Porous Crystalline Spherulite Superstructures

Full text of DOI:10.1016/j.chempr.2019.12.001

Article
Porous Crystalline Spherulite Superstructures
Liang Feng, Kun-Yu Wang,
Tian-Hao Yan, Hong-Cai Zhou
zhou@chem.tamu.edu
HIGHLIGHTS
Porous spherulites are observed
with a ‘‘Maltese cross’’ pattern
under a polarized light
Simple solvothermal synthesis
leads to sophisticated spherulite
superstructures
Evolution involves an
unconventional small-angle
branching mechanism during
MOF growth
Mechanistic effects of solvent
decomposition kinetics on the
evolution are revealed
We report the fabrication of metal-organic framework (MOF) spherulite
superstructures based on an unconventional small-angle branching mechanism.
These superstructures are constructed from bottom-up assembly of MOF nanorod
crystallites via a facile solvothermal synthesis and exhibit a ‘‘Maltese cross’’
extinction pattern typical of spherulites under a polarized light. This work provides
approaches to packing porous crystallites and fabricating sophisticated
superstructures, which are expected to serve applications from catalysis to guest
delivery and transportation.
Feng et al., Chem 6, 460–471
February 13, 2020 ª 2019 Published by Elsevier
Inc.
https://doi.org/10.1016/j.chempr.2019.12.001

Article
Porous Crystalline
Spherulite Superstructures
Liang Feng,1 Kun-Yu Wang,1 Tian-Hao Yan,1 and Hong-Cai Zhou1,2,3,
*
SUMMARY
The Bigger Picture
Spherulites are radially
Assembly of crystallites into three-dimensional hierarchical superstructures is
vital for the design of multicomponent architectures as capsules and reactors
for storage, delivery, and catalysis. However, the development of these assem-
blies, for example, spherulite superstructures, has mainly been hampered by
limited control over nucleation, orientational growth, and stability. In this
work, we observed a hierarchical evolution from metal-organic framework
(MOF) nanofibrils into spherulite superstructures. Under a polarized light, these
superstructures exhibited a ‘‘Maltese cross’’ extinction pattern typical of spher-
ulites, which is the first time this has been observed in porous materials. We
demonstrated that by tuning the evolution kinetics via a mixed-solvent
approach, varying morphologies could be obtained as a result of small-angle
branching. Isoreticular expansion of MOF spherulites and incorporation of mul-
tiple components could also be achieved. This work provides a fresh avenue to
pack porous crystallites into sophisticated superstructures, which are expected
to serve applications from catalysis to guest delivery and transportation.
polycrystalline aggregates with an
external spherical envelope. The
hierarchical assembly of
sophisticated spherulites
architectures is a ubiquitous
phenomenon in nature, such as in
polymers, metals, minerals, and
inorganic crystals. However,
further exploring the assembly
process in porous materials and
uncovering the mechanisms
remain a sustainable challenge.
Here, we present an
unprecedented case of metal-
organic framework (MOF)
spherulite assembly by carefully
controlling solvent
INTRODUCTION
decomposition, spherulite
nucleation, and directional
growth. Instead of traditionally
observed MOF single crystals,
porous spherulite superstructures
with well-organized crystallites
and tunable pore environments
were obtained after solvothermal
reaction. This research exhibits
the beauty of porous hierarchical
superstructures and also provides
a synthetic approach to pack
porous crystallites into
Evolution of crystallites into hierarchical superstructures is critical for the design of
multicomponent architectures as capsules and reactors for storage, delivery, and
catalysis.1 Spherulite superstructures refer to radially polycrystalline aggregates
with an external spherical envelope, which has been widely observed in crystalliza-
tion processes of polymers, metals, minerals, and inorganic crystals.2–9 For instance,
rod-like cellulose crystallites are regulated to lie along the radius or along the
circumference to form spherulites.2 Some proteins can also be self-associated into
high-order spherical structures.8 The fundamental characteristics of spherulites
have mainly been deduced by a typical ‘‘Maltese cross’’ pattern of light extinction
from polarized light microscopy. For example, the spherulite crystallization of insu-
lin, a highly flexible polypeptide system, could be observed by polarized optical mi-
croscopy, while the Maltese cross extinction pattern disappeared after drying and
rehydration, indicating the fragile nature of protein spherulite structures.8 Addition-
ally, inorganic minerals including alkaline feldspars, plagioclase, chalcedony, and
malachite have been widely found, both in laboratory and in nature, in the form of
spherulite superstructures.2
sophisticated superstructures for
catalysis, delivery, and
transportation.
Strangely, the evolution of spherulite superstructures from porous materials
including metal-organic frameworks (MOFs), zeolites, porous carbon, or silica has
not been widely explored yet, given the rapid progress in the self-assembled spher-
ulite of metals, polymers, proteins, and other materials. MOFs are a class of porous
crystalline materials assembled from organic linkers and inorganic clusters.10–14 The
intrinsic tunability over pore environments promises MOF’s wide applications
460 Chem 6, 460–471, February 13, 2020 ª 2019 Published by Elsevier Inc.

