Characterization of Defects and Surface Structures in Microporous Materials by HRTEM, HRSEM, and AFM

Abstract

High-resolution transmission (HRTEM) and high-resolution scanning electron microscopy as well as atomic force microscopy (AFM), X-ray diffraction, and electron diffraction were used for studying the zeolites MFI, MEL, and the MFIMEL intergrowth system. All three zeolites consisted of individual particles having a size in the range of approximately 0.5 m to 5 m. The particle habits varied from rather cubelike to almost spherelike with many intermediate habits. Typically, the particles of these three zeolites were assembled by many individual blocks that differed in the dimension from about 25 nm to 140 nm as well as in the shape from very frequently almost rectangular (for MFI, MEL, and MFIMEL) to sometimes roundish or irregular habits (mainly for MFIMEL). An estimate shows that some 104 up to more than 106 densely packed blocks typically may assemble each individual zeolite particle or, related to the corresponding unit cell dimension, about 108 up to 1010 unit cells. The fine surface structure of zeolite particles was terracelike with steps between adjacent terraces typically in the range of 20 nm to 60 nm; the minimum step measured was approximately 4 nm. A detailed study of the surface topography was performed by AFM, detecting organic molecules at the block intersections. The presence of topological defects was observed by HRTEM and electron diffraction.

Evolution of Surface Morphology with Introduction of Stacking Faults in Zeolites

“…Specific SDAs such as [18]crown-6 (18C6) and [15]crown-5 (15C5) are used to synthesise the pure forms of EMT and FAU, respectively. By using a mixture of SDAs specific for each system in the synthesis gel (MFI-MEL [23,[28][29][30] and FAU-EMT [31][32][33] ), it is possible to vary the amount of intergrowths by varying the SDA compositional ratios. Such studies also help to develop an understanding of the role of organic templates in the crystal growth of zeolites.…”

“…[29] Catalytic cracking efficiency of the pure and the faulted MFI and MEL has also been compared. [35] HRTEM studies have provided useful insights into the internal structure of MFI-MEL faults in ZSM-5 [28,36,37] and boron-MEL [38] and also FAU-EMT faults in zeolite Y. [32,40,41] There have been only a few attempts in the literature to study the surface structure of faulted crystals.…”

Incidence and properties of nanoscale defects in silicalite

“…For comparison, we also calculated the energy of forming a regular stacking fault between MFI and MEL, which gives rise to a continuous structure. This class of defect is readily observed in crystal samples 4,9 and has been discussed in some detail by Thomas and Millward. By comparison, the stacking fault energy is just 27 mJ m 22 , clearly suggesting that such a defect could be formed more readily than the interrupted structures we have studied.…”

Study of template interactions in MFI and MEL zeolites using quantum methods

“…A model of anion and cation distribution is proposed here for the MFI zeolite. (d) A very important issue to be considered in upcoming modeling studies is the formation of intergrowths [35][36][37], because it may produce changes in the zeolite reactivity. In addition, solvent effect should be taken into account because it is very important to stabilize components that participate in the zeolite structure formation [38].…”