Metal-Specific Interactions of H2 Adsorbed within Isostructural Metal–Organic Frameworks

Direct observation of highly effective hydrogen isotope separation at active metal sites by in situ DRIFT spectroscopy

In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was developed for the first time to observe the hydrogen isotope separation behavior at active Cu^I sites within Cu^I-MFU-4l, and clear evidence of the preferential adsorption of D₂ over H₂ was directly captured. More importantly, our results show direct spectral proof to clarify the chemical affinity quantum sieving mechanism of hydrogen isotope separation within porous adsorbents.

Symmetry Breakings in the interactions of Molecular Hydrogen with Solids

The following conference report considers hydrogen gases with odd and even rotational quantum number as two separate gases, the ortho and para varieties which do not interconvert in absence of a catalyst. The physical catalysis of hydrogen is interpreted in terms of symmetry breakings introduced by the solid to pass round the peculiar selection rules of the molecular hydrogen assigned by the Pauli Principle. The catalytic effect presents the striking effect of reducing drastically the interconversion time, longer than the age of the universe for isolated molecules, to a few seconds or minutes when an hydrogen sample (gaseous or liquid) is brought into contact with an efficient catalyst. In the present report, the variety of new optical and electronic devices, measurements and interpretations that have been reported since the turning of the new century are reviewed. New experiments on non-magnetic catalysts measuring hydrogen conversion on the time scales of one-ten minutes turned upside down the previous theory, established in 1933, of the absolute necessity of a magnetic catalyst to break the Pauli Principle. The o-p catalyzed reaction is discussed for hydrogen molecules adsorbed on electric surfaces, or in confining porous structures or inside nanocages. New concepts and new electromagnetic conversion channels that interpret these experimental renewals are described in terms of how the hydrogen nuclei feel the solid-molecule electron cloud complex. The described channels differentiate one another owing to the catalyst and owing to the electronic path followed in the configuration space by the o-p reaction.

Hydrogen isotope separation on nickel-containing metal-organic framework@gama-alumina-based multicomponent composite packed column: Identification of individual role of each component

A new optimized multicomponent composite, Na₂Cr₂O₇/Na₂CrO₄/NaCl/MOF-74(Ni)@γ-Al₂O₃ (S₁/S_2/S₃/MOF-74(Ni)@γ-Al₂O₃. Na₂Cr₂O₇ = S₁, Na₂CrO₄ = S₂, NaCl = S₃), was prepared and used as a gas chromatography stationary phase for the separation of H₂ and D₂ isotopes. Under the optimal chromatographic separation conditions, the resolution of the packed column for the separation of H₂/D₂ was 2.87, and the separation time was 7.15 min at 77 K. The control experiments showed that in the multicomponent composite, MOF-74(Ni), which has a chemical affinity quantum sieving effect, played a major role in the separation of H₂/D₂. As the support of MOF-74, γ-Al₂O₃ enhanced the mechanical strength of MOF-74 and reduced the gas resistance. The presence of Na₂CrO₄ in the column increased the H₂/D₂ separation resolution, while the presence of NaCl reduced the separation time, produced more symmetrical and narrow chromatographic peaks of Gaussian distribution. Furthermore, by optimizing the ratio of NaCl and Na₂CrO₄, (S₂/S₃/MOF-74(Ni)@γ-Al₂O₃) composite with a NaCl/Na₂CrO₄ mass ratio of 0.7:1 was synthesized and used to realize the high-resolution separation of H₂/D₂ (R = 2.56) with a short separation time (t = 5.91 min). Both composites also showed excellent repeatability/reproducibility for separation.