including storage, separation, catalysis, and sensing.15–19 So far, only a few studies
have illustrated the potential to fabricate porous spherical MOF superstruc-
tures.14,20,21 For instance, Maspoch and co-workers reported a spray-drying strategy
for the synthesis of spherical hollow superstructures. These hierarchical assemblies
enable the encapsulation of guest species and can function as reactors for delivery
and catalysis.22 Xu and co-workers also reported a chestnut-shell-like MOF-74(Zn)
spherical superstructure composed of MOF nanorods on the shell formed under hy-
drothermal conditions with urea as a modulator.23
The development of porous self-assembled spherulite superstructures has mainly
been precluded by limited control over nucleation, directional growth, and
framework stability. In this work, we describe the observation and mechanism
study of MOF spherulite superstructures assembled through evolution of MOF
nanofibrils. To the best of our knowledge, this is the first time that an optical Mal-
tese cross pattern was observed in the porous material world including MOFs, ze-
olites, and others. The formation of MOF spherulites is further characterized and
studied by optical microscopy, scanning electron microscopy (SEM), and trans-
mission electron microscopy (TEM). We demonstrate that the resulting compact
MOF spherulite architectures are assembled from fibrous crystallites, which
further undergo Ostwald ripening to make interface coarsened. This work pro-
vides a fresh avenue to pack porous crystallites and fabricate porous sophisti-
cated superstructures, which are expected to serve applications from catalysis
to transportation.
Our findings originated from our previous studies into MOF-74-II (Figures S1–S5).24
Due to the existing open metal sites along its 1D channels, MOF-74 shows great po-
tential in gas storage, separation, and catalysis.25–28 It should be noted that varying
reaction conditions of MOF-74 can generate diverse morphology evolution behav-
iors. For example, Dai and co-workers demonstrated the template-free synthesis of
hierarchically porous MOF-74 by etching the framework with various solvents for
different periods.29 Xu group also observed the formation of superlong single-crys-
tal Co-MOF-74 nanotubes with a diameter of ꢀ70 nm and length of 20–35 mm
through a recrystallization approach.30 In addition, Zn-MOF-74 could also be pre-
pared directly from a metal oxide without bulk solvent through mechanochemistry.31
More interestingly, Zhou group recently discovered a temperature-controlled evolu-
tion from MOF-74 crystallites to hierarchically porous superstructures including sin-
gle-crystalline hollow tubes, ripened hierarchically porous multichannel tubes or he-
lical tubular structures. It was found that N,N-diethylformamide (DEF) solvent could
decompose slowly under solvothermal conditions, modulating the assembly of
these superstructures.
RESULTS
Self-Assembly of Porous Crystalline MOF-74-III Spherulite Superstructures
Herein, we judiciously choose N,N-dimethylformamide (DMF) solvent as the reac-
tion medium to accelerate the assembly of hierarchical superstructures. Strikingly,
the reaction of zinc nitrate and organic linker III in a mixture of DMF, ethanol, and wa-
ter by conventional solvothermal conditions yields colorless MOF-74-III spherulites
within a few hours (Figure 1A). MOF-74-III contains one-dimensional hexagonal
channels with a channel size of about 2.9 nm, and powder X-ray diffraction (PXRD)
patterns indicate its phase purity (Figure S8A). Nitrogen sorption experiments on
the MOF-74-III verify its mesoporosity (Figure S17). Optical microscopy reveals
that the materials consist of spherical particles ranging in size from around 20 to
1Department of Chemistry, Texas A&M
University, College Station, TX 77843-3255, USA
2Department of Materials Science and
Engineering, Texas A&M University, College
Station, TX 77843-3003, USA
3Lead Contact
*Correspondence: zhou@chem.tamu.edu
https://doi.org/10.1016/j.chempr.2019.12.001
Chem 6, 460–471, February 13, 2020 461

