CsF and alumina: A mixed homogeneous–heterogeneous catalytic system for the transesterification of sunflower oil with methanol

The activity and nature (i.e. heterogeneous and/or homogeneous) of catalysts based on CsF supported on α-Al2O3 were investigated for the transesterification of vegetable oil with methanol. The effect of the activation temperature, CsF loading and the reusability in a recirculating reactor were first studied. CsF/α-Al2O3 exhibited the highest activity for a CsF loading of 0.6 mmol/g and when activated at 120 °C. An important aspect of this study is the effect of CsF leaching into the reaction mixture, which is attributed to the high solubility of CsF in methanol, leading to a complete loss of activity after one run. It was identified that the activity of the catalyst resulted from a synergy between alumina and dissolved CsF, the presence of both compounds being absolutely necessary to observe any conversion. The use of an alumina with a higher surface area resulted in a far greater reaction rate, showing that the concentration of surface site on the oxide (probably surface hydroxyl) was rate-limiting in the case of the experiments using the low surface area α-Al2O3. This work emphasizes that combined homogeneous–heterogeneous catalytic systems made from the blending of the respective catalysts can be used to obtain high conversion of vegetable oil to biodiesel. Despite the homogeneous/heterogeneous dual character, such a catalytic system may prove valuable in developing a simple and cost-effective continuous catalytic process for biodiesel production.