Method for preparing lactic acid by catalyzing hemicellulose with monatomic catalyst

Abstract

The invention belongs to the technical field of catalyst preparation of chemicals, and relates to a method for preparing lactic acid by catalyzing hemicellulose with a monatomic catalyst, which comprises the following steps: preparing the monatomic catalyst, fully mixing hemicellulose, the monatomic catalyst and deionized water, carrying out catalytic reaction, cooling, and collecting lactic acid reaction liquid. The biomass carbon-based monatomic catalyst provided by the invention has the advantages of high specific surface area, high contact property, low metal content, high atom utilization rate, abundant active sites and high catalytic efficiency, can realize the lactic acid yield up to 66%, and is a production and preparation process of the catalyst with low metal loading capacity, high catalytic activity and recoverability; the preparation method is green, mild and efficient, and is simple in process and safe to operate. In the whole process steps, various complex technologies are avoided, the yield of lactic acid is high, and the repeated use effect of the catalyst is good.

Description

Technical Field

[0001] The present invention relates to the technical field of preparing high - value - added chemicals by chemical catalysis of hemicellulose with a catalyst, and particularly relates to a method for preparing lactic acid by catalyzing hemicellulose with a single - atom catalyst. Background Art

[0002] A single - atom catalyst is a catalyst in which metal atoms are dispersed at the atomic level. The single - atom catalyst combines the advantages of homogeneous catalysts and heterogeneous catalysts, has high catalytic activity, selectivity and stability, and can significantly reduce metal consumption and economic costs while ensuring high activity. Biomass carbon materials have abundant metal - atom anchoring sites, can promote the dispersion of metal atoms, and have high stability, and are an ideal ligand material for single - atom catalysts. In recent years, single - atom catalysts have attracted extensive attention in the conversion of biomass - derived platform chemicals due to their high atomic utilization rate and high catalytic efficiency.

[0003] Lactic acid is one of the three important organic acids recognized worldwide. As an important biomass - derived intermediate platform molecule, it can be used to produce fine chemicals and is widely used in fields such as energy, daily chemicals, medicine, chemical engineering and surface materials, and has high application value. In addition, lactic acid can be polymerized to form polylactic acid, which is used to produce biodegradable plastics and can effectively solve the pollution problem caused by global plastics. At present, the market demand for lactic acid is increasing day by day, but the production cost of the traditional fermentation method is relatively high, and using biomass raw materials to produce lactic acid is an effective way.

[0004] Hemicellulose is the main component of lignocellulose, with rich reserves and low acquisition cost. Hemicellulose is mainly composed of polysaccharides composed of pentoses and hexoses, has a low molecular weight and an amorphous structure, and can be converted into important platform compounds such as lactic acid and furfural. Screening catalysts with excellent catalytic performance and excellent stability and optimizing reaction conditions are important challenges for the mild and efficient conversion of hemicellulose into lactic acid.

[0005] In view of this, it is urgent to develop a catalyst with high catalytic activity and good selectivity to achieve the efficient conversion of hemicellulose to prepare lactic acid. Summary of the Invention

[0006] The purpose of the present invention is to develop a preparation method of a biomass - carbon - based single - atom catalyst with excellent catalytic performance to achieve the efficient conversion of hemicellulose into lactic acid, aiming to improve the selectivity and efficiency of the conversion of hemicellulose into lactic acid under the conditions of no external pressure and no external acid or base, and to achieve the efficient and mild conversion of hemicellulose into lactic acid.

