Patent Application
20250073691
One of the biggest problems in manufacturing catalysts with multiple components is to locate all of the components in proximal position to one another so as to fully utilize the synergistic properties that result from cooperative interaction of the different components. For instance, ethylene oxide production uses silver (Ag) catalysts in conjunction with promoters that are added to improve reaction selectivity to ethylene oxide. Elements such as cesium (Cs), rhenium (Re), and molybdenum (Mo) have been shown to be effective promoters either alone or in various combinations with one another. Typical synthesis of promoted silver catalysts use simple impregnation of precursors into a support (e.g., alumina (α-Al2O3)) with no control for targeting the promoter element to be in a proximal, adjacent position relative to the Ag surface sites.
What is needed in the art are catalyst formation methods that can provide promoted catalysts with improved positioning of promoters relative to the catalytic metal. Resulting catalysts can exhibit improved activity as compared to previously known catalysts with randomly located materials.
According to one embodiment, disclosed are methods for forming promoted catalysts. A method can include combining a solution with a porous support material. The solution can include a catalyst metal and a metal oxide promoter. The metal oxide promoter can have a surface free energy of about 100 ergs/cm2 or less, and the catalyst metal can have a surface free energy of about 1000 ergs/cm2 or more. Upon the combination, the catalyst metal and the metal oxide promoter can be co-impregnated in the porous support material, with the impregnated porous support material comprising the metal oxide promoter at a concentration of about 1.2 micromoles per gram catalyst (μmol/g-cat) or greater. A method can also include a first calcination of the impregnated support material at a temperature of about 250° C. or greater and a second calcination step of the impregnated support material at a temperature that is higher than that of the first calcination step. The formation method can provide preferential deposition of the promoter on the catalyst metal and provide a promoted catalyst with improved activity.
A full and enabling disclosure of the present subject matter, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which:
Reference will now be made in detail to various embodiments of the disclosed subject matter, one or more examples of which are set forth below. Each embodiment is provided by way of explanation of the subject matter, not limitation thereof. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present disclosure without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, may be used in another embodiment to yield a still further embodiment.
The technology described in this disclosure uses thermodynamics, one of the basic forces of science and nature, to drive the formation of multi-component catalysts with key components in the correct positions. More specifically, disclosed methods utilize a combination of relative surface free energies of catalyst and promoter materials, promoter loading concentrations, and calcination temperatures to encourage deposition of promoters at preferred locations on the catalyst materials, and in particular, to encourage thermodynamically preferential diffusion of promoters on catalyst metals. This disclosed approach can form a superior catalyst compared to currently known catalysts.
In one embodiment, disclosed formation methods and catalysts formed thereby can improve the selectivity to form ethylene oxide relative to formation of CO2, a greenhouse gas. Thus, in addition to using the expensive ethylene feed gas more efficiently, disclosed methods and catalysts can lower greenhouse gas emissions.
Disclosed methods take advantage of surface free energy differences between catalyst metals and metal oxide promoters to preferentially locate a promoter in proximity to the catalyst metal sites, thereby creating truly promoted catalysts. At relatively high loading level for the promoter metal oxide (e.g., about 1.2 μmol/g-cat or greater) and upon thermal treatment of the promoted catalysts, disclosed systems with a relatively large differential in surface free energies between the catalyst and promoter metal oxide can reconfigure to form a lower overall surface free energy—i.e. the metal oxide with a relatively low surface free energy can preferentially migrate toward the surface of the catalyst metal with the relatively high surface free energy, and upon calcination, the promoters can readily diffuse onto the surface of the catalyst metal.
According to the present disclosure, a formation method can include forming a solution that includes a catalytic metal and a metal oxide promoter. In general, a solution formation process can include dissolving salts of the catalytic metal and the metal oxide in a suitable solvent in addition to any other components that may be of benefit to an impregnation process or the formed supported catalyst material.
The catalyst metal is not particularly limited, and can encompass any catalytic metal having a surface free energy of about 1000 ergs/cm2 or greater, such as from about 1000 ergs/cm2 to about 4000 ergs/cm2, from about 1000 ergs/cm2 to about 3000 ergs/cm2, or from about 1000 ergs/cm2 to about 2000 ergs/cm2 in one embodiment. Table 1, below, provides surface free energies and melting point values of several exemplary metals.
In one embodiment, the catalyst metal can be a Group IB metal, e.g., silver, gold, copper, or any combination thereof.
