The First Stage of Microporous Silicate Growth:
Characterization of the Silica Nanoparticle Precursor and Analysis of its Role in the Growth Mechanism
Joseph Fedeyko, Jeffrey D. Rimer, Dionisios G. Vlachos, Raul F. Lobo
Department of Chemical Engineering, Center for Catalytic Science and Technology,
University of Delaware, 150 Academy St., Newark, DE 19716, USA.
Microporous silicates (i.e. zeolites) are synthesized through a molecular self-assembly
process involving the organization of a silica framework around a structure-directing agent.
Silicalite-1 is the most investigated zeolite synthesis, yet much remains to be learned to develop
growth models capable of predicting both the morphology and surface properties of zeolites for
novel applications in nanotechnology, such as thin-film membranes. A unique aspect of the
growth is the formation of stable silica nanoparticles (2–5 nm), which exist not only at room
temperature but throughout the growth of silicalite-1 (70–150 oC). We have discovered that
these subcolloidal silica particles are in fact a new class of self-assembled entities akin to the
aggregates that are formed with classical surfactants, i.e., micelles.
Using tetrapropylammonium hydroxide (TPAOH) as our working system, we have
investigated the formation of the silica nanoparticles at room temperature using conductance, pH
and x-ray and neutron small angle scattering (SAXS and SANS). We have found both in
conductance and pH studies a critical point (near SiO2:TPA+ of 1:1) above which nanoparticles
are observed, similar to the critical micelle concentration (CMC) in surfactant systems. SANS
and SAXS measurements reveal that the particle size and morphology is very uniform. Analysis
of the SAXS pattern measures smaller particles than the particles observed using SANS, which is
evidence that these particles have a core of mostly silica covered by a shell of TPA. We will
present further investigations of the formation and structure of these particles using various TAA
and alkali metal cations to understand the driving forces for nanoparticle self-assembly.
The presence of nanoparticles during the growth has led to competing theories for the growth
mechanism: growth by direct nanoparticle addition to a zeolite surface or nanoparticle
dissolution followed by the addition of smaller silica species (i.e. monomers and oligomers). We
have characterized the growth as a function of pH, temperature, and both electrolyte and silica
concentration using seeded growth experiments in which the particle size is measured as a
function of time by dynamic light scattering. From these studies we have extracted activation
energies and growth rates, which were compared to a proposed growth model based on DLVO
theory. Recent experiments using SAXS have shown that the morphology and size of the
nanoparticles significantly change with pH and temperature. We will present both experimental
and theoretical analyses for the evolution of nanoparticles during zeolite growth – the results of
which will be used to further investigate the growth mechanism.
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