Unraveling Thermally Regulated Gating Mechanisms in TPT Pore-Partitioned MOF-74: A Computational Endeavor

In the pursuit of advanced gas adsorption materials, we investigate the thermally activated gating mechanism of functionalized TPT (2,4,6-tri(4-pyridyl)-1,3,5-triazine) ligands within the one-dimensional channels of Mg-MOF-74. These ligands induce the formation of ultramicroporous windows, whose sizes dynamically respond to substituent groups and temperature. Gas-phase DFT calculations reveal molecular rotator behavior in the TPT-X (X = H, F, CH₃, Cl) ligands showing an inverse steric relationship between the functional group size and the barrier for rotation, indicating a propensity for larger functional groups to facilitate molecular rotation. Once incorporated into the channel of MOF-74, the larger functional groups result in energy landscapes marked by higher barriers, suggesting that portions of the landscape are only accessible at higher temperatures. These findings suggest temperature-driven pore size changes, hinting at thermally regulated gating. Molecular dynamics (MD) calculations corroborate these findings, showing an increase in the pore size of the -Cl and -CH₃ functionalized systems with increasing temperature. Comparative analysis highlights TPT-X pore partitioned MOF-74's (mTPT-X) superiority over other ultramicroporous MOFs, making them promising for gas adsorption and separation applications. At lower (10% maximum) loading, the materials demonstrate ultramicroporosity with pore volumes akin to MOF-74, potentially leading to temperature-driven high capacity gas storage and gas separation. Conversely, higher loading (50% maximum) leads to a loss of pore volume and a subsequent loss of capacity, leading to lower feasibility for gas storage but potentially enhancing gas separation processes.