WO2007092216A2

WO2007092216A2 - Oxidation catalyst

Info

Publication number: WO2007092216A2
Application number: PCT/US2007/002585
Authority: WO
Inventors: Natasha Dropka; David Linke; Debra Ann Ryan
Original assignee: Celanese International Corporation
Priority date: 2006-02-07
Filing date: 2007-01-30
Publication date: 2007-08-16
Also published as: TW200803983A; WO2007092216A3; AR059342A1

Abstract

The present invention is directed to a procedure for preparing an oxidation catalyst for a fluid bed reactor which will not show detrimental performance due to the technique used to support the active phase. The catalyst of the present invention contains a novel oxide phase that forms over the surface of the spherical catalyst particles. This phase is more porous than the catalyst precursor and is composed of some of the chemical elements present in the catalyst precursor in addition to the chemical elements of the support material.

Description

OXIDATION CATALYST

BACKGROUND OF THE INVENTION ioooi I Catalysts for the gas phase oxidation of hydrocarbons to useful products such as olefins and/ or carboxylic acids have been known for many years. To offer commercial viability, oxidation catalysts must be able to demonstrate good hydrocarbon conversions at process conditions wherein the selectivity to useful products is high and the production of carbon oxides is minimized. Poor catalyst activity is often compensated for by increasing the severity of the process conditions, particularly by increasing the reaction temperature to improve the reaction rate. This can lead to poor efficiencies to desirable products.

100021 On the other hand, high catalyst activity can itself be detrimental if the removal of heat from the reactor is poorly managed, as this may result in high localized reaction temperatures which may also lead to poor selectivities. Hydrocarbon oxidations are highly exothermic reactions and control of reaction temperature can be especially challenging, Use of fluid bed reactors instead of fixed bed reactors has been discussed as a means of better controlling the exotherm. (US 5300684, WO 00/14047, US 2004/0133039) While fluid bed reactors have been described which can effectively control the reaction temperature, other performance issues remain. In attempts to improve the mechanical stability of oxidation catalysts so that they can be used in a fluidized bed reactor, other performance issues such as reduced activity or decreased selectivity to desired products can result. Improving oxidation catalysts for their specific use in a fluidized bed, while maintaining activity and selectivities comparable to a fixed bed is therefore an area of continuing interest.

100931 Presently several techniques exist for supporting an active catalytic material for fluid bed applications. Fluid bed catalysts can be prepared from a catalyst precursor by using one of three primary methods: impregnation on an attrition resistant support, encapsulation by an attrition resistant coating, or embedding individual particles of the catalyst precursor in an attrition resistant matrix. Impregnation is one of the most commonly used techniques for fluid bed catalyst preparation. The impregnation method generally involves filling the pores of a preformed support with a solution or slurry of the catalyst or catalyst precursor. Generally speaking, the catalyst or catalyst precursor is dissolved or slurried into a solvent, and the attrition resistant support is added to the catalyst/ catalyst precursor solution. Encapsulation is another preparation technique where the mechanical properties of the catalyst are improved by encapsulating the catalytic active particle with a porous attrition-resistant shell, usually a silica or an alumina. US 4677084 to Bergna and US 6107238 discuss techniques for encapsulation of catalysts. Finally, the embedding method involves embedding or binding catalyst particles with a porous abrasion resistant matrix of a non-catalytic material, such as titania, zirconia, or boron phosphate. While each of these preparation methods are vastly different, each method is directed to maximizing attrition resistance of the fluid bed catalyst, while the catalyst is in operation in a fluid bed reactor. However, performance data of the resulting fluidized bed catalyst prepared by the different techniques can vary greatly, even resulting in uneconomical technologies.

SUMMARY OF THE INVENTION iooo4i The present invention is directed to a new procedure for preparing an oxidation catalyst for a fluid bed reactor which will not show detrimental performance due to the technique used to support the active phase. It was discovered, while processing a catalyst precursor using the methods of the present invention, that a novel oxide phase forms over the surface of the spherical catalyst particles. This new phase is more porous than the catalyst precursor and is composed of some of the chemical elements present in the catalyst precursor in addition to the chemical elements of the support material. The formation of this new phase contrasts this catalyst composition from what would be achieved in a traditional embedding procedure, where the intact catalyst particles would be found interspersed in a matrix of the pure 'embedding' component. This new phase further contrasts with an encapsulated catalyst where catalyst particles would be coated by a pure shell of the encapsulating compound. A supported catalyst prepared in this manner shows improved performance compared to the catalyst precursor.

