Hydrothermal synthesis of zeolite A from K-feldspar and its crystallization mechanism

• The K-feldspar powders were pretreated to obtain amorphous materials.
• The effect of reaction times on the conversion of the pretreated K-feldspar to zeolite A was investigated.
• The conversion mechanism of pretreated K-feldspar to zeolite A was discussed.

Zeolite A was indirectly synthesized from K-feldspar using hydrothermal conversion method. In order to gain active SiO2 and Al2O3, the K-feldspar powders were pretreated with KOH solution followed by H2SO4 solution to form X-ray amorphous materials (AS). The effects of reaction time on the conversion of the obtained AS to zeolite A were investigated. X-ray diffraction (XRD), wet chemistry analysis, scanning electron microscope (SEM), Fourier Transform Infrared spectroscopy (FTIR), and 29Si and 27Al magic-angle-spinning nuclear magnetic resonance (29Si MAS-NMR and 27Al MAS-NMR) were used to characterize the nucleation and crystal growth processes. The results indicated that the AS was dissolved completely and abundant nuclei were formed during the first hour of the crystallization stage. With the crystallization time increasing, the nuclei were rapidly grown into zeolite A crystals and relative crystallinity of pure zeolite A reached its maximum when crystallization time reached to 3 h. An excess time resulted in further dissolution of zeolite A and replacement by hydroxyl sodalite. The mechanism of synthesis of zeolite A from pretreated K-feldspar can be described by three main stages: (1) the dissolution of amorphous aluminosilicate releasing [SiO2(OH)2]2− and Al(OH)4−; (2) the formation sodium aluminosilicate gel as zeolite precursor; (3) the crystallization of zeolite A.

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