Synthesis and characterisation of pure phase ZSM-5 and sodalite zeolites from coal fly ash

The global availability of energy demands the continual combustion of coal to produce electricity. However, this results in the generation of coal fly ash, raising several environmental concerns. Hence, the use and recycling of the coal fly ash waste material is foremost to alleviating the environmental burden caused by its disposal. In this study, an alkaline leaching process was used to extract silica from coal fly ash (CFA), and the resultant solid residue was thereafter transformed into a sodalite zeolite. The extracted silica was further treated with water and used as feedstock for the synthesis of pure phase ZSM-5 without the additional aluminosilicate source. The water treatment step removed a considerable amount of Na from the silica extract, which improved the property and yield of the ZSM-5 zeolite product: The untreated silica extract (UFSE) resulted in a mixed phase of ZSM-5 and Na-P zeolite (H-ZU), with a yield of 9.00 g/kg of CFA and a Si/Al ratio of 5.58. While the water-treated silica extract (TFSE-H2O) resulted in a pure phase ZSM-5 zeolite (H-ZH2O), with a yield of 89.98 g/kg of CFA and a Si/ Al ratio of 182.48. The micropore and mesopore surface area in H-ZU was 315 m2/g and 84 m2/g, while the micropore and mesopore surface area in H-ZH2O was 240 m2/g and 126 m2/g respectively. The innovative use of the water treatment step to adjust the Si/Al ratio and remove excess Na prior to the hydrothermal process would significantly reduce the production cost of CFA-based zeolites, thus leading to a viable synthesis process. This study provides a holistic approach to using CFA to synthesise sodalite and pure ZSM-5 zeolite with zero-solid waste, which in addition to addressing the environmental problem associated with its disposal, presents CFA as a useful resource that could be commercialised.

Automated summary

In this study, an alkaline leaching process was used to extract silica from coal fly ash, which was further transformed into a sodalite zeolite. The extracted silica was then treated with water and used to synthesize pure phase ZSM-5 zeolite. The innovative use of water treatment step before the hydrothermal process significantly reduced the production cost and led to a viable synthesis process. This study provides a holistic approach to using coal fly ash as a useful resource for synthesizing zeolites.