Aluminosilicate products, adsorption

Hydr)oxide and aluminosilicate products of rock weathering comprise much of the available surface area in soils and sediments. Such inorganic solids can be either amorphous or (micro)crystalline. In the presence of vapor or liquid water, surfaces are hydrated. In adsorption experiments, it is common to compare adsorption onto two inorganic solids that are chemically distinct yet possesses the same physical characteristics (e.g., surface area and sur ce charge). Similarly, the adsorption behavior of two chemically distinct solutes... 

Considering all we know up to now, the specific properties of zeolites can be summarized as follows. Zeolites are aluminosilicates with defined microporous channels or cages. They have excellent ion-exchange properties and can thus be used as water softeners and to remove heavy metal cations from solutions. Furthermore, zeolites have molecular sieve properties, making them very useful for gas separation and adsorption processes, e.g., they can be used as desiccants or for separation of product gas streams in chemical processes. Protonated zeolites are efficient solid-state acids, which are used in catalysis and metal-impregnated zeolites are useful catalysts as well. 

Erionite has been synthesized at i00°-I50°C from a (Na,K) aluminosilicate gel with Si02/AUOs = 10. X-ray and electron diffraction results on the product show intergrowths of the related offretite structure, which is a large-pore zeolite. Adsorption capacity for n-hexane is consistent with the density but adsorption rates are far slower than for zeolite A. Adsorption rates for n-octane are even slower but still better than for natural erionite. Hydrocracking tests on a C /Cq naphtha show strong selectivity for converting normal paraffins to Cf gas, particularly propane. As temperature is increased, other components of the naphtha feed are cracked and selectivity decreases. 

Our discussion of mineral surfaces will be restricted to simple oxide and hydroxide minerals that are widely used in adsorption studies (Table 9-1). Under strong weathering conditions, these minerals may comprise a substantial fraction of the available surface area in soils and aquifers. More complex minerals, including parent material and partially weathered products (especially aluminosilicates), are of equal or greater importance in most other subsurface environments. Excellent reviews of the equilibrium and reaction chemistry of aluminosilicate surfaces are available (Voudrias and Reinhard, 1986 Mortland, 1970). 

The short-term potential for solid solution formation is low for minerals with small solubility products, such as aluminum oxides and aluminosilicates, because spontaneous dissolution and recrystallization is very slow in these minerals. Without recrystallization, trace metals cannot be incorporated into the mineral structures. Movement of metal ions into these mineral crystals by solid diffusion is not possible on the time scale of adsorption experiments ionic diffusion into most crystalline solids is negligibly slow at all but extremely high temperatures. Nevertheless, metals could diffuse into imperfect solids along interstices, pores, or other structural defects. 

Although adsorption processes represent an extremely large application of molecular sieves (49), applications in the area of heterogeneous catalysis have received the most attention for borosilicate molecular sieves. Due to the inherently weaker acidity of borosilicates relative to aluminosilicates, a number of advantages in using borosilicates have been reported due to improved product distributions or reaction selectivities. 

The development of new or improved processes in catalysis and adsorption were in many cases induced by the development of new catalytic materials and adsorbents. In this context, the synthesis of new aluminosilicates is a continuing challenge in zeolite science. The present review, discussing the synthesis principles of selected more recent zeoUtes, has shown that there is still much room for innovation in this field. It can be expected that by the use of new classes of templates (one recent example is that of the metallocenes) new structures wiU be synthesized in the future. Moreover, with the availability of more and more sophisticated tools for modelling zeolite and template structures and their interactions, it will probably be possible to tailor templates for a given (or a theoretical) zeolite structure. Finally, beside the exploration of new templates and new reaction compositions, the influence of the synthesis conditions on the products should not be overlooked, e.g. changing the reaction parameters from subcriti-cal to supercritical conditions could well have an influence on the materials which are formed. 

To date, potentiometric titration is still a main approach to study the surface acid base chemistry of clay minerals. Only some papers deal with the dissolution of a solid matrix resulting in various hydrolyzed aluminum species, silicic acid and their product hydrous aluminosilicates, though their interaction with a clay surface should be considered in the modeling description. The surface complexation model (SCM) was successfully applied in a recent paper [6] to interpret surface acid-base reactions involving the dissolution of illite clays during prolonged titration. Voluminous literature on ion adsorption and surface complexation... 

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