Unveiling unique structural features of the YNU-5 aluminosilicate family

Highlights

• YNU-5 family of zeolites have been investigated by advanced electron microscopy methods.

• The structures were solved by 3D-Electron diffraction tomography obtaining a higher symmetry that the one already reported.

• Atomic resolution imaging of the surface revealed different terminations associated to the dealumination process.

• An additional tetragonal minor phase, solved by electron diffraction as MSE framework type, was identified.

Abstract

YNU-5 (YFI type) is the first zeolite reported with interconnected 12-, 12-, and 8-ring pores showing a remarkable catalytic potential towards the dimethyl ether (DME)-to-olefin reaction. In this work, the structures of the as-synthesized, calcined and dealuminated YNU-5 zeolites, were investigated by various techniques with special emphasis on advanced electron microscopy methods. The frameworks of the three materials were solely determined by three-dimension electron diffraction tomography, and the space group for the three of them was determined to be Cmmm, which is of higher symmetry than the previous reported result. Rietveld refinement was performed against synchrotron Powder X-ray diffraction data in order to obtain precise information of the framework and to locate the organic species, cations and water. Additionally, spherical aberration-corrected scanning transmission electron microscopy was employed to study the local fine structure and to indicate surface reconstruction associated to the displacement of the vacancies through the dealumination process. Finally, a minor phase, whose structure was solved by electron microscopy was found to be MSE framework type, appeared in all the three YNU-5 materials.

Overall, the electron microscopy analyses reported in the present work provide additional information regarding the YNU-5 structure in terms of space group determination, additional surface terminations and the identification of a minor phase.