WO2021038027A1

WO2021038027A1 - Process for preparing an epoxidation catalyst

Info

Publication number: WO2021038027A1
Application number: PCT/EP2020/074057
Authority: WO
Inventors: Andrey Karpov; Wolfgang Ruettinger; Christian Walsdorff; Christian Bartosch; Mauricio Grobys
Original assignee: Basf Se
Priority date: 2019-08-28
Filing date: 2020-08-28
Publication date: 2021-03-04

Abstract

A process for preparing a catalyst for producing ethylene oxide by gas-phase oxidation of ethylene comprises i) impregnating a porous alumina support having a packed tube density in the range of 100 to 450 g/L; a Log differential pore volume distribution curve, 5 as measured by mercury porosimetry, having at least one peak in a pore diameter range of 0.01 to 5.0 µm; and a BET surface area in the range of 1.5 to 30.0 m2/g; with a silver impregnation solution, preferably under reduced pressure; and optionally subjecting the impregnated porous alumina support to drying; and ii) subjecting the impregnated porous alumina support to a calcination process; wherein steps i) and ii) are optionally repeated, 10 to yield a catalyst comprising at least 25 wt.-% of silver, relative to the total weight of the catalyst. The catalyst shows high activity and selectivity. The invention further relates to a catalyst for producing ethylene oxide by gas-phase oxidation of ethylene, obtainable by a process as defined above. The invention also relates to a catalyst for producing ethylene oxide by gas-phase oxidation of ethylene, comprising at least 25 wt.-% of silver, 15 relative to the total weight of the catalyst, deposited on a porous alumina support, the support having i) a packed tube density in the range of 100 to 450 g/L, preferably 150 to 450 g/L, more preferably 200 to 400 g/L; ii) a Log differential pore volume distribution curve, as measured by mercury porosimetry, having at least one peak in a pore diameter range of 0.01 to 5.0 µm; and iii) a BET surface area in the range of 1.5 to 30.0 m2/g, 20 preferably 1.5 to 20.0 m2/g, more preferably 2.0 to 10.0 m2/g, most preferably 2.5 to 8.0 m2/g, in particular 3.0 to 7.0 m2/g. The invention further relates to a process for producing ethylene oxide by gas-phase oxidation of ethylene, comprising reacting ethylene and oxygen in the presence of a catalyst as defined above.

Description

Process for preparing an epoxidation catalyst Description The present invention relates to a process for preparing a catalyst effective in the oxidative conversion of ethylene to ethylene oxide, a catalyst obtained by the process, and a process for preparing ethylene oxide by gas-phase oxidation of ethylene by means of oxygen in the presence of the catalyst. Ethylene oxide is produced in large volumes and is primarily used as an intermediate in the production of several industrial chemicals. In the industrial oxidation of ethylene to ethylene oxide, heterogeneous catalysts comprising silver are used. To carry out the heterogeneously catalyzed gas-phase oxidation, a mixture of an oxygen-comprising gas, such as air or pure oxygen, and ethylene is generally passed through a plurality of tubes which are arranged in a reactor in which a packing of shaped catalyst bodies is present.

Catalyst performance is characterized by selectivity, activity, longevity of catalyst activity, and mechanical stability. Moreover, the performance in the reactor tubes is characterized by the packing density of the catalyst in the volume of the tubes and pressure drop across the catalyst bed. Selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. Even small improvements in selectivity and the maintenance of selectivity over longer time yield huge dividends in terms of process efficiency.

In an effort to optimize these various properties several approaches have been suggested, including variation of support pore BET surface area, support pore volume, support pore size distribution and support pore architecture.

US 9,776,169 describes a porous body comprising at least 80 percent alpha alumina and having a pore volume from 0.3 mL/g to 1.2 mL/g, a surface area from 0.3 to m²/g to 3.0 m²/g, and a pore architecture that provides at least one of a tortuosity of 7 or less, a constriction of 4 or less and a permeability of 30 mdarcys or greater.

In “Fundamentals and Applications of Structured Ceramic Foam Catalysts”, Ind. Eng. Chem. Res. 2007, 46, 4166 f., the use of foamed silver-based catalysts in the production of ethylene epoxidation is mentioned. The invention relates to a process for preparing a catalyst for producing ethylene oxide by gas-phase oxidation of ethylene, comprising i) impregnating a porous alumina support having a packed tube density in the range of 100 to 450 g/L, preferably 150 to 450 g/L, more preferably 200 to 400 g/L; a Log differential pore volume distribution curve, as measured by mercury porosimetry, having at least one peak in a pore diameter range of 0.01 to 5.0 pm; and a BET surface area in the range of 1 .5 to 30.0 m²/g, preferably 1 .5 to 20.0 m²/g, more preferably 2.0 to 10.0 m²/g, most preferably 2.5 to 8.0 m²/g, in particular 3.0 to 7.0 m²/g; with a silver impregnation solution, preferably under reduced pressure; and optionally subjecting the impregnated porous alumina support to drying; and ii) subjecting the impregnated porous alumina support to a calcination process; wherein steps i) and ii) are optionally repeated, to yield a catalyst comprising at least 25 wt.-% of silver, relative to the total weight of the catalyst.

The invention also relates to a catalyst obtainable by the process described above.

The alumina support has a BET surface area in the range of 1 .5 to 30.0 m²/g, preferably 1.5 to 20.0 m²/g, more preferably 2.0 to 10.0 m²/g, most preferably 2.5 to 8.0 m²/g, in particular 3.0 to 7.0 m²/g. The BET method is a standard, well-known method and widely used in surface science for the measurement of surface areas of solids by physical adsorption of gas molecules. The BET surface is determined according to DIN ISO 9277 herein, unless stated otherwise.

Without wishing to be bound by theory, it is believed that a BET surface area in the range defined above allows for the deposition of high silver amounts on the support while avoiding confluence of the silver particles on the surface of the support. Hence, a better dispersion of silver particles is achieved, and more active sites are maintained during catalyst operation.

Mercury porosimetry is used to measure the porosity of a material by applying controlled pressure to a sample immersed in mercury. External pressure is required for mercury to penetrate into the pores of a material due to high contact angle of mercury. The amount of pressure required to intrude into the pores is inversely proportional to the size of the pores. The larger the pore the smaller the pressure needed to penetrate into the pore. The mercury porosimeter generates volume and pore size distributions from the pressure versus intrusion data generated by the instrument using the Washburn equation. The pore diameter distribution can be determined by gradually changing the pressure, measuring the volume of mercury having penetrated into the pores at that time, that is, measuring the pore volume V, and drawing the relationship between the micropore diameter D calculated according to the above-mentioned equation and the micropore volume. For the purposes herein, the vertical axis is the Log differential coefficient dV/d(Log D) of the relational curve and the horizontal axis is the pore diameter D. This graph is referred to as a Log differential pore volume distribution curve. Mercury porosimetry may be performed using a Micrometries AutoPore IV 9500 mercury porosimeter (140 degrees contact angle, 485 dynes/cm Hg surface tension, 60000 psia max head pressure). The Hg porosity is determined according to DIN 66133 herein, unless stated otherwise.

