RESEARCH PAPER
PCCP
Jian Jiao,a Johanna Kanellopoulos,b Wei Wang,a Siddharth S. Ray,c Hans Foerster,d
Dieter Freudeb and Michael Hunger*a
www.rsc.org/pccp
Characterization of framework and extra-framework aluminum
species in non-hydrated zeolites Y by 27Al spin-echo, high-speed
MAS, and MQMAS NMR spectroscopy at B0 = 9.4 to 17.6 T
a
Institute of Chemical Technology, University of Stuttgart, 70550 Stuttgart, Germany.
E-mail: [email protected]
b
Abteilung Grenzflächenphysik, Universität Leipzig, 04103 Leipzig, Germany
c
Indian Institute of Petroleum, Dehardun, 248005, India
d
Bruker Biospin GmbH, 76287 Rheinstetten, Germany
Received 13th June 2005, Accepted 7th July 2005
First published as an Advance Article on the web 26th July 2005
27
Al spin-echo, high-speed MAS (nrot ¼ 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of
B0 ¼ 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework
positions in non-hydrated zeolites Y. Non-hydrated g-Al2O3 and non-hydrated aluminum-exchanged zeolite Y
(Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed
zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a
superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and
framework aluminum atoms compensated in their negative charge by aluminum cations (diso ¼ 70 10 ppm,
CQCC ¼ 15.0 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum
atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extraframework aluminum oxide clusters (diso ¼ 65 5 ppm, CQCC ¼ 8.0 0.5 MHz). The residual two signals were
attributed to aluminum cations (diso ¼ 35 5 ppm, CQCC ¼ 7.5 0.5 MHz) and octahedrally coordinated
aluminum atoms in neutral extra-framework aluminum oxide clusters (diso ¼ 10 5 ppm, CQCC ¼ 5.0 0.5
MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of
aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this
material was determined.
DOI: 10.1039/b508358c
Introduction
Brønsted and Lewis acid sites in zeolites play an important role
as active sites in heterogeneous catalysis. Brønsted acid sites of
zeolite catalysts are bridging hydroxyl protons in the vicinity of
tetrahedrally coordinated framework aluminum atoms.1 Often,
Lewis acid sites are caused by extra-framework aluminum
species formed upon calcination or steaming of the zeolite
catalyst.2 Until now, the nature and number of Lewis acid sites
occurring in zeolite catalysts are still a matter of debates.2,3 A
number of different extra-framework aluminum species, such
as Al31, Al(OH)21, Al(OH)21, AlOOH, and Al(OH)3, are
suggested.3 In the past decades, solid-state 27Al NMR spectroscopy was demonstrated to be an important tool for the study
of aluminum species in zeolites.4–9 This spectroscopic method
is suitable for investigating the oxygen coordination, local
symmetry, and concentration of aluminum species at framework and extra-framework positions. For most of these solidstate 27Al NMR studies, however, fully hydrated zeolite samples were used.4–9 In the case of zeolites with high aluminum
contents and extra-framework aluminum species, the rehydration of the calcined materials may be accompanied by changes
of the coordination and nature of aluminum species occurring
in these catalysts. Therefore, investigations of the calcined
zeolite catalysts in the non-hydrated state are of general
interest.
An important parameter describing the strength of the
quadrupolar interaction of quadrupole nuclei with a spin of
I 4 1/2, such as 27Al nuclei with I ¼ 5/2, is the quadrupole
coupling constant CQCC ¼ e2qQ/h. Here, eQ corresponds to
the electric quadrupolar moment of the nucleus, eq is the
z-component of the electric field gradient at the position of
the nucleus, and h denotes the Planck constant.10 In the case of
hydrated zeolite catalysts, aluminum atoms give rise to 27Al
NMR signals at isotropic chemical shifts of 0 to 60 ppm
characterized by quadrupolar coupling constants of CQCC ¼
2 to 6 MHz.5–9 In non-hydrated zeolite catalysts, the CQCC
values of aluminum species at extra-framework and framework
positions cover a range of 6 to 16 MHz accompanied by a
strong broadening of the corresponding signals.11,12a,13,14
A suitable approach for 27Al NMR studies of non-hydrated
zeolites is the spin-echo technique.11,12a,13 However, the resolution of this technique is very limited and a simulation of spinecho NMR spectra requires spectroscopic data obtained by
other methods. More modern techniques are high-speed MAS
and MQMAS NMR spectroscopy in high magnetic fields such
as B0 ¼ 17.6 T.
