Effects of de-fluorination on hydrophobicity and on CO2 and CH4 adsorption under high humidity in MeMOFs, methylated analogues of FMOFs

Abstract

Herein, we report carbon dioxide (CO₂) and methane (CH₄) adsorption behavior in MeMOFs, methylated analogues of FMOFs with −CF₃ groups replaced with −CH₃, utilizing grand canonical Monte Carlo (GCMC) simulations at 288, 298, and 308 K and P ≤ 40 bar and density functional theory (DFT) computations of adsorption energies. Isosteric heats of adsorption (Q_st), Henry's constants (K_H) and interaction energies were used to analyze the adsorbate–adsorbent interaction strengths and gas uptake of guest molecules. The Q_st of CO₂ was found to be 1.29–1.73 times higher in MeMOF-1 than in FMOF-1, vs. 1.30–1.47 times for CH₄, hence demonstrating higher guest affinity to MeMOF-1 than to the FMOF-1 polymorph. Simulated isotherms were further fitted with Langmuir, Langmuir–Freundlich, and Tóth models to calculate the isosteric heat of adsorption at infinite dilution (Q_st0) using the Clausius–Clapeyron equation. The data were then compared with those obtained from force-field-based Monte Carlo (MC) simulations to determine the consistency. The Tóth model presented excellent characterization of CO₂ and CH₄ adsorption, implying both FMOF-1 and MeMOF-1 materials have inhomogeneous surfaces. The order of the Q_st0 values obtained using the Clausius–Clapeyron equation was consistent with that obtained from MC simulations and confirmed the higher uptake of CO₂ and CH₄ in MeMOF-1 as predicted by GCMC. The presence of H₂O vapor, up to 80% relative humidity, did not affect the CO₂ and CH₄ adsorption in MeMOF-1 structures, as was observed in the analogous FMOF-1 parent structure. The larger pore size and surface area upon substituting −CF₃ groups with −CH₃ groups allow for significantly greater CO₂ and CH₄ uptake in MeMOFs compared to FMOFs with no water uptake even at high humidity. These simulations were applied upon MeMOF analogues of multiple FMOF-1 polymorphs known to date and are thus expected to hold for MeMOF analogues of other FMOF and MOFF structures reported by the Omary and Miljanić teams, respectively. Experimental data have validated the superhydrophobic nature of the MeMOF-1 composition via a polymorphic form with a different topology, MeMOF-2, attaining an ∼100° increase in water-drop contact angles, from ∼74° for a control plastic substrate to ∼172° upon dry-coating it with MeMOF-2. Experimentally synthesized MeMOF-2 possesses the same {Ag(3,5-(CH₃)₂-1,2,4-triazolate)} empirical formula as that of simulated MeMOF-1 structures, albeit with a different crystal structure and lower porosity.