Crystallographic Visualization of Postsynthetic Nickel Clusters into Metal-Organic Framework

Article abstract of DOI:10.1021/jacs.9b06711

Postsynthetic metalation (PSM) has been employed as a robust method for the postsynthetic modification of metal-organic frameworks (MOFs). However, the lack of relevant information that can be obtained for the postsynthetically introduced metallic ions has hindered the development of PSM applications. Thanks to the advancement in single-crystal X-ray diffraction (SCXRD) technology, there have been a few recent examples in which successful postsynthetic introduction of single metal ions into MOFs occurred at the defined chelating sites. These works have provided useful explanations about the complicated host-guest chemistry involved in PSMs. On the other hand, there are only limited examples with crystallographic snapshots of the postsynthetic installation of metal clusters into the pores of MOFs using an ordinary SCXRD due to the loss of crystallinity of parent matrix during the PSM process. Herein, by the careful selection of starting materials and controlling the reaction conditions, we report the first crystallographic visualization of metal clusters inserted into Zr-based MOFs via PSM. The structural advantages of the parent Zr-MOF, which are inherited from the stable Zr₆ cluster and triazole-containing dicarboxylate ligand, ensure both the preservation of high crystallinity and the presence of flexible coordination sites for PSM. Furthermore, PSM of metal clusters in a MOF pore space enhances stability of the final samples while also imparting the functionality of a successful catalyst toward ethylene dimerization reaction. The related construction ideas and structural information detailed in this work can help lay the foundation for further advancements using the postmodification of MOFs as well as open new doors for the utilization of SCXRD technology in the field of MOFs.

Full text of DOI:10.1021/jacs.9b06711

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Article
Crystallographic Visualization of Post-synthetic
Nickel Clusters into Metal-Organic Framework
Xiao-Ning Wang, Peng Zhang, Angelo Kirchon, Jialuo Li, Wen-
Miao Chen, Yu-Meng Zhao, Bao Li, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06711 • Publication Date (Web): 09 Aug 2019
Downloaded from pubs.acs.org on August 10, 2019
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Crystallographic Visualization of Post-synthetic Nickel Clusters into
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,‡
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Xiao-Ning Wang, Peng Zhang, Angelo Kirchon, Jia-Luo Li, Wen-Miao Chen, Yu-Meng Zhao ,
,
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,2,3
Bao Li,* and Hong-Cai Zhou*
1
Key laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and
Engineering, Huazhong University of Science and Technology
Chemical
Hubei Key Laboratory of Bioinorganic Chemistry & Materia Medica, Wuhan, Hubei 430074, People’s
2
Republic of China. Department of Chemistry, Texas A&M University, College Station, Texas 77843-
3
255, United States. 3 Department of Materials Science and Engineering, Texas A&M University,
College Station, Texas 77842, United States.
These authors contribute equally
‡
Supporting Information Placeholder
ABSTRACT: Post-synthetic metalation (PSM) has been employed as a robust method for the post-
synthetic modification of Metal-Organic Frameworks (MOFs). However, the lack of relevant
information that can be obtained for the post-synthetically introduced metallic ions has hindered the
development of PSM applications. Thanks to the advancement in single-crystal X-ray diffraction
(SCXRD) technology, there have been a few recent examples in which successful post-synthetic
introduction of single metal ions into MOFs occurred at the defined chelating sites. These works have
provided useful explanations about the complicated host-guest chemistry involved in PSMs. On the
other hand, there are only limited examples with crystallographic snapshots of the post-synthetic
installation of metal clusters into the pores of MOFs using an ordinary SCXRD due to the loss of
crystallinity of parent matrix during the PSM process. Herein, by the careful selection of starting
materials and controlling the reaction conditions, we report the first crystallographic visualization of
metal clusters inserted into Zr-based MOFs via PSM. The structural advantages of the parent Zr-MOF,
which are inherited from the stable Zr cluster and triazole-containing dicarboxylate ligand, ensure both
6
the preservation of high crystallinity and the presence of flexible coordination sites for PSM.
