Storage of CO2 into Porous Coordination Polymer Controlled by Molecular Rotor Dynamics.

Design to store gas molecules, such as CO2 , H2 , and CH4 , under low pressure is one of the most important challenges in chemistry and materials science. Herein, we describe the storage of CO2 in the cavities of a porous coordination polymer (PCP) using molecular rotor dynamics. Owing to the narrow pore windows of PCP, CO2 was not adsorbed at 195 K. As the temperature increased, the rotors exhibited rotational modes; such rotations dynamically expanded the size of the windows, leading to CO2 adsorption. The rotational frequencies of the rotors (k≈10-6  s) and correlation times of adsorbed CO2 (τ≈10-8  s) were elucidated via solid-state NMR studies, which suggest that the slow rotation of the rotors sterically restricts CO2 diffusion in the pores. This restriction results in an unusually slow CO2 mobility close to solid state (τ≥10-8  s). Once adsorbed at room temperature, CO2 is robustly stored in the PCP under vacuum at 195-233 K because of the steric hindrance of the rotors. We also demonstrate that this mechanism can be applied to the storage of CH4 .