Coupling of chemical, electrochemical and ultrasonic energies for controlled generation of hydroxyl radicals - Direct synthesis of phenol by benzene hydroxylation

Hydroxyl radicals HO. are generated under Fenton-like (Fe2++H2O2-->HO.+OH-+Fe3+) catalytic conditions using chemical, electrochemical and ultrasonic energies in combination. The resulting hydroxyl radicals are utilised in a model reaction in which benzene is hydroxylated directly to produce phenol. Chemical energy alone using a FeCl2 catalyst supported on silica gel gives synthetic yields of ca. 2% (calculated on benzene initially present), which are increased to 13-15% when irradiated with ultrasound. Coupled chemical and electrochemical synthesis in which the Fe2+ catalyst is generated in situ through controlled dissolution of an iron electrode produces yields of 10-13%. Higher yields (15-20%) are obtained when ultrasonic energy is coupled with the chemical-electrochemical method. The merits and limitations of these different methods and the significance of the yields of the model reaction in terms of the efficiency of radical generation are discussed. it is concluded that the combined chemical-electrochemical-ultrasonic approach offers more effective control of energy input for hydroxyl radical generation appropriate to various synthetic reactions. The direct synthesis of phenol approaches a limiting yield of ca. 20% when competing side reactions, such as further oxidation, occur. (C) 2000 Elsevier Science B.V. All rights reserved.