BRIEF DESCRIPTION OF THE DRAWINGS iooo5| The figures refer to the gas phase oxidation of ethane using the modified catalysts of the invention. "Precursor' in this context refers to the non-modified catalyst.

(0006J Figure 1 shows the space-time yield of ethylene at three different temperatures for precursor and catalysts treated with zirconia.

100071 Figure 2 shows the space-time yield of acetic acid at three different temperatures for precursor and catalysts treated with zirconia.

₁0008) Figure 3 shows the space-time yield of ethylene at three different temperatures for precursor and catalysts treated with titania. iooo9i Figure 4 shows the space-time yield of acetic acid at three different temperatures for precursor and catalysts treated with titania.

(ooioi Figure 5 shows the space-time yield of ethylene at three different temperatures for precursor and catalysts treated with A12O3. looπj Figure 6 shows the space-time yield of acetic acid at three different temperatures for precursor and catalysts embedded with A12O3.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

10012) As used throughout this description, the present invention is illustrated by describing the preparation of a modified oxidation catalyst which could be used in a fluid bed reactor for the production of ethylene and/or acetic acid from ethane and/or ethylene by gas phase oxidation. However, it should be noted that the disclosure contained herein is applicable to any mixed oxide catalyst used for any oxidation process, and is therefore not limited to catalysts for the oxidation of ethane and/or ethylene. Furthermore, while the description is directed toward the use of a catalyst in a fluid bed reactor, the disclosed catalyst could also be advantageously used in a fixed bed reactor. _loon] Generally, a catalyst precursor is first prepared according to normal procedures for the specific precursor. While the embodiments disclosed herein are directed to a single catalyst precursor, it should be known that any catalyst precursor, particularly an oxidation catalyst precursor, would benefit from the present invention. Once the catalyst precursor is formed, it can optionally be calcined prior to the modification described herein. The catalyst precursor is next modified by the use of inorganic sols and then dried to create the catalyst of the present invention. While the examples used herein were dried using a spray drying process, the important aspect is that the catalyst precursor slurry be formed into dry particles of the required shape and size for fluidization. Acceptable alternatives to spray drying would include freeze drying and vacuum drying, both of which are known in the art.

(ooi4| The modified procedure, also an embodiment of this invention, is described below in the form of several specific examples.

Preparation of the Catalyst Precursor

(ooi5| In one embodiment of the present invention, an oxidation catalyst precursor having the composition Mθ|.ooVo.₅₅Nbo.o₉Sbo.oiCao.₀iPdo.ooo₇₅θ_x was prepared according to the procedure described in U.S. Patent No. 6,852,877, incorporated herein by reference in its entirety. Three separate solutions were prepared, a first solution comprising 80g ammonium molybdate in 400ml water, a second solution comprising 29.4g ammonium metavanadate in 400ml water, and a third solution comprising 19.01g niobium ammonium oxalate, 1.92g antimony oxalate, and 1.34g calcium nitrate in 200ml water. The three solutions were stirred separately at 70°C for 15 minutes. The third solution was then combined with the second solution and stirred at 70⁰C for another 15 minutes, after which the combined second and third solutions were added to the first solution. Thereafter, a fourth solution of 0.078g palladium(II)acetate in 200ml ethanol was added to the mixture of the first three solutions. The resulting mixture was evaporated to obtain a remaining total volume of 800ml. This mixture was spray-dried at 180⁰C followed by drying the powder in static air at 120⁰C for 2 hours and was then calcined at 300⁰C for 5 hours in static air.

(0016_J Subsequently, the calcined catalyst precursor was modified with sol precursors of ZrO≥, TiO₂ or Al₂O₃, generated in situ from their corresponding metal alkoxides, as described below, so as to improve both the physical and chemical properties of the catalyst.

