Synthesis and Characterization of Mesoporous Chromium-Containing Silica Tube Molecular Sieves CrMCM-41

Abstract

A series of mesoporous chromium-containing silica tube molecular sieves (CrMCM-41) with variable Si/Cr ratios have been synthesized and characterized by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance UV−visible spectroscopy (UV−vis), electron spin resonance (ESR), electron spin-echo modulation (ESEM), Raman spectroscopy, 29Si magic-angle-spinning nuclear magnetic resonance (MAS NMR), and N2 adsorption measurements. XRD, EPMA, UV−vis, and ESR show that the as-synthesized CrMCM-41 materials have the MCM-41 structure and contain only atomically dispersed Cr(III). FTIR, UV−vis, and Raman reveal that Cr(VI) monochromate exists in calcined CrMCM-41. 29Si MAS NMR and N2 adsorption show that part of the chromium is incorporated into the MCM-41 structure. ESR shows that Cr(VI)−O2- exists in calcined CrMCM-41 and transforms to Cr(V) after evacuation above 150 °C. The interaction of Cr(V) with O2, CO, C2D4, ND3, CD3OH, and D2O was studied. Cr(V) occurs at two different sites. At one site, Cr(V) coordinates with one molecule of O2, CO, and D2O to form square pyramidal complexes and reacts with C2D4, CD3OH, and ND3 to be reduced to Cr(III). At the other site, Cr(V) is inert to O2, CO, and C2D4 while it coordinates with two molecules of ND3 to form six-coordinated complexes and coordinates with one molecule of CD3OH.