Method for preparing lactic acid by using reactor with porous solid electrolyte

Abstract

The invention discloses a method for preparing lactic acid by using a reactor with a porous solid electrolyte, which comprises the following steps of: catalyzing hexose to react by using alkali liquor to generate a lactate solution, then introducing the lactate solution into a middle chamber of the reactor with the porous solid electrolyte to separate, and separating the lactate under the action of an electric field to obtain the lactic acid. Hydrogen ions generated by the anode chamber of the reactor can penetrate through the proton exchange membrane to enter the middle chamber and are combined with lactate to generate lactic acid; meanwhile, positive ions in the lactate migrate to the cathode chamber and are combined with hydroxyl ions generated by a cathode hydrogen evolution reaction to be regenerated into an alkali solution, lactic acid is obtained in the middle chamber, and meanwhile the alkali solution is obtained at the cathode through recovery. According to the method, efficient preparation and separation of lactic acid are achieved, meanwhile, the obtained alkali liquor can be reused for pretreatment of hexose, material circulation is achieved, waste water and waste salt are not discharged, separation energy consumption is low, waste of alkali is avoided, the green circular economy concept is met, and carbon emission in the whole process is low.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of electrochemical reaction, and particularly relates to a method for preparing lactic acid by using a reactor with a porous solid electrolyte. BACKGROUND

[0002] Lactic acid is a vital bio-based chemical, especially as the core raw material of degradable plastic polylactic acid (PLA). PLA is an environmentally friendly material derived from renewable resources and can be completely biodegraded, and its market demand continues to grow rapidly.

[0003] The production process of lactic acid is mainly through microbial fermentation. This technology uses lactic acid bacteria (such as lactobacillus) to convert sugars such as glucose into lactic acid under mild conditions. However, lactic acid bacteria produce lactic acid during fermentation, resulting in a sharp drop in the pH of the fermentation broth. The acidic environment can inhibit or even kill the bacteria themselves. In order to maintain the optimal activity of the bacteria, it is necessary to continuously add alkaline neutralizing agents such as calcium hydroxide or calcium carbonate during fermentation. Therefore, the final product obtained in the fermentation tank is not lactic acid, but lactic acid salt (such as calcium lactate). The real challenge after obtaining the lactic acid salt fermentation broth is how to efficiently, economically and environmentally convert it into pure lactic acid product.

[0004] Currently, the classic method widely used in industry is the "acidification-filtration" process. That is, an excess of sulfuric acid is added to the lactic acid salt solution to generate lactic acid and calcium sulfate (i.e., gypsum). The solubility of calcium sulfate in water is very low, and it will precipitate in the form of solid, which can be separated from the lactic acid solution by filtration. However, this step brings serious problems: for every ton of lactic acid produced, more than 1 ton of wet gypsum residue is produced as a byproduct. These waste residues contain residual organic acids, sugars and proteins, and are low in quality and difficult to handle. Large amounts of accumulation not only occupy land, but also can cause soil alkalization and water pollution. In addition, this process is long, and after acidification and filtration, the obtained crude lactic acid solution is low in concentration and contains impurities, and still needs to go through multiple refining processes such as activated carbon decolorization, ion exchange resin impurity removal, and high-energy consumption evaporation concentration and molecular distillation, to obtain a product that meets the standard. The entire process not only results in high production costs, but also causes serious resource inefficiency, and the large amount of neutralizing agent and sulfuric acid consumed cannot be recovered.

[0005] In recent years, research on the chemical conversion of sugars such as glucose to lactic acid using alkali catalysts has made progress, but lactic acid salt is still obtained after the reaction, and an acidification step is required, which does not fundamentally solve the problem of gypsum byproduct and high-salt wastewater. Therefore, developing a green lactic acid preparation technology that does not consume acid, does not discharge waste salt, and can simultaneously recover alkali, has become an important development direction for the bio-based material industry to reduce costs and increase efficiency and sustainable development. SUMMARY

[0006] The present application is proposed to overcome the shortcomings in the prior art, and aims to provide a method for preparing lactic acid by using a reactor with a porous solid-state electrolyte.