Framework aluminum atoms in the local structure of bridging OH groups in non-hydrated zeolites are characterized by a
perturbed tetrahedral oxygen coordination corresponding to
quadrupolar coupling constants of 14 to 16 MHz.11,12a,13,14
These perturbed framework aluminum tetrahedra can be considered as an intermediate state between a fourfold and a
threefold oxygen coordination.11 Tetrahedrally coordinated
framework aluminum atoms, which are compensated in their
negative charge by extra-framework cations have a CQCC value
of 6 to 8 MHz.11,12a In 27Al spin-echo NMR investigations of
dealuminated and non-hydrated zeolites ZSM-5 and Y, two
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3221
signals of aluminum species at framework and extra-framework positions could be distinguished.11,12a However, the
strong overlap of the quadrupolar patterns hinders a quantitative evaluation of the spectra. As shown by solid-state 23Na
NMR studies of sodium cations in non-hydrated zeolites,15–17
experiments performed in different magnetic fields and applying 2D techniques can help to overcome this difficulty. The first
27
Al MQMAS NMR study of dehydrated zeolite H-ZSM-5
was performed by Kentgens et al.14 This sample consisted
mainly of a single signal due to framework aluminum atoms
in the vicinity of bridging hydroxyl protons. Additional signals
were explained by a weak rehydration of the H-ZSM-5 zeolite.
The present work is the first 27Al high-speed MAS and
MQMAS NMR study of framework and extra-framework
aluminum species in non-hydrated zeolite Y performed at
B0 ¼ 17.6 T. Since strong signal broadenings due to 27Al
quadrupolar interactions have to be expected, these studies
were compared with spin-echo NMR investigations at B0 ¼
9.4, 14.1, and 17.6 T. The assignment of the different signals
obtained for dealuminated zeolite Y was supported by studies
of X-ray amorphous alumina (g-Al2O3), aluminum-exchanged
zeolite Y (Al,Na-Y) and the parent zeolite H,Na-Y as reference
materials. All investigations were performed using samples in
the non-hydrated state.
Experimental
1.
Materials
Zeolite Na-Y (nSi/nAl ¼ 2.7, Degussa AG, Hanau, Germany)
was sixfold cation-exchanged in a 1 M aqueous solution of
NH4NO3 at 353 K for 12 h. The obtained zeolite NH4,Na-Y
was washed with demineralized water until no nitrate ions were
detected. Subsequently, the powder material was dried in air at
353 K for 12 h. After this treatment, a cation-exchange degree
of 93% was reached. This exchange degree corresponds to
3.6 residual sodium cations per unit cell (u.c.) in zeolite
NH4,Na-Y. Zeolite H,Na-Y was formed by a calcination of
zeolite NH4,Na-Y at 723 K under vacuum (p r 1.5 Pa) for 12
h. The dealuminated zeolite deH,Na-Y/81.5 was prepared by
steaming zeolite H,Na-Y at 748 K for 2.5 h under a water
vapor pressure of 81.5 kPa. This sample was not rehydrated
after the steaming. Using an air lock connected to the steaming
equipment, the steamed sample was filled without contact to
air into a glass tube and sealed. The sodium and aluminum
contents of all zeolite materials were determined by atomic
emission spectrocopy (AES, Perkin Elmer Plasma 400).
