Method for producing lactic acid by coupling of electrocatalytic anode and cathode

Abstract

The application belongs to the technical field of lactic acid production, and discloses a method for preparing lactic acid by coupling electrocatalysis anode and cathode, which comprises the following steps: taking an anode catalyst as an anode, a cathode catalyst as a cathode, and assembling the anode, the cathode and an electrolyte into an electrolytic cell; adding 1,2-propanediol into the anode electrolyte and the cathode electrolyte; under the application of a certain voltage, the 1,2-propanediol is oxidized into lactic acid at the anode end; oxygen is continuously introduced into the cathode electrolyte, and a divalent iron compound is additionally added into the electrolyte to catalyze the formation of hydroxyl radicals, so that the electro-Fenton reaction is used to realize the electro-conversion of 1,2-propanediol and oxygen to prepare lactic acid. Compared with the single anode oxidation, the energy utilization rate is improved, and a new idea and method are provided for the green and efficient preparation of lactic acid under normal temperature and pressure and high current density.

Description

Technical Field

[0001] This invention belongs to the field of lactic acid production technology and relates to a method for producing lactic acid by electrocatalytic coupling of anode and cathode. Background Technology

[0002] 1,2-Propanediol is a common organic compound. With increasing demands for sustainable development and environmental protection, bio-based routes are gradually becoming an important source for 1,2-propanediol production. This method uses renewable biomass as feedstock and utilizes biocatalysis and chemical processes to produce 1,2-propanediol. Glycerol is a byproduct of biodiesel production. 1,2-Propanediol can be generated through the hydrogenation of glycerol. The selective oxidation of 1,2-propanediol has been a research hotspot; its oxidation products include formic acid, acetic acid, lactic acid, and hydroxyacetone.

[0003] Lactic acid is an important high-value-added fine chemical, widely used as a monomer for the production of the biodegradable polymer polylactic acid (PLA), and also widely applied in the pharmaceutical and agrochemical fields. Currently, microbial fermentation is the main method for lactic acid production, accounting for approximately 90% of global output; however, this method is time-consuming and subsequent separation is difficult. Thermocatalytic glycerol-to-lactic acid reaction has a faster reaction rate and higher lactic acid selectivity. However, further development of thermocatalytic strategies is limited by the high-temperature and high-pressure environment. Therefore, efficiently preparing lactic acid under mild conditions using a sustainable method remains a significant challenge.

[0004] To date, the reported electrocatalytic oxidation of 1,2-propanediol to lactic acid remains a simple cation-driven oxidation process with low energy efficiency. The most commonly used catalysts are powdered catalysts, which suffer from low current density (<100 mA cm⁻¹). -2 (ACSCatal.2015,5(11),6926–6936.), Their complex synthesis and poor stability limit their large-scale production. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of existing methods for oxidizing 1,2-propanediol to produce lactic acid. Its purpose is to provide a method for preparing lactic acid by electrocatalytic anode-cathode coupling, which achieves green, efficient, and high-current-density preparation of lactic acid under ambient temperature and pressure conditions. It has the advantage of high energy utilization and improves the overall yield of lactic acid.

[0006] The technical solution of the present invention:

[0007] A method for producing lactic acid via electrocatalytic anode-cathode coupling includes the following steps:

[0008] (1) Assemble the electrolytic cell: Use the anode catalyst as the anode and the cathode catalyst as the cathode. Place the anode and cathode in the corresponding electrolytes to assemble the electrolytic cell.

[0009] (2) Electrocatalytic reaction: 1,2-propanediol is added to both the anolyte and the catholyte; under a certain voltage, 1,2-propanediol is oxidized to lactic acid at the anode; oxygen is continuously introduced into the catholyte, and ferrous compounds are added to the electrolyte to catalyze the formation of hydroxyl radicals, thereby realizing the electroconversion of 1,2-propanediol and oxygen to prepare lactic acid.

[0010] The anode catalyst is one or more of the following: monometallic / conductive substrate, bimetallic / conductive substrate, bimetallic / hydroxide, and bimetallic / oxide. The hydroxide is one of nickel-based, cobalt-based, copper-based, and iron-based hydroxides; and the oxide is one of nickel oxide, cobalt oxide, iron oxide, and copper oxide.