Observation of an Intermediate to H2 Binding in a Metal-Organic Framework

Coordinatively unsaturated metal sites within certain zeolites and metal-organic frameworks can strongly adsorb a wide array of substrates. While many classical examples involve electron-poor metal cations that interact with adsorbates largely through physical interactions, unsaturated electron-rich metal centers housed within porous frameworks can often chemisorb guests amenable to redox activity or covalent bond formation. Despite the promise that materials bearing such sites hold in addressing myriad challenges in gas separations and storage, very few studies have directly interrogated mechanisms of chemisorption at open metal sites within porous frameworks. Here, we show that nondissociative chemisorption of H₂ at the trigonal pyramidal Cu⁺ sites in the metal-organic framework Cu^I-MFU-4l occurs via the intermediacy of a metastable physisorbed precursor species. In situ powder neutron diffraction experiments enable crystallographic characterization of this intermediate, the first time that this has been accomplished for any material. Evidence for a precursor intermediate is also afforded from temperature-programmed desorption and density functional theory calculations. The activation barrier separating the precursor species from the chemisorbed state is shown to correlate with a change in the Cu⁺ coordination environment that enhances π-backbonding with H₂. Ultimately, these findings demonstrate that adsorption at framework metal sites does not always follow a concerted pathway and underscore the importance of probing kinetics in the design of next-generation adsorbents.

Ambient-Temperature Hydrogen Storage via Vanadium(II)-Dihydrogen Complexation in a Metal-Organic Framework

The widespread implementation of H2 as a fuel is currently hindered by the high pressures or cryogenic temperatures required to achieve reasonable storage densities. In contrast, the realization of materials that strongly and reversibly adsorb hydrogen at ambient temperatures and moderate pressures could transform the transportation sector and expand adoption of fuel cells in other applications. To date, however, no adsorbent has been identified that exhibits a binding enthalpy within the optimal range of -15 to -25 kJ/mol for ambient-temperature hydrogen storage. Here, we report the hydrogen adsorption properties of the metal-organic framework (MOF) V2Cl2.8(btdd) (H2btdd, bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which features exposed vanadium(II) sites capable of backbonding with weak π acids. Significantly, gas adsorption data reveal that this material binds H2 with an enthalpy of -21 kJ/mol. This binding energy enables usable hydrogen capacities that exceed that of compressed storage under the same operating conditions. The Kubas-type vanadium(II)-dihydrogen complexation is characterized by a combination of techniques. From powder neutron diffraction data, a V-D2(centroid) distance of 1.966(8) Å is obtained, the shortest yet reported for a MOF. Using in situ infrared spectroscopy, the H-H stretch was identified, and it displays a red shift of 242 cm-1. Electronic structure calculations show that a main contribution to bonding stems from the interaction between the vanadium dπ and H2 σ* orbital. Ultimately, the pursuit of MOFs containing high densities of weakly π-basic metal sites may enable storage capacities under ambient conditions that far surpass those accessible with compressed gas storage.

Hydrogen Conversion in Nanocages

Hydrogen molecules exist in the form of two distinct isomers that can be interconverted by physical catalysis. These ortho and para forms have different thermodynamical properties. Over the last century, the catalysts developed to convert hydrogen from one form to another, in laboratories and industries, were magnetic and the interpretations relied on magnetic dipolar interactions. The variety concentration of a sample and the conversion rates induced by a catalytic action were mostly measured by thermal methods related to the diffusion of the o-p reaction heat. At the turning of the new century, the nature of the studied catalysts and the type of measures and motivations completely changed. Catalysts investigated now are non-magnetic and new spectroscopic measurements have been developed. After a fast survey of the past studies, the review details the spectroscopic methods, emphasizing their originalities, performances and refinements: how Infra-Red measurements characterize the catalytic sites and follow the conversion in real-time, Ultra-Violet irradiations explore the electronic nature of the reaction and hyper-frequencies driving the nuclear spins. The new catalysts, metallic or insulating, are detailed to display the operating electronic structure. New electromagnetic mechanisms, involving energy and momenta transfers, are discovered providing a classification frame for the newly observed reactions.

Effect of Larger Pore Size on the Sorption Properties of Isoreticular Metal-Organic Frameworks with High Number of Open Metal Sites

Four isostructural CPO-54-M metal-organic frameworks based on the larger organic linker 1,5-dihydroxynaphthalene-2,6-dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO-27 (MOF-74) materials. Desolvated CPO-54-Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N2 , H2 , CO2 and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO-27. The plateau of high heat of adsorption decreases earlier in CPO-54-Mn as a function of loading per mole than in CPO-27-Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.

