Interaction of molecular hydrogen with open transition metal centers for enhanced binding in metal-organic frameworks: a computational study

Molecular hydrogen is known to form stable, "nonclassical" sigma complexes with transition metal centers that are stabilized by donor-acceptor interactions and electrostatics. In this computational study, we establish that strong H2 sorption sites can be obtained in metal-organic frameworks by incorporating open transition metal sites on the organic linkers. Using density functional theory and energy decomposition analysis, we investigate the nature and characteristics of the H2 interaction with models of exposed open metal binding sites {half-sandwich piano-stool shaped complexes of the form (Arene)ML(3- n)(H2)n [M=Cr, Mo, V(-), Mn(+); Arene = C6H5X (X=H, F, Cl, OCH3, NH2, CH3, CF3) or C6H3Y2X (Y=COOH, X=CF3, Cl; L=CO; n=1-3]}. The metal-H2 bond dissociation energy of the studied complexes is calculated to be between 48 and 84 kJ/mol, based on the introduction of arene substituents, changes to the metal core, and of charge-balancing ligands. Thus, design of the binding site controls the H2 binding affinity and could be potentially used to control the magnitude of the H2 interaction energy to achieve reversible sorption characteristics at ambient conditions. Energy decomposition analysis illuminates both the possibilities and present challenges associated with rational materials design.     

Understanding hydrogen adsorption in metal-organic frameworks with open metal sites: a computational study

Recent experimental investigations show that the open metal sites may have a favorable impact on the hydrogen adsorption capacity of metal-organic frameworks (MOFs); however, no definite evidence has been obtained to date and little is known on the interactions between hydrogen and the pore walls of this kind of MOFs. In this work, a combined grand canonical Monte Carlo simulation and density functional theory calculation is performed on the adsorption of hydrogen in MOF-505, a recently synthesized MOF with open metal sites, to provide insight into molecular-level details of the underlying mechanisms. This work shows that metal-oxygen clusters are preferential adsorption sites for hydrogen, and the strongest adsorption of hydrogen is found in the directions of coordinatively unsaturated open metal sites, providing evidence that the open metal sites have a favorable impact on the hydrogen sorption capacity of MOFs. The storage capacity of hydrogen of MOF-505 at room temperature and moderate pressures is predicted to be low, in agreement with the outcome for hydrogen physisorption in other porous materials.     

Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organic frameworks

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.     

Molecular simulation of adsorption and diffusion of hydrogen in metal-organic frameworks

Metal-organic frameworks (MOFs) are thought to be a set of promising hydrogen storage materials; however, little is known about the interactions between hydrogen molecules and pore walls as well as the diffusivities of hydrogen in MOFs. In this work, we performed a systematic molecular simulation study on the adsorption and diffusion of hydrogen in MOFs to provide insight into molecular-level details of the underlying mechanisms. This work shows that metal-oxygen clusters are preferential adsorption sites for hydrogen in MOFs, and the effect of the organic linkers becomes evident with increasing pressure. The hydrogen storage capacity of MOFs is similar to carbon nanotubes, which is higher than zeolites. Diffusion of hydrogen in MOFs is an activated process that is similar to diffusion in zeolites. The information derived in this work is useful to guide the future rational design and synthesis of tailored MOF materials with improved hydrogen adsorption capability.     

CO2 adsorption in mono-, di- and trivalent cation-exchanged metal-organic frameworks: a molecular simulation study

A molecular simulation study is reported for CO(2) adsorption in rho zeolite-like metal-organic framework (rho-ZMOF) exchanged with a series of cations (Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), and Al(3+)). The isosteric heat and Henry's constant at infinite dilution increase monotonically with increasing charge-to-diameter ratio of cation (Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) < Al(3+)). At low pressures, cations act as preferential adsorption sites for CO(2) and the capacity follows the charge-to-diameter ratio. However, the free volume of framework becomes predominant with increasing pressure and Mg-rho-ZMOF appears to possess the highest saturation capacity. The equilibrium locations of cations are observed to shift slightly upon CO(2) adsorption. Furthermore, the adsorption selectivity of CO(2)/H(2) mixture increases as Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) ≈ Al(3+). At ambient conditions, the selectivity is in the range of 800-3000 and significantly higher than in other nanoporous materials. In the presence of 0.1% H(2)O, the selectivity decreases drastically because of the competitive adsorption between H(2)O and CO(2), and shows a similar value in all of the cation-exchanged rho-ZMOFs. This simulation study provides microscopic insight into the important role of cations in governing gas adsorption and separation, and suggests that the performance of ionic rho-ZMOF can be tailored by cations.     

