Ahmad Hamaed

H2 Storage Materials (22KJ/mol) Using Organometallic Ti Fragments as σ-H2 Binding Sites

Low-coordinate Ti (III) fragments with controlled geometries designed specifically for sigma-H2 binding were grafted onto mesoporous silica using tri- and tetrabenzyl Ti precursors. The hydrogen storage capacity was tested as a function of precursor and precursor loading level. At an optimal loading level of 0.2 mol equiv tetrabenzyl Ti the total storage capacity at -196 degrees C was 21.45 wt % and 34.10 kg/m(3) at 100 atm, and 3.15 wt % and 54.49 kg/m(3) for a compressed pellet under the same conditions. The adsorption value of this material was 1.66 wt %, which equates to an average of 2.7 H2 per Ti center. The adsorption isotherms did not reach saturation at 60 atm, suggesting that the theoretical maximum of 5 H2 per Ti in this system may be reached at higher pressures. The binding enthalpies rose with surface coverage to a maximum of 22.15 kJ/mol, which is more than double that of the highest recorded previously and within the range predicted for room temperature performance. The adsorption values of 0.99 at -78 degrees C and 0.69 at 25 degrees C demonstrate retention of 2.4 H2 and 1.1 H2 per Ti at these temperatures, respectively. These findings suggest that Kubas binding of H2 may be exploited at ambient temperature to enhance the storage capacities of high-pressure cylinders currently used in hydrogen test vehicles.

Design and synthesis of vanadium hydrazide gels for Kubas-type hydrogen adsorption: a new class of hydrogen storage materials.

In this paper we demonstrate that the Kubas interaction, a nondissociative form of weak hydrogen chemisorption with binding enthalpies in the ideal 20-30 kJ/mol range for room-temperature hydrogen storage, can be exploited in the design of a new class of hydrogen storage materials which avoid the shortcomings of hydrides and physisorpion materials. This was accomplished through the synthesis of novel vanadium hydrazide gels that use low-coordinate V centers as the principal Kubas H(2) binding sites with only a negligible contribution from physisorption. Materials were synthesized at vanadium-to-hydrazine ratios of 4:3, 1:1, 1:1.5, and 1:2 and characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption, elemental analysis, infrared spectroscopy, and electron paramagnetic resonance spectroscopy. The material with the highest capacity possesses an excess reversible storage of 4.04 wt % at 77 K and 85 bar, corresponding to a true volumetric adsorption of 80 kg H(2)/m(3) and an excess volumetric adsorption of 60.01 kg/m(3). These values are in the range of the ultimate U.S. Department of Energy goal for volumetric density (70 kg/m(3)) as well as the best physisorption material studied to date (49 kg H(2)/m(3) for MOF-177). This material also displays a surprisingly high volumetric density of 23.2 kg H(2)/m(3) at room temperature and 85 bar--roughly 3 times higher than that of compressed gas and approaching the DOE 2010 goal of 28 kg H(2)/m(3). These materials possess linear isotherms and enthalpies that rise on coverage and have little or no kinetic barrier to adsorption or desorption. In a practical system these materials would use pressure instead of temperature as a toggle and can thus be used in compressed gas tanks, currently employed in many hydrogen test vehicles, to dramatically increase the amount of hydrogen stored and therefore the range of any vehicle.

Hydride-induced amplification of performance and binding enthalpies in chromium hydrazide gels for Kubas-type hydrogen storage.

Hydrogen is the ideal fuel because it contains the most energy per gram of any chemical substance and forms water as the only byproduct of consumption. However, storage still remains a formidable challenge because of the thermodynamic and kinetic issues encountered when binding hydrogen to a carrier. In this study, we demonstrate how the principal binding sites in a new class of hydrogen storage materials based on the Kubas interaction can be tuned by variation of the coordination sphere about the metal to dramatically increase the binding enthalpies and performance, while also avoiding the shortcomings of hydrides and physisorpion materials, which have dominated most research to date. This was accomplished through hydrogenation of chromium alkyl hydrazide gels, synthesized from bis(trimethylsilylmethyl) chromium and hydrazine, to form materials with low-coordinate Cr hydride centers as the principal H(2) binding sites, thus exploiting the fact that metal hydrides form stronger Kubas interactions than the corresponding metal alkyls. This led to up to a 6-fold increase in storage capacity at room temperature. The material with the highest capacity has an excess reversible storage of 3.23 wt % at 298 K and 170 bar without saturation, corresponding to 40.8 kg H(2)/m(3), comparable to the 2015 DOE system goal for volumetric density (40 kg/m(3)) at a safe operating pressure. These materials possess linear isotherms and enthalpies that rise on coverage, retain up to 100% of their adsorption capacities on warming from 77 to 298 K, and have no kinetic barrier to adsorption or desorption. In a practical system, these materials would use pressure instead of temperature as a toggle and can thus be used in compressed gas tanks, currently employed in the majority of hydrogen test vehicles, to dramatically increase the amount of hydrogen stored, and therefore range of any vehicle.

