Structural analysis of and selective CO2 adsorption in mixed-ligand hydroxamate-based metal–organic frameworks

Koh Sugamata*, Chikaze Takagi, Keiko Awano, Teruyuki Iihama and Mao Minoura*
College of Science, Department of Chemistry, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo, 171-8501, Japan. E-mail:;

Received 24th March 2020 , Accepted 30th May 2020

First published on 1st June 2020

Two mixed-ligand metal–organic frameworks, [Zn2(BDHA)0.5(INA)3] (MOF-1: H2BDHA = benzene-1,4-dihydroxamic acid; HINA = isonicotinic acid) and [Co2(BDHA)0.5(INA)3(DMF)] (MOF-2), were solvothermally synthesized and fully characterized by single-crystal X-ray crystallography as well as N2, H2, and CO2 gas-sorption measurements. The results constitute the first detailed analysis of the bonding environment around the hydroxamates in such MOFs, which are simultaneously decorated with Lewis-basic sites from the hydroxamate moieties and metal sites predisposed for coordinative unsaturation. MOF-2 shows a desirably selective adsorption of CO2 relative to N2.

Metal–organic frameworks (MOFs) are porous coordination materials composed of organic ligands and metal ions. As their structures depend on multiple factors, including organic ligands, metal ions, and reaction conditions, MOFs have been extensively investigated in the past two decades. MOFs have been examined especially with respect to their potential to serve in the fields of gas storage and/or separation, energy storage, and catalysis.1–10 Many organic ligands in MOFs are based on carboxylates such as benzene-1,4-dicarboxylate, and the development of other organic ligands has received comparatively little attention. Another potentially interesting group of organic ligands is hydroxamates such as H2BDHA (Scheme 1), as they have been studied intensively in the context of coordination chemistry. In general, the coordination of metal ions to hydroxamic acids occurs upon deprotonation of the OH group, which induces a subsequent O,O coordination between the metal and the carbonyl oxygen atom and the deprotonated OH group. The X-ray diffraction analysis of a benzohydroxamate complex of Fe(III) revealed a chelating coordination mode between the carbonyl oxygen atom and the NHOH groups.11–14 Although MOFs with hydroxamate ligands have attracted much attention, only a very few examples have been reported so far, probably due to the difficulties associated with controlling the formation of strong reversible metal-hydroxamate coordination bonds in order to avoid the formation of amorphous materials with intrinsic disorder. Recently, two remarkable reports on hydroxamate-based MOFs have been published by the groups of Farha and Martí-Gastaldo: (i) linker-exchange synthesis of a zirconium-based MOF, wherein the benzene-1,4-dicarboxylate ligands are exchanged with benzene-1,4-dihydroxamate linkers,15 and (ii) direct solvothermal synthesis of a titanium-based MOF from benzene-1,4-dihydroxamic acid and titanium tetraisopropoxide.16 The reported Zr-based MOF exhibits improved stability upon introducing hydroxamate ligands, whose coordination properties are stronger than those of carboxylates, as the coordination moieties. Conversely, the Ti-based MOF exhibits photoelectrochemical and catalytic activity. However, the structures of these MOFs have not yet been determined unequivocally, e.g. by single-crystal X-ray diffraction analysis. On the other hand, the mixed-ligand strategy has been demonstrated to be a feasible method to obtain MOFs.17,18 Particularly, isonicotinic acid (HINA) has been widely used as an auxiliary ligand to construct MOFs.19,20 Herein, we report the synthesis of hydroxamate-based MOFs, MOF-1 and MOF-2, their detailed structural analysis by single-crystal X-ray diffraction, and their gas-sorption properties.
image file: d0dt01105c-s1.tif
Scheme 1 (a) Solvent-assisted linker exchange in Zr-based MOF UiO-66 between benzene-1,4-dicarboxylate and benzene-1,4-dihydroxamate linkers. (b) The first hydroxamate MOF obtained via direct synthesis. (c) Direct synthesis of MOF-1 and MOF-2 using mixed ligands.

