Mixed xerogels as heterogeneous catalysts for an environmentally friendly

MAGDALENA PALACIO; PAULA VILLABRILLE; GUSTAVO ROMANELLI; PATRICIA VÁZQUEZ; CARMEN CÁCERES

Rosario, Santa Fe, Argentina

Workshop; 1st Argentinean Workshop in Environmental Science; 2009

Facultad de Ciencias Bioquímicas y Farmaceúticas- Unievrsidad Nacional de Rosario

p-Benzoquinones derivatives play important roles in biosystems. Furthermore, some of them show bioactivities and some are the key semiproducts for synthesis of a number of medicines and physiologically active substances. The 2,6-dimethyl-p-benzoquinone can be used for the synthesis of trimethylbenzoquinone or for the preparation of indophenol.  Benzoquinones are usually prepared by direct oxidation of phenols. A number of oxidants are available in the literature for the preparation of quinones, but most of them produce a negative impact in the environment, for example, peroxiacids u oxidants which involves transition metals as Cr (VI). A sustaintable alternative is the use of peroxides. Among them, the hydrogen peroxide is the one that follow the green chemistry postulates almost entirely. On the other hand, mixed metal oxides have been widely investigated since they have interesting catalytic properties. The presence of a foreign element in the matrix of a pure metal oxide can greatly modified the structural, acid-base and catalytic properties which, in turn, can be modulated by a proper selection of the nature and loading of the foreign element and method the preparation. We report here that CeO2-TiO2 and V2O5-TiO2 mixed xerogels may be used as heterogenous catalysts for an efficient and selective oxidation of 2,6-dimethylphenol (2,6-DMP) to                                          2,6-dimethyl-p-benzoquinone (2,6-DMBQ) at room temperature (20°C) with aqueous hydrogen peroxide (60% water solution) as an environmentally friendly primary oxidant (Scheme 1). The mixed xerogels were prepared using titanium isopropoxide, hexahydrated cerium nitrate and vanadyl acetylacetonate as precursors, respectively. They were characterized by means of XRD, SEM and FT-IR. Their textural properties were determined by adsorption-desorption isotherms of N2 at 77 K. The best performance of CeO2-TiO2 prepared catalysts was achieved by the xerogel with 10 % w/w CeO2 calcined at 200°C; 100 % of conversion and 96 % of yield in 2,6-dimethyl-p-benzoquinone were obtained in this case after 6 hours of reaction. This catalyst was recovered and reused, only a light yield decreased, lower than 2%, was observed. Mixtures of ethanol-water (50:50; 35:65 %(v/v)) were also good solvents to perform this reaction, a conversion of 100% and a yield of 70% were obtained in 4 h with the last one mixture. However, a reaction intermediary, 3,3´,5,5´-tetramethyl-4,4´-diphenoquinone, was also identified by a low-resolution mass spectra recorded on GC-MS. When V2O5-TiO2 mixed xerogels was used as catalysts, total conversion and very good yields were also obtained in ethanol as solvent; 85% of yield after 3 hours of reaction for 5 % w/w V2O5 , calcined at 200°C, prepared xerogel. This work seeks to contribute to the search of new heterogeneous catalysts to carry out the oxidation of organic compounds in an environmentally sustainable way (low temperature, cleaner oxidizing agent, and safer solvents).