Solvolytic Deconstruction of Lignin and Upgrading to Renewable Chemicals

Field: Chemical Engineering (Physical Sciences)

Lignocellulosic biomass is gathering tremendous attention because of its abundance and potential for conversion to a wide range of chemicals and fuels. Lignin, which accounts for nearly 15- 30 wt% of all lignocellulosic biomass, is the largest source for naturally obtainable aromatic hydrocarbons. Despite its enormous potential, practical valorization is still lacking . The two fundamental challenges associated with lignin valorization are the heterogeneous structure of the lignin polymer and the high reactivity of the degraded intermediates. Due to its heterogeneity and variability, selective production of well-defined compounds from lignin is challenging. Depolymerized lignin fragments are highly reactive under reaction conditions, which leads to unwanted side reactions, including repolymerization, to form even more recalcitrant structures and char. Chapter 1 introduces lignin, its latent potential, and state-of-the-art depolymerization and upgrading strategies to chemicals and fuels. The technological challenges encountered during fractionation, depolymerization, and upgrading are illustrated. Chapter 2 in this thesis explores flow-through reactors as an alternative to commonly used batch reactors for lignin depolymerization. This configuration allows for the continuous removal of products. As the reactive intermediates formed during depolymerization remain in the reaction conditions in the batch reactor, they are prone to undesirable secondary reactions. Thus, flow-through reactors were tested as an alternative configuration for solvolytic depolymerization of technical lignin to curb secondary reactions and maximize depolymerized product yield. Chapter 3 examines the pretreatment of lignin prior to pyrolysis to increase the yield of volatile products generated for further upgrading. Char produced during lignin pyrolysis presents a significant drawback towards maximizing carbon efficiency during subsequent catalytic upgrading to platform chemicals. We first gained insights into various catalytic and solvolytic pretreatments using various complementary characterization techniques. The parameters that govern the size distribution of the lignin fragments and chemical transformations relative to raw lignin were identified. The effect of various catalytic pretreatments on product distribution and yields obtained upon fast pyrolysis are discussed in detail. Furthermore, we extended this approach to the one-pot fractionation and pretreatment of lignin from whole biomass for low-carbon-footprint chemicals production. The lignin oil generated through this pretreatment was funneled to platform chemicals, while the carbohydrate pulp was amenable to bioethanol production. As seen from the encouraging results in Chapter 2, alcohols such as methanol and ethanol can solubilize and depolymerize lignin at short reaction times. The short residence times also enabled the recovery of depolymerized lignin products before they underwent secondary reactions owing to continuous separation in this configuration. Chapter 4 delves into the use of the flow-through reactors to optimize the residence time/solvent flow rates to balance the lignin extraction vs. repolymerization rate during the lignin extraction from lignocellulosic biomass. Model compounds were used to illustrate competing reactions and the associated hurdles in balancing these reactions during biomass fractionation. The solubilized products were thoroughly characterized through various techniques such as complete mass balance, GPC, GC-MS-FID, elemental analysis to understand the structural changes as a function of the reaction conditions and flow rates. The obtained carbohydrate fraction/pulp was also be characterized to assess the yield of the resulting sugar. Flow-through reactors facilitated the delignification of biomass while retaining the sugar fraction, and the recovery of lignin was also much higher than that of batch systems. This research will also pave the way to determine the effectiveness of various alcohols, the reaction pathway, and the optimum operating conditions to obtain lignin close to its native form. Chapter 5 delivers concluding remarks, and future directions should focus on further improving the explored strategies using techno-economic analysis and flow-through reactors to deconstruct mixed plastic waste.