Comparison of Gas Adsorption Properties in Methylated and Non-Methylated Imine-Linked Nanoporous Covalent Organic Frameworks

Gas-material interactions are crucial in various industrial processes, including microchip fabrication, fuel production, and exhaust gas treatment. Covalent organic frameworks (COFs) are a class of porous, crystalline nanomaterials composed of organic building blocks linked by strong covalent bonds. Their highly tunable surface properties make them promising candidates for gas adsorption. In this study, we explored how the presence of methyl groups influences the gas adsorption properties of volatile organic compounds, i.e., probes, in stable, imine-linked COFs. Enthalpy measurements revealed that Me₃TFB-BD, a methylated COF, exhibited weaker interactions with toluene (−41.3 kJ/mol) and heptane (−45.6 kJ/mol) compared to its nonmethylated derivative TFB-BD (−50.5 kJ/mol and −54.0 kJ/mol, respectively). Partition coefficient (K) data also indicated that TFB-BD has stronger interactions with a broader set of specific probes than Me₃TFB-BD, likely due to a higher imine bond accessibility. Both COFs also showed strong interactions with polar alcohol probes, which can be attributed to their high polarizability. Analysis of Me₃TFB-PA, a COF with a lower methyl to carbon ratio, led to further reduction in the COF-probe interaction strength. All three COFs demonstrated moderate adsorption capacities, though TFB-BD showed the highest uptake for toluene (0.1 μmol/m²) and heptane (∼0.07 μmol/m²), due to its stronger interactions and smaller pore size. Additionally, selectivity analysis revealed that TFB-BD exhibited the strongest affinity for a broad range of probes. Overall, this study highlights the potential of COFs as tunable and promising materials for targeted gas sensing, gas separation, and related applications.