On the Nature of Reaction-Controlled Phase Transfer Catalysts for Epoxidation of Olefin: A31P NMR Investigation

Abstract

[π-C5H5NC16H33]3[PW4O16] was reported to be an excellent epoxidation catalyst which exhibited a unique reaction-controlled phase transfer behavior. In the paper, the composition and structural changes of the reaction-controlled phase transfer catalyst during and after reaction have been investigated by 31P NMR spectroscopy. The 31P MAS NMR confirmed that the original catalyst was a mixture of heteropoly tungstophosphates. When the catalyst reacted with hydrogen peroxide, the species PO4[WO(O2)2]4 3−, [(PO4)WO(O2)2 2WO(O2)2(H2O)]3− and [(PO3(OH))WO(O2)2 2]2− were detected by in situ 31P NMR. It was also found that the P/W ratios and quaternary ammonium cations had great influence on the composition of heteropoly tungstophosphates. Although the catalyst with [(C18H37)2N(CH3)2]+ was not a reaction-controlled phase transfer catalyst, it could be precipitated from the reaction solution when acetone was subsequently added to the solution. The 31P MAS NMR spectra of the recovered catalysts revealed that they had more low P/W ratio heteropoly tungstophosphates than fresh catalysts.