Name: synthesis of single-crystalline core-shell metal-organic frameworks
Uploaded: 2023-02-10T13:35:00
Description: Watch this Scientific Journal Video about Synthesis of Single-Crystalline Core-Shell Metal-Organic Frameworks at JoVE.com

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and the ICP (No. NRF-2020R1A2C3008908 and 2016R1A5A1009405).

CAUTION: Before conducting the experiment, thoroughly read and comprehend the material safety data sheets (MSDSs) of the chemicals used in this protocol. Wear appropriate protective gear. Utilize a fume hood for all synthesis procedures.
1. Synthesis of cubic HKUST-1
NOTE: The experimental procedure was based on a previously reported method14. For core-shell synthesis, 10 pots were synthesized at a time. Therefore, 10 pots of t...

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R., Kim, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface-deactivated+core-shell+metal-organic+framework+by+simple+ligand+exchange+for+enhanced+size+discrimination+in+aerobic+oxidation+of+alcohols.">Surface-deactivated core-shell metal-organic framework by simple ligand exchange for enhanced size discrimination in aerobic oxidation of alcohols.</a> Chemistry. 26 (34), 7568-7572 (2020).</li><li>Koh, K., Wong-Foy, A. G., Matzger, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=MOF@MOF:+microporous+core-shell+architectures.">MOF@MOF: microporous core-shell architectures.</a> Chemical Communications. (41), 6162-6164 (2009).</li><li>Luo, T. -. 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=Multivariate+stratified+metal-organic+frameworks:+diversification+using+domain+building+blocks.">Multivariate stratified metal-organic frameworks: diversification using domain building blocks.</a> Journal of the American Chemical Society. 141 (5), 2161-2168 (2019).</li><li>Tang, 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=Thermal+conversion+of+core-shell+metal-organic+frameworks:+a+new+method+for+selectively+functionalized+nanoporous+hybrid+carbon.">Thermal conversion of core-shell metal-organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon.</a> Journal of the American Chemical Society. 137 (4), 1572-1580 (2015).</li><li>Kwon, O., 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=Computer-aided+discovery+of+connected+metal-organic+frameworks.">Computer-aided discovery of connected metal-organic frameworks.</a> Nature Communications. 10 (1), 3620 (2019).</li><li>Yuan, 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=Stable+metal-organic+frameworks:+Design,+synthesis,+and+applications.">Stable metal-organic frameworks: Design, synthesis, and applications.</a> Advanced Materials. 30 (37), 1704303 (2018).</li><li>Feng, L., 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=Uncovering+two+principles+of+multivariate+hierarchical+metal-organic+framework+synthesis+via+retrosynthetic+design.">Uncovering two principles of multivariate hierarchical metal-organic framework synthesis via retrosynthetic design.</a> ACS Central Science. 4 (12), 1719-1726 (2018).</li><li>Furukawa, 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=Heterogeneously+hybridized+porous+coordination+polymer+crystals:+fabrication+of+heterometallic+core-shell+single+crystals+with+an+in-plane+rotational+epitaxial+relationship.">Heterogeneously hybridized porous coordination polymer crystals: fabrication of heterometallic core-shell single crystals with an in-plane rotational epitaxial relationship.</a> Angewandte Chemie International Edition. 48 (10), 1766-1770 (2009).</li><li>Guo, C., 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=Synthesis+of+core-shell+ZIF-67@Co-MOF-74+catalyst+with+controllable+shell+thickness+and+enhanced+photocatalytic+activity+for+visible+light-driven+water+oxidation.">Synthesis of core-shell ZIF-67@Co-MOF-74 catalyst with controllable shell thickness and enhanced photocatalytic activity for visible light-driven water oxidation.</a> CrystEngComm. 20 (47), 7659-7665 (2018).</li></ol>

In this protocol, cubic- and octahedral-shaped HKUST-1 crystals were synthesized, referring to a previously reported method14. For the synthesis of HKUST-1, H3BTC solution was added while heating and stirring the solution of Cu(NO3)2&#183;2.5H2O to prevent the precipitation of H3BTC as the temperature decreased. Subsequently, acetic acid was added immediately to prevent fast nucleation and ensure the growth of a large single crystal. As soon a...

