Copper-based catalyst as well as preparation method and application thereof

Abstract

The invention discloses a copper-based catalyst which comprises a carrier and an active component, and the carrier is silicon nitride; the active component is Cu monatomic, and the Cu monatomic is anchored on the surface of the silicon nitride through a Cu-N bond. The invention also discloses a preparation method of the copper-based catalyst and application of the copper-based catalyst in catalyzing selective hydrogenation of dimethyl oxalate to prepare methyl glycolate.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and in particular relates to a Cu single-atom copper-based catalyst using silicon nitride as a carrier, a preparation method and uses thereof. Background Art

[0002] Methyl glycolate (MG) is an important organic raw material and pharmaceutical intermediate, widely used in numerous fields, including chemicals, pesticides, pharmaceuticals, feed, fragrances, and dyes. For example, MG can be used to produce anticancer drugs and high-value-added chemicals such as polyglycolic acid (PGA). Polyglycolic acid has excellent biocompatibility and biodegradability, and has been applied in the development of biomedical materials and ecological sustained-release materials, and may even become a substitute for petrochemical plastics. Coal is a long-term primary resource in my country, and the development of coal chemical industry plays a crucial role in national resource security. Therefore, a process involving the coupling of carbon monoxide obtained from coal-to-syngas with methyl nitrite to form dimethyl oxalate (DMO), followed by selective hydrogenation to MG, has important practical applications. This process offers advantages such as environmental friendliness, mild reaction conditions, and high atom economy, making it an important non-petroleum-based route for the synthesis of MG.

[0003] Silver-based catalysts have moderate adsorption energies and low reaction barriers in DMO hydrogenation systems, facilitating the selective hydrogenation of DMO to MG. However, silver-based catalysts exhibit relatively weak hydrogen activation and dissociation abilities, resulting in low catalytic activity. Increasing the silver loading to enhance reaction activity significantly increases catalyst production costs. Non-precious copper-based catalysts, on the other hand, often achieve complete conversion of DMO in DMO hydrogenation systems and exhibit good hydrogenation activity. However, they are prone to deep hydrogenation reactions, which can lead to over-hydrogenation of the MG produced to ethylene glycol (EG) and ethanol (EtOH), resulting in reduced MG yields. Therefore, to reduce catalyst costs and increase MG yields, designing novel, highly active copper-based catalysts to achieve high selectivity and yield of MG in DMO hydrogenation reactions is key to optimizing coal-to-MG processes.

[0004] The present invention is proposed for this purpose. Summary of the Invention

[0005] To overcome the low MG selectivity and yield, particularly the low space-time yield, of copper-based catalysts in the DMO selective production of MG process, the present invention discloses a copper-based catalyst supported on silicon nitride. The copper-based catalyst of the present invention exhibits low copper loading, high catalytic activity, and high selectivity when used in the DMO hydrogenation selective production of MG. The disclosed method for preparing the copper-based catalyst utilizes a deposition precipitation method followed by an acid wash post-treatment to produce single Cu atoms anchored on the silicon nitride surface via Cu-N bonds. The hydrochloric acid wash post-treatment retains the strongly interacting single-atom Cu species, and the actual copper loading on the catalyst can be adjusted by varying the concentration of the aqueous hydrochloric acid solution.

[0006] The technical solution adopted by the present invention is as follows:

[0007] The first aspect of the present invention discloses a copper-based catalyst comprising a carrier and an active component, wherein the carrier is silicon nitride; the active component is a Cu single atom, and the Cu single atom is anchored on the silicon nitride surface through a Cu-N bond.

[0008] Preferably, the valence of copper in the copper species is between 1-2.

[0009] Preferably, in the copper-based catalyst, the loading amount of the copper species is no more than 2.3 wt.% of the total mass of the copper-based catalyst; correspondingly, the carrier accounts for no less than 97.7 wt.% of the total mass of the catalyst.

[0010] Preferably, the specific surface area of the copper-based catalyst is 37 to 41 m 2 / g, with an average pore volume of 0.15 to 0.2 cm 3 / g, and the average pore diameter is 16-22nm.

[0011] The second aspect of the present invention discloses a method for preparing the copper-based catalyst, comprising the following steps:

[0012] (1) dissolving a copper precursor salt in water to obtain a copper precursor solution; dispersing silicon nitride in the copper precursor solution and mixing the mixture uniformly to obtain a dispersion;

[0013] (2) adding a precipitant to the dispersion, adjusting the pH to no more than 8, and allowing the dispersion to settle for about 5 hours to obtain a precipitate;

[0014] (3) drying the precipitate and then calcining it to obtain a calcined solid; the drying temperature is about 75° C., and the drying time is about 12 h;

[0015] (4) adding the calcined solid to a hydrochloric acid solution for acid treatment, and then washing until neutral to obtain an acid-treated solid; drying the acid-treated solid to obtain a copper-based catalyst precursor;

[0016] (5) The copper-based catalyst precursor is reduced by hydrogen to obtain the copper-based catalyst.

