Spontaneous strain variation and thermodynamic properties through the monoclinic/orthorhombic phase transition of ZSM-5 zeolite

ZSM-5 is an aluminosilicate belonging to the pentasil zeolite family with MFI framework topology [1]. Monoclinic (P21/n) at low temperatures, the ZSM-5 structure becomes orthorhombic (Pnma) on heating. The transition temperature (Tc) can vary in a broad T-range (i.e. between 320 and 370 K), depending on the Si/Al ratio, increasing Al content decreases Tc [2]. Reported for the first time by Wu et al. in 1979 [3], such a reversible phase transition is described as a mutual shift of successive (010) pentasil layers along the c-axis, associated to a small change in the Si–O–Si bond angles and Si–O bond distances [4,5]. Furthermore, it was demonstrated that the monoclinic polymorph has the features of a ferroelastic material [6]. Although some works deal with the thermodynamic of the monoclinic to orthorhombic (m↔o) ZSM-5 phase transition [2,7,8], an investigation centred on the lattice evolution associated to the thermodynamic processes within such a phase transition is lacking. In this contribution, high-temperature X-ray diffraction data (collected in situ at ID31, ESRF) have been used to determine the lattice parameter variations of highly siliceous ZSM-5 zeolite (CBV28014, Zeolyst International, SiO2/Al2O3 ≈ 280) across the m↔o phase transition, in the temperature interval 308–573 K. The mean thermal expansion coefficients of the orthorhombic phase, calculated by means of the Fei polynomial expression [9], are in excellent agreement with those reported by Bhange and Ramaswamy [10]. The extrapolation of the orthorhombic lattice parameters into the stability field of the monoclinic phase allowed the calculation of the strain tensor components [11]. Strain analysis, based on the displacive character of the transition, reveal that the spontaneous strain behaves as the order parameter Q for the transition following Landau theory with Q ∝ |Tc–T|β, with β→¼, meaning that the data are consistent with a tricritical transition at 348 ± 1 K. A detailed comparison with thermodynamic data from literature corroborates the validity of the phase transition character.