MOF-on-MOF is a type of hybrid material comprising two or more different metal-organic frameworks (MOFs)1,2,3. Owing to the various possible combinations of constituents and structures, MOF-on-MOFs provide varied novel composites with remarkable properties, which have not been achieved in single MOFs, offering great potential in many applications4,5,6. Among the various types of MOF-on-MOFs, a core-shell structure in which one MOF surrounds another has the advantage of optimizing the characteristics of both MOFs by designing a more elaborated system5,6,7,8,9,10. Although many examples of core-shell MOFs have been reported, single-crystalline core-shell MOFs are uncommon and have been successfully synthesized mostly from isostructural pairs11,12,13. Moreover, single crystalline core-shell MOFs constructed using non-isostructural MOF pairs have seldom been reported, owing to the difficulty in selecting a pair that exhibit a well-matched crystal lattice3. To achieve seamless interfaces of the single-crystalline core-shell MOFs, a well-matched crystal lattice and chemical connection points between the two MOFs are critical. Here, the chemical connection point is defined as the spatial location where the linker/metal node of one MOF meets the metal node/linker of the second MOF through a coordination bond. In our previous reports14, the computational algorithm was used to screen for optimal targets for synthesis, and six suggested MOF pairs were successfully synthesized.
This paper demonstrates a protocol for synthesizing a single-crystalline core-shell MOF of an HKUST-1 and MOF-5 pair, which are iconic MOFs composed of totally different constituents and topologies. HKUST-1 was chosen as the core because it is more stable than MOF-5 under solvothermal reaction conditions15,16. Furthermore, because the chemical connection points between MOF-5 and HKUST-1 are well-matched in both the (001) and (111) planes, cubic and octahedral HKUST-1 crystals in which each plane is exposed were used as the core MOF. This protocol suggests the possibility of synthesizing more diverse core-shell MOFs with lattice-matching.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Acetic acid</td>
			<td>DAEJUNG</td>
			<td>1002-4400</td>
			<td>Synthesis of HKUST-1 (protocol steps 1.4, and 2.4)</td>
		</tr>
		<tr>
			<td>Copper(II) nitrate hemipentahydrate</td>
			<td>Sigma Aldrich</td>
			<td>223395-100G</td>
			<td>Synthesis of HKUST-1 (protocol steps 1.1, and 2.1)</td>
		</tr>
		<tr>
			<td>D2 PHASER</td>
			<td>Bruker AXS</td>
			<td>DOC-B88-EXS017-V3</td>
			<td>Powder X-ray diffraction&#160;</td>
		</tr>
		<tr>
			<td>Digital stirring hot plate</td>
			<td>Thermo Scientific</td>
			<td>SP131320-33Q</td>
			<td>Hotplate for heating and stirring (protocol steps 1.2, and 2.2)</td>
		</tr>
		<tr>
			<td>Direct-Q3UV water purification system</td>
			<td>MILLIPORE</td>
			<td>ZRQSVP030</td>
			<td>Deionized water (protocol steps 1.1, and 2.1)</td>
		</tr>
		<tr>
			<td>Ethyl alcohol anhydrous, 99.9%</td>
			<td>DAEJUNG</td>
			<td>4023-4100</td>
			<td>Synthesis of HKUST-1 (protocol steps 1.2, and 2.2)</td>
		</tr>
		<tr>
			<td>Forced convection oven (OF-02P/PW)</td>
			<td>JEIO TECH</td>
			<td>EDA8136</td>
			<td>Oven for heating reaction (protocol steps 1.5, 2.5, and 3.4)</td>
		</tr>
		<tr>
			<td>N,N-diethylformamide</td>
			<td>TCI</td>
			<td>D0506</td>
			<td>Synthesis of HKUST-1@MOF-5 (protocol step 3.1)</td>
		</tr>
		<tr>
			<td>N,N&#39;-Dimethylformamide</td>
			<td>DAEJUNG</td>
			<td>6057-4400</td>
			<td>Synthesis of HKUST-1 (protocol steps 1.1, and 2.1)</td>
		</tr>
		<tr>
			<td>Stereo microscopes</td>
			<td>Nikon</td>
			<td>SMZ745T</td>
			<td>Optical Microscope&#160;</td>
		</tr>
		<tr>
			<td>Terephthalic acid</td>
			<td>Sigma Aldrich</td>
			<td>185361-500G</td>
			<td>Synthesis of HKUST-1@MOF-5 (protocol step 3.1)</td>
		</tr>
		<tr>
			<td>Trimesic acid</td>
			<td>Sigma Aldrich</td>
			<td>482749-100G</td>
			<td>Synthesis of HKUST-1 (protocol steps 1.2, and 2.2)</td>
		</tr>
		<tr>
			<td>Ultrasonic cleaner</td>
			<td>BRANSONIC</td>
			<td>CPX-952-338R</td>
			<td>Sonicator with bath for dissolving solution (protocol step 3.1)</td>
		</tr>
		<tr>
			<td>Zinc nitrate hexahydrate</td>
			<td>Sigma Aldrich</td>
			<td>228737-100G</td>
			<td>Synthesis of HKUST-1@MOF-5 (protocol step 3.1)</td>
		</tr>
	</tbody>
</table>

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.

According to the two calculated structures of the HKUST-1@MOF-5 core-shell system14, in both the (001) and (111) planes, the Cu sites from the metal nodes of HKUST-1 and the oxygen sites from the carboxylates of MOF-5 are well-matched as the chemical connection points at the interface between the two MOFs (Figure 1). Therefore, cubic and octahedral crystals of HKUST-1, in which the (001) and (111) planes are exposed, respectively, were synthesized as the core MOFs for...

Watch this Scientific Journal Video about Synthesis of Single-Crystalline Core-Shell Metal-Organic Frameworks at JoVE.com

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

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. ...

