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High resolution NMR on adsorbate-adsorbent systems
Advances in Colloid and Interface Science, 23 (1985) 67-128
67
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
HIGH RESOLUTIONNMR ON ADSORBATE-ADSORBENT JANOS B. NAGY+, GiiNTER ENGELHARDT*and 'Facultes Universitaires B-5000 Namur, BELGIUM
de Namur,
SYSTEMS
DIETER MICHEL+++
Laboratoire
de Catalyse,
Rue de Bruxelles
'+Akademie der Wissenschaften der DDR, Zentralinstitut fijr Physikalische Rudower Chaussee 5, DDR-1199, Berlin, GERMAN DEMOCRATIC REPUBLIC '++Karl-Marx-Universitat Leipzig, Sektion Leipzig, GERMAN DEMOCRATIC REPUBLIC
Physik,
Linn&trasse
61,
Chemie,
5, DDR-7010
CONTENTS 1.
ABSTRACT
II.
GENERALINTRODUCTION................................................
............................................................
PART I - HIGH RESDLUTION
III.
MOLECULES
.........................
69
CONDITIONS FOR MEASUREMENTS
OF HIGHLY RESOLVED SPECTRA OF ADSORBED ...........................................................
MOLECULES
A. Practicability B. Sensitivity C. Overhauser
IV.
NMR ON ADSORBED
68
68
of 'H; 13C and 15N NMR studies
69
....................
70
...................................................... effect
69
71
(NOE) ..........................................
D. Study of anisotropy of magnetic shielding. Application of MAS techniques .......................................................
71
E. Measurement
71
of resonance
THE STATE OF ADSORBED
shifts
MOLECULES.
.. .................................
INTERACTION
WITH DIAMAGNETIC
SITES
. 79
A. Analysis
of 13C NMR shifts
.......................................
80
B. Analysis
of 15N NMR shifts
.......................................
82
C. Origin of the variation D. Variation
of coupling
OF THE ADSORBED
of the chemical
constants SPECIES
shift
upon adsorption
....................
84
..................
88
V. VI.
IDENTIFICATION OF THE SURFACE ACTIVE SITES IN PRESENCE OF ADSORBED MOLECULES ... . .......................................................
VII.
STUDY OF THE CATALYTIC
VIII.
HIGH RESOLUTION MAGIC ANGLE SPINNING 13C NMR OF STRONGLY ADSORBED MOLECULES ... . .......................................................
IX.
REFERENCES
TRANSFORMATIONS
95
..............................
101 103
.........................................................
PART II - STRUCTURAL
INVESTIGATION
OF ZEOLITES
INTRODUCTION
.......................................................
XI.
EXPERIMENTAL
PROBLEMS
A. Condition
of measurements
0
AND GENERAL
105
BY 2gSi NMR SPECTROSCOPY
X.
OOOl-8686/85/$21.70
89
.....................................
MOBILITY
FEATURES
OF THE 2gSi NMR SPECTRA
......................................
1985 Elsevier Science Publishers B.V.
109
.
109
. 109
68 29 Si chemical shifts of silicates and of (zeolites) ......................................
B. General features aluminosilicates C. Spectra XII.
assignment
APPLICATION
.........................................................
A. Silicon,
aluminum
B. Detection
ordering
C. Aluminum-deficient XIII.
ACKNOWLEDGEMENT REFERENCES
I.
ABSTRACT
structure
zeoiites
hydroxyl
groups
116
....................
............................
123
. ......... 124
.....................................................
multinuclear
NMR on adsorbed'molecules
(Part II)
of zeolites
in heterogeneous
In Part I the conditions tra of adsorbed molecules, identification
116
126
..........................................................
High resolution
from interest
114
.......................................
of Si atoms bearing
XIV.
110
...............................................
is reviewed catalytic
(Part I) and on the
both from a methodical
point of view and
systems.-
of measurements
of highly resolved
the state of the adsorbed
of the surface.active
126
13C and 15N NMR spec-
molecules,
their mobility,
sites and the study of catalytic
the
transforma-
The recent results on high resolution magic-angle tions are thoroughly discussed. 13 spinning C NMR are only included for the sake of completeness. These latter, together
with the results on more mobile molecules,
of the admolecule-support
and different
silicon-aluminum properties
II.
applications detection
of aluminum-deficient
GENERAL The subject
netic resonance quantum
ordering,
number
29
Si magic-angle
of this method of silicon
spinning
are discussed,
atoms bearing
NMR spectra
including
hydroxyl
groups and
zeolites.
INTRODUCTION of this paper is to review the study of highly resolved spectra of adsorbed
molecules
and of solid adsorbents
l/2 (13C, 15N, *'Si) and the discussion
NMR which are based on different molecules
a better understanding
interaction.
In Part II general features of solid-state of zeolites
provide
studied
properties
by NMR has already
of the nuclei.
been reviewed
point of view (ref. 1,2) or from interest
of possible
nuclear magfor spins with
applications
The behavior
of
of adsorbed
either from a purely theoretical
in heterogeneous
catalytical
systems
(ref.
3- 10). In a comprehensive in interaction discussion different
study of a solid adsorbent
with adsorbed
of possible properties
cules at different (ref. 2,4-6); of adsorbed
molecules,
applications
of nuclei
temperatures
acting as a heterogeneous aspects
of NMR investigations
(ref. 5,11,12);
(ref. 7,13,14);
catalyst
should be included: which are.based
of the surface active
of the adsorbed
I) a
on the
2) the state of the adsorbed
(ref. 7); 3) the mobility
4) the identification
molecules
different
mole-
species
sites in the presence
5) the study of catalytic
transformations
69 (ref. 15-18);
6) the study of the adsorbent
itself by multinuclear
8,19);
and 7) the mobility
of protons and counterions
Points
1 to 5 are included
in Part I of this review dealing
adsorbed
molecules.
Here we restrict
ourselves
NMR (ref. 4,5,
at the surface
(ref. 2,4,6).
with the behavior
to the more recently
of
developed
study of 13C and 15N NMR.
In Part II Point 6 is reviewed, especially of 2g Si NMR.
With respect
taking into account the application 27 to Al NMR studies and to proton mobility on surfaces
(Point 7), we refer to the review of Freude
PART I - HIGH RESOLUTION CONDITIONS
III.
A. Practicability Highly
NMR spectra of adsorbed
molecules
Hence, among the different
13C and I5N are of importance, dances
MOLECULES
OF HIGHLY RESOLVED
SPECTRA
OF ADSORBED
MOLECULES
of 'H, 13C and 15N NMR studies
resolved
with spins l/2.
NMR ON ADSORBED
FOR MEASUREMENTS
(ref. 19).
of which.are
nuclei of organic molecules
the different
well known.
may only be studied for nuclei
magnetic
properties
mainly
and natural
'Ii,
abun-
If we put, e.g. for IH, the product of relative
NMR intensity: Ir = $ I(I+l)(v,oo/loo)3
Bo= 2.34869 T (corresponding
for a value
to a proton resonance
frequency
of "IO0 =
100 MHz) and the natural abundance equal to 100, then the respective values for 13 C and l5 N NMR studies are 1.8. 10m2 and 3.8. 10s4. This comparison reveals the 13 poor sensitivity of C and 15N NMR studies. In spite of the suitable measuring conditions, however, reason
proton
resonance
is the strong line broadening
to dipolar
proton-proton
(impurities), exchange
the resolution
with structural
(with respect and/or
(ref. 25) and on measurements
be mentioned
averaging
internal
proton-proton
of the adsorbate
fields and to proton
More general
there are no strong
line widths
are based on an exchange
because ofI$he
of the asorbed
was mainly
(ref. 26).
achieved
by
remaining
limitations
with respect
shifts.
of
high internal mobility
C and l5N NMR specta of adsorbed
of
of resonance
(see
to increase
of NMR line narrow-
at very high frequencies
For studies
in the 13C and l5 N NMR spectra
intervals
attempts
fields of the sample but not by a reduction
interaction
molecules.
shift) due
centers
with proton spin relaxation
that in Ref. 25 a line narrowing
inhomogeneous
studied, The
This is the reason why most of the
in this volume). spectra
with paramagnetic
of magnetic
OH groups.
were successfully (ref. 5,20-23).
to the small chemical
coupling
are concerned
of proton resonance
It should
species,
and Michel
molecules
resolution
with the bulk liquid .(ref. 20.,24), on the application
ing techniques
dipolar
of adsorbed
inhomogeneities
of IH NMR techniques
the review of Winkler
molecules
interaction
due to internal
processes
applications
greater
spectra
only in a few cases due to restricted
to resolution
due to smaller
at not too low temperatures
and to
70 B. Sensitivity The strongly in general,
reduced
sensitivity
the application
tion and, additionally, The minimum
number
of 10 after z repeated
N
min
where
for
Nmin of nuclei necessary measurements
I, is the relative
NMR intensity,
time in s, 6~ is the observed
in cm3.
receiver
having natural
,&.
values
half width
system
without
~~)1’~ “cl/2
vloo is the resonance
abundance
for the remaining
of 13C nuclei
parameters
effect enhancement.
density
2. 102' cavities
per cm3), we arrive
zeolites
frequency
in MHz if
of 13C NMR signals (1.1%).
leading
are:
Taking
coil
for adsorbed
vloo= 25.1 MHz,
that:
TI - 0.1 s, 6w= 100 s-l (-20 Hz),
to:
i: 5.6. 102’
Overhauser
of nuclei
,
in K, Tl is the longitudinal relaxation -1 of the NMR signal in s , AV is the band
measurements
with an apparent
Nmin(13C)
ratio
in Hz and Vc is the volume of the receiver
Aw= 500 Hz and z= lo4 accumulations,
Nmin(13C)
a signal-to-noise
is given by (ref. 5):
T= 300 K, Vc= 1 cm3 and I,= 1.59. 10m2, it follows
Typical
requires,
temperature
As an example, we consider
molecules
to produce
(accumulations) B-03/2 1;
B,= 2.34869 T, T is the sample
width of the whole
IH NMR measurements
of Fourier transform techniques and signal accumula15 N NMR studies, the use of enriched materials.
1018 1;I (&0)i'2(&)3'2
I
as compared with
Practically,
for faujasite
Pa I: 0.4 gem -3 and about 4. 102' cavities at a minimum
type zeolites per g (i.e.,
number:
- 3
per large cavity.
This is in agreement
with experiments
of butenes
in
NaY (Fig. 1).
Fig. 1. Number of accumulations as a function of number of molecules per large cavity for 1-butene in zeolites NaY for a signal- o-noise ratio (SNR) of ca. 6 in C NMR spectra (+: natural abundance;O: 55% 15 C enrichment in group= CH-).
71 C. Overhauser
effect
(NOE)
The signal enhancement (index S) to the nuclei
due to magnetization investigated
transfer
from the coupling
(index I) is mostly
protons
less than the optimum
factor: n=l+_.--_yS
\ 3 for 13C
TII TIIS
y1
(3)
[ -3.9 for 15N
with y the gyromagnetic
ratio
(S: IH; I: 13C, l5N), the longitudinal
and the cross-relaxation time TIIS through dipolar coupling TII The reduced effect is due to: 1) restricted molecular mobility polar I-S coupling, of the factor
viz., for (w~-w,$T~
l/2 for short correlation
of other relaxation rary correlation
mechanisms
rather
of magnetic
studies
(liquid-like
shielding
spectra)
than heteronuclear
dipolar
1) for adsorbed
Application
molecules
fixed molecular
sieves
fragments
on solid surfaces
In these cases of a strongly means
of MAS techniques
E. Measurement Chemical macroscopic
in the majority
average
of the adsorbed
of possible
1. Correction
shifts,
6
of chemical
susceptibility
real
-6
obs =
adsorbate
mobility Examples
shifts)
generally
2) for molecules
also
a line narrowing
are discussed
by
in Part VII.
have to be corrected
interaction.
processes
(ref. 37,38)
Furthermore,
bound
+ k)(x,(ref)
to obtain
complexes,
molecules
the
a thorough
for adsorbent
can be used for the measurement
the observed
shift area,.
to liquid
is necessary.
shifts of adsorbate references
either
for
in order to eliminate
in order
in adsorption
resonance
of the adsorbate-adsorbent
resonance
(ref. 28,29);
(cf., e.g., Ref. 31).
compounds
molecule
it is necessar.y to correct
real (corrected)
As=
exchange
Since only external
magnetic
for the following
(ref. 33,34) and are compared
reference
of van der Waals
true resonance
of
value of the
shifts
(or other resonance
or to gaseous
the influence
bility.
reduced
for arbit-
[ref. 301); and 3) for rigidly
(ref. 14,30,32).
volume susceptibilities
(ref. 35,36)
analysis
occurs
of resonance
shifts
at low temperatures
(e.g., mordenites
coupling
of MAS techniques
are bell-shaped
and hence only the isotropic
molecules
<< 1 instead
are given in Part V.
tensor is available but not the anisotropy. 13 of C NMR shielding tensors has been measured
in small pore molecular
I and S spins
even for purely di-
The anisotropy cases:
time
times ~~ (ref. 27); and 2) the predominance
shielding.
13C and I5 N NMR lines of adsorbed
between
5 1 we find the value TII/TIIs
times ~~ (ref. 11). Further examples
D. Study of anisotropy
relaxation
system
The correction
- x,(sample))
suceptiof chemical
shift dabs for the bulk in order
to obtain
the
is (ref. 33,34,39):
,
(4)
72
taking the same geometry for the reference probe (denoted by "ref") and the adsorbate-adsorbentsystem (denoted by "sample") which is characterized by the demagnetizationfactor ~1. The factor k describes the influence of the local magnetic field which is averaged to zero (kz 0) under normal conditions of relatively fast isotropic reorientationaland/or translationalmotion (with -9 mean life times of r&O s in their equilibrium positions at room temperature). x,(ref)
and x,(sample) are the respective volume magnetic susceptibili-
ties. If the external reference is the corresponding gas at very low pressure, we may put x,(ref)= 0.
Further, according to Wiedemann's rule we assume that the
total volume susceptibilityx,(sample) can be written as the weighted average: x,(sample) = x,(solid)t B[x,(admolecule)-x,(solid)l
,
(5)
where B is the volume fraction of the adsorbed molecules. Now if molecules are
adsorbed for which G,_~~, +O for very small fractions B+O,
the extrapola-
tion of fiobsvalues to zero coverage then yields the susceptibility correction for the solid (ref. 4,33,40): x,(solid) . For the usual form of the sample (viz.,cylindricaltubes perpendicular to the external magnetic field B, with a value of ca. 3 for the ratio between the filling height and the sample diameter), the demagnetizationfactor is: u = 1.8 K (ref. 34). This method which was first proposed by Fraissard et al. (ref. 33,40) leads to a good agreement with direct measurements of volume magnetic susceptibilities on the basis of an analysis of proton resonance shifts. The usual correction varies between 0.4 and 1.5 ppm (ref. 33,34). However, for 13C NMR studies, chemical shift and volume susceptibilitymeasurements (ref. 34) gave essentially the same result only when larger molecules such as n-butane, cyclohexane or TMS are used while significant differences arise with small molecules like methane.
Obviously, the supposition Greal = 0 for 6 = 0 is no longer
fulfilled here and the resonance shifts due to van der Waals interaction are greater than those due to the bulk susceptibility. 2. Chemical shifts as referred to either the liquid or the gaseous state. The elimination of the influence of van der Waals interactions is closely connected with the difference of resonance frequencies between the gaseous and the liquid state. Hence, a fundamental question concerns the state of the adsorbed compound, namely whether it is closer to the gaseous or the liquid state. Tables 1 and 2 give a few examples of chemical shifts referenced either in the liquid or the gaseous state.
13
0.0
0.0
4’1
10.6
+O.L
19
to.1
+o.*
49
-0.6 -0.6 to.2
-1.3 -1.1 -0.2
:: 13
-0.2
-0.9
II
-0.1
-0.8
13
to.1
-0.7
13
+I.,
-0.2
13
74
TABLEZ-A 13
C NMR chemical
shifts Sims (ppm) of different support
Compound
___--__. CH4 gas
__
(Ref TMS/CD4)
N.3Y
co gas
cs* liq
molecules
Covera9d
in the gaseous.
T'C
Cl
1 atm 100 Torr
38 38
SiO*
-8.9012.631
38
__
183
N?IY
179.36[-3.641
30
NaX
180.261-2.741
38
Pd/Al203
177.611-5.391
38
Pt/Al203
177.5[-5.51
38
NaA
22
181.81-1.21
63
NaX
22
178.9[-4.1,
63
NaY
22
179.0[-4.01
63
DeNaY
22
216[+331
63
__
192.5
104
NIY
153.71[-38.81
38
154.741-37.81
38
__
1
22
132.2
103
NaA
1
22
132.2101
63
NaX
1
22
131.1[-1.01
63
NaY
1
22
130.7[-1.51
63
OeNaY
1
22
130.41-1.81
63
SiD*
O.lD
22
130.71-1.51
105
119.11
38
2.3 M
__
122.94[3.81
38
NaY
2.DlN.a 0. l/Na
124.25~5.21 124.05[4.91
38 38
1 atm 0.14
SiO*
28
38 38
124.0[4.9]
38
115.03[-4.11
38
122.5[3.4]
38
118.4[-0.71
105
70
38
NaY
71.2811.281
38
NaX
73.11~3.111
38
__
117.7(0.3)
104
LiNaX (80)
22
118.2(0.5)
106
LiKX
22
122.9(5.2)
106 106
Rb.NaX
(71)
22
122.2 (4.5)
Cs,NaX
(50)
22
121.8(4.1)
106
Ag.NaX
(20)
22
119.7(2.0)
106
Ag,NaX
(60)
22 110-450
13x __
“5%010
30
107 38 15.25
38
114.12[-1.321
131.5010.301
17.7[2.45]
38
Z.l/Na
114.231-1.211
135.73I4.59
17.53[2.28]
38
O.VNa
115.91*0.46l
137.55I6.351
19.30[4.057
38
116.0[+0.561
136.3815.181
19.35[4.10]
38
Lay
per supercage
104
48.5(-0.5) -7.64 131.20
2.6 M
USbOS
106
115.44
RT
NaY
120.7(3.0) 49.0
-4.613.01
NaY
CH2=CH-CH3 9% (1 atm)
7 'number of molecules
123.714.61 124.65c2.41
__
CH3OH llq
In COCl3
38 38 38
-9.4712.061
in benzene
g,as
-11.53 -9.80[1.73] -9.81
AgY
AgY
liq
Ref.
C5
38
HY
H3C-CsN
C4
-8.4113.121
5. 10-4/Na
gas
rdagas(ppm)l
C3
-9.2412.291
HY
WCH
Cz
NaX
CH2-CH2 gas (1 atm) in CDCl3
State
(nalis(pp m));
HY
NaX co* gas
liquid and adsorbed a(ppm);
-_._
__ __ (zeolites)
25 -a7 25 -87 or statistical
monolayers
(130.5)[-0.71 (129.7)[-1.51
74 74
(132.1)[0.9] (137.7)[6.5]
::
(sio 2, A1203)
if not stated otherwise.
75
TABLEZ-8 13C NMR chemical
shifts
bTMS (ppn) of different
Compound
molecules
Coverage+
support
in the 9aseous.
-2.03
38
85.56
NaX
__
84.4
SiO2
67.49[+5.19]
82.22
15.4
16.11
38
15.54[0.141
16.11[0.0]
38
-2.561-0.531
38 38
-1.58[+0.45]
38
38
Jiq
27.913.61
38
N?lX
24.61[0.31]
38
HV
24.19[-o.llJ
38
5102
23.25[-1.051
liq SiO
2
Yf,' SiO
2
N&i02 CH3-CH2-CH2-CH3
0.22 0.47 0.82 0.71
28
gas liq s102
H2C=CH-CH2=CH2
1.93 !J ml m-2
gas 1iq SiO2
H3C-CH2-C'CH
2.05umolm
-2
gas liq SiO2
1.96”mlm-2
38 X.3
202.8
37.3
6.0
206.0(3.2)
38.4(1.1)
6.0(0.0)
24.1
197.5
28.1[4.OJ
205.11t7.61
102
27.9(-1.1) 27.1(-1.0) 27.8(-0.3)
216(+11) 215.2(+10.5) 214.3(+9.2) 212.2(+7.1)
43 105 105 105
27.0(-1.1)
211.8(+6.7)
66
10.9
24.4
37
13.112.21
24.9IO.51
104
11.810.91
20.4lO.11
104
112.8
136.2
37
116.313.51
136.9
37
116.1[3.3]
136.810.61
108 67
37
28
62.8
81.8
10.7
11.6
37
28
67.014.21
84.712.91
12.3[1.61
13.812.2~
104
28
66.613.91
87.615.81
11.5[0.81
12.010.41
37 37
gas
28
70.2
liq
28
73.613.41
104
75.515.31
37
CH2=F-CH-CH2
CH3-CH2-CH2-CH2-CH=CH2 Jiq Cl-CH2-CH2-Cl
sio2
2.16
25
116.4
142.2
139.6
113.2
17.5
WA1203
10~ moln-2
25
114.3(-2.1)
140.8(-1.4)
138.8(-0.8)
112.1(-1.1)
14.5(-3.0)109
liq
CH3
liq
28
NaV -
11.61
5 u no1 III-’
25
113.8(-2.6)
141.3(-0.9)
138.5(-1.1)
-
14.8(-2.7)109
25
117.4(1.0)
144.1(2.1)
141.3(-1.7)
114.5(1.3)
lS.O(O.5)
113.5
137.8
102
1.0
60
112.4(-1.1)
142.6(+4.8)
49
0.48
HCOOH liq TiO2
25
Jiq ST02
102
51.2(-0.5)
105
liq
'number Of m~lecqler
110
176.9
20.8
175.8(-1.1)
104
180.1
18.2(-2.6) 27.5
a.7
178.3(-l.@.)
-
26.3(-1.2)
6.8(-1.9)
179.3
36.0
18.2
111 104 111 13.1
104
510.
177.3(-2.0)
35.2(-0.8)
17.3(-0.9)
ll.O(-2.1)
-
179.4
33.8
26.7
21.7
13.2
177.3(-2.1)
32.4(-1.4)
25.9(-0.8)
20.3(-1.4)
10.9(-2.3)lll
L
CH3-CH2-CH2-CH2-COOH
104
4100(+3934)
SiO2
liq
109
51.7
166
CH3-CH2-CH2-COOH
109
5 y mol m-‘+ o2 (traces) -
Si02
CH3-CH2-COOH
Ref. C5
24.3
CH3-CH2-CHO
CH3COOH
5
C4
gas
CH3
H3C-CsC-CH3
[66g= (Ppm)l
68.16[+5.861
NlV
HC3-C-F3
state
(Ppm)).
NaV
gas
CH3-YH-CH3
liq
62.30
gas
CH3-CH2-CH3
(A6
3
5 HCX-CH3
liquid and adsorbed 6 (pp);
TqC
0.24 0.48 0.74
27.8 27.4 -0.4) 27.0 -0.8) ZE.l(t0.3)
Per ruperc:a9e (zeolites) or statistical
monolayers
111 104
102 105 105 105
I
(S102. Al20,)
if not stated othewfse.
TABLE 2 -C 13
C NMR chemical
shifts
6TMS (ppm) of different
molecules
Coverage+
support
Canpound
in the gaseous,
Cl
-
@.w
liquid and adsorbed s(ppm);
TqC
state
(b61'q(ppm));
CP
Ref
rA6gas(ppm)l
C3
C4
C5
102
128.5
Si02
127.7(-0.8)
111
co2+ Aerosil 0.7 (0.60%)
60
139.5(+11.0)
93
Ni2+ Aew;'
0.7
25
163.5(+35)
94
0.7
25
163.5(+35)
94
0.25
25
154.5(+26)
94
;.;
25
0:7
2':
151.5 +23) M;.;(+9’;) I . +
;“4 94
N,2+
: Aeros, (0%)
SiO N1"
(0.9%)
144.1
sio*
125.6
21.3
137~8
129.3-128.5
102 102
(+1.8)
20.0(-1.3)
60
139.8(2.0)
(+2.0)
21.3(0.0)
60
OeNaY
0.8
140
140.9(3.1)
(+1.1)
21.1(-0.2)
60
sio2
0.49
138.6(0.8)
(0.1)
19.8(-1.5)
105
19.4(-2.1)
111
0.70
28
126.6(+1.0) 125.1
140.9(3.1)
128.6-128.2
36.9
19.0
102
35.7(-1.2)
15.5(-3.5)
113
47.5
Si02
0.51
0.61
NaY US-Ex
28
H in H2S04
28
13.8
55.6(-2.6)
9.3(-4.5)
111
55.9(-2.3)
9.5(-4.3)
113
149.9
124.0
136.0
114
157
150.2(0.3)
124.3(-0.3)
137.9(1.9)
114
60
149.7(-0.2)
123.0(-1.0)
135.0(-1.0)
114
0.77
28
148.8(-1.1)
124.0(0.0)
136.2(+0.2)
113
A1203
0.86
60
149.7(-0.2)
37
142.0
128.7
148.2
114
NaY
157
141.5(-0.5)
127.5(-1.2)
147.0(-1.2)
114
US-Ex
157
140.6(-1.4)
127.3(-1.4)
145.9(-2.3)
114
142.1(0.1)
129.1(+0.4)
148.3(+0.1)
115
118.4
108.0
104
118.3(-0.1)
106.9(-1.1)
113
;+, 2
28 28
147.7
116.1
129.8
119.0
104
sio2
0.51
28
145.2(-2.2)
118.3(+2.1)
129.0(-0.8)
120.6(+1.6)
113
A1203
0.9
147.1(-0.6) 80
liq
:'02-A'203
'.'
116.5 119.7(+3.2)
... C6
102
104
120.0(+6.6)
129.4(-0.7)
122.4(+9.0)
129.7(-0.4)
-
150.9
112.8
129.3
116.7
40.3
Si02
150.7(-0.2)
109.2(-3.6)
128.2(-1.1)
119.8(+3.1)
41.3(+1.0)111
57.3
17.9
102
58.3(+1.0)
105
67.4
16.4(-1.5) 17.1
67.7(+0.3)
15.7(-1.4)
105
5102
28
0.74
sio2
'number of nwlecules
130.1
147.9(-0.7) 0.70
liq
liq
113 113.4
149.0(+0.4)
s102
CH3-CH2-O-CH2-CH3
113
143.3(-4.4) 148.6
SiO2
liq
113
0.91
_
4&2_C;3)2
102
37
liq
40-NH211q
113
58.2
sio2 (1:s) -
@
102
45.5(-2.0)
sio2
CH3CH20H
5~.;4~~.5’11’
140.7(+2.9)
liq
4@!NK:3)2 3 2
125.4(-0.5)
127.6(-0.4) 137.8
140
lfq
&NH
127.7(-0.8)
144.4(+0.3) 113.5
104
140
Si02
6
29.2 C6 15.8
0.8
so2
4 @
125.9
0.8
llq
N(CH,-CH3)3
128.5
NaMgY(68%Mg)
so2 NW+3
128.0
NCIY
so* CH3-CH2-NH2
112
128.5(0.0)
NaY
per supercage
0.68
(zeolltes) or statistical
monolayers
(Si02, A1203j if not stated otherwise.
102
The gas-liquid are more exposed C4 of 1-butene respect
shifts
to intermolecular
adsorbed
to the gas.
the methinic
Similar
on the terminal
interaction
on NaGeX zeolite
The C3 carbon
C2 carbon
other carbon atoms, cule.
are more significant
(ref. 34,37).
experience
experiences
conclusions
a specific
atoms which
The carbons
a small high field shift, while (low field shift) than the
interaction
at this site of the mole-
can be drawn from comparison
with the liquid state,
where all Cl, C3 and C4 carbon atoms show a high field shift, smaller tude than that of C2 (low field shift). zeolite, almost
the chemical
identical
shift variations
However,
specific
interactions
methinic
C2 carbon atom.
in Table
1.)
In conclusion, is somewhat erencing between
if the comparison
are clearly
intermediate
the admolecule interaction
for the pure liquid
adsorbed
molecules
state are
to a general
intermolec-
change
another
physical
In general,
the interaction
is here eliminated
included
state which
however,
the ref-
(Tables 1 and 2)
and the surface at low surface coverage similar molecules
in the
in Table 2 and 1-butene/TlX
between gas and liquid.
between
on the same
is made with liquid trans-2-butene,
constitute
state better reveals
in magni-
to the gaseous
shown again by the greater
(See also l-butyne/Si02
the adsorbed
to the gaseous
der Waals
In trans-2-butene as represented
for Cl and C2, which could correspond
ular interaction.
Cl and
a small low field shift with
atom is much more influenced
suggesting
carbon
because
the van
in the chemical
in the first approximation
shift
(except CH4,
see Ref. 34). In contrast, actions
at higher coverages
has to be corrected
molecules
in the liquid states
the surface
on the resonance
3. Exchange
effects.
(see, for example,
adsorbed
nature of the counterion
(1-butene coverage
When the surface
coverage
This implies
adsorbed
increases,
the existence
the variation
are also populated
of the different
fication
is also necessary
between
distributed
signal
of their resonance
in the case of an exchange
netically
adsorbed
molecules.
shifts becomes
sites.
the adsorption
is significantly
for a study of complexes
shifts
sites of
shift (which is a weighted modified.
This modi-
with paramagnetic
since here the line widths may be so broad and the signals determination
Ag
adsorp-
the different
molecules,
and the chemical
contributions)
of the chemical
of heterogeneously
of adsorbed
amount
or Si02), the
on NaAgX zeolite with different
Indeed, with increasing
average
of
(Tables 1 and 2).
of admolecules
energies
influence
shifts vary with the nature of the surface on NaY and NaGeX zeolites
tion sites and a rapid exchange
weaker
inter-
shift for the
if one wants to study the specific
content)
decreases.
of molecule-molecule
the resonance
shift of the admolecules.
The chemical
1-butene
and the surface
the contribution
by using as reference
possible
with a sufficiently
sites
so weak that a
only via the resulting
great number of diamag-
Also, for interactions analysis
of resonance
cules is necessary adsorption
shifts measured
the experimental
are not generally
identical
NA from these experiments physisorbed
molecules
(MA) (ref. 41-43) experimental
M+AA
(MA]
bound in complexes
One way .to derive
is to consider
between
of decomposition
6c and
the purely
sites (A), and the complexes of sites
(number
to zero coverage
the quantities
the equilibrium
shifts as a function
constant
of the number N of admole-
shifts 6 extrapolated
and to derive the number
If the probability
by its stability
with 6c.
sites, a careful
number NA of sites for physical
shifts 6c of molecules resonance
(M), the adsorption
resonance
adsorption
as a function
in order to determine,the
and the resonance
NC) because
cules.
with purely diamagnetic
formed
(NA) for the fit of the
of the total number of the surface
(N) of mole-
complex
is determined
k only (model l), we can write:
(7)
.
Since the number of empty adsorption which are not bound in complexes
sites is NA-NC and the number of molecules
is N-NC, the constant
k is given by:
(8)
Combining
this equation
with respect
for the constant
to the line position
k with the resonance
of purely physisorbed
shift 6 measured
molecules
(6M=O),
NC
6
(9)
T=x=T
we arrive at the quadratic 1+ kNA JL NA + x --l=O %A
(l-x)x
For the treatment
X ’
equation:
(10)
.
of experiments,
we define an experimental
quantity:
6
=-
6m where
(11) ’
am is a maximum
N+O.
shift value obtained
from a simple extrapolation
of 6 for
According. to Eq. 10, we find:. 6m
Xm=6c= Inserting suitable
kNA l+kNA
x = x' form:
*
(12)
- x, into Eq. 10, we may transform this equation into the more
79
Nxt2 _=-'__<
1
1-x
Nx'
x,
NA
l-x
,
(13)
which allows one to extract Nx' l_x' *
the quantities
NxL2
xm and NA from a plot of -versus
In Ref. 43, a further model is treated, taking into account that the decomposition of the surface
complex
to it for physical
adsorption
Then the equilibrium M+Ax
will proceed
only if there is an empty siteu
next
(model 2).
in the system can be written
in the form:
[MAI +L.J,
and instead
(14)
of Eq. 13, we arrive
at:
(15) It is important
to note that the value xm and, hence, the real shift of the
line in the NMR spectra
(determined
used for the characterization the criterion
from unity
For the treatment is termed
to be strong
data, two special
if kNA>>l
IV.
shifts
stricting
sites are discussed The significance
mainly
47).
i.e.,
concentration
For a weak complex,
shift 6m may deviate
INTERACTION
basically
WITH DIAMAGNETIC
the static NMR parameters constants
developed
(Studies of interactions
appreciably
SITES such as the
J (in Hz), re13 C and
study of
with surface acidic
in Part VII.) of the change of coupling results concern
"s" character
(ref. 38). Theoretical results
to zero adsorbate
to the more recently
molecules.
yet; the only available in carbon
MOLECULES.
we examine
of
cases are of importance:
Then we find x,=1,
6 (in ppm from TMS) and the coupling
ourselves
15N NMR of adsorbed
change
N+O
and the extrapolated
THE STATE OF ADSORBED In this section,
chemical
holds.
insnediately the real shift 6c in a complex.
kN
Obviously,
is the deviation
(ref. 43).
of experimental
6m= bc, and from a simple extrapolation we obtain
on the surface.
for the choice of one of these two models
the value k'/(k'-1)
A complex
from this plot) does not depend on the model
of the equilibrium
constants
ethylene
has not been assessed
and propylene
and/or bond length variations
calculations
on simple models
and throw some light on the structure
for which
have been reported
help to understand
of adsorbed
some
molecules
the NMR
(ref. 45-
80 A. Analysis
of 13C NMR shifts
The chemical collected
in Tables
trans-2-butenes Numerous resonance
shifts of the different 1 and 2.
carbon atoms in various molecules
Most thoroughly
studied are 1-butene,
investigations
of molecules
adsorbed
due to interactions
and the exchangeable
cations.
interactions
butenes and silver cations
between
in zeolites between
(Fig. 2) which showed that the electronic and in solutions
(ref. 13).
structure
was strongly
state will be discussed
found in Ref. 52 for toluene, zeolite
o-xylene
(with 50% of its Na+ replaced
both from thermodynamic
measurements
(ref. 54,55).
