American Chemical Society
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Functionalized Porous Silicas with Unsaturated Early Transition Metal Moieties as Hydrogen Storage Materials: Comparison of Metal and Oxidation State

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journal contribution
posted on 2010-05-13, 00:00 authored by Ahmad Hamaed, Hung Van Mai, Tuan K. A. Hoang, Michel Trudeau, David Antonelli
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 H2 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 H2 per Ti center and 4.74 H2 per Ti center in the case of HMS grafted with bis(naphthalene) titanium. This compares to 2.74 H2 per vanadium center in the case of HMS grafted with tris(mesityl) vanadium, and to 1.82 H2 and 2.20 H2 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 H2 to 3.2 H2 per chromium in the case of HMS treated with tris[bis(trimethylsilyl)methyl] chromium, and from 2.20 H2 to 3.5 H2 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 H2 ligands.