Influence of cations on the physicochemical and structural properties of aluminosilicate gel precursors. Part 1. Chemical and thermal properties (original) (raw)
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Journal of the American Chemical Society, 2009
All industrially available zeolites are obtained from hydrogel systems. Unfortunately the level of understanding of the events preceding zeolite crystallization is far from satisfactory. In this respect, revealing the nature of the processes taking place in the precursor gel is of paramount importance to understanding zeolite nucleation. The investigation of the gel structure, however, is a difficult task due to the complexity of the object in terms of both composition and topology. Therefore, a combination of hyperpolarized (HP) 129 Xe NMR-N 2 adsorption-high-resolution transmission electron microscopy-energy-dispersive spectrometry methods complemented by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and chemical analyses has been employed to study the changes in composition and structure of sodium hydroxide rich aluminosilicate gel yielding zeolite A. The role of each component in the system and the entire sequence of events during the induction, nucleation, and crystallization stages have been revealed. The high concentration of sodium hydroxide in the studied system has been found to control the size and structure of the gel particles in the beginning stage. During the initial polymerization of aluminosilicate species a significant part of the sodium hydroxide is expelled from the gel into the solution, which restricts extensive polymerization and leads to formation of small aluminosilicate particles with open pore structure. The induction period that follows is marked by incorporation of Na back in the bulk gel. The combined action of the Na ion as a structure-directing agent and the hydroxyl group as a mobilizer results in partial depolymerization of the gel and formation of voids with mesopore sizes. The nucleation maximum coincides temporally with development of pores with sizes in the range of 2-5 nm. The amorphous gel undergoes into crystalline zeolite only after these pores have disappeared and the chemistry of the gel has evolved to reach the stoichiometric zeolite composition. It was established unambiguously by high-resolution transmission electron microscopy and HP 129 Xe NMR that the nucleation of zeolite occurs in the solid part of the system and the succeeding crystallization commences only after the nuclei are released into the liquid, which is consistent with the autocatalytic mechanism. Also this investigation has demonstrated the unrivaled sensitivity of HP 129 Xe NMR that is capable of identifying presence of small amounts of crystalline zeolite material in amorphous medium with detection limit extending below 1 wt %. Figure 6. TEM micrographs of gel treated under hydrothermal conditions at 90°C for (A) 60 and (B) 120 min.
Influence of Alcalinity on Synthesis of Zeolite a
2011
Two different systems were examined: (1) homogeneous (optically clear solution saturated with Na + , aluminate, silicate and alumosilicate species) and (2) heterogeneous (hydrogel – saturated solution with precipitated gel). Hydrothermal transformation of samples in both systems was made by heating of HDPE reactors (flasks) with samples at 80°C. All systems were prepared adding of the basic sodium silicate solution into the basic sodium aluminate solution. Both solutions were prepared by dissolving of appropriate amounts of sodium silicate (Fluka, > 97% Na2O SiO2 × 5H2O) or sodium aluminate (Riedel de Haën, 54% Al2O3; 41% Na2O) with NaOH (Kemika, 98% NaOH) in deionized water. In order to remove impurities from the sodium aluminate solutions, they were centrifuged and only clear supernatant was used for preparation of starting system. After preparation and ageing for 24 h, starting solution of systems 1 (1a, 1b, 1c) and hydrogel of systems 2 (2a, 2b, 2c) were divided among needed ...
Journal of Crystal Growth, 2008
Heating of the clear aluminosilicate solution (33.33 Na 2 O Al 2 O 3 3.33 SiO 2 400 H 2 O) from the room temperature (23 1C) to that the reaction (40, 50 and 60 1C) causes a spontaneous precipitation of an amorphous aluminosilicate (gel) and simultaneous formation of zeolite nuclei in the gel matrix. Measuring of the mass, m s , of the precipitated amorphous aluminosilicate and simultaneous analysis of the change in the concentrations, C Al , of aluminium and, C Si , of silicon in the liquid phase, showed that aluminium and silicon contained in amorphous aluminosilicate represent only a small fraction of the total amounts of aluminium and silicon in the system. The amorphous aluminosilicate tends to dissolve immediately after the formation, which causes the releasing of nuclei from the gel matrix and their growth by the reaction of monomeric and/or low-molecular aluminate and silicate species from the liquid phase on the surfaces of the growing zeolite crystals. However, the growth process and thus the crystallization of zeolite A do not start immediately after beginning of heating of the reaction mixture (e.g., at the crystallization time t c ¼ 0), but later, at a time t c ¼ t g 40. An analysis of the influence of the time of heating of the reaction mixture from the room temperature (23 1C) to the reaction one (40, 50 and 60 1C) have shown that this factor has a negligible influence on the ''delaying'' of the crystal growth and crystallization. Hence, it was concluded that the ''delaying'' of crystal growth and crystallization was determined by the time needed for the formation of amorphous aluminosilicate (gel), formation of nuclei in the gel matrix and releasing of the nuclei during the dissolution of the gel matrix. The proposed mechanism of crystallization of zeolite A from clear aluminosilicate solution was confirmed by population balance analysis of the crystallization processes at 40 and 60 1C.
Influence of alkalinity of the starting system on size and morphology of the zeolite A crystals
Materials Chemistry and Physics, 2012
The performance of zeolite crystals in different industrial processes often depends on their size and shape. Several physico-chemical parameters can have significant impact on their particulate properties (especially morphology). Amongst the chemical parameters the Si/Al ratio is one of most important variable together with the Na + ions (alkalinity) content. The present study is devoted to the effect of Na on the morphology of zeolite A (LTA-type) crystals. Initial hydrogels were heated at 80 • C in reactors made of HDPE, under static conditions. After separation by centrifugation, samples of solid and liquid phase were characterized using several analytical methods such as: atomic absorption spectroscopy, Xray diffraction, laser light scattering, optical, scanning and transmission electron microscopy. The results show that the increase of alkalinity in starting system causes two effects: (a) increase of the number of nuclei (crystals) in system through additional nucleation at surface and subsurface area of gel particles, and (b) the growth of crystals which have more irregular shape (rounded edges).
The crystal morphology of zeolite A. The effects of the source of the reagents
Microporous Materials, 1997
This study investigates the effects of the variation in reagents on the synthesis of zeolite A. The synthesis was attempted under autogenic conditions and at temperatures of 100 (±1)°C and 70 (±1 )°C, using various aluminium and silicon sources. Preparation of the gels used silica and aluminium to form mixed systems of organic and inorganic reagent sources. Products formed were analysed and characterised instrumentally using X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. Systems using aluminium isopropoxide consistently produced uniformly sized crystals, each having deep, chamfered edges. Sodium reagent sources produced sharp edged crystals. The system using tetraethyl-orthosilicate with aluminium powder produced a hexagonal morphology. The change in the hydroxide concentrations, relative to the systems using sodium silicate and sodium aluminate sources was also monitored, and while it had a marked effect on crystallinity and stability, the crystal morphology remained consistent. © 1997 Elsevier Science B.V.