CN113617353B - Preparation method of silicon carbide-based silver nanocatalyst and its application in ethylene oxide synthesis - Google Patents

Abstract

The invention relates to a preparation method of a silicon carbide-based silver nano catalyst and application thereof in ethylene oxide synthesis, comprising modification of a SiC carrier and size regulation of silver nano particles, wherein a proper amount of boron source and phosphorus source are added to carry out B, P bulk doping on SiC in the preparation process of a SiC precursor; then CeO is adopted ₂ 、ZrO ₂ 、Al ₂ O ₃ The oxide containing oxygen vacancies modifies the SiC surface; finally, taking silver nitrate with different amounts as a precursor, adopting a liquid phase reduction method to load metallic silver on the surface of the SiC carrier, and obtaining the load silicon carbide-based silver nano catalyst after suction filtration and drying. The catalyst prepared by optimizing the catalyst carrier property and the metal loading condition has the characteristics of controllable silver particle size, uniform dispersion, good thermal conductivity, good stability and the like. The catalyst is used in the reaction of preparing ethylene oxide by ethylene epoxidation, and can show excellent catalytic activity, product selectivity and stability.

Description

Preparation method of silicon carbide-based silver nanocatalyst and its use in ethylene oxide synthesis Applications

Technical field

The invention relates to the technical field of catalyst preparation, in particular to a preparation method of a silicon carbide-based silver nanocatalyst and its application in the synthesis of ethylene oxide.

Background technique

Ethylene oxide is an important organic chemical raw material and is widely used in washing, pharmaceutical, printing and dyeing and other industries. Conventional preparation methods of ethylene oxide include chlorohydrin method and oxidation method. The chlorohydrin method is mainly divided into two steps: the first step is to pass ethylene and chlorine into water to react to generate 2-chloroethanol; the second step is to use alkali (commonly used lime milk) to react with 2-chloroethanol to generate epoxy Ethane product. However, the ethylene unit consumption during the chlorohydrin production process is high and causes serious environmental pollution, so its use has been gradually stopped.

At present, the industrial production of ethylene oxide mainly uses the direct oxidation method, that is, the epoxidation of ethylene based on oxygen to prepare ethylene oxide. Most of the existing industrial ethylene oxide production devices use Ag-based nanocatalysts. After years of development, although the catalytic performance of Ag-based nanocatalysts has been improved, there are still certain shortcomings. The first is the selectivity of the catalyst. One of the main reasons why it is difficult to improve the product selectivity is that the oxidation reaction releases a large amount of heat, and there is a bottleneck in the heat removal rate. The core factor is that the traditional carrier material α-alumina has a low thermal conductivity; secondly The α-alumina carrier material used in traditional technology has a small specific surface area, poor dispersion of loaded Ag, and is easy to agglomerate; in order to ensure activity, a high loading of Ag is required. In addition, if a material with acidic sites on the surface is used as a catalyst carrier, the reaction network will be more likely to proceed toward the path of isomerization of ethylene oxide to acetaldehyde, seriously reducing the EO selectivity. Therefore, surface inert materials are required as carriers. Therefore, developing an inert inorganic material with a high specific surface area and good thermal conductivity as a carrier will effectively improve the selectivity and stability of Ag-based catalysts.

Silicon carbide is a semiconductor material with high temperature resistance, corrosion resistance, high mechanical strength, good chemical stability, and especially good electrical and thermal conductivity. In addition, when metal is loaded onto the SiC surface, electron transfer can occur between the metal and the SiC carrier, allowing it to exhibit unique activity and selectivity in catalytic reactions. As a new type of carrier, high specific surface area silicon carbide has shown obvious advantages in the fields of catalysis, photocatalysis and electrocatalysis. Therefore, using SiC as an ethylene epoxidation catalyst carrier will make it possible to break through the technical bottlenecks of low product selectivity and high Ag loading faced by the existing Ag/α-Al ₂ O ₃ catalytic system, thereby improving the epoxidation efficiency. The reaction had a disruptive effect.

