A method for preparing a lignocellulose-based diatomic catalyst and its application in electrocatalytic reaction

Abstract

The application discloses a preparation method of a diatomic catalyst based on lignocellulose and application of the diatomic catalyst in an electrocatalytic reaction. 2+ The application uses ammonium chloride as a nitrogen source to modify lignocellulose, and through freeze-drying, programmed annealing treatment and Cu 2+ Coordination, an atomically dispersed Cu1Zn1 diatomic catalyst can be obtained, which exhibits excellent activity and stability in an electrocatalytic biomass molecule upgrading reaction system, can convert 5-hydroxymethylfurfural into high-value-added 2,5-hexanediol, and the selectivity is more than 90% and the yield is more than 190 μ mol mg cat ‑1 h ‑1 Technical and economic analysis results show that the reaction system for preparing high-value-added chemicals through electrochemical reduction ring opening has commercial application prospects.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of inorganic material synthesis and electrocatalysis, and particularly relates to a preparation method of a diatomic catalyst based on lignocellulose and application thereof in electrocatalytic reactions. BACKGROUND

[0002] Compared with traditional nanometer catalysts, the advantages of atomically dispersed catalysts mainly include three aspects: (1) the fully exposed active metal atoms can realize maximum atom utilization efficiency, thereby reducing the cost of the catalyst, especially the noble metal catalyst; (2) by changing the coordination environment of the metal active site, the electronic and spatial structures of the metal can be effectively adjusted, thereby regulating the catalytic activity, selectivity and stability of the metal; (3) the precisely designed structure of the catalytic center is conducive to the establishment of a calculation model, and provides a good platform for the study of the catalytic mechanism of the target reaction with the aid of theoretical calculation. Based on the above advantages, atomically dispersed catalysts are widely used in various types of catalytic reaction systems.

[0003] In recent years, with the rapid growth of the world economy, the development of the chemical and energy industries increasingly relies on the exploration and exploitation of oil, which not only accelerates the depletion of fossil resources on earth, but also causes serious environmental pollution. Using renewable biomass as raw material to produce high-value-added chemicals can effectively alleviate the dependence of human society on fossil resources and reduce environmental pollution. As a carbon-neutral resource with abundant earth reserves, renewability and high processability, 5-hydroxymethylfurfural (HMF) is considered to be one of the most ideal alternatives for the sustainable production of biofuels / bio-derived chemicals. Using renewable energy conversion electrical energy as driving force, converting 5-hydroxymethylfurfural into various high-value-added chemicals and intermediates under mild conditions is a green way to realize HMF conversion. 2,5-hexanediol is an important high-value-added fine chemical, which can be prepared by electrochemical reductive ring-opening of 5-hydroxymethylfurfural. However, the previously reported catalysts cannot achieve high electrochemical activity. Therefore, designing an efficient and stable diatomic catalyst for the electrochemical reductive ring-opening reaction to prepare 2,5-hexanediol is of great significance for the upgrading of biomass molecules. SUMMARY

[0004] Based on the design concept of electrocatalysts, the present application provides a preparation method of a diatomic catalyst based on lignocellulose and application thereof in electrocatalytic reactions. The present application uses lignocellulose as a substrate to prepare a diatomic catalyst, thereby improving the adsorption of reactants and intermediates on the surface of the catalyst, and effectively improving the activity and stability of the electrocatalytic reductive ring-opening of 5-hydroxymethylfurfural.

[0005] The application is based on a preparation method of lignocellulose-based diatomic catalyst, using ammonium chloride as a nitrogen source to modify lignocellulose, and obtaining an atomically dispersed diatomic catalyst through freeze-drying, programmed annealing treatment and metal ion coordination. Specifically, it includes the following steps:

[0006] Step 1: Mix lignocellulose nanocrystals with a 0.1 M ammonium chloride solution, stir for 10-12 hours, and then obtain nitrogen-doped lignocellulose through freeze-drying and programmed temperature treatment.

[0007] Step 2: Mix 100 mg of nitrogen-doped lignocellulose with a metal salt solution, adsorb metal ions onto the surface of nitrogen-doped lignocellulose through coordination, and then obtain a lignocellulose-based diatomic catalyst through high-temperature annealing treatment in a protective atmosphere.