Furthermore, PSM of metal clusters in a MOF pore space enhances stability of the final samples while
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also imparting the functionality of a successful catalyst toward ethylene dimerization reaction. The
related construction ideas and structural information detailed in this work can help lay the foundation
for further advancements using the post-modification of MOFs as well as open new doors for the
utilization of SCXRD technology in the field of MOFs.
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Introduction
The technology of single-crystal X-ray diffraction (SCXRD) is well known as one of the most powerful,
credible, and popular characterization methods for probing structural information at the atomic and
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molecular level. However, the main limitation of this technology is its strict requirement for high
quality, crystalline-state samples, which must be arranged in a regular atomic structure with a long-range
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ordered electron density distribution. Metal-Organic Frameworks (MOFs), as extremely robust
materials, have shown fascinating applications in various fields due to their excellent structure-property
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relationships. The components or secondary building units (SBUs) of metal nodes and organic linkers
in MOFs exhibits a very regular arrangement through coordination bonds, allowing for the detailed
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structure of MOFs to be characterized by SCXRD. Thanks to the assistance of SCXRD technology, the
structural advantages of MOFs, such as diversity, adjustability, and tunability, have been well illustrated
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and utilized. However, due to the strict limitations of SCXRD technology, guest molecules either that
reside within or that are post-introduced into the pores of the parent MOFs are very difficult to be
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crystallographically determined because of their incomplete occupancy and disordered arrangement. It
is well known that MOFs could be excellent parental carriers towards the wide scope of guest substances,
but the deficiency of explicit structural information has significantly limited the development of host-
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guest chemistry related to MOFs. Although it is possible to fit or analyze the host-guest interaction
through neutron diffraction, high-precision XRD, or theoretical methods, these methods are difficult to
generalize as conventional characterization routes because of the expensive requirements of equipment
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and professional theory. Therefore, exploring the host-guest chemistry of MOFs by virtue of standard
SCXRD technology is still a great challenge.
Over the last several years, developments of stable MOFs bring new opportunities to perform post-
synthetic modification by means of covalent reaction on the organic linkers, metal cations/organic
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linkers exchange, capture and release, and many other means. For instance, the stable Zr-MOFs can
serve as versatile platforms to allow the incorporation of terminal ligands, dicarboxylate linkers, metal
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cations, and metal−organic complexes onto the coordinative unsaturated Zr clusters. Most recently,
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post-synthetic metalation (PSM) has become a standardized method for the post-synthetic modification
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of MOF materials. PSM has helped us realize that MOF materials can serve as advanced heterogeneous
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catalysts for better gas adsorption or separation performances. In contrast to the strategy of metathesis
onto metal nodes, anchoring metal ions to the framework of MOFs via their coordination vacancies and
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pore characteristics is more popular and suitable for most MOF materials. Previous studies have shown
that the modification of nitrogen-based coordination sites plays an important role in the uniform and
regular introduction of different metal ions in MOFs. It is well-known that metal ions can coordinate
with nitrogen vacancies and attach onto the MOF skeleton, but there is an extremely small amount of
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evidence that clearly shows the true form of metal ions after attachment to the framework. By virtue of
SCXRD technology, the limited results of the ordered metalation onto MOF skeletons have been
reported and illustrated with explicit structural information of the host-guest chemistry occurring within
the framework.
