Electrocatalysis has gained tremendous attention as a versatile and greener approach for achieving challenging transformations for energy-related applications (such as CO<sub>2</sub> reduction and water splitting) and, more recently in synthetic organic chemistry. The unique abilities of electrocatalytic hydrogenation (ECH) allow the generation of reactive intermediate species under mild conditions. It inspired the development of novel electrochemical lignin valorization to produce aromatic commodity chemicals and building block sources, which is a promising alternative to fossil-fuel-based refineries. Lignin is the most abundant natural phenolic biopolymer bonded by various types of C-O and C-C linkages; it possesses great potential for value-added aromatics and biofuels if effective depolymerization can be achieved. Currently, catalytic hydrogenolysis of lignin C-O bonds often requires the use of a precious metal catalyst or/and harsh conditions such as elevated temperatures and pressurized hydrogen gas. Previous attempts of C-O cleavage using electrochemical approaches still involved using precious metals catalysts (Pt, Ru) or unsafe Raney-nickel prepared via an energy-intensive process. Herein, we established a novel, highly selective electrochemical protocol to reductively cleave the benzylic C-O bond of the lignin model compound benzyl phenyl ether (BPE) at room temperature and ambient pressure without applying external H<sub>2</sub>. It was found that with the presence of the earth-abundant and cost-effective nickel chloride hexahydrate in the electrocatalytic system, BPE can be selectively converted to toluene and phenol with outstanding yields (up to 90%) simply using inexpensive Ni foam and/or carbon paper as electrode materials. Cyclic voltammetry study suggests a possible formation of the Ni (II) complex with substrates, which catalyzed the reductive cleavage of the C-O bond effectively. The effects of substituted benzyl ethers on C-O bond cleavage had also been explored. To our best knowledge, this is the first investigation of electrochemical selective C-O cleavage of lignin model compound using green and low-cost electrode materials with earth-abundant Ni salts catalyst. This work opens a new avenue for biomass feedstocks conversion using a sustainable electrochemical approach and presents a strategy to reduce the chemicals’ dependence on petroleum-based refineries.

Effective and selective Ni-catalyzed electo-reductive cleavage of lignin model compounds: A green alternative methodology for aromatic building blocks production

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ACS Fall 2022

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