Improving the activity of fly ash as a solid base catalyst through the pozzolanic reaction under mild conditions

Fly ash is a chemically promising material composed of several metal oxides, including SiO2, Al2O3, MgO, and Fe2O3. However, its chemical properties have not been successfully exploited in catalysis applications. One of the reasons for this is that the effective surface area of fly ash is extremely small because vitrification proceeds through combustion in a furnace under high-temperature conditions. In this study, a pozzolanic reaction was applied to increase the surface area of fly ash and improve its performance as a solid base catalyst. Microcrystals of calcite (CaCO3) and tobermorite (Ca5Si6O18H2·4H2O) were formed on the surface by reacting fly ash with calcium hydroxide in water at 40 °C. After 11 days of the reaction, the surface area of the fly ash (22 m2 g−1) increased to almost 11 times that of the raw fly ash (2 m2 g−1). The fly ash–based catalyst was applied as a solid base catalyst to the Knoevenagel condensation reaction of malonic acid with benzaldehyde. Although the catalytic activity of the raw fly ash for the Knoevenagel condensation reaction was considered acceptable, the catalytic activity of the fly ash–based catalyst increased by approximately fivefold after 11 days of reaction with calcium hydroxide. The changes in the physical and chemical properties of fly ash induced by the reaction with calcium hydroxide were discussed.