Impact of aqueous impregnation on the long-range ordering and mesoporous structure of cobalt containing MCM-41 and SBA-15 materials
Résumé
The effect of modification with cobalt via aqueous impregnation on the structure of MCM-41 and SBA-15 materials was studied using X-ray diffraction (XRD) at low scattering angles and low-temperature nitrogen adsorption. It was shown that the introduction of small amounts of cobalt resulted in dramatic changes in the structure of MCM-41 silica. The low angle XRD peaks characteristic of long-range ordering disappeared while the BET surface area and the total pore volume decreased considerably. Only micropores smaller than 20 Å were detected in the cobalt containing MCM-41 materials. A much smaller impact of impregnation with cobalt on the hexagonal structure and porosity was observed for SBA-15 materials. The periodic structure, surface area and total pore volume of SBA-15 mesoporous silica remained almost unchanged on impregnation with cobalt nitrate, catalyst drying and calcination. Low angle XRD patterns typical of hexagonal structure were still well resolved and intense in the SBA-15 materials containing up to 20 wt. % of cobalt. Analysis of XRD intensities suggested that after aqueous impregnation and drying a considerable amount of cobalt ions was located in the SBA-15 mesopores. Nitrogen adsorption-desorption data indicated plugging a part of SBA-15 mesopores in cobalt containing samples. Due to the high stability, SBA-15 silica appears to have a good potential for its use as a support for metal and oxide catalysts.