Separation of Oxygen from Air Using Coordination Complexes: A

I n d . Eng. Chem. Res. 1994,33, 755-783

755

REVIEWS Separation of Oxygen from Air Using Coordination Complexes: A Review G u a n g Qing Li and Rakesh Govind' Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221

Dioxygen is produced in tonnage quantities by the distillation of air a t cryogenic temperatures. In recent years, alternative technologies have emerged that employ coordination complexes t o offer technical and economic potential for air separation which yet may prove competitive with cryogenic separation. New transition metal complexes that can bind dioxygen reversibly and with high specificity may provide the basis for even better processes for dioxygen recovery. In this paper, we review the structure and property of synthetic dioxygen complexes containing the elements iron, cobalt, and manganese. We describe the application of these complexes in air separation. 1. Introduction Synthetic reversible oxygen carriers have been of interest as model compounds in the study of the reversible oxygenation mechanisms and as a means of separating oxygen from air. The reversible reactionsof dioxygen with protein complexes are of critical importance to advanced and primitive forms of animal life because these proteins can bind, transport, store, and release dioxygen. Naturally, the details of dioxygen binding and transport in these proteins are of great interest to biochemists and numerous reviews of these and related subjects are available (Senozan, 1974, 1976;Weissbluth, 1974;Wang et al., 1979; Hayaishi, 1962;Bannister, 1977;Eichhorn, 1973;Sigel, 1978;Antonini and Brunori, 1971;Freedman et al., 1976; Kuntz et al., 1977). The structures and properties of these proteins, which are also of interest to chemists who design and study metal complexes as models for dioxygen transport proteins, have been reported in detail (Collman, 1977;Chang and Traylor, 1973b; Collman and Suslick, 1978;Bayer and Holzbach, 1977;Wang, 1970;Niederhoffer et al., 1984;Henrici-Olive and Olive, 1974;Fallab, 1967; Jones et al., 1979;Gubelmann and Williams, 1983;Basolo, 1974; Martell, 1982, 1988; Vatentine, 1973; Choy and O'connor, 1972/73;Erskine and Field, 1976;McLendon and Martell, 1976;Basolo et al., 1975;Vogt, 1963;McAuliffe, 1988;Coleman and Taylor, 1980;Martell and Sawyer, 1988). The study of the model dioxygen carrier complexes is critically important to biological systems and industrial processes. In biochemistry, monooxygenases can selectively catalyze alkyl and aromatic hydrocarbon hydroxylation, olefin epoxidation, and ketone lactonization (Hayaishi, 1974) by molecular oxygen and dioxygenases are able to catalyze oxidation scission for a wide range of aromatic hydrocarbons under mild conditions (Que, 1980). Most of these enzymes contain a transition metal as the active center. In recent years, considerable headway has been made in understanding the interaction of dioxygen with transition metal complexes and many model systems have been established (Jones et al., 1979;McLendon and Martel, 1976;Basolo and Burwell, 1973). These achieve-

* Author to whom correspondence should be addressed.

ments indicate a hopeful prospect for artificially mimicking the procedure of biological oxidation. The industrial preparation for the production of many oxygen-containing compounds is generally based on complex chain reactions or heterogeneous catalysis. Selectivity appears to be especially critical. Thus, it would certainly be a rewarding goal to use oxygen fixed at soluble transition metal species for catalytic and specific oxidation of substrates, which could be introduced into the metal dioxygen complex as ligands. Several approaches of this kind have already been undertaken (Henrici-Olive and Olive, 1974). In addition to the significance as models of natural oxygen carriers, synthetic dioxygen complexes have promising, though yet undeveloped application for separation of oxygen from air (Adduci, 1976;McAuliffe et al., 1979; Nelson et al., 1986;Kawakami et al., 1982;Imamura and Lunsford, 1985;Motekaitis and Martell, 1988;Smith et al., 1977). Eventual success of dioxygen complexes for oxygen separation depends on several critical factors, which includes (1) long-term reversibility of oxygen uptake, (2) fast complexation and regeneration of the complex, (3) stability in the presence of moisture, (4)high equilibrium capacity for oxygenuptake,and (5) capability for the ligand to be synthesized easily from low-cost precursors, which impacts the cost of the dioxygen complex. 2. Dioxygen and Its Active Forms

The structure of the molecular oxygen is best described by the molecular orbital theory (Gray, 1967). According to the theory, the bond between the oxygen atoms is due to the interaction of 2p electrons and consists of one twoelectron u bond (2pu2) and two three-electron ?r bonds ( 2 p ~ ) 2(2pr*)'. Each three-electron bond constitutes a combination of a pair of bonding electrons and one antibonding electron in the same interaction plane. Hence, molecular oxygen is paramagnetic, having a triplet YZgground state (Figure 1). The two lowest electronic excited statesof the molecular oxygen are the singlet states, '$and 'Eg+.They are formed by different arrangements of the two electrons in the 2pr* orbital. Some of the salient physical data for 0 2 (Ei-Ichiro, 1977) are summarized in Tables 1 and 2. According to the molecular orbital theory, the atomic orbitals of the two oxygen atoms combine to form four

o a a a - ~ a a ~ ~ ~ ~ ~ ~ s0~1994 ~ - American o ~ ~ ~ Chemical ~ o ~ . Society ~o~o

756 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994

reported values for v0-0 11103cm-' for MbOz (Maxwell et al., 1974) and 1107 cm-l for HbO2 (Barlow et al., 1973)l together with single-crystal X-ray structural studies (Jameson et al., 1978) of model compounds make this almost a certainty. Synthetic q1 metal dioxygen complexes have been prepared from the reaction of 0 2 with metal complexes containing Cr(II), Fe(II),and Co(I1). In terms of simple valence bond theory, the bonding in these complexes can be described in two way depending on whether the configuration selected has an even number (even oxygen) or an odd number (odd oxygen) of electrons on the coordinated 0 2 . The first approach, that of even oxygen,f i s t proposed by Pauling (Pauling, 1964)to explain the diamagnetism of HbO2, assumes an even number of electrons about dioxygen and is represented as aresonance hybrid of the structures A and B.In this case, following

zpd

..FP-0, ..

0

0

02

0

Figure 1. Molecular orbital diagram for 02.

A

bonding and four antibonding molecular orbitals with a vacancy for the addition of a single electron in both antibonding orbitals. Thus the addition of one or two electrons to the partially vacant antibonding orbital 2pr* of 0 2 results in the formation of the superoxide (02-1 and peroxide (02%) anions, respectively, with a decrease in the 0-0 bond order so that 0 2 % contains a single bond only. This is also reflected in the increasing of the bond length and the decreasing of the dissociation energy and the vibrational frequency for oxygen. The removal of electron from molecular oxygen strengthens the 0-0bond (eq 1).

%+-

-e- o,

-- +e-

%-

+e-

eL'- +2e-

202-

(1)

3. Properties of Dioxygen Complexes 3.1. Bonding Types in Dioxygen Complexes. Although the transition metal dioxygen complexeshave been intensively studied, the nature of their bond formation has been quite controversial. This problem arises because the ligation of dioxygen to a metal complex differs from the ligation of a simple neutral or anionic species, such as ammonia or the chloride anion, to a metal center. The addition of molecular oxygen to a metal complex could involve a formal oxidation of the metal center with the reduction of the coordinated dioxygen. Vaska (Vaska, 1976) showed that almost all currently known metal dioxygen complexes can be divided into two types according to the characteristics of the dioxygen ligand, these being either superoxo (I) or peroxo (11) complexes. The a or b classification distinguishes complexes where the dioxygen is bound to one metal atom (type a) or bridges two metal atoms (type b). Gubelmann and Williams (Gubelmann and Williams, 1983) uses a "hapto"nomenclature in which the structures are classified by the number of atoms of dioxygen bound to the metal ion. A comparison of the 0-0 stretching frequencies, obtained from metal dioxygen complexes (Table 3) shows that the metal dioxygen adducts have 0-0 stretching frequencies similar to those obtained from compounds aolltaining ionic superoxide or peroxide species,suggesting a substantial transfer of electron density from the metal center to the coordinated dioxygen. It has long been delivered that the bonding of dioxygen in oxyhemoglobin and oxymyoglobin is of type 91. The

B

the conventional method of assigning oxidation states, the canonical forms A and B have valencies corresponding to M"(O2) and MnW(0z2-),respectively. The second approach, proposed by Weiss (Weiss, 1964) for bonding of 0 2 in HbO2, assumes an odd number of electrons on the coordinated dioxygen and is represented by the structures C and D. In this case, following the rule

:i?m ?-*, ..

+?m

-2.,

..

C

..

0:

2: D

and assigningthe electron pair to the coordinated dioxygen results in a metal having a +I11 oxidation state and a dioxygen which is formally superoxide-like 02-. For this bonding model, the unpaired spins on M(II1) and 0 2 - are reputedly coupled, thus allowing for the observed diamagnetism of the M-02 system. Other bonding approaches are also possible. Figure 2 shows a simplified molecular orbital scheme for the F e 0 2 bond. From this scheme, two possible ground-state electronic configurations are possible: When the energy separation, A, is greater than the electron pairing energy, the spin-paired electronic configuration (u)2(dyz)2(d,,)2(d,,I2, which is equivalent to the FeTO2) FeW(O&) resonance pair suggested by Pauling, is obtained. However, for A smaller than the pairing energy, the electronic configuration (u)2(dyz)2(d,,)z(d,,)1(r*)', having two unpaired electrons, is obtained. This configuration can be considered similar to, but not equivalent to, the Fe"(02-) valent bond pair C and D. A large number of q2 metal complexes have been prepared, and the chemistry of q2 complexes containing group VI11 metals has been reviewed (Choy and O'connor, 1972173). The salient differences between this type of bonding and the bonding observed in q1 complexes are the coordination geometry, with the dioxygen being bound to the metal center in a symmetric triangular fashion, similar to that proposed by GrWith (Griffith, 1956)for the bonding of 0 2 in oxyhemoglobin, and the v0-0 values, which are similar to those observed for free peroxide ion. There exist four metalloporphyrin complexes which have the dioxygen bond in a symmetricallytriangular fashion. These are the complexes Mo[T@-CH3)PPl(O2)2(Chevrier et al., 19761, Ti(TPP)(021, Ti(0EP)( 0 2 1 , and Ti(M0EP)( 0 2 ) (Guilard et al., 1978). The onlysynthetic dioxygen carriers

-

Ind. Eng. Chem. Res., Vol. 33, No. 4,1994 757 Table 1. Some Properties of the Dioxyeen Moiety activated energy, oxygen species kcal/mol

type

bond bond energy, bond v-, order kcal/mol length, A cm-1

electronic configuration"

ground state of dioxygen excited state of dioxygen

lO(lD) lO('S) 4

45.4 96.6

The z-axis is parallel to the 0-0 bond.

