Coordination Polymer Ligands (CPLs) are nanoporous-crystalline materials consisting of layers formed by copper and ionic pyrazine-2,3-dicarboxylate that are separated by organic pillar ligands. CPLs are promising materials for carbon capture applications because they have higher adsorption affinity for CO2 than for other small molecules, such as O2 and N2. Previous works, however, have reported that the structure of these materials change upon CO2 adsorption, and while changes in the crystallographic unit cell dimensions have been estimated to quantify those changes,[1-2] the actual structural transformation mechanism is still unclear.
In this work, we use grand canonical Monte Carlo (GCMC) simulations to predict CO2 adsorption loadings and get insights into the structural transformation of CPL-2, CPL-4, and CPL-5 during adsorption. First, we calculated CO2 adsorption isotherms based on the crystallographic structures and unit cell dimensions reported by Matsuda et al. and Riascos et al., and compared them with experimental data available in the literature. However, the calculated isotherms do not correctly describe the experimental data at high pressures even if unit cell expansion as reported by Matsuda et al. and Riascos et al. is considered. Interestingly, when we consider the potential rotation of the organic pillar ligands as the pressure increases the agreement with experimental data is improved significantly. Indeed, we were able to reproduce experimental adsorption isotherms at high pressures through a superposition of adsorption isotherms in which different degrees of rotation of the organic ligands were induced on the CPLs structures, strongly suggesting that this kind of transformation occurs in the studied CPLs as CO2 loadings increase.
 R. Matsuda, R. Kitaura, Y. Kubota, T. C. Kobayashi, M. Takata, and S. Kitagawa, “Incommensurate guest adsorption in bellows-shaped one-dimensional channels of porous coordination polymers,” Microporous Mesoporous Mater., vol. 129, no. 3, pp. 296–303, 2010.
 K. Riascos-Rodríguez, A. J. Schroeder, M. R. Arend, P. G. Evans, and A. J. Hernández-Maldonado, “Hysteretic adsorption of CO2 onto a Cu2(pzdc)2(bpy) porous coordination polymer and concomitant framework distortion,” Dalton Trans., vol. 43, no. 28, pp. 10877–10884, Jul. 2014.