[0OI7J The catalyst modification procedure employed by the present invention was a modification of the embedding procedure described by Martin (Martin, F. Entwicklung und kinetische Untersuchung eines Wirbelschichtkatalysators fur die Maleinsaureanhydrid- Herstellungaus n-Butan. Ph.D. Dissertation, Erlangen-Nurnberg, 1989), incorporated herein by reference. However, as explained within, the modified procedure does not produce an 'embedded' catalyst but results in a novel surface modification of the catalyst precursor particles. 10018] The modified procedure, also an embodiment of this invention, is described below.

Modification with Zirconia iooi9| To generate a zirconia sol in situ, Zr(O-nPr)₄ was chosen as the sol precursor. The oxidation catalyst precursor was created as described above and the proportion of the catalyst precursor to water, as well as the proportion of solvent to Zr(O-IiPr)₄, were calculated according to the instructions provided by the Martin reference. However, in the catalyst preparation technique of the present invention, iso-propanol, as used in Martin, was replaced with n- propanol. Only 1/3 of the calculated volume of solvent was n-propanol , while water replaced the remaining 2/3 of the calculated volume of solvent.

100201 The procedure for preparing the suspension used for the treatment is as follows. First, the oxidation catalyst precursor described above was mixed with water for 10 minutes using an ultrasound stirrer, forming Solution A. Then, the calculated volume of solvent, of which 2/3 was water as described above, was mixed with the Zr(OnPr)₄ solution (a commercially available 70% Zr(OnPr)₄ in n-propanol) in a glass vessel cooled to O⁰C using an ultrasound stirrer, giving Solution B. Solution A was slowly added drop wise into Solution B. During the addition, the mixture was continuously stirred and maintained at O⁰C. Spray drying ensued immediately after the two solutions were completely mixed. The spray dryer feed slurry was maintained at O⁰C and stirred continuously with a magnetic stirrer. The spray dryer was operated at an inlet temperature of 220⁰C. After spray drying, all samples of the modified catalyst were calcined at 300⁰C for five hours under static air in a muffle furnace.

100211 Four different samples were developed to obtain varying amounts of zirconia incorporation, calculated as ZrO₂, in a range of 6-12 wt % of the total weight of the modified catalyst formulation (Table 1).

Table 1

Amounts of chemicals used for preparation of modified catalysts at various amounts of zirconia in final sample. All catalysts were calcined before modification and after.

10022J The zirconia-modified catalysts were tested in fixed-bed reactors to determine activity and selectivity for the oxidation of ethane in the gas phase to form ethylene and acetic acid. The modified catalysts were used in the particle size range that was obtained in the spray-drying process. The catalysts were all diluted with 7 times the amount of quartz before testing. The selectivities of the different catalysts are summarized in Table 2. The catalyst productivities as represented by space time yield (STY) to the desired products (ethylene and acetic acid) are shown graphically in Figures 1 and 2. Table 2

Conversions (X) and selectivities(S) for ZrO₂ modified catalyst compared with the non-modified catalyst precursor. Reactor feed: C₂H₆:θ2:N^' ₂=40:8:52. P=J6 bar. Total feed flow= 16

Nml/min; catalyst mass = 200 rng

|0023| As can be seen by examination of the data in Table 2, the catalysts that were modified with zirconia show that the modification actually improves catalytic performance compared to the catalyst precursor. The modification with zirconia increases selectivity to ethylene and CO at the expense of acetic acid and CO₂, which is desirable if ethylene is a more desired product than acetic acid.

(00241 Modifying the catalyst precursor with zirconia in general increases the ethane conversion. Furthermore, as shown in Figures 1-2, modifying the catalyst precursor with zirconia in general increases the catalyst productivity. This effect is even more significant if it is taken into account that in each experiment, the same mass of catalyst was used, which in the case of the modified catalyst means lower amount of "active mass." For the cited examples, the optimal concentration of zirconia in the sample based on catalytic performance is 12 wt% ZrO₂. This sample has the highest conversion of ethane and the highest space-time yield, STY, of ethylene and acetic acid and produces minimal amount of CO₂ at all temperatures. 12 wt% ZrO₂ was the highest level examined. Because the trend was still increasing at this level, it is likely that higher levels ol ZrO2 may give even better performance. Modification with Titania

