Effect of Thermal Treatment on the Catalytic Performance of Magnesium Oxide-Hydroxide Systems in Biodiesel Production from Jatropha Oil: Homogeneous versus Heterogeneous Catalytic Action

Abstract

A series of magnesium oxide-hydroxide catalysts was prepared and tested for their efficiency in catalyzing the transesterification of raw Jatropha oil into biodiesel. Thermal treatment was found to play a crucial role in determining the catalytic activity and calcination of Mg(OH)₂ for two hours at 350^oC was found to give mixed Mg(OH)₂ and MgO phases with high activity  as rather small amounts of this catalyst (only 0.25 g per 20 g oil) could achieve in three hours of transesterification with methanol at 60^oC a conversion of 75%. The solubility of the above system in methanol was determined to be 1.53×10^-4 M, suggesting that its catalytic action is both of homogenous (dissolved hydroxide ions) as well as of heterogeneous (basic surface sites) nature. Thermal treatment at 500^oC results in a less active system (biodiesel yield 47%) which could be related to decreased amount of soluble fraction (solubility in methanol 6.7×10^-5 M) and to decreased strength of basic surface sites as suggested by the CO₂ desorption experiments. Nevertheless, the system resulting from the thermal treatment at 500^oC is believed to be the better one in terms of heterogeneity and thus recyclability; higher amounts must however be used for longer periods of time in order to achieve higher yields