Cu(1) Acetylacetonate complexes for CO separation

Carbon monoxide is selectively removed from a gas stream by a process which comprises contacting the gas stream with an absorbent solution containing the reaction product of a Cu(I) compound and a halogenated acetylacetone of the formula: ##STR1## where X is a halogen, R.sub.1 is CX.sub.3, linear or branched C.sub.1 to C.sub.8 alkyl, C.sub.4 to C.sub.6 heterocycle containing O, S or N or C.sub.6 to C.sub.10 aryl, R.sub.2 is hydrogen or C.sub.1 to C.sub.6 alkyl, or R.sub.1 and R.sub.2 are joined together to form a C.sub.6 ring.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for absorbing carbon monoxide from a gas 
stream. In particular, the absorbing agent is the reaction product of a 
Cu(I) compound and a halogenated acetylacetone. 
2. Description of the Prior Art 
It is well-known that cuprous salt solutions will absorb carbon monoxide 
(CO). A review of the early literature relating to this topic may be found 
in J. Appl. Chem. (London), 15, 17-28 (1965). 
U.S. Pat. Nos. 3,401,112 and 3,517,079 relate to methods of separating 
olefins and vinyl aromatics using cuprous salts of fluoro-substituted 
carboxylates, fluoroborates and fluorophosphates. A process for separating 
CO from gas mixtures using copper (I) salts of sulfonic acids or dialkyl 
phosphates is disclosed in U.S. Pat. No. 4,042,669. U.S. Pat. No. 
4,048,292 teaches a method for preparing high purity CO from CO.sub.2 
-free gas streams using a copper ammonium C.sub.1-2 acetate as the 
absorbent medium. 
While many Cu(I) compounds are known to form carbon monoxide-containing 
complexes, they suffer from one or more disadvantages such as high 
corrosivity, low reactivity to CO, high energy cost to regenerate CO, low 
selectivity to CO and instability of the absorbent system. It would be 
highly desirable to have a method for selectively and efficiently removing 
CO from a gas stream while at the same time being able to regenerate the 
absorbent system and recovering CO under mild conditions. 
SUMMARY OF THE INVENTION 
It has been discovered that carbon monoxide can be selectively removed from 
a gas stream by a process which comprises contacting the gas stream with 
an absorbent solution containing an inert solvent and a Cu(I) salt, said 
salt being the reaction product of a Cu(I) compound and a halogenated 
acetylacetone of the formula: 
##STR2## 
where X is F or Cl, R.sub.1 is CX.sub.3, linear or branched C.sub.1 to 
C.sub.8 alkyl, C.sub.4 to C.sub.6 heterocycle containing O, S or N, or 
C.sub.6 to C.sub.10 aryl, R.sub.2 is hydrogen or C.sub.1 to C.sub.6 alkyl, 
or R.sub.1 and R.sub.2 are joined together to form a C.sub.6 ring. 
The process of the invention has the advantage that CO is selectively 
removed from the gas stream. Also, the carbonyl-containing complex which 
is formed upon CO absorption is readily decomposed under mild conditions, 
thereby regenerating the absorbing medium and CO at low energy cost. It is 
generally not necessary to pretreat the gas stream to remove gases such as 
olefins and CO.sub.2. 
DETAILED DESCRIPTION OF THE INVENTION 
The Cu(I) salts which react with CO to yield CO-containing complexes are 
formed by reacting a Cu(I) compound with halogenated acetylacetone. In a 
preferred embodiment, a basic cuprous compound such as Cu.sub.2 O, CuOH, 
CuOR or CuR where R is alkyl or aryl is treated with halogenated 
acetylacetone and the resulting acid-base reaction forms the Cu(I) salt. 
Another method making Cu(I) salts involves reacting the halogenated 
acetylacetone with a base such as NaOH and adding the resultant salt to a 
cuprous compound such as CuCl, CuCN or Cu.sub.2 SO.sub.4. 
Preferred Cu(I) salts for absorbing CO are the reaction products of a Cu(I) 
compound and a fluorinated acetylacetone of the formula: 
##STR3## 
where R.sub.1 is CF.sub.3, linear or branched C.sub.1 to C.sub.6 alkyl, 
C.sub.6 to C.sub.10 aryl, C.sub.4 to C.sub.5 heterocycle containing O, S 
or N, R.sub.2 is hydrogen, or R.sub.1 and R.sub.2 together form a C.sub.6 
ring. Especially preferred fluorinated acetylacetones include: 
##STR4## 
Preferred inert solvents for the Cu(I) salts are ethers, ketones, esters, 
alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, and halogenated 
derivatives thereof. 
Cu(I) salt concentration for absorbing CO are not critical and can vary 
over wide ranges. In order to maximize CO absorption, higher 
concentrations of Cu(I) salts of at least 0.001 M are preferred, 
especially 0.01 to 5 M. 