According to the invention, the Log differential pore volume distribution curve of the porous alumina support has at least one peak in a pore diameter range of 0.01 to 5.0 pm. A pore diameter in that range is suitable from the viewpoint of substance mobility that a reactant penetrates into the depth of the catalyst structure and reacts, and the reaction product moves toward the outside of the catalyst structure.

In one embodiment, the value of the vertical axis in the peak having a pore diameter in a pore diameter range of 0.01 to 5.0 pm, that is, the value of Log differential coefficient dV/d(Log D) at the peak, is 0.3 mL/g or more. A higher value indicates that many pores having a predetermined size region exist inside the structure, and indicates that the structure has voids suitable for reaction. From the viewpoint of mobility of the reactants and reaction product, Log differential pore volume at the peak is preferably 0.5 mL/g or more, more preferably 1.0 mL/g or more, or 2.0 mL/g or more, such as 3.0 mL/g or more, or 5.0 mL/g or more. From the viewpoint of the compatibility with the amount of the silver metal carried per unit volume of the catalyst structure, the Log differential pore volume at the maximum peak is preferably 12 mL/g or less. In one embodiment, the porous alumina support has a total pore volume of more than 1.0 mL/g, preferably more than 1.3 mL/g, as measured by mercury porosimetry. The inventive alumina support is preferably in the form of individual shaped bodies. The size and shape of the individual shaped bodies and thus of the catalyst is selected to allow a suitable packing of the shaped bodies in a reactor tube. The shaped bodies suitable for the process of the invention are preferably used in reactor tubes with a length from 6 to 14 m and an inner diameter from 20 mm to 50 mm. In general, the support is comprised of individual bodies having a maximum extension in the range of 3 to 20 mm, such as 4 to 15 mm, in particular 5 to 12 mm. The maximum extension is understood to mean the longest straight line between two points on the outer circumference of the support.

According to the invention, the support has a packed tube density in the range of 100 to 450 g/L, preferably 150 to 450, more preferably 200 to 400 g/L. The packed tube density is understood to be the density per liter of a cylindrical tube with an inner diameter of 39 mm packed with support. The packed tube density may be determined according to the method described below.

In order to meet the above requirements, the porous alumina support has a foam-like structure with a plurality of cellular pores partitioned by cellular walls and intergranular pores formed in the cellular walls. It is considered that the intergranular pores formed in the cellular walls are assignable to the Log differential pore volume distribution peak in a pore diameter range of 0.01 to 5.0 pm; and the cellular pores are assignable to a Log differential pore volume distribution peak in a pore diameter range of more than 6.0 to 300 pm. Typically, the cellular pore diameter at the peak is 2 to 1000 times greater than the intergranular pore diameter at the peak. In one embodiment, 30 to 99% of the total pore volume, as measured by mercury porosimetry, is provided by intergranular pores.

The pore diameter of pores, in particular the cellular pores, may also be determined by scanning electron microscopy. In one embodiment, the cellular pores of the alumina support have pore diameters in the range of 10 to 500 pm, preferably 10 to 300 pm, more preferably 10 to 200 pm, as determined by scanning electron microscopy.

The basic structure of an alumina foam is composed of solid walls and the cellular pores surrounded by them. If the alumina phase surrounds entire cells so that each cell is isolated from its adjacent ones, it is called as closed cell structure. If all cells are connected to each other with ceramic phase only in cell edges, it is called an open structure. In fact, alumina foams often exist in a semi-open structure between the two ideal structures. A common method for manufacturing alumina foams is the polymeric sponge replication method, with the products sometimes designated as reticulated porous ceramics. In this method, a polymeric sponge with open pores is immersed in an alumina slurry, and after removing redundant slurry, the coated sponge is dried and pyrolyzed, leaving only the porous alumina structure. Subsequently, the resultant foam is sintered for final densification to get required mechanical strength.

Another technique for producing alumina foams is the direct foaming method. Alumina foams are produced by incorporating air into a suspension or liquid media, which is subsequently set in order to keep the structure of air bubbles created. In one embodiment, direct foaming occurs in a continuous process, e.g., in a device shown in Fig. 1 of “Characteristics of a Continuous Direct Foaming Technique”, Luthardt et al., Int. J. Appl. Ceram. Technol. 2015, 12, S3, E133 to E138. Subsequently, the consolidated foams are sintered at high temperature to obtain high-strength foams. This method can result in full dense struts without defects as obtained by the polymeric sponge replication method. Flence, the mechanical strengths of the products are generally higher than those of reticulated porous ceramics. Characteristically, foams obtained by this technique have a closed or semi-open cell structure, depending on the air bubbles incorporated.

The foaming of alumina slurries involves dispersing gas in the form of bubbles into an alumina suspension. There are two basic approaches for achieving this: (1 ) incorporating an external gas by mechanical frothing, or injection of a gas stream and (2) formation of a gas in situ. In order to stabilize the bubbles developed within the slurry, the surface tension of the gas-liquid interface needs to be reduced, typically by adding surfactant, or sometimes by adding hydrophobic particles. In some cases, water-soluble polymers are added into the slurry to modify the viscosity, which will affect the foaming results and the stability.

The foam is essentially a metastable system, with some bubbles shrinking and others gathering. It is important to consolidate the foams within a certain period of time, so as to preserve the cellular structure during further heating. Freezing is one of the practical methods to consolidate the foamed slurry. The foamed slurries are poured into molds and cooled using liquid nitrogen for instant freezing of the porous structure. Some natural polymers from animal and plant sources have the properties of liquid-solid transition due to denaturation which has potential applications in the consolidation of foamed slurries. The consolidated wet foams are a mixture of gas, liquid, and solid, which need to be dried before sintering to obtain the final ceramic foams.

Suitable methods for the manufacture of alumina foams are described in “Fabricating Porous Ceramics”, Porous Materials 2014, Chapter 5, p. 221 f., “Processing and Properties of Advanced Porous Ceramics: An Application Based Review”, Ceramics International 2014, 40, p. 15351 to 15370, “Alumina Ceramic Foams as Catalyst Supports”, Catalysis, 2016, 28, p. 28 to 50, “Porosity and Cell Size Control in Alumina Foam Preparation by Thermo-Foaming of Powder Dispersions in Molten Sucrose”, Journal of Asian Ceramic Societies, 2016, 4, 344 to 350, and “Fundamentals and Applications of Structured Ceramic Foam Catalysts”, Ind. Eng. Chem. Res. 2007, 46, 4166 f. Further general information on foams and their manufacture may be found in

“Characteristics of a Continuous Direct Foaming Technique”, Luthardt et al., Int. J. Appl. Ceram. Technol. 2015, 12, S3, E133 to E138 and “Hybrid Foams - A New Approach for Multifunctional Application”, Reinfried et al., Advanced Engineering Materials 2011, 13, No. 11, 1031 to 1036.

