Zn-loaded catalyst forming body as well as preparation method and application thereof

Abstract

The invention relates to the field of chemistry and chemical engineering and catalysis, in particular to a Zn-loaded catalyst forming body, a preparation method and application thereof, and more particularly relates to a Zn-loaded catalyst forming body, a preparation method and a method for preparing acetaldehyde through catalytic dehydrogenation of ethanol. The catalyst is formed by loading a catalytic active component Zn on a nitrogen-doped carbon sphere matrix with a honeycomb-shaped pore structure, the structural formula of the catalyst is Zn-N / C, the catalyst has a honeycomb-shaped pore channel microstructure and is in a millimeter-level spherical form, and the diameter of the catalyst is 0.1-2 mm; in the reaction of preparing acetaldehyde through catalytic dehydrogenation of ethanol, the ethanol conversion rate is less than or equal to 93%, and the acetaldehyde selectivity is less than or equal to 95%; the novel Zn catalyst is obtained through special raw materials and proportions and a specific preparation process, and a new choice is provided for preparation of acetaldehyde through catalytic dehydrogenation of ethanol.

Description

Technical Field

[0001] The present invention relates to the fields of chemistry, chemical engineering and catalysis, and particularly to a supported Zn catalyst formed body, a preparation method and an application thereof, and more specifically to a supported Zn catalyst formed body, a preparation method and a method for catalytic dehydrogenation of ethanol to acetaldehyde. Background Art

[0002] At present, ethanol, as a key biomass platform molecule, has a wide range of applications. Ethanol molecules have rich chemical bonds and are the most representative oxygen-containing molecules, which endow ethanol with diverse conversion paths and can be used to prepare a series of high-value-added chemicals such as diethyl carbonate, 2,3-butanediol, acetal, ethyl acetate, n-butanol, N-ethylamine compounds, 1,3-butadiene, etc. Among them, the direct dehydrogenation of ethanol to acetaldehyde is not only a key starting step for subsequent conversion to acetal, ethyl acetate, n-butanol, N-ethylamine compounds, 1,3-butadiene, etc., but also can co-produce hydrogen. Hydrogen, as the core carrier of future green energy, is of great significance. Therefore, in-depth exploration and development of efficient processes for preparing acetaldehyde from ethanol have great scientific research value and broad industrial application prospects.

[0003] CN117380275A discloses a Cu-based organic hybrid material and a preparation method thereof. The Cu-based organic hybrid material is prepared based on a soluble copper salt precursor, a polybasic acid or polybasic aldehyde, a polyamine and an alcohol solution, and is applied to the catalytic dehydrogenation reaction of ethanol to acetaldehyde. The reaction lasts for 200 h, the ethanol conversion rate is 33.2%, and the acetaldehyde selectivity is 95.2%, showing certain reaction activity and a long service life. However, there are still areas for improvement in this technology: firstly, fine adjustment of multiple factors such as catalyst composition, structure and reaction conditions is required to further improve the reactant conversion rate; secondly, there is still a large room for improvement in the service life and stability of the catalyst.

[0004] A Cu / SiO 2 catalyst reported by Yuan et al. is used for the acceptorless catalytic dehydrogenation reaction of ethanol. This method successfully prepares highly dispersed Cu particles by means of ammonia evaporation method. Under the condition of pure ethanol feed and without any equilibrium gas, it shows strong thermal stability. Due to the active component Cu and the carrier SiO 2The strong interaction between them endows the catalyst with relatively excellent catalytic performance. By optimizing the reaction conditions, an ethanol conversion rate of 40% and an acetaldehyde selectivity of 95% are achieved at 250°C. However, this method still faces many problems: in terms of catalyst life, the activity of most Cu-based catalysts rapidly declines at low temperatures in a short period. Even if there are catalysts with better performance, such as those prepared by the ammonia evaporation method, which can operate stably for 400 h, they still do not meet the ideal long-life requirements, which will increase the industrial application cost; in terms of stability, the aggregation of Cu particles and carbon deposition often lead to catalyst deactivation, thereby causing problems such as low acetaldehyde selectivity and low space-time yield. The selectivity of the target product and the conversion rate of the catalyst need to be further improved.

