V-MCM-22: Synthesis and characterization of a novel molecular sieve

This work presents the synthesis and spectroscopic characterization of V-MCM-22, a novel molecular sieve. The synthesis was performed under static hydrothermal crystallization, using VOSO₄ as source of vanadium at SiO₂/V₂O₃ = 66 and hexamethyleneimine as structure-directing agent. Aluminum was also incorporated in the structure at SiO₂/Al₂O₃ ratios (SAR) of 50 and 80, named V-MCM-22 (50) and V-MCM-22 (80) respectively. Only V-MCM-22 (50) showed higher crystallinity than the parent MCM-22. The insertion of vanadium ions in the framework sites was confirmed by diffuse reflectance (DR) UV-Vis and FTIR spectroscopy, the latter using CO adsorption at 100K. The V-MCM-22 (50) sample presented V in the 5+ state after calcination of the template; two families of vanadium sites were found: a family of distorted tetrahedral oxovanadium (SiO)₃V=O species absorbing at 290, 250 and 330 nm which underwent a reduction to V^IV (d-d transition at 550 nm) after treatment in H ₂ at 500°C, and a family of tetrahedral oxovanadium with a lower distortion degree, which showed bands at 225, 245 and 265 nm. The latter species were stable after reduction. Hydroxyls bound either to the Lewis vanadium centers or to partially extra-framework Al ions were detected by FTIR and their acidity monitored by CO adsorption. The stretching frequency of these hydroxyls showed a red-shift of ca. 200 cm^-1 by CO adsorption which suggests an acidity intermediate between silanols (90 cm^-1) and bridged SiO(H)Al groups (320 cm^-1). This material is a good candidate for selective oxidation reactions of organic molecules.