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. 2021 Jan 21;27(5):1724-1735.
doi: 10.1002/chem.202003304. Epub 2020 Dec 14.

Increasing the Complexity in the MIL-53 Structure: The Combination of the Mixed-Metal and the Mixed-Linker Concepts

Johannes Bitzer  1 , Milada Teubnerová  1 , Wolfgang Kleist  1
Affiliations

Increasing the Complexity in the MIL-53 Structure: The Combination of the Mixed-Metal and the Mixed-Linker Concepts

Johannes Bitzer et al. Chemistry. .

Abstract

The isoreticular mixed-component concept is a promising approach to tailor the material properties of metal-organic frameworks. While isoreticular mixed-metal or mixed-linker materials are commonly synthesized, the combination of both concepts for the development of isoreticular materials featuring both two metals and two linkers is still rarely investigated. Herein, we present the development of mixed-metal/mixed-linker MIL-53 materials that contain different metal combinations (Al/Sc, Al/V, Al/Cr, Al/Fe) and different linker ratios (terephthalate/2-aminoterephthalate). The possibility of changing the metal combination and the linker ratio independently from each other enables a large variety of modifications. A thorough characterization (PXRD, ATR-IR, TGA, 1 H NMR, ICP-OES) confirmed that all components were incorporated into the framework structure with a statistical distribution. Nitrogen physisorption measurements showed that the breathing behavior can be tailored by adjusting the linker ratio for all metal combinations. All materials were successfully used for post-synthetic modification reactions with maleic anhydride.

Keywords: breathing behavior; isoreticular; metal-organic frameworks; mixed-metal/mixed-linker; physisorption.

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Conflict of interest statement

The authors declare no conflict of interests.

Figures

Figure 1
Figure 1
Models of isoreticular a) mixed‐metal, b) mixed‐linker and c) mixed‐metal/mixed‐linker metal–organic frameworks and d) a schematic representation of the diamond‐shaped pores of the MIL‐53 structure in the np‐form.
Scheme 1
Scheme 1
Schematic representation of the synthesis of MIL‐53(Al0.8M0.2)‐NH2(X) materials (M=Sc, V, Cr, Fe).
Figure 2
Figure 2
ATR‐IR spectra of MIL‐53(Al0.8M0.2)‐NH2(X) materials. The substitution of terephthalate for 2‐aminoterephthalate could be confirmed qualitatively based on the increasing and decreasing band intensities of the corresponding benzene ring vibrations of both linkers and the amine vibration of 2‐aminoterephthalate.
Figure 3
Figure 3
Powder X‐ray diffraction patterns of MIL‐53(Al0.8M0.2)‐NH2(X) materials showing typical reflections for the MIL‐53 structure in the np‐form and, for higher amounts of terephthalate, also the lp‐form. Characteristic reflections of the lp‐form are indicated by vertical lines.
Figure 4
Figure 4
Results of the thermogravimetric analysis (a–d) and the corresponding first derivatives of the TG curves (e–h) of MIL‐53(Al0.8M0.2)‐NH2(X) materials performed in synthetic air. The thermal stability of these materials increased with an increasing amount of terephthalate in the frameworks for all metal combinations.
Figure 5
Figure 5
a) XANES, b) EXAFS and c) Fourier‐transformed EXAFS spectra of MIL‐53(Al0.8Fe0.2)‐NH2(X) materials recorded at the Fe K‐edge. The incorporation of terephthalate did not have any influence on the oxidation state of iron and had only a small influence on the local environment.
Figure 6
Figure 6
Nitrogen physisorption isotherms of MIL‐53(Al0.8M0.2)‐NH2(X) materials recorded at 77 K. The incorporation of terephthalate resulted in significant changes of the adsorption isotherms and, thus, in the breathing behavior of the MIL‐53 structures during nitrogen physisorption.
Figure 7
Figure 7
High‐resolution powder X‐ray diffraction patterns of MIL‐53(Al0.8Fe0.2)‐NH2(X) materials recorded in situ at different temperatures under nitrogen at ambient pressure (X=a) 100, b) 80, c) 60 or d) 40).
Figure 8
Figure 8
Powder X‐ray diffraction patterns of MIL‐53(Al0.8V0.2)‐NH2(X) materials recorded in situ at different temperatures under nitrogen at ambient pressure with a lab powder X‐ray diffractometer (X=a) 100, b) 80, c) 60 or d) 40).
Scheme 2
Scheme 2
Schematic representation of the post‐synthetic modification of MIL‐53(Al0.8M0.2)‐NH2(X) materials performed with maleic anhydride.
Figure 9
Figure 9
Powder X‐ray diffraction patterns of post‐synthetically modified MIL‐53(Al0.8M0.2)‐NH2(X)‐Mal materials (X=100, 80, 60, 40).
Figure 10
Figure 10
Nitrogen physisorption isotherms of post‐synthetically modified MIL‐53(Al0.8M0.2)‐NH2(X)‐Mal materials (X=100, 80, 60, 40) measured at 77 K.
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