Adsorption of Gases by Amine and Phosphine Complexed Mn(II) and Compounds

The adsorption of selected gases from a fluid stream provides a useful method of removing impurities from the fluid stream where the selected gases are impurities and a means to collect desired gases where the selected gas is useful in other processes. In one particular example, the selective adsorption of a gas from a fluid stream can be used to remove sulfur dioxide from an effluent gas stream such as the tower emissions from a coal-burning power plant. Selective adsorption can also be used to collect oxygen, lower alkenes, carbon monoxide, nitrogen oxides and other gases.

To be of any worthwhile economic value, it is highly critical that the adsorption be reversible, requiring minimal energy input. Compounds are known which reversibly adsorb gases from a fluid stream. For example, Cobalt (II), known as the Salen chelate and Fluomine chelate complexes of the Vaska type takeup oxygen reversibly. The adducts do not, under moderate conditions, release oxygen to regenerate the sorbent complex sufficiently readily to enable oxygen to be produced or nitrogen to be purified on a commercial scale.

McAuliffe et al U.S. Pat. No. 4,251,452 and continuation 4,323,543 disclose the use of manganese (II) complexes to reversibly adsorb oxygen, sulfur dioxide, hydrogen and ethylene. The manganese compounds disclosed have the following formula:

wherein L represents a mono-dentate ligand having the formula: ##STR2## in which formula R.sub.1, R.sub.2 and R.sub.3, which may be identical or different, represent substituted or unsubstituted alkyls, cycloalkyl or aryl groups or hydrogen, and X is a species capable of existing as an anion representing Cl.sup.-, Br.sup.-, I.sup.-, CN.sup.-, NO.sup.-.sub.2, NO.sup.-.sub.3, 0H.sup.-, NCS.sup.- or NCO.sup.-. Because of the reactivity of the ligand towards water, it is necessary in the preparation and use of these manganese (II) complexes to maintain anhydrous conditions. The complexes react quickly with water and are irreversibly inactivated. For this reason, these compounds are totally useless in an ambient environment or under the rigorous conditions of a flue stack where high concentrations of moisture vapors may be encountered.

Many different manganese compounds have been reported. Various compounds formed from hydrated manganese and various nitrogen containing ligands are known. For example, in Vol. 35 of The Journal of the Indian Chemistry No. 10,1958 several different compounds formed from manganese chloride and ammonia are disclosed. The compounds formed and discussed in this article are formed in an alcohol solution which prevents formation of Mn(II) compounds. In Zhurnal Neorganicheskoi Khimii, 20, 1852-1856 (1975) there is disclosed compounds of manganese (II) thiocyanate reacted with aniline or a toluidine. Since the manganese compound is not anhydrous, the formed compound was not a Mn(II) compound.

The Jour. and Proc. Inst. Chem. Vol. 37, November, 1965 reports compounds formed from manganese chloride (dehydrated) and dilute solutions of amines in an alcohol solvent. The compounds are formed in dry air. Oxygen present during the formation prevents formation of an Mn(II) compound. These references do not teach any method of adsorbing gas. Further none of these reported compounds adsorb oxygen or any other gases.

The present invention is premised upon the realization that a manganese (II) complex having the following general formula:

wherein L represents either a diphosphine ligand or a ligand which includes a nitrogen atom which can complex with the Mn(II) atom and which is sensitive to oxygen, nitrogen oxides, sulphur dioxide, carbon dioxide, carbon monoxide or lower alkenes will reversibly adsorb oxygen, sulfur dioxide, nitrogen oxides, carbon monoxide, carbon dioxide, lower alkenes or other gases. These compounds are not water sensitive.

In comparison to the compounds disclosed in McAuliffe et al, the compounds of the present invention adsorb more gas and act at a faster rate.

The above compounds must be formed by reacting an anhydrous manganese II compound such as a manganese halide with the ligand in an appropriate inert solvent free of water and alcohol. The reaction must be conducted in an inert environment. The formed product is capable of adsorbing and desorbing gases at relatively low temperatures and pressures and is not moisture sensitive.

These advantages as well as others will be appreciated in light of the following detailed description.

