Oxidation of olefins is a valuable reaction type in the production of fine chemicals.However,traditional reaction processes rely on the use of toxic or hazardous oxidants.To replace conventional oxidants with environmentally friendly hydrogen peroxide(H2O2)or oxygen(O2)and construct a"green"catalytic oxidation system has become a development trend.However,H2O2 or O2 with olefin substrates will form a multiphase reaction system,and the gas/liquid and liquid/liquid interfaces constrain both the mass and heat transfer processes of the reaction,as well as hinder the effective contact between polyoxometalates(POMs)and reactants,thereby inhibiting the reaction’s progress.This paper explores a new approach to overcome the limitations of the liquid/liquid interface on the H2O2/olefin multiphase reaction system by constructing organic hybrid POMs with both surface/interface activity and catalytic activity.A series of polyethylene glycol(PEG)hybrid POMs based on covalent bonding were prepared with POMs as hydrophobic head groups and PEG as hydrophilic tail segments.Based on the structural characterization of the hybrid POMs,the surface/interface activity and emulsification properties were investigated,and the catalytic performance in the H2O2/olefin multiphase reaction system was evaluated.The main research contents are as follows:(1)Synthesis and structural characterization of PEG-hybridized POMs(TMA-POM-Si-C3-S-mPEG).The silane-hybridized POMs(TMA-POM-Si-C3)were synthesized by reacting mercaptopropyl silane with mono-lacunary Keggin-type Na7PW11O39.Ethylene-modified PEG monomethyl ether(mPEG-allyl)was synthesized.POMs hybrids with different lengths of PEG chain were prepared by thiol-ene click reaction.The PEG-hybridized POMs were confirmed by FT-IR,FT-Raman,1H NMR,13C NMR and 31P NMR.(2)Surface/interface activity of TMA-POM-Si-C3-S-mPEG.The surface/interface properties,aggregation state in aqueous solution and emulsification ability of POMs hybrids with different lengths of PEG chain were evaluated.As the length of the PEG chain increases,the hybrid POMs’surface/interface activity and emulsification ability increase.TMA-POM-Si-C3-S-mPEG5000 with the longest PEG chain can reduce the surface tension of water to 38.92 m N/m,and form aggregates with a particle size of 138.5 nm in aqueous solution.In the styrene-water system,it can reduce the interfacial tension to 1.02 m N/m and form emulsion with droplets size of 10.39μm.(3)Catalytic performance of TMA-POM-Si-C3-S-mPEG in H2O2/olefin multiphase reaction system.Compared with pristine POMs,the POMs hybrids showed enhanced catalytic performance,and the catalytic performance increased with the surface/interface activity and emulsification ability of POMs hybrids.Under the conditions of catalyst concentration=0.0019 mol/L,VOlefin/V=1/10,and reacted at 60℃ for 7 hours,TMA-POM-Si-C3-S-mPEG5000 can achieve a styrene conversion of 73.09%and benzaldehyde selectivity of 68.08%,while the pristine POMs showed a styrene conversion of 2.63%and benzaldehyde selectivity of 45.40%.The enhanced catalytic activity of TMA-POM-Si-C3-S-mPEG was attributed to its adsorption at the H2O2/olefin interface,which not only promoted the formation of emulsions,expanding the interfacial area and improving the mass and heat transfer efficiency of the system,but also effectively increased the contact of olefins,H2O2,and the catalyst at the interface,leading to the enhanced catalytic activity of the POMs hybrids. |