The aluminum-exchanged zeolite Y (Al,Na-Y) was prepared
by exchanging zeolite Na-Y in an aqueous solution of 1.0 M
Al(NO3)3 at 293 K for 4 h. The pH value of the solution was
adjusted to 4 to avoid a dealumination or framework destruction. According to the chemical analysis by AES, 69% of the
sodium cations (36 Na1/u.c.) were replaced by 18 aluminum
cations per unit cell in zeolite Al,Na-Y. The above-mentioned
amount of introduced extra-framework aluminum cations
indicates a divalent nature of these species.
X-Ray amorphous g-Al2O3 with a specific surface area of
150 m2 g1 is a product of Merck KGaA, Darmstadt, Germany. Prior to the NMR investigations of zeolite Al,Na-Y and
g-Al2O3, these materials were dehydrated at 723 K under
vacuum (p r 1.5 Pa) for 12 h and sealed in glass tubes. Before
the NMR experiments, the non-hydrated samples were filled
into 2.5 mm or 4 mm MAS rotors and sealed with gas tight
rotor caps inside a glove box purged with dry nitrogen. 1H
MAS NMR spectroscopy verified that no residual water was
present on the samples under study.
2.
Spectroscopic characterization
27
Al spin-echo NMR experiments were performed in magnetic
fields of B0 ¼ 9.4 to 17.6 T on a Bruker MSL 400 (9.4 T), a
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Phys. Chem. Chem. Phys., 2005, 7, 3221–3226
Bruker Avance 600 (14.1 T), and a Bruker Avance 750 (17.6 T)
spectrometer. These investigations were carried out using 4 mm
MAS NMR probes without sample spinning. A p1-t1-p2-t2
spin-echo sequence with p1 ¼ p2 ¼ 0.61 ms, t1 ¼ 10 ms, t2 ¼ 9 ms
and a repetition time of 2 s were applied.18 The number of
scans was 4000–6000 at B0 ¼ 9.4 T, 1024 at B0 ¼ 14.1 T, and
640 at B0 ¼ 17.6 T. 27Al high-speed MAS NMR experiments
were performed at B0 ¼ 17.6 T using a 2.5 mm MAS NMR
probe with a sample spinning frequency of nrot ¼ 30 kHz, a
single-pulse p/12 excitation with a pulse duration of 0.34 ms,
and a repetition time of 2 s. 27Al MQMAS NMR spectra were
obtained applying the split-t1 echo pulse sequence14,19 with
hard pulses of 3.3 and 13.7 ms, a soft pulse of 47 ms and a
repetition time of 2 s. A 2.5 mm MAS NMR probe with an rf
field of 120 kHz and a sample spinning frequency of nrot ¼ 30
kHz were utilized. The 27Al NMR shift is given with respect to
the standard reference.20 The Bruker software WINNMR,
WINFIT, and XWINNMR were applied for the deconvolution and simulation of 1D NMR spectra and the transformation evaluation of 2D MQMAS spectra.
In systematic studies of the total 27Al NMR intensities of
zeolites H,Na-Y and different zeolites deH,Na-Y in the nonhydrated state, the hydrated parent zeolite Na-Y was used as
an external intensity standard. It was found that the correct
total amount of aluminum atoms in the different non-hydrated
zeolites Y can be observed using a repetition time of 2 s. For
the calculation of the numbers of aluminum species, however,
the total amounts of aluminum atoms obtained by AES were
utilized.
Results and discussion
1.
27
Al spin-echo NMR investigation of non-hydrated c-Al2O3
Fig. 1 shows the 27Al spin-echo NMR spectra of g-Al2O3
recorded at B0 ¼ 9.4, 14.1, and 17.6 T. The spectra consist of
two quadrupolar patterns at isotropic chemical shifts of 68 and
12 ppm. With an increasing magnetic field, the resolution of
27
Al spin-echo NMR signals is significantly improved due to
the decrease of the second-order quadrupolar broadening. The
spectroscopic parameters used for the simulation of the spectra
are summarized in Table 1. According to the above-mentioned
isotropic chemical shift values, the low-field signal is due to
tetrahedrally coordinated aluminum species (cluster AlIV),
while the high-field signal is caused by octahedrally coordinated aluminum species (cluster AlVI). This finding agrees with
the structure of g-Al2O3 consisting of aluminum oxide clusters
Fig. 1 27Al spin-echo NMR spectra of non-hydrated g-Al2O3 recorded at B0 ¼ 9.4 (a), 14.1 (b), and 17.6 T (c). The experimental
spectra (top) are compared with the simulated spectra (bottom).
