MgO-based catalysts for selective delignification of lignocellulosic waste and carboxylic acids production under mild hydrothermal conditions

Abstract

Lignocellulosic biomass offers a sustainable alternative to traditional raw materials. However, its complex structure, and particularly the presence of lignin, presents a significant challenge for its conversion into valuable products. This study explores the catalytic performance of magnesium oxide (MgO) based materials for the selective delignification of lignocellulosic waste and concomitant carboxylic acids production. Different MgO samples have been synthesized through various synthetical methods with the purpose of promoting the selective delignification of a lignocellulose waste while minimizing the degradation of other polymers (cellulose and hemicellulose). The resultant lignin-free holocellulose solid after selective depolymerization might be used in subsequent fermentation processes. The catalytic results demonstrated that MgO synthesized by a sol-gel method exhibited the highest catalytic activity, achieving ca. 90 % lignin conversion with minimal degradation of cellulose and hemicellulose and outstanding production of carboxylic acids (ca. 30 % of the carbon of the degraded polymers was converted into carboxylic acids). The high concentration of medium-strength base sites combined with significant macroporosity are crucial for enhancing the catalytic performance of MgO-based catalysts. Importantly, these results were achieved under mild conditions (120 °C) using water as a solvent and without addition of external oxidant agents. Furthermore, the study observed a significant production of valuable (di)carboxylic acids, such as fumaric acid during the depolymerization process over optimized MgO catalyst. This research provides valuable insights into the potential of MgO as a sustainable catalyst for the selective conversion of lignin present in lignocellulosic biomass into valuable chemicals and biofuels.