An unprecedented Cu₆ cluster-based bimetallic MOF with multiple open sites for high CO₂ capture and efficient CO₂ conversion

Abstract

Based on the soft–hard acid–base theory, a novel bimetallic MOF (JLU-MOF108) has been successfully synthesized through a one-pot synthesis method. JLU-MOF108 is a mixed metal-based MOF, constructed from an unprecedented hexanuclear copper cluster (12-connected), indium metal center (4-connected) and a tetrazole-containing carboxylic acid ligand (3-connected), that exhibits a (3, 4, 12)-connected xag topology. In order to effectively enhance the CO₂ capture and cycloaddition catalytic performance, abundant open metal sites (OMSs), Lewis basic sites (LBSs) and specific pore sizes have been implanted in the JLU-MOF108 structure. Fascinatingly, the expected mixed metal-based MOF material has a high OMS density and suitable pore volume. The results show that JLU-MOF108 exhibits high CO₂ adsorption performance and excellent epichlorohydrin (ECH) catalysing capability in cycloaddition reactions. More importantly, the yield of ECH can reach up to 97%, which surpasses many reported MOF materials. Therefore, JLU-MOF108 can be a good candidate material for cycloaddition reactions and CO₂ storage.

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Introduction

Along with the increasing progress of society and industrial development, humans demand more and more energy consumption. Due to industrial manufacturing, a high level of carbon dioxide (CO₂) appears in the air, becoming a main cause of global warming. So far, the efficient capture and utilization of CO₂ has become a major concern for the world.^1–5 The presence of high CO₂ concentrations in industrial production can affect the complete combustion and conversion efficiency of fuels, leading to huge energy waste.^6–9 Over the past few years, the method of CO₂ cycloaddition reaction has garnered significant interest for its gentle and efficient approach for capturing CO₂, and the conversion from CO₂ and epoxides to cyclic carbonate through a cycloaddition reaction is an effective way of CO₂ utilization.^4,10–13 In order to successfully achieve carbon peak and neutrality targets and transform CO₂ into more valuable feedstocks, researchers have investigated various materials to capture, convert and utilize CO₂.^14–18

Metal–organic frameworks (MOFs) are usually composed of inorganic secondary building units (SBUs) of metal ions or metal clusters with multidentate organic ligands through coordination bonds in a specific coordination mode, also known as porous coordination polymers, and usually possess large surface areas, tunable pore sizes, adjustable chemical functionalities, and well-defined metal nodes.^19–21 The above advantages endow them with diverse potential applications especially in catalysis. With the continuous development of industry, the targeted synthesis of MOFs applied in specific fields is particularly important. For catalysis, it is necessary to design and synthesize MOF-based catalysts with a large specific surface area and suitable pore size to provide an appropriate reaction environment, and it is also necessary to introduce corresponding active sites to ensure efficient non-homogeneous catalytic reactions.^22–27 Several key properties of MOFs can enhance their CO₂ capture capability: (1) ultra-high specific surface area and controllable pore size.^28,29 (2) By the design and synthesis of nitrogen atoms with specific functional sites, such as the introduction of Lewis basic sites (LBSs), etc.^30,31 (3) Open metal sites (OMSs) can be seen as Lewis acid sites (LASs) and they can polarize gas molecules such as carbon dioxide, thus enhancing the interaction of gases such as carbon dioxide with MOFs.^32–34 However, many catalysts have been reported, including some MOF-based materials, which usually need high work pressures (up to 3 MPa or higher) and high temperatures. Thus, designing and synthesizing MOFs capable of efficiently capturing and converting CO₂ under mild conditions remains a challenge.