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Table 1 Isotropic chemical shifts diso, quadrupolar coupling constants CQCC, asymmetry parameters ZQ, and relative intensities I of aluminum
species in non-hydrated g-Al2O3 and non-hydrated zeolite Al,Na-Y determined by simulation of the 27Al spin-echo NMR and 27Al high-speed MAS
NMR spectra recorded in magnetic fields of B0 ¼ 9.4 to 17.6 T
Assignment
g-Al2O3
1
Cluster AlIV
g-Al2O3
2
Cluster AlVI
Al,Na-Y
1
AlIV/Alx1
Al,Na-Y
2
AlIV/Na1
Al,Na-Y
3
Alx1 cat.
diso/ppm
CQCC/MHz
ZQ
I/%
68 5
8.5 0.5
0.8
60
12 5
5.5 0.5
0.7
40
70 10
14.5 1.0
0.3
48
60 5
5.5 0.5
0.8
28
35 5
6.0 0.5
0.7
24
Sample signal
with a cubic packing of oxygen atoms, which form tetrahedral
and octahedral holes filled with aluminum atoms.21
Comparing the spectroscopic data given in Table 1 with
those published for hydrated g-Al2O3,8 no significant change of
the isotropic chemical shift of tetrahedrally and octahedrally
coordinated aluminum species upon dehydration was observed. However, an increase of the CQCC value from ca. 4
MHz in the hydrated state to a value of 5.5 to 8.5 MHz occurs
as a result of dehydration.
g-Al2O3 is the low-temperature phase of solid aluminum
oxides and consists of small clusters of ordered aluminum
oxide being X-ray amorphous.21 In the further work, therefore,
the spectroscopic parameters of the cluster AlIV and cluster
AlVI species, summarized in Table 1, are utilized for the
description of solid-state 27Al NMR signals of neutral extraframework aluminum oxide clusters in non-hydrated zeolites Y
(Schemes 1a and 1b).
2. Solid-state 27Al NMR investigations of non-hydrated zeolite
Al,Na-Y
In zeolite Al,Na-Y, dehydrated at 723 K, the occurrence of
bridging OH groups, i.e. of framework aluminum atoms,
which are compensated in their negative charge by hydroxyl
protons (AlIV/H1, Scheme 1c) can be neglected.12b Therefore,
the solid-state 27Al NMR spectrum of this sample consists of
signals of framework aluminum atoms in the vicinity of extraframework sodium cations (AlIV/Na1, Scheme 1d), framework
aluminum atoms in the vicinity of extra-framework aluminum
cations (AlIV/Alx1, Scheme 1e) and extra-framework aluminum cations themselves (Alx1 cat., Scheme 1f).
Fig. 2 shows the 27Al MQMAS NMR spectrum of nonhydrated zeolite Al,Na-Y recorded at B0 ¼ 17.6 T. In this
spectrum, three signals occur at isotropic chemical shifts of
ca. 70 ppm (signal 1), 60 ppm (signal 2), and 35 ppm (signal 3)
in the F1-dimension. Along the F2-dimension, these signals are
shifted due to the second-order quadrupolar shift.10 These
second-order quadrupolar shifts amount to ca. 35 ppm for
signal 1 and 5 ppm for signals 2 and 3 corresponding to CQCC
values of ca. 15 MHz and 5.5 MHz, respectively. Because of
limitations in the signal/noise ratio, no simulation of 1D slices
obtained at the above-mentioned isotopic chemical shifts was
performed.
