Hierarchical porous zeolite synthesis from coal fly ash via microwave heating

Zeolites that contain mesopores with a diameter of 20–100 nm were hydrothermally synthesized from coal fly ash using a microwave heating method. Due to the uniform and rapid heating involved, numerous zeolite nanoparticles were formed in the early stages of heating. These particles aggregated and crystallized on the coal fly ash, leading to the formation of mesopores in the interparticle gaps. To investigate the influence of the heating method, zeolites were synthesized using the oil bath heating method. Microwave heating resulted in a higher crystallization rate than that obtained by oil bath heating. Scanning electron microscopy images and pore size distribution measurements obtained from nitrogen adsorption–desorption isotherms revealed that the products obtained using microwave heating consisted of larger pores than those obtained using oil bath heating. The effect of pore structure on adsorption performance was evaluated through adsorption experiments using three types of ions with different hydration ion radii. The zeolites obtained from both synthesis methods had the same saturated ion adsorption performance. However, microwave heating resulted in a greater ion adsorption rate than that obtained by oil bath heating. The ion adsorption rate differed depending on the ion species owing to different hydrated ion radii.