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Anal. Chem. 1985, 57, 1572-1577
Selective Retention of Oxygen Using Chromatographic Columns Containing Metal Chelate Polymers John N. Gillis' and Robert E. Sievers* Department of Chemistry and Cooperative Institute for Research i n Environmental Sciences, University of Colorado, Boulder, Colorado 80309 Glenn E. Pollock N A S A - A m e s Research Center, Moffett Field, California 94035 Porous polymers containing various metal chelates bonded to nltrogen functionalitles on the surface of the polymer have been synthesized and found to bind oxygen reverslbiy. The stationary phases containing [5,5'-( 1,2-ethanediyidinltrlio)bis( 2,2,7-trimethyi-3octanonato)]cobalt( I I ) were found to be the most suitable of the phases investigated for separating oxygen from argon, nitrogen, and carbon monoxide. At ambient temperatures, near 25 O C , the reversible interaction of molecular oxygen with the transition-metal complex bonded to the statlonary phase results in a marked increase in the retention time of oxygen, relative to species that have similar retentlon times In columns that do not contain the metal chelate. The stationary phase can be used alone to achieve the separatlon of low molecular weight gases or in series with another column. The metal chelate stationary phase is selective for oxygen and little change in the retention time of oxygen Is observed after hundreds of lnlections over a severai-month period, indicating that no appreciable degradation of the statlonary phase had taken place under these conditions.
The gas chromatographic separation of oxygen from nitrogen and argon at room temperature cannot be easily accomplished by methods previously reported. Although this separation can be accomplished using a molecular sieve column at subambient temperatures (1,2) or with stationary phases containing catalysts, which convert oxygen to water (3,4), use of a new sorbent that can achieve the separation at ambient temperature would have advantages over existing methods of separation. One possible way to achieve such a separation is to use a metal chelate complex that is incorporated into the stationary phase of the chromatographic column and selectively interacts with oxygen. The use of metal ions and metal complexes in chromatographic systems is well-known, and several reviews describe examples of the use of such systems (5-8). The reversible interaction of certain compounds or classes of compounds with the metal ion to form metal complexes serves as the mechanism to selectively increase the retention of these ligands. The possibility of using metal chelate compounds to selectively increase the retention of oxygen contained in a gas sample is attractive for several reasons. Important factors are the small amount of column "bleed" expected from these relatively nonvolatile metal chelates, the selective nature of the complexation of oxygen with the metal complex, and the increased likelihood of achieving the separation of oxygen from argon and nitrogen isothermally a t ambient temperature (eliminating the need for operating columns at subambient temperatures). The interaction of oxygen with synthetic cobalt(I1) complexes has been recognized for many years (9-11). The reversible interactions of cobalt(I1) Schiff base compounds, Present address: U.S.Fish and Wildlife Service, Denver Wildlife Research Center, Denver, CO 80225.
similar to those shown in Figure 1,with oxygen in a 1:1ratio in solution is well-known (12-17) and these compounds have been studied as possible models for biological oxygen-carrying systems (18-21). Reversible oxygen uptake is limited, however, to experimental systems operated at subambient temperatures; irreversible and nonstoichiometric oxygen uptake has usually been observed in solution at ambient temperature (13).It has also been noted that an axial base, such as pyridine, imidazole, or an alkylamine must be present in order for oxygen binding to be observed and the magnitude of the oxygen affinity is dependent on the axial base present (16). The axial base activates the metal chelate by increasing the energy of the d,z orbital of cobalt(I1) relative to that of the d,, orbital, upon going from a four-coordinate square planar geometry to a five-coordinate square pyramid geometry (22-24). The unpaired electron in the resulting d7 low-spin complex is in an orbital of suitable geometry to facilitate interaction with an unpaired electron in the T* orbital of molecular oxygen. Christensen et al. (25)have prepared a polymer to which Co(acacen) was added and the resulting mixture was heated. Heating causes additional cross-linking of the polymer, physically trapping the metal chelate within the polymer matrix. Pyridine is added and the resulting solid mixture was used as a stationary phase in a gas chromatographic system. The use of this stationary phase resulted in some separation of oxygen from nitrogen, but the peak shape for oxygen exhibited a considerable amount of tailing. The pyridine forms an adduct with the metal chelate, activating it for complex formation with oxygen. Pyridine can be lost over time by volatilization, because it is not bonded to the polymer. The new sorbent was synthesized from a porous polymer containing pyridyl functional groups that are part of the polymer matrix. The use of such a porous polymer provides a functional group that can serve as an axial base for the metal chelate, resulting in a coordinate bond being formed between the metal chelate and the polymer. This serves to activate the metal complex for oxygen coordination as well as to immobilize and isolate the metal centers. This is also intended to prevent the interaction of oxygen with more than one metal center at one time, so that the probability of the formation of peroxy-bridged species is decreased or eliminated. The resulting nonvolatile sorbent, composed of the integral parts described, can be used effectively as the stationary phase in a gas chromatographic system. The selective and reversible interaction of molecular oxygen with this stationary phase results in an increase in retention time for oxygen at room temperature, facilitating chromatographic separations of mixtures containing oxygen.
