Investigations on ionic liquid pre-treatment of lignocellulosic biomass for the production of biofuels and platform chemicals

<p>Lignocellulosic biomass is a carbon-neutral and renewable feedstock and thereby has gained significant attention for the production of fuels, chemicals, and other value-added products. However, the complex and rigid lignocellulosic structure, particularly the presence of glue-like lignin causes significant inhibition to the conversion of biomass into biofuels and biochemicals. Apart from this, the production of specific chemicals from biomass is a challenging task due to the heterogeneous structure of lignocellulosic biomass. The research presented in this thesis focuses on an integrated approach to overcome the existing barriers of bioprocessing for producing platform chemicals from lignocellulosic biomass. The integrated process proposed here includes 1) pre-treatment of lignocellulosic biomass using ionic liquids (ILs) to separate cellulose-rich material (CRM) and lignin from the lignocellulosic structure and 2) pyrolysis of CRM and lignin for the selective production of platform chemicals such as furans, furfural, levoglucosenone, and phenolic rich oil. In particular, the research aims to explore the role of cation and anion of ILs on lignin extraction efficiency and the effect of IL pre-treatment on the selective production of platform chemicals from pyrolysis. In the first investigation, three imidazolium cation-based ILs (i.e., 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium acetate and 1-butyl-3-methylimidazolium acetate) were compared with two organic solvents (i.e., methanol and ethanol) for the pre-treatment of biomass exploring the variations of degree of delignification and structural changes in the regenerated cellulose-rich material (RCRM). It was observed that ILs and organic solvents removed more than 20% lignin at low-temperature pre-treatment (i.e., 70 °C for 3 h) of river red gum. The XRD, FTIR, TGA and SEM analyses suggested that imidazolium cation-based ILs pre-treatment reduced crystallinity and thermal stability and increased porosity of RCRM, which were not observed in the case of organic solvents pre-treatment. This was due to the transformation of crystalline cellulose I to cellulose II with amorphous structure during imidazolium cation-based ILs pre-treatment. The higher delignification performance of IL is associated with higher hydrogen-bond basicity of anion and shorter alkyl chain length of cation. In the second investigation, two renewable choline cation-based ILs (i.e., choline glycinate and choline chloride) were synthesised and compared their delignification performance with that of imidazolium cation-based ILs for the pre-treatment of biomass. The pre-treatment of sugarcane straw (SCS) employing choline cation-based ILs at 100 ºC for 5 h recovered approximately 78.1-83.7% of lignin. The analysis concluded that the cation structure predominantly influenced the lignin dissolution in IL and identified hydrogen-bond formation between -OH group of choline cation and the oxygen atom of lignin as the dominant mechanism for lignin dissolution than the π-π interaction of imidazolium cation and lignin polymer. In the third investigation, biomass dissolution (i.e., SCS in 1-ethyl-3-methylimidazolium acetate and oak wood in aqueous choline chloride) reaction mechanism and kinetics were investigated and a diffusive mass transfer correlation was developed for the IL-biomass pre-treatment system. The delignification reaction was greatly influenced by the mass transfer of the reactant species and followed the product layer diffusion mechanism. Finally, the pyrolysis of RCRM and recovered lignin obtained from the IL pre-treatment was investigated, looking at the production of platform chemicals, for instance, furans, levoglucosenone, furfural, and phenol. The pyrolysis kinetic study of RCRM from imidazolium cation-based IL (i.e., 1-ethyl-3-methylimidazolium acetate) pre-treatment exhibited lower activation energy of pyrolysis and the TGA-FTIR analysis showed higher production of furans due to the presence of higher content of cellulose II with amorphous structure in RCRM compared to untreated SCS. Additionally, IL pre-treatment cleaved the ß-O-4 ether bonds of lignin, which reduced the activation energy of pyrolysis and increased the production of phenol-rich pyrolysis oil compared to that of alkali lignin. The biochar produced from the pyrolysis was analysed using SEM and FTIR. Pre-treatment of SCS using low-cost choline cation-based ILs increased the crystallinity of RCRM. The pyrolysis of RCRM samples obtained from choline cation-based ILs pre-treatment using TGA-FTIR-GC/MS suggested the increase in the production of furfural and levoglucosenone from pyrolysis of RCRM compared to that of untreated SCS. This was mainly because of the presence of higher content of crystalline cellulose in RCRM, which enhanced in the depolymerisation and dehydration reaction during the pyrolysis of RCRM. A techno-economic model was developed for the co-production of furfural, levoglucosenone along with lignin, hemicellulose, and biochar from IL pre-treatment integrated pyrolysis process. The economic analysis suggested minimum furfural selling price and minimum levoglucosenone selling price of around 1640 and 3590 AU$/tonne, respectively. The payback period was estimated about 15.4 years. These exciting findings establish that ILs pre-treatment can play an important role in the future pyrolysis-based bioprocessing industry.</p>