Synthesis of Dual‐Mesoporous Silica Using Non‐Ionic Diblock Copolymer and Cationic Surfactant as Co‐Templates

Abstract

Ordered mesoporous materials, ever since their discovery, have attracted considerable interest owing to their outstanding physicochemical properties (such as high surface area, large pore volume, variety of pore structures, easily modifiable surface, diverse framework compositions) and broad potential applications in, for example, adsorption, separation, catalysis, drug delivery and fuel cells. Considerable efforts have recently been devoted to fabricating ordered porous materials with hierarchical pore structures, such as macro-/meso-, meso-/micro-, and macro-/microporous materials. Additionally, in order to further increase the surface area for enhanced interactions with adsorbents and reduced transport limitations, much research has been conducted on the synthesis of ordered mesoporous materials with a bimodal pore structure which are highly desirable for applications in catalysis, sensing, and drug delivery. According to the templating synthesis concept for mesoporous materials, the bimodal mesopore system can be achieved by using two templates of different molecular weights that lead to pores of corresponding pore sizes. In fact, various template pairs have been employed to synthesize mesoporous materials with bimodal pores, including a nonionic copolymer and an ionic surfactant, a non-ionic copolymer and a non-ionic surfactant, an ionic block copolymer and an ionic surfactant, and mixed templates of two block copolymers with different molecular weights. However, in most cases dual-mesoporous materials with welldefined pore arrangements (specifically, with small mesopores surrounding the large mesopores) and tunable pore sizes could not be synthesized because of difficulties in controlling the assembly process in the dual-templating system. From a general thermodynamic point of view, the creation of a hierarchical micellar system is usually unfavorable; the two template molecules tend to form either mixed micelles or they form separate macroscopic phases. In light of this theoretical consideration, to achieve a hierarchical mesostructure, it is necessary for one set of template molecules to form stable micelles first before interaction with the second set of template molecules during the synthesis process. Consequently, to obtain ordered dual-mesoporous materials with two well-arranged sets of mesopores, two obstacles must be overcome. One is forming distinct large micelles of high-molecular-weight block copolymers and small micelles of low-molecular-weight surfactants. The second obstacle is triggering the independent co-assembly of the two kinds of micelles into an organized “alloy” phase, with small surfactant molecules filling the interstitial voids of the ordered mesostructure built up by large block copolymers. However, to date, it is a challenge to control the assembly process in the synthesis of dual-mesoporous silica with two kinds of template molecules. We report herein on a solvent-evaporation-induced stepby-step aggregating approach for the synthesis of ordered dual-mesoporous silica materials by using poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMMA) and alkyltrimethylammonium bromide (CnTAB) as co-templates and tetraethyl orthosilicate (TEOS) as a silica source in an acidic tetrahydrofuran (THF)/H2O solution. It is found that, at the early stage of THF solvent evaporation, PEO-bPMMA copolymers with associated silicate oligomers first formed stable composite micelles in the solution. With the further evaporation of THF, the increased concentration and strong electrostatic attraction induce CnTAB molecules to move toward the negatively charged spherical composite micelles. Further evaporation of THF causes the composite micelles to assemble into large particles (0.5–6.0 mm) with a face-centered-cubic (fcc) structured mesophase. Meanwhile, the ultrahigh concentration of CnTAB molecules located in the interstitial space of the aggregated composite micelles assemble into curving rod-shaped micelles, resulting in a unique ordered mesostructure constructed of spherical large micelles and wormlike small micelles bound with silicate species (Scheme 1). After the templates were removed by simple calcination in air, unique ordered dual-mesoporous silica materials were obtained with large mesopores (ca. 20 nm) packed in an Fm 3m structure and small wormlike mesopores (ca. 2.5 nm) homogeneously distributed in the wall of the large pores. Because the non-ionic EO125-MMA174 has a very long hydrophobic segment, a mixture of THF and aqueous hydrochloric acid solution (3:1 v/v) was used to dissolve this template, the ionic surfactants, and the silica source (TEOS), yielding a homogeneous starting solution. It was found that, similar to our previous report, as the THF evaporated, the reaction system underwent a dramatic change from a clear solution, to a blue colloidal dispersion, to a white turbid [*] J. Wei, Q. Yue, Z. K. Sun, Prof. Dr. Y. H. Deng, Prof. Dr. D. Y. Zhao Department of Chemistry and Advanced Materials Laboratory Fudan University, Shanghai 200433 (P.R. China) E-mail: yhdeng@fudan.edu.cn dyzhao@fudan.edu.cn Homepage: http://www.mesogroup.fudan.edu.cn/