Enhancing CO2 Adsorption and CO2/N2 Separation Performance by Incorporating Calcium in MIL-53 (Al)

Abstract Global warming is primarily driven by the rapid accumulation of carbon dioxide (CO₂) in the atmosphere. Metal–organic frameworks (MOFs) have emerged as promising materials for mitigating CO₂ emissions due to their tunable porosity, large surface area, and structural flexibility. Among them, MIL-53(Al) has attracted widespread interest owing to its excellent thermal and chemical stability. Recent studies show that modifying MOFs with secondary metals can significantly enhance their CO₂ adsorption performance. In this work, a one-pot synthesis method was employed to incorporate calcium (Ca), functioning as a Lewis basic metal, into the MIL-53(Al) framework to produce a series of bimetallic materials: MIL-53(Al, Ca)-1, -2, and − 3. These materials were further treated with NaOH to yield MIL-53(Al, Ca, Na)-1, -2, and − 3. Introducing Ca into the framework increased the affinity toward CO₂—a Lewis acidic gas—thereby enhancing adsorption. All materials were characterised and compared with pristine MIL-53(Al). CO₂ adsorption was evaluated at 273 K under both low pressure (up to 100 kPa) and high pressure (up to 980 kPa), while N₂ adsorption was measured at 273 K and 100 kPa. N₂ uptake decreased significantly with increasing Ca content, from 0.7177 mmol·g⁻¹ for MIL-53(Al) to 0.173 mmol·g⁻¹ for MIL-53(Al, Ca)-3 and MIL-53(Al, Ca, Na)-3. Moderate Ca incorporation improved CO₂ adsorption: at 100 kPa, MIL-53(Al, Ca)-1 and MIL-53(Al, Ca, Na)-1 achieved 4.09 and 3.56 mmol·g⁻¹, respectively, compared with 2.19 mmol·g⁻¹ for MIL-53(Al). However, excessive Ca loading (samples − 3) reduced CO₂ uptake due to structural distortion and reduced surface area. Despite this, the highly Ca-modified samples (-3) exhibited superior CO₂/N₂ selectivity. These findings demonstrate that controlled Ca incorporation can tune the adsorption behaviour and separation efficiency of MIL-53-type MOFs, offering a promising pathway for designing high-performance adsorbents for CO₂ capture and CO₂/N₂ separation. Graphical Abstract