A Thermodynamically Consistent Approach to Molecular Simulations of Adsorption-Induced Deformation and Structural Transitions in MOFs.

Abstract

Flexible metal-organic frameworks (MOFs) exhibit coupled adsorption-deformation behavior that cannot be captured in molecular detail by conventional rigid-framework simulations. We present an iterative hybrid GCMC/MD methodology that explicitly couples grand canonical Monte Carlo sampling of adsorbate configurations with isothermal-isobaric molecular dynamics relaxation of the framework structure, using a Metropolis acceptance criterion in the osmotic ensemble to ensure thermodynamic consistency. Applied to argon adsorption on ZIF-8 at 87.3 K, the method quantitatively reproduces the experimental stepped isotherm characteristic of the gate-opening transition between low pressure (LP) and high pressure (HP) conformations and predicts nonmonotonic strain isotherm with initial contraction (∼0.5%) at low loadings followed by expansion (∼0.7%) during gate-opening. In addition, the elastic modulus variation upon loading is calculated from the volume fluctuations. The simulations reveal and quantify the molecular mechanism of gate-opening through cooperative linker reorientation from a unimodal swing angle distribution centered at 0° to a bimodal distribution peaked at ± 23°. The proposed approach is computationally efficient, yielding converged strain and compressibility isotherms within an accessible number of iterations, and provides a general framework for predicting adsorption-induced structural transitions in flexible porous materials without a priori knowledge of end point structures.