Coordination of Cu2+ to Oxygen Six-Rings of Zeolites - ACS

Chapter

13 2+

Coordination of Cu to Oxygen Six-Rings of Zeolites D. Packet and R. A. Schoonheydt Laboratorium voor Oppervlaktechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3030 Leuven (Heverlee), Belgium Downloaded by FUDAN UNIV on March 20, 2017 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch013

An analysis of the ESR parameters and the d-d absorption bands of Cu , coordinated to oxygen six-rings of dehydrated zeolites, is presented in the framework of the ligand field theory and the angular overlap model theory. Two six-rings are considered : type I', on the hexagonal prism and type II, on the supercages. The ideal C point symmetry of the sites is distorted by a dynamic Jahn-Teller effect and spin-orbit coupling, the former being much more important than the latter. The sum of the ligand field stabilisation energy and the Jahn-Teller stabilisation energy is 7545 cm for type II six-rings in CuNaA and 7669 cm for type I' six-rings in Cu-chabazite. With these numbers the site 2+

3v

-1

-1

2+

preference of Cu for six-rings sites over hexagonal prisms can be explained but not the experimentally found preference for site I' over site II in zeolites X and Y. There is a non-negligible contribution of the 3 oxygens in the second coordination sphere of the six-rings to the overall ligand field. The coordinative bond of the 3 oxygens in the first coor dination sphere has a strong σ-donor character and a weakπ-donorcharacter. The coordinative bond of the 3 oxygens in the second coordination sphere has a more pronounced π-donor character. For

about

20 y e a r s

coordination to

zeolites

and

Cu(II)

has been

of z e o l i t e s .

used

spin

(1-11).

The t e c h n i q u e s resonance The

used

extent,

were

(ESR) and d i f f u s e

results

as a probe

t o study the

A t t e n t i o n has been p r i m a r i l y

X and Y and, t o a l e s s e r

Mordenite.

electron (DRS)

sites

o f these

zeolite

X-ray

Chabazite

diffraction

reflectance

techniques

A,

given

(XRD),

spectroscopy

are not

comparable, because e.g. s m a l l C u - l o a d i n g s have t o be used

directly to obtain

0097-6156/88/0368-0203$06.00/0 © 1988 American Chemical Society

Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

204

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE

w e l l r e s o l v e d ESR s p e c t r a , w h i l e h i g h l o a d i n g s a r e r e q u i r e d f o r DRS and,

especially, We

XRD.

have been a b l e t o match the ESR d a t a and the DRS

s p e c t r a on

a l a r g e number o f z e o l i t e s by t a k i n g the s p e c t r a on samples w i t h t h e same C u - l o a d i n g s , w h i c h underwent cell

b e f o r e r e c o r d i n g the s p e c t r a (12-14).

p o s s i b l e t o determine

constants

Downloaded by FUDAN UNIV on March 20, 2017 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch013

It almost

A and d-d t r a n s i t i o n s

type

of

coordination important oxygen I , in

i t has been

coordinated

t o the oxygen

of z e o l i t e ,

parameters

(Table I) are

the S i : A l

ratio

and the

cation.

i . e . the p l a c e

c o o r d i n a t i o n types

six-ring

o f Cu

experimental

o f the type

co-exchanged

I n t h a t way,

zeolites.

t u r n s out t h a t these independent

i n the same

unambiguously the g - v a l u e s , h y p e r f i n e c o u p l i n g 2+

s i x - r i n g s of dehydrated

f

the same p r e t r e a t m e n t

They reflect only the type of 2+ o f Cu i n the s t r u c t u r e . The 2 most 2+ o f Cu

a r e (12-14) : type

o f the l a r g e c a v i t i e s

I I , a t the

i n z e o l i t e s A, X and Y;

a t the oxygen s i x - r i n g , which forms p a r t o f the h e x a g o n a l

type

prisms

z e o l i t e s Y and C h a b a z i t e . T a b l e I . ESR parameters and d-d t r a n s i t i o n e n e r g i e s o f CuNaA and Cu-chabazite

Sample

8// mT

CuNaA Cu-chab. CuNaY II r CuNaX II III CuNa-mord. VI 1

d-d

8± mT

transitions l

(cm

)

2.386 2.340

12.6 16.0

2.064 2.073

0.25 2.00

10500 10700

12200 12900

15100 14800

2.397 2.328

11.9 15.5

2.070 2.065

1.50 1.90

10400 10700

12600 12600

15000 15000

2.384 2.354

12.7 14.3

2.074 2.068

1.20 1.50

10300 11100

12500 12500

15100 15100

2.327

15.5

2.068

1.50

13680

14460

T a b l e I g i v e s the ESR parameters and d-d t r a n s i t i o n s o f Cu in 2+ 2+ d e h y d r a t e d CuNaA, e x e m p l i f y i n g Cu on type I I s i x - r i n g s and o f Cu 2+ in

dehydrated

This

data

Chabazite,

s e t i s used

exemplifying

Cu

for interpretation

on

type

I

?

six-rings.

i n the framework

a n g u l a r o v e r l a p model (AOM) t h e o r y and the l i g a n d f i e l d

o f the

(LF) t h e o r y .


