Imine-Linked Covalent Organic Cage Porous Crystals for CO2 Adsorption

In this study, covalent organic cages (COC-MI, COC-PI and COC-PA) with covalently imine linked molecules were developed for CO2 adsorption. By varying the organic linkers (iso-phthalaldehyde to para-phthalaldehyde) tuned the structure and extrinsic porosity such as specific surface area of the organic cages was demonstrated. Additionally, COC-PI (3.8 cm(3)/g to 5.7 cm(3)/g) and COC-PA (0.43 cm(3)/g to 0.48 cm(3)/g) showed the significantly enhancement in CO2 adsorption capacity with increase in temperature from 273 K to 298 K at 1 bar. On the other hand, COC-MI (1.17 cm(3)/g to 1.1 cm(3)/g) cage showed the significant reduction in CO2 adsorption capacity with increase in temperature from 273 K to 298 K at 1 bar. These unusual enhancements of CO2 adsorption capacity within COC-PI and COC-PA is elucidated by powder X-ray diffraction (PXRD), and it is shown that cooperative diffusion and induced crystal lattice expansion in these molecular crystals can enhance the CO2 adsorption capacity. Computational modelling on the COC cages with CO2 reveals the higher binding energy and rigidity at the C=N group during intermolecular interactions enhances the overall CO2 adsorption capacity in the bulk.