The potential role of amines in biological and biomimetic silica precipitation.

摘要

The aim of this study was to advance the understanding of amine-catalyzed biomimetic silica precipitation. Silica is produced in nature with a remarkable degree morphological control, under mild pH and temperature conditions, by organisms such as diatoms and sponges. The enzymes that act to precipitate silica contain amine-moieties, either as polylysine modifications to amino acids such as in diatoms, or as the histidine-containing active site of the protein found in some sponges. Consequently, for biomimetic purposes, the fundamental questions of interest were whether and how small monoamine and polyamine molecules influence the kinetics of reactions that are involved in silica precipitation, namely organosilicate hydrolysis and condensation.; Several monoamines and polyamines, representing a range of potential reactivities, were evaluated for their ability to catalyze organosilicate hydrolysis and condensation at mildly acidic (pH ∼6) conditions and room temperature. A 29Si NMR spectroscopic method (RINEPT+) was evaluated and employed in order to obtain quantitatively the effect of amines on reaction rates. A monofunctional organosilicate starting material, trimethylethoxysilane, was chosen as a model compound such that the reaction progress could be measured unambiguously. It was found that mildly acidic amines were more effective catalysts than basic amines. Variable temperature and pH kinetic studies, however, indicated faster rates at pH 6.; The possible hydrolysis mechanisms were evaluated by combining experimental results with ab initio quantum mechanical calculations. Two different mechanisms were demonstrated as potential pathways for the reaction. At pH pKa, the pathway involves proton transfer followed by SN2 substitution. The alternative mechanism, at pH > pKa, involves direct coordination of the amine to silicon, resulting in hydrolysis via a pentacoordinate intermediate and a hexacoordinate transition structure, and is named "nucleophlie-assisted nucleophilic substitution". The particular pathway taken depends on the ambient pH of the system, the nature of the organosilicate, and the amine hybridization.; Finally, the effect of amine-catalysts in influencing silica morphology produced via sol-gel synthesis from a tetrafunctional organosilicate starting compound (TEOS) was examined qualitatively using SEM. Particle size decreased with increasing amine acidity, as anticipated from the quantitative NMR results for model TMES hydrolysis and condensation rates.