Effect of the nature of the templating surfactant on the synthesis and structure of mesoporous V-Mg-O

The mesostructured V-Mg-O was synthesized at room temperature using vanadium acetylacetonate and magnesium chloride as source materials. The effects of a number of factors, such as the nature of the surfactant, the pH, and the vanadium/magnesium molar ratio in the formation of mesoporous V-Mg-O phases were investigated for a fixed molar concentration of surfactant. The synthesized samples were examined by X-ray diffraction, Brunauer-Emmett-Teller analysis, transmission electron microscopy, and energy dispersive spectroscopy. The anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) generated amorphous structures, while the cationic surfactant cetyltrimethylammonium bromide (CTAB) generated lamellar mesoporous structures with various degrees of crystallinity. Equimolar mixtures of an anionic surfactant, SDS, and a cationic surfactant (dodecyltrimethylammonium bromide (DTAB), myristyltrimethylammonium bromide (MTAB), cetylpyridium bromide (CPB), and cetyltrimethylammonium bromide (CTAB)), particularly the mixture SDS + CTAB, generated ordered lamellar structures with high crystallinity. It was found that the ordered arrangement ("crystallinity") of the pores was strongly dependent on the vanadium/magnesium molar ratios in the initial batches; too large or too small ratios were unfavorable to the crystallinity and generated distorted hexagonal (wormlike) mesostructures. The optimum vanadium/magnesium molar ratio regarding the crystallinity was found to be in the range 1:1 to 1:1.5, and the mesoporous phase had a lamellar structure. For both the single and the mixtures of anionic and cationic surfactants, the higher the pH, the more favorable was the formation of a lamellar-structured mesoporous V-Mg-O phase.