Spontaneous Formation of Silica Nanoparticles in Basic Solutions of Small Tetraalkylammonium Cations

Abstract

We have found that the spontaneous formation of silica nanoparticles is a general phenomenon in basic solutions of small tetraalkylammonium (TAA) cations. The nanoparticle formation and structure have been investigated using conductivity, pH, and small-angle scattering methods. The particles have a core−shell structure with silica at the core and the TAA cations at the shell. The particle core size is nearly independent of the size of the TAA cation but decreases with pH, suggesting electrostatic forces are a key element controlling their size and stability. The nanoparticle formation is a reversible process at low temperatures, in several ways similar to surfactant aggregation into micelles. These silica nanparticles may be a connection between the synthesis of zeolites and ordered mesoporous silicas such as MCM-41.

Inorganic molecular sieves: Preparation, modification and industrial application in catalytic processes

“…Researchers from Johnson Matthey Inc. presented a recent surface adsorption FTIR spectroscopy study on the differences in transient response and the effect of NO:NO2 ratio on Fe or Cu exchanged zeolites in the SCR of NOx with urea [367]. Still, the main drawback concerning the industrial use of zeolite based SCR catalyst is their long term hydrothermal stability [368][369][370][371].…”

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

“…First TEOS was hydrolyzed in 40 wt % TPAOH aqueous solution at ambient conditions under vigorous magnetic stirring for approximately 1 h. Appropriate amounts of water were then added subsequently when it was necessary. The compositions were chosen to be in the range of those used in silica precursor particle studies [14,25,61] and silicalite-1 growth experiments [59]. All samples were allowed to equilibrate for a minimum of three days before NMR measurement.…”

“…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].…”

“…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].…”

Internal structure of a gel leading to NBA-ZSM-5 single crystals

“…It is noteworthy that the presence of an amorphous phase in the form of colloidal nanoparticles is observed during the synthesis of silicalite-1, the aluminum-free version of ZSM-5, suggesting that the presence 1 3 of a gel precursor is also required as an intermediate stage for crystallization from clear solutions [6]. These primary silica particles that act as precursor species for crystallization [7] are amorphous and in the range of 2-6 nm [8][9][10][11]. The primary particles evolve to larger particles in the form of aggregates, in which nucleation and subsequent growth take place [12][13][14].…”