Structural changes of synthetic paulingite (Na,H-ECR-18) upon dehydration and CO₂ adsorption

The structure of dehydrated calcined ECR-18, synthetic paulingite, topology type PAU, unit cell composition Na₁₃₂H₂₈Si₅₁₂Al₁₆₀O₁₃₄₄, has been determined by Rietveld refinement against synchrotron X-ray powder diffraction data. Upon dehydration the symmetry of Na,H-ECR-18 changes from Im3m to I 43m, with a corresponding decrease of cubic unit cell a parameter from 34.89412(1) A to 33.3488(3) A. This occurs as the framework distorts to afford closer coordination of Na⁺ cations by framework O atoms in 8-ring window sites of the seven cage types present. Na+ cations in 8R sites block the access of N₂ molecules to the internal pore space at 77 K but CO₂ adsorption at 308 K is observed, and is postulated to occur via a 'trapdoor' mechanism. In situ PXRD during CO₂ adsorption at pressures up to 10 bar show reversible broadening of diffraction peaks that is attributed to local crystallographic strain.