Oxidative fractionation of lignin and depolymerization from palm kernel shell with Co and Cu based heterogeneous catalysts

Renewable energy sources, particularly lignocellulosic biomass, have significant potential for conversion into valuable bio-based chemicals. This research investigates the oxidative fractionation and depolymerization of lignin derived from palm kernel shell (PKS) biomass using novel cobalt and copper metal based heterogeneous catalysts, aiming at sustainable conversion of various bio-based chemicals such as phenolics, aldehydes, and carboxylic acids. The oxidative approach presents a promising method for catalytically breaking down lignin polymers into lignin monomers, and oligomers under mild reaction conditions. To facilitate the oxidative breakdown of lignin, this study employs heterogeneous cobalt (Co) and copper (Cu) metal catalysts due to their strong potential for oxidation processes, and the reaction is carried out in an isopropanol-water solvent, with hydrogen peroxide serving as the oxidizing agent. In the first phase, monometallic catalysts cobalt (Co) and copper (Cu) supported on Zeolite HY were developed and evaluated for their efficiency in lignin depolymerization. Co/Zeolite HY exhibited higher selectivity towards aldehydes and carboxylic acids, while Cu/Zeolite HY favored the production of phenolic compounds. According to the results, the 10%Co/Zeolite HY catalyst attained a lignin-derived yield of 32.85% when operated at 180 °C for 2 hours with a 5% catalyst loading relative to the feedstock mass. Gas chromatography-mass spectrometry (GC-MS) analysis confirmed that the major products were phenol derivatives and aldehydes. Notably, butylated hydroxytoluene, a derivative of phenol, was present in significant quantities, while syringaldehyde emerged as the most promising aldehyde among those detected. Building on these findings, the second phase introduced a bimetallic Co-Cu/Zeolite HY catalyst to enhance catalytic performance through synergistic effects. Various Co:Cu molar ratios were examined, with the 4:1 ratio demonstrating the maximum catalytic efficiency over conventional zeolite HY support, achieving a lignin-derived yield of 34.25%, surpassing the increase of around 4.27% observed with the monometallic catalysts under an atmospheric pressure environment with an optimal reaction conditions of 180 °C, 2h. Due to the synergistic effect between the metals, the bimetallic system improved redox activity, leading to higher selectivity towards targeted oxygenated compounds. Comprehensive catalyst characterization, BET surface area analysis, and NH3-TPD analysis revealed enhanced surface area, and improved acidity of the synthesized catalysts, which contributed to higher catalytic efficiency. In the final phase, the catalytic support was further optimized by modifying zeolite HY through aluminum doping and desilication to fine-tune acidity and pore structure. The desilicated zeolite-supported catalyst (10%Co4-Cu1/DS-HY) significantly enhanced the performance of the reaction, resulting a maximum 37.62% yield of lignin-derived compounds and 86.75% biomass conversion under optimal conditions. This represents a 9.83% improvement compared to a conventional zeolite HY-supported metal catalyst, attributed to the increased surface properties, and acidity resulting from desilication. Furthermore, catalyst reusability tests demonstrated that after four reaction cycles, the optimized catalyst retained over 90% of its initial activity, confirming its stability and potential for industrial applications. The insights gained from this study are expected to drive progress in lignin-based biorefinery technologies and encourage the circular economy by leveraging waste biomass.