Valorising polysaccharide-rich waste into value-added chemicals in green solvents

Abstract

The anticipated transition of our society from a fossil resource-dependent linear economy to a bio-based circular economy requires considerable research efforts on valorising waste-based feedstock such as lignocellulosic biomass and food waste for greener production of value-added products, energy, and chemicals. These wastes are enriched in polysaccharides such as cellulose and starch that are deemed suitable and potential for value-added chemical production. However, it is crucial to develop cost-effective and time-efficient valorisation technologies for complex waste substrates. This thesis focused on the development of facile and efficient technology to valorise polysaccharide-rich waste to value-added chemicals through catalytic conversion under diverse green solvent systems. Solvent is a significant parameter that influences catalytic conversion through interactions with reactants and regulation of product selectivity in the system. Therefore, appropriate solvents could enhance the conversion of polysaccharide-rich waste to bio-based chemicals. However, high-performance industrial solvents often pose direct and indirect risks to health, safety, and environmental components, which should be addressed by finding green yet reactive solvents for biorefinery applications. This research demonstrated that cellulosic paper wastes and starchy food waste could be efficiently valorised into sugars, levulinic acid (LA), hydroxymethylfurfural (HMF), and other valuable chemicals through tandem hydrolysis and dehydration/rehydration using the dilute sulphuric acid catalyst in green solvent systems under microwave heating. Gamma-valerolactone (GVL) (used as a co-solvent, GVL/H₂O) as a bio-derived, renewable, and green solvent demonstrated excellent performance as a reaction medium for LA production (32 mol%) from highly cellulosic paper towel waste. Further, diverse paper waste substrates with variable properties (feedstock characteristics originated due to varied paper manufacturing operations) were also deemed viable for platform chemical production under GVL/H₂O solvent generating up to 27 wt% LA yield along with a considerable amount of HMF, and furfural, which is attractive to foster paper waste valorisation at industrial scale. Such notable performance of GVL solvent could be attributed to its positive enhancement in cellulose solvation, maintaining the solubility of the desired products, while preventing polymerisation/condensation of derivatives and by-product formation. Depolymerisation, which is a bottleneck for the efficient valorisation of cellulosic waste, could be enhanced by using CO₂-derived organic carbonate solvents (i.e., propylene carbonate (PC), ethylene carbonate (EC), and dimethyl carbonate (DMC)) as reaction medium. This research demonstrated the efficiency of these emerging green solvents for sugar production, up to 25 mol% in PC/H₂O compared to 11 mol% in H₂O-only solvent under mild reaction conditions (130 °C, 20 min). Enhanced sugar yield could be attributed to the higher availability of reactive protons in the catalytic system comprising carbonate solvents that facilitated efficient acid hydrolysis of recalcitrant cellulosic fibres. Moreover, a substantial build-up of auto-generated pressure in PC/H₂O might be favourable for cellulose depolymerisation. Furthermore, the valorisation of starch-rich food waste was optimised based on the knowledge acquired regarding catalytic conversion green solvent systems. This research highlighted that solvent effects, reaction pressure, and phase separation could be the critical factors for determining levulinic acid production from bread waste. The presence of polar aprotic solvents such as GVL and PC were instrumental in maintaining proton reactivity during catalytic conversion. In PC/H2O solvent, CO2-derived pressure generated by PC decomposition favours starch hydrolysis and subsequent dehydration (15-20 mol% LA, 130 °C, 20 min, 0.5 M H₂SO₄). Besides, LA yield was significantly increased by using biphasic GVL/H₂O solvent (28 mol%, 150 °C, 15 min), promoted by NaCl as the phase modifier. These research efforts elucidated the roles and performance of green solvents, which could be potential alternatives to conventional industrial solvents for valorising polysaccharide-rich wastes in biorefineries.