The manganese complexes used in the present invention are complexed manganese (II) salts. The general formula of these compounds is:

wherein L represents a diphosphine ligand or gas sensitive ligand, i.e., which is reactive with oxygen, nitrogen oxides, sulfur dioxide, carbon monoxide, carbon dioxide or lower alkenes at about 200.degree. C. or less and which is represented by either of the following general formulas:

wherein R.sub.1 generally represents one or more substituted or unsubstituted alkyl, cycloalkyl, aryl, arylalkyl or hydrogen group or a combination of the above and n represents an integer equal to 1-3. R.sub.2 or R.sub.3 also represents the same or different radicals including substituted and unsubstituted alkyl, cycloalkyl, aryl, arylalkyl or hydrogen. Alkyl also includes alkenyl and alkylene radicals. These radicals may be substituted with halogens, phosphine amines and alkoxy, ketone or aldehyde containing moieties but should not be substituted with oxidizing moieties such as esters, alcohols and carboxylic acid containing radicals.

Where L represents a diphosphine ligand it has the following general formula: ##STR3## wherein R.sub.4 generally represents the same or different radical selection from substituted and unsubstituted alkyl, cycloalkyl, aryl, arylalkyl or hydrogen group or a combination of the above. Alkyl also includes alkenyl and alkylene radicals. These radicals may be substituted with halogens, phosphines amines and alkoxy, ketone or aldehyde containing moieties but should not be substituted with oxidizing moieties such as esters, alcohols and carboxylic acid containing radicals. Preferably R.sub.4 is C.sub.1 --C.sub.5 alkyl; R.sub.5 represents substituted or unsubstituted alkylene and arylene groups and may be substituted with the same moieties listed above. Preferably R.sub.5 is C.sub.2 -C.sub.5 alkylene.

X is an anion or counter ion preferably representing C1.sup.-, Br.sup.-, I.sup.-, SCN.sup.- or OH.sup.-. Larger anions and anions which form strong close bonds with Mn(II), such as, for example, F.sup.-, do not function as well as the preferred species. This is perhaps due to steric hindrance.

More specifically, L can represent a variety of nitrogen containing ligands varying from NH.sub.3 and including such compounds as methyl amine, dimethyl amine, trimethyl amine, ethyl amine, diethyl amine, triethyl amine, N-propylamine, di-N-propylamine, tri-N-propylamine, isopropylamine, N-butyl amine, isobutyl amine, sec-butyl amine, tert-butyl amine, benzyl amine, phenyl ethylamine, ethylene diamine, 2-(N-methylamino) heptane, N-methyl-N-ethylene aniline, dipthalamine, aniline, methyl aniline, diphenyl amine, triphenyl amine, aminobenzoic acid, 1,2-diamino cyclohexane, diethylene triamine, tetramethyl cyclohexane-1, -2, diamine, 1,4,7,10-tetraazadecane, N,N-di-n-hexylethylenediamine, N-(.alpha.-methylbenzilidene) methylamine, o-toluidine, p-toluidine, N-phenyl-benzylamine, p-phenetidine, quinoline, quinoxaline, phenylhydrazine, dibenzylamine, N,N-dimethyl-m-toluidine, N,N-dimethyl-p-toluidine, 5-aminoquinoline, 6-aminoquinoline HCl, m-anisidine, p-anisidine, benzylamine, dibenzylamine, N-ethylaniline, 8-hydroxyquinoline, 1-napthylamine, .alpha.-(1-napthyl)ethylamine, p-aminodiphenylamine, o-phenylene diamine, m-phenylene diamine, p-phenylenediamine, 2,3-dimethylaniline, N,N-diethyaniline, tribenzylamine, piperazine, 2-(2-diethylaminoethyl) pyridine, di-n-butylamine, benzidine, 2,4-diaminodiphenylamine, tetraethylene pentamine, N-phenyl benzylamine, 2,4-diaminodiphenyl amine, o-dianisidine, N-butyfluorocarbon, as well as others. Higher molecular weight amines are preferred because they are more stable and decompose or evaporate at higher temperatures. Compounds formed from 1,4,7,10-tetraazadecane and tetraethylene pentamine demonstrate a superior rate of oxygen uptake.

Preferred diphosphine ligands include bis(alkyl phosphino) alkane wherein the alkane is C.sub.1 -C.sub.4 alkylene and the alkyl is C.sub.1 -C.sub.5 alkyl. Particularly suitable due to availability is 1,2bis(diethyl phosphino) ethane.

The compounds of the present invention are prepared by treating an appropriate manganese (II) compound with the ligand L preferably in a solvent.