[0017] Preferably, in step (1), the copper precursor salt is copper nitrate; the silicon nitride is amorphous silicon nitride; the amount of copper precursor salt and silicon nitride added is such that the loading of copper species in the final copper-based catalyst does not exceed 10.0 wt.% of the total amount of the copper-based catalyst; the precipitant in step (2) is an aqueous solution of sodium carbonate; and the calcination temperature in step (3) is 300-500° C., the calcination time is 3-5 h, and the calcination atmosphere is nitrogen.

[0018] Preferably, the concentration of the hydrochloric acid solution in step (4) is 1 to 5 wt%, and the calcined solid is mixed with the hydrochloric acid solution in a ratio of 1 g: 30 mL; the hydrochloric acid solution is added for acid treatment so that the loading amount of the copper species in the final copper-based catalyst does not exceed 2.3 wt.% of the total amount of the copper-based catalyst.

[0019] Preferably, the hydrogen reduction treatment conditions in step (5) are: reduction temperature 300-400° C., and reduction time 3-5 h.

[0020] The third aspect of the present invention discloses the use of the copper-based catalyst for catalyzing the selective hydrogenation of dimethyl oxalate to produce methyl glycolate.

[0021] Preferably, the reaction conditions for the selective hydrogenation of dimethyl oxalate to methyl glycolate catalyzed by a copper-based catalyst are as follows: introducing the mixed gasified dimethyl oxalate and hydrogen into a reactor encapsulated with the copper-based catalyst for reaction; the reaction pressure is 2.5 MPa; the reaction temperature is 200-240° C.; the mass space velocity of dimethyl oxalate is 0.1 h -1 ; The hydrogen ester molar ratio is 150.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. Based on the literature (see Guo Lingling, Study on the "Structure-Activity Relationship" of Copper-Based Catalysts in the Selective Oxidation of Propylene to Prepare Acrolein, Doctoral Dissertation of the Chinese Academy of Sciences, April 2020), the present invention uses a precipitant to adjust the particles of the copper species, and then uses a hydrochloric acid solution to wash away the large particles, retaining the Cu single atoms, and making the loading amount of the copper species no more than 2.3 wt.% of the total amount of the copper-based catalyst; then hydrogen reduction treatment is carried out to obtain a copper-based catalyst in which the Cu single atoms are anchored on the silicon nitride surface through Cu-N bonds.

[0024] 2. In the preparation method of the copper-based catalyst of the present invention, a precipitant is used to adjust the pH of the precipitation. When the pH exceeds 8, the Cu loading is high, but the total amount of Cu atoms is relatively low, as shown in Comparative Examples 5 and 6. When the pH is below 6, the Cu loading is low, and the total amount of Cu atoms is also low. Therefore, the pH of the precipitation adjusted by the precipitant in the present invention is generally controlled between 5 and 8, preferably between 6 and 7.

[0025] 3. In the Cu-based catalyst of the present invention, Cu atoms are anchored on the silicon nitride surface through Cu-N bonds. The silicon nitride carrier is rich in amino groups, which makes the copper atoms dispersed on the silicon nitride carrier, which is conducive to the formation of strong Cu-N interaction between the silicon nitride carrier and the copper atoms, and the active center is Cu δ+ (δ is between 1 and 2). The present invention uses silicon nitride as a carrier to increase the oxidation state of copper species, which is beneficial to the heterolytic hydrogenation reaction process of single-atom Cu.

[0026] 4. The copper-based catalyst of the present invention is used for DMO hydrogenation reaction and has good selectivity for MG. Among them, the copper-based silicon nitride catalyst with a loading of 2.3 wt.% of Cu single atom as the active center has a DMO conversion rate of 99.3% and an intermediate product MG selectivity of 84.5% when the reaction temperature is 235°C, the reaction pressure is 2.5 MPa, and the feed hydrogen ester molar ratio is 150 (Example 3). The copper-based silicon nitride catalyst with a loading of 0.8 wt.% of Cu single atom as the active center has a MG space-time yield of 7.4 h / min per unit mass of copper species under the above reaction conditions. -1 (Example 1) The MG space-time yield of copper species per unit mass of copper-based catalysts reported in the prior art is mostly 1 to 3 h -1 , rarely reaches 7h -1 Therefore, the copper-based catalyst of the present invention is used for DMO hydrogenation reaction, and its MG space-time yield per unit copper species ranks among the best among the currently reported copper-based catalysts, showing excellent catalytic performance.

