The heterogeneous binding of oxygen

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Title:
The heterogeneous binding of oxygen the preparation and characterization of cobalt cyanide complexes inside zeolite Y
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vii, 183 leaves : ill. ; 28 cm.
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English
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Taylor, Robert J., 1963-
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Subjects / Keywords:
Oxygen   ( lcsh )
Adsorption   ( lcsh )
Ligand binding (Biochemistry)   ( lcsh )
Zeolites   ( lcsh )
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bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1989.
Bibliography:
Includes bibliographical references (leaves 178-181).
Statement of Responsibility:
by Robert J. Taylor, Jr.
General Note:
Typescript.
General Note:
Vita.

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University of Florida
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All applicable rights reserved by the source institution and holding location.
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oclc - 22479956
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Full Text












THE HETEROGENEOUS BINDING OF OXYGEN:
THE PREPARATION AND CHARACTERIZATION OF
COBALT CYANIDE COMPLEXES INSIDE ZEOLITE Y

















BY

ROBERT J. TAYLOR, JR.


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1989
























TO SHARON, MY WIFE AND BEST FRIEND















ACKNOWLEDGMENTS


I would like to thank the many people who have been a

great help to me throughout my career at the University of

Florida. First, I would like to thank Dr. Russell Drago for

his leadership and encouragement over the years. His

excitement about chemistry and unending optimism has been an

inspiration. I would also like to thank his wonderful wife,

Ruth, for her kindness and hospitality.

Second, I would like to thank the people who have

directly contributed to this work. Dr. Iwona Bresinska's

initial work was invaluable. Dr. Jim George's comments and

discussions as well as his experimental diligence were also

much help.

Next, I would like to thank my co-workers in the Drago

group for their friendship through the years. Former group

members Mark Barnes, Cindy Bailey, Ken Balkus, Rich Riley,

Jeff Clark, Andy Griffis, Shannon Davis and Pete Doan always

lent a hand when needed. Present group members Larry

Chamusco, Ngai Wong, Tom Cundari, Alan Goldstein, Mike

Naughton, Don Ferris, Steve Showalter, and Steve Petrosius

have been a valuable source of knowledge and advice over the

years. I would like to especially thank my true friend


iii









through it all, Jerry Grunewald. From start to finish his

friendship and encouragement was always there. I also want

to thank the second-in-command of the Drago gro' l, Maribel

Lisk, for lending her assistance when needed. Her greatest

accomplishment during my years at UF was to keep Doc busy

and out of the labs so work could get done. To the newest

group members, John Hage and Doug Patton, I would like to

offer a word of advice. Take advantage, whenever possible,

of the priceless resource available to you while you are at

UF, the Drago group.

Most importantly, I would like to thank my wife,

Sharon, for her unending love and encouragement through

thick and thin. Her caring and understanding have made even

the darkest days a little brighter. I would also like to

thank my parents, Joel and Becky, for their guidance through

my early years and their love and support for a lifetime.
















TABLE OF CONTENTS


ACKNOWLEDGMENTS .......... ......................... iii

ABSTRACT ......................................... vi

CHAPTERS

1 INTRODUCTION ................................... 1

2 REVIEW OF LITERATURE ......................... 7

Introduction ................................... 7
Zeolites ............................. ........ 8
Transition Metal Coplexes Inside Zeolites .... 13
Cyanide Complexes ............................ 18

3 EXPERIMENTAL ................................... 22

Synthesis ...................................... 22
Characterization ............................. 25

4 RESULTS AND DISCUSSION ....................... 30
2-
Co(CN)42- Inside Zeolite Y ................... 30
3-
Co(CN)53- Inside Zeolite Y .................. 74

5 SUMMARY AND CONCLUSIONS ...................... 110


APPENDIX A CALCULATIONS AND PROCEDURES ........ 113

APPENDIX B COMPUTER PROGRAMS .................. 123

APPENDIX C QUATTRO SPREADSHEETS ............... 160

REFERENCES .......................................... 178

BIOGRAPHICAL SKETCH ............ ................. 182















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of
the Requirements for the Degree of Doctor of Philosophy

HETEROGENEOUS BINDING OF OXYGEN:
THE PREPARATION AND CHARACTERIZATION OF
COBALT CYANIDE COMPLEXES INSIDE ZEOLITE Y

By

Robert J. Taylor, Jr.

December 1989


Chairman: Russell S. Drago
Major Department: Chemistry

Cobalt complexes that reversibly bind dioxygen are

available in a large variety of ligand systems. The main

drawback to the utilization of these materials in catalysis

or oxygen enrichment from air arises from the fact that very

few complexes bind oxygen in the solid state while in

solution dimerization and irreversible oxidation of the

complexes occur. Synthesizing these cobalt complexes inside

the cage of a zeolite has the potential of eliminating these

undesirable properties and several zeolite encapsulated

metal complexes that reversibly bind dioxygen have been

reported. These compounds all involve neutral ligands and

though effective for separating oxygen from air,

coordination of water as a sixth ligand or oxidation of the

ligand limits their utility.









In this study, these problems have been eliminated by

preparing anionic cobalt cyanide complexes that are more

stable than the cationic complexes previously reported. The

cyanide ligand is very stable to oxidation and the zeolite

prevents dimerization to form a g-peroxo complex. Both

CoCN)2- and Co(CN)53- have been isolated inside zeolite-Y
Co(CN)4 and have

and shown to reversibly bind oxygen. The former is bound to

the zeolite wall through a lattice oxide while the later is

formed in the large cavity of the zeolite. Electron

paramagnetic resonance spectroscopy, EPR, has been used to

characterize both the oxygenated and deoxygenated forms of

these two complexes. Quantitative gas uptake measurements

have shown the host zeolite materials to exhibit a high

selectivity for oxygen over nitrogen or argon. These uptake

measurements have been used to determine the equilibrium

constant for binding oxygen to both active species and the

enthalpy and entropy of 02 binding have also been determined

for the latter. The effect of supporting a transition metal

complex inside a zeolite on its oxygen binding properties is

discussed.


vii















CHAPTER 1
GENERAL INTRODUCTION



The past two decades have seen a tremendous growth of

gas adsorption processes that have made adsorption systems a

key separations tool in chemical and petrochemical

industries.1 This growth is a result of significant

improvements in both adsorbents and adsorption cycles. The

invention of synthetic zeolites2 and, more recently, carbon

molecular sieves3 has provided new materials which are both

versatile and efficient. These materials have, in turn,

prompted the development of more efficient processes, such

as the pressure swing adsorption (PSA) cycle.

The work presented in this dissertation is concerned

with the preparation of a new adsorbent for the separation

of oxygen from nitrogen. The goal is to entrap a transition

metal complex that reversibly binds oxygen inside a zeolite.

Such a material would have a high affinity for oxygen and

have certain advantages over the nitrogen selective

adsorbents used today in both gas purification and bulk gas

separations.

The separation of oxygen from nitrogen or the

production of oxygen-enriched air is an important industrial











process,4 with 37 billion pounds of oxygen being produced in

the U.S. in 1988.5 This large scale production ranks oxygen

third among all chemicals produced in the United States.

Oxygen and oxygen-rich air find a variety of uses in such

processes as steel production, chemical oxidations, waste

water treatment, and medical/life support applications.6 It

can be seen that the adsorbents and adsorption processes

used for oxygen production are very important to the

chemical industry and the search for ways to improve this

separation is worthy of further study.

There are several methods for producing pure oxygen and

oxygen-rich streams from air, including cryogenic fractional

distillation,7 pressure swing adsorption (PSA),1 and
8-9
membrane separation. Cryogenic separation--that is,

liquefaction followed by distillation--remains the most

frequently used process for large scale production of pure

oxygen or nitrogen. However, as PSA processes are improved

and new adsorbents are discovered, PSAs share of the

separations task will increase.1 In applications where high

purity oxygen is not needed, membrane separation is also

being used. Recently, these membrane systems are being used

in conjunction with the cryogenic air separation processes

resulting in hybrid systems which are economically

attractive for production of high purity oxygen or

nitrogen.10











Many of the PSA processes used in the separation of 02

and N2 employ adsorbents which preferentially adsorb N2 over

02. Figure 1-1 shows a simple flow diagram for a PSA
device. In a simple PSA cycle, a bed of adsorbent is

charged with air and nitrogen is selectively adsorbed.

Oxygen is concentrated in the gas phase and is removed as

the product stream. Nitrogen is then removed and collected

by lowering the pressure over the adsorbent. This process

performs well if the feed air is free of gases which could

concentrate and contaminate the oxygen-rich stream. In the

case where the oxygen product is to be used in medical or

life support applications, contaminants in the air can cause

major problems. Nitrogen selective adsorbents in pressure

swing process do not, even in principle, allow oxygen to be

separated from the contaminants, resulting in a higher

contaminant concentration in the oxygen product stream than

in the inlet air.

This type of contamination problem could be overcome if

a material which preferentially adsorbs 02 over N2 and the

contaminants were available. In a cycle using this material

the oxygen would be adsorbed by the sorbent, leaving the

nitrogen and contaminants together in the gas phase. This

would allow the contaminants to pass through the system with

the N2, resulting in a pure oxygen stream.

An oxygen selective system would also have other

advantages. First, the ability of these new materials to












































PRODUCT
SURGE
TANK


Figure 1-1.


Flow diagram for a pressure swing adsorption
device.









5
simply replace molecular sieves in existing pressure swing

devices would eliminate to need for construction of new

devices. Second, since such an adsorbent would adsorb

oxygen (21% in air) instead of nitrogen (78% in air), the

adsorbent bed size could be one-fourth the size of a

conventional bed. This would result in a saving of weight

and space in an era when smaller and more portable oxygen

generation systems are desired.

The main impediment to such an oxygen extraction system

is the production of an oxygen selective material. Nearly

all substances that reversibly bind oxygen decompose upon

repeated oxygenation and deoxygenation cycling. The few

systems with better stability are extremely moisture

sensitive and are deactivated by water. This work reports

the preparation of a material that overcomes both of these

drawbacks by entrapping a stable, moisture resistant complex

that reversibly binds oxygen inside a zeolite cavity. The

material has a high affinity for 02 and is stable to

repeated cycling of oxygenation and deoxygenation.

This work is also concerned with the chemistry involved

in the preparation and characterization of these zeolite

entrapped complexes and on the effect their presence has on

the gas uptake characteristics of the zeolite material.

