Characterization of the textural properties of metal loaded ZSM-5 zeolites

The textural properties of metal ion over-exchanged zeolites, Co-, Cu-, Ni- ZSM-5, were compared with those of the parent ZSM-5 by nitrogen adsorption measurements and photoelectron spectroscopy. The treatment of the adsorptio n isotherms permitted a thorough characterization of the microstructure of the samples. A first estimation of the microporous volume was made by the D ubinin-Radushkevich method. The "t-plot" method was used to determine the e xternal surface and to obtain another estimation of the microporous volume. Effective micropore size distribution was obtained with the Horvath-Kawazo e approach. The Dubinin-Radushkevich method overestimated the microporous v olume of the zeolites, in particular when the solid had an important extern al surface. It is shown that Cu-ZSM-5 had micropore size and micropore size distribution very close to those of ZSM-5, whereas Co-ZSM-5 and Ni-ZSM-5 h ad lower microporous volumes and larger external surfaces. The external to internal (microporous) surface ratios, r(s), and the mesoporous to micropor ous volume ratios, r(v), are proposed as practical parameters to classify t he solids with respect to their microstructure. ZSM-5, a pure microporous s olid, has low values of both r(s) and r(v) parameters. Ni- and Co-ZSM-5 hav e high r(s) ad r(v) parameters, indicative of some degree of mesoporous cha racter. Cu-ZSM-5 has intermediate characteristics, typical for micro-mesopo rous solid. XPS measurements confirmed that copper ions penetrated to a gre ater extent in the channels of ZSM-5 zeolite than cobalt and nickel ions, w hich merely deposited on the external surface of ZSM-5 zeolite as oxidic ag gregates. (C) 1999 Elsevier Science B.V. All rights reserved.