Table 2. Some Prowrties of Activated Oxygen Species activated oxygen energy, tme species kcal/mol electronic configuration reduced forms 0" (HOO') of dioxygen 02" (HOO-) 0' (HO') 0% (HO-) other 02+

bond order

bond energy, kcal/mol

bond length,A

v-,

cm-1

O+ 03

Table 3. Structural Classification and Promrties of Dioxygen . _ Complexes structure structure Vaska MO tme designation claesification $1 dioxygen 1:l Ia A0

bond length, A 1.10-1.30

1130-1195

q 2 dioxygen

1.42-1.45

800-932

ql:qldioxygen

1.24-1.36

10751122

ql:qldioxygen

1.31-1.49

790-884

range of v u , cm-1

' 0

I

M

IIa

1:1

21

of biological interest in which this type of bonding has been implicated are the porphyrinatomanganese dioxygen complexes Mn(por)(Oz). These complexes will be discussed in more detail in a subsequent section. The chemistry of q1:ql complexes (type Ib) has been extensivelyreviewed (Fallab, 1967;Erskine and Field, 1976; McLendon and Martell, 1976;Vogt, 1963;Sykes and Weil, 1970; Wilkins, 1971). The only complexes of type Ib characterized are those that contain cobalt. These com, their pounds are paramagnetic with peff ca. 1.6 p ~ and EPR spectra are consistent with there being two equivalent cobalt nuclei having the majority of the electron density residing on the dioxygen bridge (Weiland Kennaird, 1967). The chemistry of +:q1 complexes (type IIb) has been reviewed several times (Fallab, 1967;Erskine and Field, 1976;McLendon and Martell, 1976;Vogt, 1963;Sykes and Weil, 1970;Wilkins, 1971). The bonding of dioxygen in IIb complexesis believed to be similar to that of the natural systems of hemerythrin and hemocyanin. Bonding of dioxygen in these natural systems involves two metal

dioxygen

1.49

905

T+:$ dioxygen

1.44

845-857

q1 dioxygen gl:+ dioxygen

1.33 1.49

1145 842

+:q2

centers with the bound dioxygen being in a reduced peroxide-like state with a concomitant one-electron oxidation of both metal centers. Most synthetic IIb complexes contain cobalt centers. The X-ray and infrared data on the 0-0 bond lengths and u0-0 of IIb complex are consistent with the bound dioxygen being regarded as peroxo-like. The 0-0 bond lengths vary between 1.31 and 1.49 A and the vo-0 span a range of 790-884 cm-l. The q2:q2 structure with a "sideways" bound dioxygen bridging two metal atoms has been suggested for the complexes [Rh(diene)l2(02) (Sakurai et al., 1980) and [Cu(HB(3,5-RzPZ)3)12(02) (R = i-Pr and Ph) (Kitajima et al., 1992). The crystal structure showing this geometry has been reported for the uranium complex (U02ClhO21' (Boeyens and Haegele, 1977) and a complex of La3+ (Bradley et al., 1977). The q1:q2complex is known only for [RhCl(PPh3)2(02)12 (Bennett and Donaldson, 1977). The structure may be described as a dimer having two trigonal bipyramidal subunits, the bridge being formed by one oxygen atom of

758 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994

(a 1 (b) Figure 2. A qualitative MO scheme for the bondingin iron-dioxygen complexes.

each subunit having a third bond to the second subunit. The dioxygen molecules thus have features similar to those of both ?r-bonded ligands and chelating peroxo groups. Rhodium is at present the only transition metal known to form an +:v2 complex. 3.2. Natural Dioxygen Carriers. Natural dioxygen complexes play a significant role in the absorption, transport, and storage of dioxygen for respiration in biological systems. Natural dioxygen carriers can be divided into two main types: the heme-containing proteins such as hemoglobin and myoglobin and the non-heme proteins such as hemerythrins and hemocyanins. These dioxygen carriers are generally dioxygen complexes in which dioxygen is coordinated to ametal ion firmly bound in a multidentate ligand. The oxygen-free metal complex has been named a “prosthetic group”. Hemoglobin and myoglobin consist of an iron(I1) protoporphyrin [iron(II) heme or ferrohemel prosthetic group associated with a protein part called globin. All vertebrates have hemoglobin in their blood cells and myoglobin in muscular tissue. Hemoglobin is also found in the root nodules of leguminous plants. Hemerythrin and hemocyanin, which are non-heme dioxygen-carrying proteins containing iron and copper, respectively, in their corresponding prosthetic groups, are found in various types of terrestrial and marine invertebrates. A survey (Hayaishi, 1962)of the composition and properties of some natural dioxygen carriers is given in Table 4. 3.2.1. Homoglobin and Myoglobin. Hemoglobin and myoglobin are the dioxygen carriers in vertebrates. Their capacity to bind dioxygen depends on the presence of a heme group which consists of an organic part, protoporphyrin IX, and an iron atom. There are six coordinate sites about the iron atom, four of which are satisfied by a porphyrin moiety while the fifth axial position is occupied by an imidazole group of a histidine residue of the polypeptideglobin chain; the sixth site is available to bind an extra ligand. Hemoglobin has a quaternary structure made up of four subunits, each having one polypeptide chain and one heme

prosthetic group (Figure 3), while myoglobin is a conjugated protein consisting of a single polypeptide chain of about 153 amino acid residues associated with the heme group (Figure 4). Of great significance are certain differences between the oxy and deoxy forms of hemoglobin. The deoxy form is paramagnetic, with the magnetic moment corresponding to four unpaired electrons per heme, a high-spin (S = 2) configuration (Pertuz, 1970; Weiss, 1958), while the iron atom lies above the plane of the nitrogen donors toward the imidazole axial ligand with an Fe-N bond length of 2.9 A. The oxy form is low spin, is diamagnetic, and has the iron atom in the plane of the porphyrin ring with a Fe-N bond length of 2.0 A, due to the smaller size of the low-spin ion. The high-spin d6form is too large to fit in the “hole” in the porphyrin. This change in the size of the iron atom on absorbing dioxygen is then magnified through the interactions in hemoglobin so that substantial conformational changes occur. The X-ray structure of myoglobin shows the globin part to be in the form of a helices with axial repeats of about 5.4 A. Each segment of the right-hand CY helices contains between 7 and 20 amino acid residues (Weiss, 1958). Hemoglobin is an ellipsoid of dimensions 64 X 55 X 50 A, and the four iron hemes are present as symmetrically related pairs lying at the corners of an irregular tetrahedron with distances of 33.4 and 36.0 A between the pairs. The polypeptide chain appears as four separate units that are identical in the symmetrical pairs. The iron-oxygen distances is close to 2.1 A, and that between iron and the bound imidazole nitrogen atom is 1.9 A. The difference between hemoglobin and myoglobin is that hemoglobinis an allosteric protein whereas myoglobin is not. This difference is clearly expressed in the shape of the dioxygen-binding curve of hemoglobin which is sigmoidal,whereas that of myoglobin is hyperbolic (Figure 5). The sigmoidalcurve shows that the binding of dioxygen to hemoglobin is cooperative (Baldwin, 1975). This phenomenon arises from what is known as heme-heme interactions, which arise indirectly from conformational changes induced by the binding of dioxygen. Perutz (Perutz, 1970), following the suggestion of Williams (Williams, 1961), using both experimental results from studies on hemoglobin combined with studies on model heme systems, has postulated a “trigger” mechanism for the cooperative interaction. On oxygenation, the heme iron that is originally above the porphyrin plane undergoes a spin change and moves into the mean plane of the porphyrin ring. This is accompanied by the movement of the covalently linked proximal F8 histidine residue of the globin. The movement of this residue causes a change in the structure of the protein (Figure 6). The function of the protein environment is to sheath the dioxygen-binding site, preventing the close approach of two heme rings and consequent irreversible oxidation via the p-peroxocomplex. The peptide chain also stabilizes the FeL02 species by enclosing the porphyrin in a hydrophobic pocket to which access by protons is inhibited. In addition, neutron (Phillips and Schoenborn, 1981)and X-ray diffraction (Shaanan, 1982) studies have indicated that stabilization of the iron-oxygen bond in oxyhemoglobin and oxymyoglobin may in part be due to hydrogen bonding between the terminal oxygen atom and the imidazole of the distal histidine (His-E7), 3.2.2. Hemerythrin. Hemerythrin is a non-heme iron protein which serves as a reversible dioxygen carrier (York and Bearden, 1970). It is an octamer and does not contain a porphyrin moiety; each subunit contains two iron atoms and binds one molecule of 02 per subunit.

Ind. Eng. Chem. Res., Vol. 33. No. 4, 1994 759 Table 4. Properties of Natural Dioxygen Carrion

molecular hemoglobin

weight 67 OOO

metal %

Fe 0.335

nature of prosthetic gmup heme

metakdioxygen

absorption bands of oxygenated product. nm

heat of oxygenation, keal/mol

414 541 511

-12

-11

1:l

myoglobin

17 250

Fe 0.345

heme

1:l

hemerythrin

So00

Fe 0.84

non-heme

absorbs at slightly longer wavelengths than oxyhemoglobin

21

335 365

hemocyanin

300OOO-

10 million

Cu 0.250,0.175

non-heme

-16

m 21

218

345 555

20

0

40

-12

€4

so

IW

oxygen pressure (Ion)

Figure 3. Three-dimensional structure of hemoglobin.

