Controlling the spatial distribution of aluminum in ZSM-5 crystals

The spatial distribution of aluminum over ZSM-5 crystals was systematically studied using electron microprobing on polished crystals. Crystals synthesized with TPABr as template exhibit a pronounced enrichment of aluminum in the crystal rim, essentially irrespective of the aluminum source employed, although aluminum sources with organic anions are favoring less inhomogeneous profiles. With 1,6-hexanediol or from totally inorganic reaction gels, crystals with completely homogeneous aluminum profiles are obtained, even if the crystals grow larger than 50 μm. In the 1,6-hexanediol system the homogeneous profiles could be changed to profiles similar to the TPABr system by addition of KNO3. The results are explained with the competition between the different cations for silicate and alumosilicate species. As long as alkali ions are present in the synthesis gel, they interact favorably with the alumosilicate species. In TPA-based reactions, the TPA primarily induces the structure, incorporating predominantly silicate species, because the alumosilicate species are blocked by alkali ions. In the absence of TPA, i.e., in hexanediol-based syntheses or in inorganic reaction gels, the structure is induced by Na+ ions which also interact favorably with the alumosilicate species, thus incorporating them into the structure already at a very early stage of the synthesis.