Products Cited in Publications Worldwide Mechanistic insights into the mild reductive depolymerization of hydrolysis lignin: effects of catalyst, temperature and reaction time

Mechanistic insights into the mild reductive depolymerization of hydrolysis lignin: effects of catalyst, temperature and reaction time

Deroma, Matteo; Lauwaert, Jeroen; Thybaut, Joris W; De Clercq, Jeriffa; Verberckmoes, An

DOI:

Abstract

The valorization of lignin via mild reductive depolymerization has gained increasing interest, but various knowledge gaps regarding the underlying reaction mechanism continue to hinder the rational optimization of the process conditions. In order to fill these gaps, this work investigates both solvolytic and catalytic mild reductive depolymerization of a hydrolysis lignin at different reaction times (15 min – 10 h) and temperatures (150 °C – 200 °C). Novel insights into the depolymerization of (technical) lignins were achieved by investigating the modification of native end groups (NEGs), defined as para-substituted side chains attached to the end of the lignin polymers before depolymerization such as formyl and 3-oxo-1-propenyl. These were found to be stable in solvolysis but highly reactive in catalysis with Pd/γ-Al2O3. Furthermore, this work introduces a new catalytic reaction mechanism, whereby β-O-4-derived propanol side chains are converted into ethyl side chains at elevated temperatures. At 150 °C, these propanol groups remained largely intact, whereas at 200 °C, up to 40 % were transformed into ethyl chains. The findings further highlight distinctive depolymerization pathways for solvolysis and catalysis. Solvolysis proceeds via a “peeling-off” mechanism, while catalysis involves random cleavage of β-O-4 linkages. This results in a higher degree of depolymerization and monomer yield under catalytic conditions (up to 9 wt% compared to approximately 3 wt% for solvolysis). In both cases, the majority of depolymerization occurs within the first 3 h of reaction. Finally, it was proven that the Pd catalyst plays a primordial role in the modification of β-β and β-5 linkages, which remain stable under solvolytic conditions.

Keywords

Lignin ; Mild reductive depolymerization ; Heterogeneous catalysis ; Reaction mechanisms ; Bio-aromatics

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