Waste to wealth: Sustainable heterotrophic chlorella sorokiniana micro-algae oil biodiesel production in renewable energy development

Chlorella micro-algae species are promising feedstocks alternatives in commercial biodiesel production because of their fast-growth rates, substantial lipids accumulation and ability to survive under different cultivation dynamics. This study examines the biodiesel yield and fuel properties of Chlorella sorokiniana micro-algae that was synthesized through transesterification process using calcium oxide (CaO) catalyst from waste materials on microporous zeolite, mesoporous silicon dioxide and ionic solid (sodium oxide) supports. The chlorella microalgal cells were cultured heterotrophically in a 20L- closed vertical bubble column bioreactor with a sterile BG-11 growth medium under nitrogen-deficient conditions to enhance carbon storage as lipids and extraction was done in a Soxhlet apparatus using hexane solvent. An Agilent HP 6890 Gas Chromatograph with a Flame Ionization Detector (FID) and a 6890 auto sampler was used for the fatty acids analyses. The CaO/ZSM-5 zeolite and CaO/Na2O catalysts were synthesized by wet impregnation and calcination of powdered waste snail shell on zeolite and sodium oxide supports. While CaO/SiO2 catalyst was prepared from calcination of waste bamboo leaves at 700°C and precipitation with calcium hydroxide powder (Ca(OH)₂). The supported catalysts crystallographic, morphological and functional group characteristics were determined by using an Agilent GV1000 X-ray diffractometer, an Agilent 8500 FE-SEM and an Agilent 5500a FTIR machine respectively. The micro-algae provide a sustainable feed stock for biodiesel production with a lipid content of 57.4% and 32.73%, 50.74% and 16.53% saturated, mono and polyunsaturated fatty acids components respectively under heterotrophic conditions. The CaO/ZSM-5 catalyst achieved the highest biodiesel yield of 97.82% and superior ester qualities. This is due to enhanced structural bonding and arrangement, larger pores and more efficient close-packed structure as evident in its XRD and SEM images. This results in larger surface area, increased number of accessible active sites, improved reactants diffusion and superior catalytic performance. Whereas, relatively lower methyl ester yields of 95.26% and 87.95% were obtained from the CaO/SiO2 and CaO/Na2O supported catalysts respectively during the first cycle of the transesterification process. The CaO/ZSM-5 catalyst has better recovery and reusability tendencies with a high yield of 94.1% at the end of the 7th reaction cycle due to enhanced stability and minimal leaching of its active sites. Unlike the CaO/Na2O system with ridiculously lower ester yield of 64.58% after the 7th cycle because of the substantial leaching of its active sites into the reaction mixture due to its high basicity and solubility in methanol. Finally, due to their enhanced thermal, hydrothermal and mechanical stability, zeolites supported CaO catalysts exhibit prolonged catalytic activity and reusability, making them highly suitable for large-scale biodiesel production processes that demand energy efficiency and sustainable development.