It was noted that acid de-alumination of zeolites left a number of nest silanols as structural defects in the vacancies resulting from aluminum removal from the channel structure. The resultant siliceous zeolite was therefore unexpectedly hydrophilic and sorbed water for over 15% of its mass. The zeolite protons associated with the remaining aluminum gained more freedom. Sorbed water H-bonds to the formed nest silanols, and formed a lake streaming over the zeolite surface. This high mass of sorbed water greatly assisted the proton mobility and hence the protonic conduction, so that the dealuminated zeolite gains a two order of magnitude rise in its electric conductivity. A regular rise in temperature should then result in a regular loss of sorbed water, unless other structural factors were involved. An irregular drop in water-assisted conduction was rather shown at the middle of the measurement, in the temperature range 293–373K, at 333K. This was associated with a concurrent channel contraction taking part around 333K. Such a structural shift would force sorbed water faster out of the channel structure, which could explain the irregular drop in water-assisted protonic conduction. Complementary analysis for the results of γ-irradiation at integrated doses revealed contrasting effects. Low dose irradiation with ionizing γ-rays assists the proton mobility at the low temperature of 308K, inducing an extra rise by one order of magnitude in conductivity in addition to the two orders gained due to sorbed water. At variance, high dose irradiation reduces the conductivity at the same temperature to even lower values than for the non-irradiated zeolite. This dramatic loss in protonic conduction was attributed to a relevant loss in the assisting role of sorbed water via two mechanisms: (i) general loss due to γ-induced dehydration; and (ii) specific loss due to γ-assisted contraction in the zeolite channels initiated at even lower temperatures. These observations (figure 18) were discussed in the light of ac conductivity analysis for the changes in frequency-dependent electric permittivity, impedance and phase angle with the zeolite water content and the measurement temperature.

Contrasting Effects of γ-Irradiation on the Role of Sorbed Water Assisting the Protonic Conduction in US-HY Zeolite. M.B.Sayed: Microporous and Mesoporous Materials, 2000, 37[1–2], 107–16