Framework charge balance

Table II. Chemical conpceition, synthesis efficiency, pore filling and framework charge balance of seme pure Nu-10 sarples, synthesized under various experimental...

Sarrple Unit cell conpceiticn of Synthesis efficiency Pore filling Framework charge balance ... 

Flanigen believed that the components of alkali metal cations of the synthetic system would play an important role in two aspects in the synthesis of zeolites [55] (1) supply of base, i.e., OH and (2) structure directing. Recently, Dutta and colleagues concluded that the hydrated cations can stabilize aluminosilicate ions via static electric and steric interactions.[56] In a word, alkali metal cations or alkaline earth metal cations mainly play the roles of structure direction, framework charge balancing, and pH adjustment in the synthesis of aluminosilicate zeolites. 

The regular crystal structures of microporous solids have led, directly and indirectly, to their widespread study and application. The starting point in understanding their behaviour is the architecture of the repeat unit, or unit cell, which controls their properties. The well-defined pore size permits discrimination between molecules (and transition states in catalytic reactions) to better than 0.1 A activities for oxidation and solid acid catalysis are determined by the structural constraints on the local environments of framework cations and protons within the structure and the regular distribution of extra-framework, charge-balancing cations is responsible for cation exchange and gas adsorption and separation behaviour. 

The location of extra-framework species can be more difficult to determine than framework atom positions, and is best performed in conjunction with accurate chemical analysis. The usual approach is via difference Fourier syntheses, in which comparison of observed reflection intensities and those predicted on the basis of the nearly complete structural model enables the residual electron density (in the case of X-ray diffraction) to be located. This can then be interpreted in terms of the species present and their expected distances from the framework. Extra-framework charge-balancing cations may occupy sites at less than unit occupancy, that is they may be either present or absent from that site in any given unit cell, and the situation is further complicated in mixed cation systems, where different cations may partition between different sets of extra-framework sites. 

In addition to extra-framework charge-balancing cations, a variety of other elements and compounds can be included within the pores, usually from the vapour. These include volatile organic and inorganic molecules, which are described in the next chapter, but also species such as metals and non-metals. 

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