Direct Synthesis of Hybrid Organic−Inorganic Nanoporous Silica Microspheres. 1. Effect of Temperature and Organosilane Loading on the Nano- and Micro-Structure of Mercaptopropyl-Functionalized MSU Silica

Abstract

The preparation of a series of organically functionalized wormhole-motif microspherical MSU-X silicas mesostructures was achieved by a direct synthesis process involving the addition of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) to mildly acidified solutions of a structure-directing nonionic surfactant (Igepal CA-720), followed by fluoride-mediated hydrolysis/cross-linking and surfactant extraction. The influence of organosilane content (MPTMS/TEOS ratio) and synthesis temperature on the framework order, pore structure, and particle size of the resulting mesostructures was investigated. The lattice spacings, pore volumes, and pore diameters were found to systematically increase as a function of increasing synthesis temperature. Although very poorly ordered and microporous when prepared at lower synthesis temperatures, the framework ordering and pore channel diameters of the microspheres dramatically improved when these were synthesized at temperatures above 40−45 °C. Moreover, increasing the synthesis temperature reduced the average particle sizes of the microspheres, although a sudden swelling of the particle sizes was observed in the threshold temperature zone where improved framework order and mesoporosity in the microspheres was evidenced. Incorporation of organic functional groups (MPTMS) inside the structures appeared to have little influence on the structure and morphology of the mesostructures, save for slight lattice and pore diameter contractions upon increasing organosilane group loading. By systematically varying the organosilane content and the synthesis temperature of the microspheres, functional MSU-X materials with accurately fine-tuned pore dimensions and particle diameters could therefore be obtained.