Mixed metal cluster MOFs, which are composed of one or more organic ligands and a variety of inorganic SBUs, can provide new ideas for the design and synthesis of functional MOFs. Most of the reported mixed metal cluster-based MOFs are synthesized by post-synthesis methods.^35–39 However, these methods are complicated and limit the diversity and topological structures of MOFs to a certain extent. In contrast, the one-pot synthesis method of MOFs based on the soft–hard acid–base theory not only enriches the variety of structural elements, connection modes and framework structures, but also brings broader application prospects for mixed metal cluster-based MOFs with their novel pore size.^40–44 Combining the superiorities of mixed metal cluster MOFs, the one-pot synthesis method and outstanding performances in catalysis, bimetallic MOFs are expected to be a new type of efficient catalyst.^45–47 On the one hand, bimetallic MOFs possess more diverse active sites than monometallic MOFs, which makes them exhibit excellent performance in catalysis.^{41,45,47–49} On the other hand, organic ligands can covalently bond with two metal sites in a single node, which is conducive to improving their thermal and chemical stability.^50,51

Considering all the problems above, a novel Cu–In-MOF material (JLU-MOF108) constructed from a hetero-tetrazole-containing carboxylic acid ligand H₃TZPA (H₃TZPA = 5-(4-(tetrazol-5-yl)phenyl)isophthalic acid) and mixed copper and indium metals was successfully synthesized by a one-pot solvothermal method.

Remarkably, the structure of JLU-MOF108 possesses an unprecedented hexanuclear copper cluster [Cu₆(μ₆-Cl)(N–N)₁₂] SBU, which has not been found so far in copper-based SBUs (Scheme 1).^52,53 The versatile coordination modes and stable oxidation states of metallic copper facilitate the formation of multi-nuclear copper cluster motifs. The common copper clusters include the typical paddlewheel [Cu₂(CO₂)₄],⁵⁴ the triangular trinuclear copper cluster [Cu₃(μ₃-OH)N₆],⁵⁵ the square planar tetranuclear copper halide cluster [Cu₄Cl]⁷⁺,⁵⁶ and the bicrown hexanuclear copper cluster [Cu₆N₁₂Cl₆] and so on.^57–59 In contrast, the hexanuclear copper cluster in JLU-MOF108, with twelve connections, is distinct from the commonly observed 12-connected hexanuclear Zr clusters or rare earth (RE) clusters.^60–64 While hexanuclear Zr/RE clusters are linked by carboxylate oxygen, the hexanuclear copper clusters in JLU-MOF108 are linked by N–N bonds in the tetrazole ligands, representing exceedingly rare formation, which is similar to the recently reported triazole-ligand linked Cd₆(μ₆-Cl)(N–N)₁₂Cl₆ cluster.⁶⁵

Scheme 1 Examples of Cu cluster-based MOFs with different connections and different hexanuclear metal cluster-based MOFs with 12 connections.

The hexanuclear copper clusters in JLU-MOF108 can be viewed as 12-connected nodes and linked with 3-connected ligands and 4-connected indium SBUs to form a (3, 4, 12)-connected xag topology. Notably, JLU-MOF108 has a large number of OMSs (considered as LASs) and LBSs (N atoms on the ligand and exposed in the pore channel). Benefiting from these active sites, JLU-MOF108 achieves a 97% conversion rate of epichlorohydrin (ECH) at 1 atm under rather mild conditions, which exceeds many reported MOF materials. Moreover, JLU-MOF108 exhibited high CO₂ uptake at 273 and 298 K.

Experimental

Materials and methods

The organic ligands and other chemicals were commercially obtained without further purification. The analyses of C, H and N elements were carried out using a Vario MICRO Cube analyzer. Thermogravimetric analysis (TGA) was performed using a TGA Q500 machine at a heating rate of 10 °C min⁻¹ under atmospheric pressure from room temperature to 800 °C. Powder X-ray diffraction (PXRD) data were collected through a Rigaku D/MAX2550 diffractometer, and Cu Kα radiation (λ = 1.5418 Å) was in the 2θ range of 3–40°. X-ray photoelectron spectroscopy (XPS) measurements were executed on an ESCALAB 250 X-ray photoelectron spectrometer and Mg Kα X-rays were used as the excitation source. IR spectra were recorded in the range of 400–4000 cm⁻¹ on a Nicolet Impact 410 FTIR spectrometer using KBr pellets. The inductively coupled plasma (ICP) data were collected with a PerkinElmer Optima 3300 DV spectrometer.