[0027] 5. The copper-based catalyst of the present invention exhibits high directional selectivity for MG when used in the hydrogenation of DMO to MG. Compared to the existing copper-based silica catalyst (Comparative Example 1), the addition of silicon nitride increases the selectivity from 71% to 86%, reducing the energy required for product separation. The abundant amino groups on the silicon nitride support, compared to the hydroxyl groups on the silica support, repel MG. Steric hindrance forces MG away from the active sites of the adsorbed copper species, preventing deep hydrogenation and thus improving MG selectivity.

[0028] 6. When the copper-based catalyst of the present invention is used for the DMO hydrogenation reaction to prepare MG, it has a higher directional selectivity for MG compared to copper-based silicon nitride catalysts loaded with larger Cu atomic clusters and Cu nanoparticles. The adsorption effect of its single atomic copper species on MG is weaker than the repulsive effect of the surface chemical environment (ammonia-rich chemical environment on the surface of the silicon nitride support) on MG, and has a higher MG selectivity.

[0029] 7. The preparation method of the copper-based catalyst of the present invention has readily available raw materials, mild reaction conditions, a simple and controllable process, and strong operability; it has good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 For comparative example 1 ( Figure 1 a) and Example 1 ( Figure 1 b) spherical aberration-corrected transmission electron microscopy image of the prepared copper-based catalyst; Figure a is a spherical aberration-corrected transmission electron microscopy image of the single-atom Cu catalyst sample loaded on silicon dioxide as a carrier in Comparative Example 1, and Figure b is a spherical aberration-corrected transmission electron microscopy image of the single-atom Cu catalyst sample loaded on silicon nitride as a carrier in Example 1.

[0031] Figure 2 X-ray absorption fine structure spectra of the Cu single-atom catalysts prepared in Comparative Example 1 and Example 1, as well as metallic copper, cuprous oxide, and cupric oxide standards; Figure a is the X-ray absorption near-edge structure spectrum, and Figure b is the first-order derivative function diagram of the X-ray absorption near-edge structure spectrum.

[0032] Figure 3 The XPS graphs of the catalysts obtained in Comparative Example 1 and Example 1 are shown.

[0033] Figure 4 MG-TPD-MS results of methyl glycolate in Comparative Example 1 and Example 1.

[0034] Figure 5 These are DFT simulation diagrams of copper-based catalyst samples, where (a) is the DFT simulation diagram of Example 1, and (b) is the DFT simulation diagram of Comparative Example 1; the left figure is the geometric structure of the catalyst, the right figure is the adsorption configuration of methyl glycolate molecules, and the inset is the corresponding electron density difference diagram.

[0035] Figure 6 This is a graph showing the performance evaluation results of the Cu single-atom silicon nitride catalyst prepared in Example 1 for the hydrogenation of dimethyl oxalate to methyl glycolate.

[0036] Figure 7 This is a graph showing the performance evaluation results of the Cu single-atom silica catalyst prepared in Comparative Example 1 for the hydrogenation of dimethyl oxalate to methyl glycolate.

[0037] Figure 8 The graph is a comparison of the space-time yield of Example 1 and the currently reported copper-based catalysts for the hydrogenation of dimethyl oxalate to methyl glycolate. 1 ,Cu / RGO 2 ,Cu3 / CeO2 3 ,0.1Ca-8Cu / AC 4 ,Cu / AC-673 5 ,0.2Sr-8Cu / AC 4 ,Cu@NC 6 ,SP-Cu / SiO2 7 ,20Cu / HAP 8 See the following literature:

[0038] 1. Liu, Z.; Li, Z.; Ma, Z.; Xie, J.; Wen, X.; Chen,

[0039] 2.Abbas,M.;Chen,Z.;Chen,J.,Shape-and size-controlled synthesis of Cunanoparticles wrapped on RGO nanosheet catalyst and their outstandingstability and catalytic performance in the hydrogenation reaction of dimethyloxalate.Journal of Materials Chemistry A 2018,6(39),19133-19142.

[0040] 3.Yao,D.;Wang,Y.;Li,Y.;Li,A.;Zhen,Z.;Lv,J.;Sun,F.;Yang,R.;Luo,J.;Jiang,Z.;Wang,Y.;Ma,X.,Scalable synthesis of Cu clusters for remarkableselectivity control of intermediates in consecutive hydrogenation.NatureCommunications 2023,14(1),1123.

[0041] 4.Rong,Z.;Shen,W.;Fang,Y.,Alkaline earth modified activated carbonsupported Cu catalysts with enhanced selectivity in the hydrogenation ofdimethyl oxalate to methyl glycolate.RSC Advances 2024,14(17),11849-11861.

