Silver catalyst and preparation method thereof

Abstract

The invention provides a silver catalyst and a preparation method thereof, and the preparation method comprises the following steps: step 1, mixing a silver precursor, a dispersing agent and water to form a mixed solution, and then mixing the mixed solution with a reducing agent; step 2, performing phase splitting on the mixture obtained in the step 1, wherein the lower layer is nano Ag sol; step 3, mixing and drying the nano Ag sol and SiC, and then performing thermal shock treatment in a Joule heating device to obtain a silver catalyst; wherein the dispersing agent is at least one of polyvinylpyrrolidone, carboxymethyl cellulose and polyvinyl alcohol; the reducing agent is at least one of sodium borohydride, diisobutylaluminium hydride and lithium aluminum hydride. Nano Ag sol is loaded on a SiC carrier, and thermal shock treatment is combined. The number of active sites on the carrier is increased by controlling the size and distribution of Ag particles. In addition, the SiC carrier can conduct out heat generated in the ethylene epoxidation process in time, and aggregation of the active component silver is further avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of olefin cyclization, in particular to a silver catalyst and a preparation method thereof. BACKGROUND

[0002] Ethylene oxide is mainly used for the manufacture of ethylene glycol and other important chemical raw materials, which can be used for the synthesis of detergent, surfactant, antifreezing agent and other chemical products. Ethylene oxide is an important organic chemical product in ethylene industry, whose importance is only next to polyethylene and polyvinyl chloride, and its annual output value can reach billions of dollars, and it is widely used in the synthesis of various alcohols. There are mainly two production processes of ethylene oxide: chlorohydrination method and ethylene direct oxidation method. The chlorohydrination method is gradually eliminated due to many synthesis steps, more byproducts, serious equipment corrosion and environmental pollution. The ethylene direct oxidation method is a method for preparing ethylene oxide by directly oxidizing ethylene with air or oxygen through a catalyst under certain temperature and pressure in gas phase. At present, 90% of the ethylene oxide process in China belongs to the ethylene direct oxidation method.

[0003] Currently, Ag / Al2O3 catalyst is mainly used in ethylene epoxidation reaction in industry, but the reaction and related side reactions are all strong exothermic reactions, and the active Ag particles on the surface of the catalyst are agglomerated by heat, thereby reducing the performance of the catalyst. The performance of the catalyst is not only crucially important to the composition of the catalyst, but also closely related to the performance of the carrier and the preparation method. In the development process of ethylene oxide technology, developing high-performance catalysts is the core. The activation energy of ethylene to generate ethylene oxide and carbon dioxide is 63 and 84 kJ / mol respectively, so the ethylene epoxidation reaction first requires high catalyst activity, which can reduce the reaction temperature, and reducing the reaction temperature is more beneficial to the main reaction. Secondly, the selectivity should be good, the side reaction should be reduced, and the yield of ethylene oxide should be improved. In addition, the service life of the catalyst should be long.

[0004] For the reaction of ethylene epoxidation to prepare ethylene oxide, the performance of the catalyst carrier is crucial, and currently α-Al2O3 is mostly used as the carrier, Ag is used as the active component, and elements such as rhenium and cesium are added as auxiliary elements. Ethylene epoxidation reaction is a strong exothermic reaction, and the heat released by the reaction needs to be promptly discharged by the material, and the material itself should have certain heat resistance and thermal conductivity. In recent years, various elements have been doped into the carrier material, and various pretreatment processes have been used to improve the chemical properties and physical structure of the carrier. At the same time, the exploration and discovery of new carrier materials have also been the focus of research.

[0005] Among them, silicon carbide (SiC) is a semiconductor material, which is resistant to high temperature, corrosion, has high mechanical strength, good chemical stability, especially has good electrical conductivity and thermal conductivity, and is also a potential catalyst carrier material. As a catalyst carrier, the biggest difference between SiC and Al2O3, SiO2 and other carriers is that SiC is a semiconductor, and its work function is 4.0eV, which is lower than the commonly used catalytically active metal. Due to the Mott-Schottky effect, the SiC conduction band electrons will transfer to the metal, and a synergistic effect will be generated at the metal and SiC surface, which shows unique activity and selectivity in some catalytic reactions. At the same time, the specific surface area of silicon carbide can be adjusted between 1-50m 2 / g, and high specific surface area can make metal particles uniformly dispersed and not react with active components. In addition, SiC has stable chemical properties and is resistant to acid and alkali corrosion, and does not change in structure and properties during the catalytic process, is simple to recover, can be reused, and is safe and reliable during use.