Moreover,
systems gives direct evidence by metal
data
later.
of silver-olefin the molecular
Similar
silver,
for hydrocarbons.
This
of infrared
of proton relaxation
of adsorption
complexes
(ref. 51,56).
C NMR spectra of 1-butene: a) free liquid; b) in Fig’2.‘H1;,CHC, 3; c) in AgNaX zeolite (60% Ag+); d) in NaY 2
AgBF4
in-a 50 AgNaX Besides
(ref. 53) and the results an analysis
of
Details
results could be
adsorbed
centers
complexes
mobility
(ref. 51).
by Agt ions) at 390K.
for the existence
cations and hydrocarbons
Here a direct CM-
restricted
and p-xylene
other metal cations may also act as adsorption follows
carbons
(ref. 48- 50) was possible
was the same, although
in AgNaX and AgNaY zeolites
of the electronic
showed that the
the unsaturated
A direct proof could be given in the case of
parison with the results for silver salt solutions
olefins
cis- and
(Table 1).
shifts are mainly
in zeolites
are
solution zeolite.
of
in such formed
81 The adsorption of olefins in zeolites with monovalent cations like Li+, Nat, KS, Rb+, Cst and Tl" has been studied in great detail by means of 13C NMR. Because a direct comparison with ion solutions is not possible here, it is necessary to check systematicallywhether other sites may also be responsible for the 13C NMR shifts observed for these zeolites (ref. 36,57). The various steps of this proof, e.g., treated for I-butene and isobutene molecules as adsorbates (ref. 49,58), are: 1) to check the role of Si- and O-atoms by comparing the resonance shifts found for NaX and NaY types with Y zeolites having a very low content of Al and Na (Si/Al=47);
2) to check the influence of the small number of
structural OH groups by measuring the variation of resonance shifts with the number of admolecules; and 3) to check the role of aluminum atoms of the lattice by changing the type of monovalent cations (Tl+, Cs'). It could be shown that the 13 C NMR shifts measured for olefins in NaX and NaY zeolites are due to interactions with Nat ions. Moreover, by means of proton spin relaxation of n-butenes (ref. 51) and benzene (ref. 59), a stronger interaction with Na+ ions at S-III sites rather than with those at S-II
sites was detected. The adsorption of ben-
zene and its derivatives in X and Y zeolites leads to such a strong restriction of their molecular mobility (ref. 59) that in general no highly resolved 13C NMR spectra can be observed at room temperature (ref. 46,60). The shift for the 13C resonance of benzene in NaX and NaY zeolites measured at higher temperatures is of the order of the experimental error. For the methylbenzene, a shift of 2...4 Ppm to lower fields occurs for ring carbon atoms to which the methyl group is attached which can be explained by the polarizing effect of the Nat ions. For toluene adsorbed in a CsX zeolite, a strong anisotropy of about 130 ppm for the l3C NMR line of this ring carbon line was observed which was explained (ref. 61) by a restricted rotation of the toluene molecule about its C2-axis (with a correlation time of about 10-3s). No such anisotropy occurs for a NaX zeolite (ref. 61,62). In the case of acetone, the observed I3C NMR shifts for the carbonylic carbons are larger than the resonance shifts of the olefins adsorbed in the same zeolites. In contrast to that, the 13C NMR shifts of CO and CO2 adsorbed in NaX, NaY and NaA (ref. 63) zeolites are not very different from the values for the gaseous state. The same results hold for these adsorbate-adsorbent systems if Nat is replaced by other alkali cations or Tl+.
From the temper-
ature dependence of the observed anisotropy of the 13C NMR signal of CO, CO*, COS and CS2 molecules adsorbed in mordenites, a model for the thermal reorientation of these molecules was developed (ref. 64). Also, in these experiments 13 the isotropic part of the C NMR shifts was small. Very large shifts to lower fields were observed for CO adsorbed on decationated zeolites. The values were largest for deep-bed treated specimens but even for shallow-bed zeolites, they are much greater than for zeolites containing alkali cations (ref. 63). No such effects were observed for COR,
82 13
C NMR shifts of hydrocarbons
compared
with zeolites
of the effects
adsorbed
on silica gels are relatively
so that an interpretation
observed
with certain
and an unambiguous
types of adsorption
oxygen atoms of the silica gel, lattice defects, Several
attempts
partially.
were undertaken
to overcome
centers
small
correlation
like OH groups,
etc., is rather difficult. these difficulties,
at least
In Ref. 93 the resonance shifts of isobutenes were measured both
for partially
dehydroxylated
it has been concluded molecules
and methylated
that in contrast
with the oxygens
silica gels.
From this comparison,
to Ref. 66 the interaction
in siloxane
bridges
has no influence
of isobutene
on the 13C NMR
shifts. Significant
shifts were observed
sorbed on silica gel (ref. 67). droxylated
and methylated
ated from the coverage
dependence
of centers N,= (1.4+0.2) ylated
hydroxyl
is in accordance
between
and the clear dependence
acetone molecules
of this group with the surface
group of acetone molecules
The strong difference
specimens
the total number of adsorbed
This suggestion
for the C=O
indicates
partially
ad-
dehy-
of the shifts on
a strong
interaction
groups.
with the number of adsorption
sites evalu-
of the shifts by means of Eq. 13.
nmV2 and NA= (0.320.1)
and m'e‘thylated silica gels, respectively,
nmm2 for partially are in reasonable
with the number of OH groups for these samples.
The number dehydrox-
agreement
The same conclusion
was re-
cently drawn in Ref. 43. The formation
of hydrogen
cules and surface
OH groups of Si02has
and 15N NMR measurements; cated an appreciable the nitrogen indicate
bonds between
a strong
influence
atom, whereas
that no specific
the nitrogen
been studied
atoms of pyridine mole-
by means of combined
1
'H
low field shift in the 15N NMR spectra
of adsorption
only small changes interaction
indi-
sites on the electronic density at 13 C NMR spectra observed in the
of the r-electron
system with surface
sites occurs.
B. Analysis
of 15N NMR shifts
In the first detailed nitrogen-15 trimethylamine, were measured
pyridine
carried
nuclei
special
interest
resonance
transform
were employed
the nitrogen
atoms of a variety
zeolites
NMR techniques.
Moreover,
molecules
of molecules
in the formation molecules
may be directly protonated at the nitrogen 15 between the N NMR shifts of protonated
the difference
spectra of ammonia,
in various
which were enriched
spectra of adsorbed
which may participate
giving rise to strong 15N NMR shifts. acetonitrile
sorbed
In with
(ca. 95%).
because
electrons
Fourier
out, substances
A study of the nitrogen-15
lone-pair
(ref. 42,68),
molecules
at 9.12 MHz by conventional
all measurements nitrogen-15
NMR study
and acetonitrile
of hydrogen
is of possess bonds,
like pyridine
atom.
and
Consequently,
and nonprotonated
83 species may be considerably
larger
carbon-13
in the molecule.
spins and protons
be more favorable addition, molecule
through
prevents
resonance
their accurate
depends
characterized
in more detail than with proton
complete
strongly
pore filling
(e=l)
a strongly
dealuminated
not change
as a function
molecules
adsorbed
on the coverage
resonance
shifts
of liquid ammonia and gaseous
Y-type
zeolite,
ammonia
for adsorbed
amnonia mole-
(18 ppm) if we have nearly (0 ppm) for zero coverage.
factor.
resonance
shift does
for liquid ammonia.
of the resonance
shifts with decreasing
coverage
values measured
for the liquid and the gas can be explained
the association
of the ananonia molecules
with the Na+ ions.
is increasingly
The interaction,
influence
on the electron
influence
of the Na+ ions on the adsorption
due to the
lead to an al-
at the nitrogen
of adsorption-heat
the
by the fact that
must
though a strong
from results
density
however,
to gaseous
between
prevented
most negligible
inferred
For
Its value is about the
Fig. 3. '5 N NMR shift 6 of ammonia in NaY zeolites (in ppm referred NH3) as a function of the pore-filling factor e at 300K.
can be clearly
because
The plot (cf. Fig. 3) is
the nitrogen-15
of the pore-filling
same (16 ppm) as that reported
interaction
In
in Nay, NaX, NaA and Na-
(ref. 68).
of the resonance
characteristic
The variation
should
measurement.
by the approach
cules to values
15N NMR measurements
shifts are small and often less than the line widths which
The 15N NMR shift for ammonia mordenite
Hence,
shifts for the adjacent
for a study of the acidic sites on a surface (see below). 15 a study of the N NMR, the electronic state of the ammonia
can be investigated
the proton
than the resonance
atom, al-
behavior
measurements
of amnonia (ref. 69).
Moreover,
the same value for the indirect
was measured
at low coverages
phase (61.6 Hz), indicating has not been changed Nitrogen-15
NMR resonance
that the geometrical
shifts of acetonitrile
Fig. 4, the resonance
From the whole
pretation
of zeolites.
can be favorably
(see below).
cations
Nat ions.
of Eq. 13, a number of
in agreement
with the simple
a strong adsorption
complex.
sites is equal to about twice the number of accessible
ions (about 3.3 Nat at S-II
of
As is shown in
for CH3CN in NaY as long as the
of N by means
can be calculated,
cules are not protonated
Hz)
and a value hc= -19.5 ppm for the shift of
of the plot in Fig. 4, assuming
number of active
molecules
(62-5
of the NH3 molecule
in Na-forms
is less than the number of accessible
sites per supercage molecules
sites
remain constant
plot of 6 as a function
active
the complexed
shifts
molecules
structure
constant
in the gas or liquid
with the exchangeable
(e.g., Na+) and with Lewis-acid
number N of adsorbed
6+0.5 -
of interactions
coupling
for ammonia
in the course of adsorption
used in the characterization the zeolite
spin-spin
as that obtained
sites).
In decationated
zeolites,
but interact with the structural
interThe sodium
the CH3CN mole-
OH groups
via hydrogen
bonds.
_
-..
-+---
10 N
Fig. 4. I5N NMR shifts 6 (in ppm referred to liquid CH3N02) of acetonitrile in Complete pore-filling a= 1 corzeolite: a) NaX; b) Nay; c) in NaX for e=1.2. responds to N=lO molecules per large cavity.
C. Origin of the variation For a more detailed of interaction, tum chemical tribution
of the chemical
characterization
the observed
calculations.
(0 para)
shifts have to be interpreted For nuclei different
to the screening
netic one (adia ) (ref. 70):
shift
of the nature of bonds and the geometry
constant
in the light of quan-
from 'H, the paramagnetic
con-
is far beyond that of the diamag-
85 ' = 'dia + 'para
(16)
’
with: -. 'para =
where
'AA +A&
Q,, is a function
the bond order matrix.
Various
to calculate
the NMR chemical
the simplest
methods
volved
quantum
shifts
surface
interactions active
of alkenes
site interaction
its isomerization
The main interaction
NaX and NaY zeolites,
between
the charge distribution redistribution
to unoccupied
overestimated
in comparison
and on silicas.
here preferentially
between
the n-orbitals
in the 1-butene-Na+
out to l-butene-
because
(ref. 15 -18,
complex:
influenced
from occupied
of the olefin
have been found to 1) an intra-
by the electric
orbitals
field
of the 1-butene
of the Na+ ion (ref. 74,78).
yield
energies
74).
were carried
by 13C NMR spectroscopy
about the same total amount
(ref. 78), overestimated
The stabilization
only
l-butene and Nat ion (the active site on
transfer
orbitals
CNDO/Z and PCILO methods fer (0.2 electron)
in zeolites
of the charge density
and 2) a charge
molecules
(ref. 45,46,60,72-
of the Na+ ion. Two mechanisms
influence
molecule
studied
to
applied
of the large number of atoms in-
will be reviewed
molecular
of the cation;
and Q,, is related were already
(ref. 76) calculations
see above) occurs
orbitals
methods
(ref. 71) but for adsorbed
or arenes
was extensively
and the unoccupied
of charge density chemical
interaction
(ref. 75) as well as PCILO
describe
CNDO/Z
(17)
’
have been used because
in the admolecule-surface
CNDO
79).
1
AE is the mean electronic excitation energy, stands for the mean radius
of carbon 2p orbital,
77).
QAB
for the
of charge trans-
by both semi-empirical 1-butene-Na+
with experimental
values
methods
supermolecule (ref. 47,78,7g),
(ref.
are also the
values being much more unrealistic.
In order to choose the best structure for the molecular complex, theoretical 13C NMRshifts were compared. An empirical relationship between and experimental the variation
of chemical
the variations the chemical
of u-
shifts
shift from the free to the adsorbed
state
to the electron
densities
(ref. 72):
Au = 150 Ap, + 300 BP= . The experimental in Table different
3, where
(Au) and
(Ap,) and n- (a~~) electron densities were used to fit
and the theoretical
(18) chemical
shift variations
the CNDO as well as the PCILO results
(the cis and skew) conformations
of 1-butene.
are reported
are included
for two
86 TABLE 3 Comparison of calculated l3C NMR chemical shifts with experimental data of 1-butene adsorbed on NaX zeolite (ref. 47) Compound
4bliq(ppm) Cl
CH3-CH2-CH=CH2 (e=O.15) cis CNDO PCILO skew CNDO PCILO
C2
c3
c4
+0.2
10.1
0.9
1.6
1.65
6.25
-1.4
-0.5
-18.3
20.8
-1.8
-0.3
0.6
8.3
-2.0
-0.1
-0.5
9.4
0.5
-1.1
The most stable association is a r-complex where the.Na+ ion is localized near the r-orbital of 1-butene in a region opposite to the methyl group of the molecule in skew conformation (PCILO method) (ref. 78):
For the influence of the mean excitation energy on the chemical shift (AE), there is very little data available (ref. 45); a variation of 0.066 eV corresponds approximately to a change of 1 ppm in
Au.
Together with the charge den-
sities variations, the effect of the surface on the mean excitation energy may have important implications in catalysis (ref. 38). A lowering of this energy makes the molecules more reactive. The changes in electron densities may involve a better stabilization of either negatively or positively charged intermediates (ref. 80-83). The influence of the electric field being quite large in zeolites (l- 5 volt/W) (ref. 83) deserves particular attention. Horsley and Sternlicht proposed the relationship between the variations of the chemical shifts and the electric field (ref. 85):
"C-H
= 2.9
x
lo-llE
(cgs);
Au~_~
=
5.1 x 10-llE (cgs).
(19)
Following these ideas, a maximum variation of about 80 ppm can be accounted for.
In a molecule, however, when the contribution of all the different bonds
87 is calculated, polarization
the local electric effects
in neighboring
cated in the plane of an ethylene center
field is rather
of the C-C bond produces
bonds.
For example,
molecule
at a distance
an electric
(INCO calculations
ter of the molecule
about 4 ppm in the 13C NMR chemical calculations
yield
low due to compensating
fluenced
interaction
one, in the 1-butene-Agt
larger change
in chemical
of X or Y zeolites ing is a two-way pied r-orbital
shifts
of the adsorbed
tal); and 2) a backbonding antibonding
s*-orbital
the C2 carbon atom is the more in-
interaction
Cl shows a diamagnetic
transfer
to the 5s empty orbital
uses d xy + p, hybrid orbitals
(n-bonding)
occurs
bond-
from the occu-
of Agt ion (u type orbi-
occurs from the dxy orbital
of the olefin
and a
on the Ag+ content
It is known that the olefin-Agt
(ref. 88): 1) a charge
of the olefin
molecule
site.
(ca. -12 ppm), depending
(ref. 13) (Table 1).
bonding
to a change of
We can see that the theoretical
not only the best conformations
in the l-butene-Na+
lo-
- lo5 esu at the cen-
(ref. 86)) corresponding
shift.
charge
of 5 8 from the
field of 0.76
but also shed some light on the nature of the active While
a positive
of Ag+ to the empty
(ref. 89).
for Agt ion (ref. 90,91).)
(Another model
This model could be
Y
tx
‘XY adequate'for 2-butene.
the description
of the structure
Even in these cases,
the Agt ion is not equidistant (ref. 89,91). tron density account
In 1-butene,
however,
u-bonding
crystallographic
from the two carbons
the double
is higher on Cl than C2.
the polarization
of a symmetric
of the a-bond, would
be:
n-bonding
results
such as
show that
at the site of interaction
bond is already Therefore,
olefin
polarized
a better model,
and the electaking
into
The u-bonding would be responsible for a paramagnetic shift while a diamagnetic shift would accompany the a-backbonding formation. The change in hybridization from sp2 to sp3 could also partially explain the rather important diamagnetic shift (ref. 48). Detailed quantum chemical calculations are not available for the moment for a better understanding of the different contributions. The 1-butene-Agt ion complex can also be formed in solution. The variation of the chemical shifts in water is quite similar to those in the adsorbed state: A6(c1)
= -
14.6 ppm; AS(CP) = -1.7;
As(C3)
=
t1.1
ppm; and
Ao(C4)
= +
1.0 ppm
(ref. 48). The chemical shifts are independent of the nature of anions and only slightly dependent on solvent effects (ref. 48,50). When strong solvation of the Agt ion occurs, as in
DMSO, the specific interaction of l-butene with Ag+
is impeded (ref. 50). Quantum chemical calculations were also carried out on the following complexes: iso-butene-Nat (ref. 72,74,92), cis-2-butene-Nat (ref. 45,46,60,74), trans-2-butene-Na+(ref. 74), benzene-Nat (ref. 45,46), benzene-Mgtt (ref. 46, 60), benzene-OH (ref. 45, 46) and toluene with Na+, Mg++ and OH (ref. 45, 46, 60).
In a more realistic model containing Al or Si atoms, seven 0 and seven
H atoms were included in the aluminosilicatel-butene interaction (ref. 80) but the NMR parameters were not computed. Finally, a promising method for the study of admolecule-activesite interaction is the use of paramagnetic ions (Co2', Ni2*...) where contact and pseudocontact interactions can lead to quite large chemical shift variations (ref. 52,93,94). D. Variation of coupling constants upon adsorption 13 C-H coupling constants of ethylene and propylene upon The variation of adsorption on NaY zeolite has been reported (ref: 38). The small changes (Table 4) are interpreted as due to the variation of "s" character of the C-H bond which is also reflected in the change of C-H distance rCH (-ref.95): rcH(fi)= 1.1597 - 4.17 x 10m4 JCH
.
(20)
The influence of the solvent CDCL3 is larger than that of the surface, which shows the rather small interaction of these molecules with the NaY zeolite sur15 N-H coupling constant in ammonia adsorbed on NaY zeolite measured face. The at low pore-filling factors (6255 Hz) is the same as was obtained in the gas or liquid phase (61.6 Hz) (ref. 42). This fact emphasizes that the geometric structure of the ammonia molecule has not been changed during the adsorption. At higher pore filling factors (a,0.3), the multiplet splitting disappeared due to an increasing proton exchange rate.
89 TABLE 4 13
C-H coupling
constants
Compound
in the adsorbed e
Support
state
J(Hz)
T°C Cl
CH2=CH2gas
c2
38
156.2
104
in CDC13
159.4
38 105
Si02
-
159.4
NaY
0.14
159.4
CH3-CH2-OH
USb05
25
“Sb30 10
25
liq
125.7
38
153.3
148.4
129.3
38
CH30H liq
13x
sorbed on pyrogenic tional preference
silica
coupling
(ref. 96).
25
140
107
25
140
107
constants,
constants
The JHH coupling
constant
It is concluded
MOBILITY Despite
and is more stabilized
of 4.5 Hz increases
in polar solvents,
temperature
effect
effect are able to explain
that the 1,1,2-trichloroethane
read-
for the conforma-
constant
rota-
result-
(ref. 97).
to 5.5 Hz with increasing
temper-
(t 0.15 Hz per the observed
is adsorbed
varia-
preferentially
(ref. 96).
OF THE ADSORBED
SPECIES
the fact that l3 C nuclei are essentially
contributions
mention
It has been shown that the gauche
dipole moment
in the gauche conformation
we also
in 1,1,2-trichloroethane
JHH for solvents with a higher dielectric a simple
105 105
ing in a smaller
Neither
104
127
mer has a higher
1OO'C) nor a "substituent-like"
74
126.9
Evidence was provided
at the solid surface.
ature from 40 to 8DOC.
154.9
104
of data on the coupling
on proton-proton
74
122
0.24
of the paucity
149.9
127
liq
sults reported
155.5
0.74
Si02
38
160.6
140.3
Si02
Because
38 147.1
gas
NaY
V.
-
159.9
Si02
in CDC13
tion.
c3
liq
CH3-CH=CH2
0
Ref.
relaxed by intramolecular
simplifies the subsequent data treatments, few 13 studies have been devoted to the C NMR relaxation measurements because of 13 the low sensitivity of 13C nucleus and the cost of C-enriched compounds. These compounds ation
(ref. 34), which
are indispensable
times at low surface
times (Tl in s),together
in order
coverage.
to get reliable
The available
with NOE effects,
values of the relax-
longitudinal
are reported
relaxation
in Table 5.
.
90
TABLE 5 Longitudinal relaxation times T, (5) and NOE factors measured in the gaseous, liquid and adsorbed state "(MHZ) suoport
Compound
et
T'C
Tl (s) (NOE)++ Cl
CH4 gas
CH,=CH, gas c
Ref. c3
C2
c4
c5 38-
20
-
23
0.021(1.0)
20
NaY
23
0.030
38
23
0.050(1.0)
38 38
20
L
in CDC13
CH3-CH=CH2 gas
20
2.3 M
23
7.500
20
NaY
23
0.450
20
in CDC13
CH3-CH2-CH=CH2
CH3-CH=CH-CH3 trans
38
0.095(1.0)
20
2.6 N
23
59.900
58.700
65.200
38
20
NaY
23
0.81
1.6
0.81
38
25.1
LiX
:::I:::]
0.9(1.5) Z.l(l.5)
36 36
0.2 0.8
0.3 1.2
0.6 2.2
:i
;.;n;.;j
y{;.;j .
:::
25.1
CH3-CH2-CH=CH2
38
23
NaX
5.7/cage 2.3/cage
::
b.O/c?.ge 1.5/cage
z: 23 23
25.1
KX
3.0/cage 1.3fcage
25.1
RbX
5.4/cage 2.1/cage
:: 23 23
25.1
csx
4.9lcage l.z/cage
25.1 20.1 25.1 20.1 20.1
NaGeX
0.5
;.$.;]
.
.
.
1.8(1.3) 3.2(1.9)
X:$
0.3(1.6) 1.4Cl.4)
0.5(1.5) 1.7(1.4)
O.Z(l.4) l.E(Z.3)
0.2( 1.3) 2.3(2.0)
:::I:::]
:“6
0.38(2.2) 0.33(2.2) 0.17(1.9)
99 * 100 99,100 99,100 99,100 99,10D 99,100 99,100
.
.
0.24(2.0) 0.22(2.0) 0.32tl.6) 0.20(2.0) ~.;p~‘;~’
0.9
20
NaY
22.63
NaY
20.1
NaGeX
0.9
0.8
20
0:71+++
K:Ii:iI 0.14$1.7) 0.35 ++
ii
0.18
0.71
0.35
1.32
0.049(2.7)
0.060
0.056(2.7)
0.075
20.1
0.53(2.0)
0.24(2.0)
99,100
99,100 11
25.2
Si02
1.10
28
0.33
0.16
105
CH3-CO-CH3
15.0
s102
0.83
28
0.15
0.09
105
CH3COOH liq
15.0 15.0
SiD2
-
RT RT
30.0 0.72
8.1 0.58
111 111
CH3-CH2-COOH liq
15.0
CH3-CH2-CH2-CDOH liq
CH3-CH2-CH2-CH2-COOH liq
32.8
6.6
6.3
sio2
RT
0.51
0.17
o.lJ7
15.0
-
RT
24.4
3.3
4.0
5.7
15.0
sio2
RT
0.42
0.13
0.22
0.77
15.0
-
RT
9.7
2.0
2.5
4.0
4.5
111
RT
0.37
0.14
0.22
0.33
0.92
111
15.0 @liq
4@-CH2-Ca3 3 2.5
3
111
RT
15.0
111
liq
N(CH2-CH3j3 liq
4@N(Ci?,Cfi,), 3 2
liq
111
sio2 -
RT
23.3
15.0
sio2
RT
2.2
15.0
-
RT
41.1
15.0
sio,
RT
2.51
15.0
-
RT
31.2
18.1
17.7
15.0
sio2
RT
1.18
0.91
0.91
15.0
-
RT
12.4
11.0
111
15.0
sio,
RT
0.06
0.40
111
L
111 14.1
13.6
10.1
0.58
0.58
0.46
‘
11.6 c6:7.4 7.4 C6:1.37
111
15.3
12.7
111
0.60
1.05
111
111
15.0
-
RT
41.4
7.84
7.99
6.49
11.5
111
15.0
sio2
RT
0.70
0.19
0.46
0.24
0.28
111
15.0
-
RT
32.2
3.4
3.5
2.5
2.2
15.0
sio2
RT
0.65
0.15
0.19
0.15
0.16
'pore filling factors (reolitesl or statistical monolayers (S102) if not stated othenrise. ++NOE factors in parenthesis (II):ratio of signals with and without NOE +++pse"do-liquid
111
15.0
2
4@CH3
111
3.6 0.61
111 111
91 The sole complete study reported on both T1 and 12 (transverse relaxation times derived from linewidth and from spin-echo measurements) variations with temperature concernsthesystem n-butanol I3C labeled at Cl adsorbed on NaY zeolites (ref. 27). For a pore filling of e=0.8 T2 values are reported (Fig. 5).
(4.4 molecules per large cavity), the T1 and T1 values were determined from pure exponen-
tial functions obtained in the inversion-recoveryT-T-;
pulse sequence. The
large (T1/Tq)min* 9 ratio obtained at the minimum of the longitudinal relaxation time can be explained by a model involving a distribution of correlation times (ref. 2,27).
(Because of nearly equal values for T1 and T2 at high temperature,
the model of exchange between two regions with different correlation times cannot account for the experimental results.)
Fig. 5. 13C-spin relaxation times of -CH OH of n-butanol as a function of temperature (e=0.8, V= 25.2 MHz (ref. 27)). ?, derived from linewidths (0) and from decay of Hahn's spin echo (+).
The
I3C-relaxation times can be explained in terms of
the dipolar coupling
between carbons and protons. A log normal distribution is supposed for the spectral density function: m
Jb,i
=
'TC
F(z) 1+
-co
dz ,
(“c’c)2
(21)
where ~~ is the correlation time and wc is the Larmor frequency of 13C-nucleus. The distribution function is given by: F(t) = ---$-exp
?I
(22)
92 with
z= In -, 'cm tion parameter.
where
T Cm
the mean correlation
is
time and 6 is the distribu-
If one replaces the spectral density function in the expressions laxation
of the re-
rates:
3J(wc)+6J(wH+wc)+J(wH-WC)
, (23)
4J(0)t3J(wC)+6J(oH)+6J(wH+OC)+J(WH-WCJ with n the number of protons
in a CH,-group,
culated.
between
The best agreement
tained for B= 3, corresponding to 2.10-10
s at 286 K.
be obtained
is essentially
to an interval
enhancements
than the dipole coupling
with adjacent
times cannot be attributed
jump lengths or to an anisotropic
with n-butanol
with surface molecules
times (ref. 27). 13 C NMR linewidths surements
The T1 measurements
were performed
of Brevard
stored
in the same memory,
Finally,
FID.
instrumental
The integrals errors
uniformly
The different are therefore
error on the Tl values being the intensity
is about 5%.
coverage
on the sample with curred because
is 0.5.
errors
with
T values is by ZOO- 300 Hz from this unique
etc.) being dis-
The NOE measurements
The variation
shift"
the systematic
in FT processing,
obtained
the average
standard
of In (S,- S) versus 'I (S,
relaxation
A two-exponential
condensation
on NaGeX were
frequency
are obtained
lo- 20%, while
to NOE mea-
(ref. 7,99,100).
being shifted
(T= 4s) and S the inten-
by a single exponential
e= 0.9 at 250 K.
of butene
adsorbed
the "repetitive
signals
of the line at complete
sity at time T) is characterized
molecules
of
pores
of correlation
times in addition
much more reliable,
through all data points. are reproducible
of the zeolite
and temperature
frequency
of cor-
only the heterogeneity
Each FID with different
the carrier
(drift, round-off
from gated decoupling
the surface
following
rather
motion with small
for this distribution
coverage
on the surface
mechanisms
The broad distribution
relaxation
et al. (ref. 116).
from one FID to the other.
tributed
motion.
are responsible
method
summed
protons.
sites and an uneven filling
of surface
for the n-butanol/NaY
of the molecule
to either a translational
and longitudinal
as functions
rate curves can
of other relaxation
of the 1-butene and trans-2-butene
determined
is ob-
times from 8.10q8 s
relaxation
(1.8- 1.2) observed
due to the reduced mobility to the occurrence
the interaction
of correlation
can be cal-
values
of Ref. 27.)
and cannot be attributed
relation
rate curves
and experimental
(Useful tables of simulated
from the authors
The small Overhauser system
theoretical
theoretical
’
behavior
behavior
In this case, however, on the surface
as long as
was clearly
demonstrated
a phase change oc-
(ref. 117).
93 Fig. 6 shows adsorbed ature
on NaGeX zeolite
(250 K) results
linewidth between uith
the variation
(Table 6).
shows a greater
ents determined difference torr-l)
intensity
interaction
in mobility
with
of AH with
temperature
for trans-2-
The adsorption
and kinetic
data support
times Tl of carbons
suggests
coeffici-
entirely
this
8.2. 10W2
in 1-butene
for carbons
This comparison
resemblance
of these carbons
5. 10m2 tort--l; trans-2-butene:
(1-butene:
(Table 5).
interaction
temper-
In the
molecules.
there exists a greater
order C4> C3> C2> Cl at 300 K; they are similar C4 at 250 K (e=0.5)
for 1-butene
of e= 0.5; the decreasing
the surface.
isotherms
The relaxation
(ref. 17).
temperature
of the adsorbed
denote a greater
The variation
from adsorption
with
coverage
variations,
1, 2 and 3 which
the surface
butene
at a surface
in a lower mobility
and relative
carbons
of the linewidth
vary in the
Cl and C3,and C2 and
a n-complex
nature
of
-CH,cli2=
-UC+&
C NMR spectra
of 1-butene
as a function
of temperature
(e= 0.5; v=
::%36MH:;.
TABLE 6 13c NMR linewidth
(AH in Hz)of gaseous,liquid
Compound 1-butene
e
T°C
and adsorbed Cl
c2
1-and'trans-2-butenes c3
c4
Ref.
gas (p= 2.5 atm)
28
5
5
5
5
37
liq
20
0.5
0.5
0.5
0.5
103
23;0
0.9
25 -23 30 2 -23
33:
35 140 30
35 110 $55
90
;I:
45
Z 20 25 30
99,100 99,100 99,100 99,100 99,100
-
28
5
5
37
20
0.5
0.5
103
29 -15 -26 -38 -59
30 65 65 100 300
94:
99,100 99,100 99,100 99,100 99,100
NaGeX
trans-2-butene
0.5
gas (p= 2.0 atm) Iiq NaGeX
0.8
120 700 1300
94
the adsorbed species, between the 1-butene molecule and the surface at 300 K, where Cl is more influenced than C2 and this influence decreases in the carbon chain. At 250K, the different carbon atoms are closer to the surface and the carbon atoms 2 and 4 are relaxed at comparable rates, respectively. This low temperature species can be characterized as a cyclic model (Fig. 7).
In this
complex, the molecule is quasi-parallel to the surface and the two adjacent interacting sites could be an acid site (Na+) linked to Cl and a basic site (oxygen atom) interacting with one proton of C3. ,CH,-CH, H,CyCH
A
,CH, ,‘% CH HzC ,
&& &,
n - complex --
cyclic
complex
Fig. 7. s-complex (high temperature) and cyclic complex (low temperature) models of l-butene adsorbed on NaGeX zeolfte. At 8=0.9, the TI values of C2 show an approximate minimum at 275 K: 0.12 s. The TI/T; ratio is rather high (-10) and is characteristicof a distribution of correlation times (ref. 27,118). From this minimum, a mean correlation time ~~~ 10 ns can be calculated. The NOE factors are close to the value of two in all carbon atoms, except the methyl group at 250 K, where it is equal to 2.6.
The dipole-dipole carbon-
hydrogen interaction probably constitutes the main relaxation mechanism. The low NOE factors can again stem from the lower mobility of the adsorbed molecule. Nevertheless, a contribution from spin-rotation interactions cannot be excluded (ref. 36). The NOE factor is smaller at lower temperature for carbons Cl, C2, and C3, which are closer to surface than C4, which is always less influenced and rematns quasi-free. In the case of rather high NOE effects (close to the maximum value) for the methyl group, one could attribute the basic mechanism of relaxation to the internal rotation of the methyl group (ref. 119).
From the value of the correla-
tton time, one can easily compute the energy barrier of rotation (ref. 120). The energy barrier of rotatfon could be a useful parameter to characterize the fnteractlon between the admolecule and the surface (ref. 38). At 20.1 MHz and 300 K, the relaxatton times of the different carbon atoms of 1-butene are independent of surface coverage. Oppositely, they do depend on surface coverage at lower temperature (250 K), where a two-exponentialbehavior was observed for carbons 2, 3 and 4.
The low TI values at e= 0.9 are similar
to those determined at e=0.5 and characteristicof the adsorbed phase. The
95 higher T1 values show the formation as a pseudo-liquid
for which
of a new phase in the adsorbed
the interaction
with
the surface
layer such
is still signif-
icant (ref. 99,100). The comparison particularly
of the T1 values of gaseous, The T1 relaxation
interesting.
in the gaseous NaY zeolite
phase,
in solution
(ref. 38).
phase, where
relaxation
mechanism
times characterize
play a role and the total relaxation
magnitude
The corresponding
respectively.
