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The synthesis of high silica zeolites in many cases implies the usage of organic structural direction agents (SDA). However, to manifest their functionalities, the SDA occluded inside the channels of the as-synthesized structure should be removed, usually by a high temperature treatment (calcination). In this paper, the positron annihilation lifetime spectroscopy (PALS) was used to monitor the development of accessible spaces, their sizes and distributions in MFI zeolites, ZSM-5 and silicalite-1 in order to give an additional insight in the process of the SDA removal. For that purpose, a conventional PALS setup with 22Na positron source was applied. It was established that there is a pronounced difference between positron annihilation data for these two zeolites of the same structural type. The samples were additionally analysed by X-ray powder diffraction at room temperature with a crystal structure refinement and thermogravimetry.

Crystallization of zeolite ZSM-5 from a diluted heterogeneous system (12.5Na2O-Al2O3-8TPABr-60SiO2-4000H2O) was investigated by various experimental methods such as, X-ray diffraction (XRD), electron diffraction (ED), infrared spectroscopy (FTIR), X-ray fluorescence (XRF), scanning-electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), thermo-gravimetric analysis (TG-DTG), particle size analysis (PSA) and Inductive Coupling Plasma (ICP) Emission Spectrometry. The crystallization process is characterized by a very long “induction period” (95 % of the entire reaction time) and very fast transformation (5 % of the entire reaction time) of amorphous to crystalline phase (zeolite ZSM-5) at the end of crystallization process. Analysis of the obtained results have shown that the crystallization process takes place by a chain of processes: (i) formation of “primary” amorphous aluminosilicate precursor (gel) at room temperature, (ii) formation of “secondary” amorphous aluminosilicate precursor (“worm-like” particles – WLPs), (iii) formation of “tertiary” amorphous aluminosilicate precursor (condensed aggregates – CAs) by aggregation of the WLPs and densification (condensation) of aggregates and (iv) formation of nuclei and their growth in the matrixes of CAs ; these processes result in the formation of fully crystalline zeolite ZSM-5 in the form of polycrystalline aggregates.

Influence of the simultaneous change of batch alkalinity ( A = [Na 2 O/H 2 O] b ) and total batch silica concentration ( tsc = [SiO 2 /H 2 O] b ) of the reaction mixture, at two characteristic batch molar ratios, y ≡ [SiO 2 /Al 2 O 3 ] b (1.3 and 2.0), on the duration of crystallization, reaction yield and the particulate properties (size and shape) of the crystalline end product (zeolite A) was investigated. Sharp decrease of the duration of crystallization process with simultaneous increase of A and tsc is caused by simultaneous increase of the number of nuclei formed in gel matrix, rate of their release from the gel matrix and their growth with the increase of the alkalinity of the reaction mixture. Although the reaction yield, Y R , is proportional to the total batch silica concentration, tsc , the ratio Y R / tsc slightly decreases with increasing alkalinity as a consequence of the increasing solubility of zeolite A with the increase of the alkalinity. Specific number of crystals (number of crystals per unit mass of the crystalline end products), N s increases with simultaneous increase of A and tsc as a consequence of the increase of the number of nuclei formed in the gel matrix with increase of alkalinity of the reaction mixture. On the other hand, a complex dependence of the crystal size distribution on A , tsc and y , is a consequence of influence of the mentioned chemical factors not only on the number of nuclei, but also on their distribution in the gel matrix. Four different morphological forms of zeolite A, namely, (i) regular cubic crystals with “sharp” edges and apexes, (ii) cubic crystals with truncated edges, (iii) cubic crystals with “rounded” edges and apexes and (iv) pseudo-spherical, face-less particles are obtained during hydrothermal treatment of investigated hydrogels. Influence of the chemical factors on the formation of particular morphological forms of zeolite A are discussed and the possible mechanisms of their formation are considered.

Changes of the concentrations, CAl, of aluminum and, CSi of silicon in the liquid phase as well as of the dimension, Lm, of the largest crystals were measured during crystallization of zeolite A from the amorphous aluminosilicate precursors dispersed in alkaline solutions containing different amounts of Na+ ions. Analysis of the measured data have shown that the growth rate of zeolite A crystals is size-independent, and that Na+ ions, together with aluminate and silicate anions from the liquid phase take part in the surface reaction of the crystal growth of zeolite A. Kinetic equations which describe the change of Lm as a function of the change in concentrations of Na, Al and Si in the liquid phase are derived and evaluated by correlation with the measured changes of Lm.

