Selective Electrochemical sp3 C-O Bond Hydrogenolysis: From Model Compounds to Lignin

Abstract

[EN] The valorization of lignin into aromatic products is currently one of the most relevant challenges in sustainable chemistry. Here, we show that the electrochemical hydrogenolysis of model substrates and organosolv lignin occurs selectively on the sp(3) C-O bond, to give phenols as main products, without deterioration of the aromatic units. The electrochemical system uses stoichiometric amounts of water (H2O) as the only hydrogen source, and inexpensive and widely available magnesium as a sacrificial electrode, to achieve a 98% phenol yield and an overall 98.8% Faradaic efficiency after optimization. Mechanistic studies based on reactive, kinetics, and isotopic experiments, among others, reveal the necessary presence in the system of tri(pyrrolidin-1-yl)phosphine oxide (TPPA) as a protecting agent against electrode passivation and lithium bromide (LiBr) as a supporting electrolyte, enabling the breaking and separation of organosolv lignin into its aromatic and alkyl organic parts. These results open the way to directly obtain the aromatic compounds present in lignin with water and electricity.