One-Step Synthesis of Hybrid Core–Shell Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.201710019

Epitaxial growth of MOF-on-MOF composite is an evolving research topic in the quest for multifunctional materials. In previously reported methods, the core–shell MOFs were synthesized via a stepwise strategy that involved growing the shell-MOFs on top of the preformed core-MOFs with matched lattice parameters. However, the inconvenient stepwise synthesis and the strict lattice-matching requirement have limited the preparation of core–shell MOFs. Herein, we demonstrate that hybrid core–shell MOFs with mismatching lattices can be synthesized under the guidance of nucleation kinetic analysis. A series of MOF composites with mesoporous core and microporous shell were constructed and characterized by optical microscopy, powder X-ray diffraction, gas sorption measurement, and scanning electron microscopy. Isoreticular expansion of microporous shells and orthogonal modification of the core was realized to produce multifunctional MOF composites, which acted as size selective catalysts for olefin epoxidation with high activity and selectivity.

Full text of DOI:10.1002/anie.201710019

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: One-Step Synthesis of Hybrid Core-Shell Metal-Organic
Frameworks
Authors: Xinyu Yang, Shuai Yuan, Lanfang Zou, Hannah Drake,
Yingmu Zhang, Junsheng Qin, Ali Alsalme, and Hongcai Zhou
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710019
Angew. Chem. 10.1002/ange.201710019
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10.1002/anie.201710019
Angewandte Chemie International Edition
COMMUNICATION
One-Step Synthesis of Hybrid Core-Shell Metal-Organic
Frameworks
Xinyu Yang[a]§, Shuai Yuan[a]§, Lanfang Zou[a], Hannah Drake[a], Yingmu Zhang[a], Junsheng Qin[a], Ali
Alsalme[c] and Hong-Cai Zhou*[a][b]
Abstract: Epitaxial growth of MOF-on-MOF composite is an evolving
research topic in the quest for multifunctional materials. In previously
reported methods, the core-shell MOFs were synthesized via a
stepwise strategy that involved growing the shell-MOFs on top of the
preformed core-MOFs with matched lattice parameters. However, the
inconvenient stepwise synthesis and the strict lattice-matching
requirement have limited the preparation of core-shell MOFs. Herein,
we demonstrate that hybrid core-shell MOFs with mismatching lattices
can be synthesized under the guidance of nucleation kinetic analysis.
A series of MOF composites with mesoporous core and microporous
shell were constructed and characterized by optical microscopy,
powder X-ray diffraction, gas sorption measurement, and scanning
electron microscopy. Isoreticular expansion of microporous shells and
orthogonal modification of the core was realized to produce
multifunctional MOF composites, which acted as size selective
catalysts for olefin epoxidation with high activity and selectivity.
functionalization.[10] With the PSM approach, both metal nodes
and organic linkers can be functionalized without significantly
compromising the overall structural integrity. This approach is
adopted to build topologically identical but functionally diverse
frameworks. Furthermore, the complexity can also be enhanced
by encapsulating other materials to form MOF composites that
combine the advantages of both.[11] Recently, Tang and
coworkers synthesized
a series of MOF composites by
incorporating Pt nanoparticles into MIL-101 to serve as an
efficient catalyst for the selective hydrogenation of α, β-
unsaturated aldehydes.[12]
The complexity of MOF materials can be further escalated by
the core-shell strategy to combine the functionalities of both
MOFs.[13] The core-shell MOF self-assembly method was
demonstrated by Kitagawa and coworkers in the synthesis of Cu-
based MOFs on a substrate of Zn counterpart.[13b] Another
representative example was reported by Rosi and coworkers who
prepared bio-MOF-11@bio-MOF-14 composite by stepwise
synthesis strategy. This composite exhibited higher CO2/ N2
selectivity and water stability than bio-MOF-11.[13d] Another
example was given by Oh and coworkers who reported two MOF-
templated core-shell composites, MIL-68@MIL-68-Br and MIL-
68@MIL-68-NDC, in which the secondary MOF showed different
growth directions.[13a]
To date, core-shell MOFs were mostly synthesized via a
stepwise strategy in which the core was initially synthesized and
subsequently used as a seed for the shell crystal growth. Lattice
matching (similar crystallographic parameters) was usually
required for the core-shell MOF crystal growth as the metal
secondary building units (SBUs) of the core were aligned with
those of the shell. However, the inconvenient stepwise synthesis
and the strict lattice-matching requirement have limited the
development of core-shell MOFs. To address this issue, we
recently explored the one-pot synthesis of hybrid core-shell MOFs
(PCN-222@Zr-BPDC) with mismatching lattices. The design and
synthesis of hybrid core-shell MOFs were guided by nucleation
kinetics (Figure 1). Linkers with high connectivity (TCPP, H4TCPP
= tetrakis (4-carboxyphenyl)porphyrin) bind strongly with metal
cations, allowing for fast homogeneous nucleation in solvothermal
reaction conditions. In contrast, the low connectivity linkers
(BPDC, BPDC = biphenyl-4,4′-dicarboxylate) usually take longer
time to form crystals than TCPP under similar conditions.
