Electron crystallography of MWW zeolites – filling the missing cone
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Douglas L. Dorset
Abstract
Given that many zeolites crystallize in habits that lead to preferred orientation, and that there is a ±60° tilt limitation in many electron microscope goniometer stages, collection of a three 3-D electron diffraction data set from these materials is severely restricted. To overcome this, preparative measures must be taken to produce other sample orientations. Using a number of MWW framework zeolites (MCM-22, MCM-22P, MCM-49, ITQ-1) as examples, thin-sectioned views onto the edge stacking of lamellar layers can be prepared by a number of methods. While ultamicrotomy is a traditional approach to this problem, sections are often very thick and sparsely distributed. Other extreme mechanical processing techniques, such as extrusion and jet milling produce numerous thin crystals with the desired orientation. Dealumination, initiated by steaming, calcination or low pH, also yields a similar result. Additional intensity data from the ‘missing cone’ in addition to those obtained by tilting hexagonal plate crystals provides a more complete data set and hence a more easily interpreted structure analysis. Crystallographic phase determination by maximum entropy and likelihood, followed by Fourier refinement, yields a structural model very close to that of the known MWW framework.
© by Oldenbourg Wissenschaftsverlag, Milford, New Jersey, Germany
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