Microporous materials, single molecule

USE OF SINGLE MOLECULES AS TEMPLATES TO SYNTHESIZE MICROPOROUS MATERIALS... 

The use of single molecules or ions as templates results in materials with the smallest pore sizes (typically less than Inm), and hence they are called microporous according to the lUPAC definitions. Most microporous materials synthesized to date are zeolites. 

The current PEP setup allows two types of experiments to measure diffusion in microporous materials, hi the first type, labeled molecules are injected as a small pulse into a steady-state feed stream of either an inert carrier gas or of unlabeled molecules of the same kind. The propagation of the pulse through the reactor is followed using the PEP detector. Information about the diffusive processes can be obtained from the delay and broadening of the pulse, and quantitative information can be obtained by analysis of the measurements using an appropriate model, as will be discussed in more detail in the next section. This type of experiments is especially suited for diffusion measurements under zero loading conditions. A drawback of this method is that it is limited to the determination of single-component diffusion coeffi-... 

Organic cations are able to complex silicate anions to form silicate complexes. These complexes can condense to form ultimately microporous zeolite structures. Different templat-ing molecules may form different microcavities. Hence zeolite synthesis is an example of template-directed synthesis. Mesoporous materials can be prepared by using nanometersized micelles preorganized as hquid crystals as a template rather than a single molecule template. 

In all cases the key question is whether the ultimate silica particles are sufficiently separated so that the pores between them are accessible to the adsorbed ions or molecules and so that the silica surface can become covered with at least a single layer of adsorbed material. There are numerous instances, as in microporous gels, where water molecules or OH ions can penetrate where nitrogen -or other large molecules cannot. It will be assumed that the pores of the solids on which surface areas are being determined are such that the surface is generally accessible to the material being adsorbed. 

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