Synthesis and Characterization of Functionalized Metal-organic Frameworks

Summary

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.

Abstract

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.

Introduction

Metal-organic frameworks (MOFs) are a class of crystalline coordination polymers consisting of metal-based nodes (e.g., Zn²⁺, Zn₄O⁶⁺, Zr₆O₄(OH)₄¹²⁺, Cr₃(H₂O)₂OF⁶⁺, Zn₂(COO)₄) connected by organic linkers (e.g., di-, tri-, tetra- and hexacarboxylates, imidazolates¹, dipyridyls; see Figure 1).² Their highly ordered (and thus amenable to high levels of characterization) structures, combined with their exceptional surface areas (reaching 7,000 m²/g)³ endow them with the potential as attractive candidates for a slew of applications, ranging from hydrogen storage⁴ and carbon capture^5,6 to catalysis,^7,8 sensing^9,10 and light harvesting.¹¹ Not surprisingly, MOFs have elicited a great amount of interest in the science and materials engineering communities; the number of publications on MOFs in peer-reviewed journals has been increasing exponentially over the past decade, with 1,000-1,500 articles currently being published per year.

The synthesis of MOFs with desirable properties, however, poses a series of challenges. Their principal point of attraction, namely their exceptional porosity, in fact may present, for specific MOFs, one of the greatest obstacles towards their successful development. The large empty space present within the frameworks of these materials detracts from their thermodynamic stability; as a result, when MOFs are synthesized de novo (i.e., by solvothermally reacting the metal precursors and organic linkers in one step), their constituent building blocks often tend to assemble into denser, less-porous (and less desirable for some applications such as gas storage) analogues.¹² After the procedure to reproducibly obtain the framework of desirable topology has been developed, the MOF needs to be treated in order to enable its application in processes that require gas sorption. Since MOFs are synthesized in a solution, the cages and channels of the newly grown MOF crystals are typically full of the high-boiling solvent used as the reaction medium; the removal of the solvent without inducing the collapse of the framework under the capillary forces requires a series of specialized procedures known as “MOF activation”.¹³ Finally, to ensure the purity of the end product and to enable conclusive studies of fundamental properties, MOFs need to be rigorously characterized upon their synthesis. Given the fact that MOFs are coordination polymers, which are highly insoluble in conventional solvents, this process often involves several techniques developed especially for this class of materials. Many of these techniques rely on X-ray diffraction (XRD), which is uniquely suited to provide high-level characterization of these crystalline materials.

Typically, MOF synthesis in the so-called de novo fashion employs one-pot solvothermal reactions between the metal precursors (inorganic salts) and the organic linkers. This method suffers from multiple limitations, as there is little control over the arrangement of the MOF components into the framework, and the resulting product does not always possess the desired topology. An easy to implement approach that allows circumventing the problems associated with the de novo MOF synthesis is solvent-assisted linker exchange (SALE, Figure 2).^14-16 This method involves exposing easily obtainable MOF crystals to a concentrated solution of the desired linker, until the daughter linkers completely replace those of the parent. The reaction proceeds in a single crystal-to-single crystal fashion — that is, despite the replacement of the linkers within the framework, the material retains the topology of the original parent MOF. SALE essentially allows synthesis of MOFs with linker-topology combinations that are difficult to access de novo. So far, this method has been successfully implemented to overcome various synthetic MOF challenges, such as control over catenation,¹⁷ expansion of MOF cages,^18,19 synthesis of high energy polymorphs²⁰, development of catalytically active materials^20,21 and site-isolation to protect reactive reagents.²²

Freshly synthesized MOFs almost always have channels filled with the solvent used during their synthesis. This solvent needs to be removed from the frameworks in order to take advantage of their gas sorption properties. Conventionally, this is achieved by a) exchanging the solvent in the channels (usually a high boiling solvent like N,N’-dimethylformamide, DMF) with a more volatile solvent like ethanol or dichloromethane by soaking the MOF crystals in the solvent of choice, b) heating the MOF crystals under vacuum for prolonged times to evacuate the solvent, or c) a combination of these two techniques. These activation methods are, however, not suitable for many of the high-surface thermodynamically fragile MOFs that may suffer from framework collapse under such harsh conditions. A technique that allows solvent removal from the cages of the MOF, while avoiding the onset of extensive framework collapse, is activation through supercritical CO₂ drying.²³ During this procedure, the solvent inside the MOF structure is replaced with liquid CO₂. The CO₂ is subsequently heated and pressurized past its supercritical point, and eventually allowed to evaporate from the framework. Since supercritical CO₂ does not possess capillary forces, this activation treatment is less forcing than conventional vacuum heating of MOFs, and has enabled access to most of the ultrahigh Brunauer-Emmett-Teller (BET) surface areas that have been published so far, including the MOF with the champion surface area.^3,24,25