[0008] In step 1, the lignocellulose nanocrystals are prepared by the following method:

[0009] Using a paper pulp board as the raw material, it is crushed by a pulverizer, 64 wt% sulfuric acid solution is added, and hydrolysis is carried out at 50°C for 6 hours, followed by dialysis with distilled water for 5-7 days to obtain lignocellulose nanocrystals.

[0010] In step 1, the programmed temperature treatment is set as follows: heating to 520°C at a rate of 2°C / min under an argon atmosphere and holding for 1 hour, and then heating to 800°C at a rate of 5°C / min and holding for 1 hour.

[0011] In step 1, the nitrogen content in the nitrogen-doped lignocellulose is 4.5-5.5 wt%.

[0012] In one embodiment, the nitrogen content in the obtained nitrogen-doped lignocellulose is 5.1 wt%.

[0013] In step 2, the metal salt is copper nitrate and zinc nitrate. The concentration of the metal salt solution is 10 mg / mL.

[0014] In step 2, the protective atmosphere is argon. The high-temperature annealing treatment is to heat the obtained solid after centrifugation to 750°C at a rate of 5°C / min under an argon atmosphere and hold for 2 hours; then the obtained product is washed with hydrochloric acid, and calcined again, specifically heating to 900°C at a rate of 5°C / min under an argon atmosphere and holding for 2 hours.

[0015] In step 2, the content of metal atoms in the lignocellulose-based diatomic catalyst is 0.6-0.9 wt%.

[0016] In one embodiment, the obtained lignocellulose-based copper-zinc diatomic catalyst has a copper content of 0.68 wt% and a zinc content of 0.72 wt%.

[0017] In the present application, the metal sites are anchored on the surface of the lignocellulose-based carbon material through N coordination, and exhibit the characteristics of an atomically dispersed catalyst.

[0018] The present application relates to the application of a lignocellulose-based diatomic catalyst in an electrocatalytic hydrogenation reaction.

[0019] Specifically, the lignocellulose-based copper-zinc diatomic catalyst is mixed with a nafion solution and uniformly coated on carbon paper, with a loading amount of 1 mg / cm 2 , to obtain a working electrode for an electrocatalytic reaction. The effective area of the working electrode is 1 cm 2 . With the working electrode as the cathode, a glassy carbon electrode as the anode, and a Hg / HgO electrode as the reference electrode, 5-hydroxymethylfurfural is dissolved in an alkaline electrolyte solution to form an electrolysis reaction device, so as to realize the electrocatalytic hydrogenation reduction of 5-hydroxymethylfurfural to prepare 2,5-hexanediol.

[0020] The changes of reactants and products are analyzed by nuclear magnetic resonance hydrogen spectrum.

[0021] The intermediates in the reaction process are detected by an in-situ reaction device to determine the reaction path.

[0022] The present application uses an H-type electrolytic cell as the reaction device, which comprises a working electrode, a reference electrode, a counter electrode, an electrolyte and a proton transmission film. By comparing the reaction activity of the catalyst in the alkaline electrolyte and the electrochemical activity after the addition of HMF, it is determined that the applied potential of the reaction is-0.4 V vs. RHE; by comparing the reaction activity of different types of catalysts, the structure of the catalyst is optimized; by detecting the reactants and products, the optimal reaction conditions are determined.

[0023] Specifically, the following steps can be performed:

[0024] (1) An H-type electrolytic cell is used as the reaction device, 50 mL of electrolyte (1.0 M KOH solution) is added to each of the cathode and the anode, and argon is introduced into the electrolytic cell before the reaction to remove the oxygen dissolved in the electrolyte.

[0025] (2) A carbon rod is used as the working electrode for the anode, and the prepared diatomic catalyst is used as the working electrode for the cathode, and Hg / HgO is used as the reference electrode, the reaction conditions are normal temperature and pressure, linear voltammetry test is performed, the potential range is-1.5 to-0.8 V vs. Hg / HgO, and the scan rate is 10 mV / s.

