A surface silver-rich low-silver-content ethylene oxide catalyst, a preparation method and application thereof

Abstract

This invention belongs to the field of catalyst preparation technology, and discloses a low-silver-content ethylene oxide catalyst with a silver-rich surface, its preparation method, and its application. The method employs an economical and industrially applicable low-silver-content high-efficiency catalyst preparation system, innovatively using a "nebulized spray fluorine impregnation-calcination" process. By precisely controlling the penetration depth of fluorine on the alumina support surface through atomized solution, the calcined support forms a gradient structure of "surface lamellar alumina - internal dense alumina," providing support for the preparation of the surface-rich silver catalyst. Utilizing the high specific surface area and adsorption activity of the surface lamellar structure of the support, combined with the low adsorption of the internal dense structure, silver is preferentially enriched on the support surface during impregnation, forming a "surface-rich silver - internal-silver-poor" distribution state. This overcomes the technical bottleneck of low internal silver utilization caused by the uniform distribution of silver in traditional catalysts.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, specifically relating to a silver-rich, low-silver-content ethylene oxide catalyst, its preparation method, and its application. Background Technology

[0002] Ethylene oxide (EO) is a globally important basic chemical, widely used in the production of high-value-added products such as ethylene glycol, surfactants, pharmaceuticals, and plastics. Currently, the direct oxidation of ethylene is the most common industrial method for its production, with silver-based catalysts being the core material. Due to the exothermic nature of the reaction system and process requirements, industrial EO catalysts typically use large-particle alumina supports (meeting requirements for mechanical strength and bed permeability). However, large-particle catalysts present significant reaction kinetic limitations; the reaction mainly occurs on the outer surface of the catalyst, while the deeper internal regions, due to limited reactant diffusion, hardly participate in the effective reaction. Therefore, how to reduce silver usage and improve silver utilization while ensuring catalytic activity and selectivity has become the core research and development direction for EO catalyst technology.

[0003] The core characteristics of current mainstream industrial EO silver catalysts and related technologies are as follows: (1) Support and loading system: α-alumina is generally used as the support. Silver elements are loaded on the surface and inside of the support by impregnation. Some technologies will add alkali metals, alkaline earth metals or rare earth elements as co-catalysts to optimize catalytic selectivity and stability. In recent years, low silver content catalysts have become a research hotspot. For example, Sinopec's YS-8840 catalyst has reduced the silver content by one-third, and Shell's S-895 catalyst has replaced traditional high silver products with a low silver content solution. Both aim to balance cost and performance.

[0004] (2) Fluorine-modified related technologies: There are existing technologies for modifying alumina supports with fluorine, but they are mainly used in the field of ozone catalysts for water pollution treatment. By uniformly doping fluorine into the overall structure of alumina, the Lewis acidity of the support is enhanced to improve ozone decomposition activity. The core purpose is to optimize the overall catalytic performance of the support, rather than to regulate the loading distribution of silver.

[0005] (3) Current status of performance indicators: The EO selectivity of high-selectivity silver catalysts (such as YS-8810 and YS-9010) has reached over 90%, and high space-time yield catalysts (such as YS-9110) can be adapted to over 95% of industrial equipment. However, these catalysts still have problems such as high silver content or unreasonable silver distribution, and there is still room for improvement in the activity and stability of low silver content products.

[0006] The core flaws of existing technology: (1) Low silver utilization and high cost: Traditional catalysts use a uniform silver loading mode, and silver particles are uniformly distributed on the surface and inside of the support. However, the deep internal region of the large particle support hardly participates in the reaction, resulting in this part of silver being completely idle and having extremely low utilization. Even with existing low silver content catalysts, the problem of "internal silver waste" has not been completely solved, and the cost of precious metals is still the main expenditure for catalyst preparation.

[0007] (2) Mismatch between support structure design and reaction characteristics: Most existing alumina supports are homogeneous structures (fully dense or fully porous), which cannot be adapted to the kinetic characteristics of "surface region-dominated reaction" of large particle catalysts. Fully dense supports have a small specific surface area and poor silver dispersion on the surface; fully porous supports cause excessive silver penetration into the internal ineffective area, further aggravating silver waste.

[0008] (3) Balancing the challenges of low silver content and high activity: In existing technologies, reducing silver content often leads to a decrease in catalytic activity or selectivity, which requires compensation through a complex co-catalyst system, resulting in complicated preparation processes and increased costs. At the same time, the poor compatibility between large particle supports and low silver content further limits the industrial application range of low silver catalysts. Summary of the Invention

