THERMODYNAMIC-BASED METHODS FOR FORMATION OF PROMOTED METAL CATALYSTS

Abstract

Methods are described for formation of a promoted catalysts. Promoted catalysts include metal oxide promoted silver catalysts for use in ethylene oxide production. Methods utilize a combination of relative surface free energies of catalyst and promoter materials, promoter loading concentrations, and calcination temperatures to encourage deposition and diffusion of promoters on catalyst metals of supported catalysts.

Claims

1. A method for forming a promoted catalyst comprising: combining a solution with a porous support material, the solution comprising a catalyst metal and a metal oxide promoter, the metal oxide promoter having a surface free energy of about 100 ergs/cm.sup.2 or less, the catalyst metal having a surface free energy of about 1000 ergs/cm.sup.2 or more, wherein upon the combination, the catalyst metal and the metal oxide promoter are co-impregnated in the porous support material, the impregnated porous support material comprising the metal oxide promoter in a concentration of about 1.2 mol per gram catalyst or greater; calcinating the impregnated support material in a first calcination at a temperature of about 250 C. or greater; and calcinating the impregnated support material in a second calcination at a temperature that is greater than the temperature of the first calcination.

2. The method of claim 1, wherein the catalyst metal comprises a Group IB metal.

3. The method of claim 1, wherein the catalyst metal comprises silver.

4. The method of claim 1, wherein the metal oxide promoter comprises a rhenium oxide, a molybdenum oxide, a tungsten oxide, a sulfur oxide, or any combination thereof.

5. The method of claim 4, wherein the metal oxide promoter comprises ReO.sub.4.sup., MoO.sub.4.sup.2, WO.sup.2, SO.sub.4.sup.2, or any combination thereof.

6. The method of claim 4, wherein the metal oxide promoter comprises as rhenium sesquioxide, rhenium dioxide, rhenium trioxide, perrhenate, rhenium heptoxide, or any combination thereof.

7. The method of claim 1, the solution further comprising an alkali metal salt or hydroxide, an alkaline earth metal salt or hydroxide, or any combination thereof.

8. The method of claim 7, wherein the alkali metal salt or alkaline earth metal salt comprises a nitrate salt or a halide salt.

9. The method of claim 7, the impregnated porous support material comprising the alkali metal salt or hydroxide or the alkaline earth metal salt or hydroxide in a concentration of about 2 mol/g-cat or greater.

10. The method of claim 1, the porous support material comprising an alumina, a silica, an aluminosilicate, a zirconia, a titania, or any combination thereof.

11. The method of claim 1, wherein the porous support material has a surface free energy that is between the surface free energy of the catalyst metal and the surface free energy of the metal oxide promoter.

12. The method of claim 1, wherein the second calcination is at a temperature of from about 270 C. or higher.

13. The method of claim 1, wherein the second calcination is at a temperature of from about 275 C. to about 300 C.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0005] 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:

[0006] FIG. 1 presents a hydrogen titration pulse chart for 12 wt. % Ag/-Al.sub.2O.sub.3 particles.

[0007] FIG. 2 presents temperature programed reduction (left) and pulsed hydrogen titration at 170 C. (right) for 3 wt. % Cs/-Al.sub.2O.sub.3 (top), 3 wt. % Re/-Al.sub.2O.sub.3 (middle) and 4 wt. % Mo/-Al.sub.2O.sub.3 (bottom).

[0008] FIG. 3 presents the fractional coverages of Cs on Ag and -Al.sub.2O.sub.3 as a function of promoter levels.

[0009] FIG. 4 presents hydrogen uptake plotted against mol promoter loading on 12 wt. % Ag/-Al.sub.2O.sub.3.

DETAILED DESCRIPTION

[0010] 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.

[0011] 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.