Adsorption isotherms of single component gases are measured

here. Complex adsorption processes or the use of complex

gas mixtures is more of an applied engineering concern. The









6

primary interest of this work involves formation of the

active complexes, their characterization, their influence on

the adoption characteristics of the zeolite in which they

are entrapped, and the zeolites influence on their binding

constants.















CHAPTER 2
REVIEW OF THE LITERATURE



Introduction



The synthesis and characterization of transition metal

complexes inside zeolites have been the subject of much

research.11-17 Zeolites are of interest as a matrix for

synthesizing coordinatively unsaturated complexes as

reactive intermediates in heterogeneous catalysis and as an

interesting medium for carrying out coordination and

transition metal chemistry.14 Zeolites not only function as

solid supports for encapsulated complexes, but can also

serve as solvents or ligands. In many cases they serve one

or more of these functions simultaneously.

The three-dimensional pore structure of zeolites

provides the possibility of preparing complexes that are

unstable in solution. In the case of zeolites with the

faujasite structure, such as zeolite X and Y, this can

result by trapping complexes in discrete cavities that would

otherwise dimerize in solution.18 This occurs because

zeolite cavities are often larger than the connecting

channels, allowing the formation of a "ship-in-a-bottle"











type complex.19 In addition to the ability to physically

isolate complexes, zeolites can also influence the stability

of complexes with their unique solvating and lighting

properties. This stabilization can result in the formation

of complexes with unique coordination numbers14 or oxidation

states20 that cannot be prepared in solution. Acting as a

ligand, zeolites can also stabilize complexes by anchoring
21-22
them to the lattice through framework oxygens.21



Zeolites


Before looking at some of the complexes that have been

synthesized inside zeolites, a brief introduction to the

structure and properties of zeolites is needed.

Zeolites2 are crystalline aluminosilicates that have a

structure based on tetrahedral aluminum and silicon

oxygenates. As seen in Figure 2-1, when these Si and Al

tetrahedra are linked by mutually sharing oxygens, a three-

dimensional framework can result. Figure 2-1(b) and 2-1(c)

both represent a sodalite unit, which is the basic building

block of zeolites A, X and Y; for simplicity the latter is

shown as a line drawing. Using this representation, a

tetrahedral atom (Si or Al) is located at each vertex and

oxygen atoms are located between the tetrahedral atoms.

When these sodalite units are connected together through

half of the hexagonal faces, in a tetrahedral arrangement,

























0 0 QCYGENN

AWUMINIUM
OR SIUCON


Figure 2-1.


Structural representations of alumino-
silicates. (a) Silicon or aluminum tetraheral
oxygenates; (b) ball and stick representation
of a sodalite unit; (c) line drawing of
sodalite unit; (d) line drawing of faujasite
structure.











the faujasite structure results, as seen in Figure 2-1(d).

This is the structure of both zeolite X and Y.

Much of the work described here focuses on zeolite Y,

which was chosen because of its large pore openings, large

cavities and three-dimensional pore structure. The only

difference between zeolite X and Y is the ratio of Si to Al

atoms present in the framework, with zeolite X having the

Si/Al ratio of near 1 and zeolite Y between 2 and 3. The

higher Si/Al ratio for zeolite Y results in a smaller charge

density. Zeolite Y was chosen over zeolite X because the

smaller charge density allows the metal cations to be more

mobile and more available to coordinate with ligands rather

than oxygen atoms of the framework.12

Zeolites have a molecular formula based on the number

of Si and Al tetrahedra present in the framework. A unit

cell of zeolite Y contains 192 (Si,Al)O2 tetrahedra and,

with a Si/Al ratio of 2.4, has a molecular formula of

Na56[(AlO2)56(SiO2)136]. Due to the polar nature of the

covalent bonds between oxygen and Si or Al and the overall

negative charge of the framework, zeolites are very

hydrophilic. This results in a typical unit cell of

zeolite Y containing 250 water molecules.2

Having Al atoms in place of Si atoms gives the zeolite

framework an overall negative charge. This negative charge

is balanced by the presence of cations located throughout

the framework. As shown in Figure 2-2, these cations can be



























































Figure 2-2.


Cation sites available in zeolite Y.











located in many different sites. The smallest of these

sites is the hexagonal prisms (Site I) located between two

connecting sodalite units. The opening to this site is 2.2

A and the internal diameter is only 2.4 A; therefore, only

unsolvated cations can enter this site. The sodalite cage

or beta cage is next in size. It has a pore opening of 2.2

A and an internal diameter of 6.6 A and contains two types

of cation sites, Site I' and II'. With this size pore

opening only small ligands, such as H20 or NH3, can enter

this site. The large cavities, formed when these sodalite

units are linked together, are called alpha cavities or

supercages and contain many sites along their internal

surface (Site II, III and III'). The opening to the

supercage is 8 A and, therefore, allows much larger ligands

to enter. Moreover, these supercages have an internal

diameter of 13 A, which is large enough for a reasonably

sized complex to form.

The location of these charge balancing cations in the

zeolite is important in understanding their behavior.17

Since they are only electrostatically bound to the

framework, they are able to migrate within the structure and

their distribution in the different sites depends on several

factors.2 These factors include (a) the size of the cation,

(b) the extent of hydration of the zeolite, (c) the presence

of molecules or ions that serve effectively as ligands, and

(d) the nature of other cations that may be present. For











example, large cations like rubidium and cesium are unable

to enter Site I positions in zeolite Y due to its small 2.2

A pore opening. Multivalent cations, such as Mg2+ and Ca2+

prefer Site I over other sites due to the strong solvation

provided by the lattice oxygens in this site. Lunsford

proposedl2 that even a cation which might be in an

inaccessible site (e.g. Site I) at one moment will be

available for complex formation in the large cavity at some

later time.



Transition Metal Complexes Inside Zeolites


The ability to exchange transition metal cations into

zeolites has allowed the synthesis of many zeolite entrapped

transition metal complexes and at least seven review

articles have been devoted to this work.11-17 The

characterization of these complexes is made almost entirely

with spectroscopic data.15 X-ray diffraction has not been

effective in locating ligands for these samples since they

are generally polycrystalline. Therefore, one can see the

importance of a complex having an informative spectroscopic

handle in order to be studied inside a zeolite.

As mentioned in Chapter 1, this work is concerned with

the preparation of a zeolite entrapped transition metal

complex that reversibly binds oxygen. This is not a new

idea, Lunsford and co-workers have worked in this area for











some time.18,21,23 His early work focused on preparing

cobalt amine complexes inside zeolite Y.18 Howe and Lunsford

were able to prepare and characterize a series of cobalt

amine oxygen adducts with the formula [CoL502 2+, where L=

NH3, CH3NH2, and n-CH3CH2CH2NH2. Characterization of these

1:1 adducts by EPR spectroscopy yielded similar results to

those of analogous adducts in solution. Also similar to the

solution adducts was their tendency to dimerize. When the

ligand is NH3 or CH3NH2, prolonged exposure to oxygen

resulted in the formation of the superoxo dimer,

[L5CoO2CoL5]5. The most interesting result of this early

work occurred with the n-propylamine complex, where only the

monomeric oxygen adduct was formed. This was the first

report of using the cavities of a zeolite to sterically

inhibit dimerization.

Another interesting complex which came out of this

early work21 was the square planar Co(en)2+ (en =

ethylenediamine), shown in Figure 2-3. In this complex the

axial coordination site is occupied by a lattice oxygen.

When exposed to molecular oxygen the monomeric oxygen adduct

is formed, with no evidence of dimer formation. It is also

stable in the presence of oxygen up to 70 OC. This complex

is similar to cobalt complexes formed in solution, where the

in-plane ligand is a Schiff base or porphyrin and the sixth

coordination site is occupied by a coordination base such as

pyridine.2425 However, these solution analogues dimerize










b0


N --


NSl


Figure 2-3.


Co

ANl

0 \Al


Proposed structure21 of Co(en) 22+ inside
zeolite Y.


--N


f/'











and decompose under most conditions. The remarkable

stability of the supported complex compared with the

solution analogues is probably a consequence of its

coordination to the zeolite lattice, which inhibits dimer

formation. In this case it is not the steric restraints of

the cavity that inhibit dimer formation because the

ethylenediamine adduct is no larger than the methylamine-

cobalt adduct. Instead, this stabilization is most likely

due to the immobilization of the complex by anchoring it to

the framework.

The most recent zeolite entrapped oxygen carrier

reported by Lunsford is [CoII(bpy)(terpy)]2+ (bpy =

bipyridine and terpy = terpyridine).23 With this complex,

the formation of the oxygen adduct is completely reversible

at 25 OC and the complex is thermally stable in the presence

of oxygen up to 70 OC. Unlike the previous amine complexes,

Lunsford found that the preparation of this complex was not

trivial. To get the mixed ligand complex, cobalt exchanged

zeolite had to be exposed to both bipyridine and terpyridine

vapor simultaneously. This lead to the formation of

[Co(bpy) ]2+ and [Co(terpy)2]2+, with the desired complex

only being formed in low concentrations, only 5.6 x 1018

spins/g.

This [CoII (bpy)(terpy)]2+-Y material was a-so found to

be effective for separating oxygen from air, with a

separation factor (02/N2) of 12.3.26 The major drawback of











this material for use in any practical application is its

extreme sensitivity to moisture. Water can occupy the sixth

coordination site of the complex and prevent oxygen from

binding. This effect is reversed, however, by evacuating

the sample to remove the water. In the presence of both

water and oxygen, the complex is completely deactivated

towards binding oxygen. This effect is irreversible and

reported to be caused from oxidation of the ligands by a

hydroperoxy radical formed from the reaction of the oxygen

adduct with water.26

This early work has established the utility of

synthesizing cobalt complexes inside a zeolite cavity in

order to prevent dimer formation in the presence of oxygen.

The steric restrictions of the zeolite cavity allowed both

[Co(n-C3H7NH2)5]2+-Y and [Co (bpy)(terpy)]2+-Y to form

stable monomeric oxygen adducts, with no tendency to

dimerize. [Co(en)2] 2+Y was also found to form a stable 1:1

oxygen-adduct due to its axial coordination to a lattice

oxygen, which anchors it to the framework. Unfortunately,

none of these complexes are stable under the conditions

needed to be useful in practical applications. Therefore,

it appears a new ligand system is needed that is both strong

field enough to form a low-spin cobalt(II) configuration and

oxidatively stable enough to withstand both oxygen and

water. It was for this reason that the work reported here

was initiated.












