CA1180725A - Process for stabilizing carbonylation catalyst in soluble form - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides for an improved process wherein an olefin, an alcohol, or an ester, halide or ether derivative of said alcohol is reacted with car-bon monoxide in a liquid phase in the presence of a cata-lyst system that contains (a) a rhodium component, and (b) an iodine or bromine component. By passing at least a portion of the liquid reaction mass from the reaction zone to a separation zone of substantially lower CO par-tial pressure, at least a portion of the carbonylation products, as well as unreacted carbon monoxide, inert gases and unreacted olefin, alcohol or alcohol derivatives are vaporized and can be withdrawn from the separation zone. Precipitation of the rhodium catalyst under cata-lyst under carbon monoxide deficient conditions is pre-vented or retarded by addition to the system of a stabi-lizer which is a compound of tin germanium, antimony or an alkali metal.

Description

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2~-19-1266 BACKGROUND OF THE INVENTION
The present invention relates to a carbonyla-tion process improvement. More particularly, this inven-tion relates to an improved process scheme wherein at least a por~ion of the reaction mass from a carbonylation process can be withdrawn from the reactor and separated at a lower pressure from a catalyst-con`taining stream which is recycled to the reactor. In this processing scheme the catalyst is stabilized in soluble form and any of the catalyst which may have precipitated is reconverted to a soluble form.
Recently, processes for producing carboxylic acids and esters by carbonylating olefins, alcohols, esters, halides and ethers in the presence of homogeneous catalyst systems that contain rhodium and halogen components such as iodine components and bromine components have been dis-closed and placed into commercial operations. These re-cently developed processes represent a distinct improve-ment over the classic carbonylation processes wherein such feed materials have been previously carbonylated in the presence of such catalyst systems as phosphoric acid, phos-phates, activated carbon, heavy metal salts and metal car-bonyls such as cobalt carbonyl, iron carbonyl and nickel carbonyl. All of these previously known processes require $~

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the use of extremely high partial pressures of carbon monoxide. These previously known carbonylation systems also have distinc-t disadvantages in that they require higher catalyst concentrations, longer reac~ion times, higher temperatures to obtain substantial reaction and conversion rates that all result in larger and more costly processing equipment and higher manufacturing costs.
The discovery that rhodium and iodine or bro-mine containing catalyst systems will carbonylate such feed materials as olefins, alcohols and esters, halide or ether deriva~ives of the alcohols at relatively mild pres-sure and temperature conditions was a distinct contribu-tion to the carbonylation art. In spite of the vast superiority of these newly developed catalyst systems, it has been found that conventio~al processing schemes for separation of the carbonylation products from the liquid reaction mass has posted problems of catalyst inactiva-tion and precipitation from carbon monoxide-deficient streams.
It has been disclosed to U.S. Patent 3,845,121 that by withdrawing a portion of the liquid reaction mass from the reactor and passing it to a separation zone of substantially lower pressure, without the addition of heat, at least a portion of the carbonylation products can be vaporized and passed on to purification equipment with much reduced decomposition of the carbonylation catalyst system.
According to ~his scheme, the carbonylation reaction is carried out in the reaction zone at a temperature o~ from about 50 to about 500C and a pressure of from about 345 to about 10340 kPa. By withdrawing a portion of the liquid reaction mass and passing it to a separation zone that is maintained at a pressure that is substantially lower than the pressure in the reactor, at least a portion of the car-bonylation products will vaporiæe with much reduced decom-position of the liquid catalyst system. This vaporization will take place without the addition of heat to the re-action mass. The unvaporized liquid in the separation zone .3 containing the catalyst system can be recycled to the reactor.
Using this processing scheme, it has been found that catalyst precipitation may occur, though to a reduced degree, from liquid streams which are deEicient in carbon monoxide.
Such streams include the stream of reaction mass withdrawn from the reaction zone, in which CO has been consumed by reaction, and the liquid cycle stream returned from the separation zone to the reaction zone.
From U.S. Patent 3,818,060 it is known th~t penta-valent nitrogen and phosphorous compounds of the form ~NR3 orXP~3 wherein X is oxygen or sulfur may be used as stabilizers for rhodium catalysts in the liquid phase carbonylation of ethylenically unsaturated compounds. Also, from U.S. Patent

3,579,552 it is known that, inter alia, phosphines, amines and trihalostannate compounds form coordination complexes with rho-dium and carbon monoxide which remain soluble in the carbonyla-tion of ethylenically unsaturated compounds.
Accordingly, it is an object of this invention to pre-vent precipitation of the soluble catalyst system from CO-deficient streams.
Additional objects of this invention will becomeapparent from the following discussion of the invention.

