Process for preparing an epoxidation catalyst

Abstract

A process for preparing a silver-containing catalyst for the selective oxidation of ethylene to ethylene oxide comprising the steps of: (a) providing a multimodal support, (b) preparing an impregnation solution comprising a silver component, (c) impregnating the multimodal support of step (a) at least once with the silver-containing impregnation solution of step (b) to form an impregnated support; (d) subjecting the impregnated multimodal support from step (c) at least once to a removal device such as a centrifuge for a time sufficient to remove the impregnated silver impregnation solution from the multimodal support and to control the amount of silver of the multimodal support by selectively removing the impregnated silver impregnation solution from a set of larger pores of the multimodal support; (e) calcining the multimodal support at least once after the step (d); (f) optionally, repeating the impregnation step (c); (g) optionally, repeating the centrifuging step (d); and (h) optionally, repeating the calcining step (e).

Description

Technical Field

[0001] This invention relates to a method for preparing an epoxidation catalyst. More specifically, this invention relates to a method for preparing a silver-containing catalyst that can be used to epoxidize olefins. Background Technology

[0002] The production of ethylene oxide via the catalytic epoxidation of ethylene in the presence of oxygen and a silver-based catalyst is known. Typically, the epoxidation reaction involves contacting a feed containing at least ethylene and oxygen with a supported silver-containing catalyst to produce the corresponding ethylene oxide (EO). Commercially known catalysts for the production of EO contain silver (Ag) particles supported on an alumina support.

[0003] While previous efforts have attempted to improve the selectivity (synonymous with efficiency), activity, and stability of olefin epoxidation catalysts by employing multimodal supports, none of the existing techniques disclose a method for controlling the Ag particle size in each type of pore within the multimodal support. Numerous studies have demonstrated that Ag particle size has a significant impact on catalyst activity and selectivity. Figure 1 This is reproduced in the work of Goncharova et al., “Size effects in ethylene oxidation on silver catalysts. Influence of support and Cs promoter.” (Applied Catalysis A: General, 126(1), 67-84); and Figure 1 The effect of particle size on the selectivity of several catalysts, including Ag supported on α-Al₂O₃, SiO₂, and bulk Ag, is shown. Goncharova et al. demonstrated that, within the studied range, selectivity increases with the particle size of the supported catalyst (20 nm to 100 nm). Furthermore, bulk Ag exhibits lower selectivity than Ag supported on α-Al₂O₃, suggesting that large aggregates are less efficient in epoxidation reactions.

[0004] Current technology does not allow for the control of Ag particle size as a function of pore size, especially for supports with multi-peak pore size distributions. For example, using known methods for preparing catalysts, large agglomerates can be readily formed on the macropores of multi-peak supports; and therefore, optimal catalyst performance is achieved using such multi-peak supports with large agglomerates. Summary of the Invention

[0005] One embodiment of the present invention relates to a method for preparing a silver-containing catalyst for oxidizing ethylene to ethylene oxide (EO), comprising the following steps:

[0006] (a) A porous multi-peak carrier is provided having at least a first set of carrier pores of a first size range and at least a second set of carrier pores of a second size range, wherein the second size range of the second set of carrier pores is smaller than the first size range of the first set of carrier pores.

[0007] (b) Providing an impregnation solution, such as an impregnation solution including a silver component, to impregnate the impregnation solution into the at least first set of carrier pores and the at least second set of carrier pores of the porous multi-peak carrier;

[0008] (c) Impregnate the porous multi-peaked carrier at least once (i.e., at least once) with at least a portion of the impregnation solution from step (b);

[0009] (d) Before fixing silver onto the carrier, the impregnation solution is selectively removed from the at least first set of carrier pores of the porous multi-peaked catalyst support by subjecting the impregnated porous multi-peaked support to a removal device at least once after the impregnation step (c) for a duration sufficient to selectively remove at least a portion of the impregnated silver-containing impregnation solution contained in the at least first set of carrier pores of the porous multi-peaked catalyst support.

[0010] In another embodiment, the method for preparing the silver-containing catalyst may include the following additional steps:

[0011] (e) The impregnated catalyst support component from step (d) is calcined (i.e., roasted) at a time and temperature sufficient to form a silver-containing catalyst that can be used to epoxidize olefins at least once.

[0012] In yet another embodiment, the selective removal device in step (d) may include a centrifuge for centrifuging the impregnated catalyst support member to remove at least a portion of the impregnated solution contained in the at least first set of support pores.

[0013] In an optional embodiment, the impregnation step (c) or the removal step (d) may be repeated at least once more for a duration sufficient to further remove the Ag impregnation solution contained in the catalyst support member.

[0014] In a further embodiment, the removal step (d) may be performed at least twice before the calcination step (e). The centrifugation step (d) is advantageously performed at a certain centrifugal force and for a period of time, which is sufficient to remove the Ag solution from the desired pores and, after calcination, to provide a more uniform distribution of Ag particle size in the multi-peaked pores of the catalyst support.

[0015] In even other embodiments, any one or more of the above steps (a) to (e) may optionally be repeated any number of times as desired and sufficient to control the amount of Ag deposited in the pores of the catalyst support.

[0016] This application also provides a reaction system for producing ethylene oxide from a feed comprising ethylene and oxygen over an ethylene oxide catalyst prepared by the method of the present invention. Attached Figure Description

[0017] The accompanying drawings illustrate a non-limiting embodiment of the invention, in which:

[0018] Figure 1 This is a graphical illustration of the effect of Ag particle size on the selectivity of ethylene oxide, taken from Goncharova et al.'s "Size effect of ethylene oxidation on silver catalysts. Effect of support and Cs promoter." (Applied Catalysis, Series A: General, 126(1), 67-84). Figure 1 The symbols shown are as follows: Ag / α-Al2O3 (☆); Ag / SiO2 (★) and Ag powder (■). The epoxidation conditions of the method include the following: 230°C, 1 bar, 2% C2H4, and 7% O2 in N2.

[0019] Figure 2 This is a schematic flowchart illustrating the synthesis of a catalyst according to the method of the present invention.

[0020] Figure 3 This is another schematic flowchart illustrating the catalyst synthesis according to the method of the present invention, which provides two centrifugation steps.

[0021] Figure 4 This is another schematic flow chart illustrating the catalytic synthesis according to the method of the present invention, which provides a series of an impregnation step, a centrifugation step and a calcination step, followed by a second series of second impregnation steps, a second centrifugation step and a second calcination step in sequence.

[0022] Figure 5aThese are scanning electron micrographs (SEM) of the fracture cross-sections of Ag particles in a larger pore pattern of a multi-peaked EO catalyst support, where Ag impregnation was performed using conventional methods and did not include a centrifugation step. Unless otherwise stated, pores smaller than 5 micrometers are considered “small” pores in this document; and pores larger than 5 micrometers are considered “large” pores.

[0023] Figure 5b These are SEM cross-sectional images of Ag particles in the larger diameter pore mode of a multi-peaked EO catalyst support, where the Ag-impregnated sample underwent centrifugation after being impregnated in Ag solution within the support. Figure 2 As stated above.

[0024] Figure 6a This is a SEM cross-sectional image of Ag particles in the larger diameter pore mode of a multi-peaked EO catalyst support, where Ag impregnation was performed via a single impregnation step (a conventional synthesis without a centrifugation step).

[0025] Figure 6b This is a SEM image of the fracture cross-section of Ag particles found in the larger diameter pore pattern of a multi-peaked EO catalyst support, where Ag was impregnated in a single step followed by a centrifugation step, as shown in the image. Figure 2 As stated above.

[0026] Figure 7a This is a SEM cross-sectional image of Ag particles in a smaller diameter pore pattern of a multi-peak EO catalyst support, where Ag impregnation was performed via a single impregnation step (a conventional synthesis without a centrifugation step).

[0027] Figure 7b This is a SEM image of the fractured cross-section of Ag particles found in the smaller diameter pore pattern of a multi-peaked EO catalyst support, where Ag was impregnated in a single step followed by a centrifugation step, as shown in the image. Figure 2 As stated above.

[0028] Figure 8a This is a SEM fracture cross-sectional image of Ag particles found in the larger diameter pore mode of a multi-peaked EO catalyst support, where Ag impregnation was performed in two consecutive steps without centrifugation (a conventional synthesis without a centrifugation step).

[0029] Figure 8b This is a SEM image of the fracture cross-section of Ag particles found in the larger diameter pore pattern of a multi-peaked EO catalyst support, where Ag was impregnated in two consecutive steps, with a centrifugation step performed after each step and before calcination, as shown. Figure 4 As stated above.

[0030] Figure 9aThis is a SEM image of the fracture cross-section of Ag particles found in the smaller diameter pore mode of a multi-peaked EO catalyst support, where Ag impregnation was performed in two consecutive steps without centrifugation (a conventional synthesis without a centrifugation step).

[0031] Figure 9b This is a SEM image of the fracture cross-section of Ag particles found in the smaller diameter pore pattern of a multi-peaked EO catalyst support, where Ag was impregnated in two consecutive steps, each followed by centrifugation.

