Solvate-supported proton transport in zeolite H-ZSM-5 was studied by means of complex impedance spectroscopy. The zeolite shows enhanced proton mobility in the presence of NH3 and H2O that depends on the concentration of the solvate molecule, temperature (298-773 K), and the SiO2/Al2O3 ratio of the zeolite (30-1000). In general, proton conductivity in H-ZSM-5 is most effectively supported in the presence of NH3 and H2O at high concentrations, low temperatures, and low SiO2/Al2O3 ratios (< or = 80). For the aluminum-rich samples desorption measurements reflect different transport mechanisms that depend on the respective temperature range. Up to about 393 K a Grotthus-like proton transport mechanism is assumed, whereas at higher temperatures (393-473 K) vehiclelike transport seems to dominate. The activation energies for NH4+ and H3O+ vehicle conductivity depend on the SiO2/Al2O3 ratio, and the values are in the range of 49-59 and 39-49 kJ mol-1, respectively, and thus significantly lower than those for "pure" proton conduction in solvate-free samples.

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Activation barriers and jump rates for translational proton motion in zeolite H-ZSM-5 are calculated by a combined quantum mechanics-interatomic potential function approach (QM-Pot). The potential energies of the stable intermediate proton positions and the transition structures for proton jumps between two neighboring Brønsted-sites, spatially separated by 14 A ˚, show an almost symmetrical trend, which reaches a maximum value midway between these sites. The highest barrier is 210 kJ mol À1 above the initial and final state energies. The energy barrier for the initial step of the translational motion (leaving the AlO 4 À site) is 127, 119 and 83 kJ mol À1 for Al-Al distances of 14, 8 and 6 A ˚, respectively. Thus, decreased separation of Brønsted sites leads to decreased energy barriers for proton jumps due to increased interaction of their Coulomb potentials. Proton jump rates calculated by classical transition state theory vary over wide ranges, 10 À5 to 10 8 s À1 and 10 3 to 10 10 s À1 , at temperatures of 373 and 673 K, respectively, depending on the particular proton path and the Al-Al distance.