Direct synthesis of an aliphatic amine functionalized metal–organic framework for efficient CO2 removal and CH4 purification

Abstract

Porous materials bearing free aliphatic amine groups are powerful adsorbents for carbon dioxide but generally have to be synthesized by complicated, multi-step post-synthetic modification methods due to the high activity of these functional groups. We report a crystal-engineering strategy for such metal–organic frameworks. Reactions of Zn(II) ion with diversified coordination modes and bis(4-carboxy-benzyl)amine (H₂bcba) with a secondary aliphatic amine bridge at different temperatures yielded a pair of isomeric frameworks with very similar framework structures but distinctly different local structures, namely α-[Zn(bcba)] (MCF-51 or α-Zn) with all amino groups coordinated and β-[Zn(bcba)] (MCF-52 or β-Zn) with uncoordinated amino groups. Single-component gas adsorption and CO₂/CH₄ mixture breakthrough experiments at ambient conditions showed that, the CO₂ adsorption ability and CH₄ purification performances of β-Zn, in terms of CO₂ adsorption enthalpy (42 kJ mol⁻¹), CO₂/CH₄ selectivity (32) and CH₄ purity/productivity (1.49 mmol g⁻¹ at 99.999%+), are much higher than those of α-Zn (CO₂ adsorption enthalpy of 17 kJ mol⁻¹, CO₂/CH₄ selectivity of 2.9 and CH₄ purity/productivity of 0.088 mmol g⁻¹ at 99%+).