Kinetics of nucleation, crystal growth and entire process of crystallization of zeolite A in the presence of different sodium salts were analyzed by population balance method. Both the rate of crystallization and rate of crystal growth of zeolite A increase with increasing concentration of sodium sulfate dissolved in the liquid phase of the crystallizing system, but equimolar amounts (with respect to Na + ions) of different sodium salts influences the rate of crystal growth only on the extent which can be attributed to experimental errors characteristic for this type of experiments. Thus, it was concluded that only sodium ions and not the associated anions influence the rate of crystal growth of zeolite A. Insensibility of the particulate properties of the crystalline end products on the presence of different anions supports this conclusion was explained by the “memory”effect of specially prepared amorphous aluminosilicate gel precursors.

The principle of the “memory” effect of amorphous aluminosilicate gel precursors, namely that crystal size distribution in the crystalline end product depends exclusively on the number and distribution of nuclei in the gel matrix, previously evidenced theoretically by simulation of zeolite crystallization under various conditions, was evaluated experimentally by analysis of the data obtained from specially designed kinetic experiments of zeolite crystallization. It was found that particulate properties (specific number of crystals, crystal size distribution, size of the largest zeolite crystals) considerably depend on the conditions of gel preparation, and thus on the specific number of nuclei and their distribution in the gel matrix. On the other hand, despite considerable differences in the rates of nucleation and crystal growth during crystallization from appropriate precursor, the particulate properties of the crystalline end products as well as number and distribution of nuclei in the gel matrix are not affected, or are affected very slightly by the crystallization condition, and thus by the rates of the relevant critical process (gel dissolution, nucleation, crystal growth). Hence, it was shown that invariability of the size distribution of zeolite crystals in the crystalline end products (zeolite) crystallized from the amorphous aluminosilicate precursors prepared in the same way, but differently treated prior and during crystallization is a consequence of the constancy in the number and distribution of nuclei in the matrix of appropriate gel precursor. This result is in accordance with the result obtained previously by simulation, and thus the principle of the “memory” effect of amorphous aluminosilicate gel precursors was evidenced not only theoretically but also experimentally.

A critical analysis was carried out for the purpose of understanding the role of subcolloidal (nanosized) (alumino)silicate precursor species in the early stage of crystallization of zeolites in heterogeneous systems (hydrogels). The formation and evolution of these subcolloidal species in both the solid and the liquid phases were investigated by various experimental methods such a scanning electron microscopy (SEM, FE-SEM), transmission electron microscopy, atomic force microscopy, particle size analysis, pH measurement, atomic absorption spectroscopy, and dynamic light scattering, after careful separation of intermediates from reaction mixture by two-step centrifugation treatment. The results revealed that a chain of processes (i) the formation of low-molecular-weight (LMW) silicate species, by dissolution of Al-enriched amorphous silica, and their aggregation into about 3 nm sized primary precursor species (PPSs), (ii) the formation of larger (∼3 to ∼15 nm sized) silicate precursor species (LSPSs) by a rapid aggregation/coalescence of PPSs, (iii) the formation of "gel" (primary amorphous precursor) by a random aggregation of LSPSs at room temperature, and (iv) the formation of the worm-like particles (secondary amorphous precursor) occurred in the solid phase during heating of the reaction mixture (hydrogel) from room temperature to 170 °C. It is interesting that almost the same processes occur in the liquid phase but with decreased rate according to the relative low concentration of LMW silicate species. With the above described findings, it is highly expected that the manipulation of crystallization pathway through controlling the formation/evolution of precursor species in the initial stage of the process can be achieved.

The kinetics of the crystal growth of zeolite A were measured during the crystallization of zeolite A at 80°C from an amorphous aluminosilicate precursor (1.03Na 2 O·Al 2 O 3 ·2.38SiO 2 ·1.66H 2 O) dispersed in 1.2 M (system I), 1.4 M (system II), 1.6 M (system III), 1.8 M (system IV) and 2 M (system V) NaOH solutions. Analysis of the experimental results (changes in the concentrations C Al of aluminum and C Si of silicon in the liquid phase as well as of the dimension L m of the largest crystals of zeolite A during crystallization) shows that the growth rate of zeolite A crystals is independent of size, and that the growth is governed by the reaction of monomeric and/or low-molecular aluminate, silicate and aluminosilicate anions from the liquid phase on the surfaces of the growing zeolite crystals. The crystal growth takes place in accordance with the model of Davies and Jones for growth and dissolution, and taking into consideration the particularities of crystallizing zeolite systems, the kinetics of crystal growth of the zeolites can be defined as d L/ d t c =k g [C Al −C Al ∗][C Si −C Si ∗], where C Al ∗ and C Si ∗ are the concentrations of aluminum and silicon in the liquid phase which correspond to the solubility of zeolite A at given crystallization conditions, and k g is the rate constant of the linear growth of zeolite A crystals.

