The
crystallization of zeolites, a disorder-to-order transformation
of aluminosilicates, has not been thoroughly understood because the
nucleation events in the amorphous matrix are difficult to recognize
from the diverse structural changes, especially for the dense hydrogel
systems. Therefore, relationships between the synthesis conditions,
the generated amorphous species, and the crystallization behavior
of zeolites remain unclear. Herein, by comparatively investigating
the structural evolution of the aluminosilicate matrix in a dense
hydrogel system when different Si reactants (fumed silica and silicate
solution) are employed, we demonstrate that the reactivity of the
reactants and the kinetics of the condensation reaction is critical
to the formation of short-range order in an amorphous matrix, which
greatly influences the nucleation frequency of zeolites. It was revealed
that an amorphous solid containing plentiful Al-rich four-membered
rings and Si-rich six-membered rings could be produced when fumed
silica gradually reacted with sodium aluminate solution at 80 °C.
It is considered that the interaction between these rings promotes
the construction of the essential building units of zeolite X (FAU). In contrast, a complex aluminosilicate matrix was formed
immediately when sodium silicate solution was mixed with sodium aluminate
solution due to the intense condensation reaction. Furthermore, this
complex matrix became more stable when the reactant mixture was hydrothermally
treated at 80 °C, which significantly impedes the crystallization
process. Aging the reactant mixture at ambient temperature before
heating, instead, facilitated the formation of short-range order in
the amorphous matrix, which increases the nucleation frequency of
zeolites.