Bis(benzene) and Bis(cyclopentadienyl) V and Cr Doped Mesoporous Silica with High Enthalpies of Hydrogen Adsorption

Hexagonally packed mesoporous silica synthesized using a dodecylamine template was treated with bis(cyclopentadienyl) chromium, bis(cyclopentadienyl) vanadium, and bis(benzene) vanadium. The obtained composite samples were characterized by nitrogen adsorption, X-ray powder diffraction, and X-ray photoelectron spectroscopy. These materials were then studied for hydrogen adsorption and storage capacities to determine the effect of metal doping and the degree of Kubas binding to the metals in these systems. By studying the hydrogen adsorption at two temperatures of 77 and 87 K, the heats of adsorption were also obtained by fitting the data in Clapeyron-Clausius equation. The composite samples all possess higher adsorption and storage capacities than pristine silica, and all show the same rising trend of adsorption enthalpies as a function of surface coverage observed in previous studies from our group on mesoporous oxides with reduced transition metal centers on the surface. The highest enthalpy recorded was 18.43 kJ mol H2 -1 in the case of bis(cyclopentadienyl) chromium treated silica, which is the second highest enthalpy of hydrogen binding ever reported for a porous material behind benzyltitanium grafted silicas also studied by our group, which show enthalpies of 23 kJ mol H2 -1. Unlike these benzyltitaium materials, which decompose over several weeks in an inert atmosphere at room temperature to gradually lose their adsorption capacity, this Cr-grafted sample retains its adsorption capacity after 3 months in inert atmosphere. As suggested by the higher enthalpies of adsorption, substantial degrees of performance of the chromium metal centers were retained at -78 °C (71%) and room temperature (RT) (18%) in comparison with the performance of this sample at 77 K.