Preparation of Highly Ordered Vanadium-Substituted MCM-41: Stability and Acidic Properties

Abstract

Vanadium-substituted mesoporous molecular sieves (MCM-41) were prepared and the effects of water concentration, anti-foaming agent addition, pH adjustment, and Si/surfactant mole ratio were investigated. The hydrothermal and mechanical stability were tested for the pH-adjusted sample (Si/surfactant = 3.67 and 7.34). Catalysts were characterized by XRD, N2 physisorption, ICP, and 51V NMR. The acidic properties of synthesized V−MCM-41 were also examined by in-situ FTIR and TPD after pyridine adsorption. The increase of water concentration in the synthesis solution did not affect the physical properties of the resulting samples. The anti-foaming agent was very effective in improving the reproducibility of sample structure. The degree of vanadium substitution in the silica framework was increased with increasing water concentration and surfactant chain length. All of the incorporated vanadium was tetrahedrally coordinated in the silica framework. When the pH of the synthesis solution for V−MCM-41 was adjusted to 11 (Si/surf. = 7.34), the hydrothermal and mechanical stability were enhanced. The vanadium-substituted MCM-41 formed Lewis and Brönsted acid sites. Brönsted acid sites increased with increasing vanadium content, but the density of Lewis acid sites was constant for all samples. It is hypothesized that there is formed a constant and saturated density of isolated tetrahedral coordinated vanadium, and these make weak Lewis acid sites. Brönsted acid sites can be formed by a combination of incorporated vanadium with a hydroxyl group of a free silanol and/or a hydroxyl group produced by dissociation of water when the sample was pretreated at high temperature.