Parametric optimization of calcined and Zn-doped waste egg-shell catalyzed biodiesel synthesis from Hevea brasiliensis oil

The utilization of waste sources for energy development is increasingly gaining popularity because of its two-fold benefit to the environment. The present study discusses the synthesis of biodiesel from commercially available rubber seed oil using two catalysts: CaO derived from calcined waste egg-shells and Zn-CaO prepared by doping ZnSO4 onto the calcined egg-shells through wet impregnation. An L9 Taguchi matrix was used to optimize the process, and can accomodate 4 parameters at 3 levels each. Data obtaind was used for a comparative evaluation of their catalytic performance. During catalyst characterization studies, morphological analyses showed the effectiveness of calcination in development of a porous structure that was capable of adsorbing Zn in high quantities, which was further verified through spectral analyses. Using CaO the optimal biodiesel yield noted was 80.2%, while Zn-CaO enabled a biodiesel yield of 94.12%. The optimal conditions were a reaction temperature of 55°C, methanol to oil molar ratio of 12:1, catalyst concentration of 5% w/w for both catalysts within a reaction duration of 1.5 h (for CaO) and 2 h (for Zn-CaO). Fuel characterization tests through ASTM prescribed procedures showed that the obtained product has acceptable physico-chemical properties and is thus suitable for use as biodiesel. Reusability studies (tested up to 4th reuse) indicated that CaO loses its catalytic efficiency much faster compared to Zn-CaO, primarily due to the vacant active sites in CaO being blocked by increased adsorption of organic molecules during the transesterification phase compared to Zn-CaO.