Anodic electrochemical C–C bond cleavage co-catalyzed by ionic liquids and FeNi@C for lignin upgrading†

Abstract

Electrochemical oxidation of lignin for the production of high-value aromatic aldehydes, while co-generating hydrogen, represents a promising strategy to achieve dual benefits. However, the selective electrochemical cleavage of the nonpolar and robust C–C bonds in lignin presents tremendous challenges. Here, FeNi@C derived from metal–organic frameworks (MOFs) is designed for the high-selectivity electrochemical oxidation of the lignin model compound veratrylglycerol-β-guaiacyl ether (VG) to achieve C–C bond cleavage, resulting in the production of veratraldehyde (VAld). Moreover, a trace amount of an ionic liquid (IL) additive is incorporated into the electrolyte to further optimize the selectivity for VAld. In an anion exchange membrane (AEM) single cell, the FeNi@C anode, under the synergistic catalytic effect of BmpyrroCl, demonstrates a remarkable conversion efficiency of up to 89.8% for VG, with the yield and selectivity of VAld reaching 63.9% and 70.1%, respectively. Notably, this approach demonstrates exceptional performance in lignin upgrading, achieving an aromatic aldehyde yield of 23.5 wt%, with VAld showing a yield and selectivity of 7.3 wt% and 31.1%, respectively, highlighting its significant practical potential. In situ electron spin resonance (ESR) analysis and density functional theory (DFT) calculations reveal that the exceptional selectivity of FeNi@C for VAld is attributed to the ability of Fe to serve as an electronic conversion switch, regulating the valence state of Ni. Specifically, it enhances the formation of highly active Ni^3+δ at 1.4 V, fostering C–C bond dissociation while mitigating the continuous oxidation of Ni^3+δ to higher valence states, thereby inhibiting the undesired conversion of VAld to veratric acid (VAc). The Gaussian calculation results indicate that the strong hydrogen bond between the ILs and C_α–OH promotes the preactivation of VG, thus exerting a synergistic catalytic effect on FeNi@C.