[0026] (3) Add HMF in the cathode electrolyte solution, stir to disperse it evenly, and then continue to perform linear voltammetry test, the potential range is -1.5 V to -0.8 V, and the scan rate is 10 mV / s. By comparing with the linear voltammetry curve in (2), the reaction potential is optimized. By comparing the linear voltammetry curves of different types of working electrodes, the catalyst with the optimal performance is determined.

[0027] (4) Perform constant potential electrolysis in the reaction potential range, take 100 μL of electrolyte, mix with 100 μL of internal standard solution, then add 500 μL of heavy water, mix evenly, and then perform nuclear magnetic resonance hydrogen spectrum test to determine the content of reactants and products in the electrolyte, and then determine the optimal reaction condition.

[0028] (5) Perform stability performance test under the optimal reaction condition, detect the yield of the product and the Faraday efficiency in 50 times of cyclic electrolysis experiments, and perform physical characterization on the working electrode after the cyclic experiment to determine the structure stability of the catalyst and the long cycle stability of the reaction system.

[0029] The present application uses 5-hydroxymethylfurfural with low added value as a reactant to be dissolved in an alkaline electrolyte solution, applies a potential of -0.4 V vs. RHE for constant potential electrolysis, and detects the changes of the reactants and products by nuclear magnetic resonance hydrogen spectrum. The product is mainly 2,5-hexanediol, the selectivity is more than 90%, and the yield is 190 μmol mg cat -1 h -1 , which can maintain stable 50 times of cyclic electrolysis.

[0030] The beneficial technical effects of the present application are embodied in the following aspects:

[0031] 1. Compared with the traditional thermal chemical technology, the electrocatalytic reduction ring-opening reaction system provided by the present application realizes electrochemical reduction ring-opening preparation of high value-added chemicals from low value-added biomass-derived molecules under the reaction conditions of normal temperature and pressure, using renewable energy conversion electric energy as driving force, water as proton source, and renewable biomass-derived diatomic catalyst as working electrode.

[0032] 2. The electrocatalytic reduction ring-opening reaction system constructed by the present application can convert HMF into high value-added 2,5-hexanediol, the selectivity of the reaction product can be more than 90%, and the generation rate can reach 190 μmol mg cat -1 h -1 , which is much higher than the previously reported thermal chemical and electrochemical conversion systems.

[0033] 3、The electro-catalytic reduction open ring reaction system constructed by the application can be found to have certain commercial application prospects through technical and economic analysis and performance comparison, and meets the strategic demand of national green development. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a transmission electron microscope image of the lignocellulose prepared in Example 1 of the application. Figure 1 As can be seen from the figure, the lignocellulose exhibits a nanofiber electron microscope structure.

[0035] Figure 2 is an atomic force microscope image of the lignocellulose prepared in Example 1 of the application. Figure 2 As can be seen from the figure, the surface structure of the lignocellulose is relatively uniform.

[0036] Figure 3 is a transmission electron microscope image of the N-doped lignocellulose prepared in Example 1 of the application. Figure 3 As can be seen from the figure, the electron microscope structure of the lignin does not change significantly after N doping.

[0037] Figure 4 is an atomic force microscope image of the N-doped lignocellulose prepared in Example 1 of the application. Figure 4 As can be seen from the figure, the surface structure of the lignin is still relatively uniform and does not change significantly after N doping.

[0038] Figure 5 is a comparison chart of X-ray photoelectron spectroscopy of the N-doped lignocellulose and the lignocellulose prepared in Example 1 of the application. Wherein a chart is an XPS total spectrum chart of the N-doped lignocellulose and the lignocellulose, b chart is a high-resolution XPS C 1s spectrum chart of the N-doped lignocellulose and the lignocellulose, c chart is a high-resolution XPS O 1s spectrum chart of the N-doped lignocellulose and the lignocellulose, and d chart is a high-resolution XPS N 1s spectrum chart of the N-doped lignocellulose and the lignocellulose. Figure 5 As can be seen from the figure, N is successfully introduced into the lignocellulose by N doping.