10025₁ To generate a titania sol in situ, Ti(O-iPr)₄ was chosen as the sol precursor. The procedure for the preparation of the suspension for modifying the oxidation catalyst precursor with titania was the same as for the preparation of the zirconia modified catalyst. The oxidation catalyst precursor was prepared as described above. The proportion of catalyst to water, as well as the proportion of iso-propanol to Ti(O-iPr)₄ were calculated according to the instructions provided by the Martin reference. Unlike with the zirconia samples, iso-propanol, as used by Martin, was not replaced with n-propanol. After the appropriate amount of iso-propanol was calculated, only 1/3 of the volume of iso-propanol was used, while the rest of the iso-propanol was replaced with water. Four different samples were developed to obtain varying amounts of titania incorporation, calculated as TiO₂, in a range of 6-12 wt % of the total weight of the modified catalyst formulation (Table 3).

Table 3

Amounts of chemicals used for preparation of modified catalysts at various amounts of titania in final sample. All catalysts were calcined before modification and after.

100261 The titania-modified catalysts were tested in fixed-bed reactors to determine activity and selectivity for the oxidation of ethane in the gas phase to form ethylene and acetic acid. The modified catalysts were used in the particle size range that was obtained in the spray drying process. The catalysts were diluted with 7 times the amount of quartz.

|0027| The catalytic performance of the samples modified with titania is shown in Table 4 and in Figures 3-4. Table 4

Conversions(X) and selectivities(S) far TiO₂ modified catalyst compared with the non-modified catalyst precursor. Reactor feed: C₂H₆:O₂:N₂= 40:8:52. P—16 bar.. Total feed flow= 16

Nml/min; catal st mass = 200 m .

|0028| Titania modified catalysts generally convert less ethane and produce less ethylene, but more acetic acid and CO_2j in comparison to the zirconia modified catalysts under similar conditions. However, ethane conversions are higher for the titania modified catalysts on an equal "active mass" basis when compared to the unmodified catalyst precursor. The 6 wt% TiO2 sample performed similar to the catalyst precursor, and can be an alternative to zirconia modified catalysts if desired. As presented in Figures 3-4, the 6 wt% TiO2 sample generally has lower ethylene STYs and higher acetic acid STYs than the unmodified catalyst precursor sample.

Modification with Alumina ioo29| To generate an alumina sol in situ, Al(OC₄H^)₃ was chosen as the sol precursor. The catalyst precursor was created as described above, calcined, and then proportions of catalyst to water as well as i-propanol to A1(OC₄H₉)₃ were calculated according to the instructions provided by the Martin reference. After the appropriate amount of iso-propanol was calculated, only 1/3 of volume of iso-propanol was used, while the rest of the iso-propanol was replaced with water. Four different samples were developed to obtain varying amounts of AI2O3 in a range of 6-12 wt% of the total weight of the modified catalyst formulation (Table 5).

Table 5

Amounts of chemicals used for preparation of treated catalysts at various amounts of alumina in a final sample. Samples were calcined before modification and after.

|0030| The alumina-modified catalysts were tested in fixed-bed reactors to determine activity and selectivity for the oxidation of ethane in the gas phase to form ethylene and acetic acid. The modified catalysts were used in the particle size range that was obtained in the spray-drying process. The catalysts were diluted with 7 times the amount of quartz. The catalytic performance of the alumina modified samples is presented in Table 6 and in Figures 5-6.

Table 6

ConversionsζX) and selectivities(S) for AI2O3 modified catalysts compared with the non- modified catalyst precursor. Reactor feed:

16 Nml/min; catalyst mass — 200 mg.

|003i| Alumina modified catalysts generally showed increased of selectivity to ethylene at the expense of acetic acid and CO₂ compared to the catalyst precursor.

Comparison of Modifiers

10032| Modification of the catalyst precursor with zirconia, titania, or alumina in the range 6-12 % by weight has a positive impact on catalytic performance of these catalysts. Modifying the oxidation catalyst precursor with these oxides did not damage selectivity or activity, and in some cases, improved selectivity and activity were observed. For maximum ethane efficiency, the optimal loading for each group of treated catalysts were found to be 12 wt% Zrd, 12 wt% AI₂O₃ and 6 wt% TiO₂. The zirconia catalyst has higher conversion of ethane and higher selectivity to acetic acid and COx than the alumina treated catalyst. Despite being less active and selective in comparison to zirconia and alumina treated catalysts, 6 wt% Tiθ₂-treated catalyst behaves similarly to the precursor, and can be an alternative to zirconia and alumina treated catalysts if desired.