[0042] 5.Cui,Y.;Wang,B.;Wen,C.;Chen,X.;Dai,W.L.,Investigation of Activated-Carbon-Supported Copper Catalysts with Unique Catalytic Performance in theHydrogenation of Dimethyl Oxalate to Methyl Glycolate.ChemCatChem 2015,8(3),527-531.

[0043] 6.Zhang,H.;Song,Y.;Wu,S.;Yin,S.;Zhao,J.;Ren,J.,Highly stable Cucatalyst embedded in N-doped carbon microsphere for hydrogenation of dimethyloxalate to methyl glycolate.Applied Catalysis A:General 2024,677,119710-119719.

[0044] 7. Sun, J.; Yu, J.; Ma, Q.; Meng, F.; Wei,

[0045] 8. Wen, C.; Cui, Y.; Chen, DETAILED DESCRIPTION

[0046] The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the following examples are illustrative and not restrictive, and the scope of protection of the present invention cannot be limited by the following examples. The raw materials required in the following examples are all commercially available.

[0047] The methods for online reduction of the catalyst and evaluation of the catalytic effect in the examples and comparative examples are as follows:

[0048] In the present invention, the dimethyl oxalate hydrogenation reaction is carried out in a fixed-bed reactor. The catalyst precursor mixtures from the Examples and Comparative Examples that were not subjected to reduction treatment were pressed into tablets and sieved to obtain 40-60 mesh particles. 0.5 g of the mixture was weighed and loaded into the constant temperature section of the fixed-bed reactor. The mixture was then reduced at 300°C in a 2.5 MPa hydrogen atmosphere at a gas flow rate of 80 mL / min for 4 hours.

[0049] After the reduction is completed, the temperature is lowered to the reaction temperature, the dimethyl oxalate solution is vaporized and mixed with hydrogen and then enters the reaction system. The reaction pressure is 2.5 MPa, the hydrogen-ester ratio is 150, and the mass space velocity is 0.1 h -1 The hydrogenation reaction is carried out under the conditions of ; the collected products are analyzed by gas chromatography to calculate the conversion rate of dimethyl oxalate and the selectivity of each product.

[0050] The catalyst preparation conditions and copper species particle size of the comparative examples and examples are shown in Table 1 below, and the activity data are shown in Table 2.

[0051] Comparative Example 1: Preparation of copper-silicon catalyst. The preparation of a copper-based catalyst sample in which single Cu atoms are supported on a silica carrier is as follows:

[0052] Dissolve 0.0276g of copper nitrate in 100mL of deionized water and stir evenly for 10 minutes. Add 0.9g of commercially available hydrophilic fumed silica powder to the copper nitrate aqueous solution and stir evenly for 10 minutes. Then, add 0.5mol / L sodium carbonate aqueous solution dropwise for precipitation until the mixed solution reaches a pH of 7. After aging for 5 hours, filter and wash the solid. Dry the washed solid in a 75°C vacuum drying oven for 12 hours to remove residual moisture. Finally, heat the catalyst precursor to 400°C in an air atmosphere and calcine for 4 hours to obtain the catalyst precursor. Then, reduce it at 300°C in a 2.5MPa hydrogen atmosphere for 4 hours to obtain the catalyst.

[0053] Comparative Examples 2-4: Copper-based silicon nitride catalyst.

[0054] Comparative Examples 2-4 are preparations of copper-based catalyst samples using silicon nitride as a carrier to support Cu nanoparticles of 1.7 nm, 3.5 nm, and 7.2 nm, as follows:

[0055] 0.0276 g of copper nitrate was dissolved in 100 mL of deionized water and stirred evenly for 10 minutes. 0.9 g of commercially purchased amorphous silicon nitride powder was added to the copper nitrate aqueous solution and stirred evenly for 10 minutes. 0.5 mol / L sodium carbonate aqueous solution was added dropwise for precipitation until the pH of the mixed solution was 7, 8, and 9 to control the size of the nanoparticle copper species in the copper-based catalyst. Three copper-based catalyst samples were prepared; after aging for 5 hours, the solid was filtered and washed, and the washed solid was placed in a 75°C vacuum drying oven and dried for 12 hours to remove residual moisture; finally, the catalyst precursor was heated to 400°C under a nitrogen atmosphere and calcined for 4 hours to obtain a catalyst precursor; and then reduced at 300°C for 4 hours in a 2.5 MPa hydrogen atmosphere to obtain a catalyst.

[0056] Examples 1-3: Copper-based silicon nitride catalyst.