Computational Study of Silica-Supported Transition Metal Fragments for Kubas-type Hydrogen Storage

To verify the role of the Kubas interaction in transition metal grafted mesoporous silicas, and to rationalize unusual rising enthalpy trends with surface coverage by hydrogen in these systems, computational studies have been performed. Thus, the interaction of H2 with the titanium centers in molecular models for experimentally characterized mesoporous silica-based H2 absorption materials has been studied quantum chemically using gradient corrected density functional theory. The interaction between the titanium and the H2 molecules is found to be of a synergic, Kubas type, and a maximum of four H2 molecules can be bound to each titanium, in good agreement with previous experiments. The average Ti-H2 interaction energies in molecules incorporating benzyl ancillary ligands (models of the experimental systems) increase as the number of bound H2 units increases from two to four, in agreement with the experimental observation that the H2 adsorption enthalpy increases as the number of adsorbed H2 molecules increases. The Ti-H2 interaction is shown to be greater when the titanium is bound to ancillary ligands, which are poor π-acceptors, and when the ancillary ligand causes the least steric hindrance to the metal. Extension of the target systems to vanadium and chromium shows that, for molecules containing hydride ancillary ligands, a good relationship is found between the energies of the frontier molecular orbitals of the molecular fragments, which interact with incoming H2 molecules, and the strength of the M-H2 interaction. For the benzyl systems, both the differences in M-H2 interaction energies and the energy differences in frontier orbital energies are smaller than those in the hydrides, such that conclusions based on frontier orbital energies are less robust than for the hydride systems. Because of the high enthalpies predicted for organometallic fragments containing hydride ligands, and the low affinity of Cr(III) for hydrogen in this study, these features may not be ideal for a practical hydrogen storage system.

Kubas-Type Hydrogen Storage in V(III) Polymers Using Tri- and Tetradentate Bridging Ligands

Oxalic acid, oxamide, glycolic acid, and glycolamide were employed as 2-carbon linkers to synthesize a series of one-dimensional V(III) polymers from trismesityl vanadium(III)·THF containing a high concentration of low-valent metal sites that can be exploited for Kubas binding in hydrogen storage. Synthesized materials were characterized by powder X-ray diffraction (PXRD), nitrogen adsorption (BET), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), Raman spectroscopy, thermogravimetric analysis, and elemental analysis. Because each of these organic linkers possesses a different number of protons and coordinating atoms, the products in each case were expected to have different stoichiometries with respect to the number of mesityl groups eliminated and also a different geometry about the V(III) centers. For example, the oxalate and glycolate polymers contained residual mesityl groups; however, these could be exchanged with hydride via hydrogenolysis. The highest adsorption capacity was recorded on the product of trismesityl vanadium(III)·THF with oxamide (3.49 wt % at 77 K and 85 bar). As suggested by the high enthalpy of adsorption (17.9 kJ/mol H(2)), a substantial degree of performance of the vanadium metal centers was retained at room temperature (25%), corresponding to a gravimetric adsorption of 0.87 wt % at 85 bar, close to the performance of MOF-177 at this temperature and pressure. This is remarkable given the BET surface area of this material is only 9 m(2)/g. A calculation on the basis of thermogravimetric results provides 0.88 hydrogen molecule per vanadium center under these conditions. Raman studies with H(2) and D(2) showed the first unequivocal evidence for Kubas binding on a framework metal in an extended solid, and IR studies demonstrated H(D) exchange of the vanadium hydride with coordinated D(2). These spectroscopic observations are sufficient to assign the rising trends in isosteric heats of hydrogen adsorption observed previously by our group in several classes of materials containing low-valent transition metals to the Kubas interaction.

Cyclopentadienyl chromium hydrazide gels for Kubas-type hydrogen storage

Cyclopentadienyl chromium hydrazide gels were synthesized from the protonolysis reaction between bis(cyclopentadienyl) chromium and hydrazine. The amorphous products containing low valent chromium species are exploited as substrates for Kubas-type hydrogen storage. These materials demonstrate enthalpies that rise from 10 to 45 kJ mol(-1) and show a retention of 49% of the adsorption capacity at 298 K relative to 77 K, compared to values of 10-15% for most MOFs and amorphous carbons.

The Kubas interaction in M(II) (M = Ti, V, Cr) hydrazine-based hydrogen storage materials: a DFT study.

