Solid-State NMR Study of Various Mono- and Divalent Cation Forms of the Natural Zeolite Natrolite

Abstract

Here we present the one-dimensional Si-29 and Al-27 MAS NMR and two-dimensional Al-27 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 Si-29 MAS NMR were found to be in good agreement with those determined by crystallographic analyses. However, Al-27 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.