Experimental and theoretical investigation of synergistic effects in a binary ionic liquid system for the selective production of benzaldehyde from lignin model compound

Lignin is an abundant and cost-effective aromatic polymer, which makes the production of high-value aromatic aldehydes from it highly significant. However, a challenge is that these aldehydes can easily over-oxidation into aromatic acids during aerobic oxidation. Therefore, preventing over-oxidation is crucial. In this study, a binary ionic liquid system ([Emim][Ac] and [Bmim][FeCl4]) was employed for synergistic catalysis to mitigate this issue. Notably, density functional theory analyses have revealed that [Ac]- and [FeCl4]- can engage in hydrogen bonding and van der Waals interactions with the beta-O-4 linkages in lignin. These interactions can effectively reduce the bond energy of the C beta-H bonds, facilitating its homolysis to generate free radicals. Furthermore, under mild conditions, these free radicals can readily break C-C bonds, producing aromatic aldehydes while minimizing over-oxidation. In this system, the lignin model compound 2-Phenoxy-1-phenylethanone (PP) was cleaved to produce benzaldehyde (46.4 %) and phenol (73.4 %), enzymatic lignin was degraded to yield aromatic products like vanillic aldehyde, all without over-oxidation. To meet the growing demand for biomass upgrading, this work introduces a green and controlled conversion technology capable of cleaving inert C-C bonds under mild conditions, establishing a foundation for future advancements in renewable energy production.