Mesocellular Siliceous Foams with Uniformly Sized Cells and Windows

Abstract

Molecular sieves with uniform large pores are desirable for chemical reactions and for use in separations involving large molecules.1 Periodic cubic and hexagonal mesoporous silica phases with uniform large pores have been synthesized by using nonionic triblock and star diblock copolymers as templates.2 Control over the pore size is achieved by adjusting the hydrophobic volumes of the self-assembled aggregates.2,3 In this paper, we describe how adding a sufficiently large amount of an organic cosolvent induces a phase transformation from the highly ordered p6mm mesostructure of SBA-15-type mesoporous silicas to remarkable mesostructured cellular foams (mesocellular foams, MCFs) composed of uniformly sized, large spherical cells that are interconnected by uniform windows to create a continuous 3-D pore system. The interconnected nature of the large uniform pores makes these new mesostructured silicas promising candidates for supports for catalysts and in separations involving large molecules, and they may be of interest in low-dielectric applications. The MCFs have been synthesized in aqueous acid by using dilute Pluronic P123 solutions in the presence of 1,3,5-trimethylbenzene (TMB) as organic cosolvent.4 X-ray diffraction (XRD) experiments5 reveal well-resolved peaks at small angles, as shown in Figure 1 for a sample with a cell diameter of 33 nm. Careful analyses of the scattering data for MCFs show that the higher order peaks cannot be indexed to any plane or space group (e.g., p6mm) or to a lamellar diffraction pattern. In fact, after subtraction of the background,6 the X-ray data are in good agreement with simulated scattering7 due to monodisperse spheres (cells) of diameter D (see Table 1), while attempts to fit the X-ray data to