Preparation method of bifunctional cobalt-based catalyst and application of bifunctional cobalt-based catalyst in preparation of methyl glycolate by selective hydrogenation of dimethyl oxalate

Abstract

The invention discloses a preparation method of a bifunctional cobalt-based catalyst and application of the bifunctional cobalt-based catalyst in preparation of methyl glycolate by selective hydrogenation of dimethyl oxalate. The bifunctional cobalt-based catalyst comprises a carrier, cobalt and other auxiliaries, the cobalt and the other auxiliaries are loaded on the carrier, the carrier is selected from one of activated carbon, apatite hydroxide, carbon nitride and silicon dioxide, the other auxiliaries are selected from one of nickel, copper, silver, molybdenum, phosphorus and sulfur, and the molar ratio of the cobalt to the other auxiliaries is (2-4): 1. The cobalt-based catalytic system provided by the invention accords with the green chemistry principle, the generation of harmful byproducts is reduced, the economical efficiency of the reaction is improved, a cleaner and more sustainable production process is realized, more effective technical support is provided for the industrial production of biodegradable plastic polyglycolic acid (PGA), and the development of the environment-friendly material industry is promoted.

Description

Technical Field:

[0001] The present invention relates to the technical field of catalysts, and particularly relates to a preparation method of a bifunctional cobalt-based catalyst and its application in the selective hydrogenation of dimethyl oxalate to methyl glycolate. Background Art:

[0002] As the biodegradable plastic with the fastest degradation rate, polyglycolic acid (PGA), and methyl glycolate (MG) as the key raw material, the diversity of its synthesis methods directly affects the production efficiency and environmental friendliness of PGA. At present, the synthesis routes of MG include chloroacetic acid hydrolysis, formaldehyde carbonization esterification, coupling of methyl formate and formaldehyde, and coupling of glyoxal and methanol, etc. However, these methods rely on petrochemical resources, and the raw materials are toxic and costly, which limits the large-scale production of PGA. Syngas (CO / H2), as a source of non-petroleum energy and chemicals, can be obtained from resources such as coal, biomass, organic waste, natural gas, and carbon dioxide, which helps to reduce the dependence on petroleum resources and achieve the goal of carbon neutrality. In recent years, the technology of converting syngas into dimethyl oxalate (DMO) and selectively hydrogenating it to ethylene glycol (EG) has attracted wide attention. In fact, this process is a complex consecutive reaction, in which methyl glycolate (MG) is the key intermediate product of selective hydrogenation. Compared with ethylene glycol, MG is generated by selectively hydrogenating one carboxyl group in the DMO molecule, rather than completely hydrogenating two carboxyl groups, retaining more of the functional group structure of DMO and becoming the key monomer raw material of PGA. Therefore, developing an efficient catalytic reaction system to convert DMO into MG is the key to improving atom economy and has significant commercial potential and environmental benefits.

[0003] In the research on catalysts for the hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG), Cu-based and Ag-based catalysts have attracted much attention due to their excellent performance. Research shows that by selecting suitable carriers (such as hydroxyapatite, activated carbon, carbon aerogel, etc.) and adopting new preparation methods such as ammonia-assisted one-pot synthesis method and ammonia evaporation-impregnation method, the dispersion degree and valence state distribution of Cu species can be effectively regulated, thereby optimizing the catalytic performance. In addition, introducing doping elements such as B and N can adjust the electronic structure and dispersion state of Cu species and further improve the catalytic performance. However, under hydrogen-rich conditions, Cu + species are easily reduced to Cu 0 species, and Cu 0The low Hüttig temperature of the species leads to its easy migration and aggregation, which in turn exacerbates the deactivation and sintering carbon deposition of the catalyst, ultimately limiting its thermal stability (J. Catal. 2010, 271, 1 - 10). In contrast, due to its weak hydrogen molecule dissociation adsorption ability, Ag-based catalysts show unique advantages in the selective hydrogenation to methyl glycolate (MG). Using porous supports (such as SiO2, SBA-15) and carbon-based supports (such as CNTs, nitrogen-doped activated carbon) can improve the dispersion of Ag and regulate the electronic structure, significantly enhancing the catalytic performance. Ag nanoparticles with a particle size controlled within 2 - 5 nm exhibit the best activity, and introducing promoters such as B, Ni, Au, etc. further improves the catalytic performance through electron transfer or synergistic effects. However, Ag-based catalysts are costly and prone to agglomeration at high temperatures, affecting long-term stability. Silicon dioxide (SiO2) is widely used in industrial Cu-based catalysts due to its low cost, but it will generate tetramethoxysilane in methanol solvent, resulting in silicon loss and catalyst deactivation. To solve these problems, researchers have gradually explored non-silica-based supports.

