Different Kinds of Tetrahedral V Species in Vanadium-Containing Zeolites Evidenced by Diffuse Reflectance UV−vis, Raman, and Periodic Density Functional Theory
The combined use of DR-UV?vis and Raman spectroscopies allows one to evidence three kinds of tetrahedral VV with different structure in VxSiBEA zeolite. In all cases, vanadium species possess distorted tetrahedral configuration. At very low vanadium content (0.05 wt %), the majority of tetrahedral V(V) species are strongly distorted and have a nonhydroxylated pyramidal structure (SiO)3V═O with V═O stretching at 1054 cm?1 whereas the remaining tetrahedral V(V) species are less distorted with a hydroxylated pyramidal structure (SiO)2(HO)V═O with V═O stretching at 1018 cm?1. In contrast, at higher vanadium contents (0.2?2 wt %), the main distorted V(V) species possess a nonhydroxylated pyramidal structure (SiO)3V═O with V═O stretching at 1033?1036 cm?1. Periodic density functional theory (DFT) calculations confirm the presence of at least two different types of V site in V-containing zeolites. The experimental Raman bands recorded for VxSiBEA with low and high V content were confirmed on the basis of DFT calculations related to seven vanadium model sites. The difficulty to assign the vibrations in V-containing silica materials is due to the overlap of the different critical vibrational fingerprints (V═O, and Si?O?V vibration modes). Moreover, it was found that H-bond formation with a V═O group is not energetically favorable.