Scheme 1
In Fig. 3, the 27Al spin-echo NMR and 27Al high-speed
NMR spectra of non-hydrated zeolite Al,Na-Y recorded at
B0 ¼ 9.4, 14.1, and 17.6 T are depicted. The simulation of all
these spectra was possible assuming three signals with the
isotropic chemical shifts and the CQCC value determined by
MQMAS NMR spectroscopy. A summary of the spectroscopic
parameters used for these simulations is given in Table 1.
According to the chemical analysis of zeolite Al,Na-Y by
AES, this material contains 70 aluminum atoms per unit cell.
In this case, there are 16 framework aluminum atoms per unit
cell compensated in their negative charge by extra-framework
sodium cations (AlIV/Na1, Scheme 1d), 36 framework aluminum atoms per unit cell compensated by extra-framework
aluminum cations (AlIV/Alx1, Scheme 1e), and 18 aluminum
atoms per unit cell existing as extra-framework cations (Alx1
cat., Scheme 1f). In the solid-state 27Al NMR spectra of nonhydrated zeolite Al,Na-Y, signal 1 is the strongest component
with a relative intensity of 48% corresponding to 34 Al per u.c.
Therefore, this signal was assigned to framework aluminum
atoms compensated by extra-framework aluminum cations
(AlIV/Alx1). Signal 2 is typical for framework aluminum atoms
compensated by extra-framework sodium cations (AlIV/Na1)
as observed in earlier studies.11,12a,13 Hence, signal 3 must be
due to extra-framework aluminum cations (Alx1 cat.). The
resonance position of signal 3 of 35 ppm could be an indication
of a fivefold oxygen coordination of these species as suggested
by Bhering et al. for extra-framework Al(OH)21 species.22 The
extra-framework aluminum species coordinate to framework
oxygen atoms in the vicinity of framework aluminum atoms.22
This coordination could be the reason for the strong
Fig. 2 27Al MQMAS NMR spectrum of non-hydrated zeolite
Al,Na-Y recorded at B0 ¼ 17.6 T with nrot ¼ 30 kHz and the split-t1
echo pulse sequence.19
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Phys. Chem. Chem. Phys., 2005, 7, 3221–3226
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Fig. 3 27Al spin-echo NMR spectra of non-hydrated zeolite Al,Na-Y
recorded at B0 ¼ 9.4 (a), 14.1 (b), and 17.6 T (c). The spectrum in (d)
was recorded with MAS (nrot ¼ 30 kHz) at B0 ¼ 17.6 T. The
experimental spectra (top) are compared with the simulated spectra
(bottom).
quadrupolar broadening observed for signal 1 (AlIV/Alx1:
CQCC ¼ 14.5 MHz) in the spectra of non-hydrated zeolite
Al,Na-Y.
3. Solid-state 27Al NMR investigations of non-hydrated zeolite
H,Na-Y
Fig. 4 shows the 27Al MQMAS NMR spectrum of nonhydrated zeolite H,Na-Y recorded at B0 ¼ 17.6 T. This
spectrum consists of at least two signals occurring at isotropic
chemical shifts of ca. 75 ppm (signals 1 0 ) and 60 ppm (signal 2 0 )
in the F1-dimension. Interestingly, the broad signal 1 0 is split
into two peaks. No signal of octahedrally coordinated aluminum is observed. Along the F2-dimension, a second-order
quadrupolar shift of ca. 40 ppm for signal 1 0 and 5 ppm for
signal 2 0 was found. These second-order shifts correspond to
CQCC values of ca. 16 and 5.5 MHz, respectively. The splitting
of the broad signal 1 0 could be an indication that this signal is
Fig. 5 27Al spin-echo NMR spectra of non-hydrated zeolite H,Na-Y
recorded at B0 ¼ 9.4 (a), 14.1 (b), and 17.6 T (c). The spectrum in (d)
was recorded with MAS (nrot ¼ 30 kHz) at B0 ¼ 17.6 T. The
experimental spectra (top) are compared with the simulated spectra
(bottom).
due to a superposition of at least two quadrupolar patterns.
