Syntheses of two Mg-based metal-organic frameworks by a coordination competitive strategy and the selective CO2 capture

Citation:  Yu-Ao DONG, Zhe FENG, Dun-Ru ZHU. Syntheses of two Mg-based metal-organic frameworks by a coordination competitive strategy and the selective CO2 capture[J]. Chinese Journal of Inorganic Chemistry, 2023, 39(1): 181-190.

## 配位竞争策略制备的两个镁基金属有机骨架及其选择性CO2捕集

###### 通讯作者: 朱敦如, zhudr@njtech.edu.cn
• 基金项目:

国家自然科学基金 21476115

### English

• Recently, porous metal-organic frameworks (MOFs) have attracted much attention in recent two decades because of their potential applications in many areas including gas separation, catalysis, and proton conduction[1-4]. In particular, the potential utility of MOFs in gas storage and gas separation of important industrial gases such as CO2/CH4, C2, and C3 hydrocarbons has been demonstrated[5-8]. This is because of the designability and tunability of functional sites and the pore size/shape of MOF materials[9]. To efficiently construct MOFs for gas storage and related applications, a better understanding of the function and connectivity of the ligands is important. For instance, Kitagawa et al. developed a solid solution strategy via a fine ligand matrix for gate opening of the flexible MOFs[10-11]. Bai and co-workers reported a series of highly porous MOFs based on the ligands with inserting amide group for selective CO2 capture[12-13]. Notably, some workers prefer to adopt two or more ligands for preparing 3D porous MOFs[14]. However, understanding the coordination competition between these ligands, particularly for the matrix with sharply different sizes, remains a difficulty since the complex reactions and self-assembly of building blocks occur almost simultaneously.

To study the coordination competition, the selection of suitable solvents is important as they act as the media to dissolve the ligands and metal salts or as the templates to induce the self-assembly of the metal salts and ligands[15]. In addition, solvents may also work as a co-ligand to take part in coordination[16-17]. However, some solvents may be subjected to decompose under hydrothermal conditions[3]. For example, N, N-dimethylformamide (DMF), a commonly used solvent for hydrothermal reactions, often decomposes to the dimethylamine cation and formate anion under solvothermal conditions. It is worthwhile to note that the dimethylamine cation can be used for the charge balance of the negative MOF network[4, 18], while the small HCO2- anion can be applied as the ligand for the in-situ synthesis of MOFs. Inspired by these observations, herein we report two Mg-MOFs via coordination competitive strategy (Scheme 1). The HCO2- anions generated from DMF decomposition reacted directly with Mg2+ to form a 3D dia topological MOF, [Mg3(HCO2)6]·DMF (1). However, under the same conditions but with a competing ligand H4L (1,1': 3',1''-terphenyl-3,3'',5,5''-tetracarboxylic acid), a new 3D sra topological Mg-MOF, [Mg2(L)(H2O)3]·2H2O·2CH3CN·DMF (2), was obtained. This result indicates that the short formic acid cannot meet the coordination requirement of Mg2+ when a large-sized ligand H4L is involved. Gas adsorption studies reveal that 1 has a good ability for selective CO2 capture from CH4 contained mixture.

## Scheme 1

Scheme 1.  Syntheses of two Mg-MOFs based on coordination competitive strategy

All commercially available chemicals were of analytical grade and used without further purification. H4L was purchased from Shanghai Kaiyulin Pharmaceutical Co., Ltd. The C, H, and N elemental analyses were performed on a PerkinElmer 240 micro analyzer. The FT-IR spectra were performed on a Nicolet 380 FT-IR spectrometer with KBr pellets. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance diffractometer under Cu radiation (λ = 0.154 06 nm) at 40 kV and 30 mA in a range of 5°-40° Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449C thermal analyzer under an N2 atmosphere with a heating rate of 10 ℃·min-1.

Mg(NO3)2·6H2O (1.08 g, 4.21 mmol) and HNO3 (0.25 mL) were added into a mixed solution (7 mL) of DMF and CH3CN (5:1, V/V) and stirred for ca. 10 min at room temperature (RT). The solution was transferred and sealed in a 20 mL Teflon-lined autoclave, and then heated at 110 ℃ for 48 h. After cooling to RT, colorless crystals of 1 were isolated by filtration, washed with ethanol, and dried in air. Yield: 39.2% (based on Mg2+). Anal. Calcd. for C9H13Mg3NO13(%): C, 25.98; H, 3.15; N, 3.37. Found(%): C, 25.81; H, 3.01; N, 3.25. FT-IR (KBr discs, cm-1): 1 674 (s), 1 609 (vs), 1 353 (s), 1 096 (w), 709 (m).