[0011] The metal on the anode catalyst is one or a combination of two of the following: platinum, gold, silver, ruthenium, palladium, and copper.

[0012] The conductive substrate is one of carbon cloth, carbon paper, nickel foam, nickel mesh, copper foam, and copper mesh.

[0013] The divalent iron compound is at least one of ferrous sulfate, ferrous nitrate, and ferrous oxalate.

[0014] The anolyte is an aqueous solution of any one of sodium hydroxide, potassium hydroxide, lithium hydroxide, potassium bicarbonate, potassium carbonate, sodium bicarbonate, and sodium carbonate, with a concentration of 0.1–2 M.

[0015] The cathode electrolyte is at least one of perchloric acid, sulfuric acid, and sodium bisulfate solution, with a concentration of 0.1–1 M.

[0016] The electrocatalytic reaction in step (2) is carried out at room temperature and pressure, with an anode voltage of 0.3V to 1.5V vs. RHE.

[0017] The concentration of the ferrous compound applied in step (2) is 0.05 to 50 mM, preferably 2 mM.

[0018] In step (2), the oxygen flow rate is 10-50 mL / min, preferably 20 mL / min.

[0019] The beneficial effects of this invention are as follows: This invention provides an electrocatalytic anode-cathode coupling method for producing lactic acid. A monometallic / bimetallic catalyst is constructed as the anode and cathode catalysts, and water molecules are used as the oxygen source in the oxidation reaction. Using an electrocatalytic method driven by renewable energy, 1,2-propanediol is oxidized to lactic acid at the anode, while hydroxyl radicals generated by the electro-Fenton reaction are used at the cathode to oxidize 1,2-propanediol. This achieves the co-conversion of oxygen and 1,2-propanediol to produce lactic acid, and the efficiency of co-conversion of anode and cathode to produce lactic acid is higher than that of single-anode conversion.

[0020] In summary, the present invention can be used to produce lactic acid through anode-cathode coupling, and has broad application prospects. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the principle of electrocatalytic anode-cathode coupling for lactic acid production in this invention.

[0022] Figure 2 This is a scanning electron microscope image of the catalyst in Example 1 of the present invention;

[0023] Figure 3 This is a linear polarization curve of the electrocatalytic oxidation of 1,2-propanediol by different bimetallic anode catalysts in Example 1 of the present invention;

[0024] Figure 4 This is the NMR spectrum of the 1,2-propanediol oxidation product at the cation end in Example 1 of this invention.

[0025] Figure 5 This is the NMR spectrum of the product of the co-conversion of 1,2-propanediol and oxygen at the cathode in Example 1 of this invention.

[0026] Figure 6 This is a scanning electron microscope image of the catalyst in Example 2 of the present invention;

[0027] Figure 7 This is a linear polarization curve of the electrocatalytic oxidation of 1,2-propanediol by the catalyst in Example 2 of the present invention.

[0028] Figure 8 This is a scanning electron microscope image of the catalyst in Example 3 of the present invention;

[0029] Figure 9 This is the XRD pattern of the catalyst in Example 3 of this invention;

[0030] Figure 10 This is a linear polarization curve of the electrocatalytic oxidation of 1,2-propanediol by the catalyst in Example 3 of this invention.

[0031] Figure 11 This is a performance diagram of the catalyst in Example 3 of the present invention at different oxidation potentials for the electrocatalytic oxidation of 1,2-propanediol at the cation extreme.

[0032] Figure 12 This is a comparison chart of lactic acid production rates in embodiment 4 of the present invention, showing the interaction between anode-cathode coupling and simple anodic oxidation. Detailed Implementation

[0033] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0034] Example 1

[0035] Electrocatalytic anode-cathode coupling for lactic acid production:

[0036] (1) Preparation of catalyst

[0037] ① Electroplating method for preparing nickel foam supported AuPt alloy catalyst

[0038] First, the nickel foam is pretreated by ultrasonicating it in acetone for 15 minutes, rinsing it with deionized water, then ultrasonicating it in dilute hydrochloric acid (3M) for 15 minutes, rinsing it with deionized water, and finally ultrasonicating it in deionized water for 15 minutes and then drying it quickly to obtain the pretreated nickel foam.

