NMR and SAXS Analysis of Connectivity of Aluminum and Silicon Atoms in the Clear Sol Precursor of SSZ-13 Zeolite

Abstract

We report the first study of the hydrolysis of tetraethyl orthosilicate (TEOS) in an aqueous solution of N,N,N-trimethyl-1-adamantammonium (TMAda) hydroxide, the clear sol precursor for the preparation of the high-silica zeolite SSZ-13 (CHA). The initial stages of the hydrolysis of TEOS were monitored by quantitative 29 Si and 27 Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). 29 Si NMR allowed quantitative characterization of Si in nanoparticles and dissolved oligomers and measuring the average Si−O−Si connectivity. The average Si connectivity increases when hydrolysis advances, and at a [Si]/[TMAdaOH] ratio of one, nanoparticles are detected. The average connectivity of nanoparticles reached 3.1. This is similar to what has been observed during TEOS hydrolysis with other organic bases, i.e., tetrapropylammonim hydroxide (TPAOH) and tetrabutylammonium hydroxide (TBAOH) used for silicalite-1 and silicalite-2 syntheses, confirming that it is a general phenomenon independent of the structure of the organocation. 27 Al NMR shows that the connectivity of Al increases as well with increasing [Si]/[TMAdaOH] ratio. Aluminum atoms are in tetrahedral coordination to four silicate units SiO 4 4− and are located exclusively in the nanoparticles.

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

“…If the Si/OH ratio exceeds 1, a critical aggregation concentration is reached and oligomers aggregate into micellar hydrated ion pairs, designated as nanoaggregates (NAs). Historically, such NAs have often been called primary nanoparticles . Upon thermal treatment, the NAs evolve progressively into secondary nanoparticles (sNPs) with a slightly higher degree of internal condensation and sizes up to 4–6 nm.…”

“…These larger, amorphous, but topologically highly structured objects are usually called tertiary nanoparticles (tNPs). Nucleation typically starts from within these tNPs, forming crystallites growing at the expense of the noncrystalline part of the tNPs …”

“…The aim of the present contribution is to study the early stages of the formation of zeolite beta from clear sols using TEAOH as the SDA in the presence or absence of NAs in the reaction mixture by controlling the initial Si/OH ratio. Although the mechanism of zeolite beta crystallization has been the subject of several studies in the past, the current contribution uses a combination of methods to register species in precursor solutions/sols. In addition, the evolution of these objects during crystallization under hydrothermal conditions is monitored.…”

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

“…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].…”

Nucleation in complex multi-component and multi-phase systems: general discussion

“…[1][2][3] Signicant scientic progress has been made in the understanding of zeolite formation from precursor clear solutions with organic SDA, a system which contributed signicantly to the elucidation of the molecular aspects of this type of zeolite assembly process. [10][11][12] These suspended primary nanoparticles measuring 1-2 nm have been characterized using 29 Si NMR and scattering techniques such as SAXS and DLS. These optically transparent solutions are typically prepared by hydrolysis of a monomeric silicon source such as tetraethylorthosilicate (TEOS) in the presence of tetrapropylammonium hydroxide (TPAOH).…”