X-ray crystallography

Single-crystal X-ray diffraction measurements for JLU-MOF108 were obtained with a Bruker Apex II CCD diffractometer using graphite monochromatic Mo-Kα (λ = 0.71073 Å) radiation under atmospheric pressure and at room temperature. The structure of JLU-MOF108 was solved by direct methods and refined by full-matrix least squares on F² in SHELXL-2014.⁶⁶ All non-hydrogen atoms were determined anisotropically. The formula was rooted in the crystallographic data associated with CHN elemental analyses and TGA data. The detailed crystallographic data and selected bond lengths and angles for the JLU-MOF108 are listed in Tables S1 and S2.† Topology information for JLU-MOF108 was obtained using TOPOS 4.0 software.⁶⁷

Synthesis

CuCl₂·2H₂O (2 mg, 0.010 mmol), In(NO₃)₃·4H₂O (6 mg, 0.015 mmol) and H₃TZPA (3 mg, 0.010 mmol) were well dispersed in N,N-dimethylformamide (DMF) (1.0 mL) in a 20 mL vial. Then, deionized water (300 μL) was added to the solution and sonicated for 3 min. Finally, the mixture was heated at 115 °C for 24 h. Brown polyhedral single crystals were obtained and washed with DMF several times (74% yield based on the H₃TZPA ligand). Elemental analyses calculated (%): C, 44.67; H, 4.96; N, 15.05. Found (%): C, 44.39; H, 4.69; N, 15.42.

Gas adsorption measurements

The as-synthesized JLU-MOF108 samples were exchanged by methanol solution 12 times in 2 days and then activated at 70 °C for 10 hours. The N₂ adsorption/desorption isotherm at 77 K was carried out on a Micromeritics ASAP 2420 instrument. The adsorption/desorption isotherms of CO₂, CH₄, C₂H₆, and C₃H₈ at 273 and 298 K were performed on a Micromeritics 3Flex.

Reactions of CO₂-epoxide cycloaddition

In a general atmospheric pressure catalytic reaction, activated JLU-MOF108, all epoxides (20 mmol) and co-catalysts (TBAB, 5 mol%) were placed in a clean 15 mL Schlenk tube. The tube was emptied of air before carbon dioxide was injected. A total of three evacuations were required to ensure that the tube was fully filled with carbon dioxide. The mixture was stirred under relatively mild conditions at 25 °C for propylene oxide (PO) and 80 °C for ECH, styrene oxide (SO) and other epoxides. The amount of catalyst in each catalytic reaction was calculated based on the number of OMSs. The catalysts were recovered after centrifugal separation in preparation for the next catalytic cycle. The final yields were determined and calculated by ¹H-NMR (n-dodecane was selected as an internal standard for PO).

Results and discussion

Structural description

The X-ray single-crystal structure reveals that JLU-MOF108 crystallizes in the cubic crystal system, specifically belonging to the Im3̄m space group. As shown in Fig. 1 and Fig. S1,†JLU-MOF108 is a bimetallic MOF material constructed from mixed-metal copper clusters and indium units, which contains three types of SBUs: a rare hexanuclear copper cluster structural motif, a mononuclear indium structural motif, and an organic structural motif with N and O donors. The hexanuclear copper cluster is an octahedron composed of six copper atoms, one Cl atom located at the centre of the cluster, and twelve N–N fragments at the bottom of twelve tetrazole ligands. The edge of the octahedron is connected by twelve N–N fragments, making it a hexacore copper cluster structure with twelve connections. This hexanuclear copper cluster SBU has not been found so far, and it is similar to the Cd₆ clusters in the reported PMo₁₂@CdMOF.⁶⁵ The nitrogen-containing portion of the H₃TZPA ligand is coordinated to copper, and the carboxylic oxygen at the other end is coordinated to the indium centre. Each In³⁺ chelated to four carboxylic oxygen atoms from different ligands, forming a 4-connected [In(COO)₄] structural motif. The organic ligand H₃TZPA containing a tetrazole group with a resorcylic acid moiety can be viewed as a 3-connected triangle node. From a topological viewpoint, JLU-MOF108 shows a (3, 4, 12)-connected xag topology with the point symbol {6²⁴·8¹²·10²⁴·12⁶}{6³}₁₂{6⁴·8²}. Although indium has no OMSs to assist in catalysis, the structure cannot form a three-dimensional structure when the mononuclear indium structural motif is removed, and thus the indium center acts as a connecting bridge.

