However, as more benzene was adsorbed,
the flexibility of the pore channels decreased,
and the shapes tended to resemble an ellipse
with a greater curvature (Fig. 3, C and D). For
example, when one benzene molecule was
adsorbed per unit cell, the meanDmaxand
Dminwere 9.6 and 8.6 Å, respectively. The
deviation betweenDmaxandDminincreased
(to ~9.9 and 8.3 Å, respectively) when four
benzene molecules were confined inside each
MFI zeolite unit cell. Moreover, the standard
deviation of the pore size distribution tended to
decrease as more benzene adsorbed, indicating
decreasing flexibility in the Si 10 opening pores
(Fig. 3A and table S1). Overall, as the amount
of adsorbed benzene increased, the deforma-
tion of MFI straight channels increased, which
was consistent with our experimental images.
Structurally, zeolites are composed of corner-
sharing nearly perfect tetrahedral TO 4 , con-
nected by hinges at the oxygen atoms (fig. S1).
Hence, the different structures of zeolites are
mainly determined by three structural param-
eters:a(Si-O-Si bond angle),b(O-Si-O bond
angle), andd(Si-O bond length) (Fig. 3E). To
further investigate the origin of MFI zeolite
flexibility, we performed detailed statistical
analyses on the bond lengths and angles in
the 10-membered rings (10MRs). With benzene
adsorption, the main contribution to channel
deformation came from the increase ina(Fig. 3F
and fig. S15), whereasbanddwithin tetrahedral
SiO 4 were almost unchanged (Fig. 3, G and H,
and figs. S16 and S17). The most substantial
change came from the second and seventha,
which were exactly vertical to the plane of the
benzene molecule (Fig. 3F). The local stress
was mainly concentrated in these two Si-O-Si
bonds and nearby areas, causing the Si-O-Si
angle to be flattened and distant in the [010]
projection (fig. S18).
Previous studies have found that thermo-
dynamics and kinetics are sensitive to the dif-
ference in the local chemical environment,
especially the T-O-T angle at the acid site, which
can be stretched to influence the intrinsic
acidity, stabilize the protonated transition
state, and thereby control the reaction path-way in ZSM-5 ( 47 , 48 ), which would have a
profound impact on understanding the molec-
ular reaction mechanism. In contrast, the dis-
tributions ofbanddat different loadings were
nearly identical and near the theoretical value
of a perfect SiO 4 tetrahedron (109° and 1.66 Å)
(Fig. 3, G and H). We further examined the
distributions of six O-Si-O angles within a single
tetrahedral SiO 4 in a 10MR straight channel to
confirm the rigidity of the SiO 4 tetrahedron (fig.
S19). Thus, we concluded that the local flexibility
of the zeolite framework originated from the
topologically soft Si-O-Si hinges between adja-
cent tetrahedral SiO 4 , whereas the tetrahedral
SiO 4 remained rigid and nearly perfect.MFI zeolite framework with subcell topological
flexibility and overall rigidity
The above structural analysis and theoretical
calculation results revealed the local flexibility
in the MFI framework, which allows extensive
local deformation of the straight channels to
accommodate larger guest molecules. When
benzene and other organic sorbates, with a
comparable molecular size to the opening pores,
adsorb from the gas phase into the pore-
confined phase, the molecules fall into a deep
potential well. The movement and vibrations of
confined molecules are restricted, and the images
of confined molecules resemble their molecu-
lar dimensions, behaving as one-dimensional
oriented single crystals, such that the mole-
cules cannot simply be treated as spheres. Thus,
the opening pores will be stretched according
to the molecular dimensions of the confined
molecules, and the maximum change in pore
diameter can reach 0.63 Å in the [010] projec-
tion (Fig. 4, A to C, and figs. S20 and S21).
Despite severe subcell deformation, mono-
crystalline MFI zeolite could still maintain
an intact crystal structure, and the unit cell
parameters from the [010] projection were
basically unchanged (Fig. 4, D and E). Here,
we used profile analysis to study the unit cell
parameters (a,c) and their included angle
from the [010] projection. We only saw slight
variance in the overall cell parameters (from
2.033 ± 0.002 nm × 1.364 ± 0.001 nm × 90.5° to
2.043 ± 0.002 nm × 1.365 ± 0.001 nm × 90.1°).
The changes in cell size and included angle
were < 0.5%, indicating little structural change
in the bulk material, which was consistent
with previous work ( 18 , 20 , 23 ).
This considerable difference between local
and global structural changes may originate
in the symmetry of the zeolite framework.
Although part of the TO 4 tetrahedrons was
twisted by host-guest interactions to form local
deformation in opening pores, thePnma
space group symmetry of the MFI framework
makes the subcell deformation of adjacent
straight channels symmetrical, and the changes
inDmaxandDminappear to alternately compen-
sate for each other (Fig. 4, F and G). Thus, theSCIENCEscience.org 29 APRIL 2022¥VOL 376 ISSUE 6592 495
a
c
γ3 x 3 cellsSingle cell: 2.033 x 1.364 nm^2 90.53 x 3 cellsSingle cell: 2.043 x 1.365 nm^2 90.1Orientation
AD EF GB CFig. 4.Pnmaspace group symmetry of MFI zeolite brings subcell topological flexibility and overall
rigidity.(AtoC) IDPC images of a straight channel before and after benzene adsorption show severe
local deformation, with a high aspect ratio of 1.17. (DandE) iDPC images of the MFI framework before
and after benzene adsorption from the [010] projection with nearly identical cell parameters, from the
statistical data of 10 × 10 cells. (FandG) Corresponding structures of the MFI framework before (F)
and after (G) benzene adsorption in [010] projection. The yellow ellipses present the shapes of straight
channels. Scale bar, 500 pm [(A) and (B)], 2 nm [(D) and (E)].
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