Single-crystal X-ray diffraction is the straightforward and principal method for direct visualization of
the three-dimensional structures before and after the structural transformation. Thus, single-crystal to
single-crystal (SCSC) transformation is highly desirable to follow the structural changes. However,
analysis of the extensive research associated with PSM of MOFs reveals that reported SCXRD
information and current examples are not enough.16 The difficulties in achieving useful SCXRD
information after PSM is usually caused by the low initial crystallinity of the parent MOFs, the inability
to retain this crystallinity after the post-synthesis process, the inevitable disorder associated with the
high symmetry of MOFs, or the fabrication of saturated coordination environments of the introduced
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metal ions. Consequently, single-crystal to single-crystal (SCSC) transformation is hard to occur and is
of particular interest and importance. To resolve these issues, the existing work focuses on adding bi- or
tri-dentate chelating sites onto the framework of MOFs and visualizing evidence of these post-
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synthetically introduced single metal ions via SCXRD. However, more challenging tasks, such as the
achievement and detection of the PSM of metal clusters onto MOF skeleton via SCXRD technology, are
seen as extremely valuable yet elusive. Differing from the facile pre-design strategy for chelating single
metal ions onto MOF skeleton or the post-modification of nano-metal clusters into the pores of MOFs,
how to effectively anchor entire metal clusters onto a MOF skeleton while monitoring with SCXRD
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technology is still a huge challenge. This work has remained elusive due to the difficulties of pre-
designing rational chelating sites for entire metal clusters and the inability to control the cluster
formation in a defined and uniform manner. Although there are multiple difficulties and restrictions,
more sophisticated work on this area is urgently needed to uncover the mysterious veil of PSM.
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To truly observe the PSM of metal clusters onto the MOF skeleton by ordinary X-ray single-crystal
diffractometer in our laboratory, a new Zr-MOF, [Zr O (OH) (H O) (L) ] ( HUST-1, HUST =
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Huazhong University of Science and Technology, H L = 4,4’-(4-Amino-4H-1,2,4-triazole-3,5-
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diyl)dibenzoic acid ), fabricated by the combination of zirconium ion and the specially designed V-
shaped di-carboxylic acid containing triazole unit, is utilized for the following reasons. Firstly, thanks to
the strong Zr-O bond and stable Zr O cluster, Zr-MOF can exhibit extremely high kinetic stability and
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maintain high crystallinity suitable for diffraction studies, even under extreme conditions. Secondly,
the vacant azole unit has excellent coordination abilities to form stable metal clusters. Thirdly, different
from the normal ligands used in Zr-MOF, herein, the purpose of the utilization of V-shaped ligand is to
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eliminate the possible problem of disorder caused by the high symmetry of Zr-MOF parent. As a result,
a combination of the selected components would fabricate the anticipated MOF, which would exhibit a
rigid skeleton and reserve the flexible chelating coordination sites for PSM (Figure 1). In terms of these
structural advantages described above, the fabricated Zr-MOF should be an excellent parent for PSM
and might maintain the complete crystallinity to carry out the SCXRD studies. Fortunately, in
accordance with our assumption, the definite PSM of different metal clusters onto MOF skeleton have
been newly presented via SCXRD studies, which could fill the gap of the related research field. The
related construction idea and structural information in this work have laid the foundation for further
promoting the post-modification of MOFs, and opened a new door for the utilization of SCXRD
technology on the field of MOFs.
Figure 1. The illustration of the structural advantages of the pre-designed Zr-MOF material.
