Zeolites crystal design

A 1969 paper presented a mathematical crystallization model for the continuous crystallization of zeolite A [174]. The successful implementation of continuous synthesis of zeoHtes must accommodate the relatively slow crystallization rates with the reactor design to allow sufficient residence time at the necessary digestion temperature. A recent patent publication describes continuous zeolite synthesis using microwave heating, which couples the often significant advantages of faster zeolite crystallization under microwave radiation with a continuous synthesis, dewatering and work-up process [175],... 

The combination of synthesis and modification techniques gives us a chance to rationally design or tailor zeolite structures. For example, we can increase shape selectivity by modifying or eliminating active sites on the external surface of zeolite crystals. Although this outside surface may represent only 2-5 % ot the total surface area, acid sites located there are more accessible to reacting molecules than acid sites in the pores. As these catalytic sites are not shape selective, they catalyze a disproportionate amount of non-shape selective reactions. 

In addition, several other experiments designed by Koizumi and coworkers showed that the growth of zeolite crystal would not be immediately affected by the change of the composition of the liquid phase. For example, a small amount of liquid was taken from the synthetic system of zeolite Y crystallized for one, two, three, and four days, respectively. The sodium silicate solution was used to tune the composition of this solution to that for the crystallization of zeolite P. The resultant liquid phase was continually crystallized under the original crystallization conditions. The growth of zeolite NaY crystals did not stop immediately but continued for a considerable length of time as shown in Figure 5.19. 

Microporous ceramics are being designed to extend the concept of zeolites by building structures that do not necessarily have the well-defined walls of a zeolite crystal but still have the large cavities an ordered alignment of the cavities can make it appear that the material is crystalline. The lUPAC definition is that a microporous material... 

The systematic analysis of the parameters governing the crystallization of zeolite ZSM-5 also makes a chance to define the crystallization mechanism of ZSM-5 zeolites in SDA-free system. To achieve the controllable synthesis of ZSM-5 zeolites with designed particulate and chemical properties, the thorough understanding of the critical processes during the crystallization is the only way. Such goal is achieved by both efforts, the kinetic analysis of crystallization processes and the alkaline, post-treatment of final products. 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

Thorium metal, 24 759-761 in alloys, 24 760-761 preparation of, 24 759-760 properties of, 24 760-761 reactions of, 24 761 Thorium nitrate, 24 757, 766 Thorium oxalates, 24 768-769 Thorium oxide, 21 491 Thorium oxides, 24 757, 761-762 Thorium oxyhalides, 24 762 Thorium perchlorate, 24 764 Thorium phosphates, 24 765-766 Thorium pnictides, 24 761 Thorium sulfate, 24 764 Thorium-uranium fuel cycle, 24 758-759 Thorocene, 24 772 Thorotrast, 24 775-776 3A zeolite. See Zeolite 3A Three-boiling beet sugar crystallization scheme, 23 463-465 Three-color photography, 19 233-234 3D models, advantages of, 19 520-521 3D physical design software, 19 519-521 3D QSAR models, 10 333. See also QSAR analysis... 

Probably the only general statement which can be made about the experimental studies on zeolites is that the majority of published data is inapplicable directly to natural minerals. This is due either to the excessively high temperatures under which the experiments are performed, outside of the physical limits of zeolite stability, or to short time spans of observation which do not allow the silicates to come to equilibrium with the fluids of the experiments. Those studies designed to determine zeolite stability indicate that the most silica-poor alkali zeolite, analcite, is not stable above 180°C. More silica-rich species will be found below this temperature. However, the reasons for the crystallization of one or another of the silica-rich alkali zeolites are not yet elucidated. 

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