(PFG) NMR diffusometry

Report
Ion and water mobility in zeolite Li-LSX
studied by 1H, 6Li and 7Li NMR
spectroscopy and diffusometry
Investigation of the water and lithium ion mobility in a crystalline porous material
by Dieter Freude, Steffen Beckert, Frank
Stallmach, Jörg Kärger, Jürgen Haase
Universität Leipzig
Institute für Experimentelle Physik
Linnéstraße 5, 04103 Leipzig, Germany
rotor with sample
in the rf coil
B0 = 9  18 T
zr
rot  10 kHz
θ
The study was recently published in
Microporous and Mesoporous Materials
172 (2013) 174–181 (15 May 2013).
Reprints are available on request now
or by E-mail to [email protected]
gradient coils for
pulsed field gradients,
maximum 1 T / m for MAS,
but 35 T / m for PFG NMR
Pulsed field gradient (PFG) NMR diffusometry
Spin recovery by Hahn echo without diffusion of nuclei:
p/2
p
r.f. pulse
t
gradient pulse
gmax = 25 T / m
magnetization y
t
d
free induction
Hahn echo
D
B0
z
B0
z
y
y
M
x
t
D
B0
1
5 4
z
y
2
B0
y
5
3
1
z
4
2
3
M
x
PFG NMR: signal decay by diffusion of the nuclei
PFG NMR diffusion measurements base
on radio frequency (rf) pulse sequences.
They generate a spin echo,
like the Hahn echo (two pulses), or
the stimulated spin echo (three pulses).
At right, a sequence for alternating
sine shaped gradient pulses and
longitudinal eddy current delay (LED)
consisting of 7 rf pulses, 4 magnetic field gradient pulses of duration d, intensity
g, observation time D, and 2 eddy current quench pulses is presented.
The self-diffusion coefficient D of molecules is obtained from the decay of the
amplitude S of the FID in dependence on the field gradient intensity g by the equation
2

d  d
 4d g   

S  S0 exp  D 
  pp   S0 exp D k 
 D 
2

 p  


High-resolution solid-state MAS NMR
Fast rotation (1-60 kHz) of the sample about an
axis oriented at the angle 54.7° (magic-angle)
with respect to the static magnetic field
zr
removes all broadening effects with an
rot angular dependency of
B0
3 cos2   1
.
2
θ
  arccos
1
 54.7o
3
Chemical shift anisotropy,
internuclear dipolar interactions,
first-order quadrupole interactions, and
inhomogeneities of the magnetic susceptibility
are averaged out.
It results an enhancement in spectral resolution
by line narrowing for solids and for soft matter.
The transverse relaxation time is prolonged.
MAS PFG NMR  diffusometry with spectral resolution
6 5 4 3 2
d / ppm
Spectral resolution is necessary for studies of systems consisting of proton
species with different mobility. The spectrum shows water molecules which
are located in the sodalite cages (signal at 3.8 ppm) having a small mobility
and water molecules in the large cavities (signal at 4.9 ppm) having a high
mobility in the hydrated zeolite Li-LSX at 373 K (observation time is 100 ms).
NMR exchange spectroscopy (EXSY)
p/2
p/2
t1
p/2
tmix
t2
time
1
0
δ F1 / ppm
0
1
0
δ F2 / ppm
2D 6Li MAS NMR exchange spectrum
of the zeolite Li-LSX obtained at
373 K with a mixing time of 1000 ms.
Exchange spectroscopy is a two-dimensional
NMR experiment. The free induction signal is
monitored as a function of t2. Consecutive
experiments give the dependence on t1. After a
two-dimensional Fourier transform, we obtain
cross peak intensities, which depend on the
exchange between the different locations of
the nuclei as a function of the mixing time tmix.
MAS NMR spectroscopy and MAS PFG NMR diffusometry
1H
and 6Li MAS NMR spectroscopy
and 1H MAS PFG NMR diffusometry
were performed in a wide-bore magnet
with the external magnetic field of
17.6 Tesla.
PFG NMR diffusometry
7Li
PFG NMR measurements were
carried out by means of the homebuilt PFG NMR spectrometer FEGRIS
in the external field of 9.4 Tesla.
The spectrometer is able to provide
pulsed field gradient amplitudes up
to gmax = 39.3 Tm-1.
Microporous zeolite Li-LSX
Faujasite crystallite
Lithium ion
Water molecule
Faujasite cage
The lithium form of the low-silica X type zeolite (Li-LSX) has good properties
for N2/O2 separation processes, cleaning liquid nuclear waste, CO2 capture
from the atmosphere, and hydrogen storage. Li-X zeolites were also used as
model systems for the investigation of the electrical properties of nano-scale
host/guest compounds. The commercial zeolite Li-LSX consists of crystallites
with about 3 µm diameter. A zeolite Li-LSX with a diameter of about 10 µm was
used for the present study.
1H
and 6 Li MAS NMR spectroscopy
and the results of exchange spectroscopy
Signals from species which are located
in the large cavities
and
in the sodalite cages
6.0
1H
5.0
4.0
d / ppm
3.0
MAS NMR spectrum of
the hydrated Li-LSX at 373 K
1.0
2.0
0.0
d / ppm
-1.0
6Li
MAS NMR spectrum of
the hydrated Li-LSX at 373 K
2D 1H MAS exchange spectroscopy yields for a 91% lithium exchanged zeolite
Li-LSX a value of 40 ms for the mean residence time of a water molecule in the
sodalite cage before jumping into the supercage. By 2D 6Li MAS NMR, the
mean residence time of a lithium ion on SIc position in the sodalite cage before
exchange with a SIIc position is estimated to be 150 ms. The lithium ions on
SIIc positions are in much faster exchange with all cations in the supercage.
1H
MAS PFG NMR and 1H PFG NMR
Semi logarithmic plot of the decay of the
intensity of the 1H MAS PFG NMR (open
squares) and 1H PFG NMR (filled squares)
signals as a function of the applied gradient
strength (-value) for an observation time
of 10 ms at a temperature of 373 K.
 2d
 4d g  