However, heterogeneous nucleation takes place far more quickly
than homogenous ones since the seed crystal act as a template
for the growth of the second MOF. The one-step approach thus
eliminates the necessity of creating a new surface and reduces
Metal-Organic Frameworks (MOFs), also known as Porous
Coordination Polymers (PCPs), are an emergent class of
crystalline porous materials, which have received considerable
attention in the past two decades.[1] Due to their structural and
functional tunability as well as their ever-expanding scope of
application, MOFs have become one of the most intriguing
classes of materials for both scientists and engineers. The high
porosity, exceptional tunability, and adequate stability of MOFs
make them promising candidates for many potential applications
including gas storage[2] and separation[3], heterogeneous
catalysis[4], sensing[5], proton conduction[6], luminescence[7], and
drug delivery.[8] Therefore, an expanding variety of strategies
have been applied to introduce desired functionalities into MOFs.
Among them, one-pot synthesis has been extensively utilized due
to the convenience of incorporating multi-functionalities in a single
step while preserving the structural integrity.[9] Yaghi and
coworkers put forward the important concept of “heterogeneity
within order”. This idea was exemplified by multivariate MOFs
(MTV-MOFs), in which mixed ligands with as many as eight
different substituents were introduced into MOF-5.[9a] Post-
synthetic modification (PSM) is another powerful tool for MOF
[a]
Xinyu Yang, Shuai Yuan, Dr. Lanfang Zou, Hannah Drake, Yingmu
Zhang, Dr. Junsheng Qin and Dr. Hong-Cai Zhou
Department of Chemistry
Texas A&M University
E-mail: zhou@chem.tamu.edu
Dr. Hong-Cai Zhou
Department of Materials Science and Engineering
Texas A&M University
Dr. Ali Alsalme
Department of Chemistry, College of Science
King Saud University
[b]
[c]
§
X. Y. and S. Y. contributed equally
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201710019
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Figure 1. Kinetically guided synthesis of a hybrid core-shell
PCN-222@Zr-BPDC
incipient surface energy requirements. Guided by kinetic control,
PCN-222@Zr-BPDC(UiO-67)
were
synthesized
by
a
solvothermal reaction of ZrCl4 with H4TCPP and H2BPDC.
From the optical microscopic images (Figure 2a), the core
and shell MOFs can clearly be distinguished. The cores consisted
of red needle-shaped crystals and the shells were colorless bulk
crystals. The SEM images (Figure 2b) from different cross
sections also indicated a core-shell structure. The composition of
the core-shell MOF was identified by powder X-ray diffraction
(PXRD) patterns (Figure 3c). All characteristic peaks from both
the PCN-222 and Zr-BPDC were found in the PXRD patterns,
demonstrating that the core is composed of PCN-222 while the
shell MOF have UiO-67 structures. The N2 adsorption isotherm
(Figure 3f, Figure S17) of the PCN-222@Zr-BPDC was very close
to that of Zr-BPDC (UiO-67), but with a noticeable hysteresis loop
around P/P0 = 0.3 due to the presence of a small amount of PCN-
222. In the pore size distribution plot (Figure S24), mesopores of
3.2 nm were observed, which is characteristic of the mesoporous
PCN-222. These results suggested that the mesopores of PCN-
222 were not blocked in the core-shell structures.