[0007] The present application provides a method for preparing lactic acid by catalyzing hemicellulose with a single - atom catalyst, comprising the following steps:

[0008] S1: Disperse the biomass in deionized water, and then add NaOH to fully dissolve the biomass to obtain a biomass solution;

[0009] S2: Fully dissolve the metal salt and zinc salt in deionized water to obtain a metal ion solution; the molar ratio of the metal salt to the zinc salt is 1:(5 - 12); the metal salt is one of chloride salts, nitrate salts, sulfate salts, and acetate salts; the zinc salt is one of chloride salts, sulfate salts, and acetate salts;

[0010] S3: According to the mass ratio of 1:1.25 of the biomass solution to the metal ion solution, slowly add the metal ion solution to the biomass solution under magnetic stirring, and then continuously stir for 0.5 - 6 h; obtain a mixed solution; the temperature of the stirring is 25 - 70 °C; the dropping rate is 1 - 50 mL / min;

[0011] S4: Let the mixed solution in S3 age statically overnight, then centrifuge and freeze-dry to collect the composite; fully grind and mix the obtained composite with the nitrogen-containing compound in a mortar; the nitrogen-containing compound includes but is not limited to one of urea, melamine, and dicyandiamide;

[0012] S5: Place the mixture in S4 in a combustion boat, pyrolyze it at 550 °C for 1 - 2 h and at 900 - 1000 °C for 1 - 2 h in a tubular furnace, with a heating rate of 2 - 15 °C / min and an N2 atmosphere; naturally cool to room temperature, fully grind and then collect to obtain a single-atom catalyst; the metal atom content of the obtained single-atom catalyst is <1%;

[0013] S6: Fully mix hemicellulose, the single-atom catalyst, and deionized water in a mass ratio of (0.3 - 0.5):(0.1 - 0.3):100 and load them into a high-temperature and high-pressure reactor; seal the high-temperature and high-pressure reactor and purge and replace the air in the reactor with an inert gas; start the catalytic reaction under the set temperature and stirring speed conditions, and keep the reaction at a constant temperature when the set temperature is reached; after the reaction ends, stop heating and stirring, cool to room temperature, release the pressure, and collect the lactic acid reaction solution.

[0014] Furthermore, the biomass in S1 is one of lignin and lignin-carbohydrate complexes.

[0015] Furthermore, the concentration of the biomass in the biomass solution in S1 is 0.3 - 1 wt%.

[0016] Furthermore, the metal ion in the metal salt in S2 is Ni 2+ 、Co 2+ 、Sn 2+ One of them.

[0017] Further, the concentration of the metal salt in the metal ion solution of S2 is 0.6 - 2 mmol / L.

[0018] Further, the mass ratio of the complex to the nitrogen-containing compound in S4 is 1:(1 - 15).

[0019] Further, the hemicellulose in S6 includes but is not limited to one of xylan and xylose.

[0020] Further, the set temperature in S6 is 240 - 260 °C and the stirring speed is 500 rpm.

[0021] Further, the constant temperature reaction time in S6 is 0.5 - 6 h.

[0022] The present invention uses a heterogeneous chemical catalyst to convert lignocellulose into lactic acid under certain conditions, which has the advantages of low cost, high product concentration, low corrosion, and can be reused repeatedly. As an important component in the field of heterogeneous catalysis, single-atom catalysts show the high activity comparable to homogeneous catalysts and the characteristics of heterogeneous catalysts being stable and easy to separate. Using biomass raw materials rich in functional groups as carriers can effectively coordinate metal ions, thus providing anchoring sites for single atoms and endowing the single-atom catalysts with high stability. Introducing zinc element as a spacer for metal atoms and introducing nitrogen element can effectively increase the specific surface area of the single-atom catalyst, improve the dispersion and stability of single atoms, and maximize the utilization rate of metal atoms, thus providing guarantee for the high yield of lactic acid. The isolated active centers in the single-atom catalyst can form an accurate adsorption configuration with the reaction sites of the substrate, and promote the desorption of the product on the catalyst surface, improving the catalytic efficiency. Under high-temperature hydrothermal conditions, single-atom metal sites can generate protons by interacting with water molecules to attack the oxygen atom in the glycosidic bond, promoting the hydrolysis reaction of hemicellulose to generate monosaccharides. In addition, the single-atom catalyst can also provide Lewis acid sites to adsorb and polarize the oxygen atom in the glycosidic bond, thus promoting the cleavage of the bond to generate monosaccharides. Monosaccharides will form an intermediate enol structure at high temperature and further isomerize into the corresponding ketoses. Metal single atoms have highly exposed active sites and unique electronic structures, which can not only stabilize the transition state but also lower the energy barrier in the process of converting monosaccharides into the corresponding ketoses. Subsequently, the metal single-atom catalyst coordinates with the hydroxyl and carbonyl groups in the sugar molecule, thereby activating the cleavage of the carbon-carbon bond, causing the ketose to undergo a retro-aldol condensation reaction to break and generate C3 intermediates (glyceraldehyde or dihydroxyacetone). Metal single atoms can provide reaction active sites for the dehydration and rearrangement of C3 intermediates, and can also stabilize the key reaction intermediates by regulating electron transfer and proton migration, thus accelerating the formation of the final lactic acid.