Cobalt Cyanide Complexes


The ligand in this study is the cyanide ion, which is

known to form low-spin complexes with cobalt27 and is very

stable to oxidation. Cobalt cyanide complexes were studied

in the 1960s as homogeneous hydrogenation catalysis because

of their ability to activate molecular hydrogen.28 This work

is, however, concerned with their ability to bind oxygen and

serve as oxygen carriers. While in solution they are not

useful for this purpose because they irreversibly

dimerize,29 inside a zeolite the monomer may be stablized

and become a useful oxygen carrier. Since cyanide is very

oxidatively stable, these complexes will also overcome the

problem of ligand degradation.

The chemistry of cobalt cyanide complexes in solution

is well known.27 Cobalt(II) cyanide is obtained as a light

brownish precipitate from solutions of cobalt(II) and

cyanide. When this material dissolves in an aqueous

solution of excess cyanide, an olive-green solution

containing Co(CN)53- is obtained. This complex is stable at

low concentration and in the absence of oxygen. At higher

concentrations dimerization occurs and ethanol precipitates

a violet solid with molecular formula K6[Co2(CN10)], known

as Adamson's salt.30 This salt can be redissolved in water

yielding the pentacyano complex. In the presence of oxygen,

however, a g-peroxo dimer is formed irreversibly.29











Formation of either the Co-Co dimer or the oxygen bridged

dimer results in complete deactivation of the complex as an

oxygen carrier. It can therefore be seen that if cobalt

cyanide complexes are to be used as oxygen carriers, one

must find a way to inhibit their dimerization.

Both the pentacyanocobalt(II) complex31-32 and its mono

adduct33 have been isolated from dimethylformamide solutions

using bulky tetraalkylammonium counterions. Thus, it is

possible to keep dimerization from occurring under the right

conditions. Carter et al.34-35 have reported the formation

of several lower coordinate cobalt(II) cyanide complexes in

aprotic media by varying the CN:Co ratio. Spectral

evidence31,35 suggests the presence of Co(CN)2(solvent)2,

Co(CN)3(solvent) Co(CN)42-, and Co(CN)53 as the ratio of

cyanide to cobalt increases. Only the pentacyano complex

forms at ratios greater than six. The tetracyano complex

has been isolated and shown to have a square-planar

structure with an oxygen atom from a solvent molecule weakly

coordinated in the axial position.34 (See Figure 2-4.)

The solution chemistry of these cobalt(II) cyanide

complexes indicates that several complexes with different

stoichiometries can be formed inside zeolite cages. The

usefulness of the solution complexes as oxygen carries is,

however, eliminated because of their facile decomposition in

the presence of oxygen. Both in aqueous3637 and





















CO) N(1)


I
I

A/
I
A/


N(2)


Proposed structure35 of (PNP)2Co(CN)4.


Figure 2-4.









21

aprotic35solution, decomposition occurs through an

intermolecular interaction. If these complexes can,

therefore, be prepared inside a zeolite cavity and isolated

from one another, they may serve as efficient, stable oxygen

carriers.















CHAPTER 3
EXPERIMENTAL



Synthesis



Solvents and Reagents



Methanol used was reagent grade and dried over

activated 3A molecular sieves. NaY used was LZY-52 powder

obtained from Linde. All chemicals were used as obtained

without further purification. CoC12'6H20 was an analytical

reagent from Mallinckrodt. NaCN was A.C.S. certified from

Fischer Scientific. Ethylenediamine-tetraacetic acid,

tetrasodium salt 98% (Na4EDTA) and Cesium Chloride (99.9%)

were obtained from Aldrich Chemical Company. Oxygen, argon,

nitrogen, and helium were obtained from Liquid Air

Corporation. All water used was distilled.



Cobalt Exchanged Zeolites



NaY was first stirred in 0.25M NaCl at room temperature

then washed with water until no precipitation was observed

when the filtrate was tested with a 0.1 M AgNO3 solution.









23

The NaY was then dried at 100 OC overnight in a vacuum oven.

CoY was then prepared by exchange of Na for Co2+ in an

aqueous CoCl2 solution at 70 OC for 24 hours. Aqueous CoC12

solutions were always less than 0.03 M and usually less than

0.01 M. There was very little cobalt remaining in the

solution after the reaction. The resulting pink solid was

then collected by filtration, washed with water until no Cl

was present in the filtrate, and dried at 150 OC in the

vacuum oven overnight. The resulting solid was deep

purple/blue.



Cesium Treatment



Cesium exchanged CoY samples were prepared by stirring

CoY in a 0.1 M CsCl or CsOH solution. These exchanges were

done at room temperature and allowed to stir a minimum of 16

hours. Often the treatment was repeated to ensure maximum

Cs+ exchange.



General Reaction Conditions



Unless otherwise specified, all reactions were carried

out in the presence of atmospheric oxygen and moisture.

Flasks were stoppered during stirring but no precaution to

exclude air was taken. Reactions done under inert

atmosphere were carried out using Schlenk techniques under











purging argon. When water was rigorously excluded, the

methanol was freshly distilled from BaO and the argon was

passed over activated sieves and NaOH. During reactions

where oxygen or water were excluded, the CoY was evacuated

at 100 OC and filled with nitrogen at least 3 times prior to

use.



Cobalt Cyanide Containing Zeolites



Dry CoY samples were reacted with CN- in a methanolic

NaCN solution (CN:Co 10:1 minimum) at room temperature for

2-4 days. The resulting solids were washed with copious

amounts of methanol and dried at 60 OC in the vacuum oven.

The resulting solids were gray-blue. The Ni(CN)-Y samples

were prepared in the same way, resulting in a yellow solid

after drying.



Successive Addition of Cyanide



Some reactions were carried out by adding cyanide to

CoY in small portions and filtering between the addition of

each portion. This was carried out using a 3-neck flask

with a fritted filter as its bottom. The NaCN/methanol

solution was added to the CoY in the special flask and the

mixture was stirred using an overhead stir motor. The

solution was then filtered off and another portion of











NaCN/methanol was added. All these reactions were done

under purging argon.



Chelate Treatment



Chelate treatment for removal of free cobalt(II) was

carried out by stirring the samples with aqueous 0.1 M

Na4EDTA at 70 oC. The solids were then washed with water

and dried at 60 OC in the vacuum oven. The resulting solids

were light yellow.



Characterization



Spectral Measurements



All IR spectra were recorded as Nujol mulls using a

Nicolet DXB FTIR spectrophotometer. X-band EPR spectra of

powder samples were recorded using a Bruker ER200D-SRC

spectrometer equipped with a variable temperature unit. For

removal or exclusion of oxygen from the EPR samples, tubes

were fitted with o-ring connectors and attached to

stopcocks, thus allowing connection to a vacuum line. EPR

spectral simulations were calculated using the "QPOW" EPR

simulation program.38 Elemental analysis of dissolved

samples were conducted using a Perkin-Elmer Plasma II

Emission Spectrometer.













Elemental Analysis



Cobalt concentrations were determined by ICP analysis

of the dissolved zeolite. A typical sample was dissolved as

follows: A 0.1 gram sample of the zeolite was refluxed in 15

ml of 2 M HC1. Next 10 ml of 6 M NaOH and 15 ml of 0.1 M

Na4EDTA were added and the mixture was refluxed again. This

treatment completely dissolved the solid. Analysis for

nitrogen content was carried out by the Microanalysis

Laboratory at the University of Florida. To ensure a

constant weight during analysis for Co and N, the samples

were allowed to equilibrate over H20 in a closed chamber for

several days prior to analysis. Water contents were

calculated from the hydrogen content of the sample and the

reported weight percent are corrected back to dry samples.

(See Appendix A.)



Quantitative EPR Measurements



When desired, signal intensities for the oxygen adducts

were determined by numerical double integration of the first

derivative spectrum. Spin concentrations were calculated by

comparison with the integrated spectrum of CuSO4'5H20.39

Typically a 0.1 gram sample of zeolite was precisely

weighted into an EPR tube and the spectrum measured. If









27

multiple samples were measured in the same experiment, care

was taken to be sure the tubes were positioned at the same

height in the cavity and the instrument settings were the

same. The spectrum for each sample and standard was

measured several times to ensure reproducibility.



Spin Concentration versus Oxygen Pressure



The EPR signal intensity of the cobalt-oxygen adduct in

Co(CN)-Y was measured at various pressures of oxygen above

the sample. A sample of Co(CN)-Y was placed in an epr

tube/stopcock assembly. This tube could be attached to the

vacuum line. The sample was evacuated until no EPR signal

was seen at room temperature. Following this, successive

amounts of oxygen were admitted to the sample and the EPR

spectrum was measured several times at each interval. Spin

concentrations were calculated by comparison to a sample of

known concentration.



Titration of CoY with Cyanide



Several samples of Co(CN)Na-Y were prepared using

different CN/Co ratios (R) during the synthesis. Potassium

bromide pellets were prepared using 0.250 grams KBr (dried

at 150 oC) and 7.5 mg of zeolite sample. The IR absorbance

of each sample was recorded from 2400 cm-1 to 1800 cm-1 for











several different orientations of each pellet. Care was

taken to ensure the absorbance range was the same in each

case. For several of the samples the preparation was

repeated and the filtrates were analyzed for Co and CN.

The test for Co using Na4EDTA and H202 in basic solution was

negative in each case. A quantitative analysis for CN- ions

was performed using the Vohard method titrationn with AgNO3

using Na2CrO4 as the indicator).



Adsorption Measurements



The adsorption isotherms were determined using a

volumetric technique. (See Figure 3-1.) A sample was placed

in a container of known volume and exposed to a known volume

of gas at a known pressure and temperature. From this the

amount of gas adsorbed by the sample was determined by

pressure differences. (See Appendix A.) Pressure

measurements were made using a MKS Baratron with a 390A

sensor head and 270B signal conditioning unit. Two sensor

heads were attached via a MKS Type 274 channel selector to

give a readable range of 10-5 1000 torr. The temperature

was monitored during each experiment and did not vary more

than 1 OC.





















to Diffusion
Pump

Inlet


Known
Volume


Figure 3-1.


Sample Gas
Container













Apparatus used for gas adsorption
measurements.