Analysis of Catalysts

(0033| The BET measurements for the zirconia, titania, and alumina modified samples (Table 7) showed that all modified catalysts have a higher BET surface area than the catalyst precursor, and also show that the modified catalysts are mesoporous with an average pore diameter about 100 Angstroms (with the exception the AI₂O₃ embedded sample with 64.4 Angstroms).

Table 7 BET measurements for ZrO 2, TiO? andAl2θ₃ treated samples in comparison to the catalyst precursor (* analysis adsorptive N₂).

|0034| In addition to BET measurements, TEM/EDX characterization was performed for the three optimal modified catalysts in each group: 12 wt% ZrO₂, 12 wt% AI₂O₃ and 6 wt% TiO₂ and compared to the catalyst precursor. For TEM characterization, the samples were ground in an agate mortar and later transferred to lacey-carbon coated grids. Transmission electron microscopy (TEM) measurements were performed using CM20 microscope (Philips) at 20OkV and energy dispersive X-ray analysis by PV 9900(EDAX). The EDX analysis was performed. 10035) In the zirconia modified catalyst, particles are covered by an outer layer of a new compositional oxide phase consisting of zirconia, molybdenum, and vanadium. The compositions of the new outer layer and the interior of the spherical catalyst particles are shown in Table 8.

Table 8 Composition of Catalyst after modification to contain 12 wt% ZrOz, determined by TEM/EDX

joo36i An outer layer covering the catalyst particles is also seen for the alumina modified catalyst, as shown in Table 9, which is compositionally similar to the outer layer in the zirconia modified catalyst (Table 8).

Table 9 Composition of Catalyst after modification to contain 12 wt% AI₂O₃, determined by TEM/EDX

10037) Finally, TEM results for the titania modified sample show that spherical catalyst particles are partially covered with a similar outer layer. The composition of the outer layer in the titania modified catalyst is shown in Table 11.

Table 10 Composition of Catalyst after modification to contain 6 wt% T1O₂, determined by TEM/EDX

(0038) The TEM/EDX studies of these treated catalysts show the presence of a new phase at the external surface of the spherical catalyst particles. This new phase is more porous and is composed of chemical elements present in the catalyst precursor as well as the chemical elements derived from the sol used for the modification. Neither the presence of a pure matrix of the sol itself, nor a pure shell composed only of the elements derived from the sol, was revealed. Therefore, the microscopy indicates that traditional encapsulation has not occurred, where the outer layer would have the composition of the 'encapsulating' agent, which in the present case would be essentially zirconia, alumina, or titania.

10039) While the above examples show results for modifications with the oxides of ZrO₂, TiO₂, and Al₂θ3, other suitable oxides can be useful in this invention, such as silica, P₂O₅, and MgO or their combinations. It is a requirement of this invention that the oxide modification occurs by treatment with the oxide precursors, or sols, as described.

10040) These results show that a catalyst prepared by the techniques described herein may show improved catalytic performance compared to the catalyst precursor. This catalyst is useful in the oxidation of alkanes or alkenes to useful products, such as the oxidation of ethane to ethylene as described above. The oxidation of ethane can be carried out in a fluidized bed or in a fixed bed reactor. The gaseous feedstock, and any recycle gas combined with said feedstock gas, contains primarily ethane, but may contain some amount of ethylene, and is fed to the reactor as a pure gas or in a mixture with one or more other gases. Suitable examples of such additional or carrier gases are nitrogen, methane, carbon monoxide, carbon dioxidej air and/or steam.. The gas containing molecular oxygen may be air or a gas which has a higher or lower molecular oxygen concentration than air, for example pure oxygen. The reaction is generally carried out at about 200 to about 500⁰C, preferably about 200 to about 400⁰C. The pressure can be atmospheric or superatrnospheric, for example about 1 to about 50 bar, preferably about 1 to about 30 bar.

