Synthesis and spectroscopic characterization of vanadosilicate mesoporous MCM-41 molecular sieves

A series of mesoporous vanadosilicate VMCM-41 molecular sieves with variable Si/V ratios have been hydrothermally synthesized at pH 11 using 2-propanol as a phase stabilizer. The chemical environment of the vanadium centers in VMCM-41 is investigated by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), framework FTIR, diffuse reflectance UV-visible spectroscopy (UV-vis), electron spin resonance (ESR), and Si-29 and V-51 NMR. XRD, EPMA, and ESR show that the solid products have the MCM-41 structure and contain only atomically dispersed vanadium consistent with framework vanadium in VMCM-41. UV-vis, ESR, and V-51 NMR reveal that most of the vanadium exists as tetrahedral V5+ ions in as-synthesized samples, but some square pyramidal VO2+ ions simultaneously occur. Both tetrahedral V5+ and square pyramidal VO2+ centers occur at two different sites inside and on the surface of the hexagonal tubular walls of MCM-41. Upon calcination and hydration, only vanadium species on the wall surfaces can be completely oxidized to tetrahedral V5+ and transformed to square pyramidal and then distorted octahedral V5+ species by additional coordination to water molecules. Vanadium species located inside the walls remain stable upon calcination and hydration. Upon thermal or CO reduction, also only the V5+ species on the wall surfaces of VMCM-41 are reversibly reduced to VO2+ species or lower valences.