In situ observation of subcell deformation in
zeolite channels
ZSM-5 zeolites are composed of corner-sharing
tetrahedral TO 4 (T: Si, Al) and have a cross-
linked channel system formed of straight
channels (5.3 Å × 5.6 Å) and sinusoidal chan-
nels (5.1 Å × 5.5 Å) (fig. S1). The straight
channels of MFI zeolite can be directly imaged
from the [010] projection at the atomic scale
and are the best imaging windows for observ-
ing opening pores and confined molecules
(fig. S2). Inspired by the synthesis strategy
for nanosized lamellar architectures ( 8 , 33 – 37 ),
we synthesized coffin-shaped MFI single crys-
tals that had a smooth surface and uniform
thickness (~40 nm) along thebaxis (figs. S3
and S4). By eliminating the influence of height
variance on STEM imaging, we could observe
in situ both adsorbed benzene molecules and
the zeolite framework in a large field of view
(25.74 nm × 25.74 nm).
Figure 1A exhibits the schematic of the in
situ STEM experimental setup. The experiments
were performed in a spherical aberration (Cs)–
corrected transmission electron microscope
equipped with an enclosed environment cham-
ber where environmental temperature, inlet
gas composition, and volume flow could be
regulated precisely. Full experimental details
are given in the supplementary materials. Be-
fore benzene adsorption, the fresh MFI zeolite
specimen was degassed under vacuum at 923
K for 5 hours. From the [010] projection, the
projected positions of Si atoms and empty
straight channels were imaged (Fig. 1B); the
O atoms were covered by nearby Si atoms
because of thermal diffuse scattering and
lattice vibrations.
To better describe the varying geometries
of straight channels, we defined the distances
of Si 1 to Si 6 and Si 4 to Si 9 as the long axis
(Dmax, red line) and short axis (Dmin,blueline),
respectively, as the periodic parameters for
straight channels (Fig. 1, C and F, and fig. S5),
which is slightly different from the definition
of the pore size (the distance of O atoms) of
straight channels ( 38 ). The magnified image
of empty straight channels showed a nearly
round pore shape (Dmax, 9.14 ± 0.11 Å;Dmin,
8.91 ± 0.16 Å) with an aspect ratio of 1.02 (Fig.
1D and fig. S6), which is highly consistent with
the standard structural model (fig. S5). This
symmetrical and uniform shape of opening
pores originates from high crystallographic sym-
metry and isostatic perfectly regular tetrahedra,
indicating no internal stress accumulation.After benzene adsorption at 473 K and 450
torr (10% benzene and 90% N 2 ), the close size
match of one C 6 -ring plane and the channels
of ZSM-5 zeolite caused the benzene molecules
to be stably confined in straight and sinusoidal
channels through van der Waals interactions
( 39 , 40 ). Following the principle of energy opti-
mization, the degrees of freedom of adsorbed
molecules would be reduced to several states
( 28 , 29 ). Thus, the confined benzene molecules
were“frozen”in zeolite channels with a fixed
orientation and critical dimensions (7.367 Å ×
6.702 Å × 3.40 Å) ( 41 , 42 ) that were different
from the kinetic diameter in the gas phase
when assumed to be a sphere ( 1 ).
The total overlapping projection of these
benzene molecules formed an elliptical spin-
dle with the smallest cross-sectional area of
benzene in each straight channel (Fig. 1E). The
unified orientation and superimposed contrast
along the [010] projection enabled us to simul-
taneously realize the imaging of small molecules
and the zeolite framework. Moreover, we ob-
served substantial local deformation of the
straight channels, from an approximate round
shape to a clear elliptical shape (Fig. 1, F and
G). The straight channels were stretched along
Dmax(9.73 ± 0.16 Å), which was parallel to492 29 APRIL 2022•VOL 376 ISSUE 6592 science.orgSCIENCE
ABCDEFG0 5 10 15 20).u
.a((^) yt
is
ne
tnI
Distance (Å)
9.73 0.16
8.31 0.12
0 5 10 15 20
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.a(
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is
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tnI
Distance (Å)
9.14 0.11
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Empty
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Fully
Filled
H
I
J
Fig. 1. Imaging the huge local deformation of zeolite channels in the in situ
STEM system during benzene adsorption.(A) Schematic of the in situ STEM
experimental setup. The benzene adsorption-desorption process was carried
out in an enclosed environment chamber made of Si 3 N 4 solid membranes in an
atmosphere system. (BtoD) iDPC-STEM images and profile analysis of an empty
MFI zeolite specimen from the [010] projection. The magnified image shows a
nearly round pore. The profile analysis gives the aspect ratio ofDmaxtoDmin,
which was used to estimate the local deformation of straight channels. (EtoG)
iDPC-STEM images and profile analysis of a saturated benzene@MFI specimen
from the [010] projection at 473 K and 450 torr (90% N 2 and 10% benzene). The
10-ring opening pore shows a distinctly elliptical shape stretched along the
parallel direction of the benzene plane. (HtoJ) Further comparison of imaging
differences between the empty (H) and saturated states (I) of benzene@MFI,
showing clearer imaging of O atoms in the MFI framework, which are highlighted
in (J). Forty opening pores were measured to obtain each statistical result. Scale
bars, 2 nm [(B) and (E)], 500 pm [(C), (F), (H), and (I)].
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