多酸插层层状硅酸盐及其在催化氧化环辛烯中的应用

Polyoxometalate intercalated layered silicates: Highly efficient catalytic epoxidation of cyclooctene

多酸基多相催化剂研究的重点和难题在于实现活性组分的良好分散和稳定负载.本文通过将催化活性组分多酸插层到离子液体修饰的层状硅酸盐限域空间内,制备了一种新型多酸基催化材料.通过扫描电子显微镜、高分辨透射电子显微镜、X射线衍射、能量色散X射线等谱学技术对催化剂进行了详细表征.结果表明,多酸在层状硅酸盐限域空间内实现了高分散、稳分散和均匀分散;层内共价修饰的离子液体有助于有机底物与多酸活性组分的可接近性,促进传质,提高反应效率.将所设计的催化材料应用在环辛烯的环氧化反应中,显示了优异的催化效率和选择性, 1 h对反应产物的收率可达98%.这为今后设计与创制高效多酸负载型催化剂提供了新的思路.

Polyoxometalates（POMs）are a class of discrete anionic metal oxides with unique physical and chemical properties.Owing to excellent redox activity and structural adjustability,POMs-based catalysts have drawn wide attention during past decades.However,homogenous POMs exhibit high crystallinity behavior,which result in aggregation and/or recycling difficulty.Therefore,appropriate supports are highly needed to achieve well dispersion and stable loading of POMs on supports.Layered silicates are a large family of two-dimensionally arranged cation-exchangeable layered materials,whose frameworks are composed of SiO4 tetrahedral and possess interlayer exchangeable cations that are often hydrated.Most importantly and interestingly,layered silicates can turn to be anion exchangeable materials through covalent modification of imidazolium salts containing terminal triethoxysilyl groups onto interlayer surface.As a result,layered silicates can generate a large variety of interesting structures and properties.Among them,the layered octosilicate（Na8Si（32）O（64）（OH）8·32H2O,Oct）is an ideal support due to its optimal arrangement and high density of Si OH/Si O^-.It is worthwhile noting that more than 80%of interlayered SiOH/SiO^-groups can be silylated with imidazolium salts generally,and are converted to anion exchangeable sites.Herein,the ionic liquid modified layered silicates（IL-Oct）have been synthesized through immobilization of1-octyl-3-（3-triethoxysilylpropyl）-4,5-dihydroimidazolium-hexafluorophosphate（Oim-PF6）on the interlayered surface of layered silicates.The cation exchangeable sites on layered silicates can be stoichiometrically converted to anion exchangeable sites after modification.Further ion exchange with K7PW（11）O（39）·12H2O（PW（11））,we are able to successfully fabricate the POM-intercalated layered silicates material that can be denoted as PW（11）-IL-Oct.The resulting PW（11）-IL-Oct has been fully characterized using a variety of spectroscopy methods including X-ray diffraction（XRD）,scanning electron microscope（SEM）,high resolution transmission electron microscopy（HRTEM）,Brunauer-Emmett-Teller（BET）gas adsorption method,inductively coupled plasma-atomic emission spectroscopy（ICP-AES）and energy dispersive spectrometer（EDS）,etc.XRD pattern of PW（11）-IL-Oct reveals the basal spacing of 2.85nm.The calculated gallery height value is 2.11 nm by subtracting the thickness of the host layer（0.74 nm）.The BET surface areas of IL-Oct and PW（11）-IL-Oct are 5.7 and 15.4 m^2/g,respectively.The loading amount of PW11 in PW（11）-IL-Oct is 7.34wt%by ICP-AES analysis.SEM images of PW（11）-IL-Oct show the square plate morphology with distortion and wrinkles on the surface.The SEM Mapping results suggest that the POMs clusters are highly dispersed and orderly arranged in the two-dimensional（2D）confined gallery spaces.Moreover,HRTEM images and EDS result of the PW（11）-IL-Oct reveal the highly dispersed black spots with the diameter approximately^1 nm,which is in good agreement with the particle size of PW（11） clusters.Epoxides are widely utilized as raw materials for sythesis of biologically important pharmaceuticals and fine chemicals.The application of epoxides is growing fast in the chemical industry,and gives rise to remarkable attention within the scientific community.In this work,by using the expoxidation of cyclooctene as model reaction,we demostrate that the PW（11）-IL-Oct exhibits excellent activity and stablity.The yield of epoxides product reaches up to 98%in 1 h with99%selectivity.Moreover,after 10 recycles,no obvious loss of activity can be observed and the loading amount of PW（11） in PW（11）-IL-Oct maintain 7.16wt%.HRTEM and Mapping results show almost no aggreaction of PW（11） clusters in the layered silicates after epoxidation reaction.This work paves a pathway for fabrication of novel POMs-based catalysts.