Mg(NO3)2·6H2O (10.8 mg, 0.042 mmol), H4 L (8.5 mg, 0.021 mmol), and HNO3 (10 μL) were added into a mixed solution (1.5 mL) of DMF and CH3CN (5:1, V/V) and stirred for ca. 10 min at RT. The solution was transferred and sealed in a 10 mL Teflon-lined autoclave, and then heated at 110 ℃ for 48 h. After cooling to RT, colorless crystals of 2 were isolated by filtration, washed with DMF, and dried in air. Yield: 15.3% (based on H4L). Anal. Calcd. for C29H33Mg2N3O14(%): C, 50.03; H, 4.78; N, 6.36. Found(%): C, 50.25; H, 4.61; N, 6.22. FT-IR (KBr discs, cm-1): 3 443 (w), 2 931 (w), 1 663 (s), 1 506 (w), 1 398 (s), 1 252 (w), 1 100 (m), 1 021 (m), 952 (w), 862 (w), 770 (m), 732 (s).

The crystal data of the MOFs were measured on a Bruker Smart Apex Ⅱ CCD diffractometer at 298 K using graphite monochromated Mo radiation (λ = 0.071 073 nm). Data reduction was made with the Bruker Saint program. The structure was solved by direct methods and refined with the full-matrix least squares technique using the SHELXTL package[19]. The coordinates of the non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were put in calculated positions or located from the Fourier maps. DMF molecule was disordered over two sites and refined with an occupancy of 0.685(17) for C7-C9, N1, O13 and 0.315(17) for C7A-C9A, N1A, and O13A. The crystallographic data are listed in Table 1, and selected bond lengths are given in Table 2.

## Table 2

The ethanol-exchanged samples were prepared by immersing as-synthesized crystals in ethanol for 3 d to remove the DMF solvent, and the extract was decanted every 8 h and fresh ethanol was replaced. The completely activated sample was obtained by heating the ethanol-exchanged sample at 120 ℃ for 24 h under a dynamic high vacuum.

In the gas sorption measurements, ultra-high-purity grade N2, CH4 (> 99.999%), and CO2 gases (99.995%) were used throughout the adsorption experiments. Low-pressure N2, CO2, and CH4 adsorption measurements were performed on Micromeritics ASAP 2020 M+C surface area analyzer. Helium was used for the estimation of the dead volume, assuming that it is not adsorbed at any of the studied temperatures. The pore size distribution was obtained from the DFT method in the Micromeritics ASAP2020 software package based on the N2 sorption at 77 K.

High-pressure adsorption of CO2 and CH4 was measured using an IGA-003 gravimetric adsorption instrument (Hiden-Isochema, UK) over a pressure range of 0-2 000 kPa at 273 and 298 K, respectively. Before measurements, about 120 mg ethanol-exchanged samples were loaded into the sample basket within the adsorption instrument and then degassed under high vacuum at 130 ℃ for 20 h to obtain about 65 mg fully desolvated samples. At each pressure, the sample mass was monitored until equilibrium was reached (within 25 min).

Ideal adsorption solution theory (IAST) was used to predict binary mixture adsorption from the experimental pure-gas isotherms[20-21]. To perform the integrations required by IAST, the single-component isotherms should be fitted by a proper model. There is no restriction on the choice of the model to fit the adsorption isotherm, but data over the pressure range under study should be fitted very precisely. The dual-site Langmuir-Freundlich equation was used to fit the experimental data:

 $q=q_{\mathrm{m} 1} \cdot \frac{b_1 p^{1 / n_1}}{1+b_1 p^{1 / n_1}}+q_{\mathrm{m} 2} \cdot \frac{b_2 p^{1 / n_2}}{1+b_2 p^{1 / n_2}}$ (1)

where p is the pressure of the bulk gas at equilibrium with the adsorbed phase (kPa); q is the adsorbed amount of the adsorbent (mol·kg-1); qm1 and qm2 are the saturation capacities (mol·kg-1) of sites 1 and 2, respectively; b1 and b2 are the affinity coefficients (kPa-1) of sites 1 and 2, respectively; and n1 and n2 represent the deviations from an ideal homogeneous surface. The R2 values for all the fitted isotherms were over 0.999 97. Hence, the fitted isotherm parameters were applied to perform the necessary integrations in IAST.

