Micrometer-sized microporous silica spheres templated by a double-hydrophilic block copolymer

Abstract

Silica spheres possessing nanometer-scale pores of tailored size, shape, and connectivity are of interest for chromatographic separation and solid-phase catalysis [1–3]. Traditionally, porous silica spheres are prepared by spray drying, spray gelling, coacervation process, and hydrolyzing silicon esters in emulsion systems [4]. However, the tailoring of the pore size, pore shape, pore volume and surface texture of the silica spheres prepared by these methods remains problematic [1]. Recently, a variety of mesoporous silica spheres exhibiting tailored pore size and pore structure has been synthesized by using surfactant assemblies as supramoleculer templates [2]. To illustrate, submicrometer-sized mesoporous silica spheres of various structures have been synthesized by the combination of the Stober approach [5], which results in nonporous silica beads, and the surfactant-templated synthesis [6]. It is also worth noting that large mesoporous silica spheres (from 0.1 to 2 mm) were prepared by an emulsion synthesis under stirred basic conditions [1] whereas micrometer-sized mesoporous silica spheres were prepared under static acidic conditions [7]. On the other hand, much effort has been devoted to the organic templating or molecular imprinting of microporous silica systems in which the templates (or imprints) interact covalently or non-covalently with silica precursors [8, 9]. The resulting microporous silicas with tailor-made pore sizes and shapes are potential candidates for shape-selective catalysts and adsorbents. It is therefore expected that microporous silica spheres templated by organic molecules are of technological importance considering that spherical particles are a typical packing material in chromatography and an easy-to-handle form for catalytic purpose. Herein, the synthesis of micrometer-sized microporous silica spheres templated by a block copolymer is described. Recently, it has been shown that the so-called doublehydrophilic block copolymers [10], which consist of one hydrophilic block interacting strongly with appropriate inorganic materials and another hydrophilic block mainly promoting solubilization in water, can exert a strong influence on the morphology and/or structure of inorganic materials such as CaCO3 [10] and BaSO4 [11]. Here the application of this kind of copolymer is extended to the silica system by using it as a molecular template. A poly(ethylene oxide) (PEO)