Kinetic modeling, mechanistic, and thermodynamic studies of HPW-MAS-9 catalysed transesterification reaction for biodiesel synthesis

Abstract

The kinetics and reaction mechanism of methanolysis of unrefined green seed canola oil using heterogeneous HPW-MAS-9 composite catalyst have been investigated. The activity coefficients of the reactant and product species were estimated using the UNIQUAC method. Eley-Rideal reaction pathway with the surface reaction of adsorbed methanol as the rate controlling step, was found to be a reliable representation of the observed kinetics. Other kinetic models deduced from pseudo-homogenous (PH) and Langmuir-Hinshelwood-Hougen-Watson (LHHW) were rejected based on statistical analysis. Ea (activation energy), ∆H‡ (enthalpy of activation), ∆G‡ (free energy of reaction), and ∆S‡ (entropy of activation) for the reaction were found to be 72.5 kJ mol−1, 68.9 kJ mol−1, 58.3 kJ mol−1, and 0.04 kJ mol−1, respectively. The positive values of ∆H‡ and ∆G‡ indicated that the reaction is endothermic and non-spontaneous. Intricate mechanistic details for the stepwise transesterification reactions were deduced from the kinetic and thermodynamic parameters. On the basis of the ΔS‡, the reaction of triglyceride to diglyceride followed the dissociative pathway (SN 1 mechanism), whereas the reactions of diglyceride to monoglyceride and monoglyceride to glycerol proceeded via the associative pathway (SN 2 mechanism).