The biodiesel produced by the transesterification of vegetable oils (VOs) has recently become more attractive due to its environmental benefits and the fact it is made from renewable resources. As the consumption of petroleum continues to increase, so does the associated air pollution and accelerated rate of global warming due to the increase of atmospheric CO2. If biodiesel was used as an alternative fuel, the production of CO2 would not increase, as the CO2 emitted from biodiesel would be recovered during the production of biomass.

Biodiesel is generally produced by the transesterification of triglyceride to methyl esters with methanol using homogeneous catalyst such as sodium or potassium hydroxide dissolved in methanol. An environmentally benign process for the production of biodiesel from VOs using heterogeneous catalyst (Na/NaOH/ã-Al2O3) has been developed recently by us. In order to reduce the production cost of the biodiesel, used vegetable oil is suggested to be used as feedstock. However, used VOs contain free fatty acids (FFA), which can react with the base catalyst if it used as a feedstock of the transesterification without any pretreatment. It brings a loss of catalyst and a production of soap as by-product. Therefore, FFA should be removed or converted to inert or reactant to overcome these problems. In this work, we tried to convert used VOs using special treatment to 100% FFA and convert the FFA using heterogeneous acid catalysts to fatty acid methyl esters (FAME), which are the components of biodiesel themselves. The activities of several heterogeneous catalysts were tested on the conversion of FFA and FAME. This reaction experimented to change the temperature, stirring speed, amount of catalyst and reaction time, even abundant 100wt% FFA was tested in optimal reaction conditions. The optimization of reaction conditions was also carried out. The characterization of heterogeneous catalyst was performed by ICP, TPD and XRD analysis.