Boosting biodiesel production from WCO utilizing marble waste powder as precursor for CaO/K₂CO₃ nanocatalyst at low temperature <i>via</i> RSM optimization

This study aims to valorize hazardous industrial marble machining and shaping waste powder as a precursor to prepare a heterogeneous nano-catalyst (CaO/K₂CO₃) employing the wet-impregnation method for producing biodiesel <i>via</i> the transesterification of waste cooking oil (WCO). Surface and morphological characterization of the thus-prepared nano-catalyst has been performed employing various analytical tools, <i>e.g.</i>, XRD, BET, CO₂-TPD, FT-IR, HR-TEM, and FE-SEM & mapping EDX. The impact of calcination treatment on the catalytic performance is investigated together with the weight percentage (wt%) of CaO compared to K₂CO₃. Response surface methodology (RSM) is used to optimize the parametric independent variables, <i>e.g.</i>, catalyst loading level, reaction temperature, and time, as well as methanol to WCO molar ratio (M : WCO) <i>via</i> the central composite design (CCD). The experimentally attained optimal reaction parameters for effective biodiesel production (92%) are: 3.2 wt% catalyst loading, operating at 40 °C for 84 min, along with a 5.8 : 1 M : WCO. The proposed nano-catalyst has been recovered and reused effectively for 5 consecutive cycles, with a slight loss in catalytic activity beginning from the 3^rd cycle. Additionally, the quality of the obtained biodiesel perfectly fits the American (ASTM D-6751) and the European (EN-14214) standard limits.