Effect of kaolin pre-treatment method and NaOH levels on the structure and properties of kaolin-derived faujasite zeolites

Abstract

Fusion with NaOH is considered effective in the conversion of kaolin to soluble aluminosilicates. However, to date, there is no consistency in the nature and type of zeolite structures prepared from kaolin via this approach, suggesting a lack of clarity on the effect of varying the fusion method. In this report, we have explored the effect of NaOH as an activator in the fusion step during the synthesis of kaolin-derived zeolites by performing partial or full fusion of the kaolin before the hydrothermal crystallization step. The content of NaOH was adjusted to maintain similar hydrogel composition, and the subsequent hydrothermal synthesis conditions were identical for the partial and full fusion protocols. The effect of the kaolin pre-treatment prior to full fusion was also explored by mixing the clay with NaOH under varied conditions. Zeolite Y and zeolite X structures were obtained from partial and full fusion, respectively. For full fusion, pre-treatment of kaolin by dry mixing before dry fusion resulted in zeolite X with some zeolite A impurity. On the other hand, wet mixing promoted homogeneity in the hydrogel with no formation of undesirable zeolite A. Wet mixing before dry fusion generated pure zeolite X with porosity (645 m² g⁻¹ and 0.24 cm³ g⁻¹) that is comparable to that of commercial zeolite 13X. However, wet mixing with subsequent wet fusion resulted in poorly crystalline zeolite X along with some small pore hydroxysodalite impurity. Our findings demonstrate the crucial role of NaOH concentration in the fusion of kaolin when used as starting material in determining the zeolite structure. We also show the need for an efficient pre-treatment method that allows the complete conversion of kaolin into suitable ingredients before the hydrothermal crystallization step. We, therefore, identify synthesis protocols that enable use of sustainable and cheap kaolin as a raw material for high quality faujasite zeolites.