Silicon-29 NMR Studies of Tetraalkylammonium Silicate Solutions. 2. Polymerization Kinetics

Abstract

The kinetics of formation of the silicate cubic octamer, Q(3)(8), in aqueous tetramethylammonium (TMA) silicate solutions was investigated by (29)Si NMR. The rate equation for solutions at pH 13.2-13.6 is d[Q(3)(8)]/dt = k(f) [H(+)](1.6)(+/-)(0.1)[TMA(+)](0.36)(+/-)(0.08)[Si](0.8)(+/-)(0.3) where k(f) = (2.2 +/- 0.8) x 10(16) mol(-)(1.8) kg(1.8) s(-)(1) at 296 K. The findings prove unequivocally that alkylammonium cations participate directly in the formation and subsequent stabilization of cagelike polysilicate anions. This implies a radically different mechanistic role than "templating" for alkylammonium cations in the synthesis of molecular sieves.

Modelling nucleation and nano-particle structures

“…For particles smaller than 2.8 nm, 29 Si NMR studies reveal some of the small structures present in aqueous silicate solutions [14,16,17]. The species found include, among others, 3-and 4-membered rings, double-4-rings and others clusters involving either 3-or 4-membered rings, or a combination of both.…”

NMR studies of biomineralisation

“…This polymerisation process has been studied extensively by 29 Si NMR spectroscopy. A variety of intermediate species such as polysilicic acid chains and rings have been identified [44,[51][52][53][54][55][56][57]. These species can easily be identified using liquid-state 29 Si NMR by means of their characteristic chemical shifts [58].…”

Nucleation and growth of zeolites and inorganic mesoporous solids: Molecular insights from magnetic resonance spectroscopy

“…The influence of tetraalkylammonium cations, which are widely used as structure-directing agents in the synthesis of zeolites with high Si/Al ratios, on the equilibria of silicate anions in solution has been studied in detail by solution-state 29 Si NMR [33,34]. The addition of tetraalkylammonium cations shifts the equilibria towards symmetric cage-like oligosilicate anions, in contrast to alkali-metal silicate solutions, where smaller asymmetric silicate anions tend to be found.…”