[0009] To address the above technical problems, the purpose of this invention is to provide a method for preparing a low-silver-content ethylene oxide catalyst with a silver-rich surface. This method employs an economical and industrially applicable low-silver-content high-efficiency catalyst preparation system, innovatively utilizing a "nebulized spray fluorine impregnation-calcination" process. By precisely controlling the penetration depth of fluorine on the alumina support surface through atomized solution, the calcined support forms a gradient structure of "surface lamellar alumina - internal dense alumina," providing support for the preparation of the surface-rich silver catalyst. Utilizing the high specific surface area and adsorption activity of the lamellar structure of the support, a high-silver-content catalyst is prepared. By leveraging the low adsorption properties of the dense internal structure, silver is preferentially enriched on the carrier surface during impregnation, forming a "silver-rich surface, silver-poor interior" distribution. This overcomes the technical bottleneck of low internal silver utilization caused by the uniform distribution of silver in traditional catalysts. Addressing the kinetic characteristic of EO catalysts with large particle sizes, where "over 80% of the reaction occurs from the outer surface to a depth of 1 mm," surface-targeted silver loading concentrates limited silver resources in the active reaction region, achieving a highly efficient match between low silver content and large particle carriers. This breaks the traditional understanding that large particle catalysts require high silver content to maintain activity. Through multi-technology synergistic innovation, this solution successfully overcomes the technical bottleneck of traditional ethylene oxide catalysts requiring high silver content to maintain high performance.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a silver-rich, low-silver-content ethylene oxide catalyst includes the following steps: S1. Gradient structure α-alumina carrier was prepared by modifying it using an atomized spray fluorine impregnation-calcination process: 1) Mix 15-35% aluminum hydroxide monohydrate and 65-85% aluminum hydroxide trihydrate according to the corresponding mass fraction ratio, add adhesive and extrude into strips, dry at 75-85℃ for 10-14 h and calcine at 500-600℃ for 3-5 h to obtain the basic alumina carrier; 2) Prepare an ammonium fluoride aqueous solution with a mass fraction of 0.7-0.9%, and spray it onto the surface of the basic alumina carrier using a high-pressure atomizing spray device with atomized particles of 45-55 μm in diameter, controlling the spray volume to be 18-22% of the carrier mass; During the spray impregnation process, the carrier temperature is maintained at 75~85℃, allowing fluorine to penetrate to a depth of 0.5~1.5mm on the carrier surface; 3) The fluorine-impregnated support is subjected to segmented calcination to complete the crystal transformation of the support and form a gradient structure, thus obtaining a gradient structure α-alumina support. The segmented roasting process is as follows: first, dry at 115~120℃ for 3~4 hours; then, heat to 1370~1380℃ at a rate of 4~5℃ / min and roast at a constant temperature for 5~6 hours. The gradient-structured α-alumina carrier has a sheet-like α-alumina structure on the surface with a thickness of 0.7~1.5 mm; an internal dense α-alumina structure; and a surface specific area of ​​1.7~1.9 m². 2 / g, surface water absorption rate 48~52wt%; internal specific surface area 1.6~1.7 m² 2 / g, internal water absorption rate is 9~10wt%; Preparation of S2, silver-ethylenediamine complex solution: Silver oxalate was used as the silver source and mixed with deionized water to form a suspension. Ethylenediamine was slowly added dropwise while stirring until the silver oxalate was completely dissolved. The pH of the system was controlled at 10.5~11.0 to obtain a silver-ethylenediamine complex solution. Add cesium nitrate and ammonium perrhenate to the silver-ethylenediamine complex solution, stir until completely dissolved, and then place in a constant temperature water bath at 25~27℃ for 25~30 min to degas. S3. Preparation of low-silver-content ethylene oxide catalyst product: 1) The gradient structure α-alumina support is placed in a silver-ethylenediamine complex solution and is immersed in a constant temperature water bath at 25~30℃ for 3~4h using an isothermal immersion-intermittent stirring mode, during which the mixture is stirred at 55~60r / min for 8~10min every 25~30min. 2) After impregnation, remove the carrier, remove excess solution from the surface by vacuum filtration, and dry in sections at 145~150℃ for 3~4 hours; then transfer it to an activation furnace at 25~30℃.3 Activation at an air flow rate of / h and a temperature of 300~330℃ for 7~8min allows the silver-ethylenediamine complex to decompose into metallic silver and form a uniform active layer on the support surface, thus obtaining a low-silver-content ethylene oxide catalyst product.

[0011] The particle size of the aforementioned basic alumina carrier is 4-6 mm, and the specific surface area is 170-350 m². 2 / g.

[0012] Preferably, in step 1) of S1, the mass fraction of aluminum hydroxide monohydrate is 25% and the mass fraction of aluminum hydroxide trihydrate is 75%. The adhesive is a nitric acid aqueous solution with a mass concentration of 5-10%, and the amount added accounts for 18-22% of the total mass of aluminum hydroxide monohydrate and aluminum hydroxide trihydrate; The basic alumina carrier was obtained by drying at 80℃ for 12 h and calcining at 550℃ for 4 h.

[0013] Preferably, in step 2) of S1, the mass fraction of the ammonium fluoride aqueous solution is 0.8%; The atomized particles have a particle size of 50 μm; The spray volume is 20% of the carrier mass; The carrier temperature is maintained at 80°C during the impregnation process; The basic alumina carrier has a particle size of 5 mm and a specific surface area of ​​215 m². 2 / g.

[0014] The spray volume mentioned above is 20% of the carrier mass. This fixed value can precisely control the penetration depth of fluorine to the target range, ensuring the fluorine content required for surface modification while avoiding excessive fluorine penetration that could damage the integrity of the carrier's gradient structure.

[0015] Preferably, in step 3) of S1, the specific surface area of ​​the gradient structure α-alumina support is 1.8 m². 2 / g, surface water absorption rate is 50wt%; internal specific surface area is 1.7 m² 2 / g, internal water absorption rate is 10wt%.

[0016] Preferably, in S2, the purity of silver oxalate is ≥99.99%; Cesium nitrate and ammonium perrhenate were added to the solution to make the cesium content in the finished catalyst 400 ppmw and the rhenium content 500 ppmw.

