Synthesis of Pure VSH-2 in Large Quantities and Its Characterization
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A method to produce pure VSH-2 in large quantities (~20 g) was developed. The reagents were silica sol, V2O5, H2SO4, CsOH, and ethanol. Despite the fact that V2O5 was used as the vanadium source the oxidation states of most of the vanadium atoms in the produced VSH-2 were 4+, indicating that ethanol acts as a reducing agent. The crystals adopted individual octahedral shapes or aggregated states depending on the gel composition. The total surface area of the pristine VSH-2 with the general formula of Cs2(VO)(Si6O14)·3H2O was only 40 m2/g, indicating that the pores are blocked by the large Cs+ ions. The surface area increased to 149 m2/g upon exchanging Cs+ with Na+. Analyses of the diffuse reflectance UV–Vis spectra of Mn+–VSH-2 (Mn+ = Cs+, Na+, Ca2+, and Pb2+) revealed that the 215, 250, and 313 nm bands arise due to the V4+ to O2− metal-to-ligand charge transfer (MLCT) and the 437, 590, and 914 nm bands arise due to the d–d transition of V4+. This reveals an unprecedented interesting situation that in dehydrated VSH-2 the framework oxide plays the role of both acceptor to V4+ and donor to Mn+. The measured atomic magnetic moment (μ) was 1.64 BM, indicating that most of the V atoms exist in V4+. The ESR spectrum of VSH-2 showed a strong signal due to V4+ with the g value of 1.959 with ΔHpp value of 168 G. The Raman spectra of Mn+–VSH-2 revealed the existence of strong V=O stretching at 960 cm−1, and other weak peaks. The V=O stretching band shifted to a higher energy region upon increasing the Sanderson’s electronegativity of Mn+. The thermogravimetric (TGA) analysis showed that VSH-2 is thermally stable up to 550 °C and above which the oxidation of V4+ occurs.