The alumina foams useful in the present invention have intergranular porosity rather than dense cell walls. Intergranular porosity develops the available surface area at the walls of the cellular pores. In order to provide a suitable concentration of intergranular pores in the porous alumina support, particulate pore-forming agents may be utilized. For example, combustible or volatile pore-forming agents are added to a suspension or liquid media, for example a ceramic slurry, which is typically mechanically frothed as described above so as to entrain a gas and subsequently set. Afterwards, the consolidated mixture is sintered at high temperature, whereby the pore-forming agents are burned out or volatilized, so as to form a porous ceramic. The porosity and pore size of the sintered bodies are typically determined by the type and amount of pore-forming agent. The pore forming agents suitably have an average particle size in the range of 0.1 to 4 pm, preferably 0.5 to 3.0 pm.

Suitable pore-forming agents include materials of which low amounts remain after calcination at a temperature of greater than 400 °C, such as less than 0.5 wt.-% based on the weight of the pore-forming agent. Examples of suitable pore-forming agents include carbon materials, such as graphite and carbon black, and organic materials consisting exclusively of carbon, oxygen, nitrogen and/or hydrogen, such as maleic acid or starch. Preferably, the pore-forming agents have a high purity and thus are essentially free of sodium, potassium, iron and silicon. The term “essentially free” is understood to mean total amounts sodium, potassium, iron and silicon of less than 2000 ppm per g of alumina support, preferably less than 1000 ppm per g of pore-forming agent. In cases where organic compounds are used as pore-forming agents, organic compounds are preferably burned off or volatilized under conditions suitable to avoid the formation of explosive gas mixtures, i.e. in the presence of nitrogen or lean air (having, e.g., less than 10 vol.-% of oxygen).

The shape of the support is not especially limited, and may be in any technically feasible form, depending, e.g., on the extrusion process. For example, the support may be a solid extrudate or a hollow extrudate, such as a hollow cylinder. In another embodiment, the support may be characterized by a multilobe structure. A multilobe structure is meant to denote a cylinder structure which has a plurality of void spaces, e.g., grooves or furrows, running in the cylinder periphery along the cylinder height. Generally, the void spaces are arranged essentially equidistantly around the circumference of the cylinder. Preferably, the support is in the shape of a solid extrudate, such as pellets or cylinders, or a hollow extrudate, such as a hollow cylinder.

The alumina foams may be shaped by extrusion through a deposition nozzle. In some embodiments, an array of deposition nozzles may be utilized for extruding the alumina foam, either simultaneously in parallel and/or sequentially in series. The extruded foam may be deposited on a substrate in a predetermined pattern. The deposition may be carried out in a controlled environment saturated with a vapor of a solvent. For instance, a mist of the solvent may be continuously sprayed onto the nozzle during deposition.

After deposition, the pattern may be subjected to heat and/or acid treatment (e.g., for drying, fugitive particle burnout/leaching, or sintering) to obtain a porous alumina support in the predetermined pattern. The heat or acid treatment may occur under varying conditions (i.e., temperature, duration, acid type) depending on the desired final result. For instance, drying may occur over a long or a short period of time ranging from about one hour or less to about one week or more, at a temperature ranging from 10 °C to about 150 °C. Fugitive particle burnout may occur over a long or a short duration ranging from about an hour or less to about 1 week or more, at a temperature ranging from about 100 °C to about 900 °C, or from about 200 °C to about 700 °C. Formation of sintered alumina foam may occur over a long or a short duration ranging from about an hour or less to about 1 week or more, at a temperature ranging from about 800 °C to about 2000 °C, preferably from 1100 °C to 1600 °C, more preferably from 1300 °C to 1550 °C.

Alternatively, the wet foam can be extruded through a piston extruder equipped with a regulate extrusion die of any suitable shape.

The extrusion of the foam may take place under an applied or injection pressure of from about 1 psi to about 1000 psi, from about 10 psi to about 500 psi, or from about 20 psi to about 100 psi. The pressure during the extrusion may be constant or varied. A variable pressure may yield extrudates having a diameter that varies along the length of the foam extrudate. The extrusion may be carried out at controlled ambient or room temperature conditions.

The porous alumina support usually comprises a high proportion of AI₂O₃, for example at least 50 percent by weight, for example at least 70 percent by weight, or at least 90 percent by weight, preferably at least 95 percent by weight, most preferably at least 97.5 percent by weight, based on the total weight of the support. Besides alumina, the porous alumina support may comprise other components, for example binders such as silicates, or other refractory oxides such as zirconia or titania.

The porous alumina support preferably does not have wash-coat particles or a wash- coat layer on its surface, so as to fully maintain the porosity and BET surface area of the uncoated support.

The catalyst comprises at least 25 wt.-% of silver, relative to the total weight of the catalyst. Preferably the catalyst has a content of at least 30 wt.-%, more preferably at least 35 wt.-%, relative to the total weight of catalyst.

For example, the catalyst may comprise 25 to 70 wt.-% of silver, relative to the total weight of the catalyst. Preferably, the catalyst comprises 30 to 60 wt.-% of silver, more preferably 35 to 50 wt.-% of silver, relative to the total weight of the catalyst. A silver content in this range allows for a favorable balance between turnover induced by each catalyst body and cost-efficiency of preparing the catalyst. Besides silver, the catalyst may comprise one or more promoting species. A promoting species denotes a component that provides an improvement in one or more of the catalytic properties of the catalyst when compared to a catalyst not containing said component. The promoting species can be any of those species known in the art that function to improve the catalytic properties of the silver catalyst. Examples of catalytic properties include operability (resistance to runaway), selectivity, activity, turnover and catalyst longevity.

The catalyst may comprise a promoting amount of a transition metal or a mixture of two or more transition metals. Suitable transition metals can include, for example, the elements from Groups NIB (scandium group), IVB (titanium group), VB (vanadium group), VIB (chromium group), VIIB (manganese group), VIIIB (iron, cobalt, nickel groups), IB (copper group), and I IB (zinc group) of the Periodic Table of the Elements, as well as combinations thereof. More typically, the transition metal is an early transition metal, i.e., from Groups NIB, IVB, VB or VIB, such as, for example, hafnium, yttrium, molybdenum, tungsten, rhenium, chromium, titanium, zirconium, vanadium, tantalum, niobium, or a combination thereof. In one embodiment, the transition metal promoter(s) is (are) present in a total amount from 150 ppm to 10000 ppm, typically 225 ppm to 7000 ppm, most typically from 300 ppm to 4000 ppm, expressed in terms of metal(s) relative to the total weight of the catalyst.

