7 JANUARY 2022 • VOL 375 ISSUE 6576 29GRAPHIC: N. DESAI/
SCIENCE
SCIENCE science.orgBy Weibin Fan and Mei DongZ
eolites as catalysts or adsorbents have
been extensively applied in petro-,
coal-, and fine-chemical industries. To
decrease the pressure drop and heat
effects in reaction and separation pro-
cesses, zeolites are typically shaped
into bars or spheres, with binders to facilitate
diffusion of reactants, adsorbates, and prod-
ucts. Inspired by this, the intrinsic adsorp-
tive and catalytic properties of zeolites are
enhanced by regulating their morphology
and size ( 1 ). On page 62 of this issue, Korde et
al. ( 2 ) report the synthesis of single-
walled zeolitic nanotubes with an
inner building layer that is topologi-
cally identical to the ZSM-5 ac plane
and an outer layer identical to that
of zeolite beta. This material has
the potential to fabricate molecular
recognition-based devices applicable
to catalysis, separations, sensing,
and drug delivery by regulating mol-
ecules’ diffusion pathway and length.
A simple method to improve zeo-
lite diffusivity is to decrease crystal
size. This can substantially increase
the catalytic stability and product
selectivity by reducing secondary
reactions and shortening diffusion
length ( 3 ). The shape-selectivity of
zeolite can be maximized through
appropriate exposure of certain crystal
facets. The large, highly intergrown, and
twinned ZSM-5 crystals exhibit p-xylene se-
lectivity of >99%, in contrast to the 24.1%
obtained on a commercial sample in tolu-
ene methylation, as a result of exposing
many more (100) planes that possess sinu-
soidal channel pore openings ( 4 ).
Another approach is the creation of meso-
or macropores in zeolite particles. With three-
dimensionally ordered mesoporous carbon
as a template, highly ordered mesoporous
aggregates of zeolite nanocrystallites are syn-
thesized by means of confined crystallization
( 5 ). Recently, an ordered macro-mesoporous
hierarchical carbon template with a face-cen-
tered cubic structure led to the formation of
macro-meso-microporous single zeolite crys-
tals ( 6 ). The ZSM-5 that was obtained showeda catalytic lifetime as long as 13 times that of
commercial microsized mesoporous ZSM-5
in a methanol-to-olefins reaction.
A more interesting strategy is the direct
synthesis of hierarchically porous zeolites
by using the soft template method because
this avoids the preparation and removal
of the carbon mold (see the figure). Two-
dimensional (2D) MFI-type nanosheets are
synthesized and assembled into 3D meso-
porous particles with house-of-cards or
90°-rotational intergrowth shape by using
di- or multi-quaternary ammonium surfac-
tants with the hydrophilic head directingzeolite crystallization and the hydropho-
bic alkyl chain suppressing crystal growth
along the b axis ( 7 , 8 ).
Assembly of the 2D nanosheets depends
on the quaternary ammonium structure.
Bolaform or triply branched cationic sur-
factants with aromatic groups in hydro-
phobic regions, such as C 6 H 5 –2N–C 6 H 4 –O–
C 10 H 20 –N+(CH 3 ) 2 –C 6 H 12 –N+(CH 3 ) 2 –C 6 H 13 (2Br–)
(Cazo-10-6-6), enhance formation of lamellar
micelle and 2D zeolite nanosheets through
p-p interaction and lead to the generation
of elongated octahedral crystals with or-
dered 2D square mesochannels fabricated
with MFI nanosheets expanding along the
ac plane through vertical conjunctions ( 9 ).
Further change of Cazo-10-6-6 to Cazo-12-6-6 and
Cazo-8-6-6 produces deformed rhombohedral
and tetragonal-shaped aggregates, respec-
tively ( 10 ). Additionally, alteration of the hy-
drophilic head of bolaform surfactant from
the trialkyl group to quinuclidinium creates
zeolite nanotubes, through p-p stacking ofsurfactant molecules into a hydrophobic core
along the nanotube axis and a hydrophilic
periphery with quinuclidinium groups. This
eliminates the diffusion limitation, resulting
in high activity and selectivity in reactions
that involve bulky molecules ( 1 , 7 ). But these
nanotubes exhibit low hydrothermal stabil-
ity because of the large number of surface
Si–OH groups.
Using well-crystallized zeolite crystal
building units is an effective way to increase
their hydrothermal stability. However, the
rigid framework and the presence of few
numbers of Si–OH groups on external surface
makes assembly difficult. Silicalite-1
crystals have been assembled into
1D, 2D, and 3D architectures by
selectively adsorbing ethanol and
benzaldehyde ( 11 ). Unfortunately,
the framework chemical composi-
tions, crystal size, and morphology
cannot be adjusted. In addition,
oriented assembly of zeolite crystals
into self-supported membrane along
a desired axis and further regulation
of size and thickness are required
to simultaneously achieve high
separation factor and flux upon
being supported on a porous ma-
trix. Further enhancement of its
adsorptive separation and/or cata-
lytic performance requires synthe-
sis of zeolite nanotubes with walls
assembled by perfectly arrayed crystals.
Another challenge is to tailor the macro- or
mesopore structure and size of 3D architec-
ture by modulating the construction scheme
of (hierarchical) crystals. With the emerging
of new techniques and the understanding of
crystallization mechanism, solving these is-
sues should be possible. Further rational de-
sign and assembly of hierarchical hollow par-
ticles and incorporation of metallic atoms,
clusters, and nanoparticles will offer zeolites
more interesting functions. jREFERENCES AND NOTES- L.-H. Chen et al., Chem. Rev. 120 , 11194 (2020).
- A. Korde et al., Science 375 , 62 (2022).
- Q. Zhang et al., J. Am. Chem. Soc. 141 , 3772 (2019).
- C. Wang et al., Nat. Commun. 10 , 4348 (2019).
- W. Fan et al., Nat. Mater. 7 , 984 (2008).
- M.-H. Sun et al., Matter 3 , 1226 (2020).
- M. Choi et al., Nature 461 , 246 (2009).
- K. Möller, T. Bein, Science 333 , 297 (2011).
- X. Shen et al., Angew. Chem. Int. Ed. 57 , 724 (2018).
- X. Shen et al., Chem. Eur. J. 24 , 8615 (2018).
- S. Li et al., ACS Appl. Mater. Interfaces 13 , 58085 (2021).
10.1126/science.abn2048
ZEOLITESRegulation of zeolite particle morphology
Specific particle morphology endows zeolites with improved molecular r ecognizability
State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, Chinese Academy of Sciences, 27 South Taoyuan
Road, Taiyuan 030001, China. Email: [email protected]Organic
additivesZeolite crystals Nanotubes 3D architectureCrystal assembly into designed architecture
One strategy to assemble zeolite crystals into membrane-like particles,
zeolite nanotubes, or hierarchical three-dimensional (3D) architecture
is to apply surfactant templates or organic additives through coulomb
force, hydrogen bonding, p–p interactions, and/or van der Waal forces.
Further control of crystal structure, habit, and size will produce highly
ordered, pore-tunable hierarchical zeolite materials.