Incorporation and stability of iron in molecular-sieve structures. Ferrisilicate analogues of zeolite ZSM-5

The ferrisilicate molecular sieve analogues of zeolite ZSM-5 can be prepared from ferrisilicate gels containing low-molecular-weight silica species. Precipitation of rust-red iron hydroxide at the high pH necessary for crystallization and complex formation of the iron with the amine impurities present in the crystal-directing agent are avoided by titrating the basic silica solution into the acidic iron-containing solution followed by introduction of the template. Pure white crystalline materials have been prepared in this manner over an SiO2/Fe2O3 range of 15 to 200. The presence of homogeneously dispersed iron(III) in the framework of the crystals has been confirmed. The stability of the framework iron under thermal and hydrothermal conditions is lower than the corresponding aluminosilicate.