Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Summary

Robust functional catechol coatings were produced in one step by direct reaction of the material known as HKUST with synthetic catechols under anaerobic conditions. The formation of homogeneous coatings surrounding the entire crystal is ascribed to the biomimetic catalytic activity of Cu(II) dimers on the external surface of the crystals.

Abstract

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu₂(O₂C)₄ paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science & Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of Cu^II ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites.

Introduction

Metal-organic frameworks are a class of crystalline porous materials built from inorganic metallic components, typically named secondary building units (SBUs), held together by polytopic organic ligands through coordinative bonds. The self-assembly of the these SBUs with the organic linkers enables the formation of extended 3D porous structures with very high surface areas and promising applications in the fields of gas storage and separation^1,2, catalysis and sensing³. However, the main limitation for their applicability is their poor stability in water^4,5as most of them incorporate divalent metals in their structure that results in labile coordination bonds, as those encountered in classical materials like MOF-5⁶or HKUST⁷.

Common approaches to solve this problem involve on the one hand, the creation of stronger coordination bonds by the use of highly charged metals, such as Zr or Ti(IV), basic N-donor ligands^7,8 or ligands incorporating acids and basic sites⁹. However, this method is limited to new materials and does not allow to enhance the stability of MOFs already available. On the other hand, the approaches to improve the stability of the already known materials use the post-synthetic modification methods to introduce hydrophobic moieties in the empty space by post-synthetic modification of the linker^10,11 or by chemical vapour deposition (CVD)¹². Unfortunately, the stability of these methods comes at the expenses of a drastic reduction in the porosity of the material and the use of sophisticated instrumentation. The recent use of modified phosphonic acids, such as 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA)¹³ or n-octadecylphosphonic acid (OPA)¹⁴, to impart hydrophobicity in known Zr(IV) MOFs should also be highlighted.

Catechol compounds, such as dopamine, have been extensively used to functionalize a broad range of materials through the formation of polydopamine¹⁵. However, the formation of these coatings is limited to the use of aqueous buffered solutions for slightly basic solutions which are not suitable for MOFs with labile bonds. Bortoluzzi et al. recently reported that polydopamine can be produced in solution by a binuclear Cu(II) complex featuring Cu₂(µ-O) as a catalytic¹⁶ centre which displays catecholase-like catalytic activity reminiscent of natural enzymes such as catechol oxidase¹⁷ and tyrosinase¹⁸. More recently, we have shown how a MOF based on Cu(II) paddle-wheel SBUs connected through trimesate linkers, known as HKUST, can be protected from hydrolytic degradation by the polymerization of functionalized catechols, such as 4-hepatdecyl-catechol (hdcat) or fluorinated-4-undecylcatechol (fdcat), on the surface of the crystals¹⁹. This simple method proves how efficient functional coatings can be synthesized under mild conditions regardless of the functionality of the catechol and without the use of buffer solutions that could compromise the stability of the framework, due to the biomimetic catalytic activity of the Cu(II) units. We believe that this new method could enable the formation of functional coatings that, besides protecting from hydrolytic degradation, might enable selective adsorption of chiral molecules or volatile organic compounds.

Protocol

1 . Synthetic Procedure of hdcat@HKUST

NOTE: The entire process must be performed inside a glove-box in order to avoid any contact with the ambient moisture. Accordingly, all the reagents and solvents used must be dry and stored in the glove-box.

1. Bring an open 4 mL glass vial, two spatulas and a 1 mL micropipette into the glove-box.
2. Transfer 50 mg of hdcat into the glass vial.
   NOTE: In some cases, an anti-static gun may be necessary in order to avoid the undesirable effects of static electricity.
3. Place 1 mL of anhydrous chloroform in the glass vial containing hdcat.
   NOTE: Not all hdcat may be dissolved entirely at room temperature, but it dissolves very rapidly when the vial is placed in the oven in the next steps.
4. Place 10 mg of HKUST in the chloroform solution containing hdcat and seal the vial tightly.
5. Take the vial out of the glove-box and sonicate the suspension of HKUST and hdcat in chloroform for a few seconds to homogenize the solution.
   NOTE: Do not expose the contents of the vial to ambient air as the introduction of O₂ in the reaction media could drive the polymerization of the catechol units in solution rather than on the surface of the crystals¹⁵.
6. Place the vial in the oven at 70 °C overnight. Make sure that the vial is tightly sealed in order to avoid the evaporation of the chloroform during the reaction (boiling point (CHCl₃) = 61.2 °C).
   NOTE: In some cases, a Teflon strip surrounding the screw cap may be helpful. This protocol requires a preheated oven at 70 °C. The temperature should not be higher than 70 °C, as amorphous products could be obtained otherwise.