[0017] Preferably, in step S3, the vacuum filtration conditions are: vacuum degree of -0.08~-0.09MPa; the segmented drying at 150℃ specifically involves heating to 150℃ for the first 1 hour and then maintaining a constant temperature of 150℃ for the next 2 hours.

[0018] A silver-rich, low-silver-content ethylene oxide catalyst prepared by the aforementioned method, wherein the catalyst is composed of gradient-structured α-alumina as a support, silver as the active component, and cesium and rhenium as co-catalysts; The catalyst has a total silver content of 8.1-9.1%, and the silver content within a depth of 0.5-1.5 mm on the support surface accounts for ≥92%.

[0019] Preferably, the target reaction is the epoxidation of ethylene to produce ethylene oxide. When the total silver content of the catalyst is 8.6%, after volume shrinkage correction, the reaction temperature is 220°C, the reaction pressure is 2.0 MPa, and the volume hourly space velocity is 4000 h⁻¹. -1 Under the ethylene epoxidation reaction conditions, the ethylene conversion rate was 9.0% and the ethylene oxide selectivity was 86.0%.

[0020] An application of an ethylene oxide catalyst in the direct oxidation of ethylene to ethylene oxide, wherein the catalyst is used under the following conditions: the inlet gas composition is 28-30 mol% ethylene, 5-6 mol% oxygen, and 0.5-5 ppm chloroethane, with nitrogen as the equilibrium gas; and the reaction conditions are: temperature 210-220℃, pressure 1.8-2.0 MPa, and volume hourly space velocity 3000-6000 h⁻¹. -1 The evaluation time is 950~1050 hours.

[0021] Preferably, after 1000 hours of continuous reaction, the catalyst shows no significant decrease in catalytic activity and selectivity after volume shrinkage correction.

[0022] Compared with the prior art, the present invention has at least the following technical effects: (i) This invention provides a method for preparing a silver-rich, low-silver-content ethylene oxide catalyst. This method uses a low-silver-content high-efficiency catalyst preparation system that is both economical and industrially applicable. Through multi-technology collaborative innovation, it successfully breaks through the technical bottleneck of "high silver content is required to maintain high performance" in traditional ethylene oxide catalysts.

[0023] The preparation method uses a composite raw material formulation of "15%~35% aluminum hydroxide monohydrate + 65%~85% aluminum hydroxide trihydrate". Experiments have shown that a lower ratio of aluminum hydroxide monohydrate to aluminum hydroxide trihydrate (i.e., a higher proportion of aluminum hydroxide trihydrate) is more conducive to improving catalytic performance. This formulation can precisely control the pore structure and mechanical strength of the support, providing good matrix support for subsequent gradient modification.

[0024] Based on this, an innovative process of atomized spray fluorine impregnation followed by 1380℃ high-temperature calcination is adopted. By precisely controlling the atomized particle size, spray volume, and adsorption temperature of the fluorine solution, the fluorine element penetrates only to a depth of 0.5~1.5 mm on the carrier surface (the spray volume can be flexibly adjusted to 18%~22% of the carrier mass). After high-temperature calcination at 1380℃, the crystal transformation is completed, forming a gradient structure of "surface lamellar α-alumina (thickness 0.7~1.5 mm) - internal dense α-alumina". The surface water absorption rate increases from 48 wt% to 52 wt% with the increase of spray volume, providing the core driving force for the targeted enrichment of silver element and achieving the goal of precisely controlling the silver content and catalytic performance through the spray volume.

[0025] Meanwhile, the ratio of co-catalysts was optimized, with cesium content controlled at 400 ppmw and rhenium content controlled at 500 ppmw, reducing co-catalytic costs while ensuring catalytic stability.

[0026] This preparation method has the following advantages: (1) Directional construction technology of gradient structure alumina support: The innovative “atomized spray fluorine impregnation-calcination” process is adopted. The penetration depth of fluorine on the surface of alumina support is precisely controlled by the atomized solution, so that the support forms a gradient structure of “surface lamellar alumina-internal dense alumina” after calcination, providing support for the preparation of surface silver-rich catalysts. (2) Surface-targeted silver loading mechanism: By utilizing the high specific surface area and adsorption activity of the plate-like structure on the surface of the support, combined with the low adsorption of the dense internal structure, silver elements are preferentially enriched on the surface of the support during the impregnation process, forming a distribution state of "surface rich silver - internal poor silver", which breaks through the technical bottleneck of low internal silver utilization caused by the uniform distribution of silver elements in traditional catalysts. (3) Adaptation design of low silver content and large particle support: In view of the kinetic characteristics of EO catalyst with large particle size, "more than 80% of the reaction is participated in from the outer surface to the inner 1mm depth", the limited silver resources are concentrated in the active reaction area by surface-targeted silver loading, so as to achieve efficient adaptation of low silver content and large particle support, breaking the traditional perception that large particle catalysts need high silver content to ensure activity.

[0027] (II) The catalyst prepared by this method for preparing silver-rich, low-silver-content ethylene oxide catalysts has the following advantages: (1) Reduced silver content and significantly improved silver utilization: Silver elements are concentrated on the surface of the reactive support, avoiding the waste caused by the internal silver not participating in the reaction in the traditional uniform loading. The silver content is reduced to below 10%, and the silver utilization rate is increased by more than 30% compared with the existing technology, which greatly reduces the precious metal cost of the catalyst.