Contents of the invention

The technical problem to be solved by the present invention is: in order to overcome the deficiencies in the existing technology, the present invention provides a preparation method of silicon carbide-based silver nanocatalyst and its application in the synthesis of ethylene oxide, so as to obtain uniform and stable metal dispersion. Nanocatalysts with good properties and thermal conductivity are used for the epoxidation of ethylene to prepare ethylene oxide.

The technical solution adopted by the present invention to solve its technical problems is: a preparation method of silicon carbide-based silver nanocatalyst, which has the following steps:

(1) In the preparation process of SiC precursor, boron source and phosphorus source are added to SiC for B and P bulk phase doping, and then oxides containing oxygen vacancies such as CeO ₂ , ZrO ₂ , Al ₂ O ₃ or other oxides are used. The mixture modifies the SiC surface to obtain a modified SiC carrier.

(2) Mix and stir a certain amount of silver nitrate solution and SiC carrier for a period of time, then add a certain amount of 0.1M lysine solution dropwise and continue stirring. Finally, add an appropriate amount of 0.1M sodium borohydride solution and 0.1M sodium borohydride solution to the reaction solution. hydrochloric acid solution, continue stirring the reaction, suction filtration, washing, and drying to obtain a high-efficiency silicon carbide-based silver catalyst.

Preferably, the content of bulk doped B and P in the carrier is controlled to a B/Si or P/Si molar ratio of 0.01 to 1; the surface is modified with oxygen-containing vacancy oxides such as CeO ₂ , ZrO ₂ , and Al ₂ O ₃ The content is 0.1 to 20% of the mass fraction of silicon carbide.

Preferably, the phosphorus source is phosphoric acid, phosphorous acid, sodium phosphate, ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, calcium hydrogen phosphate, calcium phosphate, calcium pyrophosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate , sodium acid pyrophosphate, sodium dihydrogen phosphate or sodium pyrophosphate; the boron source is boric acid, borane, boron trichloride or boron trifluoride.

Preferably, the carrier in the process of loading metal is an optimized and screened SiC carrier.

Preferably, the silver content in the catalyst is 0.1 to 80% of the mass fraction of silicon carbide, and the prepared silver nanoparticles have a particle size of less than 5 to 50 nanometers.

Preferably, the amount of lysine added is 0.1 to 80% of the mass fraction of silicon carbide.

Preferably, the amount of sodium borohydride reducing agent added is 0.1 to 80% of the mass fraction of silicon carbide.

Preferably, the amount of hydrochloric acid solution added is such that the pH value of the reaction solution is maintained between 9 and 10.

Preferably, the temperature is controlled at 20-50°C during the catalyst preparation process.

Preferably, the stirring time after each solution is dropped is 0.5 to 2 hours; the stirring reaction time after all the drops are completed is 1 to 48 hours.

Preferably, the stirring during the catalyst preparation process is magnetic stirring.

Preferably, the catalyst washing is using deionized water.

Preferably, the catalyst drying temperature is 50-80°C, and the drying time is 6-18 hours.

The silicon carbide-based silver nanocatalyst prepared above is used in the reaction of ethylene epoxidation to prepare ethylene oxide. The specific reaction conditions are: 0.05 to 50g of silicon carbide-based silver nanocatalyst is loaded into a fixed bed reactor. The raw material composition It is ethylene and oxygen, and the rest is nitrogen; the space velocity is 800~10000h ^-1 , the reaction pressure is 0~5MPa, the reaction temperature is 100~300℃, the selectivity of the catalyst to ethylene oxide reaches more than 80%, and the catalyst life reaches 800h above.

