Systematic modulation and enhancement of CO2 : N2 selectivity and water stability in an isoreticular series of bio-MOF-11 analogues

Abstract

An isoreticular series of cobalt-adeninate bio-MOFs (bio-MOFs-11–14) is reported. The pores of bio-MOFs-11–14 are decorated with acetate, propionate, butyrate, and valerate, respectively. The nitrogen (N₂) and carbon dioxide (CO₂) adsorption properties of these materials are studied and compared. The isosteric heats of adsorption for CO₂ are calculated, and the CO₂ : N₂ selectivities for each material are determined. As the lengths of the aliphatic chains decorating the pores in bio-MOFs-11–14 increase, the BET surface areas decrease from 1148 m² g⁻¹ to 17 m² g⁻¹ while the CO₂ : N₂ selectivities predicted from ideal adsorbed solution theory at 1 bar and 273 K for a 10 : 90 CO₂ : N₂ mixture range from 73 : 1 for bio-MOF-11 to 123 : 1 for bio-MOF-12 and finally to 107 : 1 for bio-MOF-13. At 298 K, the selectivities are 43 : 1 for bio-MOF-11, 52 : 1 for bio-MOF-12, and 40 : 1 for bio-MOF-13. Additionally, it is shown that bio-MOF-14 exhibits a unique molecular sieving property that allows it to adsorb CO₂ but not N₂ at 273 and 298 K. Finally, the water stability of bio-MOFs-11–14 increases with increasing aliphatic chain length. Bio-MOF-14 exhibits no loss of crystallinity or porosity after soaking in water for one month.