3.1 Influence of Linear Voltammetry Scanning
It can be seen from Fig. 3 that the LSV results of 7K-LCA in different electrolytes are significantly different. No matter what the electrolyte is, when there is no 7K-LCK, there is no reduction peak in the linear voltammetry curve. When the electrolyte contains 7k-LCK, an obvious reduction peak will appear in the LSV curve. Then set different scanning speeds to perform linear volt-ampere scanning. It can be seen from the following figure that as the scanning rate increases linearly, the reduction peak Ep of 7K-LCA showed a regular negative shift, and the peak current ip gradually tends to increase regularly.
As shown in Fig. 4, taking the logarithm of the increasing scanning speed as the abscissa, and the reduction peak potential as the ordinate, the mixed curves of DMI, HMPA, DMPU, DMI and HMPA are drawn into graphs. Ep and Inv in each electrolyte have a linear relationship, indicating that the reduction reaction of 7K-LCK in these four electrolytes is irreversible. So in the experiment, once the substrate is reduced to UDCA or CDCA, it is impossible to be oxidized to 7K-LCK again.
Using the square root of the set scan rate as the abscissa and the reduction current ip as the ordinate, the curves ip -v1/2 of four different aprotic solvent systems are made. As shown in the figure, the correlation coefficients of the four curves are 0.9939, 0.9970, 0.9983 and 0.9890, indicating that ip and v1/2 present a good linear relationship, indicating that the irreversible reduction reaction carried out in the experiment is affected by diffusion. Considering that the reduction reaction is affected by diffusion, the rotor was put into the electrolyte and the system was placed on the magnetic stirrer during the experiment, and the different speeds were adjusted to compare the results.
3.2 Effect of different solvents
According to the frontier orbital theory, the reactivity of a molecule is related to its highest occupied molecular orbital and the lowest empty molecular orbital. It is generally believed that a lower ELumo value indicates that the molecule has a stronger ability to act as an electron acceptor, while a high EHomo value means that the molecule has a stronger electron donating ability. Therefore, the energy gap reflects the chemical stability of organic molecules, and the lower the ΔE value, the easier it is for organic molecules to be adsorbed on the metal surface. The higher the dipole moment, the stronger the ability of organic molecules to adsorb on the metal surface.
It can be seen from Table 1 that the △E value in descending order are 8.679891, 6.662768, 6.671415, when DMI, DMPU and HMPA were used as solvents. And the dipole moment in descending order are 4.0963, 3.7477, 3.5009, when DMPU, HMPA and DMI were used as solvents, which indicates that DMPU has the strongest adsorption capacity on the surface of the metal electrode, followed by HMPA. The last is DMI. The stronger the adsorption capacity of organic molecules, the stronger the binding of the substrate molecules to the metal electrode.
Table 1
Parameters calculated by DFT for different aprotic solvent molecules.
| ELUMO(eV) | EHOMO(eV) | ΔE(eV) | µ(Debye) |
DMI | 2.181334 | -6.498557 | 8.679891 | 3.5009 |
HMPA | 0.610348 | -6.061067 | 6.671415 | 3.7477 |
DMPU | 0.405026 | -6.257742 | 6.662768 | 4.0963 |
3.3 Effect of different concentrations of supporting electrolyte
The product comparison of three different aprotic solvents as electrolytes in Table 2 shows that when only DMI is the solvent, the yield of UDCA is the highest and no by-product CDCA is generated. The reasons are as follows: the five-membered imidazole ring structure of DMI is stable, as shown in Fig. 2, 7K-LCA undergoes two nucleophilic and "Walden inversions". The chloride ion attacks the hydroxyl group at the 7-position, which can make the chiral center carbon of 7K-LCK occur. The configuration is reversed, and the hydroxide ion in solution further attacks the chloride ion, thereby stereoselectively reducing 7K-LCA to UDCA. The structures of HMPA and DMPU are relatively unstable, and chloride ions can directly attack them. 7K-LCA obtains hydrogen ions on the cathode electrode under the action of electric current and directly undergoes a reduction reaction. Compared with UDCA, CDCA has less steric hindrance, so part of 7K-LCA stereoselectively reduced to CDCA.