A solution can include a salt of the catalytic metal, e.g., a nitrate, oxalate, or carbonate salt of a catalyst metal, optionally with suitable solubilizing or reducing agents as known in the art, e.g., an alkanolamine, alkyldiamine, ammonia, or any combination thereof. In one embodiment, a method can include formation of an aqueous solution containing a metal salt of a carboxylic acid and an organic amine. By way of example, a silver oxide slurry in water can be combined with a mixture of ethylene diamine and oxalic acid to form an aqueous solution of silver oxalate-ethylene diamine-complex, to which solution can be added one or more promoter compounds as described further herein. Other amines, such as ethanolamine, may be included as well.
The solution can also include one or more promoter compounds, at least one of which can be a metal oxide promoter. In one embodiment, a solution can include a metal oxide salt containing a high valent oxyanion promoter such as, and without limitation to, a high valent rhenium oxide, molybdenum oxide, tungsten oxide, sulfur oxide, or any combination thereof. For instance, a metal oxide promoter can include ReO4−, MoO42−, WO2−, SO42−, and the like, or any combination thereof. The metal oxide can be of any form, provided the metal oxide has a surface free energy as described herein. By way of example, and without limitation, a solution can include a rhenium oxide promoter such as rhenium sesquioxide (Re2O3), rhenium dioxide (ReO2), rhenium trioxide (ReO3), perrhenate (ReO4), rhenium heptoxide (Re2O7−), or any combination thereof.
The metal oxide promoter can have a surface free energy of about 100 ergs/cm2 or less. For instance, exemplary promoter metal oxide rhenium heptoxide has a surface free energy of about 38 ergs/cm2, molybdenum trioxide (MoO3) has a surface free energy of about 97 ergs/cm2, and phosphorous pentoxide (P2O5) has a surface free energy of about 55 ergs/cm2, any one or combination of which can be preferentially deposited on a supported catalytic metal according to disclosed methods.
The solution can contain the metal oxide promoter in an amount such that the loading level of the metal oxide promoter on the porous support is about 1.2 μmol/g-cat or greater, such as from about 1.2 μmol/g-cat to about 3 μmol/g-cat in some embodiments.
In addition to one or more catalyst metals and metal oxide promoters, a solution can include one or more additional promoters as are known in the art. By way of example, a solution can include an alkali metal or alkaline earth metal salt or hydroxide promoter such as a lithium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium or barium salt or hydroxide, or any combination thereof as is generally known in the art. By way of example, a solution can include an alkali metal and/or alkaline earth metal nitrate or halide salt as an additional promoter.
When present, the solution can contain one or more additional promoters in an amount such that the loading level of the additional promoter on the porous support material is about 2 μmol/g-cat or greater, such as from about 2 μmol/g-cat to about 10 μmol/g-cat in some embodiments.
A solution can be combined with a porous catalyst support under conditions to encourage co-impregnation of the support by the catalyst metal and the promoter metal oxide (as well as any additional promoter materials, as desired) as is generally known in the art.
The porous catalyst support material can be any conventional porous support material that can function as a carrier for the active catalytic metals and promoters and any optional additives as desired. Suitable supports include, without limitation, alumina, silica, silica-aluminas, aluminosilicates, zirconia, titania, and the like. In one embodiment, a catalyst support material can include a transitional alumina, examples of which include gamma, delta, theta, and eta alumina, as well as any mixtures thereof. Mixtures of a transitional alumina with alpha alumina can also be utilized. In some embodiments, a catalyst support material can include a mixture of alumina with other support materials, such as silica, in any suitable combination.
In one embodiment, the support material can have a surface free energy that is between that of the catalyst metal and that of the metal oxide promoter, such as from about 100 ergs/cm2 to about 1000 ergs/cm2 in some embodiments. By way of example, Table 2, below, provides surface free energies and melting points of some exemplary porous support materials.
Following co-impregnation of the porous support material with the catalyst metal and the metal oxide promoter, the impregnated support material can be calcinated at a temperature of about 250° C. or greater, followed by a second calcination step of the impregnated carrier at a temperature that is higher than that of the first calcination step. By way of example, a first calcination step can include calcination of the impregnated porous support material in air (e.g., a flowing air stream, in one embodiment) at a temperature of about 250° C. or higher, such as from about 250° C. to about 300° C., or from about 260° C. to about 280° C. in one embodiment.
Following, the impregnated porous support materials can be calcinated a second time at a temperature that is higher than that of the first calcination. For instance, the materials can be calcinated a second time in air at a temperature of about 270° C. or higher, such as from about 275° C. to about 320° C. in one embodiment.
