Synthesis and characterization of zsm-5 type zeolites: III. A critical evaluation of the role of alkali and ammonium cations

Abstract

Various techniques were used to investigate the role of alkali and ammonium cations in governing nucleation and growth processes of (M I)ZSM-5 zeolites formed within the (Na 2O, M 2O)-(Pr 4N) 2O-A1 2O 3-SiO 2-H 2O synthesis mixtures (M I = Li, Na, NH 4, K, Rb, Cs). Morphology, size, chemical composition and homogeneity of the(M I) ZSM-5 crystallites depend on the competitive interaction between Pr 4N + or alkali cationic species and aluminosilicate polymeric anions at the early stages of the nucleation process. The latter, in turn, is strongly affected by intrinsic properties of the alkali cations such as their size, their structure-forming or structure-breaking role towards water and their salting-out power. Structure-breaking cations such as K +, Rb + or Cs + favour the formation of large (15–25 μm) single crystals or twins. In the presence of structure-forming cations (Li +,Na +) a rapid nucleation yields Si-rich crystallites homogeneously distributed within the 5–15 μm range. Those are coated with numerous small (1 μm) Al-richer crystallites formed by a secondary nucleation process from the Si-deficient gel. In an cases, combined PIGE (bulk) and EDX (outer shell) analyses of Si and Al reveal that Al is homogeneously distributed within the individual crystallites and that Si/Al ratio increases with the particle size. As a result, K, Rb and Cs ZSM-5 zeolites appear homogeneous in composition while Li and Na polycrystalline aggregates show an apparent Al-enriched outer rim. In presence of NH 4 + ions, large single crystals of ZSM-5 having an Al-deficient core and an Al-rich outer shell, as well as small Si-rich crystallites stemming from a delayed nucleation process, are formed. This particular role of NH 4 + is explained in terms of its preferential interaction with aluminate rather than with silicate anions, during the nucleation stage. Our various findings suggest that both solution ion transportation and gel phase transformations (surface nuc1eation) mechanisms can govern simultaneously the nucleation and growth of ZSM-5 zeolites.