Interaction of Molecular Hydrogen with Open Transition Metal Centers for Enhanced Binding in Metal-Organic Frameworks: A Computational Study

Quantum chemical modeling of hydrogen binding in metal-organic frameworks: validation, insight, predictions and challenges

A detailed chemical understanding of H2 interactions with binding sites in the nanoporous crystalline structure of metal-organic frameworks (MOFs) can lay a sound basis for the design of new sorbent materials. Computational quantum chemical calculations can aid in this quest. To set the stage, we review general thermodynamic considerations that control the usable storage capacity of a sorbent. We then discuss cluster modeling of H2 ligation at MOF binding sites using state-of-the-art density functional theory (DFT) calculations, and how the binding can be understood using energy decomposition analysis (EDA). Employing these tools, we illustrate the connections between the character of the MOF binding site and the associated adsorption thermodynamics using four experimentally characterized MOFs, highlighting the role of open metal sites (OMSs) in accessing binding strengths relevant to room temperature storage. The sorbents are MOF-5, with no open metal sites, Ni2(m-dobdc), containing Lewis acidic Ni(II) sites, Cu(I)-MFU-4l, containing π basic Cu(I) sites and V2Cl2.8(btdd), also containing π-basic V(II) sites. We next explore the potential for binding multiple H2 molecules at a single metal site, with thermodynamics useful for storage at ambient temperature; a materials design goal which has not yet been experimentally demonstrated. Computations on Ca2+ or Mg2+ bound to catecholate or Ca2+ bound to porphyrin show the potential for binding up to 4 H2; there is precedent for the inclusion of both catecholate and porphyrin motifs in MOFs. Turning to transition metals, we discuss the prediction that two H2 molecules can bind at V(II)-MFU-4l, a material that has been synthesized with solvent coordinated to the V(II) site. Additional calculations demonstrate binding three equivalents of hydrogen per OMS in Sc(I) or Ti(I)-exchanged MFU-4l. Overall, the results suggest promising prospects for experimentally realizing higher capacity hydrogen storage MOFs, if nontrivial synthetic and desolvation challenges can be overcome. Coupled with the unbounded chemical diversity of MOFs, there is ample scope for additional exploration and discovery.

Structural, Thermodynamic, and Spectroscopic Evolution in the Hydration of Copper(II) Ions, Cu2+(H2O)2-8

Gas-phase clusters of the hydrated Cu(II) cation with 2-8 water molecules were investigated using ab initio quantum chemistry. Isomer structures, energies, and vibrational spectra were computed across this size range, yielding a qualitative picture of this ion as an intact Cu2+ hydrate that also partially oxidizes the surrounding water network at equilibrium. At sufficient cluster sizes, these ion hydrates also become thermodynamically preferred over competitive Cu(II) hydroxide hydrates. Competitive coordination environments were found to exist at some cluster sizes, due to both hydrogen-bonding and d-orbital chemical effects, and the dominant coordination number was found in some cases to be temperature-dependent. Clear spectral signatures of the ion's coordination environment were computed to exist at each cluster size, which should make experimental verification of these computational predictions straightforward. Through comparison to recent studies of hydrated CuOH+, the effective charge on the metal center was shown to converge to approximately +1.5 in both cases, despite qualitatively different behavior of their radical spin densities. Therefore, nominally Cu(II) ions exhibit considerable electronic, chemical, and structural flexibility. The electronic origins of this flexibility─including key roles played by the water network itself─are investigated in this work and should provide a conceptual foundation for future studies of copper-based, water-oxidation catalysts.

A Long-Term Stable Sensor Based on Fe@PCN-224 for Rapid and Quantitative Detection of H2O2 in Fishery Products

Hydrogen peroxide (H2O2) has been reported to be used for the illegal treatment of fishery products in order to obtain "fake" freshness. Residues of H2O2 in food may be of toxicology concern. In this study, a nonenzymatic sensor was developed based on Fe@PCN-224 metal-organic frameworks wrapped by Nafion to detect H2O2 concentration. The hybrid structure of Fe@PCN-224 was fabricated by incorporated free FeIII ions into the center of PCN-224, which was ultra-stable due to the strong interactions between Zr6 and the carboxyl group. Scanning electron spectroscopy images exhibited that Nafion sheets crossed together on the surface of Fe@PCN-224 nanoparticles to form a hierarchical and coherent structure for efficient electron transfer. Electrochemical investigations showed that the Fe@PCN-224/Nafion/GCE possessed good linearity from 2 to 13,000 μM (including four orders of magnitude), low detection limits (0.7 μM), high stability in continuous monitoring (current remained nearly stable over 2300 s) and in long-term measurement (current decreased 3.4% for 30 days). The prepared nanohybrid modified electrode was effectively applied to H2O2 detection in three different fishery products. The results were comparable to those measured using photometrical methods. The developed electrochemical method has a great potential in detecting the illegal management of fishery products with H2O2.

The effect of co-adsorbed solvent molecules on H2 binding to metal alkoxides

The introduction of metal alkoxides has been proposed as an attractive option to enhance hydrogen binding energies in porous materials such as metal-organic frameworks (MOFs) for room-temperature hydrogen storage applications. The presence of residual solvent molecules from MOF synthesis can, however, affect the performance of these functional groups. We perform quantum chemical calculations to predict solvent binding energies onto the metal-alkoxides and the temperatures required to drive off the solvent molecules and successfully activate porous materials with these moieties. Calculations are performed for Li, Mg, Zn, Cu, and Ni alkoxides and chloroform (CHCl3), dimethylformamide (DMF), ethanol, methanol, and water solvent molecules. We identify CHCl3 as a promising solvent that can be removed from these alkoxides at mild temperatures, whereas DMF binds strongly to the metal alkoxides and removal would require temperatures above the present upper bound of thermal stability in MOFs. As a second objective, we calculated the binding energies of hydrogen to metal alkoxide-solvent complexes to explore the effect of any solvent molecules that cannot be removed.

