Ionothermal synthesis of phosphonitrilic-core covalent triazine frameworks for carbon dioxide capture

Abstract

Covalent Triazine Frameworks (CTFs) are a class of porous organic frameworks with tunable properties obtainable through rational design targeting a wide range of applications. In the present work, we developed a series of CTFs with phosphazene core for CO2 adsorption. The CTFs were synthesized using building blocks of hexakis(oxy)hexabenzonitrile phosphazene (HCPz) under ionothermal conditions and ZnCl2 as catalyst. The effect of ZnCl2/monomer ratio and the ionothermal reaction temperature and synthesis procedure on the porosity of the Pz-CTFs were systematically explored. The Pz-CTF synthesized using a molar ratio of ZnCl2/monomer of 10 with gradient ionothermal reaction conditions (400 °C/25 h; 450 °C/13 h; 500 °C/1h; 600 °C/1h) yielded an amorphous, predominantly ultra-microporous material (Pz-CTF6) with high surface area (SBET of 1009 m2 g−1). The primary aspect of the applied gradient ionothermal reaction scheme is that it induces a simultaneous reversible and irreversible trimerization of nitriles, allowing restructuring of the triazine units, thus resulting in an extended microporous network. Besides, the highly crosslinked, electron-rich phosphazene core provided high structural and thermal stability (up to 500 °C) and enhanced CO2-philicity of the synthesized CTFs. As a result, Pz-CTF6 showed a CO2 uptake capacity of 4.19 and 2.47 mmol g−1 at 273 and 298 K, respectively, at 1 bar. In addition, it exhibited a high CO2/N2 selectivity of 147 for a feed containing 85% of CO2 at 50 mbar and 298 K, as determined by the IAST method. Furthermore, the developed adsorbents exhibited enhanced hydrophobicity, causing a rather mild reduction in CO2 capacity when humidity conditions were applied. These findings show that the synthesized Pz-CTFs are promising for CO2 capture and adsorptive separation.