ADSORPTION STUDY OF SURFACE AND STRUCTURAL-PROPERTIES OF MCM-41 MATERIALS OF DIFFERENT PORE SIZES

Nitrogen adsorption measurements were performed over a wide range of r elative pressures (10(-6)-0.995) for a series of siliceous MCM-41 samp les obtained using alkyltrimethylammonium surfactants with different c hain length. Both high- and low-pressure adsorption data were analyzed . The pore size was shown to increase in a regular way with the chain length of the surfactant used. Moreover, a very good correlation betwe en the pore size and the interplanar spacing of the MCM-41 samples was observed. Methods used to calculate the pore diameter were critically compared, and a new procedure to estimate the pore size of MCM-41 mat erials was proposed. This new procedure is based on geometrical consid erations of the ratio of the pore volume to the pore wall volume for a n infinite hexagonal array of cylindrical pores. Adsorption measuremen ts showed that the amount adsorbed in the low-pressure region increase s with a decrease in the pore size of the samples probably because of the enhancement of the gas-surface interactions caused by an increase in the curvature of pore walls. The adsorption energy distributions we re found to be similar for all samples studied and quite close to thos e previously reported for porous silica gels. However, the adsorption energy distributions for the MCM-41 samples were exceptionally numeric ally stable.