It is extremely important that the manganese compound be anhydrous to enable formation of an Mn(II) compound. For use in the present invention, the manganese compound is preferably dried at 60.degree.-70.degree. C. for 8-12 hours by passing hot dry nitrogen over the manganese compound. The manganese compound will generally have the formula Mn(II)X.sub.2

The compound for use in the present invention is formed by contacting the ligand L with the anhydrous manganese compound in an appropriate solvent at room temperature under dry nitrogen conditions or other inert conditions. The ligand as stated above can be selected from a wide variety of nitrogen containing compounds or diphosphine compounds and generally should be selected from compounds which are sensitive to oxygen, nitrogen oxides, sulfur dioxide, carbon monoxide, carbon dioxide or lower alkenes, in other words compounds which react with any of these gases in the temperature range of less than 230.degree. C. Preferably the ligand is one which reacts with oxygen, i.e., is oxygen sensitive at temperatures less than 100.degree. C. and preferably less than 30.degree. C. The ligand must also be an anhydrous ligand or dehydrated and can be purchased in this form.

The solvent can be any inert organic solvent which can dissolve both the manganese compound MnX.sub.2 and the ligand. Further the solvent must be dry and alcohol free. Alcohols are unsuitable for use as solvents in the present invention since they will react with the MnX.sub.2 compound and prevent the formation of the compound of the present invention. Suitable solvents include tetrahydrofuran (THF) or toluene which have been dried by refluxing with sodium for 8-12 hours.

The reaction is conducted by combining the ligand with the MnX.sub.2 compound in solution under a blanket of nitrogen. Generally 15-20 milliliters of solvent per gram of MnX.sub.2 compound will be employed. A slight molar excess of ligand up to about 10% relative to the manganese present is used in the present reaction. A greater excess of ligand interferes with the formation of the final solid compound in that it has to be removed to obtain a functional compound.

The reaction is conducted at room temperature by mixing the MnX.sub.2 compound with the ligand and solvent under dry nitrogen. This forms a solution which is mixed for 20-40 minutes and allowed to sit for about 2 hours.

When L is an amine, a precipitate should immediately form. This precipitate is usually not the compound of the present invention. The formed precipitate is filtered off under nitrogen. The solvent in the resulting solution is then removed by evaporation under vacuum at room temperature. A solid compound will remain. This is the compound of the present invention having the formula Mn(II)LX.sub.2.

In certain cases, the formed product Mn(II)LX.sub.2 is insoluble in the selected solvent and itself precipitates out. Under these circumstances, no other precipitate is formed and the product, i.e., the precipitate, can be recovered by filtration and drying. For example, MnBr.sub.2 compounds which are insoluble in THF include compounds formed with the following ligands: tetramethyl cyclohexane-1,2,diamine,1,4,7,10tetraazadecane, N,N di-n-hexylethylenediamine, N-(.alpha.methylbenzyilidene) methylamine, p-phenetidine, quinoline, N,N-dimethyltoluidene, 7-aminoquinoline.HCl, anisidine, benzylamine, dibenzylamine, tribenzylamine, 1-naphthylamine, phenylene diamine, piperazine, 2-(2-diethylaminoethyl) pyridine, N-phenyl benzylamine, and 2,4diaminodiphenylamine. Spectrographic analysis of the precipitate provides a means to determine if the Mn (II)LX.sub.2 compound remains in solution or precipitates from solution.

Where L is a diphosphine no precipitate forms. The Mn(II) LX.sub.2 compound remains in solution until the solvent is chilled to 5.degree.-10.degree. C. or the concentration is increased by evaporating a portion of the solvent. The compound then precipitates as a crystalline solid and is collected by filtration. Because the diphosphine ligand as purchased may contain monophosphine impurities, Mn(II) complexed to the monophosphine may be present. These are removed by placing the collected crystals in THF at room temperature. This would immediately dissolve any Mn(II) complexed to the monophosphine. The crystals of the diphosphine complex do not readily redissolve in THF and can be collected by filtration and dried.

In an alternate embodiment the compound Mn(II)LX.sub.2 can be formed by depositing the composition on a solid support preferably porous aluminum oxide (Al.sub.2 O.sub.3) beads. In this embodiment dried aluminum oxide beads are added to the solution of ligand and Mn(II)LX.sub.2 after the initial precipitate has been filtered off. The solvent is removed by evaporation causing the aluminum oxide beads to be coated with the formed Mn(II)LX.sub.2 compound. This is preferred where L is an amine. In this operation the upper and lowermost level of the aluminum oxide beads are discarded or marked because they generally have surface deposition of the complex.

