New Reticular Chemistry of the Rod Secondary Building Unit: Synthesis, Structure, and Natural Gas Storage of a Series of Three-Way Rod Amide-Functionalized Metal-Organic Frameworks

Article abstract of DOI:10.1021/jacs.1c04946

Reticular chemistry and methane storage materials have been predominately focused on finite metal-cluster-based metal-organic frameworks (MOFs). In contrast, MOFs constructed from infinite rod secondary building units (SBUs), i.e., rod MOFs, are less developed, and the existing ones are typically built from simple one-way helical, zigzag, or (mixed)polyhedron SBUs. Herein, inspired by a recent unveiled structure of Zn6(H2O)3(BTP)4 and by means of an amide-functionalized preliminary single tricarboxylate, a subsequent mixed tricarboxylate, and dicarboxylate linkers, an intricate three-way rod MOF and the next three isoreticular three-way rod MOFs have been successfully realized, namely, 3W-ROD-1 and 3W-ROD-2-X (X = -OH, -F, and -CH3), respectively. The structural analyses disclosed that the four compounds were constructed from unprecedented three-way invariant nonintersecting trigonal rod-packing SBUs cross-linked via the noncovalent-interaction-driven self-assembly of pseudo hexacarboxylates with the original tricarboxylate or different functional ditopic linkers, resulting in cage-like pore geometries accessible via ultramicroporous apertures concomitant with the complex topology transitivity, namely, 18 42 and 18 44. Sorption studies show that the apparent surface areas of these materials are among the most highly porous materials for rod MOFs. Due to the presence of favorable pocket sites created by X, ketone, and proximal amide groups as revealed by Monte Carlo molecular dynamics (MCMD) computational calculations, the MOFs exhibit impressive methane storage working capacities, outperforming the well-known rod Ni-MOF-74 and representing the highest values among rigid rod MOFs.

Full text of DOI:10.1021/jacs.1c04946

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New Reticular Chemistry of the Rod Secondary Building Unit:
Synthesis, Structure, and Natural Gas Storage of a Series of Three-
Way Rod Amide-Functionalized Metal−Organic Frameworks
Yu-Feng Zhang, Zong-Hui Zhang, Logan Ritter, Han Fang, Qian Wang, Brian Space, Yue-Biao Zhang,
Dong-Xu Xue,* and Junfeng Bai*
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ABSTRACT: Reticular chemistry and methane storage materials
have been predominately focused on finite metal-cluster-based metal−
organic frameworks (MOFs). In contrast, MOFs constructed from
infinite rod secondary building units (SBUs), i.e., rod MOFs, are less
developed, and the existing ones are typically built from simple one-
way helical, zigzag, or (mixed)polyhedron SBUs. Herein, inspired by a
recent unveiled structure of Zn6(H2O)3(BTP)4 and by means of an
amide-functionalized preliminary single tricarboxylate, a subsequent
mixed tricarboxylate, and dicarboxylate linkers, an intricate three-way
rod MOF and the next three isoreticular three-way rod MOFs have
been successfully realized, namely, 3W-ROD-1 and 3W-ROD-2-X (X
= −OH, −F, and −CH3), respectively. The structural analyses
disclosed that the four compounds were constructed from unprecedented three-way invariant nonintersecting trigonal rod-packing
SBUs cross-linked via the noncovalent-interaction-driven self-assembly of pseudo hexacarboxylates with the original tricarboxylate or
different functional ditopic linkers, resulting in cage-like pore geometries accessible via ultramicroporous apertures concomitant with
the complex topology transitivity, namely, 18 42 and 18 44. Sorption studies show that the apparent surface areas of these materials
are among the most highly porous materials for rod MOFs. Due to the presence of favorable pocket sites created by X, ketone, and
proximal amide groups as revealed by Monte Carlo molecular dynamics (MCMD) computational calculations, the MOFs exhibit
impressive methane storage working capacities, outperforming the well-known rod Ni-MOF-74 and representing the highest values
among rigid rod MOFs.
