X-ray diffraction study of the zeolite natrolite: T–PH2O phase diagram and phase transitions during dehydration/rehydration

The response of the natrolite crystal structure (Na₁₆Al₁₆Si₂₄O₈₀ · 16H₂O) to dehydration/rehydration was evaluated as a function of P_H2O with X-ray powder diffraction (XRD) data measured in situ from 23 to 400 °C. Dehydrated natrolite at a low-P_H2O appears as a mixture of two anhydrous phases, the previously described α1-metanatrolite (F112, a = 16.177(1) Å, b = 16.943(1) Å, c = 6.4370(4) Å, γ = 89.685(2)°, V = 1764.3(2) ų), and a new phase, α2-metanatrolite (Fdd2, a = 17.576(1) Å, b = 18.163(1) Å, c = 6.3704(4) Å, V = 2033.7(2) ų). 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_H2O. Both metanatrolite phases occur simultaneously during dehydration under low-P_H2O conditions and they re-adsorb H₂O molecules at different rates during rehydration. α2-metanatrolite adsorbs H₂O 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_H2O illustrate the sensitivity of the natrolite framework and its extraframework Na cations to hydration state, with α1- and α2-metanatrolite forming under low-P_H2O conditions and only α1-metanatrolite forming under high-P_H2O 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.