Micro- and Mesoporous Materials
Introduction
Porous inorganic materials have found numerous new applications in both Atraditional@ areas like sorbents, catalysts and ion exchangers, but also in unexpected areas like luminescent electronics, laundry detergents and biosensors. The inorganic porous materials can be divided into three classes:
1. Disordered pores: contains a wide range of forms and sizes. Classical sorbents like activated carbon and catalyst supports like aluminas and TiO2(B).
2. Ordered micropores: Well defined pores, channels and cavities in the range 0.1-1 nm based on the atomic crystal structure of the material. Examples are solid ion conductors like β-alumina, and natural and synthetic zeolites.
3. Ordered mesopores: A new class of materials with well defined pores, lamellas and cavities in the range 1-10 nm, formed by combining inorganic materials with organic surfactant molecules, acting as templates. Examples are MCM-41, MCM-48 and MCM-50.
Microporous zeolitic materials,
have had a commercial breakthrough as a catalyst in petroleum based industry and as a large-scale laundry detergent additive in addition to the established use as molecular sieves in gas streams and vacuum-conserving devices. Zeolites occur naturally as minerals (e.g natrolite, chabazite, sodalite, faujasite, mordenite), but some are easily produced synthetically (faujasite, mordenite) and are commercially available. A number of new zeolites (e.g Linde A, ZSM-5, zeolite beta) are not found in nature, and have in some cases been successfully designed by using structure-directing agents, like tetrapropylammonium ions, during synthesis.
Zeolites are crystalline aluminosilicates that have pore sizes in the range 0.2-1.0 nm. For each type of zeolite, the apertures of the channels are well defined. E.g. Zeolite A can adsorb linear paraffins while rejecting branched hydrocarbons. It can thus discriminate a difference in kinetic diameter of 0.3 Å for n-butane versus isobutane.
Mesoporous materials
Ordered mesoporous oxides was first described in 1992, when researchers at the Mobil Oil Company presented a synthesis of MCM-41 [1]. The principle for synthesis of this type of compounds is simple; long-chain surfactants and suitable alkylated silica precursors are mixed in liquid phase. Together they form a three-dimensional liquid crystal, which act as a casting mould. The system gelatises and the silica is crosslinked at just below the boiling point, (95-150EC, pressurised). The silica network is solidified, and the organic remains are removed by e.g. calcination at 300-550EC. It should be noted that the silica is amorphous, and that the only x-ray reflections are at low angles, with a first-order reflection corresponding to between 30 and100Å, depending on the synthesis conditions.
The project aims at synthesising zeolitic and mesoporous inorganic materials, characterising and modifying them for specific functionality.