Simultaneous enhancement of CO2 adsorption capacity and kinetics on a novel micro-mesoporous MIL-101(Cr)-based composite: Experimental and DFT study

Abstract

MIL-101(Cr), a class of metal-organic framework, is a potential candidate for CO₂ capture applications because of its high capacity of adsorption and separation capability. However, the intrinsic microporous structure of this nanomaterial poses limitations on its adsorption kinetics. Techniques employed to enhance its adsorption kinetics often adversely impact its adsorption capacity at equilibrium. Herein, as a new approach, we prepared amine-functionalized FAC@MIL-101(Cr) composites with adjustable micro-mesoporous structure and tunable nitrogen content by embedding different ratios of amine-functionalized activated carbon throughout the framework of MIL-101(Cr). This led to a simultaneous improvement in both kinetics and adsorption capacity for CO₂. The best adsorbent, FAC-6@MIL-101(Cr), has excellent textural properties with a high surface area (1763.1 m².g⁻¹), great pore volume (1.29 cm³.g⁻¹), and suitable nitrogen content (4.7 wt%). The adsorption analysis revealed that the modification of MIL-101(Cr) improved its CO₂ adsorption capacity from 3.21 to 5.27 mmol/g under standard conditions of 1 bar and 25 °C. Furthermore, the FAC-6@MIL-101(Cr) adsorbent demonstrated fast CO₂ adsorption kinetics (three times more relative to the pure MIL-101(Cr)), high CO₂/N₂ selectivity, and remarkable cyclic stability. The results confirmed that hybridization enhanced the polarizability of FAC@MIL-101(Cr) samples, causing more robust CO₂-adsorbent surface interactions. Simultaneously, the existence of mesopores in the structure facilitated the transport of CO₂ into the interior pores, resulting in a more efficient contact of CO₂ molecules with all of the amine sites and a faster adsorption rate as well as more efficient regeneration. According to density functional theory (DFT) calculations, hybridization process induces significant changes in composites’ electronic structure, enhancing their capacity to interact with CO₂ molecules more effectively. On the other hand, DFT calculations confirm that N₂ molecule is less activated on the FAC@MIL-101(Cr) as evidenced by calculated small adsorption energy and charge-transfer values.