In the case of the highly crystalline parent zeolite H,Na-Y,
a distribution of chemical shifts and quadrupolar parameters
of the signals of framework aluminum species can be excluded.
In Fig. 5, the 27Al spin-echo NMR and 27Al high-speed
NMR spectra of non-hydrated zeolite H,Na-Y recorded at
B0 ¼ 9.4, 14.1, and 17.6 T are depicted. The simulation of the
high-speed MAS NMR spectrum recorded at B0 ¼ 17.6 T
requires three signals: Two quadrupolar patterns at ca. 70 ppm
with CQCC values of ca. 16 MHz (signal 1) and 14 MHz (signal
2) and one weak signal at ca. 60 ppm with a CQCC value of
ca. 5.5 MHz (signal 3). A summary of the spectroscopic
parameters of these signals is given in Table 2.
Due to the low intensity and the spectroscopic parameters of
signal 3, this component was assigned to framework aluminum
atoms compensated by sodium cations (AlIV/Na1, Scheme
1d).11,12a,13 The strong quadrupolar broadenings of signals 1
and 2 indicate that these components are caused by framework
aluminum atoms in the vicinity of bridging hydroxyl protons
(AlIV/H1, Scheme 1c). In earlier 27Al spin-echo NMR studies
of non-hydrated zeolites H-Y and H-ZSM-5, only one quadrupolar pattern similar to that of signals 1 and 2 was used to
describe the signal of AlIV/H1 species.11,12a,13 The simulation
of the 27Al high-speed MAS NMR spectrum of non-hydrated
zeolite H,Na-Y/81.5, however, requires at least two patterns
with similar isotropic chemical shifts but slightly different
quadrupolar coupling constants. These two quadrupolar patterns may be due to framework aluminum atoms in the vicinity
of bridging hydroxyl protons (AlIV/H1) pointing into supercages and sodalite cages. This assumption is supported by the
Table 2 Isotropic chemical shifts diso, quadrupolar coupling constants
CQCC, asymmetry parameters ZQ, and relative intensities I of aluminum
species in non-hydrated zeolite H,Na-Y determined by a simulation of
the 27Al spin-echo NMR and high-speed MAS NMR spectra recorded
in magnetic fields of B0 ¼ 9.4 to 17.6 T
Fig. 4 27Al MQMAS NMR spectrum of non-hydrated zeolite
H,Na-Y recorded at B0 ¼ 17.6 T with nrot ¼ 30 kHz and the split-t1
echo pulse sequence.19
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Phys. Chem. Chem. Phys., 2005, 7, 3221–3226
Signal
Assignment
1
AlIV/H1
2
AlIV/H1
3
AlIV/Na1
diso/ppm
CQCC/MHz
ZQ
I/%
70 10
16.0 0.5
0.3
47
70 10
14.0 0.5
0.3
47
60 5
5.5 0.5
0.8
6
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intensity ratio of 1 : 1 found for the signals 1 and 2 (see Table
2), since 1H MAS NMR spectroscopy of highly cation-exchanged zeolites H,Na-Y indicates the same intensity ratio for
bridging OH groups in supercages and sodalite cages.23
4. Solid-state 27Al NMR investigations of non-hydrated zeolite
deH,Na-Y/81.5
Fig. 6 shows the 27Al MQMAS NMR spectrum of nonhydrated zeolite deH,Na-Y/81.5 recorded at B0 ¼ 17.6 T. This
spectrum shows up to four signals occurring at isotropic
chemical shifts of ca. 80 ppm (signals 1), 65 ppm (signal 2),
35 ppm (signal 3), and 10 ppm (signal 4) in the F1-dimension.
Along the F2-dimension, second-order quadrupolar shifts of
ca. 35 ppm for signal 1, 10 ppm for signal 2, and 5 ppm for
signals 3 and 4 occur. These second-order quadrupolar shifts
correspond to CQCC values of ca. 15 MHz, 8 MHz, and 5.5
MHz, respectively.
