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
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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 .
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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.
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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.
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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
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206
PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
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
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
oxygen
13.
PACKET & SCHOONHEYDT
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
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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
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
5*5 to
208
PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
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
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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
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
13.
J+
Coordination of Cu
PACKET & SCHOONHEYDT
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;
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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
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
210
PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
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
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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
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
13.
+
Coordination of CxS to Ο Six-Rings
PACKET & SCHOONHEYDT
Ε kcnï
11
Downloaded by FUDAN UNIV on March 20, 2017 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch013
•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.
Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
211
212
PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
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