Ionothermal ¹⁷O enrichment of oxides using microlitre quantities of labelled water

We present an ionothermal-based method for the simple and low-cost enrichment in 17O of oxide materials. This is demonstrated for the case of SIZ-4, an ionothermally-prepared aluminophosphate framework with the CHA topology. A preliminary study of unenriched samples of SIZ-4 highlights the importance of the careful choice of template in order to obtain an ordered structure. We then show how an ionothermal synthesis procedure incorporating microlitre quantities of 17O-enriched H2O enables as-prepared and calcined samples of SIZ-4 to be obtained with 17O enrichment levels that are sufficient to enable the recording of high-quality 17O solid-state NMR spectra. While second-order quadrupolar-broadened resonances are unresolved in 17O MAS NMR spectra, 17O double-rotation (DOR) and multiple-quantum (MQ)MAS NMR spectra reveal distinct resonances that are partially assigned by comparison with NMR parameters derived using first-principles calculations. The calculations also enable an investigation of the dependence of 17O NMR parameters on the local structural environment. We find that both the 17O isotropic chemical shift and quadrupolar coupling constant show clear dependencies on Al–O–P bond lengths, and angles and will therefore provide a sensitive probe of structure and geometry in aluminophosphate frameworks in future studies.