Novel Fe-based complex oxide catalysts for hydroxylation of phenol

Novel catalysts for the hydroxylation of phenol, Fe-Si-O, Fe-Mg-O and Fe-Mg -Si-O complex oxides, have been synthesized by a coprecipitation method. X- ray diffraction studies show that MgFe2O4 crystallites with spinel structur e are formed in Fe-Mg-Si-O and Fe-Mg-O complex oxides and the crystallite s ize of the metal oxide or complex oxide is reduced after addition of Si. In the hydroxylation of phenol with hydrogen peroxide, Fe-based complex oxide s exhibit high activities after a short induction period. The phenol conver sion is improved when silicon is introduced into the Fe-based complex oxide s, and formation of MgFe2O4 crystals with spinel structure in the catalysts increases the diphenol selectivity. The addition of a little acetic acid t o the reaction liquid can shorten the induction period effectively. Under t he same reaction conditions, phenol conversion and diphenol selectivity ove r the Fe-Mg-Si-O catalyst are close to those over TS-1, and furthermore, th e reaction time is more than ten times shorter as compared to TS-1. The rea ction mechanism of the hydroxylation of phenol on the catalysts has been st udied, and a free-radical mechanism initiated by the formation of phenoxy f ree radicals is suggested.