[0032] Figure 10 The catalytic performance of two catalysts, one with and one without centrifugation prior to calcination, was compared. Ag was impregnated in a multi-peak support in one step, such as... Figure 6a and Figure 6b As stated above. Detailed Implementation

[0033] In this article, "catalyst" refers to a substance that increases the rate of a chemical reaction.

[0034] In the epoxidation reaction, in the presence of a supported Ag catalyst, ethylene reacts with oxygen or oxygen-containing gas in the reactor to form ethylene oxide. The epoxidation reaction is characterized by its "activity," "selectivity," and / or "productivity."

[0035] For example, the “selectivity” of an epoxidation reaction, synonymous with “efficiency,” refers to the relative amount (expressed as a fraction or percentage) of ethylene oxide that has been converted or reacted to form the corresponding ethylene oxide product. The terms “efficiency” and “selectivity” are used interchangeably herein. For example, “efficiency to ethylene oxide” refers to the percentage of ethylene converted or reacted on a molar basis to form ethylene oxide. The “yield” of ethylene oxide is the net number of moles of ethylene oxide produced by the method divided by the net number of moles of ethylene added to the method at any given time.

[0036] The "activity" of a catalyst, such as in a fixed-bed reactor, is typically defined as the rate of reaction of the desired product per unit volume of catalyst in the reactor. Activity involves the total number of available active sites on the catalyst and the reaction rate at each site.

[0037] Furthermore, the "activity" of an epoxidation reaction can be quantified in several ways. One is the molar percentage of ethylene oxide in the reactor effluent relative to the reactor influent (the molar percentage of olefin oxides in the influent is typically, but not necessarily, close to zero) while the reactor temperature is kept substantially constant. Another is the temperature required to maintain a given rate of ethylene oxide production. In some cases, activity is measured as the molar percentage of ethylene oxide produced at a specified constant temperature over a period of time. Alternatively, the "activity" of an epoxidation reaction can be measured based on the temperature required to maintain ethylene oxide production at a specific rate under given conditions such as the feed pressure and total molar number.

[0038] A “promoter,” sometimes called an “inhibitor” or “modifier,” is a material that enhances catalyst performance by increasing the rate at which ethylene oxide is desired to form and / or inhibits the undesirable oxidation of ethylene or ethylene oxide to carbon dioxide and water relative to the desired formation of ethylene oxide.

[0039] The terms "reaction temperature," "epoxidation temperature," or "epoxidation reaction temperature" refer to any selected temperature that directly or indirectly indicates the catalyst bed temperature of the reactor. In some embodiments, the reaction temperature may be the catalyst bed temperature at a specific location within the catalyst bed. In other embodiments, the reaction temperature may be the numerical average of several catalyst bed temperature measurements taken along one or more catalyst bed dimensions (e.g., along length). In still other embodiments, the reaction temperature may be the reactor outlet gas temperature. In a further embodiment, the reaction temperature may be the reactor inlet or outlet coolant temperature.

[0040] As used herein, the term "reaction product" includes unreacted feed components and those components produced by the chemical reaction. For example, in an ethylene oxide production process, "reaction product" includes ethylene oxide products; and, if present, any byproducts (such as carbon dioxide) and / or unreacted feed components (such as ethylene, oxygen, and / or chlorides).

[0041] In this document, “silver loading” refers to the weight percentage or weight fraction of silver catalyst based on calcined catalyst. Silver loading can be determined by X-ray fluorescence, titration, or other methods known to those skilled in the art (neutron activation analysis, NAA).

[0042] In this article, “silver particle size” refers to the average particle size (nanometers) as measured by dynamic pulsed CO chemisorption, scanning electron microscopy (SEM) or other instruments known to those skilled in the art.

[0043] Dynamic pulsed CO chemisorption can identify exposed silver sites and, based on the reaction of CO on oxidized Ag surfaces, generate CO2. The average particle size of the CO titration experiment can be determined by the following expression:

[0044]

[0045] Where σ Ag ρ is the average atomic surface density of silver. Ag It is the density of silver, N A It is an Avogadro number, n Ag n is the number of moles of Ag present in the sample, and n CO2 This represents the total number of moles of CO2 produced. The stoichiometry between surface atoms and adsorbed gas is considered to be equal to 1.

[0046] Referring to catalyst supports, the term "multi-peak pore size distribution" in this article means that the pore size of the support is a continuous probability distribution, wherein at least two different modes are determined by methods known to those skilled in the art (such as the Hg porosity determination method).

[0047] Referring to catalyst supports, the "bimodal pore size distribution" in this paper refers to the continuous probability distribution of pore size in two different pore patterns.

[0048] Referring to particle size, the term “control” (or “controlling”) in this document primarily refers to the concentration of silver in the impregnation solution used in conjunction with impregnation, calcination, and selective removal devices to provide silver particles of the desired particle size.

[0049] In its broadest scope, this invention includes a novel method for synthesizing silver-based catalysts, the method comprising controlling the Ag particle size in a support having a multi-peaked pore size distribution. The silver-based catalyst of this invention can be used for the epoxidation of ethylene to form ethylene oxide.

[0050] The broad general procedure of the present invention for preparing silver-based catalysts comprises at least the following steps: preparing an impregnation solution comprising an Ag-containing component, said Ag-containing component being used to impregnate a multi-peaked support member, contacting and impregnating the multi-peaked support member with the prepared impregnation solution, and then selectively removing at least a portion of the impregnation solution from at least a set of pores of the multi-peaked support.

[0051] In a preferred embodiment, the method of the present invention may include, for example, a five-step method, such as... Figure 2 The schematic flowchart illustrates the following steps:

[0052] (a) Provide a porous, multi-peak catalyst support component.

[0053] (b) Provide an impregnation solution comprising the Ag component.

[0054] (c) Impregnate the porous catalyst support component with at least a portion of the impregnation solution.

[0055] (d) Selectively removing the Ag component solution from at least one set of pores of the multi-peaked porous catalyst support component by subjecting the impregnated porous catalyst support component to a removal device such as a centrifuge after impregnation step (c) under conditions and for a time sufficient to remove at least a portion of the impregnated silver impregnation solution from at least one set of pores of the porous catalyst support component; and

[0056] (e) After step (d), the porous catalyst support component is calcined (i.e., roasted) to convert the Ag component into Ag metal and deposit Ag on the inner and outer surfaces of the porous catalyst support component to form a catalyst; and to provide a uniform distribution of Ag particles with a uniform average particle size.

[0057] In other embodiments, the method of the present invention may comprise one impregnation step or a sequence of two or more impregnation steps. The impregnated carrier from the impregnation step may then be treated by a removal device or undergo a removal device, such as a centrifugation step, before a calcination step is performed. Any impregnation, centrifugation, or calcination step of the method of the present invention may be performed once (e.g., Figure 2 (as shown), or if desired, and in some embodiments, any or all steps may be performed two or more times (as shown). Figures 3 to 4 (As shown).

[0058] refer to Figure 3 This illustrates another embodiment of the method of the present invention, wherein the catalyst is impregnated with an Ag component solution in a single step, and then the impregnated catalyst is subjected to two centrifugation steps, namely, a first centrifugation step and a second centrifugation step performed thereafter.

[0059] exist Figure 4 In another embodiment of the method of the present invention, after the silver impregnation solution is prepared, the catalyst is subjected to a first impregnation step, a first centrifugation step, and a first calcination step in sequence, followed by a series of second impregnation steps, second centrifugation steps, and second calcination steps in sequence. Figure 4 As a result of the method shown, the catalyst undergoes essentially two consecutive impregnation, centrifugation, and calcination steps to produce the catalyst product.

[0060] In one embodiment (not shown), the method of the present invention for preparing a silver-containing catalyst for olefin epoxidation may include the following steps: (a) providing a porous multimodal support having at least two pore size distribution patterns; (b) providing a silver impregnation solution to impregnate the pores of the porous multimodal support; (c) impregnating the porous multimodal support with the impregnation solution from step (b) to provide the porous multimodal support with a first amount of impregnation solution; (d) centrifuging the impregnated silver-containing impregnation solution from the porous multimodal support to provide the porous multimodal support with a second amount of impregnation solution retained in the pores of the support; (e) calcining the silver-containing impregnation solution from step (b). (d) the impregnated porous multi-peaked support; (f) impregnating the porous multi-peaked support with the impregnation solution from step (b) to provide the porous multi-peaked support with a third amount of impregnation solution; (g) centrifuging the silver-containing impregnation solution from step (f) to provide the porous multi-peaked support with a fourth amount of impregnation solution, wherein the fourth amount of impregnation solution is retained in the pores of the support; and (h) calcining the impregnated porous multi-peaked support of step (g) for a time and temperature sufficient to form a silver-containing catalyst suitable for olefin epoxidation; and wherein centrifugation steps (d) and (g) are performed under different centrifugation conditions.

[0061] In yet another embodiment, not shown, the impregnated centrifugal catalyst may undergo two calcination steps in sequence, namely a first calcination step, and then a second calcination step performed in sequence at the same or different temperatures to provide a catalyst product.