1.6 and 0.81 s.
iate behavior
between
values
the gas
and spin-rotation
rate is about three orders of
in the adsorbed
The adsorbed
gaseous
phase on
59.9, 58.7 and 65.2 s for Cl, C2 and C3,
lower than in the gas phase;
are 0.81,
is
investigated
(NOE factor: n= 1) is very effective
In the liquid, both dipole-dipole
(ef. 36,38); 0.095 s for Cl. mechanisms
molecules
of CDC13 (2.6 M) and in the adsorbed
The shortest
the spin-rotation
liquid and adsorbed
times of propene were
molecules
and liquid,
state on NaY zeolite
show, therefore,
as is also suggested
an intermed-
by chemical
shift
data.
In addition to the dipole-dioole paramagnetic viously
centers
for the behavior
was nicely demonstrated
tem (ref. 36).
for the system
on NaY zeolite
propene
of relaxation
relaxation
Nevertheless,
be necessary
adsorbed
with
pre-
(ref. 11) and it
on NaY zeolite
(ref. 36). 3+ Fe
time was found with increasing
in order
NOE effects,
could also play a role in the propene-NaX
a more detailed to unveil
As the distribution
mechanism.
a NOE value smaller
VI. IDENTIFICATION
mechanism
ACTIVE
of the surface
in heterogeneous
in decreasing
one cannot by itself be a
(ref. 27).
IN PRESENCE OF ADSORBED MOLECULES
SITES
Although
unknown,
by a combined
sys-
study
of this relaxation
times also results
active sites
catalysis.
sites in most cases remains can easily be studied
the participation
than the maximum
OF THE SURFACE
The identification
temperature-dependence
of correlation
proof in favor of the spin-rotation
problems
adsorbed
the interaction
It has been suggested
of the zeolite.
The spin-rotation
would
mechanisms,
importance.
of 1-butene
For this latter, a decrease content
relaxation
is of paramount
is one of the most exciting
the exact nature of the active
the Broensted multinuclear
acid sites of the surface
NMR technique
(ref. 42,114,
121- 123). The behavior ferently
of pyridine
treated Y zeolites
The l3C NMR spectra The chemical
molecules
of pyridine
shift difference
upon protonation.
It was,
amount of the pyridinium
and of pyridinium
is systematically and pyridinium
between
therefore,
ions.
studied
ions adsorbed
on dif-
by 13C, 15N and 'H NMR.
ions are reported
in Fig. 8.
carbon 2 and 3 shows a great variation chosen for the computation
Following
a fast equilibrium
of the relative
between
pyridine
I
-
PYRIDINE
PYRIDINIUM
Fig. 8. I3C NMR spectra of pyridine and pyridinium ions. Pyridfne: A) pure liquid; B) adsorbed on Nay; C) adsorbed on USEx (both 3 molecules per large cavity). Pyridinium ions: A) in H SO4 (1:3 molar ratio); B) coadsorption of pyridine and HCl on NaY (3:6, numi5 er of pyridine:HCl); C) similar to B on USEx (adapted from Ref. 114). and pyridinium ion, it can be shown (ref. 114):
I62-b3fobs =
(I- Ppy+) 25.9+ 13.3 ppyt
(24)
,
where 25.9 and 13.3 ppm stand for the difference in chemical shifts for the pyridine and pyridinium ion. It has to be emphasized that the chemical shift difference is the same whatever system was used; it seems, therefore, that thfs technique is of general applicabflity for the study of the acidity of solid surfaces (ref. 113). The relevant data on.dlfferent decatfonized samples are collected in Table 7 which shows that less than one-half of the total number of OH groups give rise to the formation of pyridinium ions.
From the total number of pyridine molecules
large cavity
amount of pyridfnium
(n), the relative
number of OH groups (nOHI as detained
by wide
per
fons (p,y+) and the total
line 'H NMR measu~n~,
the
relative amount of acidic OH groups is easily computed. These data are in good agreement
with the number of OH groups
determined
by IR measurements (ref. 124).
The use of l5N-labeled pyridine Is even more interesting shifts depend
very strongly
on the nature of the adsorption
tions are much larger than in the case of analogous
42).
because sites;
the chemical their varia-
13 C NMR measure~nts (ref.
91 TABLE 7 Computation of the number of acidic measurements (ref. 42,114) n+
Support 70 OeNaYtttttt 85 OeNaY 85
(670)
(670)
DeNaY (570)
88 HY
'number
of pyridine
ttnumber
ttttnumber
tttttt70%
decationized
9.
&15N ohs
where
=
shifts
, tttt
"OH
3-4
0.50
1.5
0.70
2.1
3.0
6-7
0.60
1.8
2.1
0.912++ttt
3.0
0.847ttt+t
4.6
0.650ttttt
per large cavity (from wide
line 'H NMR)
ions per large cavity
zeolite pretreated
of pyridine
at 670 K for 20 h
and pyridinium
ions are reported
versus CH3N02 are shown in Table 8.
the observed
(I-
-it:
from both 13C and l5N NMR measurements
l5N NMR spectra
Chemical
region,
pPy+
5-6
of acidic OH groups values
t-t
3.0
per large cavity
of pyridinium
tttttsame
Typical
"OH
shift
3.0
molecules
of OH groups
tttfraction
sites from 13C and 15N NMR chemical
PPyt)(-90)
chemical
in Fig.
In the fast exchange
shift "ohs is given by:
+ ppyt(-180)
,
(25)
-90 ppm and -180 ppm are the chemical shifts of pyridine and pyridinium, 15 The agreement between the N and l3C NMR measurements is very
respectively.
good (Table 7), showing Broensted
acidity
clearly
the potentiality
of the method
to study the
of the solid surfaces.
C,H,NH*in solution
p -50
-100
-150 -
6 (mm)
Fig. 9. l5N NMR spectra of pyridine molecules adsorbed on Nay: A) 2.4 per large cavity (300 K); B) 5.6 (300 K); C) 5.6 (380 K) and on 88 DeNaY zeolite; D) 4.6 (380 K) (chemical shifts in ppm referred to CH3N02) (adapted from Ref. 42).
98 The complementary interesting
use of ammonia
observations.
on Nay, NaX, NaA, Na-mordenite phase value
value
different
organic
and US-Ex zeolites
the chemical
(ref. 42,68)
shift of which
(Table 8).
strong interaction
the protonated
15N NMR study of
is thus open to deter-
of the surface acidic
with Lewis acid sites was also evidenced
sites.
zeolites
(88 DeNaY) the CH3CN molecules
with the structural
OH groups via hydrogen
higher fields with respect
the values measured
molecules
adsorbed
for the pure sodium forms.
zeolites,
chemical
on silica-gel
In contrast,
shifts to lower fields appear
be used in the characterization
of Lewis acid sites created
of adsorbed
pyridine
the longitudinal
coverage
at 313 K, while coverage
l-
lo-SC 1
I
I
’
0 2
,
’
strongly
At low surface
and T2 increases
(4 molecules
during
These
results
I
1
’ 3
’
,
I
’
’
4 l/T
~10~
I
’
5
de-
C NMR results
The behavior
on the number
with increasing
temperature.
of
of adsorbed per large At higher
the TI curve shows a minimum higher than the low surface
have been interpreted
two regions of different
13
(2 molecules
per large cavity),
and to
the stabili-
coverage
the T2 values are systematically
values.
between
ions (ref. 121,122,125).
surfaces
for stabilized
quite well the 15N and
time TI depends
(Fig. 10).
TI decreases
surface
change
and pyridinium
relaxation
pyridine molecules cavity),
data complement
order of magni-
(Table 8) which can
ation process. The 'H NMR relaxation
decat-
but interact
The shifts which are to
to the gas phase are of a comparable
tude to those for acetonitrile
cationated
are not protonated
bonds.
The
by a large 116 ppm
low field shift relative to the gaseous state. 15 N NMR chemical shifts of acetonitrile reveal that even in strongly ionated
anznon-
is close to that of aqueous
The way of combined
but also the strength
leads to
form of alrmonia
is close to that of the gas
bases with a large range of pKA values
mine not only the number
acidity
shift of the adsorbed
In the case of 88 OeNaY zeolite,
(see above).
ium form was detected, solution
for the study of surface
The chemical
in terms of a rapid ex-
thermal mobility
which are attributed
I
’
-
6
(K-l)
Fig. 10. IH NMR relaxation times for pyridine adsorbed on 85 DeNaY zeolites as function of temperature: A) 4 molecules; and B) 2 molecules per large cavity (adapted from Ref. 121).
99
TABLE 8 15N NMR chemical shifts of adsorbed molecules (ref. 42,6B) Support
Compound
et
NH3 gas
ToC $H3N02 27
(ppm)
-399.9 -382.0
Tig NH; Cl- (1 M in 10 M HCl)
-349.92
-
NH3
NaY
0.1
-394.9
NaX
0.5
-388.9
NaA
0.8
-383.9
USEx
0.1
-383.9 -383.9 -383.9
CH3CzN
88 DeNaY
20.5 mol/cavity 9.0 mol/cavity 1.9 mol/cavity
-368.9 -361.9 -360.9
70 HY
1 mol/cavity ID mol/cavity
-293.9 -292.9
88 DeNaY
1 mollcavity 5 rollcavity 10 mollcavity
-160 -152 -147
USEx
1 mol/cavity 5 mol/cavity 10 mol/cavity
-148 -139 -137
NaY
1 5 10
27 ;:
-156 -156 -148
;:
-164 -154 -148
(BO)LiNaX
2
22
- 158.6
(BD)Li KX
2
22
-153.4
(71)Rb NaX
2
22
-149.1
(5D)Cs NaX
2
22
-148.7
(20)Ag NaX
2
22
-172.4
(60)Ag NaX
2
22
-183.4
NaX
k 10
27
CH3-CeN gas
-126.5
Tie
-136.4 -239
CH3-CeNH+ CH3-CeN
70 DeNaY (stabilized) sio2
@
27
-109 - 146 -144
:::
-57.6
gas RT
liq sol (4.3 mol% in H,O)
_
-63.9 -179.6
NaY
2.4 mol/cavity 5.6
107 107
-90.7 -89.2
sio2
0.08 0.16 0.79 1.35
107 107 107
-89.3 -89.9 -85.7 -70.9
107
'pore filling factors (zeolites) or statistical monolayers (Si02) if not stated otherwise
100 to adsorbed
pyridine
molecules
time for the thermal motion magnitude
and pyridinium
of the pyridine
as the values for adsorbed
The correlation nificantly
time for adsorbed
benzene
pyridinium
larger than that for adsorbed
by one or two orders of magnitude
protons
(10S5 s).
These apparent
is of the same order of -8 10 s at 313 K.
ions (5. 10S7 s at 313 K) is sig-
dinium
to another
pyridinium
have been explained
above room temperature.
framework.
the proton of the hydroxyl At temperatures
cules are bound to the solid surface The total amount of OH groups
values. 2rigid the minimum of the
OH groups oxygen
These
OH groups
bond. from the
1 H NMR signal
(n,) is computed
to transverse
from the T2min
relaxation
times at
curve and in the region of rigid lattice is used as a relative
nA/T
group to another
if T is the mean residence
measure
time of a hydroxyl
behavior,
for the acidity
of
proton at a lattice
(ref. 122).
Quite recently,
a very valuable method of hydrogen
It was also possible mean size.
on platinum
to determine
In the absence
was explained
using 12'Xe NMR was proposed particles
of chemisorbed
oxygen,
6Pt and "NaY stem from the collisions
probabilities.
NaY sites occurs.
Because
from the chemical
these components neighbors,
is very important
shift values
particles
between
the Pt and of the 12'Xe
and precise data can be
two different
to Xe atoms striking
with chemisorbed
hydrogen.
proves that the Xe atoms exchange
but they do not diffuce
par-
(ref. 127).
of a small amount of hydrogen,
The new NMR line is attributed
and platinum
shift
vPt and vNaY are the correspond-
of the large value of rSpt, the variation
obtained
zeolite
chemical
of Xe atoms with the platinum
In all cases, a fast exchange
shift with Xe concentration
After chemisorption
the observed
and their
at low xenon coverage:
respectively;
chemical
observed.
particles
to probe
(ref. 126).
’
ticle and with the zeolitic wall, ing collision
in NaY zeolites
the number of platinum
as the sum of two contributions
obs = VPt bPt ' "Nay 'Nay
where
via a hydrogen
latter correspond
log T2=f
The ratio
the chemisorption
6
molecule
below 295 K, pyridine mole-
is easily determined
the number of acidic
and T
respectively.
as a pyridine
After
group and then being again converted into a pyri-5 after an average time of ca. 10 s, a jump of the
ion occurs which carries
while
by the fol-
hydroxyl
of the zeolitic
intensity,
time of hydroxyl
comes into contact with a hydroxyl
ion is formed at temperatures
ion. Less often,
oxygen
They are, however,
than the correlation
the mean lifetine of 5. 10S7 s at 313 K, it is desorbed hopping
The correlation
or cyclohexadiene,
discrepancies
When a pyridine molecule
group, a pyridinium
molecules
pyridine molecules.
shorter
lowing model:
ions (ref. 121).
lines are
the wall of the
The existence
of
only with their nearest
rapidly across several
supercages.
The
101 former
line decreases,
hydrogen
concentration
this proportionality that chemisorption At higher
size.
while
to Pt particles
the new line increases
up to 2 hydrogen
of hydrogen
conclusion
concerns
corresponds
Based on
on all particles
of similar
which is attributed
atoms.
the size of platinum
particles
The mean number of Pt atoms per particle
this indirect method. type of particle
particle.
a third NMR line appears,
hydrogen
with increasing
narrow NMR lines, it was concluded
occurs homogeneously
content
with 4 adsorbed
An important
atoms per platinum
and on the relatively
hydrogen
in intensity
to a solid developed
mainly
obtained
is ca. 8.
in two dimensions
by This (ref.
128).
VII.
STUDY OF THE CATALYTIC
The heterogeneous desorbed
reactants
situ the catalytic ferentiating behavior
TRANSFORMATIONS
catalytic
reactions are usually followed by analyzing the 13 C NMR spectroscopy enables one to study -in and products. transformations.
between
different
of the adsorbed
state
In addition,
it has the advantage
carbon atoms and of identifying
of dif-
the specific
(ref. 100).
Table 9 shows the different is thus proven
unambiguously
systems investigated in the adsorbed phase. It 13 C NMR can be used to provide quantitative that
TABLE 9 Reactions
studied
by 13C NMR in the adsorbed
state
support
reaction CH3-CH2-W=CH2
Isomerlzation
B
NaCaY (67% Ca)
0.7
A’203
0.x-
1
NaX
CH3-CH=CH-CH3
CH3-CH2-CHO CH3-CO-CH3
ds
Olfgomerlzation Aldol condensarion
131 35
300
17
2
24
77
NaCaY (67% Ca)
0.7
107
16
SNSbO
1.3- 2
24
132
NaGeX
0.8
NaGeX *'2'3
0.25-
300 32
18 17
1.3- 2
24
77
sio2
4x 10-6 nml " -2
zo- 120
108
A1203
4.4
22- 322
133
250- 300
134
150- 350
100.135
350
136
H-2%5
CH30H+
H-ZW5
CH2=CH2
Dehydration-polymerization
H-ZSH-5
CH30H Poly~rization-cracking Polynerizatlon-cracking
CH30H Pyrolysis
1.3-
SnSbO
CH30H Oehydration-polymerization
CH2'CH2+
18
25
0.8-
NaGeX
CH3-CH20H
32
1.4
HY
CH30H Dehydration
CO Dehydration-polymerization
Ref.
15,16,49,129,130
160- 165
SnSbO Polyl!wiratlon
T'C 37- 60
1
0.08 ml/g
0.08 ml/g
150- 350
100,135
H-ZSU-5
25- 350
83,100,137
H-ZSU-5
zo- MO
87,138
NaX
420
158
400
159
csx Parafolmaldehyde
Pyrolysis
NIX csx CSBX
CH3-CH-CH2 CH2=F-CH3 CH3
Polynwlzatlon Dimriration
H-ZSM-6 NaCaY
27- 327
138 130
102 data on catalytic
reaction
a suitable
for the determination
state.
method
kinetics.
In such conditions,
state must differ
It is also demonstrated of equilibrium
the observed
equilibrium
from the corresponding
ences are explained
constants
constants
in the adsorbed
in the
value in the gas phase.
by the nature of the surface
values of the adsorption
that 13C NMR is
constants
complex
for both the reactants
adsorbed
These differ-
and the respective and the products
(ref.
17,77,139). The most thoroughly
studied
reaction
NaCaY (67%Ca) zeolite (ref. 15,16,49), and mixed
tin-antimony
oxides
function
time intervals.
From the variation
constants,
are strictly
logarithmic
(ref. 171, alumina
adsorbed
kc _t , can be determined.
kl_*,and
The apparent
as a
kinetic
by the good linearity
rate
of the
plots.
The kl_2 constant are equal,
simultaneously. 1-butene
of
on NaGeX
the geomet-
Fig. 11. Double bond shift of l-butene and geometric i r~$$n,&$;; trans-2-butenes adsorbed on NaGeX zeolite followed by (3OO'C). The spectra are similar as in previous work (ref. 16) where 13C-enriched butenes were adsorbed.
E-butenes
on
(ref. 18)
of the line intensities
shift kinetic constants,
first order as evidenced
of 1-butene
Fig. 11 shows the evolution
and cis- and trans-2-butenes
of time, the double-bond
ric isomerization constants
NaGeX zeolite
(ref. 77) (SnSbO).
the l3C NMR spectra of 1-butene at various
is the isomerization
also
for the disappearance
of 1-butene and the formation
showing
that no side reaction
Similar
unambiguously
conclusions
on SnSbO catalysts.
were reached
A comnon
feature
of
takes place
for the isomerization
of
of both NaGeX and SnSbO catalysts
103 concerns higher
the cis/trans equilibriumconstants
than the corresponding
greater isomer
tendency
cis/trans
(ref. 82). various
SnSbO catalysts
intervene
Moreover,
the difference
the two types of catalyst of an anionic
of kl_2 constants
also supports
in the appar-
is the high initial
transition
with the surface
the hypothesis
the 'TI-and cyclic-complex
as real intermediates
isomerizations.
to the trans
state because
is known to be more stable than its trans conformation
The variation
ition state.
account
between
ratio which is characteristic anion
stems from the
with respect
is ca. 0.5 kcal mol -' (ref. 140).
similarity
the cis-n-ally1
state which are
This difference
of the cis-Z-butene
Indeed, on most catalysts
ent heats of adsorption The second
gas phase value.
for adsorption
(ref. 17,77,100).
of the adsorbed
Finally,
the nature of the adsorbed
mechanismcould
of the
of an anionic-type
detected
in both the double-bond
a detailed
basicity
trans-
on the surface
can
shift and the geometric
be proposed
which takes into
species and that of the transition
states
(ref. 77,100). In the methanol-to-hydrocarbon zeolite,
the role of methanol
importance
of carbenium
polymerization
process
tion, it was concluded
conversion
on the shape selective
as an autocatalytic
ions was assessed (ref. 135).
on the H-ZSM-5
zeolites
VIII.
HIGH RESOLUTION MAGIC-ANGLE
agent was established;
as intermediates
and methanol
the
in the dehydration-
From the ethylene-methanol
that ethylene
dently
H-ZSM-5
conjunct
are reacting
reac-
quite indepen-
(ref. 100,83).
.. High resolution to investigate
allowed
surface
side the zeolitic in ZSM-5 zeolite
channels
species
Large molecules
opens new ways of studying
and strongly
Tetrapropylamnonium
the low power
13
proton enhancement,
intersections in ZSM-5 zeo-
information
incor-
growth mechanisms
C NMR with the high power MAS technique valuable
ions
The progressive
and crystal
in-
are shown to be
the channel
of ZSM-11 zeolite.
the nucleation
which are
can also be trapped
ions in ZSM-11 zeolite They occupy
motion
studied.
molecules
in the linear and zig-zag channels channels
(ref. 146,158,159). and without
systems
as a result of pore plugging
frameworks.
lite or in the perpendicular
in which molecular
hydrocarbon
during crystallization.
and their alkyl chains extend
By combining
betweeen
(ref. 141).
framework
species
10 shows the different
and tetrabutylamnonium
in their respective
poration
adsorbed
Table
a distinction
inside the zeolite
chemisorbed
intact
of strongly
(ref. 100).
This technique trapped
magic-angle
the nature
is highly reduced
SPINNING "C NMR OF STRONGLY ADSORBED MOLECULES 13 spinning C NMR appears as a superior technique
can be obtained
with
on the
behavior
of adsorbed molecules (ref. 62,160,161). First applications have alSO 15 N NMR MAS measurements (pyridine on v-alumina, Ref. 162). been reported for It is expected
that magic-angle
a strong reduction
of adsorbate
spinning
at low temperatures
mobility
occurs,
(ref. 1631, where
will open new ways for a study
104 TABLE 10 High resolution proton enhanced magic angle spinning 13C NMR spectra of adsorbates Compound A) With MAS Small molecules entrapped on coking
Support H-ZSM-5
T°C 23
Ref. 141
H-Mordenite (H-MDR) K-ZSM-5
142
Cs-ZSM-5 K-MOR Cs-MOR Polyethylenes
H-ZSM-5
23
141,143,144
H-MOR Polypropylenes
H-ZSM-5
Polyisobutylene
H-ZSM-5
Tetrapropylamnonium
ZSM-5
23
146.147
Tetrabutylamnonium
ZSM-11
23
146,148
Tetrabutylphosphonium
ZSM-11
23
146
Vinylmethyldichlorosilane
asbestos
149
Ethylbenzene
NaX
61
Toluene
csx
144,145 144
CsBX Formic Acid
NH4Y
150
HYUS
co2
Na-MOR
107
Acetoacetamide
SiO2
151
Organosilanes
SiO2
147.152
Aminosilanes
SiO2
153
CH3OH
MgO
154
Chrysotile
155
B) Without MAS Organic derivatives
Asbestos
coz
H-MOR
0.2-l
107
NaY NaA
Benzene
Charcoal
28
SiO2 co,co2,CS2 and OCS
Na-MOR
64
H-MOR K-MOR Cs-MOR NH4-MOR co
Rd/A1203
156,157
105 of adsorbed developed
molecules.
here because
The high resolution other articles
MAS 13C NMR (Table 10A) will not be
are devoted
to this important
subject
(ref.
164,365).
IX 1 2 3 4
65 7
8 9 10
16 17 :: 20 21
REFERENCES K.J. Packer, Progress in NMR Spectroscopy, J.M. Erisley, J. Feeney and L.H. Sutcliffe, eds., Vol. 3, Pergamon, London, p. 87, 1967. H. Pfeifer, in: NMR-Basic Principles and Progress, P. Diehl, E. Fluck and R. Kosfeld, eds., Vol. 7, Springer, Berlin, 1972, p. 53. H.A. Resing, Adv. Mol. Relaxation Processes, 1(1967/68)109. E.G. Derouane, J. Fraissard, J.J. Fripiat and W.E.E. Stone, Catal. Revs., 7(1972)121. H. Pfeifer, Physics Reports (Section C), 26(1976)293. J. Tabony, Progress in NMR Spectroscopy, 14(1980)1. J.B. Nagy, M. Guelton and E.G. Derouane, in: Magnetic Resonance in Colloid and Interface Science, J.P. Fraissard and H.A. Resing, eds., D. Reidel, Dordrecht, p. 583, 1980. H. Pfeifer, W. Meiler and D. Deininger, Annual Reports on NMR Spectroscopy, X(1983)291. J.J. Fripiat, J. Phys. (Paris), 38(1977)44. W. Derbyshire, Specialist Periodical Report on NMR, Vol. 9, G.A. Webb, ed., The Chemical Society, London, p. 256, 1981. D. Michel, Surf. Sci., 42(1974)453. D. Michel, Z. Phys. Chemie (Leipzig), 252(1973)263. 0. Michel,W. Meiler and D. Hoppach, Z. Phys. Chemie (Leipzig), 255(1974)509. T.M. Duncan and R.W. Vaughan, J. Catal., 67(1981)49. D. Michel, W. Meiler, H. Pfeifer and H.J. Rauscher, Proceedings of the Symposium on Zeolites, Acta Physica et Chemica, Nova Series, Szeged,
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1507. 22
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:z
76
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PART II - STRUCTURAL X.
INVESTIGATION
OF ZEOLITES
BY 2gSi NMR SPECTROSCOPY
INTRODUCTION The first application
spinning
(MAS-NMR)
made by Lippmaa,
to the investigation
Engelhardt
2), they showed clearly for 2g Si MAS-NMR responding
of high-resolution
of various
to the different
importantly,
they showed
structure
that the isotropic chemical
it through
and aluminosilicates
and well resolved
Si environments
shifts could
peaks could be observed
in the silicate
paper
bridging
framework.
and used for investigations
in terms of the short-range Si, Al order. 29 shift of Si is strongly dependent (notably upon the number of neighboring
oxygen
other aluminosilicates.
atoms) has been used in numerous
Some review articles
reveals
numerous
group of catalysts
XI.
EXPERIMENTAL
A. Condition
properties
new insights
further
papers for
of zeolites
were published
into the structure
upon its local
and
(ref. 5-7). solid-state
2gSi
of these important
and sorbents.
PROBLEMS
AND GENERAL
FEATURES
OF THE 2gSi NMR SPECTRA
of measurements
To measure
2g Si NMR spectra
usual high-resolution Gibby and Waugh usually
The observation
Al atoms bound to
It is the aim of this paper to explain how high-resolution MAS-NMR
corMost
(ref. 4) that characteristic
be assigned
of bulk, surface and chemical
was
paper (ref.
and aluminosilicates,
chemical
environments
an investigation
NMR with magic-angle
In a pioneering
solid silicates
in a subsequent
ranges of these isotropic of zeolite
of silicates
et al. (ref. l-3).
that distinct
spectra
solid-state
in solid silicates
NMR techniques
(ref. 8) are combined
and aluminosilicates,
of rare nuclei as developed with magic-angle
there are no 'H nuclei covalently
spinning
bonded to the framework
the
by Pines, (MAS).
Since
of these
110 inorganic
solids as zeolites
(except some OH groups within
the zeolitic
cavi-
ties or on the surface of amorphous materials), a high-power decoupling (of 29 Si-lH dipolar interactions) is rendered unnecessary and cross-polarization can often not be used.
This reduces
the experiment
the sample which can easily be carried NMR spectrometer tageous
modified
for MAS technique.
to apply the CP-technique
in lattice defect the zeolite performing
It is, however,
to investigate
sites at the surface
Furthermore,
high-resolution sometimes
the presence
or in amorphous
(see Part XII-C).
structure
to one of simple MAS of
out using a conventional
advan-
of SiOH groups
materials
occluded
in
there are advantages
in
the experiment
at as high a magnetic field strength as possible. For 29 nuclei with a spin one half, such as Si (I = l/2), there is an essential gain in resolution superior
due to a greater
field homogeneity
To obtain correct
spread of resonance
and stability
signal intensities
times of 5 ...30 s were applied. 29 Si chemical
negative
sign for high-field
6. General
features
of
29
Si chemical
shifts of silicates
the total range of "Si
-60 to -120 ppm) with an analytically istic ranges for monosilicates
solid silicates
cross-link;:
framework
set of shift data is discussed single to double
tetrahedra,
the 2g Si nucleus.
parallels
tetrahedra.
such as electrostatic
bond strength
found to cause significant operative
changes
atoms in zeolites
carry the designation fy the nomenclature,
In both cases there
to the number of neigh-
distortion
shifts.
nomenclature notation
in
have been
These factors
thus leading to overlapping
in anion condensation
factors,
of the bond angles
are
of the
(ref. 9).
belong to a three-dimensional
Q4 in the silicate
of
in the liquid-
in the SiOSi bond angles
we use in the following
and finally
shielding
to this, other structural
in the chemical
in solid silicates,
shift ranges caused by differences The silicon
of cations,
from the
structures
diamagnetic
(ref. 10).
In addition
and changes
condensation
the same phenomenon
solutions
study of
in Ref. 2 and an extended
layered
useful ranges according
boring silicon-oxygen
tetrahedra
Increasing
(PI).
(Q3) and the
The first thorough
leads to increasing
This behavior
the silicon-oxygen
(Q4).
sites
(from
into character-
and chain end groups
to chains and cyclic
frameworks
and analytically
subdivision
was reported
in Ref. 9.
state 2gSi NMR spectra of silicate
especially
200 using
and aluminosilicates
shifts is appreciable
(Q2), chain branching
Si MAS-NMR
by means of
to three-dimensional
chemical
significant
(Q'), disilicates
in chains and cycles
three-dimensional
are distinct
vary between
to tetramethylsilane
^^
In silicates
groups
repetition
shifts.
(zeolites)
middle
rates normally
shifts are related
with the
solenoid magnets.
in the single pulse experiments,
Accumulation
and 10,000 scans.
lines in connection
of superconducting
framework
as mentioned. Si(nA1)
and
To simpli-
instead of Q4(nAl)
111 with n=o zeolite
to 4 for the five possible
in Table
shifts of a series of natural
1 and of synthetic
distinctly
of Si atoms.
zeolites
on the number
in Tables
From the data in Tables vious that quite
Si(nA1)
fects on the central
shifts for the Si(oA1)
ively
properties
observed
for distinct
the framework
AL Cl AlO&OAl 0 Al
Sil3Al)
Si(LAI1
5112 Al
1
[F
1
-a5
-90
-95
29
structural
shifts for a effect:
(ref. 2) to -116.6 ppm for sili-
induced
shift of about 9 ppm is exclus-
arrangements Similar
of the Si04 tetrahedra shift differences
have been
crystallographic
sites of
Al 0 SiOSiOSl 0 SI
Si 0 SiO&OSl 0 SI
Si(lAl)
S~ioAll
SillAll
r -80
of the shift ef-
(ref. 4,13,14,15).
Al 0 AiOsOSl 0 SI
0 Al
Additional
units in nonequivalent
zeolites
Al 0 AlOgOSi
shifts ap-
of the TO, tetrahedra. The T groups of aluminum-free Si02 polymorphs range,
silica framework.
Si(nA1)
of various
to sili-
in Fig. 1, it is ob-
that independently
by the following
Hence, a structure
the corresponding
bridges
arrangements
due to different crystallographic
forming
sphere
shift values of
may cause additional
units.
from -107.4 ppm for low quartz
(ref. 12).
these shifts
ranges of chemical
This suggests
Si atom of the Si(nA1)
crystallographic
are given
and about -85 to -90 ppm for Si(4Al).
overlapping
units.
unit may be induced
1. Different
calite
via oxygen
1,2,4 and 5 which are summarized
of Al atoms, other structural
for instance,
Obviously,
Al atom connected
large and strongly
pear for the various
chemical
2-5.
shift of about 5 ppm is induced with typical
about -105 ppm to -110 ppm for Si(oA1)
given Si(nA1)
zeolite minerals
of Al atoms in the second coordination
For each tetrahedral
con, a low-field
number
units of the
framework.
The *' Si chemical
depend
(.S10)4_nSi(OAl)n building
- 100
1 SiloAL -105
- 110
-115
Si chemical shifts of Si(nA1) Fig. 1. Ranges of versus TMS, high-field shifts negative).
120
d
units in zeolites
(s-values
112 TABLE
1
*'Si Chemical
shifts of natural Typical
Zeolite
zeolite minerals
Unit-Cell
. 2NaCl
Sodalite
Na6(Si6A160p4)
Thomsonite
Na4Ca8(Si20A120080)
(in ppm versus TMS)
Si(4Al)
Composition
24H20
Si(3Al
Si(2Al)
Si(lA1)
Si(OA1)
(Si/Al)
Ref.
-85.4
1.0
4
-83.5
1.0
4
-89.9+
1.0
14
1.0
14
Gismondine
Ca4(Si8A18032)
lcH20
Cancrinite
Na6(Si6A16024).
CaC03. 2H20
-85.4
Davyne
Na6(Si6A16024).
2CaS04
-91.6++
1.0
14
Bicchulite
Ca2(SiA1206)(0H)2
-86.8
0.5
11
Natrolite
Na16(Si24A11608,,)
Scolecite
Ca8(Si24A116080).
Gmelinite
Na8(A18Si16048)
Chabazite
Ca2(Si8A14024).
Analcime
Na16(Si32A1160g6).
Leucite
K(Si2A106)
24H20 13H20
1650 -81.0
Laumontite
Ca4(Si16A18048)
Stilbite
Na2Ca4(Si26Al
Harmotone
Ba2(Si12A14032)
Heulandite
Ca4(Si28A18072)
Clinoptilolite
Na3K3(Si30A16072)
Mordenite
Na8(Si40A180q6).
'Weak signal
16H20 24H20
-87.7
-95.4
1.5+ttt
4
-86.3; -89.1
-95.8
1.5tttt
14
-92.0
-97.2
2.1tttt
4
2.6++++
4
-99.4
-104.8
-91.8
-96.5
-101.7
-85.2
-91.6
-97.4
16H20
1o072)
-102.5
-94.0
-110
2 ltttt -101.0
-92.2
28H20
12H20
. 24H20
14
2.0
14
-108
2.6
4
-98
:;;::;it+
-95
-98.6; -102.6
- 108
3
4
-95
-99.0 -105.3+++
- 108
3.5
4
-100.6
-106.qttt -112.8
5
4
-105.2
-112.!, -.
4.7tttt
14
. 24H20 -97
24H20
4,ll
1.9tttt
at -96.9 ppm
++Weak signal at -83.3 ppm +++Tentative tttt(Si/Al)
assignment ratio from 2gSi NMR
TABLE 2 2qSi Chemical
shifts of faujasite
Zeolite
(S;i;l)
type zeolites
(in ppm versus TMS)
(S;:Al)
Si(4Al)
Si(3Al)
Si(2Al)
Si(lA1)
Si(OA1)
Ref.
NaKX
1.0
-84.6
NaX
1.14 1.17 1.24 1.36 1.6
1.18 1.27 1.4 1.5
-83.9 -84.6 -84.4 -84.8 -85.3
-88.1 -89.0 -88.9 -89.3 -89.5
-93.1 -94.2 -93.3 -94.2 -94.9
-97.9 -98.8 -98.7 -99.1 -99.5
-102.4 -103.1 -102.6 -103.4 -103.6
26 18 18
2.02 2.45 2.82
2.0 2.5 2.85
-84.8 -83.8 -
-89.5 -89.2 -89.2
-94.0 -94.5 -95.0
-99.6 -100.0 -100.2
-105.8 -105.5 -105.6
18 18.43 18.
