A Spectroscopic Study On The Heteropolyoxo-tungstophosphates For Olefin Epoxidation

Zuwei Xi et al. reported a reaction-controlled phase transfer catalyst composed of quaternary ammonium heteropolyoxotungstates [π-C5H5N(CH2)15CH3]3[PW4O 15], which is active and selective for epoxidation of propylene and other olefins (such as linear terminal olefms, internal olefms, cyclic olefms, styrene, and allyl chloride) with 30% hydrogen peroxide. This catalyst is insoluble solid and can dissolve into the reaction solution when it reacts with hydrogen peroxide, and the catalyst becomes insoluble solid and precipitate again after the reaction with olefms. Therefore the catalyst can be easily separated from the reaction system. However, the mechanism of the reaction-controlled phase transfer of this catalyst is not clear. To understand the changes of the structure and species of the catalyst during and after the epoxidation reaction, the fresh catalyst, the catalyst under reaction condition and the used catalysts were characterized by FT-IR, Raman and 3IP NMR spectroscopy.It is found that the fresh catalyst [π-C5H5N(CH2)15CH3]3[PW4O16] is a complexmixture, which can form small active species, [(PO4){WO(O2)2}4]3-(PW4), [(PO4){WO(O2)2}2{WO(O2)2(H2O)}]3- (PW3), and [(PO3(OH)){WO(O2)2}2]2-(PW2) containing the common peroxo group ^202(^-02)2(02)2] via the reaction with hydrogen peroxide, these small hydrophilic anions can be extracted and transport into the organic medium by proper lipophilic cation. The active oxygen in the peroxo group [W2O2(n-O2)2(O2)2] subsequently reacts with olefms to produce the epoxides and the corresponding W-Ob-W(corner-sharing) and W-Oc-W( edge-sharing) bonds are formed, the peroxo group [W2O2(u-O2)2(O2)2] can be regenerated after the W-Ob-W and W-Oc-W bonds react with hydrogen peroxide again. The small species polymerize into the large compounds throughthe formation of the W-Ob-W and W-Oc-W bonds that the lipophilic quaternary ammonium cation can not extract and transport into the reaction system, as a result, the catalyst becomes insoluble solid and precipitates from the reaction medium after the hydrogen peroxide is consumed up. With the increase of the reaction cycles, the catalyst tends to gradually transforms into a more stable [PW12O40]3- polyanion under the reaction condition. The fresh Q3[PW12O40] (Q= [π-C5H5N(CH2)15CH3] catalyst almost can return to its original structure and precipitate from the reaction system after the epoxidation reaction.In addition, a novel ionic liquid-1-octyl-3-methyl-imidazolium heteropoly-oxotungstophosphates was synthesized and characterized by FT-IR, 31P NMR and DTA. It is found that the ionic liquids has a wide liquid range and thermally stables up to 300癈. The epoxidation of olefins with hydrogen peroxide or urea hydrogen peroxide (UHP) was performed with the ionic liquid as both solvent and catalyst, the conversions were 53.9, 32.1 and 12.6% based on hydrogen peroxide and the selectivities were 50.6, 32.1 and 9.2% for cyclohexene, styrene and 1 -hexene, respectively.