Benzene-1,4-dihydroxamic acid (H2BDHA) was synthesized according to a previous report.21 MOF-1 and MOF-2 were synthesized solvothermally by treating a mixture of linkers H2BDHA and HINA with Zn(NO3)2·6H2O (MOF-1) or Co(NO3)2·6H2O (MOF-2) for 24 h in N,N-dimethylformamide (DMF) at 120 °C (Scheme 1). MOF-1 and MOF-2 were characterized by single-crystal and powder X-ray diffraction analysis, elemental analysis, thermogravimetric analysis (TGA), IR spectroscopy, and gas-sorption measurements. The single-crystal X-ray diffraction analysis of MOF-1 revealed that it crystallizes in the monoclinic space group P21/n, and the asymmetric unit is shown in Fig. 1. The crystal structure of MOF-1 consists of a binuclear [Zn2(BDHA)0.5(INA)3] unit. There are two distinct Zn ions in MOF-1. Zn1 is tetra-coordinated by four oxygen atoms from one BDHA ligand and by three oxygen atoms from three INA ligands. The four Zn1–O bonds are similar in length (1.943(3)–1.986(3) Å) and form a tetrahedron. Zn2 is hexa-coordinated by one oxygen atom from a BDHA ligand, two oxygen atoms from an INA ligand, and three nitrogen atoms from three INA ligands in a slightly distorted octahedral coordination geometry. The Zn–O distances range from 2.080(3) to 2.104(3) Å. The hydrogen bond H1⋯O8 (2.356(4) Å) enhances the stability of the crystal structure. The single-crystal X-ray diffraction analysis of MOF-2 revealed that it crystallizes in the monoclinic space group C2/c. The structure of MOF-2 consists of a binuclear [Co2(BDHA)0.5(INA)3(DMF)] complex (Fig. 2). Each Co cation in MOF-2 is hexa-coordinated. Co1 is coordinated by one oxygen atom from a BDHA ligand, three oxygen atoms from three INA ligands, and two nitrogen atoms from two INA ligands. Co2 is coordinated by two oxygen atoms from a BDHA ligand, two oxygen atoms from two INA ligands, one oxygen atom from a DMF molecule, and one nitrogen atom from an INA ligand. Therefore, MOF-2 has metal sites with potentially and easily accessible coordination sites, which might be activated by removing one of the coordinated solvent molecules from the metal center. The Co–O distances range from 2.063(3) to 2.127(3) Å, and the hydrogen bond H1⋯O4 (1.896(4) Å) enhances the stability of the crystal structure. The coordination in these MOFs is accordingly of the A type (Fig. 3). The O,μ2-O-type chelating/bridging mode is also often encountered in hydroxamate transition-metal complexes.22,23 In the majority of the hitherto reported hydroxamate metal complexes, the O,O chelating mode B was observed. For both crystals, the void volumes were calculated using PLATON.24 MOF-1 does not contain a solvent-accessible void, while MOF-2 contains a void space of 3260.4 Å3, i.e., the void space occupies 41.5% of the unit-cell volume once all solvent molecules were removed.

image file: d0dt01105c-f1.tif
Fig. 1 (a) Crystal structure of MOF-1. (b) Structure of the zinc cluster moiety of MOF-1. (c) 3-D polyhedron framework of MOF-1.

image file: d0dt01105c-f2.tif
Fig. 2 (a) Crystal structure of MOF-2. (b) Structure of the cobalt cluster moiety of MOF-2. (c) 3-D polyhedron framework of MOF-2.

image file: d0dt01105c-f3.tif
Fig. 3 Coordination modes of MOF-1 and MOF-2.

The thermal stability of MOF-1 and MOF-2 was evaluated using TGA (from r.t. to 550 °C) (Fig. S7 and S8). MOF-1 exhibits high thermal stability up to 320 °C. Conversely, the TGA curves of MOF-2 indicate a continuous mass loss below 500 °C. The first weight loss (<100 °C) is due to the removal of solvent molecules in the pores. During the second weight loss (225–250 °C), MOF-2 loses 3.5% of its mass, which corresponds to the loss of a water molecule (calcd 3.0%). This result indicates that the cobalt centers in MOF-2 are coordinated to water instead of DMF upon activation. The elemental analysis of MOF-2 also suggested the presence of coordinated water instead of DMF (cf. ESI).

