Aluminosilicate surface, nature

Naturally, structures (d) and (f) do not exhaust all possible states of low-coordinated A1 atoms on the surface of the oxides considered. The calculations, however, seem quite sufficient to suggest that water molecule coordination by a LAS is energetically less favorable for aluminophosphate than for aluminosilicate surfaces. This conclusion is also in accordance with IR data, which indicate that LASs of the both oxides quite similarly interact with pyridine, whereas the LASs of aluminophosphates do not coordinate C02 molecules (136). Indeed, in the case of a sufficiently strong base (pyridine), adsorption interaction appears stronger than the structural coordination and therefore stabilizes the A1 atom in the adsorption state. On the contrary, for C02, which is certainly a very weak base, the interaction is strong enough in the case of aluminosilicates but is insufficient for the adsorption stabilization of aluminum in aluminophosphates. 

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

In addition to stabilizing organic products by reaction with metal-exchanged clays, as indicated above, aluminosilicate minerals may enable the preparation of metal organic complexes that cannot be formed in solution. Thus a complex of Cu(II) with rubeanic acid (dithiooxamide) could be prepared by soaking Cu montmorillonite in an acetone solution of rubeanic acid (93). The intercalated complex was monomeric, aligned with Its molecular plane parallel to the interlamellar surfaces, and had a metal ligand ratio of 1 2 despite the tetradentate nature of the rubeanic acid. 

Sorption processes are influenced not just by the natures of the absorbate ion(s) and the mineral surface, but also by the solution pH and the concentrations of the various components in the solution. Even apparently simple absorption reactions may involve a series of chemical equilibria, especially in natural systems. Thus in only a comparatively small number of cases has an understanding been achieved of either the precise chemical form(s) of the adsorbed species or of the exact nature of the adsorption sites. The difficulties of such characterization arise from (i) the number of sites for adsorption on the mineral surface that are present because of the isomorphous substitutions and structural defects that commonly occur in aluminosilicate minerals, and (ii) the difference in the chemistry of solutions in contact with a solid surface as compound to bulk solution. Much of our present understanding is derived from experiments using spectroscopic techniques which are able to produce information at the molecular level. Although individual methods may often be applicable to only special situations, significant advances in our knowledge have been made... 

Only a few materials can be successfully studied in this way. One of them is the layered natural aluminosilicate crystal, muscovite mica, which is available in large crystals and can be cleaved in a controlled manner to produce two molecularly smooth new surfaces. 

Silica and aluminosilicate fibers that have been exposed to temperatures above 1100°C undergo partial conversion to mullite and cristobalite (1). Cristobalite is a form of crystalline silica that can cause silicosis, a form of pneumoconiosis. IARC has determined that cristobalite should be classified as 2A, a probable carcinogen. The amount of cristobalite formed, the size of the crystals, and the nature of the vitreous matrix in which they are embedded are time- and temperature-dependent. Under normal use conditions, refractory ceramic fibers are exposed to a temperature gradient, thus only the hottest surfaces of the material may contain appreciable cristobalite. Manufacturers Material Safety Data Sheets (MSDS) should be consulted prior to handling RCF materials. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

The use of surfactant-modified zeolite (SMZ) as a permeable barrier sorbent may offer several unique advantages when dealing with mixed contaminant plumes. Zeolites are hydrated aluminosilicate minerals characterized by cage-like structures, high internal and external surface areas, and high cation exchange capacities. Both natural and synthetic zeolites find use in industry as sorbents, soil amendments, ion exchangers,... 

However, the multicipliclty of bands and relatively intense character of these concerted vibrational modes are not observed in the V loaded gels under study. Thus, significant development of long ribbons of V205.nH20, or a polymeric nature of this surface species, is not ejqiectecrto exist in the V-loaded aluminosilicates under study. 

Natural and synthetic zeolites, a family of aluminosilicates with pores and cavities in the range 0.4-1.5 nm, are well-known heterogeneous catalysts and sorbents. Zeolite-incorporated cellulosic fibers and membranes could be suitable for medical antibacterial materials, deodorizers, absorbent pads, sanitary napkins, gas separators, ion exchangers, and so forth however, the complete and continual use of the whole zeolite surface is not easy in the... 

Mintova et al. studied deposition of zeolite A on various cellulose fibers pretreated chemically and/or mechanically [151]. It was shown that the amount of zeolite deposited was controllable by suitable fiber pretreatment with ball-milling or with diethyl ether under ultrasonic action. The reactive high-concentration hydroxyl groups on the structurally loosened celluloses seem to interact with aluminosilicate species and thus promote the formation of nuclei for zeolite crystallization. Pretreatment of natural cellulose fibers with alkali provides another simple route for anchoring preformed zeolite crystallites onto the cellulose surface. 

By October 1949, I started experimenting with crystallization at 100 C, reasoning that the higher water content zeolites with larger pore volumes and, presumably, larger pore sizes, would be more likely to crystallize at temperatures lower than 200 -300 C. In nature the anhydrous aluminosilicates were formed at relatively high temperatures, and the hydrous ones were believed to have been formed later as the earth s surface cooled. Not surprisingly, when we first tried low temperature synthesis with relatively insoluble silica and alumina in mildly alkaline solutions, there was no reaction in reasonable time periods. 

Of great interest is the question of the role of trigonal aluminum, which is usually assumed to act as a LAS. Such a center should be quite typical of A1203, where it may appear as a result of surface oxygen vacancy formation. These vacancies may either develop due to dehydroxylation or be of a biographical nature. A similar situation may take place in the case of such mixed oxides as amorphous aluminosilicates. Uytterhoeven, Cristner, and Hall 123) have concluded that trigonal aluminum could also appear as a LAS upon dehydroxylation of H forms of zeolites. Their scheme was criticized, however, by Kiihl 124), who has undertaken X-ray fluorescence studies of the dehydroxylated forms of faujasites and found that the dehydroxylation was accompanied by dealumination of a zeolite framework with formation of extralattice aluminum which could also exhibit the Lewis acidity. 

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