Abstract
The oxidative functionalization of organic substrates to valuable chemicals has attracted global interest from both fundamental and practical viewpoints. One of the promising approaches for the C-H oxidative functionalization is an aqueous electrochemical method because it can operate under mild conditions without toxic oxidants or flammable solvents. However, there are two major challenging issues for the further dissemination of electrochemical hydrocarbon oxidation in aqueous solution. The first one is the low solubility of raw organics in aqueous media. Here, we focus on gas diffusion electrodes (GDEs) that have been originally developed for hydrogen/oxygen fuel cells in an attempt to overcome this problem. The other major issue is that highly reactive species on an electrocatalyst are required for the activation of stable C−H bonds of hydrocarbons. Although high-valency ruthenium-oxo species (RuIV=O or RuV=O) are known to catalyze O-atom insertion into stable C-H bonds, Ru-based molecular electrocatalysts generally exhibit poor robustness and also catalyze the competitive oxygen evolution reaction (OER) in aqueous solutions. Our group has recently reported that singly isolated Ru-doped covalent triazine frameworks (Ru-CTF) effectively oxidize benzyl alcohol in aqueous electrolytes without the OER[1]. Besides, Ru-CTF displayed higher stability than an organometallic complex immobilized on an electrode because of the covalently cross-linked structure of CTF. In this study, we attempt to realize the electrochemical oxidation of vaporized hydrocarbons in aqueous solutions using a GDE that supports Ru-CTFs (Ru-CTF/GDE).
A mixture of 1.0 g ZnCl2, 100 mg 2,6-dicyanopyridine, and 100 mg Ketjenblack was filled into a vacuum glass tube and then heated at 400 °C for 40 hours to prepare a CTF. Ru atoms were then impregnated into the CTF using 0.91 mM RuCl3 ethanol solution for 6 h at 90 °C. A catalyst ink was prepared by dispersing 4.0 mg Ru-CTF in 38 μL 5% Nafion solution and 400 μL ethanol, which was then added dropwise onto the GDE.
Ethylbenzene (EB)-saturated (1.2×103 Pa) argon gas was supplied from the backside of GDE as a model substrate. Reaction products in liquid phases after electrolysis were quantitatively analyzed using high-performance liquid chromatography. When the GDE electrode carrying Ru-CTF was applied at 1.5V vs. RHE, acetophenone was generated as the product of ethylbenzene oxidation. The amount of generated acetophenone reached up to 4.7 µmol after 6 h electrolysis. In contrast, generated acetophenone was almost negligible in the absence of ethylbenzene or for bare CTF (without Ru). These results indicated that the Ru atoms in CTF served as the active center for ethylbenzene oxidation. For comparison, electrochemical measurements of Ru-CTF/GDE with 0.2 mM EB in the electrolyte instead of gaseous EB were also conducted using the same GDE electrochemical cell. In this case, the gas chamber was filled with pure argon gas. Although the EB concentration of 0.2 mM is about one-seventh lower than the saturated concentration (solubility of EB in water: 1.4 mM), the amount of acetophenone generated from gaseous EB was over 40-fold larger than that from the electrolyte. This result demonstrates that a GDE is effective for the aqueous electrochemical oxidation of hydrophobic organic compounds. The stability of Ru-CTF on GDE was investigated by the repeated tests in which an EB oxidation reaction was performed over four consecutive cycles with the replacement of the electrolyte every 12 h. Ru-CTF on GDE had negligible deactivation for both current-time profiles and acetophenone generation with repeated use. These results indicate that Ru-CTF on GDE has good stability at 1.5 V vs. RHE.
In summary, gaseous EB was successfully oxidized to acetophenone in aqueous electrolytes using a GDE that supported Ru-CTF. Singly isolated Ru-atoms coordinated with nitrogen atoms in Ru-CTF were converted to high-valency Ru=O by water oxidation, and O-atom insertion into the C-H bond of EB was facilitated. Considering that Ru-CTF is highly stable during the long-term electrolysis in aqueous solution, Ru-CTF is a novel platform as a heterogeneous electrocatalyst for partial hydrocarbon oxidation reactions. This is the first demonstration of electrochemical hydrocarbon oxidation in aqueous electrolytes using GDEs. The three-dimensional microstructures in the GDE maximized the transportation of gaseous hydrocarbons, and the oxidation reaction occurred at the triple-phase boundary. The present results clearly demonstrate that GDEs are a promising approach for the electrochemical oxidative upgrading of hydrophobic organics.
Reference:[1] S. Yamaguchi et al. Chem. Commun. 2017, 53, 10437-10440.
Figure. (a) Schematic illustration of GDE carrying Ru-CTF (left), Molecular structure of Ru-CTF (right), (b) The amount of acetophenone generated as a function of time with gaseous EB (Black), with 0.2 mM aqueous EB (Gray).
Figure 1