Confinement effects in the hydrogen adsorption on paddle wheel containing metal-organic frameworks

The confinement effects upon hydrogen adsorption in Cu(II)-paddle wheel containing metal-organic frameworks (MOFs) were evaluated and rationalized in terms of the structural properties (cavity types and pore diameters) of PCN-12, HKUST-1, MOF-505, NOTT-103 and NOTT-112. First-principles calculations were employed to identify the strongest adsorption positions at the paddle wheel inorganic building unit (IBU). The adsorption centres due to confinement were located through analysis of 3D occupancy maps obtained from the hydrogen trajectories computed via molecular dynamics simulations. It was found that the confinement enhances the adsorption on the weakest adsorption centres around the IBU in regions close to the narrowest windows and promotes the formation of new adsorption regions into the small cavities. Our results indicate that at low pressure, the high H(2) uptake in these materials is partly due to the presence of small cavities (5.3-8.5 ?) or narrow windows where the long-range contribution to the adsorption becomes important. Conversely, confinement effects in cavities with diameters >12 ? were not observed.     

Early stages in the degradation of metal-organic frameworks in liquid water from first-principles molecular dynamics

IRMOF-1 structures are known to suffer lattice break-up when exposed to water-rich environments, a limiting factor in their everyday use. To shed light on the underlying mechanism of disruption, the role of the metal in the secondary building unit (SBU) has been systematically investigated, and the global behaviour of IRMOF-1-type structures with the three metals Zn, Mg, and Be studied by Born-Oppenheimer Molecular Dynamics in liquid water. Results show that fully hydrated Be based compounds are stable up to 500 K while the equivalent structures with Mg or Zn break down already at 300 K. The reasons behind this instability are in the tendency of the metal atom to form penta- and hexa-coordination spheres and in the strength of the M-O bond. These are the key factors that generate unique breaking patterns for Mg and Zn IRMOF-1 analogues, as well as the reason for the high hydrothermal stability of the Be-IRMOF-1.     

Modeling adsorption in metal-organic frameworks with open metal sites: propane/propylene separations

We present a new approach for modeling adsorption in metal-organic frameworks (MOFs) with unsaturated metal centers and apply it to the challenging propane/propylene separation in copper(II) benzene-1,3,5-tricarboxylate (CuBTC). We obtain information about the specific interactions between olefins and the open metal sites of the MOF using quantum mechanical density functional theory. A proper consideration of all the relevant contributions to the adsorption energy enables us to extract the component that is due to specific attractive interactions between the π-orbitals of the alkene and the coordinatively unsaturated metal. This component is fitted using a combination of a Morse potential and a power law function and is then included into classical grand canonical Monte Carlo simulations of adsorption. Using this modified potential model, together with a standard Lennard-Jones model, we are able to predict the adsorption of not only propane (where no specific interactions are present), but also of propylene (where specific interactions are dominant). Binary adsorption isotherms for this mixture are in reasonable agreement with ideal adsorbed solution theory predictions. We compare our approach with previous attempts to predict adsorption in MOFs with open metal sites and suggest possible future routes for improving our model.     

Alkali metal cation effects on hydrogen uptake and binding in metal-organic frameworks

A 2-fold interwoven metal-organic framework has been chemically reduced and doped with Li(+), Na(+), and K(+). At low pressures and temperatures, the reduced and doped materials exhibit enhanced H2 uptakeup to 65% higher than for the neutral framework. Notably, at similar doping levels, H2 binding is strongest with Li(+) and decreases as Li(+) > Na(+) > K(+). However, the uptake increases in the opposite order. We attribute the behavior to structural changes accompanying framework reduction.     

Hydrogen storage in metal-organic frameworks by bridged hydrogen spillover

The possible utilization of hydrogen as the energy source for fuel-cell vehicles is limited by the lack of a viable hydrogen storage system. Metal-organic frameworks (MOFs) belong to a new class of microporous materials that have recently been shown to be potential candidates for hydrogen storage; however, no significant hydrogen storage capacity has been achieved in MOFs at ambient temperature. Here we report substantially increased hydrogen storage capacities of modified MOFs by using a simple technique that causes and facilitates hydrogen spillover. Thus, the storage of 4 wt % is achieved at room temperature and 100 atm for the modified IRMOF-8. The adsorption is reversible, and the rates are fast. That has made MOFs truly promising for hydrogen storage application.     

Grand canonical Monte Carlo simulation study on the catenation effect on hydrogen adsorption onto the interpenetrating metal-organic frameworks

Among recently synthesized isoreticular metal-organic frameworks (IRMOFs), interpenetrating IRMOFs show high hydrogen adsorption capacities at low temperature and under ambient pressure. However, little is known about the molecular basis of their hydrogen binding properties. In this work, we performed grand canonical Monte Carlo (GCMC) simulations to investigate the effect of catenation on the interactions between hydrogen molecules and IRMOFs. We identified the adsorption sites and analyzed the adsorption energy distributions. The simulation results show that the small pores generated by catenation can play a role to confine the hydrogen molecules more densely, so that the capacity of the interpenetrating IRMOFs could be higher than that of the non-interpenetrating IRMOFs.     

