Pressure- and Temperature-Induced Monoclinic-to-Orthorhombic Phase Transition in Silicalite-1

Abstract

The thermal, mechanical, and volumetric behavior of silicalite-1, an all-silica Mobil Five (MFI) zeolite, is elucidated by atomistic simulations. A flexible force field was selected and validated from a set of force fields to capture the intramolecular interactions of the crystal lattice. This force field accounts for realistic bond, angle, and torsional interactions among atoms of the framework alongside with conventional Lennard-Jones and Coulomb interactions. By monitoring the behavior of silicalite-1 as a function of pressure and temperature, a fully reversible monoclinic-to-orthorhombic phase transition (polymorphism) was revealed in accordance with experimental data. Thermodynamic considerations dictate that this is a second-order phase transition in the Ehrenfest classification. Additionally, reversible pressure-induced amorphization was captured by our model and was associated with the formation of linear zones of increased distortion running parallel to the straight and sinusoidal channels of this zeolite. Remarkably high isothermal compressibility (small bulk modulus) was calculated for orthorhombic silicalite-1, in excellent agreement with experimental data, rendering silicalite-1 as the most compressible zeolite known to date. The rigid unit mode model was identified as the dominant structural mechanism for negative thermal expansion (NTE), typically observed over a wide temperature range in MFI zeolites. Better understanding of the monoclinic-to-orthorhombic phase transition and molecular mechanisms associated with energy dissipation and NTE in zeolites provides control over the framework microstructure, allowing for enhanced molecular sieving, tunable selectivity in separation processes, mechanical stability, and substantially amplified catalytic efficiency in petrochemical applications.

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

“…To improve the predictions for small pore (8-ring) zeolites Boulfelfel et al modified the popular Hill-Sauer force field for silicas and zeolites [117] using fully periodic dispersion-corrected DFT calculations for reference silica zeolites [116]. The new force field provides reliable unit cell and window dimensions and even predicts the monoclinicto-orthorhombic phase transition in siliceous MFI zeolite [118]. For the zeolite-adsorbate interactions Fang et al developed a first-principles-derived force field for CH 4 in silica zeolites by using periodic DFT calculations and the DFT/CC (coupled cluster) method [115].…”

The elastic behavior of zeolitic frameworks: The case of MFI type zeolite under high-pressure methanol intrusion

“…The MOPT has been investigated by several authors, who underlined its dependence to different variables, among those: temperature, pressure, the zeolite crystal chemistry, or the nature and concentration of the sorbate [12,17,44,18,19,[21][22][23][24][25][26]. In this study, due to the relatively high number of experimental points at P < 1 GPa, we could accurately bracket the MOPT between 0.3(1) and 0.5 (1) GPa in all but the Na-Fe MFI sample (in which the transition occurs between 0.40(5) and 0.70(5) GPa) and the Na-B MFI, for which an unambiguous split of the above-mentioned diagnostic peaks could not be detected, and the Pnma space group was, therefore, always assigned (e.g., Fig.…”

“…In the last decades, zeolites have been extensively employed in the oil and gas industries as catalysts in the cracking of hydrocarbons due to their unique key features, such as the efficient shape selectivity coupled to moderate synthesis costs and eco-friendliness [2,6,10,11]. The ZSM-5 belongs to the structure-type code Mobil Five (MFI) and are characterized by an Al-doped siliceous framework with the Si/Al ratio ranging from 10 to ∞; when this ratio exceeds 1000, the zeolite is known as "Silicalite-1" [12,13]. Indeed, MFI-type porous materials comprise of several synthetic phases, whereas the only mineral known with a MFI-framework topology is mutinaite [14].…”

“…The MOPT may be governed by different conditions such as the nature and concentration of the sorbate, temperature, pressure and the concentration of structural defects, and it is reported to be very sensitive to the chemical nature of the framework components [12,17,[19][20][21][22]. For instance, the addition of a given fraction of aluminum (Si/Al less than 100) or of other trivalent dopants, such as iron and boron, in the tetrahedral framework is reported to lower the MOPT transition temperature below the ambient conditions [23,24].…”

Controlling Reaction-Induced Loss of Active Sites in ZnOx/Silicalite-1 for Durable Nonoxidative Propane Dehydrogenation

“…This structure changes to the MFI polymorph with increasing amounts of Zn-content in MgO-free samples, as clearly indicated by the changes of diffraction data in the 2θ range between 23 and 25° (Figure a–c). Such changes could be caused by the presence of a foreign cation or a structure-directing agent, temperature, pressure, and so on. , The Zn-related structural changes in our samples should not be caused by the incorporation of Zn into the framework of the zeolite due to the following reasons. As reported in a previous study dealing with Mn-ZSM-5, the Bragg peaks in the 2θ range of 7–10° shift to lower values when manganese is inside of the framework .…”