Zn2(C9H3O6)(C4H5N2)(C4H6N2)3 MOF as a highly efficient catalyst for chemical fixation of CO2 into cyclic carbonates and kinetic studies

Accepted Manuscript Title: Zn2 (C9 H3 O6 )(C4 H5 N2 )(C4 H6 N2 )3 MOF as a highly efficient catalyst for chemical fixation of CO2 into cyclic carbonates and kinetic studies Authors: Yuanfeng Wu, Xianghai Song, Jiahui Zhang, Siquan Xu, Ningning Xu, Hongmei Yang, Yanan Miao, Lijing Gao, Jin Zhang, Guomin Xiao PII: DOI: Reference:

S0263-8762(18)30564-1 https://doi.org/10.1016/j.cherd.2018.10.034 CHERD 3404

To appear in: Received date: Revised date: Accepted date:

18-7-2018 7-9-2018 17-10-2018

Please cite this article as: Wu, Yuanfeng, Song, Xianghai, Zhang, Jiahui, Xu, Siquan, Xu, Ningning, Yang, Hongmei, Miao, Yanan, Gao, Lijing, Zhang, Jin, Xiao, Guomin, Zn2(C9H3O6)(C4H5N2)(C4H6N2)3 MOF as a highly efficient catalyst for chemical fixation of CO2 into cyclic carbonates and kinetic studies.Chemical Engineering Research and Design https://doi.org/10.1016/j.cherd.2018.10.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Zn2(C9H3O6)(C4H5N2)(C4H6N2)3 MOF as a highly efficient catalyst for chemical fixation of CO2 into cyclic carbonates and kinetic studies Yuanfeng Wu, Xianghai Song, Jiahui Zhang, Siquan Xu, Ningning Xu, Hongmei Yang, Yanan Miao,

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Lijing Gao, Jin Zhang and Guomin Xiao*

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

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E-mail:[email protected]; Tel./fax: +86-25-52090612.

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Graphical Abstract

into

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One-pot conversion of carbon dioxide Zn2(C9H3O6)(C4H5N2)(C4H6N2)3 MOF

Highlights 

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98.93% conversion of epichlorohydrin and 98.32% selectivity to chloropropene carbonate was obtained. Zn-BTC-2MeIm compound was stable in catalytic activity for CO2 conversion. Zn-BTC-2MeIm also can enhance cycloaddition of CO2 with other epoxides. The law of CO2 coupling with ECH was coincident with the first order kinetic.

Abstract Zn2(C9H3O6)(C4H5N2)(C4H6N2)3 (Zn-BTC-2MeIm) was hydrothermally synthesized and employed as a highly efficient catalyst for CO2 coupling with

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Keywords: Zn-BTC-2MeIm, CO2, Epichlorohydrin, Epoxides, Kinetic

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epichlorohydrin (ECH). Various techniques such as XRD, FT-IR, XPS, TG-DTG, NH3/CO2-TPD were employed for characterizing the compound. Interestingly, TG profile illustrated the Zn-BTC-2MeIm was stable above 200 oC. The highest catalytic activity of 98.93% conversion of ECH and 98.32% selectivity to chloropropene carbonate was observed under the optimum conditions (100 oC, 120 mesh, 1000 rpm, 3.0 MPa, 6 h, 0.75 wt.% of ECH). Besides, the recyclability result exhibited Zn-BTC2MeIm compound can be reused no less than three times with a slight reduction in its catalytic ability. Moreover, coupling result of CO2 with other epoxides showed this compound can efficiently convert various epoxides into cyclic carbonates. Finally, the investigation of the kinetic exhibited the law of CO2 coupling with ECH was coincident with the first order kinetic and the activation energy (Ea) was to be 113.38kJ/mol.

