Hydrothermal synthesis of zeolites using silica extracted from tropical volcanic ash

Abstract

This study addresses the reliance on costly aluminosilicate sources for the synthesis of zeolites by exploring the use of silica extracted from tropical volcanic ash. The volcanic ash contained CaO, in the form of calcite polymorph of CaCO₃, and silica at 47.09 wt% and 18.38 wt%, respectively. High purity silica was obtained by simply washing the volcanic ash with HCl to remove the calcite, and was then used, with additional commercial aluminate, in the synthesis of varieties of zeolites via the fusion method. The type of zeolite prepared, Na–X, Na–P or hydroxysodalite (Na-HS), was determined by the SiO₂ : Al₂O₃ : Na₂O : H₂O molar ratios and in particular the amount of SiO₂ and Na₂O. The Na–X product had the highest surface area (766 m² g⁻¹), micropore surface area (644 m² g⁻¹) and pore volume (0.26 m³ g⁻¹), and these textural values were comparable to those of commercial molecular sieve 13X. This work demonstrates that (i) high purity silica can be obtained from abundant and inexpensive volcanic ash by simple acid treatment, and (ii) the silica can be used in the production of high purity zeolites, whose quality is dependent on the hydrogel composition.