Table 2
Influence of cathode materials on the stereoselective electroreduction of 7K-LCA.
Sovents | LiCl(%) | C(%) | Yu(%) | Yc(%) |
HMPA | 0.1M | 94.2 | 10.7 | 21.1 |
0.2M | 98.5 | 12.3 | 24.8 |
0.3M | - | - | - |
DMPU | 0.1M | 39.3 | 8.2 | 9.7 |
0.2M | 50.2 | 11.8 | 12.4 |
0.3M | 62.4 | 11.3 | 19.6 |
DMI | 0.1M | 34 | 22 | - |
0.2M | 43.6 | 28.4 | - |
0.3M | 30.2 | 18.2 | - |
Theoretically, the greater the amount of electrolyte in the electrolysis system, the better the conductivity, and therefore the better the effect of substrate electrolysis. If more electrolyte is added to the electrolysis system, at the same time, a smaller voltage value can be set. It can be seen from the Table 2 that adding different amounts of electrolyte to DMI, HMPA, and DMPU will affect the electrochemical reduction results of 7K-LCK, and the best LiCl concentration is 0.2M. In the DMPU system, although the substrate conversion rate is becoming higher with the increase of the electrolyte concentration, the product yield were not raised significantly, and the by-product output was increased. In the HMPA system, 0.3M LiCl cannot be dissolved, and the overall conversion rate of 7K-LCK is very high. Among the three electrolysis systems, the DMI system can obtain the largest amount of UDCA, and the effect was best when 0.2M LiCl is added. At this time, the yield of UDCA is 28.4%, and by-product CDCA wasn’t produced.
3.3 Effect of the ratio of HMPA to DMI
The yields of UDCA and CDCA produced by electrochemical reaction in aprotic solvent are not equal to the conversion rate of 7K-LCK, and intermediate products were generated during the reaction. The initial conjecture is 3,7-ketolithocholic acid, UDCA loses hydrogen ions on the anode, the -OH at the 3rd position and the -OH at the 7th position can be oxidized to become = O, and 3,7-ketolithocholic acid can be produced, Hydrogen ions are obtained at the cathode to generate UDCA and CDCA, which are not completely reduced. And in the molecular simulation content of this chapter, it is known that the binding force between DMI and HMPA and the metal electrode surface is relatively small in the three aprotic solvents. The essence of this experiment is that the substrate undergoes a reduction reaction on the cathode electrode, so these two solvents relatively does not affect the reaction of the substrate 7K-LCK with the metal electrode. Based on the above content, DMI and HMPA are mixed as the electrolyte, 0.2M electrolyte is added, and the two aprotic solvents are mixed in different ratios for experimental comparison. As shown in Table 3 above, with the increase of the HMPA solvent content in the electrolyte, DMI solvent content decreases, the conversion rate of 7K-LCK is getting higher and higher, the yield of by-product CDCA is also getting higher and higher, the yield of target product UDCA continues to increase due to the increase of the conversion rate of substrate 7K-LCK, the optimal mixing ratio of DMI and HMPA solvent is 1:1. At this time, the conversion rate of substrate 7K-LCK was 94%, the yield of target product UDCA was 67.8%, and the yield of by-product CDCA was 25.3%.
Table 3
Influence of the ratio of DMI to HMPA on the stereoselective electroreduction of 7K-LCA.
