The design of a novel and resistant Zn(PZDC)(ATZ) MOF catalyst for the chemical fixation of CO₂ under solvent-free conditions

Abstract

A well-designed 3D metal–organic framework with accessible Lewis acid–base sites, [Zn₂(PZDC)(1/2ATZ)₂(H₂O)₂·2.5H₂O] (H₂PZDC = 3,5-pyrazoledicarboxylic acid, 5-ATZ = 5-aminoterazole) abbreviated as Zn(PZDC)(ATZ), was successfully prepared by incorporating zinc(II) ions, 3,5-pyrazoledicarboxylic acid and nitrogen-rich 5-aminoterazole. The existence of abundant Lewis bases can increase the chemical affinity to carbon dioxide, thus resulting in their high CO₂ capture ability. Additionally, the heterogeneous Zn(PZDC)(ATZ) catalyst facilitated the CO₂ coupling reaction of various epoxides into cyclic carbonates with the aid of Bu₄NBr under solvent-free conditions (90 °C, 1 MPa, 6 h); besides, a high TON (131.0) and TOF (21.8 h⁻¹) of propylene carbonates were obtained. Furthermore, the Zn(PZDC)(ATZ) catalyst also showed good chemical stability and recyclability for seven cycles without a significant catalytic loss. What's more, a plausible catalytic mechanism of synergistic effects derived from Zn(PZDC)(ATZ)/Bu₄NBr catalysts for CO₂ conversion to cyclic carbonate was proposed.

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Introduction

The greenhouse effect has already become an intractable environmental problem due to the excessive emission of carbon dioxide caused by human activities from the large-scale use of fossil energy.^1,2 Carbon capture and utilization (CCU) could transform CO₂ as carbon feedstock into high value-added chemicals so as to limit CO₂ destabilization;³ therefore, the development of the capture and conversion of CO₂ has drawn considerable interest. The main challenge for CO₂ utilization is the high kinetic and thermodynamic stability of the CO₂ molecule, which requires a large amount of energy to be activated. The effective and industrial application method for CO₂ utilization is the conversion of CO₂ into cyclic carbonates (Scheme 1), which is attractive for its 100% atom-economical reaction in terms of sustainable and green chemistry.⁴ Furthermore, the cyclic carbonates can be employed as reaction intermediates for chemicals, precursors of polymers, electrolytes and so on.⁵

Scheme 1 Synthesis of cyclic carbonates from CO₂ and epoxide.

Numerous catalysts for coupling CO₂ and epoxides into cyclic carbonates have been reported in the past few decades. For example, homogeneous catalysts showed exceptional properties under mild conditions, such as ionic liquids (ILs),⁶ metal complexes⁷ and organic bases;⁸ however, the problems of product separation and recyclability decreased the demand of these homogeneous catalysts and limited industrial production. Therefore, more and more attention has been focused on the development of heterogeneous catalysts like functional zeolites⁹ and porous organic polymers.¹⁰ In particular, a series of self-assembled MOFs with an ordered structure, tunable architecture, high adsorption capacity and modifiable groups have emerged as novel catalysts for the fixation of CO₂. For instance, MIL-68(In)-NH₂,¹¹ ZIF-78(Zn),¹² and BIT-103(Zn)¹³ have been developed and presented high catalytic performance at high temperature. Particularly, in order to meet the needs of activity and selectivity, organic solvents were added into the reaction system and the preparation of catalysts was complex and expensive.^14,15 Even some MOFs such as MOF-892(Zr) exhibited catalytic performance at ambient CO₂ pressure over 60 h.¹⁶ Moreover, only a few studies have considerably concentrated on the stability of catalysts, for example, MOFs based on Zn ions and organic carboxylates are usually subjected to hydrolysis in the presence of moisture.^17,18 Obviously, these disadvantages restrict their applications in the industrial production. For the CO₂ cycloaddition reactions, it is generally regarded that the synergistic effects of Lewis acid–base sites and the auxiliary effect of nucleophilic reagents from the catalysts are favorable to CO₂ cycloaddition to epoxide smoothly.^19,20 Furthermore, the amino group and water existed in MOFs as hydrogen bond donors (HBD) are conducive to the activation of propylene oxide (PO) and the abundant nitrogen units in MOFs can improve the affinity of CO₂ molecules.²¹ Taking these into account, it is imperative to design the metal organic framework catalyst with Lewis acid–base sites that are inexpensive, efficient, stable, and highly resistant to the synthesis of cyclic carbonates.

Herein, a novel zinc-based MOF catalyst constructed from the mixed ligands of 3,5-pyrazoledicarboxylic acid and 5-aminoterazole has been reported, the developed Zn(PZDC)(ATZ) not only contains Lewis acid–base sites with a three-dimensional framework in crystals but also shows great chemical affinity to CO₂ due to the existing abundant N atoms. What's more, this Zn(PZDC)(ATZ) with unique structures displays fascinating catalytic properties for the cycloaddition reaction of epoxides and CO₂, and can be easily recycled for seven runs. Remarkably, the Zn(PZDC)(ATZ) catalyst possesses excellent hydrothermal and structural stability in a wide range of pH values. Finally, a possible synergistic mechanism catalyzed by Zn(PZDC)(ATZ)/Bu₄NBr catalysts under solvent-free conditions was proposed. The developed Zn(PZDC)(ATZ) can both capture and transform carbon dioxide, and its performance is comparable or superior to the reported MOFs in the literature (as shown in Table 3).

