Fully Inverse Adsorption Enables One-Step High-Purity C₂H₂ Separation from Ternary C₂ Mixtures in a Robust Porous Crystal
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Directly harvesting ultrahigh-purity acetylene (C<sub>2</sub>H<sub>2</sub>) from ternary C2 mixtures remains a significant challenge due to the intrinsic adsorption trend of conventional porous materials (C<sub>2</sub>H<sub>2</sub> > ethylene (C<sub>2</sub>H<sub>4</sub>) > ethane (C<sub>2</sub>H<sub>6</sub>)). To overcome this, we propose an adsorption-steering strategy that exploits steric pore-environment engineering in microporous crystals. By using the pyrazole carboxylate ligand functionalized with trifluoromethyl/methyl groups, we developed a series of MOF-5 analogs with robust structures and finely-tuned pores. One of them, named<b> NTU-98,</b> featuring dual cages (~5.6 and 10.4 Å in pore size), fully reverses the adsorption trend of C2 hydrocarbons (C<sub>2</sub>H<sub>6</sub> > C<sub>2</sub>H<sub>4</sub> > C<sub>2</sub>H<sub>2</sub>), enabling direct production of ultrahigh-purity C<sub>2</sub>H<sub>2</sub> (>99.99%) from ternary feeds at room temperature, in one step. Density functional theory calculations and gas-loaded crystallographic analyses reveal that the methyl groups strategically positioned within both cages synergistically enhance host-guest interactions with the more saturated C<sub>2</sub>H<sub>4</sub> and C<sub>2</sub>H<sub>6</sub>, while concurrently weaken the affinity for the more polarizable C<sub>2</sub>H<sub>2</sub>. This work not only presents the first porous crystal capable of directly purifying C<sub>2</sub>H<sub>2</sub> from ternary C2 feeds, but also offers a generalizable blueprint for engineering microporous environments for challenging gas separations.
