Novel catalysts for eco-compatible anisole acylation

FUCHS, V. M. PIZZIO, L. R., BLANCO, M. N.

Dresden, Alemania

Conferencia; 1st International IUPAC Conference on Green-Sustainable Chemistry; 2006

German Chemical Society

In the present paper novel materials were prepared by immobilizing stoichiometric aluminium or copper salts of tungstosilicic acid in a polymeric matrix in order to avoid its solubilization in the reaction medium during reaction. The matrix was constituted by polyvinyl alcohol and polyethylene glycol and was prepared with spherical shape by the freeze-thawing method. It was observed by Fourier transform infrared spectroscopy that the salts retain the Keggin primary structure of the heteropolyanion during their synthesis and immobilization. By potentiometric titration with n-butylamine it was estimated that the immobilized salts present very strong acid sites, being the acid strength of the catalyst based on the aluminium salt higher than that of the copper salt. X-ray diffraction patterns of the solid samples did not present any line corresponding to crystalline structures of the bulk salts. This may be attributed to a high dispersion of the salts in the polymeric matrix. Besides, by scanning electron microscopy with an energy dispersive X-ray analysis system it was observed that, when the salts are included in the polymeric blend, they are almost constant distributed along the sphere width. The catalytic activity of the polymer-immobilized salts was proved in the acylation of anisole with acetic anhydride in liquid phase at 100 ºC, as a test reaction previous to its use in the 2-methoxynaphtalene acylation. It was selected acetic anhydride as acylating agent instead of acetyl chloride to avoid hydrogen chloride emission. The main product of the reaction was p-methoxyacetophenone, with selectivity higher than 90%. Only monoacylated products were detected. It was found that the conversion of the catalyst obtained from the aluminium salt is higher than that of the copper salt, that could be related to its higher acid strength. It must be also considered that the Al3+ cations have absolute hardness values higher than Cu2+ cations, so they are more effective to form the acylium ions. This type of catalyst can be used in liquid phase acylation reactions as eco-compatible catalyst, so it is attractive for many processes in replacement of conventional homogeneous catalysts at temperatures below that of the decomposition of the support (200 ºC).