A virial-type expression comprising the temperature-independent parameters ai and bi was employed to calculate the enthalpies of adsorption for CH4 and CO2 (at 273 and 298 K) on 1. In each case, the data were fitted using the following equation:

 $\ln p=\ln N+\frac{1}{T} \sum\limits_{i=0}^m a_i N^i+\sum\limits_{i=0}^n b_i N^i$ (2)

where p is the pressure (Torr), N is the adsorbed amount (mmol·g-1), T is the temperature (K), ai and bi are virial coefficients, and m and n represent the number of coefficients required to adequately describe the isotherms (m and n were gradually increased until the contribution of extra added a and b coefficients were deemed to be statistically insignificant towards the overall fit, and the average value of the squared deviations from the experimental ones was minimized).

 $Q_{\mathrm{st}}=-R \sum\limits_{i=0}^m a_i N^i$ (3)

where Qst is the coverage-dependent isosteric heat of adsorption and R is the universal gas constant.

Under solvothermal conditions, MOF 1 was synthesized by adding only Mg(NO3)2·6H2O to DM/CH3CN solution in the presence of HNO3. The HCO2- ligand comes from the decomposition of DMF at high temperature, autoclave high pressure, and special acid-ic conditions. This simple synthetic approach is quite different from the reported methods earlier for the formate-based MOFs (Mn2+, Co2+, and Ni2+) in which the HCO2H was directly used as an organic linker[22-26]. In addition, the present synthetic route can be easily scaled up to prepare MOF 1 in gram grade at a time. Under the same condition, MOF 2 was prepared after adding H4L and Mg(NO3)2·6H2O to DMF/CH3CN solu-tion in the presence of HNO3.

Although the crystal structure of MOF 1 is known[25], the synthetic methods are quite different. 1 crystallizes in the monoclinic P21/n space group (Fig. 1, Table 1), which is also different from another formate-based Mg-MOF with the Pbcn space group[26]. Of particular interest is that there is a pentanuclear Mg5 cluster consisting of Mg1, Mg2, Mg3, Mg3i, and Mg4 ions, which can be viewed as a [Mg4@Mg2] tetrahedron with the Mg2 ion in the center to act as a secondary building unit (SBU). The SBUs are further connected by the formate anions to form a neutral 3D dia net topology (Fig. 1e). 1 possesses 1D channels along the b-axis with a diameter of about 0.44 nm (Fig. 1d). The channels are filled with DMF molecules, which form two intermolecular hydrogen bonds with the H atoms of HCO2- anions (C2⋯O13 0.354 6(2) nm, C5iii⋯O13 0.313 1(2) nm, Fig.S1, Supporting information). Interestingly, all the H atoms of HCO2- anions point to the channels in 1 (Fig. S2), which may also provide strong interactions with CO2 after removing the DMF, reflecting high selective CO2 capture.

## Figure 1

Figure 1.  Structure of MOF 1: (a) OPTEP drawing of the asymmetric unit with 50% thermal ellipsoids probability; (b) a pentanuclear Mg5 cluster consisting of Mg1-Mg3, Mg3i, and Mg4 ions; (c) a [Mg4@Mg2] tetrahedron with the Mg2 ion in the center; (d) 1D channels along the b-axis with a diameter of about 0.44 nm; (e) corresponding dia topology

All H atoms are omitted for clarity; Symmetry code: i 3/2-x, y-1/2, 3/2-z

MOF 2 crystallizes in the monoclinic P21/c space group with relatively large unit cell parameters. In this asymmetric unit, two Mg2+ ions, one L4- ligand, and three coordinated water molecules are observed (Fig. 2a, Table 1). However, the HCO2- anion was not observed in 2, despite that the synthetic condition was the same as that of 1. This result demonstrates that there is a coordination competition between H4L and formate acid. The small-sized formate ions cannot meet the coordination requirements of Mg2+ in the presence of a large-sized H4L ligand. Further analysis shows that the Mg-O distances in both 1 and 2 are all in a normal range (0.196 4(4)-0.228 4(4) nm). In MOF 2, each L4- ligand is connected by six Mg2+ ions with a distorted [MgO6] octahedron. Due to the chelate coordination nature of two carboxylate groups in L4-, two Mg2+ ions can be viewed as a binuclear cluster, which is bridged by four different L4- ions. Interestingly, this connection mode makes 2 show the obvious 1D channels with dumbbell window aperture along the a-axis. The window size is 1.42 nm (Fig. 2d). Further packing of these channels forms a 3D porous framework (Fig. 2e). To better understand this structure, topology analysis was performed. Each L4- linker can be viewed as a 4-connected node (Fig. 2b) and the binuclear Mg2 cluster can be described as another 4-connected node (Fig. 2c). Thus, 2 shows a 3D sra topology[27-29]. In addition, the ideal porosity of 2 is as high as 49.2%, making it a highly porous MOF material.