Of the transition metal promoters listed, rhenium (Re) is a particularly efficacious promoter for ethylene epoxidation high selectivity catalysts. The rhenium component in the catalyst can be in any suitable form, but is more typically one or more rhenium- containing compounds (e.g., a rhenium oxide) or complexes.

Preferably, the catalyst comprises 100 to 3000 ppm by weight of rhenium, relative to the total weight of the catalyst. It is preferred that the catalyst comprises 250 to 2000 ppm by weight of rhenium, more preferably 500 to 1500 ppm by weight of rhenium, relative to the total weight of the catalyst.

In some embodiments, the catalyst may include a promoting amount of an alkali metal or a mixture of two or more alkali metals. Suitable alkali metal promoters include, for example, lithium, sodium, potassium, rubidium, cesium or combinations thereof. The amount of alkali metal, e.g. potassium, will typically range from 50 ppm to 5000 ppm, more typically from 300 ppm to 2500 ppm, most typically from 500 ppm to 1500 ppm expressed in terms of the alkali metal relative to the total weight of the catalyst. The amount of alkali metal is determined by the amount of alkali metal contributed by the porous alumina support and the amount of alkali metal contributed by the impregnation solution described below. Combinations of heavy alkali metals like cesium (Cs) or rubidium (Rb) with light alkali metals like lithium (Li), sodium (Na) and potassium (K) are particularly preferred. Cesium is an especially preferred alkali metal promoter. Preferably, the catalyst comprises 100 to 2000 ppm by weight of cesium, relative to the total weight of the catalyst. It is preferred that the catalyst comprises 400 to 1750 ppm by weight of cesium, more preferably 600 to 1500 ppm by weight of cesium, relative to the total weight of the catalyst.

Preferably the catalyst contains at least two light alkali metals, selected from sodium, potassium and lithium. Most preferably the catalyst contains sodium, potassium and lithium. Preferably, the catalyst comprises 40 to 1170 ppm by weight of potassium, relative to the total weight of the catalyst. It is preferred that the catalyst comprises 100 to 1000 ppm by weight of potassium, most preferably 140 to 500 ppm by weight of potassium. The amount of potassium is determined by the amount of potassium contributed by the porous alumina support and the amount of potassium contributed by the impregnation solution described below.

Preferably, the catalyst comprises 100 to 2000 ppm by weight of lithium, relative to the total weight of the catalyst. It is preferred that the catalyst comprises 150 to 1500 ppm by weight of lithium, most preferably 300 to 1000 ppm by weight of lithium. The amount of lithium is determined by the amount of lithium contributed by the porous alumina support and the amount of lithium contributed by the impregnation solution described below.

Preferably, the catalyst comprises 10 to 1000 ppm by weight of sodium, relative to the total weight of the catalyst. It is preferred that the catalyst comprises 20 to 500 ppm by weight of sodium, most preferably 30 to 250 ppm by weight of sodium. The amount of sodium is determined by the amount of sodium contributed by the porous alumina support and the amount of sodium contributed by the impregnation solution described below.

The catalyst may also include a Group 11 A alkaline earth metal or a mixture of two or more Group IIA alkaline earth metals. Suitable alkaline earth metal promoters include, for example, beryllium, magnesium, calcium, strontium, and barium or combinations thereof. The amounts of alkaline earth metal promoters can be used in amounts similar to those used for the alkali or transition metal promoters. The catalyst may also include a promoting amount of a main group element or a mixture of two or more main group elements. Suitable main group elements include any of the elements in Groups IIIA (boron group) to VI I A (halogen group) of the Periodic Table of the Elements. For example, the catalyst can include a promoting amount of sulfur, phosphorus, boron, halogen (e.g., fluorine), gallium, or a combination thereof.

The catalyst may also include a promoting amount of a rare earth metal or a mixture of two or more rare earth metals. The rare earth metals include any of the elements having an atomic number of 57-103. Some examples of these elements include lanthanum (La), cerium (Ce), and samarium (Sm). The amount of rare earth metal promoters can be used in amounts similar to those used for the transition metal promoters.

In order to obtain a catalyst having high silver contents, steps i) and ii) can be repeated several times. In that case it is understood that the intermediate product obtained after the first (or subsequent up to the last but one) impregnation/calcination cycle comprises a part of the total amount of target Ag and / or promoter concentrations. The intermediate product is then again impregnated with the silver impregnation solution and calcined to yield the target Ag and / or promoter concentrations.

The aluminium oxide support may comprise impurities, such as sodium, potassium, iron, silica, magnesium, calcium, zirconium in an amount of 100 to 10000 ppm, based on the total weight of the support. Any known impregnation processes known in the art can be used. Examples of suitable impregnation processes are described, e.g., in WO2013/066557 or WO2018/044992. Preferably, impregnation is conducted at a pressure of less than 250 mbar, more preferably at a pressure of less than 100 mbar. Any calcination processes known in the art can be used. Suitable examples of calcination processes are described in US 5,504,052, US 5,646,087, US 7,553,795, US 8,378,129, US 8,546,297, US2014/0187417, EP 1893331 or W02012/140614. Further provided is a catalyst for producing ethylene oxide by gas-phase oxidation of ethylene, comprising at least 25 wt.-% of silver, relative to the total weight of the catalyst, deposited on a porous alumina support, the support having i) a packed tube density in the range of 100 to 450 g/L, preferably 150 to 450 g/L, more preferably 200 to 400 g/L; ii) a Log differential pore volume distribution curve, as measured by mercury porosimetry, having at least one peak in a pore diameter range of 0.01 to 5.0 pm; and iii) a BET surface area in the range of 1.5 to 30.0 m²/g, preferably 1.5 to 20.0 m²/g, more preferably 2.0 to 10.0 m²/g, most preferably 2.5 to 8.0 m²/g, in particular 3.0 to 7.0 m²/g.

It is understood that all embodiments of the process for preparing the catalyst described above also apply to the catalyst, where applicable. For example, the porous alumina support is preferably characterized and obtainable by the embodiments described above.

It should be noted that the packed tube densities (PTD) of the support and of the catalyst obtained from the support essentially differ by the weight of silver comprised in the catalyst. Hence, the following equations can be used to estimate the packed tube density (PTD) of the support if the PTD of the catalyst is known, and vice versa: wt. % Ag (catalyst)

Catalyst PTD = Carrier PTD x 1+ lOOwt. % — wt. % Ag(catalyst) wt % Ag (catalyst)

Carrier PTD = Catalyst PTD ÷ 1+ lOOwt. % — wt. % Ag(catalyst) It is considered that the presence of at least one peak in the pore diameter range of 0.01 to 5.0 pm is not affected by silver deposition. Further, it is considered that the BET surface area of the catalyst is to the great extent determined by the BET surface area of the support, with the BET surface of the catalyst generally being about 20% higher than the support from which it is prepared.

