Bio-oil is a renewable liquid fuel, having negligible contents of sulfur, nitrogen, and ash, and is widely recognized as one of the most promising renewable fuels that may one day replace fossil fuels. Fast pyrolysis of biomass technologies for the production of bio-oil have been developed extensively in recent years, which is usually carried out by the rapid (in a few seconds) raising of temperature to around 450-550 °C under atmospheric pressure and anaerobic conditions. The bio-oils are of high oxygen content of high viscosity, thermal instability, corrosiveness, and chemical complexity. These characteristics limit the applications of bio-oils, pre­cluding it from being used directly as a liquid fuel [100, 101]. Therefore, bio-oils need to be upgraded to improve its fuel properties. In order to develop a mild pyrolysis process with higher selectivity to favored compounds, the ILs-based technology was also introduced into this area. Several reports have demonstrated that ILs could be used as solvents or catalysts for this purpose. For example, Sheldrake et al. reported that dicationic molten salts were used as solvents for the controlled pyrolysis of cellulose to anhydrosugars [102]. It was demonstrated that the use of serials of dicationic ionic liquids for the pyrolysis of cellulose gave levoglucosenone as the dominant anhydrosugar product at 180 °C. An acidic dicationic IL were prepared and used as the catalyst to upgrade bio-oil through the esterification reaction of organic acids and ethanol at room temperature [103]. It was found that no coke and deactivation of the catalyst were observed. The yield of upgraded oil reached 49 %, and its properties were significantly improved with higher heating value of 24.6 MJ/kg, an increase of pH value to 5.1, and a decrease of moisture content to 8.2 wt%. The data showed that organic acids could be success­fully converted into esters and that the dicationic IL can facilitate the esterification to upgrade bio-oil. It is also found that microware irradiation could promote the pyrolysis of rice straw and sawdust with 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium tetrafluoroborate ILs as catalysts, and the bio-oil yield from rice straw reached 38 % and that from sawdust reached 34 % [104]. However, due to the high cost of ILs, and thermo stability during the pyrolysis, the use of ILs for the pyrolysis of biomass will not be the right direction.

7.3 Conclusion and Prospects

Biodiesel and bioalcohols are major biofuels that will offer many advantages over traditional fossil fuels and chemicals. In consideration of the unique properties of ILs and the key issues of biodiesel production from lipids, and bioalcohols separa­tion from fermentation process, ILs including functionalized acidic and basic ILs, switchable ILs and deep eutectic solvents have been used for more efficient production of biodiesel and bioalcohols. Although satisfactory results have been achieved in terms of lipids extraction, catalytic conversion of lipids and fatty acids, biodiesel purification, and bioalcohols separation, major challenges remain in this area in terms of lowering the costs, improving recyclability and environmental compatibility of ILs. In the future, the effect of possible residual ILs on the quality of biofuel products and downstream application need to be addressed. Bearing all of this in mind, new switchable ionic liquid systems may have great potential in application because of their unique properties, such as easy preparation and good recyclability. It is expected that ILs will be applied in a wider and more integrated way for biofuel production from various raw materials.