The Cr(II) binding sites of an experimentally realised hydrazine linked hydrogen storage material have been studied computationally using density functional theory. Both the experimentally determined rise in H(2) binding enthalpy upon alteration of the ancillary ligand from bis[(trimethylsilyl)methyl] to hydride, and the number of H(2) molecules per Cr centre, are reproduced reasonably well. Comparison with analogous Ti(II), V(II) and Mn(II) systems suggests that future experiments should focus on the earliest 3d metals, and also suggests that 5 and 7 wt% H(2) storage may be possible for V(II) and Ti(II) respectively. Alteration of the metal does not have a large effect on the M-H(2) interaction energy, while alteration of the ancillary ligand bound to the metal centre, from bis[(trimethylsilyl)methyl] or hydride to two hydride ligands, THF and only hydrazine based ligands, indicates that ancillary ligands that are poor π-acceptors give stronger M-H(2) interactions. Good evidence is found that the M-H(2) interaction is Kubas type. Orbitals showing σ-donation from H(2) to the metal and π-back-donation from the metal to the dihydrogen are identified, and atoms-in-molecules analysis indicates that the electron density at the bond critical points of the bound H(2) is similar to that of classical Kubas systems. The Kubas interaction is dominated by σ-donation from the H(2) to the metal for Cr(II), but is more balanced between σ-donation and π-back-donation for the Ti(II) and V(II) analogues. This difference in behaviour is traced to a lowering in energy of the metal 3d orbitals across the transition series.

Direct Observation of Activated Hydrogen Binding to a Supported Organometallic Compound at Room Temperature

Current interest in the use of hydrogen as a transportation fuel has driven extensive research into novel gas storage materials. Although physisorption materials can possess technologically viable storage capacities, their isosteric heats are generally below 10 kJ mol , limiting these materials to cryogenic temperatures. Chemisorbers, such as metal hydrides or complex hydrides, can store large amounts of hydrogen but require elevated temperatures to release the gas; isosteric heats for hydride systems are typically larger than 40 kJ mol . The optimum conditions for viable room-temperature hydrogen storage require materials that possess isosteric heats of adsorption in between that of standard physisorbers and chemisorbers, typically in the 20– 30 kJ mol 1 regime. Theoretical work has shown that the incorporation of transition-metal atoms onto a porous support can provide such binding energies with multiple hydrogen molecules adsorbed. However, despite the very large number of theoretical papers, there is no direct experimental proof of these predictions yet. An early experimental example is the gas-phase hydrogen reaction with Ti–ethylene complexes, where the gravimetrically measured hydrogen uptake agrees well with theoretical predictions but details of structure, dynamics, and the local chemistry are absent. Herein, we present direct experimental evidence for dihydrogen–Ti binding on a silica-supported Ti organometallic complex (hereafter referred to as Ti-HMS) using detailed sorption and inelastic neutron scattering (INS) measurements. Our experimental findings are further supported by extensive first-principles DFT and reaction path calculations. We show that the Ti ion is essential for the formation of a dihydrogen complex, and its presence is confirmed by EPR spectroscopy (see Figure S1 in the Supporting Information). Surprisingly, we discover that the H2–Ti binding is a thermally activated process; exposing the supported organometallic to hydrogen below 150 K results in only physisorption, while near room temperature it forms H2–Ti moieties that are stable for extended periods of time. Such an activation barrier was missed in earlier DFT calculations, which predicted only the formation of dihydrogen complexes. Though this particular sample does not represent a viable storage material due to its modest uptake and nonoptimized support, it does offer a useful benchmark for understanding the underlying hydrogen coordination chemistries. The sorption performance of the activated Ti-HMS sample measured immediately prior to INS studies (Figure 1) shows an uptake of approximately 12 mg g 1 at 30 bar and 77 K, similar to the bare HMS. This is comparable but somewhat lower than that previously measured and indicates that some of the active titanium adsorption sites have been lost compared to the as-synthesized complex. This is not unexpected as Ti alkyls are highly reactive [a] Dr. J. M. Simmons, Dr. T. Yildirim NIST Center for Neutron Research National Institute of Standards and Technology Gaithersburg, MD 20899 (USA) Fax: (+1)301-921-9847 E-mail : jason.simmons@nist.gov taner@nist.gov [b] Dr. T. Yildirim Department of Materials Science and Engineering University of Pennsylvania Philadelphia, PA 19104 (USA) [c] Dr. A. Hamaed, Prof. D. M. Antonelli Department of Chemistry and Biochemistry University of Windsor, Windsor, ON, N9B 3P4 (Canada) [d] Prof. D. M. Antonelli Sustainable Environment Research Centre University of Glamorgan Pontypridd CF37 1DL (UK) [e] M. I. Webb, Prof. C. J. Walsby Department of Chemistry, Simon Fraser University Burnaby, BC, V5A 1S6 (Canada) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102658. Figure 1. Excess hydrogen isotherms showing reversible physisorption for Tdose <150 K and irreversible adsorption at 300 K (inset).