Figure 5. The dioxygen isotherms of hemoglobin and myoglobin. H,Sld".

re

- - -I \

Fe

I

Fe-

t

"*n*.l1l.

0,

Figure 6. The "trigger" mechanism.

Figure 4. The myoglobin molecule.

The nature of the dioxygen binding in hemerythrin has been the subject of controversy. Deoxyhemerythrin is a paramagnetic, high-spin ( S = 2) iron(I1) species, but oxyhemerythrin may be formulated in a number of ways depending upon the extent to which electron transfer to thedioxygen hasoccurredand theresultingoxidationstate of the metal ion. It has been suggested that oxyhemerythrin contains iron(II1) and peroxide and alternatively iron(I1) and dioxygen. Many structural representations

have been suggested for oxyhemerythrin, but the mode of dioxygen binding is stilluncertain (Figure 7) (Kurtzetal., 1977). Compared to hemoglobin, hemerythrin has been found to have much stronger oxygen affinity and to bind dioxygen with little or no cooperativity as shown by the Hill coefficient (1.2-1.4). The best models for the iron sites in hemerythrin are found to be the iron-EDTA complexes (Loehr et al., 1980). 32.3. Hemocyanin. Hemocyanins are non-heme c o p per-containing proteins found in the hemolymph of various mollusks and arthropods. The hemocyanin proteins consist of a number of subunits, and the size of these subunits is phylum dependent. As for hemerythrins, the mannerinwhichthemetalcentersareboundtotheprotein is not well understood, nor has the structure of the metaldioxygen complex been well established. The copper in deoxyhemocyanin is present as Cu(I), and hence the deoxygenated protein is almost colorless.

760 Ind. Eng. Chem. Res., Vol. 33, No. 4,1994

?

Fell'

/"\/ 0

Fe"

Fen'

Fe"'

p-(mono oxygenlbridged

Fell'

p(di0xygen)bridged

____ ____

i

Fd"

0

Figure 7. Suggested structures for FemOz unit in oxyhemerythrin.

Upon oxygenation, hemocyanin has a deep blue color, having absorption maxima in the visible at 345 nm. The similarity between these bands and those of known cupric proteins supports a Cu(I1) center in oxyhemocyanin. These absorptions have been attributed to d-d transitions of Cu(I1) in a distorted tetragonal ligand field (Moss et al., 1973). Hemocyanins absorb one 0 2 for every two copper atoms in the molecule. From a recent resonance Raman study on oxyhemocyanin using unsymmetrically labeled l60180, it has been suggested that the Cu202 moiety adopts a nonplanar, p-dioxygen confirmation (Karlin and Gulneh, 1987;Thamann et al., 1977).

cun -0

'o-cun

3.3. Synthetic Dioxygen Carriers. Certain transition metal complexesare able to absorb dioxygen without either the metal (M) or the ligand (L) being irreversibly oxidized and will, under appropriate conditions, release the absorbed dioxygen to yield the metal complex (eq 2). In

nM(L) + 0

2 nmlor2

[M(L)In(02)

(2)

practice the reverse reaction can be initiated by lowering the partial pressure of 0 2 , by heating the adduct, or by adding a ligand capable of replacing bound dioxygen. The study of dioxygen complexes of the transition metals is generally accepted to have begun with the report of the oxygenated ammoniacal salts of cobalt in 1852. A satisfactory explanation of the results had however to await the development of a general theory of coordination compounds, and the dioxygen bridged complexesof cobalt(111)figured among the many complexes studied by Werner (Werner, 1910) at the turn of the century. In the 1930s the mechanism of the autoxidation of metal ions was studied and the first synthetic oxygen carriers were discovered. Pauling and Coryell (Pauling and Coryell, 1936) proposed the first of many theoretical models to explain the iron-dioxygen interaction in hemoglobin. The increasing availability of physical methods allowing the ready characterization of dioxygen complexes and the determination of their molecular structures, coupled with a better understanding of the electronic structures, has given considerable encouragement to the study of these compounds in recent years. The early work tended to concentrate on specific types of complex, and may distinguish three basically different areas of research: (i)

complexes of cobalt with Schiff base and nitrogencontaining ligands; (ii) complexes of group VI11 metals in low oxidation states; (iii) biological systems where transition metals are known to be intimately involved in reactions with molecular oxygen. This field includes innumerable simpler model complexes, and covers systems which act as dioxygen carriers as well as those acting as redox systems. 3.3.1. Iron Dioxygen Carriers. Because of their ubiquitousness and the variety of their natural functions, heme proteins have been investigated on multi- and interdisciplinary levels. These proteins, all containing an iron porphyrin as the prosthetic group, are responsible for oxygen transport and storage (hemoglobin and myoglobin (Ho, 1982),electron transport (cytochromesb, c) (Mathews, 1985), oxygen reduction (cytochrome oxidase) (Miller, 1982), hydrogen peroxide utilization and destruction (peroxidases and catalases) (Frew and Jones, 19841,and hydrocarbon oxidation (cytochrome P-450) (Ortiz de Montellano, 1986). The active site in each case contains an iron porphyrin, the nitrogens of the porphyrin ring occupying four essentially planar coordination sites of the metal. Therefore their diversity of function must be dictated by the number and nature of the axial ligands, the spin and oxidation state of the iron, and the nature of the polypeptide chain. The discovery that iron(I1) porphyrin complexes can reversibly bind dioxygen has caused a great deal of interest in this case of complex and has been of interest in an attempt to understand the structure and functions of the natural heme proteins hemoglobin and myoglobin. Historically, much of the research on metalloporphyrins has focused on the mechanism of reversible dioxygen binding to hemoglobin and myoglobin. Dioxygen binding heme proteins are five-coordinate high-spin ( S = 2) iron(I1) species, which upon oxygenation become six-coordinate low spin ( S = 0). The difficulty in reproducing this behavior is dominated by twoproblems: (i) the irreversible oxidation of iron(I1) porphyrins on exposure to dioxygen; (ii) the difficulty in obtaining well-defined five-coordinate iron porphyrins. Simple iron(I1) porphyrins cannot reversibly bind oxygen, except a t low temperature. At room temperature and in the absence of a large excess of a sixth ligand, formation of the six-coordinate iron(I1) dioxygen species is immediately followed by attack of a second fivecoordinate iron(I1) complex to give the p-peroxo-bisiron(111)complex. This rapidly breaks down, presumably via a ferry1intermediate to give a p-oxo-bisiron(II1)complex in which the iron has been irreversibly oxidized to the ferric form (Scheme 1; B = base) (Kao and Wang, 1965; Hammond and Wu, 1968;Cohen andcaughey, 1968;LatosGrazynski et al., 1982). The second major problem in studying simple iron porphyrins is the preference of the metal for six-coordination. For example, in solution containing strongly coordinating N-donor ligands, six-coordination is favored over five-coordination; i.e., for the equilibria in eq 3, K2 > K1 KI

Fe"(por)

+ B +Fe"(por)B + B

KP F=

Fe'(por)(B),

(3)

(KdK1 = 10-30 in aprotic solvent at 25 "C) (Brault and Rougee, 1974). The values of K1 and K2 are obviously controlled by the spin state of iron. The four-coordinate iron porphyrin is in an intermediate-spin state (S = 1); addition of one ligand gives the high-spin (S= 2) fivecoordinate complex which adds a second ligand to form the low-spin ( S = 0) six-coordinate species with a gain in crystal field stabilization energy (Scheidt and Reed, 1981).

Ind. Eng. Chem. Res., Vol. 33, No. 4,1994 761

Scheme 1 B

addition of a second ligand allows formation of sixcoordinate mixed ligand systems. On the other hand, for strong ligands, e.g., imidazole and pyridine, chelation provides a built-in 1:l baselporphyrin stoichiometry. As long as dimerization, to form mixtures of six- and fourcoordinate (eq 51, is prevented this approach produces five-coordinate complexes (Traylor, 1981).

B

I

I

-Fen-

+B

-Fen-

I

B B

B

I

+ 4 =-Fen-I I

-Fen-

0 2

B I -Fen-

B

I

+ -Fen-

d

I

0 2

I -0' -Fen' I I B- Fe"=O

I

0 -Feu

+

I

-

-B

I

I

kn-B

I

I I

B-Fenl

2

I 0 -Fen' I

-0'

B

-Fen-I

I

I

B

I -FenI

co

B hv. KBW

I

L-Fen-

+ CO.K Z o

-

B

% I

+ -Fen-

-Fen-

I

I

B

B

I I

-&, Kco2

I Fe" I

B-

I

0 -Fern

I

-0'

-6

However, in this case, one is limited to studying competitive dioxygen binding to six coordinate hemes. B

I I

-Fen-

0 2

+

O2

I I

-Fen-

B

+

B

B

E. Low Temperatures. Iron(II)-O2 porphyrin complexes are stable a t low temperatures, where the irreversible oxidation reactions are slowed down. Again one is reduced to studying competitive dioxygen binding as KJKl increases as temperature decreases (Anderson et al., 1974; Almog et al., 1974;Brinigar and Chang, 1974;Wagner and Kassner, 1974).

B

I -Fe" 7 + 4. KnQ

(5)

Fe"

Fen' -0 -FeIn

Numerous approaches have been used to control oxidation and coordination in model porphyrin systems. A. Metal Substitution. Replacement of iron with cobalt (Basolo et al., 1975;Hoard and Smith, 1975) or ruthenium (Tsutsui, 1979) leads to metalloporphyrins which are more inert to oxidation and possess different coordination properties. Such an approach is applicable since apoproteins may be reconstituted with cobalt and ruthenium porphyrins. In the case of cobalt, reconstituted cobalt hemoglobin exhibits cooperative oxygen binding although to a diminished extent. B. Kinetic Measurements. Fast spectroscopic methods may be used to observe reversible oxygen binding even under conditions where irreversible oxidation will occur. Traylor et al. (Traylor et al., 1979) has exploited the stability of imidazole-heme-CO complexes toward oxidation. A solution of Im-Hm-CO, equilibrated with a mixture of oxygen and carbon monoxide, is subjected to a short laser pulse which dissociates the carbon monoxide. The deoxyheme then reacts preferentially with dioxygen at a fast but measurable rate. In 103-10 s, the Im-Hm0 2 complex dissociates and returns to the Im-Hm-CO complex. B

--

B

D. Excess Ligand. The presence of excess base will minimize the concentration of five-coordinate heme and reduce p-peroxo complex formation.

