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

• Published in: Microporous and mesoporous materials 2017-03, Vol.241, p.115-122
• Main Authors: Kobera, Libor, Abbrent, Sabina, Holcova, Lenka, Urbanova, Martina, Kolousek, David, Dousova, Barbora, Brus, Jiri
• Format: Article
• Language: English
• Subjects: Adsorbents; Aluminosilicate; Modification; Paramagnetic NMR shift; Solid-state NMR
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2016.12.030

Post-synthetic modifications of zeolites enable their possible use as adsorbents in separation and purification processes. Here, we present a multinuclear (29Si, 27Al, 31P) magnetic resonance study of solid samples of zeolite A (LTA) and hydroxysodalite (HS), their post-synthetic modification using FeSO4 solution, and the results of their analysis by NMR after the anion (PO43−) adsorption process. High-resolution 29Si and 27Al MAS NMR spectra provide valuable structural information on the oxidation of Fe2+ ions to Fe3+ species during the modification process, whereas 27Al 3Q/MAS solid-state NMR spectra of LTA and HS unveil a simultaneous dealumination process characteristic of acid leaching in aluminosilicate systems. 31P 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 FePO4 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 27Al and 29Si 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). [Display omitted] •Post-synthetic modification of zeolites allowing anion adsorption is presented.•VF/MAS NMR approach to structural insight into modified zeolites was demonstrated.•Distinct mechanisms of phosphate anions (PO43−) adsorption were revealed.