Results and discussion

Zn(PZDC)(ATZ) catalyst characterization

Crystal structure description. The compound Zn(PZDC)(ATZ) crystallizes in an orthorhombic system with the space group Pnnm and displays a three-dimensional (3D) framework with one-dimensional (1D) channels. The asymmetric unit of Zn(PZDC)(ATZ) consists of two crystallographically independent Zn(II) centers, one PZDC²⁻ anion, two halves of ATZ⁻ anions and two coordinated water molecules (Fig. 1a). Both zinc centers locate in a slightly distorted octahedral coordination geometry and are coordinated by three carboxylate oxygen atoms from three different PZDC²⁻ anions, two nitrogen atoms from PZDC²⁻ and ATZ⁻ anions, respectively, and one coordinated with water molecules. The bond distances range for Zn–O from 2.035(3) to 2.347(2) Å and for Zn–N from 2.062(3) to 2.104(3) Å. The H₂PZDC ligand in Zn(PZDC)(ATZ) loses three protons to form the PZDC²⁻ anion, in which two protons arise from carboxyl groups and one proton from the pyrazole ring. The PZDC²⁻ anion adopts a μ6-κN,O:κO:κO′:κO′′:κO′′′:κO′′′,N′ coordination mode, in which both carboxylate groups coordinated with three zinc centers by the η1:η2:μ3 coordination mode (Fig. 1b). Thus, the zinc centers are connected by the PZDC²⁻ anion into a 2D sheet in the ab plane (Fig. 2a). The ATZ⁻ anions coordinated to the Zn center of adjacent 2D sheets with a Zn⋯Zn distance of 6.3358(2) Å. Thus, the 2D sheets are further pillared by the ATZ⁻ anions to construct the 3D framework. A feature of Zn(PZDC)(ATZ) is that there are 1D rectangular tunnels with dimensions of ca. 6.34 Å × 7.84 Å, which are filled by the lattice water molecules (Fig. 2b). Furthermore, the 1D tunnels in the neighboring sheets are interlaced with each other at an angle of ca. 68.12 degrees.

Fig. 1 (a) A view of the asymmetric unit and some symmetry-related atoms in Zn(PZDC)(ATZ). [Symmetry codes: (i) x, y, −z; (ii) −x, −y, z; (iii) −0.5 − x, 0.5 + y, 0.5 − z; (iv) x, y, 1 − z]. (b) The coordination mode of the PZDC²⁻ anion.

Fig. 2 (a) View down the c axis, showing the 2D sheet structure built up of Zn centers connected by the PZDC²⁻ anion. (b) A view of the 3D framework of Zn(PZDC)(ATZ).

PXRD and FT-IR characterization. The probed powder diffraction patterns had good agreement between the simulated and measured samples in Fig. 3a, which confirmed the structural integrity and phase homogeneity.

Fig. 3 (a) PXRD patterns of the as-synthesized Zn(PZDC)(ATZ) and simulated, (b) FT-IR spectra of Zn(PZDC)(ATZ).

Furthermore, the stretching vibration of the primary amine was observed at 3458 and 3453 cm⁻¹ in the FT-IR spectrum (Fig. 3b). The absorption bands at 792 and 755 cm⁻¹ were attributed to the N–H flexural vibrations of the primary amine,²² and a broad band that appeared at around 3180 cm⁻¹ was ascribed to the O–H stretching vibration,²³ coming from the water molecules contained in Zn(PZDC)(ATZ). Moreover, the absorption band at around 1652 cm⁻¹ belonged to the C=O stretching vibration.²⁴ The possibility of the involvement of the tetrazolium and pyrazole carboxylic acid ligands in the coordination was confirmed in Zn(PZDC)(ATZ).

XPS analysis. XPS was assigned to investigate the surface photo-electron states of elements in the Zn(PZDC)(ATZ) sample as shown in Fig. 4. The survey spectrum indicated that it consisted of C, N, O, Zn signals. The Zn 2p XPS high resolution spectrum in Fig. 4b was deconvoluted into four peaks, two peaks at 1021.4 and 1026.7 eV attributed to Zn 2p_3/2, while the Zn 2p_1/2 region contained the peaks at 1044.8 eV and 1049.7 eV.

Fig. 4 XPS spectra of (a) the survey spectrum and (b) the Zn 2p XPS core level spectrum of Zn(PZDC)(ATZ).

