Predicting the impact of functionalized ligands on CO2 adsorption in MOFs: A combined DFT and Grand Canonical Monte Carlo study

Density Functional Theory (DFT) calculations and Grand Canonical Monte Carlo (GCMC) simulation techniques are used to study CO2 adsorption in (CH3)2-, (OH)2-, NH2- and COOH-functionalized MIL-53(Al3+)lp. The study provides a comparative analysis of CO2 adsorption in the parent and functionalized structures, in terms of heats of adsorption, isotherms and CO2 binding sites. For each functionalized MIL-53 material, DFT calculations of CO2⋯framework binding energies at different CO2 locations within the porous framework provide detailed information about the nature and strength of the CO2⋯site interactions. The multi-technique computational approach allows a direct comparison between adsorption sites from DFT calculations (at 0 K) and those observed in GCMC at 303 K and in the 0.01–25 bar pressure range. The results demonstrate the influence of confinement on the occurrence of synergic CO2–ligand interactions, and also highlight the strong link between the nature of CO2 adsorption sites and adsorption properties at low pressure, in particular the beneficial impact of polar functional groups on CO2 uptake. At high pressures, radial distribution functions of CO2 distances from the pore centres, and snapshots of the density and energy distributions of CO2 in the MOF materials provide important insights into the dependence of the adsorption process upon structural parameters, including surface area and free volume. At very high pressures, physical properties of the functional groups govern the adsorption of CO2. These counterbalancing influences play an essential role in CO2 capture and are key factors in designing new MOFs for selective capture of CO2.

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► The adsorption of CO2 on MIL-53 metal–organic frameworks was modelled by DFT and GCMC.
► The enhancement of CO2 capture by –CH3, –NH2, –OH and –COOH functionalised ligands is predicted and quantified.
► The enhanced affinity of CO2 by polar ligands, and the reduced volume available for adsorption are counterbalancing factors.
► The effect of confinement, together with synergistic effects from neighbouring functional groups are found to be important.