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.

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.