Sustainable production of renewable carbon-based fuels and chemicals remains a necessary but immense challenge in the fight against climate change. Bio-oil derived from lignocellulosic biomass requires energy-intense upgrading to produce usable fuels or chemicals. Traditional upgrading methods such as hydrodeoxygenation (HDO) require high temperatures (200–400 °C) and 200 bar of external hydrogen. Electrochemical hydrogenation (ECH), on the other hand, operates at low temperatures (<80 °C), ambient pressure, and does not require an external hydrogen source. These environmental and economically favorable conditions make ECH a promising alternative to conventional thermochemical upgrading processes. ECH combines renewable electricity with biomass conversion and harnesses intermediately generated electricity to produce drop-in biofuels. This review aims to summarize recent studies on bio-oil upgrading using ECH focusing on the development of novel catalytic materials and factors impacting ECH efficiency and products. Here, electrode design, reaction temperature, applied overpotential, and electrolytes are analyzed for their impacts on overall ECH performance. We find that through careful reaction optimization and electrode design, ECH reactions can be tailored to be efficient and selective for the production of renewable fuels and chemicals. Preliminary economic and environmental assessments have shown that ECH can be viable alternative to convention upgrading technologies with the potential to reduce CO2 emissions by 3 times compared to thermochemical upgrading. While the field of electrochemical upgrading of bio-oil has additional challenges before commercialization, this review finds ECH a promising avenue to produce renewable carbon-based drop-in biofuels. Finally, based on the analyses presented in this review, directions for future research areas and optimization are suggested.

Hydrothermal liquefaction (HTL) is a promising strategy for the conversion of energy-dense waste streams to fuels. Mixed-feed HTL aggregates multiple feed streams to achieve greater scales that capitalize on local resources, hence lowering costs. The potential for new pathways and products upon feedstock blending becomes a compounding level of complexity when unlocking emergent chemistries. Food and green waste streams were evaluated under HTL conditions (300 °C, 1 h) to understand the effect of feed molecular composition on product distributions and mechanisms. Thousands of emergent chemical compounds were detected via Fourier transform ion cyclotron resonance mass spectrometry, ultimately leading to the emergence of two dominant outcomes. First, the presence of small amounts of food waste into green waste results in substantial decarboxylation and subsequent polymerization to biocrude than chars. Second, in the other limit, small amounts of green waste promote the capping of oxygenates into the biodiesel range, such as with the emergence of fatty acid methyl esters.

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ZSM-5 was evaluated for chemical production in a reaction mixture consisting of palmitic acid and water at conditions near the critical point of water (400 °C, 23 ± 2 MPa). Two types of ZSM-5, a microscale variety with particle diameters determined by scanning electron microscopy in the range from 1.66 to 2.56 μm (micro-ZSM-5) and a nanoscale variety with 350–730 nm diameters (nano-ZSM-5), and three water loadings (0, 15, and 65 wt %) were evaluated for their effects on conversion and product selectivity. Palmitic acid conversion and yields of one-ring aromatics, including toluene and xylenes, were greatest for the combination of nano-ZSM-5 and 15 wt % water loadings, showing that reducing particle size and optimizing water content help achieve desired reaction outcomes. Subsequently, the use of nano-ZSM-5 combined with 15 wt % water loading was studied in greater detail, finding that the catalyst could be reused up to four times at these conditions without reduction of aromatic yields and while retaining a fraction of the original acid sites. Time-resolved studies and molecular-level analysis using two-dimensional gas chromatography and isotopic resolution mass spectrometry provided information on the reaction pathway, which consists of a combination of homogeneous and heterogeneous steps. The results of this study motivate future work on water-promoted catalytic cracking of oils to produce valuable chemicals.