Zeolites clay conversion process

The manufacture of molecular sieve adsorbents has been reviewed by Breck > and more recently by Roberts. The steps in the process are shown schematically in Figure 1.12. A variety of different starting materials may be used. In the hydrogel process the reagents are added in soluble form as sodium silicate and sodium aluminate, whereas in the clay conversion process the alumina is added as a clay mineral, usually metakaolin. Formation of the desired zeolite depends on maintenance of the correct conditions of pH, temperature, and concentration. Seeding may be used to promote crystalliza-... 

Although pillared clays could generate low cost fluidized cracking catalysts (FCC) with unique selectivity properties, they have not yet been accepted by the petroleum industry. In fact, refiners (to date) have been reluctant to field test these new catalysts because, in addition to a high tendency for coke generation, they exhibit hydrothermal stability inferior to that of those zeolites used in hydrocarbon conversion processes. The physicochemical properties of pillared clays have been reviewed elsewhere (1,2). 

Catalytic cracking is a process for the conversion of heavy petroleum cuts into gasoline. The catalyst is constituted of microspheres of about 60 /zm diameter contains 10 to 40 % of an acid Y zeolite (REY, USHY...) dispersed in a matrix (clay + binder). This catalyst circulates rapidly in the unit, its contact time with oil in the riser reactor (T 530 0) being of several seconds. The coked catalyst (about 1 wt % coke) is transported to the regenerator where the introduction of air makes possible, in a few minutes the combustion of coke at high temperature (about 700 C). The use of heavier and heavier feeds creates several problems during the regeneration steps ... 

Stable nano/mesoporous materials with mixed oxides such as zeolites, clays, and other minerals are widely used in various fields catalysis, adsorption, ion-exchange, separation, etc. because of their catalytic activity in acid/base and redox (e.g., materials with titania phase) reactions, ability to sorb selective molecules of diverse types, participate in ion-exchange reactions, providing sieve effects, etc. (Tanabe 1970, Grandjean and Laszld 1989, Rocha and Anderson 2000, Cundy and Cox 2005, Tao et al. 2006). The most important processes that utilize the selective properties of these materials are alkylation and isomerization of aromatic hydrocarbons as well as conversion of methanol to hydrocarbons, and some other reactions. Silicalite is an extreme type of the materials with the ZSM-5 zeolite structure but whose aluminum content is negligible. Therefore, unlike conventional zeolites, silicalite does not possess ion-exchange properties, and its surface has a weak affinity to water. 

The hydrothermal conversion of modified clay mineral, for example, metakaolin, is another manufacturing route to zeolites. Although the raw material costs may be as much as 15% lower than that of the hydrogel route, the major shortcoming of the process is that iron impurities in the raw material may cause an undesirable coloring of the zeolite product [15,18,99-101,104,108,116,122,123,125,126]. 

For all processes that involve the adsorption step, such as physical processes of separation or catalytic transformations, the usage of solid materials with optimised activity as adsorbents and catalysts is necessary. Various solids, such as porous materials (zeolites—molecular sieves with hierarchical porosities and natural clays), activated carbons, mesoporous silica-based materials, pillared clays and metal oxides, have shown the ability to act as adsorbents or as catalysts for the conversions of previously mentioned atmospheric pollutants. Solid materials are also used for the removal of pollutants that can be found in wastewaters. The possibilities to remove polyaromatic hydrocarbons (PAHs) and heavy metal particles using the adsorptive characteristics of activated carbon and porous materials from wastewaters have been proven [15-17]. The same classes of solids are used for the elimination of organic pollutants form wastewaters by heterogeneous catalytic oxidation processes one of the most important tasks is to eliminate phenolic compounds [13]. 

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