I1 -FeIV-

- II

7 3

_J" =-Fe" I

-6

0

2

-Fen

I

02

The kinetics are described by

C. Chelated Hemes. Covalent attachment of the ligand to the porphyrin periphery allows one to control the extent of coordination. For poor ligands such as thiolate or phenoxide, covalent attachment increases the local concentration and the likelihood of coordination to the metal without the necessity of a large excess of external ligand (eq 4). As long as displacement does not occur,

B

02

F. Immobilization. This approach attempts to prevent irreversible oxidation by anchoring the porphyrin to a solid support. In Wang's classicexperiment (Wang, 1970) a heme diethyl ester was embedded in a matrix of polystyrene and 1-(2-phenylethyl)imidazole. The matrix not only prevented close approach of two hemes but also provided a hydrophobic environment. Reversible oxygen uptake was observed. Alternatively, either the porphyrin or the ligand may be covalently attached to a rigid support. Leal et al. (Leal et al., 1975) has undertaken the latter approach and prepared silica gel support which contained 3-imidazolylpropylgroups attached to the surface. Reaction with Fen(TPP)(B)2 coordinated the prophyrin, and heating in flowing helium removed the sixth axial base to give the five-coordinate iron(I1) prophyrin. However, attempts to observe reversible dioxygen binding was obscured by the physisorption of dioxygen by the silica support.

762 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 “Picket-fence”

Oxygenbinding

Pocket

I-;

I -

7 PP R i n g Unse

‘R

Figure 9. The ‘picket fence” porphyrin.

Y Figure 8. The “tailed” porphyrin.

G. Steric Encumbrance. By sterically blocking one or both faces of the prophyrin, close approach of two porphyrin rings and therefore p-oxo bridge formation may be prevented. The approach most closely mimicking the natural system is to enfold the prophyrin ring in a polymer chain. This approach has been vigorously pursued in an attempt to prepare compounds capable of reversible dioxygen binding in aqueous solution at room temperature. However, difficulty in reversing oxygenation has made this approach less fruitful. In contrast, prophyrins have been synthesized in which one or both faces of the ring are obstructed by some groups covalently bound to the ring. The function of the steric hindrance is 2-fold: (i) to direct base binding to the open face, ensuring five-coordination, and (ii) to allow 02 to bind on the hindered face, with steric hindrance preventing p-oxo bridge formation. Fivecoordination may also be ensured in these systems by using bulky axial bases which cannot bind on the protected face. The approach has been used by many groups to produce a wide variety of architecturally different model prophyrins. a. The “Tailed” Porphyrins. The most obvious approach to the synthesis of heme protein models is to reproduce the local environment of the heme active site by covalently attaching various peptide fragments to a suitable porphyrin. If the peptide fragments contain suitable amino acids, reproduction of the coordination sphere of the heme protein may be possible (Figure 8) (Traylor et al., 1977, 1979, 1981; Hashimoto et al., 1983; Chang and Traylor, 1973a,c;Brinigar et al., 1974; Geibel et al., 1975; Collman and Groh, 1982; Warme and Hager, 1970; Tabushi and Sasaki, 1982; Tabushi et al., 1985; Collman et al., 1980; Mashiko et al., 1981; Santon and Wilson, 1984,Kong and Loach, 1980;Maiya and Krishnan, 1983). The model can only bind dioxygen reversibly at low temperature (Chang and Traylor, 1973b;Momenteau et al., 1976; Molinaro et al., 1977; Traylor et al., 1979, 1981). b. The “PicketFence”Porphyrins. Perhaps the most successful of the heme protein active site models is the “picket fence” porphyrin. Synthesis of a substituted iron(11) tetraphenylporphyrin having four pivalamido groups located on the same side of the porphyrin ring would give a “protected pocket”, a ligand could bind to the metal on the open face but could not penetrate the pocket, thereby ensuring five-coordination even in the presence of excess ligand, the much smaller molecular oxygen would not be sterically encumbered, and a six-coordinate complex could form (Collman et al., 1974a-c, 1975; Anzui and Hatano, 1984;Valiotti et al., 1981;Gunter et al., 1980;Schappacher

Figure 10. The ‘tailed picket fence” porphyrin.

et al., 1987). This oxygenated complex should be stable since the bulky pivalamide groups should prevent irreversible oxidation through close approach of two porphyrins and formation of a p-peroxo complex (Figure 9). Reviewson dioxygen binding to the picket fence porphyrin and related systems have been reported (Collman, 1977; Spiro, 1980; Xavier, 1985; Collman et al., 1974a-c). c. The “Tailed Picket Fence” Porphyrins. The model is a combination of the “tailed” and “picket fence” models. Although the direct oxygenation of five-coordinate Fe(I1) picket fence was observable in the solid state, such studies in solution were not possible since the picket fence could not prevent six-coordination in the presence of excess sterically unhindered base. To control coordination, Collman et al. (Collman et al., 1980, 1983a-c) adopted the “tailed” approach. Dispensing with external ligand, the base was covalently attached to the o-phenyl position of TPP, and so constrained into a position promoting intramolecular binding to the porphyrin metal. The other three meso-phenyl rings carry the “pickets” necessary to prevent irreversible oxidation. This model bind dioxygen reversibly at room temperature (Figure 10) (Collman et al., 1978a,b, 1979, 1983a-c). d. The “Capped”Porphyrins. The direct condensation of aromatic aldehydes and pyrrole to form tetraphenylporphyrins was exploited to prepare “capped” porphyrins (Ellis et al., 1980; Almog et al., 1975, 1981). In these molecules a benzene ring was covalently attached to all four ortho-positions of the meso-phenylrings, enclosing a volume of space above one face of the porphyrin ring. If the cap was sufficientlytight, the binding of bases should be prevented on the enclosed face, and binding on the open face would result in a five-coordinate species. On the other hand, the smaller oxygen molecule would be able to fit under the cap, which should provide a physical barrier to pox0 complex formation (Figure 11). e. The “Pocket”and “Tailed Pocket” Porphyrins. Collman et al. (Collman et al., 1981, 1983a-c) have used a combination of the “picket fence” and “capped” porphyrin approaches to prepare a series of “pocket” por-

Ind. Eng. Chem. Res., Vol. 33,No. 4,1994 763

Figure 13. The “strapped” porphyrin.

0 0

Figure 11. The “capped“ porphyrin.

phyrin. A benzene ring is used to provide steric encumbrance a t one face of the porphyrin, but in this case it is linked to only three rneso-phenyl groups, leaving an open side. Dioxygen may bind within the pocket by orientation of the bent Fe-0-0 unit toward open side (Figure 12). f. The “Strapped“Porphyrins. The strapped porphyrin class of heme protein models embraces all those compounds in which same group is covalently linked to two comers (usually diagonally opposite) of a porphyrin macrocycle and binds dioxygen reversibly only at low temperature (Figure 13) (Baldwin et al., 1976;Morgan and Dolphin, 1987;Gunter and Mander, 1981;Uemori and Kyuno, 1987;Ogoshi et al., 1984). g. The “Bridged”Porphyrins. In this system dioxygen is allowed to shelter under a bridge constructed across the porphyrin macrocycle (Figure 14) (Chang and Kuo, 1979;Battersby and Hamilton, 1980; Battersby et al., 1976). h. The “Crowned”Porphyrins. Two approaches have been developed in the model (Chang, 1977;Richardson et al., 1985;Hamilton et al., 1984). Firstly, the nonporphyrin part and porphyrin precursors are built onto the two ends of this structure; secondly, functional groups are introduced to the two diagonally substituted side chains (Figure 15). i. The “Cofacial”Diporphyrins. Several groups of workers have prepared dimeric porphyrins covalently

Figure 12. The “pocket” and “tailed pocket” porphyrins.

LL. Figure 14. The “bridged“ porphyrin. c o - 0 -

N

HN

0

Figure 15. The “crowned” porphyrin.

linked either by ‘pillared” (Changand Abdalmuhdi,1983), amido (Chang et al., 1977;Collman et al., 19771,or ester (Ogoshi et al., 1977)bridges in the cofacial configuration (Figure 16)and confirmed the formation of the p-superoxo bridge in the dioxygen adduct by using EPR spectroscopy (Chang, 1977). There are only two completelysyntheticnon-porphyrin

764 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994

\ A Figure 16. The 'cofacial" diporphyrin.

B Figure 19. Type A and B Schiff base complexes,

W Figure 17. The first synthetic non-porphyriniron dioxygen carrier of Baldwin.

