Electrocatalytic Upgrading of Biomass Fast Pyrolysis Oil

Abstract

Attempts to electrocatalytically upgrade bio-oil have been made in recent years. Mostly, the ECH (Electrocatalytic Hydrogenation) experiments of bio-oil were done using fixed bed electrode configurations at room temperatures and low current densities (<100 cm-2). The main goal was to demonstrate the viability of ECH as a bio-oil stabilization strategy, mainly through reduction of carbonyl contents (aldehydes and ketones) which could cause polymerization. High temperature may promote condensation polymerization of fast pyrolysis oil, low-temperature ECH is therefore considered a promising approach to stabilize pyrolysis oil. However, recent advances in ECH of fast pyrolysis oil have shown the limited reductive upgrading chemistry (hydrogenation, hydrogenolysis, or hydrodeoxygenation) through this approach. This was mainly attributed to the complexity of fast pyrolysis oil and the restricted electrolysis operating conditions. Most of the published works in ECH of pyrolysis oil reported the reduction of carbonyl content into alcohols in the substrate rather than the hydrodeoxygenation of aromatics or phenolics. In this study, ECH of fast pyrolysis oil (FPO) in acidic electrolytes using SSER (Stirred Slurry Electrocatalytic Reactor) configuration was conducted for the first time. A compositional analysis suggested that the FPO sample contained water (29 wt.%) and detected monomers (38.5 wt.%), which comprise of carbohydrate derivatives (35.2 wt.%) and lignin derivatives (3.3 wt.%). Balance of water and monomers must be oligomers. Three polar organic solvents (e.g., ethanol, isopropanol, and acetone) were tested in a mixed aqueous and organic electrolyte with MSA solution. The electrolysis experiments were carried out for 4–30 h at constant cathodic current densities (I = -218 to -255 mA cm-2) and temperatures (50–60 oC). In all cases, decreases in the weight average molecular weight (Mw) and the number average molecular weight (Mn) were observed with prolonged reaction times. Temperature and catalyst loading contributed to faster depolymerization of the oligomers. Using different organic solvents (e.g., ethanol, isopropanol, acetone), the degree of depolymerization was about 33–37% after nearly 20 h reactions. Color changes in the catholyte FPO samples were observed in all cases after 18–25 h, turning from dark brown to light yellow. This observation supports the GPC results showing the molecular weight reduction after the ECH. In this study, compound distributions were determined based on ten functional groups, such as acids, alcohols, aldehydes, ketones, esters, furans, alkanes, aromatics, phenols, and guaiacols. In all cases, the reduction of ketones, aromatics, and guaiacols was noticed, while the alkanes content slightly increased. The most dramatic increases in esters content were observed when ethanol (17–25%) or isopropanol (19–32%) was used. This was attributed to esterification of carboxylic acids (mainly acetic acid) in the FPO with alcohols in the presence of MSA, which catalyzed the reaction. Reduction of p-eugenol to cerulignol (4-propylguaiacol) was the most noticeable in all cases, implying that the unsaturated bond in the allyl group is easily reduced under the ECH conditions. Significant increases in the cerulignol contents were achieved with all the different solvents: ethanol (0.5–4.9%), isopropanol (1.4–6.9%), and acetone (1.4–5.4%), based on the composition calculation normalized by the lignin-relevant compounds detected in the catholyte samples. ECH of FPO in SSER with organic solvents addition has demonstrated mild depolymerization of FPO oligomers, esterification, and reduction of carbohydrate and lignin derivative monomers which could potentially upgrade the quality of FPO for synthesis of hydrocarbon fuels.