title: 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.
authors.affiliation: National Creative Research Initiative Center for Ordered Nanoporous Materials Synthesis, Department of Chemical Engineering and School of Environmental Science and Engineering, POSTECH, Pohang 790-784, Korea.
authors.forename: Min Bum
authors.initials: MB
authors.lastname: Park
authors.forename: Aurélie
authors.initials: A
authors.lastname: Vicente
authors.forename: Christian
authors.initials: C
authors.lastname: Fernandez
authors.forename: Suk Bong
authors.initials: SB
authors.lastname: Hong
mesh_terms: D064751:Ammonium Compounds / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008672:Metals, Alkali / Q000737:chemistry*; D008673:Metals, Alkaline Earth / Q000737:chemistry*; D049329:Nanostructures / Q000737:chemistry*; D017641:Zeolites / Q000737:chemistry*
chemical_list: D064751:Ammonium Compounds; D002412:Cations; D008672:Metals, Alkali; D008673:Metals, Alkaline Earth; D017641:Zeolites