Theoretical and Practical Aspects of Zeolite Crystal Growth

A significant role of the particulate properties (size, shape, size distribution) of zeolites in the mode and efficiency of their application, and a possibility of the controlling the particulate properties through a knowledge on the mechanism and kinetics of crystal growth as well as the influence of crystallization conditions on the crystal growth of zeolites is outlined in the Introductory section (Section 1.). Analysis of the crystal growth kinetics during crystallization of different types of zeolites from both hydrogels and clear (alumino)silicate solutions (Section 2.) showed the general feature of zeolite crystal growth does not depend on the type of zeolite, and variety of conditions under even a single type of zeolite may be synthesized: The size, L, of zeolite crystals increases linearly during the main part of crystallization process, starts to decrease (decline from the linear rate) near the end of the crystallization process. The crystals attain their final (maximal) size when the amorphous aluminosilicate precursor is completely dissolved and/or the concentrations of reactive silicate, aluminate and aluminosilicate species reach the values characteristic for the solubility of zeolite formed under the given synthesis conditions. Three characteristic profiles of the growth curves with respect to the origin of the crystal growth process, i.e., L = Lm = 0 at tc = 0, L = Lm = 0 at 0 0 at tc = 0 are discussed and rationally explained in accordance with the synthesis conditions. The crystal growth kinetics of zeolites synthesized under specific synthesis conditions and/or by special methods may deviate from those characteristic profiles. Influence of the most important crystallization conditions (temperature, ageing, seeding) and composition-dependent parameters (alkalinity, dilution, ratio between Si and other tetrahedron-forming elements, presence of inorganic cations, and organic template concentration) on the kinetics of crystal growth and/or particulate properties (size, shape) of different types of zeolites are presented, and rationally explained whenever possible (Section 2.2.). Existing models of the crystal growth of zeolites are critically evaluated in accordance with the known growth theories, taking into consideration the particularities of zeolite crystallizing systems (Section 3.). Based on the findings that:  ; ; ; a linear relationship between tc and L caused by a layer-by-layer growth of zeolites cannot be expected for diffusion-controlled crystal growth,  ; ; ; the activation energies (30-130 kJ/mol) obtained by measuring the linear growth rates of different types of zeolites are considerably higher than the activation energy (12-17 kJ/mol) of diffusion, and  ; ; ; besides the chemical interactions between the reactive species from the solution and the surface of growing crystals (dehydration, condensation), rearrangements of the reactive species on the crystal surface and repulsive forces between the reactive species and crystal surface may also contribute the relatively high apparent activation energy of zeolite crystal growth, most authors consider surface reaction (surface integration step) as the rate-limiting step of the crystal growth of zeolites. Analysis of the interactions between different species (TPA-Si inorganic-organic composite species, primary 2.8 nm species formed by aggregation of the inorganic-oragnaic composite species, and secondary aggregates (10 nm) of the primary species) existing in the reaction mixtures during crystallization of siliceous zeolites (silicalite-1, Si-BEA, Si-MTW), leads to the conclusion that the crystal growth of these zeolites occurs by the first-order surface integration of the precursor species (inorganic-organic composite species and/or primary 2.8 nm species) to the growing zeolite crystals. On the other hand abundant finding that the crystal growth rate of aluminum-rich zeolites depends on the concentrations of both silicon and aluminum in the liquid phase leads to an assumption that different aluminate, silicate and aluminosilicate species from the liquid phase participate the surface reactions. Analysis of the kinetics of crystal growth of zeolite A in accordance with the existing growth theories shows that the crystal growth rate of zeolite A is proportional to the fluxes of aluminum and silicon in the liquid phase, and thus that the growth of zeolite crystals (at least zeolite A) is governed by the reactions of monomeric and/or low-molecular aluminate, silicate and aluminosilicate anion from the liquid phase on the surfaces of growing zeolite crystals in accordance with the Davies and Jones model of dissolution and growth. In Section 4. was shown that due to manifold interdepences between critical processes of zeolite crystallization (gel dissolution, nucleation and crystal growth of zeolites), only the population balance methodology enable the modeling and simulation of crystallization processes using different mechanisms of gel dissolution, nucleation, and crystal growth of zeolites based on fundamental theories of the particulate processes which occur during the crystallization. Based on the general principles of the population balance, modeling and simulation of crystallization of zeolites A and ZSM-5 from hydrogels, with special emphasis to crystal growth kinetics and influence of the heating rate of the reaction mixture on the crystal growth, are shown as examples. Although the general and many specific principles of the crystal growth of zeolites are known, as it is elaborated in this Chapter, some very important question such as: (i) which species (silicate monomers, inorganic-organic composite species and/or primary 2.8 nm species) are real precursors for the growth of siliceous zeolites, (ii) what is (are), among different aluminate, silicate and aluminosilicate species in the liquid phase, key precursor(s) for the crystal growth of different aluminum-rich zeolites, and (iii) how in the reality occur(s) the reaction(s) between the reactive species from the liquid phase and the surface of growing zeolite crystals, are still open, and are excellent challenges for the continuation of the work in this exciting area.