In this paper, we describe the de novo synthesis of a representative easily accessible MOF that serves as a good template for SALE reactions — the pillared-paddlewheel framework Br-YOMOF.²⁶ Its long and relatively weakly bound N,N’-di-4-pyridylnaphthalenetetracarboxydiimide (dpni) pillars can be readily exchanged with meso-1,2-di(4-pyridyl)-1,2-ethanediol (dped) to produce an isostructural MOF SALEM-5 (Figure 2).¹⁸ Furthermore, we outline the steps that need to be taken to activate SALEM-5 by supercritical CO₂ drying and to successfully collect its N₂ isotherm and obtain its BET surface area. We also describe various techniques pertinent to MOF characterization, such as X-ray crystallography and ¹H NMR spectroscopy (NMR).

Protocol

1. Synthesis of the Parent MOF (Br-YOMOF)

1. Weigh out 50 mg Zn(NO₃)₂ × 6 H₂O (0.17 mmol), 37.8 mg dpni (0.09 mmol) and 64.5 mg 1,4-dibromo-2,3,5,6-tetrakis-(4-carboxyphenyl)benzene (Br-tcpb, 0.09 mmol). Combine all solid ingredients in a 4-dram vial.
2. Add 10 ml of DMF measured with a graduated cylinder to the vial with the solid ingredients. Then, using a 9’’ Pasteur pipet, add one drop (0.05 ml) of concentrated HCl (CAUTION! Corrosive to eyes, skin and mucous membrane. Handle with gloves.).
3. Tightly cap the vial and thoroughly mix the ingredients using an ultrasonication bath for ~15 min. Observe the contents of the vial as they form a suspension.
4. Place the vial in an oven at 80 ºC for two days. On day 1, check the vial to ensure that its contents have completely dissolved, forming a yellow clear solution. On day 2, observe yellow tear-shaped crystals on the walls and the bottom of the vial. Following the formation of the crystals, remove the vial from the oven.
5. Allow the vial to cool to RT. Then use a spatula to gently push the crystals off the vial walls, so that they all collect on the floor of the vial. Let the vial stand for ~5 min to ensure that all the crystals have settled on the floor.
6. Using a 9’’ Pasteur pipet, gently remove the reaction solution from the vial, while avoiding sucking up the crystals into the pipet. Leave just enough solution so that the crystals are completely covered, to prevent the framework from drying out.
7. Add ~5 ml fresh DMF to the vial with the crystals. Soak the MOF crystals in fresh DMF for at least one day in order to remove the acidic reaction solution and any unreacted ingredients trapped in the MOF pores. For best results, periodically replace the DMF with fresh batches (~3 times over the course of the first hour, then every 6-12 hr).
8. Store the Br-YOMOF crystals in DMF at RT until further use.

2. Characterization by Powder X-ray Diffraction (PXRD)

1. Prepare a 0.7 mm diameter borosilicate glass capillary for the experiment by carefully cutting off the closed end so that the top 3 cm of the capillary (with the funnel top) remain.
2. Heat regular beeswax until it is melted and dip the narrow (cut) end of the capillary into the melted wax. Remove the capillary and let the wax solidify as a plug in the bottom of the capillary.
3. Support the capillary in a small amount of modeling clay.
4. Using a Pasteur pipet, draw up several milliliters of crystals in solution. Carefully transfer the crystals and solution to the capillary though the funnel opening. Use a paper towel or tissue to wick away excess solvent. Avoid spilling solvent or crystals on the outside of the capillary.
5. Allow the crystals to settle into a small plug (approximately 2-5 mm in length). Use a very small piece of modeling clay to seal the top (funnel) end of the capillary.
6. Remove any mounting devices from the goniometer head (brass pins, magnetic mounts, etc.) and place your capillary supported by modeling clay on top of the goniometer head.
7. Center the capillary in the X-ray beam to ensure that the plug of crystals does not precess as it rotates.
   NOTE: The volume of crystalline material will exceed the beam size of most standard laboratory X-ray sources.
8. Using your diffractometer’s software, prepare a series of 180° φ scans, in overlapping increments of 2θ. For example, using a kappa-geometry diffractometer equipped with an Apex2 detector set at 150 mm (dx), we collect a series of 10-sec, 180° φ scans with the parameters as per Table 1.
9. Once the frames have been collected, use your diffractometer’s software to combine all images and integrate over the resultant diffraction pattern.

