A Quantum Mechanical Description of the Diffusion Properties of C1-C2 Hydrocarbon Molecules in MOF-74-Mg

Among the numerous metal organic framework (MOF) families, of significant importance is the MOF-74-M series (M = metal, -also known as CPO-27-M), characterized by a high density of not fully coordinated open metal site centers; this peculiarity has been demonstrated to translate into a higher hydrocarbons’ separation potential than other known MOFs and zeolites. To provide a more comprehensive description of the behavior of the hydrocarbon molecules within the MOF-74 cavity, diffusion processes and corresponding diffusion barriers of small hydrocarbons in the MOF-74-Mg were modeled. This work provides a description of the molecular transport processes of CH4, C2H2, C2H4 and C2H6 within and along the cavity of MOF-74-Mg; in addition, the influence of pre-adsorbed water molecules is also addressed. Density functional theory (DFT), as implemented within a plane-wave (PW) approach under periodic boundary conditions (PBC), has been used to investigate the diffusion mechanisms using the climbing-image nudge elastic band (CI-NEB) method, coupled with the van der Waals functional (vdW-DF) and ultra-soft pseudopotentials. Two transport mechanisms were identified: M1, referring to the molecular migrations within the MOF cavity; and M2, referring to the molecular migrations along the MOF longitudinal channel. The M1 transport mechanism was further analyzed to address the molecular migration from one metal atom to its adjacent one (M1a), and with respect to the second metal over (M1b). All the transport mechanisms considered show that the diffusion of paraffin molecules in MOF-74-Mg is energetically more favorable than that of olefin molecules. An interesting trend is observed across all the diffusion mechanisms where the stronger the molecule binds to the open metal site, the higher the diffusion barrier it needs to overcome. For all the small hydrocarbons considered in this study, transport mechanism M1a is significantly more energetically favorable than M1b, showing that it is easier for molecules to drift along the longitudinal cavity than to remain trapped cross-sectionally within the cavity itself. This work shows how this computational approach can be successfully applied not only to reveal the molecular transport in other MOF-74 isostructural species, but also in the fundamental understanding of the screening of MOFs and other nano-porous materials for gas separation applications.