DEALUMINATION OF LARGE CRYSTALS OF ZEOLITE ZSM-5 BY VARIOUS METHODS

Large crystals of ZSM-5 have been dealuminated by several methods in order to compare the effectiveness of the various treatments and their influence on the crystal structure. Any acidic treatment always leads to a partial dealumination of the aluminosilicate framework. The extracted aluminium cannot be completely removed from the crystals. Some of it remains inside the pores, presumably as aloxo compounds. The largest fraction of aluminium can be removed from the crystals of ZSM-5 by leaching with an acid followed by steaming and a subsequent final acid leaching. The steaming treatment apparently causes a re-ordering within the crystal structure by removing the framework vacancies by the T-jump mechanism. Ion exchange of Al3+ by using aqueous solutions of Al salts is impossible.