New insight into cation relocations within the pores of zeolite Rho: In situ synchrotron X-ray and neutron powder diffraction studies of Pb- and Cd-exchanged Rho

Upon heating, certain cations exchanged into zeolite RHO undergo large shif ts in their positions within the pores. In several of these materials, nega tive thermal expansion is observed in conjunction with these cation relocat ions. Rather than being the purely temperature-driven effects presumed in p revious reports, a combination of in situ time-resolved synchrotron X-ray a nd neutron powder diffraction studies indicates that the cation relocations and framework distortions observed in Pb- and Cd-exchanged zeolite rho are mediated by the presence of water in specific sites in the pores of RHO. R ietveld refinements using these data reveal that the initial unit cell cont raction (50 degreesC less than or equal to T less than or equal to 100 degr eesC) is due to the loss of unbound water in the -cages of rho. Water molec ules in the double eight-ring (D8R) building units persist after this step, bound to the extraframework cations. The framework then contracts as water molecules are gradually removed (200 degreesC :less than or equal to T les s than or equal to 400 degreesC). During this period, the extraframework ca tions migrate from the single eight-ring (S8R) site to the double eight-rin g (D8R) sites in a concerted manner with the dehydration at the D8R. Upon c omplete removal of bound waters (400 degreesC less than or equal to T less than or equal to 500 degreesC), lead and cadmium cations experience differe nt rearrangements. Cd2+ ions relocate from the D8R and S8R sites to the sin gle six-ring (S6R) site, while all Pb2+ ions migrate from the S8R site to t he D8R site. Neither transition is reversible upon cooling to room temperat ure in vacuo although both are reversible in the presence of water vapor. T he role of water in these samples appears to determine the coordination env ironment of the extraframework cations uniquely since other sorbates. such as Kr, methanol, and CO. do not cause significant changes in either extrafr amework cation or framework atomic positions.