ZIF-8-derived nitrogen-doped carbon-loaded copper catalyst as well as preparation method and application thereof

Abstract

The invention particularly relates to a ZIF-8-derived nitrogen-doped carbon-loaded copper catalyst as well as a preparation method and application of the ZIF-8-derived nitrogen-doped carbon-loaded copper catalyst. The method comprises the following steps that copper acetate, zinc acetate and methyl alcohol are subjected to ultrasonic dissolution and mixed with a methyl alcohol solution of 2-methylimidazole, then stirring, heating, centrifugal washing, freeze drying and roasting reduction are conducted, the Cu / NC catalyst, namely the ZIF-8 derived nitrogen-doped carbon-loaded copper catalyst is prepared, and the loading capacity of copper is 5-20 wt%; according to the invention, products MM and MG are obtained in a dimethyl oxalate hydrogenation path on the prepared ZIF-8 derived nitrogen-doped carbon-loaded copper catalyst for the first time; a metal-nitrogen-carbon synergistic effect exists between the copper nanoparticles and the nitrogen-doped carbon carrier, the chemical state of a Cu component can be effectively adjusted through interaction of copper sites and nitrogen doping sites, on one hand, adsorption and dissociation of DMO molecules are promoted, on the other hand, adsorption and dissociation of hydrogen can be promoted, and then hydrogenation efficiency and product selectivity are improved.

Description

Technical Field

[0001] The invention relates to the technical field of catalyst preparation and application, and in particular to a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst and a preparation method and application thereof. Background Art

[0002] Polyglycolic acid (PGA), also known as polyglycolic acid or polyglycolide, is the simplest linear aliphatic polyester and the fastest degradable aliphatic polyester polymer material. PGA has good biocompatibility, gas barrier properties, and machinability. It can be completely degraded into non-toxic and harmless CO within a few months. 2 Therefore, it will not cause pollution to the environment when it is discarded, and is expected to replace petroleum-based plastic materials, which has great practical significance and commercial value for the sustainable development of the ecological environment.

[0003] Polyglycolic acid is polymerized with glycolic acid (GA) or its precursor methyl glycolate (MG) as raw materials. The main synthesis methods include solution polycondensation, formaldehyde and carbon monoxide polycondensation, glycolic acid (ester) direct polycondensation and ring-opening polymerization. Early polyglycolic acid was mainly prepared by polycondensation of glycolic acid (GA). As the monomer of PGA, its synthesis methods mainly include chloroacetic acid hydrolysis, formaldehyde carbonylation and cyanidation. Chloroacetic acid hydrolysis will produce a large amount of volatile hydrochloric acid during the reaction, which is highly corrosive to equipment and causes serious environmental pollution. In addition, the yield of GA is low, so it cannot be mass-produced. Although the formaldehyde carbonylation method has low process raw material cost, it has high reaction temperature and high technical difficulty. In addition, there is strong liquid acid in the reaction process, which seriously corrodes the equipment and has high requirements for equipment. The cyanidation method uses formaldehyde and hydrocyanic acid as raw materials and is synthesized by adding cyanide, but cyanide is highly toxic and the safety pressure in the production process is relatively high. The glycolide ring-opening polymerization method is to first hydrolyze and polycondense MG to obtain glycolic acid oligomers, and then heat and cyclize to depolymerize glycolide. The high molecular weight polymer PGA is obtained by ring-opening polymerization of glycolide. It is a promising PGA industrial production route. In recent years, the preparation of methyl methoxyacetic acid methyl ester by carbonylation of methylal and the generation of methyl glycolate by hydrolysis have attracted widespread attention. However, on the one hand, methylal is prone to disproportionation reaction, with more by-products, difficult post-treatment, and not conducive to environmental protection requirements; on the other hand, the carbonylation reaction of methylal is greatly affected by the acid strength. Stronger acids have stronger catalytic activity, but strong acids corrode equipment severely. In summary, although there are many routes for synthesizing polyglycolic acid, due to harsh reaction conditions, complex reaction process, more by-products, easy corrosion of equipment, and environmental pollution, etc. Therefore, it is urgent to develop environmentally friendly, efficient and low-pollution PGA synthesis technology.