CHAPTER 4
RESULTS AND DISCUSSION


2-
Co(CN)42- Inside Zeolite Y



Preparation of Co(CN)Na-Y



The reaction of cobalt(II) exchanged into zeolite Y

with cyanide solutions produces entrapped cobalt cyanide

complexes.40-41 This reaction is unique because it requires

a negatively charged ligand to enter into a negatively

charged framework and form an anionic complex. All the

previously reported transition metal complexes synthesized

inside a zeolite framework have been either cationic or

neutral.11-17 In this case, the driving force to overcome

the charge repulsions between the ligand and framework is

likely the large formation constant for the cobalt cyanide

complexes. Cyanide does not enter the framework when

stirred under similar synthesis conditions with zeolites

exchanged with Group IA and IIA cations. Furthermore,

cobalt cyanide complexes, such as Co(CN)53- and Co(CN)63-

prepared in solution, do not enter into Na-Y under synthesis

conditions.


30'











Since charge balance inside the zeolite must be

maintained during this reaction, a cation must also be

incorporated into the zeolite when a cyanide enters.

Elemental analysis of these Co(CN)Na-Y materials shows

increased sodium ion content.

Solvent effects are important in the reaction of CoNaY

with NaCN. Table 4-1 lists the dielectric constants and

Gutman acceptor numbers42 for several solvents used in this

work. When water is the solvent more than 80% of the cobalt

is removed from the zeolite to form the cobalt cyanide

complexes in solution. When, however, methanol (CH30H) is

the solvent, very little cobalt is lost from the zeolite

during the reaction with cyanide. Formamide (HCONH2) gives

similar results to methanol. When dimethyl sulfoxide

(DMSO), N,N-Dimethyl formamide (DMF), and N,N-Dimethyl

Acetamide (DMA) are used as the reaction media, very little

cobalt is lost during the cyanide reaction but very little

cyanide is coordinated to the cobalt in the zeolite. The

effectiveness of the solvent is dependent on its ability to

solvate the cyanide ion and correlates well with the

acceptor numbers, AN, listed for each solvent.42 Water is

good at solvating ions but extracts cobalt from the zeolite

to form complexes in solution. The best solvent for the

formation of cobalt cyanide complexes inside zeolite Y must

have a balance between the ability to solvate cyanide and to

extract cobalt.













Table 4-1. Dielectric constants and
acceptor numbers for various solvents.

Solvent E ANa

Water 78.5 54.8

MeOH 32.6 41.3

HCONH2 109.6 39.8

DMSO 46.6 19.3

DMF 37.8 16.0

DMA 36.7 13.6

aReference 42.













Characterization of Co(CN)Na-Y



Infrared and EPR characterization of Co(CN)Na-Y

The IR spectrum of Co(CN)Na-Y exhibits a C-N stretching

vibration, vCN, at 2129 cm-1. Table 4-2 lists IR data for

complexes prepared here as well as for several known metal

cyanide complexes. These results suggest that the major

species formed in the zeolite is Co(CN)63-; however, the

zeolite lattice may influence the frequency and other

complexes with similar vCN may be masked by this large peak.

The species formed are not cobalt dimers or oxygen bridged

dimers, both of which have multiple cyanide stretching

frequencies.48-49

The EPR spectrum of Co(CN)Na-Y consists of a broad

signal near g=2. (See Figure 4-1.) The EPR parameters

(Table 4-3) are characteristic of a wide variety of low-spin

Co-02 adducts, including several cationic and neutral

adducts which have been synthesized in zeolite Y.18,19,21,23

Hyperfine splitting resulting from 59Co (S=7/2) is small, as

predicted by the spin pairing model for Co-02 adducts in

which the unpaired electron resides predominately on the

dioxygen molecule.21,50-52 The spin pairing model of binding

dioxygen to a low-spin cobalt(II), see Figure 4-2, can be

viewed as a free radical reaction in which the lone unpaired

electron on cobalt(II) combines with one of the 7*electrons










Table 4-2. IR Data for Cobalt Cyanide Complexes

Compound CN,(cm- )

Co(CN)Na-Y 2129

(Et4N)3Co(CN)5 2080

(Et4N)3Co(CN)5(02)a 2120

(PNP)3Co(CN)5b 2072

(PNP)2Co(CN)4b 2095

K3Co(CN)6 2129
13 d
K3Co(13CN)6 2082

Cs2Li[Co(CN)6]e 2142

Cs2Na[Co(CN)5(H) ] 2113

Co3[Co(CN) 612 2176

K6[(CN)5Co-Co(CN)5 h 2130(m),2100(s),2073(vs)

K6[(CN)5Co-02-Co(CN)5]1 2146, 2132, 2125, 2120

a Reference 31 b Reference 34 c Reference 43
d Reference 44 e Reference 45 Reference 46
g Reference 47 h Reference 48 Reference 49















100 G


H


g = 2.022
g1 = 2.075
A.= 7.5 G
All= I 1.4 G


Figure 4-1.


X-Band EPR spectrum of [Co(CN) (O0) 2--Y.
(A) experimental and (B) simulated.















Table 4-3. EPR parameters for cobalt-oxygen adducts.


Co-02 ADDUCTS gz gy gx az ay ax

(G) (G) (G)


Co(CN)4(02)Na-Y

(Et N)Co(CN)5(02) a
(solid)

(Et4N)3Co(CN)5(2 )a
(acetone)


2.075 2.022

2.02-2.03


2.02


Co(CN) (0 )3b 2.007
(water)

Co(NH3)5(02)-Yc 2.084

Co(en)2(O2)-Yd 2.084

Co(SALEN) (2)-Ye 2.078

Co(bpy)(terpy) (O2)-f
11.0


2.010

1.998

2.020

2.063


b Reference 29 Reference


a Reference 31 b Reference 29 c Reference
eReference 19 Reference 23


49 d Reference 21


11.4


7.5


10.5


9.8


17.8

20

21.0

1.998


2.000

1.992

2.020

2.007


12.0

10

11.3

15.6


12.5

13

11.3

11.0


























b,(xy)-4- -


e(xz~yz) -+


S.
S- n,sp

:w& 7r, V
X
~~-*1-0



jQ-Q


Figure 4-2.


Molecular obital diagram for the dioxygen
adduct of Co(II).











of the dioxygen molecule to form a a-bond. The other 7

electron remains unpaired and resides essentially on the

oxygen with cobalt hyperfine arising from spin polarization

of the cobalt-oxygen a-bond.5152

This model allows an interpretation of the EPR

parameters which produces the cobalt(II) contribution to the

a bonding molecular orbital, a,2Co


2 -4 -1
C o-O [ Aaniso (CoO2) + 1.0 x 104 cm1
3 4-1
2.44 x 10 cm


The partial negative charge on the bound 02 is referred to

as the extent of electron transfer from cobalt(II), E.T.,

and is given by the formula



E.T. = 2(1 a'2) 1 4-2



Such an analysis produces an electron transfer value of

0.7 e- for the cyano complex trapped in the zeolite.

The spin concentrations for the Co-02 adduct in

Co(CN)Na-Y(1) and Co(CN)Na-Y(2) are 4.8 x 1018 and

1.8 x 1018 spins/g respectively. These correspond to 8.0

and 3.0 umoles/g respectively, which is less than 1% of the

total cobalt present.

The Co-02 adduct can be deoxygenated under vacuum

yielding a species with an EPR spectrum which is quite










different from that of the original Co-02 adduct. (See

Figure 4-3.) The much larger 59Co hyperfine coupling and

reversal in magnitude of g1 and g_ indicates that the

unpaired electron density is located mostly in the dZ2

orbital of the cobalt(II) ion.53 (See Table 4-4.) The gl

values are close to those of the 4-coordinate complex formed

in a 3:1 mixture of CN to Co2+ in acetonitrile solvent.31

The relatively large anisotropy in the g tensors supports

the contention that the complex is not Co(CN)53 but rather

square-planar Co(CN)42- with axial coordination positions

occupied by zeolite framework oxygens.

The IR spectrum for square planar (PNP)2[Co(CN)4] (PNP

= bis(triphenylphosphine) nitrogen(+l) cation) is reported

to contain a vCN at 2096 cm-1, but the oxygen adduct of this

square planar complex has not been reported.34 By assuming

the same shift of vCN upon oxygenation as seen when

Co(CN)53- (2080 cm-1) forms Co(CN)5(02)3- (2120 cm1), 31 we
2- -1
can estimate vCN for Co(CN)4(02- to be about 2136 cm

The VCN frequency for metal cyanide complexes is reported to

increase as the oxidation state of the metal increases54 and

is consistent with the removal of electron density from

cobalt(II) due to the binding of oxygen.51 Due to the low

concentration of the Co-02 adduct and the presence of the

intense vCN (2129 cm-1) for Co(CN) 3-in the sample, the vCN

for the Co(CN)4(02)2- is not observed.



















200 G
|---


\g.L= 2.248
V g = 2.001
A.= 15 G
+***+ All= 95 G


4- + + + + + + +


Figure 4-3.


2-
X-Band EPR spectrum of [Co(CN)4 ]-Y.
(A) experimental and (B) simulated.









41


Table 4-4. EPR parameters for low-spin cobalt(II) complexes.


NON-ADDUCTS g g a1 a l

(G) (G)


Co(CN)4Na-Y 2.248 2.001 15 95
3-
Co(CN)53 a 2.157 1.992 28 87

Co(CN)3- b 2.20 2.00
3-
Co(CN)5 c 2.18 2.00 29 87

Co(CN)3(NCCH3) c 2.28 2.00 14 112

Co(CNCH3) 6-d 2.087 2.000 72 68

Co(CNCH3)5-Yd 2.163 2.003 32 89

Co(bpy)(terpy)-Ye 2.250 2.012 15 101


a Reference 53 b Reference 29 c Reference 31 d Reference 22
e Reference 23
eReference 23











The active species is stable to repeated cycling

experiments in the presence of atmospheric levels of

moisture. A sample of Co(CN)Na-Y(1) was alternately

oxygenated (atmospheric oxygen) and deoxygenated at room

temperature through 510 cycles over a 6-week period without

losing its oxygen absorption capacity. Analysis by EPR

performed at the end of the experiment indicated that no

decomposition had occurred.

It can be concluded from the EPR characterization of

this material that a low-spin, d7 cobalt(II) complex is

formed during the reaction of CoNaY with CN- in methanol.

