Direct Synthesis of Hybrid Organic−Inorganic Nanoporous Silica by a Neutral Amine Assembly Route: Structure−Function Control by Stoichiometric Incorporation of Organosiloxane Molecules

Abstract

The co-condensation of tetraethoxysilane (TEOS) and organotrialkoxysilane molecules by neutral alkylamine surfactant assembly (S 0 I 0 ) is a convenient synthesis methodology for the one-step preparation of organically functionalized mesostructured materials. Two distinct approaches were considered for the formation of such hybrid inorganic−organic compounds, denoted as the template substitution and direct addition pathways. The template substitution pathway, involving the replacement of both surfactant and TEOS by an equivalent amount of organosilane (up to 11% substitution), was convenient for the functionalization of mesostructures with moieties whose size is comparable to that of the structure-directing amine surfactant. The direct addition method, involving the equimolar replacement of TEOS with organosilane (up to 9% of total silicon) while keeping the surfactant amount constant in the reaction mixture, was successful for the incorporation of organic groups whose sizes were smaller than that of the templating amine surfactant. Thus, the functionalization of mesostructured silica was demonstrated using a wide range of organosilane species, including octyltriethoxysilane, mercaptopropyltrimethoxysilane, phenyltriethoxysilane, butyltrimethoxysilane, and propyltrimethoxysilane. Short-chain functional groups such as ethyltriethoxysilane, however, appeared to become occluded within the framework walls of HMS silica, resulting in a lowering of the framework order and the porosity of the hybrid materials. A sufficiently long carbon chain (at least three methylene units) in the functional group of the organosilane allows an interaction with the hydrophobic core of the structure-directing micelle, thereby permitting the successful incorporation of the organosilane into the pore walls of the mesostructure.