Beta zeolite anisole acylation

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

Both materials were tested as catalysts in the anisole acylation (Scheme 1). The conventional Beta sample showed a slightly higher activity than the Beta (PHAPTMS). At 3 hours, the conversions were 26.8 and 22.8 % for the conventional and seed silanized catalysts, respectively. This behavior is explained as a consequence of the relatively small size of the anisole molecule, which allows this compound to diffuse without significant hindrances through the zeolitic micropores, and of the slightly weaker acidity of the Beta (PHAPTMS) sample. In both cases, p-methoxyacetophenone (p-MAP) was the main reaction product, being obtained with a high selectivity (> 97%). 

Beers, A. E. W., van Bokhoven, J. A., de Lathouder, K. M., Kapteijn, F. and Moulijn, J. A. Optimization of zeolite Beta by steaming and acid leaching for the acylation of anisole with octanoic acid a structure-activity relation, J. Catal., 2003, 218, 239-248. 

In contrast to anisole, the acylation of toluene with HPA is far less efficient than that with H-Beta. These results can be explained by the well-known strong affinity of bulk HPA towards polar oxygenates, which would lead to the preferential adsorption of acetic anhydride on HPA, blocking access for toluene to the catalyst surface. It appears that the hydrophobic zeolites with high Si/Al ratios less strongly differentiate the adsorption than the hydrophilic HPA and, therefore, are more suitable catalysts for the acylation of nonpolar aromatics like toluene. 

For example, the Friedel-Crafts-acylation of anisole with acetic anhydride with the acidic zeolite H-beta leads to p-methoxyacetophenone in fixed-bed operation (Eq. 8-26). Compared with the traditional process, using more than one equivalent of AICI3 (Eq. 8-27), the number of unit operations could be reduced from 12 to 2 by Rhone-Poulenc. 

---