Monoclinic–Orthorhombic Phase Transition in ZSM-5 Zeolite: Spontaneous Strain Variation and Thermodynamic Properties

Abstract

Modeling the ferroelastic properties of the monoclinic−orthorhombic phase transition of ZSM-5 zeolites is very relevant to the understanding of the effects of lattice strain on adsorption and diffusion properties of these microporous materials widely used in catalysis and water treatment. Using very accurate synchrotron X-ray diffraction data, we report here the analysis of spontaneous strain variation across the transition. According to the Landau theory, the behavior of the order parameter reveals the tricritical character of this transition from ferroelastic to paraelastic phase in ZSM-5. Combination of results from Landau analysis with calorimetric data allows the calculation of thermodynamic quantities which are in good agreement with the experimentally derived ones.

Temperature-Induced Desorption of Methyl tert-Butyl Ether Confined on ZSM-5: An In Situ Synchrotron XRD Powder Diffraction Study

“…After the full desorption of organics, a contraction of the unit cell volume is observed until 600 °C. This phenomenon is explained as a thermal negative expansion (NTE), already observed not only in MFI-type materials [41,[45][46][47][48], but also in other microporous materials [22][23][24][25][26][27][48][49][50][51], thus attesting the relaxation of framework distortions induced by host molecules which diffuse through the zeolite channels during the heating process. The desorption process occurred without any significant zeolite crystal.…”

“…Figure 2 shows the evolution of the investigated ZSM-5 sample close to the expected transition temperature, Tc, in the 3.70-4.30 and 5.60-6.50 2θ range. Therefore, recent works [41][42][43] reported this phenomenon both in the unloaded ZSM-5 as well as in the same samples after organics adsorption. The evolution of refined unit cell parameters as a function of temperature is illustrated in Figure 2.…”

“…The automatic indexing of the peaks, carried out by the High Score Plus v. 3.0 software [40], revealed the gradual overlapping of groups of peaks (i.e., 131 + 13 − 1 and 311 + 31 − 1 in the first angular range, and 133 + 13 − 3 and 313 + 31 − 3 in the second range) attesting the monoclinic to orthorhombic phase transition, with a Tc close to 100 ± 5 • C. Figure 2 shows the evolution of the investigated ZSM-5 sample close to the expected transition temperature, Tc, in the 3.70-4.30 and 5.60-6.50 2θ range. Therefore, recent works [41][42][43] reported this phenomenon both in the unloaded ZSM-5 as well as in the same samples after organics adsorption. The evolution of refined unit cell parameters as a function of temperature is illustrated in Figure 2.…”

Influence of downsizing of zeolite crystals on the orthorhombic ↔ monoclinic phase transition in pure silica MFI-type

“…In conventional zeolites (micro-or nanosized) for which the crystal thickness contains tens, hundreds or thousands of unit cells, monoclinic and orthorhombic domains may coexist. It has been shown from single crystals that, upon cooling, the orthorhombic form of HZSM-5 obtained above the critical temperature switches into a combination of twin domains with a monoclinic symmetry [18,31]. In the case of nanosheets the presence of multidomains is not possible, monoclinic, orthorhombic or other, and could explain the absence of transition in the temperature range used in this study.…”

“…Note that this orthorhombic 4 monoclinic phase transition was also observed at low temperature (~175 K) on highly siliceous as-synthesized MFI-type zeolite [17]. Recently, in situ high-temperature synchrotron X-ray powder diffraction data have demonstrated a tricritical phase transition between the ferroelastic [14] (monoclinic) and the paraelastic (orthorhombic) phase in conventional ZSM-5 zeolite [18]. This last experimental fact was connected to the previously discovered strain as an unusual deformation field distribution determined by coherent X-ray diffraction imaging [19].…”