X-ray diffraction study of the zeolite natrolite: TPH2O phase diagram and phase transitions during dehydration/rehydration

Abstract

The response of the natrolite crystal structure (Na 16 Al 16 Si 24 O 80 · 16H 2 O) to dehydration/rehydration was evaluated as a function of P H 2 O with X-ray powder diffraction (XRD) data measured in situ from 23 to 400 °C. Dehydrated natrolite at a low- P H 2 O appears as a mixture of two anhydrous phases, the previously described α1-metanatrolite ( F 112, a = 16.177(1) A, b = 16.943(1) A, c = 6.4370(4) A, γ = 89.685(2)°, V = 1764.3(2) A 3 ), and a new phase, α2-metanatrolite ( Fdd 2, a = 17.576(1) A, b = 18.163(1) A, c = 6.3704(4) A, V = 2033.7(2) A 3 ). The structure of α2-metanatrolite was solved using a combination of powder pattern indexing, distance least-squares modeling, and Rietveld refinement. The structures of α1- and α2-metanatrolite provide an excellent context for describing the behavior of natrolite phase transitions and the effects of P H 2 O . Both metanatrolite phases occur simultaneously during dehydration under low- P H 2 O conditions and they re-adsorb H 2 O molecules at different rates during rehydration. α2-metanatrolite adsorbs H 2 O molecules and reconverts to natrolite more rapidly than α1-metanatrolite because of the larger channel size in the α2-metanatrolite framework. The path dependence of natrolite phase transitions and the dependence on P H 2 O illustrate the sensitivity of the natrolite framework and its extraframework Na cations to hydration state, with α1- and α2-metanatrolite forming under low- P H 2 O conditions and only α1-metanatrolite forming under high- P H 2 O conditions. The flexibility of the natrolite framework makes the natrolite- to- α1-/α2-metanatrolite-to-natrolite reaction reversible, although strain-induced mosaicity resulting from the phase transition is not reversible.