Determination of pore size and pore wall structure of MCM-41 by using nitrogen adsorption, transmission electron microscopy, and X-ray diffraction

Abstract

Highly ordered WCM-41 materials were synthesized using single or mixed alkyltrimethylammonium and alkyltriethylammonium surfactants of alkyl chain lengths from 12 to 22. As determined using combined adsorption/powder X-ray diffraction (XRD) analysis, the obtained samples exhibited pore sizes from 3.2 to 4.8 nm with about 0.3 nm increments. Nitrogen adsorption isotherms for the samples under study exhibited remarkably sharp capillary condensation steps. The adsorption isotherm for the highly ordered 4.8 nm MCM-41 featured a narrow hysteresis loop with parallel adsorption and desorption branches. This behavior is probably a feature of nitrogen adsorption in ideal cylindrical pores of this size and is distinctly different from the previously reported behavior of MCM-41 with pore sizes in the range 4-5.5 nm, which had triangular hysteresis loops on their nitrogen adsorption isotherms. All MCM-41 materials had remarkably narrow pore size distributions, which showed that fine-tuning of the pore size is easily achievable for the synthesis procedure used. The pole wall thickness was found to increase as the pore size increased. The pore wall thickness determined using transmission electron microscopy (TEM) was somewhat larger than that obtained using the combined XRD/adsorption analysis, which was attributed to inherent features of these determination methods. TEM images showed that pores of MCM-41 samples were approximately hexagonal rather than circular, thus providing structural information especially important for application of MCM-41 as a model adsorbent. Low-pressure adsorption studies of uncalcined ethnnol-washed MCM-41 samples revealed that external surfaces of their particles were covered by relatively dense layers of surfactant ions. This behavior is consistent with the generally accepted mechanism of the MCM-41 formation via self-assembly of silicate-surfactant ion pairs. Comparison of adsorption isotherms for uncalcined and calcined samples allowed us to exclude the possibility of any appreciable structural degradation during calcination, although TEM provided some indication of the lowering of structural ordering upon calcination.