English

• Recently, the development of feasible carbon dioxide capture and storage/sequestration technologies has received widespread attention to reduce greenhouse gas emissions^[1-8]. Also, CO₂ is considered to be a more useful and environmentally friendly source of C1 than other C1 feedstocks in industry^[9-14]. Besides capturing and storing atmospheric CO₂ by using functional nanoporous materials such as activated carbons^[15-16], zeolites^[17], metal-organic frameworks (MOFs) ^[18-20], and porous organic polymers^[21], the direct cycloaddition of CO₂ with epoxides into value-added chemicals such as cyclic carbonate may be a more attractive strategy for reducing and utilizing CO₂^{[8, 22]}. Although many hetero-geneous catalysts such as zeolite, metal oxide, and functional graphene have been applied to carbon dioxide cycloaddition reaction system, most catalysts have low catalytic activity, and the reaction conditions often require high temperature (> 100 ℃) and high pressure (> 3 MPa)^[23-27]. Therefore, it is pertinent to explore new and efficient heterogeneous catalysts that can effectively convert CO₂ under mild conditions.

  As a novel type of heterogeneous catalysts, MOFs have been widely explored in the cycloaddition of CO₂ with epoxides, due to their specific advantages such as high specific surface area, and multifunctional structure^[28-36]. However, MOF catalysts still have various practical problems in CO₂ cycloaddition, such as low activity, poor stability, difficult recycling, and harsh reaction conditions, which directly put the hurdle for the application of MOF materials^[37-38]. Therefore, it is imperative to explore stable, efficient, and recyclable MOF catalysts, especially those capable of converting CO₂ into cyclic carbonate under mild conditions.

  In our previous work, a new 2D Ag-MOF with the open Lewis acid silver site was obtained, and it could effectively promote the CO ₂ cycloaddition reaction of N-methylpropargylamine to generate the product of 3-methyl-5-methyleneoxazolidin-2-one under relatively mild conditions (75 ℃ and 101.325 kPa) ^[39]. Also, the Ag-MOF can be recycled at least three times and still maintain the stability of the structure. Following this research line, a rigid bis(triazole) ligand (L) has been combined with Zn²⁺ to achieve two novel MOFs by using mixed-ligand strategy in the presence of aromatic carboxylic acid ligands, namely {[Zn₂(L) (TP)₂(H₂O)·H₂O]}_n (1) and [Zn(L)(HTMA)]_n (2), where L=4, 4′-(3, 3′-dimethyl-(1, 1′-biphenyl)-4, 4′-diyl)bis(4H-1, 2, 4-triazole), H₂TP=terephthalic acid, H₃TMA=1, 3, 5-benzenedicarboxylic acid (Scheme 1). Structural analysis reveals that 1 displays a 3, 6-connected 2D structure with a new topological point symbol of (4²·6)₂(4⁸·6⁶·8), while 2 presents a 2D sql topology structure. Interestingly, due to the good stability and open Lewis acid Zn sites, 2 exhibits efficient catalytic activity for CO₂ conversion into cyclic carbonates under mild conditions. Moreover, 2 can be recycled at least three times without significant loss of catalytic activity and retain the original frame structure.

  Scheme1

  

  Scheme1.  Structures of ligand L (base) and different auxiliary ligands (acid)

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  1.   Experimental

  1.1   Materials and measurements

  All the chemicals were received as reagent grade and used without further purification. IR spectrum was recorded on a Varian 640 FT/IR spectrometer with KBr powder in a 4 000-500 cm^-1 range. Powder X-ray diffraction (PXRD) patterns were collected on a DX-2600 spectrometer with Mo Kα radiation (λ=0.071 073 nm) at 296 K (U=40 kV, I=30 mA, 2 θ=5°-50°). Thermal gravimetric analyses (TGA) were carried out under N₂ flow on an SDT 2960 differential thermal analyzer at a heating rate of 10 ℃·min^-1. The concentration of leaching zinc ions was determined by the ICP-MS of Agilent 7800. ¹H NMR spectrum was recorded on a Varian (400 MHz) spectrometer. And the conversion was calculated by ¹H NMR.

