Preparation Of Phosphotungstic Acid-Functionalized Pickering Emulsifier And Its Catalytic Application For Olfein Epoxidation Reaction

Oxidation reaction was one of the most fundamental and important reactions in organic synthesis reaction with wide application in industry.Traditionlly,this process utilized over stoichiometric amount of ingorganic metal oxides or salts as oxidants,such as manganese dioxide,potassium permanganate,potassium dichromate and so on,which generated a large amount of metal-containing wastes and led to serious environmental issues.From the viewpoint of green chemistry,aqueous hydrogen peroxide represented one of the most ideal oxidants for oxidation reaction since water was the sole by-product.However,the direct use of hydrogen peroxide for oxidation reaction was infeasible,and a suitable catalyst was generally needed to promote the reaction.Polyoxometallates are a class of inorganic transition metal oxide clusters that can be used as catalyst for H₂O₂-involved oxidation reaction.But the catalytic efficiency of such catalyst system is often far from ideal due to the immiscibility of organic reactants and aqueous H₂O₂ solution.In this paper,phosphotungstic acid-functionalized Pickering emulsifier was prepared,which was incombinated with aqueous 30%H₂O₂ solution as the oxidant and a mixture of ethyl acetate and acetonitrile as the solvent to form Pickring emulsion.Then,the epoxidation of olefins in this emulsion was investigated.The detailed research content was as following:Part I:A series of hydrophilic and hydrophobic silane coupling agents were prepared from triethoxysilane and their structures were characterized by NMR and GC-MS.Also,several nano-sized silica particles with different particles sizes were prepared from tetraethyl orthosilicate in certain ammonia solution.The morphologies of prepared nano-sized silica particles were characterized by transmissioin electron microscopy?TEM?.The results showed that these nano-particles had good dispersion.The silane coupling agents L-2 and L-5 were chosend to modify the surface of nano-silica by adjusting the ratio of the silane coupling agents L-2 and L-5.The modified nano-silica particles were characterized by FT-IR.H₃PW₁₂O₄₀.xH₂O was loaded onto the surface of the modified nano-silica by ion-exchanged method.Finally,the obtained H₃PW₁₂O₄₀.xH₂O-functinalized Pickering interface catalyst was used for the olefin epoxidation reaction with aqueous H₂O₂ solution as oxidant.The 30%H₂O₂ solution was used as an aqueous phase and mixed with different organic solvents.The prepared solid catalysts were added to the mixed solvent,and the emulsion was observed after homogenizing at high stir speed?12000 rpm/s?.After that,the epoxidation of olefins in the formed emulsion was studied.Using cyclooctene as a model substrate,various reaction conditions,including the kind of organic solvents,reaction temperature and reaction time were optimized and the resulted-optimal reaction conditions were as following:0.6 mL of ethyl acetate,0.4mL of acetonitrile,5.0 equivalents of H₂O₂?30 wt.%?as both oxidant and aqueous phase,0.3 mol%of catalyst loading and at 60 ^oC,for 6 h.Under this reactioin conditions,the conversion of cyclooctene can reach to 98%with the TON number up to 327.The propsed catalytic system were also efficient in the epoxidation of cyclohexene,cyclopentene.Unfortunately,linear olefins did not react in this system.