Carbon monoxide absorption can occur over a broad temperature range of from 
-80.degree. to +100.degree. C., preferably -10.degree. to +40.degree. C. 
The carbon monoxide partial pressure can be from 0.001 atm to 1000 atm, 
preferably 0.1 to 10 atm. Generally, as one increases the temperature of 
the absorbing medium, it is preferred to operate the subject process at 
higher ranges for the CO partial pressure. The increased CO partial 
pressure at higher temperatures helps to stabilize the CO-containing Cu(I) 
complex and also increases efficiency of absorption. 
The gas stream from which CO is to be removed may contain other gases such 
as N.sub.2, O.sub.2, CO.sub.2, alkanes, alkenes, alkynes, aromatics, 
H.sub.2 0, H.sub.2, SO.sub.2, SO.sub.3, H.sub.2 S, NH.sub.3 and nitrogen 
oxides. Gases such as olefins and acetylenes which might be expected to 
compete with CO do not affect the present process even in relatively high 
concentrations, nor is it poisoned by CO.sub.2 or H.sub.2 O. 
In order to desorb CO from the Cu(I) carbon monoxide-containing complex in 
solution, the solution may be purged with a CO-free inert gas, heated to 
temperatures of from 30.degree. to 200.degree., preferably 50.degree. to 
150.degree. C. in the absence of CO, or both purged with inert gas and 
heated. The nature of the inert purging gas is not critical so long as it 
is CO-free. Air and N.sub.2 are preferred purging gases. 
While not wishing to be bound to any particular theory, it appears that 
when CO is absorbed by the solution of Cu(I) salt containing a halogenated 
acetylacetone as ligand, an equilibrium is established which in the 
presence of CO lies far to the right, i.e., in favor of the CO-containing 
complex. The precise identity of the Cu(I) salt and CO complex is 
difficult to determine due to the complexity of Cu(I) solution chemistry. 
By purging with inert gas, heating or both, the equilibrium is shifted to 
favor the desorption process which results in the original Cu(I) salt and 
free CO. 
The process of the invention may be batchwise or continuous. One preferred 
embodiment for practicing the subject process is described as follows. The 
gas stream containing CO is contacted with the absorbing solution of Cu(I) 
salt. Typically, a countercurrent extractor may be employed wherein the 
absorbent solution is passed downwardly through the extractor while the 
gas stream flows upwardly. The gas stream exiting the countercurrent 
extractor is monitored for CO content and the flow rates of gas and/or 
absorbent solution may be adjusted accordingly. 
The solution leaving the bottoms of the countercurrent extractor contains 
the Cu(I) CO-containing complex and is conducted to a desorption stage. CO 
may be freed from the complex by heating preferably under reduced pressure 
to facilitate decomposition. The CO thus obtained is pure and may be 
further processed as desired. Alternatively, the solution from the 
countercurrent extractor may be purged with a CO-free inert gas such as 
air or N.sub.2 in a second countercurrent extractor. The solution from 
which CO has been desorbed is then recycled to the first countercurrent 
extractor for further CO absorption.

The process of the invention is further exemplified by the following 
examples. 
EXAMPLE 1 
A suspension of 10 mmole Cu.sub.2 O in 100 ml tetrahydrofuran was prepared 
and 20 mmole hexafluoroacetylacetone was added to this mixture. After 
stirring, most of the Cu.sub.2 O reacted to form a clear solution 
containing a Cu(I) hexafluoroacetylacetonate salt. A gaseous mixture with 
the composition 18.9% CO, 42.4% H.sub.2, 14.4% CH.sub.4 9.6% C.sub.2 
H.sub.6 and 14.7% CO.sub.2 was then bubbled through the solution at room 
temperature and a complex was formed with a CO:Cu(I) salt ratio of 1:1. 
Analysis of the gas mixture over the solution indicated an essentially 
complete removal of CO. 
In order to recover CO and regenerate the Cu(I) salt, N.sub.2 was passed 
through the solution. Alternatively, the solution was heated to 50.degree. 
C. until CO evolution ceased. 
EXAMPLE 2 
The procedure of Example 1 was repeated except that tetrahydrofuran is 
replaced with a solvent mixture of 90 ml of tetrahydrofuran and 10 ml 
water. Identical results were obtained showing that the absorption of CO 
is not appreciably affected by relatively large amounts of water. 
EXAMPLE 3 
The procedure of Example 1 was repeated except that the gaseous mixture 
bubbled through the Cu(I) salt solution contained about 50% N.sub.2, 30% 
CO and 20% C.sub.2 H.sub.4. An analysis of the solution and gas mixture 
over the solution indicated the formation of a 1:1 CO:Cu(I) complex and 
the nearly complete removal of CO from the gas. This demonstrates that the 
absorption of CO can take place in the presence of relatively large 
amounts of olefins.