[0004] In view of this, although the optimization strategies have improved the performance of Cu-based and Ag-based catalysts, there is still a need to develop highly efficient and low-cost non-Cu / Ag-based catalysts to meet the industrial demand for the hydrogenation of DMO to MG. Summary of the Invention:

[0005] The present invention solves the problems existing in the prior art and provides a preparation method of a bifunctional cobalt-based catalyst and its application in the selective hydrogenation of dimethyl oxalate to methyl glycolate. In the present invention, a cobalt phosphide thin film is in-situ formed on the surface of metallic cobalt by stepwise impregnation (taking the promoter phosphorus element as an example). Metallic cobalt still exhibits excellent hydrogen adsorption and activation performance, while cobalt phosphide shows excellent performance in the adsorption and activation of dimethyl oxalate (DMO). The hydrogen spillover between the two promotes the coordination between them; the novel cobalt-based catalyst proposed by the present invention effectively inhibits the over-hydrogenation reaction of methyl glycolate (MG) and improves the selectivity of MG.

[0006] The first object of the present invention is to provide a bifunctional cobalt-based catalyst, including a support and cobalt and other promoters loaded on the support. The support is selected from one of activated carbon, hydroxyapatite, carbon nitride, and silicon dioxide, and the other promoters are selected from one of nickel, copper, silver, molybdenum, phosphorus, and sulfur. The molar ratio of cobalt to other promoters is 2 - 4:1.

[0007] Preferably, based on the mass of the support, the loading amount of cobalt is 5 - 20 wt%. Further preferably, based on the mass of the support, the loading amount of cobalt is 15 wt%.

[0008] The second object of the present invention is to provide a preparation method of the bifunctional cobalt-based catalyst, which realizes the coupling presence of bifunctional active centers on the catalyst surface through stepwise impregnation, and includes the following steps:

[0009] S1. Oxidize the carrier in an acid solution to open the pores to obtain a pretreated carrier;

[0010] S2. First, impregnate the cobalt source on the pretreated carrier in an equal volume, then dry it, calcine it, and then reduce it in a hydrogen atmosphere at 500 °C - 700 °C for 2 - 4 h to obtain a cobalt-carbon catalyst. Then, further impregnate and load other additives on the basis of the cobalt-carbon catalyst in an equal volume to obtain a catalyst precursor;

[0011] S3. Calcinate the catalyst precursor in an inert atmosphere to obtain the cobalt-based catalyst.

[0012] The preparation method of the novel cobalt-based catalyst proposed by the present invention is divided into three steps: The first step is the pretreatment of the carrier. The carrier is pickled with nitric acid to improve the surface activity and metal loading capacity of the carrier; the cobalt source is impregnated on the carrier in an equal volume, and calcined and pre-reduced in a mixed atmosphere containing hydrogen, so that a stable metal cobalt or cobalt oxide structure is formed by the decomposition of cobalt salt on the catalyst surface; the second step is the doping of additives (for example: phosphorus). Nickel, copper, silver, molybdenum, phosphorus or sulfur (taking phosphorus as an example, ammonium dihydrogen phosphate is added) are doped in the above cobalt-based catalyst by equal-volume impregnation, and then the catalyst is dried to obtain a catalyst precursor; the third step is calcination in a nitrogen atmosphere, and then the calcined catalyst is put into a reduction furnace for reduction treatment in a hydrogen atmosphere, and finally a cobalt-based catalyst (taking phosphorus as an example, Co2P@Co / AC catalyst) is obtained. It is found that the doping of phosphorus changes the electronic structure of the catalyst, and a thin film of partial Co2P phase is formed on the surface of metallic cobalt. The formation of this interfacial phase structure weakens the hydrogenation ability of cobalt and inhibits the deep hydrogenation of MG, thereby improving the selectivity of MG.

[0013] Preferably, the specific steps of the pretreatment in step S1 are: oxidize the carrier in nitric acid with a mass fraction of 8% - 12% to open the pores to obtain a pretreated carrier. More preferably, oxidize the carrier in nitric acid with a mass fraction of 10% to open the pores to obtain a pretreated carrier, and the carrier is coconut shell charcoal.

[0014] The present invention uses modified coconut shell charcoal as the carrier, combined with nitric acid pickling treatment, which enhances the specific surface area, metal loading capacity and thermal stability of the catalyst. Research shows that after high-temperature calcination and appropriate reduction treatment, the cobalt-based catalyst can still maintain high activity and selectivity after multiple cycles of use, and cobalt phosphide has strong anti-sintering ability.

