Tetrazole-Decorated Ultramicroporous MOF Exhibiting Inverse CO ₂ Selectivity for Hydrocarbon Purification

Efficient CO 2 capture from hydrocarbon-rich gas mixtures, such as acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), and methane (CH 4 ), 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 CO 2 . Gas sorption measurements combined with dynamic breakthrough experiments demonstrate an unusual inverse selectivity, with Cu-btz favoring CO 2 over C 2 H 2 . 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 CO 2 adsorption, while additional weak C–H···O contacts further stabilize CO 2 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 CO 2 capture and hydrocarbon stream purification.