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,

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 di....

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