The complex of the present invention can be used to selectively adsorb gases from a fluid stream by simply contacting the fluid stream with the Mn(II)LX.sub.2 complex at effective temperatures and pressures. Gases which can be selectively adsorbed include oxygen, sulphur dioxide, nitrogen oxides, carbon monoxide, carbon dioxide and lower (C.sub.2 -C.sub.5) alkenes. The gases are selectively adsorbed by contacting the mixture of gases with the manganese complex at a selected temperature and pressure. By controlling the temperature and pressure any one of these gases can be selectively adsorbed and desorbed from the manganese complex to provide a means to effectively separate selected gases from a stream of gas.

The temperature at which the adsorption will occur will range generally from about 30.degree. C. to about 220.degree. C. at about 1 atmosphere. The adsorption is reversed by increasing the temperature generally to 10.degree. to 15.degree. C. above the temperature at which the gas was adsorbed.

The adsorption can be further increased by increasing the pressure. Generally adsorption can occur at 1 atmosphere or higher and desorption can occur at less than 1 atmosphere down to a total vacuum.

With respect to the adsorption of oxygen, the most efficient adsorption temperature is from about 30.degree. C. to about 50.degree. C. at about 50 to about 75 psi. However, some compounds can adsorb oxygen at much lower temperatures such as -10.degree. C. The amount of oxygen adsorbed will depend on the adsorption temperature and pressure and particular compound used. At room temperature and pressure the amount of oxygen adsorbed will generally equal from about 0.5% to about 8% of the weight of the manganese complex. Maximum uptake is one mole of gas per mole of complex.

Sulphur dioxide is preferably adsorbed at substantially higher temperatures. For some compounds sulphur dioxide adsorbtion at atmospheric pressure is maximized at temperatures from about 200.degree. C. to about 220.degree. C. These are the typical stack temperatures encountered in a coal burning power plant. At atmospheric pressure desorption occurs at about 230.degree. C. Sulphur dioxide adsorption can occur at lower temperatures but preferential adsorption of SO.sub.2 in a mixed fluid stream generally requires higher temperatures.

When adsorption and desorption of SO.sub.2 and other gases is conducted at these higher temperatures, the Mn(II)LX.sub.2 compound must be one which is stable at these higher temperatures. More specifically the compound must be one wherein the ligand L as well as the MnX.sub.2 salt are stable at these temperatures. MnCl.sub.2 is preferred. Nitrogen containing ligands which are stable at 230.degree. C. are known and include 1-napthylamine, and quinoxaline. These ligands are particularly suitable because they are sensitive (i.e., react) with SO.sub.2 at 200.degree. C.-230.degree. C.

At atmospheric pressure nitrogen oxides, carbon monoxide, carbon dioxide and lower alkenes are adsorbed at temperatures generally between 0.degree. C. and 200.degree. C. again with desorption at about 15.degree. C. above the adsorption temperature. This again will vary depending on the compound used. Generally for any particular compound the minimum gas adsorption temperature will be lowest for oxygen, highest for sulfur dioxide adsorption. The minimum adsorption temperatures for nitrogen oxides, carbon monoxide, carbon dioxide and the alkenes will be above the minimum adsorption temperature for oxygen and below the minimum adsorption temperature for sulfur dioxide.

The adsorption and desorption temperature not only varies with respect to the gas being adsorbed but also with respect to the particular Mn(II)LX.sub.2 complex formed. When the complex is coated onto the aluminum oxide beads adsorption is substantially improved however the temperature of adsorption remains about the same.

The manganese complex of the present invention facilitates a wide variety of gas separations. For example, the complex can simply be used as a solid complex and contacted with the gas to be adsorbed at the effective temperatures and pressures. Alternately it can be dissolved in an appropriate solvent and adsorb gas by bubbling the gas through or over the solvent.

Adsorption and desorption of gas can also be conducted by simply varying the pressure and not actively changing the temperature with high pressure promoting adsorption and low pressure promoting desorption.

The present invention will be further appreciated by reference to the following examples in which a manganese (II) complex was contacted with a fluid stream having a defined oxygen or sulphur dioxide content, the oxygen or sulphur dioxide uptakes were measured.

