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 adsorption isotherms permitted a thorough characterization of the microstructure of the samples. A first estimation of the microporous volume was made by the Dubinin-Radushkevich method. The "t-plot" method was used to determine the external surface and to obtain another estimation of the microporous volume. Effective micropore size distribution was obtained with the Horvath-Kawazoe approach. The Dubinin-Radushkevich method overestimated the microporous volume of the zeolites, in particular when the solid had an important external 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 had lower microporous volumes and larger external surfaces. The external to internal (microporous) surface ratios, rs, and the mesoporous to microporous volume ratios, rv, are proposed as practical parameters to classify the solids with respect to their microstructure. ZSM-5, a pure microporous solid, has low values of both rs and rv parameters. Ni-and Co-ZSM-5 have high rs ad rv parameters, indicative of some degree of mesoporous character. Cu-ZSM-5 has intermediate characteristics, typical for micro-mesoporous solid. XPS measurements confirmed that copper ions penetrated to a greater extent in the channels of ZSM-5 zeolite than cobalt and nickel ions, which merely deposited on the external surface of ZSM-5 zeolite as oxidic aggregates.