[0007] The present application is realized by the following technical solutions: The present application is realized by the following technical solutions: S1, hydrolyzing glucose and / or fructose with an alkaline solution to obtain a lactate solution; The method for hydrolysis specifically comprises: adding glucose and / or fructose into an alkaline solution and heating and stirring for a period of time; The alkaline solution is a LiOH aqueous solution, a NaOH aqueous solution, a KOH aqueous solution, or a CsOH aqueous solution; The molar ratio of glucose and / or fructose to alkali in the alkaline solution is (0.2 ~ 1): 1; The concentration of the alkaline solution is 0.1 M ~ 8 M; The pH of the alkaline solution is 12 ~ 14; The hydrolysis temperature is 60 °C ~ 100 °C; The reaction time is 6 h ~ 24 h; S2, passing the lactate solution into the middle chamber of a solid-state electrolyte reactor to perform electrolytic separation, obtaining lactic acid in the middle chamber, and recovering the alkaline solution at the cathode.

[0008] The solid-state electrolyte reactor can adopt a mature commercial three-chamber reactor, which can be a three-chamber flow cell for gas-phase electrocatalytic reaction evaluation or a solid-state electrolytic tank, as long as it meets the three-chamber structure and the middle chamber is filled with solid-state electrolyte; During the electrolytic separation process, the anode of the solid-state electrolyte reactor undergoes oxygen evolution reaction or hydrogen oxidation reaction; when the anode of the solid-state electrolyte reactor undergoes oxygen evolution reaction, circulating water is introduced into the anode, and the flow rate is 0.5 mL·min -1 ~ 30 mL·min -1 ; when the anode of the solid-state electrolyte reactor undergoes hydrogen oxidation reaction, dry hydrogen is introduced into the anode, and the flow rate is 10 sccm ~ 60 sccm; During the electrolytic separation process, the cathode of the solid-state electrolyte reactor undergoes hydrogen evolution reaction, and the cathode is introduced into single-pass or circulating deionized water, and the flow rate is set to 0.01 mL·min -1 ~ 30 mL·min -1 ; The cathode of the solid-state electrolyte reactor is a cathode catalyst loaded on a conductive substrate, and the cathode catalyst is ruthenium oxide or platinum carbon, and the loading amount is 0.2 mg·cm-2 5 mg·cm -2 ; The anode of the solid-state electrolysis reactor is an anode catalyst supported by a conductive substrate, the anode catalyst is platinum carbon, platinum oxide or iridium oxide, and the loading amount is 0.2 mg·cm -2 5 mg·cm -2 ; The conductive substrate is carbon paper, titanium felt or foamed nickel; The cathode and the anode of the solid-state electrolysis reactor are prepared by using a catalyst-coated membrane or a catalyst-coated substrate; The cation exchange membrane of the solid-state electrolysis reactor is any one of Nafion N115, Nafion N117, CMB, CXP-S or SIN-D117; The proton exchange membrane of the solid-state electrolysis reactor is Nafion N115 or Nafion N117; The gas diffusion electrode of the solid-state electrolysis reactor is a commercially available gas diffusion electrode subjected to hydrophobic treatment; the gas diffusion electrode is carbon paper or titanium felt subjected to hydrophobic treatment; The intermediate chamber of the solid-state electrolysis reactor is filled with a porous solid-state electrolyte; the porous solid-state electrolyte is an ion exchange resin with high conductivity; the porous solid-state electrolyte is Amberlite IR120, Amberlite IRN77 or Dowex 50WX2; The lactic acid salt solution is introduced into the intermediate chamber of the solid-state electrolysis reactor in a single-pass or circulation mode, and the flow rate is 0.005 mL·min -1 15 mL·min -1 ; The method of electrolytic separation is a constant voltage method or a constant current method; The voltage of the constant voltage method is 1 V ~ 5 V; The current density of the constant current method is 50 mA·cm -2 500 mA·cm -2 ; The reaction time of the electrolytic separation is 100%-400% of the theoretical reaction time; The temperature of the electrolytic separation is 10 °C ~ 80 °C; The alkali solution recovered by the cathode in the step S2 is used for the alkali hydrolysis process of glucose and / or fructose in the step S1, so that a circular economy is realized.

[0009] The beneficial effects of the present application are: The application provides a method for preparing lactic acid by using a reactor with a porous solid-state electrolyte, is a green lactic acid preparation technology without acid consumption, without solid waste discharge, and with simultaneous recovery of lye, ingeniously utilizes electrochemical driving to realize direct and efficient separation of lactic acid salt in the reactor, avoids the use of sulfuric acid and the generation of gypsum waste from the source, and simultaneously recovers high-value lye while producing lactic acid, which can be directly returned to the hydrolysis process for recycling, forming a perfect material closed loop, greatly reducing raw material consumption and waste treatment cost.

