The selective dissolution of rice hull ash to form [OSiO1.5]8[R4N]8 (R = Me, CH2CH2OH) octasilicates. Basic nanobuilding blocks and possible models of intermediates formed during biosilicification processes

Abstract

Rice hull ash (RHA) silica (80–98 wt.% amorphous, >25 m² g⁻¹ silica, 2–20% porous amorphous C) can be depolymerized in aqueous alcohol with [NR₄]₈OH (R = Me, CH₂CH₂OH) under ambient conditions with the selective formation of octasilicate anions, [NR₄]₈[OSiO_1.5]₈. Dissolution kinetics were studied as a function of base and water concentration and temperature. Dissolution rates were determined by conversion of the octaanion to [HMe₂OSiO_1.5]₈, OHS. Activation energies for dissolution were 5 ± 1 kcal mol⁻¹, much lower than for typical base-promoted silica dissolution. Furthermore, dissolution was not catalytic in base as found for other base-promoted silica dissolution reactions. Reaction rates were dependent on ammonium base and water concentrations and temperature. Dissolution was optimal at approximately one equivalent of [NR₄]₈OH and three equivalents of water for choline hydroxide and five equivalents of water for [NMe₄]₈OH. A single crystal study of the octacholine [Me₃NCH₂CH₂OH]₈ derivative indicates that the compound crystallizes with three equivalents of water per SiO_1.5 suggesting that the rate limiting step in the dissolution process may be formation of the octaanion which is in accord with theoretical E_a’s calculated for the condensation of polyhydroxyl siloxanes. Octasilicate anions offer access to novel polyfunctional silsesquioxane platforms with each functional group occupying a single octant in Cartesian space. These platforms offer potential as precursors to dendrimers and hyperbranched polymers, and as nanobuilding blocks for the formation of nanocomposites. Furthermore, choline is structurally similar to: (1) ε-N,N,N-trimethyl-γ-hydroxy-lysine, (2) the oligomeric N-methylpropylamine components found in silafins, and (3) possibly the hydroxyamino acid units found in sponge filament proteins; all of which are thought to play a role in silica accretion, transport and deposition in diatoms and sponges. Thus, the octasilicate structure may reflect the structure(s) of species involved in silica transport and/or deposition in biosilicification processes.