Dramatic Effect of the Electrostatic Parameters on H₂ Sorption in an M‑MOF-74 Analogue

Simulations of H₂ sorption were performed in Cu-MOF-74, a recent addition to the M-MOF-74 series. Electronic structure calculations revealed that the Cu²⁺ ions exhibit an unusually low partial positive charge distribution in Cu-MOF-74, which is a direct consequence of the Jahn–Teller effect. This is in contrast to the charge environment for the metal ions in some of the other M-MOF-74 variants as determined in previous work [Pham, T.; J. Phys. Chem. C 2015, 119, 1078−1090]. Because of the low magnitude of the partial charges of the Cu²⁺ ions in Cu-MOF-74, this MOF displays the lowest H₂ uptake and <i>Q</i>_st values of the M-MOF-74 series, which is consistent with what was observed experimentally for H₂ sorption in this series of MOFs. Control simulations of H₂ sorption in a nonphysical Cu-MOF-74 variant were performed in which a set of calculated partial charges, appropriate for one of the other M-MOF-74 analogues, were used. These simulations utilize a much higher partial positive charge for the metal ions and, as a result, a different shape for the simulated H₂ sorption isotherms was obtained compared to that using the normal force field. This shape was not representative of the experimental isotherm for Cu-MOF-74, and thus, confirms the notion that the electrostatic parameters on the metal ions are the key to understanding the H₂ sorption behavior in this MOF. Examining the distribution of the induced dipoles and the Cu²⁺–H₂ distance via simulated annealing and executing two-dimensional quantum rotation calculations have also verified that the H₂–metal interaction in Cu-MOF-74 is the weakest in the M-MOF-74 series. This study shows the power of using computational modeling to explain certain experimental observables and trends in a series of MOFs

Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology

The adsorption of H₂ in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn²⁺ ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H₂ adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H₂ adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (<i>Q</i>_st) value for H₂ in ZIF-76 was determined to be 7.7 kJ mol^–1, which is comparable to that for other ZIFs and is fairly high for a material that does not contain open-metal sites. Simulations of H₂ adsorption in one of these structures resulted in <i>Q</i>_st values that are in very good agreement with experiment within the loading range considered. Two notable H₂ binding sites were discovered from simulations in both structures of ZIF-76; however, the preferential regions of H₂ occupancy are reversed for the two structures. The inelastic neutron scattering (INS) spectra for H₂ adsorbed in ZIF-76 contain several peaks that arise from transitions of the hindered H₂ rotor, with the lowest energy peak occurring in the range of 6.0–7.2 meV. Two-dimensional quantum rotation calculations for H₂ adsorbed at the considered sites in both structures yielded rotational transitions that are in good agreement with the peaks that appear in the INS spectra. Despite the large degree of disorder in the ZIF-76 crystal structure, the overall environment in the ZIF still gives rise to interconnected INS features as discerned from our calculations. This study demonstrates how important details of the H₂ adsorption mechanism in a ZIF with structural disorder can be obtained from a combination of experimental measurements and theoretical calculations

Capturing the H₂–Metal Interaction in Mg-MOF-74 Using Classical Polarization

Grand canonical Monte Carlo (GCMC) simulations of H₂ sorption were performed in Mg-MOF-74, a metal–organic framework (MOF) that displays very high H₂ sorption affinity. Experimental H₂ sorption isotherms and isosteric heats of adsorption (<i>Q</i>_st) values were reproduced using a general purpose materials sorption potential that includes many-body polarization interactions. In contrast, using two models that include only charge–quadrupole interactions failed to reproduce such experimental measurements even though they are the type normally employed in such classical force field calculations. Utilizing the present explicit polarizable model in GCMC simulation resulted in a Mg²⁺–H₂ distance of 2.60 Å, which is close to a previously reported value that was obtained using electronic structure methods and comparable to similar experimental measurements. The induced dipole distribution obtained from simulation assisted in the characterization of two previously identified sorption sites in the MOF: the Mg²⁺ ions and the oxido group of the linkers. The calculated two-dimensional quantum rotational levels for a H₂ molecule sorbed onto the Mg²⁺ ion were in good agreement with experimental inelastic neutron scattering (INS) data. Although the H₂–metal interaction in MOFs may be thought of as a quantum mechanical effect, this study demonstrates how the interaction between the sorbate molecules and the open-metal sites in a particular highly sorbing MOF can be captured using classical simulation techniques that involve a polarizable potential

Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology

The adsorption of H₂ in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn²⁺ ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H₂ adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H₂ adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (<i>Q</i>_st) value for H₂ in ZIF-76 was determined to be 7.7 kJ mol^–1, which is comparable to that for other ZIFs and is fairly high for a material that does not contain open-metal sites. Simulations of H₂ adsorption in one of these structures resulted in <i>Q</i>_st values that are in very good agreement with experiment within the loading range considered. Two notable H₂ binding sites were discovered from simulations in both structures of ZIF-76; however, the preferential regions of H₂ occupancy are reversed for the two structures. The inelastic neutron scattering (INS) spectra for H₂ adsorbed in ZIF-76 contain several peaks that arise from transitions of the hindered H₂ rotor, with the lowest energy peak occurring in the range of 6.0–7.2 meV. Two-dimensional quantum rotation calculations for H₂ adsorbed at the considered sites in both structures yielded rotational transitions that are in good agreement with the peaks that appear in the INS spectra. Despite the large degree of disorder in the ZIF-76 crystal structure, the overall environment in the ZIF still gives rise to interconnected INS features as discerned from our calculations. This study demonstrates how important details of the H₂ adsorption mechanism in a ZIF with structural disorder can be obtained from a combination of experimental measurements and theoretical calculations

A General Protocol for Determining the Structures of Molecularly Ordered but Noncrystalline Silicate Frameworks

A general protocol is demonstrated for determining the structures of molecularly ordered but noncrystalline solids, which combines constraints provided by X-ray diffraction (XRD), one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles quantum chemical calculations. The approach is used to determine the structure(s) of a surfactant-directed layered silicate with short-range order in two dimensions but without long-range periodicity in three-dimensions (3D). The absence of long-range 3D molecular order and corresponding indexable XRD reflections precludes determination of a space group for this layered silicate. Nevertheless, by combining structural constraints obtained from solid-state ²⁹Si NMR analyses, including the types and relative populations of distinct ²⁹Si sites, their respective ²⁹Si–O–²⁹Si connectivities and separation distances, with unit cell parameters (though not space group symmetry) provided by XRD, a comprehensive search of candidate framework structures leads to the identification of a small number of candidate structures that are each compatible with all of the experimental data. Subsequent refinement of the candidate structures using density functional theory calculations allows their evaluation and identification of “best” framework representations, based on their respective lattice energies and quantitative comparisons between experimental and calculated ²⁹Si isotropic chemical shifts and ²<i>J</i>(²⁹Si–O–²⁹Si) scalar couplings. The comprehensive analysis identifies three closely related and topologically equivalent framework configurations that are in close agreement with all experimental and theoretical structural constraints. The subtle differences among such similar structural models embody the complexity of the actual framework(s), which likely contain coexisting or subtle distributions of structural order that are intrinsic to the material

Understanding the H₂ Sorption Trends in the M‑MOF-74 Series (M = Mg, Ni, Co, Zn)

Electronic structure calculations and simulations of H₂ sorption were performed in four members of the M-MOF-74 series: Mg-MOF-74, Ni-MOF-74, Co-MOF-74, and Zn-MOF-74. Notable differences were observed in the partial charge and polarizability of the metal ions derived from the electronic structure calculations. The modeling parameters obtained from the electronic structure calculations were found to influence certain features in the experimentally observed H₂ sorption trends in the M-MOF-74 series. The simulations were performed with the inclusion of explicit many-body polarization, which was required to reproduce the experimental H₂ sorption observables (i.e., sorption isotherms and isosteric heats of adsorption (<i>Q</i>_<i>st</i>)) and the H₂–metal interaction in all four MOFs using classical molecular simulation. Consistent with experimental measurements, the simulations captured the following trend for the H₂–metal interaction strength: Ni-MOF-74 > Co-MOF-74 > Mg-MOF-74 > Zn-MOF-74. The calculations revealed that stronger H₂–metal interactions within the M-MOF-74 series corresponded to shorter H₂–metal distances and higher induced dipoles on the metal-sorbed H₂ molecules. In addition, it was observed that there was a strong correlation between the H₂–metal interaction and the polarization contribution. Although Mg-MOF-74 has the highest calculated partial charge for the metal ion within the series, the Mg²⁺ ion has a very low polarizability compared to the other M²⁺ ions; this explains why the H₂–metal interaction in this MOF is weaker compared to those for Ni-MOF-74 and Co-MOF-74. The sterics interactions, reflected in the crystal structure for all four MOFs, also played a role for the observed H₂ sorption trends. Zn-MOF-74 has the lowest H₂ uptakes and <i>Q</i>_<i>st</i> within the series due to an unfavorable geometric environment for the Zn²⁺ ions within the ZnO₅ clusters. Lastly, the two-dimensional quantum rotational levels were calculated for the H₂–metal interaction in all four MOFs using the potential energy function employed herein and the calculated transitions were in good agreement with the corresponding peaks that were observed in the experimental inelastic neutron scattering (INS) spectra for the respective MOFs. This observation serves both to provide atomistic resolution to the spectroscopic experiments and to validate the molecular force field

