Synthesis of Single-Crystalline Core-Shell Metal-Organic Frameworks

Summary

Here, we demonstrate a protocol for the two-step synthesis of single-crystalline core-shells using a non-isostructural metal-organic framework (MOF) pair, HKUST-1 and MOF-5, which have well-matched crystal lattices.

Abstract

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. However, the synthesis of single-crystalline core-shell MOFs is very challenging, and thus a limited number of examples have been reported. Here, we suggest a method of synthesizing single-crystalline HKUST-1@MOF-5 core-shells, which is HKUST-1 at the center of MOF-5. Through the computational algorithm, this pair of MOFs was predicted to have the matched lattice parameters and chemical connection points at the interface. To construct the core-shell structure, we prepared the octahedral- and cubic-shaped HKUST-1 crystals as a core MOF, in which the (111) and (001) facets were mainly exposed, respectively. Via the sequential reaction, the MOF-5 shell was well-grown on the exposed surface, showing a seamless connect interface, which resulted in the successful synthesis of single-crystalline HKUST-1@MOF-5. Their pure phase formation was proved by optical microscopic images and powder X-ray diffraction (PXRD) patterns. This method presents the potential of and insights into the single-crystalline core-shell synthesis with different kinds of MOFs.

Introduction

MOF-on-MOF is a type of hybrid material comprising two or more different metal-organic frameworks (MOFs)^1,2,3. Owing to the various possible combinations of constituents and structures, MOF-on-MOFs provide varied novel composites with remarkable properties, which have not been achieved in single MOFs, offering great potential in many applications^4,5,6. Among the various types of MOF-on-MOFs, a core-shell structure in which one MOF surrounds another has the advantage of optimizing the characteristics of both MOFs by designing a more elaborated system^5,6,7,8,9,10. Although many examples of core-shell MOFs have been reported, single-crystalline core-shell MOFs are uncommon and have been successfully synthesized mostly from isostructural pairs^11,12,13. Moreover, single crystalline core-shell MOFs constructed using non-isostructural MOF pairs have seldom been reported, owing to the difficulty in selecting a pair that exhibit a well-matched crystal lattice³. To achieve seamless interfaces of the single-crystalline core-shell MOFs, a well-matched crystal lattice and chemical connection points between the two MOFs are critical. Here, the chemical connection point is defined as the spatial location where the linker/metal node of one MOF meets the metal node/linker of the second MOF through a coordination bond. In our previous reports¹⁴, the computational algorithm was used to screen for optimal targets for synthesis, and six suggested MOF pairs were successfully synthesized.

This paper demonstrates a protocol for synthesizing a single-crystalline core-shell MOF of an HKUST-1 and MOF-5 pair, which are iconic MOFs composed of totally different constituents and topologies. HKUST-1 was chosen as the core because it is more stable than MOF-5 under solvothermal reaction conditions^15,16. Furthermore, because the chemical connection points between MOF-5 and HKUST-1 are well-matched in both the (001) and (111) planes, cubic and octahedral HKUST-1 crystals in which each plane is exposed were used as the core MOF. This protocol suggests the possibility of synthesizing more diverse core-shell MOFs with lattice-matching.

Protocol

CAUTION: Before conducting the experiment, thoroughly read and comprehend the material safety data sheets (MSDSs) of the chemicals used in this protocol. Wear appropriate protective gear. Utilize a fume hood for all synthesis procedures. 1. Synthesis of cubic HKUST-1 NOTE: The experimental procedure was based on a previously reported method14. For core-shell synthesis, 10 pots were synthesized at a time. Therefore, 10 pots of t…

Representative Results

According to the two calculated structures of the HKUST-1@MOF-5 core-shell system14, in both the (001) and (111) planes, the Cu sites from the metal nodes of HKUST-1 and the oxygen sites from the carboxylates of MOF-5 are well-matched as the chemical connection points at the interface between the two MOFs (Figure 1). Therefore, cubic and octahedral crystals of HKUST-1, in which the (001) and (111) planes are exposed, respectively, were synthesized as the core MOFs for…

Discussion

In this protocol, cubic- and octahedral-shaped HKUST-1 crystals were synthesized, referring to a previously reported method¹⁴. For the synthesis of HKUST-1, H₃BTC solution was added while heating and stirring the solution of Cu(NO₃)₂·2.5H₂O to prevent the precipitation of H₃BTC as the temperature decreased. Subsequently, acetic acid was added immediately to prevent fast nucleation and ensure the growth of a large single crystal. As soon a…

Materials

Acetic acid	DAEJUNG	1002-4400	Synthesis of HKUST-1 (protocol steps 1.4, and 2.4)
Copper(II) nitrate hemipentahydrate	Sigma Aldrich	223395-100G	Synthesis of HKUST-1 (protocol steps 1.1, and 2.1)
D2 PHASER	Bruker AXS	DOC-B88-EXS017-V3	Powder X-ray diffraction
Digital stirring hot plate	Thermo Scientific	SP131320-33Q	Hotplate for heating and stirring (protocol steps 1.2, and 2.2)
Direct-Q3UV water purification system	MILLIPORE	ZRQSVP030	Deionized water (protocol steps 1.1, and 2.1)
Ethyl alcohol anhydrous, 99.9%	DAEJUNG	4023-4100	Synthesis of HKUST-1 (protocol steps 1.2, and 2.2)
Forced convection oven (OF-02P/PW)	JEIO TECH	EDA8136	Oven for heating reaction (protocol steps 1.5, 2.5, and 3.4)
N,N-diethylformamide	TCI	D0506	Synthesis of HKUST-1@MOF-5 (protocol step 3.1)
N,N'-Dimethylformamide	DAEJUNG	6057-4400	Synthesis of HKUST-1 (protocol steps 1.1, and 2.1)
Stereo microscopes	Nikon	SMZ745T	Optical Microscope
Terephthalic acid	Sigma Aldrich	185361-500G	Synthesis of HKUST-1@MOF-5 (protocol step 3.1)
Trimesic acid	Sigma Aldrich	482749-100G	Synthesis of HKUST-1 (protocol steps 1.2, and 2.2)
Ultrasonic cleaner	BRANSONIC	CPX-952-338R	Sonicator with bath for dissolving solution (protocol step 3.1)
Zinc nitrate hexahydrate	Sigma Aldrich	228737-100G	Synthesis of HKUST-1@MOF-5 (protocol step 3.1)