STRUCTURAL CASE-STUDIES OF INCLUSION PHENOMENA IN ZEOLITES - XE IN RHO AND STILBENE IN ZSM-5

The development and increased availability of brighter synchrotron sou rces has stimulated investigations of zeolite-inclusion complexes usin g powder X-ray diffraction data. This is illustrated with two examples of studies facilitated by these sources. Both involve the use of Riet veld refinement to obtain structure from data collected at a versatile synchrotron beamline. The first example describes the structural chan ges commensurate with Xe desorption from a fully Xe-loaded powder samp le of Cd-exchanged zeolite rho. After exposure to the atmosphere, data were collected in real-time as Xe desorbed from the sample over a fou r hour period. The results of Rietveld refinement indicate that Xe is not lost continuously; instead within the first hour, Xe desorbs compl etely 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 com petition with H2O, which prefers the 8-ring site. In the second exampl e, the high resolution and signal-to-background discrimination afforde d the synchrotron powder X-ray diffraction experiment, was used to adv antage to study the stilbene-ZSM-5 complex. Stilbene was located, usin g modeling techniques, in the straight channels with one phenyl ring a t the intersection of the two channel system. The structure is pseudo- tetragonal (a a b) at room temperature but distorts at lower temperatu res to give b > a. Distinguishing these subtle structural changes is f acilitated by the superior resolution available at a beamline configur ed with both incident and diffracted beam monochromators.