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1. Introduction

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Carbon dioxide (CO2) is one of the prevalent greenhouse gases, its rapid increase from fossil energy consumption has triggered some negative change such as global warming and weather fluctuant(Li et al., 2016; Liu et al., 2015; North et al., 2010; Tantikhajorngosol et al., 2017). Therefore, the issue of CO2 utilization is of relative significance both for academic viewpoint and sustainable development, which has motivated several researchers to undertake the exploration of the converting processes(Ravi et al., 2017; Sarfraz and Ba-Shammakh, 2016). Additionally, Carbon dioxide as an inexpensive, abundant, non-toxic C1 resource can be widely used as a starting raw for synthesis of fine chemicals (Cho et al., 2016; Darensbourg and Wei, 2012; Jing et al., 2007; Kongpanna et al., 2015; Zhi et al., 2016). In this regard, the use of CO2 to produce value-added chemicals is of growing interest, especially for energy supply and carbon management (Shen et al., 2015). One of the promising conversion for CO2 utilization is to synthesize five membered cyclic carbonates via CO2 coupling with epoxides (Jawad et al., 2017; Ji et al., 2018; Kelly et al., 2017)(scheme 1). Interestingly, the cyclic carbonates were especially important in industry, commonly employed as precursors or/and intermediate for fine chemicals synthesis, importantly pharmaceutical compounds and polyester (polycarbonates and polyurethanes) (Ema et al., 2012; Peng et al., 2015; Ren et al., 2014). In the recent years, a series of catalysts have been found with the catalytic activity for acceleration of CO2 conversion into cyclic carbonates, including the homogeneous catalysts (quaternary ammonium or phosponium salts(Buckley et al., 2011; Monassier et al., 2013; Ren and Shim, 2013), functional polymer(Ma et al., 2015; Xie et al., 2014), ionic liquids(Appaturi and Adam, 2013; He et al., 2014; Yang et al., 2012) and alkali metal halides(Tharun et al., 2013; Wu et al., 2013)) and heterogeneous catalysts (metal oxides(Kazuya Yamaguchi, 1999) and metal salen complexes(Buonerba et al., 2015; Roy et al., 2016; Tian et al., 2012)). However, usual requirement of the catalytic systems

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2. Experimental

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was the additives, organic solvents under the harsh reaction conditions, which has urged some researchers in the recent years to undertake the investigation of a promising catalyst for carbon dioxide conversion. Importantly, a series of the metal-organic frameworks (MOFs) have been found with the catalytic activity for enhancement of CO2 coupling with epoxides(Babu et al., 2016; Gao et al., 2016; Song et al., 2017). For example, Mn-BTC MOF has been investigated as the highly efficient catalyst for CO2 with epichlorohydrin (98% conversion of epichlorohydrin and 96.05% selectivity to chloropropene carbonate)(Wu et al., 2018). MOF-5 was also explored to promote propylene carbonate synthesis from CO2 and propylene epoxide in the presence of a cocatalyst (n-Bu4NBr) (Song et al., 2009). Synthesis of chloropropene carbonate (CPC) via chemical fixation of CO2 were also respectively enhanced by ZIF-8 and Cu3(BTC)2 without any additives(Macias et al., 2012; Miralda et al., 2012). Besides, aminemodified MIL-101(Jang et al., 2015) and MIL-125(Kim et al., 2013) were also applied as the highly effective catalysts for enhancing conversion of CO2 to give chloropropene carbonate, respectively. In addition, the catalytic activity was associated with the coordinated metal ions which as Lewis acid sites can facilitate the cycloaddition of CO2 with epoxides. The metal organic frameworks, Zn2(C9H3O6)(C4H5N2)(C4H6N2)3 (Zn-BTC2MeIm), was found with a large number of Lewis acid sites in its framework, which was not explored as a heterogeneous catalyst for CO2 coupling with epoxides. Hence, the present work aimed to investigate the catalytic activity of Zn-BTC-2MeIm for CO2 conversion. Various parameters including, string speed, particle size, initial CO2 pressure, reaction temperature, catalyst amount, reaction time were studied with the use of epichlorohydrin as coupling reagent. What’s more, other coupling reactions were also performed with the use of Propylene oxide, Allyl glycidyl ether, and 1, 2epoxybutane as substrates. Finally, the kinetic law of CO2 conversion was also investigated.

2.1Chemicals

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Zn(NO3)2.6H2O (>99.0%); 1, 3, 5-benzenetricarboxylic acid (H3BTC, >98.0%), 2Methylimidazole (2MeIm, >99.0%); Dimethylformamide (DMF, >99.5%); Epichlorohydrin (ECH, >99.8%) and Propane oxide (PO, >99.5%) were purchased from Sinopharm Chemical Reagent Co. Shanghai, China. Allyl glycidyl ether (AGE, >99.0%) were purchased from sigma-Aldrich Corporation (shanghai, China). 1, 2-epoxybutane (1,2EB, >99.0%) was purchased from Tokyo Chemical Industry (shanghai, China). All the reagents were of analytical grade without further purification. 2.2 Catalyst Synthesis Zn-BTC-2MeIm sample was synthesized under the modified hydrothermal method reported by Maniam(Maniam and Stock, 2011). In a typical synthesis, 6 mmol Zn(NO3)2.6H2O and 3 mmol H3BTC, 9 mmol NaOH, 15mmol 2-Methylimidazole and 250 mL deionized water were respectively charged into 300 ml Teflon-lined stainless autoclave. The autoclave was sealed after vigorously stirring for 20 min, and