[0023] The beneficial effects of the present invention are as follows:

[0024] 1. The present invention successfully prepared a biomass carbon-based single-atom catalyst by pyrolysis using biomass as a carrier, a dispersant, and a ligand, which improved the atomic dispersion, enhanced the reaction activity of the catalyst, and reduced the amount of metal required. Functional groups such as hydroxyl, carboxyl, and sulfonic acid groups in biomass can coordinate with metal ions to form complexes. Biomass carbon has abundant metal ion anchoring sites and a large specific surface area, with good dispersion and high stability.

[0025] 2. The biomass carbon-based single-atom catalyst provided by the present invention has a high specific surface area, high accessibility, low metal content, high atomic utilization rate, abundant active sites, and high catalytic efficiency, and can achieve a lactic acid yield of up to 66%. The method of the present invention stabilizes metal single atoms through nitrogen doping coordination, improves the stability and performance of the catalyst, further enhances the recycling service life of the catalyst, reduces the demand for fresh catalysts, and lowers the catalytic reaction cost. The lactic acid preparation system does not require external pressure, acids, or bases, is green, mild, and efficient, is beneficial to environmental protection and sustainable production, and has great application potential in industrial production, and can achieve low-cost and high-yield lactic acid production to meet market demand. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] By describing the exemplary embodiments of the present application in more detail in conjunction with the accompanying drawings, the above and other objects, features, and advantages of the present application will become more apparent. Among them, in the exemplary embodiments of the present application, the same reference numerals generally represent the same components.

[0027] Figure 1 It is a transmission electron microscope image of the biomass carbon-based single-atom catalyst of the present invention;

[0028] Figure 2 It is an XRD pattern of the biomass carbon-based single-atom catalyst of the present invention;

[0029] Figure 3 It is a graph showing the effect of different reaction temperatures on the preparation of lactic acid in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The embodiments of the present application will be described in more detail below with reference to the accompanying drawings. Although the embodiments of the present application are shown in the drawings, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present application more thorough and complete, and to fully convey the scope of the present application to those skilled in the art.

[0031] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application.

[0032] Example 1

[0033] S1: Disperse lignin in deionized water, then add NaOH to fully dissolve the biomass to obtain a biomass solution with the biomass concentration in the biomass solution being 1 wt%.

[0034] S2: Fully dissolve cobalt nitrate and zinc sulfate in deionized water to obtain a metal ion solution, where the concentration of the metal salts is 2 mmol / L; the molar ratio of the metal salts to the zinc salts is 1:9.

[0035] S3: Slowly add the metal ion solution to the biomass solution under magnetic stirring at a mass ratio of the biomass solution to the metal ion solution of 1:1.25, and then continuously stir for 2 h to obtain a mixture; the temperature of the stirring is 60 °C; the dropping rate is 10 mL / min.

[0036] S4: Static-cure the mixture in S3 overnight, then centrifuge and freeze-dry to collect the composite; fully grind and mix the obtained composite with urea in a mortar at a mass ratio of 1:9 to obtain a mixture.