Our protocol suggests the possibility of synthesizing diverse lattice-matched, core-shell MOFs, and can be extended to multiple pairs. Non-isostructural MOF pair can be integrated into single-crystalline core-shell MOFs with a seamless interface by the solvothermal reaction. By using MOFs with different pro-environment, core-shell materials for efficient gas separation or catalytic application can be designed and synthesized.To begin, add 4.72 grams of copper nitrate hemi-pentahydrate to a 100-milliliter Erlenmeyer flask and dissolve in 60 milliliters of deionized water and DMF mixture. Swirl the flask manually. Place six milliliters of this solution into a 20-milliliter vial.Next, add 1.76 grams of 1, 3, 5-benzenetricarboxylic acid and 22 milliliters of ethanol to a 50-milliliter Erlenmeyer flask, and stir the solution at 90 degrees Celsius on a heated hot plate until dissolution. Add 2.2 milliliters of this solution to the vial containing the solution prepared earlier and immediately add 12 milliliters of acetic acid. Close the lid of the vial and place it in a convection oven heated to 55 degrees Celsius for 60 hours.Decant the mother liquor. Wash the crystals by first adding fresh ethanol to the vial using a dropper, and then removing it. Wash the crystals two more times.For core-shell synthesis, store the cubic crystals of HKUST-1 in a 20-milliliter vial full of N, N-Diethylformamide, or DEF, solvent. Add 0.760 grams of zinc nitrate hexahydrate and 10 milliliters of DEF in a 20-milliliter vial and dissolve it using a sonicator. Similarly, dissolve 0.132 grams of terephthalic acid in 10 milliliters of DEF in a 20-milliliter vial.Mix the total volume of both solutions in a 35-milliliter glass jar. Quickly weigh the filtered HKUST-1 crystals and place them in the glass jar containing the mixed solution. Seal the jar tightly with a silicon cap.After spreading the HKUST-1 crystals well on the bottom of the glass jar, place the jar in a convection oven and heat at 85 degrees Celsius for 36 hours. Decant the mother liquor and wash the resultant crystals by first adding fresh DEF to the jar, and then removing the DEF. Wash the crystals two more times.For the solvent exchange, discard the storing solvent and DEF from the vial containing HKUST-1@MOF-5, and add dichloromethane into the vial. Shake it manually for effective exchange and change the dichloromethane solvent three to four times every four hours. The computational structure models for the HKUST-1@MOF-5 system at the 001 and 111 plane are shown here.The optical microscope images of cubic-shaped HKUST-1, octahedral-shaped HKUST-1, and cubic and octahedral-shaped HKUST-1@MOF-5 core-shell are presented in this figure. The HKUST-1 crystal is located in the center of the colorless MOF-5 crystal, with a seamless interface to provide a core-shell structure. Photographs of HKUST-1@MOF-5 in diethylformamide and dichloromethane and the corresponding optical images of the cores-shell MOF using cubic and octahedral-shaped HKUST-1 are shown here.The images represent the PXRD patterns of HKUST-1, HKUST-1@MOF-5 with cubic and octahedral-shaped HKUST-1, and the simulated patterns of HKUST-1 and MOF-5. These PXRD measurements proved the phase purity of the core-shell crystal. placing the glass jar in the convection oven where the expression of the core MOF is important for synthesizing a singly-grown core-shell MOF.PCN-68@MOF-5, UiO-66@MIL-88B, and others can be synthesized by following this procedure, which means that it is expandable to other core-shell pairs. This technology can provide a systematic pathway to design and synthesize core-shell MOFs to improve performance in a specific application.

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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, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

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 ...

Synthesis of Core-shell Lanthanide-doped Upconversion Nanocrystals for Cellular Applications

Lanthanide-doped upconversion nanocrystals (UCNs) have attracted much attention in recent years based on their promising and controllable optical properties, which allow for the absorption of near-infrared (NIR) light and can subsequently convert it into multiplexed emissions that span over a broad range of regions from the UV to the visible to the NIR. This article presents detailed experimental procedures for high-temperature co-precipitation synthesis of core-shell UCNs that incorporate different lanthanide ions into nanocrystals for efficiently converting deep-tissue penetrable NIR excitation (808 nm) into a strong blue emission at 480 nm. By controlling the surface modification with biocompatible polymer (polyacrylic acid, PAA), the as-prepared UCNs acquires great solubility in buffer solutions. The hydrophilic nanocrystals are further functionalized with specific ligands (dibenzyl cyclooctyne, DBCO) for localization on the cell membrane. Upon NIR light (808 nm) irradiation, the upconverted blue emission can effectively activate the light-gated channel protein on the cell membrane and specifically regulate the cation (e.g., Ca2+) influx in the cytoplasm. This protocol provides a feasible methodology for the synthesis of core-shell lanthanide-doped UCNs and subsequent biocompatible surface modification for further cellular applications.

A protocol is presented for the synthesis of core-shell lanthanide-doped upconversion nanocrystals (UCNs) and their cellular applications for channel protein regulation upon near-infrared (NIR) light illumination.

Lanthanide-doped upconversion nanocrystals (UCNs) have attracted much attention in recent years based on their promising and controllable optical ...

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.

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. ...

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 ...

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 ...