Evolution of Aromatic Structures during the Low-Temperature Electrochemical Upgrading of Bio-oil

Abstract

The electrochemical upgrading of bio-oil is a potential renewable approach toward generating liquid biofuels or industrial chemicals under mild reaction conditions (≤80 °C and ambient pressure). The aromatic structural evolution in biooil is a key consideration in bio-oil application. In this study, a bio-oil sample produced from the fast pyrolysis of rice husk at 500 °C and its lignin-derived oligomers were electrolyzed in an electrolytic cell with platinum electrodes. The samples at discrete time intervals were extracted and analyzed using ultraviolet fluorescence spectroscopy, gas chromatography−mass spectrometry, and Fourier transform ion cyclotron resonance−mass spectrometry (FT-ICR MS). Results showed that aromatic compounds with one and two benzene rings decreased with a prolonged processing time. The unsaturated aromatic compounds were hydrogenated and converted into saturated compounds. Species with more than two aromatic rings were the main compounds detected by FT-ICR MS. The lignin-derived oligomers contained the most phenolic compounds with more than two aromatic rings of the bio-oil. However, the evolution of these phenolic compounds showed different trends between the electrolysis of bio-oil and the lignin-derived oligomer fraction. This phenomenon was attributed to the presence of the light components derived from cellulose/hemicellulose species in the bio-oil. These species were reactive and able to produce radicals that enhanced the hydrogenation reactions. Accordingly, interactions among bio-oil compounds occurred during electrochemical treatment.

Electrochemical Lignin Conversion

“…The products have lower C : O and H : C ratios, requiring upgrading (through deoxygenation and hydrogenation) before it can be used as biofuel. [166][167][168][169] However, such hydrogenation is typically run at temperatures over 100°C, high enough to facilitate bio-oil polymerization. [170] To circumvent this, efforts have focused on using electrocatalytic hydrogenation as a mild process to upgrade ligninderived bio-oil compounds such as phenol, 4-phenoxyphenol, syringol, and guaiacol using hydrogen and heterogeneous catalysts to form chemicals and fuels.…”

“…[170] To circumvent this, efforts have focused on using electrocatalytic hydrogenation as a mild process to upgrade ligninderived bio-oil compounds such as phenol, 4-phenoxyphenol, syringol, and guaiacol using hydrogen and heterogeneous catalysts to form chemicals and fuels. [141,142,169,[171][172][173] For the effects of various reaction parameters (temperature, applied potential/current, initial concentration of reactants, electrolyte pH, and catalyst nature) on electrochemical upgrading of ligninoil compounds, one can refer to a review by Carneiro and Nikolla. [174] Figure 25 depicts an electrocatalytic hydrogenation cell for upgrading lignin-oil compounds, highlighting the electrode material, electrolyte, and catalyst.…”

“…[174] Figure 25 depicts an electrocatalytic hydrogenation cell for upgrading lignin-oil compounds, highlighting the electrode material, electrolyte, and catalyst. The anode material, individually depicted in Figure 25 (top left), can include cobaltphosphate, [175] platinum wire, [143,169,171,176] platinum sheet, [142] and platinum mesh, [148,177,178] of which the platinum electrode is mostly used. Meanwhile, carbon is electrocatalytically inactive and serves only as a mechanical support.…”

“…Therefore, carbonsupported electrocatalysts, such as Ru/ACC, [172,176] Pt/ graphite, [142] Pt/carbon, [148,173,177,178] Pd/carbon, [177,178] Rh/ carbon, [143,177,178] and Ni/carbon [178] are generally used as cathodes in the electrocatalytic hydrogenation process. In addition, various electrolytes (anolyte/catholyte) including BK 3 O 3 buffer/ BK 3 O 3 buffer, [175] phosphate buffer/HCl solution, [176] acetate buffer/acetate buffer, [143,177,178] H 2 SO 4 /HClO 4, [173] LiCl/LiCl, [169] and H 3 PO 4 /SiW 12 [148] were utilized, as shown in Figure 25 (bottom left).…”

Investigation into Properties of Carbohydrate Polymers Formed from Acid‐Catalyzed Conversion of Sugar Monomers/Oligomers over Brønsted Acid Catalysts

“…Cellulose and hemicellulose are the major components in biomass, the hydrolysis or pyrolysis of which in aqueous medium could produce sugar monomers and oligomers, the major components in bio‐oil . These sugars are the feedstocks for production of the value‐added chemicals, such as 5‐hydroxymethylfurfural (HMF), furfural, levulinic acid (LA), and so on .…”