Raman Spectroscopy of Vanadium Oxide Species Immobilized at Surface Titanium Centers of Mesoporous Titanosilicate TiMCM-41 Molecular Sieves

The vanadium oxide species immobilized at surface titanium centers of mesoporous titanosilicate TiMCM-41 molecular sieves have been studied by Raman spectroscopy to establish a better structural understanding of these vanadium oxide species and their transformations. Siliceous MCM-41 gives a Raman spectrum similar to that of amorphous SiO2 except that a band at 978 cm-1 from surface silanol groups is much more intense. This supports a previous finding that the amorphous silica walls of MCM-41 contain abundant silanol groups. Incorporation of titanium into the MCM-41 framework dramatically reduces the Raman intensity due to strong fluorescence and reduction of the local symmetry arising from a longer Ti−O bond relative to a Si−O bond. Vanadium oxide supported on siliceous MCM-41 in its calcined form shows an intense Raman band at 1033 cm-1 characteristic of terminal vanadyl groups. This suggests that V5+ is present in siliceous MCM-41 as an isolated tetrahedral vanadate species possessing one normal terminal VO bond and three V−O−Si bonds to the support. In contrast, such a Raman band is not observed in vanadium oxide supported on TiMCM-41 with low vanadium loading. This is suggested to be due to the affinity of titanium for oxygen to produce a stronger Ti−O bond than a SiO bond and thus a stretched terminal VO bond. As the vanadium loading on TiMCM-41 increases beyond a critical V/Ti ratio of 0.33, a broad feature in the 800−900 cm-1 region is resolved and assigned to bridged V−O−V chain vibrations of polymeric vanadium oxide species. This feature shifts to higher frequency as the vanadium loading increases. Simultaneously, the 1033 cm-1 band observed in vanadium oxide supported on siliceous MCM-41 appears, indicating additional bonding between vanadium species and titanium-free surface sites. Prolonged laser illumination of samples with high vanadium loading results in the disappearance of the broad feature in the 800−900 cm-1 region as well as a band near 960 cm-1 together with an intensity increase in the band at 1033 cm-1. This indicates that surface dehydration and dehydroxylation occur under laser illumination together with decomposition of the polymeric vanadium oxide species immobilized at titanium centers which migrate to titanium-free surface sites.