Structure charge-compensating cations

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

In the case of subsurface cation exchange, charge compensation cations are held in the solid phase within crystals in interlayer positions, structural holes, or surface... 

How exactly the molecules are oriented inside the channels depends on their specific shape and on the adsorption interaction between the dyes and the channel walls or charge compensating cations. Because of the dye s oblongness, a double-cone-like distribution in the channels is a reasonable model. This distribution is illustrated in Fig. 19a. The arrows represent the transition moments of the dyes and a describes the half-opening angle of the double cone. The hexagonal structure of the zeolite L crystal hence allows six equivalent positions of the transition moments on this double cone with respect to the channel axis. 

Rutile pigments, prepared by dissolving chromophoric oxides in an oxidation state different from +4 in the rutile crystal lattice, have been described (25,26). To maintain the proper charge balance of the lattice, additional charge-compensating cations of different metal oxides also have to be dissolved in the rutile structure. Examples of such combinations are Ni2+ + Sb5+ in 1 2 ratio as NiO + Sb2Os, Cr3+ + Sb5+ in 1 1 ratio as Ci C + Sb2Os, and Cr3+ +... 

Subsequently, it is possible to consider that the adsorbate-adsorbent interaction field inside these structures is characterized by the presence of sites of minimum potential energy for the interaction of adsorbed molecules with the zeolite framework and charge-compensating cations. A simple model of the zeolite-adsorbate system is that of the periodic array of interconnected adsorption sites, where molecular migration at adsorbed molecules through the array is assumed to proceed by thermally activated jumps from one site to an adjacent site, and can be envisaged as a sort of lattice-gas. 

In zeolites, the rate of molecular diffusion depends on the position of charge-compensating cations in the pore network and the structure of the framework [77-81], Since mass transport in micropo-rous media takes place in an adsorbed phase [82,83], this transport can be envisaged as activated molecular hopping between fixed sites [60,82,84] (for more details, see Section 5.9.1). 

The framework charge-compensating cations in a zeolite, which for synthetic zeolites are normally sodium ions, can be exchanged for other cations of different type and/or valency. However, care must be taken during ion exchange to avoid strongly acidic solutions which can lead to proton exchange with the zeolite metal cations or even structure collapse. For example, zeolites A, X, and Y decompose in 0.1 N HCI. The more silica-rich zeolites such as mordenite are, however, stable under such conditions. Acidity can be introduced into a zeolite in a number of different ways ... 

Schematic 1. The structure of 2 1 layered silicates. M is a monovalent charge compensating cation in the interlayer and x is thedegree of isomorphous substitution, which for the silicates of interest is between 0.5 and 1.3. The degree of isomorphous substitution is also expressed as a cation exchange capacity (CEC) and is measured in milli-equivalents/g.

The charge compensating cations (see above) may be exchanged by NH ions. A thermal treatment at temperature higher than 600K leads to the removal of NH.i and consequently to the formation of structural hydroxyl groups as follows ... 

The natural zeolite community has followed a different path towards a systematisation of its nomenclature. An historically-determined nomenclature in which the crystal structure, the nature of the charge-compensating cation and some thermal properties were taken into account in a non-systematic manner [41] has been replaced by a nomenclature in accordance with the rules of the International Mineralogical Association (IMA) [42], in which both the structure and composition of the mineral are considered. This has led to a substantial increase... 

Clearly, investigation of zeolites with a small unit cell is computationally advantageous. However, with the exception of purely silicious structures and zeolites with Si/Al=l, the periodicity of the zeolite is not absolute since the distribution of framework Al is expected to be more random. The same applies for the position of charge-compensating cations. Special care must be taken when investigating adsorption or chemical reactivity. Calculations using a unit cell where at least one dimension is small can lead to potential problem. It is always very valuable to perform some test calculations with the unit cell doubled along the shortest cell dimension. 

The well-defined porous structure of zeolitic materials makes these materials true shape-selective molecular sieves. The presence of charge-compensating cations such as alkaline and alkaline earth cations, protons, etc. within the inorganic frameworks adds ion exchange and catalytic properties. Moreover, the hydrophobic nature of pure silica zeolites or the hydrophilic nature of aluminosilicates makes these solids useful as specific adsorbents of organic molecules or water in the gas or liquid phase. 

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