Thermal analysis. The TG-DSC revealed the thermal stability and decomposition of the skeleton construction of Zn(PZDC)(ATZ). As shown in Fig. 5, the weight loss could be decomposed into four steps, the initial 10.2% weight loss before 170 °C was attributed to the loss of existing H₂O in Zn(PZDC)(ATZ). From 170 °C to 300 °C, a small weight loss of 10.3% came from the coordinated water with Zn atoms. Whereas, about 11.7% of Zn(PZDC)(ATZ) that disintegrated within 300–450 °C was assigned to the subsequent decomposition of organic ligands under an N₂ atmosphere. Ultimately, the residual mass of 48.5% was caused by a complete collapse. Thus, the Zn(PZDC)(ATZ) sample was dried at 120 °C before the catalytic test and Zn(PZDC)(ATZ) possessed thermostability during the process of CO₂ fixation reactions conducted at 90 °C in the following.

Fig. 5 TG-DSC curves of the as-synthesized Zn(PZDC)(ATZ).

Gas adsorption properties. The incorporation of nitrogen-rich groups into Zn(PZDC)(ATZ) was expected to result in high CO₂ sorption behaviors. As illustrated in Fig. 6, its CO₂ adsorption capacity reached 119 mg g⁻¹ at 273 K and 80 mg g⁻¹ at 298 K under 1 atm. The CO₂ uptake over Zn(PZDC)(ATZ) at 298 K was comparable to USTC-253(Al) which possessed adsorption sites (79 mg g⁻¹, 1 atm)²⁵ and higher than several reported MOFs such as MOF-892(Zr) (38 mg g⁻¹, 0.8 atm),¹⁶ MOF-177(Zn) (35 mg g⁻¹, 1 atm),²⁶ and ZnGlu (5 mg g⁻¹, 1 atm).²⁷ Based on the adsorption isotherms at 273 and 298 K, the isosteric heat of adsorption (Q_st) was calculated to be ∼19.8 kJ mol⁻¹ (ESI, Fig. S1†),¹⁵ which suggested the ease of desorption and adsorbent regeneration. Meanwhile, Zn(PZDC)(ATZ) showed a slightly higher Q_st value than Zn₄O(BDC-NH₂)₃ (IRMOF-3, 19 kJ mol⁻¹);²⁸ but less than those of [Zn₂(NH₂BDC)₂(dpNDI)]_n (46.5 kJ mol⁻¹)²⁹ and [{Cd₂(L-glu)₂(bpe)₃(H₂O)}·2H₂O] (40.8 kJ mol⁻¹).³⁰ However, on the basis of the Zeo++ calculation,³¹ it has been suggested that the aperture window size and maximum pore diameter were 2.23 Å and 4.90 Å, since the corresponding pore window size was smaller than the kinetic diameter of N₂ (3.64 Å), and therefore, no significant N₂ adsorption isotherms could be obtained. In contrast, the Zn(PZDC)(ATZ) framework pores decorated with highly polar NH₂ groups were favourable to the quadrupole moment of CO₂, and thus Zn(PZDC)(ATZ) showed selective adsorption properties of CO₂ over N₂.

Fig. 6 CO₂ adsorption isotherms over Zn(PZDC)(ATZ) at 273 K and 298 K.

CO₂ and epoxide cycloaddition reaction

The cocatalyst screening for CO₂ cycloaddition to propylene oxide. CO₂ and propylene oxide (PO) were selected as the model substrates for the optimization of the reaction conditions (Table 1). It was obviously observed that both Zn(PZDC)(ATZ) and Bu₄NBr showed low activities when used alone (entries 1 and 2). Noticeably, Zn(PZDC)(ATZ) combined with tetrabutyl ammonium bromide (Bu₄NBr) exhibited a significant catalytic activity at 94% propylene carbonate (PC) yield and 99% selectivity at 90 °C and under 1.0 MPa for 6 h without any additional organic solvent (entry 3). The enhanced activity was attributed to the existence of anion nucleophilicity from Bu₄NBr, which was conducive to the ring-opening step in the synthesis of PC. This will be discussed in the tentative reaction mechanism section. Simultaneously, two kinds of common cocatalyst halides were investigated including tetrabutylammonium halides (Bu₄NX with X = Cl, Br, I) and potassium halides (KX with X = Cl, Br, I). To our delight, Zn(PZDC)(ATZ) with a Bu₄NBr cocatalyst displayed a much better synergistic effect than Bu₄NI and Bu₄NCl (entries 3, 5 and 7). This result was ascribed to the higher nucleophilicity of bromide than iodide and to its better leaving ability in comparison with chloride.³² However, only a small number of target products were detected by GC with KX surveyed (entries 4, 6 and 8). During the catalytic reaction, Bu₄NBr was decomposed into tributylamine, which could activate CO₂, thus both the anion and cation from Bu₄NBr played vital roles in the CO₂ cycloaddition reaction.³³ Therefore, all of the following investigations were carried out with Bu₄NBr as the cocatalyst. Even at a low temperature of 50 °C, Zn(PZDC)(ATZ)/Bu₄NBr still showed a moderate 74% catalytic activity (entry 9). Furthermore, the concentration of CO₂ in industrial waste gas is generally lower than 15%,³⁴ and it is important to study the cycloaddition reaction of CO₂ at a lower concentration. The result indicated that the yield of PC only reached 7% at room temperature and 1 atm pressure with mixed gases of 15% CO₂ and 85% N₂ (entry 11).

