Tailoring the framework and textural mesopores of HMS molecular sieves through an electrically neutral (S degrees I degrees) assembly pathway

Water:ethanol solvent mixtures of differing polarity have been used to tailor the framework and textural mesopores of HMS molecular sieve silicas through an electrically neutral ((SIO)-I-O) assembly pathway (S-O = dodecyl- or tetradecylamine; I-O = tetraethyl orthosilicate). Mesostructure assembly from a water-rich solvent mixture, water:ethanol = 90:10 (v/v), afforded wormhole-like framework structures with a complementary textural pore volume equal in magnitude to the framework pore volume. An ethanol-rich mixture, water:ethanol = 35:65 (v/v), also formed wormhole-like frameworks, but the textural porosity was less than 20% of the framework pore volume. HMS derivatives with high textural porosity were composed of mesoscale fundamental particles that aggregate into larger particles. In contrast, HMS mesostructures with low textural porosity were assembled into much larger aggregates of macroscale spheroid to disk-shaped fundamental particles. The differences in particle textures were attributed to differences in I-O hydrolysis rates and (SIO)-I-O nucleation and growth rates in the two solvent systems. The presence of mesitylene in the reaction mixtures resulted in an expansion of the framework pores under water-rich conditions. Pore contraction, however, was observed with mesitylene present under ethanol-rich conditions. This versatile structure-modifying property of mesitylene in (SIO)-I-O assembly is explained by the solvent-dependent binding of the aromatic molecules to two structurally distinct and size-altering ''dissolved'' and ''adsorbed'' states at the centers and interfacial surfaces of the surfactant micelle, respectively. Thus, both the framework and the textural pores of HMS silica can be readily tailored to the needs of a particular materials application through (SIO)-I-O assembly by a judicious choice of an appropriate solvent and an auxiliary structure modifier.