A method for the catalytic-free electrolytic reduction of vanillin to produce vanillyl alcohol

Abstract

The application discloses a non-catalyst electrolytic reduction method for preparing vanillin alcohol from vanillin, belonging to the non-metal electrolysis field, which comprises the following steps: dissolving vanillin in a solvent to obtain an electrolyte; the solvent is selected from 0.05-0.15 M sulfuric acid aqueous solution or a mixed solvent of 0.05-0.15 M sulfuric acid aqueous solution and alcohol solvent; and the electrolyte is placed in a single-chamber electrolytic cell equipped with an anode and a cathode to perform electrolysis, so that vanillin is hydrogenated and reduced into vanillin alcohol; wherein the cathode is Fe, and the anode is selected from Fe or Al. The application has excellent performance for preparing vanillin alcohol from vanillin through hydrogenation reaction, under the optimal conditions, the conversion rate of vanillin can reach 91.8%, and the selectivity of vanillin alcohol can reach 97.7%, which is higher than that of the existing catalyst; the device used is simple, no additional hydrogen gas is needed, no catalyst is needed, the reaction can be carried out at normal temperature, the reaction condition is mild, and the energy consumption cost is low.

Description

Technical Field

[0001] This invention belongs to the field of non-metallic electrolysis, specifically relating to a method for preparing vanillin by catalyst-free electrolytic reduction of vanillin. Background Technology

[0002] Vanillin, as an important high-value-added fine chemical intermediate, possesses both excellent bioactivity and chemical reactivity, and is widely used in pharmaceutical synthesis, food flavoring, and cosmetic additives. Its efficient preparation technology has always been a research hotspot in the field of biomass conversion. Currently, the industrial preparation of vanillin mainly relies on the selective hydrogenation reaction of vanillin. The core of this reaction is to reduce the aldehyde group (-CHO) to the hydroxyl group (-CH2OH) while retaining functional groups such as the benzene ring and ether bond. Therefore, the selectivity and stability of the catalytic system are crucial to determining the reaction efficiency. Existing catalytic systems for the hydrogenation of vanillin to vanillin mainly focus on noble metal catalysts and non-noble metal catalysts. While noble metal catalysts have high catalytic activity, their high cost and resource scarcity limit their large-scale industrial application. Non-noble metal catalysts have become a research focus due to their cost advantage. These catalytic hydrogenation systems typically react under high-pressure hydrogen conditions, selectively hydrogenating the aldehyde group to obtain the target product. However, these catalytic systems still face the following technical challenges. 1. Poor selectivity and severe side reactions: Vanillin molecules contain multiple functional groups, including aldehyde, benzene ring, and methoxy (-OCH3). Existing non-precious metal catalysts (such as single Ni-based or Cu-based catalysts) struggle to accurately identify the hydrogenation reaction of the aldehyde group, easily triggering side reactions such as benzene ring hydrogenation and ether bond cleavage, resulting in low selectivity and limited yield of the target product. 2. Insufficient catalyst stability and poor cycle performance: Existing supported non-precious metal catalysts mostly use traditional supports such as activated carbon and Al2O3. The interaction between the metal active component and the support is weak, making it prone to metal particle sintering and agglomeration or metal leaching under the high temperature and pressure conditions of hydrogenation reactions. This leads to rapid decay of catalyst activity, with conversion rates decreasing by more than 30% after 3-5 cycles. 3. Difficult catalyst separation and recovery, and high cost: Traditional catalysts are mostly in powder or granular form, requiring solid-liquid separation methods such as centrifugation and filtration after the reaction. This process is complex, energy-intensive, and results in catalyst loss rates as high as 10-15%, failing to meet the high-efficiency separation requirements of continuous industrial production. 4. Harsh reaction conditions and poor industrial adaptability: To improve reaction efficiency, most existing non-precious metal catalytic systems require reactions at temperatures above 150°C and pressures above 5 MPa under hydrogen conditions. This places extremely high demands on the pressure and temperature resistance of the reaction equipment, significantly increasing equipment investment and operating costs for industrial production. To overcome these technical bottlenecks, developing a synthesis system that combines high selectivity and high stability is a key requirement for the industrial upgrading of the vanillin hydrogenation to vanillinol technology. Summary of the Invention

[0003] This invention addresses the technical shortcomings of existing processes for preparing vanillin alcohol by hydrogenation of vanillin, such as poor selectivity, insufficient catalyst stability, difficulty in separation and recovery, and harsh reaction conditions. It provides a catalyst-free method for the electrolytic reduction of vanillin to prepare vanillin alcohol, offering a feasible technical solution for the large-scale green industrial production of vanillin alcohol.