[0015] Preferably, the calcination conditions in step S2 are: calcination temperature 450°C - 550°C, calcination time 1 - 2 h. Further preferably, the calcination conditions in step S2 are: calcination temperature 500°C, calcination time 1.5 h.

[0016] Preferably, the molar ratio of cobalt to other promoters in step S2 is 2 - 4:1, the cobalt source is Co(NO3)2·6H2O, and the other promoters are selected from one of nickel, copper, silver, molybdenum, phosphorus, and sulfur, with the salts of the other promoters as raw materials. Further preferably, the molar ratio of cobalt to other promoters in step S2 is 3:1.

[0017] Preferably, the calcination conditions in step S3 are: calcination temperature 450°C - 550°C, calcination time 1 - 2 h. Further preferably, the calcination conditions in step S3 are: calcination temperature 500°C, calcination time 1.5 h.

[0018] The third object of the present invention is to provide the application of the described bifunctional cobalt-based catalyst in the selective hydrogenation of dimethyl oxalate to methyl glycolate.

[0019] The fourth object of the present invention is to provide a method for the selective hydrogenation of dimethyl oxalate to methyl glycolate, comprising the following steps: putting the described bifunctional cobalt-based catalyst into a reaction vessel, introducing hydrogen under atmospheric pressure conditions, maintaining the temperature in the reaction vessel at 500°C - 700°C, and heating for 1 - 3 h for pretreatment; after the pretreatment is completed, cooling the described bifunctional cobalt-based catalyst to the reaction temperature, adding a methanol solution of dimethyl oxalate with a mass fraction of 5% - 30% to react to prepare methyl glycolate, and the reaction conditions are: reaction temperature 160°C - 250°C, reaction pressure 2.0 - 3.0 MPa, liquid phase space velocity 0.1 - 5.0 h-1, and molar ratio of hydrogen to dimethyl oxalate 115 - 125:1.

[0020] The reaction conditions are: reaction temperature 170°C - 190°C, reaction pressure 2.0 - 3.0 MPa, liquid phase space velocity 0.2 - 0.3 h-1, and molar ratio of hydrogen to dimethyl oxalate 120:1. Further preferably, the described reaction conditions are: reaction temperature 180°C, reaction pressure 2.5 MPa, liquid phase space velocity 0.25 h-1, and molar ratio of hydrogen to DMO 120:1.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. Aiming at the problem that traditional copper (Cu) and silver (Ag) catalysts are prone to deactivation under high temperature and hydrogen-rich conditions, the present invention aims to develop a new type of cobalt-based catalyst that can maintain high activity and selectivity under harsh reaction conditions, significantly improve the conversion efficiency and selectivity of DMO to MG, and achieve higher product yields.

[0023] 2. Given that the high cost of silver catalysts limits their industrial applications, the present invention is dedicated to developing an economically viable alternative catalyst that can significantly reduce production costs while ensuring high catalytic performance, thereby enhancing the economic feasibility of the process.

[0024] 3. Aiming at the problem that existing catalysts are prone to structural changes and deactivation under high-temperature conditions, the present invention aims to design a catalyst with excellent thermal stability that can maintain its activity for a long time at higher temperatures, extend the service life of the catalyst, reduce the frequency of catalyst replacement, and improve production efficiency.

[0025] 4. Aiming at the problem that the SiO2 support is prone to degradation in methanol solvent, resulting in the loss of catalytic active sites, the present invention aims to develop a new catalyst system that can maintain structural stability under reaction conditions and avoid the loss of active components.

[0026] 5. Considering the environmental impact in traditional catalytic processes, the present invention aims to develop a catalytic system that conforms to the principles of green chemistry, reduces the generation of harmful by-products, improves the economic efficiency of the reaction, and realizes a cleaner and more sustainable production process. It provides more effective technical support for the industrial production of the biodegradable plastic polyglycolic acid (PGA) and promotes the development of the environmental protection material industry. Description of the Drawings:

[0027] Figure 1 Catalytic hydrogenation performance of cobalt-based catalysts prepared with different supports for Examples 1-4;

[0028] Figure 2 Catalytic hydrogenation performance of cobalt-based catalysts doped with different phosphorus contents for Examples 1 and 11-14;

[0029] Figure 3 Catalytic hydrogenation performance of the P-Co / AC catalyst obtained in Examples 1 and 15-17 at different reduction temperatures;

