Design of synthetic zeolites as catalysts in organic reactions

Acylation of anisole with phenylacetyl and phenylpropanoyl chlorides and phenylacetic and 3-phenylpropionic acids was carried out in a batch reactor at 50–115°C over a series of zeolites Y at 21, 50 and 100% Na+ exchange, with a framework Si-to-Al ratio from 9 to 24 and on HZSM-5 and H-β zeolites. The products observed were the corresponding acids and the para-substituted aryl ketones. Only trace amounts of the ortho isomers could be detected, and phenol and cresols were absent from the reaction mixtures. When phenylpropanoyl chloride and phenylpropionic acid were used as the acylating agent, 1-indanone was also formed. From the relative adsorption constants of phenylacetyl chloride and anisole on HY-100 zeolite, and the absence of any anisole demethylation and rearrangement products, it was concluded that the interaction of the acid sites with the anisole oxygen must not be very strong. On the other hand, the influence of the level of Na+ exchange on the initial rate of formation of 1-(4-methoxyphenyl)-2-phenyl-phenylethanone shows that the activity of the catalyst is independent of its acid strength distribution. This was confirmed by studying the influence of the framework Si-to-Al ratio on the turnover numbers (TON). The relatively small TON differences could be related to the hydrophobicity of the samples. By using zeolites with different pore size and configuration it was observed that while Y and 5β zeolites show similar behaviour, the medium-pore ZSM-5 fails to catalyse the acylation of p-cresol with phenylacetic acid for steric reasons. Finally, it was observed that zeolites give a much higher ratio of inter- to intramolecular acylation than a typical Lewis acid such as AlCl3.