CO2 adsorption and separation by fullerene encapsulated MOF: A multiscale insight combining molecular dynamics and density functional theory

Abstract

Composites encapsulating fullerenes into metal–organic frameworks (MOFs) have great potential as efficient CO₂ adsorption and separation materials. However, the microscopic structure–function relationship and underlying mechanisms for such materials remain unclear. By multiscale simulations combining molecular dynamics (MD) and density functional theory (DFT), we have investigated the effect of adding fullerene C₆₀ to a typical MOF-177 on CO₂ adsorption and separation from equilibrium and kinetics perspectives and explained their microscopic mechanisms. It is found that the adsorbed amount and gas adsorption energy in the low-pressure range increases with increasing C₆₀ number, and the adsorption isotherm shifts from the Henry to the Langmuir model, not only due to the increase in surface area, but also the synergistic effect of co-adsorption of C₆₀ with CO₂. Although the addition of C₆₀ attenuates CO₂ adsorption kinetics and diffusion, the enhancement of equilibrium adsorption capacity predominates, resulting in an CO₂ permeability increase of 38 % under optimal C₆₀ incorporating numbers in the range of 5 ∼ 10 per unit cell. The adsorption selectivity of CO₂/H₂ can be increased by an order of magnitude when C₆₀ is saturated. Overall, the selectivity improvement of mixtures of CO₂ and lighter gases is more obvious by C₆₀ encapsulation. Particularly, we demonstrate from a kinetic point of view that the addition of C₆₀ achieves separation of the otherwise inseparable H₂S/CO₂ mixture. The competition between different adsorbed molecules dominates the change in adsorption selectivity after the addition of C₆₀. Our results provide deep insights into CO₂ adsorption and separation by MOF incorporated with carbon nanomaterials.