Characterization of high-quality MCM-48 and SBA-1 mesoporous silicas

Abstract

MCM-48 and SBA-1 samples were characterized using powder X-ray diffraction and nitrogen adsorption in a wide range of relative pressures. X-ray diffraction provided direct evidence for the high degree of pore structure ordering for the materials under study. Nitrogen adsorption showed that highly ordered MCM-48 materials exhibited sharp steps of capillary condensation and narrow pore size distributions, whereas capillary condensation steps for highly ordered SBA-1 samples were broad, which may be related to a cage-like pore structure of these materials. Surface properties of MCM-48 and SBA-1 with respect to nitrogen molecules were found to be similar to those of macroporous silica gels and thus close to those of MCM-41. High values of the surface area and mesopore volume for MCM-48 silicas indicate that both of the enantiomeric channel systems separated by the silica wall in the structure are fully accessible for nitrogen molecules. Moreover, the absence of detectable microporosity indicates that there are no holes on the nanometer length scale in the pore walls of MCM-48. It was demonstrated that MCM-48, MCM-41, and SBA-1 samples, which exhibited capillary condensation at approximately the same relative pressure, have markedly different ratios for the amount adsorbed on the pore surface to the total adsorption capacity (or alternatively, different pore volume/surface area ratios). As this behavior was attributed to differences in porous structures of these materials, such ratios may provide information which would allow one to differentiate between pore geometries of ordered mesoporous materials.