[0028] (2) Maintaining high catalytic activity under low silver content: The synergistic effect of the surface plate structure and the silver-rich distribution ensures that the EO selectivity and conversion rate of the ethylene epoxidation reaction are still no lower than those of existing high silver content catalysts when the silver content is reduced, thus balancing economy and catalytic performance.

[0029] (3) Excellent cost performance in industrial applications: The low silver content reduces the cost of catalyst preparation, and the high utilization rate and long life reduce the overall cost in industrial operation. Combined with high activity and high selectivity, the catalyst cost per unit of EO product is reduced by 30% to 40% when the catalyst is used in the industrial unit for ethylene oxidation to EO, which significantly improves the economic benefits of enterprises. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the catalyst prepared in Example 1. Detailed Implementation

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention will be briefly introduced below in conjunction with the accompanying drawings and descriptions of the embodiments or the prior art. Obviously, the following description of the structure of the accompanying drawings is only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. It should be noted that the description of these embodiments is for the purpose of helping to understand the present invention, but does not constitute a limitation of the present invention.

[0032] One specific embodiment of the present invention is as follows: This experiment uses the epoxidation of ethylene to produce ethylene oxide (EO) as the target reaction. The technical solution of this application is demonstrated through examples. The comparative example uses a traditional catalyst preparation technique. The silver content and catalytic performance of the two are compared to verify the superiority of the technology of this application.

[0033] In the experiment, a fixed-bed reactor was used to evaluate the catalyst performance. The reaction conditions were as follows: reaction temperature 220℃, reaction pressure 2.0 MPa, inlet gas composition of ethylene 30 mol%, oxygen 6 mol%, chloroethane 0.5~5 ppm, nitrogen as the equilibrium gas, and volumetric hourly space velocity 4000 h⁻¹. -1 Evaluation time: 1000 hours.

[0034] The measurement results need to be corrected for volume shrinkage. The selectivity (S) is calculated using the formula S=ΔEO / (ΔEO+0.5*ΔCO2)*100%, where ΔEO is the concentration difference of ethylene oxide in the inlet and outlet gases of the reactor, and ΔCO2 is the concentration difference of carbon dioxide in the inlet and outlet gases of the reactor. The carrier raw materials are uniformly aluminum hydroxide monohydrate and aluminum hydroxide trihydrate, which are prepared by extrusion molding and then processed in subsequent steps. During the silver loading process, the addition of cesium (Cs) is precisely controlled at 400 ppmw and the addition of rhenium (Re) at 500 ppmw to ensure stable catalytic effect and controllable cost.

[0035] I. Implementation Examples Example 1: Preparation and performance of gradient structure support and surface silver-rich catalyst 1.1 Preparation of Gradient Structured Alumina Support 25 kg of aluminum hydroxide monohydrate and 75 kg of aluminum hydroxide trihydrate were mixed evenly, and 40 kg of 10% nitric acid aqueous solution was added as a binder. The mixture was then extruded, dried at 80℃ for 12 h, and calcined at 550℃ for 4 h to prepare a basic alumina carrier with a particle size of 5 mm (specific surface area 215 m²). 2 / g).

[0036] Gradient structure carriers were prepared by modification using a process of "atomized spray fluorine impregnation-calcination". Prepare an ammonium fluoride aqueous solution with a mass fraction of 0.8%, and spray it onto the surface of the base carrier using a high-pressure atomizing spraying device (atomized particle size of 50μm). Control the spraying volume to be 20% of the carrier mass. Maintain the carrier temperature at 80℃ during the impregnation process to ensure that the fluorine element only penetrates to a depth of 0.5~1.5 mm on the carrier surface.

[0037] The process then proceeds with segmented calcination: first, the substrate is dried at 120℃ for 4 hours to remove moisture; then, the temperature is increased to 1380℃ at a rate of 5℃ / min and calcined at a constant temperature for 6 hours to allow the substrate to complete its crystal transformation and form a gradient structure, ultimately yielding a gradient structure α-alumina substrate.

[0038] Testing revealed that the carrier surface formed a sheet-like α-alumina structure (approximately 1.1 mm thick), while the interior consisted of a dense α-alumina structure, with a surface area of ​​1.8 m². 2 The surface water absorption rate is approximately 51 wt%; the internal specific surface area remains at 1.7 m² / g. 2 With an internal water absorption rate of approximately 10 wt%, the difference in water absorption rates between the surface and the interior provides a key driving force for the targeted enrichment of silver.

[0039] 1.2 Preparation of surface-targeted silver-supported catalysts To prepare the silver complexing solution, high-purity silver oxalate (purity ≥99.99%) was selected as the silver source. The silver oxalate was mixed with deionized water and stirred to form a suspension. Then, ethylenediamine was slowly added dropwise while stirring until the silver oxalate was completely dissolved, resulting in a stable silver-ethylenediamine complexing solution. The pH of the system was controlled at 10.5~11.0 throughout the process to ensure that the complexation reaction was complete and no precipitate was formed.

[0040] Cesium nitrate and ammonium perrhenate were precisely added to the solution to control the cesium content in the finished catalyst to be 400 ppmw and the rhenium content to be 500 ppmw. After stirring until completely dissolved, the solution was placed in a constant temperature water bath at 25°C for 30 minutes to remove air bubbles and prevent air blockage in the carrier pores during impregnation.

