Crystallization Kinetics of Zeolites

Crystallization fields of zeolites growing from homocationic Na gels at around 100 °C have been studied extensively at a number of laboratories (cf. 3,11, 21). Sand and coworkers recorded the coexisting phases for different reaction times (19). The kinetics of zeolite crystallization in the Na system also has received attention (12, 14, 15). Studies of low-temperature crystallizations from Li gels, on the other hand, have been much more limited (13). 

Prior to discussing the kinetics of zeolite crystal nucleation and growth it is beneficial to consider several thermodynamic aspects which have bearing on the phase transformation process. Reports on this topic are not abundant in the zeoHte crystallization hterature, but several papers will serve to illustrate various issues which should be important in these systems. 

Kinetics of Zeolite Formation. Since the D5R synthesis mechanism, if operative, is clearly disturbed by DMSO (vide supra), we also carried out a number of time-dependent syntheses in the absence of DMSO in the hope of being able to relate the kinetics with the D5R concentration present. Starting from a molar composition of 40 Si02 1 A OjX 5 NaOH 1000 H20 X at T - 190 °C, various organics and/or seed crystals (X) were added and the solid products were characterized after various synthesis times. Properties of the products obtained when the highest crystallinities were reached are summarized in Table II. 

In contrast to more or less well defined kinetics of the crystal growth (5,6,12-16), various nucleation mechanisms have been proposed as zeolite particles forming processes. Most authors explained the formation of primary zeolite particles by nucleation in the liquid phase supersaturated with soluble silicate, aluminate and/or aluminosilicate species (1,3,5,7,16-22), with homogeneous nucleation (1,5,7,17,22), heterogeneous nucleation (5,2 1), cell walls nucleation (16) and secondary nucleation (5) as dominant processes of zeolite particles formation, but the concepts dealing with the nucleation in the gel phase are also presented in the literature (2,6,11,12,1 1,23-25). 

Equation (5), respectively, during the main parts of the crystallization processes (autocatalytic stage) indicates that Equation (1) is equivalent to Equation (5) and that both kinetic forms are suitable for the kinetic analysis of the autocatalytic stage of zeolite crystallization. 

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). 

Fig 11 Crystallization kinetics of zeolite Beta, from reference hydrogels, at... 

Figure 5.Crystallization kinetics of zeolite ZSM-48 in presence of Li+, Na+ and K+ ions, without A1 (a) and in presence of 0.5 mole AI2O3 in the gel (b) 5Na20 2.5HMBr2 (0 or 0.5)Al2C>3 60SiO2 3000H20.

Twenty-eight kinetics of crystallization of different types of zeolites have been analysed using kinetic equation fz = K t KQt /(1 - Ka t ) (fzis the fraction of zeolite formed at crystallization time tc, and K, K0, KQ, and q are constants). The analysis showed that the exponent q is a function of the ratio between number Na of the particles formed by the growth of nuclei released from the gel during its dissolution (autocatalytic nucleation) and the number Nq of particles formed by the growth of heteronuclei. Functional dependences between q, K, K0, KQ, NQ, Nq, Kg (growth rate constant) and other factors relevant to zeolite crystallization have been established and the influences of these factors on the characteristics of zeolite crystallizing systems have been discussed. 

In some zeolite crystallization in heterogenous aluminosilicate systems, temperature is mainly a kinetic factor whereas the nature and composition of crystals are determined by concentrations and correlations of components. Studies of their chemical structure and the peculiarities of crystallization kinetics show that a solubility equilibrium exists between the solid and liquid phases of gels. Gel crystallization on heating is preceded by increased component concentrations in the liquid phase, which determines the composition of the zeolite crystals formed. Decreases in liquid component concentrations are compensated by dissolution of the solid phase of gels. This explains the linear rate of crystal growth during the first part of the crystallization process. Crystal size distribution for zeolite A and the linear rate of crystal growth indicate the autocatalysis of zeolite crystallization. 

Studies on the mechanism of zeolite crystallization have received extensive attention from many investigators (4, 16, 22, 24, 29, 30, 33, 37, 39). However, the main questions of the mechanism of crystallization referring to kinetics, such as the autocatalysis of the crystallization process, alkalinity effect on the rate of crystallization, the nature of the induction period, and seeding effects, have been insufficiently studied. The question of the role of solid and liquid phases in the formation of zeolite crystals in the heterogenous aluminosilicate systems also is still being discussed (16,22,25, 30,31,37,39). 

During recent years, new data have been obtained and published on the influence of temperature and composition of the initial mixtures as well as seeding and addition of complexing reagents upon the kinetics and final results of the process of zeolite crystallization (16, 20, 23, 31, 32, 33,34,35,36,37,39, 41). 

The pecularities of zeolite crystallization reported in several works (6, 13, 16, 20, 24) are the induction period, autocatalytic nature of the process, growth of crystallization rate with increasing alkali concentration, and effects of seeding on the kinetics. 

Figure 12. Crystallization kinetics of zeolite X from pumices at 3 different finenesses, treated at 65°C with S/L = 1/25 and 18% NaOH solution Fraction passing at 320 mesh ...

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