Crystallization kinetics from aged

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... 

Equation (10) shows that the value of the exponent q in Equation (1) is the function of the ratio Na-mo(te)/N0-ma(te) indicating that the numerical value of the exponent q increases with the increasing Na/N0 ratio, as evidenced experimentally by the analysis of the kinetics of crystallization of zeolite A from differently aged gels (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. 

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). 

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.

From kinetic studies, the optimum conditions for the synthesis of pure phase and highly crystalline n-DBA-VPI-5 sample were found out as (a) aging time of 20-30 hours (b) crystallization time of 20-30 hours (c) crystallization temperature in the range 413-418K and (d) the composition of the synthesis gel in terms of moles of oxides as,... 

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