Solids, microporous

W, D J Willock, C R A Catlow, J M Thomas and G J Hutchings 1996. De Novo Design of icture-directing Agents for the Synthesis of Microporous Solids. Nature 382 604-606. 

D J, D W Lewis, C R A Catlow, G J Hutchings and J M Thomas 1997. Designing Templates for Synthesis of Microporous Solids Using De Novo Molecular Design Methods. Journal of Molecular alysis A Chemical 119 415-424. 

Type 1 isotherms, as will be demonstrated in Chapter 4, are characteristic of microporous adsorbents. The detailed interpretation of such isotherms is controversial, but the majority of workers would probably agree that the very concept of the surface area of a microporous solid is of doubtful validity, and that whilst it is possible to obtain an estimate of the total micropore volume from a Type I isotherm, only the crudest guesses can be made as to the pore size distribution. 

In the simplest case, adsorption in a microporous solid leads to an isotherm of Type I consequently it is convenient to approach the subject by a discussion, from a classical standpoint, of Type I isotherms. 

Perhaps the most direct method of evaluating microporosity is to fill up the micropores with some suitable adsorbate whilst leaving the mesopores, macropores and external surface free. The use of n-nonane as a preadsorbate was proposed by Gregg and Langford on the basis of earlier work on the adsorption of n-alkanes C, to C, on ammonium phos-phomolybdate, a microporous solid. This work had shown that the rate at... 

This second sub-region will give rise to a rounded knee to the isotherm. Thus a purely microporous solid which contains both categories of micropore will give rise to a Type 1 isotherm, having a very steep initial... 

Types III and V isotherms arc characteristic of weak gas-solid interactions, the Type III isotherm being given by a nonporous or macroporous solid and the Type V isotherm by a mesoporous or microporous solid. 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

We have previously shown (ref. 1) that microporous solids are useful in the controlled bromination of aromatic substrates. In particular, we showed how a reagent system comprising V-bromosuccinimide (NBS) and silica is useful for the bromination of reactive aromatic systems such as indoles (Fig. 1) (ref. 2), carbazoles and iminodibenzyls (Fig. 2) (ref. 3). 

Figure 9.1 illustrates a variety of different stractures. This selection is by no means all-inclusive a host of related stractures such as colloids, microstrands, thin films, microporous solids, microemulsions, and gels could also have been shown. The parts of each of these stractures are distinguished by the zones—interfaces—between them, which often seem to be... 

The problem of accessibility in microporous solids is extreme in zero-dimensional zeolite structures such as clathrasils, that is, zeolite-related materials consisting of window-connected cages. The pore openings in these caged structures are restricted to six-membered rings of [Si04] units at most, which corresponds to pore diameters of approximately 0.2 nm [58]. These pores are too small for the removal of templates and, afterward, are impenetrable to typical sorptive molecules for characterization such as N2 and Ar or reactants such as hydrocarbons. Therefore, the intrinsic... 

Karger, J Ruthven, DM, Diffusion in Zeolites and Other Microporous Solids Wiley New York, 1992. 

Base catalysis is another area which has received a recent stimulus from developments in materials science and microporous solids in particular. The Merk company, for example, has developed a basic catalyst by supporting clusters of cesium oxide in a zeolite matrix [13]. This catalyst system has been developed to manufacture 4-methylthiazole from acetone and methylamine. 

Zeolites form a class with tremendous variety. Besides the microporous solids described in the above, mesoporous materials have been synthesized. A breakthrough were the MCM-41 mesoporous zeolites with pores of typically 3 nm. Later, many related materials have been reported allowing fine-tuning of pore sizes. A recent example is the synthesis of materials with pores in the lOnm range with satisfactory uniformity and stability (Sun etai, 2001). 

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

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