Figure 18. The 'lacunar" ligand and complex of Buech.

iron dioxygen-carrying complexes. The first one is that reported by Baldwin and Huff (Baldwin and Huff, 1973) and which only attained reversibility at -85 OC while suffering irreversible oxidation at -50 "C in toluene/ pyridine (Figure 17). The second is that of Herron and Busch (Herron and Busch, 19811,known as the "lacunar" complex, [Fe(L)Cl]PFs, where L is the "lacunar" ligand shown in Figure 18. It reacts reversibly with dioxygen at -35 OC in a combination of acetone/water/axial base as solvent and decomposes rapidly a t 20 O C , producing iron(111)-containingspecies. 3.3.2. Cobalt Dioxygen Carriers. Since the initial discovery that Schiff base chelates of cobalt(I1) are dioxygen carriers, there has been a continued interest in the property of these complexes. The binding of dioxygen to cobalt(I1)chelate complexes of Schiff base and cyclidene lacunar has been studied extensively. A number of papers, reviews, and books have in detail described the chemistry of cobalt dioxygen carriers (Norman et al., 1988a,b; Stephenson et al., 1991;Lin et al., 1991;D z a a n and Busch, 1990;Niederhoffer et al., 1984;Henrici-Olive and Olive, 1974; Fallab, 1967; Jones et al., 1979; Gubelmann and Williams, 1983;Basolo, 1974; Martell, 1982, 1988;Vatentine, 1973;Choy and O'connor, 1972/73;Erskine and

Field, 1976; McLendon and Martell, 1976;Basolo et al., 1975;Vogt, 1963;Martel and Sawyer, 1988;Wilkins, 1971; Werner, 191R Pauling and Coryell, 1836; Calvin and Martell, 1952). Cobalt(I1) Schiff base complexes are of two types (A and B in Figure 19). The dioxygen-binding characteristics of these complexes are very dependent upon the preparation and pretreatment of the sample, as well as the purity of the starting materials. Complexes of type A absorb up to 0.5 mol of Odmol of cobalt, the oxygenated complex forming p-peroxo dimeric species with the dioxygen being peroxo-like. Type B complexes can absorb up to 1.0mol of dioxygenlmol of cobalt, the oxygenated complex being monollieric and the dioxygen moiety being superoxo-like. Both type A and B complexes are paramagnetic in the unoxygenated state. On oxygenation, type A complexes become diamagnetic and type B complexes reduce their paramagnetism from three unpaired electrons to one. Four-coordinated Schiffbase and macrocyclic complexes of cobalt(I1) are very poor dioxygen binders, whereas their corresponding five-coordinated Schiff base and macrocyclic complexes of cobalt(I1) readily bind dioxygen at ambient pressures of oxygen. The binding of an axial fifth ligand leads to a square-pyramidal arrangement, raising the d,a orbital above the d,, orbital (Figure 20) and thus resulting in the configuration (d,,)2(dy,)2(d,y)2(d~a)~. This latter configuration appears to be a necessary prerequisite of oxygenation. With the discoveryof the 1:l dioxygen adducts of cobaltSchiff base complexes, it was apparent that parallel studies on cobalt porphyrin compounds (Wayland and Mahajer, 1971;Stynes et al., 1973a,b;Walker, 1973;Scheidt and Ramanuja, 1975;Takayangi et al., 1975;Walker et al., 1976;Durham et al., 1977;Hoffman and Petering, 1970; Chien and Dickinson, 1972;Spilburg et al., 1972;Gibson et al., 1973; Uemori et al., 1988) were in order, since metalloporphyrins are utilized in biological systems. 3.3.3. Manganese Dioxygen Carriers. The interaction of manganese with dioxygen has been considered to be important in a number of processes of biological and industrial importance, although the nature of these interactions is not well understood. Therefore, model manganese compounds have been employed in order to study their interaction with dioxygen and these studies have led to the discovery of many dioxygen carriers.

Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 765

A

Figure 22. Manganese(I1) Schiff base complexes.

L4Co( II)

BL4Co(ll)

Figure 20. Energy level diagram denoting the changes in energy levels for the axial ligation of CoL,.

and then by a slow dioxygen absorption to Mn:On cu. 0.5. On the basis of these experimental data, a four-step mechanism for solid state/suspension dioxygen uptake is proposed (Figure 23): (i) diffusion of dioxygen into the polymer lattice; (ii) formation of Mn(II1) ions with p-oxo bridges; (iii) slow diffusion of additional dioxygen into the polymer; (iv) reaction to form manganese(1V) with di-poxo bridges between manganese atoms. Coleman and Taylor (Coleman and Taylor, 1977) reported the preparation and characterization of some manganeae(I1)complexesemploying a series of Schiff base pentadentate O2N3 ligands. These complexes quickly reacted with dioxygen or air when dissolved or suspended ina solvent. Heating these oxygenated complexesin uucuo at cu. 110 "C liberated the oxygen with the generation of the manganese(I1) precursors. The following scheme has been suggested for the oxygenation process:

Scheme 2 M#L Lh4nm-02-

Figure 21. Proposed structures for the oxygenation complexes.

A. Manganese-Schiff Base Complexes. The chemistry of tetradentate Schiff base manganese(I1) complexes and their reaction with dioxygen have been extensively studied (Colemanand Taylor, 1980;Ebbs and Taylor, 1974; Wilkinson et al., 1987). M n I W e n ) reacts with dioxygen to yield MnIII(salen)OH, for which Lewis et al. (Lewis et al., 1968)proposed a polymeric structure including unitary MnI*l(salen)-O-Mnl"(salen).HzO on the basis of the magnetic property. Recently these oxygenation products have been reexamined (Yarino et al., 1970; Matsushita et al., 1973). Three types of complexes were postulated (Figure 21). X-ray photoelectron spectroscopic data (Burness et al., 1974) was in good agreement with a p-peroxo structure. Similar complexes have also been reported either by oxygenation of Mn(I1) (Coleman and Taylor, 1978) or by addition of superoxide ion to Mn(I1) complexes (Coleman and Taylor, 1982). Titus et al. (Titus et al., 1979) prepared a series of manganese(I1) complexes with different numbers of CH2 units in the ligand bridge (Figure 22) and found the oxygenation reaction to be a function of the methylene carbon chain length so that only C6-clO derivatives react with dioxygen both in solution and in the solid state while all the others react only when moist, dissolved in pyridine, or suspended in dimethyl sulfoxide. Dioxygen uptake measurements as a function of time both in the solid state and in solution have suggested a two-step process where an initial absorption of dioxygento Mn:02 = 0.25 occurred, followed by a period of no measurable dioxygen absorption,

+ 0,

+ LMtf

= LMnmO,LMnnxQ2-Mtf%

Although the nature of these reversibly formed oxygenation materials is still uncertain, these oxygenation products are documented p-peroxo-manganese(II1) compounds with a suggested structure similar to that found for [Co(salrdpt)l2(02).C6H~OH(Lindblom et al., 1971). The manganese(I1) complexes of other potentially pentadentate ligands with donor seta of OzN3,03Nz, and OzSN2 derived from substituted aldehydes and polyamines have also been studied (Coleman and Taylor, 1977; Coleman et al., 1981). The rate of dioxygen uptake by these complexes is a function of the substituent on the central nitrogen donor as well as of that on the salicyaldehyde aromatic ring. B. Tris(3,5-di-tert-butylcatecholato)manganese Complexes. The first manganese system that involves solely oxygen donor groups from the ligand is believed to be tris(3,5-di-tert-butylcatecholato)manganese(III),[Mn(III)(dtbc)sgl, an oxidation product of which Mn"*(cat)2(semiquinone)2(0H)g was reported to bind dioxygen reversibly in dimethyl sulfoxide solutions (Magers et al., 1978). At room temperature, similar results are obtained in Nfl-dimethylformamide and Nfl-dimethylacetamide but the reversibility is optimal in acetonitrile (Scheme 3).

Scheme 3

+

I (02)Mn1*'((ca1)2(semi~~~)d~)3-

Magers et al. (Magers et al., 1980) has investigated this interesting system and found that this reaction was affected by concentration, acidity, dioxygen partial pressure, moisture, and light. Spectrophotometric and electrochemical data indicated that, for low metal to oxygen

766 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994

I

I

I

I

Figure 23. Proposed mechanism for oxygenation of Mn(salen) in the solid state or dimethyl sulfoxide suspension.

ratio, a 1:l dioxygen adduct is formed, while, when this ratio is increased and with high concentrations of the manganesecomplex,the extent of the irreversibleoxidation of the coordinated catechol ligands by 02 increases. EPR spectra indicated that the primary step in the oxygenation reaction is the irreversible formation of a manganese(II1) mixed catechol-semiquinone complex which reversibly binds molecular oxygen. Further studies indicated that a solution of [MnIV(dtbc)3]2- in acetonitrile has a magnetic moment of 3.94 p ~ which , is consistent with a high-spin (d3) manganese(IV)complex. The EPR spectrum of this complex in frozen acetonitrile is identical with that for a S = 3/2 system in an axial field with a large zero-field splitting (Richens and Sawyer, 1979). Such a pattern provides additional support for the formulation of the blue complex as [MnIv(dtbc)3I2-. This complex was found to bind dioxygen reversibly at room temperature to form a red-brown complex [MnrV(dtbc)a(sq-) (02-)12-, (sq = semiquinone). The formation constant, Kf, for the dioxygen adduct, determine spectrophotometrically, is 2.9 atm-1 at 298 K and 3.2 atm-' at 251.5 K. Hill plots (Ibers et al., 1974)yield a straight line with an average slope of 0.8 which is indicative of a 1:l dioxygen adduct. However, Cooper and Hartman (Cooper and Hartman, 1982),reproducing the experiments, claimed that [MnIV(dtbc)3I2- does not bind dioxygen reversibly but upon exposure to 02 generates a steady-state concentration of the red-brown semiquinone which is formed continuously by oxidation and is removed by precipitation of polymerized semiquinone. This system has been reconfirmed by more spectrophotometric results (Chin and Sawyer, 1982)together with some recent studies (Chin et al., 1983). All the evidence supports the fact that dioxygen is reversibly bound by dilute concentrations of [Mn"(dtbc)sl2- in slightly alkaline acetonitrile according to the reaction (Scheme 4). The Mn(IV)-dtbc complexrepresents an interesting new category of dioxygen carriers in which electron transfer from the catecholato ligand to molecular oxygen is necessary for binding. It also represents an important model system for the biological interaction of oxygen with manganese especially in the terminal oxygen releasing step in photosystem I1 (Wieghardt, 1989). C. Manganese Phthalocyanine Complexes. The phthalocyanine complexesof manganese have been studied

Scheme 4

+

[Mn'"(dtb~)3~- 02

I

+ OH-

+

Mn"(dtb~)~(o,-)(OH)~- dlbq

for many years. Barrett et al. (Barrett et al., 1936) first reported that a pyridine solution of Mn(I1)phthalocyanine absorbed dioxygen, the color changing from olive green to dark blue. Boiling the solution regenerated the olive green color of MnIIPc, and once cool the solution would absorb further amounts of dioxygen. The cycle could be repeated many times with little destruction of the pigment. These observations were investigated further by Elvidge and Lever (Elvidge and Lever, 1959),who found that oxidation of pyridine solutions of MnIIPc with molecular oxygen resulted in precipitation of crystals they formulated as MnIVPc(Py)O. This was, however, shown to be in error by Vogt et al. (Vogt et al., 1967) who showed the material to be [MnIn(Pc)(Py)lzO. The recent work has established the formation of a superoxo complex on oxygenation of MnPc in N,Ndimethylacetamide solution (Lever et al., 19811, and a similar result has been obtained for a tetrasulfonated phthalocyanine derivative (Moxon et al., 1981). The solution of the dioxygen adduct can be converted into the p-oxo species by the addition of either imidazole or N-methylimidazole and has postulated the following mechanism for the reaction (Scheme 5).