[0039] Then, a layer of nickel hydroxide was first loaded onto the surface of the nickel foam. Specifically, 50 mg of nickel chloride hexahydrate, 50 mg of urea, and 15 ml of deionized water were transferred to a 50 ml hydrothermal reactor and kept at 150°C for 12 hours. After removing the nickel foam, it was rinsed with deionized water to obtain (Ni(OH)2 / Ni foam). Then, AuPt alloy was loaded. Specifically, a mixed solution of 5 mM chloroauric acid and 5 mM chloroplatinic acid was prepared, and 50 mM hexadecyltrimethylammonium bromide was added. The pretreated nickel foam was used as the working electrode, the nickel foam as the counter electrode, and a saturated calomel electrode as the reference electrode, forming a three-electrode system. The system was then subjected to a 5 mA cm⁻¹ test. -2 Constant current cathodic electrodeposition was performed at a current density of 20 minutes to obtain an anode nickel-supported AuPt alloy (AuPt / Ni(OH)2). This was then polarized in 1M KOH to generate nickel hydroxyl oxide. Specifically, cyclic voltammetry was used on an electrochemical workstation at a scan rate of 100 mV / s. -1 The mixture was cycled 20 times within a voltage range of 0V to 0.8V versus Ag / AgCl to obtain (AuPt / NiOOH).

[0040] The scanning electron microscope image of the obtained catalyst AuPt / NiOOH is shown below. Figure 2 As shown, by Figure 2 It can be seen that the AuPt alloy has a granular morphology.

[0041] (2) Preparation of electrolyte

[0042] The electrolyte at the anode end is prepared as a mixed solution of 50 mM 1,2-propanediol and 1 M potassium hydroxide.

[0043] The cathode electrolyte was prepared as a mixed solution of 50 mM 1,2-propanediol, 0.3 M sodium bisulfate, and 2 mM ferrous sulfate.

[0044] (3) Electrocatalytic reaction

[0045] An oxidation potential of 1.1V vs. RHE is directly applied to the anode. Figure 3 This is a linear polarization curve of the electrocatalytic oxidation of 1,2-propanediol using the anolyte catalyst. The curve shows that at an oxidation potential of 1.1 V vs. RHE, the AuPt / NiOOH catalyst can achieve an oxidation potential greater than 250 mA cm⁻¹. -2 The current density.

[0046] Oxygen is introduced into the cathode at a flow rate of 20 mL / min.

[0047] (4) Product testing

[0048] After the reaction was complete, 300 μL of electrolyte was drawn from each of the anode and cathode, followed by 100 μL of deuterium water, then 200 μL of water, and finally 50 μL of DMSO (2 mM). The mixture was then placed in an NMR tube for NMR analysis. The NMR results were... Figure 4 This indicates that lactic acid is generated in all anolyte products. Figure 5 This indicates that lactic acid is also generated in the cathode products.

[0049] Example 2

[0050] (1) Preparation of catalyst

[0051] ① Hydrothermal preparation of nickel foam supported Au catalyst

[0052] First, the nickel foam is pretreated by ultrasonicating it in acetone for 15 minutes, rinsing it with deionized water, then ultrasonicating it in dilute hydrochloric acid (3M) for 15 minutes, rinsing it with deionized water, and finally ultrasonicating it in deionized water for 15 minutes and then drying it quickly to obtain the pretreated nickel foam.

[0053] Then, prepare 20mM chloroauric acid. In a 20ml glass bottle, add 3ml of the prepared chloroauric acid, 30mg of polyether, 50µL of ethanol and 50µL of hydrochloric acid (6M), and then put in 1cm*2cm*1mm nickel foam. Place it at room temperature for 12h, take it out and rinse it repeatedly with deionized water and ethanol, and quickly vacuum dry it for later use to obtain Au / Nifoam catalyst.

[0054] The scanning electron microscope image of the obtained Au / Ni foam anode catalyst is shown below. Figure 6 As shown, by Figure 6 It can be seen that the Au / Nifoam catalyst has a dendritic morphology.