3. Performing Solvent-assisted Linker Exchange (SALE) on Br-YOMOF Crystals

1. Weigh out 21 mg of dped (0.095 mmol) and dissolve it in 5 ml DMF in a 2-dram vial with ultrasonication.
2. Using a 6’’ Pasteur pipet, collect the Br-YOMOF crystals and filter them on a Buchner funnel. Then weigh out ~30 mg of the crystals; return the rest of the crystals to the vial with Br-YOMOF.
3. Disperse the crystals in the previously prepared dped solution. Place the resulting SALE mixture in an oven at 100 ºC for 24 hr.
4. On the next day, check the progress of the SALE reaction with ¹H NMR. With a spatula or a 6’’ Pasteur pipet, remove approximately 2-5 mg of the MOF crystals from the reaction DMF solution. Rinse these crystals by submerging them in a small amount of clean solvent (low boiling solvent such as dichloromethane, or same solvent as the reaction medium — in this case DMF) in a 1.5-dram vial.
5. Add ~1 ml deuterated dimethyl sulfoxide (d₆-DMSO) in a separate 1.5-dram vial. Filter out the crystals from the cleaning solution and disperse them in d₆-DMSO. Dissolve the crystals by adding 3 drops of deuterated sulfuric acid (D₂SO₄) to the mixture. Thoroughly sonicate the capped vial to obtain a homogeneous solution.
6. Transfer the resulting NMR sample to an NMR tube with a Pasteur pipet and collect the NMR spectrum. Perform 64 scans, since the solution is relatively dilute due to the low solubility of the MOF crystals.
7. Interpret the spectrum by verifying that all the dpni has been replaced by dped, and that the dped:Br-tcpb ratio is 1:1.
   NOTE: If dpni is still present in the crystals, return the vial with the reaction mixture to the oven and keep monitoring the reaction with ¹H NMR until the desired product is obtained.
8. If all dpni has been replaced by dped, stop the reaction by decanting the reaction solution with a 9’’ Pasteur pipet and replacing it with fresh DMF. Perform additional characterization of the SALEM-5 crystals by collecting their PXRD pattern; then store the crystals in DMF until further use.

4. Activating SALEM-5 Crystals with Supercritical CO₂ Drying

1. Prior to activation, exchange all the DMF from the MOF cages with ethanol, which is miscible with liquid CO₂ and compatible with the supercritical dryer. Perform solvent replacement by decanting the DMF from the MOF vial with a 9’’ Pasteur pipet and replacing it with a small amount of ethanol (enough to completely submerge the crystals).
2. Continue the solvent exchange for 3 days, replacing the ethanol with a fresh batch every day. Ensure that all the DMF has been removed from the crystal pores by collecting a ¹H NMR spectrum of the crystals.
3. Check that a tank with sufficient liquid CO₂ is connected to the supercritical dryer.
4. Transfer the MOF crystals to an activation dish using a 6’’ Pasteur pipet. Then remove as much of the ethanol as possible with a 9’’ Pasteur pipet while avoiding sucking the crystals up into the pipet.
5. Remove the lid of the activation chamber by unscrewing the three bolts and inspect the chamber for residual MOF debris (if those are present, wipe the chamber clean with a Kimwipe). Using a pair of forceps, insert the activation dish with the MOF into the chamber and screw the lid back into its place.
6. Turn the dryer on and open the CO₂ tank. Adjust the temperature knob to achieve a temperature between 0 and 10 ºC. Maintain that temperature range throughout the activation process to keep CO₂ in its liquid state.
7. Once the temperature is in the correct range, turn up the “fill” knob slowly. Observe liquid CO₂ pouring into the activation dish through the glass window on the chamber lid. At the same time the pressure reading on the gauge should increase until it reaches 800 psi.
8. Perform the first “purge”, that is the first replacement of the activation solvent with a fresh batch. First turn the “fill” knob up to the mark that reads 15. Then slowly turn up the “purge” knob until a jet of solvent shoots out from the tube on the side of the instrument. Let the purge go on for ~5 min; then close the “purge” knob and turn the “fill” knob down to the mark that reads 5.
9. Continue the supercritical drying for 8 hr, performing a “purge” every 2 hr.
10. After 8 hr, turn all the knobs off and flip the “heat” switch on. Wait until the temperature and pressure exceed the supercritical point (31 ºC and 1,070 psi).
11. Connect a flowmeter to the tube on the side of the instrument and open the “bleed” knob. Adjust the flow to 1 cm³/min; then remove the flowmeter and let the CO₂ slowly bleed from the sample (which typically takes place O/N).
12. The next day, check that the pressure has dropped to 0 psi; if it has not, turn up the “bleed” knob until you achieve the desired pressure drop. Close the “bleed” knob and turn off the “heat” and power switches on the instrument.
13. Remove the sample from the activation chamber. Cap the activation dish tightly and wrap it in Parafilm. Store the activated SALEM-5 in a glove box until further use. Make sure that no ethanol is present in the sample by collecting a ¹H NMR spectrum.