[0057] Examples 1-3 are the preparation of three samples of single-atom Cu catalysts loaded with silicon nitride at 0.8 wt.%, 1.4 wt.%, and 2.3 wt.%, as follows:

[0058] 0.3800 g of copper nitrate was dissolved in 100 mL of deionized water and stirred evenly for 10 min. 0.9 g of commercially purchased amorphous silicon nitride powder was added to the copper nitrate aqueous solution and stirred evenly for 10 min. 0.5 mol / L sodium carbonate aqueous solution was added dropwise for precipitation until the pH of the mixed solution reached 7. After aging for 5 h, the mixture was filtered and washed. The washed solid was placed in a vacuum drying oven at 75 ° C and dried for 12 h to remove residual moisture. Finally, the catalyst precursor was heated to 400 ° C under a nitrogen atmosphere and calcined for 4 h. Then, the catalyst precursor was added. The samples were pickled at room temperature for 4 h in 30 mL of a 1-5% hydrochloric acid solution. The loading of Cu single atoms in the copper-based catalyst was controlled by adjusting the hydrochloric acid concentration (5% in Example 1, 3% in Example 2, and 1% in Example 3). The resulting mixed solution was filtered and washed until the solution became neutral. The washed precipitate was dried in a vacuum drying oven at 75°C for 12 h to obtain a catalyst sample precursor. The catalyst was then reduced in a 2.5 MPa hydrogen atmosphere at 300°C for 4 h to obtain three catalysts.

[0059] Comparative Examples 5-6: Copper-based silicon nitride catalyst.

[0060] Comparative Examples 5-6 are the preparation of two samples of copper-based catalysts using silicon nitride as a carrier to support Cu atomic clusters, as follows:

[0061] 0.3800 g of copper nitrate was dissolved in 100 mL of deionized water and stirred evenly for 10 minutes. 0.9 g of commercially purchased amorphous silicon nitride powder was added to the copper nitrate aqueous solution and stirred evenly for 10 minutes. 0.5 mol / L sodium carbonate aqueous solution was added dropwise for precipitation until the pH of the mixed solution was 8 (Comparative Example 5) and 9 (Comparative Example 6); to control the size of the Cu atomic clusters in the copper-based catalyst, a copper-based catalyst sample was prepared; after aging for 5 hours, it was filtered and washed, and the washed solid was placed in a 75 ° C vacuum drying oven and dried for 12 hours to remove residual moisture; finally, the catalyst precursor was heated to 400 ° C under a nitrogen atmosphere and calcined for 4 hours; then 30 mL of hydrochloric acid solution (5%) was added and pickled at room temperature for 4 hours, the resulting mixed solution was filtered and washed until the solution was neutral, and the washed precipitate was placed in a 75 ° C vacuum drying oven and dried for 12 hours to obtain a catalyst precursor; then reduced at 300 ° C in a 2.5 MPa hydrogen atmosphere for 4 hours to obtain two catalysts.

[0062] Example 4: Copper-based silicon nitride catalyst.

[0063] Example 4 is the preparation of a 0.5 wt.% single-atom Cu catalyst sample supported on silicon nitride, as follows:

[0064] 0.3800 g of copper nitrate was dissolved in 100 mL of deionized water and stirred evenly for 10 minutes. 0.9 g of commercially purchased amorphous silicon nitride powder was added to the copper nitrate aqueous solution and stirred evenly for 10 minutes. 0.5 mol / L sodium carbonate aqueous solution was added dropwise for precipitation until the pH of the mixed solution was 6. After aging for 5 hours, the mixture was filtered and washed, and the washed solid was placed in a 75°C vacuum drying oven and dried for 12 hours to remove residual moisture. Finally, the catalyst precursor was heated to 400°C under a nitrogen atmosphere and calcined for 4 hours. Subsequently, 30 mL of a 5% hydrochloric acid solution was added and pickled at room temperature for 4 hours. The obtained mixed solution was filtered and washed until the solution was neutral. The washed precipitate was placed in a 75°C vacuum drying oven and dried for 12 hours to obtain a catalyst sample precursor. The catalyst was then reduced at 300°C in a 2.5 MPa hydrogen atmosphere for 4 hours to obtain a catalyst.

[0065] Figure 1 For comparative example 1 ( Figure 1 a) and Example 1 ( Figure 1 b) spherical aberration correction transmission electron micrograph of the prepared copper-based catalyst; wherein Figure a is a spherical aberration correction transmission electron micrograph of sample 1 of the comparative example 1 using silicon dioxide as a carrier to support the single-atom Cu catalyst, and Figure b is a spherical aberration correction transmission electron micrograph of sample 2 of the embodiment 1 using silicon nitride as a carrier to support the single-atom Cu catalyst. Figure 1 It can be seen that both Comparative Example 1 and Example 1 present a single-atom distribution of Cu. In Example 1, amorphous silicon nitride is used as a carrier, and the surface of silicon nitride contains a large amount of amino groups, which can form a strong interaction with single-atom Cu.

