Showing papers by "Juergen Eckert published in 2009"

Zeolite-like Metal−Organic Frameworks (ZMOFs) as Hydrogen Storage Platform: Lithium and Magnesium Ion-Exchange and H2-(rho-ZMOF) Interaction Studies

Abstract: Zeolite-like metal−organic frameworks (ZMOFs) are anionic, have readily exchangeable extra-framework cations, and can be constructed with a variety of organic linkers. ZMOFs therefore can be regard...

Exceptional Stability and High Hydrogen Uptake in Hydrogen-Bonded Metal−Organic Cubes Possessing ACO and AST Zeolite-like Topologies

Abstract: Herein we detail a novel approach for targeting zeolite-like metal-organic frameworks (ZMOFs) that utilizes metal−organic cubes, which are regarded as double four-membered rings (d4Rs) and are comp...

Enhanced H2 adsorption enthalpy in the low-surface area, partially fluorinated coordination polymer Zn5(triazole)6(tetrafluoroterephthalate)2(H2O)2·4H2O

Abstract: A porous coordination polymer containing zinc, 1,2,4-triazolate, and tetrafluoroterephthalate displays a high physisorptive hydrogen adsorption enthalpy of 8 kJ/mol, as a result of fluorine atoms exposed to the pore surface and the structure's small pore size.

Mechanistic aspects of propene epoxidation by hydrogen peroxide. Catalytic role of water molecules, external electric field, and zeolite framework of TS-1.

Abstract: We have assessed various aspects of the epoxidation of propene by hydrogen peroxide, a reaction of considerable industrial importance, and elucidated some of the important factors that govern its mechanism. Quantum chemical calculations on the reactants, products, and transition states were performed both in the gas phase and using models to represent the TS-1 (titanosilicalite-1) catalyst. The reaction energy for the uncatalyzed process is computed as -52.6 kcal/mol with a barrier of 35.2 kcal/mol in the gas phase using the B3LYP hybrid density functional and a 6-31+G(d,p) basis set. The reaction appears to occur via a concerted mechanism. The competing reaction of ionic addition of hydrogen peroxide to the double bond to form a hydroperoxopropane is computed to have a reaction energy of only -17.1 kcal/mol with a barrier of 34.8 kcal/mol and is therefore expected not to be thermodynamically preferable. Introduction of water molecules to the model is calculated to reduce the reaction barrier to 25.7 kcal/mol in the case of a single molecule but did not significantly affect the reaction energy. The competing addition reaction barrier appears to be significantly less sensitive to the presence of water molecules, suggesting that the concerted epoxidation reaction is also kinetically favored in the polar environment. Introduction of additional water molecules does not result in a noticeable enhancement. The water molecules appear to mediate proton transfer between the peroxide oxygens in the rate determining step of the concerted epoxidation reaction. The introduction of a background solvent field was also found to reduce the activation energy. For example, a model with a single explicit water molecule and the solvent field gives an activation barrier of 16.9 kcal/mol. A similar effect is observed if an external electric field is applied to the model with the dipole component directed along the O-O bond direction. Calculations were also performed on the same reaction occurring in the vicinity of a model for the active site of the TS-1 catalyst using a cluster model. The activation barrier for the cluster model is calculated to be 25.8 kcal/mol with a reaction energy of -55.5 kcal/mol, which is comparable to the gas phase model with a single water molecule added. No significant changes are observed with the addition of water molecules in this model.

Hydrogen Adsorption in the Ti-Doped Mesoporous Silicate SBA-15

Abstract: While metal-doped mesoporous silicate SBA-15 has been studied extensively for its catalytic properties, the adsorption of molecular hydrogen in this system has not been considered. We report herein that the titanium-doped material (10% Ti) adsorbs nearly twice the amount of hydrogen at 77 K and a pressure of 25 bar than pure SBA-15, Hydrogen adsorption isotherms also show that binding energies in the Ti-doped material are more than 50% greater than in SBA-15. Neutron vibrational spectroscopy shows clear evidence for interaction of hydrogen with Ti, which is responsible for part of the increase in hydrogen sorption in the material.

Methane molecule confined in the small and large cages of structure I clathrate hydrate: Quantum six-dimensional calculations of the coupled translation-rotation eigenstates.

Abstract: We report fully coupled quantum six-dimensional (6D) calculations of the translation-rotation (T-R) energy levels of CH4 molecule inside the small dodecahedral (512) and large tetracaidecahedral (51262) cages of the structure I clathrate hydrate. The quantum dynamics of the three translational and three rotational degrees of freedom of CH4 are treated rigorously, while the guest molecule and the host cavities are taken to be rigid. The matrix of the full 6D T-R Hamiltonian is diagonalized in the product basis of contracted translational and angular basis functions, generated by solving two reduced-dimension (3D) eigenvalue problems. A pairwise additive CH4-cage 6D potential energy surface (PES) is employed, constructed using the anisotropic CH4H2O pair potential which was utilized previously in the molecular dynamics simulations of methane hydrate. Our calculations elucidate the key features of the T-R energy level structure of the nanoconfined CH4. The rotational levels of methane exhibit an elaborate p...

Mode of Adsorption of (CH3)2Au(acac) onto Partially Dehydroxylated Silica

Abstract: The deposition of volatile cis-(CH3)2Au(O,O′-acac) onto silica partially dehydroxylated at 400 °C leads to molecular dispersion of the organogold complex. X-ray absorption near-edge structure demonstrates that the gold retains its oxidation state upon binding. IR and 1H magic-angle spinning NMR spectra suggest that the molecular framework also remains intact. Computational models involving hydroxyl-terminated octasilsesquioxane cube clusters to represent silica predict the most energetically favorable interaction to be hydrogen-bonding between two surface hydroxyls and the oxygen donor atoms of the chelated acetylacetonate ligand. This mode of adsorption was confirmed by analysis of the IR and the Au LIII edge extended X-ray absorption fine structure. The surface hydroxyls most likely to participate in the attachment of the gold complex to silica are vicinal silanol pairs that are not involved in hydrogen bonding to other silanols.