MOFs, e.g., IRMOF-74, COMOC-2, STA-12/16, and the MIL-
140 series.8,11−14 According to the pattern features of rod
packing SBUs, the rod MOFs could be tentatively classified into
three categories (Figures 1 and S1). The first category
characterizes one-way straight or helical rod SBUs that point
in one direction, which dominate the rod MOFs and feature 1D
channels. An exception is the recently reported MOF-910
constructed from a unique linker with three nonidentical
coordinating groups, whish displays 3D channels.15 While the
propagating directions have increased, new rod MOFs have
arisen; however, only a few have been reported.6,16−19 For
instance, MOF-77 is the first two-way rod MOF where the rods
propagate along the a- and b-axes, but is regrettably a dense
INTRODUCTION
■
Metal−organic frameworks (MOFs) in combination with metal
ions or clusters and polytopic organic linkers via coordination
bonds have been proven to be great accomplishment for
reticular chemistry par excellence in the last two decades.1,2
Thanks to the well-defined and intrinsic geometries (i.e.,
secondary building units (SBUs)) derived from metal ions or
clusters and organic ligand-based molecular building blocks
(MBBs), diverse topological networks have been experimentally
isolated according to the dedicated reticular chemistry structure
resource (RCSR) database.3−5 Among them, the majority of
MOFs are predominately built from finite or discrete metal-
cluster-based MBBs. In contrast, MOFs based on infinite rod-
shaped building blocks, i.e., rod MOFs, are less predicted, most
probably because of their hardly controllable rod SBUs.6,7
Rod MOFs are renowned for the forbidden catenation and
breathing effect. Since the first two pioneering discoveries of
MIL-47(V) and MOF-69 in 2002 and their systematic and
extensive study in 2005,6,8,9 a great number of rod MOFs have
been documented;7,10 however, the isoreticular platform has
been less developed in comparison with metal-cluster-based
Received: May 13, 2021
Published: July 30, 2021
https://doi.org/10.1021/jacs.1c04946
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Figure 1. Timeline of representative rod MOFs according to the rod SBU patterns.
polymeric material. Consequently, CAU-8 and Zn3(BTP)2
represent two principally porous two-way rod MOFs. As for
three-way rod MOFs of the third category, astonishingly there is
only one example, i.e., Zn6(H2O)3(BTP)4, which was
unexpectedly obtained upon heating the aforementioned two-
way rod MOF of Zn3(BTP)2 in boiling water or a concentrated
basic solution and was unfortunately underestimated until the
further disclosure by Yaghi et al. recently.7,16 The scarcity of
three-way rod MOFs is probably attributed to the specified
requirement that the spacing radio along the rods (translation)
and between the rods (determined by the length of the linkers)
must be fixed as one in order to acquire three-way rod MOFs.6
Methane, the principal component of natural gas (NG), has
been regarded as clean energy and an alternative to fossil fuels
due to its low carbon emissions, high research octane number
(RON = 107), and abundance. Therefore, increasing interest
has been dedicated to the exploitation of methane for fueling
automobiles, other than the currently conventional applications
for electric power plants, industry, and household. In contrast to
the traditional utilizations of high-cost liquefied natural gas
(LNG) and potentially hazardous compressed natural gas
(CNG), adsorbed natural gas (ANG) appears to be an emerging
and promising technology for on-board transportation applica-
tions that can be both cost-effectively and safely applied under
ambient temperatures and moderate pressures, i.e., 35, 65, or 80
bar. The key for ANG technology is seeking high NG storage
materials, especially the working capacities (methane uptake
between 5 and 35, 65, or 80 bar). So far, various porous materials
have been investigated for methane storage, and the perform-
ances still fall significantly short of the U.S. Department of
Energy targets, i.e., a volumetric capacity of 263 cm3 (STP) cm−3
(without considering the adsorbent material-packing density
loss) and a gravimetric capacity of 0.5 g g−1.20 MOFs have gained
intensive attention toward methane storage due to their high
surface areas, high pore volumes, and tunable structures. So far,
several state-of-the-art MOF materials exhibit impressive
achievements, e.g., HKUST-1,21,22 MOF-519,23 UTSA-76a,24
Al-soc-MOF-1,25 and Co(bdp).26 Nevertheless, the examined
porous materials are predominately discrete metal-cluster-based
MOF compounds that are accompanied by less rod MOFs,
which is due to their limited surface areas and simple 1D straight
pore geometries.21,22,27,28
In consideration of the above-mentioned finding for reticular
chemistry and the methane storage status of MOF materials, it is
time to explore and develop a new generation of infinite MBB-
based robust polymeric structure platforms to enrich the rod
MOF family and boost the NG storage working capacities.
Inspired by the rare structure of Zn6(H2O)3(BTP)4, which is
constructed from three-way invariant nonintersecting cubic rod
packing patterns followed by BTP linkers positioned along the
four threefold symmetry axes (Figure 2), we thought to employ
semirigid amide-functionalized tritopic ligands instead of the
predicted only ditopic ones to initiate the exploration and
development of three-way rod MOFs, since they have more
ligand conformers, are more adaptable, and have opportunities
to constitute novel organic building blocks via noncovalent
interactions (NCIs).29−32 In this regard, we herein report a
novel discovery and development of a series of rod MOFs for the
first time, termed 3W-ROD-1 and the isoreticular 3W-ROD-2-X
(X = −OH, −F, and −CH3), which were assembled by an
amide-functionalized single tricarboxylate linker and mixed
tricarboxylate and dicarboxylate ligands, respectively. The
MOFs feature unprecedented three-way trigonal invariant rod
packing SBUs and two types of cage-like pore geometries
concomitant with complex transitivity, namely, 18 42 and 18 44,
respectively, resulting in highly apparent surface areas compared
to rod MOFs, e.g., 2742 m2 g−1. Finally, 3W-ROD-2-OH totally
adsorbs methane 220/254 cm3 (STP) cm−3 at 80 bar and 298/
273 K, respectively, and exhibits the working capacity of 184/
198 cm3 (STP) cm−3 between 5 and 80 bar, outperforming the
well-known rod Ni-MOF-74 and even some highly porous
metal-cluster-based MOFs.