[0012] In one embodiment, disclosed formation methods and catalysts formed thereby can improve the selectivity to form ethylene oxide relative to formation of CO.sub.2, a greenhouse gas. Thus, in addition to using the expensive ethylene feed gas more efficiently, disclosed methods and catalysts can lower greenhouse gas emissions.

[0013] 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 mol/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 energyi.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.

[0014] 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.

[0015] The catalyst metal is not particularly limited, and can encompass any catalytic metal having a surface free energy of about 1000 ergs/cm.sup.2 or greater, such as from about 1000 ergs/cm.sup.2 to about 4000 ergs/cm.sup.2, from about 1000 ergs/cm.sup.2 to about 3000 ergs/cm.sup.2, or from about 1000 ergs/cm.sup.2 to about 2000 ergs/cm.sup.2 in one embodiment. Table 1, below, provides surface free energies and melting point values of several exemplary metals.

TABLE-US-00001 TABLE 1 Melting point Surface free energy Metal ( C.) (ergs/cm.sup.2 surface) Silver (Ag) 962 1302 Gold (Au) 1064 1626 Copper (Cu) 1083 1934 Palladium (Pd) 1554 2043 Nickel (Ni) 1453 2364 Platinum (P)t 1772 2691 Cobalt (Co) 1495 2709 Rhodium (Rh) 1966 2828 Molybdenum (Mo) 2617 2877 Iron (Fe) 1535 2939 Niobium (Nb) 2468 2983 Rhenium (Re) 3180 3109 Iridium (Ir) 2410 3231 Ruthenium (Ru) 2310 3409 Tungsten (W) 3410 3468

[0016] In one embodiment, the catalyst metal can be a Group IB metal, e.g., silver, gold, copper, or any combination thereof.

[0017] 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.

[0018] 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 ReO.sub.4.sup., MoO.sub.4.sup.2, WO.sup.2, SO.sub.4.sup.2, 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 (Re.sub.2O.sub.3), rhenium dioxide (ReO.sub.2), rhenium trioxide (ReO.sub.3), perrhenate (ReO.sub.4), rhenium heptoxide (Re.sub.2O.sub.7.sup.), or any combination thereof.

[0019] The metal oxide promoter can have a surface free energy of about 100 ergs/cm.sup.2 or less. For instance, exemplary promoter metal oxide rhenium heptoxide has a surface free energy of about 38 ergs/cm.sup.2, molybdenum trioxide (MoO.sub.3) has a surface free energy of about 97 ergs/cm.sup.2, and phosphorous pentoxide (P.sub.2O.sub.5) has a surface free energy of about 55 ergs/cm.sup.2, any one or combination of which can be preferentially deposited on a supported catalytic metal according to disclosed methods.

[0020] 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 mol/g-cat or greater, such as from about 1.2 mol/g-cat to about 3 mol/g-cat in some embodiments.

[0021] 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.

[0022] 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 mol/g-cat or greater, such as from about 2 mol/g-cat to about 10 mol/g-cat in some embodiments.

[0023] 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.

[0024] 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.

[0025] 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/cm.sup.2 to about 1000 ergs/cm.sup.2 in some embodiments. By way of example, Table 2, below, provides surface free energies and melting points of some exemplary porous support materials.

TABLE-US-00002 TABLE 2 Melting point Surface Free Energy Support material ( C.) (ergs/cm.sup.2 surface) Carbon 3550 506 Silica (SiO.sub.2) 1600 605 Titania (TiO.sub.2) 1843 670 Alumina (Al.sub.2O.sub.3) 2072 805

[0026] 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.

[0027] 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.

[0028] 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.

[0029] The present invention may be better understood by reference to the Example provided below.

