Reaction Optimization and Production of Value-added Products in Lignin Oxidation Reaction

Abstract

When carbon resources are becoming limited there is a great interest to use renewable resources and replace biomass as a feed stock for different industries. It is necessary to investigate methods that isolate biomass resources and convert it into value added materials safely, efficiently, and effectively. Lignin is a natural polymer and valuable starting chemical with heterogeneous sources in nature. Different lignin pre-treatments methods in pulping and bleaching often provide different lignin streams with variable chemical characteristics. Among the different conversion strategies for lignin, oxidation has shown reliable results and valuable products. There are various methods to perform oxidation, and, in this study, free radical oxidation at mild condition was utilized. However, Lignin's complexity and structural diversity make it challenging to acquire high yield and conversion. The work developed in this thesis intends to study the production of vanillin and other value-added chemicals by oxidative depolymerization of lignin from lignin-rich side streams of pulp and paper industry. Characterization of lignin before and after oxidation shows physical and chemical changes in lignin. More oxygenates formed during the oxidation reaction, and addition of hydroxyl, methoxyl and carbonyl group can be determined in the product of oxidation. Oxidation of lignin was performed in a batch reactor and compared with the microfluidic reactor regarding conversion and product achieved. Increased oxygen availability in the microfluidic reactor alongside enhanced mixing and mass transfer leads to fast conversion of lignin (within 1-minute residence time) and a high vanillin retention rate that is close to the vanillin formation in the batch experiment at 130C. In batch reactor degradation of long-chain aromatic hydrocarbon results into the formation of mono aromatics in primary oxidation and a significant amount of aliphatic compound including acids and other smaller compounds due to overoxidation. Low oxygen availability in the batch reactor could potentially lead to increased formation of addition products since there is no sufficient oxygen to drive free radical oxidation to the final stage. In a continuous microfluidic experiment, oxidation of aromatics leads to the formation of new mono aromatics with a lower percentage of organic acids. The results from this study can be employed in the pulping and bleaching section in paper making industry to substitute the oxidation process with chlorine-based strategies. Using greener oxidants like oxygen and hydrogen peroxide during oxidation will reduce the environmental impacts and toxic effects of bleached pulp mill effluent.