Kinetic and thermodynamic studies on the esterification of oleic acid ‏with methanol over sulfonated biochar catalyst derived from waste tea dregs

Abstract

Tea dregs, an abundant and easily found bio-waste, were utilized as a precursor to developing acid biochar (SO3H-TDAC) that was used as a catalyst in producing biodiesel through oleic acid (OA) esterification with methanol. SO3H-TDAC catalysts were prepared through the consecutive carbonization–sulfonation two-step method. The catalyst with the highest sulfonic acid density was obtained at a carbonization temperature of 300 °C, carbonization time of 1 h, sulfonation temperature of 125 °C, and sulfonation time of 5 h and was characterized by multiple characterization methods. The catalyst performance was optimized, and the maximum OA conversion of 95.4% was obtained under the optimum conditions of temperature, time, catalyst concentration, and MeOH/OA molar ratio of 70 °C, 2.5 h, 7.5 wt% to OA, and 12/1, respectively The reaction was well-described by pseudo-first-order kinetics, and the activation energy was calculated as 29.93 kJ mol−1. Moreover, thermodynamic parameters of enthalpy change, entropy change, and Gibbs energy change were computed as 27.6 kJ mol−1, −0.19 kJ mol−1 K−1, and 95.171 kJ mol−1, respectively, indicating the reaction to be endergonic and thermodynamically unfavorable. Finally, after four cycles, the catalyst retained a high OA conversion efficiency of 86.5%, and the original efficiency can be regained via re-sulfonating the spent catalyst.