Surface heterogeneity analysis of MCM-41 metallosilicates by using nitrogen adsorption data

MCM-41 silicas with frawework-incorporated aluminum and boron heteroatoms were studied using nitrogen adsorption at 77 K over a wide range of relative pressures. It was shown that despite significant differences in surface areas, pore sizes, and widths of pore size distributions, the framework-substituted MCM-41 materials studied exhibited very similar low-pressure relative adsorption curves and adsorption energy distributions. This indicates that the presence of heteroatoms does not lead to significant changes in the surface properties with respect to nitrogen molecules. However, small but detectable and systematic changes in the low-pressure adsorption were observed for two series of samples with different contents of heteroatoms prepared under similar conditions. It was concluded that low-pressure nitrogen adsorption exhibits rather low sensitivity toward framework-incorporated aluminum and boron heteroatoms, but it may provide some information about contents of heteroatoms for samples prepared under the same conditions. Because of the similarity of nitrogen adsorption processes on surfaces of siliceous ordered mesoporous materials with and without framework-substituted heteroatoms, the methods of micropore analysis (e.g., the comparative plot method) and mesopore size analysis developed and/or calibrated for pure-silica materials are expected to be fully applicable for samples with different kinds of incorporated heteroatoms.