Critical evaluation of lignin obtained from different lignocellulosic biomasses and using different delignification processes

Abstract

Lignocellulosic biomasses are renewable, abundant, and low-cost resources that are generated from various industrial processes. They are mainly composed of cellulose, hemicellulose, and lignin. Lignin is an aromatic polymer complex that has attracted considerable attention in recent years due to its potential to replace phenols obtained from the petroleum-based industry for application in the most diverse areas. However, its complex structure makes its valorization difficult. The objective of this work was to evaluate the effects of steam explosion in the acetosolv process, to optimize the lignin extraction process by ethanosolv, as well as to provide a critical evaluation of the lignins characteristics from coconut fiber (CF), oil palm mesocarp fiber (OPMF), and sugarcane bagasse (SCB) towards of their potential applications. The chemical, structural, and thermal characteristics of the lignins were determined by klason lignin, 2D-HSQC, FTIR, GPC, TGA/dTG, and DSC. Biomasses were pretreated by steam explosion at 168 °C and reaction time of 10 min. Steam explosion followed by acetosolv increased lignin yield by approximately 15% and 17% in OPMF and SCB, respectively. Furthermore, the steam explosion reduced the reaction time of acetosolv in four-fold while maintaining the same extraction yield only for SCB. Our findings demonstrate that steam explosion is efficient in improving lignin yield and/or decreasing the severity of the extraction process. Furthermore, ethanosolv was optimized and lignin yield was maximized for all biomasses. Ethanol concentration and temperature have great significance in the delignification process. Under the conditions studied, the reaction time did not significantly influence and the shortest reaction time was enough to maximize the lignin yield. Regarding the characteristics of lignins, the processes used were able to extract lignins with high purity and high quality. Therefore, it can be concluded that the sugarcane bagasse lignins, especially the one extracted by ethanosolv, presented the best characteristics in general compared to the other lignins, due to the high extraction yield, high purity, high thermal stability, and higher content of reactive sites for polymerization reactions being the most suitable for technological applications. However, CF and OPMF lignins probably only need purification to be applied as building blocks.