Example

[0030] Catalysts were prepared by co-impregnation. Ag.sub.2C.sub.2O.sub.4 was dissolved in an aqueous ethylenediamine (EN) (Millipore Sigma, 99%) solution using a 3:1 ratio for EN:Ag.sub.2C.sub.2O.sub.4. The impregnation solution was prepared using 5% excess pore volume of the carrier (Saint Gobain SA5562 -Al.sub.2O.sub.3, 8-mm rings, BET Surface area 0.73 m.sup.2/g by Kr BET, with water accessible pore volume of 0.32 cm.sup.3/g support). Promoter salts were added using co-impregnation of NH.sub.4ReO.sub.4 (Millipore Sigma, 99%), CsNO.sub.3 (Millipore Sigma, 99.99% trace metal basis) and (NH.sub.4).sub.2MO.sub.4 (Millipore Sigma, 99.98% trace metal basis) dissolved at the same time with Ag.sub.2C.sub.2O.sub.4 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.

[0031] -Al.sub.2O.sub.3 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.

[0032] 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 m that were loaded into a Micromeritics Autochem 2920 equipped with a thermal conductivity detector and reduced in situ in flowing 10% H.sub.2 balanced helium (bal He) at 280 C. for 2 hrs. Following reduction, samples were cooled to 170 C. in flowing 10% H.sub.2 bal He, purged with He for 30 min, and exposed to 10% O.sub.2 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% H.sub.2/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/-Al.sub.2O.sub.3 sample, H.sub.2 uptake was measured for three different samples; each sample was analyzed three times to give a total of nine H.sub.2 titrations. This was done to determine reproducibility and obtain accurate values for the base sample.

[0033] 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 -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 H.sub.2, cooled to 170 C. in 20 sccm Ar and then O.sub.2 exposure was performed for 30 min in 20 sccm of flowing 10% O.sub.2 bal He. The catalyst was then cooled to 40 C. in 20 sccm of Ar and the feed was switched to 20 sccm 10% H.sub.2 bal Ar before TPR was performed up to 500 C. at a ramp rate of 20 C./min.

[0034] Following pretreatment, the reproducibility of H.sub.2 uptake on unpromoted 12 wt. % Ag/-Al.sub.2O.sub.3 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% H.sub.2/bal Ar. All samples were then purged with Ar at 280 C. for two hours, and then cooled to 170 C. where dissociative O.sub.2 adsorption occurred; after a second purge in flowing Ar, pulsed H.sub.2 titration was done. The pulsed hydrogen titration results are shown in FIG. 1 for the different 2.0 g samples. The calculated standard deviation of H.sub.2 uptake was +0.004 cm.sup.3 H.sub.2/g-sample, indicating a stable Ag surface with reproducible H.sub.2 uptakes.

[0035] The Ag active site concentration determined by H.sub.2 titration was 2.310.sup.18/g cat, which is approximately half as much as the value of 4.210.sup.18/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, 2.310.sup.18/g cat was used as the concentration of accessible Ag sites. For 12 wt. % Ag/-Al.sub.2O.sub.3, this corresponds to an Ag dispersion of 0.0034.

[0036] In order to determine reaction, or titration, trends of the promoters in the absence of Ag, a series of experiments using -Al.sub.2O.sub.3 promoted with higher loadings of NH.sub.4ReO.sub.4, CsNO.sub.3, or (NH.sub.4).sub.2MoO.sub.4 than traditionally used for authentic ethylene oxide catalysts were conducted to determine whether H.sub.2 adsorption or promoter reduction might occur to give hydrogen consumption during H.sub.2 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 H.sub.2 was performed from 50 C. to 500 C. following dissociative oxygen adsorption at 170 C. (TPR parameters: 50-500 C., 20 sccm 10% H.sub.2 bal Ar, ramp rate: 20 C./min). Results are shown in FIG. 2 (left hand side). As indicated, the 3 wt. % Cs sample showed no H.sub.2 uptake below 350 C., only minor uptake between 350 C. and 400 C., and significant uptake above 400 C. Given the extremely low ionization potential of Cs.sup.0, it is difficult to rationalize reduction of Cs.sup.+ at these conditions. The 3 wt. % Re/-Al.sub.2O.sub.3 sample followed a similar trend although H.sub.2 consumption began at a lower temperature of 275 C. with minor consumption occurring between 250 C. and 300 C. and substantial consumption between 300 C. and 400 C. with continued uptake to 500 C. 4 wt. % Mo/-Al.sub.2O.sub.3 showed no signs of hydrogen uptake until 425 C. To confirm the absence of H.sub.2 consumption at 170 C., fresh samples of each catalyst were subjected to a series of 10% H.sub.2/balance Ar pulses and results are shown on the right hand side of FIG. 2. These experiments confirmed that the oxides of Cs, Re, and Mo do not undergo reduction at the temperature of 170 C. used for AgO titration and also that they should be stable at temperatures up to at least 270 C.

