(527a) Adsorption-Induced Transformations in Flexible Microporous Frameworks: Insight from Phonon Spectra Analysis

Adsorption induced transformations of the pore structure have received recently broad attention in view of discovery of novel flexible adsorbents. Low frequency lattice vibrations (phonons) can be used as indicators of a variety of structural transformations. In particular, conformational changes in flexible metal-organic frameworks (MOFs) can be successfully explained by analyzing materials phonon spectrum in frequency range below 200 cm^-1 [1,2]. It has been shown that such lattice vibrations are responsible for pressure- and adsorption-induced gate-opening in zeolitic imidazolate frameworks (ZIFs), and pore breathing in materials from MIL-53 family.

In this work, the density functional theory (DFT), Grand Canonical Monte Carlo (GCMC) and adsorption stress model (ASM) [3] were used to analyze low-frequency phonons and their impact on adsorption-related structural transformations.

We analyze adsorption-induced transformations in ZIF-8 and MIL-53. The step-like adsorption isotherm observed in this ZIF-8 was interpreted in the literature as a signature of gate-opening (enlargement of the pore free volume) induced by the increasing number of guest molecules entering the pore. To analyze this phenomenon at the microscopic level, we first calculated and visualized normal modes of the lattice vibrations. For that, density functional theory was used. Only the modes which fulfill two criteria: (i) their frequency is below 200 cm^-1, and (ii) the related lattice deformation leads to increase of the pore free volume, have been further analyzed in GMCM adsorption simulations. For MIL-53 we used ASM to analyze softening of the vibrational modes related to breathing transformation.

Studies of low-frequency vibrations in MOFs provide important, qualitative insight into the phenomena of transformations in MOFs. These dynamical fluctuations may become a precursor to a static deformation under any external stimuli such as temperature (thermal energy), pressure (mechanical energy) or adsorption (intermolecular interactions) which overcomes the energy barrier of the transition. The main focus is made on particular vibrations which are related to breathing and gate-opening transitions in studied MOFs.

[1] MR Ryder, et al., Physical Review Letters, 2014 113, 215502
[2] F Formalik, et al., Microporous and Mesoporous Materials, 2019
[3] A Neimark, et al., Journal of Physical Chemistry C, 2010, 114, 22237