Table 1 Screening of the cocatalyst for the CO₂ cycloaddition reaction^a

Entry	Catalyst	Cocatalyst	T (°C)	Time (h)	Y (%)	S (%)
a Reaction conditions: PO (34.5 mmol), Zn(PZDC)(ATZ) (0.25 mmol), cocatalyst (100 mg), 1.0 MPa. b Based on the GC analysis. c Bu4NBr (400 mg), 1.0 MPa. d Bu4NBr (400 mg), CO2 concentration of 15% at 1 atm.
1	Zn(PZDC)(ATZ)	—	90	6	5	99
2	—	Bu4NBr	90	6	28	98
3	Zn(PZDC)(ATZ)	Bu4NBr	90	6	94	99
4	Zn(PZDC)(ATZ)	KBr	90	6	2	99
5	Zn(PZDC)(ATZ)	Bu4NI	90	6	86	96
6	Zn(PZDC)(ATZ)	KI	90	6	16	99
7	Zn(PZDC)(ATZ)	Bu4NCl	90	6	59	98
8	Zn(PZDC)(ATZ)	KCl	90	6	4	99
9^c	Zn(PZDC)(ATZ)	Bu4NBr	50	24	74	98
10^c	Zn(PZDC)(ATZ)	Bu4NBr	30	24	35	99
11^d	Zn(PZDC)(ATZ)	Bu4NBr	25	24	7	97

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Effects of reaction parameters. It was worth pointing out that temperature, cocatalyst loadings, CO₂ pressure, and time variables had effects on the catalytic activities. Obviously, the temperature was a vital factor in the cycloaddition reaction. When the temperature rose from 60 °C to 100 °C (Fig. 7a), PC was obtained with 97% yield, and the selectivity was excellent. This result was due to that the high temperature promoted the effective collisions between the reaction substrate and the active centers of the catalyst. In consideration of energy utilization, the following reactions were performed at 90 °C.

Fig. 7 Effects of different reaction condition parameters on the PC synthesis. (a) Effect of reaction temperature, conditions: PO (34.5 mmol), Zn(PZDC)(ATZ) (0.25 mmol), Bu₄NBr (100 mg), 1.0 MPa, 6.0 h; (b) effect of CO₂ pressure, conditions: PO (34.5 mmol), Zn(PZDC)(ATZ) (0.25 mmol), Bu₄NBr (100 mg), 90 °C, 6.0 h; (c) effect of Bu₄NBr loading, conditions: PO (34.5 mmol), Zn(PZDC)(ATZ) (0.25 mmol), 90 °C, 1.0 MPa, 6.0 h; (d) effect of reaction time, conditions: PO (34.5 mmol), Zn(PZDC)(ATZ) (0.25 mmol), Bu₄NBr (100 mg), 90 °C, 1.0 MPa.

Remarkably, when the pressure was increased from 0.5 to 1.0 MPa at 90 °C, the yield of PC had a dramatic increase (Fig. 7b); however, the PC yield had little changes in the pressure range of 1–3 MPa, which was attributed to CO₂ saturation in the reactive phase with the pressure increasing. What's more, as shown in Fig. 7c, when 100 mg Bu₄NBr was used, the PC yield was up to 94%. However, the yield increased slightly with the further addition of the cocatalyst.

Additionally, on the basis of the above optimization conditions, the effect on the reaction time was investigated (Fig. 7d). The yield of PC increased notably in the first 4 h. When the reaction time was further extended and PC increased steadily, apparently, PO was almost completely converted into a PC product with good selectivity.

Recyclability and stability

Encouraged by the superior catalytic performance of Zn(PZDC)(ATZ), its recyclability was also investigated. As shown in Fig. 8a, Zn(PZDC)(ATZ) could be recycled for seven cycles without any significant catalytic deactivation and the selectivity was still above 99%. From the XRD pattern (Fig. 8b), the spent catalyst was identical to the fresh sample, suggesting that Zn(PZDC)(ATZ) possessed remarkable stability. Additionally, a major difference in the peak intensity suggested that the crystallinity of the spent sample was poor through the reaction runs. Meanwhile, it was pointed out that the structure and active sites of the catalyst were not destroyed. Thus, the good catalytic performance of the catalyst could be mainly attributed to the synergistic effects of the active metal sites/amino of the frameworks exposed for interactions with the substrates. In order to further confirm the structural stability, SEM images of the fresh and used Zn(PZDC)(ATZ) catalysts are shown in Fig. S2,† and no obvious morphological changes were observed after seven runs. Simultaneously, the structural stability had been systematically assessed by immersing Zn(PZDC)(ATZ) at different pH values in Fig. 9a. To our surprise, powder X-ray diffraction patterns had no changes, and the unusual chemical stability of Zn(PZDC)(ATZ) might have resulted from the unique structure composed of a high six-coordinated Zn(II) ion subunit and Zn–N coordination interaction, which was stronger than Zn–O cluster interaction,¹⁸ thus the high strength of the metal–nitrogen bond prevented its complete destruction under acidic–alkaline conditions. Generally, MOFs based on Zn ions and organic carboxylates are usually sensitive to moisture, only a few known MOFs show such an excellent chemical stability. So the synthesized Zn(PZDC)(ATZ) was immersed in water for 24 h at different temperatures (100 °C, 120 °C, 150 °C), and then dried at room temperature. Fortunately, no significant peak changes could be observed from the PXRD patterns in Fig. 9b, which indicates the excellent hydrothermal stability of Zn(PZDC)(ATZ).

