Solids, mesoporous

There is great variability among mesoporous silicas, and even among different samples of nominally the same structure type. These differences arise because of different gel compositions, preparation conditions, surfactant templates and post-synthetic treatments, and manifest themselves as different unit cell repeats, pore and window sizes and wall thicknesses and silicon connectivities within the walls. In this respect they are very different from the crystalline zeolites. A mesoporous solid is usually identified primarily by its symmetry and its lattice dimensions. On this basis, at least seven different types have been synthesised, some with very different pore sizes. For example, whereas MCM-41 and MCM-48 have different symmetries (and struetures), they have similar 

The proposed mechanisms of synthesis are discussed in more detail in Chapter 5 at this stage the structures of the as-prepared inorganic-organic composites are simply considered as surfactant micelle arrays interpenetrated by amorphous and incompletely condensed silica walls, in which silicon has local coordination Si(OSi)4 n(OH)n, where n is mainly 0 (Q ), 1 (Q ) or 2 (Q ). 

Symmetry Materials References Structure Pore space arrangement Channel or cage diameter window size of cages A 

Ia-3d MCM-48, AMS-4 ° Pore structure comprises two interpenetrating but unconnected branching networks of cylinders/rods Branching cylinders 20-40 A in diameter 

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A Type II isotherm indicates that the solid is non-porous, whilst the Type IV isotherm is characteristic of a mesoporous solid. From both types of isotherm it is possible, provided certain complications are absent, to calculate the specific surface of the solid, as is explained in Chapter 2. Indeed, the method most widely used at the present time for the determination of the surface area of finely divided solids is based on the adsorption of nitrogen at its boiling point. From the Type IV isotherm the pore size distribution may also be evaluated, using procedures outlined in Chapter 3. 

As already indicated in Section 3.1, the study of mesoporous solids is closely bound up with the concept of capillary condensation and its quantitative expression in the Kelvin equation. This equation is, indeed, the basis of virtually all the various procedures for the calculation of pore size... 

It follows therefore that the specific surface of a mesoporous solid can be determined by the BET method (or from Point B) in just the same way as that of a non-porous solid. It is interesting, though not really surprising, that monolayer formation occurs by the same mechanism whether the surface is wholly external (Type II isotherm) or is largely located on the walls of mesopores (Type IV isotherm). Since the adsorption field falls off fairly rapidly with distance from the surface, the building up of the monolayer should not be affected by the presence of a neighbouring surface which, as in a mesopore, is situated at a distance large compared with the size of a molecule. 

Bray KL (2001) High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. 213 1-94 Bronstein LM (2003) Nanoparticles Made in Mesoporous Solids. 226 55-89 Bronstrup M (2003) High Throughput Mass Spectrometry for Compound Characterization in Drug Discovery. 225 275-294... 

More recently, bis(oxazoline)-metal complexes supported in micro- and mesoporous solids have been used as catalysts of hetero-Diels-Alder and ene reactions. 

Stein, A. (2003) Advances in microporous and mesoporous solids -Highlights of recent progress. Advanced Materials, 15, 763-775. 

Yiu, H.H.P. and Wright P.A. (2005) Enzymes supported on ordered mesoporous solids a special case of an inorganic-organic hybrid. Journal of Materials Chemistry, 15, 3690-3700. 

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