One-step synthesis and structural characterization of mesoporous vanadosilicates and super-microporous vanadosilicates

This paper reports the one-step ambient-temperature synthesis of porous vanadosilicates by using tetraethyl orthosilicate as a silicon source, V2O5 powder as a vanadium source, and long-chain n-alkylamine (C8–C12) as a structure directing agent. The texture of the materials was characterized by powder X-ray diffraction, N2 adsorption–desorption isotherms, transmission electron microscopy, and thermogravimetric and differential thermal analysis. The pore size of the vanadosilicates depended largely upon the chain length of n-alkylamine. The n-alkylamine with longer carbon chain was advantageous for the formation of the larger pores. The Si/V ratio was another key factor affecting pore size of the vanadosilicates. When n-dodecylamine was employed as a structure directing agent, only mesopore structure vanadosilicates were obtained. However, when n-decylamine was used, the pore diameter of the vanadosilicates underwent a change from mesoporous scale to super-microporous scale through changing the Si/V molar ratio. The vanadosilicates exhibit a large BET specific surface area and a good thermal stability. The Si/V molar ratios of both mesoporous and super-microporous vanadosilicates with the relatively uniform pore channels could be reached 10.0 or even below. The vanadosilicates with a Si/V ratio in the range of 8.5–12 synthesized by using either n-dodecylamine or n-decylamine have a high thermal stability beyond 823 K. The nature of vanadyl species in the vanadosilicates was investigated with the use of several characterization techniques. It was determined by Raman, H2-TPR and ESR that both VVOx and VIVOx species were coexisted in the calcined vanadosilicates. The VVOx species occurred mainly in a form of monomeric tetrahedral vanadyl species on the pore wall, having one VO bond and three bridging VOSi bonds linked with the framework. The V4+ centers were in an isolated octahedrally-coordinated VIVOx mode. It was interesting to find that only a small part of VIVOx species existed in the as-synthesized vanadosilicates, while a large part of VIVOx species were formed upon calcination in air.