RESULTS AND DISCUSSION
■
Synthesis and Structure. Indeed, the solvothermal
reaction between the amide-functionalized ligand of H3L1 and
In(NO3)·4H2O in the presence of N,N-dimethylformamide
(DMF) and nitric acid yielded transparent polyhedral single
crystals, which were formulated by the single-crystal X-ray
diffraction (SCXRD) technique as [In5(OH)6(L1)3(H2O)]
x(solv), namely, 3W-ROD-1. The phase purity of the bulk
crystalline materials for 3W-ROD-1 was independently
confirmed by similarities between the calculated and as-
synthesized powder X-ray diffraction (PXRD) patterns.
SCXRD reveals that 3W-ROD-1 crystallizes in the trigonal
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Figure 2. Schematic representation showing the assembly of Zn6(H2O)3(BTP)4 (top) and 3W-ROD-1 (down), which were constructed from the
respective infinite zinc- or indium-based MBBs, further propagated into three-way invariant nonintersecting cubic (top) or trigonal (down) rod
patterns, and subsequently interconnected via the organic linkers of BTP3− or L13− from different directions to generate both three-periodic three-way
rod MOFs.
space group R-3. There are six crystallographically unique In3+
ions in the asymmetric unit, all of which are octahedrally
coordinated by six oxygen atoms. Among them, four of the In3+
ions are surrounded by four different carboxylate groups in a bis-
monodentate fashion and two different bridging hydroxyl
groups, while the other two In3+ ions are coordinated from
three different carboxylate groups, two different bridging
hydroxyl groups, and one terminal H2O or OH−. A long rod-
shaped MBB of [In11(OH)13(O2C−)20(H2O)2]∞ that is
associated with a translation distance of 33.889 Å in contrast
to that of 18.547 Å in Zn6(H2O)3(BTP)4 (Figures S2 and S3) is
generated in situ. Subsequently, such MBBs propagate and
surround the threefold roto inversion and screw axes along the c-
axis in three directions rather than the commonly observed one
way, resulting in three-way invariant nonintersecting trigonal
rod patterns rotated by 76.168° with respect to each other. It is
noted that the three-way cubic rod packing in
Zn6(H2O)3(BTP)4 is rotated by 90° with respect to each
other (Figure 2). Interestingly, two kinds of pseudo
hexacarboxylic acids are formed by two pairs of slightly different
dimers of [L1a···L1b] (i.e., H6D1) and [L1c···L1d] (i.e., H6D2),
respectively, through π−π stacking interactions (i.e., 3.87 and
3.64 Å, respectively) as well as hydrogen bonds (ranging from
approximately 3.384 to 3.570 Å) (Figures S4−S6). Finally, the
three-way trigonal rod-shaped MBBs are cross-linked via
6−
deprotonated L13−, D16−, and D2 linkers to produce an
3−
intricate three-periodic network. Among them, the L1 and
6−
D1 linkers are arranged perpendicular to the c-axis via an
6−
alternate and opposite manner, while the D2 linkers are
positioned along the [1−12] direction and surround the
threefold roto-inversion axes to individually furnish six-
coordinated In3+ metal ions. The structure of 3W-ROD-1
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Figure 3. Deconstruction of 3W-ROD-1 for topological analysis. (a) Simplification the lengthy MBB of [In11(OH)13(O2C−)20(H2O)2]∞ into a pair of
opposite corner- and edge-sharing rectangles (green), two triangles (green), a quadrangle (green), and a tetrahedron (orange) with another centered
6−
3−
rectangle (green). (b) Pseudo hexacarboxylates of D1 and D26− are regarded as distorted octahedral SBUs, and one single L1 is regarded as a
triangle SBU. Two dimers are formed through π−π stacking interactions (3.64 and 3.87 Å, purple lines) as well as hydrogen bonds (ranging from 3.384
to 3.570 Å, black lines) in 3W-ROD-1. (c) Topological views along the a-axis and (d) the c-axis.
encompasses two types of cages with closer sizes, i.e., the first
one (ca. 16 × 22 Å2) and the other one (ca. 17 × 21 Å2), which
are delimited by two L13− and six pseudo D26− and two pseudo
imply that the tritopic ligands of H3L1 associated with the
formed in situ pseudo hexacarboxylic acids of H6D1 and H6D2
driven by NCIs play important roles during the self-assembly of
such unique three-way rod MOFs.