[0041] 1.3 Preparation of the finished catalyst: The aforementioned gradient structure α-alumina support was placed in a silver-ethylenediamine complex solution and immersed in a 30°C constant temperature water bath for 4 hours using an "isothermal immersion-intermittent stirring" mode. During this period, the solution was stirred at 60 r / min for 10 minutes every 30 minutes. By utilizing the difference in water absorption rate between the surface (50 wt%) and interior (10 wt%) of the support, the adsorption and aggregation of silver complex ions on the surface was enhanced. At the same time, by utilizing the difference in adsorption activity between the surface lamellar structure and the interior dense structure, silver elements were preferentially enriched on the support surface, preventing diffusion inward.

[0042] After impregnation, the carrier was removed and excess solution was removed from the surface using vacuum filtration (vacuum degree -0.08~-0.09MPa). It was then dried in stages for 3 hours in a 150℃ hot air drying oven (the first hour was heated to 150℃, and the next 2 hours were kept at a constant temperature). Finally, it was transferred to an activation furnace and dried at 30 m... 3 Activation was performed for 7 minutes at an air flow rate of / h and a temperature of 330 ℃ to remove organic impurities and residual ethylenediamine, causing the silver-ethylenediamine complex to decompose into metallic silver and form a uniform active layer on the support surface, thus obtaining the target catalyst (denoted as Cat-1). A schematic diagram of the specific structure is shown below. Figure 1 As shown.

[0043] Testing revealed that Cat-1 contained a total silver content of 8.6%, with 92% of the silver content located within a 1.1 mm depth on the surface of the carrier. The silver content inside the carrier was extremely low, which fully validated the targeted enrichment effect of the gradient structure α-alumina carrier.

[0044] 1.4 Catalyst performance evaluation results The performance evaluation data of Cat-1 has been included in the performance comparison table of Example 2. As the sample with the lowest silver content, it shows the corresponding level of conversion rate and selectivity, providing a basic reference for the positive correlation between silver content and catalytic performance. Moreover, its performance is stable after volume shrinkage correction under the above fixed-bed reaction conditions.

[0045] Example 2: Performance Comparison of Gradient Supported Catalysts with Different Silver Contents Using the same carrier formulation as in Example 1 (25 kg aluminum hydroxide monohydrate + 75 kg aluminum hydroxide trihydrate), basic carrier preparation process (drying at 80°C for 12 h after extrusion and calcining at 550°C for 4 h), and silver complexation auxiliary system, the total silver content was controlled by adjusting the spraying amount of atomized spray fluorine impregnation (the spraying amount directly affects the penetration depth of fluorine and the water absorption rate of the carrier surface, thereby controlling the silver loading). Gradient structure catalysts with total silver content of 8.6% (Cat-1, sample of Example 1, spraying amount is 20% of carrier mass), 8.1% (Cat-2, spraying amount is 18% of carrier mass), and 9.1% (Cat-3, spraying amount is 22% of carrier mass) were prepared respectively.

[0046] The fluorine impregnation and subsequent calcination process was optimized as follows: A 0.8% ammonium fluoride aqueous solution was prepared and sprayed onto the surface of the base carrier using a high-pressure atomizing sprayer (50 μm atomization particle size). The spray amounts of Cat-2 and Cat-3 were controlled to be 18% and 22% of the carrier mass, respectively, and the carrier temperature was maintained at 80℃ during the impregnation process. Subsequently, both were dried at 120℃ for 4 hours and then calcined at a constant temperature of 1380℃ for 6 hours at a rate of 5℃ / min to obtain a gradient structure α-alumina carrier. The surface water absorption rate of Cat-2 carrier was approximately 48 wt%, and that of Cat-3 was approximately 52 wt%, both maintaining a surface area of ​​1.8 m². 2 Approximately 1.7 m³ / g, internal specific surface area 1.7 m² / g. 2 Approximately 10 wt% of water is absorbed by the container.

[0047] The preparation and impregnation steps of the silver complex solution were standardized and optimized: Silver-ethylenediamine complex solutions were prepared separately. Silver oxalate and deionized water were mixed and stirred into a suspension. Ethylenediamine was added dropwise until the silver oxalate was completely dissolved. The pH value was controlled at 10.5~11.0. The amount of cesium and rhenium added was fixed at 400 ppmw and 500 ppmw, respectively. The solution was allowed to stand for degassing for 30 min. The corresponding gradient structure α-alumina support was placed in the complex solution and impregnated in a constant temperature water bath at 30℃ for 4 h. The intermittent stirring parameters were the same as in Example 1. Silver element was targeted enriched by relying on the difference in water absorption rate on the surface of the support. After impregnation, the liquid was removed by vacuum filtration, and the solution was dried in stages at 150℃ for 3 h. Then it was transferred to an activation furnace and activated at 330℃ for 7 min at an air flow rate of 30 m³ / h. The other preparation parameters remained unchanged.