Hydrogen Storage in New Metal–Organic Frameworks

Five new metal–organic frameworks (MOFs, termed MOF-324, 325, 326 and IRMOF-61 and 62) of either short linkers (pyrazolecarboxylate and pyrazaboledicarboxylate) or long and thin alkyne functionalities (ethynyldibenzoate and butadiynedibenzoate) were prepared to examine their impact on hydrogen storage in MOFs. These compounds were characterized by single-crystal X-ray diffraction, and their low-pressure and high-pressure hydrogen uptake properties were investigated. In particular, volumetric excess H₂ uptake by MOF-324 and IRMOF-62 outperforms MOF-177 up to 30 bar. Inelastic neutron-scattering studies for MOF-324 also revealed strong interactions between the organic links and hydrogen, in contrast to MOF-5 where the interactions between the Zn₄O unit and hydrogen are the strongest. These data also show that smaller pores and polarized linkers in MOFs are indeed advantageous for hydrogen storage

Polymorphism of Paracetamol: A New Understanding of Molecular Flexibility through Local Methyl Dynamics

This study focuses on the interplay of molecular flexibility and hydrogen bonding manifested in the monoclinic (form I) and orthorhombic (form II) polymorphs of paracetamol. By means of incoherent inelastic neutron scattering and density functional theory calculations, the relaxation processes related to the methyl side-group reorientation were analyzed in detail. Our computational study demonstrates the importance of considering quantum effects to explain how methyl reorientations and subtle conformational changes of the molecule are intertwined. Indeed, by analyzing the quasi elastic signal of the neutron data, we were able to show a unique and complex motional flexibility in form II, reflected by a coupling between the methyl and the phenyl reorientation. This is associated with a higher energy barrier of the methyl rotation and a lower Gibbs free energy when compared to form I. We put forward the idea that correlating solubility and molecular flexibility, through the relation between p<i>K</i>_a and methyl rotation activation energy, might bring new insights to understanding and predicting drug bioavailability

Investigating H₂ Sorption in a Fluorinated Metal–Organic Framework with Small Pores Through Molecular Simulation and Inelastic Neutron Scattering

Simulations of H₂ sorption were performed in a metal–organic framework (MOF) consisting of Zn²⁺ ions coordinated to 1,2,4-triazole and tetrafluoroterephthalate ligands (denoted [Zn­(trz)­(tftph)] in this work). The simulated H₂ sorption isotherms reported in this work are consistent with the experimental data for the state points considered. The experimental H₂ isosteric heat of adsorption (<i>Q</i>_st) values for this MOF are approximately 8.0 kJ mol^–1 for the considered loading range, which is in the proximity of those determined from simulation. The experimental inelastic neutron scattering (INS) spectra for H₂ in [Zn­(trz)­(tftph)] reveal at least two peaks that occur at low energies, which corresponds to high barriers to rotation for the respective sites. The most favorable sorption site in the MOF was identified from the simulations as sorption in the vicinity of a metal–coordinated H₂O molecule, an exposed fluorine atom, and a carboxylate oxygen atom in a confined region in the framework. Secondary sorption was observed between the fluorine atoms of adjacent tetrafluoroterephthalate ligands. The H₂ molecule at the primary sorption site in [Zn­(trz)­(tftph)] exhibits a rotational barrier that exceeds that for most neutral MOFs with open-metal sites according to an empirical phenomenological model, and this was further validated by calculating the rotational potential energy surface for H₂ at this site

Oxidation as A Means to Remove Surface Contaminants on Cu Foil Prior to Graphene Growth by Chemical Vapor Deposition

One of the more common routes to fabricate graphene is by chemical vapor deposition (CVD). This is primarily because of its potential to scale up the process and produce large area graphene. For the synthesis of large area monolayer Cu is probably the most popular substrate since it has a low carbon solubility enabling homogeneous single-layer sheets of graphene to form. This process requires a very clean substrate. In this work we look at the efficiency of common pretreatments such as etching or wiping with solvents and compare them to an oxidation treatment at 1025 °C followed by a reducing process by annealing in H₂. The oxidation/reduction process is shown to be far more efficient allowing large area homogeneous single layer graphene formation without the presence of additional graphene flakes which form from organic contamination on the Cu surface