6−
D1 and six pseudo D26−, respectively (Figure S7). The
Encouraged by the initial discovery and considering 3W-
ROD-1’s terminal coordinating hydroxyl groups and water
molecules in the structure and tedious activation using dry
supercritical CO2 (vide infra), we speculated building a terminal
coordinating-ligand-free structure and improving the stability by
virtue of a mixed-linker strategy. Exhilaratingly, the solvothermal
reaction between amide-functionalized mixed linkers of
tricarboxylate-based H3L1 and dicarboxylate-based H2L2−OH
and In(NO3)·4H2O in the presence of DMF and nitric acid
yielded transparent polyhedral single crystals, which were
formulated by synchrotron single-crystal X-ray diffraction
(SSCXRD) as [In5(OH)5(L1)8/3(L2−OH)]·x(solv), namely,
3W-ROD-2-OH. As revealed by SSCXRD, 3W-ROD-2-OH
crystallizes in the same space group R-3 and a unit cell similar to
that of 3W-ROD-1. In 3W-ROD-2-OH, there are six crystallo-
graphically unique In3+ ions in the asymmetric unit. All the In3+
ions are octahedrally coordinated by six oxygen atoms from four
different carboxylate groups in a bis-monodentate fashion and
two different bridging hydroxyl groups. Afterward, a lengthy rod-
shaped MBB of [In11(OH)11(O2C−)22]∞ without terminal
ligated moieties is produced, which is associated with translation
distance of 33.995 Å (Figure S9). Such MBBs propogate into
three different directions to generate three-way invariant
nonintersecting trigonal rod packing patterns. Interestingly
and similarly, two kinds of pseudo-hexacarboxylic acids are also
formed by two pairs of slightly different dimers of [L1e···L1f] (i.e.,
H6D3) and [L1g···L1h] (i.e., H6D4) through π−π stacking
interactions (i.e., 3.72 and 3.76 Å, respectively) as well as
hydrogen bonds (ranging from 2.981 to 3.374 Å) (Figures S10−
apertures to access the interior open space are approximately
5.5−6.4 Å along the a- or b-axis, which are sufficient for the
adsorption of small gas molecules, e.g., N2, H2, CO2, CH4, etc.
The solvent-accessible free volume for 3W-ROD-1 was
estimated to be 71.1% by summing voxels more than 1.2 Å
away from the framework using PLATON software.33
As a unique three-way rod MOF of the third category, it is
essential to further investigate the structure of 3W-ROD-1 by
means of a topology analysis (Figure 3). According to the
deconstructing principle proposed by O’Keeffe et al.7 and
considering the carbon atoms of the carboxylate groups as points
of extension, the MBB of [In11(OH)13(O2C−)20(H2O)2]∞ in
3W-ROD-1 is defined as a relatively lengthy SBU composed of a
pair of opposite corner- and edge-sharing rectangles, two
triangles, a quadrangle, and a tetrahedron with another centered
rectangle (Figure 3a). Meanwhile, considering the single C3-
3−
symmetrical L1 as a triangular SBU at the same time both
6−
pseudo hexacarboxylates of C3-symmetrical D1 and C1
6−
symmetrical D2 are judiciously regarded as distorted
octahedral SBUs, an intricate net with the exceptional
transitivity 18 42 is generated afterward. It is worth noting
that 3W-ROD-1 crystallizes in the highly symmetrical space
group of R-3, which is reminiscent of the most complicated net
of rn-11, namely, CAU-17, an extraordinary bismuth-based
MOF with 54 unique nodes, 135 edges, and a trigonal P-3 space
group.34 The consequence ultimately indicates the uniqueness
and scarceness of such a three-way rod MOF. Moreover, the
attempts to isoreticulate 3W-ROD-1 by means of extra
functional H3L1-X3 (X = −OH, −F, −Cl, and −CH3) linkers
along with X groups located in the α-positions relative to
carboxylate moiety failed (Figure S8). The results fundamentally
S12). Subsequently, three-way rod-shaped MBBs are linked via
fully deprotonated C3-symmetrical D36−, C1-symmetrical D4
,
6−
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and L2−OH2− ligands to generate an intricate three-periodic
three-way rod network once again (Figure 4). Among them, the
estimated to be 70.2% by summing voxels more than 1.2 Å away
from the framework using PLATON software.33
Analogously, the topological analysis was carried out for 3W-
ROD-2-OH (Figure S15). According to the same principles, the
MBB of [In11(OH)11(O2C−)22]∞ in 3W-ROD-2-OH was
simplified as a relatively prolonged SBU composed of a pair of
opposite edge-sharing rectangles, a tetrahedron, two quad-
rangles, and another tetrahedron with one more centered
6−
rectangle. Meanwhile, both pseudo hexacarboxylates of D3
and D4
and the dicarboxylate-based L2−OH2− were
6−
judiciously regarded as distorted octahedral and linear SBUs,
respectively, resulting in an intricate topological net with the
exceptional transitivity 18 44. Motivated by the second isolation
of such an unprecedented three-way rod MOF, reticular
chemistry par excellence was enthusiastically attempted to
fabricate such three-way trigonal rod MBBs into an isoreticular
MOF platform despite its scarceness and complexity. As a matter
of fact, combinations of extra functional H3L1-X3 (X = −OH,
−F, −Cl, and −CH3) with the static ditopic linker of H2L2−OH
afforded either a clear solution or an amorphous product (Figure
S16). Once the original H3L1 was constantly retained while
H2L2−OH was varied to H2L2-F and H2L2-CH3, two more
isoreticular three-way rod MOFs were successfully attained,
termed 3W-ROD-2-F and 3W-ROD-2-CH3, respectively
(Figure S17). These structures were precisely validated by the
excellent matching PXRD patterns between the as-synthesized
bulk materials and the calculated 3W-ROD-2-OH (Figure S20).