[0048] The performance evaluation results are shown in Table 1 below:

[0049] As shown in Table 1, when the spray volume varies within the range of 18% to 22% of the carrier mass, the EO selectivity and ethylene conversion of the catalyst show a mild positive correlation with the increase of spray volume (corresponding to the increase of silver content): when the spray volume increases from 18% to 22%, the silver content increases from 8.1% to 9.1%, the conversion rate increases from 8.8% to 9.2% (difference of 0.4%), and the selectivity increases from 85.5% to 86.5% (difference of 1.0%), and both are controlled below 87%. Compared with the traditional high-silver catalyst Ref-Cat-1 (the specific preparation method is shown in Comparative Example 1, with a selectivity of 87.9% and a conversion rate of 10.1%), the technical sample of this application has similar performance at low silver content and has a significant cost advantage.

[0050] This trend stems from the fact that the higher the spray volume, the more fully the fluorine penetrates, the higher the water absorption rate on the carrier surface, and the higher the silver loading and active site density. It also highlights that the technology of this application can achieve controllable adjustment of silver content and catalytic performance by precisely controlling the spray volume. The precise ratio of cesium and rhenium (400 ppmw and 500 ppmw) provides stable support for this mild positive correlation trend.

[0051] Example 3: Effect of different ratios of aluminum hydroxide monohydrate and aluminum hydroxide trihydrate on performance Three different ratios of raw materials were used to prepare basic carriers by extrusion. After extrusion, all carriers were dried at 80℃ for 12 hours and calcined at 550℃ for 4 hours. Other processes (gradient modification: calcination at 1380℃ to obtain a gradient-structured α-alumina carrier with a specific surface area of ​​1.8 m²) were also performed. 2 Approximately 0.5 g / g, water absorption rate of 50 wt%, and internal specific surface area of ​​1.7 m². 2 The total silver content was approximately 7.3%, the water absorption rate was 10 wt%, and the silver loading and co-catalyst addition were consistent with those in Example 1. Catalysts Cat-5 and Cat-6 were prepared, with the total silver content controlled at approximately 7.3%, the cesium content at 400 ppmw, and the rhenium content at 500 ppmw.

[0052] The silver impregnation process was the same as in Example 1. The focus was on the effect of the ratio of aluminum hydroxide monohydrate to aluminum hydroxide trihydrate on the catalytic performance, with particular attention paid to the correlation between the ratio and the performance.

[0053] The results are shown in Table 2 below:

[0054] As shown in Table 2, when the content of aluminum hydroxide monohydrate is in the range of 15% to 35%, the catalyst performance exhibits a clear pattern: the lower the ratio of aluminum hydroxide monohydrate to aluminum hydroxide trihydrate (the higher the proportion of aluminum hydroxide trihydrate), the better the ethylene conversion rate and EO selectivity, and the overall performance difference is small and controllable.

[0055] The 15% aluminum hydroxide monohydrate formulation (Cat-5, 85% trihydrate) achieved a conversion rate of 8.9% and a selectivity of 85.7%, slightly lower than the control sample Cat-1; the 35% aluminum hydroxide monohydrate formulation (Cat-6, 65% trihydrate) had slightly lower performance (conversion rate 8.7%, selectivity 85.3%); the control sample Cat-1 (25% / 75%, silver content 8.6%) had the best performance (conversion rate 9.0%, selectivity 86.0%), and a relatively high decay rate.

[0056] This pattern stems from the fact that aluminum hydroxide trihydrate is more likely to form a uniform porous structure during calcination, which is beneficial for the dispersion of active sites on the carrier surface and the adsorption of silver. This proves that composite raw materials with a lower aluminum hydroxide monohydrate ratio are more suitable for gradient modification processes. At the same time, the increased silver content also helps Cat-1 to perform better. This formulation range has good process adaptability and stability.

[0057] II. Comparative Example Comparative Example 1: Traditional uniformly supported silver catalyst (high silver content, conventional support) 1.1 Catalyst Preparation Industrial α-alumina carrier (8 mm particle size, no gradient structure, specific surface area 1.8 m²) was selected. 2 / g (commercially available industrial-grade finished product), the catalyst was prepared using the traditional equal-volume impregnation method. The optimized silver complex solution preparation and impregnation steps are as follows: Prepare a silver-ethylenediamine complex solution by mixing silver oxalate and deionized water to form a suspension. Add ethylenediamine dropwise until the silver oxalate is completely dissolved. Adjust the pH to 10.5~11.0. Add cesium nitrate and ammonium perrhenate to control the cesium content in the finished product to 400ppmw and the rhenium content to 500ppmw, consistent with Example 1. After stirring and dissolving, let stand for degassing for 30 min. Place the carrier in the solution and impregnate in a constant temperature water bath at 30℃ for 6 h (extending the impregnation time is suitable for 8mm large-particle-size carriers to ensure full penetration of silver elements). During this period, stir continuously at a rate of 80r / min to ensure that silver elements are evenly distributed inside and on the surface of the carrier, avoiding local enrichment. After impregnation, allow the surface solution to drain naturally (without vacuum filtration, a characteristic of traditional processes), dry at 150℃ for 3 h, and then transfer to an activation furnace at 30 m 3 A conventional catalyst (denoted as Ref-Cat-1) was prepared by activation at 330℃ for 7 minutes under an air flow rate of / h. The total silver content was 14.2%, and the silver was uniformly distributed inside and on the surface of the support. The silver content in the surface depth of 0.5~1.5 mm accounted for only 25%.