[0004] In recent years, the selective hydrogenation of dimethyl oxalate to methyl glycolate has received widespread attention. As an intermediate product in the coal-to-ethylene glycol process, dimethyl oxalate can also be used to prepare methyl glycolate by controlling the degree of hydrogenation, and glycolic acid can be obtained after hydrolysis. This process is in line with my country's energy structure with abundant coal and natural gas resources, and no serious pollution by-products are generated during the reaction, which is in line with the economic and environmental development route. At present, the hydrogenation pathway of dimethyl oxalate on copper-based catalysts is: dimethyl oxalate (DMO) → methyl glycolate (MG) → ethylene glycol (EG) → ethanol.

[0005] However, it is difficult to balance the conversion of DMO and the selectivity of MG on copper-based catalysts, that is, the yield of MG is relatively low, and a small amount of methyl methoxyacetate (MM) may be generated during the hydrogenation process. Since the selectivity of MM is very low or even absent, MM is considered to be a by-product, and MG can be generated by hydrolysis of MM.

[0006] Therefore, how to hydrogenate dimethyl oxalate to produce methyl methoxyacetate (MM) by regulating the surface properties and structure of the catalyst, and then further hydrolyze it to produce methyl glycolate is expected to become a highly competitive PGA synthesis route. Summary of the invention

[0007] The present invention aims to solve the technical problems that it is difficult to balance the conversion rate of DMO and the selectivity of MG on a copper-based catalyst in the existing dimethyl oxalate hydrogenation pathway, the yield of MG is relatively low, the catalyst activity is low, the MM selectivity is low or even zero, and the stability is poor. A preparation method of a ZIF-8-derived nitrogen-doped carbon-supported copper catalyst is provided.

[0008] The present invention realizes for the first time that the products MM and MG are obtained in the hydrogenation pathway of dimethyl oxalate on the prepared ZIF-8 derived nitrogen-doped carbon-supported copper catalyst, and for the first time realizes that the hydrogenation pathway changes. MM can generate MG by hydrolysis reaction, so both the conversion rate of DMO and the selectivity of MG are taken into account.

[0009] A method for preparing Cu / NC catalyst was conducted using Cu-ZIF-8 as a precursor by doping and calcination reduction. Due to its high specific surface area and porous structure, it can provide more active sites, thereby increasing the contact area between the catalyst and the reactants, which is helpful for the mass transfer of the reactants and the diffusion of the products. Nitrogen-doped carbon materials derived from metal-N-ZIF-8 usually have good thermal stability and can remain stable at higher reaction temperatures, thereby improving the stability of the catalyst and the selectivity of the product.

[0010] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst, comprising the following steps: ultrasonically dissolving copper acetate, zinc acetate, and methanol and mixing them with a methanol solution of 2-methylimidazole, and then stirring, heating, centrifugal washing, freeze-drying and calcination reduction to prepare a Cu / NC catalyst, i.e., a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst.

[0011] As a further limitation of the technical solution of the present invention, the copper loading in the ZIF-8 derived nitrogen-doped carbon-supported copper catalyst is 5 to 20 wt%.

[0012] As a further limitation of the technical solution of the present invention, the mass ratio of copper acetate, zinc acetate and methanol is 1:4:79, the mass ratio of 2-methylimidazole and methanol is 1:4, and the molar ratio of 2-methylimidazole to zinc acetate is 8:1.

[0013] As a further limitation of the technical solution of the present invention, the stirring time is 1 hour; the heating is carried out in a hydrothermal kettle, the reaction temperature of the hydrothermal kettle is 120° C., and is maintained for 4 hours.

[0014] As a further limitation of the technical solution of the present invention, the centrifugal washing speed is 8000r / min, and the centrifugal washing is performed twice.

[0015] As a further limitation of the technical solution of the present invention, the freeze-drying time is 48 hours.

[0016] As a further limitation of the technical solution of the present invention, the roasting reduction conditions are: roasting temperature 900°C, roasting time 4h; reduction temperature 400°C, reduction time 2h; the roasting reduction atmosphere conditions: roasting is N 2 atmosphere, reducing atmosphere is H 2 -N 2 Mixed atmosphere.