[0005] Generally speaking, the mainstream catalyst for the direct dehydrogenation of ethanol to acetaldehyde at present is the Cu catalyst. However, it is extremely prone to aggregation and deactivation during the reaction process. This not only greatly reduces the catalyst life and stability but also significantly increases the frequency of catalyst replacement, causing serious negative impacts on production efficiency and economic benefits. In view of this, there is an urgent need to explore other alternative products. Such products need to have catalytic activity and selectivity similar to or even better than those of the Cu catalyst, be paired with a more excellent carrier and advanced preparation process to enhance stability. Only by solving these problems can this technology be widely popularized and deeply applied in the industrial field. Summary of the Invention

[0006] To solve the above problems, the present invention provides a supported Zn catalyst. The catalyst is composed of a nitrogen-doped carbon sphere matrix with a honeycomb pore structure supporting the catalytic active component Zn. The nitrogen-doped carbon sphere matrix with a honeycomb pore structure can provide a large specific surface area and a rich pore structure. This unique microstructure is conducive to the adsorption and diffusion of reactant ethanol molecules, enabling ethanol molecules to come into contact with the catalytic active component Zn more fully, thereby improving the reaction efficiency. The introduction of nitrogen atoms may adjust the electron cloud density of Zn, changing its adsorption and activation ability for ethanol molecules, which is more conducive to promoting the cleavage of the C-H bond in ethanol molecules, thereby increasing the conversion rate of the reaction and the selectivity of acetaldehyde. Its honeycomb pore structure can also resist sintering and aggregation to a certain extent, keeping the active sites of the catalyst exposed and dispersed, thereby maintaining catalytic activity for a long time. In summary, the supported Zn catalyst of the present invention has the advantages of good stability, high conversion rate of reactants and selectivity of products, and long life, and is suitable for industrial application, providing an important alternative product for the catalyst for the catalytic dehydrogenation of ethanol to acetaldehyde. The specific technical solutions are as follows: In order to solve the above problems, the present invention ingeniously introduces a supported Zn catalyst, which is exquisitely constructed on a nitrogen-doped carbon ball matrix with a honeycomb pore structure, and the matrix is ​​loaded with a catalytically active ingredient Zn; the unique nitrogen-doped carbon ball matrix, with its honeycomb pore structure, can expand a larger specific surface area and construct a rich pore system, and the special microstructure unexpectedly creates excellent conditions for the adsorption and diffusion of the reactant ethanol molecules, enabling the ethanol molecules to contact the catalytically active ingredient Zn in all directions and in depth, thereby significantly improving the reaction efficiency; at the same time, the nitrogen atoms cleverly adjust the electron cloud density of Zn, causing it to change its adsorption and activation ability for ethanol molecules, more effectively promote the breaking of the CH bond in the ethanol molecules, and help improve the conversion rate of the reactants and the selectivity of acetaldehyde from the electronic level; furthermore, the honeycomb pore structure can effectively resist sintering and agglomeration to a certain extent, ensuring that the active sites of the catalyst are always fully exposed and evenly dispersed, thereby maintaining the catalytic activity for a long time.

[0007] In summary, the supported Zn catalyst of the present invention has many advantages such as good stability, high reactant conversion rate, excellent product selectivity and long life, which is highly consistent with the needs of large-scale industrial applications, and provides a valuable new direction for the selection of catalysts in the field of ethanol catalytic dehydrogenation to acetaldehyde and hydrogen. The specific technical scheme is described in detail as follows: A supported Zn catalyst, comprising a nitrogen-doped carbon sphere matrix with a honeycomb pore structure and a catalytically active component Zn supported thereon, wherein the structural formula is Zn-N / C; the catalyst has a microstructure of honeycomb pores, presents a millimeter-scale spherical shape, and has a diameter of 0.1-2 mm; in the catalyst, based on the catalyst mass as a percentage basis, the Zn loading amount is 4-20% in terms of metal element.

[0008] In the XRD spectrum of the catalyst, a diffraction peak of amorphous carbon was observed at a 2θ angle of 20-30°, and there were no other peaks except the characteristic peak of amorphous carbon, which proved that Zn existed in a single atomic form.