13. Our

goals

gain

(1)

to

insight into

the

ligand

the on Cu

are

field

obtain Cu-0

d e t a i l s of

stabilisation

2 +

I I and

I

1

the

coordinative energy

d - o r b i t a l energy s p l i t t i n g types

t o the

s i x - r i n g s and

to

site

bond;

(LFSE)

to Ο Six-Rings geometry;

(3)

to

i . e . the

205

(2)

the

the

contribution

o v e r a l l l a t t i c e energy o f deduce

to

calculate

site

of 2+ Cu

preference

of

.

Geometricl Model.


z+

Coordination of Cu

PACKET & SCHOONHEYDT

model and

The

CuO.

coordination second

unit

1).

(8-11) are

is

constructed

3 oxygens i n the first 2+ r . from Cu and 3 oxygens i n the 2+

sphere a t a d i s t a n c e

coordination

(Figure

The

sphere a t

geometrical

shown i n

6 oxygens are axis,

methods of c a l c u l a t i o n

a

with

distance

r ^ from Cu

parameters,

derived

with

from

the

r^ > r^ XRD

T a b l e I I . They s u p p o r t our h y p o t h e s i s , 2+

l o c a t e d i n one

common to the

2 sets

plane.

Cu

i s s i t u a t e d on

data

that

the

the t r i g o n a l

o f e q u i l a t e r a l t r i a n g l e s of oxygens.

The

p o i n t symmetry o f the CuO u n i t i s C« . I t i s an e x t e n s i o n o f T a b l e I I . Bond a n g l e s and bond d i s t a n c e s of Cu i n dehydrated c h a b a z i t e and z e o l i t e A Sample

r (nm)

CuA Cu-chab.

Klier's point are

0.214 0.197

CuO^

complex, a l s o w i t h p o i n t

lattice

therefore,

to o b t a i n AOM

is

and

f a r as

LF

AOM

and,

more

AOM

119 120

10 11

(15) . Because CuO^

parameters

about

the

generally,

i t influences

first

réf.

the

and

CuO^

derived

from

oxygens of

the

of

the

coordinative

c a l c u l a t i o n s on

the

same u n i t . In

calculated

to

look

the for

second p a r t second

the

the

on

bond.

CuO^

CuO^

coordination

effects.


the

complete

p a r a m e t e r s , which are d i r e c t l y comparable w i t h

parameters o f explicitly

as

v

Thus,

sphere

present

symmetry C^

c a l c u l a t i o n s on

information

θ (°) 2

same, the

contain

coordination

zeolitic will,

LF

identical.

calculations

second

the

(nm)

2

0.285 0.283

119 119

symmetries are

formally

CuO^

r

0

e^ )

x

We

complex Klier's complex sphere


206


F i g u r e 1. G e o m e t r i c a l six-ring i n zeolites.

model

o f Cu

coordinated

a t an


oxygen

13.


Coordination of Cxf* to Ο Six-Rings

A n g u l a r O v e r l a p Model.

In AOM

ligand

i n the d - o r b i t a l b a s i s

field

according

potential

to

the

number

the m a t r i x elements V

of

ligands

and

m e t a l - l i g a n d i n t e r a c t i o n s i n the complex, V

pq

= LFJ q

L

Ν Σ

=

Ν

p

(V

207

[aF (d , L . ) F (d ,L.) +

number

of

ligands

and

T

F

F

( d , L ) F ( d , L )]

(1)

p

i s the

fraction

of

maximum

π a r e t h e energy

changes


σ,ττ overlap for

i n the p a r t i c u l a r

geometry,

maximum o v e r l a p . F o r the C ^

are

explicitly

calculated

p o i n t group the m a t r i x elements V

v

by

σ and

Vanquickenborne

a u t h o r s , as w e l l as K l i e r

e t a l . (15) and 2+ shown t h a t the g r o u n d - s t a t e o f Cu i n C^

with the JT

p o i n t group i s d i s t o r t e d t o C

v

effect

observed

and

ESR

signal

(12-14). Our

measurement, 110K, out

(JT) d i s t o r t i o n .

the d o u b l y degenerate v i b r a t i o n s C^

with

3

g

o//:

*

complex

( 1 9 ) . In the case o f a

g-values

JT e f f e c t

There

That i s , i t c o u p l e s

e o f the C u 0

hypothesis i s that

a dynamic

a l . (18).

