Tungsten oxide supported cage-type mesoporous silicate: W-KIT-5
Anand Ramanthan1 , Wenjuan Yan1 , Rajamanickam Maheswari1 and Bala Subramaniam1,2 *
1Center for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, KS 66047, USA 2Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, KS 66045, USA *email@example.com
Introduction Tungsten has been incorporated on different supports and the resulting materials have been studied as catalysts for olefin metathesis , acid catalyzed reactions such as isomerization  and dehydration , and oxidation of alkenes  . Among various supports, three dimensional cage-type mesoporous silicates are considered to be advantageous due to their interconnected cage-type pores that can accommodate bulky reaction intermediates. KIT- 5 is a highly ordered cubic Fm3m closely packed symmetry with tunable cage-type pores . Based on these advantages, we present herein the preparation of tungsten incorporated KIT-5 materials with detailed characterizations of structural properties and the nature of tungsten coordination within the KIT-5 framework. We also show that such catalysts show ethylene epoxidation activity. Materials and Methods Tungsten was incorporated into cubic cage type mesoporous silicate, KIT-5, via hydrothermal synthesis method using a Pluronic F-127 triblock copolymer as the structure directing agent. Four W-KIT-5 samples with Si/W ratio of 100, 40, 20 and 10 were prepared. Calcined W-KIT-5 samples were characterized by SAXS, XRD, elemental analysis, N2 sorption, HR-TEM, Raman, DR-UV-Vis, H2-TPR and NH3-TPD. W-KIT-5 samples are tested in the epoxidation of ethylene in ethylene expanded liquid phase with 50% H2O2 as oxidant (8 g) and methanol (20 g) as solvent. Acetonitrile was employed as internal standard. The reaction is carried out in ethylene expanded liquid phase (50 bars) in a 50 mL Parr reactor at 35 °C, stirred at 1,400 rpm to ensure the absence of mass-transfer resistances. Results and Discussion Cubic Fm3m symmetry of KIT-5 samples was confirmed by the presence of reflections due to (111) and (200) planes in low angle XRD spectra. The values obtained for unit cell parameter is similar to those observed in KIT-5 materials. N2 sorption of W-KIT-5 samples showed type IV adsorption isotherm with a sharp capillary condensation step between 0.60 – 0.70 P/P0 and a broad H2-type hysteresis loop with a desorption occurring at ~0.47 P/P0 typically of ordered mesoporous materials possessing large uniform cage-like pores. The BET surface area drastically decreased from 964 m2 /g (for Si/W = 100) to 425 m2 /g (for Si/W = 10) and the pore volume decreased from 0.68 to 0.42 cm3 /g respectively. A typical Fm3m structure was also noticed in HR-TEM for W-KIT-5(20) (Fig. 1). An intense band at 212-220 nm observed in Diffuse reflectance UV-Vis spectra is due to ligand to-metal charge transfer in isolated [WO4] tetrahedral species  and is a direct proof for the framework incorporation of tungsten species in silica framework (Fig. 1). Existence of partially polymerized W species in octahedral coordination with low nuclearity is also observed in these spectra. This is further confirmed by Raman and high angle XRD spectra. XPS studies suggest the coexistence of tungsten as both W5+ and W6+ in the surface of KIT-5. In addition, W-KIT-5 samples are shown to possess acid sites of low to medium strength. Figure 1. Diffuse reflectance UV-Vis spectra of W-KIT-5 (Si/W ratio) samples compared with Si-KIT-5 and crystalline WO3. Inset: HR-TEM image of W-KIT-5(20). Table 1. Ethylene epoxidation over W-KIT-5 catalyst W-KIT-5 (Si/W =) 10 20 40 100 EO productivity mg EO/g W/h 8.4 13.4 19.4 43.4 Significance The results represent the first reported synthesis and characterization of W-KIT-5 materials. W-KIT-5 shows ethylene epoxidation activity with hydrogen peroxide without formation of CO2. This opens up the possibility of further developing these catalysts as selective and relatively inexpensive epoxidaiton catalysts.
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