A mixed-linker approach towards improving covalent triazine-based frameworks for CO2 capture and separation

Abstract

Carbon dioxide separation and capture/sequestration are important problems not only in the context of global warming but also for optimization of industrial processes. Porous covalent triazine-based frameworks (CTFs) are a class of promising materials for adsorptive separation of CO2, which could be an effective alternative to chemisorption based approaches. We report nine novel CTFs based on mixed building-block approach, synthesized under ionothermal conditions (ZnCl2, 400 °C). Angular 1,3-bis(4-cyanophenyl)adamantane (Ad2), trigonal-pyramidal 1,3,5-tris(4-cyanophenyl)adamantane (Ad3) and tetrahedral 1,3,5,7-tetrakis(4-cyanophenyl)adamantane (Ad4) were processed together with the linear terephthalonitrile (L1) and 4,4'-biphenyl dicarbonitrile (L2) as well as the planar trigonal 1,3,5-benzenetricarbonitrile (L3) targeting maximization of surface area and void volume. The thermally (>500 °C) and acid stable CTFs showed a specific surface area (SBET) in the range of 747–1885 m2 g−1, with the highest surface area achieved for Ad4L2, reflecting the generally observed synergism of elongation of L-building blocks with branching of the Ad-building blocks. The CO2 uptake correlates empirically with the product of BET surface area, micropore volume fraction (V0.1/Vtot) and CO2-accessible micropore volume (Vmicro(CO2)). At low pressure (up to 1 bar) and 273 K, Ad2L1 - Ad4L3 adsorb significant amounts of CO2 (40–76 cm3 g−1) and CH4 (14–26 cm3 g−1), which are generally in line with other reported CTFs, proving the viability of the mixed building-block approach for continuous tuning of properties in the case of CTFs. The obtained CTFs exhibit good ideal selectivity in the range of 17–30 for CO2 over N2 (for a 85% N2/15% CO2 mixture at 273 K, 1 bar), and represent a prototypal ‘tunable’ series for efficient adsorption separations. Water vapour sorption by the novel CTS indicates that the adamantane based CTFs are largely hydrophobic.