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 mesaporous titanosilicate TiMCM-41 molecular sieves have been studied by Raman spectroscopy to establish a better structural understanding o f these vanadium oxide species and their transformations. Siliceous MC M-41 gives a Raman spectrum similar to that of amorphous SiO2 except t hat a band at 978 cm(-1) from surface silanol groups is much more inte nse, This supports a previous finding that the amorphous silica walls of MCM-41 contain abundant silanol groups. Incorporation of titanium i nto the MCM-41 framework dramatically reduces the Raman intensity due to strong fluorescence and reduction of the local symmetry arising fro m 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 suggest s that V5+ is present in siliceous MCM-41 as an isolated tetrahedral v anadate species possessing one normal terminal V=O bond and three V-O- Si bonds to the support. In contrast, such a Raman band is wt observed in vanadium oxide supported on TiMCM-41 with low vanadium loading. Th is is suggested to be due to the affinity of titanium for oxygen to pr oduce a stronger Ti-O bond than a Si-O bond and thus a stretched termi nal V=O bond. As the vanadium loading on TiMCM-41 increases beyond a c ritical V/Ti ratio of 0.33, a broad feature in the 800-900 cm(-1) regi on is resolved and assigned to bridged V-O-V chain vibrations of polym eric vanadium oxide species. This feature shifts to higher frequency a s the vanadium loading increases. Simultaneously, the 1033 cm(-1) band observed in vanadium oxide supported on siliceous MCM-41 appears, ind icating additional bonding between vanadium species and titanium-free surface sites. Prolonged laser illumination of samples with high vanad ium loading results in the disappearance of the broad feature in the 8 00-900 cm(-1) region as well as a band near 960 cm(-1) together with a n intensity increase in the band at 1033 cm(-1). This indicates that s urface dehydration and dehydroxylation occur under laser illumination together with decomposition of the polymeric vanadium oxide species im mobilized at titanium centers which migrate to titanium-free surface s ites.