Eggshell waste as an eco-friendly and low-cost catalyst for the synthesis of α, β-unsaturated compounds

Three different eggshells, chicken eggshell, duck eggshell and quail eggshell, were used as the catalysts for the Knoevenagel reaction to produce α, β-unsaturated compounds in this work. To obtain the interrelationship among the factors of compositions, structure and catalytic activity, the eggshells were systematically characterized by X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectrophotometry, scanning electron microscopy, and the Hammett indicator method. Eggshells showed the hierarchical and periodic three-dimensional network morphology and calcite crystal phase with the weak alkalinity. All eggshells were active for Knoevenagel reaction, and chicken eggshell showed the highest catalytic activity among them. The reaction conditions were optimized, and the optimal reaction condition was that aromatic aldehyde could efficiently react with active component methylene compound in the presence of 0.5 g chicken eggshell waste in water-ethanol (1:1, volume ratio) at room temperature. Various aromatic aldehydes and active component methylene compounds could be converted into the desired products in high yields, suggesting the high universality of different substrates in the presence of eggshell waste catalyst. In particular, the reaction of 4-cyanobenzaldehyde with malononitrile could be accomplished in 0.7 min with a yield of 99%. Eggshell waste catalysts could be used four times without significant loss in activity, demonstrating their high recyclability and eco-friendliness for the Knoevenagel reaction.