Gel aging

Zeolites. A large and growing industrial use of aluminum hydroxide and sodium alurninate is the manufacture of synthetic zeoHtes (see Molecular sieves). ZeoHtes are aluminosiHcates with Si/Al ratios between 1 and infinity. There are 40 natural, and over 100 synthetic, zeoHtes. AH the synthetic stmctures are made by relatively low (100—150°C) temperature, high pH hydrothermal synthesis. For example the manufacture of the industriaHy important zeoHtes A, X, and Y is generaHy carried out by mixing sodium alurninate and sodium sHicate solutions to form a sodium alurninosiHcate gel. Gel-aging under hydrothermal conditions crystallizes the final product. In special cases, a small amount of seed crystal is used to control the synthesis. 

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... 

The sol is made of a stable suspended solution of metal salts or solvated metal precursors containing solid particles of nanometer diameter. Polycondensation or polyesterification results in the appearance of particles in a new phase called the gel . Aging, drying and dehydration are steps required to achieve solid-form ultra-fine particles. Coarsening and phase transformation occur simultaneously with aging. Gel drying is associated with the... 

Figure 1. Spectral gain envelope for coumarin 153 in ethanol and in aged gels. Triangles, ethanol solution crosses, gel aged for one day inverted triangles, gel aged for eleven days.

Figure 1 shows the four key steps in taking a precursor to a particular product form via sol-gel preparation formation of a gel, aging of a gel, removal of solvent, and heat treatment. The versatility of this preparative approach lies in the number of parameters that can be manipulated in each of these steps. 

Now, knowing the strong influence of particles (nuclei-I) on the overall crystallization kinetics, the increase of the crystallization rate with the gel ageing at ambient temperature (6,12,13,44,45) can be explained by the increase in the number of nuclei-I and/or the number of nuclei-II, respectively, during the gel ageing (11,12). 

The analysis of the kinetics of crystallization of different types of zeolites from aluminosilicate gels points to the conclusion that the crystallization takes place by the simultaneous growth of the constant number N0 of nuclei-I present in the system at the very start of the crystallization process and the number Na of nuclei-II released from the gel disolved during the crystallization process. Some characteristics of the crystallization systems such as the duration of the "induction period", the shortening of the "induction period" and the increase of the crystallization rate, respectively, with the gel ageing and the bimodal size distributions in the specific cases have been discussed and explained in relation to the ratio Na/N0 of particles (nuclei)-II and particles (nuclei)-I present in the crystallizing systems. 

It is well known that the low-temperature ageing of aluminosilicate gel precursor markedly influences the course of zeolite crystallization at the appropriate temperature (1-10). The primary effects of the gel ageing are the shortening of the induction period and the acceleration of the crystallization process (1-5), but in some cases the gel ageing also influences the type(s) of zeolite(s) formed (1,6,7,10). 

Thus, in many syntheses the gel ageing (8-11) or the addition of the "crystal direction agent" (aged, X-ray amorphous aluminosilicate gel) (7,12-14) is a necessary step needed for the obtaining of the desired type of zeolite at the desired reaction rate. 

Figure 1 shows the kinetics of the crystallization of zeolite X (Figure 1A) and zeolite Na-Pc (Figure 1B), respectively, at 80°C from the gels aged at 25°C for 1, 3, 5, 7 and 10 days. In all cases, zeolite X appears as the first crystalline phase, thereafter zeolite Na-Pc co-crystallizes with zeolite X. After the maximal yield of zeolite X crystallized has been attained, the fraction f of zeolite X slowly decreases as the consequence of the spontaneous transformation of zeolite X into more stable zeolite Na-Pc (17). 

The induction periods of both zeolite X and zeolite Na-Pc shortens and the maximal yield of zeolite X increases, respectively, with the increased time of gel ageing. All kinetics of zeolite X and zeolite Na-Pc, respectively, can be mathematically expressed by the simple kinetic equation (5,23-26),... 

Table I. Numerical values of the constants K and q which correspond to the kinetics of crystallization of zeolite X and zeolite Na-Pc, respectively, from aluminosilicate gels aged for various times ta...

Figure 1. Kinetics of crystallization of zeolite X (Figure A) and zeolite Na-Pc (Figure B) at 80°C, from the aluminosilicate gels aged for tQ = 1 d ( ), tQ = 3 d (A), ta = 5 d (A), ta = 7 d ( ) and ta = 10 d (o) at 25°C. f and fpc are mass fractions of zeolite X and zeolite Na-Pc crystallized at crystallization time tc. Solid curves represent the kinetics of crystallization, calculated by Equation (1) and the corresponding values of the constants K and q from Table I.

Figure 2. The changes in A. mass fractions of zeolite X (o), fpc of zeolite Na-Pc ( ) and fg of gel (A), B. concentrations C [(L) of aluminum ( ) and Cgj(L) of silicon (o) in the liquid phase and C. molar ratio [ Si/Al]s in the solid phase (o), with the crystallization time tc, during the crystallization from the gel aged for ta = 1 d.

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