Anodic Degradation of Lignin at Active Transition Metal‐based Alloys and Performance‐enhanced Anodes

Abstract

Electrochemical oxidative degradation is one of the most promising methods for generation of phenolic fine chemicals from the renewable feedstock lignin. High selectivity, no reagent waste, as well as cost efficiency are major advantages of this particular process. Application of Ni‐ and Co‐based anode materials led to the best results in respect to product yield and selectivity. Interestingly, repeated use of Ni foam electrodes for electrochemical oxidative degradation resulted in significantly increased yields of vanillin, indicating a modification of the electrode surface. In particular, activation of the electrodes by electrochemical treatment of black liquor enabled an activation which further increased the electrocatalytic activity as well as the yield of the aroma chemical vanillin up to more than 100 % compared to non‐activated Ni foam electrodes. Additionally, this activated electrode surface was analyzed via flowing atmospheric pressure afterglow surface desorption mass spectrometry (FAPA‐MS). The measurement revealed diaminotoluene as a major compound in this adsorption layer, which indicates that this compound is partly responsible for the activation process. Most likely, electrochemical induced deposition of such an organic surface layer enhances the lipophilicity of the electrode surface and increases the accessibility of relevant structural features of lignin particles to the anodic surface, resulting in a higher yield of the desired degradation product vanillin.

Greener Routes to Biomass Waste Valorization: Lignin Transformation Through Electrocatalysis for Renewable Chemicals and Fuels Production

“…[110] In particular, activation of the electrodes by electrochemical treatment in depleted BL improved the electrocatalytic activity for vanillin production;0 .6 wt %v anillinf or untreated Ni foam versus 1.6 wt %v anillinf or the treated Ni foam were reported. [110] The analysis and subsequentt esting of the electrode surfacea ctivated by diaminotoluene (possibly formed in situ reductively from BL) revealed that this compound plays a major role in forming an adsorption layer that enhances the lipophilicity of the electrode surface and increases thea ccessibility of relevant structural features of lignin to the anodic surface, resulting in ah igher yield of the desired degradation products. [110] To tackle the issue of product overoxidation that is often reported, Stiefel et al investigatede lectrooxidation of Kraft lignin in an electrochemical membrane reactor (ECMR) with a tubular ceramic membrane (an activel ayer of TiO 2 supported on a-Al 2 O 3 ), Ni anode,a nd Ni felt cathode separated by an anion exchange membrane.…”

“…[110] The analysis and subsequentt esting of the electrode surfacea ctivated by diaminotoluene (possibly formed in situ reductively from BL) revealed that this compound plays a major role in forming an adsorption layer that enhances the lipophilicity of the electrode surface and increases thea ccessibility of relevant structural features of lignin to the anodic surface, resulting in ah igher yield of the desired degradation products. [110] To tackle the issue of product overoxidation that is often reported, Stiefel et al investigatede lectrooxidation of Kraft lignin in an electrochemical membrane reactor (ECMR) with a tubular ceramic membrane (an activel ayer of TiO 2 supported on a-Al 2 O 3 ), Ni anode,a nd Ni felt cathode separated by an anion exchange membrane. [111] The use of the nanofiltration membrane resultedi nd ouble the amount of low molecular weightp roducts recovered despite the relativelyl ow surface area of the Ni electrode.…”

“…The Waldvogel group and the Staser group both explored the use of Ni, Co, and Ni-Co bimetallic electrodes and found that both Ni-and Co-based materials show exceptional selectivity towards the formation of low molecular weightp henolics [110] whereas Ni-Co electrodes showedi mproved electrochemicala ctivity ands tability. [116] Schmitt et al reported the use of Ni-andC o-based alloy electrodes at 80 8Ca nd 1.9 mA cm À2 to selectively convert Kraft lignin to 0.7-1.8 %v anillin and some acetovanillone.…”

“…[117] Similarly,Z irbes et al reportedh igher yields of vanillin (up to 1.8 wt %) during Kraft lignin electrooxidation on Co-based materials compared to Ni-, Fe-, and Sn-basede lectrodes;s till Co exhibitedl ow corrosion resistance. [110] Nickel, however,w as more resistant to corrosion and could withstand ag reater current densities, which shortened the reaction durationb ut made it less selectivet owards vanillin (< 1% yield) and less efficient compared to Co. [117] Using Ni foam electrode and increasingt he current density to 38 mA cm À1 at 80 8Cl ed to an increased yield of 1.2 %vanillin.…”

“…Other methods such as modification and organic activation of electrode surfaces can also be beneficial in addressing this issue. [110] v. Due to variability in feed materialand product slates, current protocols for product recovery,i dentification, product quantification,a nd other procedural variabless uch as current efficiency and mass balances are not standardized for variouselectrochemical degradation studies, making comparisonofd ifferent studies challenging. vi.…”