Modeling-Assisted Elucidation of the Organosolv Lignin Depolymerization: Lessons Learned from β-Ether Cleavage over Ni/C
Tina Ročnik Kozmelj, Edita Jasiukaitytė‐Grojzdek, Matej Huš, Miha Grilc, Blaž Likozar
Abstract
High Resolution Image Download MS PowerPoint Slide The complexity of lignin is a major challenge to overcome in order to develop a complete biorefinery concept for the biobased community. Therefore, the lignin model compound 2-phenoxy-1-phenylethanol was used to design lignin depolymerization. We proposed a two-step mechanism involving predehydrogenation at the C α -position, removal of the OH group, and subsequent cleavage of the β- O -4 bond at the C β -position into phenol and ethylbenzene. The study was supported by density functional theory and kinetic modeling to evaluate the activation barriers for the cleavage of the β- O -4 bond in the dimeric lignin compound. The activation energies for predehydrogenation and cleavage at the C β -position of phenethoxybenzene were predicted to be 71 kJ mol –1 and 9 kJ mol –1, respectively, suggesting that the predehydrogenation is beneficial for the cleavage of the β- O -4 bond as it lowers the activation energy. Additionally, the removal of the OH group at the C α -position increased the reaction rate constant for the β- O -4 bond cleavage to 0.68 min –1 . By comparing lignin depolymerization and the cleavage of the β- O -4 bond in the dimeric lignin compound, the study provided mechanistic insights and suggested process- and structure-dependent correlations. Similarities were found in the process mechanism of aliphatic OH group removal and cleavage at the C β -position, while the temperature increase contributed more to the enhanced cleavage of the β- O -4 bond in the lignin model compound compared to the lignin macromolecule. On the other hand, the reaction conditions affected the structural characteristics of the products after lignin depolymerization, especially the molecular weight and functionality of the oligomeric fragments. We have found that using a lignin model component is beneficial for fundamental research, but correlating the results with the real lignin sample is essential to improve the potential of lignin in the biorefinery concept.