The hydrothermal synthesis of zeolites: precursors, intermediates and reaction mechanism

An account is presented of the mechanistic aspects of hydrothermal zeolite synthesis. The introduction provides a historical and experimental perspective and is followed by a summary of proposed mechanisms and associated modelling studies. The central section of the review contains a description of the most probable mechanistic pathways in zeolite formation. In this, the reaction stages of the induction period, nucleation and crystal growth are examined in chronological sequence. Finally, particular aspects of the synthesis process such as the constitution of growth species, template–framework interactions and the nature of zeolite solubility are treated in more detail. Emphasis is placed upon the chemical basis of zeolite synthesis. Fundamental to this are the TOT bond-making and bond-breaking reactions which establish the equilibration between solid and solution components. The consequent generation of order, driven by energy differences and strongly moderated by kinetic limitations, is essentially one of continuous evolution. However, the discreet step of nucleation provides a discontinuity in which isolated regions of local order are superceded by the establishment of a periodic crystal lattice, capable of propagation. Crystal growth occurs through an in-situ, localised construction process from small, mobile species ordered by the participating cations. The process of hydrothermal zeolite synthesis can be most adequately explained by a mechanism based upon the solution–mediation model, whether or not there is a visible liquid phase. The common presence of mobile species emphasises the overall similarity of zeolite synthesis reactions so that the need to distinguish any separate “gel rearrangement” or “solid-phase transformation” mechanism becomes unnecessary.