Synthesis of covalent triazine-based frameworks with high CO2 adsorption and selectivity

Abstract

Four covalent triazine-based frameworks (PCTF-1 to PCTF-4) with triphenylamine as the core were synthesized by a consolidated ionothermal reaction between aromatic nitriles under the catalysis of ZnCl₂. The Brunauer–Emmett–Teller (BET) specific surface area values of PCTF-1 (853 m² g⁻¹), PCTF-2 (811 m² g⁻¹) and PCTF-3 (391 m² g⁻¹) are gradually decreased when increasing the length of branched arm. It indicates that PCTF using monomers with longer branches can be packed more efficiently, resulting in higher density and lower surface area. PCTF-4, compared with PCTF-2, just has the middle benzene of the branches replaced with benzothiadiazole, however N₂ adsorption isotherms showed that its BET specific surface area value (1404 m² g⁻¹) is the highest among the PCTFs, almost two times that of PCTF-2. The nitrogen-rich characteristics of C₃N₃ triazine rings result in the frameworks’ strong affinity for CO₂ and thereby high CO₂ adsorption capacity. In particular PCTF-4 with benzothiadiazole exhibited the highest CO₂ uptake (20.5 wt%) at 273 K and 1 bar, this value is one of the highest reported values for covalent triazine-based frameworks. The results demonstrate that the introduction of strong polar groups (benzothiadiazole) into a polymer skeleton is an efficient strategy to produce CO₂-philic microporous organic polymers with enhanced binding affinity to CO₂ molecules. In addition, such PCTFs with high physical–chemical stability and comparable BET surface areas exhibited ideal CO₂/N₂ selectivities (14–56) and CO₂/CH₄ selectivities (11–20) at 273 K, showing that these materials are potential candidates for gas storage and separation. However, in the presence of water vapor, the CO₂ uptake of all polymers decreased probably due to the formation of hydrogen bonds with water, suggesting that materials that perform well under dry conditions may deteriorate under practical conditions.