The combustion of fossil-derived liquid hydrocarbon fuels constitutes the primary means for powering the transportation sector in today’s world. However, the twin challenges of reducing carbon dioxide (CO¬2) emissions to combat climate change and ensuring energy independence and security have prompted a shift towards renewable energy in recent years. Electrification of the road and rail transport segments represents a promising strategy to address these challenges. However, aviation and heavy-duty road and maritime transport remain hard to decarbonize by electrification. Lignocellulosic biomass resources constitute an underutilized feedstock for production of high energy density liquid hydrocarbon fuels to power the hard-to-electrify segments and chemicals.The thermal deconstruction of lignocellulosic biomass via fast pyrolysis and condensation of the resulting vapors yields a liquid product known as bio-oil. The unfavorable characteristics of bio-oil, attributed to its high oxygen content, prevent its utilization as a transportation fuel. Even so, bio-oil may be upgraded by catalytic cracking or catalytic hydrotreatment to produce liquid hydrocarbon fuels. To lower raw material transportation costs, the processing of biomass to upgraded bio-oils at regional depots near biomass production areas has been proposed. Electrocatalytic hydrotreatment (ECH) has been presented as a technology to upgrade bio-oil at the regional depots. ECH presents several advantages over conventional thermocatalytic treatments. First, ECH can be implemented at mild conditions (temperatures <80 °C under atmospheric pressure). Secondly, the hydrogen equivalents required for ECH are produced in situ by splitting water. Finally, ECH may be powered by renewable electricity derived from solar and wind energy. The ECH upgraded bio-oils from multiple depots would then be transported for further treatment and purification at a centralized facility, such as a petroleum refinery, to produce liquid hydrocarbon fuels and chemicals. The present studies seek to derisk bio-oil ECH by investigating: the conversion of bio-oil model compounds and their mixtures, the yields of desired products and the faradaic efficiency of the process using noble metal electrocatalysts. The first two studies focused on ECH of furfural (derived from hemicellulose) and oxygenated aromatics (e.g. 4-propylguaiacol, derived from lignin), respectively. Furfural ECH yielded tetrahydrofurfuryl alcohol (THFA), a specialty solvent and chemical intermediate. Analysis of variance from a duplicated factorial design on an Ru electrocatalyst indicated that acidity of the catholyte had the strongest effect on THFA yield. Meanwhile, ECH of oxygenated aromatics yielded C7-C9 cycloalkanes that could be considered for blending into jet fuels. It was observed that bimetallic (RuPt) electrocatalysts outperformed the monometallic (Ru and Pt) electrocatalysts. Furthermore, it was shown that deoxygenation preceded the aromatic ring saturation during cycloalkane formation from the oxygenated aromatics. Bio-oil is a complex mixture of several hundred compounds; therefore, in the third study ECH of synthetic binary mixtures was investigated to identify possible synergistic and inhibitory effects that are likely to be observed during ECH of “whole” bio-oils. Importantly, this study showed that on RuPt electrocatalysts phenolic compound reduction was inhibited in the presence of aldehydes, particularly furfural. This inhibitory effect was partially suppressed by reagent-based reduction of the formyl group to a hydroxymethyl group prior to ECH. The final study sought to estimate the kinetic parameters for furfural ECH using a Pt rotating disk electrode. Polarization curves were obtained by linear sweep voltammetry at different electrode rotation speeds. The kinetic current densities at different potentials were estimated using the Koutecký–Levich equation. The kinetic current densities were then used to estimate kinetic parameters (exchange current density and cathodic charge transfer coefficient) using the Tafel approximation for the Butler-Volmer equation. Overall, these studies showed that ECH represents a promising technology for upgrading bio-oils to valuable fuels and chemicals under mild conditions. The outcome of this work should inform further development of the ECH technology enabling its scale up and integration with both upstream biomass processing and downstream fuel and chemical production.
Read
