Abstract

The desilicated forms of the Na-X, Na-Y, the ammonium and sodium forms of the ZSM-5 zeolite were prepared by a process which involves the selective removal of silicon atoms from the parent zeolite in an aqueous basic solution. This treatment has the effect to decrease the Si/Al ratio while keeping the zeolite structure almost unmodified. In all cases, we noticed an enhancement of the ion-exchange capacity. This property could be exploited industrially since the desilicated forms have higher total ion removal rates of calcium and magnesium ions from a solution of hard water. As for two ZSM-5 zeolites, having quite different Si/Al ratio, the preparation of highly thermally stable zeolite materials with smaller micropores at 4.9 A or larger at 5.5 A than those of the parent ZSM-5 zeolites was possible. These final materials were obtained by a process which involved the following additional steps: (i) reinsertion into the desilicated ZSM-5 zeolite of a well defined amount of silicon species (sodium orthosilicate and pyrosilicate) which have been selectively removed from the zeolitic framework during the desilication process, and then (ii) activation of the resulting materials in air at high temperatures (550$\sp\circ$C). The amount of the reinserted silicon containing species depends on the pore size wanted. The zeolite materials so obtained still have quite high surface areas and pore volumes. This method has the great advantage of being very reproducible, giving us a micropore size which is much smaller or much larger by $\pm$0.4A than the size of the parent zeolite with a quite homogeneous micropore distribution, and the latter characteristic must result in selectivity in catalysis or adsorption.