This complex reversibly binds oxygen and is stable during

repeated cycling, however, spin concentration measurements

show this active complex is only present in low

concentrations. It appears from IR data that the major
3-
complex formed in this reaction is Co(CN)6 This complex

is diamagnetic and, therefore, not observed in the EPR

spectrum for this material. Its IR spectrum shows a single,

strong vCN at 2129 cm-1. The vCN for the active complex is

not observed due to its low concentration and the presence

of the band for the major complex.



Gas adsorption characterization of Co(CN)Na-Y

The gas adsorption properties of these Co(CN)Na-Y

materials were studied by volumetric gas uptake measurements

to determine if the presence of this active cobalt complex









43

produces an oxygen selective material. Figure 4-4 shows the

equilibrium adsorption isotherms for N2, 02, and Ar obtained

for Na-Y at 298 K. The enhanced affinity of Na-Y for N2

over 02 and Ar is due to the quadropole interaction of the

N2 molecule with the ions present inside the framework.2

This enhanced affinity for nitrogen is a general property of

zeolites and explains why these materials are used in PSA

processes to separate 02 and N2. As mentioned in Chapter 1,

the separation of 02 and N2 in the PSA process is based on

the difference in the extent of physical adsorption of 02

and N2 by the adsorbent. Gases for which these quadropole

interactions are minimal, such as 02 and Ar, show very

similar adsorption isotherms resulting in the material

having no selectivity for either gas. Consequently, argon

is used as the blank when trying to determine any increase

in oxygen uptake resulting from the presence of an active,

oxygen binding complex.

Figure 4-5 shows the adsorption isotherms for 02 and Ar

on Co(CN)Na-Y(1) and Co(CN)Na-Y(2). From this we see that

the presence of the active complex indeed has an influence

on the gas uptake properties of this zeolite material. The

presence of this active complex results in a material which

has a high selectivity for oxygen. At 100 torr, more than

twice as much 02 is adsorbed as Ar on Co(CN)Na-Y(1). A

similar increase for 02 uptake over that of Ar is shown for

















SN

-Id





< Oa



0 ,







200 400 600
Pressure (torr)









Figure 4-4. Gas adsorption isotherms for Na-Y measured at
298 K.














1.0



0.8
o



00.6



0.4 -
m







0 or
0



0.0

0.0
0









Figure 4-5.


s


/ /


/,


40
Pressure


(torr)


Comparison of 02 and Ar gas adsorption
isotherms for Co(CN)Na-Y materials. -, o
- Co(CN)Na-Y(1), 0 ; -, Co(CN)Na-Y(1),
Ar; -, Co(CN)NaY(2), 02; -
Co(CN)Na-Y(2), Ar.











Co(CN)Na-Y(2), however, it is not as large as in the

Co(CN)Na-Y(1) sample due to the lower concentration of

active complex in the former. The decreased uptake of Ar

for Co(CN)Na-Y(1) and Co(CN)Na-Y(2) compared with NaY is due
3-
to the presence of Co(CN)3 This species, which occupies

space in the zeolite, decreases the pore volume available

for gas absorption. (See Appendix A.)

A second approach that demonstrates an increased uptake

of 02 in the Co(CN)Na-Y material involves a comparison of

its adsorption isotherm with that of a Ni(CN)-Y material

which has the same metal concentration. Square planar

nickel cyanide is inert to oxygen. Figure 4-6 shows that

the Ni(CN)-Y adsorbs the same amount of 02 as it does Ar, as

expected for a material with no enhanced affinity for either

oxygen or nitrogen. Figure 4-7 shows that this material

also adsorbs the same amount of Ar as Co(CN)Na-Y(2). When,

however, the adsorption isotherm for 02 in Ni(CN)-Y is

compared with that of Co(CN)Na-Y(2), as in Figure 4-8,

increased 02 uptake is clearly established.



Characterization of the active complex in Co(CN)Na-Y

Determination of KO2. The observed equilibrium

constant for the binding of 02 to the active cobalt can be

expressed as follows:














1.0


a0
0.8



N0.6-





0
S0.4
c3


0.2



0.0
0













Figure 4-6.


40 80
Pressure (torr)


Comparison of 02 and Ar gas adsorption
isotherms for N1(CN)-Y material.


L












1.0



0.8



M0.6
cm

m0.4
o
O3

0.2-

-/

0.0
0












Figure 4-7.


Co
4Ni


40
Pressure


Argon


torr)


Comparison of Co(CN)-Y and Ni(CN)-Y gas
adsorption isotherms for argon.














1.0



,0.8



"0.6

c,

0.4

0

0.2-



0.0
0












Figure 4-8.


/
or


40 80
Pressure Oxygen (torr)


Comparison of Co(CN)-Y and Ni(CN)-Y gas
adsorption isotherms for oxygen.













KO2 = [CoO2] 4-3

[Co] pO2

where [Co02] is the concentration of completed cobalt, [Co]

is the concentration of active cobalt which remains

uncomplexed at equilibrium, and p02 is the pressure of 02

above the zeolite. Since the total concentration of active

cobalt, [Co]T, is the sum of [Co] and [Co02], equation 4-3

can be rearranged to give equation 4-4.



[COO2] = [Co]T K02-1 [COO2] 4-4

pO2


The volume for each concentration term is the same,

therefore a plot of the amount of CoO2 formed at pO2 divided

by pO2 versus the amount of CoO2 yields a straight line,

assuming K02 is constant. The equation of this line gives

K02 and the total amount of active cobalt present.

The amount of CoO2 formed at a known pressure above the

zeolite can be determined from the data shown in Figure 4-5.

The difference between the oxygen and argon adsorption

isotherms at the same gas pressure above the zeolite

corresponds to the amount of Co02 formed at that pressure.

The same information can be obtained by determining the

amount of CoO2 formed at a pressure of 02 above the zeolite

using the intensity of its EPR signal.











Figure 4-9 shows a plot of equation 4-4 for

Co(CN)Na-Y(1) and Co(CN)Na-Y(2). The slope of these lines

are -8.5 0.5 torr and -9 1 torr respectively and

indicate the active species in both materials has the same

observed equilibrium constant for binding oxygen, KO2, of

0.12 0.01 torr-1.

If plots of equation 4-4 are extended to lower

pressures, K02 appears to change significantly. This can be

explained by the fact that at lower pressures the

distribution coefficient, Kd, for gas inside versus



Kd
[O2]outside [02inside 4-5


Kd = 102]in
4-6
[O23out


outside the zeolite increases significantly. This

phenomenon is known as the sieving effect. Equation 4-7 can

be used to relate KO2 to the true Keq and Kd'



KO2 = Kd Kq 4-7


From equation 4-7 it is seen that when Kd increases, K02

increases and the magnitude of the slope in Figure 4-9 will

decrease. This sieving effect has been described as the











8.0-






0
.6.0-



n

J4.0





*2.0





0.0
0
M












Figure 4-9.


Co(CN)Na-Y(1)
slope = -8.5 + 0.5 torr


Co(CN)Na-Y(2)
slope = -9 + I torr


10 20 30
oles CoOa/pO, (moles/g/torr x10')


A plot of equation 4-4 for Co(CN)Na-Y(1), 0 ,
determined from gas uptake data and Co(CN)Na-
Y(2), A, determined from EPR data.











ability of the zeolite to "pump" oxygen to the complex and

results comparable to ours are reported at these lower

pressures.19 This sieving of oxygen by the zeolite results

in a higher effective oxygen pressure inside the pores than

the externally applied oxygen pressure would imply, which

results in an increased Kd at lower external pressures.

The distribution coefficient for Ar on Co(CN)Na-Y(1)

and Co(CN)Na-Y(2) at 25 OC was determined to be 10 1 for

pressures of gas above the zeolite greater than 8 torr. The

Kd for 02 and Ar are the same for Na-Y and Ni(CN)-Y and

determined to be 8 1 for NaY and 10 1 for Ni(CN)-Y. One

would expect the values for NaY and Ni(CN)-Y to be the same.

The higher value of Kd for Ni(CN)-Y over Na-Y may result

because the pore volume in the former is actually smaller

than estimated by the argon ratio technique used here, see

Appendix A for details. This would result in a larger

concentration of gas being calculated for inside the zeolite

and, therefore, result in a larger Kd.

If the concentration of oxygen inside the zeolite,

[02]in, is substituted for p02 in equation 4-4, K02 becomes
Keq and the influence of Kd on the measured equilibrium

constant is corrected. The amount of argon absorbed by

Co(CN)Na-Y at a designated pressure can be assumed to be the

amount of uncomplexed oxygen present in the zeolite at that

pressure during oxygen absorption. A plot of equation 4-4

can be made with [02]in replacing p02 to produce a Kq of











240 10 M-1. Converting the K02, in units of torr1, to

units of molarity-1 gives a value for K02 of 2200 100 M-1

With a value of 10 for Kd and equation 4-7, we see that a

value of 220 results for Keq.

It can be seen from this study that the sieving effect

of zeolites will make the complete removal of 02 from the

active cobalt more difficult than that expected from the K02

measured at near atmospheric pressures. This is because the

increased Kd at lower pressures will increase the

"effective" equilibrium constant, causing it to be more

difficult to remove the oxygen.

Not only can the zeolite host affect oxygen binding of

an active complex by influencing the local oxygen

concentration, it can also influence the thermodynamics of

oxygen binding. When oxygen binds to cobalt, a partial

negative charge on the oxygen results. This negative charge

could have an unfavorable interaction with the negatively

charged zeolite framework, resulting in lower equilibrium

binding constants for complexes inside zeolites. The active

complex could also be in a restrictive binding site where

steric interactions are important. Recent workl9 has

reported this to be the case with Co(SALEN)2+ prepared

inside zeolite Y. In this work Herron found the P1/2 value

to be 306 torr for Co(SALEN)'py2+ inside Na-Y compared to a

value of 10.5 torr in solution. It was reported that inside

the zeolite, this lower binding constant is due to a reduced









55

exothermicity for oxygen binding. This effect is consistent

with the oxygen being bound inside the zeolite cavity at a

restrictive binding site where sterics and electrostatic

interaction of the bound oxygen with the zeolite walls are

important.

The P1/2 value for a cobalt complex is the pressure of

oxygen required to oxygenate half the active complexes.