Scheme 5 Mn"Pc Mn(Pc)(O,)

+ 0, + Mn(Pc)(O,)

+ Mn"Pc

+ (Pc)Mn"'-O,-Mn"'(Pc)

(Pc)Mn"'-0,-Mn"'(Pc) Mn"(Pc)O

+ Mn"Pc

+ 2MnW(Pc)O

+ (Pc)Mn"'-O-Mn'"(Pc)

Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 767 under It2

._.......- ......

u n d a r Ot

Figure 24. Visible spectrum of Mn(TPP)(Py) in toluene at -79

OC.

The formation of a dioxygen complex of MnIIPc in poly(Zmethyl-l-vinylimidazo1e)-NJV-dimethylformamide solution has also been demonstrated by comparison of electronic spectra and EPR measurements of MnIIPc and its oxygenated partner (Uchida et al., 1978). Since the solution spectra is very similar to those of typical MnlIIPc mononuclear species (Engelsma et al., 1962) and the infrared spectrum established the superoxide ion (Lever et al., 1979),the most probable formulation of this adduct would be (dmf)Mnu1(Pc)(02-). Lever et al. (Lever et al., 1980) has extended this investigation and presented a molecular orbital treatment based on the superoxo formulation with a bent end on bound oxygen molecule. D. Manganese Porphyrin Complexes. (meso-Tetraphenylporphyrinato)manganese(II) complexes of the form MnII(TPP)(L), where L represents a coordinating ligand, have been shown to act as reversible dioxygen carriers in toluene solution at -78 "C (Hoffman et al., 1976). The reaction of dioxygen with a series of parasubstituted (meso-tetrapheny1porphyrinato)manganese(11) complexes Mn"[T@-X)PP] (B) containing an axial base B in toluene has been studied (Jones et al., 1978). Spectrophotometric titrations of toluene solutions of these complexes at -78 "C with molecular oxygen confirm that an equilibrium between the five-coordinated monoligated species Mn[T@-X)PP](B)and the dioxygen complex Mn[T@-X)PP](Oz) is established. The binding of dioxygen to manganese(I1) porphyrins greatly modifies their optical and EPR spectra (Hoffman et al., 1976) as shown in Figures 24 and 25. Thus optical spectra of these five-coordinated complexes are transformed from the normal to the "hyper" type with a split Soret band, while the EPR spectra indicate a change from high spin (S = 5/2)to intermediate spin (S = 3/2)upon binding of dioxygen. Moreover, analysis of the EPR data (Hoffman et al., 1978)supports a MdV(0z2-)formulation for the dioxygen adduct with the 0 2 molecule bound to an out-of-plane manganese in the Griffith geometry, whereas ab initio calculations (Dedieu and Rohmer, 1977) favor a Mn=(02-) formulation with 0 2 bound in the Pauling mode. In order to obtain a better understanding of the EPR results and to resolve the apparent contradictions concerning the electronic and geometric structures of oxymanganese, charge interactive extended Huckel, restricted Hartree-Fock plus configuration interaction, and generalized molecular orbital plus configuration interaction

Figure 25. Low-field portion of the ESR spectrum of Mn(TPP)(Py) in toluene.

calculations were performed on both Griffith and Pauling models of oxymanganese porphyrins (Hanson and Hoffman, 1980; Newton and Hall, 1985). The authors concluded that all the results can be explained in terms of a d3 configuration and Griffith binding of the dioxygen. Additional evidence for the Griffith mode of coordination has also been obtained by Urban et al. and Jones et al. (Urban et al., 1982; Jones et al., 1979). Using matrix isolation techniques, they were able to record the infrared spectra of the dioxygen adduct MnTPP(02). They assigned the voZbands for both '802 and 160z adducts at 933 and 983 cm-l, respectively. The authors also carried out an oxygen isotope scrambling experiment to provide further evidence on the structure of the dioxygen adduct. They observed three bands at 983,958, and 933 cm-l when MnTPP was condensed with a mixture of 1 6 0 2 , l60l8O, and 'BO2. The bands at 983 and 933 cm-l corresponded to yoz of the l 6 0 2 and l a 0 2 adduct, respectively. The remaining band at 958 cm-l was assigned to the 1601*0 species. Since only one band was observed for this species, the 0 2 moiety must be side-on coordinated. End-on coordination would lead to splitting of the central band into a doublet due to the pressure of both Mn-160-180 and Mn-180-160 species (Kozuka and Nakamoto, 1981). E. Manganese Tertiary Phosphine Complexes. A large number of manganese(I1) tertiary phosphine complexes of generalformulaMnLX2 (L = tertiary phosphine, but not PPh3, and X = C1, Br, I, NCS) have been prepared and characterized (McAuliffe et al., 1979; Brown et al., 1980;Green, 1982). These compounds can bind dioxygen reversibly in a manner that resembles the properties of hemoglobin or myoglobin in blood. The MnLX2 complexes are pale in color, but on exposure to dioxygen they rapidly become deeply colored, the color being dependent upon the halide as shown in Table 5. In addition to the MnLXz complexes, complexes with the general formula MnLzX2 have been prepared (Li, 1989; Challita, 1983; Hebendanz et al., 1984; Kulkarni and

768 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 5. Colors of the MnLXz and MnLXz(02) Complexes X c1

Br I

NCS

MnLXz white off-white pale-orange yellow

MnLXdOz) purple blue green red

Govind, 1988, 1989, 1992). These complexes were also shown to bind dioxygen reversibly in both the solid state and in solution, and the reactivity toward dioxygenappears to be dependent on the phosphine. a. Synthesis of Manganese(I1) Phosphine Complexes. Under stringent anhydrous conditions, manganese(I1) salts react with tertiary phosphines to give complexes with a Mn-P bond. Three factors (a-bonding, a-bonding, and steric effects) are involved in the formation of the bond between the metal and phosphorus atoms (Wilkinson et al., 1987). The fundamental characteristic of all PR3 compounds is the presence of a lone pair of electrons on P. It therefore can behave as both bases and nucleophiles. The Mn-P bond is a donor covalent bond. The phosphorus behaves as a Lewis base, and the lone pair of electrons on the phosphorus atom is responsible for bond formation between the metal and the phosphine. The stability of the metal-phosphorus bond increases with the increasing of pK, value. ?r-bonding arises from a transfer of charge from the metal to the phosphorus. Steric effects dominate the reactivity of many metal phosphine complexes. The formation of a bond between a metal and a phosphine generally involves a change in the phosphorus coordination number from three to four, i.e., from pyramidal geometry (p3) to tetrahedral geometry (sp3). The primary Mn-P bond is the a-bond formed between this sp3 hybrid phosphorus orbital and a d2sp3 manganese hybrid orbital. The importance of the steric demands of the substituents lies in their effect on the phosphorus geometry before and after bond formation. Sterically demanding phosphines interact strongly with other ligands in the coordination sphere and can cause restricted rotation. To relieve steric strain, the metal often remains coordinatively unsaturated. b. Spectrophotometric and Magnetic Properties. The infrared and far-infrared spectra of the MnLX2 complexes show the absorption bands which are consistent with those observed for both bridging and terminal Mn-X linkages. This suggests that these complexes are at least dimeric. Because a V C N in the infrared spectrum was taken as a guide to predict structure/reactivity relationships, a series of manganese(I1) isothiocyanate complexes Mn(NCS)2(PR3) (R = alkyl or phenyl) was prepared and characterized (McAuliffe et al., 1983). Their ability to bind dioxygen was studied. Complexes with trialkylphosphine ligands were found to bind dioxygen reversibly in the solid state in 1:l ratio to give the dioxygen adduct, while those complexes with phosphines containing one or two phenyl groups were inactive toward molecular oxygen in the solid state. The presence of a V C N band at ca. 2140 cm-I indicated the inactivity of the complex toward dioxygen binding. In solution, the inactive complexes changed into active complexes. This change was accompanied by the disappearance of the V C N band at 2140 cm-'. The infrared spectra of the dioxygenated complexes showed bands at 1402-1430 cm-I which were assigned to neutral dioxygen (McAuliffeet al., 1979). Infrared studies by Burkett et al. (Burkett et al., 1983,1984)on the complex MnBrz(PMe3) which was prepared as film on infrared windows have led to their assigning both vo-0 and v h - 0 bands. When exposed to dioxygen, bands appeared at 1132 and 570 cm-' in the spectra of the resulting MnBr2-

