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

Abstract 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 −ΔHads 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.