5. Collecting a N₂ Isotherm of the MOF to Obtain Its BET Surface Area

1. Obtain a sorption tube equipped with a filler rod and a seal frit and accurately weigh it. Weigh it at least two times and make sure the two balance readings agree with each other within ±0.01 mg.
2. Transfer the pre-weighed tube to the glove box and load the activated SALEM-5 sample inside the tube. We recommend using a clean, dry funnel, as activated MOF samples stored in the glove box are often electrostatically charged and difficult to handle. Remove the seal frit and the filler rod from the tube and fit it with a funnel; then rapidly invert the activation dish over the funnel, making sure the sample slides down the tube.
3. Reinsert the filler rod and the seal frit into the tube loaded with SALEM-5. Slide the filler rod slowly and carefully into the tube to avoid spilling the sample and/or breaking the tube. Remove the tube from the glove box.
4. Weigh the loaded tube accurately, using the same technique (and the same balance) that you used to load the empty tube.
5. Place an isothermal jacket on the tube, load the tube on the sorption instrument and set up the sorption file by entering the masses of the empty tube and the tube with the sample. Adjust the file to evacuate the sample on the instrument for 1 hr prior to the collection of the isotherm.
6. Ensure that the Dewar used to store the liquid nitrogen is free from ice and/or water by filling it with water to allow the ice to thaw and then wiping it dry. Fill the Dewar with liquid nitrogen to the appropriate mark and start the measurement. The measurement lasts anywhere from 4 to 12 hr, depending on the amount and the porosity of the material.

Results

The use of HCl during MOF synthesis is often beneficial for the growth of high quality MOF crystals. As it slows down the deprotonation of the carboxylate (and the binding of the linkers to the metal centers), it promotes growth of larger crystals and prevents formation of amorphous and polycrystalline phases, which may form if the reaction is allowed to proceed more rapidly. In fact, as it can be seen in Figure 3, the pillared-paddlewheel MOFs that are produced during this reaction form large, yellow cr...

Discussion

MOF crystallization is a delicate procedure that can be inhibited by even slight variations in the multiple parameters that describe the synthetic conditions. Therefore, special care needs to be taken when preparing the reaction mixture. The purity of the organic linkers should be confirmed by ¹H NMR prior to the onset of the synthesis, as the presence of even small amounts of impurities is known to prevent crystallization altogether or result in the formation of undesired crystalline products. Polar, high-boi...

Materials

Name	Company	Catalog Number	Comments
6’’ Pasteur pipet	VWR	14673-010	For transferring MOF crystals
9’’ Pasteur pipet	VWR	14673-043	For separating liquid solution from MOF crystals
1-dram vials	VWR		For preparation of NMR samples
2-dram vials	VWR	66011-088	For small-scale SALE reactions
4-dram vials	VWR	66011-121	For de novo pillared-paddlewheel MOF synthesis
NMR tube Grade 7	VWR	897235-0000
NMR instrument Avance III 500 MHz	Bruker	N/A
Oven	VWR	414004-566	For solvothermal MOF reactions
Sonicator	Branson	3510-DTH
Balance	Mettler-Toledo	XS104
Superctitical CO2 dryer	Tousimis™ Samdri®	8755B	For activation of pillared-paddlewheel MOFs
Activation dish	N/A	N/A
Tristar II 3020	Micromeritics	N/A	For collection of gas isotherms/measurement of BET surface area
X-ray diffractometer	Bruker	N/A	Kappa geometry goniometer, CuKα radiation and Powder-diffraction data collection plugin.
Capillary tubes	Charles-Supper	Boron-Rich BG07	Thin walled Boron Rich capillary 0.7 mm diameter
Beeswax	Huber	WAX	sticky wax for specimen fixation
Modeling Clay	Van Aken	Plastalina
CO2 (l)	N/A	N/A
N2 (l)	N/A	N/A
N2 (g)	N/A	N/A
DMF	VWR	MK492908	For MOF reactions and storage
Ethanol	Sigma-Aldrich	459844	For solvent exchange before supercritical drying
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Zn(NO3)2 × 6 H2O	Fluka	96482
dped	TCI	D0936
dpni			Synthesized according to a published procedure
Br-tcpb			Synthesized according to a published procedure
D2SO4	Cambridge Isotopes	DLM-33-50	For MOF NMR
d6-DMSO	Cambridge Isotopes	DLM-10-100	For MOF NMR