(PMe3)(02) complex. With use of 1 8 0 2 , these bands were shifted to 1095and 545 cm-1, respectively, and the authors assign these bands to YO-0and v ~ n - 0vibrations. They formulate the dioxygen adduct as a manganese(II1) superoxo species, MnIX1Br2(PMe3)(O2-).However, recent infrared and isotopic labeling studies (Burkett and Worley, 1989) show that MnBrz(PEt3)(02) complex has a side-on peroxo variety. Electronic spectral measurements have shown that, upon contact with dioxygen, colorlesssolutions of MnLX2 begin to color intensely as MnLX2(02) forms. There is a linear relationship between the magnitude of the absorption coefficient and the degree of dioxygenation for these complexes. These colors essentially reflect the nature of the halide ion and not the phosphine; i.e., the chloro complexes are purple and exhibit band maxima at 395 and 530 nm, the bromo complexes are blue and exhibit band maxima at 414 and 570 nm, and the iodo complexes are green and exhibit band maxima at 455 and 620 nm. The spectral profiles (band maxima) of any particular complex are solvent independent, but the absorption coefficients do vary with solvent, being largest in noncoordinating solvents. Cyclicvoltammetric studies on some manganese tertiary arylphosphine complexes show a one-electron-transfer process which becomes more reversible as the temperature is lowered, thus indicating an electrochemical-chemical mechanism, with Eo' values influenced by both the anion and phosphine ligands (Li et al., 1992). The absorption of dioxygen is stoichiometric and completelyreversible. A mass spectral analysis (McAuliffe and Al-Khateeb, 1980) carried out on mixtures of ISOz/ l B 0 2 absorbed and desorbed by a number of the manganese(11)phosphine complexes showed no evidence for 180-160 molecules. The absence of these scrambled dioxygen molecules, and the fact that the 1802:1602 ratio remained constant throughout the experiment, indicated that the dioxygen molecule undergoes no bond breakage during the oxygenation reaction. EPR studies have shown evidence for solvent interaction in the coordination of dioxygen. The spectra can be readily interpreted in terms of a 6S state (I = 6/2). In the deoxygenated state, the EPR spectra indicate that the complexes are polymeric in toluene and monomeric in tetrahydrofuran, whereas in the oxygenated state the species present are monomeric in both of these solvents. In contrast to the solution chemistry in which ad6system appears to be present regardless of oxygenation, in the solid state the magnetic moments of the MnL,X2 (n = 1 or 2) complexes are close to 5.92 PB,indicating a system containing five unpaired electrons. c. X-Ray Crystal Structures. The structures of monophosphinemanganese complexes have been crystallographically characterized (Beagley et al., 1984;Godfrey et al., 1991; McAuliffe et al., 1992). Typically, MnI2(PPhMe2) complex has an infinite halide-bridged MnI2 polymer with two phosphine ligands bound to each alternate manganese to give a repeating o h , Td, Oh,Td, etc. pattern. There are distinct spaces above and below the coordinatively unsaturated manganese sites. Thus, the small molecules can be bound on the Td manganese atom. These spaces can be fine-tuned by either changing the bridging halide or by changing the steric requirements of the phosphine group. The structures of bisphosphine- and diphosphinemanganese complexes have also been crystallographically determined (Li, 1989; Challita, 1983; Hebendanz et al., 1984;Kulkarni and Govind, 1989,1992). These complexes

Ind. Eng. Chem. Res., Vol. 33, NO. 4, 1994 769

b

a

C

0

s 8 3

c

J

p (02) torr

a = [ Mn(PPhMez)Bri] b = [Mn(PPhEtz)Brz] c = [Mn(PBu,)(NCSh(THF)2] Figure 26. Some solid-state dioxygen isotherms for the MnLXz complexes.

have either pseudotetrahedral or pseudooctahedral arrangement of ligands around the manganese. d. Dioxygen Uptake Measurements. Reaction of the manganese(I1) phosphine complexes with dioxygen in the solid state is almost instantaneous a t the surface. Complete dioxygenation is diffusion controlled and can take 2-24 h to complete, depending upon particle size. For dioxygen uptake measurements in solution, before quantitative measurements of 0 2 absorption by the MnLX2 complexes in solution were recorded, the quantity of 02 absorbed by solvent alone was measured by a gas buret. The amount of 02 absorbed by the solution of complexes was then recorded. By subtracting the blank value for solvent alone,the quantity of 02absorbed by the complexes could be calculated. That MnLXz complexes bind dioxygen in solution is further evidenced by plots of dioxygen absorbed against complex concentration. The slopes of the plots are close to 1.0, indicative of a 1:l complex, MnLX2(02), in solution. The value of the intercepts is equivalent to that of a blank under the same conditions. e. Isotherms and Equilibrium Constants. Solidstate isotherms for the MnLX2 complexes have a remarkable similarity to those of the natural dioxygen carriers myoglobin and hemoglobin, and show that changing either the phosphine ligand or the halide changes the dioxygen binding characteristics of the complexes. Thus, changing L from PPhMe2 to PPhEtz not only causes a shift of the isotherm, but also causes a change in shape from hyperbolic to sigmoidal as shown in Figure 26. In solution the intensity of the color developed upon exposure to dioxygen is proportional to the concentration of MnLXz(02) present, and this made it possible to construct isotherms spectrophotometrically and to calculate the equilibrium constants for these reactions. A large number of KoZvalues for the equilibrium Kol

[MnX2(PR3)l+ 0,+ [MnX2(PR3)(02)l have been reported (McAuliffe et al., 1983;McAuliffe and Al-Khateeb, 1980; Barrett, 1984) in a variety of solvents (Table 6). For the complex MnBr2(PBun3)the shape of the solution isotherm has been found to be solvent dependent (Figure 27). In noncoordinating solvents the isotherm is hyperbolic, whereas in tetrahydrofuran the shape of isotherm indicates a more complex interaction, probably involving solvent participation. In general, for the same solvent and for the same manganese-bound

phosphine, affinity for dioxygen varies with halide, in the order C1 > Br > I. On the other hand, for a constant halide and solvent, the isotherm changes as the phosphine changes from alkyl to phenyl; thus dioxygen affinity decreases in the order PR3 > PPhRz > PPh2R > PPh3 (no affinity). Because of their biological and industrial applications, dioxygen carriers have received a great deal of attention. Starting with the study of the natural dioxygen carriers such as hemoglobin and myoglobin, it was then possible to synthesize many transition metal dioxygen carriers which were intensively studied and improved over the years. Recently, effort has been directed toward the development of air separation and oxygen storage technologies based on these coordination compounds that reversibly bind dioxygen, and several metal chelate systems have been developed for this purpose (Martell and Sawyer, 1988; Adduci, 1976; McAuliffe et al., 1979; Nelson et al., 1986;Kawakami et al., 1982;Imamuraand Lunsford, 1985; Motekaitis and Martell, 1988; Smith et al., 1977). It has been noted (Niederhoffer et al., 1984) that the dioxygen carriers considered good candidates for oxygen separation and transport are generally those that have low oxygenation constants. Such considerations prompted a closer look at the properties of the cobalt, iron, and manganese complexes as possible oxygen carriers for oxygen separation and recovery. Most oxygen and nitrogen is currently produced by cryogenic fractionation, or a process involving lowering the temperature of air sufficiently (to about -215 "C) to liquefy it and then using a multistage distillation process to produce pure oxygen and pure nitrogen. A major drawback of such cryogenic processes is that they require a great deal of energy and thus they are economical only for large-scale production of greater than 200 tonslday. For many applications of oxygen, high purity is not required and the daily requirement is not large. With the rising cost of energy, several separation techniques based on selective adsorption/desorption and membrane have recently emerged as cost-competitive alternatives for lower quantity application. 4. Cryogenic Air Separation

Cryogenic air separation by distillation is a mature technology and remains unchallenged as the only economical method of separating air on a large scale (Thorogood, 1991). One of the most simple air separation systems is the Linde single-column system. The entering air is compressed, the water vapor and carbon dioxide are removed, and the air is passed through a heat exchanger in which the incominggas is cooled. If the oxygen is desired as a gas, the heat exchanger is a three-channel type. The cold oxygen gas is used to help cool the incoming air. If the oxygen product is desired as a liquid, the heat exchanger is a simple two-channel type. The liquid oxygen is then withdrawn from the lower section of the column. The Linde single-column system has two serious disadvantages: (i) only pure oxygen can be produced, and (ii) large quantities of oxygen are wasted in the impure nitrogen exhaust gas. These problems can be solved by the Linde double-column system. Two rectification columns are stacked one on top of the other. The lower column is operated at a pressure on the order of 5-6 atm, and the upper column is operated at a pressure of approximately 1atm. The analysis of the double-column cycle shows that one part of the process operates best with an air expansion Lachmann air design, while another operates best when the Lachmann air is not processed

770 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 6. Eauilibrium Data for Oxygen Uptake by Mn(PRa)Xz Complexes complex solvent temp, "C log IC1,Torr' -1.054 THF 20 Mn(PEt3)Brz THF -1.073 20 Mn(PEt3)Clz -1.109 -40 THF Mn(PEt3)Iz -1.073 20 THF Mn(PPr3)Brz -0.251 20 toluene Mn(PPr3)Brz -2.707 20 THF Mn(PPr3)C12 -1.565 20 toluene Mn(PPr3)Cl -1.361 -40 THF Mn(PPr3)Iz -1.320 20 THF Mn(PBu3)Brz -0.429 20 toluene Mn(PBu3)Brz -0,418 20 1,2-EtC12 Mn(PBu3)Brz -1.472 -40 THF Mn (PBu3)12 20 -0.853 Mn(PPhMe2)Iz toluene -1.431 -40 THF Mn(PPhMe2)Iz -2.099 20 toluene Mn(PPhEb)Brz -2.105 20 1,2-EtC12 Mn(PPhEt2)Brz -2.162 20 toluene Mn(PPhEt2)Clz -2.139 20 1,2-EtC12 Mn(PPhEt2)Clz -7.300 20 toluene Mn(PPhBu2)Brz -7.219 20 toluene Mn(PPhBu2)ClZ -2.254 20 1,2-EtClz Mn(PPhBu2)Clz 100

4or eo

-e C

0

2

m

2

60

20

I

I

I

I

I

20

40

60

80

I

I

t

,

100 I20 140 160

p (0,) / Torr

Figure 27. Dioxygen binding curves for MnBrz(PBu'3). giseous phase

0

0

0

0

0 0

adsarption

c_____ n

interface

n

'-sz 1 c-3se B:SC'2$1:

. . .

. . .. .. . .

Figure 28. Basic concepts of adsorption.

(Vansant and Dewolfs, 1990). Thus, if a way could be found to combine the best features of both, then there is scope for a significantly improved process. 5. Adsorption Separation

The basic phenomena, i.e., adsorption and desorption, are schematically shown in Figure 28 to explain the basic action of adsorbents. The surface of the adsorbent comprises so-called active sites which, due to their electronic structure, can bind molecules from the gaseous or liquid phase by predominantly physical forces. This

10%.

