Thermo-catalytic conversion of GVL-extracted lignin over Ni/NiO catalysts: Support effects on monoaromatics selectivity

Abstract

[EN] This study explores the performance of Ni and NiO catalysts supported on SiO2, Al2O3, and activated carbon for producing monoaromatic hydrocarbons (MAHs) from the pyrolysis of gamma-valerolactone-extracted lignin. The extracted lignin fraction contained about 70 wt% of lignin along with residuals of sugars, which was indirectly confirmed by physicochemical characterization and analytical pyrolysis experiments (Py-GC-MS). The catalytic pyrolysis results demonstrate that product distribution in pyrolytic vapors is affected by both the oxidation state of nickel sites (Ni-0 or Ni2+) and support acidity. Ni/SiO2 yields a mixture of ketones, furans and aromatics, achieving a combined abundance of similar to 21%, which was ascribed to the activity of metal sites for activating H-containing species via hydrides and promoting hydrogenation. Meanwhile, NiO/SiO2 further increased the formation of furans, which may be associated with the presence of oxidized Ni species and their interaction with oxygenated intermediates. When supported on Al2O3, Ni and NiO, predominantly exhibited selectivity toward phenolics (similar to 19%) but showed limited selectivity for aromatic hydrocarbon formation (<5%), which was attributed to the low surface area of Al2O3 and a poor metal dispersion. The use of the Ni/C catalyst promoted demethoxylation and deoxygenation with efficient formation of low-molecular-weight aromatics, improving the selectivity to MAHs by up to 43%. In addition, the NiO/C also showed strong deoxygenation ability, improving alkylphenol and aromatic selectivity through the removal of methoxy groups and oxygen functionalities. The performance of carbon-supported catalysts for producing MAHs was ascribed to a bifunctional effect established by the support mid-strength acid density and Ni-0 and Ni2+ sites.