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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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=Remote+stabilization+of+copper+paddlewheel+based+molecular+building+blocks+in+metal-organic+frameworks.">Remote stabilization of copper paddlewheel based molecular building blocks in metal-organic frameworks.</a> Chemistry of Materials. 27 (6), 2144-2151 (2015).</li><li>Devic, T., Serre, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+valence+3p+and+transition+metal+based+MOFs.">High valence 3p and transition metal based MOFs.</a> Chemical Society Reviews. 43 (43), 6097-6115 (2014).</li><li>He, H., 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+Stable+Metal-Organic+Framework+Featuring+a+Local+Buffer+Environment+for+Carbon+Dioxide+Fixation.">A Stable Metal-Organic Framework Featuring a Local Buffer Environment for Carbon Dioxide Fixation.</a> Angewandte Chemie - International Edition. 57 (17), 4657-4662 (2018).</li><li>Nguyen, J. 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. A. F., 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=Dopamine+polymerization+promoted+by+a+catecholase+biomimetic+Cu+II(&#181;-OH)Cu+IIcomplex+containing+a+triazine-based+ligand.">Dopamine polymerization promoted by a catecholase biomimetic Cu II(&#181;-OH)Cu IIcomplex containing a triazine-based ligand.</a> Dalton Transactions. 45 (39), 15294-15297 (2016).</li><li>Koval, I. A., Gamez, P., Belle, C., Selmeczi, K., Reedijk, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthetic+models+of+the+active+site+of+catechol+oxidase:+mechanistic+studies.">Synthetic models of the active site of catechol oxidase: mechanistic studies.</a> Chemical Society Reviews. 35 (9), 814 (2006).</li><li>Yang, J., Cohen Stuart, M. A., Kamperman, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Jack+of+all+trades:+versatile+catechol+crosslinking+mechanisms.">Jack of all trades: versatile catechol crosslinking mechanisms.</a> Chemical Society Reviews. 43 (43), 8271-8298 (2014).</li><li>Castells-Gil, J., Novio, F., Padial, N. M., Tatay, S., Ru&#237;z-Molina, D., Mart&#237;-Gastaldo, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+Functionalization+of+Metal-Organic+Framework+Crystals+with+Catechol+Coatings+for+Enhanced+Moisture+Tolerance.">Surface Functionalization of Metal-Organic Framework Crystals with Catechol Coatings for Enhanced Moisture Tolerance.</a> ACS Applied Materials and Interfaces. 9 (51), 44641-44648 (2017).</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. 127 (49), 14951-14955 (2015).</li></ol>

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.

surface-functionalization-metal-organic-frameworks-for-improved

Research

JoVE Journal

Chemistry

Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high surface area microporous materials, such as metal-organic frameworks (MOFs), unique challenges present themselves for PECVD treatments. Herein the protocol for development of a MOF that was previously unstable to humid conditions is presented. The protocol describes the synthesis of Cu-BTC (also known as HKUST-1), the treatment of Cu-BTC with PECVD of perfluoroalkanes, the aging of materials under humid conditions, and the subsequent ammonia microbreakthrough experiments on milligram quantities of microporous materials. Cu-BTC has an extremely high surface area (~1,800 m2/g) when compared to most materials or surfaces that have been previously treated by PECVD methods. Parameters such as chamber pressure and treatment time are extremely important to ensure the perfluoroalkane plasma penetrates to and reacts with the inner MOF surfaces. Furthermore, the protocol for ammonia microbreakthrough experiments set forth here can be utilized for a variety of test gases and microporous materials.

Herein the procedures for plasma enhanced chemical vapor deposition of perfluoroalkanes on microporous materials such as metal-organic frameworks to enhance their stability and hydrophobicity are described. Furthermore, breakthrough testing of milligram quantities of samples is described in detail.

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high ...

