Silver-based supported porous boron nitride catalyst and preparation method thereof

Abstract

The invention relates to preparation and application of a catalyst, in particular to a preparation method of a silver-based porous boron nitride catalyst, and the catalyst prepared by the method is applied to the field of dimethyl oxalate hydrogenation and is used for preparing methyl glycolate. The preparation method comprises the following steps: carrying out ultrasonic treatment on porous boron nitride in water to form a boron nitride suspension, dissolving various metal salt precursors required for preparing the catalyst, adding a proper amount of ammonia water to form a complex after the metal salt precursors are completely dissolved in water, dropwise adding the obtained complex into the boron nitride suspension, heating in a water bath, and carrying out vacuum drying to obtain the boron nitride catalyst. The method is simple in preparation and good in process repeatability, metal silver is highly dispersed in porous boron nitride through an ammonia distillation method, and under the reaction conditions that the temperature is 200 DEG C, the pressure is 2.5 Mpa, H2 / DMO is 80 and WHSV is 0.625 h <-1 >, the dimethyl oxalate conversion rate reaches 99%, and the methyl glycolate selectivity can reach 95%.

Description

Technical Field

[0001] The present invention relates to the preparation and application of catalysts, and particularly to a method for preparing a silver-based porous boron nitride catalyst. The catalyst prepared by this method is applied to the field of hydrogenation of dimethyl oxalate for the preparation of methyl glycolate. Technical Background

[0002] With the development of industry, polyester products such as plastics are widely used in life. However, due to their non-degradability, most polyester products seriously affect our living environment. Therefore, degradable polyester products have broad application prospects, and polyglycolic acid plays an important role in degradable polyester products. With the development of coal chemical industry, the industrial route of converting coal into dimethyl oxalate through gasification has been increasingly improved. Methyl glycolate produced by the moderate hydrogenation of dimethyl oxalate can form polyglycolic acid through dehydration and polycondensation. The key step in this process is the development of a catalyst for the hydrogenation of dimethyl oxalate to methyl glycolate with high activity, high selectivity and high stability.

[0003] According to the thermodynamic equilibrium of the reaction of hydrogenating dimethyl oxalate to methyl glycolate, the reaction conditions are suitable for carrying out at high temperature and low pressure, and the raw materials and products are all gases under these conditions. Usually, a fixed-bed reactor device is used for such reactions. Since the reaction conditions are relatively harsh, the catalysts commonly used by researchers are supported catalysts, which are composed of active metals and carriers. A large number of literatures show that the activation of dimethyl oxalate and the dissociation process of hydrogen mostly occur on the metal, and the carrier mainly plays the role of dispersing the metal and changing the electronic effect of the metal.

[0004] For the active metals of the catalyst for hydrogenating dimethyl oxalate to methyl glycolate, there are mainly copper, silver, nickel and ruthenium. Among them, silver has the best catalytic activity. Therefore, silver dispersed on the carrier is a favorable choice for the reaction of hydrogenating dimethyl oxalate to methyl glycolate. For silver-based catalysts, there are mainly two problems. Firstly, since the Schöttky temperature of silver is lower than the catalyst reaction temperature, it is easy to agglomerate during the catalytic process, significantly reducing the catalyst life. Secondly, the ability of zero-valent silver to dissociate hydrogen is weak, and a high loading amount of silver is required to improve the catalytic activity, which makes the catalyst uneconomical. Therefore, the development of a catalyst with high activity and stability is crucial for the successful commercialization of the hydrogenation of dimethyl oxalate to methyl glycolate.

[0005] Since silver-based catalysts are prone to migration and aggregation on the surface of the support, researchers have modified the catalysts by using different methods. Currently, the methods reported in the literature and patents include adding promoters, adding metals to form alloys, changing the particle size of active metals, and improving the interaction between active metals and supports. For example, Chinese Patent CN114950416A reported a catalyst for the hydrogenation of dimethyl oxalate to methyl glycolate, its preparation method and application. Boric acid was dropped onto the synthesized Ag / SiO2 catalyst, and the boron oxide formed after calcination would inhibit the agglomeration of silver and improve the stability of the silver-based catalyst. However, the catalytic activity is relatively low.

