Monolithic Nanostructured Silicate Family Templated by Lyotropic Liquid-Crystalline Nonionic Surfactant Mesophases

Abstract

A family of well-defined highly ordered mesoporous silica materials (designated as HOM) was synthesized by using a high concentration of nonionic amphiphile of Brij 56 surfactant (C18EO10) as a structure-directing species. Monolithic nanostructures with regular arrays and extended periodicity were produced under acidic conditions and different ambient temperatures (25−45 °C). Liquid-crystal phases formed in aqueous/silica domains played an important role in controlling the monolithic structural morphology. The direct templating method enhanced the phase topology of Brij 56 with three-dimensional (3-D) accessible mesoporous silica of primitive-centered cubic Pn3m (HOM-7), 3-D hexagonal P63/mmc (HOM-3), and solid phase (S) with cubic Ia3d space group (HOM-5) mesophases. By adjustment of the phase behavior of Brij 56/TMOS mass ratio, ca. 35, 50, 70, and 75 wt %, the method yielded predictable mesophase structures of cubic spherical micellar Im3m (HOM-1), 2-D hexagonal P6mm (HOM-2), bicontinuous cubic Ia3d (HOM-5), and lamellar L∞ (HOM-6), respectively. A significant result was that the long-range ordered silica monoliths remained unchanged throughout the fast condensation of TMOS and removal of Brij 56 surfactant. Furthermore, all of the silica monoliths (HOM-types) had high surface area, uniform mesopore channels, well-defined morphological architectures, and large wall thickness of materials. HOM mesoporous molecular sieves were characterized by using powder X-ray diffraction, the Brunauer−Emmett−Teller method for nitrogen adsorption/desorption isotherms, transmission electron microscopy, and scanning electron microscopy. The results show that this direct template methodology can be used to successfully synthesize periodic mesoporous silica monoliths with a high degree of control over the mesopore morphology and the surfactant mesophase structures.