Ni-PC@SBA-15 derived from nano-sized Ni-MOF-74 confined in SBA-15 as a highly active catalyst for gas phase catalytic hydrodechlorination of 1,2-dichloroethane

A novel catalyst consisting of metallic Ni and porous carbon (PC) confined in SBA-15 was fabricated by the confined pyrolysis of nano-sized Ni-MOF-74 that was in situ grown in the mesopores of SBA-15. Due to the intimate contact between Ni and PC in a fine Ni-PC composite resulting from the confinement effect of mesoporous SBA-15, the catalyst displayed prominent catalytic activity and selectivity in the gas phase catalytic hydrodechlorination of 1,2-dichloroethane.

0D/2D spatial structure of CdxZn1−xS/Ni-MOF-74 for efficient photocatalytic hydrogen evolution

A novel zero-dimensional/two-dimensional Cd_xZn_1−xS/Ni-MOF-74 (CZS/NMF) heterojunction was rationally constructed via a simple hydrothermal and physical mixing method.

IRMOF-74(n)-Mg: a novel catalyst series for hydrogen activation and hydrogenolysis of C-O bonds

Metal-Organic Frameworks (MOFs) that catalyze hydrogenolysis reactions are rare and there is little understanding of how the MOF, hydrogen, and substrate molecules interact. In this regard, the isoreticular IRMOF-74 series, two of which are known catalysts for hydrogenolysis of aromatic C-O bonds, provides an unusual opportunity for systematic probing of these reactions. The diameter of the 1D open channels can be varied within a common topology owing to the common secondary building unit (SBU) and controllable length of the hydroxy-carboxylate struts. We show that the first four members of the IRMOF-74(Mg) series are inherently catalytic for aromatic C-O bond hydrogenolysis and that the conversion varies non-monotonically with pore size. These catalysts are recyclable and reusable, retaining their crystallinity and framework structure after the hydrogenolysis reaction. The hydrogenolysis conversion of phenylethylphenyl ether (PPE), benzylphenyl ether (BPE), and diphenyl ether (DPE) varies as PPE > BPE > DPE, consistent with the strength of the C-O bond. Counterintuitively, however, the conversion also follows the trend IRMOF-74(III) > IRMOF-74(IV) > IRMOF-74(II) > IRMOF-74(I), with little variation in the corresponding selectivity. DFT calculations suggest the unexpected behavior is due to much stronger ether and phenol binding to the Mg(ii) open metal sites (OMS) of IRMOF-74(III), resulting from a structural distortion that moves the Mg2+ ions toward the interior of the pore. Solid-state 25Mg NMR data indicate that both H2 and ether molecules interact with the Mg(ii) OMS and hydrogen-deuterium exchange reactions show that these MOFs activate dihydrogen bonds. The results suggest that both confinement and the presence of reactive metals are essential for achieving the high catalytic activity, but that subtle variations in pore structure can significantly affect the catalysis. Moreover, they challenge the notion that simply increasing MOF pore size within a constant topology will lead to higher conversions.

Hydrogen Isotope Separation in Confined Nanospaces: Carbons, Zeolites, Metal-Organic Frameworks, and Covalent Organic Frameworks

One of the greatest challenges of modern separation technology is separating isotope mixtures in high purity. The separation of hydrogen isotopes can create immense economic value by producing valuable deuterium (D) and tritium (T), which are irreplaceable for various industrial and scientific applications. However, current separation methods suffer from low separation efficiency owing to the similar chemical properties of isotopes; thus, high-purity isotopes are not easily achieved. Recently, nanoporous materials have been proposed as promising candidates and are supported by a newly proposed separation mechanism, i.e., quantum effects. Herein, the fundamentals of the quantum sieving effect of hydrogen isotopes in nanoporous materials are discussed, which are mainly kinetic quantum sieving and chemical-affinity quantum sieving, including the recent advances in the analytical techniques. As examples of nanoporous materials, carbons, zeolites, metal-organic frameworks, and covalent organic frameworks are addressed from computational and experimental standpoints. Understanding the quantum sieving effect in nanospaces and the tailoring of porous materials based on it will open up new opportunities to develop a highly efficient and advanced isotope separation systems.

