Versatile functionalization of de-fluorinated FMOF-1 towards enhanced carbon capture and separation: a predictive molecular simulation study

Abstract

Fluorous metal–organic frameworks, FMOFs, represent a superhydrophobic class of MOFs containing –CF₃ or –F groups in their pores. The primary objective of this research is to computationally design functionalized FMOF-1-X with X = –OCH₃, –CN, –OH, –COOH, and –NH₂ instead of –CF₃ and analyze their CO₂ adsorption and separation characteristics. Grand Canonical Monte Carlo (GCMC) simulations have been used to study the adsorption properties of CO₂, CH₄, and N₂ in all structures. Henry's constant (K_H) and isosteric heat of adsorption at infinite dilution (Q_st0) estimated from molecular Monte Carlo simulations plus the binding energy (BE) from Möller–Plesset second-order perturbation theory (MP2) quantum-mechanical simulations characterize adsorbate–adsorbent interaction strengths. Such simulations predict a systematic enhancement of all K_H, Q_st0, and BE values in X-functionalized MOFs vs. the parent FMOF-1. Among such functional MOFs, the X = –COOH structure is predicted to exhibit the largest CO₂ uptake in the low-pressure region due to the strongest CO₂/–COOH interaction strength, as supported by the largest K_H value (1.02 × 10⁻⁴ mol kg⁻¹ Pa⁻¹). In contrast, at high pressures (30 bar), the X = –OH structure is predicted to exhibit the highest CO₂ uptake. Indeed, replacing the –CF₃ groups in FMOF-1 by any aforementioned X group is expected to afford higher CO₂ uptake in the GCMC-simulated adsorption isotherms compared to the parent material. The selective adsorption of CO₂ over CH₄ and N₂ was determined using the ideal adsorbed solution theory (IAST) method at 50 : 50 and 15 : 85 CO₂/CH₄ and CO₂/N₂ binary mixtures, respectively. The X = –COOH structure amounts to the largest selectivity (59.6 for CO₂/CH₄ and 128.7 for CO₂/N₂), i.e., nearly 40× and 43× higher vs. FMOF-1 (1.5 and 3 for CO₂/CH₄ and CO₂/N₂, respectively) at 298 K and 0.1 bar. The study herein of functionalized MOFs for CO₂ separation, natural gas purification, landfill gas separation, and/or CO₂ flue gas capture suggest that X = –OH, –COOH, and –NH₂ are promising to enhance the adsorption capacity and selectivity.