Understanding the crystallisation of zinc phases from ion-exchanged zeolite precursors

Combined QEXAFS/XRD measurements have been employed to study in situ the evolution of crystalline oxide phases from zinc ion-exchanged zeolite precursors. All of the ion-exchanged zeolites were observed to first undergo dehydration, followed by amorphisation before a number of condensed oxide phases crystallized. Of the three zeolite structures studied, (with the exception of zeolite Y), the principle oxide phase to form on reaching 1000 °C was a silicate phase, Zn2SiO4, and the extent to which it formed appeared to depend on the amount of Si4+ present in the initial zeolite structure. However the formation of this phase was preceded by the crystallisation of a number of different oxidic and hydroxidic phases. For the zeolite Y sample only ZnAl2O4 crystallised at temperatures in excess of 850 °C. This unusual ability of zeolite Y to yield phase pure ZnAl2O4, despite possessing a similar Si/Al ratio to that of two other zeolite structures, may be related to the tendency of the Zn2+ ions to locate close to two Al3+ ions (as charge compensating cations) inside the porous framework thereby ensuring the correct cation stoichiometry to enable the successful crystallisation of the spinel phase. In contrast for the zeolites which lose their framework structure, random distribution of Zn2+ ions, away from Al3+ occurs, which allows the formation of other phases such as Zn2SiO4.