[0055] (2) Preparation of electrolyte

[0056] The electrolyte at the anode end is prepared as a mixed solution of 50 mM 1,2-propanediol and 1 M potassium hydroxide.

[0057] The cathode electrolyte was prepared as a mixed solution of 50 mM 1,2-propanediol, 0.3 M sulfuric acid, and 2 mM ferrous sulfate.

[0058] (3) Electrocatalytic reaction

[0059] Figure 7 This is a linear polarization curve of the electrocatalytic oxidation of 1,2-propanediol using the anolyte catalyst. The curve shows that at an oxidation potential of 1.25 V vs. RHE, the Au / Ni foam catalyst can achieve an oxidation potential greater than 150 mA cm⁻¹. -2 The current density.

[0060] Oxygen is introduced into the cathode at a flow rate of 20 mL / min.

[0061] (4) Product testing

[0062] After the reaction was completed, 300 μL of electrolyte was drawn from both the anode and cathode, 100 μL of deuterium water was added, then 200 μL of water was added, and then 50 μL of DMSO (2 mM) was added. The mixture was then placed in an NMR tube for NMR detection.

[0063] Example 3

[0064] (1) Preparation of catalyst

[0065] ① Hydrothermal preparation of nickel foam supported AuPt alloy catalyst

[0066] First, the nickel foam is pretreated by ultrasonicating it in acetone for 15 minutes, rinsing it with deionized water, then ultrasonicating it in dilute hydrochloric acid (3M) for 15 minutes, rinsing it with deionized water, and finally ultrasonicating it in deionized water for 15 minutes and then drying it quickly to obtain the pretreated nickel foam.

[0067] Then, prepare 20mM chloroauric acid and 20mM chloroplatinic acid. In a 20ml glass bottle, add 2.25ml of the prepared chloroauric acid and 0.75ml of chloroplatinic acid, add 30mg of polyether, add 50µL of ethanol and 50µL of hydrochloric acid (6M), and then put in 1cm*2cm*1mm nickel foam. Place it at room temperature for 12h, take it out and rinse it repeatedly with deionized water and ethanol, and quickly vacuum dry it for later use to obtain AuPt / Ni foam catalyst.

[0068] The scanning electron microscope image of the obtained anode catalyst AuPt / Ni foam is shown below. Figure 8 As shown, by Figure 8 It can be seen that the AuPt / Nifoam catalyst exhibits a nanoflower morphology. The XRD results of the catalyst are as follows: Figure 9 As shown in the spectrum, the catalyst corresponds to Pt. 0.2 Au 0.8 The alloy crystal phase.

[0069] (2) Preparation of electrolyte

[0070] The electrolyte at the anode end is prepared as a mixed solution of 50 mM 1,2-propanediol and 1 M potassium hydroxide.

[0071] The cathode electrolyte was prepared as a mixed solution of 50 mM 1,2-propanediol, 0.3 M perchloric acid, and 2 mM ferrous sulfate.

[0072] (3) Electrocatalytic reaction

[0073] Figure 10 This is a linear polarization curve of the electrocatalytic oxidation of 1,2-propanediol using the anolyte catalyst. The curve shows that at an oxidation potential of 0.9 V vs. RHE, the AuPt / Ni foam catalyst can achieve an oxidation potential greater than 350 mA / cm². -2 The current density.

[0074] (4) Product testing

[0075] After electrolysis at a constant potential of 0.4V vs. RHE for one hour, 300 μL of electrolyte was extracted, followed by the addition of 100 μL of deuterium water, 200 μL of water, and 50 μL of DMSO (2 mM). The solution was then placed in an NMR tube for NMR detection. The results were calculated and plotted as shown in the figure below. Figure 11 As shown, the lactate selectivity at the cation extreme is greater than 80%.

[0076] Example 4

[0077] (1) Electrode solution preparation

[0078] Control group 1: The anode electrolyte was prepared as a mixed solution of 50 mM 1,2-propanediol and 1 M potassium hydroxide. The cathode electrolyte was prepared as 1 M potassium hydroxide, and the volume of both the anode and cathode electrolytes was 30 ml.

