Monitoring of the structure of siliceous mesoporous molecular sieves tailored using different synthesis conditions

Abstract

The hexagonally ordered siliceous mesoporous molecular sieve MCM-41 and the disordered mesoporous molecular sieve designated as KIT-1 were studied by means of the X-ray diffraction (XRD), transmission electron microscopy and low temperature nitrogen adsorption over a wide pressure range. It was shown that the combination of these techniques allows for a thorough characterization of novel mesoporous materials. A repeated pH adjustment happens to be crucial in the synthesis of MCM-41 samples of high pore size uniformity. An additional influence of the salt effect (i.e. the addition of Na₄EDTA during the hydrothermal crystallization) allows one to obtain MCM-41 materials of excellent porous structures and remarkably high thermal and hydrothermal stability. The latter features may be a result of the presence of relatively thick pore walls for those mesoporous molecular sieves.

A disordered character of KIT-1, which was synthesized with an addition of Na₄EDTA to the initial synthesis gel, was supported by the fact that the sample exhibits a high pore size uniformity but yet a rather featureless XRD pattern. It was shown that comparative methods, such as the α_s-plot, are very useful for analysis of mesoporous molecular sieves, allowing for an accurate calculation of the specific surface area of the samples, which exhibit condensation in mesopores in the pressure range used in the standard BET method. A method to calculate the pore diameter for MCM-41 samples from XRD and adsorption data was shown to be convenient and useful for an accurate pore size assessment.