  1.2   Synthesis of MOFs 1 and 2

  Zn(NO₃)₂·6H₂O (14.8 mg, 0.05 mmol), H₂TP (8.5 mg 0.05 mmol), L (15.8 mg 0.05 mmol) were dissolved in a mixed solution of DMF/H₂O (1 mL/2 mL) at room temperature. The resulting solution was sonicated for 30 min, sealed in a Teflon-lined autoclave under autogenous pressure, heated to 80 ℃, and maintained for 2 d. After slowly cooling down to room temperature, colorless crystals of MOF 1 were collected and washed three times with distilled water. Yield: 63.2%, based on Zn(NO₃)₂·6H₂O). Anal. Calcd. for C₃₄H₂₈Zn₂N₆O₁₀(%): C, 50.33; H, 3.48; N, 10.36. Found(%): C, 50.24; H, 3.61; N, 10.23. IR (KBr powder, cm^-1): 3 392(m), 2 918 (w), 1 614(m), 1 574(s), 1 512(w), 1 385(s), 1 299(m), 1 033(m), 848(m), 755(s), 580(m).

  Colorless block crystals of MOF 2 were obtained through a similar procedure by using H₃TMA (7.80 mg, 0.05 mmol) instead. Yield: 75.6%, based on Zn(NO₃)₂·6H₂O). Anal. Calcd. for C₂₇H₂₀ZnN₆O₆(%): C, 54.98; H, 3.42; N, 14.25. Found(%): C, 54.26; H, 3.78; N, 13.95. IR (KBr powder, cm^-1): 3 455(m), 3 162(w), 2 923(w), 1 715(vs), 1 574(s), 1 447(m), 1 353(vs), 1 232(m), 1 038 (m), 883(m), 729(m).

  1.3   X-ray crystallography

  The single-crystal X-ray diffraction data of MOF 1 were collected on a Bruker D8 QUEST instrument equipped with graphite-monochromated Cu Kα (λ = 0.154 184 nm) at 296(2) K radiation. The suitable crystal of MOF 2 was selected and characterized on an Oxford Diffraction Gemini R Ultra diffractometer with graphite-monochromated Cu Kα (λ =0.154 184 nm) at 296(2) K. The structures were solved by SHELXS (direct methods) and refined by SHELXL (full-matrix least-squares techniques) in the Olex2 package. The topological structures of all complexes are calculated by the TOPOS 4.0 program package^[40]. Crystal data and structure refinements of MOFs 1 and 2 are provided in Table S1 (Supporting information). Selected bond lengths and angles are listed in Table S2 and S3.

  2.   Results and discussion

  2.1   Structural analysis of MOF 1

  Single-crystal X-ray diffraction analysis reveals that MOF 1 crystallizes in the orthorhombic Pbca space group. The asymmetric unit of 1 contained two Zn(Ⅱ) ions, one L ligand, two TP^2- ligands, a coordinated water molecule, and a lattice water molecule. There are two independent Zn(Ⅱ) ions (Zn1 and Zn2) with different coordination environments in 1. The Zn1 ion is four-coordinated by two oxygen atoms from two different TP^2- ligands, a nitrogen atom from the L ligand, and an oxygen atom from the coordinated water molecule to form a distorted tetrahedron configuration. The Zn2 ion is five-coordinated by three oxygen atoms from two different TP^2- ligands, and two nitrogen atoms from two different L ligands to form a trigonal bipyramid (Fig. 1a). The bond lengths of Zn—O range from 0.193 3 to 0.239 7 nm and the bond lengths of Zn—N range from 0.204 6 to 0.211 6 nm, respectively. The carboxylate groups of the deprotonated TP^2- ligands display chelating bidentate and monodentate coordination modes. The L ligand has only one coordination mode and coordinates with three Zn ions. Zn1 and Zn2 in the asymmetric unit are bridged by the L ligand. The adjacent TP^2- ligands are linked to form two different 1D chains (Fig. 1b and 1c). The coordinated L ligands are fixed between the two chains to assemble a 2D layered network (Fig. 1d). Simplify the structure of 1 to get its topological network structure (Fig. 2), in which each [Zn₂ N₄] unit is regarded as the 6-connected node, and all Zn1 are considered as 3-connected nodes. As a result, the framework is an unprecedented 3, 6-connected network with the Schläfli symbol (4²·6)₂(4⁸·6⁶·8) by software TOPOS.

  Figure 1

  

  Figure 1.  Structure of MOF 1: (a) coordination environment of Zn²⁺ ion; (b, c) two different 1D chains; (d) 2D layer

  Symmetry codes: ^ⅰ 0.5-x, 0.5+y, z; ^ⅱ 1.5-x, -0.5+y, z; ^ⅲ 1-x, 1-y, z.

