Insights into Real Lignin Refining: Impacts of Multiple Ether Bonds on the Cracking of β-O-4 Linkages and Selectivity of Products

Depolymerizing lignin to produce high-value chemicals has garnered increasing attention. Given the complex structure of real lignin, the cracking efficiency of <i>β</i>-O-4 linkages and the selectivity of depolymerization products are significantly lower than those of lignin model compounds. Meanwhile, the relationship between the structure of lignin and the <i>β</i>-O-4 linkage cracking was ignored. In this work, to well address the issue, three real lignins (corncob lignin (CL), pinus massoniana lignin (PML), and eucalyptus lignin (EL)) were employed to discuss the impacts of special ether bonds in lignin on the <i>β</i>-O-4 linkage cracking in the no-additional-hydrogen catalytic system mediated by a CoNi₂@BTC catalyst. The lignin depolymerization results showed that the ether bonding structure in the lignin significantly impacted the cracking of <i>β</i>-O-4 linkages and selectivity of the final products, resulting in a great difference among their intermediates. Notably, the methoxy groups in the real lignin greatly inhibited the further hydrogenation of phenolic compounds, resulting in the accumulation of abundant methoxy-substituted phenolic compounds and a low yield of cycloalkanes (12.37% to 14.06%). To deeply discuss the <i>β</i>-O-4 linkage cracking in the lignin depolymerization, degradation experiments with coexisting ether bond compounds were performed, and the activation energy was employed to quantitatively evaluate the impacts of other ether bonds on the <i>β</i>-O-4 linkage cracking. The results revealed that multiple ether bonds (<i>α</i>-O-4, 4-O-5, and methoxy group) significantly increased the activation energy (from 236% to 373%) of <i>β</i>-O-4 linkages, resulting in the evident decline in the <i>β</i>-O-4 model compound. In addition, the degradation of the methoxy-substituted <i>β</i>-O-4 model compound (GG) demonstrated that the methoxy-substituted aromatic ring products were resistant to further hydrogenation, resulting in the accumulation of methoxy-substituted aromatic ring products in the depolymerization of real lignin. All the findings will provide a novel perspective for the targeted high-value utilization of real lignin in chemical production.