Poorly crystalline aluminosilicates

Heterogeneous acidic materials are used widely as cracking and isomerization catalysts in the petroleum industry. These materials consist of acidic clays or synthetic, porous, crystalline aluminosilicates (molecular sieves). They appear to function as protonic acids. The counterion in these cases is the catalyst surface, which is a very poor nucleophile. 

All of the organic matter and part of the aluminosilicates, hydroxyoxides, and silica in soil exist in structures too small or too poorly crystalline to be detectable by x-ray diffraction, These amorphous materials are not well understood, but they should logically be among the most reactive of soil components, because their structure is so open and their surface area so great, They represent a transition state between unweathered parent materials and well-crystallized secondary soil minerals. 

Aluminum is an abundant element that is present as a major element in all soils. However, it appears mainly in aluminosilicate minerals (Chapter 8), gibbsite being almost the only aluminum mineral appearing in an important extent in soils (Huang et al. 2002). There are also poorly crystalline aluminum hydroxide or oxohydroxide colloids that are highly reactive, which can strongly adsorb either nutrient or pollutant ionic species. 

Isomorphorous Substitution of AlPO with Metal Cations. - The crystalline structure of AlPO-n consists of alternating alumina and phosphate tetrahedra, which is essentially the substitution of Si04 tetrahedra in aluminosilicate zeolite with a pair of Al and P " tetrahedra, as schematically shown in Figure 3. Consequently, the AlPO-n crystal is electrovalently neutral provided no extra framework cations exist and, consequently, AlPO-n exhibits no ion-exchange capacity. This is quite different from aluminosilicate zeolites which are capable of ion-exchange. Therefore, AlPO-n itself exhibits low activity for acid catalyzed reactions and poor gas adsorption capacity and its primary use is as a support for metal catalysts (surface areas approach 100 m /g). 

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