[0004] The technical solution adopted in this invention is as follows: A method for preparing vanillin alcohol by catalyst-free electrolytic reduction of vanillin includes: Vanillin is dissolved in a solvent to obtain an electrolyte; the solvent is selected from 0.05~0.15 M sulfuric acid aqueous solution, or a mixture of 0.05~0.15 M sulfuric acid aqueous solution and alcohol solvent; Electrolysis is performed in a single-chamber electrolytic cell equipped with an anode and a cathode to hydrogenate vanillin to vanillyl alcohol; wherein the cathode is Fe and the anode is Fe.

[0005] Preferably, the solvent is a 0.05~0.15 M aqueous solution of sulfuric acid.

[0006] More preferably, the solvent is a 0.1 M aqueous solution of sulfuric acid.

[0007] The alcohol solvent is selected from any one of methanol, ethanol, and isopropanol.

[0008] The material ratio of vanillin to 0.05-0.15 M sulfuric acid aqueous solution is 0.1 mmol : 2.5-5 mL.

[0009] Preferably, the material ratio of vanillin to 0.05~0.15 M sulfuric acid aqueous solution is 0.1 mmol : 3.5 mL.

[0010] The ratio of vanillin, 0.05-0.15 M sulfuric acid aqueous solution to alcohol solvent is 0.1 mmol : 2.5-5 mL : 1 mL.

[0011] Preferably, the material ratio of vanillin, 0.05~0.15 M sulfuric acid aqueous solution to alcohol solvent is 0.1 mmol : 3.5 mL : 1 mL.

[0012] The electrolysis current is 2~40 mA and the electrolysis time is 2~6 h.

[0013] Preferably, the electrolysis current is 2~5 mA and the electrolysis time is 2~4 h.

[0014] More preferably, the electrolysis current is 2 mA and the electrolysis time is 4 h.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The method of the present invention has excellent performance in the preparation of vanillin alcohol by the hydrogenation reaction of vanillin. Under the optimal conditions, the conversion rate of vanillin can reach 91.8% and the selectivity of vanillin alcohol can reach 97.7%, which is higher than that of existing catalysts. 2. The apparatus used in the method of the present invention is simple, requires no external hydrogen gas or catalyst, reduces material costs and post-processing difficulty, and is safe and reliable; 3. This invention can react at room temperature, with mild reaction conditions and low energy consumption cost. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the simplified electrolysis device invented in this invention. Figure 2 This is a comparison chart of the substrate conversion rate and product selectivity test results in Examples 1-4 of the present invention; Figure 3 This is a comparison chart of the substrate conversion rate and product selectivity test results in Examples 1, 5, and 6 of this invention; Figure 4 This is a schematic diagram showing the changes in substrate conversion rate and product selectivity with current in Example 13 of the present invention; Figure 5 This is a comparison chart of the substrate conversion rate and product selectivity test results in Examples 14-20 of this invention; Figure 6 This is a schematic diagram showing the changes in substrate conversion rate and product selectivity with electrolysis time in Example 21 of the present invention. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments. These embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0018] It should be noted that, unless otherwise stated, the technical or scientific terms used in this application should have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0019] This invention employs a self-made, simple electrolysis device, such as... Figure 1 As shown, the device includes a cathode electrode, an anode electrode, a single-chamber electrolytic cell, and a DC power supply.

[0020] Example 1

[0021] Vanillin is synthesized from vanillin using a simple electrolysis apparatus: Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell, then dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 25 mA for 4 h.

[0022] Example 2

[0023] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell, then dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution. Assemble the electrolytic cell, using Fe as the cathode and Mg as the anode. Electrolyze at a constant current of 25 mA for 4 h.

[0024] Example 3

[0025] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell, then dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution. Assemble the electrolytic cell, using Fe as the cathode and Al as the anode. Electrolyze at a constant current of 25 mA for 4 h.

[0026] Example 4

[0027] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell, then dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution. Assemble the electrolytic cell, using Fe as the cathode and Zn as the anode. Electrolyze at a constant current of 25 mA for 4 h.

[0028] The vanillin conversion rate and product selectivity of Examples 1-4 were tested, and the test results are as follows: Figure 2 As shown in Table 1. In the figure, VA is vanillin, and MMP is 4-methylguaiacol.

