Organic compounds -- part of the class 532-570 series – Organic compounds – Persulphonic acids or salts thereof
Reexamination Certificate
2002-10-23
2004-03-16
Vollano, Jean F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Persulphonic acids or salts thereof
C560S129000
Reexamination Certificate
active
06706914
ABSTRACT:
FIELD OF THE INVENTION
The field to which this invention pertains is generation and regeneration of a rhodium containing solution. This solution can be used as a catalyst precursor for the industrial manufacture of acetic acid, acetic anhydride, ethylidene diacetate and related processes utilising compounds of rhodium.
BACKGROUND OF THE INVENTION
For the industrial production of acetic acid, carbonylation of methanol has long been the preferred method. Dimethyl ether and methyl acetate and mixtures of these two components may also be used as feed, optionally admixed with methanol. These reactions are catalysed by certain transition metals together with iodide in the form of methyl iodide and/or hydriodic acid. Of the transition metals, rhodium and iridium are preferred due to their high activity and selectivity. In industrial plants, the metal catalyst exists in a dissolved form during operation of the plant. In those cases, where the catalyst is rhodium, it is generally recognised that the predominant rhodium containing specimen present under the conditions of operation is the anionic complex [Rh(CO)
2
I
2
]
−
. This complex is formed—in the form of the acid or as a salt—from virtually any rhodium source under the conditions of operation (at a temperature above 150° C. and at least 5 bar (5·10
5
Pa) CO-pressure).
The currently preferred rhodium source for start-up of acetic acid and acetic anhydride plants is rhodium iodide, RhI
3
, which in pure form appears as a black solid. It is not desirable to transfer it to the reactor zone in an undissolved form, since the dissolution of this compound under the conditions of operation takes some time, and several complications may occur before the dissolution is complete. Rhodium iodide, however, is well known to be practically insoluble at ambient conditions. Thus water, acetic acid, methanol, methylacetate and other common solvents have no effect on RhI
3
. Although it has some solubility in hydriodic acid, the solubility is low, and very high concentrations of hydriodic acid are necessary. Other rhodium compounds are more soluble, e.g. RhCl
3
, but they precipitate upon admixture with solutions of hydriodic acid due to reaction with the iodide ions present in the solution. Thus, since iodide is needed as a co-catalyst, RhI
3
precipitates in the reactor upon admixture of a RhCl
3
solution with the iodine containing co-catalyst solution. Certain complexes of rhodium are known to be more stable in the presence of iodide, but they are not preferred since they are generally costly and difficult to handle. Some of these compounds furthermore contain elements or molecular entities, the presence of which may be undesirable. Examples are sulphur, chlorine, bromine and arsenic containing complexes. Even though rhodium iodide may react with reducing agents and thus be brought into solution, such solutions are often sensitive towards air, which may re-oxidise rhodium and thus reprecipitate RhI
3
. Another problem pertains to rhodium being fairly noble, thus the admixture of rhodium iodide with strong reducing agents may potentially cause over-reduction to elemental rhodium.
It is well known that dissolution of RhI
3
is accomplished under certain circumstances by reduction in the presence of a solvent. Such reduction may be carried out with hydrogen or carbon monoxide or synthesis gas at elevated temperatures and pressure, in which case the active form of the catalyst [Rh(CO)
2
I
2
]
−
is formed directly according to the equation
RhI
3
+3 CO+H
2
O=H[Rh(CO)
2
I
2
]+CO
2
+HI
The solution thus obtained is stable towards high concentrations of iodide, but only as long as a minimum pressure of carbon monoxide is maintained and only as long as air and other oxidising agents are avoided. This is the method of current practice in the art of catalyst generation for rhodium catalysed acetic acid production.
Obviously, this method is tedious and costly since the catalyst solution must be prepared under pressure and with applied heat and also must be transferred to the methanol carbonylation reactor under pressure. Furthermore, it is well known that rhodium iodide (probably in an impure form) precipitates in certain parts of the internals of acetic acid plants during operation. Due to the very high price of rhodium, it is feasible to regenerate the catalyst from these solid precipitates. Regeneration of the catalyst solution from the solid may be carried out in the same way as described above for start-up.
A more preferable catalyst formulation would be a solution containing rhodium and iodide in high concentrations. This solution should be stable towards precipitation of RhI
3
, stable on exposure to air and water, and should not contain undesired elements such as sulphur, arsenic and the like. Such a catalyst formulation would allow for easy and less expensive transportation and simple transfer to the reactor.
As explained above, however, such a catalyst formulation is very hard to achieve. It was therefore highly surprising to discover that solid RhI
3
dissolves with hydrazine hydrate and particularly that solutions with a very high concentration of rhodium can be formed and are stable for at least several months.
The present invention thus provides a method for generating a concentrated solution of rhodium starting from solid RhI
3
and other solid rhodium sources by combination with a reducing agent, preferably hydrazine and hydrazine derivatives. Said solution does not form any precipitate of rhodium iodide or other compounds even upon admixture with large quantities of hydriodic acid. Said solution is stable at ambient conditions, very stable towards air and is easily prepared without application of external pressure and heating sources.
The use of hydrazine and other reducing agents for the activation of rhodium containing catalysts has been claimed in a number of patents: JP 63/227531 A, JP 87/60062 A, JP 62/148437 A, JP 85/289269 A, JP 88/041892 B, JP 56/144747 A, DE 3,115,032 A, U.S. Pat. No. 4,376,724 A, JP 83/018147 B, U.S. Pat. No. 4,420,420 A, DE 3,115,032 C. The known technique, however, claims use of a reducing agent for activating a solid (heterogeneous) rhodium containing catalyst without the catalyst being dissolved. Obviously, it is not the intention of the processes described in the above patents to prepare rhodium containing solutions.
Some other patents claim the use of rhodium and hydrazine in combination to form homogeneous catalysts. Thus, NO 169342 B, DK 164815 B and U.S. Pat. No. 4,550,096 teach the preparation of homogeneous hydrogenation catalysts and U.S. Pat. No. 5,051,522 teaches the use of hydrazine to prepare a hydroformylation catalyst. None of these patents pertain to the field of methanol carbonylation, however, and the stabilities of the rhodium catalysts toward iodide are not discussed.
A process for recovering Group VIII noble metals by extraction with amines, including the use of hydrazine, is claimed in U.S. Pat. No. 4,341,741. According to this patent, noble metals such as rhodium and iridium used in the carbonylation of methyl acetate and dimethyl ether, accumulate in a residue formed during the carbonylation reaction, containing typically 0-4% wt/wt rhodium. A sample containing 1% Rh was at first partly extracted with dilute hydrochloric acid and methylene chloride, leading to a 0.5% Rh content in the residue. Ten (10) miligram of the acid extracted residue was then dissolved in 5 ml methyl acetate and treated with 0.1 ml of hydrazine hydrate. It was demonstrated that more than 85% of the residue-bound Rh was extracted this way. However, the concentration of rhodium in the extract was no more than 0.01 g/l or less than 0.1·10
−3
M. Typical rhodium concentrations in an acetic acid plant reactor are 1.0·10
−3
M, and a catalyst solution should preferably be even more concentrated to facilitate transport and start-up.
The rhodium containing solutions of the present invention carry up to as much as 0.34 M rhodium, and have fu
Dickstein , Shapiro, Morin & Oshinsky, LLP
Haldor Topsoe A/S
Vollano Jean F.
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