[0039] Figure 6 is an X-ray diffraction spectrum of the CuZn diatomic catalyst and the corresponding monatomic catalyst prepared in Example 1 of the application. Figure 6 As can be seen from the figure, the CuZn diatomic catalyst and the corresponding monatomic catalyst only exhibit graphite carbon peaks.

[0040] Figure 7are specific surface area and pore size distribution of CuZn diatomic catalyst and corresponding monatomic catalyst prepared in embodiment 1 of the present application. Wherein a figure is specific surface area graph of CuZn diatomic catalyst and corresponding monatomic catalyst, and figure b is pore size distribution graph of CuZn diatomic catalyst and corresponding monatomic catalyst. From Figure 7 it can be seen that there is no obvious difference between specific surface area and pore size distribution of CuZn diatomic catalyst and corresponding monatomic catalyst.

[0041] Figure 8 is atom-resolved high-angle annular dark field scanning transmission electron microscopy graph of CuZn diatomic catalyst prepared in embodiment 1 of the present application. From Figure 8 it can be seen that CuZn diatomic catalyst shows CuZn atomic pair distribution state.

[0042] Figure 9 is element distribution graph of CuZn diatomic catalyst prepared in embodiment 1 of the present application. From Figure 9 it can be seen that Cu, Zn, C, N elements are uniformly distributed in CuZn diatomic catalyst nanofiber.

[0043] Figure 10 is X-ray absorption near-edge structure graph of CuZn diatomic catalyst prepared in embodiment 1 of the present application, wherein a, b figures are X-ray absorption near-edge structure of Cu and Zn respectively. From Figure 10 it can be seen that valence state of Cu and Zn in CuZn diatomic catalyst is close to +2 valence.

[0044] Figure 11 is Fourier transform X-ray absorption fine structure spectrum graph of CuZn diatomic catalyst prepared in embodiment 1 of the present application, wherein a, b figures are X-ray absorption fine structure spectrum of Cu and Zn respectively. From Figure 11 it can be seen that Cu and Zn in CuZn diatomic catalyst exist in atom-dispersed structure through N coordination.

[0045] Figure 12 is electrochemical reduction linear voltammetry curve of CuZn diatomic catalyst prepared in embodiment 1 of the present application. From Figure 12 it can be seen that HMF can occur electrochemical reduction reaction under the action of CuZn diatomic and Zn monatomic catalyst.

[0046] Figure 13 is electrochemical reduction open-loop reaction performance comparison graph of CuZn diatomic catalyst and corresponding monatomic catalyst prepared in embodiment 1 of the present application, wherein a, b figures are faraday efficiency and product generation efficiency of HMF electrochemical reduction open-loop to generate 2,5-hexanediol respectively. From Figure 13It can be seen from the above that the CuZn diatomic catalyst has more excellent reaction performance of electrocatalytic reduction ring-opening of HMF to prepare 2,5-hexanediol.

[0047] Figure 14 is a comparison of the performance of the CuZn diatomic catalyst prepared in Example 1 of the present application with other reaction systems. From the above, it can be seen that the CuZn diatomic catalyst has more excellent reaction performance of electrocatalytic reduction ring-opening of HMF to prepare 2,5-hexanediol. Figure 14 It can be seen from the above that the CuZn diatomic catalyst has more excellent reaction performance of electrocatalytic reduction ring-opening of HMF to prepare 2,5-hexanediol.

[0048] Figure 15 is a technical and economic analysis chart of the CuZn diatomic catalyst prepared in Example 1 of the present application in this electrochemical reduction ring-opening reaction system. From the above, it can be seen that the CuZn diatomic catalyst has more excellent reaction performance of electrocatalytic reduction ring-opening of HMF to prepare 2,5-hexanediol. Figure 15 It can be seen from the above that the CuZn diatomic catalyst has more excellent reaction performance of electrocatalytic reduction ring-opening of HMF to prepare 2,5-hexanediol.

