Physics & Astronomy

Diffuse scattering reveals framework faulting in zeolite mordenite

Microporous framework compounds are an important class of materials due to their widespread applications as molecular sieves, ion-exchangers, and heterogeneous catalysts.  The application-critical properties of these materials are highly-sensitive to the structure of the host framework.  Because framework defects grossly modify the local framework connectivity, they also alter the shape-selective host-guest interactions and the related adsorptive and catalytic properties.  Even a dilute concentration of defect sites has a significant impact, providing the motivation to investigate framework defect structure-property relationships.

Zeolite mordenite, an important industrial isomerization catalyst, has long been suspected of framework variability.  Our recent 20 keV x-ray single-crystal diffuse-scattering study of this material uncovered a novel defect architecture consisting of randomly distributed columnar fault defects and a block-mosaic network of planar stacking faults.  See J. Appl. Cryst. 37, 187-192 (2004).  These features significantly alter the wall-structure and connectivity of the mordenite channels, but do not appear to block the channels themselves.  We validated our atomistic-defect model by reconstructing two-dimensional sheets of the experimental diffuse-scattering distribution (from portions of many different area-detector images), and comparing them against computer-simulated patterns produced by collaborator Richard Welberry at Australian National University.

Image: the diffuse-scattering pattern within the  L = 5 reciprocal-space plane of a natural mordenite single-crystal.  The hazy patches, diamonds and star-like features all reveal distinct characteristics of the actual defect structure.