Figure 1. Discovery of Hierarchical MOF-74-III Spherulite Superstructures
(A) Optical images of MOF-74-III spherulites.
(B) SEM images showing that MOF-74-III spherulites contain a large number of well-defined fibrils.
(C and D) Optical image and the corresponding polarized optical images of MOF-74-III spherulites
placed in between crossed polarizers. The image displays a typical Maltese cross extinction pattern
indicating spherulite superstructures under a polarized light.
(E) TEM images of a typical MOF-74-III spherulite showing the fibril size and arrangement.
(F) TEM images of a peanut-like MOF-74-III spherulite showing the fibril size and arrangement.
Scale bar, 100 mm in (A), (C), and (D), 5 mm in (B), and 250 nm in (E) and (F).
40 mm (Figure S10). Surprisingly, the resulting material MOF-74-III displays a clear
Maltese cross extinction pattern typical of spherulite superstructures under a polar-
ized light (Figures 1C and 1D), which has never been seen in previous reports
regarding porous materials, to the best of our knowledge. This Maltese cross pattern
consists of four dark perpendicular cones diverging from the bright spherulite center
and colored brighter regions in between them (Figure 1D).
As the MOF-74-III particles were air-dried under the optical microscope, the spher-
ulites were observed to maintain its hierarchical structure, although partial deforma-
tion was observed on some particles as indicated by the cracks caused by the fragile
superstructures (Figures S6 and S7). Ultimately, the Maltese cross pattern disap-
peared once the solvents were totally released. However, infusion of the sample
back into DMF could reestablish the Maltese cross extinction pattern, indicating
the robustness of the framework and the overall hierarchical spherulite structure
(Figures S6 and S7).
The morphology of MOF-74-III was also imaged by field-emission scanning electron
microscopy (FESEM, Figure 1B). It was found that a high density of individual MOF-
74-III fibers with almost identical lengths and sizes were arranged to form a sphere.
The MOF-74-III crystallites are elongated along a principal axis of the optical indica-
trix, leading to a Maltese cross extinction between crossed polarizers. Notably, there
are discretely distributed spheres, peanut-like spherulites, and spherulites consist-
ing of multiple spheres found as the crystallization products, depending on the
nucleation position and growth rates of a specific spherulite (Figures 1C and S9–
S12). The well-aligned crystallites outside hierarchical MOF-74-III spherulites were
further confirmed by regular TEM (Figures 1E and 1F). As indicated by TEM images,
the spherulites contain a large number of well-defined fibrils ranging in diameter
462 Chem 6, 460–471, February 13, 2020