[0010] The reaction system of the application has great potential for greenization, waste reduction and resource recycling, and marks that the lactic acid production process is moving towards a more efficient, clean and sustainable important development direction. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 is a data graph of the influence of hydrolysis temperature and flow rate on the preparation of lactic acid in test example 1 of the application; Figure 2 is the influence of reaction temperature on the voltage, current efficiency and power consumption of lactic acid salt solution in the separation process of the electrochemical reactor in test example 2 of the application.

[0012] For those skilled in the art, other related drawings can be obtained from the above drawings without creative labor. DETAILED DESCRIPTION

[0013] In order to enable those skilled in the art to better understand the technical scheme of the application, the technical scheme of the application will be further described below in combination with the drawings of the specification and through specific embodiments.

[0014] In the embodiments of the application, the separation efficiency of lactic acid, the electromigration efficiency of cations and the concentration of alkali solution are calculated by liquid phase and ICP test analysis.

[0015] Before electrolytic separation, the central chamber is first supplied with potassium sulfate solution to perform cyclic voltammetry test on the reactor to activate the catalyst; the potential is set to 0 V~−5 V, the scanning rate is 100 mV / s, and the cyclic test is performed for 5 cycles.

[0016] Embodiment 1 A method for preparing lactic acid by using a reactor with a porous solid-state electrolyte, specifically comprising: S1, respectively configure 50 mL of 5 M NaOH solution and 2 M glucose solution in an 80 °C water bath, set the flow rate to 0.7 mL / min -1 The two solutions are mixed for reaction, and the outlet is connected to the middle chamber of the solid-state electrolyte reactor. S2, Pt / C catalyst was sprayed on the raw carbon paper and hydrophobic carbon paper respectively, and the loading amount was 1.5 mg cm -2 As the cathode and anode of the intermediate chamber of the solid-state electrolyte reactor, hydrogen evolution reaction and hydrogen oxidation reaction were carried out; the intermediate chamber was filled with Dowex 50WX2 as the solid-state electrolyte, Nafion 117 was selected as the proton exchange membrane, and SIN D117 was selected as the cation exchange membrane, and the reactor was assembled; The cathode was connected to circulating 100 mL deionized water, and the flow rate was set to 15 mL·min -1 ; The anode was connected to hydrogen gas, and the flow rate was 30 sccm; The solid-state electrolyte reactor was heated by a heating rod, and the temperature was set to 80 °C; S3, first, the catalyst was activated by cyclic voltammetry test, then the constant current electrolysis was carried out at 250 mA·cm -2 The current density, the intermediate chamber was directly connected to the glucose hydrolysate obtained in situ in step S1 to realize the preparation of lactic acid solution from glucose, and the separation efficiency of sodium lactate could reach 81%, and the concentration of lactic acid solution was 0.58 M; At the same time, about 2 M NaOH alkaline solution was recovered in the cathode, which could be directly used in the alkaline hydrolysis process of glucose in step S1.

[0017] Example 2 A method for preparing lactic acid by using a reactor with a porous solid-state electrolyte, specifically: S1, 50 mL of 5 M KOH solution and 2 M fructose solution were prepared respectively, the two solutions were directly mixed, and the mixture was placed in a 80 °C water bath for reaction for 0.5 h to obtain a lactate solution; S2, Pt / C catalyst was sprayed on the raw carbon paper and hydrophobic carbon paper respectively, and the loading amount was 1.2 mg cm -2 As the cathode and anode of the solid-state electrolyte reactor, hydrogen evolution reaction and hydrogen oxidation reaction were carried out; the intermediate chamber was filled with Dowex 50WX8 as the solid-state electrolyte, Nafion 117 was selected as the proton exchange membrane, and SIN D117 was selected as the cation exchange membrane, and the reactor was assembled; the cathode was connected to circulating deionized water, and the flow rate was set to 15 mL·min -1 ; The anode was connected to hydrogen gas, and the flow rate was 20 sccm; the solid-state electrolyte reactor was heated by a heating rod, and the temperature was set to 80 °C; S3, first, the catalyst was activated by cyclic voltammetry test, then the constant current electrolysis was carried out at 250 mA·cm -2The constant current electrolysis was carried out on the fructose hydrolysate obtained in situ in the intermediate chamber in step S1 at a current density, so that the preparation of a lactic acid solution from fructose was realized, the separation efficiency of sodium lactate can reach 81%, and a 0.58 M lactic acid solution is obtained; meanwhile, the alkaline solution recovered at the cathode can be directly used in the alkaline hydrolysis process of fructose.