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-3a-SUMMARY OF THE INVENTION
The present invention iæ an improvement in a carbonyl-ation process wherein at least one reactant selected from the group consisting of an alcohol, an ester derivative of said al-cohol, a halide derivative of said alcohol and an ether deriva-tive of said alcohol is reacted with carbon monoxide in a li-quid phase in a reaction zone in the presence of a catalyst system that contains (a) a rhodium component, and (b) an iodine or bromine component. Such a process further includes the steps of passing at least a portion of the liquid reaction mass in which the carbon monoxide has been depleted from the reaction zone to a separation zone and recycling the remaining liquid reaction mass from the separation zone to the reaction zone. In accordance with an embodiment oE the present inYentiOn, the improvement in the above process comprises adding to the process an amount of a compound of an element selected from tin, germanium, antimony or an alkali metal soluble in the reaction mass, said amount being sufficient to maintain the rhodium component in soluble formO
In a carbonylation process, generally at least a portion of the carbonylation products are separated from the liquid reaction mass at a reduced CO partial pressure in a separation zone. From this separa-~ S 20-19-1266 tion zone, an unvaporized liquid stream which is enriched in the catalyst system components is withdrawn and recycled ~o the reaction zone for reus~ in the carbonylation pro-ces~. A recycle pump is employed to increase the pressure of this liquid stream to enable its transfer back into the high pressure reaction zone.
Under the conditions of reduced CO partial pres-sure existing in the separation zone and piping connecting the separation zone to the reaction zone, a small portion lQ of the catalyst system may decompose, forming an insoluble rhodium containing precipitate. According to the present invention, a compo~md of tin, germanium, antimony or an alkali metal is employed as a catalyst stabilizer for rho-dium catalysts in the carbonylation of methanol. Prefer-ably, the stabilizer compound is employed in a molar ratioof at least about 0.5 to the rhodium present.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention is directed to the recently de-veloped carbonylation processes wherein oleins, alcohols and ester, halide and other derivatives of the alcohols are reacted with carbon monoxide in a liquid phase system in the presence of a homogeneous catalyst system that con-tains (a) a rhodium, and (b) an iodine or bromine compon-ent.
This invention solves the catalyst precipitation problems which may be encountered in the process of separa-tion of the carbonylation products from the liquid mass which involves withdrawing at least a portion of the liquid reaction mass from the reactor and passing it to a separa-tion zone that is maintained at a substantially lower pres-sure. The lower pressure in the separation zone results in the vaporization at at least a portion of the carbony-lation products which are then withdrawn from the separa-tion zone in the vapor form. The unvaporized liquid in the separation zone containing the stable catalyst system can then be recycled to the reactor for reuse in the carbony-lation process~ According to this invention, the rhodium carbonyl halide catalys~ complex is s~abilized by addition )'7~

of a compound of tin, germanium, antimony or an alkali metal. The stabilizer compound is preferably employed in a molar ratio of at least 0.5 to the rhodium present.
When reference is made to the "catalyst system"
throughout this disclosure of this invention, it means a catalyst system that forms on combining two distinct compo-nents in the presence of carbon monoxide. The two essen-tial catalyst precursor materials are (a) a rhodium, and (b) an iodine or bromine component while C0 is a third com-ponent.
Rhodium components suitable for use as constitu-ents in the catalyst are described and set Ollt in U.S.
Patent 3,845,121.
The iodine or bromine precursor component of the catalyst system used herein may be in combined form with the rhodium as1 for instance, one or more ligands in a co-ordination compound of the rhodium. However, it is gener-ally preferred to have an excess of the iodine or bromine present in the reaction system over the iodine or bromine that exists as ligands of the rhodium compounds. The bro-mine or iodine precursor can be in the form of elemental bromine or iodine as well as combinations of bromine or iodine such as hydrogen iodide, hydrogen bromide, alkyl iodide, alkyl bromide, aryl iodide, aryl bromide, iodide salts, bromide salts and the like. Suitable non-limiting examples of such compounds of bromine and iodine include methyl iodide, methyl bromide, ethyl iodide, ethyl bro-mide, sodium iodide, potassium iodide, sodium bromide, potassium bromide, ammonium iodide, ammonium bromide and the like.
Generally, it is preferred that the amount of iodine precursor material added to the reactlon system will be in an amount such that the atomic ratio of the iodine or bromine to the rhodium is about 2:1. Prefera-bly, the a-tomic ratio of the iodine or bromine to the rho-dium will be in a range of 5:1 to 5000:1. A more pre-ferred atomic ratio of the iodine or bromine to the rho-dium is 10:1 to 2500:1O

