Studies on the structural change of a reaction-controlled phase-transfer [π-C5H5NC16H33]3{PO4[WO3]4} catalyst during the selective oxidation of cyclopentene to glutaric acid with aqueous H2O2

The selective oxidation of cyclopentene to glutaric acid (GAC) with aqueous hydrogen peroxide was carried out over a reaction-controlled phase-transfer catalyst—[π-C5H5NC16H33]3{PO4[WO3]4}. The high GAC yield of 83.1% was obtained on the fresh catalyst, while a much higher GAC yield of 98.8% was obtained over the recovered catalyst. The fresh catalyst, the one under reaction conditions and the recovered ones were all characterized by TG, FT-IR, Raman and 31P NMR spectroscopy. It is interesting to find that the atomic content of tungsten and phosphorous as well as the molecular structure of the fresh catalyst all change after the reaction. 31P NMR results reveal that under the treatment with high concentration hydrogen peroxide the insoluble catalyst can degrade into smaller and active species, which is soluble in the reaction mixture and can transfer oxygen to the CC bond, resulting in the selective cleavage of cyclopentene to glutaric acid. After the complete consumption of hydrogen peroxide, the smaller and active tungsten species will polymerize into (PW11O39)7− and (PW12O40)3− with stable Keggin structure by forming WOcW (edge-sharing) bond. These compounds are insoluble and will precipitate from the reaction mixture after the reaction when hydrogen peroxide is used up, making it much convenient for recovering and reusing.