Synergistic Al-MIL-53-(Mn) Bimetallic Metal–Organic Frameworks: Rational Tuning of Mn/Al Composition for Enhanced CO₂ Capture

The development of bimetallic metal–organic frameworks (BMOFs) represents a promising strategy to meet critical requirements of CO₂ capture in flue gas, where CO₂ exists at low partial pressure (0.15 bar, 298–333 K) due to the synergistic effects of mixed-metal incorporation that can enhance stability and CO₂ affinity. In this study, aluminum and manganese were coordinated on terephthalic acid to produce a series of bimetallic Al-MIL-53-(Mn) with varying Mn/Al molar ratios. These samples were synthesized via a hydrothermal approach, followed by a two-step activation, and characterized using FTIR, PXRD, SEM-EDX, TGA, and the BET method to uncover fundamental CO₂ adsorption characteristics. All samples exhibited MIL-53 features, including a predominant microporous structure, the presence of hydroxyl bridging groups in the surface chemistry, a rhomboidal morphology that supports their breathing behavior, and high thermal stability. The textural properties and the crystallinity were enhanced with increased mixed-metal ratios (Mn/Al = 0.15, 0.45, 0.65), and the findings revealed structural destabilization at a high molar ratio (Mn/Al = 0.85). Besides the 77 K low-pressure N₂ isotherms, CO₂ isotherms measured at a screening temperature of 273 K demonstrated a maximum uptake of 3.03 mmol/g, which compares favorably with the 2.19–2.85 mmol/g reported in the literature for pristine MIL-53(Al). They revealed a consistent Type I Langmuir isotherm with a steep uptake at low partial pressure, which suggests the presence of strong preferential binding sites for CO₂ on the surface of bimetallic MOFs. As this is a screening study, the bulk Mn content (ICP-OES), the CO₂/N₂ selectivity derived from 273 K and 298 K isotherms, the performance at flue-gas temperatures (313–333 K) and moisture tolerance remain to be established.