|004i| The oxidation reaction produces a mixture of gases including ethylene, acetic acid, water, CO_x (CO and CO₂), unreacted ethane, and assorted heavy by-products. The product gas effluent from the reactor is preferably filtered to remove catalyst fines and is then routed to a recycle gas scrubber, which produces a top stream containing ethylene, ethane, and CO_x. The top stream from the recycle gas scrubber is routed to a fixed bed CO converter followed by a processing step that removes the CO_x from the top stream. The stream is then routed to an ethylene purification tower that provides product ethylene as a top stream and ethane as a bottom stream, which is recycled to the oxidation reactor.

[0042| The bottom stream from the recycle gas scrubber, which contains acetic acid, water, and heavy ends by-products, may be purified as known in the art to provide purified acetic acid. For example, the bottom stream may be routed to a drying column to remove water followed by a heavy ends column to remove propionic acid and other heavy components.

(0043| While the disclosure set forth herein was described for the use of a single oxidation catalyst precursor, it should be understood that the methods and procedures outlined herein may be used for any oxidation catalyst precursor, and should not be limited to the catalyst precursor described herein.

Claims

1. A process for the preparation of an oxidation catalyst suitable for the oxidation of alkanes and /or alkenes comprising: a. forming an oxidation catalyst precursor; b. slurrying the oxidation catalyst precursor in a first solvent to form a first mixture; c. mixing an oxide precursor, as its sol or sol precursor in a second solvent to form a second mixture. d. mixing the first and second mixtures to form a third mixture; e. drying the third mixture to form the modified catalyst precursor; f. calcining the modified catalyst precursor, wherein said oxidation catalyst is formed with an outer oxide layer comprised of 30-60 element % of the elements derived from the sol or sol precursor, and 40-70 element % of the elements vanadium and molybdenum, or a combination thereof.

2. The process of claim 1 , wherein the oxide is selected from the group consisting Of ZrO₂, TiO₂, AI₂O₃, SiO₂, P₂O₅, MgO, and combinations thereof.

3. The process of claim 1, wherein the sol precursor is selected from the family of metal alkoxides, M(OR)x where M is selected from the group consisting of Zr, Ti, Al, Si, P, and Mg, and R is any alkyl or aryl moiety.

4. The process of claim 1 , wherein the sol is derived from known sol precursors including hydrolyzed zirconyl nitrate, titanyl oxychloride, basic aluminum chloride, silicic acid, phosphoric acid, or magnesium acetate.

5. The process of claim 1 wherein the drying process is selected from the group consisting of freeze drying, vacuum drying, or spray drying.

6. The process of claim 1 , wherein the catalyst is a mixed metal oxide catalyst.

7. The process of claim 1 , wherein the oxidation reaction is carried out in a fluid bed reactor.

8. The process of claim 1, wherein the oxidation reaction is carried out in a fixed bed reactor.

9. A process for the catalytic oxidation of an alkane and/ or alkene to useful products, comprising reacting an alkane and /or alkene with a molecular oxygen-containing gas ai elevated temperature in the presence of an oxidation catalyst prepared according to the procedure of Claim 1.

10. The process of claim 9, wherein the metal oxide is selected from the group consisting of ZrO₂. TiC>2, Al₂θ3, silica, P₂O5, and MgO and combinations thereof.

11. The process of claim 9, wherein the catalyst is a mixed metal oxide catalyst.

12. The process of claim 9, wherein the oxidation reaction is carried out in a fluid bed reactor.

13. The process of claim 9 wherein the oxidation reaction is carried out in a fixed bed reactor.

14. A catalyst composition for use in the oxidation of alkanes and/or alkenes, comprising, a. anoxidation catalyst precursor; and b. an oxide derived from a sol or a sol precursor; wherein said oxidation catalyst comprises an outer layer comprised of 30-60 element % of chemical elements derived from the sol or sol precursor, and 40-70 element % of chemical elements derived from the oxidation catalyst precursor.

15. The composition of claim 14, wherein the oxidation catalyst precursor comprises molybdenum.

16. The composition of claim 14, wherein the oxidation catalyst precursor comprises vanadium.