Model for the Synthesis of Self-Assembling Template-Free Porous Organosilicas

High surface area solids are important materials in science and in many industrial applications but often are produced from expensive and inefficient combinations of materials and processes. New principles for the selection of molecular precursors that yield high surface area solids in simple and efficient sol–gel processes would be useful. Focusing on organosilicas, we show that an index based on rigidity theory can be used to quantify the relative strength of the gel and the level of condensation at which it is able to withstand the capillary stresses imposed by drying, thereby preventing loss of surface area. This index correctly orders precursors according to the surface area of the solid materials produced from them and provides, when correlated to a few data points, a predictive relationship between the index and the surface area. Precursor features leading to early formation of a highly connected rigid network include high ratios of nonhydrolyzing (e.g., methylene) to hydrolyzing (e.g., oxy) groups bridging silicate moieties, large SiOH/Si ratios in the hydrolyzed precursors, and low numbers of noncondensing terminal groups (e.g., methyl). These features explain the extremely high surface areas obtained from 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane and high surface areas obtained by similar materials in aqueous, nontemplated syntheses, as shown in a related publication (DOI: 10.1021/acs.chemmater.7b04480).