Method for preparing methanol through CO2 hydrogenation by using ZnO coated CuO coated ZnO core-shell catalyst

Abstract

The invention discloses a method for preparing methanol through CO2 hydrogenation by using a ZnO-coated CuO-coated ZnO core-shell catalyst, and relates to a method for preparing methanol by using a catalyst, a ZnO-coated CuO-coated ZnO sandwich core-shell structure material is synthesized through three steps, and the structure has abundant Cu-ZnO interfaces and oxygen vacancies, so that a large number of active sites can be provided, the Cu-ZnO interfaces are increased, and the CO2 hydrogenation performance is remarkably improved. The specific preparation process comprises the step of synthesizing the Zn-MOF, the Zn-MOF (at) Cu-MOF and the Zn-MOF (at) Cu-MOF (at) Zn-MOF. The ZnO-coated CuO-coated ZnO catalyst synthesized by the method has a unique core-shell structure, is simple to prepare, has good CO2 conversion rate and methanol selectivity, and has a wide application prospect.

Description

Technical Field

[0001] The present invention relates to a method for preparing methanol using a catalyst, and in particular to a method for preparing methanol using a ZnO@CuO@ZnO core-shell catalyst. 2 A method for preparing methanol by hydrogenation. Background Art

[0002] Due to the burning of fossil fuels, CO 2 The concentration of CO2 has been increasing, causing irreversible impacts on the earth's environment (climate change and ocean acidification). From 2013 to 2040, the CO2 emissions from the energy system will 2 Emissions are expected to increase by an average of 3%. Over the years, CO2 capture, storage and utilization from concentrated emission sources (natural gas and industrial processes) or from the air 2 The technology (CCUS) constitutes a major 2 To achieve a sustainable and renewable “carbon neutral” economy, artificial CO 2 Combining with chemicals and converting them into value-added chemical products has received extensive attention in the past few decades. 2 It is a C1 material with abundant reserves, low price, non-toxicity, non-flammability and renewable characteristics. 2 The main use of CO is to produce carbonates, carboxylic acids and their derivatives. With the maturity of the technology of using clean energy to electrolyze water to produce hydrogen, the use of CO 2 The method of hydrogenation reaction to achieve carbon emission reduction and convert it into value-added products has come into people's attention. 3 As the most promising clean energy, OH can be used in internal combustion engines as well as as an intermediate in chemical reactions, thus attracting great attention. 2 Catalytic conversion has become the focus of researchers. Catalysts can increase the reaction rate without changing the standard Gibbs energy of the overall reaction, making CO 2 Therefore, the development of highly efficient catalysts in terms of energy, selectivity, production efficiency and reusability is crucial for sustainable economic development.

[0003] Cu-ZnO catalysts are widely used in industrial production due to their low cost and high performance. However, they are easy to sinter, have small specific surface area, and CO 2People have been continuing to conduct in-depth research on the problems of low conversion rate and poor stability. We know that the synergistic effect of Cu and ZnO and the active sites on the surface of the formed Cu-Zn alloy are key factors in catalytic performance. In the present invention, the unique "sandwich" core-shell structure increases the Cu-ZnO interface, and the highly active Cu-ZnO interface sites formed by the migration of ZnO on Cu nanoparticles are also an important reason for the significant enhancement of catalytic activity. In addition, in terms of improving the surface area and gas adsorption capacity of the catalyst, metal-organic frameworks (MOFs) are a new functional material worthy of attention. MOFs can produce a constraint effect on active metals, manufacture customized catalysts that can adjust and control active sites, and enhance the catalytic activity of the catalyst. At the same time, the core-shell structure can not only improve the sintering of copper and improve the stability of the material, but also improve the dispersibility of the particles in the matrix and improve the overall performance of the catalyst. Summary of the invention

[0004] The purpose of the present invention is to provide a ZnO@CuO@ZnO core-shell catalyst for CO 2 A method for preparing methanol by hydrogenation. This method forms a Cu-ZnO contact interface while wrapping the Cu particles in the middle of the ZnO layer, effectively hindering the sintering of metal nanoparticles during the catalytic reaction and exposing more effective active sites. The catalyst is used to produce methanol, achieving the low-pressure CO 2 and H 2 The present invention successfully synthesized the ZnO@CuO@ZnO core-shell catalyst by room temperature synthesis and hydrothermal method, and used it for CO 2 Hydrogenation to produce methanol can improve many problems existing in traditional industrial copper-based catalysts.