The important roles of the tritopic linkers of H3L1 associated
with the in situ-formed pseudo hexacarboxylic acids of H6D3 and
H6D4, which were driven by NCIs during the self-assembly of
such a three-way rod MOFs, were reconfirmed. Interestingly, it
is notable that the spacing along the rods (translation) and
between the rods along each direction within both compounds is
exactly identical, i.e., 33.889 and 33.995 Å, respectively, but
much longer than the length of the linear L2−OH2− ligand, i.e.,
ca. 13.656 Å. Moreover, it is obviously impossible for only L2−
OH2− linkers to sustain the three-way trigonal rod packing
patterns once the D36− and D46− linkers are deleted within 3W-
ROD-2-OH (Figure S18). These findings may imply that
tritopic ligands rather than ditopic ones are the correct solution
to address the challenging self-assembly of explicit three-way rod
MOFs.
Sorption Studies. To access the methane storage capacities
of the series of three-way rod MOFs, preliminary sorption
studies were investigated in detail. After evacuating the
supercritical CO2, acetonitrile-, or dichloromethane- and
hexane-exchanged 3W-ROD-1 and 3W-ROD-2-X (X = −OH,
−F, and −CH3), nitrogen sorption isotherms were collected for
four materials at 77 K. As shown in Figure 6a and b, the four
samples all exhibit reversible isotherms. The pore size
distributions calculated according to the nonlocal density
functional theory (NLDFT) method show that the pore size is
centered on ca. 17 Å for 3W-ROD-1, which is consistent with its
closer cage sizes, while for 3W-ROD-2-X the pore sizes are
centered on ca. 13 and 20 Å, respectively, greatly matching their
crystal structures. The BET surface areas and total pore volumes
range from 2315 to 2742 m2 g−1 and from 1.00 to 1.11 cm3 g−1
due to the differing side groups on the L2-X linkers. It should be
noted that the values render 3W-ROD-1 and 3W-ROD-2-X as
having among the highest BET surface areas of rod MOFs
(Table S1). Furthermore, the series of three-way rod MOFs can
adsorb relative amounts of carbon dioxide and hydrogen, as
tabled in Table 1.
Figure 4. Schematic representation showing the assembly of
isoreticular 3W-ROD-2-X constructed from the combination of the
infinite indium-based MBB and the organic linkers of L13− and L2-X2−
to produce the three-periodic three-way MOF that encompasses two
types of cage-like pore geometries, as indicated by the yellow and pink
balls.
6−
D3 linkers are located perpendicular to the c-axis in an
6−
opposite and alternate fashion, while the D4 linkers are
positioned along [1−12] direction and the surrounding the
three-fold roto-inversion axes are accompanied by deprotonated
L2−OH2− ligands to individually complete six-coordinated In3+
metal ions. Specifically, 3W-ROD-2-OH can be overlappingly
regarded as the product of 3W-ROD-1 by the subtraction of the
L1b3− from the D16− and addition of three L2−OH2− ligands to
substitute the adjacent three pairs of OH− and H2O ligands and
fulfill the charge balance (Figure 5). It can be seen that there is
3−
basically no change from D26− to D46−; however, the single L1
in 3W-ROD-1 remains, which is renamed L1e3− and paired with
3−
3−
the L1f coming from L1a with a rearranged conformation
(Figure S13). Due to such complex relocations between both
structures along the c-axis, the structure of 3W-ROD-2-OH
encompasses two types of cages with apparently different sizes,
i.e., a smaller one (ca. 18 × 20 Å2) and a larger one (ca. 36 × 17
Å2). Both cages are independently delimited by two pseudo D3,
six pseudo D4, and six L2−OH linkers, and the considerable size
discrepancy is attributed to the differential relative distances
between both pairs of opposite L1e3− and L1f3− linkers together
with the distinct arrangements of six L2−OH2− linkers within
each cage (Figure S14). The apertures to access the interior
open spaces are approximately 4.4−5.1 Å along the a- or b-axis.