2.5
2.43
-
-89.8
-94.5
-101.1
-105.2
43+
4.9
5.0
-
-89.4
-95.9
-101.5
-106.7
18
.40
-
-101.5
-107.8
18
89.9
-95.4
-101.1
-105.4
43t
NaY
NaNH4Y
(0 to 78% NH4)
NaHY Y ultrastabilized NH4Y DE (550- 815'C) 'Mean values
2.4- 8.2
14
Further Work 20 21 23 6
25
25
113 TABLE "Si
3 Chemical
shifts of gallium
Sample+
faujasites Si/(Al+
(ref. 26)
Ga)
51(4Ga)
(WF;")
Si(3Ga)
Si(2Ga)
Si(lGa)
Si(OGa)
NaGaX (0.05: 0.95)
1.14
I.11
-77.7
-84.2
-90.3
-96.4
-102.8
NaGaX
(0.03: 0.97)
1.42
1.39
-78.0
-84.2
-90.9
-97.4
-104.2
NaGaV (0.02: 0.98)
2.17
2.06
-78.3
-84.6
-91.4
-98.2
-105.8
Ga-Sodalite
1.28
1.26
-76.0
-82.4
-89.0
-94.9
-101.3
(0.03: 0.97)
+Ratlo of A1203
to Ga203
in parentheses
TABLE 4 "51
Chemical
Zeolite
shifts of A-type
(U/Al) NMR
Si(4Al)
NaA
1
-ffl.O~l.O
ZK-4
1.13
-89.1
1.40 1.62
-89.1 -89.4
zeolites
(in ppm versus TMS)
Si(3Al)
Si(2Al)
Sl(lA1)
Si(oA1)
-93.9
-99.5
-93.9 -94.6
-99.5 -100.4
-106.1 -106.1 -106.3
-110.5 -110.7 -111.0
Reference
2,4,22,27,24
;: 28
TABLE 5 29
Si Chemlral
shifts of synthetic
zeo1ite NaK-F
(Edingtonite)+
NaK-Hydroxysodalite Losod NaK-Pl
zeolites
(Sl/Al),,,
(Gismondine)+
(except X,Y,A) Si(4Al)
(SilAllC,
Si(3Al)
Si(2Al) Si(lA1)
Si(oA1)
Reference
1.0
-85.8
14
1.0
-85.4
14
1.0
-88.9
1.9 2.5
-07.6
30 -91.9 -93.4
-97.3 -98.3
-102.4 -103.9
-107.0 -109.2
14 11
NaK-Chabarite
2.6
-93.7
-98.8
-104.4
-109.5
14.15
NaK-L (Cancrinite)+
2.6
-89.0
-96.7
-101.5
-107.4
14
NaK-Offretite
2.9
-93.5
-97.5
-101.9 -106.9
-112.5
14.15
-97.8
-102.5 -107.3
-112.3
14.15
-95 -100
-105.0 -105.5
-110 -112.1
15,31,32 33
NaK-Erionlte
3.2
Na-Mordenite
5.4 7.3?0.6
-94.5
5.5
ZSM-5
34;235
CP. -103++
:;;$
34
Ii-ZSM-5
25- 1000
ca. .103++ - 105
_113++++ _115+++++
35.31.15
Ii-ZSM-11
25 - 1000
ca. -103++ -105
ZSM-a
30
-105
ZSM-48
480
_l13++++++ _l15++++++
Sillcalite
-10,
8.2
Ferrierlte
_113++++ _115+++++ _I13++++++ _115++++++
H-ZSM-35
10.4 5
ZSM-39
54.3
++(s10),si0H;
cross-polarization
++++Sl
12 15.31
_lOg+++++++ _l15+++++++
31.37
in 5-membered
++++++Simllar
++++++++Pe~k
rings; further splittin&
-111.0.
at -114.0, -115.0.
-112.0,
-112.8.
-113.2,
116.3 ppm
IP for ZSH-5, ZSM-11 with further splittings
due to crystallographically
maxlmum;
at -1W.6,
rings; further splittings
line profiles
+++++++Splltting
spectrun
further splittings
+++++Si in 4-membered
further splittings
distinct
at -110.1,
T-sites;
-111.8.
asslgnnent
-115.9,
-116.7
atilguous PPR
37
-113
type in pwentheses
+++Peak maxima;
37
_113,6++++++++
-120 fIsomorphous
35
-114.0
mm
114 2. Effect of cations.
The effect of cations
Si(4Al) atoms is clearly
demonstrated
ratio of 1.35 and A-zeolites. about 6 ppm between
3. occurs
in synthetic
2).
This result
indicates
chemical
zeolites
rationalization
authors
character
TOT angles
related
unit depends
shift var-
(ref. 13,36,38,39). (set cr) of the
and a simple quantum-
has been given
linearly
directed
(ref. 39).
In
shift of the central
to silicon which in turn is This function,
in the interesting
as well
range of a between
140' and 160' and, therefore, an approximately linear correlation of a, 29 Si chemical shift is to be expected. This is confirmed
may be concluded, discussed silicon
from 12 silica polymorphs
therefore,
above are mainly
that the structural
caused
resonance
about 5 ppm.
It
shifts the
shift ranges, a direct assignment Si(nA1)
the 2g Si NMR spectrum
of zeolite
by
Na, K-P1 is
In this case, the signal at lowest field belongs to Si(4Al),
in the high-field
A similar consideration
direction
by Si(3Al),
may be applied
Si(2Al),
to spectra occurring
Then the latter signal can be unambiguously
attributed
For zeolites
of a
atom is straightforward
but the first one in the low field position
cations,
on chemical
of the bond angles around
lines occur in the spectrum which are separated
As an example,
shown in Fig. 2.
Si(nA1)
line to the corresponding
only if five well resolved
network
(ref. 39).
assignment
Because of the overlapping
followed
effects
for
atom under investigation.
C. Spectra
certain
by changes
and zeolites
and
P
set a with the
21 data points available
Si
on the s-hydridization
to the SiOSi bond angle u by cos a/cos(a-1).
as set C( shows a rather weak curvature
of remote Al
(ref. 38) and the mean SiOSi angles
(ref. 13) have been presented
of the oxygen bond orbital
P
Si/Al
(ref. 18,
that the chemical
shifts and the mean secants
of these correlations
structural
(ref.
lines
to Si/Al=60
the influence
the latter study, it is shown that the 2gSi chemical atom of a Si(OSi)4
of
unit.
of the average
of 2g Si chemical
SiOSi angles a of five silica polymorphs of six silica-rich
clearly
shifts for the Si(oA1)
with variations
Linear correlations
resonance
(NaX, Nay) with increasing
it has been stated by various
iations correlate
with a Si/Al
LiA and BaA zeolites
shift of the Si(oA1)
zeolites
shifts of
leads to shift differences
to 5.4 ppm on going from Si/Al =l.O
atoms on the resonance Recently,
A high-field
faujasite-type
ratio which amounts (see Table
in a series of X-zeolites
This influence
LiX and SrX (ref. 16) and between
Remote Al atoms.
19,20)
on the chemical
of isotypical
but with different
or very similar
Si/Al ratio
the spectra can be assigned
having
and Si(oA1).
less than 5 lines
at -84 ppm or below. to Si(4Al).
structure
(see, for example,
by considering
Si(lA1)
of their tetrahedral Fig. 3) or type of
systematic
changes
in the
115
Zeolite SilAl
PI = 1.9
n
S1(2AI)
r -
80
-90
- 110dlppml
- 100
Fig. 2. 2' Si NMR spectrum
84~
w
r+k
of synthetic
k..Pk.
_.._d
zeolite
Na, K-PI.
Lc,
80
-100
-120
-80
-100
-120
-80
-100
-120
-80
- 100
-120
-80
-100
-120
-80
-100
-120
Fig. 3. *' Si NMR spectra of X- and Y-zeolites with different Si/Al ratios. a) Na,K-X, Si/Al=l.O; b) NaX, Si/A1=1.4; c) Nay, Si/A1=2.5; d) NaHY, Si/Al= 3.9; e) NaHY, Si/Al=5.0; f) US-Ex (ultrastable dealuminated Y-zeolite), Si/Al=60. The numbers of the peaks designate the value of n of the corresponding Si(nA1) units. Scale numbers: 6 in ppm versus TMS.
116 resonance
shifts connected
concentration
with the total aluminum
The signal assignment
can be checked
by means of the intensities the value derived
XII.
content
or the type and
of cations.
ISi(nAl)
by comparing
the Si/Al ratio determined (see Part XII-A)
in the NMR spectrum
frcm other methods
like chemical
with
analysis.
APPLICATION
A. Silicon,
aluminum
Substitution
of silicon
tional paramagnetic considerable
ordering_ by a small cation
shift
line broadening
appears
the sharpness
of the resonance
is a measure
for the regularity
ing, however,
(like aluminum)
leads to an addi-
In some cases of aluminosilicates,
(see above).
It has been shown (ref. 2,4) that
also.
lines reflects
the degree of crystallinity
yield
spatially
and
It is worth emphasiz-
of Si, Al distribution.
that the NMR measurements
a
and temporally
averaged
Therefore, information which is related to short-range order interactions. 29 Si MAS-NMR in general do not by themselves provide direct information on zeolite crystal dination
symmetry
and framework
shell but valuable
silicate
lattice can be achieved
existing
knowledge
on framework
beyond
by combination topology
the different
of the NMR results with the
Si/Al ratio may be computed
29
aluminosilicate
using the equation
and allied
of the sample.
ISi(nAl) of the
Si(nA1) units in a tetrahedral
coor-
of the alumino-
from X-ray diffraction
From the intensities
Si/Al ratio.
the first tetrahedral
into the fine structure
as well as with the chemical composition
techniques
1.
structure
insights
Si NMR lines for framework,
the
(ref. 18):
4 (Si/Al)NMR
= f n=O
This formula Al-O-Al
linkages
Al ordering
1Si(nAl)'x0'25 n=C, follows
from a readily comprehensible
are present,
within
hood of every Al tetrahedral in a Si(nA1)
fraction
of aluminum
papers
then the imnediate
Consequently,
unit incorporates
Si/Al ratios provided
(ref. 16,19,20),
new quantitative
no Al-O-Al
linkages
The relative
for one type of struc-
is valid for all zeolites, a powerful
neighbor-
each Si-O-Al
0.25 Al atom.
is thus given by 0.25 nISi(nAl)
This equation
If no
consideration:
rule is obeyed for the Si,
framework,
site is Al(OSi)4.
structural
tural type and represents framework
i.e.,Loewenstein's
the aluminosilicate
linkage
tural unit.
"ISi(nA1)'
irrespective method
of struc-
of determining
are present.
In several
it has been found that the Si/Al ratios determined
by
chemical analysis and X-ray fluorescence measurements on the one hand and by 29 Si MAS-NMR on the other hand are in very good agreement, i.e., Loewenstein's rule clearly
holds.
117 2. Faujasite type zeolites
type zeolites.
The silicon-aluminum
(NaX, NaY) has been investigated
al. (ref. 18,21),
Klinowski
these investigators
ordering
independently
by Engelhardt
et al. (ref. 19,40) and Melchior
came to approximately
the same results
of faujasite et
et al. (ref. 22);
and conclusions.
In Ref. 18, a series of NaX and NaY zeolites with Si/Al ratios of 1.18 to 67 has been investigated. be assigned 2).
The spectra
unambiguously
be concluded
small line widths
that all specimens
Si, Al distribution.
a systematic
increase
number of aluminum
possess
Si(nA1)
a nearly regular
framework
for Si(nA1)
atoms appears which suggests
has been derived
building
units with those from theoretical
ordering
scheme developed
from interpretation
by Dempsey
of acidity
structures
from electrostatic
studies
of Si and Al atoms within
at 11.9 MHz Melchior
(ref. 22) and high-field
measurements
et al. noted that their interpretation
work
but is in good agreement (ref. 18,20,21)
Si/Al values. lational
They argued
periodicity" ordered
site crystals
flecting
is subject
a tendency
neighbors
within
culations
Vega
the restriction
the framework
rule"
reduction
such as "trans-
schemes
based on
sub-unit,
topology
Within
the sub-unit,
of the incidence
the single
in accordance
rule and an additional
fauja-
with
the Si:Al constraint
re-
of Al,Al second nearest
[ref. 411). results of Monte Carlo computer
Si:Al distributions
of‘loewenstein's
predictions
experi-
(ref. 41) for higher
ordering
in terms of an ordered
(ref. 42) has presented
of randomized
at the low values
of a criterion
and considered
to Loewenstein's
toward
("Dempsey's
Recently,
is different
It is shown that the Si, Al distribution'in
can be understood randomly
at 35.9 MHz (ref. 23).
calculations
that the application
Loewenstein's rule as the only constraint.
covering
a
a double-cubo-
with both the other published
and theoretical
is irrevelant
sub-units.
combined
distribution
spectra
and
NaX with Si/Al=1.4,
study of ordering
of Si/Al(2.0
oretical
and
in X and Y zeolites has been reported by 29 et al. on the basis of Si MAS-NMR spectra (low field measurements
Melchior
6-ring,
of the
unit has been proposed.
A similar
smaller
on
on the basis of an
(ref. 41) for Si/Al=1.18-2.0
centro-symmetrical
distribution
distribution
information
considerations
Except for zeolites
mental
with an Si/Al ratio,
in Ref. 18 by comparison model
(Table it may
groups with a small
a fairly uniform
More detailed
Si/Al= 2.43- 2.8, respectively.
octahedra
units
spectra,
As shown in Fig. 3, with increasing
of the signal intensities
ordering
structural
in the 2gSi MAS-NMR
of Si and Al atoms in the zeolite framework. silicon-aluminum
up to five signals which could
to the five different
From the relatively
ordered
exhibit
and Dempsey's
with the experimental
of more than' 50 directly
of faujasite-type
rules. A comparison
results
synthesized
zeolites
obtained
or dealuminated
Si/Al ratios of 1 up to about 70 (ref. 16,18,19,22,43)
cal-
under
of the the-
from the 2gSi NMR X and Y zeolites reveals
that
118 for Si/Al ratios in the range of about 1.5 to 4 a pronounced second nearest Al pairs exists; obeyed.
At SiJAl ratios
distribution firmed
however,
Dempsey's
tendency
rule is not completely
higher than 4, the Si.Al orderings
with the restriction
of Loewenstein's
approach
to hold true over the whole range of Si/Al ratios. 29 Si MAS-NMR spectra of gallium faujasites
hibit relative
containing
signal
intensities
of T atoms
zeolites
(T=Si,
as zeolites Ga, Al).
are deshielded
information
cal shifts are compatible
(ref. 13) ex-
NaX, Nay, indicating
However,
the signals
similar
in the gallium
by an amount nearly proportional
number of Ga atoms in the first neighbor little structural
shell
(Table 3). Although
for Ga substituted
with the general
a random
rule. The latter is con-
High resolution
distributions
to avoid
zeolites,
correlation
to the there is
the observed
chemi-
with T-O-T angle
(cf.
Part XI-B).
3. Zeolites Si/Al=l
of type A.
consist
The 2'Si MAS-NMR
spectra
of a single line with chemical
-89.7 ppm (Table 4).
This result was first observed
et al. (ref. 2,4) and confirmed
by Thomas,
previously
to Si(3Al)
NaY zeolites
because
bonds, sonance
in violation
tions.
in similar
of Loewenstein's
support
rule.
diffraction,
However,
point strongly
refinements
(ZK-4) with Si/Al>
units.
1 pointed
assignment.
the assignment
Then,
(Part XI-C).
has apparently
calcula-
of A-type
zeo-
of TlA (ref. 46-48)
in favor of the2;i(4Al)
five lines in its according
to the principles
shifts of ZK-4 reveal a rather
of the Si(4Al)
zeolites
rather than the
Si MAS-NMR
spectrum, given above
large structure
units and particularly
position
NaA, suggesting
that the single peak in NaA should be assigned
line like the single
(ref. 17) that the unusual
line in zeolite
found
by X-ray, electron
ilar high-field
Si(4Al)
Al-O-Al
of the single re-
et al. (ref. 28,29) on A-type
duced shift effect of the whole set of Si(nA1)
It has been suggested
in
involves
with perfect Si,Al ordering, consisting 29 Si MAS-NMR experiments by
is unambiguous
The resonance
for Si(3Al)
and theoretical
analysis
the
NaX and
independent
strongly
ZK-4 exhibits
of which
Si(3Al)
of zeolites
diffraction
in favor of a structure
of Si(4Al)
typical
necessarily
method
outside
like sodalite,
(ref. 44) of the X-ray structure
Thomas et al. (ref. 17) and Melchior
Si(3Al)
environment
trimethylsilylation
Engelhardt,
line had been assigned
This assignment
from investigations
lites (ref. 45) and recent neutron
exclusively
structures
Such an arrangement
line in NaA to the particular
strong independent and neutron
The single
but agreed well with the shift values
NaX, NaY and other zeolites.
by Lippmaa,
this shift value has been clearly
units observed
NaA with
in the range -88.3 to
Fyfe et al. (ref. 27), Melchior
et al. (ref. 22) and Nagy et al. (ref. 24).
range of Si(4Al)
of zeolites
shifts
NaA (ca. 5 ppm shielding
chemical
in-
a sim-
line observed
for
to Si(4Al).
shift for the
with respect
to zeolite
NaX)
119 is related
to the unusually
large average T-O-T angle of the zeolite A struc-
ture. Typical
single
ite,davyne,Na,
line spectra
appear also for sodalite,
K-X and Na, K-F (ref. 14) with Si/Al=l
in the case of NaA, the high-field attributed
to Si(4Al)
by comparing 5) which
units.
the spectrum
is isotypical
non-equivalent
tional
zeolites.
In the faujasite
(but not chemically)
signal splittings
lattice
structure
zeolites,
to different zeolite
families:
chabazite
ferrierite
5,6,14,15,35), clathrate
ZSM-11
group
attribution
(ref. 6,15),
Si,Al ordering
(ref. 15,35), ZSM-8
experimental
in zeolitic
Si/Al contents,
5. Some natural
2:l.
one Si(3Al)
of that mineral
scolecite
is isotypical
of the central distortion signal
of natural
The
agreement
Si sites in natrolite-(Table and scolecite
pentasil
region becomes
zeolites,
synthesis
dealumination
Si MAS-NMR
(ref. 6,
[ZSM-5 (ref.
(ref. 6,15) and
uncertain.
spectrum
except
results
intensity, in scolecite
1) (ref. 14).
(ref.
ratio of
structure
The aluminosilicate
with
(ref. 13).
of natrolite
line with the intensity
of the chain around
are shown in Fig. 4.
of zeolites
of the sample
with the framework
This distortion
Si sites are present
(ref. 14,15),
rmordenite
(ref. 15), ZSM-48
Typical belonging
In such cases, the unambiguous
with that of natrolite
into two lines of equal
distinct
29
(ref. 49).
Si tetrahedron
(ref. 50)).
zeolites
are developed to derive the actual 29 from Si MAS-NMR spectra: comparison
and one Si(2Al)
This is in complete
ordering
group
(ref.15)1,
the overlap
or progressive
zeolites.
is the
of both the
procedures
framework
with well known structures
4) contains
ZSM-35
feature
may overlap.
and synthetic
rerionite, offretite
[ZSM-39 (ref. 6,15,37)1.
various
important
units
sites, addi-
the shift ranges of the Si(nA1)
(ref.15)], mordenite
of an NMR line within
Therefore,
various
group
all the T sites
When Si(nA1)
distinct
frameworks
natural
with crystallog-
on the regularity
Therefore,
in various
L (ref. 14), ZSM-34
15,31-33),
Another
types of aluminosilicate
are encountered
Na, K-P1 (Table
Zeolites
equivalent.
of the line widths
and Si;Al order.
units of different examples
may be observed.
dependence
could be checked
zeolite
of the same type (equal n) occupy crystallographically
above-mentioned
As
should be
(ref. 14).
and synthetic
sites.
are crystallographically
and davyne
this assignment
with that of the synthetic
with gismondine
cancrin-
(Tables 1 and 5).
signals of gismondine
For gismondine,
4. Other zeolite minerals raphically
gismondine,
and Si;Al
framework
of
for a small rotation
the chain axis
in a splitting
(monoclinic
of the Si(3Al)
i.e., three crystallographically in contrast
to only two different
The 2gSi NMR spectra
of natrolite
120
Natroiite Si/Al=
Scolecite 1.5
SilAL- 1.5
Si (3Al)
0.
0
-60
-80
-60 -80 -lOOd(ppm~
-1OObbpm)
of natural natrolite and scolecite and skeletal dia(filled circles - Si; open circles -Al; oxygen atoms
The aluminosilicateframework of analcime consists of nearly parallel four rings containing SiO4 tetrahedra which are interconnectedthrough single Al tetrahedra (ref. 51). A regular lattice should be formed, therefore, from Si(2Al) units only. The spectrum (Fig. 5) shows the expected Si(PA1) signal but accompanied by two signals of lower intensity for Si(3Al) and Si(lAl), indicating the replacement of some aluminum atoms by silicons (ref. 4).
For
leucite, which has the same framework type and Si/Al ratio as analcime, the spectrum (Fig. 5) exhibits markedly broadened sl'gnalsof all five types of Si(nA1) (ref. 14) which confirms the highly disordered Si:Al distribution proposed already from X-ray diffraction-studies (ref. 77). Leucite
Analcime S~lpii:
SI /Al = 19
20
ICI)
lb1
-80
-1OOdippml
-80
-100 dtppml
Fig. 5. 2gSi NMR spectra of natural analcime and leucite
121 Like analcime, four-membered
the structure
Si-rings
linked together
should exist only Si(2Al) ments
of laumontite
units.
through Al tetrahedra;
The narrow
(half line width of 75 Hz) reveals
laumontite
sample
6. Chabazites. six rings
investigated
prisms)
linked by tilted four rings (see Fig. 6) is consistent
a perfect
arranged
atoms are arranged
with the proposed and a synthetic has been found
i.e., there to Si(2Al)
Si:Al ordering
framework ofchabazite
arrange-
of the
(ref. 53).
The 2gSi MAS-NMR
with an ordering
in para-position.
Si,Al ordering
scheme where
diagram
is given
atoms are
These aluminum
prism contains chabazite
For natural
the same highly ordered
and
(ref. 4,54)
two aluminum
of a typical
in Fig. 6.
ABCABC...
spectrum
prisms of a sequence.
The third hexagonal
The skeletal
Na, K-chabazite,
is formed by double
in layers in the sequence
placed in each six ring of two hexagonal
one Al atom per six ring.
line belonging
of
(Table 1) (ref. 14).
The aluminosilicate
(hexagonal
(ref. 52) is also composed
only
cavity
chabazite
Si:Al distribution
(ref. 54).
Chabazi te %/AL=26
3. 10~11: 2 -80
-100
-120 d(Ppml
29 Fig. 6. Si NMR spectrum of synthetic Na, K-chabazite and skeletal diagram with the proposed Si:Al ordering of the chabazite cavity (Al atoms are marked by black circles; Si atoms are located at the intersections of the lines not occupied by Al; oxygen atoms are not shown). The intensity ratios of the di ferent Si(nA1) groups are 3:10:12:2.
7. Zeolite
Na, K-L. The aluminosilicate
based on "cancrinite four-membered
rings.
framework
cages" which are formed
of zeolite
by five six-membered
Na, K-L is and six
122 These cavities are linked through the planes of their upper and lower sixmembered rings, thus forming vertical columns (ref. 55). Each column is crosslinked laterally to three others by oxygen bridges. The preferred Si/Al ratio of a zeolite L is about 3.
From the absence of the Si(4Al) signal, the small
Si(3Al) and Si(;$l) intensities and the fairly intense Si(2Al) and Si(lA1) signals of the
Si NMR spectrum an overall Si,Al distribution has been sug-
gested. The repeating unit consists of two cancrinite cages with different SiiAl ratios; viz., one type with 5 Al and 13 Si and another with 4 Al and 14 Si for a Si/Al ratio of 3 (ref. 54). 8. Mordenites. The tetrahedral framework of mordenites consists of complex chains of five rings which are cross-linked by four rings (ref. 56).
It has
been concluded from X-ray studies that SirAl are ordered with Al occupying half the sites of the four rings (ref. 57). This implies the presence of Si(PA1) units beside Si(lA1) and Si(oAl), with the latter two in five times higher concentration, if a typical Si/Al ratio of 5 is assumed. The *'Si MAS-NMR spectrum of a
mordenite sample from Poona, India (Table 1) is not
in agreement with that conclusion because only a rather weak shoulder signal for Si(2Al) but strong lines of Si(lA1) and Si(oA1) with at least ten times higher intensity have been observed (ref. 14). In recent papers (ref. 6,31- 33), the Si;Al ordering in the structure of synthetic mordenites has been determined using dealuminatedmaterials. The 2gSi MAS-NMR spectrum of the starting material, a Na-mordenitewith Si/Al= 5.5, shows three,lines at -95, -105 and -110 ppm, respectively (ref. 32) (Table 5). As the degree of dealumination increases, the line at -95 ppm disappears while the line intensity at -105 ppm decreases and that at -110 ppm increases. Assuming that the aluminum atoms preferentiallyoccupy the four rings, the variations of the three'line intensities may be explained by assigning the lines to Si(ZAl), Si(lA1) and Si(oAl);respectively. In the case of a completely dealuninated mordenite, three Si(oA1) lines with a shift dispersion of ca. 5 ppm and an intensity ratio of 2:1:3 are observed (ref. 13, 58) which have been attributed to the four crystallographicallyinequivalent T-sites present in the mordenite framework with the population ratio of 2:1:1:2 (ref. 13). .The last two sites coincide in one NMR peak. 9. Silicalite and ZSM-5 (Table 5). Si-licaliteis a crystalline porous silica; its framework structure is topologically closely similar (if not identical, ref. 12, 59, 60) to that of the zeolite ZSM-5. Its 2gSi MAS-NMR spectrum (ref. 12,60) (see also Ref. 5) may be simulated by a minimum of nine Gaussian signals; The chemical shift range of all peaks is characteristicof Si(OSi)4 groupings in highly siliceous materials. The observed multiplicity
123 arises from the crystallographicallynon-equivalent tetrahedral environments of the Si(OSi)4 sites. The amount of aluminum present is very small (Si/Al, 1000). However, it has been shown (ref. 5,12,59) that aluminum is tetrahedrally coordinated to oxygen. At least two distinct types of tetrahedral framework sites are occupied by the aluminum. Recently, it has been shown that the 2gSi NMR spectra of ZSM-5 samples with Si/Al of 125 and 900 are practically identical with that of silicalite and exhibit 15 clearly resolved peaks (ref. 59). The assignment of several
29 Si MAS-NMR lines and the identificationof pos-
sible aluminum sites in H-forms of ZSM-5 and ZSM-11 zeolites has been undertaken in Ref. 34,35 using samples with different Si/Al ratios from ca. 30 to 1000. All spectra show essentially three resonances at -105, -113 and -115 ppm relative to Me4Si as external reference. The signal at -105 ppm can be clearly identified as a Si(lA1) group.
Its intensity decreases with increasing Si/Al
ratio and drops to zero for an Si/Al ratio higher than 100. The decreasing intensity of this signal is directly linked to the increase of the line at -113 ppm, while the intensities of the lines near -115 ppm r-114 (partly), -115.2, -116.3 ppml remain constant in both H-ZSM-5 and H-ZSM-11. Since, additionally, the intensity ratio of these lines IZsM_11/IZsM_5=2 equals the ratio of the numbers of four-membered rings (8 in ZSM-11 and 4 in ZSM-5), the lines near -115 ppm are attributed to Si()Al) atoms located in (strained) four-membered rings. An attribution of the lines near -113 ppm L-109.6, -111.0, -112.0, -112.8, -113.2, -114 ppm (partly)] is still not clear. B. Detection of Si atoms bearing,hydroxylgroups The use of the 1H-2gSi cross-polarization(CP) MAS-NMR technique makes possible the detection of silicon atoms to which are attached one or two hydroxyl groups (ref. 4,61). By application of the CP-technique, strong enhancements of the signal intensities or even additional signals may appear for silicon atoms bearing OH groups which are sometimes difficult to detect in the U.direct"Fourier transform spectrum without polarization transfer. These resonances may then be identified in the "direct" spectrum in order to estimate their concentration. (It
should be emphasized that the cross-
polarization spectra are not quantitively reliable for this purpose.) By means of 'H-2gSi CP-technique,informationon SiOH groups in defect centers of the dealuminated structure of progressively dealuminated Y-zeolites has been obtained (ref. 61). The presence of SiOH groups is pointed out by 29 Si MAS-NMR signal at -101 ppm.
a considerable intensity enhancement of the
This method has also been applied to the identificationof silanol groups in the ZSM-5 (ref. 34,62) and in the silicalite framework (ref. 62). Maciel and Sindorf (ref. 63-65) applied the CP-technique for the characterization of silica surfaces and surface-derivatizedsilicas.
124 C. Aluminum-deficientzeolites
In some papers (ref. 18,25,43,61,66-68), 2gSi MAS-NMR has been used to investigate the structure of dealuminated zeolites Y and the process of their formation. In particular, the following problems were considered: 1) the dependence of the Si/Al ratio and Si,Al ordering in the dealuminated sample on the conditions of the thermochemicaltreatment, e.g., the degree of ammonium exchange, temperature and partial pressure of steam; 2) the influence of the dealumination procedure, e.g., thermochemical treatment, Al extraction by organic acids or by SiC14,-on the Si,Al ordering in the products; and 3) the regularities in the Si,Al ordering during the course of the step-by-step extraction of Al atoms from the faujasite lattice.
In the first step of the investigations (ref. 17,25, 61,66), it has been shown that 2gSi MAS-NMR makes it possible to follow in detail the process of removal of Al atoms from the zeolite lattice during thermochemicaltreatment, Characteristicalterations in the signal intensities of the corresponding Si(nA1) groups were observed when, during the dealumination process;some Al atoms are removed from the aluminosilicateframework. Because only aluminum 29 atoms located in the aluminosilicate lattice affect the Si NMR spectra, the presence of nonframework aluminum produced by the dealumination process of the zeolites has no effect on the spectra. A direct investigation of these aluminum sites has been performed by means of 27Al NMR studies (ref. 69,70; see also article by D. Freude in this volume). Additional information on SiOH groups in defect centers of the dealuminated structure can be obtained by lH 29 - Si CP-techniques as mentioned above. The fraction of terminal SiOH groups remains nearly constant in the dealuminated samples (ref. 43). This shows that the overwhelming majority of the vacancies created by the removal of Al from the tetrahedral framework are subsequently re-occupied by silicon. The following general conclusions could 29 Si NMR investigationof thermochemicallydealumin-
be drawn from a thorough
ated Y zeolites (ref. 43): 1. The extent of dealumination during the thermochemical treatment is mainly determined by the degree of ammonium exchange in the starting material. The number of Al atoms removable is limited by the number of NH4 cations present in the parent sample. 2. The extent of dealumination increases with the temperature applied during treatment. In the deep-bed treatment, the maximum dealumination is normally not attained. Even at 770°C the degree of dealumination is far below the degree of ammonium excha ge. Therefore, in the deep-bed treated samples 1 a considerable amount of Si-O-Al groups, the centers for the hydrolytic removal of Al atoms, is retained. It has been suggested that the hydroxyl groups adjacent to lattice aluminum are mainly responsible for the catalytic activity of such type of zeolites (ref. 71).
125 3. The promoting dealumination pressures
process
is well known.
by the available
dealumination
increases
("self-steaming
Because
amount of water.
only slightly
conditions")
ble and comparable pressure
predominantly
atoms within
samples,
conditions
successive rangement
procedure.
of the zeolite
the heating
it is quite difficult
rate, etc.
to obtain
treatment
relia-
with known
et al. (ref. 67), it has been con-
of the aluminum-deficient
The arrangement
the tetrahedral
framework
Some criteria
faujasite
is obviously
independent
for the selection
and type of
electrostatic
of external
of a definite
Si,Al
validity
of
energy
of the whole structure,
of Al by Si, the preservation
of centrosytmietric ar-
of Si and Al and net zero dipole moment
(cf. also Ref. 5).
However,
and with the Si(nA1)
The preparation
depends
of the silicon and aluminum
(ref. 67); viz., the general
rule, the minimum
replacement
themselves
of steam is not known
on the preparation
the use of thermochemical
have been considered
Loewenstein's
the
water concentration.
the pressure
study by Engelhardt
conditions.
arrangement
At higher steam pressures,
on the final Si/Al ratio and not on the conditions
the dealumination
cination)
(ref. 43) that at partial
and volume of the sample,
that the Si,Al ordering
reaction
of steam on the
of steam is recommended.
5. In a systematic cluded
pressure
with increasing
and depends
the geometry
under such undefined
partial
partial
It is shown
In the case of deep-bed treatments,
being treated,
etc.
of an increasing
of steam below 2.7 * lo4 Pa (~200 Torr) the extent of dealumination
is limited
4.
effect
these criteria
populations
in the double
sodalite
unit,
are not always compatible
derived
with
from the NMR spectrum.
of HY zeolites
by deamnoniation of NH4Y (shallow bed cal29 The Si and.27 Al NMR spectra reveal that even
has been studied.
(e.g., treatment
under very mild conditions amount of aluminum
is removed
a "pure HY" zeolite
cannot
at 300°C for one week) a certain
from the tetrahedral
be obtained
(ref. 68). 29 Si NMR study, In a detailed
lattice and, consequently,
by simple thermal
treatment
of NH4Y
zeolite
non-framework
aluminum
kaline treatment
of dealuminated
72) has to be rejected The progressive (ref. 59,60,68,74) treatment vapor,
dealumination by various
(ref. 5). spectroscopy
chemical
lattice
of
by al-
by Breck and Skeels
(ref.
in detail
of NMR methods
a further
(ref. 32,33) and ZSM-5 zeolites
procedures,
acid leaching
including
important
should be mentioned:
thermochemical
and treatment
with SiC14
using 29Si and 27A1 NMR spectroscopy
to the study of the process
by SiC14 has been reviewed Finally,
proposed
of mordenites
samples,
has been investigated
of zeolites
Y-zeolites
aluminosilicate
(ref. 73).
of NH4-exchanged
The application
it has been shown that the reinsertion
into the tetrahedral
in more detail application
.
of dealumination
by Fyfe, Thomas,
et al.
of 2gSi and 27Al NMR
viz., the study of the process
of zeolite
126 crystallizationfrom the gel phases. Engelhardt et al. (ref. 75) studied solid aluminosilicategels formed as intermediates in zeolite A synthesis and Nagy et al. studied the formation of zeolites ZSM-5 (ref. 6,76) and mordenites (ref. 31). XIII.
