Adsorption Study of Surface and Structural Properties of MCM-41 Materials of Different Pore Sizes

TLDR

Nitrogen adsorption over a wide pressure range was used to analyze MCM‑41 samples, and a new geometrical procedure based on the pore‑volume to wall‑volume ratio of an infinite hexagonal array of cylindrical pores was proposed to estimate pore size. The study found that pore size increases with surfactant chain length and correlates with interplanar spacing, that low‑pressure adsorption rises as pore size decreases due to enhanced surface curvature, and that adsorption energy distributions are similar to porous silica gels yet exceptionally stable.

Abstract

Nitrogen adsorption measurements were performed over a wide range of relative pressures (10-6−0.995) for a series of siliceous MCM-41 samples obtained using alkyltrimethylammonium surfactants with different chain 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 between 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 materials was proposed. This new procedure is based on geometrical considerations of the ratio of the pore volume to the pore wall volume for an infinite hexagonal array of cylindrical pores. Adsorption measurements showed that the amount adsorbed in the low-pressure region increases 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 were found to be similar for all samples studied and quite close to those previously reported for porous silica gels. However, the adsorption energy distributions for the MCM-41 samples were exceptionally numerically stable.