Investigation of Nanoparticles Occurring in the Colloidal Silicalite-1 Zeolite Crystallization Process Using Dissolution Experiments

Abstract

In concentrated clear sol prepared from tetraethylorthosilicate, tetrapropylammonium hydroxide, and water suitable for crystallization of Silicalite-1 zeolite, the main part of the silica is present in nanoparticles. The nature of these nanoparticles and their evolution during the induction period and the stage of early crystal growth was investigated via dissolution experiments in the presence of excess TPAOH. The dissolution process was monitored in situ using static and dynamic light scattering (SLS/DLS) and synchrotron small-angle X-ray scattering (SAXS). The complete dissolution of an individual nanoparticle was observed to occur in one step. Dissolution transformed a nanoparticle into a cluster of silicate oligomers. Larger grown nanoparticles dissolved slower. Exponential dissolution rate constants scaled inversely proportional with the volume of the nanoparticle's silica core. This experimentally observed dissolution behavior was modeled by assuming that a nanoparticle dissolved to oligomers via a series of partially dissolved nanoparticles that correspond to metastable intermediate states of increasing free energy. The resulting free energy barrier that has to be overcome by a dissolving nanoparticle could be derived from the experimental input. The idealized free energy profile provided a qualitative explanation for the apparent instantaneous disintegration of entire nanoparticles.

Zeolite Beta Formation from Clear Sols: Silicate Speciation, Particle Formation and Crystallization Monitored by Complementary Analysis Methods

“…At Si/OH ratios greater than 1, the NA population can be monitored from broad 29 Si NMR signals. Simultaneously, the concentration of oligomeric species decreases significantly as a result of their aggregation . Figure displays Q 0 to Q 4 site populations of either oligomers or NAs.…”

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

“…Typical molar composition space for silicalite-1 in the system SiO2-TPAOH-H2O occurring either in concentrated media (yellow area) as investigated by Corkery, van Santen, and Cundy et al [12,13,24,59], or dilute systems (blue area) as studied by Davis, Tsapatsis, and Shantz et al [3,22,27,28,41,42,50,51,60,61]. Both domains have also been extensively explored by Nikolakis, Lobo, and Martens et al [1,2,4,10,14,17,20,25,26,62].…”

“…Silicalite clear precursor sols are generally prepared from tetraethylorthosilicate (TEOS) hydrolysis and consist of a colloidal suspension of nanoparticles in water-ethanol mixture with many coexisting silicate oligomers [1][2][3][4][5][6][7][8]. The nature of the nanoparticles during zeolite formation by heating has been claimed as being an amorphous-like structure [9][10][11].…”

“…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]. Alternatively, at precursor formation before any heating, light diffusing nanoparticles have been demonstrated as aggregated oligomeric silicates, and not as dense silica particles [4].…”

Simple synthesis recipes of porous materials

“…At hydrothermal conditions, this process occurs on a timescale of a few hours compared to months at room temperature [12]. Concurrent with their growth, the silicate connectivity of the nanoparticle framework increases [18,[20][21] and the particles become increasingly resistant to dissolution in alkaline media [22][23]. Rimer et al suggested an evolution of nanoparticles towards zeolite structure paired with an increasing inclusion of template into the particles' cores [17].…”