Microwave assisted synthesis of zeolite A from metakaolin

Zeolites are generally synthesized by the hydrothermal reaction of sodium aluminosilicate gels prepared from pure chemicals in alkaline medium using conventional heating systems. In the present study, microwave energy is used as a novel heating tool for the synthesis of zeolite A and metakaolin (Al2O3 · 2SiO2), a calcined product of kaolin (Al2O3 · 2SiO2 · 2H2O) has been taken as a combined source for silica and alumina. Optimization studies on various reaction parameters have been conducted. Phase pure zeolite A of high crystallinity was synthesized by heating the reaction mixture for 2 h at 85 °C under microwave hydrothermal system where as conventionally it takes 6–8 h. However, initial microwave exposure for 2 min at 85 °C and 20 h ageing at room temperature was found to be a pre-requisite for the crystallization of zeolite A in this system. The products and intermediates at different stages of the reactions were characterized using XRD, IR, solid state NMR and chemical analysis. Morphology was examined using SEM. The crystallization of the zeolite was followed by measuring the water capacities of the product in each experiment. Calcium binding capacity of selected products was also determined. A comparative study of the zeolite A formation from metakaolin under microwave and conventional hydrothermal systems was also carried out.