[0062] In any of the above embodiments, a removal device can be used to remove at least 20%, at least 30%, at least 50%, or at least 80% of the silver-impregnated solution contained in at least the first set of carrier pores of the porous multi-peak carrier. In yet another embodiment, up to 100% of the silver-impregnated solution contained in at least the first set of carrier pores of the porous multi-peak carrier can be removed.

[0063] While the above embodiments illustrate variations of the method of the present invention, the method is not limited to these embodiments. Figures 2 to 4 The methods shown herein. As described above, the impregnation, centrifugation, or calcination steps may be performed once, twice, or more; and optionally, any or all of the steps may be performed twice or more. For those skilled in the art, Figures 2 to 4 Any combination and variation of the steps shown are obvious and are within the scope of the method of the invention as described in the claims of this invention.

[0064] The method for preparing the catalyst according to the invention begins with a first step of providing a multi-peaked support (also referred to as a "support") impregnated with an impregnation solution. The catalyst preparation method described herein can be applied to different types of supports having a multi-peaked pore size distribution, such as bimodal supports and other catalysts prepared by impregnation techniques. The multi-peaked support comprises a support having at least two different pore sizes. Generally, the porous multi-peaked support of the invention may have a first set of support pores with at least a first set of size ranges and a second set of support pores with at least a second set of size ranges, wherein the second size range of the second set of support pores is smaller than the first size range of the first set of support pores.

[0065] For example, in one embodiment, the first set of carrier pores may have a second pore diameter ranging from about 3 μm to about 100 μm, and in another embodiment, it may have a second pore diameter ranging from about 5 μm to about 50 μm.

[0066] For example, in one embodiment, the second set of carrier pores may have a first pore diameter ranging from about 0.01 μm to about 3 μm, and in another embodiment, it may have a first pore diameter ranging from about 0.01 μm to about 1 μm.

[0067] These carriers may include any known porous refractory structure or carrier material, provided that, regardless of the porous refractory material chosen, it is relatively inert in the presence of the chemicals and processing conditions used in the application using the carrier.

[0068] The support can be selected from a variety of inert support materials, including, for example, natural or artificial inorganic support materials such as silicon carbide, clay, pumice, zeolite, and charcoal, and alkaline earth metal carbonates such as calcium carbonate and mixtures thereof. Other embodiments may include refractory support materials such as alumina, magnesium oxide, zirconium oxide, and silicon dioxide; and mixtures thereof. In a preferred embodiment, the support material may be α-alumina. In an exemplary embodiment, Ag is deposited on the α-alumina catalyst support, and optionally, one or more promoters may also be deposited on the catalyst.

[0069] The material used for the carrier may or may not be used with an adhesive. When an adhesive is used, the adhesive may be, for example, an inorganic material.

[0070] Multimodal supports are typically prepared using well-known methods. For example, several well-known methods for preparing supports suitable for olefin oxide catalysts are described in WO 2013 / 148417 A1; and U.S. Patent Nos. 4,379,134; 4,806,518; 5,063,195; 5,384,302; and 6,831,037, which are incorporated herein by reference.

[0071] For example, an α-alumina support with at least 95% purity can be prepared by mixing (blending) raw materials, extruding, drying, and calcining at elevated temperatures. In this case, the starting materials typically comprise one or more α-alumina powders with different properties, including clay-type materials that can be added as binders to provide physical strength, and burnt materials (typically organic compounds) used in the mixture to provide the desired porosity and / or pore size distribution after being removed during the calcination step. The impurity content in the final support is determined by the purity of the raw materials used and their degree of volatilization during the calcination step. Typical impurities may include silica, alkali and alkaline earth metal oxides, and trace amounts of metallic and / or non-metallic additives.

[0072] Alumina can have very high purity grades, i.e., at least 98 wt% α-alumina, with any remaining components being silica, alkali metal oxides (e.g., sodium oxide), and trace amounts of other metal and / or nonmetallic additives or impurities. Similarly, alumina can have lower purity grades, i.e., 80 wt% α-alumina, with the remainder being amorphous and / or crystalline alumina and other alumina, silica, silica-alumina, mullite, various alkali metal oxides (e.g., potassium oxide and cesium oxide), alkaline earth metal oxides, transition metal oxides (e.g., iron oxide and titanium oxide), and one or more other metal and nonmetal oxides. Additionally, the materials used to prepare the support can include components known for improving catalyst performance, such as rhenium (e.g., rhenium salts) and molybdenum.

[0073] In one embodiment, the pore volume of the α-alumina support can be at least 0.3 cubic centimeters per gram (cm³). 3 / g), and in another embodiment approximately 0.4cm 3 / g to approximately 2.0cm 3 / g; pore size can range from about 0.1 micrometers to about 50 micrometers.

[0074] In one embodiment, the specific surface area of ​​the α-alumina support can be at least about 0.5 square meters per gram (m²). 2 / g), and in another embodiment at least about 0.7m 2 / g. In one embodiment, the surface area of ​​the α-alumina support can be less than about 10m². 2 / g, and in another embodiment less than about 5m 2 / g.

[0075] The α-alumina carriers used in this invention can have any suitable shape. Exemplary shapes of the carriers include pills, blocks, tablets, flakes, pellets, rings, spheres, cartwheels, annular rings with a star-shaped inner surface and / or outer surface, etc.

[0076] The support can be any size suitable for use in the reactor. For example, in a fixed-bed ethylene oxide reactor with multiple parallel elongated tubes (in a suitable shell) filled with catalyst, having an outer diameter of 1 inch to 3 inches (2.5 cm to 7.5 cm) and a length of 15 feet to 45 feet (4.5 m to 13.5 m), it is desirable to use an α-alumina support with a circular shape, such as spheres, pellets, rings, cross-segmented rings, pentacyclic rings, tablets, etc., with a diameter of 0.1 inch (0.25 cm) to 0.8 inch (2 cm).

[0077] The impregnation solutions used in this invention comprise silver-containing impregnation solutions. In one embodiment, a bimetallic system may be used, which may contain silver and another metal, such as copper or gold, typically impregnating only the silver into the carrier. The silver impregnation solution for impregnating the carrier comprises the silver component in the impregnation medium, such as a solvent or complexing / solvent, as disclosed in U.S. Patent No. 5,187,140, ​​which is incorporated herein by reference.

[0078] The specific silver component used in this invention may be selected from, for example, silver complexes, nitrates, silver oxide, or silver carboxylatees, such as silver acetate, oxalate, citrate, phthalate, lactate, propionate, butyrate, and higher fatty acid salts; and mixtures thereof. In another embodiment, the silver oxide complexed with an amine may be in the form of silver that can be used in the practice of this invention.

[0079] The silver component described above can be dissolved in an impregnation medium or solution of a desired concentration using a variety of solvents or complexing / solvents. The solvent can be any conventional solvent or complexing / solvent known in the art, as long as the solvent or complexing / solvent can dissolve the silver component in the impregnation solution of the desired concentration and does not adversely affect the performance of the catalyst. Solvents or complexing / solvents suitable for this purpose may contain, for example, lactic acid (as disclosed in U.S. Patent Nos. 2,477,436 and 3,501,417); ammonia (as disclosed in U.S. Patent No. 2,463,228); alcohols, such as ethylene glycol (as disclosed in U.S. Patent Nos. 2,825,701 and 3,563,914); and amines and aqueous mixtures of amines (as disclosed in all of these, by reference to and incorporated herein by reference, U.S. Patent Nos. 2,459,896; 3,563,914; 3,215,750; 3,702,259; 4,097,414; 4,374,260; and 4,321,206). Combinations of two or more of the above-described solvents or complexing / solvents may also be used in the impregnation solutions of the present invention.

[0080] Typically, the support is impregnated with an impregnation solution containing a certain amount of silver, which is a certain amount of silver capable of catalyzing the direct oxidation of olefins with oxygen or oxygen-containing gases to the corresponding olefin oxides. In preparing such a catalyst, the support can usually be impregnated (once or multiple times) with one or more silver component solutions, said silver component solutions being sufficient to allow the desired silver loading on the support.

[0081] Typically, the amount of silver component dissolved in the silver impregnation solution is greater than the amount ultimately provided on the final catalyst during each impregnation. For example, Ag₂O can be dissolved in a solution of oxalic acid and ethylenediamine to about 30 wt%. This solution is vacuum impregnated onto an α-alumina support with a porosity of about 0.7 cc / g to obtain a catalyst containing about 25 wt% silver based on the total weight of the catalyst. Therefore, to obtain a catalyst with a silver loading greater than about 25 wt%, greater than about 30 wt%, or more, the support must be subjected to at least two or more consecutive silver impregnations, with or without a promoter, until the desired amount of silver is deposited on the support. The catalyst of the present invention can be prepared by two or more impregnations to achieve the desired silver concentration in the resulting catalyst.

[0082] As is well known, the number of impregnations, the concentration of the solution used in each impregnation, and the calcination conditions are factors that can be used to fine-tune the particle size of silver in the resulting catalyst. For example, the concentration of silver salt in a later impregnation solution can be higher than that in the first impregnation solution. In other examples, approximately equal concentrations of silver can be used in each impregnation step. In further examples, a higher concentration of silver can be used in the initial impregnation than in subsequent impregnations. Calcination or other steps can be performed after each impregnation to render the silver insoluble. A further advantage of this invention over the prior art is that the method of this invention provides a more selective way to control the Ag concentration in selected pores.

