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 sign ificant 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 s maller than those of the parent zeolite, are formed during desilicatio n at the expense of the original micropores. Such a desilicated zeolit e shows an ion-exchange activity (with Ca2+) much higher than that of the parent zeolite, owing to the increase of the density of Al tetrahe dral sites. Activation of the desilicated zeolite at a temperature hig her than 300 degrees C results in the enlargement of the newly formed micropores. The final system is quite homogeneous with only one type o f micropore, ca. 0.56 nm in diameter. This is probably at the origin o f further improvements in the ion-exchange performance. There is no si gnificant change of the ZSM-5 zeolite structure in this method of seco ndary synthesis: Thus, the function of the thermal treatment is to sta bilize the framework and to homogenize the pore network of the desilic ated zeolite. (C) Elsevier Science Inc. 1997.