Solid-state NMR study of various mono- and divalent cation forms of the natural zeolite natrolite

Abstract

Here we present the one-dimensional ²⁹Si and ²⁷Al MAS NMR and two-dimensional ²⁷Al MQMAS and DQF-STMAS NMR spectra of the monovalent (Na⁺, K⁺, Rb⁺, Cs⁺ and NH₄⁺) and divalent (Ca²⁺, Sr²⁺ and Ba²⁺) cation forms of the natural zeolite natrolite (framework type NAT) with complete Si–Al ordering over the crystallographically distinct tetrahedral sites and with the same hydration state (hydrated, partially dehydrated or fully dehydrated). In the case of monovalent cation-exchanged natrolites, the differences in their crystal symmetry evidenced by ²⁹Si MAS NMR were found to be in good agreement with those determined by crystallographic analyses. However, ²⁷Al DQF-STMAS NMR spectroscopy shows the presence of two distinct Al sites in dehydrated K-NAT, Rb-NAT and NH₄-NAT, suggesting that their actual crystal symmetry is lower than the reported one (i.e., orthorhombic Fdd2). The MAS NMR results also show that the space group of hydrated Ca-NAT is lower than that (monoclinic F1d1) of hydrated scolecite, the natural calcium counterpart of natrolite, which is also the case with hydrated Sr-NAT and Ba-NAT. We believe that the unexpected diversity in the crystal symmetry of natrolite caused by exchange of various mono- and divalent ions, as well as by dehydration, may be inherently due to the high framework flexibility of this natural zeolite.