Doctor of Philosophy
Chemical and Biochemical Engineering
Xu, Chunbao (Charles)
Due to dwindling fossil resources, climate change and other environmental concerns as well as the toxicity of certain products derived from petroleum resources, there is a growing interest in exploring and utilizing the abundant biomass resources as alternative feedstocks for the production of bio-based chemicals and materials. Biomass is composed of three main macromolecules, namely: cellulose, hemicellulose, and lignin. Even though most of the initial work relating to the valorization of biomass was focused on the carbohydrate components (cellulose and hemicellulose), in the last ten years, lignin, an aromatic biopolymer, has been receiving increasingly more attention. In particular, various routes for lignin valorization such as depolymerization have been extensively investigated. However, current depolymerization/conversion strategies rely on high-temperature, high-pressure hydrogenolysis using a range of various supported metal catalysts and organic/inorganic solvents. These conditions lead to products of reduced functionality and/or difficulties in the recovery of the solid catalyst and the depolymerized lignin products after the reaction. As a result, most depolymerization processes reported in the literature are likely to be associated with high capital and operating costs as well as issues with scalability and industrial applicability. The main objective of this doctoral study was to develop cost-effective and industrially scalable approaches for the production of high–value, bio-based chemicals and materials for use in various applications. In an effort to address this objective, this study led to the development of four approaches for lignin depolymerization. A first approach demonstrated the technical feasibility of depolymerizing kraft lignin in black liquor rather than depolymerizing puriﬁed lignin itself. In this case, the presence of several well-known nucleophilic agents in black liquor was exploited to produce low-molecular weight, depolymerized lignin with a high functionality. The desired lignin MW was obtained by adjusting various process parameters such as reaction temperature, catalyst charge (NaOH), capping agent charge (phenol) or co-solvent charge (methanol). A second approach demonstrated the technical feasibility of depolymerizing biorefinery (hydrolysis) lignin using recoverable white liquor (WL) from the kraft recovery cycle at mild temperatures (150-190 °C). In this case, process scalability was demonstrated using a 20-L circulating reactor for the production of low MW, high functionality/reactivity depolymerized hydrolysis lignin (DHL). A third approach demonstrated the technical feasibility of depolymerizing softwood and hardwood kraft lignins (SKL and HKL) under ambient temperature conditions using only hydrogen peroxide (H2O2). In this case, the depolymerized kraft lignin exhibited unique properties. A fourth approach demonstrated the technical feasibility of depolymerizing SKL and HKL employing only nitric acid (HNO3) at ambient temperature. The obtained depolymerized KL products displayed a low molecular weight of about 1300 Da with a high solubility in water at low pH. The nitration of lignin was confirmed by the presence of 5 wt.% N content in the modified lignin after reaction with 35wt% HNO3 at room temperature. In summary, this work contributed new knowledge relating to kraft and hydrolysis lignin depolymerization. In particular, this work led to the development of four novel, simple, cost-effective and industrially scalable approaches for lignin depolymerization. These lignin depolymerization strategies are expected to lead to increased lignin utilization in several lignin applications including as a replacement of phenol in phenolic resins, petroleum-based polyols in polyurethane foams and bisphenol A in epoxy resins. The use of lignin in several applications will lead to reduced reliance on fossil-based resources.
Ahmad, Zaid, "Highly Efficient Depolymerization of Kraft Lignin (KL) and Hydrolysis Lignin (HL) via Hydrolysis and Oxidation" (2019). Electronic Thesis and Dissertation Repository. 6171.
Available for download on Saturday, May 01, 2021