[0049] Figure 16 is the product nuclear magnetic resonance spectrum of the CuZn diatomic catalyst prepared in Example 1 of the present application in the electrochemical reduction ring-opening reaction system. In the above, a, b and c are the nuclear magnetic resonance spectrum of HMF and the product, the standard fitting curve of HMF and the standard fitting curve of the product, respectively. From the above, it can be seen that the conversion rate of HMF and the yield of 2,5-hexanediol can be accurately detected by nuclear magnetic resonance spectrum. Figure 16 It can be seen from the above that the CuZn diatomic catalyst has more excellent reaction performance of electrocatalytic reduction ring-opening of HMF to prepare 2,5-hexanediol. DETAILED DESCRIPTION

[0050] In the embodiments of the present application, a lignocellulose-based diatomic catalyst and a preparation method thereof are provided, and by improving the adsorption of reactants and intermediates on the surface of the catalyst, the activity and stability of the electrocatalytic reduction ring-opening of 5-hydroxymethylfurfural on the surface of the diatomic catalyst are effectively improved.

[0051] The present application will be further described in detail through specific embodiments.

[0052] Embodiment:

[0053] In this embodiment, a lignocellulose-based diatomic catalyst is first prepared, and an HMF electrochemical reduction ring-opening reaction system is further assembled by using the same as the working electrode material, and the specific operation steps are as follows:

[0054] 1. Disperse 100 mg of lignocellulose into 50 mL of aqueous ammonium chloride solution (0.1 mol / L), and stir thoroughly at room temperature for 10 hours. Obtain the solid product by centrifugation, freeze-dry at -78 °C for 48 hours, and then heat to 520 °C at a rate of 2 °C / min in an argon atmosphere and keep for 1 hour, and then heat to 800 °C at a rate of 5 °C / min and keep for 1 hour, to obtain N-doped lignocellulose. The obtained N-doped lignocellulose is shown in the transmission electron microscopy (TEM) image of Figure 3 , the atomic force microscopy (AFM) image of Figure 4 , and the X-ray photoelectron spectroscopy (XPS) spectrum of Figure 5 .

[0055] 2. Disperse 50 mg of N-doped lignocellulose into 30 mL of ethanol solution, and then add 2 mL of freshly prepared copper nitrate solution (10 mg / mL) and 2 mL of freshly prepared zinc nitrate solution (10 mg / mL) into the above ethanol solution, and stir for 12 hours. Obtain the black solid by centrifugation, and heat to 750 °C at a rate of 5 °C / min in an argon atmosphere and keep for 2 hours. The obtained black product is washed with hydrochloric acid, and then calcined at a high temperature again, heated to 900 °C at a rate of 5 °C / min in an argon atmosphere and keep for 2 hours, to obtain a lignocellulose-based CuZn biatomic catalyst. The X-ray diffraction (XRD) pattern of the obtained lignocellulose-based CuZn biatomic catalyst is shown in Figure 6 , the specific surface area and pore size distribution are shown in Figure 7 , the atomic resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image is shown in Figure 8 , and the element distribution is shown in Figure 9 .

[0056] 3. Mix the above lignocellulose-based CuZn biatomic catalyst with lithium fluoride, and perform synchrotron radiation test. The X-ray absorption near-edge structure (XANES) of the obtained CuZn biatomic catalyst is shown in Figure 10 , and the Fourier-transformed X-ray absorption fine structure (FT-EXAFS) spectrum of the CuZn biatomic catalyst is shown in Figure 11 .

[0057] 4. Disperse 5 mg of catalyst in 1 mL of ethanol solution, and add 100 μL of Nafion solution. Then take 200 μL of the above mixed solution, and uniformly distribute on carbon paper (1×1 cm 2 ), and dry to obtain a working electrode. The corresponding monatomic catalyst is also loaded on carbon paper in the same way to obtain a corresponding working electrode.

[0058] 5. The H-type electrolytic cell was separated using a proton exchange membrane. 1.0 M KOH was used as the electrolyte. The electrode prepared above was used as the working electrode, the Hg / HgO electrode as the reference electrode, and the carbon rod as the counter electrode. Linear voltammetry was performed at a scan rate of 5 mV / s, without impedance compensation, with a potential range of -0.5 to 0.1 V vs. RHE. Linear voltammetry was performed without the addition of HMF. After adding HMF to the working electrode side, a significant reduction current was observed when the applied potential was below -0.2 V vs. RHE, indicating HMF reduction. Furthermore, the CuZn catalyst showed a significantly stronger current response than the Zn catalyst. The corresponding linear voltammetry curves are shown below. Figure 12 As shown.

