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

Abstract

We present an ionothermal-based method for the simple and low-cost enrichment in 17 O 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 17 O-enriched H 2 O enables asprepared and calcined samples of SIZ-4 to be obtained with 17 O enrichment levels that are sufficient to enable the recording of high-quality 17 O solid-state NMR spectra. While second-order quadrupolarbroadened resonances are unresolved in 17 O MAS NMR spectra, 17 O 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 17 O NMR parameters on the local structural environment. We find that both the 17 O 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.

Calculating NMR parameters in aluminophosphates: evaluation of dispersion correction schemes

“…This can also be seen in Figure 8, where experimental spectra of calcined AlPO-14 are compared to those simulated using the parameters calculated with DFT for the varying structure sets. The spectra obtained prior to optimisation are in very poor agreement with the experimental measurements, with a very different assignment of the 31 P spectrum, and very broad lineshapes (owing to the very large quadrupolar couplings) observed for 27 Al.…”

“…AlPOs can lead to a significant improvement in the calculated 27 Al and 31 P NMR parameters, owing to small changes in the local environment of the Al and P species.…”

“…Calculations generate the absolute shielding tensor (σ) in the crystal frame, and 27 Al). 33 Although both the sign and magnitude of C Q are generated, the latter is difficult to measure experimentally and so only the magnitude of C Q is considered here.…”

“…When the 27 Al quadrupolar coupling constant is considered, it can be seen that the agreement with experiment is considerably poorer for the unoptimised structures (R 2 = 0.787), with some improvement observed upon optimisation of only H positions. Best agreement is obtained for structure set [D] where the cell size has varied during the optimisation although the differences in regression parameters are quite small.…”

“…Figure 4 plots calculated and experimental 31 should also be noted that due to the presence of second-order quadrupolar broadening in the 27 Al MAS NMR spectra, there may be a higher uncertainty associated with the experimental values for this nucleus than for 31 P.…”