Fig. 8 (a) Recycling experiments of the Zn(PZDC)(ATZ) catalyst. Reaction conditions: PO (34.5 mmol), Zn(PZDC)(ATZ) (0.25 mmol), Bu₄NBr (100 mg), 90 °C, 1.0 MPa CO₂, 6.0 h, (b) PXRD patterns of the Zn(PZDC)(ATZ) catalyst for fresh and reuses for seven runs.

Fig. 9 PXRD patterns of Zn(PZDC)(ATZ) (a) after being immersed under acid and base conditions for 24 h, (b) treatment in boiling water.

Catalytic cycloaddition of CO₂ to various epoxides

In order to study the generality of Zn(PZDC)(ATZ)/Bu₄NBr catalysts, a series of substituted epoxides were examined, as shown in Table 2, and all could be successfully transformed into the corresponding cyclic carbonates with high yields and excellent selectivity under solvent-free conditions. The yields of cycloaddition products decreased to 94%, 91%, and 58% for 1,2-epoxy propane, 1,2-epoxybutane, and 1,2-epoxyhexane (entries 1–3), respectively. The remarkably lowered catalytic conversion of 1,2-epoxyhexane was mainly ascribed to the steric hindrance of the substituent groups; besides, on account of the large size of 1,2-epoxyhexane (∼8.198 × 3.39 Ų),¹⁵ its interaction with the active sites inside the pore was difficult. However, the yield of Cl-substituted 1,2-epoxybutane reached the highest, because the existence of an electron withdrawing group (–Cl) allowed the breakage of the C–O bond, thereby facilitating the formation of the corresponding carbonate. Nevertheless, for the cycloadditions of styrene oxide and cyclohexene with CO₂, the catalytic activities were remarkably affected (entries 5 and 6), which resulted from the steric hindrance of the substituent that makes the nucleophilic attack of the Br⁻ reaction rate reduce significantly. Fortunately, a satisfactory yield of cyclohexene carbonate could be obtained by increasing the reaction temperature or reaction time.

Table 2 Coupling reactions of CO₂ with various epoxides^a

Entry	Epoxide	Product	T(°C)	Time (h)	Reaction results^b
					Y (%)	S (%)
a Reaction conditions: PO (34.5 mmol), Zn(PZDC)(ATZ) (0.25 mmol), Bu4NBr (100 mg), 1.0 MPa CO2. b Based on the GC analysis.
1			90	6	94	99
2			90	6	91	99
3			90	6	58	89
4			90	4	98	99
5			100	6	87	98
6			100	12	43	69

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Zn(PZDC)(ATZ)/Bu₄NBr catalysts with abundant nitrogen units and Lewis acid sites exhibited high performance for coupling CO₂ into cyclic carbonates under mild conditions (90 °C, 1.0 MPa). It was worthwhile to compare the catalytic activity of our developed Zn-based MOF catalyst with other reported porous materials in the relevant CO₂ cycloaddition reaction (Table 3). Notably, Zn(PZDC)(ATZ)/Bu₄NBr systems displayed comparable activities to some reported catalysts, and it could be concluded that the Lewis acid–base sites in Zn(PZDC)(ATZ) are beneficial for the synthesis of cyclic carbonates.

Table 3 Comparisons of the catalysts of CO₂ with epoxides in previous reports

Type	Catalyst (mmol)	Cocatalyst (mmol)	Epoxide (mmol)	Reaction conditions	TON^d/TOF^e (h^−1)	Ref.
					Yield (%)
a SO, styrene oxide. b PO, propylene oxide. c ECH, epichlorohydrin. d TON (turnover number) = product (mmol)/catalyst (mmol)). e TOF (turnover frequency) = moles of product formed per mole of catalyst per hour.
MOF	Cu(HIP)2(BPY) (0.30)	—	PO^b (18.6)	120 °C, 1.2 MPa, 6 h	38/6.4	35
					62
	CuTrp (0.21)	Bu4NBr (0.21)	ECH^c (25.5)	100 °C, 1.2 MPa, 9 h	118/13.1	36
					98
	Ni(btzip)(H2btzip) (0.20)	Bu4NBr (2.0)	SO^a (20.0)	80 °C, 2.0 MPa, 4 h	40/10.0	37
					40
	Sm(BTB)(H2O) (0.10)	Bu4NBr (0.10)	SO (20.0)	80 °C, 1.0 atm, 15 h	182/12.1	38
					93
	Co(muco)(bpa)(2H2O) (0.10)	Bu4NBr (0.10)	SO (20.0)	100 °C, 1.0 atm, 12 h	171/14.2	39
					86
	Zn(PZDC)(ATZ) (0.25)	Bu4NBr (0.31)	ECH (34.5)	90 °C, 1.0 MPa, 4 h	135/33.8	This work
					98
Porous polymers	PS-HEIMBr (1.60)	—	PO (100)	120 °C, 2.5 MPa, 4 h	61/15.3	40
					98
	CBAP-1 (EDA) (0.40)	Bu4NBr (0.36)	ECH (20.0)	80 °C, 1.0 MPa, 8 h	49/6.1	41
					97
	IT-POP-1 (0.042)	—	PO (41.6)	130 °C, 1.0 MPa, 10 h	950/95.0	42
					96
Other porous materials	P-C3N4 (9.0 wt%)	Bu4NBr (0.69)	PO (28.6)	100 °C, 2.0 MPa, 3 h	38/12.7	43
					91
	NH2–Zn/SBA-15 (5.0 wt%)	—	PO (34.5)	150 °C, 3.0 MPa, 12 h	1020/85	44
					86
	AA-850 (0.27 wt%)	—	ECH (20.0)	150 °C, 4.0 MPa, 16 h	8848/553	45
					54