D



p
 
p
2 
 p  

2
b 
The two-component exponential decay reflects the fast inter-crystalline diffusion
and the slower intra-crystalline or intra-particle diffusion.
MAS PFG NMR monitors only the less important inter-crystalline effect.
The stronger gradients of the PFG NMR are necessary for the observation of the
intra-effect.
7Li
PFG NMR
Semi logarithmic plot of the decay of the
intensity of the 7Li PFG NMR signals as a
function of the applied gradient strength
(-value) for an observation time of 2 ms
at a temperature of 373 K (circles), 423 K
(triangles) and 473 K (squares)
Stronger pulsed field gradients were used for this first 7Li PFG NMR observation
of the self-diffusion of cations in zeolites. As usual, the larger crystallites favor
the measurement of the intra-crystalline diffusion.
Result
 Crystallites of zeolite LSX with a diameter of about 10 µm were synthesized.
Crystals of this size are shown to allow the simultaneous investigation of
intracrystalline mass transfer phenomena of water molecules and lithium ions in
hydrated zeolite Li-LSX by NMR diffusometry.
 By MAS NMR spectroscopy of 1H and 6Li nuclei, the water molecules and lithium
ions are found to yield two signals, a major and a minor one, which may be
attributed to locations in the sodalite cages and the supercages, respectively.
By 1H and 6Li exchange spectroscopy the mean residence times in the sodalite
cages at 373 K are found to be about 40 ms for the water molecules and about
150 ms for the lithium cations.
 PFG NMR self-diffusion measurements at 373 K yield a diffusivity of about
2.0 × 10-11 m2s-1 for the lithium ions, whereas the self-diffusion coefficient for the
water molecules amounts to D = 2.5 × 10-10 m2s-1.
This finding is not trivial, since lithium is strongly hydrated and all water molecules
belong to the hydration shells.
Conclusions
 Crystallites of zeolite LSX with a diameter of about 10 µm were synthesized.
Crystals of this size are shown to allow the simultaneous investigation of
intracrystalline mass transfer phenomena of water molecules and lithium ions in
hydrated zeolite Li-LSX by NMR diffusometry.
 By MAS NMR spectroscopy of 1H and 6Li nuclei, the water molecules and lithium
ions are found to yield two signals, a major and a minor one, which may be
attributed to locations in the sodalite cages and the supercages, respectively.
By 1H and 6Li exchange spectroscopy, the mean residence times in the sodalite
cages at 373 K are found to be about 40 ms for the water molecules and about
150 ms for the lithium cations.
 PFG NMR self-diffusion measurements at 373 K yield a diffusivity of about
2.0 × 10-11 m2s-1 for the lithium ions, whereas the self-diffusion coefficient for the
water molecules amounts to D = 2.5 × 10-10 m2s-1.
 The cation diffusivity is retarded by about one order of magnitude in comparison
with the water diffusivity. This notably exceeds the retardation of cation diffusion
in comparison with water in free solution (publication in preparation) , reflecting
the particular influence of the zeolite lattice on the guest mobility.

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