[0009] A method for preparing the catalyst according to claim 1, comprising the following steps: Step 1: Preparation of microsphere precursor Chitosan was added with deionized water, and then acetic acid was added. After stirring evenly, a chitosan solution was obtained. The obtained chitosan solution was taken, and a Zn source was added. After stirring evenly, a Zn-chitosan solution was obtained. Matrix raw materials and an alkali were added to deionized water. After stirring evenly, a matrix solution was obtained. Then, the Zn-chitosan solution was uniformly dropped into the matrix solution, aged at 60 - 100 °C for 8 - 16 h, and after filtration, washing with deionized water, and drying, a microsphere precursor was obtained. In the XRD pattern of the microsphere precursor, characteristic peaks of Zeolitic Imidazolate Frameworks (ZIF), namely ZIF-8, at 2θ angles of 7.2°, 10.4°, 12.5°, 14.58°, 16.2°, 17.7°, 21.9°, 24.4°, and 26.5° for (011), (002), (112), (022), (013), (222), (114), (233), and (134) were clearly observed, and no other diffraction peaks appeared except for the characteristic peaks of ZIF-8. Step 2: Prepare the Zn-N / C catalyst The obtained microsphere precursor was placed in a tubular furnace. Under an inert gas atmosphere, it was heated to 900 - 1200 °C at a heating rate of 5 - 10 °C / min, held for 1 - 3 h, with the inert gas flow rate of 20 - 60 mL / min. Then, it was cooled to 400 - 600 °C, and the gas was switched to H 2 and continued to be treated for 2 - 4 h, with the H 2 flow rate of 20 - 60 mL / min. Then, it was cooled to room temperature to obtain the catalyst Zn-N / C.

[0010] Furthermore, the Zn source is a soluble salt of Zn.

[0011] Furthermore, the soluble salt of Zn is any one or a mixture of two or more of zinc nitrate, zinc chloride, zinc sulfate, zinc acetate, and zinc perchlorate.

[0012] Furthermore, the matrix raw material is selected from any one of 2-methylimidazole, benzimidazole, and imidazole.

[0013] Furthermore, in Step 1, the alkali is any one of NaOH, KOH, Ca(OH) 2 , Ba(OH) 2 .

[0014] Furthermore, in Step 1, the Zn-chitosan solution was uniformly dropped into the matrix solution through a syringe. The inner diameter of the syringe needle was 0.1 - 1.0 mm, and the dropping rate was 1 - 20 mL / min.

[0015] Furthermore, in Step 2, the inert gas is N2 , any one of Ar.

[0016] A method for catalytic dehydrogenation of ethanol to acetaldehyde, the method using the catalyst described in claim 1, comprising: Load the supported Zn catalyst into a reactor, first purge the catalyst with H 2 for 1 h at a temperature of 400 - 600 °C, then switch to N 2 , pump ethanol into the vaporization chamber with an injection pump at a rate of 1 - 10 mL / h, the mass space velocity of ethanol is 1 - 50 h -1 , at a temperature of 200 - 400 °C, ethanol and N 2 After being fully mixed, enter the reactor, set the catalytic reaction temperature to 150 - 400 °C, and the reaction pressure to atmospheric pressure; analyze the product. Among them, the ethanol conversion rate ≤ 93%, and the selectivity of acetaldehyde ≤ 95%.

[0017] Furthermore, the reactor is any one of a fixed bed reactor and a shell and tube reactor.

