Mesoporous organic-inorganic hybrid materials, a new class of materials characterized by large specific surface areas and pore sizes between 2 and 15 nm, have been obtained through the coupling of inorganic and organic components by template synthesis. The incorporation of functionalities can be achieved in three ways: by subsequent attachment of organic components onto a pure silica matrix (grafting), by simultaneous reaction of condensable inorganic silica species and silylated organic compounds (co-condensation, one-pot synthesis), and by the use of bissilylated organic precursors that lead to periodic mesoporous organosilicas (PMOs). This Review gives an overview of the preparation, properties, and potential applications of these materials in the areas of catalysis, sorption, chromatography, and the construction of systems for controlled release of active compounds, as well as molecular switches, with the main focus being on PMOs.

Silica-based mesoporous organic-inorganic hybrid materials.

Nanostructured carbon materials are potentially of great technological interest for the development of electronic1,2, catalytic3,4 and hydrogen-storage systems5,6. Here we describe a general strategy for the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters (typically 6 nanometres inside and 9 nanometres outside). These structures are formed by using ordered mesoporous silicas as templates, the removal of which leaves a partially ordered graphitic framework. The resulting material supports a high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials (such as carbon black, charcoal and activated carbon fibres). The platinum cluster diameter can be controlled to below 3 nanometres, and the high dispersion of these metal clusters gives rise to promising electrocatalytic activity for oxygen reduction, which could prove to be practically relevant for fuel-cell technologies. These nanomaterials can also be prepared in the form of free-standing films by using ordered silica films as the templates.

Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles

On the Controllable Soft-Templating Approach to Mesoporous Silicates

Ordered mesoporous materials in catalysis

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Chemical strategies to design textured materials: from microporous and mesoporous oxides to nanonetworks and hierarchical structures.

We demonstrate a novel preparation method for the formation of metallic nanowires from gold, platinum, and silver using mesoporous silica SBA-15 as a template. SBA-15 controls the size and the growth direction of the nanowires and also prevents bulk aggregation of the metal. Nanowires can be isolated from the silica framework by treatment with HF.

Preparation of Noble Metal Nanowires Using Hexagonal Mesoporous Silica SBA-15

We have investigated the phase transition between two distinct mesoporous silicas:  SBA-15, comprising a hexagonally packed arrangement of cylindrical pores (6−12 nm in diameter), and mesocellular silica foams (MCF), consisting of spherical voids (22−42 nm in diameter) interconnected by “windows” of ∼10 nm. Both SBA-15 and MCF are formed using an amphiphilic triblock copolymer (Pluronic P123) as a template. The synthesis conditions for the two materials are identical, except substantial trimethylbenzene is added to form MCF. We find that the phase transition occurs at an oil−polymer mass ratio of 0.2−0.3. Although the pore structures and pore sizes change dramatically, the mean surface curvature of the system remains essentially the same throughout the transition.

Hexagonal to Mesocellular Foam Phase Transition in Polymer-Templated Mesoporous Silicas

Mesoporous silicate sequestration and release of proteins

A versatile enzyme, FeHeme chloroperoxidase (CPO) from Caldariomyces fumago, is immobilized in the mesoporous silicate material, mesocellular foam (MCF). MCF is a promising material for immobilizing enzymes, due to its large pore structure and high loading capacity compared to other mesoporous materials, such as MCM-48, SBA-16 and SBA-15. The immobilized CPO in MCF retains its activity. The optimal pH at which the maximum amount of enzyme is immobilized was determined to be pH 3.4, slightly below the isoelectric point of the enzyme. A weak ionic interaction between the enzyme and the surface of the inorganic substrate is thought to be critical in maintaining the activity of the immobilized enzyme. The loading capacity of MCF is 122 mg protein per 1 g of MCF. We demonstrate the advantage of MCF as an inorganic substrate for immobilization of enzymes.

Catalytic activity of mesoporous silicate-immobilized chloroperoxidase

In this paper we describe how pore-structure modification can be achieved in a highly ordered fashion through the use of bolaform surfactants containing a rigid unit in the hydrophobic chain. The silicate mesophase, SBA-8, synthesized using bolaform surfactants at room temperature, is a two-dimensional (2-D) pore structure with a centered rectangular lattice (space group cmm, 1 1000 m2/g). The unit cell parameter of the silicate mesostructure can be varied by adding gemini surfactants or by changing reaction temperature; a 2-D silicate mesophase (Mα, a/b > √3) and high quality hexagonal MCM-41 (p6mm, a/b = √3) can be obtained. Hydrothermal transformation of SBA-8 into MCM-41 takes place, confirming the intermediate nature of the SBA-8 mesophase.

Yong-Jin Han

Papers

Preparation of Noble Metal Nanowires Using Hexagonal Mesoporous Silica SBA-15

Hexagonal to Mesocellular Foam Phase Transition in Polymer-Templated Mesoporous Silicas

Mesoporous silicate sequestration and release of proteins

Catalytic activity of mesoporous silicate-immobilized chloroperoxidase

Novel Mesoporous Silicates with Two-Dimensional Mesostructure Direction Using Rigid Bolaform Surfactants