MODELING X-RAY-PATTERNS AND TEM IMAGES OF MCM-41

Several techniques have been applied to analyze the pore size, pore sh ape and pore structure of MCM-41. In order to assess the significance of X-ray diffraction (XRD) data and transmission electron microscopy ( TEM) images, we constructed a computer model (CERIUS2 of Molecular Sim ulations, San Diego, CA, 1996) of an amorphous silica with a unidimens ional pore system and hexagonally shaped pores. From the simulation of TEM focus series and crystal thickness series of this model, it can b e seen that the picture obtained strongly depends on imaging condition s. In addition, we used the model to simulate diffraction patterns of defect structures: destroying the hexagonal array of the pores or simu lating a broader pore size distribution in the model structure, often has only a minor influence on the simulated diffraction patterns. Alth ough these findings are not unexpected, it might be useful to keep the limitations of the methods used in mind, if conclusions concerning th e structural properties of MCM-41 are drawn from XRD or TEM data alone . (C) 1998 Elsevier Science B.V. All rights reserved.