[0005] The technical solution of the present invention is as follows: A ZnO@CuO@ZnO core-shell catalyst for CO 2 A method for preparing methanol by hydrogenation, wherein the catalyst has CuO and ZnO as active components, wherein CuO accounts for 10.4-48.6wt% and ZnO accounts for 42.3-67.8wt%; the preparation steps are as follows: (1) Preparation of Zn-MOF precursor A; (2) Take precursor A and add copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) and corresponding organic ligands, and undergo hydrothermal reaction; (3) Centrifuging, washing, drying, and cooling the reaction solution to obtain the intermediate B Zn-MOF@Cu-MOF; (4) Weigh intermediate B and dissolve it in anhydrous methanol again. Add zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) and organic ligand, stirring; (5) Centrifuging, washing and drying the stirred solution to obtain the precursor Zn-MOF@Cu-MOF@Zn-MOF, and calcining to obtain ZnO@CuO@ZnO; (6) ZnO@CuO@ZnO core-shell catalyst for CO 2 The steps of hydrogenation to produce methanol are as follows: 1) Take ZnO@CuO@ZnO sample, press it into tablets, sieve it, and mix it with quartz sand; 2) The prepared catalyst is loaded into the reaction tube and installed on a stainless steel fixed bed reactor for testing; Before testing, the catalyst needs to be pre-reduced in a reducing atmosphere; 4) After the reduction is completed and cooled to room temperature, the raw gas mixture is introduced into the reactor and pressurized, and the raw gas flow rate is set to the required fixed flow rate; 5) Then set the required temperature for the reaction and conduct the test.

[0006] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 The method for preparing methanol by hydrogenation, the specific process of step (1) is: weigh zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) and an organic ligand (such as 2-methylimidazole) were dissolved in anhydrous methanol, dispersed by ultrasonication, and then magnetically stirred at room temperature for 3-5 h, centrifuged, washed three times with methanol, and dried in vacuum for 12 h.

[0007] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 The method for preparing methanol by hydrogenation comprises the following steps: in step (1), the Zn-MOF is ultrasonically dispersed for 0.5 h to form a regular and uniform cubic structure.

[0008] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 A method for preparing methanol by hydrogenation, wherein the hydrothermal reaction conditions of step (2) are: first stirring for 20-40 minutes, transferring into a 100 mL polytetrafluoroethylene-lined stainless steel high-pressure reactor, and placing in an oven at 100-140 ° C for reaction for 12-24 hours.

[0009] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2A method for preparing methanol by hydrogenation, wherein the organic ligand used in step (2) comprises: trimesic acid (H 3 BTC), terephthalic acid (H 2 BDC).

[0010] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 In the method for preparing methanol by hydrogenation, the ratio of the Cu ions in step (2) to the Zn ions in step (1) is: Cu / Zn=0.2~1.0.

[0011] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 In the method for preparing methanol by hydrogenation, the stirring time in step (4) is 3-5 h.

[0012] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 In the method for preparing methanol by hydrogenation, the calcination conditions in step (5) are 300-600° C. for 1-3 h.

[0013] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 The method for preparing methanol by hydrogenation, wherein the roasting conditions in step (5) are: heating to 500° C. at a heating rate of 5° C. / min in an air atmosphere and maintaining for 2 h.

[0014] The ZnO@CuO@ZnO core-shell catalyst is used for CO 2 Method for preparing methanol by hydrogenation, wherein the process conditions for applying the ZnO@CuO@ZnO core-shell catalyst are: reduction time 2-6 h, reaction raw material gas H 2 / CO 2 The ratio is 1-5, the reaction pressure is 2-6 MPa, the reaction temperature is 200-500℃, the feed gas flow rate is 20-50 ml / min, and the reaction gas space velocity is 4000-12000 mL·g -1 ·h -1 .

[0015] Advantages and beneficial effects of the present invention: 1. The unique core-shell structure of ZnO@CuO@ZnO synthesized in the present invention increases the Cu-ZnO interface and a large number of active sites, which can effectively promote CO 2 and H 2 The adsorption and electron transfer of the nanocore-shell material enhance the catalytic performance of the material. The nanocore-shell material has good hydrogenation performance, good reproducibility, high conversion rate, high selectivity and good long-term stability.

[0016] 2. The present invention combines room temperature synthesis and hydrothermal methods to prepare Zn-MOF@Cu-MOF@Zn-MOF precursors, and then prepares ZnO@CuO@ZnO core-shell materials by drying and calcining. The raw materials are easy to obtain and the preparation process is simple.