[0029] Table 1

[0030]

[0031] Depend on Figure 2As shown in Table 1, the electrocatalytic reduction of vanillin to vanillinol exhibits significant differences in reaction performance under different electrode pairs. Overall, all systems achieve effective conversion of vanillin, but the yield and selectivity of the target product, vanillinol (VA), vary considerably, indicating that the electrode materials significantly regulate the reaction pathway. Taking the Mg / Fe system as an example, the vanillin conversion rate is 72.3%, the vanillinol yield is 54.8%, and the vanillinol selectivity is 75.8%, indicating that while this system promotes substrate conversion, its ability to direct the formation of the target product is generally limited. In the Al / Fe system, the vanillin conversion rate increases to 86.1%, the vanillinol yield reaches 70.9%, and the selectivity is 82.3%, indicating that the introduction of Al is beneficial for improving reaction activity and, to some extent, suppressing side reactions. Although the Zn / Fe system has the highest vanillin conversion rate of 91.9%, the vanillinol yield is only 32.8%, and the selectivity is only 35.7%, indicating that although a large amount of substrate is consumed under these conditions, more is converted into byproducts, and the system undergoes a significant deep reduction reaction. In comparison, the Fe / Fe system exhibits the best overall performance, with a vanillin conversion rate of 79.9%, a vanillin yield of 72.1%, and a vanillin selectivity as high as 90.2%. This indicates that the system can more effectively control the reaction at the step of reducing the aldehyde group to an alcohol, making it a superior electrode combination.

[0032] Example 5

[0033] The steps in this embodiment are the same as in Example 1, except that the solvent is replaced with a 0.05 M sulfuric acid aqueous solution.

[0034] Example 6

[0035] The steps in this embodiment are the same as in Example 1, except that the solvent is replaced with a 0.15 M sulfuric acid aqueous solution.

[0036] Example 7

[0037] The steps in this embodiment are the same as in Embodiment 1, except that the solvent is replaced with methanol.

[0038] Example 8

[0039] The steps in this embodiment are the same as in Embodiment 1, except that the solvent is replaced with ethanol.

[0040] Example 9

[0041] The steps in this embodiment are the same as in Example 1, except that the solvent is replaced with isopropanol.

[0042] Example 10

[0043] The steps in this embodiment are the same as in Example 1, except that the solvent is replaced with ethyl acetate.

[0044] Example 11

[0045] The steps in this embodiment are the same as in Example 1, except that the solvent is replaced with acetonitrile.

[0046] Example 12

[0047] The steps in this embodiment are the same as in Example 1, except that the solvent is replaced with tetrahydrofuran.

[0048] The vanillin conversion rate and product selectivity of Examples 5-12 were tested and compared with those of Example 1. The test results are as follows: Figure 3 As shown in Table 2:

[0049] Table 2

[0050]

[0051] Depend on Figure 3 As shown in Table 2, the sulfuric acid concentration significantly affects the reaction performance of the electrocatalytic reduction of vanillin to vanillinol. Higher sulfuric acid concentration is not always better; there is an optimal range. As the sulfuric acid concentration increased from 0.05 M to 0.1 M, the vanillin conversion increased from 53.2% to 79.9%, the vanillinol (VA) yield increased from 45.8% to 72.1%, and the selectivity increased from 86.1% to 90.2%, indicating that moderate acidity is beneficial for promoting substrate conversion and improving the selectivity of the target product. When the sulfuric acid concentration was further increased to 0.15 M, the vanillinol yield decreased to 56.3%, and the selectivity decreased to 65.0%, indicating that excessively high acidity exacerbates the deep reduction side reaction and is detrimental to the selective formation of vanillinol. Other solvents, lacking current, could not react. Therefore, under the conditions of this example, 0.1 M sulfuric acid is the optimal electrolyte concentration.

[0052] Example 13

[0053] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell, then dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 0–40 mA for 4 h. The test results are as follows. Figure 4 As shown.

[0054] Depend on Figure 4 It is evident that the current magnitude has a significant impact on the electrocatalytic reduction of vanillin. As the current increases from 0 mA to 2 mA, the vanillin conversion rate significantly increases from 23.9% to 91.8%, and the vanillin (VA) selectivity reaches 97.7%. While further increasing the current maintains a high conversion rate, the VA selectivity gradually decreases, and the proportion of 4-methylguaiacol (MMP) increases significantly, indicating that excessively high current promotes deep reduction side reactions. Under the conditions of this example, 2 mA is the optimal electrolysis current.

