Characterization of nanoparticles in diluted clear solutions for Silicalite-1 zeolite synthesis using liquid 29Si NMR, SAXS and DLS

Abstract

Clear solutions for colloidal Silicalite-1 synthesis were prepared by reacting tetraethylorthosilicate in aqueous tetrapropylammonium hydroxide solution. A dilution series with water resulting in clear solutions with a TEOS ratio TPAOH ratio H2O molar ratio of 25 : 9 : 152 up to 25 : 9 : 15,000 was analysed using liquid 29Si nuclear magnetic resonance (NMR), synchrotron small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Particle sizes were derived independently from DLS and from the combination of SAXS and NMR. NMR allowed quantitative characterization of silicon distributed over nanoparticles and dissolved oligomeric silicate polyanions. In all samples studied, the majority of silicon (78-90%) was incorporated in the nanoparticle fraction. In concentrated suspensions, silicate oligomers were mostly double-ring species (D3R, D4R, D5R, D6R). Dilution with water caused their depolymerisation. Contrarily, the internal condensation and size of nanoparticles increased with increasing dilution. SAXS revealed a decrease of effective nanoparticle surface charge upon dilution, reducing the effective particle interactions. With DLS, the reduction of nanoparticle interactions could be confirmed monitoring the collective diffusion mode. The observed evolution of nanoparticle characteristics provides insight in the acceleration of the Silicalite-1 crystallization upon dilution, in view of different crystallization models proposed in the literature.

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

“…The limiting values extrapolated to pure water are 2300, 1230, and 820 µm 2¨s´1 respectively for water [67,68], ethanol [69] and TPA [70]. DLS measurements on the same system exhibited for nanoparticles diffusion coefficients D varying between 16 and 96 µm 2¨s´1 when x was varied from 152 to 8000 [62].…”

“…The pH of the studied sols varies from ca. 13.5 to 10.0 when x changes from 152 to 9500 [62]. The moderate changes in TPA chemical shifts could reflect the changes in chemical interaction, in particular with the nanoparticles, with increasing dilution.…”

“…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].…”

“…Indeed, viscosity has been found to drop rapidly from ca. 13 to 1 mPa.s when increasing water amount in these sols [62]. In highly viscous systems, translational motion of all observed species would involve cooperative movement of one or more neighboring solvent molecules.…”

A General Perspective of the Characterization and Quantification of Nanoparticles: Imaging, Spectroscopic, and Separation Techniques

“…SAXS has been employed to observe how the silica NPs interactions are decreased by the progressive dissolution of the tetrapropylammonium hydroxide functionalized surface (decreasing the surface charge when the NPs size decreases). 139 This technique is also useful to reveal core-shell structures, e.g., in polymer coated iron oxide nanocrystals due to the high difference in electron density between the core and the polymer shell. This technique is also used to follow the growth of thiol-covered AuNPs at different reaction times.…”