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  Figure 2

  

  Figure 2.  Topological simplification diagram of MOF 1

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  2.2   Structural analysis of MOF 2

  Single-crystal X-ray diffraction analysis reveals that MOF 2 crystallizes in the monoclinic P2₁/c space group. The asymmetric unit of 2 contained a Zn(Ⅱ) ion, an L ligand, and an HTMA^2- ligand. The Zn(Ⅱ) ion is four-coordinated with a [ZnO₂N₂] coordination environ-ment and displays a tetrahedron configuration, consist-ing of two oxygen atoms (O1, O4^ⅰ) from two HTMA^2- ligands and two nitrogen atoms (N1, N4^ⅱ) from two L ligands (Fig. 3a). The bond lengths of Zn—O range from 0.194 0 to 0.195 1 nm and the bond lengths of Zn—N range from 0.203 0 to 0.203 4 nm, respectively. The carboxylate groups of deprotonated HTMA^2- form 1D chains with Zn²⁺ ions by single-dentate coordination (Fig. 3b). The coordinated L ligands are fixed between the two chains to assemble a 2D wave-type layered network (Fig. 3c). The structure of 2 is simplified by treating L and HTMA^2- as linkers, which can be viewed as a sql topology (Fig. 4).

  Figure 3

  

  Figure 3.  Structure of MOF 2: (a) coordination environment of Zn²⁺ ion; (b) 1D chain；(c) 2D layer

  Symmetry codes: ^ⅰ 1-x, y, z; ^ⅱ 1+x, 0.5+y, 0.5+z.

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  Figure 4

  

  Figure 4.  Topological simplification diagram of MOF 2

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  2.3   PXRD, FTIR spectroscopy, and TGA

  The simulated powder X-ray diffraction and experimental results of MOFs 1 and 2 are shown in Fig.S3. The main peak positions of the experimental patterns of 1 and 2 were consistent with their simulated patterns, demonstrating the single-phase purity of the products.

  IR spectra of MOFs 1 and 2 are shown in Fig.S4. The IR spectra of the MOFs showed broad bands at about 3 392 cm^-1 for 1, and 3 455 cm^-1 for 2, which can be ascribed to the O—H stretching vibration of the water molecules. No absorption peak around 1 700 cm^-1 for —COOH was observed for 1, indicating that all the carboxyl groups of H₂TP are deprotonated. By contrast, for 2, the presence of a band at 1 715 cm^-1 indicates that the HTMA^2- is protonated. The characteristic peaks of methyl groups were at 2 918 cm^-1 for 1 and 2 923 cm^-1 for 2. Characteristic bands at 1 512 cm^-1 for 1 and 1 537 cm^-1 for 2 are assigned to the ν_C=N stretching vibration of 1, 2, 4-triazole. The strong characteristic bands at 1 574, 1 385 cm^-1 for 1 and 1 574, 1 433 cm^-1 for 2 can be considered as the asymmetric and symmetric vibrations of the carboxyl groups, respectively. These IR spectra are in agreement with the results of elemental analysis and single-crystal X-ray diffraction results.

  The TGA experiments for MOFs 1 and 2 were carried out in a range of 25-800 ℃. As shown in Fig.S5a, the TG curve of 1 displayed two-step thermal decomposing behaviours. The first weight loss of 4.39% (25-285 ℃) is due to the loss of one lattice H₂O and one coordinated water molecule (Calcd. 4.44%). After 340 ℃, the skeletons of 1 began to collapse because of the decomposition of the organic ligands. As shown in Fig.S5b, the TG curve of 2 displayed high thermal stability and one-step thermal decomposing behaviours. As the temperature rose to 360 ℃, the skeletons of 2 began to collapse because of the decomposition of organic ligands. Thus, these results reveal that MOFs 1 and 2 have good thermal stability.

  2.4   Cycloaddition of CO₂ with epoxides

  Crystal structures of MOFs 1 and 2 show quite a few interesting features, for example, (ⅰ)the presence of Lewis acidic Zn²⁺ metal sites in their framework, which may activate the substrate, (ⅱ)the presence of channels that enable free diffusion of substrates to the catalytic sites, (ⅲ)the presence of a π-electron-rich framework architecture that may strengthen the structure and could facilitate the interactions between the catalytic framework and substrates/reagents. These properties provide the basis for exploring their catalytic activity for the reaction of CO₂ cycloaddition with epoxide. Initially, by using propylene oxide and CO₂ as model substrates for the studied cycloaddition reaction, 2 was tried as the catalyst to test its activity. By optimizing the reaction conditions, we have examined the influence of catalyst loading, the importance of cocatalyst, and reaction time under mild conditions (atmospheric pressure and room temperature) (Table 1).