2 . Washing Procedure of hdcat@HKUST

1. Take the vial out of the oven after being overnight at 70 °C and transfer it to the glove-box along with a 15 mL centrifuge tube.
2. Transfer the contents of the vial to the centrifuge tube inside the glove-box using fresh anhydrous chloroform.
3. Separate the coated material hdcat@HKUST by centrifugation (3354 x g, 1 min). Make sure that the centrifuge tube is tightly capped as it must be taken out of the glove-box in order to centrifuge the material.
4. Introduce the centrifuge tube rapidly in the glove-box after the centrifugation.
5. Extract the supernatant carefully using a dropper and store it in a clean 40 mL glass vial.
6. Suspend the coated material in 3 mL of anhydrous CHCl₃ in order to remove possible polymerized catechol units that are not attached to the surface of the crystals.
7. Repeat steps 2.3-2.6 three times.
8. Suspend the coated material in 3 mL of anhydrous methanol.
9. Repeat steps 2.3-2.6 three times but using anhydrous methanol in order to remove unreacted hdcat molecules.
   NOTE: Do not throw away the hdcat solutions as the product can be recovered by slow evaporation of the solutions in the glove-box and reused.
10. Transfer the washed hdcat@HKUST to a glass vial using anhydrous methanol and wait until the coated solid settles at the bottom of the vial.
11. Take out the supernatant and let the powder dry at room temperature in the glove-box.

3 . Synthetic Procedure of fdcat@HKUST

NOTE: The entire process must be performed inside a glove-box in order to avoid any contact with the ambient moisture. Accordingly, all the reagents and solvents used must be dry and stored in the glove-box.

1. Introduce an open 4 mL glass vial, two spatulas and a 1 mL micropipette into the glove-box.
2. Place 50 mg of fdcat inside the glass vial.
   NOTE: In some cases, an anti-static gun may be necessary in order to avoid the undesirable effects of the static electricity.
3. Place 1 mL of anhydrous chloroform in the glass vial containing fdcat.
   NOTE: Not all the fdcat may be dissolved entirely at room temperature, but it dissolves very rapidly when the vial is placed in the oven in the next steps.
4. Place 10 mg of HKUST in the chloroform solution containing fdcat and seal the vial tightly.
5. Take the vial out of the glove-box and sonicate the suspension of HKUST and fdcat in chloroform for a few seconds to homogenize the solution.
   NOTE: Do not expose the contents of the vial to ambient air in any case as the introduction of O₂ in the reaction media could drive the polymerization of the catechol units in solution rather than on the surface of the crystals¹⁵.
6. Place the vial in the oven at 70 °C overnight. Make sure that the vial is tightly sealed in order to avoid the evaporation of chloroform during the reaction (boiling point (CHCl₃) = 61.2 °C).
   NOTE: In some cases, a teflon strip surrounding the screw cap may be helpful. This protocol requires a preheated oven at 70 °C. The temperature should not be higher than 70 °C, as amorphous products could be obtained otherwise.

4. Washing Procedure of fdcat@HKUST

1. Take the vial out of the oven after being overnight at 70 °C and transfer it to the glove-box along with a 15 mL centrifuge tube.
2. Transfer the contents of the vial to the centrifuge tube inside the glove-box using fresh anhydrous chloroform.
3. Separate the coated material fdcat@HKUST by centrifugation (3354 x g, 1 min). Make sure that the centrifuge tube is tightly capped as it must be taken out of the glove-box in order to centrifuge the material.
4. Introduce the centrifuge tube rapidly into the glove-box after the centrifugation.
5. Extract the supernatant carefully using a dropper and store it in a clean 40 mL glass vial.
6. Suspend the coated material in 3 mL of anhydrous CHCl₃ in order to remove possible polymerized catechol units that are not attached to the surface of the crystals.
7. Repeat steps 4.3-4.6 three times.
8. Suspend the coated material in 3 mL of anhydrous methanol.
9. Repeat steps 4.3-4.6 three times but using anhydrous methanol in order to remove unreacted fdcat molecules.
   NOTE: Do not throw away the fdcat solutions as the product can be recovered by slow evaporation of the solutions in the glove-box and reused.
10. Transfer the washed fdcat@HKUST to a glass vial using anhydrous methanol and wait until the coated solid settles at the bottom of the vial.
11. Take out the supernatant and let the powder dry at room temperature in the glove-box.

Results

All the reagents and materials were stored in the glove-box and used as received without any further purification unless otherwise stated. The entire process is carried out in a glove-box in order to avoid contact with humidity that could degrade the uncoated material.

In order to ensure the reproducibility during the experiments, commercially available HKUST with an average particle size close to 40-50 µm (...

Discussion

The method reported in this work provides a simple and effective approach for the surface modification of MOF crystals by direct reaction with synthetic catechols under mild conditions regardless the functionality of the chain. Unlike the conventional approach of producing polydopamine-like coatings, this route can be performed in anhydrous and anaerobic conditions and without any base addition that could compromise the stability of the MOF. Methanol and chloroform were first chosen based on previous works

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Basolite C-300	Sigma-Aldrich	688614	Commercial HKUST
Anhydrous Methanol (99.8%)	Sigma-Aldrich	322415
Anhydrous Chloroform (>99%)	Sigma-Aldrich	288306
Mettler Toledo TGA/SDTA 851	Mettler Toledo		Thermogravimetric Analyser
Agilent Cary 630 FTIR	Agilent		FT-IR Spectrophotometer, ATR Module
PANalytical X’Pert Pro	PANalytical		Powder XRD Diffractometer
AUTOSORB-6 apparatus	Quantachrome		Nitrogen Isotherms were carried out with this equipment. Activation of the samples was carried out under dynamic vacuum at 170 °C. Performed by the technical service of Universitat d'Alacant.
K-Alpha X-ray photoelectron spectrometer system	Thermo-Scientific		Analysis were performed at the X-Ray unit of the Universitat d'Alacant
FEI Quanta 650 FEG scanning electron microscope	Fisher Scientific		Used to observe partcle morphologies and dimensions