from around 40 to 60 nm, suggesting that the spherulites are assembled from MOF
nanofibrils.
Spherulite Superstructure Evolution Mechanism
To study the evolution behavior of MOF-74-III spherulites, the solvothermal reaction
products at different periods were monitored (Figures 2B–2G). We observed that
initial spherulites around 15 mm in diameter formed within 3 h (Figure 2C). After a
6 h reaction time, continuous epitaxial growth of the MOF fibers with around
30 mm diameter was observed (Figure 2E). As the reaction time further extended,
the sizes of MOF-74-III spherulites remained almost unchanged, which indicated
the completeness of MOF growth process (Figures 2F and 2G).
In most cases, crystal growth from a typical primary nucleus into a crystallite exhibits a
discrete crystallographic orientation, producing a single crystal with well-defined shapes
and morphology as a product (Scheme 1A). Yet, there are certain systems that are un-
able to grow in a discrete crystallographic orientation and form a single crystal, for
example, spherulites. The growth of spherulites involves the branching of fiber crystal-
lites, where the crystallographic orientation of individual crystallites deviates slightly
from that of its parent fiber (Scheme 1B). As a result, a radiating array of crystalline fibers
was formed as spherulites. Notably, the formation of spherulites has been observed in a
diverse library of materials including polymer, inorganic salts, metals, and other mate-
rials. The similar morphology and spherulitic crystallization behaviors in these materials
suggest a possible general mechanism regardless of their molecular compositions. As
suggested by previous reports,2,32 due to the occurrence of the small-angle branching
and associated radial growth, spherulites consisting of radial and dense fibers are
formed. Uniform radial growth on MOF-74-III nuclei could continue in all directions,
while further secondary crystallization associated with lateral growth or coarsening of
MOF fibers proceeds subsequently (Figure 2B).
As indicated by the FESEM images of fractured spherulites, the lateral interfaces
among individual crystallites were coarsened via highly reversible coordination
bond formation and reconstruction and then proceeded to form a united superstruc-
ture (Figures 2A–2H). This has also been observed in the previous report and can be
explained by a coarsening mechanism.33–36 Since MOF-74-III crystallites grow close
to each other, the restricted assembly would contribute to transform the pre-aligned
needle-like crystallites into a dense MOF sphere via Ostwald ripening, resulting in a
robust isolated spherulite.
We further demonstrate that by tuning the kinetics of superstructure evolution via a
mixed-solvent approach, varying morphologies can be obtained (Figures 3 and S13).
It is realized that regulating the growth rate of crystallites is extremely important for
the formation of spherulite superstructures. By tuning the ratio between two solvents
with different decomposition rates, namely DMF and DEF, during the superstructure
evolution, the growth rate of MOF-74-III crystallites and the spherulite states could
be controlled. At higher DEF ratios, the slow release of diethylamine as the DEF
decomposition product could induce the slow deprotonation of linker III and growth
of MOF-74-III crystallites. As a result, needle aggregates were obtained while no
spherulites could be discerned (Figures 3C, 3G, and 3H). The aggregates were rela-
tively fragile and became individual needle crystals upon a sonicated treatment.
Conversely, at higher DMF ratios, the release of diethylamine was accelerated, ulti-
mately leading to the fast epitaxial growth of MOF-74-III crystallites in all directions
and then the formation of spherulites (Figures 3A, 3D, and 3E). It should be noticed
Chem 6, 460–471, February 13, 2020 463

Figure 2. Mechanism Study on Spherulite Superstructure Evolution
(A) Illustration of lateral coarsening between neighboring crystallites, which leads to the formation
of a fused core inside MOF-74 spherulites.
(B) Illustration of epitaxial growth of MOF-74 spherulites with 1D channels.
(C–G) Optical image and the corresponding polarized optical images of MOF-74-III spherulites
placed in between crossed polarizers showing the time-dependent superstructure evolution
process: (C) 3 h, (D) 4.5 h, (E) 6 h, (F) 8 h, and (G) 24 h.
(H) The structural mode of MOF-74 spherulites with surface fibrils and a fused core.
(I) SEM images showing well-defined fibrils on the surface of a spherulite and a fused core inside the
spherulite.
(J–L) SEM images of cracked MOF-74 spherulites; the optical image of a cracked MOF-74 spherulite
was inserted in (L). Destruction methods such as cutting and ultrasound treatment were conducted
to unveil the interior morphology information of spherulites. Scale bar, 100 mm in (C–G) and (I–L).
464 Chem 6, 460–471, February 13, 2020

