Modification of the micropore characteristics of the desilicated ZSM-5 zeolite by thermal treatment

Controlled desilication of ZSM-5 zeolite, using an aqueous solution of 0.8 M sodium carbonate and 0.01 M sodium hydroxide, results in a significant decrease of the Si/Al atomic ratio, with sodium orthosilicate and its dimer, sodium pyrosilicate, being selectively removed from the zeolite framework. Micropores of 0.49 nm, i.e., those significantly smaller than those of the parent zeolite, are formed during desilication at the expense of the original micropores. Such a desilicated zeolite shows an ion-exchange activity (with Ca2+) much higher than that of the parent zeolite, owing to the increase of the density of Al tetrahedral sites. Activation of the desilicated zeolite at a temperature higher than 300 degrees C results in the enlargement of the newly formed micropores. The final system is quite homogeneous with only one type of micropore, ca. 0.56 nm in diameter. This is probably at the origin of further improvements in the ion-exchange performance. There is no significant change of the ZSM-5 zeolite structure in this method of secondary synthesis: Thus, the function of the thermal treatment is to stabilize the framework and to homogenize the pore network of the desilicated zeolite. (C) Elsevier Science Inc. 1997.