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

Abstract

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 (aSyen) > g (||). The structural properties of active sites in the AlSCMS were characterized by means of XRD, UV-Vis, Al-27 MAS NMR, FT-IR, and energy dispersive X-ray spectrometric measurements. Upon O-2 adsorption, the Mo(V) ESR signal intensity decreased, and a new O-2 (-) 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.