Through utilization of the metal oxide promoter with a relatively low surface energy at a relatively high loading level and the two step calcination process, the metal oxide promoter can be preferentially positioned on the supported catalyst metal, thereby providing a truly promoted catalyst with improved activity as compared to promoted catalysts in which the promoter is randomly distributed over the catalyst and support materials.
The present invention may be better understood by reference to the Example provided below.
Catalysts were prepared by co-impregnation. Ag2C2O4 was dissolved in an aqueous ethylenediamine (EN) (Millipore Sigma, ≥99%) solution using a 3:1 ratio for EN:Ag2C2O4. The impregnation solution was prepared using 5% excess pore volume of the carrier (Saint Gobain SA5562 α-Al2O3, 8-mm rings, BET Surface area 0.73 m2/g by Kr BET, with water accessible pore volume of 0.32 cm3/g support). Promoter salts were added using co-impregnation of NH4ReO4 (Millipore Sigma, ≥99%), CsNO3 (Millipore Sigma, 99.99% trace metal basis) and (NH4)2MO4 (Millipore Sigma, 99.98% trace metal basis) dissolved at the same time with Ag2C2O4 in ethylenediamine solution. The rings were tumble-dried in a 250 ml round bottom flask at 200 torr pressure and 60° C. until free tumbling rings were produced. Following impregnation, 25 g portions were calcined in fast flowing air (5 L/min) at 260° C. for 6 min.
α-Al2O3 samples containing Cs, Re, or Mo, but no Ag, were impregnated using an aqueous solution at 5% excess pore volume. These samples were tumbled dried in vacuum and fast calcined using the same parameters noted for Ag containing samples.
Prior to analysis, all samples were calcined ex situ at 280° C. in flowing air for 4 hrs to remove residual ethylenediamine. Unless specified otherwise, each experiment used 2.0 g of ground and sieved particles of 400-841 μm that were loaded into a Micromeritics Autochem™ 2920 equipped with a thermal conductivity detector and reduced in situ in flowing 10% H2 balanced helium (bal He) at 280° C. for 2 hrs. Following reduction, samples were cooled to 170° C. in flowing 10% H2 bal He, purged with He for 30 min, and exposed to 10% O2 bal He for an additional 30 min to dissociatively adsorb molecular oxygen. The sample chamber was then flushed with He for 30 min and pulses of 10% H2/balance Ar were passed over the sample at 170° C. and atmospheric pressure. The oxygen pre-coverage and hydrogen titration steps were repeated in situ 2 more times and reported as an average. Good agreement was observed between all titration experiments. For the unpromoted 12 wt. % Ag/α-Al2O3 sample, H2 uptake was measured for three different samples; each sample was analyzed three times to give a total of nine H2 titrations. This was done to determine reproducibility and obtain accurate values for the base sample.
Temperature programmed desorption (TPD) was performed using a high wattage, split-tube furnace (ATS) connected to an Inficon Transpector 2™ Mass Spectrometer and a manifold of Brooks™ 5850E mass flow controllers. Following the calcination at 260° C., 0.5 g of material was loaded into a ¼-inch quartz tube supported on a quartz plug and heated to 500° C. at 10° C./min ramp rate in 20 sccm flowing Ar while monitoring m/e=1-50, particularly for ethylenediamine which has a base peak at m/e=30. Temperature programmed reduction (TPR) was performed in the same Micromeritics Autochem™ 2920 described above using 2.0 g of “fresh” material ex situ calcined at 260° C. with flowing air. The sample was reduced in situ at 280° C. for 2 hrs in 20 sccm H2, cooled to 170° C. in 20 sccm Ar and then O2 exposure was performed for 30 min in 20 sccm of flowing 10% O2 bal He. The catalyst was then cooled to 40° C. in 20 sccm of Ar and the feed was switched to 20 sccm 10% H2 bal Ar before TPR was performed up to 500° C. at a ramp rate of 20° C./min.
