Preparation of Isolated VO₄³⁻ Species in Silica Framework by Alkoxy-exchange Reactions between Metal Alkoxides and Polyethylene Glycol

Abstract

Isolated VOx species incorporated in an SiO2-framework were synthesized by alkoxy-exchange reactions between Si(OEt)4, V(Ot-Bu)3O, and polyethylene glycol (PEG). Similar alkoxy-exchange rates between alkoxides of vanadium and silicon with PEG were necessary to obtain isolated VOx species in the SiO2-framework. Gel was formed during the exchange reaction occurring with heating from 50 to 160 , liberating ethanol and t-butanol, and the obtained gel was calcined in air at 600 . The calcined samples were characterized by N2 adsorption, XRD, Raman, FT-IR, H2-TPR, and NMR techniques. Adsorption-desorption isotherms of N2 of the obtained samples exhibited H4-type hysteresis patterns, indicating the presence of narrow neck pores. In addition, the presence of micropores was confirmed. The samples had high surface areas of approximately 1000 m 2 /g, and isolated VO43-species were present in the SiO2-framework. Oxidative dehydrogenation of ethylbenzene was performed as a test reaction, using CO2 as a mild oxidant. The reaction proceeded on the isolated VO4 3-species in the silica framework with high ethylbenzene conversion and high selectivity for styrene.