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

N Lempesis and N Smatsi and VG Mavrantzas and SE Pratsinis, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 6217-6229 (2018).

DOI: 10.1021/acs.jpcc.8b00400

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.

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