Transformation of Sugar Cane Lignin into Renewable Fuel-Range Cyclo-Alkanes: In-Situ Hydrogen Release Using Earth-Abundant AlCl₃ and Biobased Mannose Triflate Support

In lignin catalytic biorefining, advanced catalysts are progressively replacing conventional options to enhance efficiency. This study utilizes a sugar-based triflate (mannose triflate) and aluminum chloride (AlCl3), leveraging the in situ hydrogen release capability of propylene glycol to achieve lignin valorization into renewable fuels. The designed catalyst, Al-OTf3-pgOH, effectively deploys triflate anions at low temperatures. Its properties were thoroughly investigated and compared with two control catalysts, Al-pgOH and pgOH. The impacts of the lignin particle size, temperature, and reaction time were examined for each catalyst. Al-OTf3-pgOH achieved a 98.5% lignin conversion with an optimal lignin particle size of 100 μm, a reaction temperature of 150 °C, and a reaction time of 5 h. The Wilkinson model indicated that lignin depolymerization was third-order in the initial 5 h and second-order over the following 25 h. Among the three catalysts, Al-OTf3-pgOH resulted in the lowest amount of oxygenates and phenolics, and the highest yield of monocyclic alkanes, showing a 56.1 wt % cycloalkane yield corresponding to 79.0% selectivity. Notably, this is the total yield from 7-ethylcyclopentane, 8-cyclohexyl cyclopropane, and 9-methylcyclopentane with individual yields of 35.5, 12.7, and 7.8 wt %, respectively. This study, therefore, demonstrates that mannose triflate’s acidic catalytic sites can effectively produce valuable renewable fuel hydrocarbons from sugarcane-derived lignin.