[0018] The beneficial effects of the present invention are as follows: 1. During the preparation of the catalyst of the present invention, chitosan is dissolved in deionized water containing acetic acid to form a chitosan solution, and then mixed with Zn to form a Zn-chitosan solution. There are a large number of active groups such as amino and hydroxyl groups on the chitosan molecular chain. These groups can form a three-dimensional network structure framework through hydrogen bonding and other interactions, providing a basic skeleton for the formation of a honeycomb pore structure, and restricting and guiding the formation direction and distribution of pores to a certain extent; during the preparation of the microsphere precursor, taking ZIF-8 as an example, when the Zn-polysaccharide solution is dropped into a matrix solution containing 2-methylimidazole and a base, ZIF-8 grows within the framework formed by chitosan. ZIF-8 itself has a regular crystal structure and porous characteristics, and it will occupy a certain space during its growth process, forming a structure similar to hanging on the pore wall within the chitosan framework, thus jointly constructing a microsphere precursor structure with honeycomb pores with the chitosan framework; the nitrogen-doped carbon sphere matrix with a honeycomb pore structure can provide a large specific surface area and a rich pore structure. This unique microstructure is beneficial to the adsorption and diffusion of reactant ethanol molecules. Compared with traditional Cu catalysts, the catalyst of the present invention enables ethanol molecules to contact the catalytic active component Zn more fully, thereby improving the reaction efficiency; 2. There is a strong coordination effect between the N atoms in the nitrogen-doped carbon sphere matrix and the active component Zn. The electronegativity of nitrogen atoms is different from that of carbon and zinc atoms. Its introduction will adjust the electron cloud density of Zn, and this change in electron cloud density makes the adsorption and activation ability of Zn for ethanol molecules change, which is more conducive to promoting the cleavage of the C-H bond in ethanol molecules. From the perspective of the reaction mechanism, when the ethanol molecule undergoes a dehydrogenation reaction at the Zn active site, the optimization of the electron cloud density reduces the activation energy of the reaction, thereby increasing the conversion rate of the reactants and the selectivity of acetaldehyde; 3. Based on the structural characteristics of the nitrogen-doped carbon sphere matrix, during the reaction process, its honeycomb pore structure can resist sintering and agglomeration phenomena to a certain extent, keep the active sites of the catalyst exposed and dispersed, and thus maintain the catalytic activity for a long time. Therefore, the Zn-N / C catalyst of the present invention has better stability; 4. The catalyst of the present invention is a shaped catalyst. The finished catalyst presents a microsphere shape with a diameter of 0.1 - 2 mm. It can be applied to the reaction of catalytic dehydrogenation of ethanol to acetaldehyde in a fixed-bed reactor without granulation and forming, which greatly improves the simplicity of use and improves the efficiency for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the SEM image of the precursor ZIF-8@CS microspheres in Example 1; As Figure 1 can be seen, the catalyst precursor ZIF-8@CS is a microsphere structure of about 2 mm. This microsphere has a honeycomb pore structure, and ZIF-8 is suspended on the pore wall, which is beneficial for subsequent Zn to be exposed on the pore wall, thus facilitating the contact and collision of ethanol; Figure 2 It is the XRD image of the precursor ZIF-8@CS microspheres in Example 1; As Figure 2 can be seen, the composition of the catalyst precursor shows a ZIF-8 structure; Figure 3 It is the SEM image of the Zn-N / C catalyst in Example 1; As Figure 3 can be seen, from the SEM image of the Zn-N / C catalyst of the present invention, it can be clearly observed that the catalyst matrix presents a honeycomb pore structure, and the diameter of the carbon material microspheres is 0.1 - 2 mm.

[0020] Figure 4 It is the XRD image of the Zn-N / C catalyst in Example 1; As Figure 4 can be seen, the characteristic diffraction peak of zinc oxide cannot be detected in the Zn-N / C catalyst of the present invention, indicating that Zn is highly dispersed in the catalyst in the form of single atoms; Figure 5TEM image of the Zn-N / C catalyst in Example 1; From Figure 5 It can be seen that the Zn-N / C catalyst of the present invention is a single-atom dispersed Zn catalyst. Detailed implementation manners

[0021] Next, in combination with specific embodiments of the present invention, the technical solutions of the present invention will be described in detail. The described embodiments are only a part of the present invention, and the present invention is not limited to the following described embodiments.