ACKNOWLEDGEMENT
The help
of Miss Angelika Friedrich in preparing themanuscript of this paper
is greatly appreciated. XIV.
7 8 9 10 11 12 13 14
REFERENCES
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67
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
HIGH RESOLUTIONNMR ON ADSORBATE-ADSORBENT JANOS B. NAGY+, GiiNTER ENGELHARDT*and 'Facultes Universitaires B-5000 Namur, BELGIUM
de Namur,
SYSTEMS
DIETER MICHEL+++
Laboratoire
de Catalyse,
Rue de Bruxelles
'+Akademie der Wissenschaften der DDR, Zentralinstitut fijr Physikalische Rudower Chaussee 5, DDR-1199, Berlin, GERMAN DEMOCRATIC REPUBLIC '++Karl-Marx-Universitat Leipzig, Sektion Leipzig, GERMAN DEMOCRATIC REPUBLIC
Physik,
Linn&trasse
61,
Chemie,
5, DDR-7010
CONTENTS 1.
ABSTRACT
II.
GENERALINTRODUCTION................................................
............................................................
PART I - HIGH RESDLUTION
III.
MOLECULES
.........................
69
CONDITIONS FOR MEASUREMENTS
OF HIGHLY RESOLVED SPECTRA OF ADSORBED ...........................................................
MOLECULES
A. Practicability B. Sensitivity C. Overhauser
IV.
NMR ON ADSORBED
68
68
of 'H; 13C and 15N NMR studies
69
....................
70
...................................................... effect
69
71
(NOE) ..........................................
D. Study of anisotropy of magnetic shielding. Application of MAS techniques .......................................................
71
E. Measurement
71
of resonance
THE STATE OF ADSORBED
shifts
MOLECULES.
.. .................................
INTERACTION
WITH DIAMAGNETIC
SITES
. 79
A. Analysis
of 13C NMR shifts
.......................................
80
B. Analysis
of 15N NMR shifts
.......................................
82
C. Origin of the variation D. Variation
of coupling
OF THE ADSORBED
of the chemical
constants SPECIES
shift
upon adsorption
....................
84
..................
88
V. VI.
IDENTIFICATION OF THE SURFACE ACTIVE SITES IN PRESENCE OF ADSORBED MOLECULES ... . .......................................................
VII.
STUDY OF THE CATALYTIC
VIII.
HIGH RESOLUTION MAGIC ANGLE SPINNING 13C NMR OF STRONGLY ADSORBED MOLECULES ... . .......................................................
IX.
REFERENCES
TRANSFORMATIONS
95
..............................
101 103
.........................................................
PART II - STRUCTURAL
INVESTIGATION
OF ZEOLITES
INTRODUCTION
.......................................................
XI.
EXPERIMENTAL
PROBLEMS
A. Condition
of measurements
0
AND GENERAL
105
BY 2gSi NMR SPECTROSCOPY
X.
OOOl-8686/85/$21.70
89
.....................................
MOBILITY
FEATURES
OF THE 2gSi NMR SPECTRA
......................................
1985 Elsevier Science Publishers B.V.
109
.
109
. 109
68 29 Si chemical shifts of silicates and of (zeolites) ......................................
B. General features aluminosilicates C. Spectra XII.
assignment
APPLICATION
.........................................................
A. Silicon,
aluminum
B. Detection
ordering
C. Aluminum-deficient XIII.
ACKNOWLEDGEMENT REFERENCES
I.
ABSTRACT
structure
zeoiites
hydroxyl
groups
116
....................
............................
123
. ......... 124
.....................................................
multinuclear
NMR on adsorbed'molecules
(Part II)
of zeolites
in heterogeneous
In Part I the conditions tra of adsorbed molecules, identification
116
126
..........................................................
High resolution
from interest
114
.......................................
of Si atoms bearing
XIV.
110
...............................................
is reviewed catalytic
(Part I) and on the
both from a methodical
point of view and
systems.-
of measurements
of highly resolved
the state of the adsorbed
of the surface.active
126
13C and 15N NMR spec-
molecules,
their mobility,
sites and the study of catalytic
the
transforma-
The recent results on high resolution magic-angle tions are thoroughly discussed. 13 spinning C NMR are only included for the sake of completeness. These latter, together
with the results on more mobile molecules,
of the admolecule-support
and different
silicon-aluminum properties
II.
applications detection
of aluminum-deficient
GENERAL The subject
netic resonance quantum
ordering,
number
29
Si magic-angle
of this method of silicon
spinning
are discussed,
atoms bearing
NMR spectra
including
hydroxyl
groups and
zeolites.
INTRODUCTION of this paper is to review the study of highly resolved spectra of adsorbed
molecules
and of solid adsorbents
l/2 (13C, 15N, *'Si) and the discussion
NMR which are based on different molecules
a better understanding
interaction.
In Part II general features of solid-state of zeolites
provide
studied
properties
by NMR has already
of the nuclei.
been reviewed
point of view (ref. 1,2) or from interest
of possible
nuclear magfor spins with
applications
The behavior
of
of adsorbed
either from a purely theoretical
in heterogeneous
catalytical
systems
(ref.
3- 10). In a comprehensive in interaction discussion different
study of a solid adsorbent
with adsorbed
of possible properties
cules at different (ref. 2,4-6); of adsorbed
molecules,
applications
of nuclei
temperatures
acting as a heterogeneous aspects
of NMR investigations
(ref. 5,11,12);
(ref. 7,13,14);
catalyst
should be included: which are.based
of the surface active
of the adsorbed
I) a
on the
2) the state of the adsorbed
(ref. 7); 3) the mobility
4) the identification
molecules
different
mole-
species
sites in the presence
5) the study of catalytic
transformations
69 (ref. 15-18);
6) the study of the adsorbent
itself by multinuclear
8,19);
and 7) the mobility
of protons and counterions
Points
1 to 5 are included
in Part I of this review dealing
adsorbed
molecules.
Here we restrict
ourselves
NMR (ref. 4,5,
at the surface
(ref. 2,4,6).
with the behavior
to the more recently
of
developed
study of 13C and 15N NMR.
In Part II Point 6 is reviewed, especially of 2g Si NMR.
With respect
taking into account the application 27 to Al NMR studies and to proton mobility on surfaces
(Point 7), we refer to the review of Freude
PART I - HIGH RESOLUTION CONDITIONS
III.
A. Practicability Highly
NMR spectra of adsorbed
molecules
Hence, among the different
13C and I5N are of importance, dances
MOLECULES
OF HIGHLY RESOLVED
SPECTRA
OF ADSORBED
MOLECULES
of 'H, 13C and 15N NMR studies
resolved
with spins l/2.
NMR ON ADSORBED
FOR MEASUREMENTS
(ref. 19).
of which.are
nuclei of organic molecules
the different
well known.
may only be studied for nuclei
magnetic
properties
mainly
and natural
'Ii,
abun-
If we put, e.g. for IH, the product of relative
NMR intensity: Ir = $ I(I+l)(v,oo/loo)3
Bo= 2.34869 T (corresponding
for a value
to a proton resonance
frequency
of "IO0 =
100 MHz) and the natural abundance equal to 100, then the respective values for 13 C and l5 N NMR studies are 1.8. 10m2 and 3.8. 10s4. This comparison reveals the 13 poor sensitivity of C and 15N NMR studies. In spite of the suitable measuring conditions, however, reason
proton
resonance
is the strong line broadening
to dipolar
proton-proton
(impurities), exchange
the resolution
with structural
(with respect and/or
(ref. 25) and on measurements
be mentioned
averaging
internal
proton-proton
of the adsorbate
fields and to proton
More general
there are no strong
line widths
are based on an exchange
because ofI$he
of the asorbed
was mainly
(ref. 26).
achieved
by
remaining
limitations
with respect
shifts.
of
high internal mobility
C and l5N NMR specta of adsorbed
of
of resonance
(see
to increase
of NMR line narrow-
at very high frequencies
For studies
in the 13C and l5 N NMR spectra
intervals
attempts
fields of the sample but not by a reduction
interaction
molecules.
shift) due
centers
with proton spin relaxation
that in Ref. 25 a line narrowing
inhomogeneous
studied, The
This is the reason why most of the
in this volume). spectra
with paramagnetic
of magnetic
OH groups.
were successfully (ref. 5,20-23).
to the small chemical
coupling
are concerned
of proton resonance
It should
species,
and Michel
molecules
resolution
with the bulk liquid .(ref. 20.,24), on the application
ing techniques
dipolar
of adsorbed
inhomogeneities
of IH NMR techniques
the review of Winkler
molecules
interaction
due to internal
processes
applications
greater
spectra
only in a few cases due to restricted
to resolution
due to smaller
at not too low temperatures
and to
70 B. Sensitivity The strongly in general,
reduced
sensitivity
the application
tion and, additionally, The minimum
number
of 10 after z repeated
N
min
where
for
Nmin of nuclei necessary measurements
I, is the relative
NMR intensity,
time in s, 6~ is the observed
in cm3.
receiver
having natural
,&.
values
half width
system
without
~~)1’~ “cl/2
vloo is the resonance
abundance
for the remaining
of 13C nuclei
parameters
effect enhancement.
density
2. 102' cavities
per cm3), we arrive
zeolites
frequency
in MHz if
of 13C NMR signals (1.1%).
leading
are:
Taking
coil
for adsorbed
vloo= 25.1 MHz,
that:
TI - 0.1 s, 6w= 100 s-l (-20 Hz),
to:
i: 5.6. 102’
Overhauser
of nuclei
,
in K, Tl is the longitudinal relaxation -1 of the NMR signal in s , AV is the band
measurements
with an apparent
Nmin(13C)
ratio
in Hz and Vc is the volume of the receiver
Aw= 500 Hz and z= lo4 accumulations,
Nmin(13C)
a signal-to-noise
is given by (ref. 5):
T= 300 K, Vc= 1 cm3 and I,= 1.59. 10m2, it follows
Typical
requires,
temperature
As an example, we consider
molecules
to produce
(accumulations) B-03/2 1;
B,= 2.34869 T, T is the sample
width of the whole
IH NMR measurements
of Fourier transform techniques and signal accumula15 N NMR studies, the use of enriched materials.
1018 1;I (&0)i'2(&)3'2
I
as compared with
Practically,
for faujasite
Pa I: 0.4 gem -3 and about 4. 102' cavities at a minimum
type zeolites per g (i.e.,
number:
- 3
per large cavity.
This is in agreement
with experiments
of butenes
in
NaY (Fig. 1).
Fig. 1. Number of accumulations as a function of number of molecules per large cavity for 1-butene in zeolites NaY for a signal- o-noise ratio (SNR) of ca. 6 in C NMR spectra (+: natural abundance;O: 55% 15 C enrichment in group= CH-).
71 C. Overhauser
effect
(NOE)
The signal enhancement (index S) to the nuclei
due to magnetization investigated
transfer
from the coupling
(index I) is mostly
protons
less than the optimum
factor: n=l+_.--_yS
\ 3 for 13C
TII TIIS
y1
(3)
[ -3.9 for 15N
with y the gyromagnetic
ratio
(S: IH; I: 13C, l5N), the longitudinal
and the cross-relaxation time TIIS through dipolar coupling TII The reduced effect is due to: 1) restricted molecular mobility polar I-S coupling, of the factor
viz., for (w~-w,$T~
l/2 for short correlation
of other relaxation rary correlation
mechanisms
rather
of magnetic
studies
(liquid-like
shielding
spectra)
than heteronuclear
dipolar
1) for adsorbed
Application
molecules
fixed molecular
sieves
fragments
on solid surfaces
In these cases of a strongly means
of MAS techniques
E. Measurement Chemical macroscopic
in the majority
average
of the adsorbed
of possible
1. Correction
shifts,
6
of chemical
susceptibility
real
-6
obs =
adsorbate
mobility Examples
shifts)
generally
2) for molecules
also
a line narrowing
are discussed
by
in Part VII.
have to be corrected
interaction.
processes
(ref. 37,38)
Furthermore,
bound
+ k)(x,(ref)
to obtain
complexes,
molecules
the
a thorough
for adsorbent
can be used for the measurement
the observed
shift area,.
to liquid
is necessary.
shifts of adsorbate references
either
for
in order to eliminate
in order
in adsorption
resonance
of the adsorbate-adsorbent
resonance
(ref. 28,29);
(cf., e.g., Ref. 31).
compounds
molecule
it is necessar.y to correct
real (corrected)
As=
exchange
Since only external
magnetic
for the following
(ref. 33,34) and are compared
reference
of van der Waals
true resonance
of
value of the
shifts
(or other resonance
or to gaseous
the influence
bility.
reduced
for arbit-
[ref. 301); and 3) for rigidly
(ref. 14,30,32).
volume susceptibilities
(ref. 35,36)
analysis
occurs
of resonance
shifts
at low temperatures
(e.g., mordenites
coupling
of MAS techniques
are bell-shaped
and hence only the isotropic
molecules
<< 1 instead
are given in Part V.
tensor is available but not the anisotropy. 13 of C NMR shielding tensors has been measured
in small pore molecular
I and S spins
even for purely di-
The anisotropy cases:
time
times ~~ (ref. 27); and 2) the predominance
shielding.
13C and I5 N NMR lines of adsorbed
between
5 1 we find the value TII/TIIs
times ~~ (ref. 11). Further examples
D. Study of anisotropy
relaxation
system
The correction
- x,(sample))
suceptiof chemical
shift dabs for the bulk in order
to obtain
the
is (ref. 33,34,39):
,
(4)
72
taking the same geometry for the reference probe (denoted by "ref") and the adsorbate-adsorbentsystem (denoted by "sample") which is characterized by the demagnetizationfactor ~1. The factor k describes the influence of the local magnetic field which is averaged to zero (kz 0) under normal conditions of relatively fast isotropic reorientationaland/or translationalmotion (with -9 mean life times of r&O s in their equilibrium positions at room temperature). x,(ref)
and x,(sample) are the respective volume magnetic susceptibili-
ties. If the external reference is the corresponding gas at very low pressure, we may put x,(ref)= 0.
Further, according to Wiedemann's rule we assume that the
total volume susceptibilityx,(sample) can be written as the weighted average: x,(sample) = x,(solid)t B[x,(admolecule)-x,(solid)l
,
(5)
where B is the volume fraction of the adsorbed molecules. Now if molecules are
adsorbed for which G,_~~, +O for very small fractions B+O,
the extrapola-
tion of fiobsvalues to zero coverage then yields the susceptibility correction for the solid (ref. 4,33,40): x,(solid) . For the usual form of the sample (viz.,cylindricaltubes perpendicular to the external magnetic field B, with a value of ca. 3 for the ratio between the filling height and the sample diameter), the demagnetizationfactor is: u = 1.8 K (ref. 34). This method which was first proposed by Fraissard et al. (ref. 33,40) leads to a good agreement with direct measurements of volume magnetic susceptibilities on the basis of an analysis of proton resonance shifts. The usual correction varies between 0.4 and 1.5 ppm (ref. 33,34). However, for 13C NMR studies, chemical shift and volume susceptibilitymeasurements (ref. 34) gave essentially the same result only when larger molecules such as n-butane, cyclohexane or TMS are used while significant differences arise with small molecules like methane.
Obviously, the supposition Greal = 0 for 6 = 0 is no longer
fulfilled here and the resonance shifts due to van der Waals interaction are greater than those due to the bulk susceptibility. 2. Chemical shifts as referred to either the liquid or the gaseous state. The elimination of the influence of van der Waals interactions is closely connected with the difference of resonance frequencies between the gaseous and the liquid state. Hence, a fundamental question concerns the state of the adsorbed compound, namely whether it is closer to the gaseous or the liquid state. Tables 1 and 2 give a few examples of chemical shifts referenced either in the liquid or the gaseous state.
13
0.0
0.0
4’1
10.6
+O.L
19
to.1
+o.*
49
-0.6 -0.6 to.2
-1.3 -1.1 -0.2
:: 13
-0.2
-0.9
II
-0.1
-0.8
13
to.1
-0.7
13
+I.,
-0.2
13
74
TABLEZ-A 13
C NMR chemical
shifts Sims (ppm) of different support
Compound
___--__. CH4 gas
__
(Ref TMS/CD4)
N.3Y
co gas
cs* liq
molecules
Covera9d
in the gaseous.
T'C
Cl
1 atm 100 Torr
38 38
SiO*
-8.9012.631
38
__
183
N?IY
179.36[-3.641
30
NaX
180.261-2.741
38
Pd/Al203
177.611-5.391
38
Pt/Al203
177.5[-5.51
38
NaA
22
181.81-1.21
63
NaX
22
178.9[-4.1,
63
NaY
22
179.0[-4.01
63
DeNaY
22
216[+331
63
__
192.5
104
NIY
153.71[-38.81
38
154.741-37.81
38
__
1
22
132.2
103
NaA
1
22
132.2101
63
NaX
1
22
131.1[-1.01
63
NaY
1
22
130.7[-1.51
63
OeNaY
1
22
130.41-1.81
63
SiD*
O.lD
22
130.71-1.51
105
119.11
38
2.3 M
__
122.94[3.81
38
NaY
2.DlN.a 0. l/Na
124.25~5.21 124.05[4.91
38 38
1 atm 0.14
SiO*
28
38 38
124.0[4.9]
38
115.03[-4.11
38
122.5[3.4]
38
118.4[-0.71
105
70
38
NaY
71.2811.281
38
NaX
73.11~3.111
38
__
117.7(0.3)
104
LiNaX (80)
22
118.2(0.5)
106
LiKX
22
122.9(5.2)
106 106
Rb.NaX
(71)
22
122.2 (4.5)
Cs,NaX
(50)
22
121.8(4.1)
106
Ag.NaX
(20)
22
119.7(2.0)
106
Ag,NaX
(60)
22 110-450
13x __
“5%010
30
107 38 15.25
38
114.12[-1.321
131.5010.301
17.7[2.45]
38
Z.l/Na
114.231-1.211
135.73I4.59
17.53[2.28]
38
O.VNa
115.91*0.46l
137.55I6.351
19.30[4.057
38
116.0[+0.561
136.3815.181
19.35[4.10]
38
Lay
per supercage
104
48.5(-0.5) -7.64 131.20
2.6 M
USbOS
106
115.44
RT
NaY
120.7(3.0) 49.0
-4.613.01
NaY
CH2=CH-CH3 9% (1 atm)
7 'number of molecules
123.714.61 124.65c2.41
__
CH3OH llq
In COCl3
38 38 38
-9.4712.061
in benzene
g,as
-11.53 -9.80[1.73] -9.81
AgY
AgY
liq
Ref.
C5
38
HY
H3C-CsN
C4
-8.4113.121
5. 10-4/Na
gas
rdagas(ppm)l
C3
-9.2412.291
HY
WCH
Cz
NaX
CH2-CH2 gas (1 atm) in CDCl3
State
(nalis(pp m));
HY
NaX co* gas
liquid and adsorbed a(ppm);
-_._
__ __ (zeolites)
25 -a7 25 -87 or statistical
monolayers
(130.5)[-0.71 (129.7)[-1.51
74 74
(132.1)[0.9] (137.7)[6.5]
::
(sio 2, A1203)
if not stated otherwise.
75
TABLEZ-8 13C NMR chemical
shifts
bTMS (ppn) of different
Compound
molecules
Coverage+
support
in the 9aseous.
-2.03
38
85.56
NaX
__
84.4
SiO2
67.49[+5.19]
82.22
15.4
16.11
38
15.54[0.141
16.11[0.0]
38
-2.561-0.531
38 38
-1.58[+0.45]
38
38
Jiq
27.913.61
38
N?lX
24.61[0.31]
38
HV
24.19[-o.llJ
38
5102
23.25[-1.051
liq SiO
2
Yf,' SiO
2
N&i02 CH3-CH2-CH2-CH3
0.22 0.47 0.82 0.71
28
gas liq s102
H2C=CH-CH2=CH2
1.93 !J ml m-2
gas 1iq SiO2
H3C-CH2-C'CH
2.05umolm
-2
gas liq SiO2
1.96”mlm-2
38 X.3
202.8
37.3
6.0
206.0(3.2)
38.4(1.1)
6.0(0.0)
24.1
197.5
28.1[4.OJ
205.11t7.61
102
27.9(-1.1) 27.1(-1.0) 27.8(-0.3)
216(+11) 215.2(+10.5) 214.3(+9.2) 212.2(+7.1)
43 105 105 105
27.0(-1.1)
211.8(+6.7)
66
10.9
24.4
37
13.112.21
24.9IO.51
104
11.810.91
20.4lO.11
104
112.8
136.2
37
116.313.51
136.9
37
116.1[3.3]
136.810.61
108 67
37
28
62.8
81.8
10.7
11.6
37
28
67.014.21
84.712.91
12.3[1.61
13.812.2~
104
28
66.613.91
87.615.81
11.5[0.81
12.010.41
37 37
gas
28
70.2
liq
28
73.613.41
104
75.515.31
37
CH2=F-CH-CH2
CH3-CH2-CH2-CH2-CH=CH2 Jiq Cl-CH2-CH2-Cl
sio2
2.16
25
116.4
142.2
139.6
113.2
17.5
WA1203
10~ moln-2
25
114.3(-2.1)
140.8(-1.4)
138.8(-0.8)
112.1(-1.1)
14.5(-3.0)109
liq
CH3
liq
28
NaV -
11.61
5 u no1 III-’
25
113.8(-2.6)
141.3(-0.9)
138.5(-1.1)
-
14.8(-2.7)109
25
117.4(1.0)
144.1(2.1)
141.3(-1.7)
114.5(1.3)
lS.O(O.5)
113.5
137.8
102
1.0
60
112.4(-1.1)
142.6(+4.8)
49
0.48
HCOOH liq TiO2
25
Jiq ST02
102
51.2(-0.5)
105
liq
'number Of m~lecqler
110
176.9
20.8
175.8(-1.1)
104
180.1
18.2(-2.6) 27.5
a.7
178.3(-l.@.)
-
26.3(-1.2)
6.8(-1.9)
179.3
36.0
18.2
111 104 111 13.1
104
510.
177.3(-2.0)
35.2(-0.8)
17.3(-0.9)
ll.O(-2.1)
-
179.4
33.8
26.7
21.7
13.2
177.3(-2.1)
32.4(-1.4)
25.9(-0.8)
20.3(-1.4)
10.9(-2.3)lll
L
CH3-CH2-CH2-CH2-COOH
104
4100(+3934)
SiO2
liq
109
51.7
166
CH3-CH2-CH2-COOH
109
5 y mol m-‘+ o2 (traces) -
Si02
CH3-CH2-COOH
Ref. C5
24.3
CH3-CH2-CHO
CH3COOH
5
C4
gas
CH3
H3C-CsC-CH3
[66g= (Ppm)l
68.16[+5.861
NlV
HC3-C-F3
state
(Ppm)).
NaV
gas
CH3-YH-CH3
liq
62.30
gas
CH3-CH2-CH3
(A6
3
5 HCX-CH3
liquid and adsorbed 6 (pp);
TqC
0.24 0.48 0.74
27.8 27.4 -0.4) 27.0 -0.8) ZE.l(t0.3)
Per ruperc:a9e (zeolites) or statistical
monolayers
111 104
102 105 105 105
I
(S102. Al20,)
if not stated othewfse.
TABLE 2 -C 13
C NMR chemical
shifts
6TMS (ppm) of different
molecules
Coverage+
support
Canpound
in the gaseous,
Cl
-
@.w
liquid and adsorbed s(ppm);
TqC
state
(b61'q(ppm));
CP
Ref
rA6gas(ppm)l
C3
C4
C5
102
128.5
Si02
127.7(-0.8)
111
co2+ Aerosil 0.7 (0.60%)
60
139.5(+11.0)
93
Ni2+ Aew;'
0.7
25
163.5(+35)
94
0.7
25
163.5(+35)
94
0.25
25
154.5(+26)
94
;.;
25
0:7
2':
151.5 +23) M;.;(+9’;) I . +
;“4 94
N,2+
: Aeros, (0%)
SiO N1"
(0.9%)
144.1
sio*
125.6
21.3
137~8
129.3-128.5
102 102
(+1.8)
20.0(-1.3)
60
139.8(2.0)
(+2.0)
21.3(0.0)
60
OeNaY
0.8
140
140.9(3.1)
(+1.1)
21.1(-0.2)
60
sio2
0.49
138.6(0.8)
(0.1)
19.8(-1.5)
105
19.4(-2.1)
111
0.70
28
126.6(+1.0) 125.1
140.9(3.1)
128.6-128.2
36.9
19.0
102
35.7(-1.2)
15.5(-3.5)
113
47.5
Si02
0.51
0.61
NaY US-Ex
28
H in H2S04
28
13.8
55.6(-2.6)
9.3(-4.5)
111
55.9(-2.3)
9.5(-4.3)
113
149.9
124.0
136.0
114
157
150.2(0.3)
124.3(-0.3)
137.9(1.9)
114
60
149.7(-0.2)
123.0(-1.0)
135.0(-1.0)
114
0.77
28
148.8(-1.1)
124.0(0.0)
136.2(+0.2)
113
A1203
0.86
60
149.7(-0.2)
37
142.0
128.7
148.2
114
NaY
157
141.5(-0.5)
127.5(-1.2)
147.0(-1.2)
114
US-Ex
157
140.6(-1.4)
127.3(-1.4)
145.9(-2.3)
114
142.1(0.1)
129.1(+0.4)
148.3(+0.1)
115
118.4
108.0
104
118.3(-0.1)
106.9(-1.1)
113
;+, 2
28 28
147.7
116.1
129.8
119.0
104
sio2
0.51
28
145.2(-2.2)
118.3(+2.1)
129.0(-0.8)
120.6(+1.6)
113
A1203
0.9
147.1(-0.6) 80
liq
:'02-A'203
'.'
116.5 119.7(+3.2)
... C6
102
104
120.0(+6.6)
129.4(-0.7)
122.4(+9.0)
129.7(-0.4)
-
150.9
112.8
129.3
116.7
40.3
Si02
150.7(-0.2)
109.2(-3.6)
128.2(-1.1)
119.8(+3.1)
41.3(+1.0)111
57.3
17.9
102
58.3(+1.0)
105
67.4
16.4(-1.5) 17.1
67.7(+0.3)
15.7(-1.4)
105
5102
28
0.74
sio2
'number of nwlecules
130.1
147.9(-0.7) 0.70
liq
liq
113 113.4
149.0(+0.4)
s102
CH3-CH2-O-CH2-CH3
113
143.3(-4.4) 148.6
SiO2
liq
113
0.91
_
4&2_C;3)2
102
37
liq
40-NH211q
113
58.2
sio2 (1:s) -
@
102
45.5(-2.0)
sio2
CH3CH20H
5~.;4~~.5’11’
140.7(+2.9)
liq
4@!NK:3)2 3 2
125.4(-0.5)
127.6(-0.4) 137.8
140
lfq
&NH
127.7(-0.8)
144.4(+0.3) 113.5
104
140
Si02
6
29.2 C6 15.8
0.8
so2
4 @
125.9
0.8
llq
N(CH,-CH3)3
128.5
NaMgY(68%Mg)
so2 NW+3
128.0
NCIY
so* CH3-CH2-NH2
112
128.5(0.0)
NaY
per supercage
0.68
(zeolltes) or statistical
monolayers
(Si02, A1203j if not stated otherwise.
102
The gas-liquid are more exposed C4 of 1-butene respect
shifts
to intermolecular
adsorbed
to the gas.
the methinic
Similar
on the terminal
interaction
on NaGeX zeolite
The C3 carbon
C2 carbon
other carbon atoms, cule.
are more significant
(ref. 34,37).
experience
experiences
conclusions
a specific
atoms which
The carbons
a small high field shift, while (low field shift) than the
interaction
at this site of the mole-
can be drawn from comparison
with the liquid state,
where all Cl, C3 and C4 carbon atoms show a high field shift, smaller tude than that of C2 (low field shift). zeolite, almost
the chemical
identical
shift variations
However,
specific
interactions
methinic
C2 carbon atom.
in Table
1.)
In conclusion, is somewhat erencing between
if the comparison
are clearly
intermediate
the admolecule interaction
for the pure liquid
adsorbed
molecules
state are
to a general
intermolec-
change
another
physical
In general,
the interaction
is here eliminated
included
state which
however,
the ref-
(Tables 1 and 2)
and the surface at low surface coverage similar molecules
in the
in Table 2 and 1-butene/TlX
between gas and liquid.
between
on the same
is made with liquid trans-2-butene,
constitute
state better reveals
in magni-
to the gaseous
shown again by the greater
(See also l-butyne/Si02
the adsorbed
to the gaseous
der Waals
In trans-2-butene as represented
for Cl and C2, which could correspond
ular interaction.
Cl and
a small low field shift with
atom is much more influenced
suggesting
carbon
because
the van
in the chemical
in the first approximation
shift
(except CH4,
see Ref. 34). In contrast, actions
at higher coverages
has to be corrected
molecules
in the liquid states
the surface
on the resonance
3. Exchange
effects.
(see, for example,
adsorbed
nature of the counterion
(1-butene coverage
When the surface
coverage
This implies
adsorbed
increases,
the existence
the variation
are also populated
of the different
fication
is also necessary
between
distributed
signal
of their resonance
in the case of an exchange
netically
adsorbed
molecules.
shifts becomes
sites.
the adsorption
is significantly
for a study of complexes
shifts
sites of
shift (which is a weighted modified.
This modi-
with paramagnetic
since here the line widths may be so broad and the signals determination
Ag
adsorp-
the different
molecules,
and the chemical
contributions)
of the chemical
of heterogeneously
of adsorbed
amount
or Si02), the
on NaAgX zeolite with different
Indeed, with increasing
average
of
(Tables 1 and 2).
of admolecules
energies
influence
shifts vary with the nature of the surface on NaY and NaGeX zeolites
tion sites and a rapid exchange
weaker
inter-
shift for the
if one wants to study the specific
content)
decreases.
of molecule-molecule
the resonance
shift of the admolecules.
The chemical
1-butene
and the surface
the contribution
by using as reference
possible
with a sufficiently
sites
so weak that a
only via the resulting
great number of diamag-
Also, for interactions analysis
of resonance
cules is necessary adsorption
shifts measured
the experimental
are not generally
identical
NA from these experiments physisorbed
molecules
(MA) (ref. 41-43) experimental
M+AA
(MA]
bound in complexes
One way .to derive
is to consider
between
of decomposition
6c and
the purely
sites (A), and the complexes of sites
(number
to zero coverage
the quantities
the equilibrium
shifts as a function
constant
of the number N of admole-
shifts 6 extrapolated
and to derive the number
If the probability
by its stability
with 6c.
sites, a careful
number NA of sites for physical
shifts 6c of molecules resonance
(M), the adsorption
resonance
adsorption
as a function
in order to determine,the
and the resonance
NC) because
cules.
with purely diamagnetic
formed
(NA) for the fit of the
of the total number of the surface
(N) of mole-
complex
is determined
k only (model l), we can write:
(7)
.
Since the number of empty adsorption which are not bound in complexes
sites is NA-NC and the number of molecules
is N-NC, the constant
k is given by:
(8)
Combining
this equation
with respect
for the constant
to the line position
k with the resonance
of purely physisorbed
shift 6 measured
molecules
(6M=O),
NC
6
(9)
T=x=T
we arrive at the quadratic 1+ kNA JL NA + x --l=O %A
(l-x)x
For the treatment
X ’
equation:
(10)
.
of experiments,
we define an experimental
quantity:
6
=-
6m where
(11) ’
am is a maximum
N+O.
shift value obtained
from a simple extrapolation
of 6 for
According. to Eq. 10, we find:. 6m
Xm=6c= Inserting suitable
kNA l+kNA
x = x' form:
*
(12)
- x, into Eq. 10, we may transform this equation into the more
79
Nxt2 _=-'__<
1
1-x
Nx'
x,
NA
l-x
,
(13)
which allows one to extract Nx' l_x' *
the quantities
NxL2
xm and NA from a plot of -versus
In Ref. 43, a further model is treated, taking into account that the decomposition of the surface
complex
to it for physical
adsorption
Then the equilibrium M+Ax
will proceed
only if there is an empty siteu
next
(model 2).
in the system can be written
in the form:
[MAI +L.J,
and instead
(14)
of Eq. 13, we arrive
at:
(15) It is important
to note that the value xm and, hence, the real shift of the
line in the NMR spectra
(determined
used for the characterization the criterion
from unity
For the treatment is termed
to be strong
data, two special
if kNA>>l
IV.
shifts
stricting
sites are discussed The significance
mainly
47).
i.e.,
concentration
For a weak complex,
shift 6m may deviate
INTERACTION
basically
WITH DIAMAGNETIC
the static NMR parameters constants
developed
(Studies of interactions
appreciably
SITES such as the
J (in Hz), re13 C and
study of
with surface acidic
in Part VII.) of the change of coupling results concern
"s" character
(ref. 38). Theoretical results
to zero adsorbate
to the more recently
molecules.
yet; the only available in carbon
MOLECULES.
we examine
of
cases are of importance:
Then we find x,=1,
6 (in ppm from TMS) and the coupling
ourselves
15N NMR of adsorbed
change
N+O
and the extrapolated
THE STATE OF ADSORBED In this section,
chemical
holds.
insnediately the real shift 6c in a complex.
kN
Obviously,
is the deviation
(ref. 43).
of experimental
6m= bc, and from a simple extrapolation we obtain
on the surface.
for the choice of one of these two models
the value k'/(k'-1)
A complex
from this plot) does not depend on the model
of the equilibrium
constants
ethylene
has not been assessed
and propylene
and/or bond length variations
calculations
on simple models
and throw some light on the structure
for which
have been reported
help to understand
of adsorbed
some
molecules
the NMR
(ref. 45-
80 A. Analysis
of 13C NMR shifts
The chemical collected
in Tables
trans-2-butenes Numerous resonance
shifts of the different 1 and 2.
carbon atoms in various molecules
Most thoroughly
studied are 1-butene,
investigations
of molecules
adsorbed
due to interactions
and the exchangeable
cations.
interactions
butenes and silver cations
between
in zeolites between
(Fig. 2) which showed that the electronic and in solutions
(ref. 13).
structure
was strongly
state will be discussed
found in Ref. 52 for toluene, zeolite
o-xylene
(with 50% of its Na+ replaced
both from thermodynamic
measurements
(ref. 54,55).