[0083] Various optional compounds or additives can be added to the impregnation solution, including, for example, one or more promoters, alkali metals, alkaline earth metals, and oxygen-containing anions, as well as mixtures thereof. In one embodiment, the promoter can be a material introduced into the catalyst during catalyst preparation (e.g., a solid-phase promoter, also referred to herein as a "catalyst promoter"). In another alternative embodiment, the promoter can be a gaseous substance introduced into the epoxidation reactor feed (a gas-phase promoter). In one example, an organohalide gas-phase promoter can be continuously added to the epoxidation reactor feed to improve catalyst efficiency. For silver-based ethylene epoxidation catalysts, both solid-phase and gas-phase promoters are typically used in one method.

[0084] If desired, a suitable alkali metal co-catalyst component can be used to form the impregnation solution. For example, the alkali metal promoter component can comprise all those promoters that are soluble in the particular solvent or solubilizer used and compatible with the other components in the impregnation solution. Therefore, inorganic and organic components of alkali metals, such as nitrates, halides, hydroxides, sulfates, and carboxylates, can be used. As an example, alkaline earth metal salts such as barium, calcium, and magnesium salts can readily dissolve in the impregnation solution and be deposited on the support according to the method of the present invention.

[0085] The order in which silver and an accelerator are impregnated or deposited on the carrier surface is optional. Therefore, the impregnation and deposition of silver and salts can be performed simultaneously or sequentially; that is, the accelerator can be deposited before, during, or after the addition of silver to the carrier. The accelerators can be deposited together or sequentially. For example, one or more salts can be deposited first, followed by simultaneous or sequential deposition of silver and other salts.

[0086] Impregnation of the catalyst support can be performed using one or more solutions containing silver and a promoter, following well-known methods, to perform deposition simultaneously or sequentially. For simultaneous deposition, after impregnation, the impregnated support is heated or chemically treated to reduce the silver component to silver metal and deposit the salt on the catalyst surface.

[0087] For sequential deposition, the support is first impregnated with silver or a promoter (depending on the order used), followed by heating or chemical treatment as described above. A second impregnation step and corresponding heating or chemical treatment are then performed to prepare the finished catalyst containing silver and the promoter.

[0088] In one embodiment of the invention, one or more promoters are added simultaneously with silver. In another embodiment, one or more promoters are added to the catalyst in the final silver impregnation step.

[0089] All components of the impregnation solution are typically mixed and dispersed in a container at a temperature capable of preparing an effective impregnation solution. For example, in one embodiment, the temperature during mixing of the above components is typically from about ambient temperature (23°C) to about 70°C, and in another embodiment, it is from about ambient temperature to about 50°C.

[0090] The preparation of the impregnation solution and / or any step thereof of the present invention can be a batch or continuous method. The type of method and apparatus used to prepare the impregnation solution can be any conventional method or apparatus known in the art. For example, a mixing vessel is used to blend (or mix) the above components: the Ag component, the solvent, and any other desired additives, such as accelerators.

[0091] Typically, methods for depositing silver catalysts and any other additives such as promoters involve impregnating a porous alumina support according to the invention with a solution comprising a solvent or solubilizer, a silver complex, and one or more promoters, if desired. Impregnation of the support is generally a preferred technique for silver deposition because it utilizes silver more efficiently than coating processes, which typically cannot achieve substantial silver deposition on the inner surface of the support. Furthermore, coated catalysts are more prone to silver loss due to mechanical abrasion.

[0092] The type of method and apparatus used to impregnate the carrier with the impregnation solution can be any conventional impregnation method or apparatus known in the art. For example, a container is used to hold the carrier, which will be impregnated with the aforementioned impregnation solution, and the impregnation solution passes through the carrier in the container.

[0093] As used herein, “surface area” refers to the surface area of ​​a support determined by the BET (Brunauer, Emmett, and Teller) method under nitrogen, as described in the Journal of the American Chemical Society, 60 (1938), pp. 309-316, which is incorporated herein by reference. “Total pore volume” refers to the pore volume of a support and is typically determined by the mercury porosimetry method. The measurements reported herein use the method described in Webb and Orr, Analytical Methods in Fine Particle Technology (1997), p. 155, using a Micromeritics Autopore IV 9520 with 60,000 psia of mercury, assuming a contact angle of 130° and a surface tension of 0.485 N / m. “Porosity” is the ratio of the non-solid volume of a material to its total volume. Total pore volume measured by the mercury porosimetry or water absorption method can be used by those skilled in the art to estimate open porosity. Although closed pores do not exhibit catalytic activity, these techniques cannot identify closed pores. In other words, porosity is defined as the volume of pores (unoccupied space) divided by the total volume of the sample. The "median pore size" refers to the pore size corresponding to a point in the pore size distribution where half of the total pore volume of the support is cumulatively measured. For multimodal supports, the overall median pore size does not fully describe the pore system but rather represents the median pore size for each pore pattern.

[0094] For a given support material, there is a significant relationship between silver loading and silver particle size. For example, multiple impregnation steps can be used to increase the Ag loading. The specific surface area of ​​silver is defined as the weight of the catalyst divided by the area of ​​silver. Preferably, the method used to determine the specific surface area of ​​silver is performed on an unpromoted catalyst. Specific techniques for determining the specific surface area of ​​silver include oxygen and carbon monoxide chemisorption, as well as microscopy combined with image analysis. One specific technique employs oxygen-selective chemisorption using dynamic pulse technology. By reference to Asterios Gavrilidis et al., “Influence of Loading on Metal Surface Area for Silver / Alpha-Alumina Catalysts” (JJCatalysis), 139(1), 41-47 (1993), which is incorporated herein by reference. Another technique uses titration to determine the number of silver surface atoms and determines the specific surface area of ​​silver by extension. M. Boudart’s “Turnover Rates in Heterogeneous Catalysis” (Chem.Rev., Vol. 95, 661-666 (1995)) is incorporated herein by reference.

[0095] After impregnating a porous catalyst support with an impregnation solution (also referred to herein as the "impregnated support"), any impregnating solution that is not impregnated into the pores of the support is referred to herein as the "unimpregnated solution". This may be contained in the solution around and on the outer surface of the support. The solution that has been impregnated or absorbed into the pores of the catalyst support is referred to herein as the "impregnated solution". A feature of the present invention is that a majority of the unimpregnated solution (if present) and at least a portion of the impregnated solution are removed from the impregnated support in separate steps.

[0096] For example, after each impregnation of a catalyst support in a silver-containing impregnation solution, it is desirable to separate any remaining, additional, or excess unimpregnated solution from the entire impregnated support using conventional separation methods such as filtration, drainage, or centrifugation. For instance, the unimpregnated solution can be drained from the impregnated support, i.e., the unimpregnated solution is physically separated from the entire outer surface of the support. As another example, when a centrifuge is used as the separation device, the unimpregnated solution is separated from the entire outer surface of the impregnated catalyst under operating conditions. Generally, drainage is most commonly used to separate any excess unimpregnated solution from the impregnated support.

[0097] Following any of the separation methods described above, the impregnated carrier may undergo a removal method to selectively and physically remove at least a portion of the impregnating solution from certain pores of the impregnated carrier, said removal method preferably including centrifugation. In one embodiment of the invention, after the non-impregnating solution is separated from the catalyst carrier, the impregnated silver-containing impregnating solution disposed in the pores of the carrier is selectively removed from certain pores of the carrier by subjecting the impregnated carrier to a centrifugal device such as a centrifuge.

[0098] When a centrifuge is used as the removal device in this invention, centrifugation conditions can be selected to selectively remove the impregnated solution not only from the entire surface of the catalyst but also from certain pores of the catalyst support. The centrifugation step is important because it separates excess unimpregnated impregnated solution from the catalyst support surface and the solution impregnated from certain pores of the catalyst support that were not removed in the drainage step. Furthermore, removing the Ag impregnated solution from the macropores by centrifugation eliminates Ag aggregation on the macropore surface and reduces the overall average particle size of silver.

[0099] Typically, important factors for performing the centrifugation steps of this invention may include, for example, rotational speed (rpm), centrifuge radius, centrifugation time, and temperature. Rotational speed and centrifuge radius determine the relative centrifugal force (RCF). The RCF can be calculated using the following equation (1):

[0100]

[0101] Where R c It is the radius of the centrifuge (mm), and rpm is the rotational speed in revolutions per minute.

[0102] Centrifugal force can be calculated using the following equation:

[0103]

[0104] Where m is the mass of the liquid inside the hole, w is the angular velocity, r is the radius of the hole, h is the height of the liquid inside the hole, and R... c Here, ρ is the centrifuge radius, and ρ is the liquid density. The height of the liquid inside the orifice is determined by the balance between surface tension and gravity.

[0105]

[0106] Where θ is the contact angle.

