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

Our protocol suggests the possibility of synthesizing diverse lattice-matched, core-shell MOFs, and can be extended to multiple pairs. Non-isostructural MOF pair can be integrated into single-crystalline core-shell MOFs with a seamless interface by the solvothermal reaction. By using MOFs with different pro-environment, core-shell materials for efficient gas separation or catalytic application can be designed and synthesized.

To begin, add 4.72 grams of copper nitrate hemi-pentahydrate to a 100-milliliter Erlenmeyer flask and dissolve in 60 milliliters of deionized water and DMF mixture. Swirl the flask manually. Place six milliliters of this solution into a 20-milliliter vial.

Next, add 1.76 grams of 1, 3, 5-benzenetricarboxylic acid and 22 milliliters of ethanol to a 50-milliliter Erlenmeyer flask, and stir the solution at 90 degrees Celsius on a heated hot plate until dissolution. Add 2.2 milliliters of this solution to the vial containing the solution prepared earlier and immediately add 12 milliliters of acetic acid. Close the lid of the vial and place it in a convection oven heated to 55 degrees Celsius for 60 hours.

Decant the mother liquor. Wash the crystals by first adding fresh ethanol to the vial using a dropper, and then removing it. Wash the crystals two more times.

For core-shell synthesis, store the cubic crystals of HKUST-1 in a 20-milliliter vial full of N, N-Diethylformamide, or DEF, solvent. Add 0.760 grams of zinc nitrate hexahydrate and 10 milliliters of DEF in a 20-milliliter vial and dissolve it using a sonicator. Similarly, dissolve 0.132 grams of terephthalic acid in 10 milliliters of DEF in a 20-milliliter vial.

Mix the total volume of both solutions in a 35-milliliter glass jar. Quickly weigh the filtered HKUST-1 crystals and place them in the glass jar containing the mixed solution. Seal the jar tightly with a silicon cap.

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 degrees Celsius for 36 hours. Decant the mother liquor and wash the resultant crystals by first adding fresh DEF to the jar, and then removing the DEF. Wash the crystals two more times.

For the solvent exchange, discard the storing solvent and DEF from the vial containing HKUST-1@MOF-5, and add dichloromethane into the vial. Shake it manually for effective exchange and change the dichloromethane solvent three to four times every four hours. The computational structure models for the HKUST-1@MOF-5 system at the 001 and 111 plane are shown here.

The optical microscope images of cubic-shaped HKUST-1, octahedral-shaped HKUST-1, and cubic and octahedral-shaped HKUST-1@MOF-5 core-shell are presented in this figure. The HKUST-1 crystal is located in the center of the colorless MOF-5 crystal, with a seamless interface to provide a core-shell structure. Photographs of HKUST-1@MOF-5 in diethylformamide and dichloromethane and the corresponding optical images of the cores-shell MOF using cubic and octahedral-shaped HKUST-1 are shown here.

The images represent the PXRD patterns of HKUST-1, HKUST-1@MOF-5 with cubic and octahedral-shaped HKUST-1, and the simulated patterns of HKUST-1 and MOF-5. These PXRD measurements proved the phase purity of the core-shell crystal. placing the glass jar in the convection oven where the expression of the core MOF is important for synthesizing a singly-grown core-shell MOF.

PCN-68@MOF-5, UiO-66@MIL-88B, and others can be synthesized by following this procedure, which means that it is expandable to other core-shell pairs. This technology can provide a systematic pathway to design and synthesize core-shell MOFs to improve performance in a specific application.