Flexibility of the Zeolite RHO Framework in Situ X-Ray and Neutron Powder Structural Characterizations of Divalent Cation-Exchanged Zeolite RHO
CALIFORNIA UNIV SANTA BARBARA DEPT OF CHEMISTRY
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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- RHO400 C, a 13.9705A Sr-RHO250 C, a 14.0451A Ba-RHO200 C, a 14. 1842A Cd-RHO350 C, a 14.4883A Na,Cs-RHO25 C, a 15.031A is a particularly sensitive function of cation and temperature. Rietveld analysis of neutron diffraction data was used to refine the structures of two samples, CaND4-RHO and Ca,D-RHO. Ca,D-RHO shows the largest reported deviation from Im3m symmetry a 13.96457A 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 results confirm one of the largest displacive distortions 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.
- Physical Chemistry