maintained at 120 oC for 48 h. After the reaction system was cooled down to room temperature, colorless crystal was obtained and washed several times with deionized water to remove the residual ions and treated at 60 oC for 8 h in static ambient to eliminate the adsorbed water molecule. 2.3 Catalyst characterization X-ray diffraction (XRD) pattern of Zn-BTC-2MeIm was recorded in 2θ range of 5-60 on the Rigaku D/max-A instrument (Cu kα, 40 kV, 20 mA, 10 o/min). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB-250 (Thermo-VG Scientific, USA) spectrometer equipped with Al Kα (1486.6 eV) irradiation. The binding energies (B.E.) value were calibrated internally based on the adventitious carbon deposit C (1s) peak at 284.8 eV. Fourier transform infrared (FT-IR) spectra of the sample was recorded in the wavenumber range of 400 to 4000 cm-1 with a 2 cm-1 resolution on a Nicolet 5700 spectrometer adopting the KBr disc method Thermo gravimetric (TG) analysis of the Zn-BTC-2MeIm was measured under nitrogen atmosphere with a linear heating rate of 10oC (30~800 oC) on a thermal analyzer SDT Q600. The overall acid and basic sites of the Zn-BTC-2MeIm were determined by temperature-programmed desorption of CO2/NH3 (CO2/NH3-TPD) performed on the TP-5056 equipment connected to a TCD detector. 0.10 g sample was first pretreated in Helium atmosphere at 200 oC for 1 h, then cooled down to 30 oC maintained in the CO2/NH3 atmosphere for 30 min and purged with He for 50 min in order to eliminate the physically adsorbed gas (CO2/NH3). Finally, the sample was heated at a linear rater of 10 oC/min to 350 oC in order to acquire the desorption curve.

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2.4 Catalytic reaction

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CO2 coupling with epoxide (scheme 1) was performed in a 100 ml stainless autoclave equipped with an electromagnetic stirrer and a thermocouple. Typically, ECH and Zn-BTC-2MeIm were charged into the stainless reactor. The system was sealed, purified with CO2 three times to remove air, and then heated to the setting point. During the catalytic process, the reaction system was stirred vigorously. When kept for 8 h, the system was cooled down to room temperature. The catalyst was collected via centrifugation, washed with ethanol to remove organic components, and dried at 60 oC for 5h for further research. The ECH conversion and CPC selectivity was analyzed by flame ionization detector Gas chromatograph (Teng Hai, Shandong, China, GC-6890) with the ethylene glycol monobutyl ether as the internal standard. 3. Results and discussion 3.1 Characterization of the compound XRD patterns of Zn-BTC-2MeIm are exhibited in Fig. 1. It can be seen that the XRD pattern of the as-prepared Zn-BTC-2MeIm matches well with that of the simulated pattern, which suggested the crystal material was successfully papered. The Basic structural unit of Zn-BTC-2MeIm without hydrogen atoms was

