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 H-2 with the titanium centers in molecular models for experimentally characterized mesoporous silica-based H-2 absorption materials has been studied quantum chemically using gradient corrected density functional theory. The interaction between the titanium and the H-2 molecules is found to be of a synergic, Kubas type, and a maximum of four H-2 molecules can be bound to each titanium, in good agreement with previous experiments. The average Ti-H-2 interaction energies in molecules incorporating benzyl ancillary ligands (models of the experimental systems) increase as the number of bound H-2 units increases from two to four, in agreement with the experimental observation that the H-2 adsorption enthalpy increases as the number of adsorbed H-2 molecules increases. The Ti-H-2 interaction is shown to be greater when the titanium is bound to ancillary ligands, which are poor pi-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 H-2 molecules, and the strength of the M-H-2 interaction. For the benzyl systems, both the differences in M-H-2 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.