Kraft lignin valorization by hydrotreatment over Mo-based sulfided catalysts
Abstract: The production of green fuels and chemicals from bio-based feedstock can suppress the dependency on fossil resources and help mitigate global climate challenges. Kraft lignin is a thermochemically modified natural lignin obtained from the pulping process as a byproduct. It is an underutilized fraction, often used to recover heat and energy in the current industrial practice. Chemically, it is highly rich in aromatics and thus has a huge potential to provide platform chemicals/fuels. However, the major challenge in the valorization of Kraft lignin is its recalcitrance to depolymerization due to the presence of strong interunit carbon-carbon linkages. Also, upon depolymerization, active monomeric fragments undergo repolymerization reactions forming undesired solid residue/char, thus making the transformation highly challenging. In this context, Mo-based sulfide catalysts being sulfur tolerant and active for removing heteroatom-such as S, N, O, metals have been modified and studied with the aim to elucidate the selective cleavage of common lignin linkages, the hydrotreating potential of Kraft lignin, and upgrading of lignin derived bio-oil. The reactivity of lignin dimers, representing common lignin linkages, shows that NiMo sulfides over ultra-stable Y-zeolite support, with a higher amount of Brønsted acidity, can efficiently cleave both etheric and carbon-carbon linkages and yield deoxygenated aromatics and cycloalkanes by hydrodeoxygenation (HDO). Such hydrogenolysis, hydrocracking, and deoxygenation activity were also found to vary with the silica/alumina ratio of the Y-zeolites. The optimum activity was obtained with catalysts having a suitable balance of acidic and deoxygenation sites (metal sulfides). Additionally, one-pot hydrotreatment of Kraft lignin with a suitably functional catalyst shows a significant reduction in the repolymerization reactions, leading to a high yield of bio-oil rich in alkylbenzene and cycloalkane, a fraction suitable for example for jet fuel applications. Characterization reveals that the key function of a suitable catalyst is hydrogen activation at a lower temperature which facilitates stabilization of the lignin fragments, the moderate acidity of the catalysts, and high HDO activity of the catalyst. Furthermore, unsupported Ni/Mo-sulfides have been synthesized and found highly active for deoxygenation reaction and Kraft lignin hydrotreatment, resulting mainly from their defect-rich morphology. Conventional Mo-based sulfide catalysts thus can be tailored to enable their effective application in the upgrading of complex biorefinery feedstocks to value added components.
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