The beneficial effects of the present invention are: the present invention uses silicon carbide as a catalyst carrier and uses a liquid phase reduction method to load active silver nanoparticles. By optimizing the properties of the silicon carbide carrier and the metal loading process, a silicon carbide-based silver nanocatalyst with uniform metal dispersion, good thermal conductivity, and high stability was prepared. The preparation process is universal and simple, and is easy to be industrially synthesized; it can be used in ethylene Epoxidation can show high activity, selectivity and cycle stability when preparing ethylene oxide.

Detailed ways

Specific embodiments of the present invention are described in further detail below. It should be understood that the specific embodiments described here are only used to illustrate and explain the technical solutions of the present invention, and are not used to limit the technical solutions of the present invention.

Embodiment 1

Directly use high-surface silicon carbide as a carrier: mix 1854mL of AgNO ₃ aqueous solution (0.01M) and 18g of silicon carbide and stir evenly, add 200mL of 0.1M lysine and stir for 1 hour, then add 100mL of 0.1M sodium borohydride aqueous solution And stir for 0.5h, finally add 100mL, 0.1M HCl to keep the pH of the solution between 9 and 10, then continue stirring at room temperature for 24h, centrifuge, wash and dry to obtain Ag/SiC with a loading capacity of 10wt%. Catalyst, in which the average particle size of silver particles is about 20nm.

Take 5g of the above catalyst and load it into the fixed bed reactor. The raw gas volume composition is 25% ethylene, 7% oxygen, and the rest is nitrogen; the space velocity is 5000h ^-1 , the reaction pressure is 1.5MPa, and the reaction temperature is 200°C. The reactor outlet gas and raw material gas were quantitatively detected by gas chromatography. The ethylene conversion rate was 5% and the ethylene oxide selectivity was 82.5%.

Embodiment 2

High-surface silicon carbide is directly used as a carrier, and then aluminum nitrate is used as a precursor to modify the surface of silicon carbide with Al ₂ O ₃ , with a content of 5% of the mass fraction of silicon carbide.

Mix 1854 mL of AgNO ₃ aqueous solution (0.01 M) and 18 g of modified silicon carbide and stir evenly, add 200 mL of 0.1 M lysine and stir for 1 h, then add 100 mL of 0.1 M sodium borohydride aqueous solution and stir for 0.5 h, and finally Add 100 mL, 0.1 M HCl to maintain the pH of the solution between 9 and 10, then continue stirring at room temperature for 30 hours, centrifuge, wash and dry to obtain an Ag/Al ₂ O ₃ -SiC catalyst with a loading capacity of 10 wt%. , where the average particle size of silver particles is approximately 13nm.

Take 8g of the above catalyst and load it into the fixed bed reactor. The raw gas volume composition is 25% ethylene, 8% oxygen, and the rest is nitrogen; the space velocity is 5500h ^-1 , the reaction pressure is 1.5MPa, and the reaction temperature is 200°C. The reactor outlet gas and feed gas were quantitatively detected by gas chromatography. The ethylene conversion rate was 5.9% and the ethylene oxide selectivity was 85%.

Embodiment 3

During the preparation process of the silicon carbide precursor, an appropriate amount of boric acid is added to make the B/Si molar ratio 0.1, and a B-doped silicon carbide carrier (B-SiC) is prepared; then Ce(NO ₃ ) ₂ ·6H ₂ O is used as the As a precursor, a single layer of CeO ₂ is modified on the surface of B-SiC (CeO ₂ is 2% of the mass fraction of B-SiC) to obtain a modified silicon carbide carrier.

Mix 2224.9 mL of AgNO ₃ aqueous solution (0.01 M) and 17.6 g of modified silicon carbide and stir evenly. Add 250 mL of 0.1 M lysine and stir for 1 h. Then add 130 mL of 0.1 M sodium borohydride aqueous solution and stir for 0.5 h. , finally add 130mL, 0.1M HCl to keep the pH of the solution between 9 and 10; then continue stirring at room temperature for 24 hours, centrifuge, wash and dry to obtain Ag/CeO ₂ -B- with a loading capacity of 12wt%. SiC catalyst, in which the average particle size of silver particles is about 15nm.

