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

Abstract

Here we present the one-dimensional (29)Si and (27)Al MAS NMR and two-dimensional (27)Al MQMAS and DQF-STMAS NMR spectra of the monovalent (Na(+), K(+), Rb(+), Cs(+) and NH4(+)) and divalent (Ca(2+), Sr(2+) and Ba(2+)) 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 (29)Si MAS NMR were found to be in good agreement with those determined by crystallographic analyses. However, (27)Al 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.

Local and Average Structural Changes in Zeolite A upon Ion Exchange

“…Similar to the exchange with NH 4 + ions, it is evident that some dealumination of the zeolite A framework has occurred upon exchange with Ca 2+ ions. This loss of aluminium is confirmed in the 27 Al NMR spectrum, which displays a broad peak centred at δ 50.11 ppm, characteristic of Al(4Si) units and also a slight peak centred at δ 9.10 ppm, which can be attributed to a distorted octahedral aluminium environment. The broadness of the peaks in the spectra can be attributed to quadrupolar interactions of 27 Al nuclei in these distorted environments.…”

“…Using Equation (1), the Si/Al ratio of NH 4 -A was calculated to be 1.04. The loss of some aluminium from the framework is further confirmed in the 27 Al MAS NMR spectrum, which displays a resonance signal at δ 1.29 ppm, attributed to extraframework octahedrally-coordinated aluminium [23,28]. This peak is broad and overlaps with the main signal at δ 58.89 ppm (for a tetrahedral Al coordination).…”

“…The only noticeable differences in the spectra are that the framework ν (O−T−O) asymmetric stretch shifts to slightly a higher wavenumber and the ν (O−T−O) symmetric stretch shifts to slightly a lower wavenumber, with increasing cation size, Li + < Na + < K + . For the parent Na-A zeolites, 29 Si and 27 Al MASNMR spectra contain one peak at δ −89.54 ppm and δ 58.41 ppm, respectively, confirming that the Si/Al ratio of the framework is one. This is in agreement with data reported by Thomas et al [24] for a single silicon, Si(OAl) 4 , and aluminium environment, Al(OSi) 4 , in Linde Type A zeolites.…”

“…This is in agreement with data reported by Thomas et al [24] for a single silicon, Si(OAl) 4 , and aluminium environment, Al(OSi) 4 , in Linde Type A zeolites. 27 Al MAS NMR spectra for the exchanged zeolites also display one sharp peak between δ 57 ppm and 60 ppm, pertaining to Al(4Si) units. Likewise, 29 Si spectra for Li-A, K-A and Rb-A are also dominated by a single sharp peak at δ −87.23 ppm, −89.83 ppm and −89.87 ppm, respectively.…”

“…In this work, we carried out aqueous ion exchange of monovalent alkali metals: Li + , K + , Rb + and NH 4 + and divalent Ca 2+ into zeolite A crystals produced from a low temperature and organic template-free synthesis and performed analysis using solid state NMR and other methods to determine the structural effects of the exchange. Previously, solid state NMR has been used to study other zeolites [20][21][22][23][24][25][26][27][28].…”