Electrocatalytic Hydrogenation of Guaiacol in Diverse Electrolytes Using a Stirred Slurry Reactor

Abstract

Electrocatalytic hydrogenation (ECH) of guaiacol was performed in a stirred slurry electrochemical reactor (SSER) using 5 wt % Pt/C catalyst in the cathode compartment. Different pairs of acid (H2SO4), neutral (NaCl), and alkaline (NaOH) catholyte–anolyte combinations separated by a Nafion® 117 cation exchange membrane, were investigated by galvanostatic and potentiostatic electrolysis to probe the electrolyte and proton concentration effect on guaiacol conversion, product distribution, and Faradaic efficiency. The acid–acid and neutral–acid pairs were found to be the most effective. In the case of the neutral–acid pair, proton diffusion and migration through the membrane from the anolyte to the catholyte supplies the protons required for ECH. Typically, the two major hydrogenation products were cyclohexanol and 2‐methoxycyclohexanol. However, ECH at constant cathode superficial current density (−182 mA cm−2) and higher temperature (i.e., 60 °C) favored a pathway leading mainly to cyclohexanone. The guaiacol conversion routes were affected by temperature‐ and cathode potential‐dependent surface coverage of adsorbed hydrogen radicals generated through electroreduction of protons.

Electrochemical Lignin Conversion

“…[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.…”

“…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).…”

“…Wijaya et al reported the electrolyte effect on guaiacol conversion, product distribution, and faradaic efficiency. [173] In an electrocatalytic hydrogenation cell, the adsorbed hydrogen generated from water or proton oxidation migrates from the anolyte to the catholyte, driven by the difference in electrode potentials, where they react to reduce the lignin-oil compounds. The key pathways for electro-upgrading of biooil molecules into hydrocarbon fuel are composed of a complex network of competitive steps (Figure 26), which undergo consecutive hydrogenation, yielding cyclohexane as final product.…”

“…The conversion routes of bio-oil molecules were affected by temperature and cathode potential-dependent surface coverage of adsorbed hydrogen radicals generated through electroreduction of protons. [173] Jackson and co-workers reported a mild electrocatalytic deoxygenation-hydrogenation process for the reduction of lignin model compounds in a simple, low-cost system that avoids the use of precious metals or costly molecular catalysts. [175] The effective source of hydrogen in electrocatalytic hydrogenation is the combination of protons from water splitting in the anode compartment with electrons at the surface of the catalytic cathode, resulting in up to 99 % selectivity to target products at high conversion rates.…”

“…Furthermore, the presence of oxygen-containing functional groups (like carboxylic acids, carbonyls, and oxygenated aromatic) in bio-oil hinders its storage and transportation. [179] Saffron and co- [142,143,178,179,148,169,[171][172][173][175][176][177] workers proposed a potential solution with localized electrocatalytic hydrogenation as an immediate measure to stabilize bio-oil via oxygen removal and carbonyl saturation. [179] Electrocatalytically stabilized bio-oil can be stored and/or transported to centralized refineries for further upgrading.…”