Direct templating assembly route for the preparation of highly-dispersed vanadia species encapsulated in mesoporous MCM-41 channel

Abstract

Understanding the nature of active sites, including the number and dispersion on the surface of a support, is essential to improve the catalytic activity. In this study, highly-dispersed and controllable quantities of vanadia species within the channels of mesoporous MCM-41 were directly prepared by a direct templating assembly method (S⁺L⁻M⁺I⁻). This method was based on the self-assembly of cationic surfactants (CTA⁺, S⁺), chelating agents (citrate ions, L⁻), vanadyl ions (VO²⁺, M⁺) and silicate oligomers (I⁻) via electrostatic and chelating interaction. First, the citrate ions were absorbed on the CTA⁺ micelles' surface by electrostatic interaction, and the vanadyl ions were subsequently anchored on their surface by chelating with citrate ions to form metallomicelles. Finally, the silicates were deposited on the metallomicelles to obtain the targeted product. The structures of the samples especially the oxidation state and surface distribution of vanadium species on the mesoporous silica were efficiently characterized with different techniques, including XRD, N₂ adsorption, SEM, TEM, UV-vis, XPS, FT-IR, ICP, and H₂-TPR. Furthermore, the samples obtained using hydroxylation of benzene as a probe reaction exhibited superior catalytic activities when compared with the post-synthesized sample.