Hydrogen storage mechanism and diffusion in metal-organic frameworks

Diffusion and storage of hydrogen molecules in metal-organic frameworks are crucial for the development of next-generation energy storage devices. By resorting to the first principles modeling, we compute the diffusion coefficient of molecular hydrogen in these systems in a range of temperatures where MOF-based devices are expected to operate. The explicit inclusion of the electronic structure shows that diffusivities are one order of magnitude smaller than those reported by classical simulations, evidencing the insufficiency of the empirical force fields used so far. We show that hydrogen is mainly rolled up around the metal oxide nodes both in MOF-5 and IRMOF-6, and partly around the carbon atoms in the case of IRMOF-6, where charged linkers are present. Metal ions embedded in the junction sites exert an electrostatic attraction toward hydrogen and the resulting distribution shows some ordering around these same sites at low temperature, whereas this tendency vanishes at room temperature. The induced polarization of hydrogen molecules generates an electrostatic interaction with charged atoms inside these nano-scaffolds and this is a key factor for the enhancement in hydrogen storage both in MOF-5 and IRMOF-6. The mechanism discussed hereby provides a novel understanding of metal-organic frameworks and acts as a guide to tune their efficiency for hydrogen storage. Moreover it paves the way to a computer-aided design of effective MOFs indicating that a fine control of the distribution of electrostatic charges inside the hydrogen hosting structure is crucial.

Signatures of Size-Dependent Structural Patterns in Hydrated Copper(I) Clusters, Cu+(H2O)n=1-10

The isomers of a hydrated Cu(I) ion with n = 1-10 water molecules were investigated by using ab initio quantum chemistry and an automated isomer-search algorithm. The electronic structure and vibrational spectra of the hundreds of resulting isomers were used to analyze the source of the observed bonding patterns. A structural evolution from dominantly two-coordinate structures (n = 1-4) toward a mixture of two- and three-coordinate structures was observed at n = 5-6, where the stability provided by expanded hydrogen-bonding was competitive with the dominantly electrostatic interaction between the water ligand and remaining binding sites of the metal ion. Further hydration (n = 7-10) led to a mixture of three- and four-coordinate structures. The metal ion was found, through spectroscopic signatures, to appreciably perturb the O-H bonds of even third-shell water molecules, which highlighted the ability of this nominally simple ion to partially activate the surrounding water network.

The remarkable ability of anions to bind dihydrogen

Anions show a noteworthy ability to bind with a large number of hydrogen molecules which can be utilized for the development of novel salt systems for hydrogen storage.

Molecular hydrogen binding affinities of metal cation decorated substituted benzene systems: insight from computational exploration

Factors affecting the binding of H₂molecule to the C₆H₆–Li⁺ complex.

Incorporating magnesium and calcium cations in porous organic frameworks for high-capacity hydrogen storage

We propose incorporating a bi-functional group consisting of magnesium or calcium cations and a 1,2,4,5-benzenetetroxide anion (C6H2O4(4-)) in porous materials to enhance the hydrogen storage capacity. The C6H2O4M2 bifunctional group is highly stable and polarized, and each group provides 18 (M = Mg) or 22 (M = Ca) binding sites for hydrogen molecules with an average binding energy of ca. 10 kJ mol(-1) per hydrogen molecule based on RIMP2/ TZVPP calculations. Two porous materials (PAF-Mg or PAF-Ca) constructed with the bi-functional groups show remarkable improvement in hydrogen uptakes at normal ambient conditions. At 233 K and 10 MPa, the predicted gravimetric uptakes are 6.8 and 6.4 wt% for PAF-Mg and PAF-Ca respectively. This work reveals that fabricating materials with large numbers of binding sites and relatively low binding energies is a promising approach to achieve high capacity for on-board storage of hydrogen.

Preferred hydrogen adsorption sites in various MOFs--a comparative computational study

Force-field based grand-canonical Monte Carlo simulations are employed to predict the hydrogen adsorption properties of seven structurally different MOFs. The performance of different parameter sets is assessed by comparison with experimental data, and the capabilities and limitations of the methodology are critically discussed, with a particular emphasis on systems with unsaturated metal sites. In addition to adsorption isotherms and isosteric heats of adsorption, the preferred adsorption sites are obtained from a detailed analysis of the calculated hydrogen density fields. Where possible, these positions are compared to the results of neutron diffraction experiments. This study highlights the capabilities of computational methods to identify the structural features which are most favourable for hydrogen adsorption, providing valuable implications for the synthesis of novel MOFs.

Hydrogen storage in metal-organic frameworks

New materials capable of storing hydrogen at high gravimetric and volumetric densities are required if hydrogen is to be widely employed as a clean alternative to hydrocarbon fuels in cars and other mobile applications. With exceptionally high surface areas and chemically-tunable structures, microporous metal-organic frameworks have recently emerged as some of the most promising candidate materials. In this critical review we provide an overview of the current status of hydrogen storage within such compounds. Particular emphasis is given to the relationships between structural features and the enthalpy of hydrogen adsorption, spectroscopic methods for probing framework-H(2) interactions, and strategies for improving storage capacity (188 references).