Adsorbents aluminum-rich zeolites

Desiccants. A solid desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or liquid) stream. The main requirements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are silica gel, activated alumina, and the aluminum rich zeolites (4A or 13X). The equilibrium adsorption isotliemis for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different applications. 

The Si/Al ratio in a zeolite is never less than 1.0 but there is no upper limit and pure silica analogs of some of the zeolite structures have been prepared. The adsorptive properties show a systematic transition from the aluminum-rich sieves, which have very high affinities for water and other polar molecules, to the microporous silicas such as silicalite which are essentially hydrophobic and adsorb n-paraffins in preference to water. The transition from hydrophilic to hydrophobic normally occurs at a Si/Al ratio of between 8 and 10. By appropriate choice of framework structure, Si/Al ratio and cationic... 

J. Turkevich (Princeton University, Princeton, N. J. 08540) In connection with Figure 3, I would like to know how the silicon-aluminum ratio is determined. The synthetic zeolites and maybe natural ones are obtained from silica-rich gels which increase in silica-alumina ratio in the mother liquid as the silicon-aluminum ratio increases in the crystals. Silica may be occluded and adsorbed as a colloidal gel on the zeolite and difficult to wash off. The silica ratio as measured will be higher and the lines asymetrically lower. 

---