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Results and Discussion
Crystal Structure of HUST-1
The pristine crystal samples of HUST-1 were obtained via the reaction of ZrCl and H L under
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solvothermal method, which were directly used for further characterization and PSM of other crystal
samples. SCXRD studies for HUST-1 had been carried out on the normal diffraction equipment in our
laboratory. Consistent with our assumption, HUST-1 crystallizes in low symmetric space group Pnnm
since the utilization of V-shaped linkers would reduce the possibility of disorder caused by the high
symmetry of MOF skeleton compared to other highly-symmetric Zr-MOFs. In HUST-1, each Zr ion
adapts an eight-coordinated square-antiprismatic sphere that is occupied by eight oxygen atoms. Six Zr
ions are inter-connected via μ -O and μ -OH units to form the eight-connected [Zr (μ -O) (μ -
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OH) (OH) (H O) ] inorganic brick, which is further capped by eight carboxyl groups of different
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ligands (Figure 2a). The adjacent symmetric Zr clusters are jointed by the deprotonated linkers to form
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the 3D distorted bcu network (Figure 6a). The bridging angle of the V-shaped linker is 152.56°, while
the dihedral angles of the benzoate plane and central triazole are 17.77° and 33.64°, respectively. In
addition, the dihedral of two carboxylate groups is 24.54°. A highly porous structure had been left in the
final product (Figure S17), which had been occupied by a large amount of guest molecules. Calculated
by PLATON program, the solvent accessible volume was determined to be 79.5 %. The central trizaole
units disposed on the surface of the framework. Importantly, thanks to the V-shaped configuration of the
di-carboxylate linker, the constructed Zr-MOF not only reserves the rigid skeleton along with an open
porous structure, but it also pre-disposes flexible chelating coordination sites originating from two
adjacent triazole units that belonged to the different di-carboxylate linkers (Figure 1). The dihedral and
center-to-center distance of two triazole groups are 63.41° and 11.74 Å. Therefore, in terms of structural
advantages, the rigid skeleton, strong coordination ability of vacant triazole unit, proper separation
distances, broad flexible degree, and open porous skeleton endow HUST-1 with great potential as an
excellent parent that could post-introduce metal clusters while reserving high crystallinity. Accordingly,
these anticipated structural advantages of HUST-1 prompt us to explore the practical possibilities of
whether the PSM of metal clusters into MOFs could be truly visualized by ordinary SCXRD technology.
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Figure 2. Crystal structures of HUST-1 (a), HUST-2 (b) and HUST-3 (c) determined by SCXRD.
In order to fulfill the anticipated PSM of metal clusters in MOFs which could be monitored by SCXRD
technology, one critical pre-requisite is the high stability of the MOF parent. Due to the strong Zr-O
bond and stable Zr cluster, the pristine Zr-MOF exhibits high chemical stability in water and in
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different concentrations of HCl solution (Figure S1). Additionally, the thermal stability of Zr-MOF
allows it to retain its structure up to 400 ℃ (Figure S23). However, the activated HUST-1 is unstable
under a vacuum condition at 80 ℃ for ten hours, which is usually observed for Zr-MOFs with eight-
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connected Zr clusters. In spite of this, the reaction process of PSM takes place in the surrounding
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solvent, and the high stability of HUST-1 in water would ensure the occurrence of PSM reaction.
Post-synthetic Metalation of Nickel Clusters
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Figure 3. The representative SCXRD images of crystal samples of HUST-1(a), HUST-2 (b) and HUST-
3(c) in four sets.
Utilizing the triazole ligands to fabricate the versatile transitional metal clusters is one of the prevalent
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routines in coordination chemistry. MOFs have been proven to be an excellent parent matrix to load
different guest molecules by virtue of their porous structure. Herein, HUST-1 inherits the advantages of
triazole groups and MOFs, which exhibits the potential application in capturing guest molecules along
with the reservation of high crystallinity. To present the crystallographic snapshot of PSM of metal
clusters in MOFs, the pristine crystal samples of HUST-1 were tentatively immersed in different nickel
salts (NiCl , NiBr , Ni(NO ) , Ni(OAc) , NiC O ) under different conditions ( temperature, solvent,
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and reaction time ). After absorbing the nickel ions, the crystallinity of parent MOFs could be still
retained validated by SCXRD and powder XRD. However, only the HUST-1 samples reacted with NiCl2
and NiBr in CH CN at 80 ℃ exhibit the definite post-modified results, which could be validated by the
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diffraction images (Figure 3). From performing the SCXRD studies, it was realized that hepta-nuclear
and tri-nuclear nickel clusters could be effectively introduced into the parent matrix of HUST-1 after the
reaction at 80 ℃ for one week. To our best knowledge, the PSM of metal clusters in MOFs monitored
via SCXRD technology have been successfully implemented, which has a very significant scientific value
in understanding host-guest chemistry at an atomic level.