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Synthesis of a Water-soluble Metal&#8211;Organic Complex Array

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as Ru(II), Pt(II), and Rh(III). The compound, named as a water-soluble metal-organic complex array (WSMOCA), is obtained through 1) the conventional solution-chemistry-based preparation of the corresponding metal complex monomers having a 9-fluorenylmethyloxycarbonyl (Fmoc)-protected amino acid moiety and 2) their sequential coupling together with other water-soluble organic building units on the surface-functionalized polymeric resin by following the procedures originally developed for the solid-phase synthesis of polypeptides, with proper modifications. Traces of reactions determined by mass spectrometric analysis at the representative coupling steps in stage 2 confirm the selective construction of a predetermined sequence of metal centers along with the peptide backbone. The WSMOCA cleaved from the resin at the end of stage 2 has a certain level of solubility in aqueous media dependent on the pH value and/or salt content, which is useful for the purification of the compound.

A potential general method for the synthesis of water-soluble multimetallic peptidic arrays containing a predetermined sequence of metal centers is presented.

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as ...

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks (MOFs)

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of the reaction site in metal-organic framework (MOF)&#8211;based catalysts in an enantioselective catalytic reaction system. Therefore, a method of validating the reaction site of MOF-based catalysts is necessary to investigate this issue. Substrate size discrimination by pore size was accomplished by comparing the substrate size versus the reaction rate in two different types of carbonyl-ene reactions with two kinds of MOFs. The MOF catalysts were used to compare the performance of the two reaction types (Zn-mediated stoichiometric and Ti-catalyzed carbonyl-ene reactions) in two different media. Using the proposed method, it was observed that the entire MOF crystal participated in the reaction, and the interior of the crystal pore played an important role in exerting chiral control when the reaction was stoichiometric. Homogeneity of the chiral environment of MOF catalysts was established by the size control method for a particle used in the Zn-mediated stoichiometric reaction system. The protocol proposed for the catalytic reaction revealed that the reaction mainly occurred on the catalyst surface regardless of the substrate size, which reveals the actual reaction sites in MOF-based heterogeneous catalysts. This method for reaction site validation of MOF catalysts suggests various considerations for developing heterogeneous enantioselective MOF catalysts.

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks MOFs

Here, we present a protocol for active site validation of metal-organic framework catalysts by comparing stoichiometric and catalytic carbonyl-ene reactions to find out whether a reaction takes place on the inner or outer surface of metal-organic frameworks.

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of ...

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

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. However, the synthesis of single-crystalline core-shell MOFs is very challenging, and thus a limited number of examples have been reported. Here, we suggest a method of synthesizing single-crystalline HKUST-1@MOF-5 core-shells, which is HKUST-1 at the center of MOF-5. Through the computational algorithm, this pair of MOFs was predicted to have the matched lattice parameters and chemical connection points at the interface. To construct the core-shell structure, we prepared the octahedral- and cubic-shaped HKUST-1 crystals as a core MOF, in which the (111) and (001) facets were mainly exposed, respectively. Via the sequential reaction, the MOF-5 shell was well-grown on the exposed surface, showing a seamless connect interface, which resulted in the successful synthesis of single-crystalline HKUST-1@MOF-5. Their pure phase formation was proved by optical microscopic images and powder X-ray diffraction (PXRD) patterns. This method presents the potential of and insights into the single-crystalline core-shell synthesis with different kinds of MOFs.

Here, we demonstrate a protocol for the two-step synthesis of single-crystalline core-shells using a non-isostructural metal-organic framework (MOF) pair, HKUST-1 and MOF-5, which have well-matched crystal lattices.

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. ...

A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural tunability. However, the progression of MOF-based photochemistry has been hindered by the difficulty in spectrally characterizing these materials. Given that MOFs are typically larger than 100 nm in size, they are prone to excessive light scatter, thereby rendering data from valuable analytical tools like transient absorption and emission spectroscopy nearly uninterpretable. To gain meaningful insights of MOF-based photo-chemical and physical processes, special consideration must be taken toward properly preparing MOFs for spectroscopic measurements, as well as the experimental setups that garner higher quality data. With these considerations in mind, the present guide provides a general approach and set of guidelines for the spectroscopic investigation of MOFs. The guide addresses the following key topics: (1) sample preparation methods, (2) spectroscopic techniques/measurements with MOFs, (3) experimental setups, (3) control experiments, and (4) post-run stability characterization. With appropriate sample preparation and experimental approaches, pioneering advancements toward the fundamental understanding of light-MOF interactions are significantly more attainable.