[0107] To selectively remove solution from a given orifice, the centrifugal force should be equal to the capillary force of the corresponding orifice. The capillary force is calculated using the following equation:

[0108] F 毛细管 =hγcosθ Equation (4)

[0109] In summary, the optimal RCF for removing solution from pores of a specific size is determined by matching capillary force with centrifugal force. Based on this, the following relationship is obtained:

[0110]

[0111] The physical properties expressed in equation (5) are temperature-dependent and may therefore affect the efficiency of the centrifugation process.

[0112] To further illustrate the present invention using the above equation (5), the following property can be used in the equation: for the properties of the impregnation liquid, γ = 7.28 x 10 -2 N / m; θ=85°; and ρ=1493kg m -3 And assuming the pore size ranges from 1,000 μm to 1 μm, the centrifugal force (RCF) can be, for example, from about 0.1 to 1,500,000.

[0113] In one embodiment, the centrifugation time can be, for example, from about 1 minute to about 20 minutes, and in another embodiment from about 5 minutes to about 10 minutes. The temperature at which centrifugation is performed can be a factor, as it can determine the properties of the impregnating solution within the well. Centrifugation can be carried out at any suitable temperature; for example, the temperature at which the solution decomposes in the centrifuge is not too high, or the temperature is not so low that the fluid cannot flow, i.e., the solution should maintain sufficient fluidity and have the necessary rheological properties to be removed from the carrier by the centrifuge. In a general embodiment, for example, the centrifugation temperature can be from about 20°C to about 80°C. Typically, the centrifugation temperature is ambient temperature. For example, in one embodiment, a silveramine oxalate solution of the type described in the examples herein can be centrifuged at a temperature from about room temperature (RT) to 40°C.

[0114] The centrifugation step and / or any step thereof of the invention may be a batch process; and the equipment used in the process may be any centrifuge and auxiliary equipment known to those skilled in the art.

[0115] The centrifugation step in the method of this invention is important because centrifugation separates excess unimpregnated impregnation solution from the surface of the catalyst support and also separates at least a portion of the impregnation solution from the pores of a specific size range in the catalyst support. In this way, the centrifugation step, combined with impregnation solution conditioning, allows control of the Ag particle size in each pore of the support by adjusting the Ag concentration in the pores to match the available surface area within the pores. By selecting the centrifugation speed to provide appropriate centrifugal force and centrifugation time, it is possible to control which pore pattern remains filled with the Ag impregnation solution, and thus control the final Ag particle size and location within the support.

[0116] Another benefit of one or more centrifugation steps is that centrifugation can reduce the amount of unimpregnated Ag solution remaining on the outer surface of the support particles, and reduce or even prevent the formation of an increased sediment load on the particle surface after calcination. Furthermore, during calcination of the Ag shell, it is assumed that excess silver on the outside will cut off the catalyst during operation, leading to fine particle accumulation and pressure. Removing excess Ag solution beforehand allows the solution to be recovered and enables better utilization of Ag in the catalyst production process, thereby reducing production costs.

[0117] Another advantage of producing catalysts in this manner is that they can have a lower overall silver loading compared to existing catalysts without sacrificing productivity. It should be recognized that this lower silver loading can reduce the cost of producing such catalysts. Furthermore, the lower Ag loading and the reduction in Ag external particle deposition can: (1) cause less pore blockage and thus reduce diffusion barriers in the catalyst, and (3) reduce subsequent Ag loss during further catalyst treatment.

[0118] After the centrifugation steps described above, the resulting centrifugal carrier is usually subjected to heat treatment, i.e., the liquid inside the carrier is calcined and evaporated at high temperature; and the silver metal salt is decomposed and reduced to metallic silver, thereby allowing silver and the promoter (if present) to be deposited on the internal and external carrier surfaces.

[0119] Various heat treatment atmospheres can be used to heat the impregnated / centrifuged carrier. For example, the carrier can be heated in air or in an inert atmosphere such as nitrogen. When heat treatment is performed in an oxidizing environment, the heat treatment can be referred to as "calcination".

[0120] Typically, the impregnated support is heat-treated at or below atmospheric pressure to remove any present solvent (or multiple solvents) and to deposit (with or without decomposition) promoter species (if present) onto the silver and support surfaces. The heat treatment can be performed at temperatures and times sufficient to remove excess solvent and convert substantially all of the silver salts into metallic silver. For example, in one embodiment, the impregnated / centrifuged support can be heated at temperatures from about 100°C to about 900°C, and in another embodiment, at temperatures from about 200°C to about 700°C for a time sufficient to convert substantially all of the silver salts into metallic silver. For example, a calcination step can be performed for a period of time from about 2 minutes to about 12 hours (hr).

[0121] Generally, the higher the temperature, the shorter the reduction time required. For example, at temperatures from about 400°C to about 900°C, reduction can be completed in about 1 minute to about 5 minutes. Other time periods for heat treatment of the impregnated support have been proposed in the art. For example, U.S. Patent No. 3,563,914 suggests heating the impregnated support for less than 300 seconds to dry rather than calcine the catalyst, thereby reducing the catalyst; U.S. Patent No. 3,702,259 discloses heating the impregnated support at temperatures from 100°C to 375°C for 2 to 8 hours to reduce silver salts in the catalyst; and U.S. Patent No. 3,962,136 suggests heating at temperatures from 100°C to 375°C for 4 to 8 hours; all of the above patents are incorporated herein by reference. Although a wide range of heating cycles can be employed in this invention, it is important that the reduction time is temperature-dependent so that the silver salts are reduced substantially completely to the metal. For this purpose, it is desirable to use a continuous or stepwise heating process. Continuous calcination of the catalyst for a short time, such as no more than 4 hours, is preferred and can efficiently prepare the catalyst of this invention. When using more than one roasting step, the roasting conditions in each roasting step need not be the same.

[0122] The roasting or calcining step of this invention, and / or any other step thereof, can be a batch or continuous process. Steps in the method of this invention involving heating the impregnated multimodal carrier can be performed using any conventional heating method. For example, the equipment used for such heat treatment can contain a static or flowing atmosphere of gas to achieve reduction.

[0123] A highly efficient silver-containing epoxidation catalyst for the selective oxidation of ethylene to ethylene oxide is derived from the above-described preparation method. The catalyst comprises at least one catalytic species deposited on a porous support; wherein the at least one catalytic species is silver; and wherein the silver deposited on the support has a controlled average particle size in the finished catalyst.

[0124] The active component of the high-efficiency catalyst produced by the method of the present invention may include Ag as a catalytic species, rhenium as a promoter, optionally rhenium co-promoter, and optionally other metals.

[0125] After calcination in the heating zone, the silver-impregnated catalyst can be weighed; and based on the weight increase relative to the material before impregnation, assuming complete removal of the solvent and precursor material, the weight of silver (by weight%) on the support can be calculated.

[0126] When the desired catalytic species includes silver, the wt% silver concentration loaded on the support can typically depend on the silver concentration in the impregnation solution, the pore volume of the support, and the number of impregnation steps used. Generally, impregnation will be sufficient to allow a suitable amount of silver to be provided on the support. For example, based on the weight of the catalyst, the amount of silver loaded on the support can be greater than about 5 wt% in one embodiment, greater than about 10 wt% in another embodiment, greater than about 15 wt% in yet another embodiment, greater than about 20 wt% in yet another embodiment, greater than about 25 wt% in yet another embodiment, greater than about 27 wt% in yet another embodiment, and greater than about 30 wt% in yet another embodiment. Based on the weight of the catalyst, the amount of silver associated with the support can typically be less than about 70 wt% in one embodiment and less than about 50 wt% in another embodiment. See, for example, US 2014 / 0371470A1, incorporated herein by reference.

[0127] In other embodiments, the wt% silver on the carrier may be from about 5 wt% to about 70 wt% in one embodiment, from about 5 wt% to 50 wt% in another embodiment, from about 15 wt% to about 40 wt% in yet another embodiment, and from about 15 wt% to 35 wt% in yet another embodiment.

[0128] The catalyst prepared by the method of the present invention has Ag particles of a desired size in the pores and macropores of the support, which is sufficient to provide improved overall catalyst performance. Although the silver particle size in the finished catalyst is important, the silver particle size can encompass a wide range. For example, suitable silver particle sizes can be in the diameter range of about 10 angstroms to about 10,000 angstroms. In one embodiment, typically, the preferred silver particle size range can be in the diameter range of greater than about 100 angstroms to less than about 5,000 angstroms. If used, it is desirable that silver and any promoters are relatively uniformly dispersed within, throughout, and / or on the alumina support. By the method of the present invention, the silver particles also have a relatively uniform particle size throughout the support, i.e., a relatively narrow particle size distribution.

[0129] The particle size of Ag metal deposited on the support is a function of the catalyst preparation process employed. Therefore, the specific selection of solvents and / or complexing agents, Ag salts, heat treatment conditions, and catalyst supports can influence the size of the resulting Ag particles to varying degrees. Importantly, this invention provides an additional method for optimizing Ag particle size when the support has a multi-peaked pore size distribution.