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When HUST-1 was immersed in the CH CN solution containing NiCl ·6H O salt for one week, a new
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crystal sample, HUST-2, was obtained. Thanks to the structural advantages of the pre-designed Zr-MOF,
high crystallinity of the new crystal state was still preserved, even after the PSM process. Detailed
information about the collection and refinement of crystal data had been listed in supporting
information (Table S1).
Compared to HUST-1, the diffraction images of HUST-2 exhibit higher resolution due to the
incorporation of hepta-nuclear Ni clusters on the surface of the framework. The regular arrangement of
post-modified hepta-nuclear clusters not only increases the electron density of the whole framework, but
also enhances diffraction intensity of the crystal sample, compared to the low density of the original
MOF with a large porous structure (Figure 3a, b). After integration and scaling of the whole data for
HUST-2, there was no obvious change in cell parameters after PSM treatment, reflecting the rigid
skeleton of the original framework. The main difference between HUST-1 and HUST-2 focuses on the
flexible chelating coordination sites. In order to fit the kinetic requirement for stabilizing the post-
introduced hepta-nuclear clusters, the adjacent triazole units must rotate to the proper positions and
chelate onto the guest clusters. For HUST-2, the dihedral and the center-to-center distance of two
triazole groups is 69.23° and 8.63 Å. Apart from this change, there are the other changes in the
configuration of the whole linker. For example, the bridging angle of V-shaped linker is 142.37°; the
dihedral angles of the benzoate plane and central triazole are 84.34° and 77.87°; the dihedral of two
carboxylate groups is 36.32°. Comparably, there are slight differences occurring in the Zr inorganic
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brick. These variations adequately illustrate the important role of the disposed flexible chelating
coordination sites in stabilizing the extra clusters.
Through the presentation of SCXRD technology, the post-modified hepta-nuclear clusters exhibit the
mirror-symmetric configuration. Ni1, Ni2, and Ni3 adopt the octahedral coordination sphere, whilst the
coordination environment of Ni4 is a square pyramid. Ni3 atom is located in the central portion of the
cluster, whose periphery is decorated by other six nickel ions via three μ -OH and three μ -Cl bridges. In
3
3
the peripheral ring: Ni1 and Ni2 are connected via triazole unit, and μ -OH and μ -Cl bridges; Ni2 and
3
2
Ni4, Ni1 and its symmetric Ni1A, are bound together by μ -Cl and μ -Cl bridges; the connection mode
3
2
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between Ni4 and its symmetric Ni4A are μ -Cl and μ -Cl bridges. For Ni1 and Ni2, the vacant
3
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coordination sites are occupied by water molecules, whilst a chloride ion occupies the remanent site of
Ni4 (Figure 5a). Compared to the original position of triazole in HUST-1, the obvious torsion has been
presented in order to cater to the requirement of the post-modified cluster. Viewed from the packing
mode, the post-modified clusters regularly distribute on the surface of the skeleton, and the specific
porosity is still remained in the final framework (Figure 6b and Figure S18). Calculated by PLATON
program, the solvent accessible volume in HUST-2 had been determined to be 62.6 %. In addition, the
existence and ratio of nickel and chloride ions could be also validated by the comprehensive results of
SEM (Figure 4b), XPS (Figure 4e, Figure S24 and S29) and ICP (Table S5).
0
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0
Figure 4. Photo and SEM images of HUST-1(a), HUST-2 (b), HUST-3(c) and HUST-4 (d) along with
the EDX mapping image of Zr, Ni, Cl and Br elements; XPS spectra of Ni element in HUST-2(e) and
HUST-3(f).
