Pressure-Induced Amorphization of ZIF-8: Deciphering the Role of Atomic Charge Assignments and Adsorbed CH₄ Molecules through Atomistic Insights

Revealing the gas adsorption properties of amorphous metal organic frameworks (aMOFs) remains both vexed and vital as novel adsorbents demand an accurate computational description of disordered MOFs. This work aims to examine the effect of atomic charge assignment methods and the presence of confined gas molecules on the pressure-induced amorphization process of crystalline zeolitic imidazolate framework-8 (cryZIF-8). We performed molecular dynamics (MD) simulations using a flexible and transferable force field, ZIF-FF, with four charge models, namely, Qeq, PACMAN, PACMOF, and DDEC. Our results show that the crystalline-amorphous transition (intermediate phase) and the onset pressures for amorphization and crystallization exhibit minimal dependence on the assigned atomic charges. The crystallinity of ZIF-8 is preserved up to 0.8 GPa, but further pressure increases result in the loss of pair distribution function (PDF) peaks from similar to 7 to similar to 30 & Aring;, indicating the degradation of long-range order and the transition to an amorphous state. Incorporating a moderate number of adsorbed CH4 molecules (<= 54) into cryZIF-8 does not affect the initial amorphization pressure. However, when the number of adsorbed CH4 molecules increases to a saturated loading (400), the amorphization onset shifts to similar to 1.0 GPa. We also evaluated the CO2/CH4 separation performances of selected aZIF-8 structures at vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions using grand canonical Monte Carlo (GCMC) simulations. Several adsorbent performance evaluation metrics, including adsorption selectivity, working capacity, adsorbent performance score (APS), and regenerability, were computed at 298 K and compared between aZIF-8 and cryZIF-8. Our results show that aZIF-8 outperforms cryZIF-8 in terms of adsorption selectivity and APS, highlighting its potential for gas adsorption applications.