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Tentative reaction mechanism

Previous studies have shown that the coupling reaction of CO₂ with epoxides can be promoted by the activation of both CO₂ and epoxides. Epoxides are usually activated by forming hydrogen bonds through functional groups such as NH₂ and coordinated water, or the Lewis acid metal sites such as Zn²⁺ could also form M–O adducts to activate epoxides, which give rise to the polarization of C–O of epoxide, availing the subsequent of ring-opening process.^46,47 Moreover, Lewis bases such as the nitrogenous sites can activate CO₂. Likewise, when the developed Zn(PZDC)(ATZ) catalyst was treated with CO₂ at room temperature and 1 MPa, a new absorption peak at 1785 cm⁻¹ was found as shown in Fig. 10, which belonged to the asymmetric stretching C=O vibration of the carbamate salt,⁴⁸ and it indicated that the CO₂ molecule was activated by basic sites with the formation of a carbamate salt (Scheme 2). Besides, the physical absorption of CO₂ appeared as a peak at 2334 cm⁻¹.⁴⁹ Therefore, a tentative mechanism for the chemical fixation reaction of CO₂ is proposed in Scheme 2. First, the cycloaddition reaction was triggered by the activation of the oxygen atoms of epoxides through the Zn(PZDC)(ATZ) catalyst, which made the ring-opening procedure of the epoxide and the subsequent CO₂ insertion easy. Meanwhile, the Br⁻ nucleophile from Bu₄NBr attacked the less hindered β-carbon atom of the epoxides to assist in the ring-opening process. Finally, the intermediate was formed rapidly by the interaction of activated CO₂ with the oxygen anions of the opened epoxy ring, which was further converted into the corresponding cyclic carbonates through the ring-closing step and the catalysts were regenerated for the next run.

Fig. 10 FT-IR spectra of fresh Zn(PZDC)(ATZ) and its adsorption of CO₂ treated at room temperature and 1 MPa.

Scheme 2 Possible reaction mechanism for CO₂ coupling over Zn(PZDC)(ATZ)/Bu₄NBr catalysts. HBD stands for NH₂ and water in the Zn(PZDC)(ATZ) catalyst.

Conclusions

A novel Lewis acid–base 3D zinc-based MOF was successfully prepared under hydrothermal conditions, and the structure was composed of Zn²⁺ and functional NH₂ groups for the purpose of achieving high activity and adsorption. The abundant nitrogen-containing groups presented excellent adsorption to CO₂ molecules at 1 bar (119 mg g⁻¹ at 273 K and 80 mg g⁻¹ at 298 K). By screening the effect of a halide salt cocatalyst, the catalytic performance of Zn(PZDC)(ATZ) was optimized, and organic halides gave a higher activity than the corresponding inorganic halides. Remarkably, Zn(PZDC)(ATZ) showed an excellent activity cooperating with Bu₄NBr for fixing CO₂ to cyclic carbonates under solvent-free and mild conditions. In addition, the scope versatility, reusability and chemical stability were also exhibited. Zn(PZDC)(ATZ)/Bu₄NBr as effective catalysts showed great significance in the practical conversion of CO₂ to valuable chemicals under mild conditions. Also, the attractive advantages such as the better skeleton stability and abundant N atoms of the Zn(PZDC)(ATZ) catalyst might be further extended to other application fields.

Experimental section

Preparation of the Zn-MOF catalyst

The synthesis of [Zn₂(PZDC)(1/2ATZ)₂(H₂O)₂·2.5H₂O] (abbreviated as Zn(PZDC)(ATZ)): a mixture of (0.2 mmol) Zn(NO₃)₂·6H₂O, (0.1 mmol) 3,5-pyrazoledicarboxylic acid, and (0.2 mmol) 5-aminoterazole was dissolved in mixed solvents of H₂O (7.5 mL) and DMF (2.5 mL), and then heated at 120 °C for 33 h, and after the completion of the reaction, the mixture was cooled down to room temperature. The crystals were obtained and dried at 120 °C overnight before use.