The 27Al spin-echo NMR and 27Al high-speed NMR spectra
of non-hydrated zeolite H,Na-Y/81.5 recorded at B0 ¼ 9.4,
14.1, and 17.6 T are depicted in Fig. 7. In agreement with the
signals found in the 2D 27Al MQMAS spectrum in Fig. 6, the
simulation of the 1D spectra was performed assuming four
signals at isotropic chemical shifts of 70, 65, 35, and 10 ppm
with CQCC ¼ 15.0, 8.0, 7.5, and 5.0 MHz, respectively. A
summary of the spectroscopic parameters of these signals 1 to 4
is given in Table 3.
For the assignment of signals 1 to 4 in the solid-state 27Al
NMR spectra of non-hydrated zeolite deH,Na-Y/81.5, the
spectroscopic parameters and assignments of the signals obtained for non-hydrated g-Al2O3 and non-hydrated zeolites
Al,Na-Y and H,Na-Y were utilized. Considering the large
CQCC value and the isotropic chemical shift of signal 1 in the
spectrum of non-hydrated zeolite deH,Na-Y/81.5, this signal
was attributed to framework aluminum atoms in the vicinity of
bridging hydroxyl protons (AlIV/H1) as well as framework
aluminum atoms compensated by extra-framework aluminum
cations (AlIV/Alx1). Signal 2 was assigned to framework
aluminum atoms compensated by extra-framework sodium
cations (AlIV/Na1) as well as tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (cluster AlIV). Finally, signals 3 and 4 are attributed to
extra-framework aluminum cations (Alx1 cat.) and octahedrally coordinated aluminum atoms in neutral extra-framework
Fig. 7 27Al spin-echo NMR spectra of non-hydrated zeolite deH,NaY/81.5 recorded at B0 ¼ 9.4 (a), 14.1 (b), and 17.6 T (c). The spectrum
in (d) was recorded with MAS (nrot ¼ 30 kHz) at B0 ¼ 17.6 T. The
experimental spectra (top) are compared with the simulated spectra
(bottom).
aluminum oxide clusters (cluster AlVI), respectively. A summary of the spectral parameters and the signal assignments is
given in Table 3.
The chemical analysis of zeolite deH,Na-Y/81.5 by AES (52
Al per u.c., 3.6 Na1 per u.c.) and the relative intensities of the
solid-state 27Al NMR signals (last line in Table 3) allows a
quantitative discussion of the distribution of aluminum species
in this material in the non-hydrated state. The relative intensity
of signal 1 (48%) corresponds to 25 Al per u.c. By 1H MAS
NMR it was determined that this zeolite catalyst exhibits 17
bridging OH groups per unit cell (AlIV/H1).12b Hence, 8 Al per
u.c. occur as framework aluminum atoms compensated by
extra-framework aluminum cations (AlIV/Alx1). The relative
intensity of signal 2 (27%) corresponds to 14 Al per u.c. By
AES, the number of 3.6 sodium cations per unit cell (AlIV/
Na1) was determined. Therefore, 10.4 Al per u.c. occur as
tetrahedrally coordinated aluminum atoms in neutral extraframework aluminum oxide clusters (cluster AlIV). The relative
intensities of signals 3 (21%) and 4 (4%) indicate the presence
of 11 Al per u.c. as extra-framework aluminum cations (Alx1
cat.) and 2 Al per u.c. as octahedrally coordinated aluminum
atoms in neutral extra-framework aluminum oxide clusters
(cluster AlVI).
