Ecofriendly liquid phase oxidation with hydrogen peroxide of 2,6-dimethylphenol to 2,6-dimethyl-1,4-benzoquinone catalyzed by TiO2-CeO2 mixed xerogels

Abstract

Titania–ceria mixed xerogels were prepared with 10% (w/w) CeO2 and different pH values during the sol–gel process. The gels were dried in air and calcined at different temperature. Moreover, a sample without cerium, 100% (w/w) TiO2, was prepared to be kept as reference material. The prepared solids were characterized by means of XRD, SEM, DRS, FT-IR, TEM and EDX. Their textural properties were determined by adsorption–desorption isotherms of N2 at 77 K. The xerogels were tested as catalysts in the liquid phase oxidation of 2,6-dimethylphenol at 20 °C, using ethanol as solvent and aqueous hydrogen peroxide as a clean oxidizing agent. The 2,6-dimethyl-p-benzoquinone yield achieved was of 85–96% using the mixed xerogels as catalysts, and of 49% when the catalyst of titania without cerium was used. The best performance was achieved by the mixed xerogel synthesized at pH 4 and calcined at 200 °C; 100% of conversion of 2,6-dimethylphenol and 96% of yield in 2,6-dimethyl-p-benzoquinone were obtained in this case after 6 h of reaction. The reactions were again carried out with the catalyst recovered after its first use. Only a light yield decrease, lower than 2%, was observed. The use of an ethanol–water mixture as solvent (35:65% (v/v)) in the reaction was studied in order to make the reaction more environmentally friendly. Total conversion of the initial reactant, after 4 h of reaction at 20 °C, with 70% yield in 2,6-dimethyl-p-benzoquinone (2,6-DMBQ), was observed. Under these conditions a red by-product, insoluble in the reaction solution, was formed. This by-product was isolated and characterized and it was possible to identify it as 2,6-DMBQ dimmer (3,3′,5,5′-tetramethyl-4,4′-diphenoquinone). This by-product was observed in very few amounts when ethanol (96% (v/v)) was used as solvent.