Electron spin resonance study of Mo(V) ion species incorporated into aluminosilicate nanospheres with solid core/mesoporous shell structure

Silica spheres with sub-micrometer sized solid core and mesoporous shell (SCMS) structure were synthesized, and aluminum was incorporated into the mesoporous shell framework by impregnation method to generate SCMS aluminosilicate (AlSCMS) nanospheres. The impregnation of aluminum into the SCMS spheres generates the acid sites on the framework due to the presence of Al³⁺ ions. The AlSCMS was then used to support molybdenum ion species on the mesoporous shell framework. A solid-state reaction of MoO₃ with AlSCMS followed by thermal reduction generated paramagnetic Mo(V) species. The dehydration produced a Mo(V) species that is characterized by electron spin resonance with g_e > g_⊥ > g_∥. The structural properties of active sites in the AlSCMS were characterized by means of XRD, UV-Vis, ²⁷Al MAS NMR, FT-IR, and energy dispersive X-ray spectrometric measurements. Upon O₂ adsorption, the Mo(V) ESR signal intensity decreased, and a new O₂ ^- radical was generated. The Mo species in the dehydrated Mo-AlSCMS is found to exist as oxo-molybdenum species, (MoO₂)⁺ or (MoO)³⁺. Since the AlSCMS has a low framework negative charge, the MoO₂ ⁺ with a low positive charge can be easily stabilized and thus seems to be more probable in the AlSCMS framework.