## Figure 2

Figure 2.  Structure of MOF 2: (a) OPTEP drawing of the asymmetric unit with 30% thermal ellipsoids probability; (b) connection of L4-; (c) connection of Mg2 cluster; (d) a twisted window aperture along the a-axis with a size of 1.42 nm; (e) packing view of the 3D framework; (f) corresponding sra topology

All H atoms are omitted for clarity

PXRD patterns of as-synthesized samples were in good agreement with their simulated results, revealing the high purity of the bulk products (Fig. 3a). Additionally, activated 1 possessed identical PXRD peaks to the simulated ones, indicating good framework stability of activated 1. However, after guest removal, nearly no diffraction peaks were observed on activated 2 (Fig. 3b), reflecting that the framework of 2 collapses. In addition, the TGA curve of 1 shows that the weight loss of 18.0% between RT and 200 ℃ can be assigned to the removal of one DMF molecule (Calcd. 17.6%, Fig. 3c). For 2, the first weight loss of 28.2% from RT to 145 ℃ is ascribed to the removal of two CH3CN molecules, two lattice water molecules and one DMF molecule (Calcd. 27.5%). The second weight loss of 7.1% until 245 ℃ is ascribed to the loss of three coordinated water molecules (Calcd. 7.8%, Fig. 3d). Compared with the decomposition temperatures of 390 ℃ for 1 and 300 ℃ for 2, it is worthwhile to note that the short linker prefers to form a more stable porous MOF material.

## Figure 3

Figure 3.  PXRD patterns (a, b) and TGA curves (c, d) of MOFs 1 and 2

The permanent micro-porosity of MOFs 1 and 2 was evaluated by N2 adsorption isotherm at 77 K (Fig. 4a and S3). The N2 adsorption isotherm of 1 shows a quick uptake with a type-Ⅰ behavior at low pressure and a total uptake of 104.5 cm3·g-1 at p/p0=1. The Brunauer-Emmett-Teller (BET) and Langmuir surface areas were calculated to be 342 and 378 m2·g-1, respectively. As shown in Fig. 4b, the pore size centered at about 0.40 nm, which was very close to the value determined from the crystal structure (Fig. 1d). However, nearly negligible uptake was found in 2, which agrees with the decomposition of the framework after the removal of guest (Fig. 3b).

## Figure 4

Figure 4.  (a) N2 adsorption isotherms of MOF 1 at 77 K; (b) Pore size distribution of 1; (c) Single gas adsorption isotherms of 1; (d) IAST selectivity of 1; (e) Qst of 1 for CO2 and CH4; (f) PXRD patterns of treated 1

Due to the micro-porosity of MOF 1, pure gascomponent sorption isotherms of CO2 and CH4 were collected at 273 and 298 K, respectively (Fig. 4c). With reversible type-Ⅰ isotherms, 1 exhibited a higher CO2 uptake (mass fraction) of 2.4% (0.53 mmol·g-1) at 298 K and 15 kPa, the partial pressure of CO2 in the flue gas. This value was higher than that of NJU-Bai50 (2.11%) [27], FZU (2.01%) [30] and approaching to that of ZIF-78 (3.3%)[31]. Interestingly, by reducing the adsorption temperature to 273 K, the uptake at 15 kPa increased by more than two times (5.3%), which makes 1 a good CO2 collector. In addition, the excess CO2 uptake reached 11.7% (2.6 mmol·g-1) at 273 K and 100 kPa, while the unsaturation CO2 uptake was as high as 17.2% (3.9 mmol·g-1) at 2 000 kPa. With a nearly similar adsorption trend, the maximum CO2 uptake was about 14.7% at 298 K and 2 000 kPa. Although the CO2 uptake at 2 000 kPa was limited by the volume of the micropore, the uptake value of 1 at 100 kPa was higher than those of the known microporous MOFs[32]. However, corresponding CH4 uptakes of 1 at 2 000 kPa were only 4.4% at 273 K and 4.0% at 298 K. This adsorption difference indicates the high potential of 1 for selective CO2 capture from CH4-contained mixture.