Solvents | Conversion of 7K-LCA(%) | Yield of UDCA (%) | Yield of CDCA(%) |
10%HMPA + 90%DMI | 73.7 | 28.3 | 13.5 |
15%HMPA + 85%DMI | 82.2 | 31.6 | 16,4 |
20%HMPA + 80%DMI | 86.7 | 34.3 | 20.7 |
30%HMPA + 70%DMI | 93.8 | 39.4 | 22.4 |
40%HMPA + 60%DMI | 93.9 | 50.4 | 23.1 |
50%HMPA + 50%DMI | 94 | 67.8 | 25.3 |
60%HMPA + 40%DMI | 92 | 58.9 | 27 |
70%HMPA + 30%DMI | 95 | 58.6 | 28.4 |
80%HMPA + 20%DMI | 96.7 | 58.5 | 30 |
90%HMPA + 10%DMI | 97.7 | 47 | 35.6 |
3.4 Effect of different mixing speeds
According to the linear voltammetry curve in Fig. 5, the outcome of the electroreduction reaction is influenced by diffusion. Therefore, in the electrolysis process of DMI, DMPU, HMPA and the mixed solution of DMI and HMPA as the electrolyte, the rotor was added to set different rotational speeds, and the results are shown in Fig. 7. After the reduction reaction of the four systems, within 1000 r/min, with the increase of rotation speed, the conversion rate of 7K-LCK gradually increased, and the yield of product UDCA also gradually increased, but the yield of by-product CDCA also showed an overall upward trend. By making the curve obtained by fitting the ip and v1/2 scatter diagram, it can be seen that the linear relationship between ip and v1/2 is very good, then it can be known that the reduction reaction is controlled by diffusion,
And it can be clearly seen that the conversion rate of substrate 7K-LCK and the yield of UDCA are the highest in the HMPA and DMI mixed solution electrolysis system, reaching 94.8% and 70% when the rotation speed is 1000 r/min.
3.5 Effect of electrode materials on electroreduction
It can be seen from the above that the type of solvent, the amount of electrolyte, and the rotational speed during electrolysis all have an effect on the electrochemical reduction of the substrate 7K-LCK. Since the electrochemical reduction reaction occurs on the surface of the cathode electrode, the influence of different metals as the cathode electrode on the electrolysis reaction should be continuously explored. In an aprotic solvent mixed with 50% HMPA and 50% DMI, 0.2 mol of LiCl was added as the electrolyte, and four metal sheets of Cu, Hg-Cu, Pb and Ni were used as the cathode electrode for electrolysis. When Cu is used as the cathode, the yield of UDCA is the highest, reaching 74.3%, while the by-product CDCA is the least, only 16.8%. It can be seen that the Cu electrode is the best electrode.
Molecular dynamics (MD) simulation can explore the change of motion state of particles in a certain statistical mechanics system as time progresses to obtain the physical and chemical properties of the system. MD simulation can find the interaction between molecules and metal electrodes. This experiment investigates the 7K electro-reduction results under different electrodes. Since the reaction is carried out on the electrode surface, electrons are finally obtained from the cathode electrode to generate the final product. The simulation can caculate the binding energy of 7K-LCK molecules with different metal surfaces. According to the method, the model was built with Materials Studio software as shown in the Fig. 8, and the 7K-LCK was calculated in 50% DMI and 50% HMPA solvents and different electrodes.
Binding energies, as shown in Table 3, the calculated binding energies from small to large are Ni, Hg-Cu, Pb, Cu, which are − 49.353789, -65.204288, -69.127444, -124.32943 kcal/mol, respectively, binding to the substrate 7K-LCK in the system The biggest one is the copper electrode. As shown in Table 4, four kinds of metals were used as cathode electrodes. When Cu was used as cathode electrode, the 7K-LCK conversion rate and UDCA yield were the highest. They are 74.3% and 16.8% respectively, which are consistent with the simulation results, indicating that the binding energy between the substrate and the electrode also affects the experimental results.
Table 4. The influence of electrodes on electrochemical reduction (a: Cu, b: Pb, c: Hg-Cu, d:Ni)
Electrode
|
Conversion of 7K-LCA(%)
|
Yield of UDCA (%)
|
Yield of CDCA(%)
|
Cu
|
91
|
74.3
|
16.8
|
Pb
|
94
|
67.8
|
20.3
|
Hg-Cu
|
91
|
64.6
|
22.7
|
Ni
|
93.7
|
62.9
|
25.7
|
Table 5. Binding energy of substrate and different electrodes in molecular simulation