[0006] The support also plays an important role in improving the catalytic activity of the reaction for the hydrogenation of dimethyl oxalate to methyl glycolate and inhibiting the aggregation of metal particles. Generally, a neutral support with a high specific surface area is selected. For example, in Chinese Patent CN112387306A, the selectivity of methyl glycolate was improved by utilizing the characteristics of short pore diameter and high specific surface area of silica spheres. However, the structural stability of silica spheres after long-term reaction remains to be investigated.

[0007] Therefore, it is particularly important to select metallic silver and a support with both high specific surface area and neutrality, such as non-metal oxides or composite metal oxides, for developing a catalyst with low cost and stable performance for the hydrogenation of dimethyl oxalate to methyl glycolate. Summary of the Invention

[0009] The present invention provides a silver-based porous boron nitride catalyst prepared by an ammonia evaporation method, and this catalyst is applied to the reaction of hydrogenating dimethyl oxalate to methyl glycolate.

[0010] A silver-based porous boron nitride catalyst is prepared as follows: Ultrasonicate porous boron nitride in water for 1 hour to form a boron nitride suspension. Dissolve various metal salt precursors required for preparing the catalyst in an appropriate amount of water. After all the metal salt precursors are dissolved in water, add an appropriate amount of ammonia water to form a complex. Drop the obtained complex into the boron nitride water suspension drop by drop, and complete the dropping within 3 minutes. Then heat in a water bath or oil bath for 5 - 8 h, take it out and dry at a suitable temperature and calcine for an appropriate time to obtain a fresh catalyst with the molecular formula M x (BN) 1-x (M is a transition metal, x = 0.05 - 0.15) The transition metal is one of copper, nickel, palladium, platinum, rhodium, ruthenium, gold, and silver; The metal salt precursor is one of sulfates, chlorides, nitrates, and carbonates, preferably nitrates; The boron nitride is one of hexagonal boron nitride, cubic boron nitride, and wurtzite boron nitride, preferably cubic boron nitride; The complexing agent described above is ammonia water or ammonium chloride, preferably ammonia water.

[0011] The described M x (BN) 1-x The mass fraction of M in it is 5% - 20%, preferably 7.5% - 15%, and the mass fraction of BN is 20% - 95%.

[0012] For the water bath or oil bath described in the present invention, the stirring temperature can be 30 - 100 °C, preferably 70 - 90 °C; the drying temperature is 80 - 200 °C, preferably 90 - 120 °C, and the drying time is 3 - 15 h, preferably 9 - 13 h; Its calcination temperature is 200 - 600 °C, preferably 300 - 500 °C, and the time is 1 - 7 h, preferably 3 - 5 h.

[0013] The catalyst prepared above is used for performance evaluation test after grinding and screening. The particle size of the catalyst used for evaluation is 5 - 80 mesh, the reaction temperature is 180 °C - 240 °C, the pressure is 1.5 - 3.5 Mpa, and the space velocity is 0.3 - 0.9 h -1 , and the reaction raw materials are dimethyl oxalate and hydrogen, with a ratio of 1:10 - 150; the liquid after the reaction is analyzed by a gas chromatograph.

[0014] The particle size of the described catalyst is preferably 20 - 50 mesh; The described reaction temperature is preferably 170 - 230 °C; The described reaction pressure is preferably 2.0 - 3.0 MPa; The described space velocity is preferably 0.5 - 0.7 h -1 ; The described gas chromatograph uses a dual chromatographic column, with the PQ and TDX - 01 analytical columns used in series; the relative concentrations of each component are calculated by the area correction factor normalization method.

[0015] The conversion rate of dimethyl oxalate described above is higher than 99%, and the selectivity of methyl glycolate is higher than 95%.