[0030] Figure 4Spectrograms and surface element mapping images of relevant catalysts under different reaction conditions: (a) XRD patterns of cobalt-based catalysts doped with different phosphorus contents (Examples 1 and 11-14), (b) XRD patterns of P-Co / AC catalysts under different reduction temperatures (Examples 1 and 15-17), (c) TEM image and surface element distribution map of 1P-Co / AC-fresh sample, (d) TEM image and surface element distribution map of 1P-Co / AC-400, (e) TEM image and surface element distribution map of 1P-Co / AC-700, (f) Line scan results of the catalyst under low-temperature reduction (1P-Co / AC-400, right) and high-temperature reduction (1P-Co / AC-700, left) conditions, (g) XPS analysis of Co catalyst in the Co2p region under different reduction temperatures (Examples 1 and 15-17), (h) XPS analysis in the P2p region under different reduction temperatures (Examples 1 and 15-17);

[0031] Figure 5 Kinetic tests of the catalyst for DMO hydrogenation and MG hydrogenation at different reduction temperatures in Examples 1 and 15-17. Detailed implementation methods:

[0032] The following examples are further illustrations of the present invention rather than limitations thereof.

[0033] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention. Unless otherwise specified, the experimental materials and reagents in this article are all conventional commercially available products in this technical field.

[0034] A preparation method of a novel cobalt-based catalyst includes the following steps:

[0035] S1. Oxidize the carrier in an acid solution to open the pores to obtain a pretreated carrier;

[0036] S2. First, impregnate the cobalt source on the pretreated carrier in an equal volume, then dry, calcine, and then reduce in a hydrogen atmosphere at 500 °C - 700 °C for 2 - 4 h to obtain a cobalt-carbon catalyst, and then further impregnate and load other additives on the basis of the cobalt-carbon catalyst in an equal volume to obtain a catalyst precursor;

[0037] S3. Calcinate the catalyst precursor in an inert atmosphere to obtain the cobalt-based catalyst.

[0038] In the following preferred embodiments, the specific steps of the pretreatment in step S1 are as follows: The carrier is subjected to oxidation treatment in nitric acid with a mass fraction of 8%-12% to open the pores, obtaining a pretreated carrier. Further preferably, the carrier is subjected to oxidation treatment in nitric acid with a mass fraction of 10% to open the pores, obtaining a pretreated carrier, and the carrier is coconut shell charcoal.

[0039] The present invention uses modified coconut shell charcoal as the carrier, combined with nitric acid pickling treatment, enhancing the specific surface area, metal loading capacity and thermal stability of the catalyst. After high-temperature calcination and appropriate reduction treatment, the cobalt-based catalyst can still maintain high activity and selectivity after multiple cycles of use, and cobalt phosphide has a strong anti-sintering ability.

[0040] In the following preferred embodiments, the calcination conditions in step S2 are: the calcination temperature is 450°C - 550°C, and the calcination time is 1 - 2 h. Further preferably, the calcination conditions in step S2 are: the calcination temperature is 500°C, and the calcination time is 1.5 h.

[0041] In the following preferred embodiments, the molar ratio of cobalt to other promoters in step S2 is 2 - 4:1, the cobalt source is Co(NO3)2·6H2O, and the other promoters are selected from one of nickel, copper, silver, molybdenum, phosphorus and sulfur, using the salts of the other promoters as raw materials. Further preferably, the molar ratio of cobalt to other promoters in step S2 is 3:1.

[0042] In the following preferred embodiments, the calcination conditions in step S3 are: the calcination temperature is 450°C - 550°C, and the calcination time is 1 - 2 h. Further preferably, the calcination conditions in step S3 are: the calcination temperature is 500°C, and the calcination time is 1.5 h.

[0043] A method for the selective hydrogenation of dimethyl oxalate to methyl glycolate includes the following steps: Put the bifunctional cobalt-based catalyst into a reaction vessel, under atmospheric pressure, introduce hydrogen, maintain the temperature in the reaction vessel at 500°C - 700°C, and heat for 1 - 3 h for pretreatment; after the pretreatment is completed, cool the bifunctional cobalt-based catalyst to the reaction temperature, add a methanol solution of dimethyl oxalate with a mass fraction of 5% - 30% to react to prepare methyl glycolate, and the reaction conditions are: the reaction temperature is 160°C - 250°C, the reaction pressure is 2.0 - 3.0 MPa, the liquid phase space velocity is 0.1 - 5.0 h-1, and the molar ratio of hydrogen to dimethyl oxalate is 115 - 125:1.

[0044] In the following preferred embodiments, the mass fraction of dimethyl oxalate methanol solution is 10%, the reaction conditions are: reaction temperature is 170°C-190°C, reaction pressure is 2.0-3.0MPa, liquid phase space velocity is 0.2-0.3h-1, and the molar ratio of hydrogen to dimethyl oxalate is 120:1. Further preferably, the reaction conditions are: reaction temperature is 180°C, reaction pressure is 2.5MPa, liquid phase space velocity is 0.25h-1, and the molar ratio of hydrogen to DMO is 120:1.

