Langmuir 1985,1, 326-330
326
550 "C in that methylene bridges are attacked and changed and polyaromatic domains form. After this disruption, the intermediate-temperature as well as high-temperature chars exhibit the "normal" oxidation behavior found with chars derived from other precursors.
Acknowledgment. DAAG29-83-K-0063 and gratefully acknowledged.
Support by ARO Contract NSF Grant CHE-8111778 is
Registry No. (Phenol).(formaldehyde)(copolymer),9003-35-4.
Separation of Oxygen from Air by [ C o I I ( b p y ) (terpy)l2' Complexes in Zeolite Y Seiichiro Imamurat and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received November 27, 1984. In Final Form: February 1, 1985 Cobalt(I1) ions in zeolite Y react with 2,2'-bipyridine (bpy) and 2,2',2''-terpyridine (terpy) to form mixed-ligand five-coordinate [ Co"(bpy) (terpy)12+complexes. The effect of other cations in the zeolite influences the formation of the cobalt complex, with Li+ being the most effective for obtaining a high yield of [Co"(bpy)(terpy)12+. This complex adsorbs oxygen to form the [Com(bpy)(terpy)O2-l2+ complex. The stability constant for the oxygen complex, KO,,is 2.9 torr-'. The adsorption of oxygen by the complex is completely reversible at 298 K; thus the complex can be used repeatedly to separate oxygen from nitrogen in air, with a mean separation factor (02/N2)of 12.3. Water has a detrimental effect on the ability of the complex to form the oxygen adduct.
Introduction Oxygen adducts of transition-metal complexes have been intensively studied, mainly because of their role as model compounds to elucidate the function of biological oxygenation systems' or natural oxygen carriers such as hemoglobin or myoglobin.2 Cobalt-oxygen complexes have been investigated thoroughly. Most mononuclear cobalt-oxygen complexes synthesized in the liquid phase are rather unstable and are transformed into dinuclear forms3 unless they have ligands large enough to prevent dimeri~ a t i o n . Moreover, ~ some of the ligands may be oxidized by oxygen and the ability of the complexes to combine with oxygen may decrease. In order to utilize cobalt effectively by preventing dinuclear complex formation and to make a complex that is resistant to oxidative degradation, we have synthesized mononuclear [Co"(bpy) (terpy)]2+ complexes inside the large cavities of a Y-type zeolite? This complex has one vacant coordination site and adsorbs oxygen reversibly. Separation of oxygen from nitrogen or production of oxygen-enriched air is an important industrial process. There are several methods of separating oxygen from air: cryogenic fractionation, pressure swing adsorption! membrane ~ e p a r a t i o n ,and ~ composite oxide formation.8 Zeolites are used in the pressure swing adsorption method, in which separation is based upon the difference in the extent of physical adsorption of oxygen and nitrogen. At least at a fundamental level attention also had been given to transition-metal complexes that might be capable of separating oxygen from air."ll In a previous s t u d 9 it was pointed out that the [Co"(bpy)(terpy)I2+ complex in a CoNaY zeolite might be suitable for such a process; however, the degree of complex formation inside the zeolite was very low ( NH4+ > Na+. Although NH4+is large, it will be converted to the smaller H+, to some extent, in the reaction a t 523 K. Several other attempts were made to increase the degree of [Con(bpy)(terpy)lZ+formation. For example, NaY and KY zeolites were partially ion exchanged by La(II1) and heated a t 773 K under oxygen atmosphere to produce La-0-La species in the small cages of the 2e0lite.l~ This should prevent Co ions from entering the small cavities and occupying hidden sites, especially site I. It was expected that if Co ions were not located inside the small cavities, they would more easily form complexes inside the large cavities. In another experiment [Co1'(bpy)J2+ and [ C ~ ~ ( t e r p y )were ~ ] ~ first + synthesized inside a KY zeolite, and these complexes were reacted further with terpy and bpy, respectively. None of these attempts were successful.
Formation and Stability of [Co~(bpy)(terpy)Oz-]LiY( 1.2). Upon exposure of [Con(bpy)(terpy)]LiY(1.2)to 15 torr of oxygen, the [Com(bpy)(terpy)0