Scheme 1. Growth Habits of Traditional Single Crystals and Spherulites
that we did not observe the presence of MOF fibrils in the sonicated solutions, which
indicated the lateral coarsening between the MOF crystals. As indicated by SEM images
of partially damaged spherulite samples, MOF-74-III spherulites with surface fibrils and a
fused core were clearly observed (Figure 2H). The SEM images of cracked MOF-74
spherulites also revealed the internal fused core without distinguishable fibril structures
observed in the core (Figures 2J–2L and S12). These results suggest that the noncore
regions of these superstructures consist of radially arranged fibrils, while the core fibrils
undergo orientational assembly and coarsening processes. Interestingly, we captured
the intermediate state between needle aggregates and spherulites when DMF/DEF
(1:1) solvents were mixed to induce the formation of MOF-74 spherulites. The resulting
sphere structure was mainly consisted of fibril crystallites, which could be observed in
the optical images, while a weak Maltese cross extinction pattern was still observed (Fig-
ures 3B and 3F). The results of this study therefore provide enhanced understanding of
the assembly process that fabricates from mesoporous MOF fibrils to hierarchical spher-
ulite superstructures, which shall be vital for their exploitation as sophisticated porous
materials and in nanotechnology.
Isoreticular Synthesis of MOF-74 Spherulites
One well-known facet of MOF-74 is that the pore size can be systematically expanded
by elongating the linker length.28,37 We inferred that superstructures with isoreticular
MOFs could also be assembled by simply tuning the organic linker lengths under similar
conditions. To this end, we utilized linker II and IV to construct isoreticular MOF-74-II and
MOF-74-IV with 2.2 and 3.5 nm hexagonal channels, respectively (Figures 4A–4C).
Experimental observations confirmed that both MOF spherulite superstructures could
be obtained (Figures 4D–4F and S9–S11). PXRD patterns of MOF-74-II and MOF-74-
IV indicated their phase purities, and optical microscopy revealed that the materials dis-
played a clear Maltese cross extinction pattern, demonstrating the successful prepara-
tion of MOF-74 spherulite superstructures (Figures S9–S11 and 4D–4F). This unique
observation demonstrates that the porosity of MOF superstructures can be readily
tuned without altering overall arrangement of the crystallites.
Introducing Multiple Components into MOF-74 Spherulites
The highly tunable synthesis of porous crystalline spherulites enables the fabrication
of multicomponent MOF superstructures (Figure 5). To explore the possibility, a
small amount of Co(NO3)2.6H2O was incorporated into the precursors of MOF-74-III
Chem 6, 460–471, February 13, 2020 465

Figure 3. Tuning the Kinetics of Superstructure Evolution by a Mixed-Solvent Approach
(A–C) Structural illustration of superstructures during MOF-74 growth in mixed DMF/DEF solvent environments, (A) fast growth of MOF-74 in DMF
solvents leads to the formation of spherulites with surface fibrils and a fused core, (B) superstructures with assembled fibrils and a weak Maltese cross
extinction pattern were observed due to the moderate growth of MOF-74 in DMF/DEF (1:1) solvents, and (C) superstructures assembled from large
needle crystallites can be obtained after the solvothermal reaction in DEF solvents.
(D–H) Optical images and the corresponding polarized optical images showing diverse morphologies obtained by mixing DMF and DEF in varying
ratios (DMF/DEF, mL/mL), (D) 2: 0, (E) 1.5: 0.5, (F) 1: 1, (G) 0.5: 1.5, and (H) 0: 2. Note that some fibril fragments can be observed in the inserted images of
(F), highlighted by the arrows. Scale bar, 100 mm in (D–H).
(Zn) before the formation of MOF spherulites. After the reaction, bimetallic MOF
spherulites could be fabricated with a clear Maltese cross extinction pattern (Fig-
ure S14). The energy dispersive X-ray (EDX) elemental mapping indicated the
well-mixed nature of Zn and Co ions in the superstructures (Figure S14). Further
incorporating multiple metal ions including Zn2+, Co2+, and Ni2+ into MOF-74-III
spherulites was conducted (Figures 5A and 5B). The phase purity was confirmed
by the PXRD pattern of the corresponding MOF-74 (Figure S8). Optical images
and SEM-EDX analysis showed the well-maintained spherulite superstructure, and
the metal composition was analyzed by ICP-MS, showing 92.0% Zn, 2.6% Ni, and
5.4% Co dispersed in the hierarchical structure (Figures 5A and 5B). Additionally,
the incorporation of functional organic components into these frameworks provides
a facile route to prepare stable, multivariate, and hierarchical materials.15,18 For
example, doping a small amount of catalytically active ligands such as metal phtha-
locyanine Co[Pc(COOH)4] into the hierarchical MOF-74-III superstructures could also
be achieved without interrupting the formation of spherulites (Figure 5C).
The hierarchically porous superstructures of MOF-74 spherulites provide a unique meso-
scopic environment for guest storage and delivery. We further illustrated that MOF-74-III
spherulites could be utilized for dye capture (Figure 5D). Post-synthetic encapsulation of
466 Chem 6, 460–471, February 13, 2020

