Spying on Fe ions and their role in modified aluminosilicates during the sorption of anions using solid-state NMR spectroscopy

Abstract Post-synthetic modifications of zeolites enable their possible use as adsorbents in separation and purification processes. Here, we present a multinuclear ( 29 Si, 27 Al, 31 P) magnetic resonance study of solid samples of zeolite A (LTA) and hydroxysodalite (HS), their post-synthetic modification using FeSO 4 solution, and the results of their analysis by NMR after the anion (PO 4 3− ) adsorption process. High-resolution 29 Si and 27 Al MAS NMR spectra provide valuable structural information on the oxidation of Fe 2+ ions to Fe 3+ species during the modification process, whereas 27 Al 3Q/MAS solid-state NMR spectra of LTA and HS unveil a simultaneous dealumination process characteristic of acid leaching in aluminosilicate systems. 31 P solid-state NMR spectra of LTA and HS, collected at “ very-fast” spinning speed, revealed a dramatic change in the chemical shift values for the phosphates present, directly characteristic of FePO 4 in the case of LTA, and phosphates of alkali and/or alkaline earth metals in the case of HS. Moreover, density functional theoretical (DFT) computations provide satisfactory agreement with the obtained 27 Al and 29 Si experimental data. This work demonstrates possible NMR approaches to the characterization of complex framework materials and provides key insights into aluminosilicate materials affected by unpaired electron(s).