Critical Appraisal of the Pore Structure of MCM-41

Abstract

Purely siliceous and aluminosilicate types of MCM-41 were synthesized and characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen sorption (NS) and size exclusion chromatography (SEC).

Of the four Bragg reflexes obtained by XRD the first one (100) was used to calculate the pore diameter, assuming a wall thickness of 1 nm. The remaining reflexes served as an indication of the twodimensional order of the material. However XRD cannot be used to determine the exact fraction of amorphous material present.

TEM was found to be a valuable tool to assess the different phases present in a sample (hexagonal, lamellar, non-ordered), provided several micrographs of different parts of a sample were taken.

Nitrogen sorption at 77 K on materials with a pore diameter ≤ 4 nm gave a reversible curve with a steep part at p/p0 ≤ 0.4. Application of the non-local density functional theory (NLDFT) allowed to model the nitrogen isotherm assuming a cylindrical pore shape but did not explain the reversibility of the adsorption and desorption branch. The formal application of the methods based on the Kelvin equation to calculate the pore size lead to an underestimation of the pore diameter compared to that obtained by the NLDFT. The calibration curve measured by means of size exclusion chromatography revealed two linear parts. The first molecular weight fraction range reflected the pore size of the primary particles, the second the interstitial pores of the agglomerates formed by the primary particles (size about 50 to 100 nm). A comparison between the pore volume of the primary particles from sorption experiments with the experimental pore volume assessed from the SEC data indicates that only 12 % are accessible for permeation.