The crystal structures of polymorphic SUZ-4

摘要

SUZ-4 is an aluminosilicate zeolite originally synthesized at British Petroleum and known to be related somehow to ferrierite. Although a model had been suggested for the framework structure no quantitative proof of its existence has been given in previous work. It is now found to be polymorphic. The originally identified ambient, hydrated form crystallizes in space group Imma where a = 18.882(1), b = 14.872(1), c = 14.211(1) angstrom. A model constructed by trial and error refines by Rietveld methods to give R-wp = 0.1643, R(F-2) = 0.1665. Powder data collected at 100 degrees C, on the other hand, correspond to the Cmmm model proposed earlier by Lawton et al., where a = 18.790(1), b = 14.2305(8), c = 7.4511(4) angstrom. The Rietveld match gives R-wp = 0.1473, R(F-2) = 0.1508. Although the two forms crystallize in different space groups, the framework topology remains the same, as revealed by vertex symbols and coordination sequences. Evidence is seen for potassium ions that cannot be completely removed by ammonium exchange. Later, the origin of the polymorphic change was discovered to be hydration of microporous channels rather than a thermal effect. Solid-state NMR measurements of the hydrated and anhydrous forms show that a more symmetric environment around the Al species is observed without affecting the coordination of the Al sites. Synchrotron powder data collected at room temperature from anhydrous material conformed to the Cmmm structural model, where a = 18.8064(4), b = 14.2298(3), c = 7.4548(2) angstrom. Rietveld refinement, including three potassium sites gives R-wp = 0.1127, R(F-2) = 0.1296. (Elemental analysis indicates a stoichiometric assembly: K4T36O72; the Rietveld refinement finds 3.86 of the 4.00 potassium atoms.) The material represents a one-dimensional 10-membered ring framework.