Hydrogen storage in zeolites: activation of the pore space through incorporation of guest materials

Abstract

Solid state hydrogen storage materials have become a key area of research over the past 20 years. In this work, the potential of zeolites to occlude hydrogen storage materials as guests to make composite materials was explored. Lithium borohydride was successfully loaded in zeolites NaA, NaX and NaY; showing a two phased system. Desorption of hydrogen from the occluded lithium borohydride was 5$^\circ$C lower than that of bulk lithium borohydride, but with slower kinetics, implying diffusion effects of occlusion into the host. Adsorption showed reduced uptakes of hydrogen at 77 K compared with the host zeolite, which was consistent with the degree of loading. Limited hydrogenation was achieved with milder conditions at 350$^\circ$C at 15 bar hydrogen of the desorbed lithium borohydride. Heats of adsorption were estimated for the samples both before and after high temperature desorption of hydrogen. Lithium borohydride was also loaded into zeolitic carbons and lithium, copper (II) and ammonium ion-exchanged zeolites. Copper exchanged zeolite catalysed desorption of hydrogen from lithium borohydride was most promising and occurred at room temperature. Lithium and ammonium exchanged zeolite showed a 10$^\circ$C reduction in desorption temperature, the ammonium system showing the best diffusion kinetics, with a sharp desorption similar to the bulk lithium borohydride. NaA and NaX were occluded with ammonia borane and lithium borohydride amide [Li$_4$BH$_4$(NH$_2$)$_3$]. NaY containing occluded sodium was found to hydrogenate at room temperature at pressures up to 15 bar. This was accompanied with trapping of hydrogen and an increased adsorption of hydrogen at low temperature, exceeding the gravimetric absorption value for zeolite NaY