Differently, by substituting the metal salt with NiBr ·3H O, tri-nuclear nickel clusters had been
2
2
introduced onto the original framework to fabricate the new crystal sample of HUST-3. Compared to
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original HUST-1, the diffraction intensity and resolution of crystal sample HUST-3 were also enhanced,
but weaker than HUST-2 (Figure 3c). The different nucleation numbers must be responsible for the
difference in the presentation of diffraction patterns. The SCXRD studies for HUST-3 reveal that there
are some minor changes in cell parameters, which is also ascribed to the torsion of bridged linkers. The
dihedral and center-to-center distance of two triazole groups is 27.25° and 6.58 Å; the bridging angle of
V-shaped linker is 138.75°; the dihedral angles of benzoate plane and central triazole are 65.76° and
1
2
3
4
5
6
7
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9
1
1
1
1
1
1
1
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1
1
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0
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5.95°; the dihedral of two carboxylate groups is 28.59°. In addition, the bridged linker exhibits the more
bent configuration caused by the torsion in the process of dynamic PSM of clusters. The post-modified
tri-nuclear cluster also adapts the mirror-symmetry due to the same spacial constraint, in which Ni1 and
Ni2 adapt the square pyramid and octahedron coordination environments. Ni1 and Ni2 ions are bound
together via triazole and μ -OH, and two Ni1 and Ni2 ions are integrated to fabricate the typical tri-
3
nuclear nickel cluster by the stabilization of two triazole groups (Figure 5b). Due to the weak
coordination ability of bromide with nickel, the other vacant coordination sites of nickel ions are
occupied by water molecules to form the complete configuration of tri-nuclear clusters. The post-
modified clusters also regularly distribute on the surface of the skeleton viewed from the packing mode,
and the certain porosity in the final framework is retained (Figure 6c and Figure S19). Calculated by
PLATON program, the solvent accessible volume in HUST-3 was determined to be 66.8 %. In addition,
the existence and ratio of nickel and bromide ions could be also validated by the comprehensive results
of SEM (Figure 4c), XPS (Figure 4f, Figure S24 and S30) and ICP (Table S5).
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0
Figure 5. The perspective view of the hepta-nuclear clusters in HUST-2 (a) and tri-nuclear clusters in
HUST-3 (b).
The phenomenon of the successful PSM of nickel clusters onto the skeleton of the original MOF could
not be detected via SCXRD technology when the reaction temperature changed to room temperature.
The crystallinity of samples after PSM could be also retained, but no definite post-modified metal ions
could be observed, except for the slight changes of structural parameters of dicarboxyl linkers, as shown
in Figure S20-22. Other characteristic methods with high diffraction intensity might be useful to give the
definite states of post-modified nickel ions, but not the normal diffraction equipment. Therefore, the
sample loaded with nickel ions had been termed as HUST-4, in order to distinguish HUST-2 and
HUST-3. The dihedral and center-to-center distance of two triazole groups is 57.23° and 11.62 Å; the
bridging angle of V-shaped linker is 148.17°; the dihedral angles of benzoate plane and central triazole
are 29.05° and 30.65°; the dihedral of two carboxylate groups is 23.88°. There are lots of ambiguous
diffraction densities residue in the cavities of HUST-4, which could not be fixed as the definite states.
The low reaction temperature could not supply enough energy surrounding the energy barrier for
fabricating the related clusters and result in the low load amount of nickel ions in HUST-4 compared to
HUST-2 and HUST-3.
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Comprehensively, in order to achieve the desired goal of crystallographic snapshot of PSM of metal
clusters into the porosity of MOFs, the parent MOF scaffold must exhibit the following specific
characteristics: A rigid skeleton for the reservation of high crystallinity during PSM process; Uniform
distribution of a flexible chelating environment to adapt the spatial requirements for the formation of
variable metal clusters; Existence of vacant coordination positions with strong coordination abilities to
accept the post-synthetic metal ions. Herein, the constructed Zr-MOF possesses the composite
advantages and truly exhibits the single-crystal to single-crystal transformation triggered by the PSM of
metal clusters in the porosity of MOF, which imparts a unique reference for post-modifying phase-
change materials.