This value can be related to the equilibrium constant for

oxygen binding as shown in equation 4-8. Therefore the

smaller the P1/2 value, the larger the


1/2[Co] 1
K = 4-8
1/2[Co] P1/2 Pi/2


equilibrium constant for oxygen binding. Table 4-5

summarizes the P1/2 values for several cobalt(II) complexes

inside zeolites and in solution. Consistent with the spin

pairing model,51 the P1/2 value for the complex reported

here is less than that reported for CoSALEN-py2+ (SALEN =

N,N'-bis(salicylidene)ethylene diamine) prepared in NaY.19

The cyanide ligands produce a stronger ligand field than

SALEN, raising the energy of the dz2 orbital containing the

unpaired electron. (See Figure 4-2.) The higher the energy

of d 2, the more energy gained when the complex forms a

Co-02 bond and this electron drops down into the a-bonding

orbital. This results in a lower P1/2 value.


L












Table 4-5. P1/2 values for Co-O2 adducts

Complex P1/2

torr

Co(CN)-Y 9 1

CoSALEN'py2+ in NaYa 306

CoSALEN2+ in pyridinea 10.5

CoSALEN2+ in DMSOb 333

Co(3-FSALEN)2+ solid 2

Co(terpy)(bpy)2+ in NaYb 0.59

Co(terpy)(bpy)2+ in LiYc 0.34

a Reference 19
b Reference 23
C Reference 26










Comparing the active complex in Co(CN)Na-Y with

Co(terpy)(bpy)-Y23'26 (bpy = bipyridine, terpy =

terpyridine) it is seen that the active complex inside the

Co(CN)Na-Y material has a larger P1/2 value, indicating a

weaker overall ligand field. This is consistent with the

active cyanide species being a low-spin Co(CN)42- ion with

the axial position occupied by a lattice oxygen, as

suggested by the EPR parameters. In the case of

Co(terpy)(bpy)-Y, the axial position is occupied by the

stronger nitrogen donor ligand. The weaker axial base in

Co(CN)Na-Y results in a lower energy dz2 orbital and a

correspondingly larger P1/2. This is also consistent with
3-
the conclusion that the active species is not Co(CN)5

which would give a much smaller P1/2 due to the stronger

axial ligand. The dramatic influence of the axial ligand on

oxygen binding can be seen in comparing CoSALEN2+ in

solution with pyridine (P1/2 = 10.5 torr) and with DMSO

(P1/2 = 333 torr) as the axial ligands.19

Conclusions about the active complex. The EPR

parameters and equilibrium constant for oxygen binding

suggest that the active complex formed in Co(CN)Na-Y is a
2-
square-planar Co(CN)42- ion that is coordinated to the

zeolite framework through a lattice oxygen. This species is

analogous to the bis(ethylenediamine) cobalt(II) complex

prepared in zeolite Y by Lunsford.21 Unlike the Howe and

Lunsford complex, this complex is anionic and has much more








58

oxidatively stable ligands. The proposed structure for this

complex is very similar to that of (PNP)2Co(CN)4 (See

Figure 2-4.) prepared by Carter and co-workers.34 This

complex is only formed in low concentration but still has a

significant influence on the gas adsorption properties of

the zeolite. Attempts to inhibit the oxidation of Co2+

during the synthesis and increase the concentration of this

active complex are discussed later.



Characterization of the maior complex in Co(CN)Na-Y

Elemental analysis. Table 4-6 lists the elemental

analyses for the Co(CN)Na-Y materials prepared here. If the
3-
major species formed is Co(CN)63 as suggested by IR

studies, then a N/Co ratio of 6 is expected. Table 4-6

shows that the N/Co values are low even after extended

reaction times with cyanide, which suggests that there is

still free cobalt(II) present. This is not surprising in

Co(CN)Na-Y(1) since each large cavity (8 large cavities per

unit cell zeolite Y) contains one Co(CN)63- complex, leaving

no room for further complex formation.

Chelate treatment. In an attempt to remove this free

cobalt(II), the Co(CN)Na-Y samples were treated with a

chelating reagent (aqueous Na4EDTA). When Co-Y samples are

treated with this chelate solution, all the cobalt is

removed from the zeolite. When, however, a Co(CN)Na-Y











Table 4-6. Elemental analysis for Co(CN)-Y materials

material Co content N content
wt% per wt% per N/Co
unit cell unit cell


Co(CN)-Y(1) 5.9 15 4.5 48 3.2

CT-Co(CN)-Y(l)a 4.2 11 4.4 48 4.4

Co(CN)-Y(2) 3.1 7.6 3.0 30 4.1

CT-Co(CN)-Y(2)a 2.4 5.8 3.0 31 5.2

Co(CN)-Y(3) 5.9 15 3.2 33 2.3

CT-Co(CN)-Y(3)a 2.5 6.1 2.9 30 4.9





a CT prefix indicates results after chelate treatment









60

sample is given the same treatment, only part of the cobalt

is removed. For Co(CN)Na-Y(1) and Co(CN)Na-Y(2) the N

content and IR spectrum for the major complex remain

unchanged, suggesting that the major complex present is

unaffected. The removal of free cobalt(II) results in an

increase in the N/Co ratio approaching, in some cases, a

value of 6. The low values of N/Co even after chelate

treatment most likely occur due to incomplete removal of

free cobalt(II) by EDTA. The presence of the hexacyanide

complex in the large cavities may inhibit this process by

crowding or blocking the pores.

In the case of Co(CN)Na-Y(3), the chelate treatment

removes the species responsible for the vCN band at

2176 cm-1 but leaves the major Co(CN)63- species with vCN at
-1
2129 cm- This results in a corresponding decrease in the

nitrogen content due to the removal of the surface species.

Reactions with oxidizing agents. As further proof that

the major species formed is indeed a Co3+ complex, the

Co(CN)Na-Y materials were reacted with several oxidizing

reagents. (H202, Na2S208 and NaOCl). This resulted in no

change in color or in the value of uCN for the material,
3-
suggesting the major complex is Co(CN)63-

Titration of CoNa-Y with cyanide. Stuhl and co-workers

have reported the formation of several cobalt cyanide

complexes in solution by varying the CN/Co ratio during

preparation.34-35 Figure 4-10 shows selected IR






















CN/Co 1:1


2180 cm-'


2131 cm"'


Figure 4-10.


IR spectra for Co(CN)Na-Y samples prepared by
varying the CN /Co ratio during preparation.









62

spectra of samples prepared here by reacting the same CoNaY

material with solutions containing different cyanide

concentrations. These spectra show the initial formation of

a species with a iCN at 2180 cm-1. This species is formed

in low concentration, is EPR silent, and is believed to be

Co3[Co(CN)6]2 deposited on the exterior of the zeolite. As

the concentration of cyanide increases, this species is

washed off the surface. Figure 4-11 shows that the

intensity of the vCN band for Co(CN)63- increases steadily

as the concentration of cyanide increases until the CN/Co

ratio exceeds 6. Above this valve the absorbance remains

constant, indicating no more complex formation.

Conclusions about major complex in Co(CN)Na-Y. All the

results presented up to this point are consistent with the

conclusion that the major complex formed in the reaction of

CoNaY with cyanide in methanol is Co(CN)63. This is not a

surprising result in light of the fact that the reaction is

done in the presence of excess cyanide. The availability of

excess cyanide is known to have a great effect on Eo for the

Co3+ (aq) Co2+(aq) system. An E value of -0.8V shown in

equation 4-9 suggests that Co(CN) 3- must be,



E = -0.8 V
3- 3-
Co(CN)6 + e ----> Co(CN)5 + CN 4-9


thermally as well as kinetically, an extremely stable ion.

while the formation constants for Co(CN3- and CCN3-
While the formation constants for Co(CN)5 and Co(CN)6





















C)

d

0
F0





0


















Figure 4-11.


CN /


Plot of CN/Co ratio used during preparation
of Co(CN)Na-Y versus the intensity of the vCN
absorbance at 2131 cm .










3-
are not known, the value of B6 for Co(CN)6 has been
50.27
estimated to be 1050 (B 6 is the equilibrium constant for

the reverse of reaction 4-9) From this it can be seen that

there is a very large driving force for the oxidation of

Co2+ to Co3+ in the presence of excess cyanide.

This effect is further increased by the slow diffusion

of cyanide into the zeolite due to the repulsive forces of

the negatively charged ion and framework. Figure 4-12 is a

pictorial representation of this effect. It shows that even

at low total values of R there is still an excess of cyanide

at the surface which results in a large driving force for

the oxidation of cobalt.



Synthesis Modification in Preparation of Co(CN)Na-Y



The following outlines many attempts made in trying to

inhibit the oxidation of the cobalt during the reaction of

CoNaY with cyanide. The results of these attempts also give

insight into the mechanism by which this oxidation is

occurring. The reaction conditions used in preparing all

the previously mentioned samples have been the same. To

summarize, the CoNaY is dried at 150 OC prior to use. It is

then added to a methanolic solution of NaCN and stirred in a

stoppered flask. The methanol is dried over activated 3A

sieves prior to use and no special precautions are taken to






















8g0
U *O 0' 0 9
000

to 0





0C
66


I G.



~C 6
LV


' Be S
,66


a 0* ,


C 6 6
S P


CN / Co = 5


Figure 4-12.


A graphical representation of the effect
caused by the slow diffusion of cyanide. Each
small circle inside the zeolite particle
represents a supercage containing 1 cobalt.
Each dot outside the particle represents 1
cyanide.


bl
S
U
S
'
.9
9
S
0










exclude air or atmospheric moisture during the reaction.

After stirring for a minimum of 24 hours, the mixture is

filtered and the solid washed with dry methanol and dried at

60 OC under vacuum.

Any increase in the concentration of the active
2?-
complex, Co(CN)2-, would result in an increase in intensity

of the EPR signal for the 02-adduct and this was the

criterion by which the results of these attempts were

measured. Samples were also checked for the appearance of

new vCN bands in their IR spectra.



Exclusion of oxygen and water

Many reactions of CoNaY with cyanide were carried out

in the absence of oxygen. Solutions were deoxygenated by

purging with argon and zeolites were evacuated and filled

with nitrogen several times prior to use. Reactions were

done under purging argon. No increase in the concentration

of the active cobalt resulted as indicated by no increase in

the EPR spin intensity or the absence of any new vCN bands

in the IR were observed.

