Microwave synthesis of sodalite from coal fly ash as solid base catalyst for knoevenagel reaction

Abstract

Coal fly ash (CFA) is a waste product from burning of coal fuel in the electrical power plant. It contains mostly silica and alumina which in powder form is considered hazardous material but can be utilized as precursor for the synthesis of zeolite. In this study, CFA has been converted to sodalite (SOD) and tested as a base catalyst in Knoevenagel reaction. Sodalite has been synthesized using microwave heating by varying alkaline concentration and heating time to obtain optimum parameter for formation of pure sodalite. The synthesis was also conducted by conventional oven heating for comparision. All synthesized products were characterized with XRD, FTIR, FESEM and 29Si MAS NMR. The results showed that quite pure sodalite (Na-SOD) crystal has been formed in 45 min by microwave heating as compared to 5 days by conventional oven heating.29Si MAS NMR showed the synthesized SOD has Si/Al ratio of 1 while FESEM gave irregularly-shaped crystal with the size ranging between 60 – 120 nm.The synthesized Na-SOD was ion exchanged with group 1 (K, Cs) and group 2 (Ca, Sr, Ba) metal ions in order to enhance sodalite basicity. The crystallinity of ion exchanged-SOD samples has decreased to - 70% and - 30% for samples exchanged with group 1 and group 2 metal ions, respectively. The catalytic testing of the catalysts in Knoevenagel reaction of benzaldehyde and dimethyl malonate was carried out, producing dimethyl-2-benzylidemalonate, methyl cinnamate and dimethyl-2,2- bis(hydroxyl(phenyl)methyl) malonate. Based on the conversion of reactant, K-SOD has shown the most active for the Knoevenagel reaction while Ca-SOD gave the lowest conversion. For the catalyst reusability, Cs-SOD gave the best performance in long-term application. The formation of dimethyl-2,2-bis(hydroxyl(phenyl)methyl) malonate as one of the products in this Knoevenagel reaction indicate that sodalitebased catalyst was highly basic but a non-selective catalyst.