[0016] The advantages of the catalyst for hydrogenating dimethyl oxalate to methyl glycolate provided by the present invention are as follows: (1) The preparation method of the catalyst provided by the present invention is simple, which is conducive to large - scale production. (2) The selectivity of methyl glycolate is higher than that of most catalysts. (3) The reaction temperature is moderate and the hydrogen - to - ester ratio is low, which is conducive to reducing production costs. Description of the Drawings

[0018] Figure 1 It is the X - ray powder diffraction pattern of the catalyst after calcination of the catalysts in Examples 1 - 4.

[0019] Figure 2 It is the transmission electron microscopy image of the catalyst after calcination in Example 2.

[0020] Figure 3 It is the comparison of the catalytic performances of the catalysts in Examples 1 - 4 under the conditions of 180 °C, 2.5 Mpa, 0.625 h -1 , and H2 / DMO = 80.

[0021] Figure 4 It is the change of the catalyst performance with temperature of the catalyst in Example 2 under the conditions of 2.5 Mpa, 0.625 h -1 , and H2 / DMO = 80. Detailed implementation manners

[0023] The following details the specific implementation manners of the present invention, but the protection scope of the present invention is not limited thereto.

[0024] Example 1 (1) Catalyst preparation Take 4 g of porous boron nitride, add 400 ml of deionized water, and ultrasonicate for 1 h to obtain Solution A. Take 0.51 g of AgNO3, add 30 ml of deionized water, stir to completely dissolve AgNO3, and dropwise add NH3·H2O until the solution becomes clear to obtain Solution B. Drop Solution B into Solution A, complete the dropwise addition within 3 min, carry out a water bath at 90 °C for 5 h, dry, and calcine at 350 °C for 4 h to obtain fresh catalyst powder. This catalyst has a porous rod-like structure, the active metal is silver, and the carrier is boron nitride. This catalyst is denoted as 7.5Ag / p-BN.

[0025] (2) Catalyst evaluation First, take 3.5 g of the sieved catalyst with a particle size of 20 - 40 mesh and load it into the middle of a stainless steel reaction tube with an inner diameter of 12 mm. Under a reaction pressure of 2.5 MPa, reduce it in a H2 atmosphere at 300 °C for 4 h for activation. After the reduction is completed, cool it to 220 °C in a H2 atmosphere, and start to introduce DMO. The molar ratio of H2 to DMO is controlled at 80:1, and the mass liquid hourly space velocity of DMO is about 0.625 h -1 . After DMO enters the bed, start the reaction of dimethyl oxalate to methyl glycolate at 200 °C, react for 6 h and then cool to 190 °C and react for 6 h; react for 6 h at each temperature segment, and take samples for analysis of the reaction results at 2 h, 4 h, and 6 h of the reaction, and measure down to 180 °C at the lowest.

[0026] The performance test results are shown in Figure 3 . After analysis, when the mass fraction of silver is 7.5%, the conversion rate of dimethyl oxalate of the catalyst under the reaction conditions of 180 °C is 70.8%, and the selectivity of methyl glycolate is 97.2%.

[0027] Example 2 (1) Catalyst preparation Take 4 g of porous boron nitride, add 400 ml of deionized water, and ultrasonicate for 1 h to obtain solution A. Take 0.7 g of AgNO3, add 30 ml of deionized water, stir to completely dissolve AgNO3, and dropwise add NH3·H2O until the solution becomes clear to obtain solution B. Drop solution B into solution A and complete the addition within 3 min. Then, carry out a water bath at 90 °C for 5 h. The subsequent steps are the same as in Example 1, and the obtained catalyst is labeled as 10Ag / p-BN.

[0028] (2) Catalyst evaluation Use the method of Example 1 to measure its conversion rate in the reaction of hydrogenating dimethyl oxalate to methyl glycolate. The performance test results are shown in Figure 3 . After analysis, when the silver mass fraction is 10%, at a reaction temperature of 180 °C, the conversion rate of dimethyl oxalate is 88.5%, and the selectivity of methyl glycolate is 97.3%.