[0037] S5: Place the mixture in S4 in a combustion boat, pyrolyze it at 550 °C for 1 h and at 900 °C for 1 h in a tubular furnace with a heating rate of 5 °C / min under a N2 atmosphere; naturally cool to room temperature, fully grind and then collect to obtain a single-atom catalyst; the metal atom content of the obtained single-atom catalyst is <1%.

[0038] S6: Fully mix xylan, the single-atom catalyst, and deionized water at a mass ratio of 0.3:0.1:100 and load them into a high-temperature and high-pressure reactor; seal the high-temperature and high-pressure reactor and purge and replace the air in the reactor with an inert gas; start the catalytic reaction at a set temperature of 240 °C and a stirring speed of 500 rpm, and maintain a constant temperature reaction for 1 h when the set temperature is reached; stop heating and stirring after the reaction ends, cool to room temperature, release the pressure, and collect the lactic acid reaction solution.

[0039] Analyze and detect the lactic acid concentration with a high-performance liquid chromatograph. In this example, the analyzed and detected lactic acid concentration is 1.206 mg / mL.

[0040] The yield of lactic acid is determined by the following formula:

[0041]

[0042] Among them, the conversion coefficient of xylan is 0.88; the conversion coefficient of xylose is 1. The molar mass of lactic acid is 90.08 g / mol; the molar mass of xylose is 150.13 g / mol.

[0043] In this example, xylan is used as the substrate for hemicellulose. The mass of xylan is 0.3 g, and the volume of the lactic acid reaction solution is 100 mL. According to the above formula, the lactic acid yield is calculated to be 58.91%.

[0044] Example 2

[0045] S1: Disperse lignin in deionized water, and then add NaOH to fully dissolve the biomass to obtain a biomass solution with the biomass concentration in the biomass solution being 0.3 wt%;

[0046] S2: Fully dissolve nickel acetate and zinc chloride in deionized water to obtain a metal ion solution, where the concentration of the metal salt is 0.6 mmol / L; the molar ratio of the metal salt to the zinc salt is 1:5;

[0047] S3: Slowly add the metal ion solution to the biomass solution under magnetic stirring at a mass ratio of the biomass solution to the metal ion solution of 1:1.25, and then continuously stir for 0.5 h; obtain a mixture; the temperature of the stirring is 25°C; the dropping rate is 1 mL / min;

[0048] S4: Let the mixture in S3 stand overnight for static aging, then centrifuge and freeze-dry to collect the composite; fully grind and mix the obtained composite with melamine in a mass ratio of 1:15 in a mortar to obtain a mixture;

[0049] S5: Place the mixture in S4 in a combustion boat, pyrolyze it in a tubular furnace at 550°C for 2 h and at 1000°C for 1.5 h, with a heating rate of 2°C / min and in an N2 atmosphere; naturally cool to room temperature, fully grind and then collect to obtain a single-atom catalyst; the metal atom content of the obtained single-atom catalyst < 1%;

[0050] S6: Fully mix xylan, the single-atom catalyst, and deionized water in a mass ratio of 0.3:0.1:100 and then load them into a high-temperature and high-pressure reaction kettle; seal the high-temperature and high-pressure reaction kettle, and purge and replace the air in the kettle with an inert gas; start the catalytic reaction at a set temperature of 240°C and a stirring speed of 500 rpm, and maintain a constant temperature reaction for 1 h when the set temperature is reached; after the reaction is completed, stop heating and stirring, cool to room temperature, release the pressure, and collect the lactic acid reaction solution.