[0079] Control Group 2: The anode electrolyte was prepared as a mixed solution of 50 mM 1,2-propanediol and 1 M potassium hydroxide. The cathode electrolyte was prepared as a mixed solution of 50 mM 1,2-propanediol, 0.3 M sodium bisulfate, and 2 mM ferrous sulfate. The volume of both the anode and cathode electrolytes was 30 ml.

[0080] (2) Electrocatalyst

[0081] Both the anode and cathode used the nickel foam-supported AuPt alloy catalyst prepared in Example 3.

[0082] (3) Electrocatalytic reaction

[0083] A cell voltage of 1V was applied to both control group 1 and control group 2 electrolytic cells, and electrolysis was performed at a constant potential for two hours. No oxygen was introduced into the cathode of control group 1, while oxygen was introduced into the cathode of control group 2 at a flow rate of 20 ml / min for two hours. Cyclic stability tests were conducted. After the two-hour electrolysis was completed, the test was repeated five times without changing the electrodes or the electrolyte.

[0084] (4) Product testing

[0085] After two hours of constant potential electrolysis at a cell voltage of 1V, 300 μL of electrolyte was extracted from the anode and cathode of both control groups 1 and 2. Then, 100 μL of deuterium water, 200 μL of water, and 50 μL of DMSO (2 mM) were added, and the mixture was placed in an NMR tube for NMR detection. The results were then calculated and plotted as shown in the figure. Figure 12 As shown, the lactic acid production in control group 2 is greater than that in control group 1, indicating that the rate of lactic acid production by anode-cathode coupling is greater than that of simple anode reaction, which means that the anode-cathode coupling strategy can improve energy utilization efficiency.

Claims

1. A method for producing lactic acid via electrocatalytic anode-cathode coupling, characterized in that, Includes the following steps: (1) Assemble the electrolytic cell: Use the anode catalyst as the anode and the cathode catalyst as the cathode. Place the anode and cathode in the corresponding electrolytes respectively to assemble the electrolytic cell. (2) Electrocatalytic reaction: 1,2-propanediol was added to both the anolyte and the catholyte; under a certain voltage, 1,2-propanediol was oxidized to lactic acid at the anode; oxygen was continuously introduced into the catholyte, and ferrous compounds were added to the electrolyte to catalyze the formation of hydroxyl radicals, thereby achieving the electroconversion of 1,2-propanediol and oxygen to prepare lactic acid; among which, The anode catalyst is one or more of the following: monometallic / conductive substrate, bimetallic / conductive substrate, bimetallic / hydroxide, and bimetallic / oxide. The hydroxide is one of nickel-based, cobalt-based, copper-based, and iron-based hydroxides; and the oxide is one of nickel oxide, cobalt oxide, iron oxide, and copper oxide. The metal on the anode catalyst is one or a combination of two of the following: platinum, gold, silver, ruthenium, palladium, and copper. The divalent iron compound is at least one of ferrous sulfate, ferrous nitrate, and ferrous oxalate. The anolyte is any one of the following aqueous solutions: sodium hydroxide, potassium hydroxide, lithium hydroxide, potassium bicarbonate, potassium carbonate, sodium bicarbonate, and sodium carbonate. The cathode electrolyte is at least one of perchloric acid, sulfuric acid, and sodium bisulfate solution.

2. The method according to claim 1, characterized in that, The conductive substrate is one of carbon cloth, carbon paper, nickel foam, nickel mesh, copper foam, and copper mesh.

3. The method according to claim 1, characterized in that, The concentration of the anolyte is 0.1~2 M.

4. The method according to claim 1, characterized in that, The concentration of the cathode electrolyte is 0.1~1 M.

5. The method according to claim 1, characterized in that, The electrocatalytic reaction in step (2) is carried out at room temperature and pressure, with an anode voltage of 0.3 V to 1.5 V vs. RHE.

6. The method according to claim 1, characterized in that, The concentration of the ferrous compound applied in step (2) is 0.05~50 mM.

7. The method according to claim 1, characterized in that, In step (2), the oxygen flow rate is 10~50 mL / min.

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