Simulations of hydrogen, carbon dioxide, and small hydrocarbon sorption in a nitrogen-rich rht-metal-organic...

Grand canonical Monte Carlo (GCMC) simulations of gas sorption were performed in Cu-TDPAH, also known as rht-MOF-9, hereafter [1], a metal-organic framework (MOF) with rht topology consisting of Cu²⁺ ions coordinated to 2,5,8-tris(3,5-dicarboxyphenylamino)-1,3,4,6,7,9,9b-heptaazaphenalene (TDPAH) ligands. This MOF is notable for the presence of open-metal copper sites and high nitrogen content on the linkers. [1] Exhibits one of the highest experimental H₂ uptakes at 77 K/1 atm within the extant rht-MOF family (ca. 2.72 wt%) and also has strong affinity for CO₂ (5.83 mmol g^-1 at 298 K/1 atm). Our simulations, which include explicit many-body polarization interactions, accurately modeled macroscopic thermodynamic properties (e.g., sorption isotherms and isosteric heats of adsorption (Q_st)) as well as the binding sites for H₂, CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆ in the MOF. Four different binding sites were observed through analysis of the radial distribution function (g(r)) about the two chemically distinct Cu²⁺ ions, simulated annealing calculations, and examination of the three-dimensional histogram showing the sites of occupancy: (1) at the Cu²⁺ ion facing toward the center of the linker (CuL), (2) at the Cu²⁺ ion facing away from the center of linker (CuC), (3) nestled between three [Cu₂(O₂CR)₄] units in the corner of the truncated tetrahedral (T-T_d) cage and (4) straddling the copper nuclei parallel to the axis of the Cu-Cu bond within the T-T_d cage. The low-loading (initial) binding site in the MOF is highly sensitive to the partial charges of the Cu²⁺ ions that were used for parametrization. It was discovered that most sorbates prefer to sorb onto or near the Cu²⁺ ions that exhibit the greater partial positive charge (i.e., at site 1). The simulated H₂ and CO₂ sorption results obtained using a polarizable potential for the respective sorbates are in good agreement with the corresponding experimental data, especially near ambient pressure. Simulations of gas sorption were also performed in [1] using nonpolarizable potentials for the individual sorbates; these include potentials from the TraPPE force field for most sorbates.