[0130] Using the method of the present invention, Ag catalysts containing uniform or uniformly sized Ag particles can be prepared on a multi-peak support. Compared with Ag catalysts prepared on a multi-peak support, the centrifugally supported Ag catalyst of the present invention reduces the average particle size of silver without a centrifugation step and / or without Ag having a uniformly sized particle size in the catalyst.

[0131] Typically, the catalyst prepared by the method of the present invention comprises Ag particles with a particle size distribution of about 60 nm to about 300 nm in one embodiment, about 60 nm to about 220 nm in another embodiment, and about 90 nm to about 200 nm in yet another embodiment. As a result of using a centrifugation step, the formation of Ag particles larger than a certain size, such as larger than 300 nm, can be eliminated.

[0132] The catalyst according to the invention may optionally contain one or more promoters or co-promoters. For example, promoters known for silver-based epoxidation catalysts, in addition to rhenium, may include, but are not limited to, molybdenum, tungsten, sulfur, lithium, sodium, manganese, rubidium, and cesium. Rhenium, molybdenum, or tungsten may suitably be provided as oxyanions in the form of salts or acids, such as perrhenates, molybdates, or tungstates. Examples of promoters, their features, and methods for incorporating promoters as part of a catalyst are described in all of the U.S. Patent Nos. 5,187,140, ​​incorporated herein by reference, specifically in columns 11 through 15; 6,511,938; 5,504,053; 5,102,848; 4,916,243; 4,908,343; 5,059,481; 4,761,394, 4,766,105, 4,808,738, 4,820,675; and 4,833,261; patents.

[0133] When used, the concentration of the accelerator and / or co-accelerator can vary, for example, from about 0 wt% to about 1.0 wt% in one embodiment, from about 0.0005 wt% to about 1.0 wt% in another embodiment, and from about 0.005 wt% to about 0.5 wt% in yet another embodiment.

[0134] Typically, the methods of the present invention can be used to oxidize olefin compounds to oxide products. One example of the advantageous end use of the catalysts of the present invention is the epoxidation of ethylene to ethylene oxide. In such epoxidation reactions, the performance of the catalyst is typically evaluated based on its selectivity, activity, and stability during the epoxidation process. "Stability" generally refers to how the selectivity and / or activity change during the use of a particular batch of catalyst, i.e., as more ethylene oxide is cumulatively produced. The catalysts of the present invention are expected to provide advantages in selectivity, activity, and / or stability due to the Ag catalytic species loading and the uniform Ag particle size in the catalyst, which can be achieved on multimodal support materials. Another advantage also obtainable by using the methods of the present invention and by preparing the catalysts using the methods of the present invention is the reduction in production and / or equipment costs associated with said methods.

[0135] For example, after preparing the silver-supported catalyst as described above, the catalyst can be used in a method for epoxidizing ethylene to form ethylene oxide, as disclosed in US 2014 / 0323295A1, which is incorporated herein by reference. The epoxidation process of ethylene is also described, for example, in US Patent Nos. 6,511,938 and 5,057,481; and the Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, (1994) Vol. 9, pp. 915-959, which are incorporated herein by reference.

[0136] Typically, epoxidation reactions can be carried out desirably in the gas phase, wherein a feed comprising ethylene and oxygen is contacted with the epoxidation catalyst of the present invention. Typically, the catalyst is present in the form of a solid material, and more specifically, it can be present in the form of a packed bed within the desired reactor. The amount of catalyst used can be any suitable amount and depends on the application. In one embodiment, the conversion of ethylene to ethylene oxide can be carried out, for example, in a continuous process by continuously introducing a feed stream containing ethylene and oxygen or an oxygen-containing gas into a catalyst-containing reactor. The resulting ethylene oxide is separated from the reaction products and recovered using conventional methods.

[0137] Reactors that use the catalysts described herein to convert ethylene to ethylene oxide can be of various types, including fixed-bed tubular reactors, continuous stirred tank reactors (CSTRs), and fluidized-bed reactors. A wide variety of reactors are well known to those skilled in the art and need not be described in detail herein.

[0138] Oxygen can be supplied to the reaction in an oxygen-containing stream, such as air, or in the form of commercial oxygen or oxygen-enriched air. The oxygen concentration in the reactor feed stream can vary over a wide range, and in practice, flammability is often the limiting factor for oxygen concentration. Typically, the oxygen concentration in the reactor feed is at least one (1) mole percentage, preferably at least two (2) mole percentage, and more preferably at least four (4) mole percentage. The oxygen concentration typically does not exceed fifteen (15) mole percentage, preferably not more than twelve (12) mole percentage, and even more preferably not more than nine (9) mole percentage.

[0139] The concentration of ethylene in the reactor feed stream can vary over a wide range. However, it is preferably at least eighteen (18) molar percentages, more preferably at least twenty (20) molar percentages. The concentration of ethylene in the reactor feed stream is preferably no more than 50 molar percentages, more preferably no more than 40 molar percentages.

[0140] Typically, epoxidation reactions are carried out at temperatures ranging from approximately 180°C to approximately 315°C, with reactor pressures ranging from approximately atmospheric pressure to approximately 35 bar. The gas hourly space velocity (GHSV) can be greater than approximately 3,000 h⁻¹. -1 Residence time in reactors containing large-scale catalysts is typically from about 0.5 seconds to about 2.5 seconds. The epoxidation process conditions described above are generally employed, depending on the desired mass rate and productivity.

[0141] Example

[0142] The following examples further illustrate the invention in detail, but should not be construed as limiting the scope of the invention.

[0143] Example 1 and Comparison Example A

[0144] These examples illustrate the effect of centrifugation on the average particle size of Ag on the support material. Two non-promoted silver-based supported catalysts (catalyst 1 [Comparative Example A] and catalyst 2 [Example 1]) were synthesized on a support containing bimodal α-alumina produced by Saint-Gobain NorPro (“support A” in this paper). The characteristics of support A are described in Table I.

[0145] Table I – Texture Features of Carrier A

[0146] <![CDATA[Total pore volume, cm 3 g -1 > 0.55 Average pore size, μm 1μm / 40μm

[0147] Catalyst 1 (Comparative Example A) was synthesized using conventional methods according to the following general steps:

[0148] (1) Place the granules in an impregnation cylindrical container and apply a vacuum at 29 mmHg for 15 minutes using a mechanical pump;

[0149] (2) Vacuum impregnate the granules of step (1) with a standard concentration of 25 wt% to 30 wt% Ag oxalate / amine mixture solution to form the impregnated catalyst support; the support volume / solution volume ratio is 2 / 3; and the impregnation solution contains oxalic acid (16 wt%), ethylenediamine (16.3 wt%), monoethanolamine (6 wt%) and silver oxide (25 wt% to 30 wt%).

[0150] (3) Open the valve at the bottom of the impregnation container to drain excess impregnation solution, allowing the solution to drain for 15 minutes; and

[0151] (4) The impregnated and discharged granules from step (3) are roasted on a roasting belt at 500°C for 2.5 to 3 minutes to form a catalyst, with an air flow of 125 standard cubic feet per hour (scfh).

[0152] Catalyst 2 (Example 1) was synthesized using the same procedure as catalyst 1 as described above, except that a centrifugation step was performed after the discharge step (3) and before the calcination step (4). The centrifugation step was performed for 5 minutes using a Sorvall SH-3000 rotor with a radius of 182 mm (RCF approx. 204). To ensure that excess Ag impregnation solution remained separated from the catalyst particles, the container used for centrifuging the impregnated support contained porous or absorbent material at the bottom.

[0153] The pore volume is 0.55 cm³. 3 This carrier, at a concentration of / g, retains 0.57cm. 3 / g of impregnation solution, wherein approximately 0.24cm is removed by centrifugation. 3 / g.

[0154] Scanning electron microscope (SEM) fracture cross-sectional images representing inclusion and exclusion of, for example Figure 5a and Figure 5b The centrifugation steps shown are for catalysts prepared on a multi-peak support. For example, Figure 5a SEM micrographs of the larger pore pattern (uncentrifuged) of the catalyst in Comparative Example A are shown, while Figure 5b The same characteristics (centrifuged) of the catalyst in Example 1 are shown.

[0155] The Ag content of the uncentrifuged Ag-impregnated support material (Comparative Example A) and the centrifuged Ag-impregnated support material (Example 1) were 17.5 wt% and 11.2 wt%, respectively. The decrease in the percentage of Ag on the centrifuged sample (Catalyst 2) was consistent with the selective removal of the Ag-impregnating solution from the macropores and the external solution film of the particles, where capillary forces were weaker than centrifugal induction forces.

[0156] The Ag morphology and particle size of catalysts 1 and 2 were evaluated by SEM and displayed separately. Figure 5a and Figure 5b In this study, centrifugation to remove Ag from macropores using an impregnating solution eliminated Ag aggregation on the macropore surface and reduced the overall average particle size of silver.

[0157] Example 2 and Comparison Example B

[0158] This example illustrates the effect of centrifugation on the average particle size of Ag on a multimodal support. Two non-promoted silver-based supported catalysts (catalyst 3 [Comparative Example B] and catalyst 4 [Example 2]) were synthesized on support B. Support B is a bimodal α-alumina support produced by Saint-Gobain NorPro. The characteristics of support B are described in Table II.

