Highly stable nanoporous covalent triazine-based frameworks with an adamantane core for carbon dioxide sorption and separation

Abstract

Adamantane substituted with two to four 4-cyanophenyl groups was used for preparation of a new series of robust Porous Covalent Triazine-based Framework (PCTF) materials. Novel adamantane PCTFs were synthesized in good yields (>80%) by the trimerization reaction of 1,3-bis-, 1,3,5-tris- and 1,3,5,7-tetrakis(4-cyanophenyl)adamantane, respectively, in the presence of ZnCl₂ (Lewis acid condition) and CF₃SO₃H (strong Brønsted acid condition). From N₂ adsorption isotherms, the Lewis acid condition gives higher surface areas than the strong Brønsted acid condition. The amorphous nano- to microporous frameworks (>50% micropore fraction) exhibit excellent thermal stabilities (>450 °C) with BET surface areas up to 1180 m² g⁻¹. A very similar ultramicropore size distribution between 4 and 10 Å was derived from CO₂ adsorption isotherms with a “CO₂ on carbon based slit-pore model”. At 1 bar the gases H₂ (at 77 K), CO₂ (at 273 and 293 K) and CH₄ (at 273 K) are adsorbed up to 1.24 wt%, 58 cm³ g⁻¹ and 20 cm³ g⁻¹, respectively. Gas uptake increases with BET surface area and micropore volume which in turn increase with the number of cyano groups in the monomer. From single component adsorption isotherms, IAST-derived ideal CO₂:N₂, CO₂:CH₄ and CH₄:N₂ selectivity values of up to 41 : 1, 7 : 1 and 6 : 1, respectively, are calculated for p → 0 at 273 K. The adamantane PCTFs have isosteric heats of adsorption for CO₂ of 25–28 kJ mol⁻¹ at zero loading and most of them also >25 kJ mol⁻¹ over the entire adsorption range which is well above the heat of liquefaction of bulk CO₂ or the isosteric enthalpy of adsorption for CO₂ on activated carbons.