Experimental and theoretical approach to the study of CO2 adsorption in an extensive family of metal-organic frameworks

Metal org. frameworks (MOFs) are cryst. materials that contain metal-ions or metal-ion clusters as nodes and org. ligands as linkers to form 1-, 2-, and 3-D structures. In addn. to their convenient modular synthesis and chem. tunability, this class of materials has been under intense investigation for gas storage and sepn. applications due to the discovery of many 3-D frameworks with high internal surface areas. Through careful selection of the ligand and metal, which control pore size / shape and MOF-adsorbate interactions, their uptake properties, such as gas selectivity, can be tuned. Despite these desirable properties there is still a bottleneck in the field assocd. with the deliberate design of materials for targeted applications. Theor. efforts focused on the development of structure and property prediction tools could be a means this end; however, exptl. validation of the tools is necessary for them to be accurately applied to as-yet-synthesized materials in the future. As such, we have carried out a comprehensive exptl. and theor. study of the CO2 adsorption properties in a well-known series of metal-org. frameworks M2(dobdc) M= Mg, Mn, Fe, Co, Ni, Cu, and Zn). In situ structural studies carried out in tandem with gas adsorption measurements have been used 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 expts. are used to unveil the site-specific binding properties of CO2 within the materials while systematically varying both the amt. of CO2 and the temp. D. functional theory calcns. including van der Waals dispersion quant. corroborate and rationalize observations regarding intramol. CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)-CO2 adducts.

Comprehensive study of carbon dioxide adsorption in the metal-organic frameworks M-2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Miguel Gonzalez, Wendy Lee Queen, Berend Smit

Analysis of the CO2 adsorption properties of a well-known series of metal-organic frameworks M-2(dobdc) (dobdc(4-) = 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 degrees, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal-O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M-2(dobdc)-CO2 adducts.

Royal Soc Chemistry2014

Ab Initio Derived Force Fields for Predicting CO2 Adsorption and Accessibility of Metal Sites in the Metal-Organic Frameworks M-MOF-74 (M = Mn, Co, Ni, Cu)

Peng Bai, Wendy Lee Queen

Metal-organic frameworks (MOFs) are versatile nanoporous materials that have gained significant interest as low heat capacity, high selectivity sorbents for CO2 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 CO2 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 CO2 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 CO2 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.

Amer Chemical Soc2015

Nanoporous Materials Can Tune the Critical Point of a Pure Substance

Efrem Daniel Braun, Sondre Kvalvag Schnell, Berend Smit

Molecular simulations and NMR relaxometry experiments demonstrate that pure benzene or xylene confined in isoreticular metal-organic frameworks (IRMOFs) exhibit true vapor-liquid phase equilibria where the effective critical point may be reduced by tuning the structure of the MOF. Our results are consistent with vapor and liquid phases extending over many MOF unit cells. These results are counterintuitive since the MOF pore diameters are approximately the same length scale as the adsorbate molecules. As applications of these materials in catalysis, separations, and gas storage rely on the ability to tune the properties of adsorbed molecules, we anticipate that the ability to systematically control the critical point, thereby preparing spatially inhomogeneous local adsorbate densities, could add a new design tool for MOF applications.

Wiley-Blackwell2015