  Table 1

  

  Table 1.  Orthogonal experiments of CO₂ cycloaddition reactions^a

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  Entry	xcatalyst/%	xTBAB/%	Time/h	Conversionb/%
  1	0.1	0	24	0
  2	0	8.0	24	32.48
  3	0.1	1.0	24	30.30
  4	0.1	3.0	24	37.88
  5	0.1	5.0	24	52.36
  6	0.1	6.0	24	66.67
  7	0.1	7.0	24	89.30
  8	0.1	8.0	24	> 99
  9	0.08	8.0	24	69.44
  10	0.05	8.0	24	47.85
  11	0.1	8.0	18	67.57
  12	0.1	8.0	12	38.91
  13	0.1	8.0	6	18.42
  anepoxide=20 mmol; bThe conversion of the epoxide was determined by 1H NMR analysis (Fig.S8-S21).

  The reaction was conducted in a 10 mL Schlenk tube with 20 mmol of propylene oxide and CO₂ under atmospheric pressure at room temperature. In the presence of 8.0% of co-catalyst (TBAB), 0.1% loading of catalyst 2 afforded above 99% maximum conversion within 24 h (Table 1, entry 8). When the reactions were performed in the absence of TBAB or 2, no substrates or only a little were converted (Table 1, entries 1 and 2). Therefore, it was assumed that the synergistic effect of 2 and TBAB should be responsible for the high catalytic activity of the system under mild conditions. Under the same condition, when 1 was used as a catalyst, the conversion of propylene oxide was 89.28% (Table 2, entry 1). The different coordination geometry of Lewis acid Zn metal centers in 1 and 2 lead to certain differences in their catalytic effects for CO₂ cycloaddition. Additionally, control experiments indicated that other standard zinc catalysts (ZnCl₂ and Zn(NO₃)₂·6H₂O) provided low conversions with the same amount of catalysts (Table 2, entries 3 and 4).

  Table 2

  

  Table 2.  Catalytic activity of various catalysts for propylene oxide^a

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  Entry	Catalyst	Epoxide	Product	Conversionb/%	TONc
  1	1			89.28	892.8
  2	2			100.0	1 000.0
  3	ZnCl2			51.55	515.5
  4	Zn(NO3)2·6H2O			52.36	523.6
  a Reaction conditions: propylene oxide (20 mmol), xcatalyst=0.1%, xTBAB=8%, RT, atmospheric pressure (balloon), 24 h; b The conversion of the epoxide was determined by 1H NMR analysis; c TON=nproduct/ncatalyst (Fig.S22-S24).

  To explore the potential generality of the catalytic system, several typical epoxide substrates (i.e., 1-chloro-2, 3-epoxypropane, 1-bromo-2, 3-epoxypropane, and styrene oxide) were examined in this cycloaddition reaction. Under the optimized conditions, the conversions of the three substrates were 57.80%, 55.87%, and 14.53% (Table 3, entries 2-4), respectively. The lower catalytic efficiency may be associated with the large size of the substrates, resulting in ineffective binding with vacant zinc sites.

  Table 3

  

  Table 3.  Cycloaddition reactions of CO₂ and various epoxides with catalyst 2^a

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  Entry	Epoxide	Product	Conversionb/%	TONc
  1			100.0	10 00.0
  2			57.80	578.0
  3			55.87	558.7
  4			14.53	145.3
  aReaction conditions: epoxide (20 mmol), xcatalyst=0.1%, xTBAB=8%, RT, atmospheric pressure (balloon), 24 h; b The con-version of the epoxide was determined by 1H NMR analysis; c TON=nproduct/ncatalyst (Fig.S25-Fig.S27).

  To investigate the heterogeneous nature of the zinc catalyst, MOF 2 was filtered from the reaction mixture after 6 h. The resulting supernatant continued to react for another 18 h, and there were no observable catalytic conversions (Fig. S6). Additionally, the catalyst leakage in collected supernatant was tested by the ICP-MS analysis, and the result showed only 0.25 mg·L^-1 of free Zn ion, demonstrating that there was almost no leaching of Zn ion during the reaction process. Those results showed that 2 is a heterogeneous catalyst in the catalytic system. As shown in Fig.S7a, to evaluate the recyclability of 2, experiments were performed by using the recovered catalyst of 2. The obtained solid can still maintain 95.23% conversion after three recycles. The PXRD results revealed that the catalyst keeps its crystallinity after catalytic experiments (Fig.S7b).

  The catalytic mechanism was further inspected based on literature reports (Fig. 5) ^{[36-37, 41-42]}. Firstly, epoxide interacts with the Lewis acid Zn²⁺ center via the oxygen atom and activates the ternary ring. Then, the epoxide opens the ring with the help of the Zn²⁺ sites to form the O···Zn adducts (Ⅰ). Secondly, the Br^- produced by TBAB attacks as nucleophiles on the sterically less-crowded carbon atom to destroy the terpolymer epoxy ring (Ⅱ). In the third step, CO₂ is inserted and forms the alkyl carbonate anion (Ⅲ). Subsequently, the intra-molecular ring closes to produce a cyclic carbonate product (Ⅳ). Finally, the epoxy carbonate breaks away from the Lewis acid Zn²⁺ center and reproduces the catalyst, thus allowing the CO₂ cycloaddition reaction to enter the next catalytic cycle.

