Zeolite synthesis, factors influencing

This study showed that the overall crystallization processes for mor-denite, zeolite X, and zeolite A were similar. However, the physical properties of the crystallizing system determine the rate-limiting step for a particular zeolite synthesis. In the case of mordenite in which both the viscosity of the batch composition and the morphology of seed crystals were varied, it was observed that diffusion in the liquid phase was the ratedetermining step. For zeolite X the actual growth rate on the crystal-liquid interface was the rate-limiting factor as shown by identical conversion rates for the seeded and unseeded systems. For zeolite A in the system chosen, both processes influenced the conversion rate. 

The synthesis of zeolite A, mixtures of A and X, and zeolite X using batch compositions not previously reported are described. The synthesis regions defined by triangular coordinates demonstrate that any of these materials may be made in the same area. The results are described in terms of the time required to initiate crystallization at a given reaction temperature. Control of the factors which can influence the crystallization time are discussed in terms of "time table selectors" and "species selectors . Once a metastable species has preferentially crystallized, it can transform to a more stable phase. For example, when synthesis conditions are chosen to produce zeolite A, the rate of hydroxysodalite formation is dependent on five variables. These variables and their effect on the conversion of zeolite A to hydroxysodalite are described mathematically. 

The morphology of zeolites can also be strongly influenced by the variation in different synthesis parameters. Aluminium content, template/silica ratio, water content, nature of cations present, alkalinity and degree of polymerization of the silica are all major factors which can influence the crystallization and hence the morphology of zeolites [5 - 7]. These particle morphological types can generally be characterized as either spherulitic (polycrystalline spherical) or lath-shaped (polyhedral) in nature. In both cases... 

One of the most fundamental basis of the hydrothermal synthesis of zeolites is the mineralizing role of water, which is greatly cissisted by the free OH concentration in the solution / hydrogel. Apart from this basic requirement of mineralizability, other factors like. Si / A1 molar ratio, pH of the gel, aging at lower temperature, crystallization temperature and time etc., influence the type and quality of the crystalline material in rather specific ways [1]. It is clear that the enhancement of the crystallisation rate is not much dependent of the choice of counter cation (H, Na or K) of a particular oxyanion promoter, at least for high silica... 

A membrane with the same topology formed on a porous alumina substrate also showed high separation factor (220) at 28 °C and the permeation properties were slightly influenced by water addition in the feed. The water adsorbed in the zeolite pores blocks the CO2 and CH4 permeation. The DD3R presents an all silica structure, furthermore, it is expected to be affected less by water adsorption. However, the membrane used in this work was not perfectly hydrophobic (Si/Al = 980), probably owing to the support dissolution in the synthesis gel. 

The size and shape of zeolite pores and channels are not the only factors that influence their use in catalysis - the chemical composition of the framework plays a major role in determining the catalytic properties. The chemical nature of the zeolite can be altered after the initial synthesis to enhance the catalytic properties for desired processes. The composition of zeolites is often referred to by the ratio of different T-atoms that are present. This is because the assembly process is essentially random and a regular distribution is not obtained. The positions of certain atoms cannot therefore be distinguished by diffraction techniques where the disorder of atom types makes accurate assignment impossible, solid state NMR spectroscopy has been of some use with this problem as Si chemical shifts are sensitive to the neighbouring Si/Al distribution which can lead to knowledge of the ratio of various Si environments. 

Synthesis of membranes with high permeability and selectivity, that is, oriented and thin zeolite membranes. Optimal MR operation requires the membrane flux to be in balance with the reaction rate. A large number of factors - such as the support, organic additives, temperature, and profile - have a significant influence on the microstructure and overall quality of the membrane. However, the precise correlation between the synthesis procedure and conditions and the properties of the resultant zeolite membranes is not clear. In contrast, the majority of membranes synthesized so far are MFI-type zeolite membranes that have pore diameters 5 A, which are still too big to separate selectively small gaseous molecules. Zeolite membranes with pores in the 3 A range should be developed for membrane reactors, to separate small gas molecules on the basis of size exclusion. In addition, a method to produce zeolite membranes without non-zeolite pores or defects has to be found. 

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