Turning Normal to Abnormal: Reversing CO₂/C2-Hydrocarbon Selectivity in HKUST-1

Metal-organic frameworks (MOFs) can efficiently purify hydrocarbons from CO2, but their rapid saturation, driven by preferential hydrocarbon adsorption, requires energy-intensive adsorption-desorption processes. To address these challenges, an innovative approach is developed, enabling control over MOF flexibility through densification and defect engineering, resulting in an intriguing inverse CO2/C2 hydrocarbon selectivity. In this study, the densification process induces the shearing of the crystal lattice and contraction of pores in a defective CuBTC MOF. These changes have led to a remarkable transformation in selectivity, where the originally hydrocarbon-selective CuBTC MOF becomes CO2-selective. The selectivity values for densified CuBTC are significantly reversed when compared to its powder form, with notable improvements observed in CO2/C2H6 (4416 vs 0.61), CO2/C2H4 (15 vs 0.28), and CO2/C2H2 (4 vs 0.2). The densified material shows impressive separation, regeneration, and recyclability during dynamic breakthrough experiments with complex quinary gas mixtures. Simulation studies indicate faster CO2 passage through the tetragonal structure of densified CuBTC compared to C2H2. Experimental kinetic diffusion studies confirm accelerated CO2 diffusion over hydrocarbons in the densified MOF, attributed to its small pore window and minimal interparticle voids. This research introduces a promising strategy for refining existing and future MOF materials, enhancing their separation performance.