Valorization of Walnut Shells by Hydrothermal Liquefaction of Its Lignin Extracted Using the Organosolv Method

Abstract

Energy consumption reached 604 EJ in 2022 with 82% of it coming from fossil fuels and resulting in 39.3 billion tons of CO2 emission, causing twin sustainability issues of energy security and global warming. This necessitates the energy transition from fossil fuels to renewables. Biomass, which is renewable and carbon neutral from a life cycle viewpoint, has attracted huge interest as a sustainable source for fuels and biochemicals. Agro-industry wastes, particularly nutshells, are available abundantly due to the large-scale consumption of nuts, of which walnut shells contain more than 35% lignin. Walnut shells were powdered, characterized, and subjected to organosolv extraction using ethanol as a solvent to extract lignin. The X-ray diffraction (XRD) pattern of walnut shell powder confirmed the presence of crystalline cellulose, combined thermogravimetric/differential scanning calorimetry indicated its thermal stability, and the particle size ranged from 200 to 400 nm while the ultimate analysis confirmed the absence of sulphur. Hierarchical Fe-Co-Zeolite Socony Mobil-5 (Fe-Co-ZSM-5) was synthesized by steam-assisted crystallization and its characterization by XRD confirmed the ZSM-5 structure while textural analysis confirmed its hierarchical nature. From the hydrothermal liquefaction (HTL) of the walnut shell-extracted lignin, conversion and bio-oil yield obtained in the presence of hierarchical Fe-Co-ZSM-5 of Si/(Fe + Co) ratio 60 were 6% and 53.2% higher than over hierarchical Fe-Co-ZSM-5 of Si/(Fe + Co) ratio 80 due to higher crystallinity and the amount of Fe and Co of the former. Hence, it is concluded that the Fe and Co in the tetrahedral framework of hierarchical ZSM-5 may be better active sites for the HTL compared with impregnated mesoporous support catalysts.