Electron crystallography of MWW zeolites – filling the missing cone

Abstract

Given that many zeolites crystallize in habits that lead to preferred orientation, and that there is a AE60 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.

Zeolite MCM-22 Modified with Au and Cu for Catalytic Total Oxidation of Methanol and Carbon Monoxide

“…The appearance of the prepared MCM-22 crystals that can be seen in Figure 2S (TEM image), Supporting Information, shows typical characteristics. 1 They comprise thin plates, oriented almost exclusively flat-on, preventing estimation of their thickness, 45 some showing distinctly hexagonal shape. The widths vary considerably, falling in the approximate range from 50 to 500 nm.…”

Two-Dimensional Zeolites: Current Status and Perspectives

“…The mentioned treatments and operations are not only sources of problems but may also lead to an advantage, as reported for MWW crystal that were extruded, jet-milled, and subject to other potentially degrading treatments. It was observed that, in contrast to standard samples showing mainly flat-on crystal orientation, the treated ones contained many edge-on crystals . This apparently more random spatial orientation and maybe some fragmentation may contribute to high performance, for example, as for MCM-56 mentioned in the preceding paragraph.…”

“…It was observed that, in contrast to standard samples showing mainly flat-on crystal orientation, the treated ones contained many edge-on crystals. 260 This apparently more random spatial orientation and maybe some fragmentation may contribute to high performance, for example, as for MCM-56 mentioned in the preceding paragraph.…”

Layered Materials with Catalytic Applications: Pillared and Delaminated Zeolites from MWW Precursors

“…The main advantage of the mesoporous distribution generated in the MCM-36 was the easy accessibility achieved to internal acid active sites, overall for voluminous reactant molecules which take part in different catalytic processes [106]. On the contrary, the pillared materials, such as the MCM-36 zeolites, show the important inconvenient associated to the partial blockage of active sites, present in the external surface of MWW layers, due to the intercalation of silica pillars in the interlayer space [107].…”