Amorphous alumino-silicate

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

Realuminated samples R-3 and R-4 were prepared by hydrothermal isomorphous substitution (6.14) by stirring 1 g of sample D-2 in 50 ml of 0.5M and 2M KOH at 80°C for 24 hours. The treatment of amorphous alumino-silicates A-5 and A-6 in 0.5M KOH under the same conditions gave samples AR-7 and AR-8, respectively. The conditions of preparation of all samples are summarised in Table I. 

The most important feature of zeolites (and zeotypes), in the context of catalysis, is not their range of acid-base properties, since that is also available with amorphous alumino-silicates. It is the presence of a regular structure containing... 

Zeolite crystallization is a phase transformation process, since an amorphous alumino-silicate gel phase usually forms quickly after mixing the reagents in the appropriate concentrations. Clear solution syntheses have been reported, but the yield from them is typically not sufficient to generate commercial interest. It is generally agreed that the transformation from amorphous gel to crystalline zeolite occurs through the solution phase via dissolution of the amorphous gel and crystallization of the desired zeolite phase. Consequently, the normal processes of nucleation and crystal growth must occur from within the solution. 

H2O Vapor Adsorbed on Crystalline and on Amorphous Alumino-Silicates... 

Vanadyl phosphates (VPO) and multiple component molybdate (MCM) are good examples of catalysts, and alpha alumina, amorphous silica and alumino-silicates are good examples of catalyst supports that can be fabricated in the form of 45 to 150 im diameter spray dried porous spheres with attrition resistance improved by a relatively thin peripheral layer rich in amorphous silica, amorphous alumina, or phosphorus oxides. The hard phase component or precursor is selected in each case so that it will not interfere with the catalytic performance of the catalyst. 

III. The dissolution rate is controlled by diffusion through a continuously growing precipitate layer that forms on the silicate surface. Such a protective barrier has been postulated to consist of an amorphous alumino-silica phase (14) or a mono- or multi-phase crystalline alumino-silicate assemblage (15). 

The microporous alumino-silicate zeolites (Types A, X, and mordenite are frequently used) provide a variety of pore openings (3-10 A), cavity and channel sizes, and framework Si/Al ratios. They are also available in various cationic exchanged forms (Na, K, Li, Ag, Ca, Ba, Mg), which govern their pore openings and cationic adsorption site polarities. They are highly hydrophilic materials and must be dehydrated before use. The amorphous adsorbents contain an intricate network of micropores and mesopores of various shapes and sizes. The pore size distribution may vary over a wide range. The activated carbons and the polymeric sorbents are relatively hydrophobic in nature. The silica and alumina gels are more hydrophilic (less than zeolites) and they must also be dehydrated before use. 

The recently discovered mesoporous (alumino)silicates, e.g. MCM-41, consist of a regular array of uniform one dimensional pores with diameters in the range 15-100 A and have properties intermediate between those of amorphous Si02 and AljO, and microporous sieves [23]. This has considerably extended the size of molecules that can be adsorbed the immobilization of enzymes in MCM-41 has even been achieved [24]. 

Zeolites are hydrated alumino-silicates and are used widely in the chemical and environmental industries. Their activity is influenced by the ratio of silica to aluminum. Amorphous silica-alumina catalysts have a lower activity than zeolitic catalysts and are used in mild hydrocracking reactions where acidic surfaces are required. The acidity is maintained by oxygen atoms attached to the aluminum atoms. 

Systematic studies carried out at C.F.R.I. pertainining to catalytic vapour phase synthesis of pyridine bases, namely 2 4-picolines through cyclodehydrogenation reaction of acetaldehyde and anunonia have been described. Metal oxide modified amorphous silica-aliunina and crystalline alumino-silicate namely ZSM-5 zeolites were found to be active and selective catalysts towards the formation of the lower pyridines bases. The method of preparation, pretreatment vis-a-vis the acidity have been found to affect the catalytic activity and selectivity. It is interesting to observe that metal modified crystalline alumino-silicate ZSM-5 is more selective than amorphous silica-alumina for the formation of 2 4-picoline. A plausible reaction mechanism based on the findings of this study has been proposed. 

Clay minerals are a complex group of finely crystalline to amorphous hydrated silicates, mainly alumino-silicates, which were largely formed by alteration, or weathering of silicate minerals. 

A rather complex mixture is that constituted by the Italian tuffs [8, 53, 54], which contain various hydrated phases, namely three zeolites (phillipsite, chabazite and analcime, Table 1), unreacted glass (pumice, glass fragments, scoriae), hydrated ferric oxides and an X ray amorphous gel-like alumino-silicate, in addition to some non-hydrated phases, such as sanidine and biotite crystals. The most concentrated phases are phillipsite and chabazite, the total content of which usually amounts to 50% or more. Since Italian tuffs, simply because of the elevated contents of the above zeolites, are gaining a pre-eminent position in many industrial, agricultural and environmental applications, it is of great interest to have a rapid and reliable method available for evaluating zeolite content in the rock. 

As the synthesis proceeds at elevated temperatures, zeolite crystals are formed by a nucleation step, followed by a crystal growth step involving assimilation of alumino-silicate from the solution. The amorphous gel phase continues to dissolve, thereby replenishing the solution with alumino-silicate species. This process results in the transformation of amorphous gel to crystalline zeolite. 

The synthesis of most molecular sieve zeolites is carried out in batch systems, in which a caustic aluminate solution and a caustic silicate solution are mixed together, and the temperature held at some level above ambient (60-180°C) at autogenous pressures for some period of time (hours-days). It is quite common for the original mixture to become somewhat viscous shortly after mixing, due to the formation of an amorphous phase, i. e., an amorphous alumino-sdicate gel suspended in the basic medium. The viscous amorphous gel phase normally becomes less viscous as the temperature is raised, but this is not universally true, as in the case of some NH40H-based systems which remain viscous throughout the synthesis. The amorphous gel can be filtered from the solution and dehydrated by conventional drying methods. 

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