Kinetic and thermodynamic parameters in biodiesel oxidation reaction in the presence of coffee leaves and sage extracts

The biodiesel obtained from soybean oil consists mainly of esters of unsaturated fatty acids which gives it low oxidative stability. In order to inhibit the oxidative reactions, extracts of arabica coffee leaves and sage were added to the biodiesel. The induction period, the kinetic and thermodynamic parameters were determined using the Rancimat method at temperatures 110, 115, 120 and 125 degrees C. The results of induction periods containing sage extract and coffee leaves were 7.30 and 6.86 h, respectively, higher values than that obtained for biodiesel without addition of antioxidant species. Among the thermodynamic parameters it was observed that the activation entropy was negative (Delta S-double dagger < 0) for all the tests indicating the formation of activated complexes. The values of free energy of Gibbs indicated the increase of the energy necessary for the biodiesel to oxidize when in the presence of extracts of sage and coffee leaves, isolated or in mixture, which proves its efficiency as antioxidant agents.