KOH-Promoted Catalyst Derived from Coffee Husk and Eggshell Composite for Biodiesel Production from Waste Frying Oil

In this study, a strongly active and sustainable heterogeneous catalyst for producing biodiesel from waste frying oil (WFO) was successfully developed. The catalyst's synthesis involved calcining a composite of potassium hydroxide (KOH), eggshell powder (ES), and coffee husk char (CHC) at a remarkably low temperature of 700 °C, which is significantly below the typical 900-1000 °C required for the direct calcination of bare eggshells. This lower calcination temperature was crucial for forming the effective active phases (CaO and K₂O) and enhancing the catalyst's basicity. The catalyst calcined at 700 °C significantly improved the biodiesel yield to 95.33 wt %, a major increase from the 45.87% yield at 600 °C. However, increasing the calcination temperature to 800 °C resulted in a slightly lower biodiesel yield of 93 wt %. Through further optimization using central composite design (CCD) method, an overall optimal yield of 97.5% was achieved, demonstrating a promising pathway for efficiently converting waste resources into valuable biofuel. The work highlights the use of abundantly available waste materials as catalyst precursors, which not only lowers overall production expenses but also reduces the environmental footprint of the process. While the catalyst showed a progressive decrease in activity across three cycles (85, 71, and 52%), these findings underscore the need for further research into improving its durability for industrial application. In summary, this research marks a key advance toward developing a more economical and sustainable method for producing biodiesel.