Calcined oyster shell nanoparticles (COS NPs) as a novel heterogeneous nanocatalyst were prepared and fully characterized by X-ray fluorescence analysis, Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy equipped with an energy dispersive spectrometry (SEM-EDS) and transmission electron microscopy. The composition of this catalyst was determined, and it was clearly found that the particle size of COS NPs was about 100–120 nm. The nano-structured solid catalyst as an efficient and eco-friendly catalyst was used for the synthesis of 1,8-dioxo-octahydroxanthenes via the one-pot condensation reaction of 5,5-dimethylcyclohexane-1,3-dione (dimedone) with various aldehydes under solvent-free conditions. This new methodology is cost-effective and offers several advantages such as high yield, excellent purity, short reaction time, high atom economy, simple operation and a convenient workup procedure without the use of any injurious solvents. Moreover, the catalyst can be readily reusable, and considerable catalytic activity could be achieved after the six runs.

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