Study of Ion-Exchanged Microporous Lithosilicate Na–RUB-29 Using Synchrotron X-Ray Single-Crystal Diffraction and 6Li MAS NMR Spectroscopy

Abstract

Synchrotron X-ray single-crystal diffraction analyses revealed that as-synthesized and Na-exchanged RUB-29 (Cs 1− x , Na x ) 14Li 24[Si 72Li 18O 172]· yH 2O ( x=0, 0.9) displays the lattice symmetry I222. With increasing ion-exchange time, the Na cations preferentially replace Cs in the larger sites located at the intersections of the 10MR/10MR/8MR channels. The smaller Cs sites are then replaced. While Na cations are easily incorporated on the Cs sites, most of non-framework Li cations remain in the channel system. Relocation of Li cations onto new sites within the channels was observed only after 13 days of ion exchange. Using high-field (14.1 T) NMR spectroscopy, at least six separate 6Li resonances could be resolved for the first time by solid-state 6Li MAS NMR spectroscopy and assigned to Li in the framework and non-framework sites of the microporous lithosilicate materials. The fate of Li in both framework and extra-framework sites during exchange was also followed by 6Li MAS NMR spectroscopy with an Na-exchanged RUB-29 powder sample.