The solvent-accessible free volume for 3W-ROD-2-OH was
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Figure 5. Detailed comparison between (a) 3W-ROD-1 and (c) 3W-ROD-2-OH. (b) Each leaving L13− linker (yellow) in 3W-ROD-1 is supplied by
(d) three coming L2−OH2− (pink) ligands to fill the positions of six carboxylates due to the departure of one L13− linker and three pairs of terminal
OH− or H2O ligands (oxygen atoms are highlighted in pink in panel b), resulting in (d) two L13− and three L2−OH2− ligands.
Figure 6. (a) N2 sorption isotherms at 77 K for 3W-ROD-1 and 3W-ROD-2-X (X = −OH, −F, and −CH3). (b) Pore size distributions and (c) total
methane adsorption isotherms at 298 K for 3W-ROD-1 and 3W-ROD-2-X (X = −OH, −F, and −CH3).
Table 1. Low-Pressure Sorption Data Summary for 3W-ROD-1 and 3W-ROD-2-X
a
b
b
c
material
Vp (cm3 g−1
)
BET (m2 g−1
)
CH4 (mmol g−1
)
CO2 (mmol g−1
)
H2 (wt %)
3W-ROD-1
3W-ROD-2-OH
3W-ROD-2-F
1.11
1.11
1.08
2742
2552
2482
2315
0.44
0.45
0.43
0.42
1.99
2.14
1.94
2.07
1.67
1.72
1.56
1.63
3W-ROD-2-CH3
1.00
a
b
c
Vp, pore volume. Measured at 298 K. Measured at 77 K.
As shown in Table 1, the methane uptakes for the series of
High-pressure excess methane sorption isotherms for 3W-
ROD-1 and 3W-ROD-2-X (X = −OH, −F, and −CH3) were
measured up to 80 bar via a high-pressure volumetric gas
analyzer at various temperatures between 273 and 298 K and
then empirically transformed to total sorption isotherms to
evaluate the storage performance of methane (Figure 6c). The
total methane uptakes of 3W-ROD-1 and 3W-ROD-2-X
three-way rod MOFs at 1 bar and 298 K are relatively low and
range from 0.42 to 0.45 mmol g−1, which are value comparable
with those of MOF-905, functionalized MOF-905, and MOF-
177 under the similar conditions. These values indicate an
optimistic sign to have a higher methane storage working
capacity considering the practical NG utilization.35,36
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reached 137, 152, 150, and 138 cm3 (STP) cm−3 at 35 bar and
208, 220, 217, and 202 cm3 (STP) cm−3 at 80 bar and 298 K,
respectively, which are much lower than that of the well-known
rod MOF of Ni-MOF-74 that is replete with open metal sites
(OMSs) (i.e., 230 and 267 cm3 (STP) cm−3, respectively),21,22
The values of the OMS-containing dicopper paddle-wheel-
based PCN-14 (i.e., 200 and 250 cm3 (STP) cm−3) and the
OMS-free MOF of MAF-38 (i.e., 226 and 273 cm3 (STP)
cm−3),21,37 are comparable with the top-performing OMS-free
MOF-905 (i.e., 145 and 228 cm3 (STP) cm−3), MOF-950 (i.e.,
145 and 209 cm3 (STP) cm−3,), ultrahighly porous MOF-177
(i.e., 122 and 205 cm3 (STP) cm−3) and MOF-205 (i.e., 120 and
205 cm3 (STP) cm−3) and are higher than those of OMS-free
MOF-5 (i.e., 126 and 198 cm3 (STP) cm−3) and MOF-210 (i.e.,
82 and 166 cm3 (STP) cm−3) under the same conditions.35,36
However, the working capacities (desorption at 5 bar) at 80
bar are 177, 184, 181, and 168 cm3 (STP) cm−3 for 3W-ROD-1
and 3W-ROD-2-X, outperforming Ni-MOF-74 (152 cm3 (STP)
cm−3) due to its large amount of methane adsorption at 5 bar
(115 cm3 (STP) cm−3) (Figure 7).21,22 The performances for
notice that the total methane uptakes of 3W-ROD-1 and 3W-
ROD-2-X significantly reach values of 234, 254, 252, and 232
cm3 (STP) cm−3 at 80 bar as soon as the measuring temperature
decreases to 273 K, corresponding to 186, 198, 198, and 181 cm3
(STP) cm−3 given their working capacities at 80 bar. Such a
phenomenon of a temperature-dependent working capacity
increasing was certainly observed before but only seriously
concerned most recently; however, this is not universal and is
likely applicable to those MOF materials associated with a low
isosteric heat of adsorption (Qst) toward NG storage.41
Qst and Calculations. To further pinpoint the methane
storage mechanism and working capacities of 3W-ROD-1 and
3W-ROD-2-X, the Qst value at near-zero-coverage was
calculated from variable-temperature methane adsorption
isotherms by means of the Clausius−Clapeyron equation,
which resulted in very close values, i.e., 13.4, 14.6, 14.5, and 13.8
kJ mol−1. These values are much lower than those of some
reported methane storage MOFs with open metal sites, e.g., Ni-
MOF-74 (21.4 kJ mol−1), PCN-14 (18.7 kJ mol−1), and Cu-tbo-