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presented in Scheme 2. In the mixed-ligand MOFs material, Zn2+ ions linked to the N atoms of (2-MeIm)- ions and 2-MeImH molecules and were also coordinated by the (BTC)3- ions via Zn-O bonds. Besides, two different distorted tetrahedra (ZnN3O and ZnN2O2) linked to each other through BTC ligands and N:N’ bridge formed by 2-MeIm ions, and both of the tetrahedra were coexisted in its crystal structure and formed a layered structure in a plan(Maniam and Stock, 2011). Like other metal-organic crystal materials, Zn clusters in the Zn-BTC-2MeIm crystal structure can serve as the Lewis acid sites, and the Brønsted acid sites (-CO2H) can also be formed with the cleavage of Zn-O bonds. Importantly, the two species have been found with the catalytic activity of promoting the cycloaddition of CO2 with epoxides(He et al., 2016; Wen Yang Gao, 2016). Fig.2 displayed the FT-IR spectrums of Zn-BTC-2MeIm. The broad adsorption band located at 3500-3000 cm-1 was mainly due to the stretching vibrations of -OH group associated with physically adsorbed water molecules(Lyszczek, 2008; Sun et al., 2008). The bands in range of 2890-2780 cm-1 correspond to the characteristic groups of H-N that was related to 2-MeImH (Scheme 2). The adsorption bands corresponding to –COOH group were not found in the range of 1800-1680 cm-1, suggesting the complete deprotonation of BTC ligands(Sánchez-Andújar et al., 2014). While, four adsorption peaks appear in range of 1645-1550 cm-1 and 1470-1390 cm-1, associated with asymmetric vibration and symmetric vibration of the carboxylate groups(Sun et al., 2017; Zeng et al., 2016). This also implied the (BTC)3- ions have completely coordinated with Zn2+ ions(He et al., 2006). The bands at 1006 and 426 cm−1 are related to a C−H bending modes and metal oxygen stretching vibration. The bands at 1103 cm1 and 757-721 cm-1 were attributed to stretching vibrations of C-C groups and the deformation vibrations of the C-H groups in the benzene ring(Shi et al., 2015; Singh et al., 2016). No change in the FT-IR spectrum of the recovered Zn-BTC-2MeIm was observed after the recovered sample was washed with ethanol several times, elucidating the Zn-BTC-2MeIm was stable in structure when employed in the coupling reaction. The surface characteristics and chemical state of the Zn-BTC-2MeIm was investigated through XPS technology. It can be seen that the binding energies around 285, 399, 531.0, 1022 and 1045 eV in the full spectrum (Fig. 3a) correspond to the C 1s, N 1s, O 1s, Zn2p3/2 and Zn2p1/2 levels, respectively. To further illuminate the chemical state in detail, the spectrums have been fitted with several symmetrical peaks for analysis of the chemical bonds (Fig.3 b-d). The C 1s spectrum (Fig. 3b) was deconvoluted into three symmetrical peaks at 284.6, 286.1, and 288.1 eV, which were associated with the C-C/C=C, C-N, C=O functional groups, respectively(Tu et al., 2017; Wang et al., 2017). The N 1s spectrum (Fig. 3c) was fitted with three symmetrical peaks concentrated at 398.6, 399.3 and 400.3 eV, corresponding to the N-Zn(Jayalakshmi et al., 2013), N-C(Erdogan et al., 2013) and N-H(Gowthaman et al., 2017) bonds, respectively, which implies the N atoms belonging to 2-MeImH molecules/(2-MeIm)ions have coordinated with Zn2+ ions in the mixed-ligand compound. It can be observed from Fig.3d, the Zn2p1/2 and Zn2p3/2 in the Zn-BTC-2MeIm were located at the B.E. of 1044.8 eV and 1021.8 eV, respectively and the split separation energy between Zn2p1/2 and Zn2p3/2 was 23 eV, demonstrating Zn is in ionic state in Zn-BTC-2MeIm

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(Deerattrakul et al., 2017). To further confirm the chemical states of Zn in the compound, the Zn 2p3/2 was chosen and fitted with symmetrical components as shown in the inserted figure (Fig.3d). The cure-fitting analysis of the Zn 2p3/2 spectrum showed the B.E. at 1021.3 eV and 1022.1 eV were assigned to the Zn-N and Zn-O bonds (Gangil et al., 2008; Mapa et al., 2009), which further confirms that Zn ions in the compound bridge the BTC and 2-MeIm ligands (Scheme 2). The thermal behavior of the Zn-BTC-2MeIm was determined at a linear heating rate of 10 °C/min in the nitrogen atmosphere (Fig.4). It is identified that the TG profile can be generally divided into three stages on basic of DTG curve. 13.44% weight loss was detected in the first stage (60-190 °C), which was attributed to the evaporation of physisorbed water molecules from Zn-BTC-2MeIm surface. 2.41% weight loss was observed in the region of 190-269 oC, which was caused by release of the adsorbed water molecules from the internal structure of Zn-BTC-2MeIm. The third stage (8.11%) from 270 to 335 was mainly due to the structure collapse and removal of 2MeIm ligand. Approximate 36.41% weight loss was measured in the range of 335 to 515 oC, which results from the combustion of BTC ligand. According to the above analysis, Zn-BTC2MeIm were stable above 200 °C, which was a significant property for the compound when applied in enhancing the cycloaddition of CO2 with epoxides. NH3/CO2-TPD curves of the Zn-BTC-2MeIm were measured in the range of 30o 300 C with the helium flow as protecting gas. As Fig.5b presented that the profile of the Zn-BTC-2MeIm treated in the gas flow of CO2 was observed without obvious change in TCD signals when the desorption temperature was below 270 oC, which may be due to the presence of strong interaction between carbon dioxide and basic sites that the desorption of carbon dioxide required a higher temperature. Interestingly, a large desorption area appears in the range of 50 - 270oC after the adsorption gas was replaced with NH3 (Fig.5a), which represent the weak and moderate Lewis acid sites, respectively. This result also demonstrated this compound can promote the cyclic carbonates synthesis from carbon dioxide coupling with epoxides.

