Studies in bagasse fractionation using ionic liquids

Biomass is a readily available, renewable resource that is likely to become an economically viable source of starting materials for chemical and fuel production, especially with the real cost of using a depleting supply of fossil fuels becoming more widely apparent. Ionic liquids have unique advantages as solvents, and have been investigated in reactions involving biomass and carbohydrates such as cellulose and glucose. The work presented in this thesis describes the synthesis, characterisation and use of ionic liquids for the fractionation of sugarcane bagasse into useful products. Ionic liquids containing the xylenesulfonate anion were synthesised, characterised and tested for the extraction of lignin from bagasse based on previous work at Monash. The most effective ionic liquid for lignin extraction was l-ethyl-3-methylimidazolium alkylbenzenesulfonate [C2mim][ABS], which was synthesised from a technical grade of xylenesulfonate containing isomers of xylenesulfonate, ethylbenzenesulfonate, cumenesulfonate and toluenesulfonate, collectively referred to as alkylbenzenesulfonates [ABS]. Over 93% extraction of lignin was obtained using [QmimJtABS] at 190 °C for a 2 hour reaction. Other cations tested were choline [Ch], tetrabutylphosphonium [P4,4,4,4] tributylhexylphosphonium [P^.e], tetradecyltrihexyl-phosphonium [P6,6,6,i4] and tributyltetradecylphosphonium [P^ah]- Ionic liquids containing the [C^mim] cation combined with the tosylate [TS] anion, and the mesylate [MS] anion were also synthesized, characterised and tested. The fact that the process can be conducted at atmospheric pressure is significant in terms of plant and equipment requirements, and also illustrates a useful feature of ionic liquids in providing a reaction medium for hydrolysis reactions in which water is present but only VI at low activity. The isolated lignin contained some [ABS] adducts, which were detected by infrared spectroscopy and elemental analysis. The recovered ionic liquid showed no structural change, as observed through 'H NMR. Microwave heating was investigated but was found to result in the breakdown of cellulose due to the difficulty in obtaining uniform heating. Factors influencing the uneven heating were poor stirring of the ionic liquid/lignocellulose mixtures, poor heat conduction of lignocellulose, high microwave absorbance of the ionic liquid, and possible hotspots in the microwave cavity. Acid-catalysed conversion of cellulose to low molecular weight products in ionic liquids was also investigated. Efforts to distill products such as 5-hydroxymethylfurfural (HMF) and levulinic acid to avoid complex polymerisation products were complicated by the codistillation of HC1 and water, which tended to decrease the overall conversion, as both acid and water were required for the hydrolysis of cellulose.