MOF-5 (20.4 kJ mol−1),22,42 but slightly higher than those of
some OMS-free MOFs, e.g., MOF-905 (11.7 kJ mol−1), MOF-
950 (11.9 kJ mol−1), and MOF-205 (10.6 kJ mol−1), except the
outstanding MAF-38 (21.6 kJ mol−1) with a suitable pore size
and shape capable of the strong binding and close packing of
methane.35−37 Meanwhile, these Qst values for 3W-ROD-1 and
3W-ROD-2-X are in great correlation with their similar 5 bar
uptakes at 298 K, i.e., 31, 36, 36, and 34 cm3 (STP) cm−3.
Accordingly, Monte Carlo molecular dynamics (MCMD)
calculations have recognized that the −X groups within the
series of isoreticular 3W-ROD-2-X are the primary interaction
sites for CH4 with binding energies of 23.045 kJ mol−1 for −OH,
20.004 kJ mol−1 for −F, and 21.101 kJ mol−1 for −CH3, which in
combination with the proximal ketone and amide functional
groups create favorable pockets for CH4 sorption. The average
binding interaction distances around the pockets were
profoundly computed as 5.54, 5.18, and 4.46 Å for 3W-ROD-
2-X, which are much larger than the C−C distance of methane
adsorbed on the graphite surface (i.e., 3.45 Å),43 and imply the
relatively modest affinity between the methane molecule and the
MOF framework consistent with the experimental moderate Qst
values as well as 5 bar uptakes. Therefore, the 3W-ROD-2-OH
exhibits a superior working capacity compared to the high total
uptake material of Ni-MOF-74 (267 cm3 (STP) cm−3 at 80 bar)
due to its high Qst value and largely adsorbed amount of 115 cm3
(STP) cm−3 at 5 bar but is relatively inferior to the top-
performing OMS-free MOF-905 (i.e., 184 vs 203 cm3 (STP)
cm−3) despite their comparable total uptakes at 80 bar (i.e., 220
vs 228 cm3 (STP) cm−3) due to the latter’s lower Qst value and 5
bar uptake of 25 cm3 (STP) cm−3. It is certainly sensible that
some MOFs with a low Qst value display an increased methane
storage working capacity once the measurement temperature is
reduced, which is most possibly caused by the uptake increase
discrepancy between the high pressure and 5 bar.
Figure 7. Comparison of the CH4 storage working capacities at 35 and
80 bar and 298 K with some top-performing MOFs.
the series of three-way rod MOFs correlate with the respective
surface areas and pore volumes and also render 3W-ROD-2-OH
the record-high volumetric methane storage working capacity
among rod MOFs, except the flexible MOF of Co(bdp).26 The
performance discrepancy between 3W-ROD-1 and 3W-ROD-2-
OH could be attributed to their different cage sizes and the
absence of L2−OH2− linkers within 3W-ROD-1. Although the
value is still lower than the current high record holders of metal-
cluster-based MOF materials at 80 bar and 298 K, e.g., MOF-
905 (203 cm3 (STP) cm−3),36 it is comparable with those of
some high-performing MOFs for methane storage, e.g., pbz-
MOF-1 (180 cm3 (STP) cm−3) and MOF-205 (186 cm3 (STP)
cm−3), and even much higher than that of MFM-132a (i.e., 162
cm3 (STP) cm−3) with so-called slow molecular dynamics,
which has a surface area and a pore volume similar to those of
3W-ROD-2-OH.35,38,39 Moreover, the working capacity of 3W-
ROD-2-OH for methane storage is also higher than those of
some ultrahighly porous MOFs, e.g., MOF-210 (BET = 6240 m2
g−1, 154 cm3 (STP) cm−3), and NU-1501-Al (BET = 7310 m2
g−1, 174 cm3 (STP) cm−3) associated with either open metal
sites or wider pore size distributions.35,40 It is also interesting to
CONCLUSION
■
For the first time, the three-way rod SBUs have been purposely
and successfully isolated and reticulated into MOFs by virtue of
an amide-functionalized initial single tricarboxylate, a subse-
quent mixed tricarboxylate, and dicarboxylate linkers, resulting
in an intricate three-way rod MOF and then an isoreticular
three-way rod MOF platform, namely, 3W-ROD-1 and 3W-
ROD-2-X (X = −OH, −F, and −CH3), respectively. The
structures disclose not only unprecedented three-way invariant
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nonintersecting trigonal rod SBUs and the NCI-driven self-
assembly of pseudo hexacarboxylates but also cage-like pore
geometries associated with a complex transitivity of 18 42 and 18
44, respectively, in contrast to the commonly observed straight
channels and simple transitivity of most rod MOFs.
