Hydrogen Production in Aromatic and Aliphatic Ionic Liquids

The radiolytic production of molecular hydrogen in the ionic liquids N-trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide [N-1114][Tf2N]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) has been examined with gamma-rays, 2-10 MeV protons, and 5-20 MeV helium ions to determine the functional dependence of the yield on particle track structure. Molecular hydrogen is the dominant gaseous radiolysis product from these ionic liquids, and the yields with gamma-rays are 0.73 and 0.098 molecules per 100 eV of energy absorbed for [N-1114][Tf2N] and [emim][Tf2N], respectively. These low yields are consistent with the relative insensitivity of most aromatic compounds to radiation. However, the molecular hydrogen yields increase considerably on going from gamma-rays to protons to helium ions with [emim][Tf2N] while they remain essentially constant for [N-1114][Tf2N). FTIR and UV-vis spectroscopic studies show slight degradation of the ionic liquids with radiation.