HIGH-RESOLUTION NITROGEN AND ARGON ADSORPTION ON ZSM-5 ZEOLITES - EFFECTS OF CATION-EXCHANGE AND SI/AL RATIO

ZSM-5 zeolites with Si/Al ratios of 23, 72 and >400 were synthesized in the laboratory as Na+ and cation (H+, Co2+, Cu2+, Fe2+ and Zn2+)-exchanged forms and were examined by nitrogen and argon porosimetry. Na-ZSM-5 of any Si/Al ratio and high-alumina ZSM-5 (Si/Al=23) zeolites of any exchanged cation display single transition adsorption isotherms (type I isotherm) for both nitrogen and argon. However, other cation-exchanged ZSM-5 zeolites of lower alumina contents display step-like isotherms, i.e, two transitions in adsorption, and the second transition occurs between a relative pressure (P/P-0) of 0.15 and 0.2 for nitrogen and between 1 . 10(-3) and 2 . 10(-3) for argon. Calculated micropore size distributions based on argon adsorption show that the Na+ form or high-alumina ZSM-5 samples have a single pore of 0.55 nm as expected, while the other ZSM-5 samples which have a second transition of adsorption display the expected pore size of 0.55 nm but also an apparent larger pore at 0.77 nm, a non-physical result. The calculated pore size of 0.55 nm for all the ZSM-5 represents its true micropore structure. Variations in the Si/Al ratio and exchanged cations have no effect on this calculated value.