Reversible Hydrogen Fixation and Release under Mild Conditions by Poly(vinylquinoxaline)

A polymeric hydrogen carrier with reversible H2 storage capability was developed by incorporating a quinoxaline/tetrahydroquinoxaline redox couple into an aliphatic polymer chain. The quinoxaline unit was designed with a view to significantly increase the mass H2 storage density in the polymer up to 2.6 wt % calculated for the repeating unit, compared with the previously reported density of 0.9 wt % for poly(vinylfluorenone). An iridium complex-catalyzed hydrogenation of quinoxaline was characterized by its reversibility according to temperature, giving rise to tetrahydroquinoxaline under mild conditions (60 °C and 1 atm of H2), which released H2 gas by simply warming in common organic solvents. Its polymer extension allowed quinoxaline to be a polymeric hydrogen carrier in a solid state, which was characterized by the inherent advantages of safety, moldability, and handling easiness. Poly(vinylquinoxaline) was prepared by the radical polymerization of vinylquinoxaline. Poly(vinylquinoxaline) swollen with γ-butyrolactone became a gel-like solid, which quantitatively released and then fixed H2 gas under mild conditions.