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 the solution were prepared at one time and then distributed.

1. Add 4.72 g (20.3 mmol) of Cu(NO₃)₂·2.5H₂O to a 100 mL Erlenmeyer flask and dissolve in 60 mL of a deionized (D.I.) water and N,N-dimethylformamide (DMF) mixture (1:1, v/v), swirling the flask manually.
2. Add 1.76 g (8.38 mmol) of 1,3,5-benzenetricarboxylic acid (H₃BTC) and 22 mL of ethanol to a 50 mL Erlenmeyer flask, and stir the solution at 90 °C on a heated hotplate until dissolution.
3. Place 6 mL of solution 1.1 (solution prepared in step 1.1) into each 20 mL vial.
4. While stirring and heating, add 2.2 mL of solution 1.2 (solution prepared in step 1.2) to a vial containing solution 1.1, and immediately add 12 mL of acetic acid.
   NOTE: 12 mL of acetic acid should be added at once.
5. Close the lid of the vial and place it in a convection oven heated to 55 °C for 60 h.
6. After 60 h, quickly decant the mother liquor and wash the crystals by adding and removing fresh ethanol (sufficient volume to fill the vial) three times using a dropper.
7. For core-shell synthesis, store the cubic crystals of HKUST-1 in a 20 mL vial full of N,N-diethylformamide (DEF) solvent.

2. Synthesis of octahedral HKUST-1

1. Combine 4.72 g (20.3 mmol) of Cu(NO₃)₂·2.5H₂O and 30 mL of D.I. water in a 100 mL Erlenmeyer flask, swirl the flask to dissolve the solid, and add 30 mL of DMF after dissolving.
2. Add 3.60 g (17.1 mmol) of H₃BTC to 45 mL of ethanol in a 100 mL Erlenmeyer flask, and stir the solution at 90 °C on a heated hotplate until dissolution.
3. Place 6 mL of solution 2.1 (solution prepared in step 2.1) into each 50 mL vial.
4. While stirring and heating, add 4.5 mL of solution 2.2 (solution prepared in step 2.2) to a vial containing solution 2.1, and immediately add 12 mL of acetic acid.
   NOTE: 12 mL of acetic acid should be added at once without dividing.
5. Close the lid of the vial and place in a convection oven heated to 55 °C for 22 h.
6. After 22 h, quickly decant the mother liquor and wash the crystals by adding and removing fresh ethanol three times using a dropper.
7. For core-shell synthesis, store octahedral crystals of HKUST-1 in a 20 mL vial full of DEF solvent.

3. Synthesis of HKUST-1@MOF-5 core-shell

NOTE: The core-shell synthesis method is the same for both octahedral and cubic HKUST-1.

1. Dissolve 0.760 g (2.55 mmol) of Zn(NO₃)_2·6H₂O and 0.132 g (0.795 mmol) of terephthalic acid separately in 10 mL of DEF in a 20 mL vial, using a sonicator.
2. Mix the total volume of both solutions in a 35 mL glass jar.
3. Quickly weigh the filtered HKUST-1 crystals (5 mg), and place the crystals in the glass jar containing the mixed solution. To prevent static electricity, use a filter paper to weigh. Seal the jar tightly with a silicon cap.
4. After spreading the HKUST-1 crystals well on the bottom of the glass jar, place the jar in a convection oven and heat at 85 °C for 36 h.
5. After 36 h, quickly decant the mother liquor and wash the resultant crystals by adding and removing fresh ethanol three times using a dropper.

4. Solvent exchange of HKUST-1@MOF-5 core-shell

1. Discard the storing solvent, DEF, from the vial containing HKUST-1@MOF-5.
2. Add dichloromethane (DCM) (volume to fill the vial) into the vial and shake it manually for effective exchange.
3. Change the DCM solvent 3-4 times every 4 h.

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

Name	Company	Catalog Number	Comments
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)