[0066] Figure 2 The X-ray absorption fine structure spectra of Cu single atom catalysts prepared in Comparative Example 1 and Example 1, as well as standards of metallic copper, cuprous oxide, and cupric oxide; Figure a is the X-ray absorption near-edge structure spectrum, and Figure b is the first-order derivative function diagram of the X-ray absorption near-edge structure spectrum. Figure 2 It can be seen that the average valence of Cu in Example 1 and Comparative Example 1 is between +1 and +2, and the average valence of Cu in Example 1 is higher, indicating that the binding effect of the copper species on the nitrogen-containing support is stronger, so that the copper species is in a more electron-deficient state; this more electron-deficient Cu single-atom catalyst is conducive to the heterolytic dissociation of hydrogen and enhances the hydrogenation activity of the reaction.

[0067] Figure 3 The XPS graphs of the catalysts obtained for Comparative Example 1 and Example 1 are shown in FIG. Figure 3 It can be seen that in the Cu 2p XPS graph of Example 1, 932.2 eV and 952.2 eV correspond to Cu 2p 3 / 2 and Cu 2p 1 / 2 Characteristic peaks; no satellite peaks were observed between 940-944eV, Cu 2+The species are fully reduced; characteristic peaks are shown at 933.6 and 953.5 eV, which are attributed to the Cu-N interaction; this indicates that there is a strong interaction between the copper species and the support on the copper-based silicon nitride catalyst supporting Cu single atoms obtained in Example 1.

[0068] Figure 4 MG-TPD-MS results of methyl glycolate in Comparative Example 1 and Example 1. Figure 5 These are DFT simulation diagrams of copper-based catalyst samples, where (a) is the DFT simulation diagram of Example 1, and (b) is the DFT simulation diagram of Comparative Example 1; the left figure is the geometric structure of the catalyst, the right figure is the adsorption configuration of methyl glycolate molecules, and the inset is the corresponding electron density difference diagram. Figure 6 This is a graph showing the performance evaluation results of the Cu single-atom silicon nitride catalyst prepared in Example 1 for the hydrogenation of dimethyl oxalate to methyl glycolate. Figure 7 This is a graph showing the performance evaluation results of the Cu single-atom silica catalyst prepared in Comparative Example 1 for hydrogenation of dimethyl oxalate to methyl glycolate; Figure 7 As shown, the catalytic activity of the catalyst carrier using silicon dioxide in Comparative Example 1 was only 43.9%, and the yield of methyl glycolate was only 29.2%. Figure 6 In Example 1, the catalytic activity of the single-atom Cu-based silicon nitride catalyst was 89.2%, and the yield of methyl glycolate was 77.1%. In order to further understand the reasons for the differences in MG selectivity between the catalysts, the results of methyl glycolate temperature-programmed chemical desorption-mass spectrometry (MG-TPD-MS) were characterized for Example 1 and Comparative Example 1 to study the differences in MG adsorption and desorption. Figure 4 MG is ionized when entering the MS and decomposes to produce species such as methoxyl (m / z=31). Therefore, the characteristic signal of methoxyl is selected to explore the adsorption and desorption behavior of MG on the catalyst surface. Figure 4 As shown, the MG desorption peak signal range of Comparative Example 1 is 100-300 ° C; the MS signal on the catalyst of Example 1 is extremely low, indicating that the catalyst basically does not adsorb MG; it is worth noting that the copper species on the catalyst of Example 1 has a higher oxidation state. In theory, copper species with low electronic state density have stronger adsorption to intermediate species such as acetyl and methoxy, which is manifested as strong adsorption of MG and increased selectivity of its deep hydrogenation products; however, according to the MG-TPD-MS experimental results, the copper species with high oxidation state on the catalyst of Example 1 showed weak MG adsorption ability, and it is speculated that the surface chemical environment on the support has an important influence on the adsorption of MG; as the charge transfer of Cu to the support increases, the adsorption energy of MG also increases; this is because the Cu with higher oxidation state σ+ This leaves unoccupied Cu-d orbitals, which form stronger bonds with the nearest neighbor C=O orbitals; however, Cu σ+In Example 1, in which the oxidation state is relatively high, almost no desorption peak of MG is observed. Figure 5 The effect of the support on the charge distribution of the catalyst and MG adsorption is demonstrated. DFT calculations were performed on Example 1 and Comparative Example 1. The more localized electron transfer process in the catalyst of Comparative Example 1 is manifested as a significant electron redistribution between copper atoms and SiO2. In contrast, a more dispersed electron transfer process was observed in the catalyst of Example 1, with electrons transferred from copper atoms to a wider surface area. This phenomenon is consistent with the Bader charge value, where the Bader charge of Cu atoms in Example 1 is +0.98|e|, which is higher than +0.63|e| in Comparative Example 1. This result is consistent with the XPS and XAFS results, that is, the copper species supported on silicon nitride exhibit a higher degree of electron transfer at the interface and are in a more electron-deficient state. The adsorption energies of MG on the surfaces of Example 1 and Comparative Example 1 were further measured. The adsorption energy of MG on the surface of Example 1 (-0.67 eV) was lower than that of Comparative Example 1 (-0.89 eV). This was mainly due to the complex geometric structure of the silicon nitride surface. The amino groups on the support hindered the adsorption of MG on copper species, while the -Si-OH groups may have a strong adsorption effect on MG. Steric hindrance forced MG to stay away from copper atoms on the silicon nitride surface, resulting in a decrease in the adsorption energy of MG. The DFT calculation results were consistent with the MG-TPD-MS experimental results, indicating that the high MG selectivity of Example 1 can be attributed to the chemical environment on the silicon nitride support surface hindering the adsorption of copper species on MG, thereby exhibiting excellent MG selectivity.

