On the Nature of Reaction-Controlled Phase Transfer Catalysts for Epoxidation of Olefin: A ³¹P 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 2}WO(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.