[0055] Example 14

[0056] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell, then dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 2 mA for 4 h.

[0057] Example 15

[0058] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell. Dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution and 1 mL of methanol. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 2 mA for 4 h.

[0059] Example 16

[0060] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell. Dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution and 1 mL of ethanol. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 2 mA for 4 h.

[0061] Example 17

[0062] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell. Dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution and 1 mL of isopropanol. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 2 mA for 4 h.

[0063] Example 18

[0064] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell. Dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution and 1 mL of ethyl acetate. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 2 mA for 4 h.

[0065] Example 19

[0066] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell. Dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution and 1 mL of acetonitrile. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 2 mA for 4 h.

[0067] Example 20

[0068] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell. Dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution and 1 mL of tetrahydrofuran. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze at a constant current of 2 mA for 4 h.

[0069] The vanillin conversion rate and product selectivity of Examples 14-20 were tested, and the test results are as follows: Figure 5 As shown in Table 3:

[0070] Table 3

[0071]

[0072] Depend on Figure 5 As shown in Table 3, the dual-solvent system has a significant impact on the electrocatalytic reduction of vanillin. When methanol, ethanol, and isopropanol are used as co-solvents, the system still maintains high VA selectivity, reaching 97.2%, 95.8%, and 94.2%, respectively, but the conversion and yield are lower than those of the single 0.1 M sulfuric acid system. The introduction of ethyl acetate, acetonitrile, and tetrahydrofuran significantly inhibits substrate conversion, with the acetonitrile system showing a conversion rate of only 5.2%. In summary, the 0.1 M sulfuric acid system without a second solvent performs best, and is more conducive to the efficient and selective reduction of vanillin to vanillyl alcohol.

[0073] Example 21

[0074] Weigh 0.1 mmol of vanillin and place it in a single-chamber electrolytic cell, then dissolve it in 3.5 mL of 0.1 M sulfuric acid aqueous solution. Assemble the electrolytic cell, using Fe as the cathode and Fe as the anode. Electrolyze using a constant current of 2 mA. The electrolysis results at different electrolysis durations are tested, and the results are as follows: Figure 6 As shown.

[0075] Depend on Figure 6 It can be seen that when electrolyzing at a constant current of 2 mA for 2 h, the vanillin conversion rate can reach 87.5% and the vanillin selectivity can reach 99.1%; when electrolyzing for 4 h, the vanillin conversion rate is 91.8% and the vanillin selectivity is 97.7%; when the electrolysis time is further increased, the vanillin conversion rate and vanillin selectivity basically no longer change.

[0076] The present invention has been disclosed above with reference to preferred embodiments, but it is not intended to limit the present invention. All technical solutions obtained by adopting equivalent substitutions or equivalent transformations fall within the protection scope of the present invention.

Claims

1. A method for preparing vanillin alcohol by catalyst-free electrolytic reduction of vanillin, characterized in that, include: Vanillin is dissolved in a solvent to obtain an electrolyte; the solvent is selected from 0.05~0.15 M sulfuric acid aqueous solution, or a mixture of 0.05~0.15 M sulfuric acid aqueous solution and alcohol solvent; Electrolysis is performed in a single-chamber electrolytic cell equipped with an anode and a cathode to hydrogenate vanillin to vanillyl alcohol; wherein the cathode is Fe and the anode is Fe.

2. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 1, characterized in that, The solvent is a 0.05~0.15 M aqueous solution of sulfuric acid.

3. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 2, characterized in that, The solvent is a 0.1 M aqueous solution of sulfuric acid.

4. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 1, characterized in that, The alcohol solvent is selected from any one of methanol, ethanol, and isopropanol.

5. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 1, characterized in that, The material ratio of vanillin to 0.05-0.15 M sulfuric acid aqueous solution is 0.1 mmol : 2.5-5 mL.

6. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 1, characterized in that, The material ratio of vanillin, 0.05~0.15 M sulfuric acid aqueous solution to alcohol solvent is 0.1 mmol : 2.5~5 mL : 1 mL.

7. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 1, characterized in that, The electrolysis current is 2~40 mA and the electrolysis time is 2~6 h.

8. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 7, characterized in that, The electrolysis current is 2~5 mA, and the electrolysis time is 2~4 h.

9. The method for preparing vanillin by catalystless electrolytic reduction of vanillin according to claim 8, characterized in that, The electrolysis current was 2 mA and the electrolysis time was 4 h.

Images