[0058] 1.2 Performance Evaluation Results Performance of Ref-Cat-1 under the same reaction conditions: The ethylene conversion rate was 10.1%, and the EO selectivity was 87.9%. After 1000 h of reaction, the conversion rate decreased to 9.7%, and the selectivity was 87.5%. This sample used a conventional support with a large particle size of 8 mm, and the amount of silver used was double that of the technology in this application (5 mm gradient support). Although the conversion rate was slightly higher, the silver consumption increased significantly. Moreover, the large particle size easily led to an increase in mass transfer resistance, and the long-term stability was weaker than that of the sample in this application. This further highlights the synergistic advantages of the gradient structure and low silver content of the technology in this application.

[0059] Comparative Example 2: Traditional uniformly supported silver catalyst (low silver content, conventional support) 2.1 Catalyst Preparation Using the traditional preparation process and carrier formulation of Comparative Example 1 (25% aluminum hydroxide monohydrate + 75% aluminum hydroxide trihydrate), the carrier was prepared by extrusion, drying at 80℃ for 12 hours, and then calcining at a constant temperature of 1380℃ for 6 hours. The total silver content was reduced to 2% by adjusting the amount of silver complexing solution. The preparation and impregnation steps of the silver complexing solution were optimized as follows: A silver-ethylenediamine complexing solution was prepared by mixing silver oxalate with deionized water to form a suspension, adding ethylenediamine dropwise until the silver oxalate was completely dissolved, controlling the pH at 10.6, adding cesium nitrate and ammonium perrhenate to control the cesium content to 400 ppmw and the rhenium content to 500 ppmw, and allowing it to stand for degassing for 30 minutes. The carrier was then immersed in the solution at 30℃ for 4 hours with continuous stirring (80 r / min) to ensure uniform distribution of silver. After natural drainage, it was dried at 150℃ for 3 hours and then transferred to an activation furnace for further processing at 30℃. A low-silver-content conventional catalyst (Ref-Cat-2) was prepared by activation at 330℃ for 7 minutes with an air flow rate of m³ / h. The total silver content was 2%, and the silver element was uniformly distributed in the support.

[0060] 2.2 Performance Evaluation Results Performance of Ref-Cat-2 under the same reaction conditions: The ethylene conversion rate was only 2.1%, and the EO selectivity dropped to 81.5%; after 1000 h of reaction, the conversion rate further dropped to 1.6%, and the stability was extremely poor.

[0061] It is evident that under traditional technology, when the silver content is reduced to 2%, the catalytic activity decreases significantly and the selectivity declines simultaneously, failing to meet the needs of industrial production. However, the technology of this application can achieve efficient catalysis with a silver content of around 8.6%, significantly highlighting the core advantages of gradient structure and targeted silver loading process.

[0062] III. Summary of Core Performance Comparisons Between Examples and Comparative Examples Note: All data below have been corrected for volume shrinkage. The reaction conditions are uniformly set as follows: temperature 220℃, pressure 2.0 MPa, ethylene 30 mol%, oxygen 6 mol%, chloroethane 0.5-5 ppm, nitrogen as the equilibrium gas, and volume hourly space velocity 4000 h⁻¹. -1.

[0063]

[0064] Comparing Comparative Example 1 (high-silver conventional catalyst, silver content 14.2%) and Comparative Example 2 (low-silver conventional catalyst, silver content 2%), it can be seen that the technology of this application achieves a conversion rate of 9.0% and a selectivity of 86.0% when the silver content is around 8.6% (only about 60% of the conventional high-silver catalyst). Although Ref-Cat-1 has a slightly better conversion rate of 10.1% and a selectivity of 87.9%, the amount of silver used is doubled, which significantly increases the production cost. The conventional technology reduces the silver content to 2%, but the conversion rate is only 2.1% and the selectivity is 81.5%, which cannot meet the industrial requirements at all. This fully verifies the synergistic advantages of gradient structure, raw material ratio, precise spraying process and optimized co-catalyst ratio, achieving a triple balance of performance, stability and cost.