Further provided is a process for producing ethylene oxide by gas-phase oxidation of ethylene, comprising reacting ethylene and oxygen in the presence of a catalyst as discussed above. It is understood that all embodiments of the process for preparing the catalyst also apply to the process for producing ethylene oxide in the presence of the catalyst, where applicable. According to the invention, the epoxidation can be carried out by all processes known to those skilled in the art. It is possible to use all reactors which can be used in the ethylene oxide production processes of the prior art; for example externally cooled shell-and-tube reactors (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A-10, pp. 117-135, 123-125, VCH-Verlagsgesellschaft, Weinheim 1987) or reactors having a loose catalyst bed and cooling tubes, for example the reactors described in DE-A 3414717, EP 0082609 and EP-A 0339748.

The epoxidation is preferably carried out in at least one tube reactor, preferably in a shell- and-tube reactor. On a commercial scale, ethylene epoxidation is preferably carried out in a multi-tube reactor that contains several thousand tubes. The catalyst is filled into the tubes, which are placed in a shell that is filled with a coolant. In commercial applications, the internal tube diameter is typically in the range of 20 to 40 mm (see, e.g., US 4,921 ,681) or more than 40 mm (see, e.g., W02006/102189). In one embodiment, a reaction feed comprising ethylene and oxygen is subsequently reacted in at least two zones wherein the packed tube silver densities in the different zones differ from one another, the reaction feed first comes into contact with the zone having the lowest packed tube silver density. The zone having the lowest packed tube silver density preferably comprises a catalyst of the invention. The packed tube silver density is understood to be the silver density per liter of a tubular reactor packed with catalyst. The packed tube silver density can be calculated by multiplying the packed tube catalyst density with the weight amount of silver (in percent, relative to the weight of the catalyst). The packed tube catalyst density is understood to be the catalyst density per liter of a tubular reactor packed with catalyst. The other zones comprise catalysts based on conventional porous supports. The conventional porous supports have a higher bulk density than the porous alumina support having a defined pore volume distribution on which the catalyst of the invention is based.

Hence in the embodiment, only a part of the catalyst filled into the reactor is a catalyst according to the present invention, while the remaining catalyst(s) is (are) based on (a) conventional support(s). Catalysts based on conventional supports are described in, e.g., WO 2012/091898 A2, US 2015/0119590 A1, US 4,837,194, WO 2009/114411 A2, ON 102688784 A, and DE 102005019596 A. Preferably, the catalysts are filled into the reactor subsequently, so as to obtain a structured catalyst bed having at least two zones comprising silver-based catalysts. In one embodiment, the reactor is filled in a manner so that the catalyst according to the invention is present in the reactor inlet zone. In another embodiment, the reactor is filled in a manner so that the catalyst according to the invention is present in the reactor outlet zone. Preferably, the reactor is filled in a manner so that the catalyst according to the invention is present in the reactor inlet zone. More preferably, the reactor inlet zone constitutes about 5 to 50 vol.-% of the entire reactor, more preferably 10 to 25 vol.-%.

To prepare ethylene oxide from ethylene and oxygen, it is possible to carry out the reaction under conventional reaction conditions as described, e.g., in DE-A 2521906, EP-A 0014 457, DE-A 2300512, EP-A 0 172565, DE-A 2454972, EP-A 0357293, EP-A 0266 015, EP-A 0085237, EP-A 0082 609 and EP-A 0339748. Inert gases such as nitrogen or gases which are inert under the reaction conditions, e.g. steam, methane, and also optionally reaction moderators, for example halogenated hydrocarbons such as ethyl chloride, vinyl chloride or 1 ,2-dichloroethane can additionally be mixed into the reaction gas comprising ethylene and molecular oxygen.

The oxygen content of the reaction gas is advantageously in a range in which no explosive gas mixtures are present. A suitable composition of the reaction gas for preparing ethylene oxide can, for example, comprise an amount of ethylene in the range from 10 to 80% by volume, preferably from 20 to 60% by volume, more preferably from 25 to 50% by volume and particularly preferably in the range from 25 to 40% by volume, based on the total volume of the reaction gas. The oxygen content of the reaction gas is advantageously in the range of not more than 10% by volume, preferably not more than 9% by volume, more preferably not more than 8% by volume and very particularly preferably not more than 7.5% by volume, based on the total volume of the reaction gas.

The reaction gas preferably comprises a chlorine-comprising reaction moderator such as ethyl chloride, vinyl chloride or 1 ,2-dichloroethane in an amount of from 0 to 15 ppm by weight, preferably in an amount of from 0.1 to 8 ppm by weight, based on the total weight of the reaction gas. The remainder of the reaction gas generally comprises hydrocarbons such as methane and also inert gases such as nitrogen. In addition, other materials such as steam, carbon dioxide or noble gases can also be comprised in the reaction gas. The concentration of carbon dioxide in the feed (i.e. the gas mixture fed to the reactor) typically depends on the catalyst selectivity and the efficiency of the carbon dioxide removal equipment. Carbon dioxide concentration in the feed is preferably at most 3 vol.- %, more preferably less than 2 vol.-%, most preferably less than 1 vol.-%, relative to the total volume of the feed. An example of carbon dioxide removal equipment is provided in US 6,452,027. The above-described constituents of the reaction mixture may optionally each have small amounts of impurities. Ethylene can, for example, be used in any degree of purity suitable for the gas-phase oxidation according to the invention. Suitable degrees of purity include, but are not limited to, “polymer-grade” ethylene, which typically has a purity of at least 99%, and “chemical-grade” ethylene which typically has a purity of less than 95%. The impurities typically comprise, in particular, ethane, propane and/or propene.

The reaction or oxidation of ethylene to ethylene oxide is usually carried out at elevated catalyst temperatures. Preference is given to catalyst temperatures in the range of 150 to 350 °C, more preferably 180 to 300 °C, particularly preferably 190 to 280 °C and especially preferably 200 to 280 °C. The present invention therefore also provides a process as described above in which the oxidation is carried out at a catalyst temperature in the range 180 to 300 °C, preferably 200 to 280 °C. Catalyst temperature can be determined by thermocouples located inside the catalyst bed. As used herein, the catalyst temperature or the temperature of the catalyst bed is deemed to be the weight average temperature of the catalyst bodies.

The reaction according to the invention (oxidation) is preferably carried out at pressures in the range of 5 to 30 bar. All pressures herein are absolute pressures, unless noted otherwise. The oxidation is more preferably carried out at a pressure in the range of 5 to 25 bar, such as 10 bar to 20 bar and in particular 14 bar to 20 bar. The present invention therefore also provides a process as described above in which the oxidation is carried out at a pressure in the range of 14 bar to 20 bar.

