of phenol with phenyl acetate. High selectivity to p-hydroxyacetophenone is obtained by using sulfolane as a solvent, which can be explained by a better dissociation of phenyl acetate into acylium ions due to a solvation effect. However a competition between sulfolane and phenyl acetate for adsorption on the active acid sites is also demonstrated. A preliminary investigation of the effect of the acylating agent shows that generally, donor groups in aromatic acetates have a positive effect on the rate of acylation provided they do not block the access of the acetate to the acid sites of the zeolite pores. 

An efficient method for the continnons flow selective acylation of aromatics, including ethers, ovCT HBEA zeolite is reported [41, 42]. The process is performed in a fixed-bed tubnlar reactor charged with HBEA(25) zeolite deposited on a small layo- of glass pellets or directly packed without any support or mixed with alumina. The aromatic substrate and acetic anhydride are passed through the reactor heated at 90-150°C for a period of 6-7 h. Substituted acetophenones are so obtained in 63-100% yield. 

Zeolites have been used in the Friedel-Crafts acylation of (small) aromatics. While they perform well, pore-size constraints will limit their use - HBEA is generally the most active zeolite, but its largest channels are 7.6 Ax 6.4 A. With this material, acylations can be carried out with acid chlorides and anhydrides, with yields ranging from modest to veiy high. Substituting the Al centres for rare earths enhanced the activity of Y zeolites (Figure 4.3). ... 

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