[0022] Example 1 Step 1: Prepare the precursor ZIF-8@CS microspheres Add 2 g of chitosan to 94.8 g of deionized water, then add 3.2 mL of 36 wt% acetic acid, and stir evenly to obtain a 2 wt% chitosan solution; take 10 g of the chitosan solution, add 0.6 g of Zn(NO 3 ) 2 ·6H 2 O, stir evenly to obtain a Zn-chitosan solution; add 2 g of NaOH and 2.6 g of 2-methylimidazole (abbreviated as 2-MI) to 35.4 g of deionized water, stir evenly to obtain a basic solution of 2-MI, and then drop the Zn-chitosan solution into the basic solution of 2-MI through a syringe (inner diameter of the needle 0.45 mm) at a constant rate of 5 mL / min, age at 80 °C for 12 h, and obtain the precursor ZIF-8 / CS-0.45 microspheres after filtration, washing with deionized water, and drying; Step 2: Prepare the Zn-N / C catalyst Take 1 g of the ZIF-8 / CS-0.45 microspheres and place them in a tubular furnace. Under N 2 atmosphere, heat up to 900 °C at a heating rate of 5 °C / min, keep warm for 1 h, the N 2 flow rate is 40 mL / min, then cool down to 400 °C, switch to H 2 , continue to process for 2 h, the H 2 flow rate is 40 mL / min, and then cool down to room temperature to obtain the catalyst Zn-N / C-0.45-900, denoted as catalyst 1; Example 2 Step 1: Prepare the precursor ZIF-8 / CS microspheres Add 2 g of chitosan to 94.8 g of deionized water, then add 3.2 mL of 36 wt% acetic acid, and stir evenly to obtain a 2 wt% chitosan solution; take 10 g of the chitosan solution, add 0.6 g of Zn(NO 3 ) 2 ·6H2 O, stir evenly to obtain a Zn-chitosan solution; add 2 g of NaOH and 2.6 g of 2-methylimidazole to 35.4 g of deionized water, and then drop the Zn-chitosan solution into the alkaline solution of 2-MI evenly through a syringe (inner diameter of the needle 0.7 mm) at a dropping rate of 5 mL / min. Age at 80 °C for 12 h, filter, wash with deionized water, and dry to obtain precursor ZIF-8 / CS-0.7 microspheres; Step 2: Prepare the Zn-N / C catalyst Take 1 g of the ZIF-8 / CS-0.70 microspheres and place them in a tubular furnace. Under N 2 atmosphere, heat up to 900 °C at a heating rate of 5 °C / min, hold for 1 h, the N 2 flow rate is 40 mL / min, then cool down to 400 °C, switch to H 2 , continue to process for 2 h, the H 2 flow rate is 40 mL / min, and then cool to room temperature to obtain the catalyst Zn-N / C-0.7-900, denoted as catalyst 2; Example 3 Step 1: Prepare precursor ZIF-8 / CS microspheres Add 2 g of chitosan to 94.8 g of deionized water, then add 3.2 mL of 36 wt% acetic acid, stir evenly to obtain a 2 wt% chitosan solution; take 10 g of the chitosan solution, add 0.6 g of Zn(NO 3 ) 2 ·6H 2 O, stir evenly to obtain a Zn-chitosan solution; add 2.0 g of NaOH and 3.5 g of benzimidazole to 35.4 g of deionized water, stir evenly to obtain an alkaline solution of benzimidazole, and drop the Zn-chitosan solution into the alkaline solution of benzimidazole evenly through a syringe (inner diameter of the needle 0.45 mm) at a dropping rate of 5 mL / min. Age at 80 °C for 12 h, filter, wash with deionized water, and dry to obtain precursor ZIF-7 / CS-0.45 microspheres; Step 2: Prepare the Zn-N / C catalyst Take 1 g of the ZIF-7 / CS-0.45 microspheres and place them in a tubular furnace. Under N 2 atmosphere, heat up to 900 °C at a heating rate of 5 °C / min, hold for 1 h, the N 2 flow rate is 40 mL / min, then cool down to 400 °C, switch to H 2 , continue to process for 2 h, the H 2The flow rate was 40 mL / min, and then dropped to room temperature to obtain the catalyst Zn-N / C-0.45-900-(ZIF-7), which was recorded as catalyst 3; Example 4 Step 1: Preparation of precursor ZIF-8 / CS microspheres 2 g of chitosan was added to 94.8 g of deionized water, and then 3.2 mL of 36 wt% acetic acid was added and stirred to obtain a 2 wt% chitosan solution; 10 g of the chitosan solution was taken and 0.6 g of Zn(NO 3 ) 2 6H 2 O, stirring evenly to obtain a Zn-chitosan solution; adding 2g of NaOH and 2.6g of 2-methylimidazole to 35.4g of deionized water to obtain a 2-MI alkaline solution, and then uniformly adding the Zn-chitosan solution to the 2-MI alkaline solution through a syringe (needle inner diameter 0.45mm) at a dropping speed of 5mL / min, aging at 80°C for 12h, filtering, washing with deionized water, and drying to obtain precursor ZIF-8 / CS-0.45 microspheres; Step 2: Preparation of Zn-N / C catalyst 1 g of the ZIF-8 / CS-0.45 microspheres were placed in a tube furnace under nitrogen atmosphere. 