It has been found that the physical characteristics of the catalyst, especially the pore size distribution and the BET surface area, have a significant positive impact on the catalyst selectivity. The oxidation process according to the invention is preferably carried out under conditions conducive to obtain a reaction mixture containing at least 2.0 vol.-% of ethylene oxide. In other words, the ethylene oxide outlet concentration (ethylene oxide concentration at the reactor outlet) is preferably at least 2.0 vol.-%. The ethylene oxide outlet concentration is more preferably in the range of 2.2 to 4.0 vol.-%, most preferably in the range of 2.9 to 3.5 vol.-%.

The oxidation is preferably carried out in a continuous process. If the reaction is carried out continuously, the GHSV (gas hourly space velocity) is, depending on the type of reactor chosen, for example on the size/cross-sectional area of the reactor, the shape and size of the catalyst, preferably in the range from 800 to 10,000/h, preferably in the range from 2,000 to 8,000/h, more preferably in the range from 2,500 to 6,000/h, most preferably in the range from 4,500 to 5,500/h, where the values indicated are based on the volume of the catalyst. According to a further embodiment, the present invention is also directed to a process for preparing ethylene oxide (EO) by gas-phase oxidation of ethylene by means of oxygen as disclosed above, wherein the EO-space-time-yield measured is greater than 180 kg_Eo/(m³ _cath), preferably to an EO-space-time-yield of greater than 200 kg_Eo/(m³ _cath), such as greater than 250 kg_Eo/(m³ _cath), greater than 280 kg_Eo/(m³ _cath), or greater than 300 kg_Eo/(m³ _cath). Preferably the EO-space-time-yield measured is less than

500 kg_Eo/(m³ _cath), more preferably the EO-space-time-yield is less than 350 kg_Eo/(m³ _cath).

The preparation of ethylene oxide from ethylene and oxygen can advantageously be carried out in a recycle process. After each pass, the newly formed ethylene oxide and the by-products formed in the reaction are removed from the product gas stream. The remaining gas stream is supplemented with the required amounts of ethylene, oxygen and reaction moderators and reintroduced into the reactor. The separation of the ethylene oxide from the product gas stream and its work-up can be carried out by customary methods of the prior art (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A-10, pp. 117-135, 123-125, VCH-Verlagsgesellschaft, Weinheim 1987).

The invention will be described in more detail by the accompanying drawings and the subsequent examples.

Fig. 1 shows the pore size distribution of a preferred alumina support used for preparation of a catalyst of the invention (Support B1 as described below). The support has a Log differential pore volume distribution peak at about 2 pm; and a Log differential pore volume distribution peak at about 9 pm. On the x-axis, the pore size diameter [pm] is plotted. On the y-axis, the log differential intrusion [mL/g] is plotted.

Fig. 2a and 2b show micrographs depicting the cross-section of a preferred alumina support (Support B1 as described below) at a scale of 500 pm and 20 pm. Fig. 3 shows the log differential intrusion (mL/g) and cumulative intrusion (mL/g) relative to the pore size diameter (pm) of a comparative alumina support (Support B2 as described below). Examples

A. Methods Method 1 : Mercury Porosimetry

Mercury porosimetry was performed using a Micrometries AutoPore IV 9500 mercury porosimeter (140 degrees contact angle, 485 dynes/cm Hg surface tension, 60000 psia max head pressure). The Hg porosity is determined in accordance with DIN 66133.

Method 2: BET Surface Area

The BET surface area was determined in accordance with DIN ISO 9277. Method 3: Packed Tube Density

The packed tube density was determined by filling an amount of x g of support into a cylindrical glass tube with an inner diameter of 39 mm up to a marker marking an inner tube volume of y ml_. The glass tube was placed on a weighing scale and the weight increase from the filled-in support was determined as x. The density in g/L was calculated as (x/y) x 1000.

B. Preparation of Alumina Supports B1. Preparation of Alumina Support B1

1787 g of alumina powder (AKP 30; Sumitomo Chemical; dso=300nm; BET surface area about 7.5 m²/g) were dispersed in 550 ml. of deionized water at a pH value of greater than 12 (adjusted by adding aqueous sodium hydroxide) by ball milling for 1 day with yttria-stabilized zirconia milling media (5 mm diameter) to form a stock suspension.

After the ball milling process, carbon-based spherical particles (CB; Thermax N990, Cancarb LTD; dso= 280nm and BET surface area about 9.4 m²/g) were added in a step wise fashion to the stock suspension, which was mixed for 2 min at 2200 rpm in a planetary mixer (SpeedMixer DAC 600.2; FlackTek, Inc.) after each addition, thus creating a binary suspension and avoiding structural damage to the carbon particles during the milling process. CB particles were added until the alumina:CB volume ratio was 30:70. Deionized water was added to the suspension to reach a solids loading of 14 vol.-%. Subsequently, an amine surfactant (decylamine, 95%; Sigma-Aldrich) was added so as to partially hydrophobize the particles. All components were then homogenized in the planetary mixer. The specific concentration of the decylamine surfactant (obtained per unit of particle surface area) was 0.9 pmol/m². Finally, to form a binary colloidal gel, used as a precursor for the production of a wet foam, the pH value was adjusted to a value within the range of 10 to 10.5. Table 1 shows illustrative amounts of each component in 60 ml. of the described binary colloidal gel. Table 1 : Composition of a Binary Colloidal Gel

Five wet foams were obtained from five colloidal gels by mechanical frothing. 60 ml. of each of the different colloidal gels were mechanically frothed in 240 ml. glass jars to entrain air with a four-bladed impeller attached to an overhead mixer at 600, 900, 1300, 1500 and 1800 rpm for 300 seconds each.

These wet foams were used as inks for the production of extrudates through nozzles. To do so, the ink was loaded in syringe barrels and extruded through a single deposition nozzle connected to the barrel. Nuggets of foam with a size of about 5.0 mm were deposited on a tray. The extruded wet foams were slowly dried and underwent a burnout process at 700 °C for 3 h in an air atmosphere to remove carbon-based fugitive particles.