Molecular simulation of carbon dioxide/methane/hydrogen mixture adsorption in metal-organic frameworks

This work performs a systematic computational study toward a molecular understanding of the separation characteristics of metal-organic frameworks (MOFs), for which the purification of synthetic gas by two representative MOFs, MOF-5 and Cu-BTC, is adopted as an example. The simulations show that both geometry and pore size affect largely the separation efficiency, complex selectivity behaviors with different steps can occur in MOFs, and the electrostatic interactions that exist can enhance greatly the separation efficiency of gas mixtures composed of components with different chemistries. Furthermore, the macroscopic separation behaviors of the MOF materials are elucidated at a molecular level to give insight into the underlying mechanisms. The findings as well as the molecular-level elucidations provide useful microscopic information toward a complete understanding of the separation characteristics of MOFs that may lead to general design strategies for synthesizing new MOFs with tailored properties, as well as guiding their practical applications.     

Controlling the shifting degree of interpenetrated metal-organic frameworks by modulator and temperature and their hydrogen adsorption properties

For the first time, the shifting degree of pcu-type interpenetrated framework was well controlled by employing a modulator and changing the temperature, in which their evacuated samples are almost non-crystalline products with different meso- and microstructures, resulting in different hydrogen adsorption properties.     

Hierarchical modeling of ammonia adsorption in functionalized metal-organic frameworks

The adsorption of ammonia in four metal-organic frameworks modified with different functional groups (-OH, -C=O, -Cl, -COOH) was investigated using a hierarchical molecular modeling approach. To describe the hydrogen bonding and other strong interactions between NH(3) and the surface functional groups, a set of Morse potential parameters were obtained by fitting to energies from quantum chemical calculations at the MP2 level of theory. We describe a systematic force field parameterization process, in which the Morse parameters were fitted using simulated annealing to match a large number of single-point MP2 energies at various distances and angles. The fitted potentials were then used in grand canonical Monte Carlo simulations to predict ammonia adsorption isotherms and heats of adsorption in functionalized MIL-47, IRMOF-1, IRMOF-10, and IRMOF-16. The results show that ammonia adsorption can be significantly enhanced by using materials with appropriate pore size, strongly interacting functional groups, and high density of functional groups.     

Stereochemistry and spin state in four-coordinate transition metal compounds

A systematic DFT computational study of the stereochemistry associated with each spin state for first transition series four-coordinate d(n) (n = 0-10) homoleptic metal complexes is presented. The stereochemistry of [MMe4](x-) complexes in the 21 spin configurations analyzed can be predicted from the d orbital occupation in the ideal tetrahedral geometry, grouped in three families with tetrahedral, square planar, or intermediate structures that can be described in some cases as sawhorses. The effect of the following factors on the spin state and stereochemical preferences has also been studied: (a) substitution of the sigma-donor methyl ligands by pi-donor chlorides, (b) a high (+4) oxidation state of the metal, and (c) substitution of the metal atom by a second transition series one. Through those factors, low-spin tetrahedral structures can be achieved, as summarized by a magic cube.     

Rational construction of 3D pillared metal-organic frameworks: synthesis, structures, and hydrogen adsorption properties

In our efforts toward rational design and systematic synthesis of 'pillar-layer' structure MOFs, three porous MOFs have been constructed based on [Zn(4)(bpta)(2)(H(2)O)(2)] (H(4)bpta = 1,1'-biphenyl-2,2',6,6'-tetracarboxylic acid) layers and three different bipyridine pillar ligands. The resulted MOFs show similar structures but different pore volume and window size depending on the length of pillar ligands which resulted in distinct gas adsorption properties. In the three MOFs, [Zn(4)(bpta)(2)(4,4'-bipy)(2)(H(2)O)(2)]·(DMF)(3)·H(2)O (1) (DMF = N,N'-dimethylformamide and 4,4'-bipy = 4,4'-bipyridine) reveals selective adsorption of H(2) over N(2) and O(2) as the result of narrow pore size. [Zn(4)(bpta)(2)(azpy)(2)(H(2)O)(2)]·(DMF)(4)·(H(2)O)(3) (2) and [Zn(4)(bpta)(2)(dipytz)(2)(H(2)O)(2)]·(DMF)(4)·H(2)O (3) (azpy =4,4'-azopyridine, dipytz = di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) reveal pore structure change upon different activation conditions. In addition, the samples activated under different conditions show distinct adsorption behaviors of N(2) and O(2) gases. Furthermore, hydrogen adsorption properties of activated 1-3 were studied. The results indicated that the activation process could affect the hydrogen enthalpy of adsorption.     

Enhanced H2 adsorption in isostructural metal-organic frameworks with open metal sites: strong dependence of the binding strength on metal ions

Metal-organic frameworks (MOFs) with open metal sites exhibit a much stronger H2 binding strength than classical MOFs, due to the direct interaction between H2 and the coordinately unsaturated metal ions. Here we report a systematic study of the H2 adsorption on a series of isostructural MOFs, M2(dhtp) (M = Mg, Mn, Co, Ni, Zn). The experimental, initial isosteric heats of adsorption for H2 (Qst) of these MOFs range from 8.5 to 12.9 kJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni. The H2 binding energies derived from first-principles calculation follow the same trend as the experimental observation on Qst, confirming the electrostatic Coulomb attraction between the H2 and the open metals being the major interaction. We also found a strong correlation between the metal ion radius, the M-H2 distance, and the H2 binding strength, which provides a viable, empirical method to predict the relative H2 binding strength of different open metals.