CO₂ adsorption-induced structural changes in coordination polymer ligands elucidated: Via molecular simulations and experiments

Aiming to elucidate guest-induced structural changes in the coordination polymer CPL-2, grand canonical Monte Carlo (GCMC) simulations were used to predict CO₂ loadings in this material, and the results were compared with experimental isotherms. Our calculations suggest that CPL-2 exhibits more pronounced CO₂-induced structural changes than previously reported. As the initial evidence, the isotherm simulated in the previously reported CPL-2 structure (experimentally resolved from X-ray diffraction in the "as-synthesized" CPL-2) underestimated the measured CO₂ loadings at high pressure, indicating that CPL-2 might undergo structural changes that enable higher pore volumes at high pressure. GCMC simulations in CPL-2 structures considering moderate unit cell expansions reported in the literature still underestimated high-pressure experimental loadings. However, considering an incremental rotation of the CPL-2 bipyridyl pillars with increasing CO₂ pressure, we were able to trace the measured isotherm with the simulation data. Computational analysis shows that ligand rotation in CPL-2 enables higher pore volumes, which, in turn, accommodate more CO₂ as the gas pressure increases. Desorption measurements suggest that hysteresis in the CO₂ isotherm of CPL-2 may also be linked to ligand rotation, and the measured adsorption/desorption cycles show that the rotation is reversible. Based on our simulations for CPL-4 and CPL-5 and previously reported experimental data, it is likely that these materials, which differ from CPL-2 in the bipyridyl ligand, behave similarly in the presence of CO₂. Our results help understand the behavior of these materials, which present the kind of structural changes that could be potentially exploited to enhance the CO₂ working capacity of ultra-microporous materials for carbon capture applications.