Heats of adsorption of water vapour on NaX and KNaX zeolites at different temperatures

Abstract

Differential heats of adsorption, q_v, of different amounts of water vapour, n^s, have been measured in a double calorimeter at 23, 40, 60 and 100°C on KNaX zeolite and at 100°C on NaX zeolite. The mechanism of successive filling of different energy sites in the cavities of KNaX zeolite by water molecules at 23°C was shown earlier by the wave-shaped change in q_v, by the change in heat capacity and by the shift of band frequency in the i.r. spectrum of adsorbed water. The wave-shaped change in q_v is replaced by a monotonic one as the temperature rises. At 100°C, q_v smoothly decreases with increase in n^s; the q_v values for both zeolites are close. The differential heat capacities of adsorbed water, C, determined from the q_v measurements at different temperatures and n^s values, are close to those measured directly. The dependence of C on temperature has been estimated. The isotherms of water adsorption on these zeolites have also been considered.