Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Improving water stability is critical for the integration of MOFs into chemically demanding applications. Our method can help to increase the stability of MOFs that are not stable in water. The main advantage of this technique is that not only allows the modification of the hydrophobicity of the coated material, but also allows us to have control over the functionality of the coating.

This technique takes advantage of the catalytic open metal sites present in some MOFs, which can trigger the polymerization of the catechol molecules on the surface of the crystals without affecting the overall porosity of the material. First, bring a four-milliliter glass vial, two spatulas, and a one-milliliter micropipette into a glove box. Special care must be taken in order to maintain oxygen-free reaction conditions.

Add 50 milligrams of hdcat into the glass vial. Then, add one milliliter of anhydrous chloroform to the glass vial. Following this, add 10 milligrams of HKUST to the hdcat solution, and seal the vial tightly.

After removing the vial from the glove box, sonicate the suspension of HKUST and hdcat for a few seconds to homogenize the solution. Make sure the vial is tightly sealed, and place it in an oven at 70 degrees Celsius overnight. After removing the vial from the oven, transfer it to the glove box with a 15-milliliter centrifuge tube.

In the glove box, transfer the contents of the vial to the centrifuge tube using fresh anhydrous chloroform. After removing the centrifuge tube from the glove box, separate the coated material by centrifugation at 3, 354 times g for one minute. Once the centrifuge tube is returned to the glove box, carefully extract the supernatant using a dropper and store it in a clean 40-milliliter glass vial.

Next, suspend the coated material in three milliliters of anhydrous chloroform in order to remove polymerized catechol units that are not attached to the surface of the crystals. After removing the chloroform, suspend the coated material in three milliliters of anhydrous methanol in order to remove unreacted hdcat molecules. After repeating the washing step three times, transfer the washed hdcat-HKUST to a glass vial using anhydrous methanol.

Once the coated solid has settled at the bottom of the vial, remove the supernatant and allow the powder to dry at room temperature in the glove box. Bring a four-milliliter glass vial, two spatulas, and a one-milliliter micropipette into the glove box. Add 50 milligrams of fdcat to the glass vial.

Then, add one milliliter of anhydrous chloroform to the glass vial. Next, add 10 milligrams of HKUST to the fdcat solution, and seal the vial tightly. After removing the vial from the glove box, sonicate the suspension of HKUST and fdcat for a few seconds to homogenize the solution.

Make sure the vial is tightly sealed, and place it in the oven at 70 degrees Celsius overnight. After removing the vial from the oven, transfer it to the glove box with a 15-milliliter centrifuge tube. In the glove box, transfer the contents of the vial to the centrifuge tube using fresh anhydrous chloroform.

After removing the centrifuge tube from the glove box, separate the coated material by centrifugation at 3, 354 times g for one minute. Once the centrifuge tube is returned to the glove box, carefully extract the supernatant using a dropper and store it in a clean 40-milliliter glass vial. Following this, suspend the coated material in three milliliters of anhydrous chloroform in order to remove polymerized catechol units that are not attached to the surface of the crystals.

After removing the chloroform, suspend the coated material in three milliliters of anhydrous methanol in order to remove unreacted fdcat molecules. After repeating the washing step three times, transfer the washed fdcat-HKUST to a glass vial using anhydrous methanol. Once the coated solid has settled at the bottom of the vial, remove the supernatant and allow the powder to dry at room temperature in the glove box.

The surface-modified crystals show increased hydrophobicity when soaked in water. In comparison with HKUST, which immediately sinks to the bottom of the vial, hdcat-HKUST and fdcat-HKUST can stand in water for several days without sinking. Contact angle measurements confirm their superior hydrophobicity.

The FT-IR spectrum of hdcat-HKUST shows bands corresponding to alkane C-H stretching vibrations of the hdcat alkyl chain, which are not present in HKUST. For fdcat-HKUST, alkane C-F stretching vibrations are visible in the spectrum, which are not observed in HKUST. SEM images of hdcat-HKUST and fdcat-HKUST show an external corrugated layer surrounding the crystals, which suggests an effective polymerization on the crystals while respecting their morphology.

XPS measurements show the presence of copper I in hdcat-HKUST and fdcat-HKUST, which is attributed to the reaction of the catechol moieties by copper units on the surface and subsequent polymerization. The formation of catecholate coatings on HKUST proceeded with no impact on the crystalline structure of HKUST, as confirmed by powder x-ray diffraction measurements. This was also confirmed by porosity measurements at 77 kelvin, which showed that hdcat-HKUST and fdcat-HKUST retain their surface area with minor variations after the coating process.

While attempting this procedure, it’s important to maintain oxygen-free reaction conditions, as oxygen could promote the polymerization of the catechol molecules in solution rather than on the surface of the crystals. Following this procedure, we have been able to modify the wettability of MOF materials by simple functionalization of their external surfaces. Also, this technique enables us to have control over the functionality of the coating, which enables us to have novel functionalities which were not present in the bare material, such as chiral separation or VOCs capture.