Rietveld refinement of several structural models for mordenite that account for differences in the X-ray powder pattern

Abstract

The zeolite mordenite is becoming increasingly important in several catalytic processes that include paraffin isomerization, alkylations, dewaxing of heavy petroleum fractions, and shape-selective reactions of polynuclear aromatics. The catalytic activity of mordenite is related to the synthesis method and appears to be related to the X-ray powder diffraction (XRPD) pattern of the Na mordenite precursor. The differences in the XRPD pattern have been attributed to stacking errors and other effects, but the structural details are unknown. This work was conducted to define the source of these differences. Several theoretical models were developed to account for the XRPD changes. These include ordering of the cations and water molecules, c-axis faulting, and intergrowths of two different crystal forms. Rietveld refinement was used to fit the models to the experimental X-ray powder patterns. Three mordenite samples, with Si:AI ranging from 6:1 to 9.5:1, exhibit the range of differences observed in the X-ray pattern. Results indicate that a c-axis faulted structure is the best model to account for the observed XRPD behavior. The Rietveld refined fault contribution of 9–20% is consistent with other analytical and chemical evidence.