Optimization of methyl esters production from non-edible oils using activated carbon supported potassium hydroxide as a solid base catalyst

Abstract

The present investigation reports transesterification of non-edible oils, waste cooking oil (WCO) and waste fish oil (WFO) with methanol using activated carbon supported potassium hydroxide (KOH) as a solid base catalyst. Activated carbon was prepared from polyethylene terephthalate waste and loaded with KOH by wet impregnation method to prepare KOH/AC solid base catalyst. X-ray diffraction and Scanning Electron Microscopy (SEM) were utilized to characterize the resulting solid base catalyst. Retention method was utilized to measure the specific surface area of the activated carbon and its derived solid base catalyst, while Hammett indicator method was followed to determine the basic strength of the prepared solid base catalyst. The prepared catalyst has granular and porous structures with a high basicity and a superior catalytic performance for the transesterification reaction. Transesterification reaction variables were arranged to obtain the best biodiesel yield. The highest methyl ester yield from WCO (88.12% w/w with an ester content of 96.68% w/w) was obtained by employing 3.5 wt.% KOH/AC catalyst, 9:1 methanol to oil molar ratio, 65 °C reaction temperature, the reaction time of 180 min and the stirring rate of 600 rpm, while maximum methyl ester yield from WFO (92.66% w/w with 96.98% w/w ester content) was produced with 3.0 wt.% KOH/AC catalyst, 9:1 methanol to oil molar ratio at 65 °C for 150 minutes of the reaction and 600 rpm. The prepared solid base catalyst was recoverable and thermally stable giving a yield of 70 wt.% after the 5th cycle. The fuel properties of the raw oils were significantly enhanced after transesterification with methanol in the presence of KOH/AC, and were in conformity with the ASTM D 6751 limits as well. Therefore, the prepared KOH/AC composite may be considered as a promising solid base catalyst for transesterification of non-edible oils with methanol.