Modeling and optimization of two-step shea butter oil biodiesel synthesis using snail shells as heterogeneous base catalysts

Abstract

Shea butter oil (SBO) is underutilized in the biodiesel production industry in Nigeria because of its high free fatty acid (FFA) content, which reduces its biodiesel yield. This research aimed at optimizing shea butter oil biodiesel (SBOB) production using a biomass-derived heterogeneous catalyst. Crude SBO was purified and then esterified with H₂SO₄. An active heterogeneous catalyst was synthesized from snail shells via calcination at 100–900 °C. The calcined samples were characterized using XRD, SEM and EDS. The esterified SBO was transesterified using the calcined sample with the highest CaO percentage as a catalyst. Both esterification and transesterification processes were optimized using half factorial design of Design-Expert 11. The influence of the catalyst dosage, ethanol to oil ratio, reaction temperature, reaction time, and agitation speed was investigated on SBOB yields. XRD results showed that the snail shell calcined at 900 °C gave the highest yield (75%) of CaO. SEM results showed that the pore sizes ranged from 1.09 to 3.19 μm in the snail shell calcined at 100 °C and 5.30 to 9.18 μm in the sample calcined at 900 °C. EDS results showed that the calcium, oxygen and carbon contents in the snail shell calcined at 100 °C were 18.2, 44.4, and 28.0%, respectively, while their corresponding values in the sample calcined at 900 °C were 25.5, 30.1, and 23.0%, respectively. The highest FFA percentage reduction (60.36%) in SBO was obtained under optimum conditions of the 6.1 molar ratio, 78 °C reaction temperature, 120 min reaction time, 6.5 ml H₂SO₄, and 600 rpm agitation speed. The highest yield of SBOB (97.40%) was obtained under optimum conditions of the 9.1 molar ratio, 78 °C reaction temperature, 15 min reaction time, 3 wt% catalyst concentration, and 600 rpm agitation speed.