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

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

Preparation and Spectroscopic Characterization of Iron Doped Mullite

A novel method for preparation of Fe2+/Fe3+ substituted mullite is described. Aluminosilicate gels are applied as precursors instead of crystalline aluminosilicates as used in other common syntheses. The process is composed of three stages. First, iron is introduced into a homogeneous aluminosilicate gel by ion exchange. The gel is converted to a mixture of mullite and amorphous silica in a 1263 K 3 h isothermal calcination in the the second stage. Finally, in order to obtain the nano-scale pure mullite phase the formed amorphous silica is removed by a dissolution in alkaline media. The components formed in various stages of the process are characterized by 57Fe Mössbauer and Fourier transform infra red spectroscopies, X-ray diffraction method and scanning electron microscopy. Spectroscopic and diffraction methods helped the identification of the mullite phase. Mössbauer measurements revealed the presence of both Fe2+ and Fe3+ states providing a chance for perspective catalytic application of the obtained Fe-mullite

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

Synthesis of Forsterite Powder from Zeolite Precursors

Powder mixtures of NH4-exchanged zeolite A and MgO, and of NH4-exchanged mordenite and MgO were used as starting materials in the synthesis of crystalline ceramic materials. Conventional ball milling was applied in order to amorphize NH4-exchanged zeolites, reduce the particle size of MgO, and produce a homogeneous mixture. Solid-state transformation of the mixtures after heating at the temperature of their phase transformations in the interval 800 °C–1000 °C for 3 hours yielded crystalline forsterite (Mg2SiO4) with possible minor amounts of sapphirine (Mg4Al10Si2O23) in the first case, and to crystalline forsterite (Mg2SiO4) with traces of enstatite (MgSiO3) in the second case

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

Crystallization of Calcium Carbonate in Alginate and Xanthan Hydrogels

Calcium carbonate polymorphs were crystallized in alginate and xanthan hydrogels in which a degree of entanglement was altered by the polysaccharide concentration. Both hydrogels contain functional groups (COOH and OH) attached at diverse proportions on saccharide units. In all systems, the precipitation process was initiated simultaneously with gelation, by fast mixing of the calcium and carbonate solutions, which contain the polysaccharide molecules at respective concentrations. The initial supersaturation was adjusted to be relatively high in order to ensure the conditions suitable for nucleation of all CaCO3 polymorphs and amorphous phase(s). In the model systems (no polysaccharide), a mixture of calcite, vaterite and amorphous calcium carbonate initially precipitated, but after short time only calcite remained. In the presence of xanthan hydrogels, precipitation of either, calcite single crystals, porous polyhedral aggregates, or calcite/vaterite mixtures have been observed after 5 days of ageing, because of different degree of gel entanglement. At the highest xanthan concentrations applied, vaterite content was significantly higher. In the alginate hydrogels, calcite microcrystalline aggregates, rosette-like and/or stuck-like monocrystals and vaterite/calcite mixtures precipitated as well. Time resolved crystallization experiments performed in alginate hydrogels indicated the initial formation of a mixture of calcite, vaterite and amorphous calcium carbonate, which transformed to calcite after 24 h of ageing