Catalytic Performance Investigation of Alkali and Bifunctional Catalysts Derived from Lignocellulosic Biomasses for Biodiesel Synthesis from Waste Frying Oil

In this study, alkali and bifunctional catalysts were synthesized for waste frying oil methyl ester (WFOME) synthesis. Coffee husk (CH) and CH blended with Eragrostis tef straw (TS) (CH–TS) lignocellulosic biomasses (LBs) were utilized during the catalysts’ synthesis. The alkali catalysts were CH and CH–TS ashes, both modified by KNO3 impregnation. They are designated as C-45 and C-Mix, respectively. Zirconia (ZrO2) promoted CH ash catalysts via precipitation followed by impregnation (Bic-PP) and in situ precipitation–impregnation (Bic-Dm) were the bifunctional ones. CH and CH–TS chars were the supporting frameworks during the catalysts’ composite materials (CCMs) preparation. The combustion performance of LBs and CCMs was evaluated and associated with the catalysts’ physicochemical properties. Using XRD, SEM, FTIR, alkalinity, TOF, and BET surface area analysis, catalysts were characterized. The combustion performance of the LBs was in the order of TS > CH–TS > CH. Among CCMs, the highest combustion performance was for CCM-Mix (KNO3/(CH–TS char)) and the lowest was for CCM-45 (KNO3/ CH char). The C-Mix catalyst was a light green powder due to the reaction between inorganic components, whereas C-45 was dark gray due to the presence of unburned char. The CCMs for bifunctional catalysts had moderate combustion performance and yielded light gray powdered catalysts containing tetragonal ZrO2. The optimum WFOME yields were 98.08, 97, 92.69, and 93.05 wt % for C-Mix, C-45, Bic-Dm, and Bic-PP assisted WFO transesterification, respectively. The results were obtained at a reaction temperature of 65 °C, time of 1 h, and methanol to WFO molar ratio of 15:1 using catalyst amounts of 5 and 7 wt % for the alkali and bifunctional catalysts, respectively. The greatest moisture resistance was offered by the C-Mix catalyst. The best reusability was for the C-45 catalyst. Catalysts’ deactivation modes include active site leaching and poisoning.