[0045] Example 1

[0046] A method for preparing a novel cobalt-based catalyst comprises the following steps:

[0047] S1. Use coconut shell charcoal as a carrier and use 10wt% dilute nitric acid to oxidize the coconut shell charcoal to open the pores of the activated carbon, so that the dimethyl oxalate hydrogenation reaction process and the metal load can be diffused more easily. The specific steps are: take an appropriate amount of coconut shell charcoal (AC) and dry it to constant weight at 100°C. Take 10g of coconut shell charcoal, 16.6g of concentrated nitric acid with a concentration of 65% and 92g of deionized water and put them into a round-bottom flask. Condensation reflux is carried out at 70°C for 24 hours. The oxidized coconut shell charcoal is filtered in a Buchner funnel and repeatedly rinsed with deionized water until the pH value is neutral. The final coconut shell charcoal (AC) is placed in an oven at 110°C for storage.

[0048] S2, impregnating an equal volume of cobalt nitrate (Co(NO3)2·6H2O) solution on the coconut shell charcoal obtained in step S1, then drying in an oven at 100°C, calcining at 500°C for 1.5 hours, and then reducing in a hydrogen atmosphere at 600°C for 3 hours to obtain a cobalt-carbon catalyst, and then impregnating the diammonium hydrogen phosphate solution on the cobalt-carbon catalyst, stirring for 5 hours to ensure uniform dispersion, and drying at 80°C for 12 hours to remove excess moisture to obtain a catalyst precursor;

[0049] The specific steps are as follows: Take the pretreated coconut shell charcoal (AC) from step S1, and gradually add an aqueous solution of Co(NO3)2·6H2O dropwise to the coconut shell charcoal. Stir for 5 minutes and then perform ultrasonic treatment for 5 minutes. Let it stand at room temperature for 12 hours and dry at 110°C for 12 hours. The dried substance is calcined in a quartz tube of a tubular furnace. The calcination conditions are as follows: It is carried out under a 10 vol% H2 / N2 atmosphere with a flow rate of 100 mL / min. Heating conditions: Heat from room temperature to 250°C at a rate of 2°C / min and hold for 1 hour, then continue to heat to 500°C at the same rate and hold for 2 hours. After the calcination is completed, perform reduction treatment using a 1 vol% O2 / N2 atmosphere, and reduce in a hydrogen atmosphere at 600°C for 3 hours to obtain a 15Co / AC-500 catalyst. Add an aqueous solution of NH4H2PO4 to the 15Co / AC-500 catalyst, impregnate at room temperature for 12 hours, and further dry at 110°C for 12 hours to obtain a catalyst precursor.

[0050] S3. Heat the catalyst precursor in a nitrogen atmosphere from room temperature to 250°C at a heating rate of 2°C / min and hold for 1 hour, and then heat from 250°C to 500°C at a rate of 5°C / min and hold for 2 hours. Finally, obtain a bifunctional active catalyst with trace phosphidation occurring on the surface of metallic cobalt, where the cobalt loading is 15 wt% and the phosphorus doping amount is 1 wt%.

[0051] Perform a catalytic activity test on the catalyst obtained above: Mix the prepared cobalt-based catalyst (about 1 g) with quartz sand (about 3 g), and then introduce the mixture into the isothermal zone of a fixed-bed reactor. The inner diameter of the fixed-bed reactor is 10 mm, and a thermocouple is inserted into the catalyst bed to monitor the temperature in real time. Before the reaction, the catalyst needs to be pretreated in a hydrogen atmosphere. The specific steps are as follows: Under atmospheric pressure conditions, set the flow rate of hydrogen introduced at 100 mL / min, maintain the temperature at about 600°C, and heat for about 2 hours. After the pretreatment is completed, cool the catalyst to the set reaction temperature. During the reaction, use a methanol solution of dimethyl oxalate (DMO) with a mass fraction of 10% as the reactant and pump it into the reactor. The molar ratio of the reaction (hydrogen) to DMO is set at about 180, and the reaction pressure is maintained within the range of 2.5 MPa. At the start of the reaction, set the temperature at around 180°C and set the weight hourly space velocity (WHSV) of DMO at normal temperature at 0.1 h-1. During this period, regularly monitor the temperature and pressure. After the reaction is completed, quickly cool the reactor, collect the reaction product samples for analysis, and use analytical methods such as gas chromatography (GC) to quantitatively determine the product distribution.

