A broad overview of the achievements and emerging areas in the field of Green Chemistry.

Electrocatalytic hydrogenation (ECH) is an option for stabilizing or upgrading bio-oil that employs mild conditions (≤80 °C and ambient pressure) compared to hydrotreatment. In this study, phenol, guaiacol (2-methoxyphenol), and syringol (2,6-dimethoxyphenol) were chosen as model lignin-like substrates because of their abundance in bio-oil and their high resistance to hydrogenation relative to the carbonyl compounds. Cathodic reduction was catalyzed by ruthenium supported on activated carbon cloth (Ru/ACC), a novel electrocatalyst. Incipient wetness impregnation and cation exchange methods were employed to prepare the electrocatalyst using three different ruthenium precursors. Scanning electron microscopy revealed that ruthenium nanoparticles within the range of 10 to 20 nm were deposited on the support by both catalyst synthesis methods. Catalysts prepared by cation exchange were more active than those prepared using incipient wetness impregnation, presumably because of support surface functionalization by the oxidation pretreatment. When using incipient wetness impregnation, catalysts synthesized with precursor hexaammineruthenium(III) chloride showed the best activity and electrochemical efficiency, followed by catalysts prepared with ruthenium(III) chloride and ruthenium(III) nitrosyl nitrate. The Ru/ACC electrocatalyst reduced guaiacol, phenol and syringol with similar electrochemical efficiencies, but temperature was an important variable; the electrochemical efficiency for guaiacol reduction increased from 8% at 25 °C to 17% at 50 °C, but then dropped back to 10% at 80 °C. Solution pH also affected catalyst activity and product selectivity, with acidic conditions favoring guaiacol conversion, electrochemical efficiency and cyclohexanol selectivity.

Guaiacol (2-methoxyphenol) and related lignin model monomers undergo electrocatalytic hydrogenolysis/hydrogenation (ECH) to cyclohexanol with RANEY® Nickel electrodes in aqueous solution. Aryl ether (C–O) bond cleavage is followed by reduction of the aromatic ring at ambient pressure and 75 °C. Related arene-OR cleavages occur at similar rates regardless of R-group size. Protons are supplied by anodic water oxidation on a stainless steel grid coated with cobalt-phosphate catalyst, inexpensively replacing the conventional platinum anode, and remaining viable in constant current electrolyses of up to 16 hours. The overall method addresses two key barriers to energy upgrading of low specific energy biomass into fuels and chemicals: deoxygenation and hydrogenation. By directly and simply coupling energy from renewable electricity into the chemical fuel cycle, ECH bypasses the complexity, capital costs and challenging conditions of classical H₂ hydrotreating, and may help open the door to truly carbon-retentive displacement of fossil petroleum by renewables.

A sustainable bio-based future necessitates the utilization of all components of biomass including lignin, which is the second most abundant biopolymer and a source of renewable aromatics. While its use for low value fuel and power production is well established, deriving further value from it via various conversion and upgrading schemes can help enable economically and environmentally sustainable and profitable biorefineries and provide opportunities to implement the principles of Green Chemistry to minimize negative environmental impacts. As lignin is converted to phenolic monomers, dimers, and oligomers via various conversion methods, electrocatalytic upgrading of these lignin-derived intermediates offers a sustainable way to integrate renewable energy sources such as wind and solar with biomass conversion methods to make valuable products. Using this strategy, abundant low-value lignin intermediates can be used to capture excess renewable electricity in the form of chemical bonds. Furthermore, this process allows for fine tuning of selectivity via the control of electrical potential and avoids elevated temperatures and pressures, further reducing energy inputs. Herein, we present a review of recent electrochemical studies of lignin-derived model compounds. Both oxidative and reductive methods for electrocatalytic upgrading of lignin-relevant monomers are discussed as well as electrocatalytic cleavage of lignin dimers representing specific linkages. Towards framing this technology's future implementation, we also provide a perspective on the main challenges and opportunities in this growing field.