In several reactions extreme care was taken to minimize

water as well as oxygen. The complete elimination of water

from zeolite Y is very difficult and not only is water

present, but hydroxyl groups are also present on the zeolite

surface. Under the extreme conditions needed to remove

water (550 OC under vacuum) Co+2 ions are forced into hidden








67

sites inside the zeolite, making their reaction with cyanide

more difficult.

Solvents were freshly distilled and dried. Argon was

dried by passing it over activated sieves and NaOH.

Zeolites were dried up to 550 OC under vacuum and all

manipulations were carried out using Schlenk procedures in

an inert atmosphere glove bag. Again no increase in the

amount of active species was observed and oxidation to
3-
Co(CN)63- still occurred.



Reducing conditions

Attempts were made to prepare the cobalt cyanide

complexes in a reducing environment in order to retard the

oxidation process. When the cyanide reaction was carried

out in the presence of reducing agents such as sodium

borohydride or hydrazine in the solution, the major species
3-
formed was still Co(CN)6 In other attempts NaY was pre-

washed with a reducing agent and kept under inert atmosphere

during the cobalt exchange and cyanide reaction. Again only

the oxidized species was seen in the IR spectrum. Since

reactions of Co(CN)53-, with H2 are known to form hydride

species in solution,55 the CoNaY/cyanide reaction was

carried out under H2. When CoY was stirred with a

NaCN/methanol solution under 50 psig H2, no significant

increase in the concentration of active complex and no new

vCN bands in the IR spectrum were seen.












Slow addition of cyanide

Since the driving force for the oxidation of cobalt

results from the high CN/Co ratio, attempts were made to add

the cyanide very slowly and allow its concentration to

equilibrate throughout all the cobalt before more is added.

Even when the cyanide is added dropwise over a 48 hour

period leading to a final CN/Co ratio of 5, oxidation of the

cobalt is still observed and no increase in the amount of

active complex is seen.



Successive addition of cyanide

Another method used in an attempt to limit the CN/Co

ratio during the preparation of Co(CN)NaY was to add the

cyanide in small, individual portions. (CN/Co = 1 for each

portion) After each portion was added and allowed to react

and prior to the addition of the next increment, the

solution was filtered off. This allowed the total CN/Co

ratio not to exceed 1 while giving the cobalt access to a

larger amount of cyanide. These reactions were done under

argon.

Figure 4-13 shows the IR spectra for the C-N stretching

region of samples prepared using this method. After the

initial portion of cyanide is added vCN bands are























2136


2129


2129


Figure 4-13.


IR spectra for samples prepared by the
succesive addition technique. (a) after one
portion of cyanide, (b) after two portions,
(c) after 5 portions.










seen at 2177 cm-1 and 2136 cm-1 and the sample was epr

silent. These results are very similar to those shown in

Figure 4-10. The vCN band at 2177 cm-1 is believed to be

due to the formation of a complex on the outer portion of

the zeolite that is analogous to prussian blue. This

complex is reported47 and can be prepared by the reaction of

Co(CN)63- with Co2+. The IR spectrum for Co3[Co(CN)6]2

prepared in this way and the product of the reaction of

CoNaY with Co(CN)63 both have a band at 2178 cm1.

This prussian blue analogue is easily removed with

additional cyanide or by reaction with EDTA4- and is

believed to be Co3[Co(CN)612 on the surface. When Co2+
3-
reacts, the Co(CN)6 goes into solution.

The band at 2129 cm-1 is believed to be Co(CN)63

formed inside the large cavities of the zeolite. This

complex is not removed even after extensive treatments with

chelates at elevated temperatures.



Variation of CN/Co ratio

The effect of the CN/Co ratio on the concentration of

active complex is shown in Figure 4-14. In this graph the

spin concentration, as measured by EPR, is plotted versus

the CN/Co ratio used to prepare the sample. It can be seen

that the concentration of the active complex steadily

increases as the CN/Co ratio increases from 1-10 and then

levels off or even decreases as the ratio becomes larger.










2.5 -



1.0 -











(U
- 0.5
0


0.0





Figure 4-14.





Figure 4-14.


CN /


Plot of CN/Co ratio used during preparation
of Co(CN)Na-Y versus the intensity of the epr
signal for Co(CN)4(02) .












Synthesis with HCN

CoNa-Y was reacted with HCN in an attempt to prepare

active cobalt cyanide complexes and inhibit the oxidation of

cobalt. The resulting material showed C-N stretching

vibrations at 2194 cm-1 and 2136 cm- when the sample was

allowed to sit under HCN for 24 hours. After sitting for

one week a broad vCN band at 2185 cm-1 with a shoulder at

2145 cm- was observed. Neither material showed any active

cobalt when studied by EPR spectroscopy.



Conclusions About Co(CN)NaY



The major species formed in the reaction of CoY with

sodium cyanide is the Co(CN)63 ion, which can be

characterized by a uCN at 2129 cm-1. At high cobalt loading

a complex can be formed in every large cavity. Oxidation of

cobalt occurs even under conditions which exclude oxygen and

minimize water. At short reaction times and low CN:Co

ratios, a surface species believed to be Co3[Co(CN)6]2 also

forms.

The oxidation of cobalt(II) to Co(CN)63- is driven by

the presence of excess cyanide. This problem is magnified

by the slow diffusion of cyanide throughout the framework.

The mechanism for the oxidation is still unclear. Oxidation

does not appear to require the presence of 02 or large










amounts of water. Dimerization is not observed, most likely

due to the size limitations of the cavity, but this does not

rule out an intermolecular oxidation mechanism. Cobalt

complexes may interact through the connecting channels of

the large cavities without forming dimers. Surface species

on the interior walls of the zeolite, such as hydroxyl

groups, may also aid in the oxidation of cobalt. Whatever

the mechanism, it is clear that the majority of the cobalt

is readily oxidized under the conditions employed here.

The most interesting species formed in this reaction is

a low-spin cobalt(II) complex capable of reversibly binding

oxygen. The EPR parameters and equilibrium constant for

oxygen binding suggest that this active complex is a square-
2-
planar Co(CN)42- ion which may be coordinated to the zeolite

framework through a lattice oxygen. This species is stable

to repeated cycling in air and, even in low concentration,

increases the amount of oxygen absorbed by 100% over the

amount of argon at 100 torr. The maximum concentration of

this active complex is, however, limited. It may be formed

in a structural defect site in the zeolite framework or at a

very basic lattice oxide site that is only present in low

concentration. Unless this site can be identified and
2-
increased, the concentration of Co(CN)42 inside zeolite Y

is fixed at a low value.












3-
Co(CN)5- Inside Zeolite Y



Preparation of CsCo(CN)-Y Materials

2- f
Due to the limited concentration of Co(CN)42 formed in

Co(CN)Na-Y and the inability to increase this concentration,

it was realized that to produce a material containing a high

concentration of active cobalt, the formation of Co(CN)63

must be inhibited. This oxidation is driven by excess

cyanide present during the synthesis and most likely occurs

through intermolecular interaction of the cobalt centers or

interaction with surface groups present on the interior

walls of the zeolite. In order to inhibit this oxidation

the cobalt cyanide complexes need to be shielded from each

other and the zeolite walls. This shielding is especially

important when excess cyanide is present and their mobility

is increased by the presence of the solvent. It was

discovered that cesium ion exchanged into the zeolite after

cobalt produces this shielding effect. When CoNaY is

exchanged with CsOH or CsCl prior to the addition of

cyanide, the oxidation of some of the cobalt is inhibited

and a new, active cobalt complex is formed inside the

zeolite. This new complex reversibly binds oxygen and is

present in higher concentration than the previously reported

Co(CN)42- complex.
Co(CN) 4 complex.













Characterization of CsCo(CN)-Y



CsOHCo(CN)-Y

IR characterization. CsOHCo(CN)-Y is prepared by first

extensively washing CoNaY with CsOH to form CsOHCo-Y.

Elemental analysis shows that very little of the cobalt is

removed during this CsOH exchange. After drying, CsOHCo-Y

is reacted with a methanolic sodium cyanide solution to form

the cobalt cyanide complexes. The IR spectrum for the C-N

stretching region of the resulting material is shown in

Figure 4-15(a). Assignment of these bands is complicated by

the unknown influence of cesium and hydroxide on the vCN

values and the possibility of hydroxide acting as a ligand.

Deoxygenation of this material results in no change in the

IR spectrum, suggesting these bands are not due to the

active complex, but rather from an oxidized species which is

inactive. The presence of cesium has been shown4445 to

shift the C-N stretching band to higher energy. The vCN for

Cs2Li[Co(CN)6] is reported at 2142 cm-1 and precipitation of

a similar complex inside the zeolite may be to source of the
-1
band at 2138 cm1.

EPR characterization. The EPR spectrum of CsOHCo(CN)-Y

exhibits a large, broad signal at g=2.0, typical of a

cobalt-oxygen adduct. (See Figure 4-16.) A smaller signal

at g=2.2 is also present and is attributed to


















2169


2098






2122


2138


Figure 4-15.


1843


2096


2110
2138


IR spectra for CsOHCo(CN)-Y. (a) prepared in
air, (b) prepared under argon.







































X6.4



















Figure 4-16.


3-
EPR spectrum for Co(CN) (O0)3 inside
CsOHCo(CN)-Y measured at 9 K.








78

a small amount of deoxygenated cobalt complex. The presence

of this deoxygenated complex is surprising considering that

the cyanide reaction was carried out in air and may suggest

the interior pores are crowded or blocked due to the

presence of Cs+ or the formation of cobalt oxide.

The Co-02 adduct can be deoxygenated under vacuum at

elevated temperatures, which removes the signal at g=2.0 and

increases the signal at g=2.2. (See Figure 4-17.) The EPR

spectrum for the deoxygenated complex is typical of a

low-spin cobalt(II) complex and is almost identical to that

seen for Co(CN)53- in acetonitrile. (See Figure 4-18.)

Unlike solution behavior, where exposure to oxygen results

in irreversible g-peroxo dimer formation, Co(CN)53- prepared

inside the zeolite can be reversibly oxygenated and

deoxygenated. This is also in sharp contrast to solid

[NR4]3[Co(CN)5(02)] where oxygen can only be liberated by

pyrolysis, resulting in decomposition of the complex.31

Therefore, it appears the presence of cesium has stabilized

the formation of Co(CN)53- inside the zeolite by inhibiting
3-
its dimerization and further oxidation to Co(CN)6 Once

formed, the pentacyano complex is capable of reversibly

binding oxygen.