Moreover,
systems gives direct evidence by metal
data
later.
of silver-olefin the molecular
Similar
silver,
for hydrocarbons.
This
of infrared
of proton relaxation
of adsorption
complexes
(ref. 51,56).
C NMR spectra of 1-butene: a) free liquid; b) in Fig’2.‘H1;,CHC, 3; c) in AgNaX zeolite (60% Ag+); d) in NaY 2
AgBF4
in-a 50 AgNaX Besides
(ref. 53) and the results an analysis
of
Details
results could be
adsorbed
centers
complexes
mobility
(ref. 51).
by Agt ions) at 390K.
for the existence
cations and hydrocarbons
Here a direct CM-
restricted
and p-xylene
other metal cations may also act as adsorption follows
carbons
(ref. 48- 50) was possible
was the same, although
in AgNaX and AgNaY zeolites
of the electronic
showed that the
the unsaturated
A direct proof could be given in the case of
parison with the results for silver salt solutions
olefins
cis- and
(Table 1).
shifts are mainly
in zeolites
are
solution zeolite.
of
in such formed
81 The adsorption of olefins in zeolites with monovalent cations like Li+, Nat, KS, Rb+, Cst and Tl" has been studied in great detail by means of 13C NMR. Because a direct comparison with ion solutions is not possible here, it is necessary to check systematicallywhether other sites may also be responsible for the 13C NMR shifts observed for these zeolites (ref. 36,57). The various steps of this proof, e.g., treated for I-butene and isobutene molecules as adsorbates (ref. 49,58), are: 1) to check the role of Si- and O-atoms by comparing the resonance shifts found for NaX and NaY types with Y zeolites having a very low content of Al and Na (Si/Al=47);
2) to check the influence of the small number of
structural OH groups by measuring the variation of resonance shifts with the number of admolecules; and 3) to check the role of aluminum atoms of the lattice by changing the type of monovalent cations (Tl+, Cs'). It could be shown that the 13 C NMR shifts measured for olefins in NaX and NaY zeolites are due to interactions with Nat ions. Moreover, by means of proton spin relaxation of n-butenes (ref. 51) and benzene (ref. 59), a stronger interaction with Na+ ions at S-III sites rather than with those at S-II
sites was detected. The adsorption of ben-
zene and its derivatives in X and Y zeolites leads to such a strong restriction of their molecular mobility (ref. 59) that in general no highly resolved 13C NMR spectra can be observed at room temperature (ref. 46,60). The shift for the 13C resonance of benzene in NaX and NaY zeolites measured at higher temperatures is of the order of the experimental error. For the methylbenzene, a shift of 2...4 Ppm to lower fields occurs for ring carbon atoms to which the methyl group is attached which can be explained by the polarizing effect of the Nat ions. For toluene adsorbed in a CsX zeolite, a strong anisotropy of about 130 ppm for the l3C NMR line of this ring carbon line was observed which was explained (ref. 61) by a restricted rotation of the toluene molecule about its C2-axis (with a correlation time of about 10-3s). No such anisotropy occurs for a NaX zeolite (ref. 61,62). In the case of acetone, the observed I3C NMR shifts for the carbonylic carbons are larger than the resonance shifts of the olefins adsorbed in the same zeolites. In contrast to that, the 13C NMR shifts of CO and CO2 adsorbed in NaX, NaY and NaA (ref. 63) zeolites are not very different from the values for the gaseous state. The same results hold for these adsorbate-adsorbent systems if Nat is replaced by other alkali cations or Tl+.
From the temper-
ature dependence of the observed anisotropy of the 13C NMR signal of CO, CO*, COS and CS2 molecules adsorbed in mordenites, a model for the thermal reorientation of these molecules was developed (ref. 64). Also, in these experiments 13 the isotropic part of the C NMR shifts was small. Very large shifts to lower fields were observed for CO adsorbed on decationated zeolites. The values were largest for deep-bed treated specimens but even for shallow-bed zeolites, they are much greater than for zeolites containing alkali cations (ref. 63). No such effects were observed for COR,
82 13
C NMR shifts of hydrocarbons
compared
with zeolites
of the effects
adsorbed
on silica gels are relatively
so that an interpretation
observed
with certain
and an unambiguous
types of adsorption
oxygen atoms of the silica gel, lattice defects, Several
attempts
partially.
were undertaken
to overcome
centers
small
correlation
like OH groups,
etc., is rather difficult. these difficulties,
at least
In Ref. 93 the resonance shifts of isobutenes were measured both
for partially
dehydroxylated
it has been concluded molecules
and methylated
that in contrast
with the oxygens
silica gels.
From this comparison,
to Ref. 66 the interaction
in siloxane
bridges
has no influence
of isobutene
on the 13C NMR
shifts. Significant
shifts were observed
sorbed on silica gel (ref. 67). droxylated
and methylated
ated from the coverage
dependence
of centers N,= (1.4+0.2) ylated
hydroxyl
is in accordance
between
and the clear dependence
acetone molecules
of this group with the surface
group of acetone molecules
The strong difference
specimens
the total number of adsorbed
This suggestion
for the C=O
indicates
partially
ad-
dehy-
of the shifts on
a strong
interaction
groups.
with the number of adsorption
sites evalu-
of the shifts by means of Eq. 13.
nmV2 and NA= (0.320.1)
and m'e‘thylated silica gels, respectively,
nmm2 for partially are in reasonable
with the number of OH groups for these samples.
The number dehydrox-
agreement
The same conclusion
was re-
cently drawn in Ref. 43. The formation
of hydrogen
cules and surface
OH groups of Si02has
and 15N NMR measurements; cated an appreciable the nitrogen indicate
bonds between
a strong
influence
atom, whereas
that no specific
the nitrogen
been studied
atoms of pyridine mole-
by means of combined
1
'H
low field shift in the 15N NMR spectra
of adsorption
only small changes interaction
indi-
sites on the electronic density at 13 C NMR spectra observed in the
of the r-electron
system with surface
sites occurs.
B. Analysis
of 15N NMR shifts
In the first detailed nitrogen-15 trimethylamine, were measured
pyridine
carried
nuclei
special
interest
resonance
transform
were employed
the nitrogen
atoms of a variety
zeolites
NMR techniques.
Moreover,
molecules
of molecules
in the formation molecules
may be directly protonated at the nitrogen 15 between the N NMR shifts of protonated
the difference
spectra of ammonia,
in various
which were enriched
spectra of adsorbed
which may participate
giving rise to strong 15N NMR shifts. acetonitrile
sorbed
In with
(ca. 95%).
because
electrons
Fourier
out, substances
A study of the nitrogen-15
lone-pair
(ref. 42,68),
molecules
at 9.12 MHz by conventional
all measurements nitrogen-15
NMR study
and acetonitrile
of hydrogen
is of possess bonds,
like pyridine
atom.
and
Consequently,
and nonprotonated
83 species may be considerably
larger
carbon-13
in the molecule.
spins and protons
be more favorable addition, molecule
through
prevents
resonance
their accurate
depends
characterized
in more detail than with proton
complete
strongly
pore filling
(e=l)
a strongly
dealuminated
not change
as a function
molecules
adsorbed
on the coverage
resonance
shifts
of liquid ammonia and gaseous
Y-type
zeolite,
ammonia
for adsorbed
amnonia mole-
(18 ppm) if we have nearly (0 ppm) for zero coverage.
factor.
resonance
shift does
for liquid ammonia.
of the resonance
shifts with decreasing
coverage
values measured
for the liquid and the gas can be explained
the association
of the ananonia molecules
with the Na+ ions.
is increasingly
The interaction,
influence
on the electron
influence
of the Na+ ions on the adsorption
due to the
lead to an al-
at the nitrogen
of adsorption-heat
the
by the fact that
must
though a strong
from results
density
however,
to gaseous
between
prevented
most negligible
inferred
For
Its value is about the
Fig. 3. '5 N NMR shift 6 of ammonia in NaY zeolites (in ppm referred NH3) as a function of the pore-filling factor e at 300K.
can be clearly
because
The plot (cf. Fig. 3) is
the nitrogen-15
of the pore-filling
same (16 ppm) as that reported
interaction
In
in Nay, NaX, NaA and Na-
(ref. 68).
of the resonance
characteristic
The variation
should
measurement.
by the approach
cules to values
15N NMR measurements
shifts are small and often less than the line widths which
The 15N NMR shift for ammonia mordenite
Hence,
shifts for the adjacent
for a study of the acidic sites on a surface (see below). 15 a study of the N NMR, the electronic state of the ammonia
can be investigated
the proton
than the resonance
atom, al-
behavior
measurements
of amnonia (ref. 69).
Moreover,
the same value for the indirect
was measured
at low coverages
phase (61.6 Hz), indicating has not been changed Nitrogen-15
NMR resonance
that the geometrical
shifts of acetonitrile
Fig. 4, the resonance
From the whole
pretation
of zeolites.
can be favorably
(see below).
cations
Nat ions.
of Eq. 13, a number of
in agreement
with the simple
a strong adsorption
complex.
sites is equal to about twice the number of accessible
ions (about 3.3 Nat at S-II
of
As is shown in
for CH3CN in NaY as long as the
of N by means
can be calculated,
cules are not protonated
Hz)
and a value hc= -19.5 ppm for the shift of
of the plot in Fig. 4, assuming
number of active
molecules
(62-5
of the NH3 molecule
in Na-forms
is less than the number of accessible
sites per supercage molecules
sites
remain constant
plot of 6 as a function
active
the complexed
shifts
molecules
structure
constant
in the gas or liquid
with the exchangeable
(e.g., Na+) and with Lewis-acid
number N of adsorbed
6+0.5 -
of interactions
coupling
for ammonia
in the course of adsorption
used in the characterization the zeolite
spin-spin
as that obtained
sites).
In decationated
zeolites,
but interact with the structural
interThe sodium
the CH3CN mole-
OH groups
via hydrogen
bonds.
_
-..
-+---
10 N
Fig. 4. I5N NMR shifts 6 (in ppm referred to liquid CH3N02) of acetonitrile in Complete pore-filling a= 1 corzeolite: a) NaX; b) Nay; c) in NaX for e=1.2. responds to N=lO molecules per large cavity.
C. Origin of the variation For a more detailed of interaction, tum chemical tribution
of the chemical
characterization
the observed
calculations.
(0 para)
shifts have to be interpreted For nuclei different
to the screening
netic one (adia ) (ref. 70):
shift
of the nature of bonds and the geometry
constant
in the light of quan-
from 'H, the paramagnetic
con-
is far beyond that of the diamag-
85 ' = 'dia + 'para
(16)
’
with: -. 'para =
where
'AA +A&
Q,, is a function
the bond order matrix.
Various
to calculate
the NMR chemical
the simplest
methods
volved
quantum
shifts
surface
interactions active
of alkenes
site interaction
its isomerization
The main interaction
NaX and NaY zeolites,
between
the charge distribution redistribution
to unoccupied
overestimated
in comparison
and on silicas.
here preferentially
between
the n-orbitals
in the 1-butene-Na+
out to l-butene-
because
(ref. 15 -18,
complex:
influenced
from occupied
of the olefin
have been found to 1) an intra-
by the electric
orbitals
field
of the 1-butene
of the Na+ ion (ref. 74,78).
yield
energies
74).
were carried
by 13C NMR spectroscopy
about the same total amount
(ref. 78), overestimated
The stabilization
only
l-butene and Nat ion (the active site on
transfer
orbitals
CNDO/Z and PCILO methods fer (0.2 electron)
in zeolites
of the charge density
and 2) a charge
molecules
(ref. 45,46,60,72-
of the Na+ ion. Two mechanisms
influence
molecule
studied
to
applied
of the large number of atoms in-
will be reviewed
molecular
of the cation;
and Q,, is related were already
(ref. 76) calculations
see above) occurs
orbitals
methods
(ref. 71) but for adsorbed
or arenes
was extensively
and the unoccupied
of charge density chemical
interaction
(ref. 75) as well as PCILO
describe
CNDO/Z
(17)
’
have been used because
in the admolecule-surface
CNDO
79).
1
AE is the mean electronic excitation energy,
of carbon 2p orbital,
77).
QAB
for the
of charge trans-
by both semi-empirical 1-butene-Na+
with experimental
values
methods
supermolecule (ref. 47,78,7g),
(ref.
are also the
values being much more unrealistic.
In order to choose the best structure for the molecular complex, theoretical 13C NMRshifts were compared. An empirical relationship between and experimental the variation
of chemical
the variations the chemical
of u-
shifts
shift from the free to the adsorbed
state
to the electron
densities
(ref. 72):
Au = 150 Ap, + 300 BP= . The experimental in Table different
3, where
(Au) and
(Ap,) and n- (a~~) electron densities were used to fit
and the theoretical
(18) chemical
shift variations
the CNDO as well as the PCILO results
(the cis and skew) conformations
of 1-butene.
are reported
are included
for two
86 TABLE 3 Comparison of calculated l3C NMR chemical shifts with experimental data of 1-butene adsorbed on NaX zeolite (ref. 47) Compound
4bliq(ppm) Cl
CH3-CH2-CH=CH2 (e=O.15) cis CNDO PCILO skew CNDO PCILO
C2
c3
c4
+0.2
10.1
0.9
1.6
1.65
6.25
-1.4
-0.5
-18.3
20.8
-1.8
-0.3
0.6
8.3
-2.0
-0.1
-0.5
9.4
0.5
-1.1
The most stable association is a r-complex where the.Na+ ion is localized near the r-orbital of 1-butene in a region opposite to the methyl group of the molecule in skew conformation (PCILO method) (ref. 78):
For the influence of the mean excitation energy on the chemical shift (AE), there is very little data available (ref. 45); a variation of 0.066 eV corresponds approximately to a change of 1 ppm in
Au.
Together with the charge den-
sities variations, the effect of the surface on the mean excitation energy may have important implications in catalysis (ref. 38). A lowering of this energy makes the molecules more reactive. The changes in electron densities may involve a better stabilization of either negatively or positively charged intermediates (ref. 80-83). The influence of the electric field being quite large in zeolites (l- 5 volt/W) (ref. 83) deserves particular attention. Horsley and Sternlicht proposed the relationship between the variations of the chemical shifts and the electric field (ref. 85):
"C-H
= 2.9
x
lo-llE
(cgs);
Au~_~
=
5.1 x 10-llE (cgs).
(19)
Following these ideas, a maximum variation of about 80 ppm can be accounted for.
In a molecule, however, when the contribution of all the different bonds
87 is calculated, polarization
the local electric effects
in neighboring
cated in the plane of an ethylene center
field is rather
of the C-C bond produces
bonds.
For example,
molecule
at a distance
an electric
(INCO calculations
ter of the molecule
about 4 ppm in the 13C NMR chemical calculations
yield
low due to compensating
fluenced
interaction
one, in the 1-butene-Agt
larger change
in chemical
of X or Y zeolites ing is a two-way pied r-orbital
shifts
of the adsorbed
tal); and 2) a backbonding antibonding
s*-orbital
the C2 carbon atom is the more in-
interaction
Cl shows a diamagnetic
transfer
to the 5s empty orbital
uses d xy + p, hybrid orbitals
(n-bonding)
occurs
bond-
from the occu-
of Agt ion (u type orbi-
occurs from the dxy orbital
of the olefin
and a
on the Ag+ content
It is known that the olefin-Agt
(ref. 88): 1) a charge
of the olefin
molecule
site.
(ca. -12 ppm), depending
(ref. 13) (Table 1).
bonding
to a change of
We can see that the theoretical
not only the best conformations
in the l-butene-Na+
lo-
- lo5 esu at the cen-
(ref. 86)) corresponding
shift.
charge
of 5 8 from the
field of 0.76
but also shed some light on the nature of the active While
a positive
of Ag+ to the empty
(ref. 89).
for Agt ion (ref. 90,91).)
(Another model
This model could be
Y
tx
‘XY adequate'for 2-butene.
the description
of the structure
Even in these cases,
the Agt ion is not equidistant (ref. 89,91). tron density account
In 1-butene,
however,
u-bonding
crystallographic
from the two carbons
the double
is higher on Cl than C2.
the polarization
of a symmetric
of the a-bond, would
be:
n-bonding
results
such as
show that
at the site of interaction
bond is already Therefore,
olefin
polarized
a better model,
and the electaking
into
The u-bonding would be responsible for a paramagnetic shift while a diamagnetic shift would accompany the a-backbonding formation. The change in hybridization from sp2 to sp3 could also partially explain the rather important diamagnetic shift (ref. 48). Detailed quantum chemical calculations are not available for the moment for a better understanding of the different contributions. The 1-butene-Agt ion complex can also be formed in solution. The variation of the chemical shifts in water is quite similar to those in the adsorbed state: A6(c1)
= -
14.6 ppm; AS(CP) = -1.7;
As(C3)
=
t1.1
ppm; and
Ao(C4)
= +
1.0 ppm
(ref. 48). The chemical shifts are independent of the nature of anions and only slightly dependent on solvent effects (ref. 48,50). When strong solvation of the Agt ion occurs, as in
DMSO, the specific interaction of l-butene with Ag+
is impeded (ref. 50). Quantum chemical calculations were also carried out on the following complexes: iso-butene-Nat (ref. 72,74,92), cis-2-butene-Nat (ref. 45,46,60,74), trans-2-butene-Na+(ref. 74), benzene-Nat (ref. 45,46), benzene-Mgtt (ref. 46, 60), benzene-OH (ref. 45, 46) and toluene with Na+, Mg++ and OH (ref. 45, 46, 60).
In a more realistic model containing Al or Si atoms, seven 0 and seven
H atoms were included in the aluminosilicatel-butene interaction (ref. 80) but the NMR parameters were not computed. Finally, a promising method for the study of admolecule-activesite interaction is the use of paramagnetic ions (Co2', Ni2*...) where contact and pseudocontact interactions can lead to quite large chemical shift variations (ref. 52,93,94). D. Variation of coupling constants upon adsorption 13 C-H coupling constants of ethylene and propylene upon The variation of adsorption on NaY zeolite has been reported (ref: 38). The small changes (Table 4) are interpreted as due to the variation of "s" character of the C-H bond which is also reflected in the change of C-H distance rCH (-ref.95): rcH(fi)= 1.1597 - 4.17 x 10m4 JCH
.
(20)
The influence of the solvent CDCL3 is larger than that of the surface, which shows the rather small interaction of these molecules with the NaY zeolite sur15 N-H coupling constant in ammonia adsorbed on NaY zeolite measured face. The at low pore-filling factors (6255 Hz) is the same as was obtained in the gas or liquid phase (61.6 Hz) (ref. 42). This fact emphasizes that the geometric structure of the ammonia molecule has not been changed during the adsorption. At higher pore filling factors (a,0.3), the multiplet splitting disappeared due to an increasing proton exchange rate.
89 TABLE 4 13
C-H coupling
constants
Compound
in the adsorbed e
Support
state
J(Hz)
T°C Cl
CH2=CH2gas
c2
38
156.2
104
in CDC13
159.4
38 105
Si02
-
159.4
NaY
0.14
159.4
CH3-CH2-OH
USb05
25
“Sb30 10
25
liq
125.7
38
153.3
148.4
129.3
38
CH30H liq
13x
sorbed on pyrogenic tional preference
silica
coupling
(ref. 96).
25
140
107
25
140
107
constants,
constants
The JHH coupling
constant
It is concluded
MOBILITY Despite
and is more stabilized
of 4.5 Hz increases
in polar solvents,
temperature
effect
effect are able to explain
that the 1,1,2-trichloroethane
read-
for the conforma-
constant
rota-
result-
(ref. 97).
to 5.5 Hz with increasing
temper-
(t 0.15 Hz per the observed
is adsorbed
varia-
preferentially
(ref. 96).
OF THE ADSORBED
SPECIES
the fact that l3 C nuclei are essentially
contributions
mention
It has been shown that the gauche
dipole moment
in the gauche conformation
we also
in 1,1,2-trichloroethane
JHH for solvents with a higher dielectric a simple
105 105
ing in a smaller
Neither
104
127
mer has a higher
1OO'C) nor a "substituent-like"
74
126.9
Evidence was provided
at the solid surface.
ature from 40 to 8DOC.
154.9
104
of data on the coupling
on proton-proton
74
122
0.24
of the paucity
149.9
127
liq
sults reported
155.5
0.74
Si02
38
160.6
140.3
Si02
Because
38 147.1
gas
NaY
V.
-
159.9
Si02
in CDC13
tion.
c3
liq
CH3-CH=CH2
0
Ref.
relaxed by intramolecular
simplifies the subsequent data treatments, few 13 studies have been devoted to the C NMR relaxation measurements because of 13 the low sensitivity of 13C nucleus and the cost of C-enriched compounds. These compounds ation
(ref. 34), which
are indispensable
times at low surface
times (Tl in s),together
in order
coverage.
to get reliable
The available
with NOE effects,
values of the relax-
longitudinal
are reported
relaxation
in Table 5.
.
90
TABLE 5 Longitudinal relaxation times T, (5) and NOE factors measured in the gaseous, liquid and adsorbed state "(MHZ) suoport
Compound
et
T'C
Tl (s) (NOE)++ Cl
CH4 gas
CH,=CH, gas c
Ref. c3
C2
c4
c5 38-
20
-
23
0.021(1.0)
20
NaY
23
0.030
38
23
0.050(1.0)
38 38
20
L
in CDC13
CH3-CH=CH2 gas
20
2.3 M
23
7.500
20
NaY
23
0.450
20
in CDC13
CH3-CH2-CH=CH2
CH3-CH=CH-CH3 trans
38
0.095(1.0)
20
2.6 N
23
59.900
58.700
65.200
38
20
NaY
23
0.81
1.6
0.81
38
25.1
LiX
:::I:::]
0.9(1.5) Z.l(l.5)
36 36
0.2 0.8
0.3 1.2
0.6 2.2
:i
;.;n;.;j
y{;.;j .
:::
25.1
CH3-CH2-CH=CH2
38
23
NaX
5.7/cage 2.3/cage
::
b.O/c?.ge 1.5/cage
z: 23 23
25.1
KX
3.0/cage 1.3fcage
25.1
RbX
5.4/cage 2.1/cage
:: 23 23
25.1
csx
4.9lcage l.z/cage
25.1 20.1 25.1 20.1 20.1
NaGeX
0.5
;.$.;]
.
.
.
1.8(1.3) 3.2(1.9)
X:$
0.3(1.6) 1.4Cl.4)
0.5(1.5) 1.7(1.4)
O.Z(l.4) l.E(Z.3)
0.2( 1.3) 2.3(2.0)
:::I:::]
:“6
0.38(2.2) 0.33(2.2) 0.17(1.9)
99 * 100 99,100 99,100 99,100 99,10D 99,100 99,100
.
.
0.24(2.0) 0.22(2.0) 0.32tl.6) 0.20(2.0) ~.;p~‘;~’
0.9
20
NaY
22.63
NaY
20.1
NaGeX
0.9
0.8
20
0:71+++
K:Ii:iI 0.14$1.7) 0.35 ++
ii
0.18
0.71
0.35
1.32
0.049(2.7)
0.060
0.056(2.7)
0.075
20.1
0.53(2.0)
0.24(2.0)
99,100
99,100 11
25.2
Si02
1.10
28
0.33
0.16
105
CH3-CO-CH3
15.0
s102
0.83
28
0.15
0.09
105
CH3COOH liq
15.0 15.0
SiD2
-
RT RT
30.0 0.72
8.1 0.58
111 111
CH3-CH2-COOH liq
15.0
CH3-CH2-CH2-CDOH liq
CH3-CH2-CH2-CH2-COOH liq
32.8
6.6
6.3
sio2
RT
0.51
0.17
o.lJ7
15.0
-
RT
24.4
3.3
4.0
5.7
15.0
sio2
RT
0.42
0.13
0.22
0.77
15.0
-
RT
9.7
2.0
2.5
4.0
4.5
111
RT
0.37
0.14
0.22
0.33
0.92
111
15.0 @liq
4@-CH2-Ca3 3 2.5
3
111
RT
15.0
111
liq
N(CH2-CH3j3 liq
4@N(Ci?,Cfi,), 3 2
liq
111
sio2 -
RT
23.3
15.0
sio2
RT
2.2
15.0
-
RT
41.1
15.0
sio,
RT
2.51
15.0
-
RT
31.2
18.1
17.7
15.0
sio2
RT
1.18
0.91
0.91
15.0
-
RT
12.4
11.0
111
15.0
sio,
RT
0.06
0.40
111
L
111 14.1
13.6
10.1
0.58
0.58
0.46
‘
11.6 c6:7.4 7.4 C6:1.37
111
15.3
12.7
111
0.60
1.05
111
111
15.0
-
RT
41.4
7.84
7.99
6.49
11.5
111
15.0
sio2
RT
0.70
0.19
0.46
0.24
0.28
111
15.0
-
RT
32.2
3.4
3.5
2.5
2.2
15.0
sio2
RT
0.65
0.15
0.19
0.15
0.16
'pore filling factors (reolitesl or statistical monolayers (S102) if not stated othenrise. ++NOE factors in parenthesis (II):ratio of signals with and without NOE +++pse"do-liquid
111
15.0
2
4@CH3
111
3.6 0.61
111 111
91 The sole complete study reported on both T1 and 12 (transverse relaxation times derived from linewidth and from spin-echo measurements) variations with temperature concernsthesystem n-butanol I3C labeled at Cl adsorbed on NaY zeolites (ref. 27). For a pore filling of e=0.8 T2 values are reported (Fig. 5).
(4.4 molecules per large cavity), the T1 and T1 values were determined from pure exponen-
tial functions obtained in the inversion-recoveryT-T-;
pulse sequence. The
large (T1/Tq)min* 9 ratio obtained at the minimum of the longitudinal relaxation time can be explained by a model involving a distribution of correlation times (ref. 2,27).
(Because of nearly equal values for T1 and T2 at high temperature,
the model of exchange between two regions with different correlation times cannot account for the experimental results.)
Fig. 5. 13C-spin relaxation times of -CH OH of n-butanol as a function of temperature (e=0.8, V= 25.2 MHz (ref. 27)). ?, derived from linewidths (0) and from decay of Hahn's spin echo (+).
The
I3C-relaxation times can be explained in terms of
the dipolar coupling
between carbons and protons. A log normal distribution is supposed for the spectral density function: m
Jb,i
=
'TC
F(z) 1+
-co
dz ,
(“c’c)2
(21)
where ~~ is the correlation time and wc is the Larmor frequency of 13C-nucleus. The distribution function is given by: F(t) = ---$-exp
?I
(22)
92 with
z= In -, 'cm tion parameter.
where
T Cm
the mean correlation
is
time and 6 is the distribu-
If one replaces the spectral density function in the expressions laxation
of the re-
rates:
3J(wc)+6J(wH+wc)+J(wH-WC)
, (23)
4J(0)t3J(wC)+6J(oH)+6J(wH+OC)+J(WH-WCJ with n the number of protons
in a CH,-group,
culated.
between
The best agreement
tained for B= 3, corresponding to 2.10-10
s at 286 K.
be obtained
is essentially
to an interval
enhancements
than the dipole coupling
with adjacent
times cannot be attributed
jump lengths or to an anisotropic
with n-butanol
with surface molecules
times (ref. 27). 13 C NMR linewidths surements
The T1 measurements
were performed
of Brevard
stored
in the same memory,
Finally,
FID.
instrumental
The integrals errors
uniformly
The different are therefore
error on the Tl values being the intensity
is about 5%.
coverage
on the sample with curred because
is 0.5.
errors
with
T values is by ZOO- 300 Hz from this unique
etc.) being dis-
The NOE measurements
The variation
shift"
the systematic
in FT processing,
obtained
the average
standard
of In (S,- S) versus 'I (S,
relaxation
A two-exponential
condensation
on NaGeX were
frequency
are obtained
lo- 20%, while
to NOE mea-
(ref. 7,99,100).
being shifted
(T= 4s) and S the inten-
by a single exponential
e= 0.9 at 250 K.
of butene
adsorbed
the "repetitive
signals
of the line at complete
sity at time T) is characterized
molecules
of
pores
of correlation
times in addition
much more reliable,
through all data points. are reproducible
of the zeolite
and temperature
frequency
of cor-
only the heterogeneity
Each FID with different
the carrier
(drift, round-off
from gated decoupling
the surface
following
rather
motion with small
for this distribution
coverage
on the surface
mechanisms
The broad distribution
relaxation
et al. (ref. 116).
from one FID to the other.
tributed
motion.
are responsible
method
summed
protons.
sites and an uneven filling
of surface
for the n-butanol/NaY
of the molecule
to either a translational
and longitudinal
as functions
rate curves can
of other relaxation
of the 1-butene and trans-2-butene
determined
is ob-
times from 8.10q8 s
relaxation
(1.8- 1.2) observed
due to the reduced mobility to the occurrence
the interaction
of correlation
can be cal-
values
of Ref. 27.)
and cannot be attributed
relation
rate curves
and experimental
(Useful tables of simulated
from the authors
The small Overhauser system
theoretical
theoretical
’
behavior
behavior
In this case, however, on the surface
as long as
was clearly
demonstrated
a phase change oc-
(ref. 117).
93 Fig. 6 shows adsorbed ature
on NaGeX zeolite
(250 K) results
linewidth between uith
the variation
(Table 6).
shows a greater
ents determined difference torr-l)
intensity
interaction
in mobility
with
of AH with
temperature
for trans-2-
The adsorption
and kinetic
data support
times Tl of carbons
suggests
coeffici-
entirely
this
8.2. 10W2
in 1-butene
for carbons
This comparison
resemblance
of these carbons
5. 10m2 tort--l; trans-2-butene:
(1-butene:
(Table 5).
interaction
temper-
In the
molecules.
there exists a greater
order C4> C3> C2> Cl at 300 K; they are similar C4 at 250 K (e=0.5)
for 1-butene
of e= 0.5; the decreasing
the surface.
isotherms
The relaxation
(ref. 17).
temperature
of the adsorbed
denote a greater
The variation
from adsorption
with
coverage
variations,
1, 2 and 3 which
the surface
butene
at a surface
in a lower mobility
and relative
carbons
of the linewidth
vary in the
Cl and C3,and C2 and
a n-complex
nature
of
-CH,cli2=
-UC+&
C NMR spectra
of 1-butene
as a function
of temperature
(e= 0.5; v=
::%36MH:;.
TABLE 6 13c NMR linewidth
(AH in Hz)of gaseous,liquid
Compound 1-butene
e
T°C
and adsorbed Cl
c2
1-and'trans-2-butenes c3
c4
Ref.
gas (p= 2.5 atm)
28
5
5
5
5
37
liq
20
0.5
0.5
0.5
0.5
103
23;0
0.9
25 -23 30 2 -23
33:
35 140 30
35 110 $55
90
;I:
45
Z 20 25 30
99,100 99,100 99,100 99,100 99,100
-
28
5
5
37
20
0.5
0.5
103
29 -15 -26 -38 -59
30 65 65 100 300
94:
99,100 99,100 99,100 99,100 99,100
NaGeX
trans-2-butene
0.5
gas (p= 2.0 atm) Iiq NaGeX
0.8
120 700 1300
94
the adsorbed species, between the 1-butene molecule and the surface at 300 K, where Cl is more influenced than C2 and this influence decreases in the carbon chain. At 250K, the different carbon atoms are closer to the surface and the carbon atoms 2 and 4 are relaxed at comparable rates, respectively. This low temperature species can be characterized as a cyclic model (Fig. 7).
In this
complex, the molecule is quasi-parallel to the surface and the two adjacent interacting sites could be an acid site (Na+) linked to Cl and a basic site (oxygen atom) interacting with one proton of C3. ,CH,-CH, H,CyCH
A
,CH, ,‘% CH HzC ,
&& &,
n - complex --
cyclic
complex
Fig. 7. s-complex (high temperature) and cyclic complex (low temperature) models of l-butene adsorbed on NaGeX zeolfte. At 8=0.9, the TI values of C2 show an approximate minimum at 275 K: 0.12 s. The TI/T; ratio is rather high (-10) and is characteristicof a distribution of correlation times (ref. 27,118). From this minimum, a mean correlation time ~~~ 10 ns can be calculated. The NOE factors are close to the value of two in all carbon atoms, except the methyl group at 250 K, where it is equal to 2.6.
The dipole-dipole carbon-
hydrogen interaction probably constitutes the main relaxation mechanism. The low NOE factors can again stem from the lower mobility of the adsorbed molecule. Nevertheless, a contribution from spin-rotation interactions cannot be excluded (ref. 36). The NOE factor is smaller at lower temperature for carbons Cl, C2, and C3, which are closer to surface than C4, which is always less influenced and rematns quasi-free. In the case of rather high NOE effects (close to the maximum value) for the methyl group, one could attribute the basic mechanism of relaxation to the internal rotation of the methyl group (ref. 119).
From the value of the correla-
tton time, one can easily compute the energy barrier of rotation (ref. 120). The energy barrier of rotatfon could be a useful parameter to characterize the fnteractlon between the admolecule and the surface (ref. 38). At 20.1 MHz and 300 K, the relaxatton times of the different carbon atoms of 1-butene are independent of surface coverage. Oppositely, they do depend on surface coverage at lower temperature (250 K), where a two-exponentialbehavior was observed for carbons 2, 3 and 4.