Torr' 8.83 8.45 8.45 8.45 56.1 0.196 2.72 4.35 4.79 37.23 38.19 3.373 14.0 3.707 0.796 0.966 0.688 0.726 0.501 0.604 0.557

PI/I, Torr

sloDe

11.3 11.83 12.9 11.8 1.78 509 36.73 22.9 20.9 2.69 2.62 29.7 7.13 26.98 125.6 103.5 145.2 137.7 199.5 165.6 179.5

0.88 0.86 1.62 0.86 0.54 1.35 1.02 1.77 1.12 0.69 0.94 1.8 0.92 1.43 1.22 1.4 1.16 1.17 1.36 1.19 1.24

phenomenon is called adsorption. It is exothermic and takes place mostly at low temperatures. Adsorption reaches equilibrium when under the prevailing concentration and temperature conditions a definite percentage of the active sites available is occupied by the adsorbate. The adsorbent then needs to be regenerated before it can be used for a further adsorption cycle. The simplest way of regeneration is desorption of the adsorbate which, as a reversal of adsorption, is endothermic and takes place mostly at higher temperatures. Thus, in adsorption technology a desorption is obtained by a temperature increase with simultaneous decrease of the adsorptive concentration or of the total pressure. After cryogenics, the most widely used process for air separation is adsorption (Ruthven, 1984; Yang, 1987; Wankat, 1986; Vansant and Dewolfs, 1990; Rodrigues et al., 1989; Keller and Yang, 1989; Slejko, 1985). In the cryogenicair separation process,the separation mechanism depends on the difference in composition between the liquid and vapor phase. Similarly, in adsorption, a phase composition difference determines the separation but the dense adsorbed phase is created by affinity to the solid adsorbent surface. The relative and total affinities of the gas mixture are a complex function of pressure, temperature, composition, and the nature of the adsorbent. Further, the solid adsorbent is a stationary phase and the process must be operated in a cyclicmanner using multiple beds to create a continuous supply of product gas. This complexity presents an entirely different challenge to the process designer, who must now include time dependence in the process conception. Adsorption processes consist of the selective concentration (adsorption) of one or more components (adsorbates) of either a gas or a liquid at the surface of a microporous solid (adsorbent). The attractive forces causing the adsorption are generally weaker than those of chemical bonds, and by increasing the temperature of the adsorbent or reducing an adsorbate's partial pressure (or concentration in a liquid), the adsorbate can be desorbed. The desorption or regeneration step is quite important in the overall process. First, desorption allows recovery of adsorbates in those separations where they are valuable, and second, it permits reuse of the adsorbent for further cycles. One key element of an air separation process is the adsorbent used for the nitrogen-oxygen separation. Commercial adsorbents can be divided into four major classes:

Ind. Eng. Chem. Res., Vol. 33, No. 4,1994 771 molecular-sieve zeolite, activated alumina, silica gel, and activated carbon. So far all commercial processes use either an inorganic zeolite (synthetic or natural) or an organic carbon molecular sieve (CMS) adsorbent. Dessicants (alumina, silica gel) and regular active carbon are often integrated as guard or pretreatment beds within the molecular sieve beds, and these are run in conjunction with the normal pressure swing adsorption (PSA) cycle to save equipment, such as a feed dryer. The adsorptive separation is achieved by one of the three mechanisms: steric, kinetic, or equilibrium effect. The steric effect derives from the molecular sieving property of zeolites. In this case only small and properly shaped molecules can diffuse into the adsorbent, whereas other molecules are totally excluded. Steric separation is unique with zeolites because of the uniform aperture size in the crystalline structure. The two largest applications of steric separation are drying with 3-A zeolite and the separation of normal paraffins from isoparaffins and cyclic hydrocarbons using 5-A zeolite. Kinetic separation is achieved by virtue of the difference in diffusion rates of different molecules into the adsorbent. Kinetic separation is possible only with carbon molecular sieve because of a distribution of pore sizes. Kinetic separation is used commercially for nitrogen generation from air. The separation is believed to be achieved as a result of a slight difference in the kinetic diameters of nitrogen and oxygen, which results in a relatively high diffusivity for oxygen. A large majority of processes operate through the equilibrium adsorption of the mixture and hence are called equilibrium separation processes. Pressure swing adsorption is most often an equilibrium-controlled, almost isothermal process with pressure swinging between two extreme pressures. Usually, the production step is taking place at the highest pressure and the bed is regenerated by decreasing the partial pressure of the adsorbed component in the gas phase in contact with the solid, at least in part by dropping the pressure to lower pressures or eventually to the lowest limit. Several good books (Ruthven, 1984; Yang, 1987; Wankat, 1986; Vansant and Dewolfs, 1990; Rodrigues et al., 1989;Rousseau, 1987) and numerous reviews (Suzuki, 1988; Ray, 1986; Tandeur and Wankat, 1985; Riquarts andLeitgeb, 1985;Richter, 1987;Kawaiand Kaneko, 1989; Lee and Stahl, 1973; Sircar and Kratz, 1988; Sircar, 1988; Jasra et al., 1991) on PSA technology have already been published. Several PSA models have also been reported including pressure drop effects (Sundaram and Wankat, 1988), heat effect (Farooq et al., 1988), mass transfer (Raghavan and Ruthven, 1985), micropore diffusion (Doong and Yang, 1986; Raghavan et al., 19861, multicomponent mixtures (Yang and Cen, 19861,and nonlinear equilibrium (Underwood, 1986). 5.1. Zeolite. Gas separation with zeolites falls into two broad categories. Molecular exclusion or so called shapeselective separations are generally controlled by the pore size opening and separate components based on size (Breck, 1974). Generally, the larger gas is excluded from entering the zeolite. Two good examples of separations based on this mechanism are the recovery of normal paraffins from a variety of refinery streams and the separation of xylene isomers and ethylbenzene. When a zeolitic adsorbent is exposed to a gas mixture where both components are smaller than the pore opening, the equilibrium established for the different components between the bulk phase and adsorbed phase is the basis for separation. In this mode of separation, the zeolite interacts differently with each gas giving rise to the zeolite's selectivity for one gas over another. Three examples of commercial processes based

0

100

200

300

400

500

Pressure (torr)

Figure 29. Adsorption isothermsfor Nz, 02and AIon Na-mordenite at 293 K. Table 7. Selectivity of Adsorption of Nr over 0, on Na-Mordenite at 1 atm nitrogen selectivity mole fraction 02 273 K 292 K 323 K 0.10 4.81 4.27 3.92 0.30 4.91 4.45 4.12 0.50 5.02 4.66 4.36 0.70 5.12 4.89 4.63 0.90 5.24 5.15 4.95

on thermodynamic control are (i) front-end cleanup of air to remove water and carbon dioxide in the feed to a cryogenicair separation unit, (ii) the recovery of hydrogen from a variety of hydrocarbon streams and refinery offgases, and (iii) adsorption-based air separation which can be carried out using a variety of pressure conditions. Zeolites are polar adsorbents which selectively adsorb nitrogen from air because it has a larger permanent quadrupole (1.52 X 1 p 2 6 esu cm2) than that for oxygen esu cm2)or argon (0.0 X esu cm2).The (0.30 X capacity, the selectivity, and the heats of adsorption of nitrogen, oxygen, and argon can vary significantly depending on the structure of the zeolite and the cations present within them. Typically, zeolite types A, X, or mordenite, which are ion exchanged with metals of group 1A and group IIA elements such as sodium, potassium, magnesium, and calcium, are used for air separation. An example of isotherms for adsorption of N2,02 and argon on a sodium mordenite at 293 K is given in Figure 29. Nitrogen is more selectively adsorbed than oxygen and argon, whose isotherms nearly coincide, indicating that there is practically no selectivity of adsorption between them. Table 7 shows the thermodynamic selectivity of adsorption of nitrogen over oxygen as functions of equilibrium gas phase composition and temperature for this adsorbent. The selectivity of nitrogen over oxygen is defined by the ratio (nlyz/n2yl),where ni and yj are respectively the specific amount adsorbed and the equilibrium gas phase mole fractions of component i. The selectivity decreases with increasing nitrogen composition and temperature. The kinetics of adsorption of the components of air on most commercial zeolites are usually very fast at near-ambient temperatures because the zeolites are used in pelletized forms produced by binding very small (1-5 pm) crystals of the zeolite with a macroporous binder matrix which usually controls the mass-transfer resistance of the gases to the zeolitic adsorption sites. For

772 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 adsorption pores

/

I

---

admission pores

meso pore micro pore

7

sub micro pore

I-'.";

T

.

..-LAC.->.-.

y

c

m

T

"

7 I

7w F i~

Figure 31. Pore structure of C\lS adsorhents.

example, a typicaldiffusional timeconstant for adsorption ofnitrogenfromaironsodiummordeniteis0.45s-'(Kumar and Sircar, 1986). Consequently, the separation is based on thermodynamic selectivity of the zeolite for nitrogen over oxygen and argon. 5.2. Carbon Molecular Sieve. Thecarbonmolecular sieves for air separation are made by further narrowing the pore mouths of a small-pore activated carbon by depositing nascent carbon produced by thermal cracking of a hydrocarbon. This is controlled in such a fashion thataCMS withaneffectiveporemouthdiameterbetween that of nitrogen (3.64 A) and oxygen (3.46 A) molecules is produced. In CMS adsorbenta adsorption does not take place on a smooth surface but on the walls of narrow pores distributed within the material. Looking at a schematic section of anextremelyporousparticleofactivatedcarbon as shown in Figure 30, distinction can be made between the large transport pores going through the whole particle and theverysmallpores branchingofffrom thesetransport pores within the particle. The former pores are used for rapid transport of adsorptives from the external surface of the particle into the inside of the particle, and for this reason they are also called 'diffusion pores". The very fine pores, called "adsorption pores", constitute the largest part of the internal surface, and most of the adsorption takes place within these pores. According to international classification (IUPAC, 1972) further subdivision of pores is made according to their width or function, as shown schematically in Figure 31. The "macropores" for admissionldiffusion are of >500-A diameter. The diameter range from 500 down to 20 A covers the so-called "mesopores", and the range from 20 to 8 A covers the "micropores". Pores with diameters of the same order as the diameter of molecules, i.e.,