[0065] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a low-silver-content ethylene oxide catalyst with a silver-rich surface, characterized in that, Includes the following steps: S1. Gradient structure α-alumina carrier was prepared by modifying it using an atomized spray fluorine impregnation-calcination process: 1) Mix 15-35% aluminum hydroxide monohydrate and 65-85% aluminum hydroxide trihydrate according to the corresponding mass fraction ratio, add adhesive and extrude into strips, dry at 75-85℃ for 10-14 h and calcine at 500-600℃ for 3-5 h to obtain the basic alumina carrier; 2) Prepare an ammonium fluoride aqueous solution with a mass fraction of 0.7-0.9%, and spray it onto the surface of the basic alumina carrier using a high-pressure atomizing spray device with atomized particles of 45-55 μm in diameter, controlling the spray volume to be 18-22% of the carrier mass; During the spray impregnation process, the carrier temperature is maintained at 75~85℃, allowing fluorine to penetrate to a depth of 0.5~1.5mm on the carrier surface; 3) The fluorine-impregnated support is subjected to segmented calcination to complete the crystal transformation of the support and form a gradient structure, thus obtaining a gradient structure α-alumina support. The segmented roasting process is as follows: first, dry at 115~120℃ for 3~4 hours; then, heat to 1370~1380℃ at a rate of 4~5℃ / min and roast at a constant temperature for 5~6 hours. The gradient-structured α-alumina carrier has a sheet-like α-alumina structure on the surface with a thickness of 0.7~1.5 mm; an internal dense α-alumina structure; and a surface specific area of ​​1.7~1.9 m². 2 / g, surface water absorption rate 48~52wt%; internal specific surface area 1.6~1.7 m² 2 / g, internal water absorption rate is 9~10wt%; Preparation of S2, silver-ethylenediamine complex solution: Silver oxalate was used as the silver source and mixed with deionized water to form a suspension. Ethylenediamine was slowly added dropwise while stirring until the silver oxalate was completely dissolved. The pH of the system was controlled at 10.5~11.0 to obtain a silver-ethylenediamine complex solution. Add cesium nitrate and ammonium perrhenate to the silver-ethylenediamine complex solution, stir until completely dissolved, and then place in a constant temperature water bath at 25~27℃ for 25~30 min to degas. S3. Preparation of low-silver-content ethylene oxide catalyst product: 1) The gradient structure α-alumina support is placed in a silver-ethylenediamine complex solution and is immersed in a constant temperature water bath at 25~30℃ for 3~4h using an isothermal immersion-intermittent stirring mode, during which the mixture is stirred at 55~60r / min for 8~10min every 25~30min. 2) After impregnation, remove the carrier, remove excess solution from the surface by vacuum filtration, and dry in sections at 145~150℃ for 3~4 hours; then transfer it to an activation furnace at 25~30℃. 3 Activation at an air flow rate of / h and a temperature of 300~330℃ for 7~8min allows the silver-ethylenediamine complex to decompose into metallic silver and form a uniform active layer on the support surface, thus obtaining a low-silver-content ethylene oxide catalyst product.

2. The method for preparing a silver-rich, low-silver-content ethylene oxide catalyst according to claim 1, characterized in that, In S1(1), the mass fraction of aluminum hydroxide monohydrate is 25%; the mass fraction of aluminum hydroxide trihydrate is 75%. The adhesive is a nitric acid aqueous solution with a mass concentration of 5-10%, and the amount added accounts for 18-22% of the total mass of aluminum hydroxide monohydrate and aluminum hydroxide trihydrate; The basic alumina carrier was obtained by drying at 80℃ for 12 h and calcining at 550℃ for 4 h.

3. The method for preparing a silver-rich, low-silver-content ethylene oxide catalyst according to claim 1, characterized in that, In step 2) of S1, the mass fraction of the ammonium fluoride aqueous solution is 0.8%; The atomized particles have a particle size of 50 μm; The spray volume is 20% of the carrier mass; The carrier temperature is maintained at 80°C during the impregnation process; The particle size of the basic alumina carrier is 5 mm.

4. The method for preparing a silver-rich, low-silver-content ethylene oxide catalyst according to claim 1, characterized in that, In step 3) of S1, the specific surface area of ​​the gradient structure α-alumina support is 1.8 m². 2 / g, surface water absorption rate is 50wt%; internal specific surface area is 1.7 m² 2 / g, internal water absorption rate is 10wt%.

5. The method for preparing a silver-rich, low-silver-content ethylene oxide catalyst according to claim 1, characterized in that, In S2, the purity of silver oxalate is ≥99.99%; Cesium nitrate and ammonium perrhenate were added to the solution to make the cesium content in the finished catalyst 400 ppmw and the rhenium content 500 ppmw.

6. The method for preparing a silver-rich, low-silver-content ethylene oxide catalyst according to claim 1, characterized in that, In S3, the vacuum filtration conditions are: vacuum degree of -0.08~-0.09MPa; the segmented drying at 150℃ specifically involves heating to 150℃ for the first hour and then maintaining a constant temperature of 150℃ for the next 2 hours.

7. A surface-rich, low-silver-content ethylene oxide catalyst prepared by the preparation method according to any one of claims 1 to 6, characterized in that, The catalyst is composed of gradient-structured α-alumina as a support, silver as the active component, and cesium and rhenium as co-catalysts. The catalyst has a total silver content of 8.1-9.1%, and the silver content within a depth of 0.5-1.5 mm on the support surface accounts for ≥92%.

8. The silver-rich, low-silver-content ethylene oxide catalyst according to claim 7, characterized in that, The target reaction for the epoxidation of ethylene to ethylene oxide was carried out at a reaction temperature of 220°C, a reaction pressure of 2.0 MPa, and a volume hourly space velocity of 4000 h⁻¹, after volume shrinkage correction, with a total silver content of 8.6% in the catalyst. -1 Under the ethylene epoxidation reaction conditions, the ethylene conversion rate was 9.0% and the ethylene oxide selectivity was 86.0%.

9. The application of the ethylene oxide catalyst as described in any one of claims 7-8 in the direct oxidation of ethylene to prepare ethylene oxide, characterized in that, The catalyst is used under the following conditions: the inlet gas composition is ethylene 28-30 mol%, oxygen 5-6 mol%, chloroethane 0.5-5 ppm, nitrogen is the equilibrium gas, and the reaction conditions are: temperature 210-220℃, pressure 1.8-2.0 MPa, and volume hourly space velocity 3000-6000 h⁻¹. -1 The evaluation time is 950~1050 hours.

10. The application according to claim 9, characterized in that, After 1000 hours of continuous reaction, the catalyst showed no significant decrease in catalytic activity and selectivity after volume shrinkage correction.

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