[0017] The present invention also provides a ZIF-8-derived nitrogen-doped carbon-supported copper catalyst obtained by the above-mentioned preparation method.

[0018] In addition, the present invention also provides the use of the ZIF-8 derived nitrogen-doped carbon-supported copper catalyst in the reaction of hydrogenating dimethyl oxalate to prepare methyl glycolate.

[0019] In the process of hydrogenation of dimethyl oxalate, the selection of catalysts should comprehensively consider multiple factors such as catalytic performance, thermal stability, and preparation methods. In the process of preparing catalysts by doping, calcination and reduction, ZIF-8, as one of the metal organic framework compounds, has the advantages of rich nitrogen atoms, high specific surface area, excellent chemical stability and thermal stability, etc., which provides more accessible surfaces, thereby increasing the number of active sites, which is beneficial to improve the selectivity of methyl methoxyacetate / methyl glycolate.

[0020] As a further limitation of the technical solution of the present invention, the conditions for the hydrogenation of dimethyl oxalate are: reaction temperature of 210°C, reaction pressure of 2MPa, H 2 / DMO=100, weight hourly space velocity (WHSV) is 2.0h -1 .

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

[0022] (1) The present invention realizes the production of products MM and MG in the hydrogenation pathway of dimethyl oxalate on the prepared ZIF-8-derived nitrogen-doped carbon-supported copper catalyst, and realizes the change of the hydrogenation pathway for the first time. MM can generate MG by hydrolysis reaction, so both the conversion rate of DMO and the selectivity of MG are taken into account.

[0023] (2) The present invention successfully prepared the Cu / NC catalyst by doping, calcination and reduction. Its high specific surface area and porous structure can provide more active sites, increase the contact area between the catalyst and the reactants, and help improve the conversion rate of DMO.

[0024] (3) The present invention uses Cu-ZIF-8 as a precursor to prepare the Cu / NC catalyst in the process of hydrogenating dimethyl oxalate to prepare methyl glycolate. There is a metal-nitrogen-carbon synergistic effect between the copper nanoparticles and the nitrogen-doped carbon carrier. The interaction between the copper site and the nitrogen-doped site can effectively adjust the chemical state of the Cu component, which can promote the adsorption and dissociation of DMO molecules on the one hand, and promote the adsorption and dissociation of hydrogen on the other hand, thereby improving the hydrogenation efficiency and product selectivity. In addition, the acidic sites on the surface of the nitrogen-doped carbon material derived from metal-N-ZIF-8 can promote the dehydration reaction between the hydrogenation byproduct methyl acetate and methanol, improve the selectivity of methyl methoxyacetate, and methyl methoxyacetate is further hydrolyzed to prepare methyl glycolate. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 N is the Cu / NC catalyst prepared in Examples 1 and 2 of the present invention. 2 Adsorption-desorption characterization.

[0026] Figure 2 is the NH of the Cu / NC catalyst prepared in Example 1 of the present invention3 -TPD characterization. DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with specific embodiments.

[0028] The chemical materials used in this embodiment are: zinc acetate, copper acetate, methanol, 2-methylimidazole, nitrogen, and hydrogen. The combined amounts are as follows: grams, milliliters, and cubic centimeters are used as measurement units.

[0029]

[0030] Example 1

[0031] According to the amount of the above substances, 4.65g of zinc acetate dihydrate and 1g of copper acetate were completely dissolved in 150mL of methanol under ultrasound. At the same time, 13.1g of 2-methylimidazole was dissolved in 75mL of methanol and then quickly added to the above mixture under vigorous stirring at room temperature for 1 hour. Subsequently, the solution was transferred to a polytetrafluoroethylene-lined stainless steel autoclave and heated at 120°C for 4 hours. After naturally cooling to room temperature, the Cu-ZIF-8 precursor was obtained by centrifugation, washing twice with methanol and freeze drying. The Cu-ZIF-8 was then heated to 900°C in a tube furnace at a rate of 5°C / min and kept in flowing nitrogen for 4 hours. After that, it was heated at 400°C with H 2 -N 2 The mixed gas was reduced for 2 h, and the black powder collected after natural cooling to room temperature was labeled as 20Cu / NC.