Solid Mn[N(CH.sub.3).sub.3 ]Br.sub.2 was placed in the pan of a thermogravametric analyser (TGA) which was flushed with nitrogen. Dry air was introduced and maintained at 25.degree. C. for a few minutes. Oxygen was taken up by the compound in an amount equal to 0.187 mg O.sub.2 per gram of compound.

Solid Mn[N(i-C.sub.3 H.sub.7).sub.3 ](SCN).sub.2 was placed in the pan of a TGA which was flushed first with nitrogen. Dry air was introduced and the temperature was maintained at 50.degree. C. Over a period of a few minutes, 0.389 mg of oxygen were taken up per gram of compound. The oxygen was desorbed at a temperature of 120.degree. C.

Solid Mn[N(i-C.sub.3 H.sub.7).sub.3 ](SCN).sub.2 was placed in a beaker which was flushed first with nitrogen and then sulfur dioxide and sealed. Over a period of a few minutes at 50.degree. C., 0.474 mg SO.sub.2 was adsorbed per gram of Mn(II) complex. The SO.sub.2 was desorbed at 120.degree. C.

Mn[N(C.sub.2 H.sub.5).sub.3 ]Br.sub.2 was placed in the pan of a TGA which was flushed with nitrogen. Dry air was then introduced and the temperature maintained at 50.degree. C. The total oxygen uptake was 0.270 mg of oxygen per gram Mn(II) complex.

Mn[N(C.sub.2 H.sub.5).sub.3 ]Br.sub.2 was placed in a beaker which was flushed with nitrogen and then SO.sub.2 and sealed. After a few minutes at 50.degree. C., 0.123 mg of SO.sub.2 was adsorbed per gram of Mn(II) complex. Sulfur dioxide was desorbed at 120.degree. C.

Mn[C.sub.10 H.sub.7 NH.sub.2 ]Cl.sub.2 (formed from MnCl.sub.2 and napthylamine) coated on inert ceramic beads was placed in a beaker. The beaker was flushed with nitrogen and then sulfur dioxide and sealed. The temperature of adsorption was 200.degree. C. After about five minutes the pressure stabilized and about 0.18 mg of SO.sub.2 was adsorbed per gram of complex.

Mn[NH.sub.2 (CH.sub.2 CH.sub.2 NH).sub.3 CH.sub.2 CH.sub.2 NH.sub.2 ]Br.sub.2 was placed on the pan of a TGA which was flushed with nitrogen. Dry air was then passed over the pan and the temperature was maintained at 36.degree. C. After a few minutes 0.079 mg of oxygen was taken up per gram of Mn(II) complex.

Manganese (II) diisothiocyanate (Mn(SCN).sub.2) crystals were dried under vacuum at 50.degree. C., 10 mm Hg for 48 hours to remove all traces of moisture. Anhydrous chromatographic grade tetrahydrofuran (THF) from Fisher Scientific was refluxed for about 24 hours under nitrogen with potassium metal. The distilled THF was collected under nitrogen flow and used immediately during the complex preparation. The ligand 1,2-bis(diethyl phosphino) ethane was used as received in vials under argon from Strem Chemicals.

A glove box was evacuted and flushed with nitrogen several times. All chemicals were transferred to the nitrogen filled glove box directly from their respective ovens and distillation column. Five grams of Mn(SCN).sub.2 was dissolved in 300 ml of freshly distilled THF and stirred for 30 minutes. THF was added as needed to complete dissolution. The vial of ligand handled in the nitrogen glove box was transferred to a small beaker about 10 to 15 ml of THF was added to molar quantities of the ligand (relative to the Mn(SCN).sub.2) in the beaker. The mixture was stirred and added slowly to the solution of THFMn(SCN).sub.2. An immediate reaction occurs and there is a change in color from pale yellow to golden yellow. Stirring is continued for two hours under nitrogen atmosphere to ensure the completion of the reactions. Crystallization of the formed compound is promoted by evaporating a portion of the solvent at room temperature under nitrogen. The crystals are collected by filtration.

This compound adsorbs O.sub.2 at temperatures below 65.degree. C. and desorbs O.sub.2 at above 65.degree. C. at atmospheric pressure.

The complex of the present invention is useful in a variety of applications where oxygen, sulfur dioxide, nitrogen oxides, carbon monoxide, carbon dioxide or a lower alkene must be separated from a fluid. Since the compound is not sensitive to moisture, the complex can be used in ambient conditions without concern for deactivating the complex.