[0006] CN113617353A discloses a preparation method of a silicon carbide-based silver nano-catalyst and its application in the synthesis of ethylene oxide. The method includes modification of the SiC carrier and size control of silver nanoparticles. First, B and P are doped into SiC by adding appropriate amount of boron source and phosphorus source during the preparation of SiC precursor; then, the surface of SiC is modified by CeO2, ZrO2, Al2O3 and other oxides containing oxygen vacancies; finally, different amounts of silver nitrate are used as precursors, and a liquid phase reduction method is used to load metal silver on the surface of the SiC carrier. The supported silicon carbide-based silver nano-catalyst is obtained after filtration and drying. The focus of this technology is the preparation of the carrier silicon carbide, and the dispersity of silver particles still needs to be further improved.

[0007] CN111068678B discloses a preparation method of a supported silver-based multi-component nano-catalyst for ethylene epoxidation. Ti metal sheets are pretreated with nitric acid solution, acetone solution, anhydrous ethanol and deionized water under ultrasonic; the seed precursor is added to water to configure a seed precursor colloid, which is uniformly coated on the surface of the Ti metal sheet, calcined to obtain a treated Ti metal sheet; Ag precursor solution and various additive precursor solutions are added to a ZnO precursor solution containing a surfactant, and the Ti metal sheet is immersed in the above solution for reaction; a catalyst intermediate is obtained; the product is obtained after washing with deionized water, drying and calcination in an inert atmosphere. This invention uses ZnO nano-pillar array as a carrier to load Ag nanoparticles. This carrier has regular morphology and large surface area, which can better and uniformly load Ag nanoparticles and other additives; however, the preparation process of the catalyst is complicated, which is not conducive to large-scale application.

[0008] Therefore, further research is needed on catalysts for ethylene epoxidation. SUMMARY

[0009] The main object of the present application is to provide a silver catalyst and a preparation method thereof to overcome the defects of poor dispersion and easy agglomeration of the active component silver in the prior art.

[0010] To achieve the above object, the present application provides a preparation method of a silver catalyst, comprising the following steps:

[0011] Step 1, forming a mixture solution of a silver precursor, a dispersing agent and water, and then mixing with a reducing agent;

[0012] Step 2, phase separation of the mixture obtained in step 1, and the lower layer is a nano Ag sol;

[0013] Step 3, mixing the nano Ag sol with SiC, drying, and then performing thermal shock treatment in a joule heating device to obtain a silver catalyst;

[0014] In the present application, the dispersing agent is at least one of polyvinylpyrrolidone, carboxymethyl cellulose and polyvinyl alcohol; and the reducing agent is at least one of sodium borohydride, diisobutylaluminum hydride and lithium aluminum hydride.

[0015] In the preparation method of the silver catalyst, the silver precursor is a silver-containing compound; in step 1, the silver precursor is mixed with water, the dispersing agent is mixed with water, and then the mixture of the silver precursor and water and the mixture of the dispersing agent and water are mixed to form the mixture solution.

[0016] In the preparation method of the silver catalyst, the silver precursor is at least one of silver nitrate, silver acetate, silver carbonate, silver sulfate, silver nitrite and silver ammonia complex; the concentration of the silver precursor in the mixture of the silver precursor and water is 0.1-20 mg / mL; and the concentration of the dispersing agent in the mixture of the dispersing agent and water is 1-50 mg / mL.

[0017] In the preparation method of the silver catalyst, the mixing of the mixture of the silver precursor and water and the mixture of the dispersing agent and water is carried out at-10-5℃ with stirring; the stirring speed is 100-1000 r / min, and the stirring time is 5-150 min.

[0018] In the preparation method of the silver catalyst, the reducing agent is in the form of a reducing agent solution, which is added dropwise into the mixture solution in step 1 with stirring; and the concentration of the reducing agent solution is 1-30 mg / mL.

[0019] The preparation method of the silver catalyst, wherein the mass ratio of the reducing agent to the silver precursor is 0.1-10:1, and the mass ratio of the dispersing agent to the silver precursor is 1-100:1.

[0020] The preparation method of the silver catalyst, wherein the phase separation of the mixture obtained in step 1 is carried out by standing or centrifugation; the centrifugation speed is 2000-10000 r / min, and the centrifugation time is 5-60 min.