[0037] The randomness of coverage of Cs on Ag was determined using the decrease in Ag site concentration from H.sub.2 titration data and the coverage of Cs on -Al.sub.2O.sub.3, which was estimated from the exposed surface of -Al.sub.2O.sub.3 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 -Al.sub.2O.sub.3 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.

[0038] For a non-interactive support such as low surface area -Al.sub.2O.sub.3, the value of 1.210.sup.19/m.sup.2 surface was used to represent the number of potential Cs adsorption sites on -Al.sub.2O.sub.3. The exposed -Al.sub.2O.sub.3 surface was estimated to be lowered from 0.73 to 0.53 m.sup.2/g after Ag coverage of the support, corresponding to a 27% coverage by Ag. Using the decrease in Ag site concentration from H.sub.2 titration data, the amount of Cs coverage of the Ag could be estimated. The amount of Cs available for the -Al.sub.2O.sub.3 surface was thus the nominal promoter loading minus the amount on Ag. The plot of FIG. 3 shows the resulting partitioning of Cs between Ag and -Al.sub.2O.sub.3 as a function of Cs loading in ppmw. The linear dependency of Cs deposition on both surfaces shows clearly that Cs placement was random on both surfaces and not targeted to either surface.

[0039] Catalysts were also formed including a fixed Cs concentration of 350 ppmw (2.6 mol/g-cat) and Re loadings varied between 150 and 500 ppmw (0.8 and 2.7 mol/g-cat, respectively). Samples are described in Table 3, below.

TABLE-US-00003 TABLE 3 Promoters (mol/g-cat) Sample Cs Re Total A B 2.6 2.6 C 6.0 6.0 D 7.5 7.5 E 2.6 0.8 3.4 F 2.6 1.1 3.7 G 2.6 1.3 4.0 H 2.6 1.6 4.2 I 2.6 2.7 5.3

[0040] H.sub.2 titrations as described previously were then carried out to determine the effect of Re loading on the samples. Results are shown in FIG. 4. As indicated, both the Cs-promoted samples and the CsRe samples followed a linear decrease in H.sub.2 consumption for the lower concentrations of Re (points E and F), indicating random distribution on both Ag and -Al.sub.2O.sub.3. The Cs+Re points fit on the linear Cs connecting line (points A-D). However, for the higher Re loadings (points G, H, I), the H.sub.2 uptake values fell below the Cs line, suggesting targeting of Re on the Ag particle.

[0041] It is believed that this is due to the lower surface free energy of Re.sub.2O.sub.7 (or ReO.sub.4.sup. salts) compared to Ag.sup.0 and -Al.sub.2O.sub.3. Surface free energies at 200 C. are approximately 1320, 805, and 29 ergs/cm.sup.2, for Ag.sup.0, -Al.sub.2O.sub.3, and Re.sub.2O.sub.7, respectively. All high valent Re salts have exceptionally low surface energies, indicating high mobilities and as a result, for the AgReO.sub.xAl.sub.2O.sub.3 system, the ReO.sub.x will be thermodynamically driven to diffuse onto the Ag surface to lower the SFE of Ag.

[0042] 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.