The low TI values at e= 0.9 are similar
to those determined at e=0.5 and characteristicof the adsorbed phase. The
95 higher T1 values show the formation as a pseudo-liquid
for which
of a new phase in the adsorbed
the interaction
with
the surface
layer such
is still signif-
icant (ref. 99,100). The comparison particularly
of the T1 values of gaseous, The T1 relaxation
interesting.
in the gaseous NaY zeolite
phase,
in solution
(ref. 38).
phase, where
relaxation
mechanism
times characterize
play a role and the total relaxation
magnitude
The corresponding
respectively.
1.6 and 0.81 s.
iate behavior
between
values
the gas
and spin-rotation
rate is about three orders of
in the adsorbed
The adsorbed
gaseous
phase on
59.9, 58.7 and 65.2 s for Cl, C2 and C3,
lower than in the gas phase;
are 0.81,
is
investigated
(NOE factor: n= 1) is very effective
In the liquid, both dipole-dipole
(ef. 36,38); 0.095 s for Cl. mechanisms
molecules
of CDC13 (2.6 M) and in the adsorbed
The shortest
the spin-rotation
liquid and adsorbed
times of propene were
molecules
and liquid,
state on NaY zeolite
show, therefore,
as is also suggested
an intermed-
by chemical
shift
data.
In addition to the dipole-dioole paramagnetic viously
centers
for the behavior
was nicely demonstrated
tem (ref. 36).
for the system
on NaY zeolite
propene
of relaxation
relaxation
Nevertheless,
be necessary
adsorbed
with
pre-
(ref. 11) and it
on NaY zeolite
(ref. 36). 3+ Fe
time was found with increasing
in order
NOE effects,
could also play a role in the propene-NaX
a more detailed to unveil
As the distribution
mechanism.
a NOE value smaller
VI. IDENTIFICATION
mechanism
ACTIVE
of the surface
in heterogeneous
in decreasing
one cannot by itself be a
(ref. 27).
IN PRESENCE OF ADSORBED MOLECULES
SITES
Although
unknown,
by a combined
sys-
study
of this relaxation
times also results
active sites
catalysis.
sites in most cases remains can easily be studied
the participation
than the maximum
OF THE SURFACE
The identification
temperature-dependence
of correlation
proof in favor of the spin-rotation
problems
adsorbed
the interaction
It has been suggested
of the zeolite.
The spin-rotation
would
mechanisms,
importance.
of 1-butene
For this latter, a decrease content
relaxation
is of paramount
is one of the most exciting
the exact nature of the active
the Broensted multinuclear
acid sites of the surface
NMR technique
(ref. 42,114,
121- 123). The behavior ferently
of pyridine
treated Y zeolites
The l3C NMR spectra The chemical
molecules
of pyridine
shift difference
upon protonation.
It was,
amount of the pyridinium
and of pyridinium
is systematically and pyridinium
between
therefore,
ions.
studied
ions adsorbed
on dif-
by 13C, 15N and 'H NMR.
ions are reported
in Fig. 8.
carbon 2 and 3 shows a great variation chosen for the computation
Following
a fast equilibrium
of the relative
between
pyridine
I
-
PYRIDINE
PYRIDINIUM
Fig. 8. I3C NMR spectra of pyridine and pyridinium ions. Pyridfne: A) pure liquid; B) adsorbed on Nay; C) adsorbed on USEx (both 3 molecules per large cavity). Pyridinium ions: A) in H SO4 (1:3 molar ratio); B) coadsorption of pyridine and HCl on NaY (3:6, numi5 er of pyridine:HCl); C) similar to B on USEx (adapted from Ref. 114). and pyridinium ion, it can be shown (ref. 114):
I62-b3fobs =
(I- Ppy+) 25.9+ 13.3 ppyt
(24)
,
where 25.9 and 13.3 ppm stand for the difference in chemical shifts for the pyridine and pyridinium ion. It has to be emphasized that the chemical shift difference is the same whatever system was used; it seems, therefore, that thfs technique is of general applicabflity for the study of the acidity of solid surfaces (ref. 113). The relevant data on.dlfferent decatfonized samples are collected in Table 7 which shows that less than one-half of the total number of OH groups give rise to the formation of pyridinium ions.
From the total number of pyridine molecules
large cavity
amount of pyridfnium
(n), the relative
number of OH groups (nOHI as detained
by wide
per
fons (p,y+) and the total
line 'H NMR measu~n~,
the
relative amount of acidic OH groups is easily computed. These data are in good agreement
with the number of OH groups
determined
by IR measurements (ref. 124).
The use of l5N-labeled pyridine Is even more interesting shifts depend
very strongly
on the nature of the adsorption
tions are much larger than in the case of analogous
42).
because sites;
the chemical their varia-
13 C NMR measure~nts (ref.
91 TABLE 7 Computation of the number of acidic measurements (ref. 42,114) n+
Support 70 OeNaYtttttt 85 OeNaY 85
(670)
(670)
DeNaY (570)
88 HY
'number
of pyridine
ttnumber
ttttnumber
tttttt70%
decationized
9.
&15N ohs
where
=
shifts
, tttt
"OH
3-4
0.50
1.5
0.70
2.1
3.0
6-7
0.60
1.8
2.1
0.912++ttt
3.0
0.847ttt+t
4.6
0.650ttttt
per large cavity (from wide
line 'H NMR)
ions per large cavity
zeolite pretreated
of pyridine
at 670 K for 20 h
and pyridinium
ions are reported
versus CH3N02 are shown in Table 8.
the observed
(I-
-it:
from both 13C and l5N NMR measurements
l5N NMR spectra
Chemical
region,
pPy+
5-6
of acidic OH groups values
t-t
3.0
per large cavity
of pyridinium
tttttsame
Typical
"OH
shift
3.0
molecules
of OH groups
tttfraction
sites from 13C and 15N NMR chemical
PPyt)(-90)
chemical
in Fig.
In the fast exchange
shift "ohs is given by:
+ ppyt(-180)
,
(25)
-90 ppm and -180 ppm are the chemical shifts of pyridine and pyridinium, 15 The agreement between the N and l3C NMR measurements is very
respectively.
good (Table 7), showing Broensted
acidity
clearly
the potentiality
of the method
to study the
of the solid surfaces.
C,H,NH*in solution
p -50
-100
-150 -
6 (mm)
Fig. 9. l5N NMR spectra of pyridine molecules adsorbed on Nay: A) 2.4 per large cavity (300 K); B) 5.6 (300 K); C) 5.6 (380 K) and on 88 DeNaY zeolite; D) 4.6 (380 K) (chemical shifts in ppm referred to CH3N02) (adapted from Ref. 42).
98 The complementary interesting
use of ammonia
observations.
on Nay, NaX, NaA, Na-mordenite phase value
value
different
organic
and US-Ex zeolites
the chemical
(ref. 42,68)
shift of which
(Table 8).
strong interaction
the protonated
15N NMR study of
is thus open to deter-
of the surface acidic
with Lewis acid sites was also evidenced
sites.
zeolites
(88 DeNaY) the CH3CN molecules
with the structural
OH groups via hydrogen
higher fields with respect
the values measured
molecules
adsorbed
for the pure sodium forms.
zeolites,
chemical
on silica-gel
In contrast,
shifts to lower fields appear
be used in the characterization
of Lewis acid sites created
of adsorbed
pyridine
the longitudinal
coverage
at 313 K, while coverage
l-
lo-SC 1
I
I
’
0 2
,
’
strongly
At low surface
and T2 increases
(4 molecules
during
These
results
I
1
’ 3
’
,
I
’
’
4 l/T
~10~
I
’
5
de-
C NMR results
The behavior
on the number
with increasing
temperature.
of
of adsorbed per large At higher
the TI curve shows a minimum higher than the low surface
have been interpreted
two regions of different
13
(2 molecules
per large cavity),
and to
the stabili-
coverage
the T2 values are systematically
values.
between
ions (ref. 121,122,125).
surfaces
for stabilized
quite well the 15N and
time TI depends
(Fig. 10).
TI decreases
surface
change
and pyridinium
relaxation
pyridine molecules cavity),
data complement
order of magni-
(Table 8) which can
ation process. The 'H NMR relaxation
decat-
but interact
The shifts which are to
to the gas phase are of a comparable
tude to those for acetonitrile
cationated
are not protonated
bonds.
The
by a large 116 ppm
low field shift relative to the gaseous state. 15 N NMR chemical shifts of acetonitrile reveal that even in strongly ionated
anznon-
is close to that of aqueous
The way of combined
but also the strength
leads to
form of alrmonia
is close to that of the gas
bases with a large range of pKA values
mine not only the number
acidity
shift of the adsorbed
In the case of 88 OeNaY zeolite,
(see above).
ium form was detected, solution
for the study of surface
The chemical
in terms of a rapid ex-
thermal mobility
which are attributed
I
’
-
6
(K-l)
Fig. 10. IH NMR relaxation times for pyridine adsorbed on 85 DeNaY zeolites as function of temperature: A) 4 molecules; and B) 2 molecules per large cavity (adapted from Ref. 121).
99
TABLE 8 15N NMR chemical shifts of adsorbed molecules (ref. 42,6B) Support
Compound
et
NH3 gas
ToC $H3N02 27
(ppm)
-399.9 -382.0
Tig NH; Cl- (1 M in 10 M HCl)
-349.92
-
NH3
NaY
0.1
-394.9
NaX
0.5
-388.9
NaA
0.8
-383.9
USEx
0.1
-383.9 -383.9 -383.9
CH3CzN
88 DeNaY
20.5 mol/cavity 9.0 mol/cavity 1.9 mol/cavity
-368.9 -361.9 -360.9
70 HY
1 mol/cavity ID mol/cavity
-293.9 -292.9
88 DeNaY
1 mollcavity 5 rollcavity 10 mollcavity
-160 -152 -147
USEx
1 mol/cavity 5 mol/cavity 10 mol/cavity
-148 -139 -137
NaY
1 5 10
27 ;:
-156 -156 -148
;:
-164 -154 -148
(BO)LiNaX
2
22
- 158.6
(BD)Li KX
2
22
-153.4
(71)Rb NaX
2
22
-149.1
(5D)Cs NaX
2
22
-148.7
(20)Ag NaX
2
22
-172.4
(60)Ag NaX
2
22
-183.4
NaX
k 10
27
CH3-CeN gas
-126.5
Tie
-136.4 -239
CH3-CeNH+ CH3-CeN
70 DeNaY (stabilized) sio2
@
27
-109 - 146 -144
:::
-57.6
gas RT
liq sol (4.3 mol% in H,O)
_
-63.9 -179.6
NaY
2.4 mol/cavity 5.6
107 107
-90.7 -89.2
sio2
0.08 0.16 0.79 1.35
107 107 107
-89.3 -89.9 -85.7 -70.9
107
'pore filling factors (zeolites) or statistical monolayers (Si02) if not stated otherwise
100 to adsorbed
pyridine
molecules
time for the thermal motion magnitude
and pyridinium
of the pyridine
as the values for adsorbed
The correlation nificantly
time for adsorbed
benzene
pyridinium
larger than that for adsorbed
by one or two orders of magnitude
protons
(10S5 s).
These apparent
is of the same order of -8 10 s at 313 K.
ions (5. 10S7 s at 313 K) is sig-
dinium
to another
pyridinium
have been explained
above room temperature.
framework.
the proton of the hydroxyl At temperatures
cules are bound to the solid surface The total amount of OH groups
values. 2rigid the minimum of the
OH groups oxygen
These
OH groups
bond. from the
1 H NMR signal
(n,) is computed
to transverse
from the T2min
relaxation
times at
curve and in the region of rigid lattice is used as a relative
nA/T
group to another
if T is the mean residence
measure
time of a hydroxyl
behavior,
for the acidity
of
proton at a lattice
(ref. 122).
Quite recently,
a very valuable method of hydrogen
It was also possible mean size.
on platinum
to determine
In the absence
was explained
using 12'Xe NMR was proposed particles
of chemisorbed
oxygen,
6Pt and "NaY stem from the collisions
probabilities.
NaY sites occurs.
Because
from the chemical
these components neighbors,
is very important
shift values
particles
between
the Pt and of the 12'Xe
and precise data can be
two different
to Xe atoms striking
with chemisorbed
hydrogen.
proves that the Xe atoms exchange
but they do not diffuce
par-
(ref. 127).
of a small amount of hydrogen,
The new NMR line is attributed
and platinum
shift
vPt and vNaY are the correspond-
of the large value of rSpt, the variation
obtained
zeolite
chemical
of Xe atoms with the platinum
In all cases, a fast exchange
shift with Xe concentration
After chemisorption
the observed
and their
at low xenon coverage:
respectively;
chemical
observed.
particles
to probe
(ref. 126).
’
ticle and with the zeolitic wall, ing collision
in NaY zeolites
the number of platinum
as the sum of two contributions
obs = VPt bPt ' "Nay 'Nay
where
via a hydrogen
latter correspond
log T2=f
The ratio
the chemisorption
6
molecule
below 295 K, pyridine mole-
is easily determined
the number of acidic
and T
respectively.
as a pyridine
After
group and then being again converted into a pyri-5 after an average time of ca. 10 s, a jump of the
ion occurs which carries
while
by the fol-
hydroxyl
of the zeolitic
intensity,
time of hydroxyl
comes into contact with a hydroxyl
ion is formed at temperatures
ion. Less often,
oxygen
They are, however,
than the correlation
the mean lifetine of 5. 10S7 s at 313 K, it is desorbed hopping
The correlation
or cyclohexadiene,
discrepancies
When a pyridine molecule
group, a pyridinium
molecules
pyridine molecules.
shorter
lowing model:
ions (ref. 121).
lines are
the wall of the
The existence
of
only with their nearest
rapidly across several
supercages.
The
101 former
line decreases,
hydrogen
concentration
this proportionality that chemisorption At higher
size.
while
to Pt particles
the new line increases
up to 2 hydrogen
of hydrogen
conclusion
concerns
corresponds
Based on
on all particles
of similar
which is attributed
atoms.
the size of platinum
particles
The mean number of Pt atoms per particle
this indirect method. type of particle
particle.
a third NMR line appears,
hydrogen
with increasing
narrow NMR lines, it was concluded
occurs homogeneously
content
with 4 adsorbed
An important
atoms per platinum
and on the relatively
hydrogen
in intensity
to a solid developed
mainly
obtained
is ca. 8.
in two dimensions
by This (ref.
128).
VII.
STUDY OF THE CATALYTIC
The heterogeneous desorbed
reactants
situ the catalytic ferentiating behavior
TRANSFORMATIONS
catalytic
reactions are usually followed by analyzing the 13 C NMR spectroscopy enables one to study -in and products. transformations.
between
different
of the adsorbed
state
In addition,
it has the advantage
carbon atoms and of identifying
of dif-
the specific
(ref. 100).
Table 9 shows the different is thus proven
unambiguously
systems investigated in the adsorbed phase. It 13 C NMR can be used to provide quantitative that
TABLE 9 Reactions
studied
by 13C NMR in the adsorbed
state
support
reaction CH3-CH2-W=CH2
Isomerlzation
B
NaCaY (67% Ca)
0.7
A’203
0.x-
1
NaX
CH3-CH=CH-CH3
CH3-CH2-CHO CH3-CO-CH3
ds
Olfgomerlzation Aldol condensarion
131 35
300
17
2
24
77
NaCaY (67% Ca)
0.7
107
16
SNSbO
1.3- 2
24
132
NaGeX
0.8
NaGeX *'2'3
0.25-
300 32
18 17
1.3- 2
24
77
sio2
4x 10-6 nml " -2
zo- 120
108
A1203
4.4
22- 322
133
250- 300
134
150- 350
100.135
350
136
H-2%5
CH30H+
H-ZW5
CH2=CH2
Dehydration-polymerization
H-ZSH-5
CH30H Poly~rization-cracking Polynerizatlon-cracking
CH30H Pyrolysis
1.3-
SnSbO
CH30H Oehydration-polymerization
CH2'CH2+
18
25
0.8-
NaGeX
CH3-CH20H
32
1.4
HY
CH30H Dehydration
CO Dehydration-polymerization
Ref.
15,16,49,129,130
160- 165
SnSbO Polyl!wiratlon
T'C 37- 60
1
0.08 ml/g
0.08 ml/g
150- 350
100,135
H-ZSU-5
25- 350
83,100,137
H-ZSU-5
zo- MO
87,138
NaX
420
158
400
159
csx Parafolmaldehyde
Pyrolysis
NIX csx CSBX
CH3-CH-CH2 CH2=F-CH3 CH3
Polynwlzatlon Dimriration
H-ZSM-6 NaCaY
27- 327
138 130
102 data on catalytic
reaction
a suitable
for the determination
state.
method
kinetics.
In such conditions,
state must differ
It is also demonstrated of equilibrium
the observed
equilibrium
from the corresponding
ences are explained
constants
constants
in the adsorbed
in the
value in the gas phase.
by the nature of the surface
values of the adsorption
that 13C NMR is
constants
complex
for both the reactants
adsorbed
These differ-
and the respective and the products
(ref.
17,77,139). The most thoroughly
studied
reaction
NaCaY (67%Ca) zeolite (ref. 15,16,49), and mixed
tin-antimony
oxides
function
time intervals.
From the variation
constants,
are strictly
logarithmic
(ref. 171, alumina
adsorbed
kc _t , can be determined.
kl_*,and
The apparent
as a
kinetic
by the good linearity
rate
of the
plots.
The kl_2 constant are equal,
simultaneously. 1-butene
of
on NaGeX
the geomet-
Fig. 11. Double bond shift of l-butene and geometric i r~$$n,&$;; trans-2-butenes adsorbed on NaGeX zeolite followed by (3OO'C). The spectra are similar as in previous work (ref. 16) where 13C-enriched butenes were adsorbed.
E-butenes
on
(ref. 18)
of the line intensities
shift kinetic constants,
first order as evidenced
of 1-butene
Fig. 11 shows the evolution
and cis- and trans-2-butenes
of time, the double-bond
ric isomerization constants
NaGeX zeolite
(ref. 77) (SnSbO).
the l3C NMR spectra of 1-butene at various
is the isomerization
also
for the disappearance
of 1-butene and the formation
showing
that no side reaction
Similar
unambiguously
conclusions
on SnSbO catalysts.
were reached
A comnon
feature
of
takes place
for the isomerization
of
of both NaGeX and SnSbO catalysts
103 concerns higher
the cis/trans equilibriumconstants
than the corresponding
greater isomer
tendency
cis/trans
(ref. 82). various
SnSbO catalysts
intervene
Moreover,
the difference
the two types of catalyst of an anionic
of kl_2 constants
also supports
in the appar-
is the high initial
transition
with the surface
the hypothesis
the 'TI-and cyclic-complex
as real intermediates
isomerizations.
to the trans
state because
is known to be more stable than its trans conformation
The variation
ition state.
account
between
ratio which is characteristic anion
stems from the
with respect
is ca. 0.5 kcal mol -' (ref. 140).
similarity
the cis-n-ally1
state which are
This difference
of the cis-Z-butene
Indeed, on most catalysts
ent heats of adsorption The second
gas phase value.
for adsorption
(ref. 17,77,100).
of the adsorbed
Finally,
the nature of the adsorbed
mechanismcould
of the
of an anionic-type
detected
in both the double-bond
a detailed
basicity
trans-
on the surface
can
shift and the geometric
be proposed
which takes into
species and that of the transition
states
(ref. 77,100). In the methanol-to-hydrocarbon zeolite,
the role of methanol
importance
of carbenium
polymerization
process
tion, it was concluded
conversion
on the shape selective
as an autocatalytic
ions was assessed (ref. 135).
on the H-ZSM-5
zeolites
VIII.
HIGH RESOLUTION MAGIC-ANGLE
agent was established;
as intermediates
and methanol
the
in the dehydration-
From the ethylene-methanol
that ethylene
dently
H-ZSM-5
conjunct
are reacting
reac-
quite indepen-
(ref. 100,83).
.. High resolution to investigate
allowed
surface
side the zeolitic in ZSM-5 zeolite
channels
species
Large molecules
opens new ways of studying
and strongly
Tetrapropylamnonium
the low power
13
proton enhancement,
intersections in ZSM-5 zeo-
information
incor-
growth mechanisms
C NMR with the high power MAS technique valuable
ions
The progressive
and crystal
in-
are shown to be
the channel
of ZSM-11 zeolite.
the nucleation
which are
can also be trapped
ions in ZSM-11 zeolite They occupy
motion
studied.
molecules
in the linear and zig-zag channels channels
(ref. 146,158,159). and without
systems
as a result of pore plugging
frameworks.
lite or in the perpendicular
in which molecular
hydrocarbon
during crystallization.
and their alkyl chains extend
By combining
betweeen
(ref. 141).
framework
species
10 shows the different
and tetrabutylamnonium
in their respective
poration
adsorbed
Table
a distinction
inside the zeolite
chemisorbed
intact
of strongly
(ref. 100).
This technique trapped
magic-angle
the nature
is highly reduced
SPINNING "C NMR OF STRONGLY ADSORBED MOLECULES 13 spinning C NMR appears as a superior technique
can be obtained
with
on the
behavior
of adsorbed molecules (ref. 62,160,161). First applications have alSO 15 N NMR MAS measurements (pyridine on v-alumina, Ref. 162). been reported for It is expected
that magic-angle
a strong reduction
of adsorbate
spinning
at low temperatures
mobility
occurs,
(ref. 1631, where
will open new ways for a study
104 TABLE 10 High resolution proton enhanced magic angle spinning 13C NMR spectra of adsorbates Compound A) With MAS Small molecules entrapped on coking
Support H-ZSM-5
T°C 23
Ref. 141
H-Mordenite (H-MDR) K-ZSM-5
142
Cs-ZSM-5 K-MOR Cs-MOR Polyethylenes
H-ZSM-5
23
141,143,144
H-MOR Polypropylenes
H-ZSM-5
Polyisobutylene
H-ZSM-5
Tetrapropylamnonium
ZSM-5
23
146.147
Tetrabutylamnonium
ZSM-11
23
146,148
Tetrabutylphosphonium
ZSM-11
23
146
Vinylmethyldichlorosilane
asbestos
149
Ethylbenzene
NaX
61
Toluene
csx
144,145 144
CsBX Formic Acid
NH4Y
150
HYUS
co2
Na-MOR
107
Acetoacetamide
SiO2
151
Organosilanes
SiO2
147.152
Aminosilanes
SiO2
153
CH3OH
MgO
154
Chrysotile
155
B) Without MAS Organic derivatives
Asbestos
coz
H-MOR
0.2-l
107
NaY NaA
Benzene
Charcoal
28
SiO2 co,co2,CS2 and OCS
Na-MOR
64
H-MOR K-MOR Cs-MOR NH4-MOR co
Rd/A1203
156,157
105 of adsorbed developed
molecules.
here because
The high resolution other articles
MAS 13C NMR (Table 10A) will not be
are devoted
to this important
subject
(ref.
164,365).
IX 1 2 3 4
65 7
8 9 10
16 17 :: 20 21
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107
:z
76
77 78 79 80 81 :: 84 :z 87 88 89 90 91 92 93 94 95 96 97 ;98 100
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116
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PART II - STRUCTURAL X.
INVESTIGATION
OF ZEOLITES
BY 2gSi NMR SPECTROSCOPY
INTRODUCTION The first application
spinning
(MAS-NMR)
made by Lippmaa,
to the investigation
Engelhardt
2), they showed clearly for 2g Si MAS-NMR responding
of high-resolution
of various
to the different
importantly,
they showed
structure
that the isotropic chemical
it through
and aluminosilicates
and well resolved
Si environments
shifts could
peaks could be observed
in the silicate
paper
bridging
framework.
and used for investigations
in terms of the short-range Si, Al order. 29 shift of Si is strongly dependent (notably upon the number of neighboring
oxygen
other aluminosilicates.
atoms) has been used in numerous
Some review articles
reveals
numerous
group of catalysts
XI.
EXPERIMENTAL
A. Condition
properties
new insights
further
papers for
of zeolites
were published
into the structure
upon its local
and
(ref. 5-7). solid-state
2gSi
of these important
and sorbents.
PROBLEMS
AND GENERAL
FEATURES
OF THE 2gSi NMR SPECTRA
of measurements
To measure
2g Si NMR spectra
usual high-resolution Gibby and Waugh usually
The observation
Al atoms bound to
It is the aim of this paper to explain how high-resolution MAS-NMR
corMost
(ref. 4) that characteristic
be assigned
of bulk, surface and chemical
was
paper (ref.
and aluminosilicates,
chemical
environments
an investigation
NMR with magic-angle
In a pioneering
solid silicates
in a subsequent
ranges of these isotropic of zeolite
of silicates
et al. (ref. l-3).
that distinct
spectra
solid-state
in solid silicates
NMR techniques
(ref. 8) are combined
and aluminosilicates,
of rare nuclei as developed with magic-angle
there are no 'H nuclei covalently
spinning
bonded to the framework
the
by Pines, (MAS).
Since
of these
110 inorganic
solids as zeolites
(except some OH groups within
the zeolitic
cavi-
ties or on the surface of amorphous materials), a high-power decoupling (of 29 Si-lH dipolar interactions) is rendered unnecessary and cross-polarization can often not be used.
This reduces
the experiment
the sample which can easily be carried NMR spectrometer tageous
modified
for MAS technique.
to apply the CP-technique
in lattice defect the zeolite performing
It is, however,
to investigate
sites at the surface
Furthermore,
high-resolution sometimes
the presence
or in amorphous
(see Part XII-C).
structure
to one of simple MAS of
out using a conventional
advan-
of SiOH groups
materials
occluded
in
there are advantages
in
the experiment
at as high a magnetic field strength as possible. For 29 nuclei with a spin one half, such as Si (I = l/2), there is an essential gain in resolution superior
due to a greater
field homogeneity
To obtain correct
spread of resonance
and stability
signal intensities
times of 5 ...30 s were applied. 29 Si chemical
negative
sign for high-field
6. General
features
of
29
Si chemical
shifts of silicates
the total range of "Si
-60 to -120 ppm) with an analytically istic ranges for monosilicates
solid silicates
cross-link;:
framework
set of shift data is discussed single to double
tetrahedra,
the 2g Si nucleus.
parallels
tetrahedra.
such as electrostatic
bond strength
found to cause significant operative
changes
atoms in zeolites
carry the designation fy the nomenclature,
In both cases there
to the number of neigh-
distortion
shifts.
nomenclature notation
in
have been
These factors
thus leading to overlapping
in anion condensation
factors,
of the bond angles
are
of the
(ref. 9).
belong to a three-dimensional
Q4 in the silicate
of
in the liquid-
in the SiOSi bond angles
we use in the following
and finally
shielding
to this, other structural
in the chemical
in solid silicates,
shift ranges caused by differences The silicon
of cations,
from the
structures
diamagnetic
(ref. 10).
In addition
and changes
condensation
the same phenomenon
solutions
study of
in Ref. 2 and an extended
layered
useful ranges according
boring silicon-oxygen
tetrahedra
Increasing
(PI).
(Q3) and the
The first thorough
leads to increasing
This behavior
the silicon-oxygen
(Q4).
sites
(from
into character-
and chain end groups
to chains and cyclic
frameworks
and analytically
subdivision
was reported
in Ref. 9.
state 2gSi NMR spectra of silicate
especially
200 using
and aluminosilicates
shifts is appreciable
(Q2), chain branching
Si MAS-NMR
by means of
to three-dimensional
chemical
significant
(Q'), disilicates
in chains and cycles
three-dimensional
are distinct
vary between
to tetramethylsilane
^^
In silicates
groups
repetition
shifts.
(zeolites)
middle
rates normally
shifts are related
with the
solenoid magnets.
in the single pulse experiments,
Accumulation
and 10,000 scans.
lines in connection
of superconducting
framework
as mentioned. Si(nA1)
and
To simpli-
instead of Q4(nAl)
111 with n=o zeolite
to 4 for the five possible
in Table
shifts of a series of natural
1 and of synthetic
distinctly
of Si atoms.
zeolites
on the number
in Tables
From the data in Tables vious that quite
Si(nA1)
fects on the central
shifts for the Si(oA1)
ively
properties
observed
for distinct
the framework
AL Cl AlO&OAl 0 Al
Sil3Al)
Si(LAI1
5112 Al
1
[F
1
-a5
-90
-95
29
structural
shifts for a effect:
(ref. 2) to -116.6 ppm for sili-
induced
shift of about 9 ppm is exclus-
arrangements Similar
of the Si04 tetrahedra shift differences
have been
crystallographic
sites of
Al 0 SiOSiOSl 0 SI
Si 0 SiO&OSl 0 SI
Si(lAl)
S~ioAll
SillAll
r -80
of the shift ef-
(ref. 4,13,14,15).
Al 0 AiOsOSl 0 SI
0 Al
Additional
units in nonequivalent
zeolites
Al 0 AlOgOSi
shifts ap-
of the TO, tetrahedra. The T groups of aluminum-free Si02 polymorphs range,
silica framework.
Si(nA1)
of various
to sili-
in Fig. 1, it is ob-
that independently
by the following
Hence, a structure
the corresponding
bridges
arrangements
due to different crystallographic
forming
sphere
shift values of
may cause additional
units.
from -107.4 ppm for low quartz
(ref. 12).
these shifts
ranges of chemical
This suggests
Si atom of the Si(nA1)
crystallographic
are given
and about -85 to -90 ppm for Si(4Al).
overlapping
units.
unit may be induced
1. Different
calite
via oxygen
1,2,4 and 5 which are summarized
of Al atoms, other structural
for instance,
Obviously,
Al atom connected
large and strongly
pear for the various
chemical
2-5.
shift of about 5 ppm is induced with typical
about -105 ppm to -110 ppm for Si(oA1)
given Si(nA1)
zeolite minerals
of Al atoms in the second coordination
For each tetrahedral
con, a low-field
number
units of the
framework.
The *' Si chemical
depend
(.S10)4_nSi(OAl)n building
- 100
1 SiloAL -105
- 110
-115
Si chemical shifts of Si(nA1) Fig. 1. Ranges of versus TMS, high-field shifts negative).
120
d
units in zeolites
(s-values
112 TABLE
1
*'Si Chemical
shifts of natural Typical
Zeolite
zeolite minerals
Unit-Cell
. 2NaCl
Sodalite
Na6(Si6A160p4)
Thomsonite
Na4Ca8(Si20A120080)
(in ppm versus TMS)
Si(4Al)
Composition
24H20
Si(3Al
Si(2Al)
Si(lA1)
Si(OA1)
(Si/Al)
Ref.
-85.4
1.0
4
-83.5
1.0
4
-89.9+
1.0
14
1.0
14
Gismondine
Ca4(Si8A18032)
lcH20
Cancrinite
Na6(Si6A16024).
CaC03. 2H20
-85.4
Davyne
Na6(Si6A16024).
2CaS04
-91.6++
1.0
14
Bicchulite
Ca2(SiA1206)(0H)2
-86.8
0.5
11
Natrolite
Na16(Si24A11608,,)
Scolecite
Ca8(Si24A116080).
Gmelinite
Na8(A18Si16048)
Chabazite
Ca2(Si8A14024).
Analcime
Na16(Si32A1160g6).
Leucite
K(Si2A106)
24H20 13H20
1650 -81.0
Laumontite
Ca4(Si16A18048)
Stilbite
Na2Ca4(Si26Al
Harmotone
Ba2(Si12A14032)
Heulandite
Ca4(Si28A18072)
Clinoptilolite
Na3K3(Si30A16072)
Mordenite
Na8(Si40A180q6).
'Weak signal
16H20 24H20
-87.7
-95.4
1.5+ttt
4
-86.3; -89.1
-95.8
1.5tttt
14
-92.0
-97.2
2.1tttt
4
2.6++++
4
-99.4
-104.8
-91.8
-96.5
-101.7
-85.2
-91.6
-97.4
16H20
1o072)
-102.5
-94.0
-110
2 ltttt -101.0
-92.2
28H20
12H20
. 24H20
14
2.0
14
-108
2.6
4
-98
:;;::;it+
-95
-98.6; -102.6
- 108
3
4
-95
-99.0 -105.3+++
- 108
3.5
4
-100.6
-106.qttt -112.8
5
4
-105.2
-112.!, -.
4.7tttt
14
. 24H20 -97
24H20
4,ll
1.9tttt
at -96.9 ppm
++Weak signal at -83.3 ppm +++Tentative tttt(Si/Al)
assignment ratio from 2gSi NMR
TABLE 2 2qSi Chemical
shifts of faujasite
Zeolite
(S;i;l)
type zeolites
(in ppm versus TMS)
(S;:Al)
Si(4Al)
Si(3Al)
Si(2Al)
Si(lA1)
Si(OA1)
Ref.
NaKX
1.0
-84.6
NaX
1.14 1.17 1.24 1.36 1.6
1.18 1.27 1.4 1.5
-83.9 -84.6 -84.4 -84.8 -85.3
-88.1 -89.0 -88.9 -89.3 -89.5
-93.1 -94.2 -93.3 -94.2 -94.9
-97.9 -98.8 -98.7 -99.1 -99.5
-102.4 -103.1 -102.6 -103.4 -103.6
26 18 18
2.02 2.45 2.82
2.0 2.5 2.85
-84.8 -83.8 -
-89.5 -89.2 -89.2
-94.0 -94.5 -95.0
-99.6 -100.0 -100.2
-105.8 -105.5 -105.6
18 18.43 18.
