Lignin is the second most abundant component of lignocellulosic biomass after cellulose. Considering its rich aromatic structure and huge production, lignin has potential to be the major resource for renewable aromatic chemicals. However, there are three major concerns for effective lignin valorization. First, the condensation reaction of the lignin fragments during the depolymerization process. Second, the use of acid or alkali during lignin fractionation (e.g. delignification in pulping) results in the variation of the ether linkages and lignin condensation. Third, the current studies only focus on the production of chemicals from lignin but ignores the possibilities to make extra value. This dissertation focused on lignin valorization considering 1) suppressing lignin condensation by selective oxidation of active benzyl alcohol to facilitate lignin depolymerization, 2) fractionating lignin from lignocellulose for in situ depolymerization towards complete biomass valorization; and 3) simultaneous processes of lignin depolymerization to chemicals and electrolysis to hydrogen to maximize lignin utilization. In the first part, this study employed two-step oxidation strategies in two ways. In the first strategy, TEMPO was used to selectively oxidize the active Cα-OH moiety to ketone to suppress its condensation. Then POM is used as catalyst for further oxidative degradation of the preoxidized lignin. The lignin depolymerization efficiency could be remarkably improved by converting the active Cα-OH to the stable Cα=O. Under optimized conditions, 74.5% of the lignin was converted to low molecular weight compounds with 32.8 wt% of monomeric compounds. However, this process need to be conducted in two different catalysis systems (i.e. solvents, catalysts), which becomes a significant hurdle for employing this strategy in industrial practice. Herein, another two-step oxidation protocol which can be achieved in one pot by the same polyoxometalate catalysis system was conducted. First, the benzyl alcohols in technical lignin could be oxidized to ketones in methanol by POM under mild condition. Further, the preoxidized lignin was depolymerized to aromatics at an elevated temperature (e.g. 140 ̊C). Total 58.0% of technical lignin without purification can be converted to small molecules. In the second part, a new lignin-first fractionation strategy was developed to improve the biomass valorization to further improve the lignin conversion by preventing the condensation of lignin during biomass fractionation. The low concentration POM catalyst is employed to extract lignin from wood sawdust, under mild condition (100 ̊C) in alcohol, while avoiding structural lignin condensation and degradation. Then extracted lignin is further oxidized to small aromatic molecules with 86.2% conversion. The third part focused on a novel proton exchange membrane (PEM) electrolysis process in which lignin was used as the hydrogen source at anode for hydrogen production. Either POM or FeCl3 was used as the catalyst and charge transfer agent in anode. Over 90% Faraday efficiency was achieved for both systems. Compared to the alkaline water electrolysis reported in literature, the electrical energy consumption could be 40% lower with novel lignin electrolysis method. At the anode, the Kraft lignin (KL) was oxidized to aromatic chemicals by POM or FeCl3, and reduced POMs or iron ions were regenerated during the electrolysis. Structure analysis of the residual KL indicated the reduction of hydroxyl group number and the cleavage of ether bonds. Our results suggest that POM or FeCl3 mediated electrolysis process can significantly reduce the electrolysis energy consumption in hydrogen production and, simultaneously, depolymerize lignin to low molecular weight value-added aromatic chemicals. In summary, this dissertation attempts to understand the catalytic valorization of lignin mainly using polyoxometalates as catalyst in terms of its special properties, including the mechanism of condensation, process design, catalyst preparation, depolymerization of lignin (or lignocellulose), and lignin (and its products) characterization. The developed processes in this dissertation show some advantages compared with current processes, such as easy to operate, potential to scale up and adapt in current refinery. The results achieved in this dissertation can provide feasible future solutions to industry for using the industrial lignin, that is the most available and also a big waste in current biorefinery,as well as the lignocellulosic biomass to valuable outputs.
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Depolymerization of lignin for value-added products by catalytic oxidation
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