Base-catalyzed depolymerization of kraft lignin for valuable chemical production
Abstract: Lignin is one of the three predominant biopolymers that make up plants together with cellulose and hemicellulose, and it is the only plant component that is based on aromatic units. The industrial kraft pulping process generates large amounts of lignin, which is currently used to generate heat for the process. The subject of this thesis is the valorization of kraft lignin into valuable chemicals. The work started with the depolymerization of kraft lignin, followed by studies on efficient separation processes to obtain low-molecular-weight (LMW) compounds. Finally, the microbial conversion of lignin-derived monomers for the production of fine chemicals was investigated.A lab-scale continuous-flow reactor system was developed for the depolymerization of two different Kraft lignin samples: Indulin AT and black liquor retentate (BLR) using NaOH as the only catalyst. The depolymerization experiments were performed under mild conditions at temperatures less than 250 °C and residence times less than 4 min. The effect of the depolymerization temperature, residence time, NaOH concentration, and the lignin substrate loading on depolymerization was studied. Depolymerization of Indulin AT and BLR resulted in a mixture of lower molecular weight compounds. Temperature was found to be an important factor in the depolymerization of lignin, and the main phenolic compounds obtained after depolymerization were guaiacol and vanillin. An ultrafiltration GR95PP membrane with a cut-off of 2 kDa was used in membrane filtration studies. The combination of base-catalyzed depolymerization and membrane filtration was investigated in an attempt to obtain more lignin-derived LMW compounds without severe repolymerization. Ultrafiltration of the depolymerized BLR sample removed 60% of the LMW compounds. A separation process starting with membrane filtration followed by acid precipitation and low-temperature evaporation under vacuum is suggested to obtain pure guaiacol from the depolymerized lignin samples. Concentrated vanillin and other monomers were also separated and collected. Finally, bioconversion of guaiacol using an engineered strain of Pseudomonas putida KT2440 was studied. The strain had been modified for guaiacol consumption with a cytochrome P450 enzyme and ferredoxin reductase and also modified to produce muconic acid by deletion of the downstream enzymes encoded by catBC. Both pure and kraft lignin-derived guaiacol was quantitatively converted into muconic acid, which is a promising precursor to adipic acid or terephthalic acid used in the production of nylon-6,6 or bio-plastics e.g bio-PET.
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