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

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, 29 Si magic-angle-spinning nuclear magnetic resonance (MAS NMR), and N 2 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. 29 Si MAS NMR and N 2 adsorption show that part of the chromium is incorporated into the MCM-41 structure. ESR shows that Cr(VI)−O 2 - exists in calcined CrMCM-41 and transforms to Cr(V) after evacuation above 150 °C. The interaction of Cr(V) with O 2, CO, C 2 D 4, ND 3, CD 3 OH, and D 2 O was studied. Cr(V) occurs at two different sites. At one site, Cr(V) coordinates with one molecule of O 2, CO, and D 2 O to form square pyramidal complexes and reacts with C 2 D 4, CD 3 OH, and ND 3 to be reduced to Cr(III). At the other site, Cr(V) is inert to O 2, CO, and C 2 D 4 while it coordinates with two molecules of ND 3 to form six-coordinated complexes and coordinates with one molecule of CD 3 OH.