(_7) have 2 i s d o u b l y degenerate ( E)

v

and s u b j e c t e d t o a Jahn T e l l e r

et

Strome and K l i e r

i s expected

but

a t the temperature

i s o p e r a t i v e , which

and

static never of

the

averages

the n o n - a x i a l d i s t o r t i o n s . F o r our c a l c u l a t i o n s we have r e t a i n e d

only

the

Klier

linear

(7),

JT

coupling

because

the

as

explicitly

energy

discussed

contributions

a n h a r m o n i c i t y terms a r e e x p e c t e d t o be

by

Strome

of

the

and

higher

negligible.

There a r e 2 doubly degenerate v i b r a t i o n a l modes which i n t e r a c t 2 w i t h the e l e c t r o n i c s t a t e s Ε i n C ^ , a s t r e t c h i n g and a b e n d i n g . We have c o l l e c t e d k. the

The JT

JT

them f o r m a l l y

stabilisation

radius,

a

i n t o one v i b r a t i o n w i t h f o r c e c o n s t a n t 2 energy i s then E = (1/2) kR , where R i s J T

measure

of

the

distortion

of

the

complex.

At

e q u i l i b r i u m the energy s e p a r a t i o n o f the 2 energy l e v e l s , s p l i t from 2 E, i s 4E ( 2 0 ) . F o r most o f the Cu-0 complexes k i s i n the range -1 -1 50-150 Nm (21-23). We have t a k e n 70 Nm . T h i s value i s kept c o n s t a n t d u r i n g the c a l c u l a t i o n s , whereas R was T h i s i s because k and R a r e not l i n e a r l y F i n a l l y , because matrix

of

the

AOM

+

taken as a v a r i a b l e .

independent.

the ESR parameters a r e a l s o c a l c u l a t e d the JT

effect

i s extended

to

a

10*10

matrix


5*5 to

208


include the

spin-orbit

Zeeman

m a t r i x elements The are is

(SO) c o u p l i n g

energy

matrix

and

order

of perturbation,

the hyperfine

experimental parameters that are f i t t e d

not f i t t e d ,

because

low. The r e a s o n

simulate

t h e ESR

the accuracy

i s that

i n the c a l c u l a t i o n s

s p e c t r a , does

spectrum

(12-14,25).

The

theoretical

the

squares angle

(5)

the Fermi

that

values i s used

i n the

σ and π parameters; radius

R;

(2)

(4) the

, but t h i s

parameter

κ, which

quantifies

the n u c l e a r m a g n e t i c moment o f Cu a t t h e n u c l e a r calculations

adopted

suggest

We

have

then There

e q u a t i o n s , which found

10*10

value

the i s o t r o p i c

set of 6

fits

Ρ i s fixed

site.

with

Theoretical

i o n . The d i p o l a r

a t 0.036

cm

a

value

(27,28).

exists

by a l e a s t

matrix

absence

a

o f 0.43 f o r t h e f r e e

parameter

by most a u t h o r s

parameters.

is

a value

interaction

SO

due t o c o v a l e n c y and the J T e f f e c t ( 2 6 ) ;

c o n t a c t i n t e r a c t i o n o r the i n t e r a c t i o n o f t h e u n p a i r e d e l e c t r o n

hyperfine

to

transitions,

as v a r i a b l e s

λ. The f r e e i o n v a l u e i s -829 cm

contact

. Aj

ft

t h a t we

forbidden

a r e used

1 ) ; (3) t h e J T

may c o n s i d e r a b l y be reduced

and k

x

i n the p e r p e n d i c u l a r r e g i o n o f t h e

a r e : (1) the AOM

(see F i g u r e

coupling constant

program

not i n c l u d e

parameters

procedure

θ

g^ , g

o f the experimental

t h e computer

which a r e known t o be i m p o r t a n t

least

interaction

(18,24).

t h e 3 d-d t r a n s i t i o n s and t h e ESR parameters,

too


t o second

elements

experimental

a unique

solution

the e x p e r i m e n t a l squares

i s calculated

data

procedure.

data

exactly. This

F o r each parameter

and d i a g o n a l i s e d 4

o f an e x t e r n a l magnetic

and 6 a d j u s t a b l e

f o r the s e t o f normal

field

times

i n order

solution s e t the

: (1) i n t h e

t o f i t the 3 d-d

transitions;

