Tetrazole-Decorated Ultramicroporous MOF Exhibiting Inverse CO₂ Selectivity for Hydrocarbon Purification

Efficient CO₂ capture from hydrocarbon-rich gas mixtures, such as acetylene (C₂H₂), ethylene (C₂H₄), and methane (CH₄), is crucial for feedstock purification and natural gas upgrading. Ultramicroporous metal-organic frameworks (MOFs) offer promising platforms for such separations, as their narrow pores amplify molecular discrimination and host-guest interactions. Herein, we present a tetrazole-decorated MOF, Cu-btz, synthesized via a green, water-based route and featuring N-donor-rich ultramicropores that preferentially interact with CO₂. Gas sorption measurements combined with dynamic breakthrough experiments demonstrate an unusual inverse selectivity, with Cu-btz favoring CO₂ over C₂H₂. Density functional theory (DFT) calculations suggest that this behavior arises from a cooperative host-guest interaction landscape. Long-range electrostatic interactions from the nitrogen-rich pore environment facilitate CO₂ adsorption, while additional weak C-H···O contacts further stabilize CO₂ within the confined pore space. Remarkably, the framework structure effectively shields the Cu-N coordination bonds from water intrusion, endowing Cu-btz with exceptional hydrolytic stability (2 years in water at room temperature and 30 days in boiling water) and robustness across a broad pH range (pH 3-10). This combination of unusual inverse selectivity and outstanding stability renders Cu-btz a highly promising material for CO₂ capture and hydrocarbon stream purification.