[0021] The preparation method of the silver catalyst, wherein the organic solvent is added when the nano Ag sol is mixed with the SiC.

[0022] The preparation method of the silver catalyst, wherein the organic solvent is ethanol; the nano Ag sol is calculated based on Ag, and the nano Ag sol accounts for 1-20% of the total mass of the nano Ag sol and the SiC; the specific surface area of the SiC is 1-50 m 2 / g.

[0023] The preparation method of the silver catalyst, wherein the thermal shock treatment is carried out under the condition that Ar is used as the protective gas, the thermal shock temperature is 400-800℃, the time of each thermal shock is 0.1-10 s, and the number of thermal shocks is 1-50.

[0024] To achieve the above-mentioned purpose, the application further provides the silver catalyst obtained by the above-mentioned preparation method, wherein the diameter of the silver particles in the silver catalyst is 4-30 nm.

[0025] The application has the following beneficial effects:

[0026] In the application, the nano Ag sol is loaded on the SiC carrier, the number of active sites on the carrier is increased by controlling the nano size of the Ag particles, the SiC carrier has the characteristics of high specific surface area, high temperature resistance, good electrical conductivity and thermal conductivity, and stable properties, which is helpful for the further dispersion of silver, and the SiC carrier can timely conduct the heat generated in the ethylene epoxidation process, thereby further avoiding the aggregation of the active component silver. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The ultraviolet-visible spectrum of the nano Ag sol with different Ag concentrations in Example 1 of the application.

[0028] Figure 2 The transmission electron microscope image of the 5wt% Ag / SiC catalyst prepared by using the nano Ag sol in Example 1 and not subjected to thermal shock treatment.

[0029] Figure 3A transmission electron microscope image of the 5 wt% Ag / SiC catalyst prepared using the nano-Ag sol and heat shock treatment of Example 1. DETAILED DESCRIPTION

[0030] The technical solutions of the present application are described in detail below. The following embodiments are implemented on the premise of the technical solutions of the present application, and detailed implementation processes are given. However, the protection scope of the present application is not limited to the following embodiments. The structures or experimental methods not specified in the following embodiments are usually implemented according to conventional conditions.

[0031] The present application provides a preparation method of a silver catalyst, comprising the following steps:

[0032] Step 1, forming a mixture solution of a silver precursor, a dispersing agent and water, and then mixing with a reducing agent;

[0033] Step 2, phase separation of the mixture obtained in Step 1, and the lower layer is a nano-Ag sol;

[0034] Step 3, mixing the nano-Ag sol and SiC, drying, and then performing heat shock treatment in a joule heating device to obtain a silver catalyst;

[0035] The dispersing agent is at least one of polyvinylpyrrolidone, carboxymethyl cellulose and polyvinyl alcohol; and the reducing agent is at least one of sodium borohydride, diisobutylaluminum hydride and lithium aluminum hydride.

[0036] The present application first prepares a nano-Ag sol, and then loads the nano-Ag sol on a SiC carrier. By controlling the nano size of Ag particles, the number of active sites on the carrier is further increased. In addition, the SiC carrier has the characteristics of high specific surface area, high temperature resistance, good electrical and thermal conductivity, and stable properties, which is helpful for the further dispersion of silver. Moreover, the SiC carrier can timely conduct the heat generated in the ethylene epoxidation process, further avoiding the aggregation of the active component silver.

[0037] In an embodiment, the silver precursor is a silver-containing compound, for example, a silver-containing salt, including silver-containing salts soluble in water, silver-containing salts slightly soluble in water and silver-containing salts insoluble in water. In another embodiment, the silver precursor is at least one of silver nitrate, silver acetate, silver carbonate, silver sulfate and silver nitrite. The silver precursor can also be a silver ammine complex, or a combination of a silver ammine complex and a silver-containing salt.

[0038] In one embodiment, the silver precursor is first mixed with water, the dispersant is mixed with water, and then the mixture of the silver precursor and water and the mixture of the dispersant and water are mixed to form a mixture. In another embodiment, the concentration of the silver precursor in the mixture of the silver precursor and water is 0.1-20 mg / mL, and the concentration of the dispersant in the mixture of the dispersant and water is 1-50 mg / mL. In another embodiment, the mixture of the mixture of the silver precursor and water and the mixture of the dispersant and water is mixed at -10-5°C, for example, in an ice water bath, and stirring is performed; the stirring speed is 100 r / min to 1000 r / min, and the stirring time is 5-150 min. In yet another embodiment, the mass ratio of the reducing agent to the silver precursor is 0.1-10:1, and the mass ratio of the dispersant to the silver precursor is 1-100:1. The dispersant of the present application is a high molecular polymer, which has a steric hindrance effect to prevent Ag from gathering and increase the dispersibility of Ag.

