ABSTRACT
Abstract The benefits of using biodiesel as renewable fuel and the difficulties associated with its manufacturing are outlined. The synthesis via fatty acid esterification using solid acid catalysts is investigated. The major challenge is finding a suitable catalyst that is active, selective, water-tolerant and stable under the process conditions. The most promising candidates are sulfated metal oxides that can be used to develop a sustainable esterification process based on continuous catalytic reactive distillation. Introduction Developing sustainable energy sources is a key scientific challenge of the 21st century. Automotive applications consume a large fraction of the available global energy, making the implementation of sustainable fuels a critical issue worldwide. Biodiesel is an environmentally-friendly renewable fuel (Figure 1) that can be manufactured from vegetable oils, animal fats or even recycled greases from the food industry (1). It is also a viable alternative to petroleum diesel, especially following the tighter legislation on vehicle emissions. Remarkably, biodiesel is the only fuel with an overall positive life cycle energy balance. It is a green fuel, with many advantages over conventional diesel (1-4): it is safe, non-toxic, biodegradable and a good lubricant. Moreover, it has a higher cetane number and an eco-friendly life cycle. It also emits ~70% less gas pollutants and ~50% less soot particles. Moreover, it is easily blended with petroleum diesel, giving a mixture that can power regular vehicles without any changes to the engine or the fuel distribution infrastructure. Biodiesel is a mixture of fatty acid alkyl esters, derived typically from short chain alcohols. Methanol is more suitable for biodiesel manufacturing, but other alcohols can in principle also be used, depending on the feedstock available. Fatty acid methyl esters (FAME) are currently manufactured mainly by trans-esterification using a homogeneous base catalyst (NaOH). This catalyst is corrosive to the equipment and must be neutralized afterwards. Moreover, due to the presence of free fatty acids (FFA) it reacts to form soap as unwanted by-product, requiring expensive separation. In this process the biodiesel composition depends heavily on the types of fatty acid groups building the triglycerides from the raw material.
