# The Oxidation of Isobutyraldehyde to Isobutyric Acid Using a Microenactor Technology System
## 3 Results and Discussion
### Laboratory-Scale Experiments with Oxygen as Oxidizing Agent  
The system design is as shown in Figure 6. The main difference from the previous design is the addition of a reflux setup, aimed at extending the reaction time of isobutyraldehyde. Isobutyraldehyde is initially placed in a solution tank. The oxygen flow rate is controlled at 1 SLPM, with a liquid flow rate of 5 mL/min, an operating pressure of 10 bar, and an operating temperature of 20 \({}^{\circ}\)C. Sampling is performed every 12 min, and the total experimental reaction time is 2 h. The experimental results are shown in Figure 7.  
Figure 4: Pathway A for the oxidation of isobutyraldehyde to isobutyric acid [11]. Figure 5: Pathway B for the oxidation of isobutyraldehyde to isobutyric acid [11]. Figure 6: Experimental flowchart for oxidation of isobutyraldehyde by oxygen. The experimental results in Figure 7 show that after 1 h of reaction time, the conversion rate reaches its plateau. After a total reaction time of 2 h, a 93.5% conversion rate of isobutyraldehyde and a 94.4% selectivity for isobutyric acid are achieved. In the first three sampling points of the experiment, peroxyacid and isopropyl formate were detected as byproducts. After that, isopropyl formate became the main byproduct. Considering the efficiency for subsequent production, a reaction time of 1 h is considered the optimal experimental condition. The next step is to confirm the effect of temperature in the experiment. The oxygen flow rate is controlled at 1 SLPM, with a liquid flow rate of 5 mL/min, an operating pressure of 10 bar, and experimental reaction time is 1 h. Operating temperature is designed between 20 \({}^{\circ}\)C and 80 \({}^{\circ}\)C. The experimental results are shown in Figure 8.  
Figure 8: Percentage conversion/selectivity vs. temperature plot for oxidation of isobutyraldehyde by oxygen. Figure 7: Percentage conversion/selectivity vs. reaction time plot for oxidation of isobutyraldehyde by oxygen. According to the results from Figure 8, under the system temperature condition of 20 \({}^{\circ}\)C, the optimal conversion rate of isobutyraldehyde is 89.2%, and the selectivity for isobutyric acid is 94.6%, with isopropyl formate as the main byproduct. This indicates that excessively high temperatures are unfavorable for this exothermic reaction and can also reduce the solubility of oxygen in the solution. As the final step, this study will confirm the impact of pressure on the experimental results. The oxygen flow rate is controlled at 1 SLPM, with a liquid flow rate of 5 mL/min, an operating temperature of 20 \({}^{\circ}\)C, and experimental reaction time is 1 h. Operating pressure is designed between 5 bar and 15 bar. The experimental results are shown in Figure 9.  
According to the results from Figure 9, under the system pressure of 15 bar, the highest isobutyraldehyde conversion rate of 90.2% can be achieved. However, this result is only 1% higher than the experimental result under the condition of 10 bar. Furthermore, higher system pressure will lead to increased operating costs and greater instability. Therefore, operating pressure of 10 bar is considered the optimal experimental condition. At this point, the laboratory-scale experiments have obtained the optimal experimental parameters and results. The oxygen flow rate is 1 SLPM, the liquid flow rate is 5 mL/min, the operating pressure is 10 bar, the operating temperature is 20 \({}^{\circ}\)C, and the reaction time is 1 h. Under these conditions, a conversion rate of 89.2% for isobutyraldehyde and a selectivity of 94.6% for isobutyric acid can be achieved, with isopropyl formate as the main byproduct. Compared with the experimental results reported in the literature [11], the process design in this study significantly reduced the required reaction time. However, due to the limited reactor volume of the MMRS system, the current production capacity remains relatively low.