Layered host structures

Intercalation in MWCNT should be similar to intercalation in HOPG or fibres. This means that one would expect in helical samples that the intercalate would be introduced between the layers with a dramatic increase of its diameter. For concentric CNTs, it would require a large amount of defects for intercalation to occur between the planes and an important distortion of the host structure. 

A glance at the structure of graphite, illustrated in Fig. 1, reveals the presence of voids between the planar, sp -hybridized, carbon sheets. Intercalation is the insertion of ions, atoms, or molecules into this space without the destruction of the host s layered, bonding network. Stacking order, bond distances, and, possibly, bond direction may be altered, but the characteristic, lamellar identity of the host must in some sense be preserved. 

X-ray structure analysis showed that each of the cavities in the chabazite-type host structure contains two disordered SDA molecules. The lack of a superstructure suggests a random layered arrangement of the template molecules (Figure 2) similar to the crystal structure of the related compound CAL-1 [5],... 

For layered host materials as used in the lithium ion cell, the term intercalation is used for the insertion of guests into the host structure. 

A number of clay-like 2D layered and zeolite-like 3D framework host structures have been materialized using isopolycyanocadmate as the linkage unit. The 2D layered and the 3D framed hosts accommodate a variety of organic guest molecules and onium cations of different geometrical characters. 

Many of the same ionic surfactants used for the assembly of mesostructured molecular sieve catalysts [1-4] and related bulk phases [5] can be intercalated in a variety of layered host structures [6]. We have recently demonstrated that some of these mesostructure - forming surfactants retain their structure directing properties when intercalated in the galleries of smectite clays. In a manner quite analogous to bulk mesostructure formation, the intercalated surfactants direct the assembly of an open framework metal oxide (silica) structure within the constrained gallery regions of the layered host (7). The resulting porous intercalates are referred to as porous clay heterostructures (PCH). 

Figure 7.14 Stereoview of 2TMA [/Metradecane] viewed along the crystallographic caxis. The n tctradecanc molecules are highly disordered. The host structure repeats every three layers. (Reproduced with kind permission from Springer Science + Business Media from [16] 1987).

The layered arrangement makes these materials very interesting from the point of view of host-guest behaviour because ionic or molecular guest species may be inserted between one layer and another causing the layers to expand or swell. Guest intercalation is generally reversible, and it is an important characteristic of layered solids that, rather like zeolites, they can retain their layered host structure... 

Ag8(C2)(CF3C02)8(H20)2] (H20)4-(L7H2) represents a rare example of a hydrogen-bonded layer-type host structure containing C2 Ag that features... 

Figure 1. Schematic of a possible photocatalytic cycle based on the photochemistry of a multielectron photoactive center (MPC) intercalated within redox active layers of a host structure.

On the basis of these properties, the three approaches summarized in Figure 3 have been elaborated specifically for the introduction of photoactive M—— M cores into layered phosphate host structures. These are (1) the direct intercalation of solvated M—— M cores into layered phosphates wherein the phosphate groups of the layers form the ligation sphere for the bimetallic core (2) acid-base reaction of specially functionalized ligands on the bimetallic core with protons from the layers and (3) replacement of the phosphate groups with functionalized phosphonates that offer well-defined coordination sites for the M—M core. We now discuss each of these methodologies. 

Figure 3. Three basic strategies for the incorporation of multiply bonded metal-metal guest species into vanadyl and zirconium phosphate host layers, (a) The direct intercalation of solvated M—— M cores into the native layered phosphate host structure, (b) Incorporation of M—— M complexes with ancillary ligands containing a Lewis basic site, (c) Coordination of M—— M cores with ligands provided from modified phosphate layers.

Layered silicate clays intercalated by pillaring poly-oxocations are precursors to an important class of mi-croporous catalysts. Smectite clay was the only host structure known to be pillarable by purely inorganic oxo ions. Recently, layered double hydroxides (LDH) pillaring oxo ions were reported by Pinnavaia and coworkers [79, 80]. 

---