Synergistic Biomass-Eggshell Engineering of Calcium Catalysts for Efficient Biodiesel Production

The valorization of agricultural and food-processing waste offers a promising pathway to develop cost-effective heterogeneous catalysts for biodiesel production. Herein, calcium-based catalysts were prepared by pyrolyzing mixtures of rice husk (RH) and eggshell (ES) at different RH:ES mass ratios. Structural, compositional, and catalytic properties of the resulting materials were investigated for soybean oil transesterification. This was conducted under mild conditions: 65 °C, 4 h, methanol-to-oil ratio of 7:1, and 1 wt.% of catalyst. In this sense, mixed RH-ES catalysts outperformed those synthesized from the individual components (P_RH and P_ES), achieving FAME yields of 81.2% and 71.2% for P_RH:ES_1:2 and P_RH:ES_1:1, respectively. Characterization by FTIR, XRD, SEM-EDS, and BET revealed that RH can act as a carbonaceous structural modifier, promoting the transformation of CaCO3 into catalytically active species. It improves calcium dispersion and increases surface area from 0.73 to 49.56 m2 g−1. The P_RH:ES_1:1 material was selected for further evaluation to prioritize biomass loading while maintaining high catalytic performance and demonstrated good reusability with minimal activity loss. Moreover, GC-MS analysis of the resulting biodiesel confirmed that it resembles the FAME profile of conventional KOH-derived biodiesel. These results demonstrate that biomass-assisted calcium phase engineering enables efficient biodiesel production while promoting waste valorization.