Optimizing Organic Functionality in Mesostructured Silica:  Direct Assembly of Mercaptopropyl Groups in Wormhole Framework Structures

A series of mercaptopropyl-functionalized wormhole mesostructures, denoted MP-HMS, have been prepared through the S 0 I 0 assembly of alkylamine surfactants (S 0 ) and mixtures of 3-mercaptopropyltrimethoxysilane and tetraethyl orthosilicate as framework precursors (I 0 ). Unprecedented levels of organo functionalization, corresponding to at least 50% of the silicon sites, were achieved while retaining well-expressed mesostructures with pore sizes, pore volumes, and surface areas as high as 2.8 nm, 0.69 cm 3 /g, and 1225 m 2 /g, respectively. Also, up to ∼90% of the framework silicon sites could be fully cross-linked, lending exceptional hydrothermal stability to the mesostructures. The key to highly functionalized MP-HMS derivatives lies in the use of long-chain alkylamine surfactants as structure directors (e.g., octadecylamine) in combination with a relatively high assembly temperature (e.g., 65 °C) and a high-polarity water−ethanol solvent. Increasing the assembly temperature increased both the framework pore size and the degree of framework cross-linking, whereas increasing the MP content lowered the pore size while improving the framework cross-linking. The effects of assembly temperature and MP loading were attributable to changes in hydration at the H-bonded S 0 I 0 interface and concomitant changes in the surfactant packing parameter. Highly functionalized MP-HMS derivatives are promising materials for use as heavy-metal ion-trapping agents and as precursors for sulfonic-acid-functionalized mesostructured catalysts.