Fig. 1 Crystal structure of JLU-MOF108. (a) Simplification of the hexanuclear copper cluster SBU, indium SBU and organic ligand, (b) polyhedral view of the cage, (c) ball–stick and Connolly view of the channel along the [010] direction, and (d) topological view. Color scheme: carbon = gray, nitrogen = blue, oxygen = red, copper = green, indium = light blue, and chlorine = pink.

In the [010] direction, there is a triangular window with a size of about 10.0 Å × 5.1 Å. JLU-MOF108 has a cage-like structure in which each Cu₆ cluster is surrounded by six equal-sized cage-like structures. Each cage is composed of two Cu₆ clusters, four mononuclear indium units and eight ligands with a window size of about 20.0 Å × 10.0 Å. The space-filling view of JLU-MOF108 with multiple pores along the [001] and [111] directions is shown in Fig. S2.†

The above structural analysis and characterization indicated that JLU-MOF108 exhibits a high CO₂ adsorption capacity due to its high pore volume, cage structural motif and OMSs. Besides, there are a large number of OMSs and uncoordinated LBSs on the ligands, both of which can synergistically improve the CO₂ cycloaddition reaction catalytic effect.

Gas adsorption and separation behaviour

In order to explore the permanent porosity and surface area of JLU-MOF108, the nitrogen adsorption test on the sample was carried out at 77 K. Since JLU-MOF108 is stable in acetonitrile, methanol, ethanol, acetone and dichloromethane solvents according to the PXRD pattern (Fig. S3†), the TGA analysis results (Fig. S4†) showed that methanol-exchanged JLU-MOF108 could be completely activated at 70 °C. As shown in Fig. 2a, the nitrogen adsorption curve is a typical type I adsorption isotherm, which is consistent with the characteristics of microporous materials. The Brunauer–Emmett–Teller (BET) surface area is 1893 m² g⁻¹, and the experimental pore volume is 0.90 cm³ g⁻¹, which is close to the theoretical value of 0.93 cm³ g⁻¹. The calculated pore size by using nonlocal density functional theory (NLDFT) agrees well with the measured one of the JLU-MOF108 structure (Fig. S5†). Due to the cage-like structure and OMSs in JLU-MOF108, it has a high carbon dioxide adsorption capacity. At 273 K and 298 K, the CO₂ absorption capacities are 141.9 cm³ g⁻¹ (6.3 mmol g⁻¹) and 67.2 cm³ g⁻¹ (3.0 mmol g⁻¹), as shown in Fig. 2b and Fig. S6,† and exceed many of the reported materials such as NH₂-MIL-125,⁶⁸ USTC-253,⁶⁹ MAF-23,⁷⁰ JNU-2,⁷¹ and ZNU-1,⁷² (Tables S3 and S4†). Then the gas adsorption test was conducted on the main component of natural gas CH₄ and the common impurities C₂H₆ and C₃H₈ (Fig. S7–S9†). At 1 atm, 273 and 298 K, the maximum adsorption capacities of CH₄ are 29.6 and 15.8 cm³ g⁻¹, respectively, while the maximum adsorption capacities of C₂H₆ are 190.3 and 134.7 cm³ g⁻¹, and the maximum adsorption capacities of C₃H₈ are 200.8 and 177.8 cm³ g⁻¹. Correspondingly, the experimental gas adsorption isotherms at 298 K were fitted with the DSLF equation, and the theoretical selectivities of binary gas mixtures were calculated by utilizing ideal adsorbed solution theory (IAST). According to Fig. S10,† in JLU-MOF108 at 298 K and 1 atm, the adsorption selectivities of CO₂/CH₄ are 4.5 (0.5/0.5) and 4.8 (0.05/0.95). The adsorption selectivity of C₂H₆/CH₄ is 13.3 (0.5/0.5). The adsorption selectivity of C₃H₈/CH₄ reaches 248.2 (0.5/0.5) and exceeds many of the reported materials. Therefore, the results showed that JLU-MOF108 exhibited good adsorption and separation capabilities.

