QUASI-EQUILIBRIUM ADSORPTION GRAVIMETRY OF WATER ON MFI-TYPE AND FER-TYPE ZEOLITES AND ON AN AFI-TYPE ALUMINOPHOSPHATE

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 gravim etry with a quasi-equilibrium continuous adsorptive introduction. The results show that all of these samples are characterized by a continuo us distribution of strongly energetic water adsorption sites (from 60 to 120 kJ.mol(-1)) for which the adsorption is irreversible at the exp erimental temperature. This probably justifies the presence of hystere sis on desorption at very low relative pressure values. Adsorption of water in these systems firstly occurs by site. This is then followed b y 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 z eolites, silicalite-I and ZSM-5, do not accommodate duster formation w ithin its microporous network. However, an external flexible microporo us structure, containing Lewis sites, may be present for large crystal s. This flexible secondary structure may then be able to opened (swell ed) at high relative pressures. On the other hand, for the aluminophos phate AlPO4-5, it is believed that a change in the aluminium coordinat ion on the formation of a crystal hydrate together with capillary cond ensation 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.