RIETVELD REFINEMENT OF SEVERAL STRUCTURAL MODELS FOR MORDENITE THAT ACCOUNT FOR DIFFERENCES IN THE X-RAY-POWDER PATTERN

The zeolite mordenite is becoming increasingly important in several ca talytic processes that include paraffin isomerization, alkylations, de waxing 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 differen ces in the XRPD pattern have been attributed to stacking errors and ot her effects, but the structural details are unknown. This work was con ducted to define the source of these differences. Several theoretical models were developed to account for the XRPD changes. These include o rdering of the cations and water molecules, c-axis faulting, and inter growths of two different crystal forms. Rietveld refinement was used t o fit the models to the experimental X-ray powder patterns. Three mord enite samples, with Si:Al 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 observe d XRPD behavior. The Rietveld refined fault contribution of 9-20% is c onsistent with other analytical and chemical evidence.