[0069] Table 1 shows the preparation conditions and copper species particle size of each catalyst, and Table 2 shows the activity data of each catalyst for the heterogeneous hydrogenation reaction of dimethyl oxalate. It can be seen from Tables 1 and 2 that the selectivity of methyl glycolate of the single-atom Cu-based catalyst on silicon nitride prepared in Example 1 is always maintained at about 90%; while the selectivity of methyl glycolate of the single-atom Cu-based catalyst with silica as the carrier in Comparative Example 1 varies between 60% and 80%, the selectivity of the target product is relatively low, and the yield of ethylene glycol by-product gradually increases with the increase of reaction temperature. The catalyst prepared in Example 1 is used in the hydrogenation reaction of dimethyl oxalate. When the reaction temperature is 235°C, the selectivity of the intermediate product methyl glycolate reaches 86.3%, the conversion rate of dimethyl oxalate reaches 89.3%, and the space-time yield of methyl glycolate per unit mass of copper species is 7.4h -1. Therefore, the Cu single-atom silicon nitride catalyst prepared in Example 1 of the copper-based catalyst of the present invention has a space-time yield of methyl glycolate per unit copper species that ranks among the best among the currently reported copper-based catalysts, showing high activity. By adjusting the concentration of the acid washing treatment, Cu single-atom catalysts with loading amounts of 0.8wt.%, 1.4wt.% and 2.3wt.% were prepared; by increasing the metal loading amount of single-atom Cu, it is beneficial to improve the hydrogenation capacity of dimethyl oxalate; in addition, the increase in metal loading amount will partially mask the isolated amino groups on the surface of silicon nitride, resulting in a decrease in the selectivity of methyl glycolate. Among them, at a reaction temperature of 235°C, the conversion rate of dimethyl oxalate of the single-atom Cu silicon nitride catalyst with a loading amount of 2.3wt.% was 99.3%, the selectivity of methyl glycolate was 84.5%, and the yield of methyl glycolate was 83.9%.

[0070] In Example 4, a single-atom Cu catalyst with a loading of 0.5 wt.% was prepared by adjusting the pH of the precipitate to 6 using a precipitant (other conditions were the same as in Example 1). When the pH is lower than 6, the Cu loading is low, and the total amount of Cu atoms is also low. Therefore, the pH of the precipitate is adjusted using a precipitant to a value between 5 and 8, preferably between 6 and 7.

[0071] Table 1 Preparation conditions of each catalyst and copper species particle size

[0072]

[0073] Table 2 Activity data of various catalysts for heterogeneous hydrogenation of dimethyl oxalate

[0074]

[0075]