EXPERIMENTAL SECTION All reagents used were reagent grade or better and were used as received unless otherwise specified. Elemental analyses were performed by Huffman Laboratories, Wheat Ridge, Col. Gas Chromatography. A Hewlett-Packard Model 5750 gas chromatograph was used to evaluate the stationary phases. A thermal conductivity detector was used, with a bridge current of
0003-2700/85/0357-1572$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985 R
Flgure 1. Structure of Co(acacen), R = tert-butyl and R' = isobutyl.
R
= R' = CH3, and Co(toden), R
200 mA. Helium (commercial grade) was used as the carrier gas for both the sample and reference columns. The injection port, column oven, and detector were kept at room temperature (22-25 "C)unless otherwise specified. Samples were injected with a 1O-wL syringe or a loop injection system consisting of a modified Valco six-port valve. The barrel of the valve was inverted so that the sample delivered to the chromatographic system was equal to the internal volume of the valve connecting gas path (approximately 3 wL) rather than an external loop. The reproducibility of the loop injection system was 0.5% relative standard deviation based on the total area counts recorded for five replicate injections of oxygen. A Fischer Series 5000 recorder and a Hewlett-Packard Model 3390A integrator were used to record the chromatographic peaks. Sample Columns. Two types of column arrangements were used in this study. The first arrangement consisted of two columns connected in series. The first column, which was installed immediately following the injection port, was constructed of glass tubing 4 mm i.d. X 16 cm packed with the metal chelate containing stationary phase. The total amount of stationary phase in this column was about 700 mg. This was followed by a stainless steel column 2 mm i.d. X 265 cm containing 3.86 g of Porapak-N (80-100 mesh). The exit of the Porapak-N column was connected to the detector. The Porapak-N column and the glass column were connected using stainless steel reducing union tube fittings to minimize the dead volume of the system. The second analytical column arrangement used consisted of a single stainless steel column, 2 mm i.d. X 100 cm, packed with the stationary phase containing the metal chelate polymer. During the studies of the temperature dependence of retention times, the temperature was controlled by the gas chromatograph oven and monitored by an iron vs. Constantan thermocouple (reference junction = 0 OC). Preparation of 5,5'- (1,2-Ethanediyldinitrilo) bis (2,2,7-trimethyl-3-octanone), H2toden. A 1-L round-bottom flask was equipped with a magnetic stirrer and a heating mantle. To this flask was added 61.4 g (0.33 mol) of 2,2,7-trimethyl-3,5-octanone (synthesized by the method reported in ref 26 and 27) in 75 mL of absolute methanol. To this solution was added 10.0 g (0.17 mol) of 1,2-diaminoethane (Fisher). A white precipitate formed when the diamine was added. The mixture was heated to reflux, during which time all of the precipitate dissolved. Refluxing was continued for 48 h, and then the yellow liquid was allowed to cool to room temperature. The solution was kept overnight at room temperature and then placed in an ice bath. Scratching the flask at the surface of the solution initiated precipitation of a white solid, which was isolated by filtration in two crops to yield 36.4 g (56%) of a white crystalline solid, mp 110-111 "C. Recrystallization of the solid in acetone yielded a higher purity product, mp 114-115 "C. Anal. Calcd for CNH4N202: C, 73.42; H, 11.30; N, 7.14. Found: C, 73.04; H, 11.35; N, 7.15. Preparation of [5,5'-(1,2-Ethanediyldinitrilo)bis[2,2,7trimethyl-3-octanonato]]cobalt(II),Co(toden). A 100-mL flask was equipped with a magnetic stirrer, heating mantle, and nitrogen inlet. To this flask was added 0.595 g (2.5 X lo* mol) of cobalt(I1) chloride hexahydrate dissolved in 5 mL of water. Nitrogen flow was started and 0.98 g (2.5 X mol) of Hztoden dissolved in 20 mL of absolute ethanol was slowly added; during the addition of the ligand a precipitate formed. After the addition was complete, the mixture was gently heated with stiiing to 40 "C. During this heating all of the precipitate was dissolved, producing a red/brown solution. To this solution was added 0.9 mL of 5 N NaOH (4.5 X mol), at which time the solution turned dark green and a precipitate formed. The mixture was heated to 60 "C over the next 20 min and stirred at this temperature for 1.5 h. During this time an orange solid formed, which was isolated by filtration and then washed with about 25 mL of water. The
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solid was recrystallized in hexanes to yield 0.595 g (53%) of fine orange needles, mp 163-165 "C. Anal. Calcd for CoC24H42N202: C, 64.11; H, 9.43; N, 6.23. Found: C, 63.54; H, 9.30; N, 6.65. Preparation of the Porous Polymer, DVBIEVBII-VP. Prior to the polymerization reaction, the mixture of divinylbenzene and ethylvinylbenzene monomers (Dow-55,Dow Chemical Co., Midland, MI) was washed with six to seven aliquots of a 10% The NaOH solution to remove 1,2-dihydroxy-4-tert-butylbenzene. 4-vinylpyridine monomer (Aldrich) was distilled under vacuum (