[0051] Example 3

[0052] S1: Disperse lignin in deionized water, and then add NaOH to fully dissolve the biomass to obtain a biomass solution with the biomass concentration in the biomass solution being 0.5 wt%;

[0053] S2: Dissolve stannous sulfate and zinc acetate fully in deionized water to obtain a metal ion solution, where the concentration of the metal salt is 1 mmol / L; the molar ratio of the metal salt to the zinc salt is 1:12;

[0054] S3: According to the mass ratio of the biomass solution to the metal ion solution of 1:1.25, slowly add the metal ion solution to the biomass solution under magnetic stirring, and then continuously stir for 6 h; obtain a mixture; the temperature of the stirring is 70 °C; the dropping rate is 50 mL / min;

[0055] S4: Let the mixture in S3 age statically overnight, then centrifuge and freeze-dry, and collect the composite; fully grind and mix the obtained composite with dicyandiamide in a mortar according to the mass ratio of 1:1 to obtain a mixture;

[0056] S5: Place the mixture in S4 in a combustion boat, pyrolyze it at 550 °C for 1 h and at 950 °C for 2 h in a tubular furnace, with a heating rate of 15 °C / min and an N2 atmosphere; naturally cool to room temperature, fully grind and then collect to obtain a single-atom catalyst; the metal atom content of the obtained single-atom catalyst < 1%;

[0057] S6: Fully mix xylan, the single-atom catalyst, and deionized water according to the mass ratio of 0.4:0.2:100 and load them into a high-temperature and high-pressure reaction kettle; seal the high-temperature and high-pressure reaction kettle, and purge and replace the air in the kettle with an inert gas; start the catalytic reaction under the set temperature of 250 °C and a stirring speed of 500 rpm, and maintain a constant-temperature reaction for 1 h when the set temperature is reached; after the reaction ends, stop heating and stirring, cool to room temperature, release the pressure, and collect the lactic acid reaction solution.

[0058] Example 4

[0059] Same as Example 1, only replace the mass ratio of xylan, the single-atom catalyst, and deionized water in S6 with 0.5:0.3:100.

[0060] Example 5

[0061] Same as Example 1, only replace the constant-temperature reaction in S6 with 0.5 h.

[0062] Example 6

[0063] Same as Example 1, only replace the constant-temperature reaction in S6 with 6 h.

[0064] Example 7

[0065] Same as Example 1, only replace the set temperature in S6 with 260 °C.

[0066] Example 8

[0067] Same as Example 1, except that the lignin in S1 is replaced with lignin-carbohydrate complex.

[0068] Example 9

[0069] Same as Example 1, except that the xylan in S6 is replaced with xylose.

[0070] Example 10

[0071] Same as Example 1, except that the cobalt nitrate in S2 is replaced with cobalt chloride.

[0072] Comparative Example 1

[0073] Same as Example 1, delete S1 - S5, and do not use the single-atom catalyst in S6.

[0074] Comparative Example 2

[0075] Same as Example 1, replace the concentration of the metal salt in S2 with 4 mmol / L.

[0076] Comparative Example 3

[0077] Same as Example 1, except that the set temperature in S6 is replaced with 180 °C.

[0078] Comparative Example 4

[0079] Same as Example 1, except that the mass ratio of xylan, single-atom catalyst, and deionized water in S6 is replaced with 0.1∶0.025∶100.

[0080] Testing of example samples:

[0081] Measure the concentration of lactic acid with a high-performance liquid chromatograph. Calculate the ratio of the amount of substance of lactic acid to the amount of substance of hemicellulose to obtain the yield of lactic acid. The results are as follows:

[0082] Table 1 Lactic acid yields of each example and comparative example

[0083]

[0084]

[0085] According to the data in Table 1, the biomass carbon-based single-atom catalyst exhibits excellent catalytic activity for the conversion of hemicellulose into lactic acid. In Comparative Example 1, there is no catalyst in the reaction system and the lactic acid yield is extremely low, indicating that the catalyst described in the present invention has an obvious effect on the conversion of hemicellulose into lactic acid. In Comparative Example 2, increasing the metal loading of the catalyst results in a decrease in the lactic acid yield, indicating that excessive active sites at high loadings will have a negative impact on the lactic acid yield. In Comparative Example 3, the reaction system temperature is decreased, and the lactic acid yield decreases sharply with the decrease in temperature, indicating that appropriate high-temperature conditions are necessary for achieving a high lactic acid yield. In Comparative Example 4, the ratio of hemicellulose to the catalyst is increased, indicating that too little catalyst reduces the accessibility of the catalyst to the reaction substrate, thereby reducing the conversion efficiency of hemicellulose into lactic acid.

