Crystal structure of MCM-71 – a new zeolite in the mordenite group

The crystal structure of the aluminosilicate, MCM-71, was determined by a match of the powder X-ray diffraction pattern to one calculated from a theoretically predicted framework model. The unit cell contains 48 T-sites and 96 framework oxygens, where T = 41.1 Si, 6.9 Al. It crystallizes in space group Cmca, a = 7.4422(2), b = 18.5324(5), c = 19.1877(5) Å. The framework contains an elliptical 10-membered ring channel (4.3 × 6.5 Å) and an orthogonal undulating 8-membered ring channel (3.6 × 4.7 Å) to constitute a two-dimensional network of channels. Considering the aluminosilicate framework as a silicate, the crystal density is 1.77 g/cm3 and the T-site framework density is 17.8 T/1000 A3. The structure was refined against Debye-Scherrer and Bragg-Brentano powder synchrotron data by the Rietveld procedure. While non-framework oxygen atoms, consistent with water content, could be justified by TGA measurements, it was not possible to derive an accurate extra-framework model using synchrotron data from hydrated specimens. Refinement against powder data from an anhydrous specimen subsequently detected the presence of extra-framework aluminum, identified also by solid state NMR measurements.