Electrochemistry of Aluminosilicates

FIGURE 4.1 Three-dimensional representation of the framework structure of (a) zeolite A and (b) X and Y. 

For every aluminum atom in the lattice, a fixed negative charge results. This negative charge is counterbalanced by mobile cations, typically alkaline or alkaline-earth cations. In contact with aqueous solutions, these mobile cations can easily be exchanged for other cations (metal cations, cationic complexes) of appropriate dimensions, thus determining the remarkable ion exchange properties of zeolites. 

Zeolites are traditionally regarded as acid materials. There are two types of acidic centers in zeolites. First, protons held in the zeolite form bridging hydroxyl 

Type of Pore Pore Size (A) Number of Tetrahedral Units Channel Directions Examples 

Zeolite Type Unit Cell Composition Si/AI Molar Ratio Unit Cell Volume (nm ) Void Volume (cm /cm ) Pore Openings (A) Supercage Diameter (A) Kinetic Diameter (nm) tc ( C) 

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Metal nanoparticles housed in zeolites and aluminosilicates can be regarded as arrays of microelectrodes placed in a solid electrolyte having shape and size selectivity. Remarkably, the chemical and electrochemical reactivity of metal nanoparticles differ from those displayed by bulk metals and are modulated by the high ionic strength environment and shape and size restrictions imposed by the host framework. In the other extreme end of the existing possibilities, polymeric structures can be part of the porous materials from electropolymerization procedures as is the case of polyanilines incorporated to microporous materials. The electrochemistry of these types of materials, which will be termed, sensu lato, hybrid materials, will be discussed in Chapter 8. 

From the electrochemical point of view, an important class of materials is that constituted by aluminosilicates incorporating cobalt, iron, etc., centers. In the case of Fe-based zeolites with Mobil Five structure (FeZSM-5) materials, different forms of iron can coexist. These include isolated ions either in framework positions (isomorphously substituting silicon centers), isolated ions in cationic positions in zeolite channels, binuclear and oligonuclear iron complexes in extra-framework positions, iron oxide nanoparticles (size <2 nm), and large iron oxide particles (FcjOj) in a wide distribution (up to 25 nm in size) located in the surface of the zeolite crystal (Perez-Ramirez et al., 2002). The electrochemistry of such materials will be reviewed in Chapter 8. 

Domenech, A., Garcia, H., Casades, L, and Espla, M. 2004e. Electrochemistry of 6-nitro-1, 3, 3 -trimclhylspiro 2//-l-benz.opyran-2,2 -indolinc associated to zeolite Y and MCM-41 aluminosilicate. Site-selective electrocatalytic effect on lV,lVJV, 7V -tetramethylbenzidine oxidation. Journal of Physical Chemistry B 108, 20064—20075. 

From a structural point of view, porous materials can be viewed as the result of building blocks following an order of construction that can extend from the centimeter to the nanometer levels. Porous materials can range from highly ordered crystalline materials such as aluminosilicates or MOFs, to amorphous sol-gel compounds, polymers, and fibers. This text will focus on materials that have porous structures, so that ion-insertion solids having no micro- or mesoporous structures, such as the metal polycyanometalates, whose electrochemistry was reviewed by Scholz et al. (2005), will not be treated here. To present a systematic approach... 

In general, immobilization of selected species into mesoporous aluminosilicates provides opportunity for studying their electrochemistry under site isolation... 

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