~B~ 7~5 Suitable sources o:E tin stabilizer components in-clude, but are not limited to tin metal, stannous chlor-ide, stannous oxide, organo tin compounds such as tetral-kyl tin, stannic chloride, stannic oxide, stannous acetate and the like. The valence of the tin in the tin component may be 0, 2 or 4.
Other suitable stabilizer compounds, according to this invention, include but are not limited to the ha~
lides, acetates, oxides, salts and the like of germanium, antimony and al~ali metals.
The catalyst system forms by combining the fore-going rhodium and halogen in the presence of carbon mon-oxide in a liquid reaction medium. The liquid reaction medium employed may include any solvent compatible with the catalyst system and may include pure alcohols or mix-tures of the alcohol feedstock and/or the desired carboxyl-ic acid and/or esters of these two compounds. However, the preferred solvent or liquid reaction medium for the process of this invention is the desired carbonylation pro-ducts such as the carboxylic acid and/or ester of the acid and an alcohol. Water is also often in the reaction mix-ture to exert a beneficial effect upon the reaction rate.
Suitable feedstock materials for the process are set out in U.S. Patent 3,845,121.
Methanol and ethylene are two of the most pre-ferred feedstocks that are utilized in the practice of our invention.
In carrying out the carbonylation reaction, the above-mentioned feedstocks are intimately contacted with car-bon monoxide in a liquid reaction medium that contains the above-mentioned catalyst system. The catalyst system can be preformed outside of the reactor by combining the necessary catalyst precursors or it can be formed in situ.
Generally, the catalyst will be employed in such amounts as to provide a concentration of soluble rhodium in the reaction medium of from about 10 ppm to about 3000 ppm de-~ J

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pending upon ~he equipment size, desired reaction time and other factors. The carbon monoxide reactant can be sparged into the reactor in such a manner as to intimately contact the carbon monoxide with the reactants in the reaction mass. The pressure in the reactor will generally be main-tained in the range of -from 345 to about 10340 kPa. As disclosed in the prior art, the foregoing known carbony-lation process is carried out at a temperature range of from about 50C to about 500C with a preferred temperature range of from about 100 to about 250C. The optimu~ tem-perature and pressure maintained in ~he reactor will vary depending upon the reactants and the particular catalyst system utilized. The catalyst, feedstock materials and general reaction parameters set out in the foregoing dis-cussion are known inthe art.
A portion of the liquid phase reaction mass is withdrawn from the reactor and passed to a separation zone that is maintained at a pressure that is lower than the reactor pressure. This pressure reduction will cause at least a portion of the carbonylation products to vaporize and separate from the unvaporized residue of the liquid reaction mass. The aforementioned catalyst system will remain in this residue of unvaporized liquid reaction mass and can be recycled to the reactor.
Generally, it is preferred that the separation zone be maintained at a pressure of at least 138 kPa lower than the pressure in the reactor. The pressure in the reaction is usually in the range of about 345 to 10340 kPa.
Thus, the separation zone is maintained at a pressure less than 10200 kPa. It has been found that the separation zone can be maintained at very low pressure, even approach-ing a complete vacuum; however, it is usually desirable that the separation zone be maintained at a positive pres-sure to eliminate vapor compression equipment and the like in handling the vaporized carbonylation products that are withdrawn from the separation zone. By maintaining pres-sure differential of at least 138 kPa between the reactor 1:~8~ S

and the separation zone, a substantial amount of the car-bonylation products can be vaporized from the liquid reac-tion mass.
The exac~ pressure of the separation zone will vary, depending on ~he temperature and pressure maintained in the reactor. It is important that the pressure differ-ential between the separation zone and the reactor be at least 138 kPa to insure vaporization o a substantial por-tion of thP carbonylation products in the separation zone.
It is also important that the total pressure in the separa-tion zone be less than the vapor pressure of the carbony-lation products in the liquid reaction mass withdrawn from the reactor at the temperature of the liquid reac~ion mass.
For example, if at the temperature and pressure of the re-actor the carbonylation products to be vaporized have avapor pressure of 1380 kPa, the separation zone should be operated at a pressure of less than 12~0 kPa. Preferably, the separation zone of this invention will be operated at a pressure of from about 69 to 1380 kPa. Most preferably, the separation zone will be operated at a pressure of about 100 to 690 kPa.
The separation zone should be large enough ~o allow the liquid reaction mass that is passed to it from the reactor to be maintained in said separation zone for a suficient period o~ time to vaporize the desired car-bonylation products, prior to recycling the unvaporized liquid containing ~he homogeneous catalyst system back to the reactor. Usually, a residuce time of at least one minute in the separation zone is sufficient.
Following separation of the desired carbonyla-tion products, the unvaporized liquid portion of the reac-tion mass containing any precipitated catalyst decomposition products leaves the separation zone and is introduced into the suction of a recycle pump which increases the pressure of this stream sufficiently to permit its injection back into the reaction zone.

