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

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 29Si and 27Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). 29Si 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. 27Al 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 SiO44– and are located exclusively in the nanoparticles.