The utilisation of edible resources as alternative and renewable sources of energy and chemicals in the battle against climate change is unsustainable and poses a potential threat to food security. As such, the use of waste-derived resources (whether plastics, biomass, or metals) is at the forefront of sustainable technology research today, owing to the abundance of waste and the need to reduce reliance on fossil resources.
This research, therefore, exploited the potential to convert (via liquefaction) lignocellulosic biomass to valuable products that could serve as precursors to the chemical and fuel industries. Specifically, pine needles (exemplar forest waste) and sugarcane bagasse (an agricultural waste) were employed as feedstock in this work. The influence of various low-cost solvents (glycerol, ethylene glycol, and water) used as liquefaction solvents and reaction conditions (temperature, pressure, and catalyst) on both biomass conversion and product yields were investigated.
In this study, sugarcane bagasse was found to be predominantly cellulosic in nature whereas that of pine needles was more ligninic. The application of water as liquefaction solvent facilitated better degradation of cellulose and hemicellulose whereas glycerol and ethylene glycol were found to facilitate better degradation of the most recalcitrant biomass component, lignin. As such, the highest biomass conversion was achieved in the presence of glycerol (94 wt.%) followed by ethylene glycol (86 wt.%), and water (76 wt.%) at 250 °C, 1 h reaction time, 30 bar initial helium pressure, and biomass:solvent ratio of 1:10 (wt:wt). Acetic acid, glucose, levulinic acid, phenol, and various esters were the most prominent biomass-based products identified in the liquid fraction, all of which are important platform chemicals for industry.
It was also observed that the use of water enhanced the selectivity of the targeted biomass derivatives; particularly, acetic acid, phenol, and levulinic acid. Meanwhile, glycerol and ethylene glycol were found to generate additional valuable chemicals through various side reactions. Hence, a novel one-pot process was designed for the simultaneous valorisation of biomass and glycerol using acetone as a cosolvent and reagent. This process produced mainly acetic acid and phenol as biomass-based chemicals while solketal and mesityl oxide were synthesised from the excess glycerol and acetone. The one-pot valorisation of biomass and low-value solvents such as glycerol would potentially significantly reduce the number of steps needed to achieve the same results in separate processes, making future biorefineries more economical.
Finally, it was discovered in this research that bio-char formed in situ during biomass liquefaction is catalytically active towards the conversion of liquefaction solvents to value-added chemicals. The activity of the bio-char was largely attributed to its ash content. This catalytic activity was particularly observed in the conversion of acetone to mesityl oxide. Consequently, the potential role of biomass and its components; bio-char and ash as a heterogeneous catalyst for mesityl oxide synthesis was demonstrated in this thesis for the first time. The application of catalysts from renewable and environmentally friendly sources in place of fossil-derived catalyst or expensive precious metals is crucial for future research in light of the development of a sustainable industry.
