chapter  4
30 Pages

Fine-Tuning the Functionalization of Mesoporous Silica

WithHung-Ting Chen, Seong Huh, and Victor S.-Y. Lin

Directing Polymerization.............................................................. 58 4.6 Incorporation of Multiple Organic Functional Groups with Precisely

Controlled Relative Concentrations and Particle Morphology................... 63 4.7 Synergistic Catalysis by Multifunctionalized Mesoporous

Silica Nanoparticles ..................................................................................... 64 4.7.1 Gate Keeping Effect: Tuning Reaction Selectivity by

Bifunctionalized MSN Catalysts .................................................. 64 4.7.2 Cooperative Catalysis by General Acid and Base

Bifunctionalized MSN Catalysts .................................................. 67 4.8 Conclusion ................................................................................................... 71 References................................................................................................................ 71

Design and synthesis of materials with well-defined particle size, morphology, and ordered mesoporosity (2 to 10 nm in pore diameter) is a burgeoning area of current research in materials chemistry. These materials have two different surfaces, the interior pore surface and the exterior particle surface, which offer many advantages over solid particle materials. The mesoporous structure provides a size and functional group selective microenvironment that allows encapsulation of the desired molecular moieties and shelters these molecules from exposure to the external environment. The unique structural features of these mesoporous materials are important prerequisites for utilization in diverse areas, such as catalysis, chromatographic supports, controlled release of drugs or agrochemicals, development of medical implants, miniaturization of electronic devices, sensor design, and formation of semiconductor nanostructures.1-12 To realize these applications, the desired mesoporous material should also have the following features:

1. Chemically, thermally, and mechanically stable structure 2. Ordered particle and pore morphology 3. Large surface area and tunable pore size 4. Selectively functionalizable interior and exterior surfaces

A major breakthrough in fabrication of mesoporous material was the development at Mobil Corporation of the MCM family of mesostructured silicas by utilizing surfactants as structure-directing templates to generate a range of MCM-type mesoporous silica structures with tunable pore size and pore morphology, such as MCM-41 and MCM-48 silicas consisting of hexagonal channels and cubic pores, respectively.13, 14 Over the past decade, several other mesoporous silica materials with ordered porous structures, such as SBA-,15, 16 MSU-,17, 18 and FSM-type19 of mesoporous silicas, have also been developed. The typical synthesis of these structurally well-defined mesoporous silicas is based on a surfactant micelle templating approach. In an acidic or basic aqueous solution, organic surfactants, such as Pluronic P123 triblock copolymer15, 16 and cetyltrimethylammonium bromide (CTAB),13, 14 first form self-assembled micelles. These micelles serve as a structure-directing template that can interact with oligomeric silicate anions via hydrogen bonding or electrostatic interaction during the condensation reaction of tetraethoxysilane (TEOS). By either calcinations or acid extraction, the organic surfactants are removed, leaving an inorganic mesoporous silica framework. Depending on the specific synthetic condition, a disordered, hexagonal, or cubic pore structure of mesoporous silica can be obtained.