Here, we use a polymer stabilizer to prepare metal-organic framework (MOF) suspensions that exhibit markedly decreased scatter in their ground-state and transient absorption spectra. With these MOF suspensions, the protocol provides various guidelines to characterize the MOFs spectroscopically to yield interpretable data.

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural ...

Synthesis of Low-Valent MOFs from Multitopic Phosphine Linkers

Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, energy, and catalysis. Recently, low-valent MOFs (LVMOFs) have been explored for their potential use as heterogeneous catalysts, and multitopic phosphine linkers have been shown to be a useful building block for the formation of LVMOFs. However, the synthesis of LVMOFs using phosphine linkers requires conditions that are distinct from those in the majority of the MOF synthetic literature, including the exclusion of air and water and the use of unconventional modulators and solvents, making it somewhat more challenging to access these materials. This work serves as a general tutorial for the synthesis of LVMOFs with phosphine linkers, including information on the following: 1) the judicious choice of the metal precursor, modulator, and solvent; 2) the experimental procedures, air-free techniques, and required equipment; 3) the proper storage and handling of the resultant LVMOFs; and 4) useful characterization methods for these materials. The intention of this report is to lower the barrier to this new subfield of MOF research and facilitate advancements toward novel catalytic materials.

Author Spotlight: Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers  

Here, we describe a protocol for the synthesis of low-valent metal-organic frameworks (LVMOFs) from low-valent metals and multitopic phosphine linkers under air-free conditions. The resulting materials have potential applications as heterogeneous catalyst mimics of low-valent metal-based homogeneous catalysts.

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, ...

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks  

Ex situ magnetic surveys can directly provide bulk and local information on a magnetic electrode to reveal its charge storage mechanism step by step. Herein, electron spin resonance (ESR) and magnetic susceptibility are demonstrated to monitor the evaluation of paramagnetic species and their concentration in a redox-active metal-organic framework (MOF).

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs ...

Triazole and Tetrazole Functionalized Zr-MOFs Synthesis and Characterization

Synthesis of Triazole and Tetrazole-Functionalized Zr-Based Metal-Organic Frameworks Through Post-Synthetic Ligand Exchange

Metal-organic frameworks (MOFs) are a class of porous materials that are formed through coordination bonds between metal clusters and organic ligands. Given their coordinative nature, the organic ligands and strut framework can be readily removed from the MOF and/or exchanged with other coordinative molecules. By introducing target ligands to MOF-containing solutions, functionalized MOFs can be obtained with new chemical tags via a process called post-synthetic ligand exchange (PSE). PSE is a straightforward and practical approach that enables the preparation of a wide range of MOFs with new chemical tags via a solid-solution equilibrium process. Furthermore, PSE can be performed at room temperature, allowing the incorporation of thermally unstable ligands into MOFs. In this work, we demonstrate the practicality of PSE by using heterocyclic triazole- and tetrazole-containing ligands to functionalize a Zr-based MOF (UiO-66; UiO = University of Oslo). After digestion, the functionalized MOFs are characterized via various techniques, including powder X-ray diffraction and nuclear magnetic resonance spectroscopy.

Author Spotlight: Functionalizing Metal-Organic Frameworks: Advancements, Challenges, and the Power of Post-Synthetic Ligand Exchange 

Post-synthetic ligand exchange (PSE) is a versatile and powerful tool for installing functional groups into metal-organic frameworks (MOFs). Exposing MOFs to solutions containing triazole- and tetrazole-functionalized ligands can incorporate these heterocyclic moieties into Zr-MOFs through PSE processes.

Metal-organic frameworks (MOFs) are a class of porous materials that are formed through coordination bonds between metal clusters and organic ligands. ...