[0052] Examples 2 - 4

[0053] Same as Example 1, except that the carriers are: hydroxyapatite (HAP), carbon nitride (C3N4), and silica (SiO2) respectively.

[0054] Record the influence of different carriers on the performance of the catalyst, analyze the conversion rate of DMO, the selectivity of MG and EG, and the generation of side reactions. The results are as Figure 1 shown. When using hydroxyapatite (HAP) as the carrier, the conversion rate of DMO is only 37.58%, and the main product is methyl glycolate (MG) with a selectivity of 59.46%. When using silica (SiO2) as the carrier, the DMO conversion rate drops to 30.5%, and the main products are alcohols, with the selectivity of ethylene glycol being 23.5% and that of ethanol being 5.8%. When using carbon nitride (C3N4) as the carrier, the DMO conversion rate increases significantly, and the selectivities of methyl glycolate and ethylene glycol are 40.64% and 35.74% respectively. When using activated carbon (AC) as the carrier, the DMO conversion rate is the highest, reaching 97.01%, among which the selectivity of ethylene glycol is 73.57% and that of ethanol is 14.31%.

[0055] In summary, different carriers show significant differences in the hydrogenation activity of the cobalt-based catalyst. Among them, the catalyst with activated carbon (AC) as the carrier has the highest catalytic hydrogenation ability at low temperatures.

[0056] Example 5

[0057] Same as Example 1, except that:

[0058] The reaction conditions during the catalyst activity test are as follows: the molar ratio of the reaction (hydrogen) to DMO is set at about 120, and the reaction pressure is maintained within the range of 2.5 MPa. At the beginning of the reaction, the temperature is set near 180 °C, and the weight hourly space velocity (WHSV) of DMO at room temperature is set at 0.25 h -1 .

[0059] Examples 6 - 10

[0060] Same as Example 5, except that the promoters are Ni, Cu, Ag, Mo, and the non-metallic promoter S, which are added in the form of nickel nitrate, copper nitrate, silver nitrate, molybdenum nitrate, and ammonium sulfate respectively.

[0061] The influence of promoter doping on the catalytic performance of the Co-based catalyst in Examples 5 - 10 is shown in Table 1:

[0062] Table 1 Influence of promoter doping on the catalytic performance of the Co-based catalyst

[0063]

[0064]

[0065] Example 5 was analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). It was found that the presence of cobalt phosphide significantly enhanced the adsorption capacity for DMO, while reducing the adsorption intensity and activation energy for methyl glycolate (MG). This property effectively inhibited the over-hydrogenation reaction of MG and improved the selectivity of MG.

[0066] Examples 11 - 14

[0067] Same as Example 1, except that xP-Co / AC catalysts with different phosphorus contents were prepared, namely 0.5 wt%, 0.75 wt%, 2 wt% and 4 wt%.

[0068] In the same high-pressure reactor, catalysts with different P addition amounts were used to ensure consistent experimental conditions each time. Samples were collected at time intervals and analyzed for changes in their conversion rates and selectivities. Qualitative and quantitative analyses were performed using GC and mass spectrometry (MS).

[0069] Analyze the changes in the conversion rate of DMO and the selectivity of MG with phosphorus content at different P contents, as Figure 2 shown.

[0070] Examples 15 - 17

[0071] Same as Example 1, except that the same P-Co / AC catalyst was reduced at 400 °C, 600 °C and 700 °C respectively for 4 hours to ensure the formation of active sites on the catalyst at different temperatures. Evaluate the influence of different reduction temperatures on the catalyst performance.

[0072] Adjust the equipment to different reduction temperatures and maintain gas flow to ensure a stable reaction environment. React at each temperature and record the DMO conversion rate, product selectivity and by-product formation. After the reaction, quickly cool down, collect samples for analysis. Record the catalyst performance at different reduction temperatures and analyze the changes in the DMO conversion rate and MG selectivity with the reduction temperature, as Figure 3 shown.

