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 end....

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 u....

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
Name of Material/ Equipment	Company	Catalog number	Comments/Description
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 0.7mm 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
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