The unique CO2 adsorption isotherms encouraged us to further examine the capacity of MOF 1 for the selective capture of CO2/CH4 at 298 K. IAST was employed to predict multi-component adsorption behaviors from the experimental pure-gas isotherms. The predicted adsorption selectivity in 1 as a function of bulk pressure is presented in Fig. 4d, S4, and S5. The equimolar selectivity of CO2 over CH4 was very sensitive to the loading, which showed two steps in the changes of selectivity: a quick decrease from 11 to 5.2 at the low-pressure region and a slow increase from 5.2 to 6.6 following the increased pressure. Interestingly, the CO2/CH4 selectivity was also sensitive to the gas ratio, particularly at high pressure. The higher the CO2 concentration was, the higher selectivity was. To understand these results, the adsorption enthalpies were calculated by the virial method (Fig. S6 and S7). 1 exhibited a strong binding affinity (33.5 kJ·mol-1) for CO2 at zero coverage, and the enthalpy of adsorption increased to 36.5 kJ·mol-1 at about 500 kPa. The initial high value indicates that there are interactions between the H atom of the HCO2- ion and CO2 molecule, while the increased values stem from pressure-driven CO2⋯CO2 interactions. However, 1 had a relatively low enthalpy of adsorption (21.5 kJ·mol-1) for CH4.

Moreover, the framework structure of MOF 1 after the adsorption measurements and water treatment for one month was still kept, confirmed by the PXRD patterns (Fig. 4f). The convenient synthesis, high stability towards the water, good selectivity, and facile regeneration make 1 a promising porous MOF material for the separation of CO2 and CH4 for long-term use.

In summary, two Mg-based MOFs 1 and 2 were prepared by using a coordination competition strategy between formic acid generated from the decomposition of DMF and 1,1': 3',1''-terphenyl-3,3'', 5,5''-tetracarboxylic acid. MOF 1 possesses a 3D dia topological network and has a 1D channel, while MOF 2 has a unique binuclear Mg2 cluster, yielding a 3D sra topology network. These results demonstrate that ligands with the same coordinating groups and different sizes are difficult to be compatible with when reacting with Mg2+ ions. In addition, with good water stability, 1 exhibited quick CO2 uptake and good selectivity for CO2/CH4 separation in a wide pressure range at 298 K. This work permits us to envision that coordination competition strategy may be an important method for the design and preparation of MOF materials in the future.

Supporting information is available at http://www.wjhxxb.cn

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• Scheme 1  Syntheses of two Mg-MOFs based on coordination competitive strategy

Figure 1  Structure of MOF 1: (a) OPTEP drawing of the asymmetric unit with 50% thermal ellipsoids probability; (b) a pentanuclear Mg5 cluster consisting of Mg1-Mg3, Mg3i, and Mg4 ions; (c) a [Mg4@Mg2] tetrahedron with the Mg2 ion in the center; (d) 1D channels along the b-axis with a diameter of about 0.44 nm; (e) corresponding dia topology

All H atoms are omitted for clarity; Symmetry code: i 3/2-x, y-1/2, 3/2-z

Figure 2  Structure of MOF 2: (a) OPTEP drawing of the asymmetric unit with 30% thermal ellipsoids probability; (b) connection of L4-; (c) connection of Mg2 cluster; (d) a twisted window aperture along the a-axis with a size of 1.42 nm; (e) packing view of the 3D framework; (f) corresponding sra topology

All H atoms are omitted for clarity

Figure 3  PXRD patterns (a, b) and TGA curves (c, d) of MOFs 1 and 2

Figure 4  (a) N2 adsorption isotherms of MOF 1 at 77 K; (b) Pore size distribution of 1; (c) Single gas adsorption isotherms of 1; (d) IAST selectivity of 1; (e) Qst of 1 for CO2 and CH4; (f) PXRD patterns of treated 1

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• 发布日期:  2023-01-10
• 收稿日期:  2022-08-20
• 修回日期:  2022-10-06
###### 通讯作者: 陈斌, bchen63@163.com
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沈阳化工大学材料科学与工程学院 沈阳 110142

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