(634a) Lignin Aerobic Oxidation Catalyzed By Sn (?) Using Dioxygen in Biphasic System

Chao Liu, Fei Lin, Huiyan Zhang, Rui Xiao*

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, PR China

*Corresponding author. Tel.: +86-25-83795726. Fax: +86-25-57714489. E-mail: ruixiao@seu.edu.cn.

Abstract: In recent years, with the shortage of fossil energy, reasonable exploitation and utilization of renewable biomass resource have attracted wide attention in the field of energy and chemical industry. Lignin, as the second most abundant natural polymer on the earth, is an important bioresource. It accounts for 15-35% content and 40% energy content of lignocellulose biomass, and is the only renewable aromatic resource. However, so huge potential valued organic polymer was usually directly subjected to combustion as a fuel conventionally, which means the high value-added utilization of lignin has not been completely achieved. Therefore, it is necessary to develop the rational and valuable utilization of lignin in the field of biomass conversion. Herein, thermochemical conversion is regarded as the effective methods compared with biochemical conversion. However, the shortcomings of pyrolysis and hydrogenolysis, such as the complex products and harsh reaction conditions, limit their development. Thus, attentions turn to be paid on lignin oxidative degradation. In this study, a biorefinery lignin obtained from the hydrothermal treatment before cooking was selected as the rare material to investigate the aerobic oxidation characteristics of lignin using Sn (â…£) as the catalyst and dioxygen as the oxidant in biphasic system. The oxidative degradation was carried out in a bath reactor. After reaction, the gases were collected and off-line determined by gas chromatography. For further analysis, the mixture was filtrated, rotary evaporated, and then the internal standard substance (n-dodecane) was added. Results showed that the char yield was less than 5%, which was much lower than that from the monophasic system. The phenomenon suggested that the biphasic system could inhibit the repolymerization very well. Oxidative degradation products were aromatic monomers, including phenolic aldehydes, phenolic ketones, and phenolic acids. These products benefited from the creaking of Cα-Cβ and Cβ-O bonds within lignin, as well as the oxidation of the hydroxyl group at α-position. Furthermore, according to the different aromatic units, these products could be divided in p-hydroxylphenyl, guaiacyl, and syringyl compounds, which were consistent with the initial substructures within lignin. Meanwhile, demethoxylation reaction was recognized, which could be proofed by the reduction of syringyl compounds and the existence of CH₄ in gaseous products. After reaction, the molecular weight of liquid products dropped to 300 Da, which suggested the formation of dimeric and trimeric compounds. Generally, the catalytic activity of Sn (â…£) combined with the biphasic system was high than those reported V catalysts in monophasic organic systems.

Keywords: Lignin; aerobic oxidation; aromatic monomers; Sn (Ⅳ) catalyst; biphasic system