Figure 2. (a) Optical image of PCN-222@Zr-BPDC; (b) SEM
images of PCN-222@Zr-BPDC from different intersections;
(c) Time course analysis of formation of PCN-222 (red),
PCN-222@Zr-BPDC (blue) and Zr-BPDC (black)
BPDC(UiO-67) crystals as a result of the lack of crystal nucleus
(Figure 2c, black trace). This indicates the remarkably
accelerating effect of PCN-222 on the nucleation of Zr-
BPDC(UiO-67) crystals.
The crystal growth of PCN-222, Zr-BPDC, and PCN-
222@Zr-BPDC were monitored and compared to shed light on
the formation mechanism of core-shell structures. During the
crystal growth of PCN-222@Zr-BPDC(UiO-67), the nucleation of
PCN-222 was preferred at the beginning and formed the
needlelike crystal, because highly connected TCPP linkers
competitively bind with Zr4+. The PCN-222 crystals were formed
within two days (Figure 2c, red and blue traces) and subsequently
acted as seed crystals to accelerate the growth of the otherwise
slow Zr-BPDC(UiO-67) crystallization on its surface. The
formation of Zr-BPDC(UiO-67) was observed on the surface of
PCN-222 after five days (Figure 2c, blue trace). With the
extension of reaction time, the Zr-BPDC(UiO-67) shell kept
growing as the concentration of volatile TFA decreased. Indeed,
the heterogeneous nucleation takes place faster than the
homogeneous one as PCN-222 crystals act as a template for Zr-
Orthogonal modification of the core-MOF and shell-MOF can
be realized in the PCN-222@Zr-BPDC(UiO-67) system to
produce multifunctional MOF composites. The porphyrin moiety
in PCN-222 can be coordinated with a variety of metals. Different
metalloporphyrin based linkers were successfully incorporated
into PCN-222(M)@Zr-BPDC(UiO-67) (M = Mn2+, Fe3+, Ni2+, and
Zn2+) (Figure S13). Furthermore, the Zr-BPDC(UiO-67) shells
were modified with different functional groups including bipyridyl,
nitro, iodo, and methoxy groups (PCN-222@Zr-BPYDC (BPYDC
=
2,2'-bipyridyl-5,5'-dicarboxylate), PCN-222@Zr-BPDC-NO2
(BPDC-NO2=
2,2'-Dinitrobiphenyl-4,4'-dicarboxylate,
PCN-
222@Zr-BPDC-I
(BPDC-I= 2,2'-Diiodobiphenyl-4,4'-
dicarboxylate) and PCN-222@Zr-BPDC-OMe (BPDC-OMe= 2,2'-
Dimethoxybiphenyl-4,4'-dicarboxylate)) (Figure 3a, Figure S1).
The PXRD patterns (Figures S8-S11) showed the high
crystallinity of these core-shell MOF composites and all N2
adsorption isotherms (Figures S18-S21) presented noticeable
hysteresis loop near P/P0 = 0.3 due to the presence of PCN-222.
In addition, we tuned the length of the linear linker by using NDC
BPDC(UiO-67) crystal growth.
A control experiment was
conducted under synthetic conditions similar to those of the core-
shell MOF composite but without the TCPP linkers. This
experiment took much longer time (~10 days) to form the Zr-
(NDC
= naphthalene dicarboxylate) or AZDC (AZDC =
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Figure 3. Ligands (3a), PXRD patterns (b-d), and gas adsorption isotherms (e-g) of PCN-222@ Zr-NDC, PCN-222@Zr-BPDC, and
PCN-222@Zr-AZDC MOF composites
azobenzene dicarboxylate), which are shorter or longer than
BPDC, respectively. From the microscopic images (Figures S2-
S3) and SEM images (Figures S30-S32), we could easily
distinguish the core-shell structure of PCN-222@Zr-NDC and
PCN-222@Zr-AZDC. The PXRD patterns (Figures 3b, 3d) and N2
adsorption isotherms (Figures 3e, 3g) demonstrated high
crystallinity and accessibility of pores of the core-shell MOF
composites, respectively. As the core and shell components can
be easily tuned by functionalizing the linear linker and the TCPP
ligand, the PCN-222@UiO-series are an ideal platform to take
advantage of both MOFs.
