Synthesis and properties of hybrid silicas containing alkylammonium surfactants

This work describes the synthesis of silicas containing cetyltrialkylammonium and alkyltripropylammonium (CnPr3Br, n = 14-22) surfactants in their mesopores. The aqueous dispersions of these surfactants were characterized by small-angle X-ray scattering (SAXS). The hybrid silicas synthesized from the dispersions were evaluated by X-ray diffractometry (XRD), thermogravimetric analysis, nitrogen physisorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), as well as in terms of their catalytic properties. The mesoporous silicas synthesized using cetyltrialkylammonium surfactants showed increases in pore diameter and decreases in structural order as the head group of the surfactant became larger. An increase of the alkyltripropylammonium tail also resulted in silicas with larger pore diameters, but with better structural order. The mesoporous silica with the largest pore diameter (3.60 nm), which was synthesized with C22Pr3Br surfactant, exhibited a narrower pore diameter distribution and a hexagonal organization characteristic of MCM-41 silica. The catalytic properties of the hybrid silicas synthesized using the cetyltrialkylammonium surfactants were evaluated in the transesterification reaction, in which the silica containing C16Et3+ cations showed the highest catalytic performance.