Biodiesel Production via Esterification of Palm Fatty Acid Distillate Using Sulphonated Multi-walled Carbon Nanotubes as a Solid Acid Catalyst: Process Study, Catalyst Reusability and Kinetic Study

Abstract

This study reports on biodiesel production via the esterification of palm fatty acid distillate (PFAD) using sulphonated multi-walled carbon nanotubes (s-MWCNTs) as a catalyst. The process parameters studied included the methanol-to-PFAD ratio (8–30), catalyst loading (1–3 wt%), reaction temperature (80–200 °C) and reaction time (1–5 h). A fatty acid methyl ester (FAME) yield of 93.5 % was obtained at a methanol-to-PFAD ratio of 20, catalyst loading of 3 wt%, reaction temperature of 170 °C and reaction time of 2 h. The s-MWCNTs exhibited good catalytic activity, with a FAME yield higher than 75 % even after five repeated runs. Moreover, the regeneration of the spent s-MWCNTs (after five runs) with sulphuric acid was able to restore the catalytic activity to its original level. The catalyst stability and activity were enhanced by acid regeneration to achieve a FAME yield of 86.2 %, even at the fifth cycle of reaction after acid regeneration. A pseudo-homogeneous kinetic model for the esterification of PFAD with methanol using s-MWCNTs as a catalyst was then developed based on the experimental results. The pre-exponential factor, molar heat and activation energy for the esterification were found to be 1.9 × 102 L mol−1 min−1, 84.1 kJ mol−1 and 45.8 kJ mol−1, respectively.