Following pretreatment, the reproducibility of H2 uptake on unpromoted 12 wt. % Ag/α-Al2O3 was tested by twice repeating a series of pulsed titration experiments. In each case 2.0 g of fresh material supported on a bed of quartz wool was loaded into a quartz U-tube and samples were reduced in situ at 280° C. using 10% H2/bal Ar. All samples were then purged with Ar at 280° C. for two hours, and then cooled to 170° C. where dissociative O2 adsorption occurred; after a second purge in flowing Ar, pulsed H2 titration was done. The pulsed hydrogen titration results are shown in
The Ag active site concentration determined by H2 titration was 2.3×1018/g cat, which is approximately half as much as the value of 4.2×1018/g cat determined by counting and measuring particles from SEM images. This has been commonly observed when comparing chemisorption-derived active site concentrations, which are actually measured by molecules of gas uptake, with methods where active site concentrations are determined by inference (XRD line broadening, electron microscopies, XPS, etc.). The shortcomings of using electron microscopy as the basis for determining surface site concentrations include differences of particle shapes, insufficient number of particles counted, loss of surfaces due to conjoined particle faces, failure to be either spherical (sitting above the surface) or hemispherical, or assumptions that the metal particles are pristine with no surface contamination. For Ag particles such as those used for olefin epoxidation, the particles are far too large to use X-ray line broadening since the X-ray peaks are extremely sharp. It has been previously concluded that chemisorption and electrochemical surface area measurements, which count molecules and electrons, respectively, are superior to either x-ray line broadening or electron microscopy. Thus, 2.3×1018/g cat was used as the concentration of accessible Ag sites. For 12 wt. % Ag/α-Al2O3, this corresponds to an Ag dispersion of 0.0034.
In order to determine reaction, or titration, trends of the promoters in the absence of Ag, a series of experiments using α-Al2O3 promoted with higher loadings of NH4ReO4, CsNO3, or (NH4)2MoO4 than traditionally used for authentic ethylene oxide catalysts were conducted to determine whether H2 adsorption or promoter reduction might occur to give hydrogen consumption during H2 titration of the O-covered Ag surface at 170° C. Higher promoter loadings were used to ensure observation of reduction events. Temperature programmed adsorption (TPR) of H2 was performed from 50° C. to 500° C. following dissociative oxygen adsorption at 170° C. (TPR parameters: 50-500° C., 20 sccm 10% H2 bal Ar, ramp rate: 20° C./min). Results are shown in
The randomness of coverage of Cs on Ag was determined using the decrease in Ag site concentration from H2 titration data and the coverage of Cs on α-Al2O3, which was estimated from the exposed surface of α-Al2O3 after allowance was made for the fraction of surface covered by the interface of Ag particles as well as an assumption that Cs is deposited in a monodisperse, single layer coverage on both surfaces. The value of fraction of α-Al2O3 surface covered by Ag particles was determined using the particle size distribution of Ag, which was in turn determined by manually measuring 980 Ag particles in SEM micrographs. This approach was likely an oversimplification, but it does represent the most efficient use of Cs and provide a method for calculation.
For a non-interactive support such as low surface area α-Al2O3, the value of 1.2×1019/m2 surface was used to represent the number of potential Cs adsorption sites on α-Al2O3. The exposed α-Al2O3 surface was estimated to be lowered from 0.73 to 0.53 m2/g after Ag coverage of the support, corresponding to a 27% coverage by Ag. Using the decrease in Ag site concentration from H2 titration data, the amount of Cs coverage of the Ag could be estimated. The amount of Cs available for the α-Al2O3 surface was thus the nominal promoter loading minus the amount on Ag. The plot of
Catalysts were also formed including a fixed Cs concentration of 350 ppmw (2.6 μmol/g-cat) and Re loadings varied between 150 and 500 ppmw (0.8 and 2.7 μmol/g-cat, respectively). Samples are described in Table 3, below.
H2 titrations as described previously were then carried out to determine the effect of Re loading on the samples. Results are shown in
It is believed that this is due to the lower surface free energy of Re2O7 (or ReO4− salts) compared to Ag0 and α-Al2O3. Surface free energies at 200° C. are approximately 1320, 805, and 29 ergs/cm2, for Ag0, α-Al2O3, and Re2O7, respectively. All high valent Re salts have exceptionally low surface energies, indicating high mobilities and as a result, for the Ag—ReOx—Al2O3 system, the ReOx will be thermodynamically driven to diffuse onto the Ag surface to lower the SFE of Ag.
While certain embodiments of the disclosed subject matter have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the subject matter.
This application claims filing benefit of U.S. Provisional Patent Application Ser. No. 63/579,142 having a filing date of Aug. 28, 2023, entitled “Metal Oxide/Metal Catalysts Formed by Use of Differential Surface Free Energies,” which is incorporated herein by reference for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 63579142 | Aug 2023 | US |