2 The temperature was raised to 600°C at a rate of 5°C / min and kept at this temperature for 1 h. 2 The flow rate was 40 mL / min, then the temperature was lowered to 400 °C and switched to H 2 , continue to process for 2h, the H 2 The flow rate was 40 mL / min, and then the temperature was reduced to room temperature to obtain catalyst Zn-N / C-0.45-600, which was recorded as catalyst 4; Example 5 Step 1: Preparation of precursor ZIF-8 / CS microspheres 2 g of chitosan was added to 94.8 g of deionized water, and then 3.2 mL of 36 wt% acetic acid was added and stirred to obtain a 2 wt% chitosan solution; 10 g of the chitosan solution was taken and 0.6 g of Zn(NO 3 ) 2 6H 2O, stir evenly to obtain a Zn-chitosan solution; add 2 g of NaOH and 2.6 g of 2-methylimidazole to 35.4 g of deionized water, stir evenly to obtain an alkaline solution of 2-MI, and then slowly add the Zn-chitosan solution dropwise into the alkaline solution of 2-MI through a syringe (inner diameter of the needle 0.45 mm) at a dropping rate of 5 mL / min. Age at 80 °C for 12 h, filter, wash with deionized water, and dry to obtain precursor ZIF-8 / CS-0.45 microspheres; Step 2: Prepare the Zn-N / C catalyst Take 1 g of the ZIF-8 / CS-0.45 microspheres and place them in a tubular furnace. Under N 2 atmosphere, heat up to 700 °C at a heating rate of 5 °C / min, hold for 1 h, the N 2 flow rate is 40 mL / min, then cool down to 400 °C, switch to H 2 , continue to process for 2 h, the H 2 flow rate is 40 mL / min, then cool down to room temperature to obtain the catalyst Zn-N / C-0.45-700, denoted as catalyst 5; Example 6 Step 1: Prepare precursor ZIF-8 / CS microspheres Add 2 g of chitosan to 94.8 g of deionized water, then add 3.2 mL of 36 wt% acetic acid, stir evenly to obtain a 2 wt% chitosan solution; take 10 g of the chitosan solution, add 0.6 g of Zn(NO 3 ) 2 ·6H 2 O, stir evenly to obtain a Zn-chitosan solution; add 2 g of NaOH and 2.6 g of 2-methylimidazole to 35.4 g of deionized water to obtain an alkaline solution of 2-MI, and then slowly add the Zn-chitosan solution dropwise into the alkaline solution of 2-MI through a syringe (inner diameter of the needle 0.45 mm) at a dropping rate of 5 mL / min. Age at 80 °C for 12 h, filter, wash with deionized water, and dry to obtain precursor ZIF-8 / CS-0.45 microspheres; Step 2: Prepare the Zn-N / C catalyst Take 1 g of the ZIF-8 / CS-0.45 microspheres and place them in a tubular furnace. Under N 2 atmosphere, heat up to 800 °C at a heating rate of 5 °C / min, hold for 1 h, the N 2 flow rate is 40 mL / min, then cool down to 400 °C, switch to H 2 , continue to process for 2 h, the H 2The flow rate was 40 mL / min, and then it was cooled to room temperature to obtain the catalyst Zn-N / C-0.45-800, denoted as catalyst 6. Comparative Example 1: Preparation of silica-supported Cu catalyst by the initial impregnation method Weigh 0.1896 g of copper nitrate and dissolve it in 1 mL of deionized water to obtain an aqueous solution of copper nitrate. Then add the aqueous solution of copper nitrate to 1 g of silica powder, shake it well in a vortex oscillator, and place it in an oven at 120 °C for 12 h to obtain silica-supported copper nitrate. Take 1 g of the silica-supported copper nitrate and place it in a tubular furnace. Under H 2 atmosphere, heat it to 400 °C at a heating rate of 5 °C / min and hold for 1 h. H 2 The flow rate was 40 mL / min, and then it was cooled to room temperature to obtain the catalyst Cu / SiO 2 , denoted as catalyst 7.