[0076] As shown in Tables 1 and 2, the particle size distribution of the Cu atomic clusters and Cu nanoparticles supported on silicon nitride as the carrier is as follows: the Cu atomic cluster particle size is 0.5 nm in Comparative Example 5, the Cu atomic cluster particle size is 0.8 nm in Comparative Example 6, the Cu nanoparticle particle size is 1.7 nm in Comparative Example 2, the Cu nanoparticle particle size is 3.5 nm in Comparative Example 3, and the Cu nanoparticle particle size is 7.2 nm in Comparative Example 4. As shown in Table 2, as the Cu size in the copper-based catalyst increases, the DMO conversion first increases and then decreases. The change in the size of the copper species affects the number of active sites of the catalyst and the reaction hydrogenation ability. The copper-based silicon nitride catalyst with a smaller size can provide a larger specific surface area reaction active site, but its electronic state density is reduced, which is not conducive to H2 adsorption homolytic dissociation. The single-atom Cu-based silicon nitride catalyst prepared in Example 1 performs heterolytic hydrogenation, and the hydrogen activation dissociation energy barrier is low, so the reaction activity is at a relatively high level. Furthermore, the MG selectivity of copper-based silicon nitride catalysts based on Cu clusters and Cu nanoparticles depends on the reaction activity. Increased catalytic activity promotes deep hydrogenation. While increased activity favors the formation of MG at low conversions, deep hydrogenation at high DMO conversions produces byproducts. Size effects directly influence MG adsorption. As the size of the copper species increases, its adsorption strength on MG increases, indicating that the copper species' adsorption of MG gradually becomes stronger than the repulsion of MG by the ammonia-rich chemical environment on the silicon nitride support. This results in deep hydrogenation proceeding from MG to EG, decreasing the yield of the main product MG and increasing the yield of the deep hydrogenation product EG. Therefore, single-atom Cu catalysts supported on silicon nitride exhibit superior catalytic performance for DMO hydrogenation to MG compared to Cu clusters and / or Cu nanoparticles.

[0077] When the catalyst of the present invention is actually used in industry, it can be used alone for the production of methyl glycolate, which has high economic value and high demand. It can still maintain a MG selectivity of about 90% at a high reaction temperature, and the MG space-time yield per unit mass of copper is as high as 7.4g·g -1 ·h -1 ,like Figure 6 The copper-based catalyst of the present invention has a MG space-time yield per unit copper species that ranks among the highest among currently reported copper-based catalysts, demonstrating high activity. The preparation method of the copper-based catalyst of the present invention features readily available raw materials, mild reaction conditions, a simple and controllable process, and strong operability, thus possessing promising prospects for industrial application.

[0078] The above is an exemplary description of the present invention. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by other skilled in the art without expending creative labor falls within the scope of protection of the present invention.

Claims

1. A copper-based catalyst, characterized in that The invention comprises a carrier and an active component, wherein the carrier is silicon nitride; the active component is a Cu single atom, and the Cu single atom is anchored on the surface of the silicon nitride through a Cu-N bond.

2. The copper-based catalyst according to claim 1, characterized in that The valence state of Cu single atom is between 1 and 2.

3. The copper-based catalyst according to claim 1, characterized in that In the copper-based catalyst, the loading amount of Cu single atoms is no more than 2.3 wt.% of the total amount of the copper-based catalyst.

4. A method for preparing a copper-based catalyst according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) dissolving a copper precursor salt in water to obtain a copper precursor solution; dispersing silicon nitride in a copper precursor solution and mixing the mixture uniformly to obtain a dispersion; (2) adding a precipitant to the dispersion to adjust the pH to no more than 8 to obtain a precipitate; (3) drying the precipitate and then calcining it to obtain a calcined solid; (4) adding the roasted solid to a hydrochloric acid solution for acid treatment, and then washing until neutral to obtain an acid-treated solid; drying the acid-treated solid to obtain a copper-based catalyst precursor; (5) The copper-based catalyst precursor is reduced by hydrogen to obtain the copper-based catalyst.

5. The preparation method according to claim 4, characterized in that In step (1), the copper precursor salt is copper nitrate; the silicon nitride is amorphous silicon nitride; the amount of copper precursor salt added and the amount of silicon nitride added are such that the loading amount of copper species in the final copper-based catalyst does not exceed 10.0 wt.% of the total amount of the copper-based catalyst; in step (2), the precipitant is an aqueous sodium carbonate solution; in step (3), the calcination temperature is 300-500° C., the calcination time is 3-5 hours, and the calcination atmosphere is nitrogen.

6. The preparation method according to claim 4, characterized in that The concentration of the hydrochloric acid solution in step (4) is 1 to 5 wt %, and the calcined solid is mixed with the hydrochloric acid solution at a ratio of 1 g: 30 mL; A hydrochloric acid solution is added for acid treatment so that the loading amount of the copper species in the final copper-based catalyst does not exceed 2.3 wt.% of the total amount of the copper-based catalyst.

7. The preparation method according to claim 4, characterized in that The hydrogen reduction treatment conditions in step (5) are: reduction temperature 300-400° C., and reduction time 3-5 h.

8. Use of the copper-based catalyst according to any one of claims 1 to 3 for catalyzing the selective hydrogenation of dimethyl oxalate to prepare methyl glycolate.

9. The use according to claim 8, characterized in that The reaction conditions for the selective hydrogenation of dimethyl oxalate to methyl glycolate catalyzed by a copper-based catalyst are as follows: reaction pressure of 2.5 MPa; reaction temperature of 200-240°C; mass space velocity of dimethyl oxalate of 0.1 h -1 ; The hydrogen ester molar ratio is 150.

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