The present exploration and development in combination
with Zn6(H2O)3(BTP)4 may provide some clues to the rational
synthesis and construction of such unusual three-way rod
MOFs: (1) semirigid tritopic linkers other than ditopic linkers
are a better choice to assemble three-way rod MOFs, (2) the
selection for central metal ions should not only possibly generate
in situ rod MBBs but also possess long translation distances, and
(3) the distances along rods (translation) and between rods
along each direction within three-way invariant nonintersecting
rod patterns are fundamentally equal.
710062, China; orcid.org/0000-0002-6487-7085;
Email: bjunfeng@snnu.edu.cn
Authors
Yu-Feng Zhang − Key Laboratory of Applied Surface and
Colloid Chemistry, Ministry of Education, Xi’an Key
Laboratory of Organometallic Material Chemistry, School of
Chemistry & Chemical Engineering, Shaanxi Normal
University, Xi′an 710062, China
Zong-Hui Zhang − Key Laboratory of Applied Surface and
Colloid Chemistry, Ministry of Education, Xi’an Key
Laboratory of Organometallic Material Chemistry, School of
Chemistry & Chemical Engineering, Shaanxi Normal
University, Xi′an 710062, China
Logan Ritter − Department of Chemistry, University of South
Florida, Tampa, Florida 33620, United States
The apparent surface areas of the series of three-way rod
MOFs are also among the highest for rod MOFs. Due to the
integrated features of open metal sites free and relatively
appropriate pore microenvironments, the MOFs exhibit
impressive methane storage working capacities at 80 bar.
Specifically, one of the members of 3W-ROD-2-OH is able to
deliver 184 and 198 cm3 (STP) cm−3 methane at 298 and 273 K,
respectively, outperforming the well-known rod Ni-MOF-74
and representing the the highest values among rigid rod MOFs.
Finally, the series of compounds offer an interesting platform
for reticular chemistry in the successful synthesis and
construction of three-way rod MOFs. The present demon-
stration may open up a new avenue to construct scarce
frameworks and would inspire more and more researchers to
assemble elegant multiway rod MOFs toward relevant
applications, including gas storage, separation, catalysis, etc.
Han Fang − Key Laboratory of Applied Surface and Colloid
Chemistry, Ministry of Education, Xi’an Key Laboratory of
Organometallic Material Chemistry, School of Chemistry &
Chemical Engineering, Shaanxi Normal University, Xi′an
710062, China
Qian Wang − Key Laboratory of Applied Surface and Colloid
Chemistry, Ministry of Education, Xi’an Key Laboratory of
Organometallic Material Chemistry, School of Chemistry &
Chemical Engineering, Shaanxi Normal University, Xi′an
710062, China
Brian Space − Department of Chemistry, University of South
Florida, Tampa, Florida 33620, United States
Yue-Biao Zhang − School of Physical Science and Technology,
ShanghaiTech University, Shanghai 201210, China;
orcid.org/0000-0002-8270-1067
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c04946
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c04946.
■
sı
Notes
The authors declare no competing financial interest.
Experimental details, supplementary characterization
results, additional figures, data analysis, and computa-
tional calculations (PDF)
ACKNOWLEDGMENTS
■
We acknowledge the National Natural Science Foundation of
China (nos. 21771121 and 21871170), the Cheung Kong
Scholars Program, the Hundred/Thousand Talents Programs of
Shaanxi Province, and the Fundamental Research Funds for the
Central Universities (GK202001009). We also thank the staffs
from the BL-17B beamline of the National Center for Protein
Sciences Shanghai (NCPSS) at the Shanghai Synchrotron
Radiation Facility (SSRF) for assistance during data collection.
The authors gratefully acknowledge Dr. Cong-Cong Liang and
Yu Tao of ShanghaiTech University for assistance with
synchrotron single-crystal X-ray diffraction and high-pressure
methane measurements, for comparison, and others for helpful
discussion.
Rod packing views (MP4)
3W-ROD-2-OH, 3W-ROD-1, and their overlay (MP4)
Accession Codes
CCDC 2083062−2083063 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
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