[0039] In one embodiment, the reducing agent is added dropwise in the form of a reducing agent solution to the mixture of step 1, and stirring is performed during the dropwise addition. The solution gradually changes from colorless to brown, and finally to brown-black, at which point the dropwise addition is stopped. The concentration of the reducing agent solution is 1-30 mg / mL.

[0040] Then, the mixture obtained in step 1 is subjected to phase separation, and the upper layer is a clear liquid, and the lower layer is a nano-Ag sol. In one embodiment, the phase separation is performed by standing or centrifugation. The centrifugation speed is, for example, 2000 r / min to 10000 r / min, and the centrifugation time is, for example, 5-60 min.

[0041] Step 3 is mixing the nano-Ag sol and SiC, drying, and then performing thermal shock treatment in a joule heating device to obtain a silver catalyst. In one embodiment, an organic solvent is further added when the nano-Ag sol is mixed with the SiC. In another embodiment, the SiC is first mixed with an organic solvent to form a mixture, and then the mixture is mixed with the nano-Ag sol. More specifically, the nano-Ag sol is added to the mixture of the SiC and the organic solvent and stirred for, for example, 0.5-5 h, dried, to obtain a silver-loaded catalyst. In yet another embodiment, the nano-Ag sol, in terms of Ag, accounts for 1-20% of the total mass of the nano-Ag sol and the SiC; in other words, the silver in the nano-Ag sol accounts for 1-20% of the total mass of the silver and SiC in the nano-Ag sol. For example, different amounts of Ag sol and SiC carriers are weighed according to different Ag loadings, such as 10 mL of silver sol with a concentration of 5 mg / mL and 950 mg of SiC, to prepare a 5 wt% Ag / SiC catalyst.

[0042] The organic solvent is an alcohol, such as ethanol. The present invention does not particularly limit the drying method; for example, rotary evaporation can be performed in a rotary evaporator.

[0043] This invention does not impose any particular limitation on the Joule heating device; any conventional Joule heating device in the art can achieve the above objectives. In one embodiment, a graphite sample stage containing a dried silver catalyst is placed in the Joule heating device, and Ar gas is used as a protective atmosphere for thermal shock treatment. The temperature of the thermal shock treatment is 400–800°C, the duration of each thermal shock is 0.1–10 s, and the number of thermal shocks is 1–50.

[0044] In detail, Ag metal particles in the catalyst are prone to thermal vibration and displacement. The thermal shock treatment of this invention allows Ag to vibrate briefly on the SiC surface. This vibration is beneficial for Ag dispersion and prevents Ag from displacing and agglomerating significantly. Furthermore, after vibration, Ag can be fixed at defect sites on the SiC surface, forming interactions that become active sites in the epoxidation reaction, while preventing catalyst aggregation due to heat during use.

[0045] In one embodiment, the specific surface area of ​​SiC is 1–50 m². 2 / g. The diameter of the silver particles in the silver catalyst obtained by the method of this invention is 4-30 nm.

[0046] This invention prepares Ag particles into nanoscale Ag sols with controllable size and simultaneously uses high-specific-surface-area SiC as a support to prepare an Ag / SiC catalyst. The preparation of nanoscale Ag sols ensures high dispersion of the active components, while the high specific surface area of ​​the support further enhances the full dispersion of the active components on the support. In addition, the high thermal conductivity of the support further prevents the aggregation of the active components. The silver catalyst of this invention can be used in the epoxidation reaction of ethylene, exhibiting high catalytic activity and stability.

[0047] When the silver catalyst of this invention is used in the ethylene epoxidation reaction, it can be carried out in a fixed-bed reactor. In one embodiment, the ethylene epoxidation reaction conditions are as follows: 0.1-10 g of Ag / SiC catalyst is loaded into a fixed-bed reactor, and a reaction gas and an inhibitor are introduced, and the ethylene epoxidation reaction is carried out under certain temperature and pressure. The reaction gas consists of ethylene, oxygen, and a stabilizing gas, and the corresponding gas volume percentages are: ethylene: 26%, oxygen: 8%, stabilizing gas: 66%, and the inhibitor concentration is 0.01-10 ppm. The stabilizing gas is any one of nitrogen, helium, neon, and argon, and the inhibitor is a chlorinated compound, preferably 1,2-dichloroethane (EDC). In another embodiment, the pressure of the fixed-bed reactor system is 1.5-2.6 MPa, and the volume hourly space velocity of ethylene is 3000-6000 h⁻¹.-1 The reaction temperature is 180–260℃ and the reaction time is 1–1000h.