3~7~S . 20-19 -1266 The piping ~hrough which a portion of the reac-tion mass is withdrawn from the reaction zone, as well as the piping thTough which the liquid recycle stream is ~ransferred back to the reaction zone by the recycle pump, will be at substantially the pressure of the reaction zone.
As used herein, "substantially the pressure of the reaction ~one" means the reaction zone pressure plus or minus pres-sure changes caused by fluid flow through the respective lines. ,, Depending upon the temperature and pressure of the transfer piping, a minor amount of the carbonylation catalyst system according to the prior art (i.e., not in-cluding ~he stabilizer component of the present invention, may decompose and precipitate from the liquid in the piping.
The catalyst system is believed to comprise a carbonyl com-plex of the rhodium component and ~he halide component and it is further believed that carbon monoxide may be abstracted from a portion of the carbonyl'complex form of the catalyst system converting some of the catalyst to an insoluble form which may comprise a rhodium halide. Be- I
cause the rhodium component of the catalyst system is quite expensive, it is desirable to recover any traces of pre-cipitated catalyst for return to the reaction zone and re-use.
~ According to the present invention, the rhodium catalyst is maintained in soluble form in these caTbon monoxide deficient portions of the process by addition to the system of a compound of tin germanium, antimony or an alkali metal.
~' The stabilizer compound is employed in a molar ratio of at least abou* O.i to the rhodium present in the catalyst system. The stabilizer compound may be injected into the caTbonylation reaction system at any convenient point, but is pre~erably injected into the transfer piping leading from the carbonylation reactor to the separation zone, or int~o the piping which conducts the catalyst-containing recycle stream from the separation zone back to the reactor, in order to insure complete mixing of the stabilizer with the catalyst-containing liquid system.
The practice of this invention is illustrated by the following examples which should not be construed as limiting the invention in any way.
In the following examples a stock solution was prepared which simulated the liquid recycle stream which is returned from the separation zone to the carbonylation reac~or in a typical acetic acid plant. Included in this solution were traces of iron, nickel, chromium and molyb-denum which are normally found in acetic acid plants as corrosion products. The stock solution employed acetic acid as the solvent and contained the following:
Iron 0.025 moles/liter Nickel 0.02 " ~
Chromium 0.016 " "
Molybdenum 0.01 " "
Water 9.5 " "
Total iodides 0.5 " "
Labile methyl 0.35 " "
groups (methyl iodide+methanol -~methyl acetate) To establish a base run in which no tin com-ponent stabilizer was present, the following experiment was performed. About 650 milliliters of the stock solu-tion described above plus a rhodium solution and hydrogen iodide was charged into a 1500 milliliter stirred auto-clave and pressured with carbon monoxide to a pressure of 791 kPa. The contents were heated with stirr{ng and when a temperature of 150-155C was reached methanol and methyl iodide were added. The autoclave contents were then im-mediately cooled to 125-128C under a pressure of 205 kPa, refluxed, and were sampled and found to contain 444 parts per million (ppm) dissolved rhodium.

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The autoclave contents were sampled periodi-cally and analyæed for dissolved rhodium. The results of these analyses were as follows:
Time Ater Methanol ppm Dissolved % af Original Addition (minutes? _Rhodium Dissolved Rhodium This experiment clearly demo~strates tha~ in the absence of a catalyst stabilizer the rhodium rapidly precipitates from the solution in the autoclave.
EX~MPLE 2 Using the equipment and procedure of Example 1, the base case run was repeated except that the autoclave solution contained 0.00512 mols/liter of anhydrous SnC14.
Total iodine and total labile methyl groups were as in Example 1 initially. Initial dissolved rhodium was 434 ppm. Samples were taken and analyzed periodically while the temperature was maintained at 125-126C and the fol-lowing results obtained.
Time After Methanol ppm Dissolved % of Original Addition (minutes) Rhodium Dissolved Rhodium These results clearly indicate that the tin stabilizer greatly retarded the rate of rhodium precipita-tion from the autoclave solution.

