Spontaneous strain variations through the monoclinic-orthorhombic phase transition of ZSM-5 zeolite: effect of adsorbed organic molecules

Depending on composition, framework defect density, temperature, or nature and amount of guest molecules in the channels, synthetic ZSM-5 zeolites (MFI framework topology) undergo a polymorphic monoclinic (P2 1 /n) -to- orthorhombic (Pnma, Pn2 1 a, or P2 1 2 1 2 1 ) phase transition. Such a ferroelastic-paraelastic phase transition has been recently characterized through the analysis of the spontaneous strain variation in a highly siliceous ZSM-5. According to the Landau theory, the behavior of the order parameter (Q) revealed that the ZSM-5 zeolite underwent a displacive phase transition at T c = 348(1) K with a tricritical character (Ardit et al., 2015). Recent investigations have demonstrated that hydrophobic ZSM-5 is extremely promising and efficient as adsorbent for the removal of organic contaminants from waters. This issue engenders particular interest in the scientific community. Therefore, modeling the ferroelastic properties of the m↔o phase transition of ZSM-5 zeolites is very relevant to the understanding of the effects of lattice strain mechanism on adsorption and diffusion properties of these microporous materials. For these reasons, the previously characterized ZSM-5 zeolite (CBV28014, Zeolyst International, SiO 2 /Al 2 O 3 ≈ 280) has been loaded with three different organic molecules (i.e. dichloroethane (DCE), methyl tertiary-butyl ether (MTBE) and toluene (TOL), respectively), and its high-temperature behaviour monitored by means of synchrotron X-ray diffraction data (collected in situ at ID31, ESRF). Besides causing a strong variation in the m↔o phase transition temperature, i.e. T c(TOL) = 328(4) K < T c(DCE) = 346(1) K < T c(ZSM-5) =348(1) K < T c(MTBE) = 360(2) K, the presence of organic molecules within the zeolite channels affects the mechanisms of lattice strain. Since the ZSM-5 structure belongs to the "Aizu-type" mmmF2/m, the primary order parameter behaves as the symmetry-adapted strain, then a proper spontaneous strain is involved. This means that Q can be coupled with the largest component of the spontaneous strain for each sample, i.e. the shear strain tensor e 13 . Although the analysis of Q reveals that all the compared samples undergo a displacive phase transition with a tricritical character, our results suggest that the strain effects are dependent not only by the physico-chemical features of ZSM-5 and organics but also by the occurrence of host–guest and guest-guest interactions. Ardit M., Martucci A. & Cruciani G. 2015. Monoclinic-orthorhombic phase transition in ZSM-5 zeolite: spontaneous strain variation and thermodynamic properties. J. Phys. Chem. C, 119, 7351-7359.