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.