or catalyst, it is important to know how its
structure changes during the adsorption-
desorption of the gas molecules.
Structural flexibility caused by physical
or chemical stimuli has been reported in
some zeolites ( 4 ). This structural flexibility
can be induced by ion exchange, dehydra-
tion, or gas adsorption, and the flexibility
can manifest as a change in the unit-cell
volume and/or in the symmetry and pore
size. Diffraction-based techniques, includ-
ing x-ray, neutron, and electron diffrac-
tion, have served as important tools for
studying these structural flexibilities. For
example, in situ powder x-ray diffraction
has shown that the unit-cell volume can
expand by up to 25% upon ion exchange
( 5 ). The accessible pore window can be
substantially altered upon dehydration
or ion exchange, which greatly affect the
adsorption and desorption properties ( 4 ,
6 ). Three-dimensional electron diffraction
(3DED) has also been a powerful tool for
determining the structure of polycrystal-
line porous materials ( 7 ). Researchers have
used 3DED to locate carbon dioxide (CO 2 )
molecules in the zeolite CHA ( 8 ). However,
these diffraction-based techniques can
only provide structure information aver-
aged over the entire measured material.
To obtain a more detailed understanding
of the mechanisms underlying structural
flexibility, it is necessary to visualize the
host-guest interactions at the individual-
pore level.
Recent advances in imaging techniques
with atomic resolution have made it pos-
sible to visualize the guest molecules inside
the voids of porous materials. One such
method is integrated differential phase
contrast scanning transmission electron
microscopy (iDPC-STEM). Researchers
have used iDPC-STEM to observe physically
adsorbed organic molecules and chemically
bound single molybdenum oxide clusters in
the nanopores of zeolite ZSM-5 ( 9 ). When
synthesizing zeolites, organic molecules
are often used as a “structure-directing
agent” to help build the three-dimensional
structure of a zeolite. Using iDPC-STEM,
researchers have been able to track the
location of the structure-directing organic
molecules in the zeolite pores, which pro-
vides insights into the formation of the zeo-
lite material ( 10 ). iDPC-STEM imaging has
also been used to observe para-xylene mol-
ecules adsorbed inside the straight chan-
nels of the zeolite MFI ( 11 ). This revealedthat the orientation of the para-xylene mol-
ecules in the channel is correlated to the
channel geometry and therefore could be
used to detect host-guest van der Waals in-
teractions in porous materials ( 11 ).
Using an in situ transmission electron
microscopy gas-cell specimen holder,
Xiong et al. studied the gas adsorption-
desorption process in zeolite ZSM-5 us-
ing benzene as a probe molecule (see
the figure). Their imaging results show a
reversible local deformation of pore win-
dows from circular to elliptical upon the
pressure-induced adsorption-desorption
of the benzene molecules. The pores are
stretched by up to 15% in aspect ratio to
optimize the interaction with the benzene
molecules and facilitate their diffusion
into the channels. The pore structural flex-
ibility is associated with the flexibility ofthe Si–O–Si bond angles, and the intro-
duction of benzene molecules in the pores
reduces such flexibility, as predicted by ab
initio molecular dynamic simulation. The
direct observation of such a delicate in-
terplay between the zeolite host and guest
molecules allows a better understanding of
the mechanisms behind the complex ad-
sorption-desorption behavior, which will
help in the design and selection of adsor-
bents for specific applications.
Unlike zeolites, metal-organic frame-
works (MOFs) have a higher degree of
structural flexibility. Structural changes
in MOFs can be triggered using different
guest molecules for different adsorption-
desorption behaviors, such as reversible
pore opening-shrinking, or “breathing,”
and selective gate-opening ( 12 ), which has
been studied using x-ray techniques. In
earlier efforts to study the CO 2 adsorption
of MOFs, researchers have used low-dose
cryogenic electron microscopy to observe
CO 2 molecules in the channels of the MOF
ZIF-8 ( 13 ). Direct observation of sorption-
induced structural changes of the MOFs at
the individual-pore level could be achieved
by a combination of an in situ gas envi-
ronment and low–electron dose imaging
to prevent electron beam damage to the
MOFs. This knowledge is important for the
design of MOFs where structural flexibility
can be used to tailor adsorption properties
through chemical or physical interactions
between the host and guest.
The possibility of visualizing local
structural changes induced by adsorbed
molecules during an entire adsorption-de-
sorption process should serve as an inspi-
ration for applications of in situ electron
microscopy on other flexible nanoporous
materials beyond zeolites and MOFs.
Direct observation of guest molecules in
the channels using iDPC-STEM imaging
will provide an understanding of host-
guest interactions in all those nanoporous
materials, which is important for develop-
ing efficient adsorbents and catalysts with
enhanced selectivity and performance. jREFERENCES AND NOTES- M. E. Davis, Nature 417 , 813 (2002).
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- C. Baerlocher, L. B. McCusker, Database of zeolite
structures; http://www.iza-structure.org/databases/. - M. M. Lozinskaet al., J. Am. Chem. Soc. 134 , 17628
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10.1126/science.abo5434 GRAPHIC: BASED ON (2 science.org SCIENCEINSIGHTS | PERSPECTIVES
The flexible pores of a zeolite
The nanopores of a zeolite adapt their shape to fit the adsorbed molecules. Using scanning transmission
electron microscopy, Xiong et al. directly observed this structural change at the sub–unit cell scale during the
entire adsorption-desorption process.
2 nm 2 nm458 29 APRIL 2022 • VOL 376 ISSUE 6592