A Sulfonate-based Metal-organic Framework for the Transformation of CO2 and Epoxides into Cyclic Carbonates①

XUE Huan LI Tao YIN Qi HUANG Ge LIU Tian-Fu②

a (College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT An interesting metal-organic framework (MOF) PFC-31 was synthesized by a simple solvothermal reaction. X-ray crystallography reveals that PFC-31 crystallizes in monoclinic system, space group P21/c with a = 6.8410(17), b = 20.123(5), c = 7.689(2) Å, β = 104.239(7)°, V = 1026.0(5) Å3, Z = 2, C16H22CuN4O8S2, Mr = 526.03, μ = 1.322 mm–1, Dc = 1.703 g/cm3, F(000) = 542, GOOF = 1.214, R = 0.0975 and wR = 0.2324. According to X-ray analysis, PFC-31 shows one-dimensional chain constructed by Cu(II) centers coordinating with two pyridine groups, two sulfonate groups and two DMF molecules. Owing to the orderly functionalized sulfonate group and Lewis metal sites, PFC-31 exhibits good catalytic capability for CO2 cycloaddition of epoxide under normal pressure and temperature.

Keywords: metal-organic frameworks, sulfonate group, Lewis metal sites, cycloaddition;

1 INTRODUCTION

Metal-organic frameworks (MOFs) are a type of porous crystalline materials composed of organic ligands and metal ions/clusters[1-4]. Due to their tunable structures, permanent porosities[5], and large surface areas[6], MOFs have received considerable attention for many applications in such as heterogeneous catalysis[7,8], gas storage[9,10], and sensing[11,12]. Among them, the application of MOFs in CO2cycloaddition has been widely reported[13-15]. The transformation of greenhouse carbon dioxide (CO2) into other value-added chemicals such as cyclic carbonates is one of the most promising approaches for green chemistry[16-19]. This is consistent with the concept of sustainable development advocated by today's society.

In 2012, Macias et al. used Cu3(BTC)2as a catalyst for synthesizing chloropropene carbonate from CO2and epichlorohydrin. They proposed that the Lewis acid copper(II) sites in structure not only increased the carbon dioxide adsorptive capacity but also promoted the conversion of carbon dioxide to carbonate[20]. In 2016, Babu et al. reported that, at room temperature, an amine-functionalized micro- mesoporous MOF (UMCM-1-NH2) exhibits higher reactivity for converting CO2and epoxies to cyclic carbonates than the corresponding MOF (UMCM-1) without amine groups, showing that both acidity/basicity (including metal-oxide and-NH2) played important roles in this catalytic reaction[19]. These studies indicate that the introduction of suitable Lewis acids/base and high CO2-affinity groups is critical to the activity of MOF catalysts.

Herein, an interesting Cu-MOF with orderly functionalized sulfonate groups was synthesized by a simple solvothermal method. Each Cu(II) center is composed of two pyridine groups, two sulfonic groups and two DMF molecules. The obtained PFC-31 (PFC stands for the Porous materials from Fjirsm, CAS) exhibits catalytic activity toward CO2cycloaddition of epoxide to cyclic carbonates at room temperature and atmospheric pressure. Among them, PFC-31 shows low conversion but high selectivity toward some substrate with large steric hindrance, which is probably caused by the high affinity between the aromatic substrates and catalyst due to the possible p/p, cationic/p and hydrogen bonding interaction.

2 EXPERIMENTAL

2. 1 Materials and instruments

Unless otherwise mentioned, all reagents and solvents were purchased from commercial sources and used as received without further purification. Single-crystal X-ray diffraction data were collected at 100 K on a Bruker D8 Venture diffractometer using MoKα radiation (λ = 0.71073 Å). PXRD was performed on Rikagu Miniflex 600. TGA was collected by a Seiko S- Ⅱthermogravimetric analyzer from room temperature to 1000 ℃ with a heating rate of 5 ℃/min and then cooled to room temperature in N2atmosphere. The CO2gas isotherms of the samples were measured through ASAP 2020 from Micromeritics Co. Ltd. The constituents of product were determined by gas chromatography-mass spectrometry (Varian 450-GC/240-MS), and the conversion and yield were calculated based on the gas chromatography analysis (G7890A-GC).

2. 2 Structure determination

Crystallographic data of PFC-31 were collected on a Bruker D8 Venture diffractometer with MoKα radiation (λ = 0.71073 Å) at 100 K. The structure was solved by direct methods and refined by full-matrix least-squares fitting on F2using the SHELXL-2017 software package[22]. All non- hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms on the aromatic rings were located at geometrically calculated positions and refined by a riding mode. The selected bond distances and bond angles of PFC-31 are shown in Table 1.

2. 3 Synthesis of PFC-31

PFC-31 was synthesized by dissolving Cu(NO3)2·3H2O (24.1 mg, 0.1 mmol), 4,4-dithiodipyridine (6.6 mg, 0.03 mmol), biphenyl-4,4΄-dicarboxylic acid (9.2 mg, 0.03 mmol) and acetic acid (50 μL) in 2 mL of N,N-dimethylformamide (DMF). The mixture was then heated at 100 °C for 12 h. Blue crystalline was obtained. Chemical formula: [Cu(4pySO3)2(DMF)2]n(4pySO3= 4-pyridylsulfonate).

2. 4 Activation of PFC-31

Freshly prepared crystals were washed with DMF for 3 times and soaked in 8 mL acetone for 1 day over which the solvent was exchanged with fresh acetone (8 mL) every 4 h. Then, the crystals were filtrated and dried under vacuum at 150 °C for 10 hours.