Take 5g of the above catalyst and load it into the fixed bed reactor. The raw gas volume composition is 25% ethylene, 8% oxygen, and the rest is nitrogen; the space velocity is 5000h ^-1 , the reaction pressure is 2MPa, and the reaction temperature is 190°C. The reactor outlet gas and feed gas were quantitatively detected by gas chromatography. The ethylene conversion rate was 7% and the ethylene oxide selectivity was 884%.

Embodiment 4

During the preparation process of the silicon carbide precursor, an appropriate amount of ammonium phosphate is added to make the P/Si molar ratio 0.5, and a P-doped silicon carbide carrier (P-SiC) is prepared; then zirconium oxychloride is used as the precursor, and in the P-Si The SiC surface is modified with ZrO ₂ (ZrO ₂ is 1% of the mass fraction of P-SiC) to obtain a modified silicon carbide carrier.

Mix 927mL of AgNO ₃ aqueous solution (0.01mol/L) and 19g of silicon carbide and stir evenly, add 150mL and 0.1M lysine and stir for 1h, then add 50mL and 0.1M sodium borohydride aqueous solution and stir for 0.5h, and finally add 50mL , 0.1M HCl to keep the pH of the solution between 9 and 10; then continue stirring at room temperature for 24 hours, centrifuge, wash and dry to obtain an Ag/ZrO ₂ -P-SiC catalyst with a loading capacity of 5wt%, where The average particle size of silver particles is approximately 15nm.

Take 3g of the above catalyst and load it into the fixed bed reactor. The volume composition of the raw gas is 26% ethylene, 8% oxygen, and the rest is nitrogen; the space velocity is 4000h ^-1 , the reaction pressure is 2.5MPa, and the reaction temperature is 200°C. The reactor outlet gas and raw material gas were quantitatively detected by gas chromatography. The ethylene conversion rate was 6.3% and the ethylene oxide selectivity was 87.4%.

After the catalyst was used for 800 hours, the ethylene conversion rate was 5.9% and the ethylene oxide selectivity was 80%.

Embodiment 5

During the preparation process of the silicon carbide precursor, an appropriate amount of boric acid is added to make the B/Si molar ratio 0.3, and a B-doped silicon carbide carrier (B-SiC) is prepared. Then, using cerium nitrate and aluminum nitrate as precursors, CeO ₂ and Al ₂ O ₃ (oxide content is 3% of the mass fraction of B-SiC) are modified on the B-SiC surface to obtain a modified silicon carbide support.

Mix 1483 mL of AgNO ₃ aqueous solution (0.01 mol/L) and 18.4 g of silicon carbide and stir evenly, add 180 mL of 0.1 M lysine and stir for 1 h, then add 70 mL of 0.1 M sodium borohydride aqueous solution and stir for 0.5 h, and finally Add 70 mL, 0.1 M HCl to maintain the pH of the solution between 9 and 10; then continue stirring at room temperature for 24 hours, centrifuge, wash and dry to obtain Ag/CeO ₂ -Al ₂ O ₃ with a loading capacity of 8wt%. -B-SiC catalyst, in which the average particle size of the silver particles is approximately 15 nm.

Take 6g of the above catalyst and load it into the fixed bed reactor. The volume composition of the raw gas is 28% ethylene, 9% oxygen, and the rest is nitrogen; the space velocity is 8000h ^-1 , the reaction pressure is 2MPa, and the reaction temperature is 210°C. The reactor outlet gas and raw material gas were quantitatively detected by gas chromatography. The ethylene conversion rate was 7.2% and the ethylene oxide selectivity was 84.0%.

After the catalyst was used for 500 hours, the ethylene conversion rate was 6.8% and the ethylene oxide selectivity was 80%.

