Combined IR absorption and modeling study of nanoporous zeolite imidazolate frameworks (ZIFs) filled with hydrogen

A combined IR absorption and first principles modelling study of zeolite imidazolate frameworks (ZIFs) filled with hydrogen is presented. It is shown that hydrogen physisorbed in a ZIF results in a number of absorption lines at around 4131, 4121, 4480, 4700, 4880, 5100, and 5280 cm(-1), which are assigned to the Q(0), Q(1), S-1(0), S-1(1), 2S(1)(0), S-1(0) + S-1(1), and 2S(1)(1) ro-vibrational transitions of physisorbed H-2, respectively. The latter three modes represent simultaneous excitation of molecular pairs, which implies that hydrogen physisorbed in ZIF occurs at a density close to that of the liquid and/or solid state. The adsorption onset temperature defined as the maximum temperature at which localized adsorption occurs was found to be around 80 K.