[0023] The Zn-N / C catalyst is used for the reaction of catalytic dehydrogenation of ethanol to acetaldehyde Weigh 0.1 g of the aforementioned Zn-N / C catalysts 1-6 respectively, and directly load them into the middle position of a straight quartz reaction tube with a length of 800 mm and an inner diameter of 8 mm. The rest on both sides is quartz sand. Then place the reaction tube into a fixed-bed reactor. First, purge the Zn-N / C catalyst with H 2 for 1 h at a temperature of 400 °C, and then switch to N 2 , N 2 The flow rate is 20 mL / min. Pump ethanol into the vaporization chamber with an injection pump at a rate of 1 mL / h, and the mass space velocity is 8 h -1 , at a temperature of 200 °C. After mixing well with N 2 , enter the reaction tube. Set the temperature to 350 °C, and analyze the products by on-line gas chromatography. The ethanol conversion rate and acetaldehyde selectivity are shown in Table 1.

[0024] Weigh 0.1 g of catalyst 7, that is, Cu / SiO 2 catalyst. After forming and granulating it with a tablet press and sieving, load it into the middle position of a straight quartz reaction tube with a length of 800 mm and an inner diameter of 8 mm. The rest on both sides is quartz sand. Then place the reaction tube into a fixed-bed reactor. First, purge the catalyst 7 with H 2 for 1 h at a temperature of 400 °C, and then switch to N 2 , N 2 The flow rate is 20 mL / min. Pump ethanol into the vaporization chamber with an injection pump at a rate of 1 mL / h, and the mass space velocity is 8 h -1 , at a temperature of 200 °C. After mixing well with N 2After being fully mixed, it enters the reaction tube. The temperature is set at 350 °C, and the products are analyzed by on-line gas chromatography. The ethanol conversion rate and acetaldehyde selectivity are shown in Table 1.

[0025] Table 1. Performance of the Zn-N / C catalysts of the examples and comparative examples of the present invention in the catalytic dehydrogenation of ethanol to acetaldehyde.

[0026] Catalyst Ethanol conversion rate (%) Acetaldehyde selectivity (%) Catalyst 1, Zn-N / C-0.45-900 93 95 Catalyst 2, Zn-N / C-0.7-900 82 93 Catalyst 3, Zn-N / C-0.45-900-(ZIF-7) 65 93 Catalyst 4, Zn-N / C-0.45-600 72 85 Catalyst 5, Zn-N / C-0.45-700 84 95 Catalyst 6, Zn-N / C-0.45-800 90 92 <![CDATA[Catalyst 7, Cu / SiO 2 > 35 96 As can be seen from Table 1, compared with the comparative examples, the catalyst of the present invention has a significantly higher ethanol conversion rate than the comparative examples. This is because the nitrogen-doped carbon sphere matrix with a honeycomb pore structure of the catalyst of the present invention can provide a large specific surface area and a rich pore structure. This unique microstructure is beneficial to the adsorption and diffusion of the reactant ethanol molecules. Compared with the traditional Cu catalyst, the catalyst of the present invention enables the ethanol molecules to contact the catalytic active component Zn more fully, thereby improving the reaction efficiency. In addition, compared with the comparative examples, the catalyst of the present invention also has a certain advantage in the selectivity of the product acetaldehyde. Especially in Example 1 (Zn-N / C-0.45-900), a catalyst with high ethanol conversion rate and acetaldehyde selectivity was prepared by using special raw materials and ratios and a specific preparation process. This is because there is a strong coordination effect between the N atoms in the nitrogen-doped carbon sphere matrix and the active component Zn, which changes the electronic structure and the surrounding chemical environment of Zn. For example, the introduction of N atoms adjusts the electron cloud density of Zn, changing its adsorption and activation ability for ethanol molecules, which is more conducive to promoting the cleavage of the C-H bond in ethanol molecules, thereby improving the conversion rate of the reactant and the selectivity of acetaldehyde at the same time.

