The production of biodiesel through transesterification requires numerous purification steps to remove the unreacted components and unwanted products for the biodiesel grade to satisfy the international standard specifications EN 14214 and ASTM D6751. The residual catalyst (KOH) is one of those impurities which must be removed at the end of alkali-catalyzed transesterification reaction. In this work, nine Deep eutectic solvents (DESs) based on choline chloride (ChCl) as well as another nine DESs based on methyltriphenylphosphonium bromide (MTPB) salts with different compositions were synthesized and utilized as solvents for the removal of residual KOH content from palm oil-based biodiesel. The results revealed that the removal efficiency of KOH by all used DESs increased with increase in DES:biodiesel molar ratio and the mole fraction of hydrogen bond donors (HBD) in mixture of salt:HBD apart from 2,2,2-trifluoroacetamide as HBD. The ChCl:glycerol DESs and MTPB:glycerol DESs proved their superiority in removing the KOH from palm oil-based biodiesel with an average removal efficiency of 98.59% and 97.57% respectively. These DESs fulfilled both international standards specification for KOH content at all DES:biodiesel molar ratios except for the 0.75:1 (DES1:biodiesel) ratio. The optimum molar ratio of DES to biodiesel for each DES was calculated and the decontamination factors for KOH removal at the optimum molar ratios for all tested DESs were determined. The reusability of these DESs for removing KOH from palm oil-based biodiesel was also investigated. Furthermore, it was found that the tested DESs were able to decrease the biodiesel water content to a value below that stated by the international standards.
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