CO 2 Adsorption in Fe 2 (dobdc): A Classical Force Field Parameterized from Quantum Mechanical Calculations (original) (raw)

Metal−organic frameworks (MOFs) are versatile nanoporous materials that have gained significant interest as low heat capacity, high selectivity sorbents for CO 2 capture applications. Large-scale atomistic simulations for identifying high-performance MOFs are possible, but are limited to systems for which existing molecular mechanics force fields describe the interactions between the guest and framework atoms with sufficient accuracy. However, standard force fields are not applicable to cases involving coordinatively unsaturated metal centers that can strongly bind specific sorbate molecules. It has been previously shown that improved force fields can be derived from quantum mechanical calculations. In this work, we derived force fields for an isostructural series of MOFs, M-MOF-74, where M = Mn, Co, Ni, and Cu, from first principles. Monte Carlo calculations in the Gibbs ensemble were used to calculate the CO 2 adsorption isotherms in order to assess the quality of the derived force field parameters and to determine a generally applicable procedure for obtaining a reliable force field for a targeted MOF and adsorbate system. The computed CO 2 adsorption isotherms for the different M-MOF-74 members agree with experimental measurements at low loading and show that Ni-MOF-74 possesses the highest affinity toward CO 2 and Cu-MOF-74 the weakest. In addition, we explored the source of open metal site and pore inaccessibility in these materials and quantified its impact on adsorption, especially the discrepancies often observed between experiments and simulations at high loadings.

Analysis of the CO2 adsorption properties of a well-known series of metal-organic frameworks M2(dobdc) (dobdc4−= 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in-situ structural studies to identify the host-guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2adsorbs in a linear or nonlinear geometry. This has important implicatio...

The structure-activity relationship is crucial in catalytic performance and material design but still largely obscure due to the complexity of heterogeneous catalytic systems. CO activation occurs widely in Fischer-Tropsch reactions and pyrometallurgy, and it is a key to understanding carburization. Here, we investigate the structure-activity relationship in Fe nanoparticles by reactive molecular dynamics simulations. We focus on two activities, the adsorption and dissociation of CO, and four structural characteristics , morphologies, sizes, defects, and heteroatoms. The results show that CO adsorption and dissociation varies with the change of nanoparticles. Line dislocation and vacancies can strikingly boost CO dissociation, suggesting an effective way to tune the CO dissociation rate. Further analysis shows that the Eley-Rideal mechanism possibly works in the early periods, followed by the Langmuir-Hinshelwood mechanism in the later periods for CO 2 formation. Our results shed light on the mechanism and possible optimization of the carburization of iron.

Carbon capture and sequestration (CCS) is a viable strategy proposed to mitigate the negative environmental impacts derived from burning fossil fuels. Recently, nano-porous adsorbent materials such as metal-organic frameworks (MOFs) have shown to potentially provide a more energy-efficient way to separate CO2 out of flue gases. In particular, MOFs that possess unsaturated open metal sites are known to interact strongly with CO2 molecules, and thus emerging as promising materials for the separation applications. However, from the computational point of view, the strong interaction is poorly modeled by common force fields, resulting in underestimation of the adsorption properties of CO2 in Mg-MOF-74 by as much as an order of magnitude. Accordingly, a systematic methodology has been proposed to generate accurate force fields using high-level quantum chemical calculations1. In this method, interaction energies of a few selected CO2 configurations computed by quantum calculations are use...