Analysis of Reaction Rate and Product Selectivity of Ethanol Electrolytic Oxidation Reaction in the Flow Reactor
Authors: Cheng Kai Hung , Namura Keisuke , Kawase Motoaki 
Published: February 2024

Introduction

In recent years, the development of the electrolysis hydrogen production process has been vigorously promoted due to the widespread use of hydrogen energy. However, oxygen, a byproduct of the anode reaction, has not been utilized. On the other hand, chemical products are produced industrially by electro-organic synthesis, and if the anode reaction could be used directly as a chemical synthesis reaction, the overall benefit would be further enhanced. At the same time, the high selectivity and controllability of the electrochemical process make it possible to produce highly selective chemical products by anodic reactions. The objective of this study is to develop a process for simultaneous electrolytic hydrogen production at the cathode and highly selective acetaldehyde production at the anode.

Experimental

A schematic diagram of the experimental setup is shown in Figs. 1 and 2. Platinum electrodes were placed on silicone gaskets with flow channels, and the gaskets were sandwiched between silicone gaskets, PTFE plates, and stainless-steel endplates. A cation exchange membrane NR-212 was placed in the center to separate the reactions at the two electrodes. A 0 to 0.5 mol/L sulfuric acid solution containing 0.01 to 1.0 mol/L ethanol or a 0 to 0.5 mol/L sulfuric acid solution containing 0.01 to 1.0 mol/L acetaldehyde was supplied to the anode, and a 0 to 0.5 mol/L sulfuric acid solution was supplied to the cathode.

Results and Discussion

The experimental results are analyzed based on the premise of surface reaction rate limitation and steady-state conditions. As shown in Fig. 3, the ethanol conversion increased with rise in A/v (ratio of electrode area to flow rate), reaching its maximum value at A/v of 88.4 min/cm. This can be attributed to the difficulty in discharging bubbles generated from the electro-oxidation reaction as the A/v was raised. To ensure that the reactions occurred under reaction rate-limiting conditions, acetaldehyde electro-oxidation experiments were conducted with feed concentrations of 0 to 0.5 M sulfuric acid solutions containing 1.0 M acetaldehyde at the anode, and 0 to 0.5 M sulfuric acid solutions at the cathode, an A/v of 88.4 min/cm, and a potential difference of 1.0 V. The reaction rate was also affected by the reduction in the hydroxide ion concentration due to the increased sulfuric acid concentration, leading to the lower measured reaction rates. In the result of Fig. 4, it can be inferred that when the concentration of sulfuric acid was below 0.1 M, the reaction rate was governed by diffusion. Subsequently, acetaldehyde electro-oxidation experiments were performed under acetaldehyde concentrations ranging from 0.01 to 1 M, a sulfuric acid concentration of 0.1 M, the fixed A/v, and potential differences between 0.8 and 1.2 V to elucidate the impact of reactant concentration on the reaction rate. The results in Fig. 5 demonstrate that while the reaction rates increased with raising the acetaldehyde concentration, it eventually approached a plateau, attributable to the saturation of active sites with acetaldehyde on the Pt electrode.
The results of ethanol electro-oxidation experiments were shown in Figs. 6 and 7, decrease in acetaldehyde selectivity was observed with a feed concentration of a 0.1 M sulfuric acid solution, attributable to the formation of acetic acid and carbon dioxide as byproducts. Subsequently, in comparison to sulfuric acid concentrations below 0.2 M, under the conditions of 0.5 M sulfuric acid concentration and low conversion, the acetaldehyde selectivity exhibited a gradual decline as the conversion increased. This indicates that the influence of sulfuric acid concentration on the oxidation reaction of acetaldehyde to acetic acid was more significant than that on the oxidation reaction of ethanol to acetaldehyde. A plausible explanation for this phenomenon is that the oxidation of acetaldehyde is considerably inhibited by the adsorption of sulfate ions to the active sites, as it is associated with a second-order reaction of adsorbed species concentration. Consequently, altering the sulfuric acid concentration results in changes in the ethanol electro-oxidation reaction selectivity.

Conclusions

The selectivity of ethanol electro-oxidation reaction was significantly influenced by potential difference and electrolyte concentration. Notably, acetic acid was not produced at potential differences below 0.8 V. Sulfuric acid concentration affected reaction selectivity, with a greater impact on acetaldehyde-to-acetic-acid oxidation. Despite the higher selectivity observed at 0.6 V, the reaction rate was low. Employing an appropriate potential difference and electrolyte concentration can facilitate achievement both of higher selectivity and an enhanced reaction rate. Optimal conversion was achieved at an A/v of 88.4 min/cm, while the maximum acetaldehyde selectivity of approximately 95 % was observed at an applied voltage of 1.2 V and 0.5 M sulfuric acid concentration.

Figure 1