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COPYRIGHT 2002 Science Reviews Ltd.
All materials possess interatomic or interionic voids that are typically too small for any molecular species to enter. However, there is a class of crystalline materials that contain internal voids, and apertures, that are large enough for molecular species to enter and pass through. These materials are termed microporous and form a highly diverse group of compounds that may be synthesised or occur as natural minerals. The composition of microporous materials ranges from being exclusively inorganic to inorganic-organic hybrids and their applications vary from 1 Mton annual usage in detergents, to hosts for superconducting carbon nanotubes. In this contribution, new and mature aspects of the synthesis, scope, modification and application of microporous materials are covered to provide the reader with an overview of this exciting field of materials chemistry.
Introduction
Microporous materials form an extremely important class of compounds that have an annual usage of approximately 1.6 Mton per year, and as a research area is growing rapidly with many new materials, modifications, properties and applications being discovered and developed. The high degree of activity in the field is clear from the growing number of publications produced per annum that has risen to its current level of approximately 3,600.
Microporous materials are crystalline materials that contain interconnected cages/cavities or channels with pores of molecular dimensions, typically 2.5--20 A, and have an extremely narrow pore size distribution. The channels and interconnected cavities are arranged to form one-, two- or three-dimensional pore systems within the material, as is shown for the microporous aluminosilicate ZSM-5 in Figure 1 that contains two types of channels that form a three-dimensional pore system. The materials are characterised by their large internal surface area (typically > 300 [m.sup.2][g.sup.-1] and void volumes (>0.1 [cm.sup.3][g.sup.-1]). The size and uniformity of the pores in the material enable them to separate different molecules by their respective geometries, and hence microporous materials are often known as molecular sieves. This property is termed shape selectivity and determines which molecules pass through or form in the material by either controlling which reactant molecule can enter the microporous mater ial (reactant selectivity), which product may leave the material (product selectivity) or the geometry of the transition state that is formed in the material (transition state selectivity) (1). All three types of shape selectivity are shown schematically in Figure 2.
The large internal surface area of the microporous material means that highly active materials are formed when functional centres are dispersed over the surface area. These properties render microporous materials particularly useful in the areas of catalysis, ion-exchange and separation (molecular sieving and adsorption) where they find their main commercial applications.
In this contribution the synthesis, types, modification, and the current and future applications of microporous materials are discussed. The field of microporous materials is expansive and in this piece can only be discussed to a limited degree. However, it is hoped that this article should provide the reader with a basic understanding of both the mature and emerging aspects in this area. In this review, the microporous materials discussed are limited to those that are crystalline and have pore sizes in the range of 2.5-20 [Angstrom], thus excluding important non-periodic microporous systems, such as aerogels and xerogels, and mesoporous materials.
Synthesis
Microporous materials are synthesised under a wide range of experimental conditions. Typically, they are synthesised under solvothermal conditions (temperature range 80-250[degrees]C, autogenous pressures 30-9 10 bar) in Teflon-lined steel autoclaves for a few hours to ca 30 days. There are numerous variables in the synthesis that can be altered to yield different products: these include the particular starting compounds from which the microporous material is to be formed, the solubility of these starting compounds, the solvent, the pH of the synthesis mixture, whether the synthesis mixture is aged prior to reaction, the temperature and pressure of the reaction, whether the reaction mixture is agitated during the reaction, the presence of any mineralising agents, the presence and type of any charge compensating ions and the reaction time. This last parameter is particularly important as the syntheses are, in general, kinetically controlled, with different metastable phases formed over time. Microporous materi als are also formed outside these general conditions, where for example, some materials form at room temperature and ambient pressure whilst others form at far higher temperatures and pressures. One particularly useful strategy in the formation and design of new microporous materials has been the inclusion of quaternary ammonium ions (as a hydroxide or halide salt) or organic amines in the synthesis mixture. These organic guest molecules act as structure directing agents and are incorporated into the final product where they replace the usual charge compensating metal cations and other extra-framework molecules. It is the shape and size of the organic guest molecules that can lead to the formation of new microporous structures that often cannot be produced by any other methods. In this article the term structure directing agent is used to include organic guest molecules that fill the void space around which the framework of the microporous material forms but for which a particular structure can be formed by m ore than one particular type of guest molecule and guest molecules that are specific to the formation of a certain framework structure only. An example of the former is shown in Figure 3, in which the framework structure of the all-silica microporous material SSZ-23 is formed around the quaternary ammonium TMAda+ cation (2). The formation of SSZ-23 by this organic guest molecule is not unique as syntheses in the presence of an azaspiropiperidine derivative also yield this particular microporous material. The organic guest molecule must be removed from the product material to render it microporous. This is typically done by heating the material under flowing oxygen at temperatures >400[degrees]C (calcination) or in some circumstances the guest molecule can be washed out under acidic or basic conditions. The use of structure directing agents has produced many new framework materials for which the organic guest molecule cannot be removed without the collapse of the framework. Collapse occurs when the constituent bonds of the framework are too weak to maintain the structural integrity of the framework without the aid of the favourable electrostatic, hydrogen bonding or van der Waals interactions between the components of the framework and the structure directing agent. Such materials cannot be considered as microporous because the pores are never accessible to other species. Structure directing agents other than organic nitrogen compounds can be used to produce novel microporous materials, for example, the presence of the organometallic complex [[([Me.sub.5][C.sub.5]).sub.2]Co]+ ion is necessary to synthesise the high-silica microporous material UTD- 1 (3).
The synthesis of microporous materials evolves as each new material, formed by modifying some of the parameters discussed above, is reported. The synthesis of a new material is often laborious, as a wide region of phase space needs to be sampled to find the correct initial reagent proportions and conditions to form the material. One new development in the synthesis of microporous materials that is emerging, particularly in industrial laboratories, is the use of automated combinatorial or high throughput methods in which many samples (up to as many as 1000) can be prepared in a single run. This approach should increase the...
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