A supermolecular building layer approach for gas separation and storage applications: the eea and rtl MOF platforms for CO2 capture and hydrocarbon separation

Abstract

The supermolecular building layer (SBL) approach was employed to deliberately synthesize five novel metal–organic frameworks (1–5) with an exposed array of amide or amine functionalities within their pore system. The ability to decorate the pores with nitrogen donor moieties offers potential to evaluate/elucidate the structure–adsorption property relationship. Two MOF platforms, eea-MOF and rtl-MOF, based on pillaring of kgm-a or sql-a layers with heterofunctional 3-connected organic building blocks were targeted and constructed to purposely introduce and expose the desired amide or amine functionalities. Interestingly, gas adsorption properties of eea-MOF-4 (1) and eea-MOF-5 (2) showed that by simply altering the nitrogen donor position within the ligand, it is possible to relatively reduce the pore size of the related eea-MOF material and subsequently increase the associated CO₂ uptake. The slightly confined pore space in 2, relative to 1, has enabled an enhancement of the pore local charge density and thus the observed relative increase in the CO₂ and H₂ isosteric heat of adsorption (Q_st). In addition, light hydrocarbon adsorption studies revealed that 2 is more selective toward C₂H₆ and C₃H₈ over CH₄ than 1, as exemplified for C₂H₆ : CH₄ (5 : 95) or C₃H₈ : CH₄ (5 : 95) binary gas mixtures.