2. 5 Gas sorption measurements

The as-prepared sample was washed with DMF 3 times. Then the sample was allowed to soak in acetone for 24 h with the supernatant being replaced by fresh acetone several times during the process to exchange and remove nonvolatile solvates (DMF). After the removal of acetone by centri- fugation, the samples were activated under vacuum at room temperature and then dried again in the “outgas” function of instruments at 150 °C for 10 hours.

2. 6 PFC-31 thermal and chemical stability studies

About 100 mg of as-synthesized PFC-31 was fully activated, followed by incubation under the specified condition (ether, ethanol, base condition, methanol, DMF, CH2Cl2, acetone) for 24 hours. Then, the PFC-31 was isolated by filtration, followed by air-dried and PXRD studies.

2. 7 Catalysis for CO2 conversion

In each individual reaction, 10 mmol epoxide substrate along with 40 mg PFC-31 catalyst (0.076 mmol) and 325 mg tetra-n-tert-butylammonium bromide (TBAB) co-catalyst was added into a 15 mL thick-walled reaction tube. The reaction was carried out at room temperature for 48 h under constant 1 bar CO2.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

The building unit of PFC-31 is composed of four 4-pyri- dylsulfonate ligands, a Cu(II) atom and two coordinated DMF molecules. 4-Pyridylsulfonate ligands connect with Cu(II) centers as a bidentate bridging (O: N) coordinative mode to form a polymeric repeated rhomboid chain along the a-axis. Each Cu(II) center is six-coordinated to two nitrogen and two oxygen atoms from four 4-pyridylsulfonate ligands and two oxygen atoms from two monodentate terminal N,N-dimethyl- formamide ligands, forming a [4+2] octahedral geometry (Fig.1a), and O–Cu–O bond angles fall in the range of 88.6(2)～180.0º. The Cu–O bond lengths vary from 1.991(7) to 2.420(6) Å and the Cu–N bond length is 1.997 Å (Table 1). Face-to-face π-π interaction exists between two adjacent benzene rings in one-dimensional chain with the spacing between the two rings of 3.999 Å (Fig.1b).

Fig.1. (a) Representation of the structure of PFC-31. (b) Face-to-face π-π interactions in the 1D chain

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Experimental PXRD pattern of PFC-31 is in good agree- ment with the corresponding theoretical pattern calculated from the X-ray single-crystal data, which reveals the phase purity of PFC-31. In addition, PFC-31 exhibits good chemical stability in various organic solvents including pH = 12 aqueous solution, ether, ethanol, methanol, DMF, CH2Cl2and acetone (Fig.2).

Fig.2. PXRD patterns of (a) simulated PFC-31, as-synthesized PFC-31, PFC-31 after CO2 adsorption at 298 K. (b) PFC-31 after immersion in different solvents for 24 h

3. 2 Thermal and porous properties

The gas adsorption-desorption isotherms of activated PFC-31 were measured by CO2adsorption at 298 K and N2adsorption at 77 K (Fig.3). Considering the structure of PFC-31 is one-dimensional chains without pores according to the crystallographic analysis, it is not surprised that there is nearly no N2adsorption at 77 K. However, it exhibited slight adsorption for carbon dioxide at 298 K, which can be contributed to the affinity of sulfonic acid groups toward the CO2molecules[23].

Fig.3. CO2 sorption isotherm of PFC-31 at 298 K

TG analysis of activated and as-synthesized PFC-31 shows that the activation process almost completely removed the unbonded solvent in structure with showing a plateau below

200oC (Fig.4). Moreover, the consistent PXRD patterns for the material before and after gas adsorption demonstrate the structural integrity of PFC-31 upon solvent removal (Fig.2a).

Fig.4. TGA curves of (a) as-synthesized PFC-31 (black) and (b) activated PFC-31 (red)

3. 3 Catalytic properties

The CO2cycloaddition products and the yields were monitored by gas chromatography (GC). It is known that the adsorption and activation of CO2with high thermodynamic stability are critically important steps determining the reaction rate of cycloaddition. Table 2 summarizes the catalytic per- formance of PFC-31 for the CO2cycloaddition of various substrates in the atmospheric environment. Effectively transforming several typical epoxides into corresponding cyclic carbonates was observed under normal temperature and pressure.

Table 2. Cycloaddition of CO2 with Epoxides, Catalyzed by PFC-31a

Under the same catalytic condition, the epichlorohydrin and 1,2-epoxyhexane were satisfactorily converted into 4-chloro- methyl-1,3-dioxolan-2-one and 4-butyl-1,3-dioxolan-2-one with high conversion and selectivity (entries 1 and 5). The epoxide allyl glycidyl ether, glycidyl phenyl ether and 1,2-epoxyethylbenzene exhibit relatively lower conversions (39%, 54% and 42%, respectively) but high selectivity (≥ 99%) (Table 2, Entries 2, 3, 4). The epoxide propylene oxide (entry 6) shows both low conversion and selectivity.

The lower conversion of entries 2～4 probably can be attributed to the large steric hindrance of substrates, which hampers nucleophilic attack from tetra-n-butylammonium bromide (TBAB) during the catalytic reaction. However, the high carbonate selectivity of epoxides can be attributed to the interactions such as the possible p/p, cationic/p and hydrogen bonding between the substrates and catalyst.

4 CONCLUSION

In conclusion, we have synthesized a new 1D metal-organic framework with orderly sulfonate groups, showing potential Lewis acidity of the metal centers by a simple solvothermal reaction. The obtained material PFC-31 exhibits good catalytic activity toward various organic substrates under atmospheric pressure and temperature. PFC-31 shows low conversion but high selectivity toward the substrate with large steric hindrance probably due to the high affinity of the MOF catalyst toward the substrate.