Two linkers are better than one: enhancing CO2 capture and separation with porous covalent triazine-based frameworks from mixed nitrile linkers

Covalent triazine-based framework (CTF) materials were synthesized by combining two different nitrile building blocks: the tetranitrile tetrakis(4-cyanophenyl)ethylene (M) was reacted with either terephthalonitrile (M1), tetrafluoroterephthalonitrile (M2), 4,4'-biphenyldicarbonitrile (M3) or 1,3,5-benzenetricarbonitrile (M4) under ionothermal conditions (ZnCl2, 400 degrees C) to yield mixed-nitrile MM0CTFs MM1 to MM4. Comparative H-1/C-13 and F-19/C-13 CP MAS analyses of MM2(300) (synthesized at 300 degrees C) suggest that the hydrogenated and fluorinated carbon atoms are in close vicinity (< 5 angstrom) to each other and support the formulation of the MM2(300) sample as a copolymeric CTF. Systematic N-2, CO2 and CH4 gas sorption studies were performed up to 1 bar at 273 K and 293 K. The specific BET surface areas of MM1-MM4 were 1800, 1360, 1884 and 1407 m(2) g(-1), respectively. The CO2 uptake capacity of mixed-nitrile MM1, MM2 and MM4 was higher than the CO2 uptake of the respective individual single-nitrile M-or M'-CTF despite a higher surface area of the M-CTF PCTF-1 (2235 m(2) g(-1)). The synergistic increase in the CO2 uptake of the mixed-nitrile MM'-CTFs is due to the higher CO2-accessible micropore volume V-micro(CO2) and the higher micropore volume fraction V-0.1/V-tot of the MM'-CTFs compared to the M- or M0-CTFs. The surface area of porous materials does not play the most important role in CO2 storage at low pressure but the CO2-accessible micropore volume is the more decisive factor. Further, MM2 shows the second highest (of known CTFs synthesized at 400 degrees C) CO2 uptake capacity of 4.70 mmol g(-1) at 273 K and 1 bar because of its large micropore fraction (82%), which may be due to the release of fluorous decomposition products ('defluorination carbonization') during its synthesis. The CO2/N-2 adsorption selectivities of mixed-nitrile MM1, MM2 and MM4 CTFs were also higher than those of the single-nitrile component M-or M'-CTFs.