Structural and dynamic studies of substrate binding in porous metal-organic frameworks

Porous metal-organic frameworks (MOFs) are the subject of considerable research interest because of their high porosity and capability of specific binding to small molecules, thus underpinning a wide range of materials functions such as gas adsorption, separation, drug delivery, catalysis, and sensing. MOFs, constructed by the designed assembly of metal ions and functional organic linkers, are an emerging class of porous materials with extended porous structures containing periodic binding sites. MOFs thus provide a new platform for the study of the chemistry and reactivity of small molecules in confined pores using advanced diffraction and spectroscopic techniques. In this review, we focus on recent progress in experimental investigations on the crystallographic, dynamic and kinetic aspects of substrate binding within porous MOFs. In particular, we focus on studies on host-guest interactions involving open metal sites or pendant functional groups in the pore as the primary binding sites for guest molecules.

Record High Hydrogen Storage Capacity in the Metal-Organic Framework Ni2(m-dobdc) at Near-Ambient Temperatures

Hydrogen holds promise as a clean alternative automobile fuel, but its on-board storage presents significant challenges due to the low temperatures and/or high pressures required to achieve a sufficient energy density. The opportunity to significantly reduce the required pressure for high density H₂ storage persists for metal-organic frameworks due to their modular structures and large internal surface areas. The measurement of H₂ adsorption in such materials under conditions most relevant to on-board storage is crucial to understanding how these materials would perform in actual applications, although such data have to date been lacking. In the present work, the metal-organic frameworks M₂(m-dobdc) (M = Co, Ni; m-dobdc^4- = 4,6-dioxido-1,3-benzenedicarboxylate) and the isomeric frameworks M₂(dobdc) (M = Co, Ni; dobdc^4- = 1,4-dioxido-1,3-benzenedicarboxylate), which are known to have open metal cation sites that strongly interact with H₂, were evaluated for their usable volumetric H₂ storage capacities over a range of near-ambient temperatures relevant to on-board storage. Based upon adsorption isotherm data, Ni₂(m-dobdc) was found to be the top-performing physisorptive storage material with a usable volumetric capacity between 100 and 5 bar of 11.0 g/L at 25 °C and 23.0 g/L with a temperature swing between -75 and 25 °C. Additional neutron diffraction and infrared spectroscopy experiments performed with in situ dosing of D₂ or H₂ were used to probe the hydrogen storage properties of these materials under the relevant conditions. The results provide benchmark characteristics for comparison with future attempts to achieve improved adsorbents for mobile hydrogen storage applications.

Hydrogen Uptake on Coordinatively Unsaturated Metal Sites in VSB-5: Strong Binding Affinity Leading to High-Temperature D2/H2 Selectivity

We examine the adsorption of hydrogen and deuterium into the nanoporous nickel phosphate, VSB-5. On the basis of gas sorption analysis, VSB-5 exhibits one of the highest measured H₂ heats of adsorption (HOA) for hydrogen (16 kJ/mol) yet reported. This high HOA is consistent with an unusually large red shift in the Q(1) and Q(0) hydrogen vibrational modes as measured with in situ infrared spectroscopy. The HOA for D₂ is measured to be 2 kJ/mol higher than that for H₂. "Ideal adsorbed solution theory" analysis of H₂ and D₂ isotherms provides selectivities above 4 for deuterium at 140 K, suggesting that VSB-5 is a promising adsorbent for pressure-swing adsorption-type separations of hydrogen isotopes.

