THE ROLE OF POLYVALENT CATIONS IN DEVELOPING STRONG ACIDITY - A STUDY OF LANTHANUM-EXCHANGED ZEOLITES

A series of faujasite and ZSM-20 zeolites was prepared such that they had a minimum amount of extraframework aluminum, but a range of Si/Al ratios. These materials were relatively inactive for the hexane cracking reaction, which was used as a test of strong acidity. When La3+ ions had been exchanged into the zeolites, the cracking activity first increased with respect to lanthanum loading and then went through a maximum. When the maxima were plotted as a function of the framework aluminum content, Alf, for the series of zeolites, the activities increased until Alf was ca. 35 per u.c. and then decreased below a measurable level with zeolite X. The change in activity with respect to Alf paralleled that observed with steam-dealuminated zeolites, but the absolute activities of the La-exchanged samples were less. These results are consistent with a model of strong acidity in which both the Alf distribution and the presence of polyvalent cations in the β /OHM,n+ cages are important. Lanthanum ions in the form of {A figure is presented} or La(OH)2+ species are believed to be responsible for the withdrawal of electrons from the framework hydroxyl groups, thus making the protons more acidic. © 1990.