[0032] The catalyst prepared in the example was used for the performance study of hydrogenating dimethyl oxalate to prepare methyl glycolate. The results are shown in Table 1.

[0033] Table 1 is the performance study results of preparing methyl glycolate by hydrogenation of dimethyl oxalate in the embodiment of the present invention.

[0034] Table 1

[0035]

[0036] Example 2

[0037] According to the amount of the above substances, 4.65g of zinc acetate dihydrate and 0.75g of copper acetate were completely dissolved in 150mL of methanol under ultrasound. At the same time, 13.1g of 2-methylimidazole was dissolved in 75mL of methanol and then quickly added to the above mixture under vigorous stirring at room temperature for 1 hour. Subsequently, the solution was transferred to a polytetrafluoroethylene-lined stainless steel autoclave and heated at 120°C for 4 hours. After naturally cooling to room temperature, the Cu-ZIF-8 precursor was obtained by centrifugation, washing twice with methanol and freeze drying. The Cu-ZIF-8 was then heated to 900°C in a tube furnace at a rate of 5°C / min and kept in flowing nitrogen for 4 hours. After that, it was heated at 400°C with H 2 -N 2 The mixed gas was reduced for 2 h, and the black powder collected after natural cooling to room temperature was labeled as 15Cu / NC.

[0038] The catalyst prepared in the example was used for the performance study of hydrogenating dimethyl oxalate to prepare methyl glycolate. The results are shown in Table 2.

[0039] Table 2 is the performance study results of preparing methyl glycolate by hydrogenation of dimethyl oxalate in the embodiment of the present invention.

[0040] Table 2

[0041]

Claims

1. A method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst, characterized in that: The following steps are involved: Copper acetate, zinc acetate and methanol were ultrasonically dissolved and mixed with a methanol solution of 2-methylimidazole, and then stirred, heated, centrifuged, washed, freeze-dried and calcined to reduce to prepare a Cu / NC catalyst, i.e., a ZIF-8-derived nitrogen-doped carbon-supported copper catalyst.

2. The method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst according to claim 1, characterized in that: The copper loading in the ZIF-8 derived nitrogen-doped carbon-supported copper catalyst is 5-20 wt%.

3. The method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst according to claim 1, characterized in that: The mass ratio of the copper acetate, zinc acetate and methanol is 1:4:79, the mass ratio of 2-methylimidazole and methanol is 1:4, and the molar ratio of 2-methylimidazole to zinc acetate is 8:

1.

4. The method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst according to claim 1, characterized in that: The stirring time is 1 h; the heating is carried out in a hydrothermal autoclave, the reaction temperature of the hydrothermal autoclave is 120° C., and maintained for 4 h.

5. The method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst according to claim 1, characterized in that: The centrifugal washing speed is 8000 r / min, and the centrifugal washing is performed twice.

6. The method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst according to claim 1, characterized in that: The freeze-drying time is 48 h.

7. The method for preparing a ZIF-8 derived nitrogen-doped carbon-supported copper catalyst according to claim 1, characterized in that: The calcination reduction conditions are: calcination temperature 900 °C, calcination time 4 h; The reduction temperature is 400°C and the reduction time is 2h. The atmosphere conditions for the calcination reduction are: the calcination atmosphere is N2 atmosphere and the reduction atmosphere is H2-N2 mixed atmosphere.

8. The ZIF-8 derived nitrogen-doped carbon-supported copper catalyst obtained according to the preparation method of any one of claims 1 to 7.

9. Use of the ZIF-8 derived nitrogen-doped carbon-supported copper catalyst according to claim 8 in the reaction of hydrogenating dimethyl oxalate to prepare methyl glycolate.

10. The use according to claim 9, characterized in that The conditions for hydrogenation of dimethyl oxalate were as follows: reaction temperature was 210 °C, reaction pressure was 2 MPa, H2 / DMO=100, and weight hourly space velocity was 2.0 h -1 .

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