Dual-Ligand UiO-66 Metal-Organic Frameworks: Synergistic Amine-Hydroxyl Functionalization for High-Selectivity CO2/CH4 Separation in Biogas Upgrading

UiO-66 MOF was functionalized with dual ligands namely BDC-NH 2 and BDC-(OH) 2 . • Selective CO 2 /CH 4 adsorption were investigated at atmospheric and high pressures. • The highest CO 2 /CH 4 selectivity of 35.60 was achieved with 10%H-ZrMOF-A adsorbent. • CO 2 adsorption capacity of the adsorbent remained over 91.5% after five reuses. Selective CO 2 /CH 4 separation is essential for biogas valorization, yet conventional adsorbents typically face a trade-off between adsorption capacity and selectivity. In this work, a dual-ligand engineering strategy is implemented on UiO-66-NH 2 (ZrMOF-A) by co-incorporating 2,5-dihydroxyterephthalate (BDC-(OH) 2 ) to create a series of mixed-ligand MOFs denoted X %H-ZrMOF-A ( X = 5%–50% (mass)). The materials were synthesized via a solvothermal route and systematically characterized by N 2 physisorption, powder X-ray diffraction, FTIR spectroscopy, and SEM to elucidate structure-property relationships. Increasing hydroxyl content progressively modified surface area, pore volume, and morphology, with excessive BDC-(OH) 2 loading (>30% (mass)) inducing pronounced structural degradation. Among the series, 10%H-ZrMOF-A exhibited the most favorable balance between textural properties and functional site density, achieving a CO 2 uptake of 1.352 mmol·g –1 and a CO 2 /CH 4 selectivity of 35.60 at 100 kPa and 25 °C for a 40:60 CO 2 :CH 4 mixture predicted by IAST. Multi-model isotherm analysis identified the Freundlich and Elovich models as the most suitable descriptions of CO 2 adsorption, confirming a heterogeneous adsorption landscape arising from the coexistence of amine and hydroxyl sites. Complementary DFT calculations showed that dual functionalization strengthens CO 2 binding while weakening CH 4 interactions, consistent with the experimentally observed enhancement in selectivity. Pressure-dependent measurements revealed that high CO 2 /CH 4 selectivity at low pressures (≤100 kPa) diminishes at elevated pressures as non-selective pore filling becomes dominant, indicating that 10%H-ZrMOF-A is particularly suited for low-pressure biogas upgrading and post-combustion CO 2 capture. Five consecutive adsorption-desorption cycles demonstrated 91.5% capacity retention, confirming moderate short-term recyclability under mild thermal regeneration conditions. Download: Download high-res image (204KB) Download: Download full-size image