Fig. 2 (a) N₂ adsorption–desorption isotherm of JLU-MOF108 at 77 K and (b) CO₂ adsorption–desorption isotherms of JLU-MOF108 at 273 K and 298 K.

CO₂ cycloaddition reactions

Based on the high CO₂ adsorption capacity of JLU-MOF108, further cycloaddition reactions were performed. First, the smallest propylene oxide (PO) was selected to explore its cycloaddition reaction performance (Table S6†). A series of exploratory experiments were carried out to find the optimal conditions for the catalytic reaction, as shown in Table 1. Firstly, 20 mmol PO was added, and the yield was only 15% after 12 hours at 1 bar and 25 °C without a catalyst. Then, when TBAB and JLU-MOF108 were added alone, the yield also remained 25% and 20%, respectively. This indicates that the simultaneous presence of the co-catalyst and JLU-MOF108 would be more favorable to the reaction. Therefore, the optimal conditions of JLU-MOF108 for catalyzing the cycloaddition reaction of PO were obtained by varying the amounts of catalyst and co-catalyst: the addition of 5 mol% TBAB and 0.25 mol% OMSs of JLU-MOF108 for 12 h at 1 bar and 25 °C catalyzes the conversion of 20 mmol PO with a yield of 99% (Fig. S13†). As shown in Table 2 and Fig. 3, the conversion of the ECH cycloaddition reaction can reach up to 97% by adding 5 mol% TBAB and JLU-MOF108 (0.25 mol% OMSs) at 1 bar and 80 °C for 12 h (Fig. S14†). The TON (turnover number) and TOF (turnover frequency) values are 388 and 32.3 h⁻¹, which are higher than most of the reported MOF-based materials such as PCN-700-Me₂,⁷³ FJI-C10,⁷⁴ and JLU-Liu21⁷⁵ (Table S5†). However, the catalytic performances of the other four substrates were not very good, with yields ranging from 15 to 65% (Fig. S15–S18†). Therefore, the material has catalytic selectivity for small-sized substrates as illustrated in Table 3.

Fig. 3 Kinetic curve of CO₂–ECH cycloaddition catalyzed by JLU-MOF108.

Table 1 JLU-MOF108 for the CO₂ cycloaddition reaction with PO under different conditions

Entry	Catalyst	Tem./°C	P/bar	Time/h	Yield/%
a Reaction conditions: 20 mmol PO, 0.12 mol% open Cu sites of activated JLU-MOF108, and 2 mol% TBAB. b 0.25 mol% open Cu sites of activated JLU-MOF108 and 2.5 mol% TBAB. c 0.25 mol% open Cu sites of activated JLU-MOF108 and 5 mol% TBAB. Checked by ^1H NMR spectroscopy and n-dodecane was added as an internal standard.
1	None	25	1	12	15
2	Only TBAB	25	1	12	25
3	Only JLU-MOF108	25	1	12	20
4	JLU-MOF108	25	1	12	68
5	JLU-MOF108	25	1	12	73
6	JLU-MOF108	25	1	12	99

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Table 2 JLU-MOF108 for the CO₂ cycloaddition reaction with ECH under different conditions

