N-rich covalent organic frameworks with different pore size for high-pressure CO2 adsorption

Three 2D N-rich COFs linked by –C=N– bond, named COF-SDU1, COF-SDU2, COF-SDU3, have been synthesized via the Schiff-base condensation reaction. Tri-(4-formacylphenoxy)-1,3,5-triazine (TRIF) is employed as N-rich aldehyde building block, and p-phenylenediamine (PA), hydrazine hydrate, and terephthalic dihydrazide (TPDH) are as the amine building blocks, respectively, which results in high crystallinity, high BET surface area, and abundant N-atom sites for COF-SDU1, COF-SDU2, and COF-SDU3. COF-SDU1 exhibits large CO2 uptake of 741 mg g−1 at 298 K under 45 bar, while COF-SDU2 is 484 mg g−1 and COF-SDU3 is only 331 mg g−1, which are closely related to their specific surface area but irrespective of the absolute N-content. The result clearly reveals the impact of various factors, including N-content, BET surface area and pore size on CO2 storage performance. It shows that N-content determines capacities at low pressure (<1 bar), BET surface area plays the decisive role under relative low pressure (<25 bar) and large pore size helps enhance the capacity under relative high pressure (>25 bar). Among all factors, BET surface area plays the crucial role in determining the high-pressure CO2 storage capacity, which is well verified by theoretical modeling.