Synthesis and characterization of mesoporous chromium-containing silica tube molecular sieves CrMCM-41

A series of mesoporous chromium-containing silica tube molecular sieves (Cr MCM-41) with variable Si/Cr ratios have been synthesized and characterized by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), Fou rier transform infrared spectroscopy (FTIR), diffuse reflectance W-visible spectroscopy (UV-vis), electron spin resonance (ESR), electron spin-echo mo dulation (ESEM), Raman spectroscopy, Si-29 magic-angle-spinning nuclear mag netic resonance (MAS NMR), and N-2 adsorption measurements. XRD, EPMA, W-vi s, 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. Si-29 MA S NMR and N-2 adsorption show that part of the chromium is incorporated int o the MCM-41 structure. ESR shows that Cr(VI)-O-2(-) exists in calcined CrM CM-41 and transforms to Cr(V) after evacuation above 150 degrees C. The int eraction of Cr(V) with O-2, CO, C2D4, ND3, CD3OH, and D2O was studied. Cr(V ) occurs at two different sites. At one site, Cr(V) coordinates with one mo lecule of O-2, CO, and D2O to form square pyramidal complexes and reacts wi th C2D4, CD3OH, and ND3 to be reduced to Cr(III). At the other site, Cr(V) is inert to O-2, CO, and C2D4 while it coordinates with two molecules of ND 3 to form six-coordinated complexes and coordinates with one molecule of CD 3OH.