Crystal Structure of ZSM-48: A 40 Year-Old Puzzle Resolved
Lynne B. McCusker, Bernd Marler, Christian Baerlocher, Youngkyu Park, Hermann Gies
Abstract
Eight calcined ZSM-48-type materials, selected for their diversity, have been examined crystallographically to determine whether they can be described in the same way as can RUB-58 (framework type RFE ), with blocks of a single framework structure separated by shear planes. In RUB-58, only one framework structure is present, but it can adopt either of two origins related by exactly one-half of a unit cell along the channel direction. The framework blocks can connect to one another seamlessly along well-defined shear planes. The eight materials were synthesized in two different laboratories using seven different organic structure-directing agents under a variety of conditions. At first glance, their diffraction patterns are similar, but the reflection intensities and peak shapes differ. Analysis of the eight structures revealed that indeed all can be refined using the RUB-58 structural model. Only the amount of the shifted framework and the peak widths (arising primarily from different crystallite sizes) differ. Comparison of the diffraction patterns generated from several series of disorder models (using two polymorphs) with the measured ones shows that those from the RUB-58 single-framework model yield the best matches. Geometric optimization, simulations of different shear planes and the morphology suggest that the shear plane along (110) is the most likely one. These results indicate that the disorder (involving different polymorphs), which has been assumed for ZSM-48-type materials for nearly 40 years, probably does not exist.