High-resolution nitrogen and argon adsorption on ZSM-5 zeolites: effects of cation exchange and [formula omitted] ratio

Abstract

ZSM-5 zeolites with Si Al ratios of 23, 72 and >400 were synthesized in the laboratory as Na + and cation (H +, Co 2+, Cu 2+, Fe 2+ and Zn 2+)-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 o ) 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 excepted 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.