Upgrading catalytic activity of NiO/CaO/MgO from natural limestone as catalysts for transesterification of coconut oil to biodiesel

Limestone was converted to high surface area CaO/MgO catalysts via calcination-hydration-dehydration (CHD) method for transesterification of coconut oil to biodiesel. Thermal decomposition at 900°C transformed dolomite CaMg(CO₃)₂ to large crystallite and low surface area CaO/MgO. CHD treatment eliminated the large CaO/MgO aggregates and increased the surface area and the activity of CaO/MgO. The addition of polyethylene glycol (PEG) as surfactant during CHD reduced the CaO/MgO crystallite size to ~377 nm and enhanced the surface area (39 m²/g), the pore volume (0.0322 m³/g), and the basicity of the catalysts (7.8 mmol/g). Transesterification of coconut oil showed an increase in oil conversion from 16.45 to 49.27% when CaO/MgO was produced using PEG. Optimization studies at the variation of reaction temperatures, the ratio methanol:oil, and the amount of catalysts produced the optimum biodiesel yield of 81.76%. Impregnation with 5% NiO introduced acid-base functionality for esterification FFA to FAME, further improved biodiesel yield to 90%, and reduced the FFA yield. The kinematic, density, flash point, acid numbe,r and carbon residue of biodiesel from coconut oil were determined at 1.93 mm^2/s, 0.86g/cm³, 137°C, 0.27 mg KOH/g, and 0.22% respectively, and were within the ASTM D6751 standard. Graphical abstract: [Figure not available: see fulltext.]