Selection-diffusion-selection mechanisms in ordered hierarchically-porous MOF-on-MOF: ZIF-8 @NH2-MIL-125 for efficient CO2 separation

To promote CO2/N2 selectivity of MOF (metal organic framework)-on-MOF hybrids, ZIF-8 (Zeolitic imidazolate frameworks-8) @NH2-MIL-125 (Materials of Institute Lavoisier-125) with ordered hierarchically-porous structures were originally fabricated and in-situ loaded as nanofillers in mixed matrix membranes (MMMs) to efficiently separate CO2 from N2 based on selection-diffusion-selection mechanism. The guest ZIF-8 nanoparticles (theoretical pore size = 0.34 nm) were uniformly grown on surfaces of the host NH2-MIL-125 nanoparticles and acted as molecular sieves owing to its small pore size, which kept larger N2 molecules (theoretical kinetic diameter = 0.36 nm) out and allowed smaller CO2 molecules (theoretical kinetic diameter = 0.33 nm) to pass through, while the host NH2-MIL-125 with relatively large pores (theoretical pore size = 0.60 nm) provided low-resistance channels for rapid CO2 transfer. The CO2/N2 adsorption selectivity of ZIF-8 @NH2-MIL-125 heterostructures exhibited 12 % and 300 % higher than that of NH2-MIL-125 and ZIF-8, respectively. The Pebax-based MMMs loaded with ZIF-8 @NH2-MIL-125 nanofillers had an increased CO2/N2 ideal selectivity by 42 % and 61 % compared to that of [email protected]2-MIL-125 and [email protected], respectively.