[0059] 6. Constant potential electrolysis was performed at potentials of -0.15, -0.20, -0.25, -0.30, -0.35, -0.40, -0.45, and -0.50 V vs. RHE, and the product distribution was detected by nuclear magnetic resonance spectroscopy. By comparing the Faradaic efficiency and product formation rate of HMF reduction ring-opening to 2,5-hexanediol, it was found that the CuZn diatomic catalyst exhibited the best electrochemical reduction ring-opening activity. The corresponding electrochemical reduction ring-opening performance comparison is as follows: Figure 13 As shown.

[0060] 7. Comparing the HMF reduction and ring-opening reaction performance of this system with relevant literature reveals that it possesses optimal electrochemical performance and stability. Techno-economic analysis indicates that this system has promising commercial applications. Corresponding performance comparisons are as follows: Figure 14 As shown, the corresponding techno-economic analysis diagram is as follows: Figure 15 As shown.

[0061] Those skilled in the art will readily understand that the above embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1.A method for preparing lignocellulose-based diatomic catalysts, characterized in that: lignocellulose is modified using ammonium chloride as a nitrogen source, and an atomically dispersed diatomic catalyst is obtained through freeze-drying, programmed annealing treatment and metal ion coordination; comprising the following steps: Step 1: mixing lignocellulose nanocrystals with a 0.1 M ammonium chloride solution, stirring for 10-12 hours, and then freeze-drying and programmed temperature treatment to obtain nitrogen-doped lignocellulose; Step 2: mixing 100 mg of nitrogen-doped lignocellulose with a metal salt solution, adsorbing metal ions onto the surface of the nitrogen-doped lignocellulose through coordination, and then annealing at high temperature in a protective atmosphere to obtain a lignocellulose-based diatomic catalyst; In Step 1, the lignocellulose nanocrystals are prepared by the following method: using a pulp board as a raw material, crushing it with a pulverizer, adding a 64 wt% sulfuric acid solution to hydrolyze at 50℃ for 6 hours, and then dialyzing with distilled water for 5-7 days to obtain lignocellulose nanocrystals; In Step 1, the programmed temperature process is set as follows: heating to 520℃ at a rate of 2℃ / min under an argon atmosphere and holding for 1 hour, and then heating to 800℃ at a rate of 5℃ / min and holding for 1 hour; In Step 2, the metal salt is copper nitrate and zinc nitrate; In Step 2, the high-temperature annealing treatment is to heat the obtained solid after centrifugation to 750℃ at a rate of 5℃ / min under an argon atmosphere and hold for 2 hours; then washing the obtained product with hydrochloric acid and calcining again, specifically heating to 900℃ at a rate of 5℃ / min under an argon atmosphere and holding for 2 hours. 2.The method according to claim 1, characterized in that: in Step 1, the nitrogen content in the nitrogen-doped lignocellulose is 4.5-5.5 wt%. 3.The method according to claim 1, characterized in that: in Step 2, the protective atmosphere is argon. 4.The method according to claim 1, characterized in that: in Step 2, the content of metal atoms in the lignocellulose-based diatomic catalyst is 0.6-0.9 wt%. 5.The application of the lignocellulose-based diatomic catalyst prepared by any one of the preparation methods in claims 1-4 in an electrocatalytic hydrogenation reaction. 6.The application according to claim 5, characterized in that: mixing the lignocellulose-based diatomic catalyst with a nafion solution, and uniformly coating it on carbon paper to obtain a working electrode for an electrocatalytic reaction; using the working electrode as a cathode, a glassy carbon electrode as an anode, and a Hg / HgO electrode as a reference electrode, dissolving 5-hydroxymethylfurfural into an alkaline electrolyte solution to form an electrolysis reaction device, and realizing the electrocatalytic hydrogenation reduction of 5-hydroxymethylfurfural to prepare 2,5-hexanediol. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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