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Influence of the concentration of Na+ and Ca2+ ions and CO2 on the nucleation and crystal growth of zeolite A

Influence of the concentration of Na+ and Ca2+ ions as well as concentration of CO2 in the reaction mixture, on the kinetics of nucleation and crystal growth of zeolite A has been investigated. Optimized composition of the starting solutions was: 10 Na2O 0.3 Al2O3 SiO2 120 H2O. Preparation of the reaction mixture was done in dry-box under N2 atmosphere, in order to avoid influence of the CO2 from air. Reaction mixture was prepared using standard procedure: sodium silicate solution was added into stirred sodium aluminate solution at 25º C, poured into Teflon lined reactor closed and aged for 24 h. Hydrothermal syntheses were preformed at 60oC. Characterization of the solid samples was preformed by X-ray diffraction and FT-IR (phase analysis), scanning electron microscopy (morphology), and laser light scattering (particle size distribution). Concentrations of silicon and aluminum in the liquid phase were measured by atomic absorption spectroscopy. Results show that certain concentrations of Ca2+ (Na+) cations with certain concentration of CO2 are preferable for formation of ZA nuclei. Absence of Ca2+ (Na+) cations or CO2 in starting solution gives mixture of at least two solid phases during crystallization. Concentrations of the structure directing agents, such as cations and anions, also determine the number of nuclei in the system. Consequently, at the end of the crystallization, particles size distribution shows lower values for systems of higher number of the nuclei. It is interesting that in the synthesis from clear solution, kinetics of crystal growth is faster under static conditions than in systems in which stirring is applied.

nucleation; crystal growth; zeolite A

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