[0159] Table II – Texture Features of Carrier B

[0160] <![CDATA[Total pore volume V p , cm³ / g -1 > 0.52 Average pore size, μm 1μm / 40μm

[0161] Catalyst 3 was synthesized using conventional methods according to the following general steps:

[0162] (1) Place the carrier granules in an impregnated cylindrical container and apply a vacuum at 29 mmHg for 15 minutes using a mechanical pump;

[0163] (2) Vacuum impregnate the granules of step (1) with a standard concentration of 25 wt% to 30 wt% Ag oxalate / amine mixture solution to form the impregnated catalyst support; the support volume / solution volume ratio is 2 / 3; and the impregnation solution contains oxalic acid (16 wt%), ethylenediamine (16.3 wt%), monoethanolamine (6 wt%) and silver oxide (25 wt% to 30 wt%).

[0164] (3) Open the valve at the bottom of the impregnation container to drain excess impregnation solution, allowing the solution to drain for 15 minutes; and

[0165] (4) The impregnated and discharged granules from step (3) are roasted on a roasting belt at 500°C for 2.5 to 3 minutes to form a catalyst, with an air flow of 125 scfh.

[0166] Catalyst 4 was synthesized using the same procedure as catalyst 3, except that a centrifugation step was performed after the first impregnation step (2). Centrifugation was performed using a Sorvall ST-16 centrifuge. The rotor inside the centrifuge was rated to speed 5,000 rpm and had a rotor radius of 16.2 cm. Centrifugation analysis was performed at 5,000 rpm (RCF approximately 4,536) for 8 minutes. To ensure that excess Ag impregnation solution remained separated from the catalyst particles, the container used for centrifuging the impregnated support contained porous or absorbent material at the bottom.

[0167] The dispersion of Ag on catalysts 3 and 4 was evaluated by CO pulse chemisorption and SEM.

[0168] Prior to CO chemisorption, catalysts 3 (Comparative Example B) and 4 (Example 2) underwent a degassing step at 170 °C for 1 hour, followed by pretreatment at 170 °C for 1 hour with 10% (vol%) O2 / He to form a subsurface oxygen layer on the silver particles. Subsequently, the subsurface oxygen was titrated with CO pulses at 170 °C until no further CO2 formation was detected. Each CO / CO2 pulse was quantified using a mass spectrometer located downstream of the reactor.

[0169] SEM fracture cross-sectional images of catalysts 3 and 4 are shown Figure 6a and Figure 6b From Figure 6a and Figure 6b It can be seen that the centrifugation step eliminates the formation of large Ag aggregates around the macropore region.

[0170] The average equivalent circle diameters calculated from the distribution of at least 250 particles in the pore regions of catalysts 3 and 4 are shown below. Figure 7a and 7b And as shown in Table III. The average particle sizes of catalyst 3 and catalyst 4 are approximately 140 nm and 100 nm, respectively.

[0171] The average particle size of silver in centrifuged samples obtained by CO chemisorption measurements was very comparable to that obtained by microscopy; both techniques demonstrated that centrifugation reduced the Ag particle size (resulting in a higher Ag surface area and more efficient use of Ag for a given weight of Ag). These results are summarized in Table III. The Ag content of the uncentrifuged Ag-impregnated support material and the centrifuged Ag-impregnated support material was 17.4 wt% and 10.2 wt%, respectively.

[0172] Table III – Ag loading and average Ag particle size of the catalyst.

[0173]

[0174] Example 3 and Comparison Example C

[0175] This example illustrates the effect of centrifugation on the average particle size of Ag on a multi-peaked support. Two non-promoted silver-based supported catalysts, catalyst 5 (comparative example C) and catalyst 6 (example 3), were synthesized on support B.

[0176] Catalyst 5 (Comparative Example C) was synthesized using conventional methods according to the following general steps:

[0177] (1) Place the carrier granules in an impregnated cylindrical container and apply a vacuum at 29 mmHg for 15 minutes using a mechanical pump;

[0178] (2) Vacuum impregnate the granules of step (1) with a standard concentration of 25 wt% to 30 wt% Ag oxalate / amine mixture solution to form the impregnated catalyst support; the support volume / solution volume ratio is 2 / 3; and the impregnation solution contains oxalic acid (16 wt%), ethylenediamine (16.3 wt%), monoethanolamine (6 wt%) and silver oxide (25 wt% to 30 wt%).

[0179] (3) Open the valve at the bottom of the immersion container to drain the excess immersion solution and let the solution drain for 15 minutes;

[0180] (4) The impregnated and discharged granules from step (3) are roasted on a roasting belt at 500°C for 2.5 to 3 minutes to form a catalyst, with an air flow of 125 scfh.

[0181] (5) Vacuum impregnate the granules of step (4) with a standard concentration of 25 wt% Ag oxalate / amine mixture solution; the carrier volume / solution volume ratio is 2 / 3; and the impregnation solution contains oxalic acid (16 wt%), ethylenediamine (16.3 wt%), monoethanolamine (6 wt%) and silver oxide (25 wt% to 30 wt%).

[0182] (6) Open the valve at the bottom of the impregnation container to drain excess impregnation solution, allowing the solution to drain for 15 minutes; and

[0183] (7) The impregnated and discharged granules from step (6) are roasted on a roasting belt at 500°C for 2.5 to 3 minutes to form a catalyst, with an air flow of 125 scfh.

[0184] Catalyst 6 (Example 3) was synthesized using the same procedure as catalyst 5 described above, except that a first centrifugation step was performed after the first impregnation step (2), and a second centrifugation step was performed after the second impregnation step (5). Each centrifugation step was performed for 8 minutes at 5,000 rpm (approximately 4,536 RCF).

[0185] The average particle size of Ag on catalysts 5 and 6 was assessed by CO pulse chemisorption and SEM.

[0186] Prior to CO chemisorption, the catalyst underwent a degassing step at 170 °C for 1 hour, followed by pretreatment at 170 °C for 1 hour with 10 vol% O2 / He to form a subsurface oxygen layer on the silver particles. Subsequently, the subsurface oxygen was titrated with CO pulses at 170 °C until no further CO2 formation was detected. Each CO / CO2 pulse was quantified using a mass spectrometer located downstream of the reactor.

[0187] SEM fracture cross-sectional images of the larger pore regions in catalysts 5 and 6 are shown below. Figure 8a and Figure 8b From. Figure 8a and Figure 8b It can be seen that the centrifugation step increases the dispersion of Ag in the larger pore regions and eliminates the formation of large Ag aggregates.

[0188] Figure 9a and Figure 9b SEM images of the smaller diameter pore pattern are shown. The average particle size obtained from at least 230 particles indicates that the average particle size of silver in the centrifuged catalyst is reduced in the smaller pores (244 nm and 140 nm for catalysts 5 and 6, respectively, see Table IV).

[0189] Table IV – Ag loading and average Ag particle size of the catalyst

[0190]

[0191] Table IV summarizes the average particle size and Ag content of Ag obtained in Comparative Examples D and 4. The Ag content of the uncentrifuged Ag-impregnated support material and the centrifuged Ag-impregnated support material were 29.35 wt% and 14.79 wt%, respectively. Catalyst 5 was not very homogeneous and contained large Ag agglomerates (400 nm), such as... Figure 8a As shown. Both techniques indicate that the Ag particle size on catalyst 6 (Example 4) is significantly smaller than the Ag particle size on catalyst 5 (Comparative Example C).

[0192] Example 4 and Comparative Example D: Catalytic Results

[0193] In this example, the effect of centrifugation on catalytic performance was evaluated. Two non-promoting silver-based supported catalysts, catalyst 3 (comparative example B) and catalyst 4 (example 2), were synthesized on support B.

[0194] Catalytic testing was conducted on a High Pressure Reactor Module (HPRAM) II system provided by the Norwegian Institute of Technology's Foundation for Scientific and Industrial Research. The reactor feed gas contained ethylene (35 mol%), ethane (0.29 mol% to 0.58 mol%), oxygen (7.2 mol%), CO2 (1.4 mol%), chloroethane (1.3 ppm to 5.0 ppm by volume), and methane (10 mol%), with the balance being helium. The reactor temperature was maintained at 240 °C, while the reactor pressure was maintained at 11 bar.

[0195] During each run, 100 mg of catalyst (30 / 50 mesh) without inert diluent was added to the reactor. The reactor was heated under a helium flow, and all feed gases except oxygen were introduced. Oxygen was introduced approximately 2 to 3 minutes after the other feed gases were introduced into the reactor. The target gas hourly space velocity (GHSV) for each catalyst was 10,000 h⁻¹. -1 The concentration was calculated based on the actual weight of the powder and the bulk density of the intact catalyst particles (0.761 g / ml). During the test, a certain amount of chloroethane was added to the gas phase to increase the selectivity for epoxides. The catalytic chlorination efficiency (Z*) of the method was defined as follows:

[0196]

[0197] Where ECl is the ppmv concentration of ethyl chloride, and C2H6 and C2H4 are the molar percentage concentrations of ethane and ethylene in the reactor feed stream, respectively.

