Synthesis and Spectroscopic Characterization of Vanadosilicate Mesoporous MCM-41 Molecular Sieves

Abstract

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 29Si and 51V 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 51V 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 b...