[0027] The above embodiments only represent several implementation manners of the present invention, which are convenient for understanding the technical solutions of the present invention specifically and in detail, but should not be construed as limiting the patent protection scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A supported Zn catalyst, characterized in that: The catalyst is composed of a nitrogen-doped carbon sphere matrix with a honeycomb pore structure and a catalytically active component Zn, and its structural formula is Zn-N / C. The catalyst has a microstructure of honeycomb pores and presents a millimeter-scale spherical shape with a diameter of 0.1-2 mm. In the reaction of ethanol catalytic dehydrogenation to acetaldehyde, the ethanol conversion rate is ≤93% and the acetaldehyde selectivity is ≤95%; In the catalyst, the Zn loading is 4-20% based on the percentage of the catalyst mass and calculated as a single metal substance, and Zn exists in a single atomic form; in the XRD spectrum of the catalyst, a diffraction peak of amorphous carbon is observed at a 2θ angle of 20°-30°, and there are no other peaks except the diffraction peak of amorphous carbon.

2. A method for preparing the supported Zn catalyst according to claim 1, characterized in that: The method comprises the following steps: Step 1: Preparation of microsphere precursor Add deionized water to chitosan, then add acetic acid, stir evenly to obtain a chitosan solution; take the chitosan solution, add a Zn source, stir evenly to obtain a Zn-chitosan solution; add a matrix raw material and an alkali to the deionized water, stir evenly to obtain a matrix solution, then uniformly add the Zn-chitosan solution to the matrix solution, age at 60-100° C. for 8-16 hours, filter, wash with deionized water, and dry to obtain a microsphere precursor; Step 2: Preparation of Zn-N / C catalyst The microsphere precursor is placed in a tube furnace, and in an inert gas atmosphere, the temperature is increased to 900-1200°C at a heating rate of 5-10°C / min, and kept warm for 1-3h, the inert gas flow rate is 20-60mL / min, and then the temperature is reduced to 400-600°C, and switched to H2, and the treatment is continued for 2-4h, the H2 flow rate is 20-60mL / min, and then the temperature is reduced to room temperature to obtain a catalyst Zn-N / C.

3. The preparation method according to claim 2, characterized in that: In step 1, the Zn source is a soluble salt of Zn.

4. The preparation method according to claim 2, characterized in that: In step 1, the matrix material is selected from any one of 2-methylimidazole, benzimidazole and imidazole.

5. The preparation method according to claim 2, characterized in that: In step 1, the XRD spectrum of the microsphere precursor clearly observed the characteristic diffraction peaks of (011), (002), (112), (022), (013), (222), (114), (233) and (134) of the zeolite imidazolate framework at 2θ angles of 7.2°, 10.4°, 12.5°, 14.58°, 16.2°, 17.7°, 21.9°, 24.4° and 26.5°. Except for the characteristic peaks of the zeolite imidazolate framework, no other diffraction peaks appeared.

6. The preparation method according to claim 2, characterized in that: In step 1, the base is any one of NaOH, KOH, Ca(OH)2, and Ba(OH)2.

7. The preparation method according to claim 2, characterized in that: In step 1, the Zn-chitosan solution is uniformly added to the matrix solution by dripping through a syringe, the inner diameter of the syringe needle is 0.1-1.0 mm, and the dripping speed is 1-20 mL / min.

8. The preparation method according to claim 2, characterized in that: In step 2, the inert gas is any one of N2 and Ar.

9. A method for preparing acetaldehyde by catalytic dehydrogenation of ethanol, characterized in that: The method uses the supported Zn catalyst according to claim 1, comprising: The supported Zn catalyst was loaded into the reactor, and H2 was first used to purge the supported Zn catalyst for 1 h at a temperature of 400-600 °C, and then switched to N2. Ethanol was pumped into the vaporization chamber with an injection pump at a rate of 1-10 mL / h. The mass space velocity of ethanol was 1-50 h -1 , the temperature is 200-400°C, ethanol and N2 are fully mixed and then enter the reactor, the catalytic reaction temperature is set to 150-400°C, and the reaction pressure is normal pressure; the product is analyzed, among which the ethanol conversion rate is ≤93%, and the acetaldehyde selectivity is ≤95%.

10. The method for preparing acetaldehyde by catalytic dehydrogenation of ethanol according to claim 9, characterized in that: The reactor is any one of a fixed bed reactor and a shell-and-tube reactor.

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