[0017] 3. The method for preparing CO 2 The ZnO@CuO@ZnO core-shell material for hydrogenation to methanol has excellent performance and good CO 2 The conversion rate and methanol selectivity are suitable for industrial large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is the XRD spectrum of the catalyst of the present invention; Figure 2 (ab) are scanning electron microscopy and particle size distribution images of the precursors Zn-MOF@Cu-MOF and Zn-MOF@Cu-MOF@Zn-MOF of Examples 2 and 3 of the present invention; Figure 2 (cd) are scanning electron microscopy and particle size distribution images of ZnO@CuO and ZnO@CuO@ZnO prepared in Examples 2 and 3 of the present invention; Figure 3 This is a scanning electron microscope image of the product prepared in Example 1 of the present invention; Figure 4 Examples 1, 2 and 3 of the present invention are used for CO 2 Performance data chart of hydrogenation to methanol. DETAILED DESCRIPTION

[0019] The present invention is described in detail below. The following are only embodiments of the present invention and cannot be used to limit the scope of the present invention. That is, all equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope covered by the present invention.

[0020] The present invention will be further described with reference to the embodiments shown in the accompanying drawings: The preparation method of ZnO@CuO@ZnO nano core-shell material comprises the following steps: Step 1: Preparation of Zn-MOF precursor A; Step 2: Take an appropriate amount of precursor A and add copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) and corresponding organic ligands, undergoing hydrothermal reaction; Step 3: centrifuge, wash, dry and cool the solution after the reaction to obtain the intermediate B Zn-MOF@Cu-MOF; Step 4: Weigh a portion of intermediate B and dissolve it in anhydrous methanol again, add zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) and corresponding organic ligand, stirring; Step 5: Centrifuge, wash and dry the stirred solution to obtain the precursor Zn-MOF@Cu-MOF@Zn-MOF, and calcine to obtain ZnO@CuO@ZnO.

[0021] The step 1 specifically comprises: weighing an appropriate amount of zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) and an appropriate amount of organic ligand (such as 2-methylimidazole) are dissolved in anhydrous methanol, ultrasonically dispersed, and then magnetically stirred at room temperature for 3-5 h, centrifuged and washed 3 times with methanol, and vacuum dried for 12 h; The hydrothermal reaction conditions of step 2 are: first stir for 20-40 min, transfer into a 100 mL polytetrafluoroethylene-lined stainless steel autoclave, and place in an oven at 100-140 ° C for 12-24 h. The organic ligands used include: trimesic acid (H 3 BTC), terephthalic acid (H 2 BDC) etc.; The stirring in step 4 is 3-5 h; the calcination condition in step (5) is calcination at 300-600°C for 1-3 h The ZnO@CuO@ZnO core-shell catalyst of the present invention has a CO 2 The test method for producing methanol by hydrogenation comprises the following steps: Step 1: Take a certain amount of ZnO@CuO@ZnO sample, press it into tablets, sieve it, and mix a certain amount with quartz sand; Step 2: Load the prepared catalyst into the reaction tube and install it on a stainless steel fixed bed reactor for testing; Step 3: Before testing, the catalyst needs to be pre-reduced in a reducing atmosphere at a certain flow rate; Step 4: After the reduction is completed and cooled to room temperature, the raw gas mixture is introduced into the reactor and pressurized, and the raw gas flow rate is set to the required fixed flow rate; Step 5: Then set the required temperature for the reaction and conduct the test.

[0022] The application conditions of the ZnO@CuO@ZnO core-shell catalyst are: reduction time 4 h, reaction raw gas H 2 / CO 2The ratio was 3, the reaction pressure was 2 MPa, the reaction temperature was 200 °C, the feed gas flow rate was 20 ml / min, and the reaction gas space velocity was 12000 mL·g -1 ·h -1 . Example 1

[0023] Preparation of co-precipitated CuO-ZnO catalyst Step 1: 0.03 mol of copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) and 0.03 mol zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) was dissolved in 300 ml of deionized water (DI), and 53 g of sodium carbonate (Na 2 CO 3 ) was dissolved in 500 ml of deionized water; Step 2: Cu(NO 3 ) 2 ·3H 2 O、Zn(NO 3 ) 2 6H 2 O solution and Na 2 CO 3 The solutions were slowly added into 300 ml of DI water under magnetic stirring and then stirred at 60 °C for 30 min; Step 3: After stirring, the mixture was left at room temperature overnight, the blue precipitate was collected by vacuum filtration and washed with deionized water; Step 4: The product is dried at 80°C for 12 h and calcined at 350°C for 1 h, and the obtained product is co-precipitated CuO-ZnO.