Materials and characterization

All reagents and solvents were purchased commercially and used without further purification. Single crystal X-ray diffraction analysis was operated on an Agilent Technology Super Nova Eos Dual system with Mo Kα radiation (λ = 1.54184 Å) at 293 K and processed using CrysAlis^Pro.⁵⁰ The structures were solved and refined by means of SHELXS-97 and SHELXL-97.⁵¹ The patterns of powder X-ray diffraction were conducted on a PANalytical X'Pert PRO with Cu Kα radiation (40 mA, 40 kV) for phase identification. Fourier transform infrared (FT-IR) analysis was conducted on a PerkinElmer Spectrum 100 with KBr pellets in the 4000–450 cm⁻¹ region. Thermogravimetric analysis (TGA) was performed using a NETZSCH STA 449F3 thermal analyzer from 25 to 700 °C at a rate of 10 °C min⁻¹ under a N₂ atmosphere. The sample was pre-treated at 120 °C for 12 h, and then the CO₂ adsorption capacities at 273 and 298 K were measured using a Micromeritics ASAP 2020 system. GC analysis was performed on an Agilent GC-7890A equipped with a capillary column (Agilent 19091J-413) using a flame ionization detector.

Catalytic conversion of CO₂ into cyclic carbonates

All of the catalytic reactions were conducted in a 50 mL stainless-steel autoclave with magnetic stirring. In a typical run, the autoclave was first purged with CO₂ to replace the air in the autoclave, then 100 mg Zn(PZDC)(ATZ) catalyst (0.25 mmol), 100 mg cocatalyst Bu₄NBr, and propylene oxide (34.5 mmol) were added, respectively. The autoclave was heated to 90 °C followed by introducing CO₂ to 1 MPa and stirring for a certain period of time. Upon completion, the mixtures were cooled down to room temperature slowly. The products were quantitatively analyzed by GC. Furthermore, the catalysts could be recycled by centrifugation, washed with diethyl ether, dried under vacuum, and then reused for the next run.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We sincerely acknowledge the financial support from the National Natural Science Foundation of China (21673060) and the State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2017DX10).