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0
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9
0
Figure 6. The perspective view of the packing mode and topological structure of HUST-1(a), HUST-2
(b) and HUST-3(c) along b-axis direction. The rectangular, purple ball, hexagonal and triangle
represent the Zr cluster, di-carboxylate ligand, hepta-nuclear and tri-nuclear clusters, respectively.
6
The characteristic porosities in the parent MOF and derived MOFs were also significantly altered after
the inclusion of post-modified metal clusters, proven by the N sorption isotherms at 77 K. HUST-1
2
3
−1
exhibits a typical type IV isotherm morphology, with a N uptake of 13.07 cm g and a Langmuir
2
2
−1
Surface Area of 48.22 m g . The low adsorption capacity indicates the collapse of framework in
2
2
HUST-1, in consistent with the other similar Zr-MOFs consisted of 8-connected cluster. Comparably,
after the introduction of hepta-nuclear nickel cluster into HUST-1, HUST-2 exhibits a typical type I
3
−1
2
−1
isotherm morphology, with a N uptake of 100.01 cm g and a Langmuir Surface Area of 289.69 m g .
2
The larger parameters illustrate the more stable porous framework in HUST-2, and further manifest the
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important role of the PSM metal clusters to the reservation of porous framework. However, the N2
adsorption isotherm of HUST-3 gives the slightly parameters compared to the ones of HUST-1, which
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might be caused by the unstable Ni cluster. Therefore, all of the comparable adsorption phenomenons
3
adequately emphasize the important role of the post-modified metal clusters to stabilize the porous
framework.
Cooperative Catalytic Performances of Post-modified MOFs
0
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PSM of metal clusters not only provides an efficient synthetic route to obtain the post-modified
materials but also facilitates the new functionalization into MOFs. For example, by virtue of the intrinsic
catalytic activities of installed metal species and the high stability of Zr-MOF framework, the obtained
MOFs can be explored as heterogeneous catalysts towards specific reactions. Herein, we investigated the
catalytic properties of Ni-containing MOFs in ethylene dimerization reactions. The reactions were
carried out in the stainless-steel batch reactor under 40 bar of ethylene in toluene at 25 °C, and diethyl
aluminum chloride (Et AlCl) was adopted as the activator (Table 1). In every experiment, the amount
2
of catalyst was fixed to 5 mg, and the real molar amount of Ni calculated from ICP was shown in the
bracket since the Ni content is different in each catalyst. As the blank controls, we first examined the
ethylene dimerization under the activator Et AlCl alone and with the presence of the parent MOF
2
HUST-1 (entries 1 and 2). Both experiments showed no activity, further proving that the reaction
occurs on the installed Ni species. Furthermore, UiO-67-Ni (Zr O (OH) (bpydc) (NiBr ) (bpydc =
6
4
4
6
2 6
2
,2′-bipyridine-5,5′-dicarboxylic acid) as the reference (entry 3) had been utilized, since the presented
framework bearing catalytic active Ni-bipyridine moiety has been widely accepted as an effective MOF-
2
4
based catalyst for ethylene dimerization. Compared to UiO-67-Ni, the series of HUST MOFs showed
higher activity and better 1-butene selectivity (entries 4-6) due to their relatively larger pores and
facilitated mass transport inside the pores. Notably, HUST-3 shows the best activity for ethylene
dimerization, even if its Ni content is not the highest among the three catalysts after PSM treatment
-
1
(entry 5). The intrinsic activity of HUST-3 reached 4300 h for butene production based on Ni, which
is six times higher than that of HUST-2 and three times higher than that of HUST-4. Considering their
similar parameters of unit cells (Table S1), the coordination environments of Ni ion showed significant
impact on their catalytic activities. Visualized from the results of SCXRD, much more open sites had
been exposed on the surface of channels in terms of the coordinatively unsaturated nickel ions fixed in
the framework of HUST-3. The more regular arrangement of nickel ion of the hepta-nuclear cluster in
HUST-2 lead to the saturate coordination environment and less open metal sites, which is the main
reason that the lowest catalytic performance had been presented compared to HUST-3 and HUST-4.