Subsequently, the foams underwent sintering at 1500 °C for 2 h to obtain Support B1. Support B1 had a total pore volume of 2.4 mL/g, as determined by mercury porosimetry in accordance with DIN 66133. The BET surface area of Support B1 was 3.5 m²/g, as determined in accordance with DIN ISO 9277. Support B1 had a multimodal pore size distribution with the first log differential pore volume distribution peak at about 2 pm and the second log differential pore volume distribution peak in the range of from 6 to 300 pm. Fig. 1 shows the log differential intrusion of Support B1. Micrographs depicting the cross- section of Support B1 , at a 500 pm scale and at a 20 pm scale, are shown in FIG. 2a and 2b. B2. Preparation of Alumina Support B2

An aqueous alpha alumina slurry was obtained by dry mixing 100 parts by weight of alpha alumina having a particle size below 55 pm and milling the aqueous alpha alumina slurry in a ball mill to an alpha alumina particle size of less than 3 pm. Subsequently, about 40 parts by weight of an aqueous solution containing 1.5% by weight of polyvinyl alcohol of molecular weight 125,000 and 0.01% by weight of wetting agent were added to give a thick dispersion. While stirring the slurry in a mixer, polyurethane foam pellets were added and the resultant mixture kneaded for 5 min. The resultant impregnated foam pellets were then discharged from the mixer onto a tray having a mesh base. The tray was vibrated for 2 min. The impregnated pellets were then dried at about 70 °C for 24 h and subsequently heated to 1500 °C over a period of 6 h, and maintained at this temperature for 2 h so as to obtain Support B2. Support B2 had a total pore volume of 0.14 mL/g, as determined by mercury porosimetry in accordance with DIN 66133. The BET surface area Support B2 was 0.1 m²/g, as determined in accordance with DIN ISO 9277. Fig. 3 shows the log differential intrusion and cumulative intrusion of Support B2. The properties of the alumina supports are summarized in Table 2 below.

Table 2: Support Hg total pore volume [ml/g], BET surface area [m²/g] and packed tube density [g/L]

C. Preparation of Catalysts

C.1 Production of the Silver Complex Solution

Silver Complex Solution was prepared according to Production Example 1 of WO2019/154863. The resulting silver impregnation solution had a density of 1.529 g/mL and a silver content of 29.3 wt-%. C.2 Catalyst Preparation

C.2.1 Catalyst Based on Support B1 (Inventive) 9 g of support B1 listed in Table 2 was placed into a 1 L glass flask. The flask was attached to a rotary evaporator which was set under vacuum pressure of 80 mbar. The rotary evaporator system was set in rotation of 30 rpm. 11.66 g of the silver complex solution prepared according to step C.1 was mixed with 0.187 g of promoter solution I, 0.205 g of promoter solution II, and 0.354 g of promoter solution III. Promoter solution I was made from dissolving lithium nitrate (FMC, 99.3%) and ammonium sulfate (Merck, 99.4%) in deionized water to achieve Li content of 2.85 wt.-% and S content of 0.21 wt.-%. Promoter solution II was made from dissolving tungstic acid (HC Starck, 99.99%) in deionized water and cesium hydroxide in water (HC Starck, 50.42%) to achieve target Cs content of 5.28 wt.-% and W content of 3.0 wt.-%. Promoter solution III was made from dissolving ammonium perrhenate (Engelhard, 99.4%) in deionized water to achieve Re content of 3.7 wt.-%. The combined impregnation solution containing silver complex solution, promoter solutions I, II, and III was stirred for 5 min. The combined impregnation solution was added onto the support B1 over 15 min under a vacuum of 80 mbar. After addition of the combined impregnation solution, the rotary evaporator system was continued to rotate under vacuum for another 15 min. The impregnated support was then left in the apparatus at room temperature and atmospheric pressure for 1 h and mixed gently every 15 min. The impregnated material was calcined for 10 min at 290 °C under 23 m³/h flowing nitrogen in a calcination oven to yield the final catalyst C.2.1. Catalyst composition of the final catalyst C.2.1 is listed in Table 3.

C.2.2 Catalyst Based on Support B2 (Comparative)

C2.2.1. Preparation of Ag-Containing Intermediate 21.5 g of support B2 listed in Table 2 was placed into a 1 L glass flask. The flask was attached to a rotary evaporator which was set under vacuum pressure of 80 mbar. The rotary evaporator system was set in rotation of 30 rpm. 26.45 g of silver complex solution prepared according to Example C.1 were added onto support B2 over 15 min under a vacuum of 30 mbar. After addition of the silver complex solution, the rotary evaporator system was continued to rotate under vacuum for another 15 min. The impregnated support was then left in the apparatus at room temperature and atmospheric pressure for 1 h and mixed gently every 15 min. The impregnated support was calcined for 12 min at 290 °C under 23 m³/h flowing nitrogen in a calcination oven to yield Ag-containing intermediate product. C2.2.2. Preparation of Catalyst C.2.2

27.4 g of Ag-containing intermediate product prepared according to step C2.2.1 were placed into a 1 L glass flask. The flask was attached to a rotary evaporator which was set under vacuum pressure of 80 mbar. The rotary evaporator system was set in rotation of 30 rpm. 20.07 g of the silver complex solution prepared according to step C.1 was mixed with 0.86 g of promoter solution I, 0.94 g of promoter solution II, and 1.63 g of promoter solution III. Promoter solution I was made from dissolving lithium nitrate (FMC, 99.3%) and ammonium sulfate (Merck, 99.4%) in deionized water to achieve Li content of 2.85 wt.-% and S content of 0.2122 wt.-%. Promoter solution II was made from dissolving tungstic acid (HC Starck, 99.99%) in deionized water and cesium hydroxide in water (HC Starck, 50.42%) to achieve target Cs content of 5.3 wt.-% and W content of 3.0 wt.-%. Promoter solution III was made from dissolving ammonium perrhenate (Engelhard, 99.4%) in deionized water to achieve Re content of 3.7 wt.-%. The combined impregnation solution containing silver complex solution, promoter solutions I, II, and III was stirred for 5 min. The combined impregnation solution was added onto the silver- containing intermediate product C2.2.1 over 15 min under vacuum of 80 mbar. After addition of the combined impregnation solution, the rotary evaporator system was continued to rotate under vacuum for another 15 min. The impregnated support was then left in the apparatus at room temperature and atmospheric pressure for 1 h and mixed gently every 15 min. The impregnated material was calcined for 10 min at 290 °C under 23 m³/h flowing nitrogen in a calcination oven to yield the final catalyst C2.2. Catalyst composition of the final catalyst C.2.2 is listed in Table 3.

C.2.3 Catalyst Based on Support B2 (Comparative)

C2.3.1. Preparation of Ag-Containing Intermediate

21.6 g of support B2 listed in Table 2 was placed into a 1 L glass flask. The flask was attached to a rotary evaporator which was set under vacuum pressure of 80 mbar. The rotary evaporator system was set in rotation of 30 rpm. 26.58 of silver complex solution prepared according to Example C.1 was added onto support B2 over 15 min under a vacuum of 30 mbar. After addition of the silver complex solution, the rotary evaporator system was continued to rotate under vacuum for another 15 min. The impregnated support was then left in the apparatus at room temperature and atmospheric pressure for 1 h and mixed gently every 15 min. The impregnated support was calcined for 12 min at 290 °C under 23 m³/h flowing nitrogen in a calcination oven to yield Ag-containing intermediate product.

