Synthesis of basic zeolite from coal fly ash and its reactivity in Knoevenagel reaction

Abstract

Zeolite was hydrothermally synthesized from coal fly ash in NaOH solution and static condition, at various times of crystallization and treatment. Gel treated with ultrasonication before heating resulted in the formation of pure zeolite P (Na-P) while untreated gel formed a mixture of sodalite and Na-P. Pure zeolite P was produced in 5 days and stable up to 31 days of crystallization time without the trace of sodalite formed. The synthesized Na-P was then ion-exchanged with potassium (K-P) and cesium (Cs-P) ions in order to increase the basicity of the Na-P obtained. The reactivity of the catalysts was tested in Knoevenagel reaction between benzaldehyde and malononitrile, producing solid benzylidenemalononitrile as single product. The reactivity was measured by the time when the first solid product formed. K-P catalyst was found to be the most reactive followed by Cs-P and Na-P in which the reactivity depends more on the number of base sites as well as the basic strength of zeolite. In conclusion, zeolites synthesized from CFA is potential catalysts for the Knoevenagel reaction.