(2) w i t h the Zeeman terms o f t h e p a r a l l e l r e g i o n o f t h e

ESR spectrum

i n order

t o f i t g^ ; (3) w i t h

p e r p e n d i c u l a r r e g i o n o f the ESR spectrum +

the

hyperfine

calculate

Ligand

interaction

terms

t h e Zeeman terms o f t h e

t o f i t g^; (4) as under (2)

of

the

parallel

region

to

A . f/

Field

Theory.

d i s c u s s e d by K l i e r

The d e t a i l s

o f the L F c a l c u l a t i o n s

have

been

e t a l . (7,15) and need n o t be r e p e a t e d h e r e . The

m a t r i x elements o f the

L F p o t e n t i a l , V^^, i n t h e d - o r b i t a l b a s i s


13.

J+

Coordination of Cu


are

expressed

coordinate

i n terms o f L F p a r a m e t e r s , 2+

frame, c e n t e r e d

on Cu

G

?

to Ο Six-Rings 209

and G,, r e f e r r e d

. In the p o i n t

charge

to 1

approxima

tion :

G

li

=

^ T T A 3d R

r

w i t h 1 = 2 or 4 (29). wavefunctions;


frame

( r )

l

2

r l d r

(2)

J

i

R

(r)

3 d

i s the n o r m a l i z e d r a d i a l p a r t

r i s the distance

from

the o r i g i n

(= t h e Cu n u c l e u s ) t o t h e 3d e l e c t r o n

i

- 1 refers

i

= 2 t o those o f t h e second c o o r d i n a t i o n

to consider

t o t h e oxygens

K. = G

ι If

G

G

r

positions SO

parameters

G

{M r

XRD

f o r each

data

value

restricted

(3)

II,

under

to the f i t t i n g

exists

,

and

the hypothesis are fixed.

and h y p e r f i n e

c a n be that

Since

o f the unperturbed C ^

a parametric

v

the

the JT

interactions are

i n L F and i n AOM, t h e L F c a l c u l a t i o n s

i n these c a l c u l a t i o n s

theories

3

of table of

the AOM c a l c u l a t i o n s . Thus, o n l y

There

r

22

and t h e Zeeman

expressed

(30,31), t h e o f oxygens i s

2 = φ l

o f t h e oxygens o f t h e s i x - r i n g s

couplings

identically

varied

r

21 G

l

o f the 2 sets

:

and

the relevant

calculated

field

5

2

useful

t h e 6 oxygens o f t h e s i x - r i n g s have t h e same

t o t h e bond d i s t a n c e s

41 pSi = 41

sphere and

sphere. I t i s a l s o

c h a r g e , which i s u p p o r t e d by CNDO c a l c u l a t i o n s

proportional

and

coordination

7 J

zi

o f the l i g a n d

With

o f the coordinate

and r ^ t o t h e l i g a n d i ;

. 4i

we assume t h a t

partial ratio

O J

/G

o f the f i r s t

o f t h e 3d

c a n be

energy l e v e l s

from

3 independent parameters have t o be

: G^,

equivalence

and θ^. between

t h e L F and t h e AOM

:

G^

σ

±

- (4/7)

G . + (5/21)

π

±

- (3/7)

G , - (5/21) G

2

±

(4) 2j

4 i


210


These e q u a t i o n s of ir

±

the

be

used

complex

i n the

i n the

conversion

corresponding

o f the LF AOM

parameters

parameters

and

(32).

The

CuO^

All

the

model i n

AOM

c a l c u l a t i o n s are

performed

on

the

CuO^

complex

1

direct


will

CuO^

comparison w i t h K l i e r s LF c a l c u l a t i o n s ( 7 ) . The

to

make

energy

level

diagram which g i v e s the b e s t

f i t f o r CuNaA i s shown i n f i g u r e 2.

corresponding

the b e s t

parameters and

are

summarized

for

f i g u r e 2 and

Table

i n Table

We

ττ/σ

remark t h a t by

and

CuNaA

109°

for

of

the CuO^ At

expressed values

by

and

the parameter σ,

as

the

Cu-complexes.

This

contribution

section. π-donor

by

LF

The

JT

and

6

the

oxygens

of

111'

i s below

can

the

be

improved

lattice

oxygens,

later.