[0024] Structural Characterization of Coprecipitated CuO-ZnO The crystal structure of the product was characterized by XRD powder diffractometer (XRD, Shimadzu XRD-600). Figure 1 This is the XRD spectrum of the product. The diffraction peaks of the product are consistent with the standard card PDF#97-006-9094 of CuO and the standard card PDF#04-005-5076 of ZnO. The diffraction peaks are sharp and have high peak intensity. There are no other impurity peaks, indicating that it has high purity and good crystallinity.

[0025] Scanning electron microscopy (FESEM, ZEISS Ultra Plus) was used to characterize the morphology of the product. Figure 3 As shown, the CP CuO-ZnO product in the image presents an irregular morphology and has large blocks. Example 2

[0026] (1) Preparation of ZnO@CuO catalyst Step 1: Weigh 8 mmol of zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) and 64mmol 2-methylimidazole were dissolved in anhydrous methanol, dispersed by ultrasonication, and then magnetically stirred at room temperature for 4 h; Step 2: The obtained white solution was centrifuged and washed three times with methanol, and vacuum dried for 12 h to obtain sample ZIF-8; Step 3: Dissolve 0.2 g ZIF-8 in anhydrous ethanol, disperse by ultrasonic, and then add copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) and trimesic acid (H 3 BTC), after being fully stirred for 30 min, it was transferred into a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and placed in an oven at 120 °C for 24 h, and then naturally cooled to room temperature; Step 4: The reaction solution was centrifuged to obtain the reaction product, and then the product was repeatedly washed with anhydrous ethanol, and the washed product was placed in a constant temperature drying oven, dried at 60 °C overnight, and cooled after drying to obtain the sample ZIF-8@HKUST-1; Step 5: Calcine the sample ZIF-8@HKUST-1 at 500℃ for 2 h to obtain the sample ZnO@CuO.

[0027] (2) Structural characterization of ZnO@CuO The crystal structure of the product was characterized by XRD powder diffractometer (XRD, Shimadzu XRD-6100). From the XRD spectrum of the ZnO@CuO structure, it can be seen that the characteristic peaks of CuO are more prominent than those of ZnO. This is consistent with the synthesis process of this structure. Similarly, the synthesized sample has no extra impurity peaks, indicating that the synthesized sample has good purity and crystallinity.

[0028] Scanning electron microscopy (FESEM, ZEISS Ultra Plus) was used to characterize the morphology of the product. Figure 2 As shown in (a) and (c), the product presents a densely distributed small and uniform spherical particle morphology, and the average particle size histogram shows that the sample particle size distribution is in the range of 30-50 nm. Example 3

[0029] (1) Preparation of ZnO@CuO@ZnO core-shell catalyst Steps 1, 2, 3 and 4 are the same as those in Example 2.

[0030] Step 5: Weigh 0.3 g of the obtained sample and dissolve it in anhydrous methanol again, disperse it evenly by ultrasonication, and add zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) and 2-methylimidazole, with magnetic stirring at room temperature for 4 h; Step 6: centrifuge the stirred solution, wash it with methanol several times, dry it at 60 °C overnight, and then cool it to obtain ZIF-8@HKUST-1@ZIF-8; Step 7: Finally, the sample was calcined in a muffle furnace at 500 °C for 2 h to obtain the catalyst ZnO@CuO@ZnO.

[0031] (2) Structural characterization of ZnO@CuO@ZnO The crystal structure of the product was characterized by XRD powder diffractometer (XRD, Shimadzu XRD-6100). Figure 1 The XRD spectrum of the product shows diffraction peaks that are consistent with the standard card PDF#97-006-9094 of CuO and the standard card PDF#04-005-5076 of ZnO. At the same time, compared with ZnO@CuO, the intensity of the ZnO characteristic peak of the ZnO@CuO@ZnO catalyst increases, which is consistent with the structural synthesis design.

[0032] Scanning electron microscopy (FESEM, ZEISS Ultra Plus) was used to characterize the morphology of the product. Figure 2 As shown in (b) and (d), the product still presents a dense small and uniform spherical particle morphology, but Figure 2 (b) Particle size distribution histogram shows that the particle size increases to within the range of 70–80 nm, which is due to the synthesis of the outer ZnO layer.

