Zero-Point and Temperature-Dependent Vibrational Effects on Pore Stability in Amine-Appended Metal-Organic Frameworks

Metal-organic frameworks (MOFs) form an extensive group of porous materials of interest for carbon capture and gas separation. Within this class, clamped-nuclei first principles calculations with periodic boundary conditions have been widely used to predict binding sites and calculate binding energies of small molecules in crystalline frameworks. However, these methods treat finite temperature effects and zero-point energy corrections at harmonic order or neglect them entirely. In this work, we demonstrate the importance of such effects for olsalazine-based polyamine-appended MOFs. These compounds exhibit a cooperative step-shaped isotherm, allowing for complete adsorption/desorption of CO2 within small pressure/temperature swings. Exploiting the computational efficiency and near-first-principles accuracy of a custom machine-learning interatomic potential, we demonstrate the importance of such vibrational corrections including zero-point vibrational corrections to recover the experimentally probed crystal structure and predict trends in the nature of CO2 binding with temperature. We discuss the general implications of our work for the accurate prediction of the structure and dynamics of molecular adsorption in MOFs.