Entry	Catalyst	Tem./°C	P/bar	Time/h	Yield/%
a Reaction conditions: 20 mmol ECH, 0.12 mol% open Cu sites of activated JLU-MOF108, and 2 mol% TBAB. b 0.25 mol% open Cu sites of activated JLU-MOF108 and 2.5 mol% TBAB. c 0.25 mol% open Cu sites of activated JLU-MOF108 and 5 mol% TBAB. Checked by ^1H NMR spectroscopy and n-dodecane was added as an internal standard.
1	JLU-MOF108	80	1	12	53
2	JLU-MOF108	80	1	12	73
3	JLU-MOF108	80	1	12	97

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Table 3 JLU-MOF108 for different epoxides for CO₂ cycloaddition reactions under optimal conditions

Entry	Epoxide	Product	Yield (%)
Catalytic performances were checked by ^1H NMR, and n-dodecane was added as the internal standard for PO.
1			99
2			97
3			65
4			48
5			46
6			15

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There are two types of functional sites in JLU-MOF108: the unsaturated metal Cu site (regarded as LASs) and the N atom (regarded as LBSs in the H₃TZPA ligand). Therefore, according to the relevant reports,^76–78 we proposed a possible catalytic mechanism of JLU-MOF108 in the CO₂ cycloaddition reaction (Fig. 4). First, the nitrogen atoms in the epoxide can be trapped and polarized by the open Cu sites on the Cu₆ clusters, thereby activating the epoxy ring. Then, the bromide ion in TBAB acts as a nucleophile to attack the epoxide, forming an epoxide anion intermediate and activating the adjacent CO₂ molecule to form a coordination bond with its contained carbon atom. Subsequently, the oxygen in the activated CO₂ attacks the carbon coordinated to the bromide ion to complete the cyclization and generate the product. Then, the nitrogen atom of tetrazole in the ligand H₃TZPA acts as a LBS to polarize CO₂ and form a coordination bond, and the electronegative oxygen in CO₂ attacks the epoxide to activate and open the ring. Finally, the oxygen in the epoxide attacks the carbon coordinated to the nitrogen, completing the cyclization. In conclusion, LASs and LBSs in JLU-MOF108 synergistically catalyze the whole reaction process.

Fig. 4 A possible synergetic catalytic mechanism of JLU-MOF108 in the CO₂–ECH catalytic cycloaddition reaction.

The recyclability performance is also an important index for evaluating the catalytic capability. Therefore, we investigated the recyclability of JLU-MOF108 for the catalytic conversion of epichlorohydrin. The yield still remained at about 93% after four cycles. The PXRD pattern of the sample after four cycles further confirmed its stability, as shown in Fig. S11.† The IR and XPS spectra of JLU-MOF108 after recycling 4 times are shown in Fig. S12.† ICP-OES analysis revealed that there were no Cu²⁺ and In³⁺ ions leaking from JLU-MOF108 (Table S7†).

Overall, JLU-MOF108 exhibits good catalytic selectivity due to the limiting effect of pore size, which further confirms that the material is a suitable catalyst for CO₂ immobilization.

Conclusions

In conclusion, a new bimetallic copper–indium metal–organic framework (JLU-MOF108) was designed and synthesized using a one-pot synthesis strategy. The framework exhibits a rare Cu₆ cluster structural motif due to the diversity of coordination modes, which exposes a large number of OMSs. Additionally, the nitrogen-containing carboxylic acid ligand provides uncoordinated LBSs. The unique cage-like structure of JLU-MOF108 gives it excellent catalytic ability and high CO₂ capture capacity. The bimetallic mixed-MOF design demonstrates the potential for introducing numerous OMSs into the MOF structure to enhance catalytic ability. This provides guidance for future work in this field.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22171100 and U23A20360) and the ‘111 Center’ (B17020).

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Footnote

† Electronic supplementary information (ESI) available: PXRD patterns, gas adsorption measurement data, TGA analyses, ¹H NMR data, and catalytic reaction details. CCDC 2338522. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi00692e

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