MgO@CNT@K2CO3 as a superior catalyst for biodiesel production from waste edible oil using two-step transesterification process

Abstract

Due to limited fossil fuel resources and greenhouse gas emissions, the use of biodiesel is increasing. In this study, biodiesel was produced from waste edible oil (WEO), as a low-cost oil, using MgO and MgO@CNT@K2CO3 catalysts. The MgO@CNT@K2CO3 catalyst was first synthesized in this study and used to produce biodiesel. To this end, the features of these catalysts were studied using FTIR, BET, XRD, EDX, SEM, and Map analyses. The results indicated that both catalysts have high catalytic activity with mesoporous structures. Due to the high content of free fatty acid (2.43 wt%) in WEO, a two-step transesterification process was used to convert WEO to biodiesel. In the first step, hydrochloric acid homogeneous catalyst was employed to reduce the FFA content and then in the second step, MgO and MgO@CNT@K2CO3 heterogeneous catalysts were used to generate biodiesel. The highest biodiesel yields using MgO and MgO@CNT@K2CO3 were 94.26% and 98.25%, respectively, which were achieved at a catalyst concentration of 4%, temperature of 65 °C, and methanol/oil ratio of 24:1 and 20:1 after 5 and 4 h for MgO and MgO@CNT@K2CO3, respectively. The MgO@CNT@K2CO3 showed the highest biodiesel yield ever achieved from WEO. Also, the physical features of biodiesel were investigated and the results demonstrated that the biodiesel produced are in accordance with ASTM D6751 and EN14214 standards. Moreover, kinetic and thermodynamic studies showed that the transesterification reaction is endothermic and non-spontaneous. Therefore, MgO@CNT@K2CO3 is proposed as a strong and cost-effective catalyst for biodiesel generation from WEO on an industrial scale.