Heterogeneous acid catalysts for biodiesel production: current status and future challenges

The reduction of oil resources and the consequent increasing price of oil distillates as well as the environmental concerns of conventional fuels has renewed and increased interest on the preparation of biofuels from renewable resources. One of those interests is nowadays focused on biodiesel, which is usually prepared from crude and refined triglyceride containing raw materials, such as vegetable oils, animal fats and wastes—for instance waste cooking oil and yellow and brown grease. Since several commercial interests converge on this kind of feedstock, one of the priorities being crops for human food supply, the research efforts on biodiesel production are diverting towards the use of low quality triglyceride-containing raw materials. Nevertheless, all of these feedstocks feature high water and free fatty acids (FFAs) content, which strongly affects the behaviour of conventional homogeneous base catalysts. These catalysts are primarily NaOH and KOH, but also NaOCH₃ and KOCH₃ are employed—as solutions in methanol—mainly in large-scale production plants. In this context, an appropriate solid acid catalyst which could simultaneously carry out esterification of FFAs and transesterification of triglycerides would be of great interest for biodiesel production. Moreover, a heterogeneous acid catalyst could be easily incorporated into a packed bed continuous flow reactor, simplifying product separation and purification and reducing waste generation. The present review attempts to provide a wide overview on the possibility of heterogeneous acid catalysts for biodiesel production replacing the homogeneous conventional process. In this way, three aspects of solid acid catalysis for biodiesel production will be reviewed. The first section deals with the solid acid-catalyzed esterification of FFAs, the second topic relates to the transesterification of triglycerides, while the third deals with solid acid-catalyzed transformation of bioglycerol into oxygenated compounds for biodiesel formulation.