[0018] Test Example 1 Determination of reasonable range of hydrolysis temperature and flow rate: The reaction of fructose hydrolysis for preparing lactic acid was tested for a suitable hydrolysis temperature, specifically: S1, 40 mL of 5 M KOH solution and 2 M fructose solution were respectively configured; the solutions were placed in a water bath at different temperatures of 80 °C~100 °C, and the KOH and lactic acid solutions were pumped out at a flow rate of 0.7 mL min -1 ~1.0 mL min -1 to mix and react; S2, the outlet sample was directly introduced into the configured 0.5 M sulfuric acid solution for acidification, and the sample was taken for testing to quantify the effect of fructose hydrolysis for preparing lactic acid.

[0019] S3, according to the results, it was found that when the reaction was at 90 °C and the flow rate was set to 0.7 mL min -1 , the yield of fructose hydrolysis for preparing lactic acid can reach 53.3%. At the same time, the obtained lactic acid salt concentration is 0.53 M, and the results are as follows Figure 1 .

[0020] It can be seen from Figure 1 that when the reaction temperature increases from 80 °C to 100 °C, the selectivity and yield of lactic acid show a trend of first increasing and then decreasing, which is due to the fact that higher temperature leads to more side reactions; when the flow rate increases from 0.7 mL min -1 to 1.0 mL min -1 , the selectivity and yield of the reaction show a general downward trend, which is because the flow rate is too fast, resulting in incomplete conversion of part of the reactants; therefore, in order to obtain a better lactic acid yield and reactant conversion rate, it is necessary to balance the effects of temperature and reaction flow rate.

[0021] Test Example 2 Determination of flow rate range of lactic acid salt solution in the separation process of the reactor: The sodium lactate solution was introduced into the central chamber of the solid electrolyte reactor used in the application to separate the sodium lactate solution to prepare lactic acid and recover alkali, specifically: S1, Pt / C catalyst was sprayed on the carbon paper and hydrophobic carbon paper, respectively, and the loading amount was 1.5 mg cm -2As the anode and cathode of the solid-state electrolytic reactor, hydrogen evolution reaction and hydrogen oxidation reaction were carried out; the intermediate chamber was filled with Dowex 50WX2 as the solid-state electrolyte, and Nafion117 was selected as the proton exchange membrane and cation exchange membrane.

[0022] S2, deionized water was introduced into the cathode, and the flow rate was 3 mL·min -1 ; hydrogen was introduced into the anode, and the flow rate was 20 sccm; 1 M sodium lactate solution was introduced into the intermediate chamber, and the flow rate was set to 0.25-4 times the theoretical reaction flow rate to explore the effect of flow rate on separation; the theoretical reaction flow rate was calculated according to the concentration and current density; First, the catalyst was activated by cyclic voltammetry test, and then the separation test was carried out under the condition of constant current of 500 mA; It was found that the transfer efficiency of Na + could reach more than 80% under the theoretical reaction flow rate, and NaOH solution was recovered in the cathode, while the separation efficiency of sodium lactate could reach more than 80%, realizing the separation of sodium lactate into acid and base.

[0023] Test example 3 The disadvantages of the range of electrolytic separation temperature are as follows: S1, Pt / C catalyst was sprayed on carbon paper and hydrophobic carbon paper, and the loading amount was 1.5 mg cm -2 As the anode and cathode of the porous solid-state electrolytic reactor, hydrogen evolution reaction and hydrogen oxidation reaction were carried out; the intermediate chamber was filled with Dowex 50WX2 as the electrolyte, Nafion117 was selected as the proton membrane, and SIN D-117 was selected as the cation exchange membrane to assemble the reactor. Deionized water was introduced into the cathode, and the flow rate was 2 mL·min -1 ; hydrogen was introduced into the anode, and the flow rate was 30 sccm; 1 M sodium lactate solution was introduced into the intermediate chamber, and the reactor was heated by a heating rod, and the temperature was set to 25 °C, 40 °C, 60 °C and 80 °C respectively to explore the sodium lactate separation effect of the reactor under different temperatures; S2, first, the catalyst was activated by cyclic voltammetry test, and then constant current test was carried out in the range of 10-200 mA·cm -2 , and the results are shown in Figure 2 , it was found that the transfer efficiency of Na + increased, the separation efficiency of sodium lactate improved, and when the current density was 10 mA·cm -2 , the separation efficiency could be more than 95%, and the power consumption was only 0.02 kWh mol LA -1 .

