Investigation of catalyst and solvent systems for the catalytic oxidation of Kraft lignin
Tutor / director / evaluatorVon Rohr, Philipp Rudolf
Document typeMaster thesis (pre-Bologna period)
Rights accessRestricted access - author's decision
Lignin is one of the main constituents of wood together with cellulose and hemicellulose and can be used as a renewable feedstock for the production of chemicals. Lignin is an amorphous polymer which is separated from cellulose during the pulping processes in the paper industry. The Kraft process is the most extended pulping process and the so called Kraft lignin is one of the most promising sources for the obtaining of value-added products from its degradation. The catalytic oxidation of Kraft lignin under acidic conditions in the presence of oxygen for the production of value-added products has been under study during several years in the Institute of Process Engineering of the ETH Zürich (Laboratory of Transport and Reaction Processes, Prof. Dr. P. Rudolf von Rohr). The use of a polyoxometalate (POM) as a catalyst and methanol as a co-solvent were used to increase the monomeric product yield. Vanillin and methyl vanillate were the main products obtained. The increase of the product yields and the study of the influence of different parameters on the reaction are essential for the comprehension of the depolymerization reaction mechanism and for the developing of a process to obtain chemicals from a renewable feedstock. In this thesis, the reaction performance under the influence of different catalysts is studied. Different mineral acids, polyoxometalates (POMs) and conventional metal salts have been tested. The influence of the variation of some reaction parameters as pH, catalyst concentration and solvent composition have been studied as well. Different samples were taken at different time steps to monitor the progression of the reaction. The experiments were carried out in a Batch 400 mL autoclave at 170 °C and 10 bar of O2 pressure. A heat exchanger was coupled to the reactor allowing the sampling during the reaction. The reaction products were extracted with chloroform and analyzed by gas chromatography – mass spectrometry (GC/MS) for their quantification and identification. Molecular weight distributions of the samples were measured by size exclusion chromatography (SEC). Transition metal salts have been proven to be appropriate catalysts to increase the amount of products obtained and the depolymerization degree of the original lignin in comparison with the results obtained in the experiments without catalysts. Yields up to 7.3 % of quantified products (vanillin and methyl vanillate) have been obtained using CuCl2 as a catalyst. Furthermore, the production rate of these compounds was increased as well, shortening the reaction time needed to reach the maximum yields. The evolution of molecular weight distributions during the different reactions showed a higher depolymerization degree in the reaction catalyzed with metal salts. Mean molecular weights up to 500 g/mol were reached after 2 h reaction, which confirmed the higher fragmentation. Methyl dehydroabietate has been identified as a reaction product which was produced in all the experiments. Furthermore, the increase of the depolymerization caused the formation of new compounds with molecular weights in the monomeric range. Strong acidic conditions have been demonstrated to be essential for a good reaction performance. In addition, the presence of methanol as a co-solvent increased the amount of products and decreased the amount of solids found in the reactor, which confirms that methanol avoids condensation reactions between lignin fragments.