Characterization of defects and the local structure in natural and synthetic alunite (K, Na, H3O)Al3(SO4)2(OH)6 by multi-nuclear solid-state NMR spectroscopy

Abstract

The local structural environments in a series of natural and synthetic alunite samples [ideally A Al 3 (SO 4 ) 2 (OH) 6 , A = H 3 O + , D 3 O + , Na + , and K + ] have been probed by solid-state 1 H, 2 H, 23 Na, 27 Al, and 39 K NMR spectroscopy. The natural alunite [KAl 3 (SO 4 ) 2 (OH) 6 ] and synthetic hydronium alunite samples contain few structural defects, whereas the synthetic natroalunite and alunite samples have ca. 10% Al vacancies based on 27 Al NMR. A new 27 Al local environment (Al D ) was observed and assigned to Al with one Al vacancy in the first cation sphere. Three different proton environments, Al 2 -OH, Al-OH 2 , and H 3 O + are detected by 1 H and 2 H MAS NMR. The hydronium ion (H 3 O + ) is only observed in hydronium alunite, and is associated with the stoichiometric regions of the sample. It was not detected in 1 H and 2 H NMR spectra of alunite and natroalunite despite K (Na) occupancies of significantly less than 100%, as determined from elemental analysis. Thus, our NMR results suggest that the common assumption, namely that an A vacancy and an Al 3+ vacancy are compensated by adding an H 3 O + and 3 H + (creating 3 Al-OH 2 groups), respectively, is too simplistic. Instead, a significant fraction of the Al 3+ vacancies are compensated for by 4 H + ions, resulting in 4 Al-OH 2 groups per vacancy. This substitution is accompanied by the simultaneous deprotonation of a H 3 O + ion present on the A site. The resultant H 2 O molecule is unnecessary for charge balance, accounting for the A -site deficiency often observed. The presence of Al 3+ and A + vacancies appears closely correlated based on NMR.