Gas adsorption characterization. Figure 4-19 shows the

gas adsorption isotherms for 02, Ar, and N2 on CsOHCo(CN)-Y.

As seen for Co(CN)Na-Y, this material shows an enhanced 02

affinity due to the presence of the oxygen




















































++++++++
++++++++


200G
I I----


H


g.= 2.19
91g= 2.01
A = 24G
A,= 82G


Figure 4-17.


EPR spectrum for Co(CN)
Y measured at 98 K. (a)
simulated.


53- inside CsOHCo(CN)-
experimental, (b)
















200 G
I I-----


H


V V g. = 2.18
V gll= 2.00
A.1=29G
++++++++ Al= 87G
++++++++
+ + + + + + + +


Figure 4-18.


3-
EPR spectrum for Co(CN) 3 in 5:1 CN/Co
mixture in acetonitrile. (a) experimental,
(b) simulated.



















we


8


0

2.


SNo


/ ,0O


Pressure


0 0




1- N-s
*-






0 10 20 O3 40 o 0 60
Pressure (torr)


Figure 4-19. Gas adsorption isotherm for CsOHCo(CN)-Y
measured at 298 K.








82

binding complex. From these isotherms, the concentration of

Co(CN)53- can be calculated to be 18 umoles/g (1.1 x 1019

spins/g). This corresponds to 4% of the total cobalt

present and is a greater than a 2 fold increase in the

amount of active cobalt over that which is in Co(CN)Na-Y.

To ensure that CsCoY does not exhibit enhanced oxygen

affinity, an adsorption isotherm for 02 and Ar on CsOHCo-Y

was measured. The results, shown in Figure 4-20, confirm

there is no enhanced 02 affinity due to the CsOH treatment

alone. Only after CsOHCo-Y is reacted with cyanide is

oxygen selectivity observed, assuring it is the presence of

an active cobalt cyanide complex causing this selectivity.

Determination of K02. A plot of equation 4-4 for

CsOHCo(CN)-Y is shown in Figure 4-21. From this plot the

equilibrium binding constant for oxygen to the active

complex, KO2, is determined to be 4.2 0.4 torr-. This

confirms that the active complex in this material is

different from that present in Co(CN)Na-Y, where K02 was

determined to be 0.12 0.01 torr -1. The much larger K02

value is consistent with the assignment of this active
3-
complex as Co(CN)5 and results from the presence of

cyanide ligand in the axial position. Cyanide is a much

stronger axial base than a lattice oxygen, which is present

in the Co(CN)42- complex, and results in a stronger overall

ligand field. As discussed previously, a stronger ligand

field around cobalt results in a more stable Co-02 bond.













Ar
02


8-



0 6-





OT m
o/
0

0) /
0 -4





0 200 400 600 800
Pressure (torr)












Figure 4-20. Gas adsorption isotherm for CsOHCo-Y measure
at 298 K.











2.0



u-
0 1.5



UO

1.0




0.5


0.0


0.0


slope = -0.24 + 0.02 torr


Moles CoO,/pOx (moles/g/torr x10)l


Plot of equation 4-4 for CsOHCo(CN)-Y.


Figure 4-21.












Preparation under argon. The preparation of

CsOHCo(CN)-Y was repeated under Ar in an attempt to increase
3-
the concentration of Co(CN)53. The IR spectrum of the

resulting material is shown in Figure 5-15(b). The C-N

stretching bands seen in the previous sample are all present

as well as new bands at 2110 and 1843 cm1. These new bands

match those reported for Cs2Na[Co(CN)5H]46 formed from the

reaction of an aqueous solution of Co(CN)53- with hydrogen

and CsCl. No other combination of cations (including Li ,

K Rb, NH4, NMe4, NEt4, NBu4 NMe3Cetyl+, MeNH3,

Me2NH2+, and NH3OH ) resulted in the formation of this

salt.56

The presence of Co(CN)5H3- formed in the absence of

hydrogen suggests that water plays a role in the oxidation
3-
of cobalt. The slow decomposition of Co(CN)5 in water has

been shown36-37 to occur through the disproportionation with

water. (See equation 4-10)



2Co(CN)53- + H-OH ----> Co(CN)5H3 + Co(CN)53 4-10



The hydride has been detected by NMR of an aqueous solution

of cobalt cyanide.57 This is the most likely source of

Cs2Na[Co(CN)5H] seen when the cyanide reaction is done under

argon. In air the hydride species would not be expected
because it quickly reacts with to form Co(CNOOH3- 58-59
because it quickly reacts with 02 to form Co(CN)500H








86

which decomposes to the hydroxide. In the presence of free

cyanide, the next step in this process is the irreversible
37
formation of the very stable hexacyano species, as shown

in equation 4-11.


3- 3- -
Co(CN)5OH + CN ----> Co(CN)63 + OH 4-11



The rate of these reactions are accelerated as much as

60 times by the presence of alkali metal cations and cesium

has been shown37'60-61 to have the greatest effect. The

cause of this increase is unknown but its magnitude does not

logically follow the degree of hydration or the

electronegativity of the metal ion.61

These results suggest that oxidation of cobalt occurs

through an intermolecular mechanism involving two cobalt

centers. This reaction is accelerated by the large

concentration of cations present in the zeolite. Cesium

must therefore act to shield the cobalt complexes from each

other and slow the oxidation process. In doing so it
3- Oc h ecini
stabilizes the formation of Co(CN)53. Once the reaction is

completed and the methanol/CN solution is removed, the

mobility of the complex is decreased and it is further

stabilized.

The nature of the intermolecular interaction between

cobalt centers is still in question. The formation of two

Co(CN)53- complexes in one large cavity seems unlikely since











this would require a large molecule with a total charge of

-6 to be accommodated. The presence of charge balancing

cations, beyond those needed to balance the framework

charge, would also be needed and further crowd the cavity.

The size of the dimer molecules alone would push the limits

of the zeolite cage and this crowding would be worsened by

the extra cations needed to balance the -6 charge. (See

Figure 4-22.) The absence of any experimental evidence for

dimer formation in the presence or absence of 02 also

suggests this does not occur. A more reasonable explanation

is the interaction of two cobalt centers in separate large

cavities through the interconnecting channels. This would

allow the oxidation to readily occur in materials where the

mobility of Co(CN)53- is unrestricted, as is the case in

CoNaY. The presence of cesium sterically inhibits a portion

of the cobalt centers from interacting and stabilizes
3-
Co(CN)53



CsClCo(CN)-Y

IR characterization. CsClCo(CN)-Y is prepared the same

way as CsOHCo(CN)-Y except CsCl is used in place of CsOH.

Elemental analysis show that 40-50% of the cobalt is removed

during the CsCl exchange, which is much more than with CsOH.

This suggest that the formation of cobalt hydroxide keeps

the cobalt from being washed out in the CsOHCo(CN)-Y

preparation.



































12 A





















Figure 4-22. Comparison of the size of the large cavity in
zeolite Y to the size of the g-peroxo dimer.










The C-N stretching region in IR spectrum of the CsCl

material shows a single, sharp band at 2120 cm-1 which

shifts to 2127 cm-1 upon exposure to moisture. This band

appears unaffected by deoxygenation and its intensity is not

dependent on the concentration of active cobalt, suggesting

it is not due to the active complex. Wilmarth49 has

reported the formation of Ag2[Co(CN)5] and its ability to

reversibly bind water but its IR spectrum has not been

reported.

EPR characterization. The EPR spectrum for

CsClCo(CN)-Y, shown in Figure 4-23, is typical of a Co-02

adduct and very similar to that seen for CsOHCo(CN)-Y.

Figure 4-24 shows the EPR spectrum resulting from evacuation

of this material for 15 minutes at room temperature. This
3-an
spectrum shows the presence of both Co(CN)5 and
3-
Co(CN)5(02)3-. Complete deoxygenation of this material

results after evacuation for 10 min at 100 oC, yielding a

spectrum typical of a low-spin cobalt(II) which is very

similar to that seen for deoxygenated CsOHCo(CN)-Y. (See

Figure 4-25.)

Low-spin cobalt(II) complexes have a single unpaired

electron in the dz2 orbital which is primarily used in

a-bonding with the axial ligand. Overlap and mixing of

these two orbitals places spin density from the unpaired

electron directly in the ligand sigma system.62 If the axial

cyanide in Co(CN)53 contains a 13C, this spin density
cyanide in Co(CN)5 contains a 1C hssi est























120G

H


g =2.03


X200


Figure 4-23.


EPR spectrum for Co(CN) (O) 3- inside
CsC1Co(CN)-Y measured at 98 K.

















120G
'- 4


Figure 4-24.


EPR spectrum of CsClCo(CN)-Y measured at 98K
after evacuation for 15 minutes at 298K.


















9 a 2.19

A0 24G

g = 2.01

A co = 84G


+ + + + + + + +


Figure 4-25.


3-
EPR spectrum for Co(CN)5 inside CsC1Co(CN)-
Y measured at 98 K.


200G



H











results in ligand nuclear hyperfine coupling.63 The

synthesis of CsClCo(CN)-Y was repeated with Na CN and an
13 3-
entrapped Co(13CN)6 was prepared. The EPR spectrum for

this complex, shown in Figure 4-26, shows that each EPR

component is split into two lines by coupling with the axial

13CN-. Of the five cyanides, only the axial splitting is

resolved. The unresolved equatorial cyanide splitting

increases the linewidth.63

Carbon-13 hyperfine coupling constants, listed in

Table 4-7, provide good experimental estimates for the

carbon 3s and 3p spin densities on the axial cyanide. The

isotropic 13C coupling constant yields the 3s spin density

of 0.033 and the anisotropic coupling yields the 3p spin

density of 0.049. The ratio of 3p to 3s spin densities

allows the percent s character in the donor orbital to be

calculated. For the complex prepared here, the axial a

donor orbital is 40% s character, which corresponds to sp5

hybridization on the carbon. This is a reasonable value for

cyanide.

Color changes. Accompanying the oxygenation and

deoxygenation of this material is a drastic color change.

The deoxygenated sample is light blue in color, most likely

resulting from the presence of free Co2+ ions. Upon

exposure to oxygen, the color quickly changes to

yellow/green. This color change is reversible and is

further evidence of the formation of a Co-O2 adduct.