[0086] The embodiments of the present application have been described above. The above description is exemplary and not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of the terms used herein is intended to best explain the principles of the embodiments, the practical application, or the improvement of the technology in the market, or to enable other ordinary skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing lactic acid by catalyzing hemicellulose with a single-atom catalyst, characterized in that It includes the following steps: S1: Disperse the biomass in deionized water, and then add NaOH to fully dissolve the biomass to obtain a biomass solution; S2: Fully dissolve the metal salt and zinc salt in deionized water to obtain a metal ion solution; the molar ratio of the metal salt to the zinc salt is 1:(5 - 12); the metal salt is one of chloride salts, nitrate salts, sulfate salts, acetate salts; the zinc salt is one of chloride salts, sulfate salts, acetate salts; S3: According to the mass ratio of the biomass solution to the metal ion solution of 1:1.25, slowly add the metal ion solution to the biomass solution under magnetic stirring, and then continuously stir for 0.5 - 6 h; obtain a mixture; the temperature of the stirring is 25 - 70 °C; the dropping rate is 1 - 50 mL / min; S4: Let the mixture in S3 age statically overnight, then centrifuge and freeze-dry to collect the composite; fully grind and mix the obtained composite with a nitrogen-containing compound in a mortar to obtain a mixture; the nitrogen-containing compound includes but is not limited to one of urea, melamine, dicyandiamide; S5: Pyrolyze the mixture in S4 at 550 °C for 1 - 2 h, pyrolyze at 900 - 1000 °C for 1 - 2 h, with a heating rate of 2 - 15 °C / min, in an N2 atmosphere; naturally cool to room temperature, fully grind and then collect to obtain a single-atom catalyst; S6: Fully mix hemicellulose, the single-atom catalyst, and deionized water in a mass ratio of (0.3 - 0.5):(0.1 - 0.3):100, and displace the air in the autoclave with an inert gas; start the catalytic reaction under the set temperature and stirring speed conditions, and keep the constant-temperature reaction when the set temperature is reached; after the reaction ends, stop heating and stirring, cool to room temperature, release the pressure, and collect the lactic acid reaction solution.

2. The method for preparing lactic acid by catalyzing hemicellulose with the single-atom catalyst according to claim 1, characterized in that: The biomass in S1 is one of lignin and lignin-carbohydrate complex.

3. The method for preparing lactic acid by catalytic conversion of hemicellulose using the single-atom catalyst according to claim 1, wherein: The concentration of the biomass in the biomass solution in S1 is 0.3 - 1 wt%.

4. The method for preparing lactic acid by catalyzing hemicellulose with the single-atom catalyst according to claim 1, characterized in that: The metal ion in the metal salt of S2 is Ni 2+ , Co 2+ , Sn 2+ One of them.

5. The method for preparing lactic acid by catalytic conversion of hemicellulose with the single-atom catalyst according to claim 1, wherein: The concentration of the metal salt in the metal ion solution in S2 is 0.6 - 2 mmol / L.

6. The method for preparing lactic acid by catalytic conversion of hemicellulose with the single-atom catalyst according to claim 1, wherein: The mass ratio of the composite to the nitrogen-containing compound in S4 is 1:(1 - 15).

7. The method for preparing lactic acid by catalytic conversion of hemicellulose using the single-atom catalyst according to claim 1, wherein: The hemicellulose in S6 includes but is not limited to one of xylan and xylose.

8. The method for preparing lactic acid by catalytic conversion of hemicellulose using the single-atom catalyst according to claim 1, wherein: The set temperature in S6 is 240 - 260 °C and the stirring speed is 500 rpm.

9. The method for preparing lactic acid by catalytic conversion of hemicellulose with a single-atom catalyst according to claim 1, wherein: The constant-temperature reaction time in S6 is 0.5 - 6 h.

Images