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.

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.

Hexagonally packed mesoporous silica (HMS) grafted with titanium, vanadium, and chromium organometallic fragments possessing metal oxidation states between (II) and (IV) were synthesized and characterized by XRD, nitrogen adsorption, XPS, and elemental analysis. These materials were then tested for their excess hydrogen storage capacity, with the purpose in mind of subtracting out the physisorption component and establishing which transition metal fragments and oxidation states functioned as the most effective binding sites for hydrogen. By varying the metal type and the metal oxidation state, as well as the ligand environment by exchanging the alkyl group with hydride via hydrogenolysis, the effects of these variations on the H 2 adsorption capacity of the metal center as well as on the binding enthalpies of these systems were investigated. This study demonstrated that titanium is more effective at hydrogen binding than vanadium and chromium. Hence, HMS silica grafted with benzyl titanium(III) fragments can accommodate up to 4.85 H 2 per Ti center and 4.74 H 2 per Ti center in the case of HMS grafted with bis(naphthalene) titanium. This compares to 2.74 H 2 per vanadium center in the case of HMS grafted with tris(mesityl) vanadium, and to 1.82 H 2 and 2.20 H 2 per chromium center as in the HMS treated with tris[bis(trimethylsilyl)methyl] chromium and bis[(trimethylsilyl)methyl] chromium, respectively. The hydrogenation of the metal centers had no effect on the adsorption capacity of the titanium centers and only a slight effect on the vanadium centers; however, a far more pronounced effect was observed in the case of chromium as the adsorption capacity increased from 1.82 H 2 to 3.2 H 2 per chromium in the case of HMS treated with tris[bis(trimethylsilyl)methyl] chromium, and from 2.20 H 2 to 3.5 H 2 per chromium in the case of HMS treated with bis[(trimethylsilyl)methyl] chromium. The differences in binding capacity and the effect of hydrogenation were attributed to ligand environment, the availability of open binding sites, and the ability of the metal center to back-bond to the antibonding orbital of the chemisorbed H 2 ligands.