Modeling Spontaneous Formation of Precursor Nanoparticles in Clear-Solution Zeolite Synthesis

Abstract

Abstract:We present a lattice model describing the formation of silica nanoparticles in the early stages of the clear-solution templated synthesis of silicalite-1 zeolite. Silica condensation/hydrolysis is modeled by a nearest-neighbor attraction, while the electrostatics are represented by an orientation-dependent, shortrange interaction. Using this simplified model, we show excellent qualitative agreement with published experimental observations. The nanoparticles are identified as a metastable state, stabilized by electrostatic interactions between the negatively charged silica surface and a layer of organic cations. Nanoparticle size is controlled mainly by the solution pH, through nanoparticle surface charge. The size and concentration of the charge-balancing cation are found to have a negligible effect on nanoparticle size. Increasing the temperature allows for further particle growth by Ostwald ripening. We suggest that this mechanism may play a role in the growth of zeolite crystals.

Investigation of the Mechanism of Colloidal Silicalite‐1 Crystallization by Using DLS, SAXS, and 29Si NMR Spectroscopy

“…By considering the nanoparticles to be stabilized by electrostatic interactions at the negatively charged silica surface, an additional layer of organic cations of 1 nm is expected because the ionic radius of a TPA cation is approximately 0.45 nm. [33] From the DLS experiments discussed above, the nanoparticles grew to such extent, that is, from 2.4 to 6 nm. This rough simulation suggests that the 6-nm particles have a fully condensed internal silicate network terminated with Q 3 silicon atoms.…”

Hierarchical Nanomanufacturing: From Shaped Zeolite Nanoparticles to High‐Performance Separation Membranes

“…Such differences between fresh and aged precursor nanoparticles was confirmed by in situ AFM studies [175] that revealed qualitative differences in their affinity for mica surfaces as a function of particle ageing. These AFM studies, coupled with the delay in crystallization, further underscore the likely metastability of the precursor nanoparticles, [191] with transformations akin to those observed for metal hydroxide systems studied by Buyanov et al [163,164] In regards to this evolution of the particles, it is necessary to reconsider the stability of nanoparticles. It was proposed that the stability of fresh precursor nanoparticles derives from the surface adsorption of TPA molecules [167] and the higher apparent surface potential of the nanoparticles relative to TPA-silicalite-1.…”

Tetrapropylammonium Occlusion in Nanoaggregates of Precursor of Silicalite-1 Zeolite Studied by 1H and 13C NMR

“…However, the very early stages of zeolite formation lies in the pre-heating stage, at precursor formation. Since their discovery in the mid 1990s many controversial studies have attempted to derive the exact status of the very early nanoparticles of small size in the range a few nanometers [2,3,[12][13][14][15][16][17][18][19][20][21]. The question of whether these nanoparticles contain occluded organic templates or are simply organic-free silica entities has been disputed [9] and a core-shell structure with a negatively charged surface silica core surrounded by a shell of organocations, might seem to be the most accepted description [1,2,10,15,[22][23][24][25][26][27][28][29][30].…”