Immobilized 2,2,6,6-tetramethyl-piperidinyl-1-oxy catalyst on polymer microspheres and its catalytic oxidation of benzyl alcohol with molecular oxygen

Abstract

Crosslinked polymer (glycidyl methacrylate) microspheres (CPGMA) were prepared with glycidyl methacrylate as monomer and ethylene dimethacrylate as crosslinker by suspension polymerization. The ring opening reaction between the epoxy groups on the CPGMA microspheres and 4-hydroxy-2,2,6,6-tetramethyl-piperidinyl-1-oxy (4-OH-TEMPO) was carried out to immobilize TEMPO on the polymer microspheres. TEMPO-immobilized microspheres (TEMPO/CPGMA) were obtained and were characterized by several methods. The effects of the main factors in the immobilization were examined so that the reaction conditions were optimized. On this basis, a co-catalyst system was constituted with TEMPO/CPGMA and CuCl and was used in the oxidation reaction of benzyl alcohol by molecular oxygen. The catalytic activity of the co-catalyst system and the effects of the main factors were examined. The experimental results showed that the ring opening reaction of the epoxy group allowed the immobilization of TEMPO to be smoothly realized on the CPGMA polymer microspheres, on which there were many epoxy groups. The ring opening reaction of the epoxy group is a nucleophilic substitution reaction with the SN2 reaction mechanism, and N,N′-dimethyl formamide with a strong polarity is an appropriate solvent and a suitable temperature is 85 °C. The co-catalyst system consisting of the heterogeneous catalyst TEMPO/CPGMA and CuCl efficiently catalyzed the oxidation reaction of benzyl alcohol by molecular oxygen. Under mild conditions, benzyl alcohol was transformed to benzaldehyde with 100% selectivity and a yield of 90%. The suitable molar ratio of the main catalyst TEMPO/CPGMA to the co-catalyst CuCl is 1:1.2 and the suitable amount of TEMPO/CPGMA is 0.90 g. The TEMPO/CPGMA catalyst has excellent recyclability.