Notes and references

1. Z. J. Zhang, Z. Z. Yao, S. C. Xiang and B. L. Chen, Energy Environ. Sci., 2014, 7, 2868–2899.
2. L. T. Mika, E. Cséfalvay and Á. Németh, Chem. Rev., 2018, 118, 505–613.
3. P. Markewitz, W. Kuckshinrichs, W. Leitner, J. Linssen, P. Zapp, R. Bongartz, A. Schreiber and T. E. Muller, Energy Environ. Sci., 2012, 5, 7281–7305.
4. M. Alves, B. Grignard, R. Mereau, C. Jerome, T. Tassaing and C. Detrembleur, Catal. Sci. Technol., 2017, 7, 2651–2684.
5. S. Klaus, M. W. Lehenmeier, C. E. Anderson and B. Rieger, Coord. Chem. Rev., 2011, 255, 1460–1479.
6. S. Yue, P. P. Wang, X. J. Hao and S. L. Zang, J. CO₂ Util., 2017, 21, 238–246.
7. C. K. Karan and M. Bhattacharjee, Inorg. Chem., 2018, 57, 4649–4656.
8. K. R. Roshan, R. A. Palissery, A. C. Kathalikkattil, R. Babu, G. Mathai, H. Lee and D. A. Park, Catal. Sci. Technol., 2016, 6, 3997–4004.
9. C. G. Li, L. Xu, P. Wu, H. H. Wu and M. Y. He, Chem. Commun., 2014, 50, 15764–15767.
10. Y. Xie, T. T. Wang, X. H. Liu, K. Zou and W. Q. Deng, Nat. Commun., 2013, 4, 1960–1967.
11. T. Lescouet, C. Chizallet and D. Farrusseng, ChemCatChem, 2012, 4, 1725–1728.
12. Y. F. Lin, K. W. Huang, B. T. Ko and K. Y. Lin, J. CO₂ Util., 2017, 178–182.
13. X. Q. Huang, Y. F. Chen, Z. G. Lin, X. Q. Ren, Y. N. Song, Z. Z. Xu, X. M. Dong, X. G. Li, C. W. Hu and B. Wang, Chem. Commun., 2014, 50, 2624–2627.
14. H. F. Zhou, B. Liu, L. Hou, W. Y. Zhang and Y. Y. Wang, Chem. Commun., 2018, 54, 456–459.
15. N. Sharma, S. S. Dhankhar, S. Kumar, T. J. D. Kumar and C. M. Nagaraja, Chem. – Eur. J., 2018, 24, 16662–16669.
16. P. T. K. Nguyen, H. T. D. Nguyen, H. N. Nguyen, C. A. Trickett, Q. T. Ton, E. Gutiérrez-Puebla, M. Á. Monge, K. E. Cordova and F. Gándara, ACS Appl. Mater. Interfaces, 2018, 10, 733–744.
17. L. F. Liang, C. P. Liu, F. L. Jiang, Q. H. Chen, L. J. Zhang, H. Xue, H. L. Jiang, J. J. Qian, D. Q. Yuan and M. C. Hong, Nat. Commun., 2017, 8, 1233–1243.
18. S. S. Kaye, A. Dailly, O. M. Yaghi and J. R. Long, J. Am. Chem. Soc., 2007, 129, 14176–14177.
19. H. M. He, J. A. Perman, G. S. Zhu and S. Q. Ma, Small, 2016, 12, 6309–6324.
20. R. Babu, A. C. Kathalikkattil, R. Roshan, J. Tharun, D. Kim and D. Park, Green Chem., 2016, 18, 232–242.
21. M. S. Liu, L. Liang, X. Li, X. X. Gao and J. M. Sun, Green Chem., 2016, 18, 2851–2863.
22. S. Nagata, H. Sato, K. Sugikawa, K. Kokado and K. Sada, CrystEngComm, 2012, 14, 4137–4141.
23. D. Müller, R. Hochleitner and K. T. Fehr, J. Raman Spectrosc., 2015, 47, 602–606.
24. V. Krishnakumar, V. Balachandran and T. Chithambarathanu, Spectrochim. Acta, Part A, 2005, 62, 918–925.
25. Z. Jiang, H. Wang, Y. Hu, J. Lu and H. Jiang, ChemSusChem, 2015, 8, 878–885.
26. A. R. Millward and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 17998–17999.
27. A. C. Kathalikkattil, R. Roshan, J. Tharun, R. Babu, G. S. Jeong, D. W. Kim, S. J. Cho and D. W. Park, Chem. Commun., 2016, 52, 280–283.
28. R. Sabouni, H. Kazemian and S. Rohani, Environ. Sci. Pollut. Res., 2014, 21, 5427–5449.
29. S. S. Dhankhar, N. Sharma, S. Kumar, T. J. D. Kumar and C. M. Nagaraja, Chem. – Eur. J., 2017, 23, 16204–16212.
30. B. Ugale, S. S. Dhankhar and C. M. Nagaraja, Inorg. Chem. Front., 2017, 4, 348–359.
31. T. F. Willems, C. H. Rycroft, M. Kazi, J. C. Meza and M. Haranczyk, Microporous Mesoporous Mater., 2012, 149, 134–141.
32. M. Taherimehr, B. Van de Voorde, L. H. Wee, J. A. Martens, D. E. De Vos and P. P. Pescarmona, ChemSusChem, 2017, 10, 1283–1291.
33. M. North and R. Pasquale, Angew. Chem., Int. Ed., 2009, 48, 2946–2948.
34. Z. F. Dai, Q. Sun, X. L. Liu, C. Q. Bian, Q. M. Wu, S. X. Pan, L. Wang, X. J. Meng, F. Deng and F. S. Xiao, J. Catal., 2016, 338, 202–209.
35. A. C. Kathalikkattil, D. W. Kim, J. Tharun, H. G. Soek, R. Roshan and D. W. Park, Green Chem., 2014, 16, 1607–1616.
36. G. S. Jeong, A. C. Kathalikkattil, R. Babu, Y. G. Chung and D. W. Park, Chin. J. Catal., 2018, 39, 63–70.
37. Y. Z. Li, H. H. Wang, H. Y. Yang, L. Hou, Y. Y. Wang and Z. H. Zhu, Chem. – Eur. J., 2018, 24, 865–871.
38. B. Ugale, S. S. Dhankhar and C. M. Nagaraja, Cryst. Growth Des., 2018, 18, 2432–2440.
39. B. Ugale, S. S. Dhankhar and C. M. Nagaraja, Cryst. Growth Des., 2017, 17, 3295–3305.
40. J. Sun, W. G. Cheng, W. Fan, Y. H. Wang, Z. Y. Meng and S. J. Zhang, Catal. Today, 2009, 148, 361–367.
41. S. Ravi, P. Puthiaraj and W. S. Ahn, J. CO₂ Util., 2017, 21, 450–458.
42. H. Zhong, Y. Q. Su, X. W. Chen, X. J. Li and R. H. Wang, ChemSusChem, 2017, 10, 4855–4863.
43. D. H. Lan, H. T. Wang, L. Chen, C. T. Au and S. F. Yin, Carbon, 2016, 100, 81–89.
44. M. S. Liu, B. Liu, L. Liang, F. X. Wang, L. Shi and J. M. Sun, J. Mol. Catal. A: Chem., 2016, 418, 78–85.
45. X. Ma, B. Zou, M. Cao, S. L. Chen and C. Hu, J. Mater. Chem. A, 2014, 2, 18360–18366.
46. Y. H. Han, Z. Y. Zhou, C. B. Tian and S. W. Du, Green Chem., 2016, 18, 4086–4091.
47. H. Zhong, Y. Q. Su, X. W. Chen, X. J. Li and R. H. Wang, ChemSusChem, 2017, 10, 4855–4863.
48. F. Adam and M. S. Batagarawa, Appl. Catal., A, 2013, 454, 164–171.
49. A. Saad, S. Atwa, N. Yasuda and M. Fujii, Radiat. Meas., 2001, 34, 51–54.
50. A. Rothkirch, G. D. Gatta, M. Meyer, S. Merkel, M. Merlini and H. P. Liermann, J. Synchrotron Radiat., 2013, 20, 711–720.
51. G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1872616. For the ESI and crystallographic data in CIF or other electronic format, see DOI: 10.1039/c8qi01150h

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