Effects of substituent groups on methane adsorption in covalent organic frameworks†
Based on the structures of published three-dimensional covalent organic frameworks (COF-102, COF-103, and COF-105), we developed a sequence of modified COFs by replacing some H atoms on benzene rings with other substituent groups, including –Cl, –Br, –I, –CF3, –NH2, –CN, –OCH3, and –CH3. To explore the effects of the substituents on methane storage, we studied their adsorption properties by using Grand Canonical Monte Carlo (GCMC) simulation. The methane uptakes and isosteric heat of adsorption from 0 to 100 bar were simulated at room temperature. The results indicate that in COF-102 and COF-103, all these substituents are beneficial to methane storage at low pressure, but the advantage is weakened by the volume effect at medium and high pressure. In the case of COF-105 and its derivatives, all the isotherms are linear due to their large pore volumes. Among these groups, halogen groups (–Cl, –Br, and –I) and –NH2 are the best ones to improve methane uptake, whereas –CH3 and –OCH3 are of little help. Among all the covalent organic frameworks simulated in this study, COF-102-I (156 V(STP)/V of excess uptake and 169 V(STP)/V of methane delivery), COF-102-Br (153 V(STP)/V of excess uptake and 169 V(STP)/V of methane delivery), COF-102-Cl (148 V(STP)/V of excess uptake and 165 V(STP)/V of methane delivery), and COF-102-NH2 (143 V(STP)/V of excess uptake and 160 V(STP)/V of methane delivery) are found to be the most promising adsorbents for methane uptake.