Research on CO2-N2O separation using flexible metal organic frameworks

Abstract

N2O and CO2 are two linear molecules, which have almost the same molecular size and similar physical properties (polarizability and quadrupole moment). Therefore, the separation of CO2 and N2O is extremely challenging. Flexible metal organic frameworks exhibit a gate-opening phenomenon for one type of gas molecule, but not for another, thus enabling excellent selectivity for twin-like molecules. In this study, the separation of CO2/N2O by flexible metal organic frameworks with different structures was studied systematically using a combination of adsorption and breakthrough tests. The results show that the gate-opening pressure of CO2 is lower than that of N2O on Cu(bpy)2(BF4)2 (ELM-11) and Cu(bpy)2(OTf)2 (ELM-12), which are two-dimensional layered flexible MOFs, whereas N2O has a lower gate-opening pressure than CO2 on MIL-53Al, a three-dimensional breathing structure with an adsorption capacity of up to 8 mmol/g was observed at 298 K and ~5 bar, indicating its good separation properties for N2O from CO2 under high pressure. Breakthrough experiments for N2O/CO2 mixtures confirmed that MIL-53Al is capable of separating N2O and CO2 under the optimized conditions (8 bar), making it a promising material for industrial application.