[0033] The prepared ZnO@CuO@ZnO core-shell catalyst was used for CO 2 Relevant performance tests were carried out for the production of methanol by hydrogenation: In order to evaluate the superior performance of ZnO@CuO@ZnO catalyst, CP CuO-ZnO, ZnO@CuO and ZnO@CuO@ZnO were tested under the same reduction and test conditions. Before the test, all three catalysts were reduced at 240℃ and 2MPa for 4 h. After the reduction, the catalysts were tested at 200℃ and 2MPa for 2 h. Figure 4As shown, the CO of ZnO@CuO@ZnO 2 The conversion rate was 17.41%, significantly higher than ZnO@CuO (9.76%) and CP CuO-ZnO (6.28%). The methanol selectivity of ZnO@CuO@ZnO was also better than other comparative catalysts, reaching 85.7%. The traditional CP CuO-ZnO under the same conditions was only 64.7%, which was a significant improvement. Moreover, the yield and space-time yield of ZnO@CuO@ZnO were 14% and 511.56 g, respectively. MeOH kg cal h -1 , which is far superior to the CuO-ZnO catalyst prepared by the co-precipitation method. The superior performance is due to the unique structural design of the catalyst, which provides a basis for the subsequent CO 2 This provides ideas for the preparation of catalysts for hydrogenation to methanol.

Claims

1. A method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst, characterized in that: The catalyst has CuO and ZnO as active components, wherein CuO accounts for 10.4-48.6wt% and ZnO accounts for 42.3-67.8wt%; The preparation steps are as follows: (1) Preparation of Zn-MOF precursor A; (2) Take precursor A, add copper nitrate trihydrate (Cu(NO3)2·3H2O) and corresponding organic ligand in sequence, and carry out hydrothermal reaction; (3) Centrifuging, washing, drying, and cooling the reaction solution to obtain the intermediate B Zn-MOF@Cu-MOF; (4) Weigh intermediate B and dissolve it in anhydrous methanol again, add zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and organic ligand, and stir; (5) Centrifuging, washing and drying the stirred solution to obtain the precursor Zn-MOF@Cu-MOF@Zn-MOF, and calcining to obtain ZnO@CuO@ZnO; (6) ZnO@CuO@ZnO core-shell catalyst is used for CO2 hydrogenation to methanol, the steps are as follows: 1) Take ZnO@CuO@ZnO sample, press it into tablets, sieve it, and mix it with quartz sand; 2) The prepared catalyst is loaded into the reaction tube and installed on a stainless steel fixed bed reactor for testing; Before testing, the catalyst needs to be pre-reduced in a reducing atmosphere; 4) After the reduction is completed and cooled to room temperature, the raw gas mixture is introduced into the reactor and pressurized, and the raw gas flow rate is set to the required fixed flow rate; 5) Then set the required temperature for the reaction and conduct the test.

2. The method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 1, characterized in that: The specific process of step (1) is as follows: zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and an organic ligand (such as 2-methylimidazole) are weighed and dissolved in anhydrous methanol, ultrasonically dispersed, and then magnetically stirred at room temperature for 3-5 hours, centrifuged and washed with methanol for 3 times, and vacuum dried for 12 hours.

3. The method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 2, characterized in that: In the step (1), the Zn-MOF is ultrasonically dispersed for 0.5 h to form a regular and uniform cubic structure.

4. The method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 1, characterized in that: The hydrothermal reaction conditions of step (2) are: first stir for 20-40 minutes, transfer into a 100 mL polytetrafluoroethylene-lined stainless steel autoclave, and place in an oven at 100-140 ° C for reaction for 12-24 hours.

5. The method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 4, characterized in that: The organic ligands used in step (2) include trimesic acid (H3BTC) and terephthalic acid (H2BDC).

6. A method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 4, characterized in that: The ratio of the Cu ions in step (2) to the Zn ions in step (1) is: Cu / Zn=0.2~1.

0.

7. The method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 1, characterized in that: The stirring time of step (4) is 3-5 h.

8. The method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 1, characterized in that: The calcination conditions in step (5) are 300-600° C. for 1-3 h.

9. A method for preparing methanol by hydrogenating CO2 using a ZnO@CuO@ZnO core-shell catalyst according to claim 8, characterized in that: The calcination conditions in step (5) are: heating to 500° C. at a heating rate of 5° C. / min in an air atmosphere and maintaining for 2 h.

10. The method for preparing methanol by using a ZnO@CuO@ZnO core-shell catalyst according to claim 1, characterized in that: The process conditions for the application of the ZnO@CuO@ZnO core-shell catalyst are: reduction time 2-6 h, reaction raw gas H2 / CO2 ratio of 1-5, reaction pressure 2-6 MPa, reaction temperature 200-500°C, raw gas flow rate 20-50 ml / min, reaction gas space velocity 4000-12000 mL·g -1 ·h -1 .

Images