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Recycled chicken eggshells after calcination have been used as a nonconventional heterogeneous catalyst for biodiesel synthesis from karanja oil from the pongam tree (Pongamia pinnata). The catalyst was characterized on X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and differential thermal analysis/thermogravimetric analysis (DTA/TGA). The formation of the calcium oxide phase was confirmed at 900 °C after calcination. A high biodiesel yield of 95.0% and conversion of 97.4% was obtained at an 8:1 (alcohol/oil) molar ratio, 2.5 wt % catalyst, and 2.5 h reaction time at 65 ± 0.5 °C. The biodiesel conversion was determined by Fourier transform nuclear magnetic resonance (FTNMR) spectroscopy.

Solid acid (heterogeneous) catalysts have a unique advantage in esterifi cation and transesterifi cation reactions which enhances the use of high acid value oil to be used as feedstock for synthesis of biodiesel. Various solid acid catalysts such as resins, tungstated and sulfated zirconia, polyaniline sulfate, heteropolyacid, metal complexes, sulfated tin oxide, zeolite, acidic ionic liquid, and others have been explored as potential heterogeneous catalysts.The activity of the catalyst differs slightly resulting in moderate to high conversion and yield. The reuse of the solid catalyst is governed by their deactivation, poisoning, and the extent of leaching in the reaction medium. The applicability of these catalysts for synthesis of biodiesel along with their reusability aspect is discussed in this review.

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