[0198] After a 29-hour activation period, Z* was used to traverse the numerical range for each catalyst, and the ethylene oxide concentration at the reactor outlet was measured multiple times at each Z* value. The activation period was conducted at 245°C, with Z* changed to 4 and held for 4 hours, then changed to 6.0 and held for 24 hours, and then back to 4 and held for 4 hours. After the break-in period, the reaction temperature was lowered to 240°C, and Z* was adjusted to 4.0, 6, and 8.0, each held for 36 hours. During this period of each run, the reaction temperature, gas hourly space velocity, and feed gas composition were otherwise kept constant.

[0199] The catalytic performance of catalysts 3 and 4 is summarized in Table V with Z* of 8.0. Figure 10 Catalytic activity is expressed as silver time yield (STY) and defined as the number of moles of EO produced per second and per gram of silver.

[0200] As can be observed in Table V, the inherent activity of Ag on catalyst 4 is 1.7 times that on catalyst 3, demonstrating the advantage of centrifugation in improving epoxidation performance. Furthermore, the selectivity of these catalysts is very similar.

[0201] Table V – Catalysts 3 and 4 at 240°C and 11 bar in 35 vol% C2H4, 7.0 vol% O2 and Catalytic performance in the epoxidation reaction of ethylene with 1.5 vol% CO2 of ethylene and ethylene oxide

[0202]

[0203] ΔEO represents the outlet concentration of ethylene oxide, in mol%.

[0204] S EO The selectivity for ethylene oxide (EO) is expressed and calculated using the following expression:

[0205] .

Claims

1. A method for preparing a silver-containing catalyst for epoxidizing olefins, comprising the following steps: (a) Provide a porous multi-peak carrier having at least a first set of carrier pores in a first pore size range and at least a second set of carrier pores in a second pore size range; (b) Providing a first silver-containing impregnation solution to impregnate the first silver-containing impregnation solution into the at least first set of carrier pores and the at least second set of carrier pores of the porous multi-peak carrier; (c) Impregnating the porous multi-peaked support once or multiple times with the first silver-containing impregnation solution from step (b) to provide the porous multi-peaked support with a first amount of silver-containing impregnation solution; and (d) Selectively removing at least a portion of the impregnated first silver-containing impregnation solution from the porous multi-peaked carrier to provide the porous multi-peaked carrier with a second amount of the first silver-containing impregnation solution retained in the carrier pores; wherein at least a portion of the impregnated first silver-containing impregnation solution is removed from the at least first group of carrier pores of the porous multi-peaked carrier by subjecting the impregnated porous multi-peaked carrier to a removal device once or more after impregnation step (c) for a duration sufficient to selectively remove at least a portion of the impregnated first silver-containing impregnation solution contained in the at least first group of carrier pores of the porous multi-peaked carrier, wherein the removal device is a centrifuge. The selective removal step (d) removes at least 20% of the impregnated first silver-containing impregnation solution contained in at least the first group of carrier pores of the porous multi-peak carrier. The first group of pores in the carrier has a first pore diameter ranging from 3 micrometers to 200 micrometers; and the second group of pores in the carrier has a second pore diameter ranging from 0.01 micrometers to 3 micrometers, and the second pore diameter range of the second group of pores in the carrier is smaller than the first pore diameter range of the first group of pores in the carrier; and The relative centrifugal force (RCF) used in step (d) is calculated as follows: ， Where R c It is the radius of the centrifuge, in mm, and rpm is the rotational speed in revolutions per minute, and RCF is in the range of 0.1 to 1,500,000.

2. The method according to claim 1, comprising step (e): calcining once or multiple times at a time and temperature sufficient to form a silver-containing catalyst for epoxidizing olefins, the impregnated porous multi-peaked support produced in step (d).

3. The method of claim 1, wherein the selective removal step (d) is performed once or multiple times by one or more centrifuges.

4. The method of claim 1, wherein the impregnation step (c) is performed two or more times; or wherein the removal step (d) is performed two or more times and is performed under two or more different process conditions.

5. The method of claim 3, wherein the impregnation step (c) is performed two or more times prior to the selective removal step (d).

6. The method of claim 2, wherein the selective removal step (d) is performed two or more times before the calcination step (e), or wherein the calcination step (e) is performed two or more times.

7. The method of claim 1, wherein the first silver-containing impregnation solution further comprises one or more accelerators.

8. The method according to claim 1, wherein the carrier comprises α-alumina.

9. The method of claim 1, wherein at least 10% by weight of silver loading is provided on the porous multi-peaked carrier based on the total weight of the porous multi-peaked carrier.

10. The method of claim 2, wherein the method comprises the following steps in sequence: (f) Providing a second silver-containing impregnation solution to impregnate the second silver-containing impregnation solution into the at least first set of carrier pores and the at least second set of carrier pores of the porous multi-peak carrier; (g) Impregnate the porous multi-peaked carrier once or multiple times with at least a portion of the second silver-containing impregnation solution from step (f); (h) The impregnated second silver-containing impregnation solution is selectively removed from the at least first set of carrier pores of the porous multi-peak carrier once or multiple times by subjecting the impregnated porous multi-peak carrier to the removal device at least once after the impregnation step (g) for a duration sufficient to selectively remove a portion of the impregnated second silver-containing impregnation solution contained in the at least first set of carrier pores of the porous multi-peak carrier; (i) The impregnated porous multi-peak support from step (h) is calcined once or multiple times at a time and temperature sufficient to form a silver-containing catalyst that can be used to epoxidize olefins. (j) A third silver-containing impregnation solution is provided to impregnate the third silver-containing impregnation solution into the at least first set of carrier pores and the at least second set of carrier pores of the porous multi-peak carrier; (k) Impregnate the silver-containing catalyst from step (i) once or multiple times with at least a portion of the third silver-containing impregnation solution from step (j); (l) Selectively removing the impregnated third impregnation solution from the at least first set of carrier pores of the impregnated and calcined porous multi-peaked carrier from step (i) once or more by subjecting the impregnated and calcined porous multi-peaked carrier to a removal device once or more after impregnation step (k) for a duration sufficient to selectively remove at least a portion of the impregnated third silver-containing impregnation solution contained in the at least first set of carrier pores of the impregnated and calcined porous multi-peaked carrier; and (m) The impregnated porous multi-peak support from step (l) is calcined once or multiple times at a time and temperature sufficient to form a silver-containing catalyst that can be used to epoxidize olefins.

11. A silver-containing epoxidation catalyst for oxidizing ethylene to ethylene oxide, wherein the silver-containing epoxidation catalyst is prepared according to the method of claim 1; wherein the catalyst comprises at least one catalytic species deposited on a porous support; wherein the at least one catalytic species is silver.

12. A method for epoxidizing ethylene to ethylene oxide, comprising contacting ethylene and oxygen in the presence of a silver-containing epoxidation catalyst prepared by the method according to claim 1.

13. A method for preparing a silver-containing catalyst for epoxidizing olefins, comprising the following steps: (a) Provide a porous multi-peak carrier with at least two pore size distribution modes; (b) Providing a first silver-containing impregnation solution to impregnate the first silver-containing impregnation solution into the pores of the porous multi-peak carrier; (c) Impregnate the porous multi-peaked support with the first silver-containing impregnation solution from step (b) to provide the porous multi-peaked support with a first amount of the first silver-containing impregnation solution; (d) Centrifuge the porous multi-peaked carrier to impregnate it with the first silver-containing impregnation solution so that the porous multi-peaked carrier is provided with a second amount of the first silver-containing impregnation solution retained in the carrier pores; (e) Calcination of the impregnated porous multi-peaked carrier from step (d); (f) Providing a second silver-containing impregnation solution to impregnate the second silver-containing impregnation solution into the pores of the porous multi-peak carrier; (g) Impregnate the porous multi-peaked support with the second silver-containing impregnation solution from step (f) to provide the porous multi-peaked support with a first amount of the second silver-containing impregnation solution; (h) Centrifuging the porous multi-peaked carrier from step (g) to impregnate it with a second silver-containing impregnation solution so that the porous multi-peaked carrier is provided with a second amount of the second silver-containing impregnation solution retained in the pores of the carrier; and (i) Calcining the impregnated porous multi-peaked support from step (h) at a time and temperature sufficient to form a silver-containing catalyst that can be used to epoxidize olefins; Centrifugation steps (d) and (h) were performed under different centrifugation conditions. The selective removal step (d) removes at least 20% of the first silver-containing impregnation solution contained in at least the first group of pores of the porous multi-peaked carrier. The first group of pores in the carrier has a first pore diameter ranging from 3 micrometers to 200 micrometers; and the second group of pores in the carrier has a second pore diameter ranging from 0.01 micrometers to 3 micrometers, wherein the second pore diameter range of the second group of pores in the carrier is smaller than the first pore diameter range of the first group of pores in the carrier. The relative centrifugal force (RCF) used in step (d) is calculated as follows: ， Where R c It is the radius of the centrifuge, in mm, and rpm is the rotational speed in revolutions per minute, and RCF is in the range of 0.1 to 1,500,000.

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