MoO2(acac)(2) immobilized on polymers as catalysts for cyclohexene epoxidation: Effect of the degree of crosslinking.

摘要

Crosslinked poly(4-vinylpyridine-co-styrene) used as support for a immobilized oxomolybdenum complex was synthesized by radical polymerization. As crosslinking agent, 1,4-divinylbenzene (DVB) was incorporated at 2, 4, and 6 mol %. Catalysts having similar MoO2(acac)(2) loading on different resins were obtained by refluxing a solution of the complex with the polymeric supports at 70degreesC. FTIR, specific surface area, and XPS techniques were used for characterization. It was found that a significant increase in the surface area occurs as the degree of crosslinking increases, presumably due to the development of branched polymers having higher porosity. XPS showed that the complex is anchored to the pyridinic nitrogen on the resin and that that Mo was present essentially as Mo(VI) species. The Mo/C atomic surface ratio exhibits a maximum for the catalyst with medium crosslinking degree. The epoxidation of cyclohexene was studied in the temperature range 40-60degreesC by using tert-butyl hydroperoxide as oxidant agent. The catalysts were active and selective to the corresponding epoxide and almost no leaching of the active phase was observed. The highest catalytic activity of the studied solids was displayed by the one supported on the resin with intermedium degree of crosslinking. The results are explained in terms of the access to the active sites and surface composition. (C) 2004 Wiley Periodicals, Inc.