Hafnium salts of dodeca-tungstophosphoric acid catalysts for liquid phase benzylation of anisole with dibenzylether

Abstract

A series of hafnium salts of heteropoly tungstate (HfxTPA) catalysts were prepared by exchanging the counter cations (hydrogen) of heteropoly tungstate with hafnium ions. The prepared catalysts were characterized by FT-IR, Laser Raman, temperature programmed desorption of ammonia, scanning electron microscopy, and X-ray photo electron spectroscopy. FT-IR and Raman characterization results suggested retention of the Keggin ion structure of heteropoly acid. Pyridine adsorbed FT-IR spectra of the catalysts showed that generation of Lewis acidic sites was due to the presence of hafnium ions in the catalyst. Activity of these catalysts was investigated for liquid phase benzylation of anisole with dibenzylether. In the partial exchanged catalyst (Hf0.5TPA), higher conversion was due to high mobility of residual protons. The catalytic activity of HfxTPA depended on the exchangeable hafnium ions in the secondary structure of heteropoly tungstate that, in turn, relates to variation in acidity of the catalyst. Thermal stability and structural properties of the Hf0.5TPA catalyst were studied by treating it at different temperatures. Dibenzylether conversion and selectivity towards benzylated products were found to depend on the anisole to dibenzylether ratio, reaction temperature, and catalyst loading.