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Although the real nickel state could not be fixed, the separated nickel centers and open metal sites in
HUST-4 should be responsible for the higher catalytic performance than the ordered and saturated
nickel ions in HUST-2.11 In addition, although HUST-3 has no nitrogen adsorption after thermal
activation, the framework is highly robust when operated in the liquid phase, and the active sites within
the pores remain accessible. The catalyst can keep its high activity for at least three runs in the recycle
experiments without significant loss of Ni content and crystallinity, as confirmed by ICP and XRD
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(Table S5 and Figure S6).
Table 1. Ethylene Dimerization Catalyzed by Ni-Containing Catalysts a
Catalyst, Et AlCl
2
o
25 C, 40 bar
Selectivity (%)
c
C = Intrinsic Activity
4
Entry
Catalyst b
none
1-
C4=
0
2-
C4=
0
C6=
C8=
+
-
1
(
h )
1
2
0
0
0
HUST-1
(0 μmol)
0
0
0
UiO-67-Ni
3
4
5
520 ± 30
740 ± 50
59.0
72.0
77.0
29.8
17.4
11.9
11.2
10.6
11.1
(
5.3 μmol)
HUST-2
(
10.7 μmol)
HUST-3
4570 ± 230
(
5.1μmol)
HUST-4
6
1550 ± 100
77.6
14.8
7.6
(
2.6 μmol)
a
Reactions were carried out in the batch reactor under 40 bar of ethylene at 25 °C using toluene as the solvent, 5 mg of
catalyst, and 1.5 mmol Et AlCl as the activator.
2
b
Ni contents were calculated from the ICP data.
c
Calculated based on the moles of butenes generated and the Ni content. Standard deviations were calculated based on
three runs.
Conclusion
In conclusion, by the rational design of the initial components, a matrix Zr-MOF had been fabricated,
which is the excellent carrier that could effectively immobilize the dissociative metal ions into its
framework as the form of metal clusters. More importantly, the definite configuration of the PSM metal
clusters could be detected via the normal SCXRD technology. By the utilization of the different metal
salts, the hepta-nuclear and tri-nuclear states captured in the designed Zr-MOF had been presented,
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which firstly illustrates the feasibility that the detailed configuration of PSM metal clusters could be
emerged via the normal SCXRD technology. The presented results manifest HUST-1 not only would be
of the excellent platform to observe the structural configuration of the post-introduced substance, but
also illustrate the potential application as “Crystal Sponge” applicable to inorganic materials.25
Additionally, the successful examples illustrate the key role of the synthesis strategies to expand the
functional scopes and application categories of MOFs. Therefore, this work gives a fresh insights into the
fabrication of tailored MOF-based platform for sophisticated applications, especially for the structural
determination of post-introduced guest molecules.
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0
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
The detailed experimental methods, crystal data, theoretical calculation methods and results (PDF)
AUTHOR INFORMATION
Corresponding Author
*
E-mail: libao@hust.edu.cn (B.L.).
* E-mail: zhou@chem.tamu.edu (H.-C. Z.).
ORCID
Bao Li: 0000-0003-1154-6423
Angelo Kirchon: 0000-0003-1082-9739
Hong-Cai Zhou: 0000-0002-9029-3788
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
This work was supported by the financial supports of National Science Foundation of China ( 21471062
), the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research
Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, and Office of Basic
Energy Sciences (DESC0001015), Office of Fossil Energy, the National Energy Technology Laboratory
(
DE-FE0026472), and the Robert A. Welch Foundation through a Welch Endowed Chair to HJZ (A-
0030), the Fundamental Research Funds for the Central Universities (2019kfyRCPY071,
019kfyXKJC009).
2
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0
23
ACS Paragon Plus Environment