  Figure 5

  

  Figure 5.  Plausible mechanism for the cycloaddition of epoxides with CO₂ catalyzed by MOF 2

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  3.   Conclusions

  In summary, two novel metal-organic frameworks have been prepared by using a mixed-ligand strategy and structurally characterized. The presence of different acidic ligands results in two MOFs with interesting topologies. MOF 1 displays a 3, 6-connected 2D structure with a new topological point symbol of (4²·6)₂(4⁸·6⁶·8), while MOF 2 presents a 2D sql topology. Remarkably, 1 and 2 were exploited as catalytic materials in the fixation of CO₂ to cyclic carbonates under mild conditions (RT and ambient pressure). In particular, 2 has the highest catalytic efficiency (above 99%) in 24 h for cycloaddition of propylene oxide, and its structure remains stable after three cycles. In addition, 2 has certain catalytic activity for other epoxides such as 1-chloro-2, 3-epoxypropane, 1-bromo-2, 3-epoxypropane, and styrene oxide, demonstrating its generality for the catalytic reaction. This work provides a further step towards the efficient use of CO₂ under mild condi-tions and lays a foundation for achieving more heteroge-neous catalysts in this particular area.

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

  
  
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  Scheme1  Structures of ligand L (base) and different auxiliary ligands (acid)

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  Figure 1  Structure of MOF 1: (a) coordination environment of Zn²⁺ ion; (b, c) two different 1D chains; (d) 2D layer

  Symmetry codes: ^ⅰ 0.5-x, 0.5+y, z; ^ⅱ 1.5-x, -0.5+y, z; ^ⅲ 1-x, 1-y, z.

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  Figure 2  Topological simplification diagram of MOF 1

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  Figure 3  Structure of MOF 2: (a) coordination environment of Zn²⁺ ion; (b) 1D chain；(c) 2D layer

  Symmetry codes: ^ⅰ 1-x, y, z; ^ⅱ 1+x, 0.5+y, 0.5+z.

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  Figure 4  Topological simplification diagram of MOF 2

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  Figure 5  Plausible mechanism for the cycloaddition of epoxides with CO₂ catalyzed by MOF 2

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  Table 1.  Orthogonal experiments of CO₂ cycloaddition reactions^a

  Entry	xcatalyst/%	xTBAB/%	Time/h	Conversionb/%
  1	0.1	0	24	0
  2	0	8.0	24	32.48
  3	0.1	1.0	24	30.30
  4	0.1	3.0	24	37.88
  5	0.1	5.0	24	52.36
  6	0.1	6.0	24	66.67
  7	0.1	7.0	24	89.30
  8	0.1	8.0	24	> 99
  9	0.08	8.0	24	69.44
  10	0.05	8.0	24	47.85
  11	0.1	8.0	18	67.57
  12	0.1	8.0	12	38.91
  13	0.1	8.0	6	18.42
  anepoxide=20 mmol; bThe conversion of the epoxide was determined by 1H NMR analysis (Fig.S8-S21).

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  Table 2.  Catalytic activity of various catalysts for propylene oxide^a

  Entry	Catalyst	Epoxide	Product	Conversionb/%	TONc
  1	1			89.28	892.8
  2	2			100.0	1 000.0
  3	ZnCl2			51.55	515.5
  4	Zn(NO3)2·6H2O			52.36	523.6
  a Reaction conditions: propylene oxide (20 mmol), xcatalyst=0.1%, xTBAB=8%, RT, atmospheric pressure (balloon), 24 h; b The conversion of the epoxide was determined by 1H NMR analysis; c TON=nproduct/ncatalyst (Fig.S22-S24).

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  Table 3.  Cycloaddition reactions of CO₂ and various epoxides with catalyst 2^a

  Entry	Epoxide	Product	Conversionb/%	TONc
  1			100.0	10 00.0
  2			57.80	578.0
  3			55.87	558.7
  4			14.53	145.3
  aReaction conditions: epoxide (20 mmol), xcatalyst=0.1%, xTBAB=8%, RT, atmospheric pressure (balloon), 24 h; b The con-version of the epoxide was determined by 1H NMR analysis; c TON=nproduct/ncatalyst (Fig.S25-Fig.S27).

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