In summary, it can be stated that the combined application of
spin-echo, high-speed MAS, and MQMAS NMR spectroscopy
of dealuminated and non-hydrated zeolite Y in different magnetic fields allow the determination of tetrahedrally coordinated
framework aluminum species in up to three local structures
(AlIV/H1, AlIV/Na1, AlIV/Alx1) and three types of extraTable 3 Isotropic chemical shifts diso, quadrupolar coupling constants
CQCC, asymmetry parameters ZQ, and relative intensities I of aluminum
species in non-hydrated zeolite deH,Na-Y/81.5 determined by a simulation of the 27Al spin-echo NMR and high-speed MAS NMR spectra
recorded in magnetic fields of B0 ¼ 9.4 to 17.6 T
Fig. 6 27Al MQMAS NMR spectrum of non-hydrated zeolite
deH,Na-Y/81.5 recorded at B0 ¼ 17.6 T with nrot ¼ 30 kHz and the
split-t1 echo pulse sequence.19
Signal
Assignment
1
AlIV/H1
AlIV/Alx1
2
AlIV/Na1
Cluster AlIV
3
Alx1 cat.
4
Cluster AlVI
diso/ppm
CQCC/MHz
ZQ
I/%
70 10
15.0 1.0
0.3
48
65 5
8.0 0.5
0.8
27
35 5
7.5 0.5
0.7
21
10 5
5.0 0.5
0.7
4
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framework aluminum species (Alx1 cat., cluster AlIV, cluster
AlVI). Due to the overlap of some of the signals, the quantitative
evaluation of spectra obtained by different solid-state NMR
techniques with consistent spectral parameters is required.
2
3
Conclusions
27
Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy performed in magnetic fields of B0 ¼ 9.4, 14.1, and 17.6
T were demonstrated to be useful tools for the characterization
of framework and extra-framework aluminum species in nonhydrated zeolite catalysts. Non-hydrated g-Al2O3 and nonhydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite
H,Na-Y were investigated as reference materials for the assignment of solid-state 27Al NMR signals.
The solid-state 27Al NMR spectra of non-hydrated g-Al2O3
consist of two signals of tetrahedrally and octahedrally coordinated aluminum atoms in clusters of aluminum oxide
(cluster AlIV and cluster AlVI, respectively). The spectra of
non-hydrated zeolite Al,Na-Y are described by three signals of
tetrahedrally coordinated framework aluminum atoms compensated in their negative framework charges by extra-framework sodium cations (AlIV/Na1), framework aluminum atoms
compensated by extra-framework aluminum cations (AlIV/
Alx1), and extra-framework aluminum cations themselves
(Alx1 cat.). The above-mentioned signals occur at characteristic resonance positions and/or exhibit characteristic quadrupolar coupling constants. Solid-state 27Al NMR investigations
of non-hydrated zeolite H,Na-Y indicate that the aluminum
atoms in this material cause three different signals: two quadrupolar patterns with slightly different quadrupolar coupling
constants for framework aluminum atoms in the vicinity of
bridging hydroxyl protons pointing into supercages and sodalite cages (AlIV/H1), and a small pattern with a low quadrupolar coupling constant for framework aluminum atoms
compensated by extra-framework sodium cations (AlIV/Na1).
The investigation of a steamed and non-hydrated zeolite
deH,Na-Y/81.5 results in spectra consisting of at least four
signals. The assignment of these signals was performed based
on spectroscopic parameters of signals observed for non-hydrated g-Al2O3 and non-hydrated zeolites Al,Na-Y and H,NaY. Two of the signals were explained by a superposition of the
signals of framework AlIV/H1 and AlIV/Alx1 species and by a
superposition of signals of framework AlIV/Na1 species and
extra-framework cluster AlIV species. The residual two signals
are due to extra-framework Alx1 cations and extra-framework
cluster AlVI species. Based on the total amounts of aluminum
species and the relative intensities obtained by solid-state 27Al
NMR spectroscopy of non-hydrated zeolite deH,Na-Y/81.5, the
aluminum distribution in this material could be determined.
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Acknowledgements
18
Financial support by Deutsche Forschungsgemeinschaft,
Fonds der Chemischen Industrie, and Max-Buchner-Stiftung
is gratefully acknowledged. The authors thank Thomas Loeser
(w), University of Leipzig, for recording high-field spin-echo
NMR spectra.
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20
21
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