FLEXIBILITY OF THE ZEOLITE RHO FRAMEWORK - INSITU X-RAY AND NEUTRON POWDER STRUCTURAL CHARACTERIZATION OF DIVALENT CATION-EXCHANGED ZEOLITE RHO

Zeolite RHO has an unusual three-dimensional monolayer surface with a topology that gives equal access to either side of the surface. In the absence of supporting structural subunits, e.g., smaller cages or channels, RHO exhibits atypical framework flexibility with large displacive rearrangements. These have been investigated by in situ X-ray powder diffraction studies of zeolite RHO exchanged with various divalent cations. The unit cell variation (e.g., Ca, H-RHO (400 °C), a = 13.970 (5) Å; Sr-RHO (250 °C), a = 14.045 (1) °; Ba-RHO (200 °C), a = 14.184 (2) Å; Cd-RHO (350 °C), a = 14.488 (3) °; Na, Cs-RHO (25 °C), a = 15.031 (1) °) is a particularly sensitive function of cation and temperature. Rietveld analysis of neutron diffraction data was used to refine the structures of two samples, Ca, ND4-RHO and Ca,D-RHO. Ca,D-RHO shows the largest reported deviation from im3m symmetry (a = 13.9645 (7) Å) for a RHO structure. The calcium atom is located in the center of the double 8-ring, distorting the framework to generate a tetrahedral environment. The in situ X-ray studies of zeolite RHO with both monvalent and divalent cations together with the Rietveld results for the extreme end member of this structural field confirm the largest displacive distortion observed for a molecular sieve framework with a unit cell volume increase of 25% when the calcium ions of Ca,H-RHO are replaced with hydrogen ions to give H-RHO. © 1990, American Chemical Society. All rights reserved.