Water adsorption at temperatures of 286 and 296 K on silicalite-I, ZSM-5 (Si/Al=16), ZSM-48 (Si/Al=50) and AlPO4-5 is followed by gravimetry with a quasi-equilibrium continuous adsorptive introduction.
The results show that all of these samples are characterized by a continuous distribution of strongly energetic water adsorption
sites (from 60 to 120 kJ·mol−1) for which the adsorption is irreversible at the experimental temperature. This probably justifies the presence of hysteresis
on desorption at very low relative pressure values. Adsorption of water in these systems firstly occurs by site. This is then
followed by cluster formation and it is suggested that it is the ability of the adsorbent to build up these clusters within
the microporous structure which determines intracrystalline uptake. It is put forward that the zeolites, silicalite-I and
ZSM-5, do not accommodate cluster formation within its microporous network. However, an external flexible microporous structure,
containing Lewis sites, may be present for large crystals. This flexible secondary structure may then be able to opened (swelled)
at high relative pressures.
On the other hand, for the aluminophosphate AlPO4-5, it is believed that a change in the aluminium coordination on the formation of a crystal hydrate together with capillary
condensation results in a large step in the adsorption isotherm, which is itself preceded by a smaller step, revealing a brutal
densification of the adsorbed phase.