Figure 4. Isoreticular Synthesis of MOF-74 Spherulites with Tunable Pore Sizes
(A–C) Shown is a structural illustration of MOF-74-II, III, IV, and their assembled spherulite superstructures, : (A) MOF-74-II with 2.2-nm intrinsic channels,
(B) MOF-74-III with 2.9-nm intrinsic channels, and (C) MOF-74-IV with 3.5-nm intrinsic channels.
(D–F) Optical images and the corresponding polarized optical images showing the spherulite morphology of (D) MOF-74-II, (E) MOF-74-III, and (F)
MOF-74-IV. Scale bar, 100 mm in (D)–(F).
an organic dye (methylene blue) into the mesoporous channels of MOF-74-III spherulites
was achieved, while Maltese cross extinction pattern of spherulites was maintained, as
indicated by the optical images, polarized optical images, and PXRD patterns (Figures
5D and S8). The mesoporosity in these series of spherulite superstructures provides
new behaviors and rooms for guest recognition and transportation, which are extremely
difficult to achieve in the traditional spherulite superstructures.
In addition to immobilizing soluble species, other MOF seeds can be utilized to
construct hierarchical MOF spherulites by non-epitaxial growth. Guided by a retrosyn-
thetic design, chemically stable Zr-based PCN-222(Fe) crystals were chosen as core
Chem 6, 460–471, February 13, 2020 467

Figure 5. Introducing Multiple Components into MOF-74 Spherulites
(A) SEM and the corresponding EDX mapping of MOF-74-III(Zn/Co/Ni) spherulites.
(B) One-pot immobilization of multiple metal ions including Zn2+, Co2+, and Ni2+ into MOF-74-III spherulites, the corresponding optical images, and
polarized optical images.
(C) One-pot immobilization of functional species CoPc into MOF-74-III spherulites, the corresponding optical images, and polarized optical images.
(D) Post-synthetic encapsulation of an organic dye (methylene blue) into MOF-74-III spherulites, the corresponding optical images, and polarized
optical images.
(E) One-pot immobilization of PCN-222 into MOF-74-II spherulites, the corresponding optical images, and polarized optical images.
(F) PCN-222(Fe)@MOF-74-II catalyzed epoxidation of alkenes.
Scale bar, 25 mm in (A) and 100 mm in (B)–(E).
468 Chem 6, 460–471, February 13, 2020

