Mechanochemical and Thermal Transformations of Amorphous and Crystalline Aluminosilicates

Preparation of amorphous aluminosilicates precursors with defined properties is very important factor for further studies of nucleation and crystal growth of zeolites during their thermal and hydrothermal transformation to zeolites and special ceramics. In this study is presented the effect of an intensive mechanical force (ball milling) on the properties of zeolite A and zeolite A with partial exchanged of sodium ions in with other cations (Li+, K+, Cs+). It is studied the influence of different cations on the mechanical and thermal stability of the zeolite framework and the formation of amorphous phases as well their transformation to nonzeolitic crystal phases after thermal treatment

Evidence of the »Memory« Effect of Amorphous Aluminosilicate Gel Precursors by Simulation of Zeolite Crystallization Processes Using the Population Balance Method

There is abundant experimental evidence that most, or even all zeolite nuclei are formed in the aluminosilicate gel and/or gel/liquid interface by a linking of specific subunits during gel precipitation and/or ageing. Since the nuclei (particles of quasicrystalline phase) cannot grow inside the gel matrix, they start to grow after being »released« from the gel dissolved during the crystallization, i.e. when they are in full contact with the liquid phase (autocatalytic nucleation). Based on these findings it was assumed that the rate of autocatalytic nucleation depends on the rate of gel dissolution as well as on the number and distribution of nuclei in the gel matrix, but that crystal size distribution in the crystalline end product depends exclusively on the number and distribution of nuclei in the gel matrix and not on the crystallization conditions, or even on the treatment of aluminosilicate gel precursor prior to crystallization. This so called »memory« effect of amorphous aluminosilicate precursors was evidenced by simulation of zeolite crystallization under different conditions, using the population balance method

Results of hydrothermal treatment of the amorphous phases obtained by ball milling of zeolites A, X and synthetic mordenite

High-energy ball milling of zeolites A, X and synthetic mordenite for an appropriate time results in the formation of true amorphous aluminosilicate phases having the same chemical composition as the starting (unmilled) crystalline materials (zeolites). Since the solubility of thus prepared amorphous solids in hot alkaline solutions is considerably higher than the solubility of the starting zeolites under the same conditions, it can be expected that hydrothermal treatment of the amorphous solids would result in their transformation to more stable phases by solution-mediated processes. To evaluate this thesis, the X-ray amorphous solid phases obtained by high-energy ball milling of zeolites A, X and synthetic mordenite were hydrothermally treated at 80 degreesC by 2 M and 4 M NaOH solution, respectively, for 4 h. The products obtained (zeolites A, P and hydroxysodalite) were characterized by X-ray powder diffraction and particle size distribution measurements. It was concluded that the nuclei for zeolite crystallization originate from the residual nano-sized quasicrystalline particles (short-range ordering of Si and Al atoms inside amorphous regions that have not been completely destroyed during milling). Type(s) of the zeolite(s) (zeolite A, zeolite Pa) crystallized by the growth of the nuclei under the given conditions are determined by the chemical composition of the liquid phase (concentrations of Si and Al), and by the chemical composition of the precursor (determined by the type of mechanochemically amorphized zeolite) and the alkalinity of the system (NaOH concentration in the liquid phase), respectively. The results obtained are in agreement with the thermodynamic stabilities of the zeolite types that may be crystallized under the given conditions and at relative rates of crystallization

Structural and Morphological Transformations of the (NH4, Na)-exchanged Zeolites 4A, 13X and Synthetic Mordenite by Thermal Treatment

Thermal treatment of (NH4, Na)-exchanged zeolites 4A and 13X results in the formation of an amorphous phase (T < 1000 °C) and a crystalline phase of mullite at temperatures above 1000 °C. No structural changes have been noticed for the (NH4, Na)-exchanged synthetic mordenite treated under the same conditions. Scanning electron microscopy (SEM), X-ray powder diffraction, Fourier transform infrared (FT-IR) spectroscopy and particle size analysis were used to characterize the initial materials and the obtained products

Utjecaj koncentracije silicija i aluminija u tekućoj fazi na brzinu rasta mikrokristala zeolita A i X