Solid phase (gel) separated from freshly prepared sodium aluminosilicate hydrogel as well as during its hydrothermal treatment at 80 °C was analyzed by different methods such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal gravimetry (DTG), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Analysis of the obtained results have shown that the freshly prepared gel is mainly composed of disc-shaped primary particles, but also partially or even fully crystalline entities were observed by AFM analysis. AFM analysis of the solids separated from the hydrogel at various stages of its hydrothermal treatment (heating at 80 °C) indicates that the particles of the partially and/or fully crystalline phase are nuclei for further crystallization of zeolite. Although it is assumed that the nuclei have structure of faujasite rather than zeolite A, further growth of zeolite A was readily explained by the fact that regardless of the “structure” of nuclei, the type of zeolite to be formed is determined by the concentrations of silicon and aluminum in the liquid phase of the crystallizing system.

Fe-exchanged samples of zeolite A were prepared and their time-dependent properties during hydrothermal crystallization were studied by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Rutherford backscattering (RBS) and 57Fe Mossbauer spectroscopy. The relative amounts of various iron species deduced from 57Fe Mossbauer spectra of the Fe-exchanged samples are in a good correlation with the extent of ordering during the hydrothermal crystallization

Positron annihilation lifetime spectroscopy (PALS) is sensitive and proved method for material structure investigations. We used it to investigate the influence of different alkali cations on the structure of zeolite precursors in order to understand better the crystallization mechanism of zeolites. Our previous PALS investigations on Li gels show different structural features obtained for differently prepared gels, indicating the importance of the way on which cations are introduced in the gel matrix. These investigations were now extended to gels containing K, Rb and Cs cations, in correlation to their ionic radius and the way of preparation.

Analyses of the distribution of nuclei in matrices of differently prepared and treated aluminosilicate gels have shown that both a specific number (number per unit mass or volume) of nuclei and their distribution f (N) in the gel matrix depend strongly on the chemical and physical conditions under which hydrogel is prepared. The distribution of nuclei in most of gel precursors is inhomogeneous (number of nuclei decreases from the “surface” of gel particles to their “interior”); the homogeneous distribution may be a special case.

Influence of the simultaneous change of batch alkalinity (A = [Na2O/H2O]b) and total batch silica concentration (tsc = [SiO2/H2O]b) of the reaction mixture, at two characteristic batch molar ratios, y = [SiO2/Al2O3]b (1.3 and 2.0), on the kinetics of the nucleation, crystal growth and kinetics of crystallization and on particulate properties of crystallized zeolite A, was investigated. It was found that the mentioned processes during crystallization from high and medium alkaline systems (0.0165 ≤ A ≤ 0.046) takes place in accordance with previously described experimental evidences, but that the anomalous kinetics of nucleation during crystallization of zeolite A from low alkaline systems (A ≤ 0.01) was not either predicted theoretically or, till recently, evidenced experimentally. Results of the analysis of distribution of nuclei in gel matrix have shown that in low alkaline systems, most of the nuclei are concentrated in the "centers" ("cores") of gel particles and rest of nuclei are distributed between the "surfaces" and "centers" of gel particles. Population balance analyzes of the crystallization processes, showed that the nucleation in the low alkaline systems takes place in two steps as a consequence of the mentioned distribution of nuclei in the gel matrix. Another consequence of the anomalous distribution of nuclei in the gel matrix is that more than 95 wt. % of zeolite A is formed by the growth of the nuclei released from the gel matrix during the first step of nucleation, and thus that the maximum rate of nucleation in the second step is reached at the crystallization time when about 80 – 90 wt. % of gel has been transformed into zeolite A. This is, at the same time, the reason that the crystal size distributions of the products obtained by crystallization of zeolite A from low-alkaline systems are characterized by two separate crystal populations.

In porous media, positron annihilation techniques (PAT) revealed the existence of annihilation modi with long lifetimes connected to structural and extrastructural free volumes acting as trapping centres for o-Ps. Our aim in this work was to study the influence of the synthesis applied for the production of the zeolite framework and the correlation between the trapping pattern and crystallization progress. PAT lifetime (LT) measurements were performed in silicalite-1 samples in various stages of crystallization and a large change was observed in the long components. The results are discussed in connection to the crystallization and the availability of free volume.