Theoretical insight into the BH3·HCN adsorption on the Co(100) and Co(110) surfaces as hydrogen storage

Fifteen configurations and adsorption energies of the adsorption sites of BH₃∙∙∙HCN on Co(100) and Co(110) surfaces were investigated using the density functional theory. The results show that after BH₃∙∙∙HCN is adsorbed, although there is no general behavior for the H∙∙∙H distances, the adsorption energies of BH₃∙∙∙HCN are always far stronger than those of H₂ on Co surfaces, suggesting that the dihydrogen-bonded complex, one kind of prospective material for reversible hydrogen storage, can be tightly adsorbed on the surfaces of metals. Thus, the attempts to store the significant amounts of H₂ can be successful by the way that the dihydrogen-bonded complexes are adsorbed on the surfaces of metals. The stability and binding mechanism was analyzed by the Mulliken charge population and reduced density gradients (RDGs) methods. Graphical Abstract BH₃···HCN adsorption on Co surface.

Capture of heavy hydrogen isotopes in a metal-organic framework with active Cu(I) sites

The production of pure deuterium and the removal of tritium from nuclear waste are the key challenges in separation of light isotopes. Presently, the technological methods are extremely energy- and cost-intensive. Here we report the capture of heavy hydrogen isotopes from hydrogen gas by selective adsorption at Cu(I) sites in a metal-organic framework. At the strongly binding Cu(I) sites (32 kJ mol^-1) nuclear quantum effects result in higher adsorption enthalpies of heavier isotopes. The capture mechanism takes place most efficiently at temperatures above 80 K, when an isotope exchange allows the preferential adsorption of heavy isotopologues from the gas phase. Large difference in adsorption enthalpy of 2.5 kJ mol^-1 between D₂ and H₂ results in D₂-over-H₂ selectivity of 11 at 100 K, to the best of our knowledge the largest value known to date. Combination of thermal desorption spectroscopy, Raman measurements, inelastic neutron scattering and first principles calculations for H₂/D₂ mixtures allows the prediction of selectivities for tritium-containing isotopologues.

Theoretical Investigations of CO2 and H2 Sorption in Robust Molecular Porous Materials

Molecular simulations of CO₂ and H₂ sorption were performed in MPM-1-Cl and MPM-1-TIFSIX, two robust molecular porous materials (MPMs) with the empirical formula [Cu₂(adenine)₄Cl₂]Cl₂ and [Cu₂(adenine)₄(TiF₆)₂], respectively. Recent experimental studies have shown that MPM-1-TIFSIX displayed higher CO₂ uptake and isosteric heat of adsorption (Q_st) than MPM-1-Cl [Nugent, P. S.; et al. J. Am. Chem. Soc. 2013, 135, 10950-10953]. This was verified through the simulations executed herein, as the presented simulated CO₂ sorption isotherms and Q_st values are in very good agreement with the corresponding experimental data for both MPMs. We also report experimental H₂ sorption data in both MPMs. Experimental studies revealed that MPM-1-TIFSIX exhibits high H₂ uptake at low loadings and an initial H₂ Q_st value of 9.1 kJ mol^-1. This H₂ Q_st value is greater than that for a number of existing metal-organic frameworks (MOFs) and represents the highest yet reported for a MPM. The remarkable H₂ sorption properties for MPM-1-TIFSIX have been confirmed through our simulations. The modeling studies revealed that only one principal sorption site is present for CO₂ and H₂ in MPM-1-Cl, which is sorption onto the Cl^- counterions within the large channels. In contrast, three different sorption sites were discovered for both CO₂ and H₂ in MPM-1-TIFSIX: (1) between two TIFSIX groups within a small passage connecting the large channels, (2) onto the TIFSIX ions lining the large channels, and (3) within the small channels. This study illustrates the detailed insights that molecular simulations can provide on the CO₂ and H₂ sorption mechanism in MPMs.