PCN-222(Fe)@Zr-AZDC, have been prepared. The core MOF,
with Fe-porphyrin moieties, acted as the catalytic center for olefin
epoxidation reaction, while the shell with a tunable window size,
controlled the selectivity of the substrates. The two components
worked synergistically leading to a size-selective catalyst. The
compositions of the core-shell MOFs were investigated by 1H-
NMR digest experiments (Figure S33-S35). The core shell ratios,
defined as the molar ratio of the tetratopic linker to the linear linker,
were determined to be 0.01, 0.02, and 0.04 for PCN-222@Zr-
NDC, PCN-222@Zr-BPDC(UiO-67), and PCN-222@Zr-AZDC,
respectively. The catalytic performance of PCN-222(Fe)@Zr-
BPDC(UiO-67) in the epoxidation of various olefins with different
steric hindrance was evaluated. As shown in Table 1, the olefins
were converted to corresponding epoxides in different conversion
ratios due to their molecular sizes. The small olefins (Table 1,
entry 1-5) showed ideal conversions, suggesting the high
accessibility and activity of the catalytic center. Indeed, the sizes
of these olefins are smaller than the window sizes of Zr-
BPDC(UiO-67), allowing for efficient diffusion of the substrates.
Olefin size correlates very well with conversion; as olefin size
increases, the conversion decreases. This trend for decreased
olefin conversion eventually became little to no conversion under
similar experimental conditions, even with longer reaction times.
The large olefins (Table 1, entry 8-10) longer than BPDC were
blocked by the shell. The shell, though inert in catalysis, limited
Olefin epoxidation is an important reaction as epoxides are
widely used in the production of epoxy resins, paints, and
surfactants. Epoxides are also intermediates in many organic
reactions. Although many catalytic epoxidation reactions using
transition-metal
compounds
have
been
successfully
demonstrated [15], the homogeneous catalysts are somewhat
under-utilized due to difficulties in product separation from the
residual catalyst. Therefore, the immobilization of homogeneous
catalysts has attracted a lot of attention. Common polymer or
silica supported catalysts, however, give rise to poor selectivity.
Herein, core-shell MOFs have been applied as the catalyst, in
which the well-defined pores of MOFs can enhance shape and
size selectivity.
Firstly, a series of functionalized core-shell MOFs, including
PCN-222(Fe)@Zr-NDC, PCN-222(Fe)@Zr-BPDC(UiO-67), and
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the diffusion rates of the substrates and the accessibility of
catalytic centers, leading to clear size selectivity.
dramatically altered as a result of their respective steric hindrance
due to the change in window size (Figure 4). With Zr-NDC as the
shell, substrate 7 was blocked by the narrow open window,
accounting for the lower conversion. With a Zr-AZDC shell, nearly
all of the substrates can diffuse through the windows of the shell
structure to reach the catalytic center. The PXRD patterns of core-
shell MOFs recovered from catalytic reactions remained
essentially unchanged from the freshly prepared samples (Figure
S36-S38), indicating that these selective catalysts were very
stable under reaction conditions.
From Table 1, it is clear that in size, substrates 6 and 7 are
comparable to that of the open window in Zr-BPDC(UiO-67).
However, an additional methyl group in substrate 7, accounts for
a 40% decrease in conversion compared to that of substrate 6!
To further explore the reason behind this dramatic decrease in
conversion, similar epoxidation reactions were carried out by
using PCN-222(Fe)@Zr-NDC and PCN-222(Fe)@Zr-AZDC.
Consequently, the conversions of substrate 6 and 7 were
To demonstrate the generality of the kinetically controlled
strategy, three additional sets of core-shell MOFs with
mismatching lattice were constructed (PCN-134@Zr-BTB, PCN-
Table 1. PCN-222(Fe)@Zr-BPDC catalyzed epoxidation of
alkenes a
222@Nu-1000, and La-TCPP@La-BPDC, BTB
= benzene
tribenzoate). First, we show that our synthetic method can be
applied to mixed-linker MOFs and 2D-MOFs. PCN-134[16] is a
mixed-linker Zr-MOF constructed from 3-connected BTB and 4-
connected TCPP linkers with a 3D layer-pillar structure. Zr-BTB is
a 2D MOF with 6-connected Zr-cluster and BTB linker.[17] From
the optical microscopic image (Figure 5a), the core and shell
MOFs can clearly be distinguished. The cores (PCN-134)
consisted of red hexagonal crystals and the shells (Zr-BTB) were
colorless hexagonal crystals. The composition and crystallinity of
the core-shell MOF were identified by PXRD patterns (Figure 5b).