C2.3.2. Preparation of Catalyst C.2.3

26.5 g of Ag-containing intermediate product prepared according to step C2.3.1 were placed into a 1 L glass flask. The flask was attached to a rotary evaporator which was set under vacuum pressure of 80 mbar. The rotary evaporator system was set in rotation of 30 rpm. 21.05 g of the silver complex solution prepared according to step C.1 was mixed with 0.54 g of promoter solution I, 0.62 g of promoter solution II, and 1.06 g of promoter solution III. Promoter solution I was made from dissolving lithium nitrate (FMC, 99.3%) and ammonium sulfate (Merck, 99.4%) in deionized water to achieve Li content of 2.85 wt.-% and S content of 0.21 wt.-%. Promoter solution II was made from dissolving tungstic acid (HC Starck, 99.99%) in deionized water and cesium hydroxide in water (HC Starck, 50.42%) to achieve target Cs content of 5.0 wt.-% and W content of 3.0 wt.-%. Promoter solution III was made from dissolving ammonium perrhenate (Engelhard, 99.4%) in deionized water to achieve Re content of 3.7 wt.-%. The combined impregnation solution containing silver complex solution, promoter solutions I, II, and III was stirred for 5 min. The combined impregnation solution was added onto the silver- containing intermediate product C2.3.1 over 15 min under vacuum of 80 mbar. After addition of the combined impregnation solution, the rotary evaporator system was continued to rotate under vacuum for another 15 min. The impregnated support was then left in the apparatus at room temperature and atmospheric pressure for 1 h and mixed gently every 15 min. The impregnated material was calcined for 10 min at 290 °C under 23 m³/h flowing nitrogen in a calcination oven to yield the final catalyst C2.3. Catalyst composition of the final catalyst C.2.3 is listed in Table 3. Table 3: Catalyst Composition (Ag-contents are reported in percent by weight of total catalyst, promoter values are reported in parts per million by weight of total catalyst, all values are calculated)

* KADD is understood to mean the amount of potassium added during impregnation and does not include the amount of potassium comprised in the alumina support prior to impregnation

D. Catalyst Testing

An epoxidation reaction was conducted in a vertically-placed test reactor constructed from stainless steel with an inner diameter of 6 mm and a length of 2.2 m. The reactor was heated using hot oil contained in a heating mantel at a specified temperature. All temperatures below refer to the temperature of the hot oil. The reactor was filled to a height of 212 mm with inert steatite balls (1.0 - 1.6 mm), packed with the amount of catalyst indicated in table 4, and then again packed with an additional 707 mm inert steatite balls (1.0 - 1.6 mm). Prior to filling the catalyst into the reactor, the catalyst shaped bodies were gently broken into pieces of 1 to 3 mm. The inlet gas was introduced to the top of the reactor in a “once-through” operation mode.

The inlet gas consisted of about 35 vol.-% ethylene, 7 vol.-% oxygen, 1 vol.-% of CO2, and ethylene chloride (EC) moderation in the range from 1.5 to 4.1 parts per million by volume (ppmv), with methane used as a balance. The reactions were conducted at a pressure of about 15 bar and an inlet gas flow rate of 148 Nl/h. Results of the catalyst test at heating oil temperature of 250°C are shown in Table 4. Table 4: Test Reaction Results

It is evident that catalyst C2.1 shows much higher activity and selectivity than catalysts C2.2 and C2.3, despite much lower Ag-content used per catalyst bed.

Claims

1. A process for preparing a catalyst for producing ethylene oxide by gas-phase oxidation of ethylene, comprising i) impregnating a porous alumina support having a packed tube density in the range of 100 to 450 g/L; a Log differential pore volume distribution curve, as measured by mercury porosimetry, having at least one peak in a pore diameter range of 0.01 to 5.0 pm; and a BET surface area in the range of 1.5 to 30.0 m²/g; with a silver impregnation solution, preferably under reduced pressure; and optionally subjecting the impregnated porous alumina support to drying; and ii) subjecting the impregnated porous alumina support to a calcination process; wherein steps i) and ii) are optionally repeated, to yield a catalyst comprising at least 25 wt.-% of silver, relative to the total weight of the catalyst.

2. The process according to claim 1 , wherein the porous alumina support has a plurality of cellular pores partitioned by cellular walls and intergranular pores formed in the cellular walls, wherein the intergranular pores are assignable to the Log differential pore volume distribution peak in a pore diameter range of 0.01 to 5.0 pm. 3. The process according to claim 2, wherein the cellular pores have pore diameters in the range of 10 to 500 pm, as determined by scanning electron microscopy.

4. The process according to any one of the preceding claims, wherein the Log differential pore volume at the peak in a pore diameter range of 0.01 to 5.0 pm is 0.3 mL/g or more, preferably 0.5 mL/g or more, more preferably 1.0 mL/g or more, or 2.0 mL/g or more.

5. The process according to any one of the preceding claims, wherein the catalyst comprises 25 to 70 wt.-% of silver, preferably 30 to 60 wt.-% of silver, more preferably 35 to 50 wt.-% of silver, relative to the total weight of the catalyst.

6. The process according to any one of the preceding claims, wherein the porous alumina support is in the form of individual shaped bodies.

7. A catalyst for producing ethylene oxide by gas-phase oxidation of ethylene, obtainable by a process according to any one of the preceding claims.

8. A catalyst for producing ethylene oxide by gas-phase oxidation of ethylene, comprising at least 25 wt.-% of silver, relative to the total weight of the catalyst, deposited on a porous alumina support, the support having i) a packed tube density in the range of 100 to 450 g/L, preferably 150 to 450 g/L, more preferably 200 to 400 g/L; ii) a Log differential pore volume distribution curve, as measured by mercury porosimetry, having at least one peak in a pore diameter range of 0.01 to 5.0 pm; and iii) a BET surface area in the range of 1.5 to 30.0 m²/g, preferably 1.5 to

20.0 m²/g, more preferably 2.0 to 10.0 m²/g, most preferably 2.5 to 8.0 m²/g, in particular 3.0 to 7.0 m²/g.

9. A process for producing ethylene oxide by gas-phase oxidation of ethylene, comprising reacting ethylene and oxygen in the presence of a catalyst obtained by a process according to any one of claims 1 to 6 or as defined in claim 8.

10. The process according to claim 9, wherein a reaction feed comprising ethylene and oxygen is subsequently reacted in at least two zones wherein the packed tube silver densities in the different zones differ from one another, the reaction feed first comes into contact with the zone having the lowest packed tube silver density and the zone having the lowest packed tube silver density comprises a catalyst obtained by a process according to any one of claims 1 to 6 or as defined in claim 8.