Nuclear conversion theory: molecular hydrogen in non-magnetic insulators

The hydrogen conversion patterns on non-magnetic solids sensitively depend upon the degree of singlet/triplet mixing in the intermediates of the catalytic reaction. Three main 'symmetry-breaking' interactions are brought together. In a typical channel, the electron spin-orbit (SO) couplings introduce some magnetic excitations in the non-magnetic solid ground state. The electron spin is exchanged with a molecular one by the electric molecule-solid electron repulsion, mixing the bonding and antibonding states and affecting the molecule rotation. Finally, the magnetic hyperfine contact transfers the electron spin angular momentum to the nuclei. Two families of channels are considered and a simple criterion based on the SO coupling strength is proposed to select the most efficient one. The denoted 'electronic' conversion path involves an emission of excitons that propagate and disintegrate in the bulk. In the other denoted 'nuclear', the excited electron states are transients of a loop, and the electron system returns to its fundamental ground state. The described model enlarges previous studies by extending the electron basis to charge-transfer states and 'continui' of band states, and focuses on the broadening of the antibonding molecular excited state by the solid conduction band that provides efficient tunnelling paths for the hydrogen conversion. After working out the general conversion algebra, the conversion rates of hydrogen on insulating and semiconductor solids are related to a few molecule-solid parameters (gap width, ionization and affinity potentials) and compared with experimental measures.

Parameterizing and grading hydrolytic stability in metal–organic frameworks

Water stability of MOFs is reviewed including exposure techniques, characterization methods, and ultimately more consistent definitions of water stability.

Hybrid materials of Ni NP@MOF prepared by a simple synthetic method

We demonstrate a novel, simple synthetic method for metal (Ni) NPs in a MOF using the partial thermal decomposition of nickel(II) 2,5-dihydroxyterephthalate (Ni-MOF-74). The Ni NPs inside the Ni-MOF-74 are several nanometers in size, and the size can be precisely controlled by the heating conditions.

Study of van der Waals bonding and interactions in metal organic framework materials

Metal organic framework (MOF) materials have attracted a lot of attention due to their numerous applications in fields such as hydrogen storage, carbon capture and gas sequestration. In all these applications, van der Waals forces dominate the interaction between the small guest molecules and the walls of the MOFs. In this review article, we describe how a combined theoretical and experimental approach can successfully be used to study those weak interactions and elucidate the adsorption mechanisms important for various applications. On the theory side, we show that, while standard density functional theory is not capable of correctly describing van der Waals interactions, functionals especially designed to include van der Waals forces exist, yielding results in remarkable agreement with experiment. From the experimental point of view, we show examples in which IR adsorption and Raman spectroscopy are essential to study molecule/MOF interactions. Importantly, we emphasize throughout this review that a combination of theory and experiment is crucial to effectively gain further understanding. In particular, we review such combined studies for the adsorption mechanism of small molecules in MOFs, the chemical stability of MOFs under humid conditions, water cluster formation inside MOFs, and the diffusion of small molecules into MOFs. The understanding of these phenomena is critical for the rational design of new MOFs with desired properties.

H2 interaction with divalent cations in isostructural MOFs: a key study for variable temperature infrared spectroscopy

Systematic studies of H2 adsorption by variable temperature infrared (VTIR) spectroscopy have added value in the characterization of hydrogen storage materials. As a key study to describe the potential of the method, here we report VTIR spectroscopy results of H2 adsorption at isostructural MOFs CPO-27-M (M = Mg, Mn, Co, Ni, Zn). The strongest perturbation of H2 vibrational frequency is due to the interaction with an open metal site. Although ionic radius is an empirical value, the direct correlation between ionic radii of the metal cation and H2 interaction energy is found in MOFs of the same topology. The highest enthalpy of hydrogen adsorption 15 ± 1 kJ mol(-1) was found for Ni(2+). VTIR results of H2 adsorption at isostructural MOFs CPO-27-M (M = Mg, Mn, Co, Ni, Zn) were compared with data obtained from analogous studies performed on a large variety of microporous materials (MOFs and zeolites), underlining the relevance of the approach to get reliable energetic and entropic (ΔH(0) and ΔS(0)) values to be compared with computational data and isosteric heats.