[0024] The working principle of the solid electrolyte reactor for electrolytic separation in the application is as follows: The anode undergoes oxygen evolution reaction or hydrogen reduction reaction to obtain H + , H + will pass through the proton exchange membrane to the porous solid electrolyte layer, and combine with the input lactate to form lactic acid, while the cation will pass through the cation exchange membrane to the cathode under the action of the electric field, and combine with OH - generated in situ by the hydrogen evolution reaction of the cathode to obtain alkali, so as to realize the separation from lactate to lactic acid and the purpose of recovering alkali solution in the cathode.

[0025] The applicant declares that the above description is only a specific embodiment of the application, but the protection scope of the application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the application can be easily thought of by any person skilled in the art, and all fall within the protection scope and disclosure scope of the application.

Claims

1. A method for preparing lactic acid using a reactor with a porous solid electrolyte, characterized in that: Includes the following steps: S1. Hydrolyze glucose and / or fructose using an alkaline solution to obtain a lactate solution; S2. The lactate solution is passed into the intermediate chamber of the solid electrolyte reactor for electrolytic separation. Lactic acid is obtained in the intermediate chamber, and an alkaline solution is recovered at the cathode.

2. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 1, characterized in that: The alkaline solution is an aqueous solution of LiOH, NaOH, KOH, or CsOH; the concentration of the alkaline solution is 0.1 M to 8 M; the pH of the alkaline solution is 12 to 14; and the hydrolysis temperature is 60 °C to 100 °C.

3. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 1, characterized in that: During the electrolytic separation process, an oxygen evolution reaction or a hydroxide reaction occurs at the anode of the solid electrolyte reactor; Hydrogen evolution reaction occurs at the cathode.

4. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 3, characterized in that: When the oxygen evolution reaction occurs at the anode of the solid electrolyte reactor, circulating water is introduced into the anode at a flow rate of 0.5 mL / min. -1 ~ 30 mL·min -1 When the hydroxide reaction occurs at the anode of the solid electrolyte reactor, dry hydrogen gas is introduced into the anode at a flow rate of 10 sccm to 60 sccm; deionized water, either single-pass or circulating, is introduced into the cathode at a flow rate of 0.01 mL / min. -1 ~ 30 mL·min -1 .

5. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 1, characterized in that: The cathode of the solid electrolyte reactor is a cathode catalyst supported on a conductive substrate. The cathode catalyst is ruthenium oxide or platinum on carbon, and the loading of the cathode catalyst is 0.2 mg / cm³. -2 ~ 5 mg·cm -2 The anode is a conductive substrate supported on an anode catalyst, which is platinum-carbon, platinum oxide, or iridium oxide, and the catalyst loading is 0.2 mg / cm³. -2 ~ 5 mg·cm -2 The conductive substrate is carbon paper, titanium felt, or nickel foam; the cation exchange membrane is any one of Nafion N115, Nafion N117, CMB, CXP-S, or SIN-D117; the proton exchange membrane is Nafion N115 or Nafion N117; the gas diffusion electrode is hydrophobically treated.

6. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 1, characterized in that: The intermediate chamber of the solid electrolyte reactor is filled with a porous solid electrolyte, which is an ion exchange resin with high conductivity.

7. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 6, characterized in that: The porous solid electrolyte is Amberlite IR120, Amberlite IRN77, or Dowex 50WX2.

8. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 1, characterized in that: The lactate solution is introduced into the intermediate chamber of the solid electrolyte reactor via either single-pass or circulating inlet, with an inlet flow rate of 0.005 mL / min. -1 ~ 10 mL·min -1 .

9. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 1, characterized in that: The electrolytic separation method is either a constant voltage method or a constant current method; the voltage for the constant voltage method is 1 V to 5 V; the current density for the constant current method is 50 mA·cm⁻¹. -2 ~500 mA·cm -2 The reaction time for the electrolytic separation is 100% to 400% of the theoretical reaction time.

10. The method for preparing lactic acid using a reactor with a porous solid electrolyte according to claim 1, characterized in that: The alkaline solution recovered from the cathode in step S2 is used in the alkaline hydrolysis process of glucose and / or fructose in step S1.

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