Photo/thermal biorefineries towards renewable fuels and chemicals

Abstract

Lignocellulosic biomass is a renewable and earth-abundant carbon resource to produce non-fossil fuels and fine chemicals. To utilise native biomass, the depolymerisation of cellulose chains, as the initial step of biorefineries, requires stoichiometric chemical input and/or intensive energy infrastructure. Moreover, to increase the versatility of bio-based products, homogeneous/heterogeneous catalytic transformations are implemented on biomass platforms based on noble metals under high-pressure and/or high pressure. This thesis focuses on fast pyrolysis and photocatalysis – as recently emerging clean technologies¬ – to replace the conventional energy-intensive approaches. Chapter 2 probes the role of lignin-carbohydrate complex (LCC), the fundamental structure of native biomass, based on a “cellulose carboxymethylation–phase separation” strategy. The structural cross-links between lignin and carbohydrates are quantified and associated with the production of light oxygenates/furan compounds/anhydrous sugars in biomass pyrolysis. Chapter 3 shows an approach integrating biomass pretreatment and fast pyrolysis to enable the energy-efficient and environmental-friendly production of anhydrous sugars as biofuel precursors. Crude glycerol, as a surplus byproduct of biodiesel, is used in microwave solvolysis to ameliorate native biomass. Owning to the delignification, demineralisation and destruction of LCCs, kinetically controlled pyrolysis is achieved for the selective production of levoglucosan (6-fold enhanced), with an overall lowered carbon footprint in the integrated process suggested by life cycle assessment (LCA), compared to acidolysis–pyrolysis and petroleum-based ethanol production approaches. Chapter 4 reports a photocatalytic approach for the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a monomer of bio-based plastics. To resolve the poor selectivity and sluggish kinetics related to HMF adsorption, a Cu (II) porphyrin framework catalyst with side-chain urea linkages is designed and synthesized, which induces hydrogen bonding and π–π stacking with HMF to enable its flat adsorption with moderate energetics. Moreover, the selective activation of molecular oxygen and kinetically favourable pathways facilitate a FDCA selectivity of over 90% and a high turnover number of 193 mol molCu−1 in a tailored process for product purification and catalyst recovery. Chapter 5 clarifies the competing pathways, i.e. Cannizzaro (thermally driven) and photoredox reactions, in photocatalytic aqueous reforming of biomass-derived C3 oxygenates. Over TiO2, the selectivity of lactic acid (LA), a monomer of bio-based plastics, is limited to < 20% from the disproportion reaction of pyruvaldehyde driven by both Lewis acid sites and holes–electrons. Intermediate identification, in situ spectroscopies and theoretical calculations indicate an energy barrier in the shifted rate-determining step of Cannizzaro reaction (highly selective for LA) higher than those of photoredox pathways (towards multiple redox products), i.e. 1,2-hydride shit (1.49 eV) versus proton-coupled electron transfer (PCET, 0.20–0.61 eV), respectively. It is envisioned that by utilising the synergy between Lewis acid mediated hydride shit and photoredox C–H activation, a photothermal/oxygen vacancy catalyst (e.g. Au/TiO2-Ov) can remarkably increase the LA selectivity under ambient conditions. Chapter 6 and 7 discuss comparatively the key findings across the experimental chapters and with the extensive body of literature. General conclusions and future perspectives are drawn from the critical discussion and analysis to accelerate the research journey towards “net-zero” biorefineries.