To investigate the permanent porosity of MOF-1 and MOF-2, individual N2 adsorption isotherms were recorded at 77 K. MOF-1 is essentially non-porous, i.e., a significant uptake of N2, H2, or CO2 was not observed, which is consistent with the non-porous structure of MOF-1 obtained from the X-ray structural analysis. In contrast, MOF-2 is porous and able to adsorb various gases. The N2 adsorption isotherm of MOF-2 at 77 K showed a stepwise adsorption, while the desorption branch does not follow the adsorption branch, resulting in the formation of a remarkable hysteresis loop (Fig. 4). The N2 adsorption isotherms revealed the Brunauer–Emmett–Teller (BET) and Langmuir surface areas of 502 m2 g−1 and 686 m2 g−1, respectively, with a total pore volume of 0.25 cm3 g−1. The permanent porosity of MOF-2 encouraged us to further investigate the H2 and CO2 adsorption isotherms at low pressure. The H2 sorption isotherms of MOF-2 show type-I characteristics with a hysteresis, which is typical of microporous materials. The hysteresis indicates a kinetic origin of a supercritical H2 adsorption–desorption hysteresis at low temperatures.25 MOF-2 displays an uptake capacity of 107 cm3 g−1 (0.96 wt%) for H2 at 77 K and 1.0 bar, which is comparable with the capacity of previously reported MOF-5 (1.3 wt%).26 Although the specific surface area of MOF-2 is not extraordinarily high, it can store relatively large amounts of gaseous H2. This result suggests that a synergetic effect between the accessible coordinatively unsaturated metal sites and the small pore size may contribute to the retention of the H2 molecules.27 The CO2 adsorption/desorption measurements for MOF-2 indicated a maximum CO2 uptake of 49.1 cm3 g−1 (9.6 wt%) at 273 K and 1 bar, corresponding to a low-coverage isosteric heat of adsorption (Qst) of 34 kJ mol−1, which was calculated by fitting the CO2 adsorption isotherms at 258 K, 273 K, and 298 K to the van't Hoff equation (Fig S9 and S10).28 The Qst value is relatively high, but still considerably lower than those of other high-performance CO2 adsorbents such as Zeolite 13X (44–54 kJ mol−1). To predict the selectivity for a CO2/N2 binary mixture, ideal adsorbed solution theory (IAST) calculations, coupled with a dual-site Langmuir-Freundlich simulation, were performed on the basis of single-component isotherms.29 Fig. 5b shows the predicated selectivity for CO2/N2 as a function of pressure when the gas phase mole fraction is 15/85, which is a typical feed composition of flue gas. The IAST selectivity of MOF-2 for CO2 in the presence of N2 is 39 at 298 K and 100 kPa. The value of the selectivity for CO2/N2 is not as high as those of the recently reported MOFs, but higher than those of MOFs with open metal sites, such as HKUST-1.30–34

image file: d0dt01105c-f4.tif
Fig. 4 N2 and H2 sorption isotherms of MOF-2 at 77 K.

image file: d0dt01105c-f5.tif
Fig. 5 (a) Single CO2 and N2 adsorption isotherms of MOF-2 fitted with a dual-site Langmuir model (upper). (b) Calculated IAST selectivity for a range of total pressures (0–100 kPa) at a molar CO2/N2 ratio of 15/85 (bottom).

In summary, we successfully synthesized two novel mixed-ligand metal organic frameworks MOF-1 and MOF-2 from benzene-1,4-dihydroxamic acid and isonicotinic acid under solvothermal conditions. Given the moderate yet appreciable gas-adsorption performance of MOF-2, hydroxamic acid-based ligands may serve as useful ligands for MOFs with custom-tailored functionality. Moreover, the high selectivity of MOF-2 toward CO2 over N2 indicates the potential applications of this material for the separation or capture of CO2. The synthesis of hydroxamate-based MOFs using other metal ions is currently in progress in our laboratories.

Conflicts of interest

There are no conflicts to declare.


This work was supported by Nippon Soda Co., Ltd. We would like to thank Dr Nobuhiro Yasuda and Dr Kunihisa Sugimoto at JASRI and BL40XU and BL02B1 of SPring-8 (2018A1167, 2018A1405, 2018B1084, 2018B1275, 2019A1057, 2019A16772019B1129, 2019B1774, and 2019B1784).

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Electronic supplementary information (ESI) available: Synthesis and single X-ray structural analyses, powder XRD, IR, EA, TGA, and gas sorption analyses. CCDC 1971703 1971704. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/D0DT01105C

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