Kinetics of the crystal growth of zeolites A and X was measured during their crystallization, at 80 °C, from an amorphous aluminosilicate precursor dispersed in a 1.4 M NaOH solution containing different amounts of dissolved NaAlO2 or Na2SiO3. The crystallization pathway and fractions of zeolites A and X in the crystalline end product strongly depend on the composition of the liquid phase of the crystallizing system. Analyses of the changes of the concentrations of aluminum, cAl, and of silicon, cSi, in the liquid phase as well as of the dimension, Lm, of the largest crystals of zeolites A and X during crystallization, have shown that the growth rate of zeolite A crystals is size-independent, 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 growing zeolite crystals. Influence of the composition of the liquid phase of the crystallizing system on the course of crystallization process and on the growth rates of zeolite A and zeolite X crystals are discussed in terms of the possible distribution of aluminate, silicate and/or aluminosilicate anions in the liquid phase.Brzine rasta kristala zeolita A i X mjerene su tijekom kristalizacije pri 80 °C iz amorfnoga alumosilikatnoga prekursora dispergiranoga u 1,4 M otopini NaOH koja je sadržavala različite količine otopljenoga NaAlO2 ili Na2SiO3. Tijek kristalizacije i frakcije zeolita A i X u produktima kristalizacije značajno ovise o sastavu tekuće faze kristalizacijskoga sustava. Analizom promjena koncentracija aluminija, cAl i silicija, cSi u tekućoj fazi te veličine Lm, najvećih kristala zeolita A i X tijekom kristalizacije, utvr|eno je da je brzina rasta kristala neovisna o njihovoj veličini te da se rast kristala odvija reakcijom monomernih i/ili niskomolekularnih aluminatnih, silikatnih i alumosilikatnih aniona iz tekuće faze na površini rastućih kristala zeolita. Utjecaj sastava tekuće faze kristalizacijskih sustava na tijek procesa kristalizacije i brzinu rasta kristala zeolita A i X razmatran je u odnosu na moguće raspodjele aluminatnih, silikatnih i alumosilikatnih aniona u tekućoj fazi

Utjecaj koncentracije silicija i aluminija u tekućoj fazi na brzinu rasta mikrokristala zeolita A i X

Kinetics of the crystal growth of zeolites A and X was measured during their crystallization, at 80 °C, from an amorphous aluminosilicate precursor dispersed in a 1.4 M NaOH solution containing different amounts of dissolved NaAlO2 or Na2SiO3. The crystallization pathway and fractions of zeolites A and X in the crystalline end product strongly depend on the composition of the liquid phase of the crystallizing system. Analyses of the changes of the concentrations of aluminum, cAl, and of silicon, cSi, in the liquid phase as well as of the dimension, Lm, of the largest crystals of zeolites A and X during crystallization, have shown that the growth rate of zeolite A crystals is size-independent, 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 growing zeolite crystals. Influence of the composition of the liquid phase of the crystallizing system on the course of crystallization process and on the growth rates of zeolite A and zeolite X crystals are discussed in terms of the possible distribution of aluminate, silicate and/or aluminosilicate anions in the liquid phase.Brzine rasta kristala zeolita A i X mjerene su tijekom kristalizacije pri 80 °C iz amorfnoga alumosilikatnoga prekursora dispergiranoga u 1,4 M otopini NaOH koja je sadržavala različite količine otopljenoga NaAlO2 ili Na2SiO3. Tijek kristalizacije i frakcije zeolita A i X u produktima kristalizacije značajno ovise o sastavu tekuće faze kristalizacijskoga sustava. Analizom promjena koncentracija aluminija, cAl i silicija, cSi u tekućoj fazi te veličine Lm, najvećih kristala zeolita A i X tijekom kristalizacije, utvr|eno je da je brzina rasta kristala neovisna o njihovoj veličini te da se rast kristala odvija reakcijom monomernih i/ili niskomolekularnih aluminatnih, silikatnih i alumosilikatnih aniona iz tekuće faze na površini rastućih kristala zeolita. Utjecaj sastava tekuće faze kristalizacijskih sustava na tijek procesa kristalizacije i brzinu rasta kristala zeolita A i X razmatran je u odnosu na moguće raspodjele aluminatnih, silikatnih i alumosilikatnih aniona u tekućoj fazi

Kinetic Analysis of Non-isothermal Transformation of Zeolite 4A into Low-carnegieite

Kinetics of the non-isothermal transformation of zeolite 4A to low-carnegieite was investigated by the X-ray diffraction method. Changes in the fractions of zeolite 4A, amorphous aluminosilicate and low-carnegieite during zeolite 4A heating at three different heating rates (0.0833 ° s–1, 0.1667 ° s–1 and 0.333 ° s–1) showed that amorphization of zeolite 4A and crystallization of low-carnegieite take place simultaneously. Kinetic analyses of amorphization and crystallization showed that the non-isothermal transformation took place by the same mechanism as the isothermal transformation, i.e., amorphization of zeolite 4A proceeded by a random, diffusion- limited agglomeration of the short-range ordered aluminosilicate subunits formed by the thermally induced breaking of Si-O-Si and Si-O-Al bonds between different building units of zeolite framework. Crystallization of low-carnegieite occurred by homogeneous nucleation of low-carnegieite inside the matrix of amorphous aluminosilicate and was diffusion-controlled, with one-dimensional growth of the nuclei. Kinetics of non-isothermal processes was determined by the changes of the rate constants during heating and the apparent activation energies of amorphization and crystallization