* are

σ-donor

i s that

that

of

the

h i g h . F o r most

obtained.

values

in

The

octahedral

there

i n the

This

especially

U s u a l l y π/σ

SO

the t r i g o n a l a x i s

parameters

strength

0-ligands

suggests

strong

consequence

on

complexes

reason

i s that

g i v e d - o r b i t a l s p l i t t i n g s o f the same o r d e r

potential.

strength,

six-rings. The

from

overall

0.413 0.549

to O-Cu-0 bond a n g l e s

i s unusually

range 4000-6000 cm

0.0203 0.0165

respectively. This

σ-donor

κ

2+

o f Cu

geometrical

s h a l l see

Cu-chab

R (nm)

107.8 110.3

leads

Cu-chab.

the

the 3 oxygens o f CuO^

the

,

I I . These

sight,

i n the

magnitude

-287 -269

the p o s i t i o n

model as we

first

-1 (cm )

the a n g l e

Table

θ (°)

λ

0.147 0.041

characterized

values

c a l c u l a t i o n s f o r CuNaA and

)

10797 10003

first

f i t parameters o f C u - C h a b a z i t e

the d a t a o f T a b l e I I I .

σ (cm"

CuNaA Cu-chab.

The

following discussion i s pertinent

I I I . Parameters o f the AOM

Sample

with

I I I . The

a

might

second

point w i l l capacity

in

the

is

case

be

or a

tetragonal considerable

c o o r d i n a t i o n sphere

be

explored

accompanied of

Cu-Chab.

range between 0.15

and

i n the by or

0.25

the

couplings,

of

a

to

next weak

type

I*

(33,34).

sequence o f energy l e v e l s , unperturbed 2 2 2 is E < A, < E . Strome and Klier 1

0


13.

+

Coordination of CxS to Ο Six-Rings


Ε kcnï

11


•JT

2

E5

2

A°

•SO

J§j

§ L

J|i

Ej_

/

2

Ai

Aj_

J§j

E{_

2

EÎ 2

E,"

Ei

F i g u r e 2. Energy l e v e l diagram o f Cu a t an oxygen s i x - r i n g i n dehydrated CuNaA. The e f f e c t o f t h e Zeeman and h y p e r f i n e i n t e r a c t i o n i s n o t shown.


211

212


obtained

the 2

reversal shows,

of

The

JT

coupling

contribution

to

This

constants

-829 JT

bond. The

remarkable

that

value

1

for

cm"

1

is

of

assumed

Ligand

energy and,

the

0.019 force

Strome and

nm

and

its

availability

for

large

of

number

of

the

the

the

most

shown i n maxima

by

each

constants coupling

compared w i t h

contribute

covalency

of

to

the

relatively

value

of

κ i s of

Cu-0

strong

Nm

i s 0.016

f a c t o r s , the

its

and

is

the

therefore

parameter

is case

dependent on

the

P.

Cu-Chab. C l e a r l y

for

When we

take

f a c t o r s come i n t o p l a y . as

and

JT

expressed Wilson

of

by

(35)

the

JT

found

a

Cu(0D)«. W i t h

radius

calculated

our from

(7).

stabilisation 2+

distribution

adsorption

coordinative

0.51

distortion , the

of

parameter

Holuj

nm

reducibility,

free ion

decrease

i n view o f

It

course

itself,

range.

trigonal

to other

effect

70

the

the

o r even exceeds i t as i n the

i s decreased

JT

orbital

splitting

interaction

of

an

spin-orbit

contact

for a

far

quenching o f the

isotropic

the

figure

0.35,

influence

oxygens.

Jahn-Teller

determines thus,

mutually

the

(33,34).

band

lattice

expected

constant

d-d

the

κ

K l i e r ' s data

Field

the

e x p l a i n e v e r y t h i n g and

in

v

dipolar interaction

magnitude o f

radius, value

the

This ^

i s by

the h y p e r f i n e

2 effects

of

The

(28),

3. π/σ

symmetry as

f i t the

values

free ion value

six-rings.

t h i s does not The

the

t o the

chose

Ρ = 0.039

effect

effects

i s e x p e c t e d to be

strength

for

for

i n this discussion.

the

and

reason

figure

point

I I I , which have t o be

interaction

almost e q u a l I

by

The

l i t e r a t u r e values

JT C^ to

Both

in

to a c o n s i d e r a b l e

Thus,

latter

The

g - f a c t o r s and

cm

σ-donor

(7).

1

obtained

ideal

only.

shown

of Table

of

type

is

impossible

leads

the

is

the

separated

JT e f f e c t

λ : the

^A

illustrated

coupling.

been

cannot be

The

of

SO

has

SO

o t h e r and

large

and

It

admitting

value

Coordination of Cu2+ to Oxygen Six-Rings of Zeolites - ACS

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