[0048] The technical solution of the present invention will be further described below through specific embodiments.

[0049] Example 1

[0050] Add 20 mL of deionized water to beaker 1, then weigh 600 mg of polyvinylpyrrolidone (PVP) and pour it into beaker 1, stirring thoroughly until the PVP is completely dissolved. Add 5 mL of deionized water to beaker 2, then weigh 39.4 mg of AgNO3 and pour it into beaker 2 to form an AgNO3 solution. Slowly pour the AgNO3 solution from beaker 2 into beaker 1, and continue stirring at 400 rpm for 30 minutes on a magnetic stirrer to ensure the two solutions are thoroughly mixed. Then, prepare a reducing agent solution by dissolving 21 mg of sodium borohydride (NaBH4) in 10 mL of deionized water, and slowly add this solution dropwise to the above mixture. The solution gradually changes from colorless to brown, and finally to brownish-black. After stirring thoroughly for another 1 hour, the mixture was centrifuged, and the supernatant was discarded to obtain a brownish-black atomic Ag sol (concentration of 1 mg / mL). Ag sols with different Ag concentrations (3 mg / mL, 5 mg / mL, and 10 mg / mL) were prepared using a similar method, and the raw material ratios are shown in Table 1. The prepared Ag sols were then analyzed using a UV-Vis spectrophotometer, and the results are as follows: Figure 1 As shown in the figure, a distinct Ag peak appears near 350 nm, and the peak intensity decreases with increasing Ag content. This is because as the Ag content increases, the absorption of light by the Ag sol gradually increases, leading to a gradual decrease in transmittance. This indicates that the Ag sol was successfully prepared.

[0051] Table 1 Raw material formulations for Ag sols of different concentrations

[0052]

[0053] Add 20 mL of ethanol to beaker 3, and weigh out 1.98 g of SiC (with a specific surface area of ​​approximately 50 m²). 2The solution containing 1 mg / mL of Ag was poured into beaker 3 and stirred thoroughly to form a mixture. Then, 20 mL of atomic Ag sol (1 mg / mL) was slowly added to the mixture, and stirring continued for 2 hours. The mixture was then dried by rotary evaporation to form an Ag / SiC catalyst, denoted as 1 wt% Ag / SiC. Ag / SiC catalysts with different Ag contents (3 wt% Ag / SiC, 5 wt% Ag / SiC, 10 wt% Ag / SiC, 20 wt% Ag / SiC) were prepared using a similar method. Subsequently, the Ag / SiC catalyst was placed in a graphite sample stage, and the stage was subjected to thermal shock treatment using a Joule heating device. The treatment temperature was 600℃, each shock lasted 1 second, and 20 thermal shocks were performed. Figure 2 This is a transmission electron microscope (TEM) image of the 5 wt% Ag / SiC catalyst prepared using nano-Ag sol in Example 1 without thermal shock treatment. Figure 3 This is a transmission electron microscope (TEM) image of the 5 wt% Ag / SiC catalyst prepared using nano-Ag sol in Example 1 and subjected to thermal shock treatment. Figure 2 , 3 It can be seen that after thermal shock treatment, Ag particles are uniformly distributed on the surface and have a small particle size. The average particle size of Ag particles on the surface of Ag catalyst without thermal shock treatment is 8.6 nm, while the average particle size of Ag particles after thermal shock treatment is 4.8 nm.

[0054] One g of the 5 wt% Ag / SiC catalyst prepared by the above method after thermal shock treatment was added to a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane in the reaction gas was 1.5 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed a ethylene conversion rate of 9.2% and a selectivity of 85.0% for ethylene oxide.

[0055] Example 2

[0056] One g of the 1 wt% Ag / SiC catalyst prepared in Example 1 was added to a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the reaction gas pressure was maintained at 2 MPa, and the volume hourly space velocity (VHSV) of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 6.7%, and the selectivity for ethylene oxide was 73.0%.