2.5
2.43
-
-89.8
-94.5
-101.1
-105.2
43+
4.9
5.0
-
-89.4
-95.9
-101.5
-106.7
18
.40
-
-101.5
-107.8
18
89.9
-95.4
-101.1
-105.4
43t
NaY
NaNH4Y
(0 to 78% NH4)
NaHY Y ultrastabilized NH4Y DE (550- 815'C) 'Mean values
2.4- 8.2
14
Further Work 20 21 23 6
25
25
113 TABLE "Si
3 Chemical
shifts of gallium
Sample+
faujasites Si/(Al+
(ref. 26)
Ga)
51(4Ga)
(WF;")
Si(3Ga)
Si(2Ga)
Si(lGa)
Si(OGa)
NaGaX (0.05: 0.95)
1.14
I.11
-77.7
-84.2
-90.3
-96.4
-102.8
NaGaX
(0.03: 0.97)
1.42
1.39
-78.0
-84.2
-90.9
-97.4
-104.2
NaGaV (0.02: 0.98)
2.17
2.06
-78.3
-84.6
-91.4
-98.2
-105.8
Ga-Sodalite
1.28
1.26
-76.0
-82.4
-89.0
-94.9
-101.3
(0.03: 0.97)
+Ratlo of A1203
to Ga203
in parentheses
TABLE 4 "51
Chemical
Zeolite
shifts of A-type
(U/Al) NMR
Si(4Al)
NaA
1
-ffl.O~l.O
ZK-4
1.13
-89.1
1.40 1.62
-89.1 -89.4
zeolites
(in ppm versus TMS)
Si(3Al)
Si(2Al)
Sl(lA1)
Si(oA1)
-93.9
-99.5
-93.9 -94.6
-99.5 -100.4
-106.1 -106.1 -106.3
-110.5 -110.7 -111.0
Reference
2,4,22,27,24
;: 28
TABLE 5 29
Si Chemlral
shifts of synthetic
zeo1ite NaK-F
(Edingtonite)+
NaK-Hydroxysodalite Losod NaK-Pl
zeolites
(Sl/Al),,,
(Gismondine)+
(except X,Y,A) Si(4Al)
(SilAllC,
Si(3Al)
Si(2Al) Si(lA1)
Si(oA1)
Reference
1.0
-85.8
14
1.0
-85.4
14
1.0
-88.9
1.9 2.5
-07.6
30 -91.9 -93.4
-97.3 -98.3
-102.4 -103.9
-107.0 -109.2
14 11
NaK-Chabarite
2.6
-93.7
-98.8
-104.4
-109.5
14.15
NaK-L (Cancrinite)+
2.6
-89.0
-96.7
-101.5
-107.4
14
NaK-Offretite
2.9
-93.5
-97.5
-101.9 -106.9
-112.5
14.15
-97.8
-102.5 -107.3
-112.3
14.15
-95 -100
-105.0 -105.5
-110 -112.1
15,31,32 33
NaK-Erionlte
3.2
Na-Mordenite
5.4 7.3?0.6
-94.5
5.5
ZSM-5
34;235
CP. -103++
:;;$
34
Ii-ZSM-5
25- 1000
ca. .103++ - 105
_113++++ _115+++++
35.31.15
Ii-ZSM-11
25 - 1000
ca. -103++ -105
ZSM-a
30
-105
ZSM-48
480
_l13++++++ _l15++++++
Sillcalite
-10,
8.2
Ferrierlte
_113++++ _115+++++ _I13++++++ _115++++++
H-ZSM-35
10.4 5
ZSM-39
54.3
++(s10),si0H;
cross-polarization
++++Sl
12 15.31
_lOg+++++++ _l15+++++++
31.37
in 5-membered
++++++Simllar
++++++++Pe~k
rings; further splittin&
-111.0.
at -114.0, -115.0.
-112.0,
-112.8.
-113.2,
116.3 ppm
IP for ZSH-5, ZSM-11 with further splittings
due to crystallographically
maxlmum;
at -1W.6,
rings; further splittings
line profiles
+++++++Splltting
spectrun
further splittings
+++++Si in 4-membered
further splittings
distinct
at -110.1,
T-sites;
-111.8.
asslgnnent
-115.9,
-116.7
atilguous PPR
37
-113
type in pwentheses
+++Peak maxima;
37
_113,6++++++++
-120 fIsomorphous
35
-114.0
mm
114 2. Effect of cations.
The effect of cations
Si(4Al) atoms is clearly
demonstrated
ratio of 1.35 and A-zeolites. about 6 ppm between
3. occurs
in synthetic
2).
This result
indicates
chemical
zeolites
rationalization
authors
character
TOT angles
related
unit depends
shift var-
(ref. 13,36,38,39). (set cr) of the
and a simple quantum-
has been given
linearly
directed
(ref. 39).
In
shift of the central
to silicon which in turn is This function,
in the interesting
as well
range of a between
140' and 160' and, therefore, an approximately linear correlation of a, 29 Si chemical shift is to be expected. This is confirmed
may be concluded, discussed silicon
from 12 silica polymorphs
therefore,
above are mainly
that the structural
caused
resonance
about 5 ppm.
It
shifts the
shift ranges, a direct assignment Si(nA1)
the 2g Si NMR spectrum
of zeolite
by
Na, K-P1 is
In this case, the signal at lowest field belongs to Si(4Al),
in the high-field
A similar consideration
direction
by Si(3Al),
may be applied
Si(2Al),
to spectra occurring
Then the latter signal can be unambiguously
attributed
For zeolites
of a
atom is straightforward
but the first one in the low field position
cations,
on chemical
of the bond angles around
lines occur in the spectrum which are separated
As an example,
shown in Fig. 2.
Si(nA1)
line to the corresponding
only if five well resolved
network
(ref. 39).
assignment
Because of the overlapping
followed
effects
for
atom under investigation.
C. Spectra
certain
by changes
and zeolites
and
P
set a with the
21 data points available
Si
on the s-hydridization
to the SiOSi bond angle u by cos a/cos(a-1).
as set C( shows a rather weak curvature
of remote Al
(ref. 38) and the mean SiOSi angles
(ref. 13) have been presented
of the oxygen bond orbital
P
Si/Al
(ref. 18,
that the chemical
shifts and the mean secants
of these correlations
structural
(ref.
lines
to Si/Al=60
the influence
the latter study, it is shown that the 2gSi chemical atom of a Si(OSi)4
of
unit.
of the average
of 2g Si chemical
SiOSi angles a of five silica polymorphs of six silica-rich
clearly
shifts for the Si(oA1)
with variations
Linear correlations
resonance
(NaX, Nay) with increasing
it has been stated by various
iations correlate
with a Si/Al
LiA and BaA zeolites
shift of the Si(oA1)
zeolites
shifts of
leads to shift differences
to 5.4 ppm on going from Si/Al =l.O
atoms on the resonance Recently,
A high-field
faujasite-type
ratio which amounts (see Table
in a series of X-zeolites
This influence
LiX and SrX (ref. 16) and between
Remote Al atoms.
19,20)
on the chemical
of isotypical
but with different
or very similar
Si/Al ratio
the spectra can be assigned
having
and Si(oA1).
less than 5 lines
at -84 ppm or below. to Si(4Al).
structure
(see, for example,
by considering
Si(lA1)
of their tetrahedral Fig. 3) or type of
systematic
changes
in the
115
Zeolite SilAl
PI = 1.9
n
S1(2AI)
r -
80
-90
- 110dlppml
- 100
Fig. 2. 2' Si NMR spectrum
84~
w
r+k
of synthetic
k..Pk.
_.._d
zeolite
Na, K-PI.
Lc,
80
-100
-120
-80
-100
-120
-80
-100
-120
-80
- 100
-120
-80
-100
-120
-80
-100
-120
Fig. 3. *' Si NMR spectra of X- and Y-zeolites with different Si/Al ratios. a) Na,K-X, Si/Al=l.O; b) NaX, Si/A1=1.4; c) Nay, Si/A1=2.5; d) NaHY, Si/Al= 3.9; e) NaHY, Si/Al=5.0; f) US-Ex (ultrastable dealuminated Y-zeolite), Si/Al=60. The numbers of the peaks designate the value of n of the corresponding Si(nA1) units. Scale numbers: 6 in ppm versus TMS.
116 resonance
shifts connected
concentration
with the total aluminum
The signal assignment
can be checked
by means of the intensities the value derived
XII.
content
or the type and
of cations.
ISi(nAl)
by comparing
the Si/Al ratio determined (see Part XII-A)
in the NMR spectrum
frcm other methods
like chemical
with
analysis.
APPLICATION
A. Silicon,
aluminum
Substitution
of silicon
tional paramagnetic considerable
ordering_ by a small cation
shift
line broadening
appears
the sharpness
of the resonance
is a measure
for the regularity
ing, however,
(like aluminum)
leads to an addi-
In some cases of aluminosilicates,
(see above).
It has been shown (ref. 2,4) that
also.
lines reflects
the degree of crystallinity
yield
spatially
and
It is worth emphasiz-
of Si, Al distribution.
that the NMR measurements
a
and temporally
averaged
Therefore, information which is related to short-range order interactions. 29 Si MAS-NMR in general do not by themselves provide direct information on zeolite crystal dination
symmetry
and framework
shell but valuable
silicate
lattice can be achieved
existing
knowledge
on framework
beyond
by combination topology
the different
of the NMR results with the
Si/Al ratio may be computed
29
aluminosilicate
using the equation
and allied
of the sample.
ISi(nAl) of the
Si(nA1) units in a tetrahedral
coor-
of the alumino-
from X-ray diffraction
From the intensities
Si/Al ratio.
the first tetrahedral
into the fine structure
as well as with the chemical composition
techniques
1.
structure
insights
Si NMR lines for framework,
the
(ref. 18):
4 (Si/Al)NMR
= f n=O
This formula Al-O-Al
linkages
Al ordering
1Si(nAl)'x0'25 n=C, follows
from a readily comprehensible
are present,
within
hood of every Al tetrahedral in a Si(nA1)
fraction
of aluminum
papers
then the imnediate
Consequently,
unit incorporates
Si/Al ratios provided
(ref. 16,19,20),
new quantitative
no Al-O-Al
linkages
The relative
for one type of struc-
is valid for all zeolites, a powerful
neighbor-
each Si-O-Al
0.25 Al atom.
is thus given by 0.25 nISi(nAl)
This equation
If no
consideration:
rule is obeyed for the Si,
framework,
site is Al(OSi)4.
structural
tural type and represents framework
i.e.,Loewenstein's
the aluminosilicate
linkage
tural unit.
"ISi(nA1)'
irrespective method
of struc-
of determining
are present.
In several
it has been found that the Si/Al ratios determined
by
chemical analysis and X-ray fluorescence measurements on the one hand and by 29 Si MAS-NMR on the other hand are in very good agreement, i.e., Loewenstein's rule clearly
holds.
117 2. Faujasite type zeolites
type zeolites.
The silicon-aluminum
(NaX, NaY) has been investigated
al. (ref. 18,21),
Klinowski
these investigators
ordering
independently
by Engelhardt
et al. (ref. 19,40) and Melchior
came to approximately
the same results
of faujasite et
et al. (ref. 22);
and conclusions.
In Ref. 18, a series of NaX and NaY zeolites with Si/Al ratios of 1.18 to 67 has been investigated. be assigned 2).
The spectra
unambiguously
be concluded
small line widths
that all specimens
Si, Al distribution.
a systematic
increase
number of aluminum
possess
Si(nA1)
a nearly regular
framework
for Si(nA1)
atoms appears which suggests
has been derived
building
units with those from theoretical
ordering
scheme developed
from interpretation
by Dempsey
of acidity
structures
from electrostatic
studies
of Si and Al atoms within
at 11.9 MHz Melchior
(ref. 22) and high-field
measurements
et al. noted that their interpretation
work
but is in good agreement (ref. 18,20,21)
Si/Al values. lational
They argued
periodicity" ordered
site crystals
flecting
is subject
a tendency
neighbors
within
culations
Vega
the restriction
the framework
rule"
reduction
such as "trans-
schemes
based on
sub-unit,
topology
Within
the sub-unit,
of the incidence
the single
in accordance
rule and an additional
fauja-
with
the Si:Al constraint
re-
of Al,Al second nearest
[ref. 411). results of Monte Carlo computer
Si:Al distributions
of‘loewenstein's
predictions
experi-
(ref. 41) for higher
ordering
in terms of an ordered
(ref. 42) has presented
of randomized
at the low values
of a criterion
and considered
to Loewenstein's
toward
("Dempsey's
Recently,
is different
It is shown that the Si, Al distribution'in
can be understood randomly
at 35.9 MHz (ref. 23).
calculations
that the application
Loewenstein's rule as the only constraint.
covering
a
a double-cubo-
with both the other published
and theoretical
is irrevelant
sub-units.
combined
distribution
spectra
and
NaX with Si/Al=1.4,
study of ordering
of Si/Al(2.0
oretical
and
in X and Y zeolites has been reported by 29 et al. on the basis of Si MAS-NMR spectra (low field measurements
Melchior
6-ring,
of the
unit has been proposed.
A similar
smaller
on
on the basis of an
(ref. 41) for Si/Al=1.18-2.0
centro-symmetrical
distribution
distribution
information
considerations
Except for zeolites
mental
with an Si/Al ratio,
in Ref. 18 by comparison model
(Table it may
groups with a small
a fairly uniform
More detailed
Si/Al= 2.43- 2.8, respectively.
octahedra
units
spectra,
As shown in Fig. 3, with increasing
of the signal intensities
ordering
structural
in the 2gSi MAS-NMR
of Si and Al atoms in the zeolite framework. silicon-aluminum
up to five signals which could
to the five different
From the relatively
ordered
exhibit
and Dempsey's
with the experimental
of more than' 50 directly
of faujasite-type
rules. A comparison
results
synthesized
zeolites
obtained
or dealuminated
Si/Al ratios of 1 up to about 70 (ref. 16,18,19,22,43)
cal-
under
of the the-
from the 2gSi NMR X and Y zeolites reveals
that
118 for Si/Al ratios in the range of about 1.5 to 4 a pronounced second nearest Al pairs exists; obeyed.
At SiJAl ratios
distribution firmed
however,
Dempsey's
tendency
rule is not completely
higher than 4, the Si.Al orderings
with the restriction
of Loewenstein's
approach
to hold true over the whole range of Si/Al ratios. 29 Si MAS-NMR spectra of gallium faujasites
hibit relative
containing
signal
intensities
of T atoms
zeolites
(T=Si,
as zeolites Ga, Al).
are deshielded
information
cal shifts are compatible
(ref. 13) ex-
NaX, Nay, indicating
However,
the signals
similar
in the gallium
by an amount nearly proportional
number of Ga atoms in the first neighbor little structural
shell
(Table 3). Although
for Ga substituted
with the general
a random
rule. The latter is con-
High resolution
distributions
to avoid
zeolites,
correlation
to the there is
the observed
chemi-
with T-O-T angle
(cf.
Part XI-B).
3. Zeolites Si/Al=l
of type A.
consist
The 2'Si MAS-NMR
spectra
of a single line with chemical
-89.7 ppm (Table 4).
This result was first observed
et al. (ref. 2,4) and confirmed
by Thomas,
previously
to Si(3Al)
NaY zeolites
because
bonds, sonance
in violation
tions.
in similar
of Loewenstein's
support
rule.
diffraction,
However,
point strongly
refinements
(ZK-4) with Si/Al>
units.
1 pointed
assignment.
the assignment
Then,
(Part XI-C).
has apparently
calcula-
of A-type
zeo-
of TlA (ref. 46-48)
in favor of the2;i(4Al)
five lines in its according
to the principles
shifts of ZK-4 reveal a rather
of the Si(4Al)
zeolites
rather than the
Si MAS-NMR
spectrum, given above
large structure
units and particularly
position
NaA, suggesting
that the single peak in NaA should be assigned
line like the single
(ref. 17) that the unusual
line in zeolite
found
by X-ray, electron
ilar high-field
Si(4Al)
Al-O-Al
of the single re-
et al. (ref. 28,29) on A-type
duced shift effect of the whole set of Si(nA1)
It has been suggested
in
involves
with perfect Si,Al ordering, consisting 29 Si MAS-NMR experiments by
is unambiguous
The resonance
for Si(3Al)
and theoretical
analysis
the
NaX and
independent
strongly
ZK-4 exhibits
of which
Si(3Al)
of zeolites
diffraction
in favor of a structure
of Si(4Al)
typical
necessarily
method
outside
like sodalite,
(ref. 44) of the X-ray structure
Thomas et al. (ref. 17) and Melchior
Si(3Al)
environment
trimethylsilylation
Engelhardt,
line had been assigned
This assignment
from investigations
lites (ref. 45) and recent neutron
exclusively
structures
Such an arrangement
line in NaA to the particular
strong independent and neutron
The single
but agreed well with the shift values
NaX, NaY and other zeolites.
by Lippmaa,
this shift value has been clearly
units observed
NaA with
in the range -88.3 to
Fyfe et al. (ref. 27), Melchior
et al. (ref. 22) and Nagy et al. (ref. 24).
range of Si(4Al)
of zeolites
shifts
NaA (ca. 5 ppm shielding
chemical
in-
a sim-
line observed
for
to Si(4Al).
shift for the
with respect
to zeolite
NaX)
119 is related
to the unusually
large average T-O-T angle of the zeolite A struc-
ture. Typical
single
ite,davyne,Na,
line spectra
appear also for sodalite,
K-X and Na, K-F (ref. 14) with Si/Al=l
in the case of NaA, the high-field attributed
to Si(4Al)
by comparing 5) which
units.
the spectrum
is isotypical
non-equivalent
tional
zeolites.
In the faujasite
(but not chemically)
signal splittings
lattice
structure
zeolites,
to different zeolite
families:
chabazite
ferrierite
5,6,14,15,35), clathrate
ZSM-11
group
attribution
(ref. 6,15),
Si,Al ordering
(ref. 15,35), ZSM-8
experimental
in zeolitic
Si/Al contents,
5. Some natural
2:l.
one Si(3Al)
of that mineral
scolecite
is isotypical
of the central distortion signal
of natural
The
agreement
Si sites in natrolite-(Table and scolecite
pentasil
region becomes
zeolites,
synthesis
dealumination
Si MAS-NMR
(ref. 6,
[ZSM-5 (ref.
(ref. 6,15) and
uncertain.
spectrum
except
results
intensity, in scolecite
1) (ref. 14).
(ref.
ratio of
structure
The aluminosilicate
with
(ref. 13).
of natrolite
line with the intensity
of the chain around
are shown in Fig. 4.
of zeolites
of the sample
with the framework
This distortion
Si sites are present
(ref. 14,15),
rmordenite
(ref. 15), ZSM-48
Typical belonging
In such cases, the unambiguous
with that of natrolite
into two lines of equal
distinct
29
(ref. 49).
Si tetrahedron
(ref. 50)).
zeolites
are developed to derive the actual 29 from Si MAS-NMR spectra: comparison
and one Si(2Al)
This is in complete
ordering
group
(ref.15)1,
the overlap
or progressive
zeolites.
is the
of both the
procedures
framework
with well known structures
4) contains
ZSM-35
feature
may overlap.
and synthetic
rerionite, offretite
[ZSM-39 (ref. 6,15,37)1.
various
important
units
sites, addi-
the shift ranges of the Si(nA1)
(ref.15)], mordenite
of an NMR line within
Therefore,
various
group
all the T sites
When Si(nA1)
distinct
frameworks
natural
with crystallog-
on the regularity
Therefore,
in various
L (ref. 14), ZSM-34
15,31-33),
Another
types of aluminosilicate
are encountered
Na, K-P1 (Table
Zeolites
equivalent.
of the line widths
and Si;Al order.
units of different examples
may be observed.
dependence
could be checked
zeolite
of the same type (equal n) occupy crystallographically
above-mentioned
As
should be
(ref. 14).
and synthetic
sites.
are crystallographically
and davyne
this assignment
with that of the synthetic
with gismondine
cancrin-
(Tables 1 and 5).
signals of gismondine
For gismondine,
4. Other zeolite minerals raphically
gismondine,
and Si;Al
framework
of
for a small rotation
the chain axis
in a splitting
(monoclinic
of the Si(3Al)
i.e., three crystallographically in contrast
to only two different
The 2gSi NMR spectra
of natrolite
120
Natroiite Si/Al=
Scolecite 1.5
SilAL- 1.5
Si (3Al)
0.
0
-60
-80
-60 -80 -lOOd(ppm~
-1OObbpm)
of natural natrolite and scolecite and skeletal dia(filled circles - Si; open circles -Al; oxygen atoms
The aluminosilicateframework of analcime consists of nearly parallel four rings containing SiO4 tetrahedra which are interconnectedthrough single Al tetrahedra (ref. 51). A regular lattice should be formed, therefore, from Si(2Al) units only. The spectrum (Fig. 5) shows the expected Si(PA1) signal but accompanied by two signals of lower intensity for Si(3Al) and Si(lAl), indicating the replacement of some aluminum atoms by silicons (ref. 4).
For
leucite, which has the same framework type and Si/Al ratio as analcime, the spectrum (Fig. 5) exhibits markedly broadened sl'gnalsof all five types of Si(nA1) (ref. 14) which confirms the highly disordered Si:Al distribution proposed already from X-ray diffraction-studies (ref. 77). Leucite
Analcime S~lpii:
SI /Al = 19
20
ICI)
lb1
-80
-1OOdippml
-80
-100 dtppml
Fig. 5. 2gSi NMR spectra of natural analcime and leucite
121 Like analcime, four-membered
the structure
Si-rings
linked together
should exist only Si(2Al) ments
of laumontite
units.
through Al tetrahedra;
The narrow
(half line width of 75 Hz) reveals
laumontite
sample
6. Chabazites. six rings
investigated
prisms)
linked by tilted four rings (see Fig. 6) is consistent
a perfect
arranged
atoms are arranged
with the proposed and a synthetic has been found
i.e., there to Si(2Al)
Si:Al ordering
framework ofchabazite
arrange-
of the
(ref. 53).
The 2gSi MAS-NMR
with an ordering
in para-position.
Si,Al ordering
scheme where
diagram
is given
atoms are
These aluminum
prism contains chabazite
For natural
the same highly ordered
and
(ref. 4,54)
two aluminum
of a typical
in Fig. 6.
ABCABC...
spectrum
prisms of a sequence.
The third hexagonal
The skeletal
Na, K-chabazite,
is formed by double
in layers in the sequence
placed in each six ring of two hexagonal
one Al atom per six ring.
line belonging
of
(Table 1) (ref. 14).
The aluminosilicate
(hexagonal
(ref. 52) is also composed
only
cavity
chabazite
Si:Al distribution
(ref. 54).
Chabazi te %/AL=26
3. 10~11: 2 -80
-100
-120 d(Ppml
29 Fig. 6. Si NMR spectrum of synthetic Na, K-chabazite and skeletal diagram with the proposed Si:Al ordering of the chabazite cavity (Al atoms are marked by black circles; Si atoms are located at the intersections of the lines not occupied by Al; oxygen atoms are not shown). The intensity ratios of the di ferent Si(nA1) groups are 3:10:12:2.
7. Zeolite
Na, K-L. The aluminosilicate
based on "cancrinite four-membered
rings.
framework
cages" which are formed
of zeolite
by five six-membered
Na, K-L is and six
122 These cavities are linked through the planes of their upper and lower sixmembered rings, thus forming vertical columns (ref. 55). Each column is crosslinked laterally to three others by oxygen bridges. The preferred Si/Al ratio of a zeolite L is about 3.
From the absence of the Si(4Al) signal, the small
Si(3Al) and Si(;$l) intensities and the fairly intense Si(2Al) and Si(lA1) signals of the
Si NMR spectrum an overall Si,Al distribution has been sug-
gested. The repeating unit consists of two cancrinite cages with different SiiAl ratios; viz., one type with 5 Al and 13 Si and another with 4 Al and 14 Si for a Si/Al ratio of 3 (ref. 54). 8. Mordenites. The tetrahedral framework of mordenites consists of complex chains of five rings which are cross-linked by four rings (ref. 56).
It has
been concluded from X-ray studies that SirAl are ordered with Al occupying half the sites of the four rings (ref. 57). This implies the presence of Si(PA1) units beside Si(lA1) and Si(oAl), with the latter two in five times higher concentration, if a typical Si/Al ratio of 5 is assumed. The *'Si MAS-NMR spectrum of a
mordenite sample from Poona, India (Table 1) is not
in agreement with that conclusion because only a rather weak shoulder signal for Si(2Al) but strong lines of Si(lA1) and Si(oA1) with at least ten times higher intensity have been observed (ref. 14). In recent papers (ref. 6,31- 33), the Si;Al ordering in the structure of synthetic mordenites has been determined using dealuminatedmaterials. The 2gSi MAS-NMR spectrum of the starting material, a Na-mordenitewith Si/Al= 5.5, shows three,lines at -95, -105 and -110 ppm, respectively (ref. 32) (Table 5). As the degree of dealumination increases, the line at -95 ppm disappears while the line intensity at -105 ppm decreases and that at -110 ppm increases. Assuming that the aluminum atoms preferentiallyoccupy the four rings, the variations of the three'line intensities may be explained by assigning the lines to Si(ZAl), Si(lA1) and Si(oAl);respectively. In the case of a completely dealuninated mordenite, three Si(oA1) lines with a shift dispersion of ca. 5 ppm and an intensity ratio of 2:1:3 are observed (ref. 13, 58) which have been attributed to the four crystallographicallyinequivalent T-sites present in the mordenite framework with the population ratio of 2:1:1:2 (ref. 13). .The last two sites coincide in one NMR peak. 9. Silicalite and ZSM-5 (Table 5). Si-licaliteis a crystalline porous silica; its framework structure is topologically closely similar (if not identical, ref. 12, 59, 60) to that of the zeolite ZSM-5. Its 2gSi MAS-NMR spectrum (ref. 12,60) (see also Ref. 5) may be simulated by a minimum of nine Gaussian signals; The chemical shift range of all peaks is characteristicof Si(OSi)4 groupings in highly siliceous materials. The observed multiplicity
123 arises from the crystallographicallynon-equivalent tetrahedral environments of the Si(OSi)4 sites. The amount of aluminum present is very small (Si/Al, 1000). However, it has been shown (ref. 5,12,59) that aluminum is tetrahedrally coordinated to oxygen. At least two distinct types of tetrahedral framework sites are occupied by the aluminum. Recently, it has been shown that the 2gSi NMR spectra of ZSM-5 samples with Si/Al of 125 and 900 are practically identical with that of silicalite and exhibit 15 clearly resolved peaks (ref. 59). The assignment of several
29 Si MAS-NMR lines and the identificationof pos-
sible aluminum sites in H-forms of ZSM-5 and ZSM-11 zeolites has been undertaken in Ref. 34,35 using samples with different Si/Al ratios from ca. 30 to 1000. All spectra show essentially three resonances at -105, -113 and -115 ppm relative to Me4Si as external reference. The signal at -105 ppm can be clearly identified as a Si(lA1) group.
Its intensity decreases with increasing Si/Al
ratio and drops to zero for an Si/Al ratio higher than 100. The decreasing intensity of this signal is directly linked to the increase of the line at -113 ppm, while the intensities of the lines near -115 ppm r-114 (partly), -115.2, -116.3 ppml remain constant in both H-ZSM-5 and H-ZSM-11. Since, additionally, the intensity ratio of these lines IZsM_11/IZsM_5=2 equals the ratio of the numbers of four-membered rings (8 in ZSM-11 and 4 in ZSM-5), the lines near -115 ppm are attributed to Si()Al) atoms located in (strained) four-membered rings. An attribution of the lines near -113 ppm L-109.6, -111.0, -112.0, -112.8, -113.2, -114 ppm (partly)] is still not clear. B. Detection of Si atoms bearing,hydroxylgroups The use of the 1H-2gSi cross-polarization(CP) MAS-NMR technique makes possible the detection of silicon atoms to which are attached one or two hydroxyl groups (ref. 4,61). By application of the CP-technique, strong enhancements of the signal intensities or even additional signals may appear for silicon atoms bearing OH groups which are sometimes difficult to detect in the U.direct"Fourier transform spectrum without polarization transfer. These resonances may then be identified in the "direct" spectrum in order to estimate their concentration. (It
should be emphasized that the cross-
polarization spectra are not quantitively reliable for this purpose.) By means of 'H-2gSi CP-technique,informationon SiOH groups in defect centers of the dealuminated structure of progressively dealuminated Y-zeolites has been obtained (ref. 61). The presence of SiOH groups is pointed out by 29 Si MAS-NMR signal at -101 ppm.
a considerable intensity enhancement of the
This method has also been applied to the identificationof silanol groups in the ZSM-5 (ref. 34,62) and in the silicalite framework (ref. 62). Maciel and Sindorf (ref. 63-65) applied the CP-technique for the characterization of silica surfaces and surface-derivatizedsilicas.
124 C. Aluminum-deficientzeolites
In some papers (ref. 18,25,43,61,66-68), 2gSi MAS-NMR has been used to investigate the structure of dealuminated zeolites Y and the process of their formation. In particular, the following problems were considered: 1) the dependence of the Si/Al ratio and Si,Al ordering in the dealuminated sample on the conditions of the thermochemicaltreatment, e.g., the degree of ammonium exchange, temperature and partial pressure of steam; 2) the influence of the dealumination procedure, e.g., thermochemical treatment, Al extraction by organic acids or by SiC14,-on the Si,Al ordering in the products; and 3) the regularities in the Si,Al ordering during the course of the step-by-step extraction of Al atoms from the faujasite lattice.
In the first step of the investigations (ref. 17,25, 61,66), it has been shown that 2gSi MAS-NMR makes it possible to follow in detail the process of removal of Al atoms from the zeolite lattice during thermochemicaltreatment, Characteristicalterations in the signal intensities of the corresponding Si(nA1) groups were observed when, during the dealumination process;some Al atoms are removed from the aluminosilicateframework. Because only aluminum 29 atoms located in the aluminosilicate lattice affect the Si NMR spectra, the presence of nonframework aluminum produced by the dealumination process of the zeolites has no effect on the spectra. A direct investigation of these aluminum sites has been performed by means of 27Al NMR studies (ref. 69,70; see also article by D. Freude in this volume). Additional information on SiOH groups in defect centers of the dealuminated structure can be obtained by lH 29 - Si CP-techniques as mentioned above. The fraction of terminal SiOH groups remains nearly constant in the dealuminated samples (ref. 43). This shows that the overwhelming majority of the vacancies created by the removal of Al from the tetrahedral framework are subsequently re-occupied by silicon. The following general conclusions could 29 Si NMR investigationof thermochemicallydealumin-
be drawn from a thorough
ated Y zeolites (ref. 43): 1. The extent of dealumination during the thermochemical treatment is mainly determined by the degree of ammonium exchange in the starting material. The number of Al atoms removable is limited by the number of NH4 cations present in the parent sample. 2. The extent of dealumination increases with the temperature applied during treatment. In the deep-bed treatment, the maximum dealumination is normally not attained. Even at 770°C the degree of dealumination is far below the degree of ammonium excha ge. Therefore, in the deep-bed treated samples 1 a considerable amount of Si-O-Al groups, the centers for the hydrolytic removal of Al atoms, is retained. It has been suggested that the hydroxyl groups adjacent to lattice aluminum are mainly responsible for the catalytic activity of such type of zeolites (ref. 71).
125 3. The promoting dealumination pressures
process
is well known.
by the available
dealumination
increases
("self-steaming
Because
amount of water.
only slightly
conditions")
ble and comparable pressure
predominantly
atoms within
samples,
conditions
successive rangement
procedure.
of the zeolite
the heating
it is quite difficult
rate, etc.
to obtain
treatment
relia-
with known
et al. (ref. 67), it has been con-
of the aluminum-deficient
The arrangement
the tetrahedral
framework
Some criteria
faujasite
is obviously
independent
for the selection
and type of
electrostatic
of external
of a definite
Si,Al
validity
of
energy
of the whole structure,
of Al by Si, the preservation
of centrosytmietric ar-
of Si and Al and net zero dipole moment
(cf. also Ref. 5).
However,
and with the Si(nA1)
The preparation
depends
of the silicon and aluminum
(ref. 67); viz., the general
rule, the minimum
replacement
themselves
of steam is not known
on the preparation
the use of thermochemical
have been considered
Loewenstein's
the
water concentration.
the pressure
study by Engelhardt
conditions.
arrangement
At higher steam pressures,
on the final Si/Al ratio and not on the conditions
the dealumination
cination)
(ref. 43) that at partial
and volume of the sample,
that the Si,Al ordering
reaction
of steam on the
of steam is recommended.
5. In a systematic cluded
pressure
with increasing
and depends
the geometry
under such undefined
partial
partial
It is shown
In the case of deep-bed treatments,
being treated,
etc.
of an increasing
of steam below 2.7 * lo4 Pa (~200 Torr) the extent of dealumination
is limited
4.
effect
these criteria
populations
in the double
sodalite
unit,
are not always compatible
derived
with
from the NMR spectrum.
of HY zeolites
by deamnoniation of NH4Y (shallow bed cal29 The Si and.27 Al NMR spectra reveal that even
has been studied.
(e.g., treatment
under very mild conditions amount of aluminum
is removed
a "pure HY" zeolite
cannot
at 300°C for one week) a certain
from the tetrahedral
be obtained
(ref. 68). 29 Si NMR study, In a detailed
lattice and, consequently,
by simple thermal
treatment
of NH4Y
zeolite
non-framework
aluminum
kaline treatment
of dealuminated
72) has to be rejected The progressive (ref. 59,60,68,74) treatment vapor,
dealumination by various
(ref. 5). spectroscopy
chemical
lattice
of
by al-
by Breck and Skeels
(ref.
in detail
of NMR methods
a further
(ref. 32,33) and ZSM-5 zeolites
procedures,
acid leaching
including
important
should be mentioned:
thermochemical
and treatment
with SiC14
using 29Si and 27A1 NMR spectroscopy
to the study of the process
by SiC14 has been reviewed Finally,
proposed
of mordenites
samples,
has been investigated
of zeolites
Y-zeolites
aluminosilicate
(ref. 73).
of NH4-exchanged
The application
it has been shown that the reinsertion
into the tetrahedral
in more detail application
.
of dealumination
by Fyfe, Thomas,
et al.
of 2gSi and 27Al NMR
viz., the study of the process
of zeolite
126 crystallizationfrom the gel phases. Engelhardt et al. (ref. 75) studied solid aluminosilicategels formed as intermediates in zeolite A synthesis and Nagy et al. studied the formation of zeolites ZSM-5 (ref. 6,76) and mordenites (ref. 31). XIII.
ACKNOWLEDGEMENT
The help
of Miss Angelika Friedrich in preparing themanuscript of this paper
is greatly appreciated. XIV.
7 8 9 10 11 12 13 14
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