MOFs, which allow for the coating of MOF-74-II spherulites outside PCN-222(Fe).15
The resulting PCN-222(Fe)@MOF-74-II spherulites contain distinct cores, which are
different from previously described MOF-74 spherulites having undetected cores
with innumerable fibers radiating from an apparent point nucleus, as indicated by op-
tical and SEM images (Figures 5E, S15, and S16). The hierarchical PCN-222(Fe)@MOF-
74-II superstructures can function as an efficient size-selective catalyst for olefin epox-
idation reaction, because Fe-porphyrin moieties in the core PCN-222(Fe) can act as
catalytic centers while the shell MOF-74-II spherulites with a limited channel size can
control the substrate selectivity (Figures 5F and S18). It should be noted that the orien-
tation of MOF-74 channels, radiating from the spherulite center, ensures the substrate
transportation to the catalytically active PCN-222(Fe). We further studied the catalytic
performance of PCN-222(Fe)@MOF-74-II during the epoxidation reaction involving
olefins with different steric hindrance. As indicated by Table S1, small olefins including
cyclopentene and cyclohexene showed efficient conversion due to the high accessi-
bility of PCN-222(Fe) cores. When it came to larger olefins such as 1,2-diphenylethene
and (E)-1-nitro-4-styrylbenzene, the conversion after 24 h was very limited, comparing
with the PCN-222(Fe) without spherulite coating (Table S1). These results indicated the
spherulite superstructures assembled from 1D fibers could limit the diffusion of large
substrates, therefore providing a beneficial size-selective effect of hierarchical super-
structures for catalysis.
DISCUSSION
In summary, we present an initial example of porous crystalline spherulite super-
structures with well-arranged MOF crystallites. By controlling over nucleation and
growth rates via a mixed-solvent approach, the kinetics of superstructure evolution
can be well controlled, leading to the formation of various superstructures including
spherulites and needle aggregates. The Maltese cross extinction pattern typical of
spherulites under a polarized light was well-maintained during the isoreticular
expansion and multicomponent introduction. Taking advantage of this hierarchical
spherulite superstructures, we are able to control the encapsulation of guest mole-
cules into the porous channels. Accordingly, our discovery here can be used not only
to create porous MOF superstructures with tunable pore sizes but also to incorpo-
rate functional units within the hierarchical spaces of these superstructures, thereby
offering a fresh route to the design and packing of three-dimensional sophisticated
composites for applications in storage, delivery, and catalysis.
EXPERIMENTAL PROCEDURES
Synthesis of MOF-74-II Spherulite
Zn(NO3)2$6H2O (40 mg), Linker II (15 mg), EtOH (0.1 mL), H2O (0.1 mL), and DMF
(2 mL) were charged into a Pyrex vial. The mixture was heated in 100ꢁC oven for
24 h. After cooling down to room temperature, the colorless MOF-74-II spherulites
were harvested.
Synthesis of MOF-74-III Spherulite
Zn(NO3)2$6H2O (40 mg), Linker III (15 mg), EtOH (0.1 mL), H2O (0.1 mL), and DMF
(2 mL) were charged into a Pyrex vial. The mixture was heated in 100ꢁC oven for
24 h. After cooling down to room temperature, the colorless MOF-74-III spherulites
were harvested.
Synthesis of MOF-74-IV Spherulite
Zn(NO3)2$6H2O (40 mg), Linker IV (15 mg), EtOH (0.1 mL), H2O (0.1 mL), and DMF
(2 mL) were charged into a Pyrex vial. The mixture was heated in 100ꢁC oven for
Chem 6, 460–471, February 13, 2020 469

24 h. After cooling down to room temperature, the colorless MOF-74-IV spherulites
were harvested.
Experimental Procedure for Monitoring the MOF-74-III Spherulite Formation
Zn(NO3)2$6H2O (40 mg), Linker III (15 mg), EtOH (0.1 mL), H2O (0.1 mL), and DMF
(2 mL) were charged into a Pyrex vial. The mixture was heated in 100ꢁC oven for X
h (X = 3, 4.5, 6, 8, and 24 h).
Experimental Setup for the Influence of DEF on the MOF-74-III Spherulite
Formation
Zn(NO3)2$6H2O (40 mg), Linker III (15 mg), EtOH (0.1 mL), H2O (0.1 mL), DMF (0, 0.5,
1.0, 1.5, and 2.0 mL, respectively) and DEF (2.0, 1.5, 1.0, 0.5, and 0 mL, respectively)
were charged into a Pyrex vial. The mixture was heated in 100ꢁC oven for 24 h.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2019.12.001.
ACKNOWLEDGMENTS
This work was supported as part of the Center for Gas Separations, an Energy Fron-
tier Research Center funded by the U. S. Department of Energy, Office of Science,
and Office of Basic Energy Sciences under award number DE-SC0001015. The au-
thors also thank the financial support from the Robert A. Welch Foundation through
a Welch Endowed Chair to H.-C.Z. (A-0030) and the Qatar National Research Fund
under award number NPRP9-377-1-080. The authors acknowledge the valuable
assistance and disucssion from Professor Hexiang Deng (Wuhan University).
AUTHOR CONTRIBUTIONS
Conceptualization, L.F. and H.-C.Z.; Methodology, L.F.; Investigation, L.F., K.-Y.W.,
and T.-H.Y.; Writing – Original Draft, L.F.; Writing – Review and Editing, K.-Y.W. and
H.-C.Z.; Funding Acquisition, H.-C.Z.; Resources, H.-C.Z.; and Supervision, H.-C.Z.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: July 8, 2019
Revised: July 24, 2019
Accepted: November 29, 2019
Published: December 26, 2019
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