[0057] Example 3

[0058] One g of the 3 wt% Ag / SiC catalyst prepared in Example 1 was placed in a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 7.1%, and the selectivity for ethylene oxide was 76.0%.

[0059] Example 4

[0060] One g of the 10 wt% Ag / SiC catalyst prepared in Example 1 was placed in a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed a ethylene conversion rate of 9.4% and a selectivity of 84.0% for ethylene oxide.

[0061] Example 5

[0062] Using the same steps as in Example 1, but replacing the silver source with silver acetate, Ag sols with different Ag concentrations (1 mg / mL, 3 mg / mL, 5 mg / mL, 10 mg / mL) were prepared using the same process. Catalysts with Ag contents of 1 wt% Ag / SiC, 3 wt% Ag / SiC, 5 wt% Ag / SiC, 10 wt% Ag / SiC, and 20 wt% Ag / SiC were prepared using SiC as the catalyst support, and their ethylene epoxidation performance was tested.

[0063] One g of the 1 wt% Ag / SiC catalyst prepared by the above method was placed in a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.0%, 8.0%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 2 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed a ethylene conversion rate of 7.2% and a selectivity of 70.0% for ethylene oxide.

[0064] Example 6

[0065] One g of the 3 wt% Ag / SiC catalyst prepared in Example 5 was added to a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.0%, 8.0%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 2 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 6.6%, and the selectivity for ethylene oxide was 72.0%.

[0066] Example 7

[0067] One g of the 5 wt% Ag / SiC catalyst prepared in Example 5 was placed in a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.0%, 8.0%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 2 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed a ethylene conversion rate of 9.0% and a selectivity of 85.0% for ethylene oxide.

[0068] Example 8

[0069] One g of the 10 wt% Ag / SiC catalyst prepared in Example 5 was placed in a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.0%, 8.0%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 2 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 8.8%, and the selectivity for ethylene oxide was 80.0%.

[0070] Example 9

[0071] One g of the 10 wt% Ag / SiC catalyst prepared in Example 5 was placed in a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.0%, 8.0%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 2 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 220℃. Gas chromatography analysis revealed a ethylene conversion rate of 7.2% and a selectivity of 81.0% for ethylene oxide.

[0072] Example 10

[0073] One g of the 10 wt% Ag / SiC catalyst prepared in Example 5 was placed in a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.0%, 8.0%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 2 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 240℃. Gas chromatography analysis revealed a ethylene conversion rate of 9.3% and a selectivity of 80.0% for ethylene oxide.

[0074] Example 11

[0075] One g of the 20 wt% Ag / SiC catalyst prepared in Example 1 was added to a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the reaction gas pressure was maintained at 2 MPa, and the volume hourly space velocity (VHSV) of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed a ethylene conversion rate of 9.5% and a selectivity of 79.0% for ethylene oxide.

[0076] Example 12

[0077] One g of the 20 wt% Ag / SiC catalyst prepared in Example 5 was placed in a fixed-bed reactor, and a reaction gas formed by a mixture of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.0%, 8.0%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 2 ppm. Furthermore, the pressure of the reaction gas was maintained at 2 MPa, and the volume hourly space velocity of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed a ethylene conversion rate of 9.0% and a selectivity of 82.0% for ethylene oxide.

[0078] Example 13

[0079] Similar to Example 1, 300 mg of carboxymethyl cellulose was used as a dispersant and 128 mg of silver carbonate was used as the silver source. The thermal shock treatment temperature was 700 °C, the shock time was 5 s per shock, and the thermal shock was performed 10 times. The remaining processes were the same to prepare a 5 wt% Ag / SiC catalyst.

[0080] 1 g of the 5 wt% Ag / SiC catalyst was added to a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the reaction gas pressure was maintained at 2 MPa, and the volume hourly space velocity (VHSV) of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 6.6%, and the selectivity for ethylene oxide was 78.0%.

[0081] Example 14

[0082] Similar to Example 1, polyvinyl alcohol was used as a dispersant and 197 mg of AgNO3 was used as a silver source. The thermal shock treatment temperature was 500°C, the shock time was 0.5 s per shock, and the thermal shock was performed 40 times. The remaining processes were the same to prepare a 5 wt% Ag / SiC catalyst.

[0083] 1 g of the 5 wt% Ag / SiC catalyst was added to a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the reaction gas pressure was maintained at 2 MPa, and the volume hourly space velocity (VHSV) of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 8.3%, and the selectivity for ethylene oxide was 85.0%.