All characteristic peaks from both PCN-134 and Zr-BTB were
found in the PXRD patterns, demonstrating the highly crystalline
nature of core-shell MOF, PCN-134@Zr-BTB. The N2 adsorption
isotherm (Figure 5c) also confirm the core-shell structure of PCN-
134@Zr-BTB. In addition, two linkers with the same connectivity
can also form core-shell MOFs guided by nucleation kinetics.
PCN-222 and Nu-1000 are all based on tetratopic linkers and 8-
connected Zr-clusters. However, the nucleation PCN-222 is faster
than that of Nu-1000, which affords the formation of PCN-
222@Nu-1000 (Figures S4 and S15). More importantly, La-
TCPP@La-BPDC (Figures S5 and S14) were constructed
indicating that the synthetic strategy is not merely limited to Zr-
MOFs.[18] Similar to the PCN-222@Zr-BPDC system, the La-
TCPP was initially formed as the core, which induced the growth
of La-BPDC shell. Therefore, the kinetically controlled strategy is
a facile and general method to construct mismatching core-shell
MOFs. Nevertheless, the core MOF and shell MOF are required
to have the same metal and similar synthetic conditions.
a Reaction conditions: Olefin (500 mM), PhIO (10 mM), catalyst (5 mM,
based on PCN-222(Fe)) and acetonitrile (5.0 mL) sealed in a Teflon-
lined screw cap vial were stirred at room temperature.
b Conversion was determined by 1H NMR with 1,4-dibromobenzene
as the internal standard.
In conclusion, we have synthesized a series of zirconium-
based core-shell MOF composites with mesoporous cores and
microporous shells guided by nucleation kinetic analysis. The
core-shell architectures were fully characterized by optical
microscopic images, PXRD, N2 sorption isotherms, and SEM
images. Size selective catalysis is realized in the core-shell MOFs
by judicious isoreticular expansion of the shells. The strategies
developed shall lead to facile synthesis of hybrid core-shell MOF
composite with enhanced functionalities. Based on work
presented here, core-shell MOFs with the same metal can be
synthesized through the control of nucleation kinetics. However,
as shown in Zn-MOF@Cu-MOF reported by Kitagawa and
coworkers,[13b] it is possible to extend the one-pot nucleation
kinetic control strategy to synthesize hybrid core-shell MOFs with
different kinds of metals, which will enrich the synthetic methods
of core-shell MOFs. Research along this line is currently under
way.
Figure 4. Size selective olefin epoxidation using PCN-
222(Fe)@Zr-NDC,
PCN-222(Fe)@Zr-BPDC,
PCN-
222(Fe)@Zr-AZDC, and PCN-222(Fe)
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10.1002/anie.201710019
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COMMUNICATION
Department of Energy Office of Fossil Energy, National Energy
Technology Laboratory (DE-FE0026472). S. Y. also
acknowledges the Texas A&M Energy Institute Graduate
Fellowship Funded by ConocoPhillips and Dow Chemical
Graduate Fellowship. Financial support from the Robert A. Welch
Foundation in the form of an endowed chair to H.-C.Z. is much
appreciated.
Keywords: core-shell MOF • nucleation kinetic • size-selective
catalyst • olefin epoxidation
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Acknowledgements
The gas adsorption-desorption studies of this research was
supported by the Center for Gas Separations Relevant to Clean
Energy Technologies, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences under Award Number DE-
SC0001015. Structural analyses were supported as part of the
Hydrogen and Fuel Cell Program under Award Number DE-
EE0007049. The Distinguished Scientist Fellowship Program
(DSFP) at KSU is gratefully acknowledged for supporting this
work. The authors also acknowledge the financial supports of U.S.
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10.1002/anie.201710019
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This article is protected by copyright. All rights reserved.