(48h) Evaluation of Catalytic Depolymerization of Kraft and Hydrolysis Lignin to Produce Renewable Fuels

Introduction. Utilization of lignin is a huge possibility for producing large amount of renewable fuels and chemicals in the future. Many studies have reported on the depolymerization of Kraft lignin. Hydrolysis lignin is a by-product when producing bio-ethanol from waste biomass and is expected to increase in the future. However, there is much less studies performed using hydrolysis lignin. In our recent study [1], for the first time according to the best of our knowledge, we presented a study comparing the differences and similarities between Kraft and hydrolysis lignin for reductive catalytic depolymerization.

Experimental. Unsupported NiMoS was synthesized and used in all experiments [1]. The catalysts were characterized using BET, XRD, SEM, XPS and TGA. The Kraft and hydrolysis lignin feed source were characterized with TGA, elemental analysis and NMR. The reductive depolymerization experiments were performed in a 450 mL Parr reactor. The depolymerization was typically performed at 400^oC and 80 bar hydrogen using hexadecane as a solvent. The yields of gas, char and bio-oils were quantified, and the chemical composition was determined using GCxGC/MS and NMR. Finally, a reaction mechanism is proposed.

Results and discussion. The hydrolysis lignin resulted in significantly more bio-oil monomers, 64% versus 47% for Kraft lignin using the same conditions (400^oC, 80 bar, 5h,10 wt.% catalyst loading). NMR results revealed that the hydrolysis lignin consisted of residual cellulose. We propose that these small molecules originating from cellulose could act as capping agents for the lignin radicals and thereby preventing re-polymerization forming char. Indeed, the char formation is also significantly less when upgrading hydrolysis lignin, only 8.3% char in comparison with 20.6% char when valorizing Kraft lignin. To conclude, using unsupported NiMoS to utilize hydrolysis lignin is a promising future path.

References

[1] Achour, Bernin, Creaser, Olsson, Chemical Engineering Journal, 453 (2023).