[0084] Comparative Example 1

[0085] No dispersant or reducing agent was added during the preparation process. A 5 wt% Ag / SiC catalyst was prepared using the same method as in Example 1.

[0086] Subsequently, 1g of the Ag / SiC catalyst was placed in a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the reaction gas pressure was maintained at 2 MPa, and the volume hourly space velocity (VHSV) of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 3.6%, and the selectivity for ethylene oxide was 45.0%.

[0087] Comparative Example 2

[0088] The 5 wt% Ag / SiC catalyst was prepared using a method similar to that in Example 1, but without thermal shock treatment.

[0089] 1 g of the catalyst was placed in a fixed-bed reactor, and a reaction gas composed of ethylene, oxygen, and nitrogen was introduced. The volume percentages of ethylene, oxygen, and nitrogen introduced were 26.2%, 7.8%, and 66.0%, respectively, and the concentration of 1,2-dichloroethane was 1.5 ppm. Furthermore, the reaction gas pressure was maintained at 2 MPa, and the volume hourly space velocity (VHSV) of ethylene was 4000 h⁻¹. -1 The reaction temperature was 230℃. Gas chromatography analysis revealed that the conversion rate of ethylene was 6.4%, and the selectivity for ethylene oxide was 78.0%.

[0090] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

Claims

1. A method for producing a silver catalyst, characterized by, It comprises the following steps: Step 1, forming a mixture of silver precursor, dispersant and water, and then mixing with a reducing agent; Step 2, phase separation of the mixture obtained in step 1, and the lower layer is nano Ag sol; Step 3, mixing the nano Ag sol with SiC, drying, and then performing thermal shock treatment in a joule heating device to obtain a silver catalyst; The dispersant is at least one of polyvinylpyrrolidone, carboxymethyl cellulose and polyvinyl alcohol; and the reducing agent is at least one of sodium borohydride, diisobutylaluminum hydride and lithium aluminum hydride.

2. The method for producing a silver catalyst according to claim 1, characterized by, The silver precursor is a silver-containing compound; step 1 is mixing the silver precursor with water, mixing the dispersant with water, and then mixing the mixture of silver precursor and water with the mixture of dispersant and water to form the mixture.

3. The method for producing a silver catalyst according to claim 2, characterized by, The silver precursor is at least one of silver nitrate, silver acetate, silver carbonate, silver sulfate, silver nitrite and silver ammine complex; the concentration of the silver precursor in the mixture of silver precursor and water is 0.1-20 mg / mL; and the concentration of the dispersant in the mixture of dispersant and water is 1-50 mg / mL.

4. The method for preparing a silver catalyst according to claim 2, characterized by, The mixing of the mixture of silver precursor and water and the mixture of dispersant and water is performed at -10-5℃ with stirring; the stirring speed is 100-1000 r / min, and the stirring time is 5-150 min.

5. The method of making a silver catalyst according to claim 1, wherein The reducing agent is added dropwise in the form of a reducing agent solution to the mixture in step 1, and stirring is performed during the dropwise addition; the concentration of the reducing agent solution is 1-30 mg / mL.

6. The method of making a silver catalyst according to claim 1, wherein The mass ratio of the reducing agent to the silver precursor is 0.1-10:1, and the mass ratio of the dispersant to the silver precursor is 1-100:

1.

7. The method of making a silver catalyst according to claim 1, wherein The phase separation of the mixture obtained in step 1 is performed by standing or centrifugation; the centrifugation speed is 2000-10000 r / min, and the centrifugation time is 5-60 min.

8. The method of making a silver catalyst according to claim 1, wherein, An organic solvent is further added when the nano Ag sol is mixed with the SiC; the organic solvent is an alcohol.

9. The method of claim 8, wherein the silver catalyst is prepared by, The organic solvent is ethanol; the nano-Ag sol is 1-20% of the total mass of the nano-Ag sol and the SiC; the specific surface area of the SiC is 1-50 m 2 / g.

10. The method of making a silver catalyst according to claim 1, wherein, The thermal shock treatment is performed under the condition that Ar is used as a protective gas, the thermal shock temperature is 400-800℃, the time for each thermal shock is 0.1-10 s, and the number of thermal shocks is 1-50.

11. The silver catalyst obtainable by the process according to any one of claims 1 to 10, characterized in that The silver particles in the silver catalyst have a diameter of 4-30 nm.

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