Neutral frameworks

The introduction of phosphate into the framework has led to an extraordinary expansion of the possible pore diameters in framework molecular sieves. The discovery of VPI-5, the first 16-membered-ring aluminophosphate framework, was a landmark (8). Although it is a neutral framework, it... 

The characteristics of aluminophosphate molecular sieves include a univariant framework composition with Al/P = 1, a high degree of structural diversity and a wide range of pore sizes and volumes, exceeding the pore sizes known previously in zeolite molecular sieves with the VPI-5 18-membered ring material. They are neutral frameworks and therefore have nil ion-exchange capacity or acidic catalytic properties. Their surface selectivity is mildly hydrophilic. They exhibit excellent thermal and hydrothermal stability, up to 1000 °C (thermal) and 600 °C (steam). 

The as-synthesized GaPO s have essentially a neutral framework, that is, the P/Ga ratio is in-variantly around unity. Their empirical composition can be expressed as zR Ga20s 1. 0 db... 

The macrocyclization reaction described above has been used to generate a great number of catenane (12) and rotaxane (13) architectures (Scheme 10.4) using both crown ethers [preformed macrocyclic components 14 (strategy A)] and hydroqui-none-based dumbbell-shaped polyethers [preformed acyclic components 15 ( clipping )] as templates [14b, 15]. These templates are also relatively robust with regard to the substitution of different groups into both the tetracationic cyclophane and the neutral frameworks. 

The channels and cavities of zeolites with neutral frameworks may be empty. Certainly no ions are needed to balance the charge of the framework. The most common examples are zeolites of composition Si02 (the silica polymorphs) and AIPO4 (the alpo s). These zeolites... 

The contents of zeolites with neutral frameworks must themselves be neutral. Accordingly, only molecules and/or salts with no excess ions to unbalance charge may be present. Because of the weak electrostatic fields within these zeolites, there is little reason for ionization to occur in other words, they are poor solid electrolytes. For the same reason, sorption energies are small and these zeolites are relatively easy to empty. 

All of the above may occur within zeolites. Within zeolites, the reaction rates may be enhanced, the products may be different, and the equilibria may be shifted, perhaps dramatically, especially by interaction of the guest with the labile (exposed) cations which the zeolite may contain. Zeolites with neutral frameworks lack these, and if they are otherwise empty they will generally be at least much less active. (To enhance the activity of... 

Zeosils are microporous solids with tetrahedral frameworks, which are similar to those of aluminosilicate zeolites, but which are built from pure SiOz [1, 2]. With their neutral frameworks, zeosils do not show the typical properties of zeolites such as ion exchange, hydrophilicity, and catalytic activity instead, these materials are hydrophobic and non-reactive. Zeosils find their main qrplications as highly selective adsorbents for sorbing nonpolar molecules from wet gas streams or aqueous solutions. 

Fig. 11. The 3-dimensional structure of U02(C4H404)(H20) where a neutral framework is formed from hexagonal bipyramids linked through succinate groups.

Since the discovery of neutral framework aluminophosphate molecular sieves AlP04-n, a large number of anionic framework aluminophosphates have been reported. Jilin group has carried out systematic research in this field.1144-1471 Recently, they have built up a... 

Many of the layer silicate clays common in soils are based on the mica structure (shown in Figure 2.9b) in which two tetrahedral sheets sandwich a single sheet of octahedrally coordinated cations. Consequently, they are termed 2 1 layer sihcates. Conceptually, it is useful to start with the neutral framework of the talc and pyro-phyllite structures, representing the trioctahedral (Mg in the octahedral sheet) and dioctahedral (AF in the octahedral sheet) members of the 2 1 group. These have the ideal formulae given below ... 

A milestone was achieved with the discovery of aluminum phosphate molecular sieves [1]. These compounds have neutral frameworks of very low acidity. Thus, their use as catalysts and to some extent as sorbants is limited. However, it was subsequently shown that silicon could be incorporated into the aluminum phosphate framework [3-4] (SAPO s). This discovery was followed in short order by introduction of many metals (Mg2+, Mn +, Fe -, Cq2+, Zn2+) [5-6] into the framework by isomorphous... 

The coupled derivatives are represented by AIPO4, GaP04, AlAs04, FeP04, and others which yield neutral frameworks with similarities to various silica crystal structures. The formation of an AlP04-type compound in the zeolite synthesis gel appeared to be a reasonable approach to the synthesis of phosphorus-substituted zeolites. Experiments therefore were initiated to attempt the isomorphous substitution of [PO4] in zeolite structures using the low-temperature and pressure hydrothermal gel systems generally employed in zeolite synthesis in the Union Carbide laboratories (4). 

The group of Jacobson has reported a microporous copper silicate that contains silicate layers linked by Cu04(0H2)2 groups, themselves arranged as corner-sharing chains. The structure has a neutral framework, in which alkali metal hydroxide species reside after synthesis, and possesses pores bounded by large 12MRs. So far it has not been possible fully to remove species from within the pores, so that the potential porosity of the solid has not been achieved, but it can be reversibly dehydrated. 

Scheme 6.2 Equations for the thermal generation of neutral frameworks from tem-plated microporous solids.

The adsorption of molecules by microporous solids is discussed here within six categories, according to the nature of the adsorption sites, and without considering adsorption by metal clusters introduced within the pores. Tables 7.3-7.5 give enthalpies of adsorption for examples within these categories, which vary from the interaction of neutral frameworks with small molecules to strong adsorption on acid or basic sites. 

A number of molecular sieves ean be prepared as defect-free, neutral frameworks. Traditional, inorganic examples inelude pure silicas and aluminophosphates most of the organic-inorganie MOFs also fall into this eategory. In these cases the interactions of adsorbed moleeules are dominated by dispersive interactions of the adsorbed molecules with the atoms of the framework and so can be modelled by interatomic potential models that have been parametrised according to experimental data. Because these inorganic systems are so well defined and well characterised, they make exeellent model systems for ealculations. From an... 

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