Melamine-Modified MIL-101(Cr) with Enhanced CO₂ Affinity for Low-Pressure Capture

MIL-101-(Cr) was post-synthetically modified with melamine to investigate the effect of nitrogen-rich functionalities on low-pressure CO₂ adsorption behavior. Melamine incorporation partially reduced the accessible surface area and pore volume of the framework while preserving its crystallinity and structural stability. Despite the decrease in porosity, the modified MIL-101-(Cr)-MEL exhibited enhanced CO₂ adsorption performance at low pressures and elevated temperatures compared with pristine MIL-101-(Cr). At 60 °C and 1 bar, the CO₂ uptake of MIL-101-(Cr)-MEL was more than twice that of the pristine material, while the working capacity in the 0.08-0.14 bar range increased fourfold. IAST calculations revealed significantly enhanced CO₂/N₂ selectivity at low pressures, reaching selectivity values close to 90 below 0.05 bar. Dynamic breakthrough experiments under both dry and humid CO₂/N₂ gas streams further confirmed the preferential CO₂ adsorption of MIL-101-(Cr)-MEL, with only a 13.4% reduction in CO₂ adsorption capacity under humid conditions. The modified material also exhibited higher isosteric enthalpies of adsorption at low CO₂ coverage, indicating enhanced CO₂ affinity relative to pristine MIL-101-(Cr). However, the moderate -ΔH_ads values suggest that adsorption is governed by reversible non-covalent CO₂-melamine interactions rather than by the formation of strongly bound CO₂-amine adducts, providing an effective balance between CO₂ affinity and regenerability. DRIFTS analyses confirmed reversible CO₂ adsorption without evidence of carbonate or carbamate formation, supporting a mechanism dominated by moderate-strength non-covalent interactions. Overall, these results demonstrate that melamine functionalization provides a simple and effective strategy to enhance low-pressure CO₂ capture performance under thermally demanding conditions.