~0 To show the variety of tin components which may be used ~s stabilizers, the experiment of Example 1 was repeated except that the autoclave solution contained the tin components shown in the following Table as a stabi-lizer. The initial total iodine level and the initial total labile methyl groups were as in Example 1 and the initial dissolved rhodium was as shown in the Table. Peri-odic samples for dissolved rhodium gave the results shown.
In all cases, the tin component was present in a concen-tration of 0.0045 moles/liter, except in Example 3 th~
tin component concentration was 0.0046 moles/liter.

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20-19~1266 Usîng the equipment and procedure of Example 1, the base case run was repeated except that the autoclave solution contained 0.0045 mols/liter of GeI4. Total io-dine and total labile methyl groups were as in Example 1initially. After 85 minutes at a temperature of 128-129C, the solution was analyzed and found to contain 379 ppm dissolved rhodium or 85% of the original dissolved rhodium.
These results clearly indicate that the stabili-zer greatly retarded the rate of rhodium precipitation fromthe autoclave solution.

The experiment of Example 1 was repeated except that the autoclave solution contained 0.2 mols/liter of lithium acetate and 0.7 mols/liter total iodide. Auto-clave temperature was 126-128C. Periodic sampling for dissolved rhodium gave the following results:
Time After Methanol ppm Dissolved % of Original Addition (minutes) Rhodium Dissolved rhodium C~

The experiment of Example 1 was repeated using as a stabilizer 0.2 mole/liter of potassium iodide. After 124 minutes of refluxing at 128-129C, the solution was analyzed and found to contain 228 ppm or 51% of the origi-nal dissolved rhodium.

To establish another base run under different conditions in which no stabilizer was present, the follow-ing experiment was performed. About 650 milliliters of astock solution containing 0.027 moles/liter iron, 0.019 moles/liter nickel, 0.014 moles/liter chromium, 0.007 moles/liter molybdenum, 1.40 moles/liter total iodide, 8.9 moles/liter water and 1.44 moles/liter labile methyl groups using acetic acid as the solvent plus a rhodium so-lution and hydrogen iodide was charged into a 1500 mil-liliter stirred autoclave and pressured with carbon monox-ide to a pressure of 791 kPa. The contents were heated with stirring and when a temperature of 185C was reached methanol and methyl iodide were added. Temperature was maintained at 185C.
The autoclave contents, which ini~ially con-tained 416 ppm dissolved rhodium, were sampled and ana-lyzed for dissolved rhodium after 61 minutes. Dissolved rhodium ran 168 ppm or 40% of the original.
Again, this experiment clearly demonstrates that in the absence of a catalyst stabilizer the rhodium rapidly precipitates from the solution in the autoclave.

The procedure of Example 11 was repeated using the stock solution of that Example and using the stabili-zer compounds shown in the following Table. The results were as shown below.

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a carbonylation process wherein at least one reactant selected from the group consisting of an al-cohol, an ester derivative of said alcohol, a halide de-rivative of said alcohol and an ether derivative of said alcohol is (1) reacted with carbon monoxide in a liquid phase in a reaction zone and in the presence of a catalyst system that contains (a) a rhodium component, and (b) an iodine or bromine component, (2) passing at least a portion of the liquid reaction mass in which the carbon monoxide has been depleted from the reaction zone to a separation zone, and (3) recycling the remaining liquid reaction mass from the separation zone to said reaction zone, the improvement which comprises adding to the process an amount of a compound of an element selected from tin ger-manium, antimony or an alkali metal soluble in said reac-tion mass, said amount being sufficient to maintain the rhodium component in soluble form.

2. The process of Claim 1 wherein the catalyst system contains a rhodium component and an iodine compon-ent.

3. The process of Claim 2 wherein methanol is carbonylated to acetic acid.

4. The process of Claim 3 wherein the stabi-lizer compound is the halide or acetate of the element selected from tin germanium, antimony or an alkali metal.

5. The process of Claim 3 wherein said stabi-lizer is present in a molar ratio of at least 0.5 to the rhodium component.

6. The process of Claim 5 wherein the stabili-zer is GeI4.

7. The process of Claim 5 wherein the stabili-zer is SbCl3.

8. The process of Claim 5 wherein the stabili-zer is lithium acetate.

9. The process of Claim 5 wherein the stabili-zer is potassium iodide.

10. The process of Claim 5 wherein the tin component is tin metal

11. The process of Claim 5 wherein the tin component is SnC14.

12. The process of Claim 5 wherein the tin component is SnO.

13. The process of Claim 5 wherein the tin com-ponent is K2SnO3.

14. The process of Claim 5 wherein the tin com-ponent is stannous acetate.

15. The process of Claim 5 wherein the com-ponent is stannous linoleate.