Structural case studies of inclusion phenomena in zeolites: Xe in RHO and stilbene in ZSM-5

The development and increased availability of brighter synchrotron sources has stimulated investigations of zeolite-inclusion complexes using powder X-ray diffraction data. This is illustrated with two examples of studies facilitated by these sources. Both involve the use of Rietveld refinement to obtain structure from data collected at a versatile synchrotron beamline. The first example describes the structural changes commensurate with Xe desorption from a fully Xe-loaded powder sample of Cd-exchanged zeolite rho. After exposure to the atmosphere, data were collected in real-time as Xe desorbed from the sample over a four hour period. The results of Rietveld refinement indicated that Xe is not lost continuously; instead within the first hour, Xe desorbs completely from the 8-ring and is replaced by H₂O. Following rearrangement of the remaining Xe between the available 6-ring sites, it is lost as the sample hydrates fully. This behavior is probably the result of competition with H₂O, which prefers the 8-ring site. In the second example, the high resolution and signal-to-background discrimination afforded the synchrotron powder X-ray diffraction experiment, was used to advantage to study the stilbene-ZSM-5 complex. Stilbene was located, using modeling techniques in the straight channels with one phenyl ring at the intersection of the two channel system. The structure is pseudo-tetragonal (a a b) at room temperature but distorts at lower temperatures to give b>a. Distinguishing these subtle structural changes is facilitated by the superior resolution available at a beamline configured with both incident and diffracted beam monochromators.