Vanadium-incorporated MCM-48 materials: Optimization of the synthesis procedure and an in situ spectroscopic study of the vanadium species

Highly crystalline and porous vanadium-incorporated MCM-48 materials were prepared using gemini surfactants as structure-directing agents and vanadyl sulfate pentahydrate as the source of the heteroelements. Materials with Si/V ratios varying from 20 to 100 were synthesized without loss of the typical cubic MCM-48 structure. The synthesis conditions were optimized to yield reproducible V-MCM-48 materials of high quality. The resulting materials are thoroughly characterized by W-vis diffuse reflectance, electron spin resonance, Raman, and Fourier transform infrared spectroscopies. It was proven that the V ions in the MCM-48 are present as isolated surface species and as incorporated species in the silica matrix. For the first time, both chemical and spectroscopic tools were employed to distinguish the incorporated V sites (which are present inside the silica walls) and species that are situated externally on the surface. A fairly low amount (1 wt %) of V species is really incorporated, but these species are extremely stable, and do not leach, whereas the surface species are easily lost in aqueous media.