[0029] Example 3 (1) Catalyst preparation Take 4 g of porous boron nitride, add 400 ml of deionized water, and ultrasonicate for 1 h to obtain solution A. Take 0.9 g of AgNO3, add 30 ml of deionized water, stir to completely dissolve AgNO3, and dropwise add NH3·H2O until the solution becomes clear to obtain solution B. Drop solution B into solution A and complete the addition within 3 min. Then, carry out a water bath at 90 °C for 5 h. The subsequent steps are the same as in Example 1, and the obtained catalyst is labeled as 12.5Ag / p-BN.

[0030] (2) Catalyst evaluation Use the method of Example 1 to measure its conversion rate in the reaction of hydrogenating dimethyl oxalate to methyl glycolate. The performance test results are shown in Figure 3 . After analysis, when the silver mass fraction is 12.5%, at a reaction temperature of 180 °C, the conversion rate of dimethyl oxalate is 89.5%, and the selectivity of methyl glycolate is 97.5%.

[0031] Example 4 (1) Catalyst preparation Take 4 g of porous boron nitride, add 400 ml of deionized water, and ultrasonicate for 1 h to obtain solution A. Take 1.11 g of AgNO3, add 30 ml of deionized water, stir to completely dissolve AgNO3, and dropwise add NH3·H2O until the solution becomes clear to obtain solution B. Drop solution B into solution A and complete the addition within 3 min. Then, carry out a water bath at 90 °C for 5 h. The subsequent steps are the same as in Example 1, and the obtained catalyst is labeled as 15Ag / p-BN.

[0032] (2) Catalyst evaluation The conversion rate in the reaction of hydrogenating dimethyl oxalate to methyl glycolate was determined by the method of Example 1, and the performance test results are shown in Figure 3 . After analysis, when the silver mass fraction was 15%, the conversion rate of dimethyl oxalate was 92.4% and the selectivity of methyl glycolate was 98.2% at the reaction temperature of 180 °C for the catalyst.

Claims

1. A method for preparing a silver-based supported porous boron nitride catalyst, characterized in that: The porous boron nitride is ultrasonically treated in water for a certain period of time to form a boron nitride suspension, and various metal salt precursors required for preparing the catalyst are dissolved in water. After the metal salt precursors are completely dissolved in water, ammonia water is added to form a complex, and the obtained complex is dripped into the boron nitride suspension drop by drop; then heated in a water bath or an oil bath, taken out, dried, and calcined to obtain a fresh catalyst with a molecular formula of M x (BN) 1-x , M is a transition metal, x=0.05-0.15).

2. The preparation method according to claim 1, characterized in that: The transition metal is one of copper, nickel, palladium, platinum, rhodium, ruthenium, gold and silver; the metal salt precursor is one of sulfate, chloride, nitrate and carbonate; the boron nitride is one of hexagonal boron nitride, cubic boron nitride and wurtzite boron nitride; the complexing agent is ammonia water or ammonium chloride.

3. The preparation method according to claim 2, characterized in that: The metal salt precursor is nitrate.

4. The preparation method according to claim 2, characterized in that: The boron nitride is cubic boron nitride; and the ultrasonic time is 1 hour.

5. The preparation method according to claim 1, characterized in that: The dropwise addition was completed within 3 minutes.

6. The preparation method according to claim 1, characterized in that: The M x (BN) 1-x The mass fraction of M is 5%-15%, and the mass fraction of BN is 75%-95%.

7. The preparation method according to claim 1, characterized in that: The water bath or oil bath has a temperature of 30-100°C and a heating time of 5-8h; the drying temperature is 80-200°C and the drying time is 3-15h; the roasting temperature is 200-600°C and the time is 1-7h.

8. A silver-based supported porous boron nitride catalyst prepared by the method according to any one of claims 1 to 7.

9. Use of the silver-based supported porous boron nitride catalyst as claimed in claim 8 in methane dry reforming reaction.

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