[0073] To further study this, three catalysts were observed by transmission electron microscopy (HRTEM) and mapping analysis: the precursor of the 1P-Co / AC catalyst, 1P-Co / AC-400 and 1P-Co / AC-700 (see Figure 4)。Initially, the catalyst particles in the precursor were uniformly distributed with a particle size of approximately 7.6 ± 1.8 nm. HRTEM and Fourier transform analysis indicated the coexistence of CoO and metallic Co, with lattice spacings of 0.241 nm (CoO (111) plane) and 0.207 nm (Co (111) plane) respectively, confirming CoO as the main phase. For the 1P-Co / AC-400 catalyst reduced at a lower temperature, the particle size of the metallic Co particles slightly increased to 9.3 ± 1.5 nm. The disappearance of the CoO(111) phase confirmed the effective reduction of Co, providing favorable conditions for the hydrogenation reaction, thus promoting the hydrogenolysis of DMO and MG to produce EG. In contrast, at 700 °C, significant particle aggregation was observed, with the particle size increasing to 31.8 ± 8.6 nm. The lattice spacing indicated the simultaneous formation of Co2P with metallic Co, showing phosphidation at high temperatures. This led to an increase in the selectivity of hydrogenation from DMO to MG, possibly due to the influence of cobalt phosphide species, which may inhibit the original hydrogenation activity of metallic Co. The mapping images of the catalyst precursor (see Figure 4 b) showed that the Co and P elements were uniformly distributed on the carbon support without aggregation. After reduction at 400 °C, the Co element continued to be uniformly dispersed, but partial aggregation of Co particles began to occur, while the distribution of the P element remained uniform on the support. In contrast, at 700 °C, significant aggregation of Co particles occurred, forming larger clusters. In particular, the P element was mainly associated with the Co particles and was rarely distributed on the carbon support. To further understand these changes, line scan analysis was performed on the catalyst particles reduced at 400 °C and 700 °C (see Figure 4 c). The results at 400 °C showed that the catalyst consisted only of cobalt particles, and no overlap of C, O, or P elements was detected. However, after reduction at 700 °C, the line scan results showed the simultaneous presence of Co and P elements, and the content of Co was higher, strengthening their uniform overlap and dispersion inside the particles. Combining XRD and TEM analysis, it was shown that at low-temperature reduction, cobalt oxide was effectively reduced to metallic cobalt, while phosphorus remained amorphous and was uniformly dispersed on the carbon support. At high-temperature reduction, the P compound migrated on the carbon surface and formed a stable cobalt phosphide phase on the larger cobalt particles, resulting in the formation of an interfacial structure between Co and P. This unique structure changed the activity and selectivity of hydrogenolysis.

[0074] The DMO hydrogenation process involves a series of reactions from DMO to MG, MG to EG, and potentially EG to ethanol (EtOH). To study the effect of the reduction temperature on this process, kinetic experiments were conducted to measure the activation energy of the catalysts activated at different temperatures. As Figure 5As shown, the activation energies of the catalyst reduced at 400 °C for the conversion of DMO to MG and MG to EG are similar, being 103.7 kJ / mol and 103.8 kJ / mol respectively. This indicates that after reduction at 400 °C, there is no obvious distinction on the catalyst surface between these two hydrogenation steps, leading to challenges in controlling the selectivity of the intermediate product and potentially resulting in over-hydrogenation of MG to form EG. At 500 °C, the activation energies of DMO-MG and MG-EG are 76.0 kJ / mol and 89.9 kJ / mol respectively. The lower activation energy of DMO-MG compared to MG-EG indicates that the catalyst can inhibit the deep hydrogenation of MG, resulting in an increase in the selectivity of MG in the experiment. When the catalyst is further reduced at 600 °C, a significant decrease in the activation energy of the first-step reaction is observed. The activation energies of DMO-MG and MG-EG are 40.2 kJ / mol and 80.45 kJ / mol respectively. This significant difference in activation energy ensures that the step from DMO to MG proceeds faster, thereby enhancing the selectivity of MG in the sequential reaction. The lower activation energy is beneficial for efficient reactions, especially at lower temperatures, enabling the catalyst reduced at high temperatures to effectively promote the selective hydrogenation of DMO to MG while preventing over-hydrogenation of MG. This provides important kinetic basis for understanding the reaction mechanism of the DMO hydrogenation process and the selectivity of the catalyst.

[0075] Example 18

[0076] Same as Example 5, except that: the specific steps of the pretreatment in Step S1 are: oxidizing the support in nitric acid with a mass fraction of 8% to open the pores to obtain the pretreated support. In Step S2, pre-reduction treatment is carried out at 450 °C for 2 h, and then reduction is carried out in a hydrogen atmosphere at 500 °C for 4 h to obtain the cobalt-carbon catalyst, and the molar ratio of cobalt to other additives is 2:1. In Step S3, the calcination conditions are: calcination temperature 450 °C, calcination time 2 h; the catalyst is reduced at 500 °C in a hydrogen atmosphere for 4 h.

[0077] The reaction conditions for the hydrogenation of dimethyl oxalate to methyl glycolate are: adding a methanol solution of dimethyl oxalate with a mass fraction of 30% to react to prepare methyl glycolate, the reaction temperature is 160 °C, the reaction pressure is 2.0 MPa, the liquid-phase space velocity is 0.1 h-1, and the molar ratio of hydrogen to dimethyl oxalate is 115:1.