Embodiment 6

During the preparation process of the silicon carbide precursor, an appropriate amount of boric acid is added to make the B/Si molar ratio 0.25 to prepare a B-doped silicon carbide carrier (B-SiC); then, cerium nitrate and zirconium nitrate are used as precursors in B -SiC surface modification with CeO ₂ and ZrO ₂ (oxide content is 4% of the mass fraction of B-SiC) to obtain a modified silicon carbide carrier.

Mix 742mL of AgNO ₃ aqueous solution (0.01M) and 19.2g of silicon carbide and stir evenly, add 100mL of 0.1M lysine and stir for 2h, then add 50mL of 0.1M sodium borohydride aqueous solution and stir for 1h, and finally add 30mL, 0.1M HCl keeps the pH of the solution between 9 and 10; then continue stirring at room temperature for 48 hours, centrifuge, wash and dry to obtain an Ag/CeO ₂ -ZrO ₂ -B-SiC catalyst with a loading capacity of 4wt% , where the average particle size of silver particles is about 10nm.

Take 10g of the above catalyst and load it into the fixed bed reactor. The volume composition of the raw gas is 28% ethylene, 8% oxygen, and the rest is nitrogen; the space velocity is 7000h ^-1 , the reaction pressure is 2.3MPa, and the reaction temperature is 200°C. The reactor outlet gas and feed gas were quantitatively detected by gas chromatography. The ethylene conversion rate was 6.5% and the ethylene oxide selectivity was 88%.

After the catalyst was used for 200 hours, the ethylene conversion rate was 6.1% and the ethylene oxide selectivity was 80%.

Embodiment 7

During the preparation process of the silicon carbide precursor, an appropriate amount of phosphoric acid is added to make the P/Si molar ratio 0.19 to prepare a P-doped silicon carbide carrier (P-SiC); then, cerium nitrate and zirconium nitrate are used as precursors, in P -SiC surface modification with CeO ₂ and ZrO ₂ (oxide content is 10% of the mass fraction of B-SiC) to obtain a modified silicon carbide carrier.

Mix 5562 mL of AgNO ₃ aqueous solution (0.0 mol/L) and 34 g of silicon carbide and stir evenly. Add 600 mL of 0.1 M lysine and stir for 1 hour. Then add 350 mL of 0.1 M sodium borohydride aqueous solution and stir for 2 hours. Finally, add 350 mL. , 0.1M HCl to keep the pH of the solution between 9 and 10; then continue stirring at room temperature for 48 hours, centrifuge, wash and dry to obtain Ag/CeO ₂ -ZrO ₂ -B-SiC with a loading capacity of 15wt%. Catalyst, wherein the average particle size of silver particles is approximately 25nm.

Take 30g of the above catalyst and load it into the fixed bed reactor. The volume composition of the raw gas is 30% ethylene, 9% oxygen, and the rest is nitrogen; the space velocity is 10000h ^-1 , the reaction pressure is 1.5MPa, and the reaction temperature is 180°C. The reactor outlet gas and raw material gas were quantitatively detected by gas chromatography. The ethylene conversion rate was 6.8% and the ethylene oxide selectivity was 85.3%.

After the catalyst was used for 800 hours, the ethylene conversion rate was 6.5% and the ethylene oxide selectivity was 82%.

In the present invention, P and B are doped into the silicon carbide carrier to increase the thermal conductivity of the carrier, accelerate the heat removal rate of the catalyst, and effectively improve the selectivity of ethylene epoxidation to produce ethylene oxide; the surface of the catalyst is increased by modifying oxides on the surface of the catalyst oxygen vacancies, thereby effectively stabilizing the loaded silver nanoparticles and improving the stability of the catalyst; the catalyst metal prepared by optimizing the properties of the silicon carbide carrier and the metal loading process is uniformly dispersed, has good thermal conductivity and stability; the catalyst preparation process is universal The silicon carbide-based silver nanocatalyst prepared has high activity, selectivity and cycle stability for the epoxidation of ethylene to produce ethylene oxide.

