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

Abstract

PCTF-4 with benzothiadiazole exhibited the highest CO ₂ uptake (20.5 wt%) and CO ₂/N ₂ selectivity (56) among the reported covalent triazine-based frameworks.

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 ZnCl ₂. The Brunauer–Emmett–Teller (BET) specific surface area values of PCTF-1 (853 m ² g ⁻¹), PCTF-2 (811 m ² g ⁻¹) and PCTF-3 (391 m ² 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 N ₂ adsorption isotherms showed that its BET specific surface area value (1404 m ² g ⁻¹) is the highest among the PCTFs, almost two times that of PCTF-2. The nitrogen-rich characteristics of C ₃N ₃ triazine rings result in the frameworks’ strong affinity for CO ₂ and thereby high CO ₂ adsorption capacity. In particular PCTF-4 with benzothiadiazole exhibited the highest CO ₂ 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 CO ₂-philic microporous organic polymers with enhanced binding affinity to CO ₂ molecules. In addition, such PCTFs with high physical–chemical stability and comparable BET surface areas exhibited ideal CO ₂/N ₂ selectivities (14–56) and CO ₂/CH ₄ 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 CO ₂ 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.