[0078] Example 19

[0079] Same as Example 5, except that: the specific steps of the pretreatment in Step S1 are: oxidizing the support in nitric acid with a mass fraction of 12% to open the pores, obtaining the pretreated support. In Step S2, calcine and pre-reduce at 550 °C for 1 h, and then reduce in a hydrogen atmosphere at 700 °C for 2 h to obtain the cobalt-carbon catalyst, and the molar ratio of cobalt to other additives is 4:1. In Step S3, the calcination conditions are: the calcination temperature is 550 °C and the calcination time is 1 h; the catalyst is reduced at 700 °C in a hydrogen atmosphere for 2 h.

[0080] The reaction conditions for the hydrogenation of dimethyl oxalate to methyl glycolate are: adding a methanol solution of dimethyl oxalate with a mass fraction of 5% to react to prepare methyl glycolate, the reaction temperature is 250 °C, the reaction pressure is 3.0 MPa, the liquid-phase space velocity is 5.0 h-1, and the molar ratio of hydrogen to dimethyl oxalate is 125:1.

[0081] The description of the above examples is only used to help understand the technical solution and its core idea of the present invention. It should be noted that for those skilled in the art of this technology, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A bifunctional cobalt-based catalyst, characterized in that: The invention comprises a carrier and cobalt loaded on the carrier and other additives, wherein the carrier is selected from one of activated carbon, hydroxide apatite, carbon nitride and silicon dioxide, and the other additives are selected from one of nickel, copper, silver, molybdenum, phosphorus and sulfur, and the molar ratio of cobalt to the other additives is 2-4:

1.

2. The bifunctional cobalt-based catalyst according to claim 1, characterized in that Based on the mass of the carrier, the loading amount of cobalt is 5-20wt%.

3. The method for preparing the bifunctional cobalt-based catalyst according to claim 1 or 2, characterized in that: The coupling of bifunctional active centers on the catalyst surface is achieved by step-by-step impregnation, including the following steps: S1, oxidizing the support in an acid solution to open the pores to obtain a pretreated support; S2, firstly impregnate an equal volume of a cobalt source on a pretreated carrier, then dry, calcine, and then reduce in a hydrogen atmosphere at 500°C-700°C for 2-4h to obtain a cobalt-carbon catalyst, and then further impregnate and load other additives in equal volumes on the cobalt-carbon catalyst to obtain a catalyst precursor; S3, calcining the catalyst precursor under an inert atmosphere to obtain the cobalt-based catalyst.

4. The preparation method according to claim 3, characterized in that: The specific steps of the pretreatment in step S1 are: oxidizing the carrier in nitric acid with a mass fraction of 8%-12% to open the pores, thereby obtaining a pretreated carrier.

5. The preparation method according to claim 3, characterized in that: The calcination conditions in step S2 are: calcination temperature 450° C.-550° C., and calcination time 1-2 h.

6. The preparation method according to claim 3 or 5, characterized in that: In step S2, the molar ratio of cobalt to other additives is 2-4:1, the cobalt source is Co(NO3)2·6H2O, and other additives are selected from one of nickel, copper, silver, molybdenum, phosphorus and sulfur, and the salt of other additives is used as raw material.

7. The preparation method according to claim 3, characterized in that: The calcination conditions in step S3 are: calcination temperature 450° C.-550° C., and calcination time 1-2 h.

8. Use of the bifunctional cobalt-based catalyst according to claim 1 or 2 in the selective hydrogenation of dimethyl oxalate to produce methyl glycolate.

9. A method for preparing methyl glycolate by selective hydrogenation of dimethyl oxalate, characterized in that: The method comprises the following steps: placing the bifunctional cobalt-based catalyst according to claim 1 or 2 into a reaction container, introducing hydrogen under atmospheric pressure, maintaining the temperature in the reaction container at 500° C.-700° C., and heating for 1-3 hours for pretreatment; after the pretreatment, cooling the bifunctional cobalt-based catalyst to the reaction temperature, adding a methanol solution of dimethyl oxalate with a mass fraction of 5%-30% to react and prepare methyl glycolate, and the reaction conditions are: the reaction temperature is 160° C.-250° C., the reaction pressure is 2.0-3.0 MPa, and the liquid phase space velocity is 0.1-5.0 h -1 , the molar ratio of hydrogen to dimethyl oxalate is 115-125:

1.

10. The method according to claim 9, characterized in that The reaction conditions are as follows: reaction temperature is 170° C.-190° C., reaction pressure is 2.0-3.0 MPa, liquid phase space velocity is 0.2-0.3 h-1, and the molar ratio of hydrogen to dimethyl oxalate is 120:1.

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