Name: surface functionalization of metal-organic frameworks for improved moisture resistance
Uploaded: 2018-09-05T15:30:00
Description: Watch this Scientific Journal Video about Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance at JoVE.com

This work was supported by the EU (ERC Stg Chem-fs-MOF 445 714122), Spanish MINECO (Unit of Excellence MDM-2015-0538), and the Generalitat Valenciana 447 (Grant GV/2016/137). C.M.-G. and J.C.-G. thank the Spanish 448 MINECO for a Ramo&#769;n y Cajal Fellowship and FPI Scholarship 449 (CTQ2014-59209-P), respectively. N.M.P. thanks the Junta de 450 Andaluc&#237;a for a postdoctoral fellowship P10-FQM-6050. F.N. and 451 D.R.M. are also grateful to the &#64257;nancial support offered by 452 Project MAT2015-70615-R from the Spanish Government and 453 by FEDER funds. The ICN2 is funded by the CERCA programme/Generalitat de Catalunya and supported by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, grant no. SEV-2013-0295).

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.
<ol>
	<li>Bring an open 4 mL glass vial, two spatulas and a 1 mL micropipette into the glove-box.</li>
	<li>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 undesirabl...

<ol>
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G., Cohen, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Moisture-resistant+and+superhydrophobic+metal-organic+frameworks+obtained+via+postsynthetic+modification.">Moisture-resistant and superhydrophobic metal-organic frameworks obtained via postsynthetic modification.</a> Journal of the American Chemical Society. 132 (13), 4560-4561 (2010).</li><li>Sun, Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imparting+amphiphobicity+on+single-crystalline+porous+materials.">Imparting amphiphobicity on single-crystalline porous materials.</a> Nature Communications. 7, 13300 (2016).</li><li>Decoste, J. B., Peterson, G. W., Smith, M. W., Stone, C. A., Willis, C. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enhanced+stability+of+Cu-BTC+MOF+via+perfluorohexane+plasma-enhanced+chemical+vapor+deposition.">Enhanced stability of Cu-BTC MOF via perfluorohexane plasma-enhanced chemical vapor deposition.</a> Journal of the American Chemical Society. 134 (3), 1486-1489 (2012).</li><li>Wang, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface-specific+functionalization+of+nanoscale+metal-organic+frameworks.">Surface-specific functionalization of nanoscale metal-organic frameworks.</a> Angewandte Chemie - International Edition. 54 (49), 14738-14742 (2015).</li><li>Sun, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+molecular-level+superhydrophobic+external+surface+to+improve+the+stability+of+metal-organic+frameworks.">A molecular-level superhydrophobic external surface to improve the stability of metal-organic frameworks.</a> Journal of Materials Chemistry A. 5 (35), 18770-18776 (2017).</li><li>Saiz-Poseu, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Versatile+Nanostructured+Materials+via+Direct+Reaction+of+Functionalized+Catechols.">Versatile Nanostructured Materials via Direct Reaction of Functionalized Catechols.</a> Advanced Materials. 25 (14), 2066-2070 (2013).</li><li>de Oliveira, J. 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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<sup class="xre...

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 separation1,2, catalysis and sensing3. However, the main limitation for their applicability is their poor stability in water4,5as 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 ligands7,8&#160;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 linker10,11&#160;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&#160;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(&#181;-O) as a catalytic16&#160;centre which displays catecholase-like catalytic activity reminiscent of natural enzymes such as catechol oxidase17&#160;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.

<table border="0" cellpadding="0" cellspacing="0" style="width:572px;" width="573">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Basolite C-300</td>
			<td style="width:71px;">Sigma-Aldrich</td>
			<td align="right" style="width:119px;">688614</td>
			<td style="width:167px;">Commercial HKUST</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Anhydrous Methanol (99.8%)</td>
			<td style="width:71px;">Sigma-Aldrich</td>
			<td align="right">322415</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Anhydrous Chloroform (&#62;99%)</td>
			<td style="width:71px;">Sigma-Aldrich</td>
			<td align="right">288306</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Mettler Toledo TGA/SDTA 851</td>
			<td style="width:71px;">Mettler Toledo</td>
			<td style="width:119px;"></td>
			<td>Thermogravimetric Analyser</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Agilent Cary 630 FTIR</td>
			<td style="width:71px;">Agilent</td>
			<td style="width:119px;"></td>
			<td>FT-IR Spectrophotometer, ATR Module</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">PANalytical X&#8217;Pert Pro</td>
			<td style="width:71px;">PANalytical</td>
			<td style="width:119px;"></td>
			<td>Powder XRD Diffractometer</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">AUTOSORB-6 apparatus</td>
			<td style="width:71px;">Quantachrome</td>
			<td style="width:119px;"></td>
			<td>Nitrogen Isotherms were carried out with this equipment. Activation of the samples was carried out under dynamic vacuum at 170 &#176;C. Performed by the technical service of Universitat d&#39;Alacant.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">K-Alpha X-ray photoelectron spectrometer system</td>
			<td style="width:71px;">Thermo-Scientific</td>
			<td style="width:119px;"></td>
			<td>Analysis were performed at the X-Ray unit of the Universitat d&#39;Alacant</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">FEI Quanta 650 FEG scanning electron microscope</td>
			<td style="width:71px;">Fisher Scientific</td>
			<td style="width:119px;"></td>
			<td>Used to observe partcle morphologies and dimensions</td>
		</tr>
	</tbody>
</table>

surface functionalization of metal-organic frameworks for improved moisture resistance

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&#160;paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science &#38; 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 CuII 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.

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 &#181;m (<strong class="x...

Watch this Scientific Journal Video about Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance at JoVE.com

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

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.

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. ...

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.

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