The Impact of Metal Centers in the M-Mof-74 Series on Carbon Dioxide and Hydrogen Separation

The series of metal-organic frameworks M-MOF-74 gained popularity due to numerous, highly reactive open-metal sites. As effective adsorbent, the M-MOF-74 series is an alternative to expensive liquid absorbent processes in the field of capture and separation of CO2. The description of the enhanced interactions between guest molecules and open-metal sites without accounting polarization effects is challenging, but can reduce the computational cost of simulations. In this study, we propose a non-polarizable force field for CO2, and H2 adsorption in M-MOF-74 (M = Ni, Cu, Co, Fe, Mn, Zn), adjusted by scaling the Coulombic interactions of M-MOF-74 atoms, and Lennard-Jones interaction potentials between the center of mass of H2, and the open-metal centers. The presented force field is based on UFF and DREIDING parameters, characterized by high transferability and efficiency. The quantum behavior of H2 at cryogenic temperatures is taken into account by incorporating Feynman-Hibbs quantum corrections. To validate the force field, the experimental isotherms of CO2 at 298 K, 10−1 - 102 kPa, isotherms of H2 at 77 K, 10−5 - 102 kPa, the corresponding enthalpy of adsorption, and binding geometries in the M-MOF-74 series were reproduced using Monte Carlo simulations in the grand-canonical ensemble. The computed loadings and heats of CO2 and H2 adsorption in M-MOF-74 are in a very good agreement with the experimental values. The relative differences between the simulated binding geometries and literature data are lower than 10%. The temperature transferability of the force field from 77 K to 87 K, and 298 K was shown for adsorption of H2. The validated force field was used to study the adsorption and separation of CO2/H2 mixtures at 298 K. The adsorption of H2 practically does not occur when CO2 is present in the mixture. For all the studied feed mole fractions of CO2, the breakthrough time of CO2 was found to be the longest in Ni-MOF-74, and the shortest in Cu-MOF-74. Increasing the feed mole fraction of CO2 from 0.1 to 0.9 speeds up the breakthrough time. The application of the non-polarizable force field allows full investigation of the capture and separation of CO2 in M-MOF-74, and can be expanded to study multi-component mixtures or industrial reactions in the future research.