Taking the above-mentioned ideal embodiments of the present invention as inspiration and through the above description, relevant workers can make various changes and modifications without departing from the scope of the technical idea of the present invention. The technical scope of the present invention is not limited to the content in the description, and must be determined based on the scope of the claims.

Claims (7)

1. A preparation method of a silicon carbide-based silver nano catalyst is characterized by comprising the following steps: the preparation method comprises the following steps: the modified SiC is used as a carrier, silver salt is used as a metal precursor, a low-temperature liquid phase reduction method is used for loading metallic silver, and the silicon carbide-based silver nano catalyst is obtained after suction filtration and drying;

the modified SiC carrier is SiC subjected to surface modification or bulk phase doping treatment;

the surface modification of the SiC adopts oxide CeO with oxygen vacancies ₂ 、ZrO ₂ Or Al ₂ O ₃ Or a mixture thereof, for modifying the SiC surface, said oxygenThe content of the compound or the mixture is 0.1-20% of the mass fraction of silicon carbide;

the bulk doping is to add a boron source and a phosphorus source to carry out B, P bulk doping on SiC in the preparation process of the SiC precursor, wherein the molar ratio of B/Si or P/Si is 0.01-1, and then carrying out carbothermic reduction to obtain the bulk B, P doped modified SiC.

2. The method for preparing the silicon carbide-based silver nano catalyst as set forth in claim 1, wherein: the modification method of the oxide to the SiC is to prepare the oxide by adopting an impregnation method, adding a proper amount of SiC powder into a solution of cerium salt, zirconium salt, aluminum salt or mixed salt serving as a precursor, stirring for a certain time, then drying at 110 ℃ for 12-24h, and then calcining at 400-600 ℃ for 4-10 h in a muffle furnace to obtain the oxide modified SiC.

3. The method for preparing the silicon carbide-based silver nano catalyst as set forth in claim 1, wherein: the phosphorus source is phosphoric acid, phosphorous acid, sodium phosphate, ammonium phosphate, diammonium phosphate, monoammonium phosphate, calcium hydrogen phosphate, calcium pyrophosphate, monopotassium phosphate, dipotassium phosphate, sodium acid pyrophosphate, sodium dihydrogen phosphate or sodium pyrophosphate; the boron source is boric acid, borane, boron trichloride or boron trifluoride.

4. The method for preparing the silicon carbide-based silver nano catalyst as set forth in claim 1, wherein: the liquid phase reduction comprises the following steps: mixing a certain amount of silver salt solution and a modified SiC carrier, stirring for 0.5-2 h, then dropwise adding 0.1M lysine solution into the reaction solution, continuously stirring for 0.5-2 h, and finally dropwise adding a proper amount of 0.1M sodium borohydride solution and 0.1M hydrochloric acid solution into the reaction solution in sequence, and continuously stirring for 1-48 h.

5. The method for preparing the silicon carbide-based silver nano catalyst according to claim 4, which is characterized in that: in the liquid phase reduction process, the amount of the silver salt solution is controlled as follows: the silver content is 0.1-30% of the silicon carbide mass fraction, and the particle size range of the prepared silver nano particles is 5-50 nm.

6. Use of the silicon carbide-based silver nano-catalyst prepared by the preparation method according to claim 1 in the synthesis of ethylene oxide, which is characterized in that: the catalyst is applied to the reaction for preparing ethylene oxide by ethylene epoxidation, and the specific reaction conditions are as follows: filling 1-200 g of silicon carbide-based silver nano catalyst into a fixed bed reactor, wherein the composition of raw material gas is as follows: 25-30% of ethylene, 6-9% of oxygen and the balance of nitrogen; airspeed of 800-10000 h ^-1 The reaction pressure is 0-5 MPa, and the reaction temperature is 150-250 ℃.

7. The use of the silicon carbide based silver nano-catalyst in the synthesis of ethylene oxide according to claim 6, wherein: the ethylene oxide selectivity reaches more than 80 percent.