Polyoxometalate intercalated layered silicates: Highly efficient catalytic epoxidation of cyclooctene

Abstract

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 (Na8Si32O64(OH)8·32H2O, Oct) is an ideal support due to its optimal arrangement and high density of SiOH/SiO−. 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 of 1-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 K7PW11O39·12H2O (PW11), we are able to successfully fabricate the POM-intercalated layered silicates material that can be denoted as PW11-IL-Oct. The resulting PW11-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 PW11-IL-Oct reveals the basal spacing of 2.85 nm. 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 PW11-IL-Oct are 5.7 and 15.4 m2/g, respectively. The loading amount of PW11 in PW11-IL-Oct is 7.34wt% by ICP-AES analysis. SEM images of PW11-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 PW11-IL-Oct reveal the highly dispersed black spots with the diameter approximately ~1 nm, which is in good agreement with the particle size of PW11 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 PW11-IL-Oct exhibits excellent activity and stablity. The yield of epoxides product reaches up to 98% in 1 h with >99% selectivity. Moreover, after 10 recycles, no obvious loss of activity can be observed and the loading amount of PW11 in PW11-IL-Oct maintain 7.16wt%. HRTEM and Mapping results show almost no aggreaction of PW11 clusters in the layered silicates after epoxidation reaction. This work paves a pathway for fabrication of novel POMs-based catalysts.