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3.2 Cycloaddition of CO2 with epichlorohydrin

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In the preliminary studies, the conversion of ECH and the selectivity to CPC were regarded as the determining parameters which serve as a guidance for optimization of various parameters in acceleration of carbon dioxide coupling with ECH. Effects of particle size (80, 100, 120 mesh) on the ECH conversion was measured and the result was shown in Fig.6a. For all samples, the conversion of ECH firstly increases obviously, and then maintained a constant. When Zn-BTC-2MeIm in 120 mesh was adopted in promoting the coupling reaction, the conversion of ECH reaches the top value with the shortest time, which suggested the 120 mesh sample can easily eliminate the effect of internal diffusion. Effects of stirring speed on the ECH conversion was elucidated in Fig.6b. When the stirring speed was set at 400 rpm, the conversion of ECH continuously increased with the reaction time prolongation. After the stirring speed was altered above 1000 rpm, the ECH conversion was very close to a constant when the reaction time was above 5h. Therefore, to eliminate the effects of internal and external diffusion, the optimized

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parameters (120 mesh, 1000 rpm) were employed for facilitating the cycloaddition of CO2 with ECH. The effect of reaction temperature on the ECH conversion and CPC selectivity was investigated in the range of 90-120oC (Fig.7a). The conversion of ECH was continuously enhanced from 90.13% (90 oC) to 98.75% (120 oC) with the reaction temperature increase, while the CPC selectivity presented an opposite change. The highest yield was observed with 97.77% conversion of ECH and 98.7% selectivity to CPC (100 oC). This can be explained with the follow reasons: low reaction temperature cannot offer enough energy for the cycloaddition of CO2 with ECH in the fixed time, and the excessive reaction temperature made the adsorbed water release from the internal structure of Zn-BTC-2MeIm, which facilitated hydrolysis of ECH into 3Chloro-1,2-propanediol. Therefore, considering the energy cost and product purity, 100 o C was considered as the optimized temperature for the successive research. The effect of catalyst amount on the yield of CPC was presented in Fig.7b. The conversion of ECH was rapidly accelerated from 88.49% (0.25 wt.%) to 98.93 % (0.75 wt.%) and exhibited a slight fluctuation. The selectivity to CPC decreased from 98.62% (0.25 wt.%) to 97.71% (1.0 wt.%), which was attributed to the increase of ECH hydrolysis, in a good agreement with the temperature effect (Fig.7a). Therefore, 0.75wt.% catalyst of ECH was optimal for achievement of the product with a higher purity. Initial pressure is an important parameter for investigation of CO2 coupling with epoxides. The effect of initial pressure on the CO2 coupling with ECH was investigated in the range of 2.0 MPa to 4.0MPa (Fig.7c). ECH is in its liquid and gas form at 100 oC and the coupling reaction should be perfectly promoted with the increase in CO2 initial pressure(Wu et al., 2008). The conversion of ECH was continuously facilitated from 88.03% (2.0MPa) to 98.93 % (3.0MPa) and then decreased to 95.39% (4.0 MPa). This can be perfectly explained by the following reason: low pressure can’t offer enough efficient molecular for ECH conversion and excess amounts of carbon dioxide in the reactor could inhibit the access of the ECH to the active sites(Kuruppathparambil et al., 2016). Therefore, initial pressure of 3.0 MPa was perfect in promotion of CO2 cycloaddition with ECH in this fixed system. The effect of reaction time on the coupling reaction of CO2 with and ECH was investigated under the optimized reaction conditions (100 oC, 3.0 MPa and 0.75%wt. catalyst of ECH) and the result was elucidated in Fig.7d. The conversion of ECH was obviously enhanced in the first 6 h and maximized to 98.93%, and then maintained a constant with time, while the CPC selectivity changed oppositely with a slight drop. This may be due to the following reason: the adsorbed water molecular derived from the internal structure has reacted with ECH increasingly, which was in a good agreement with the result of catalyst amount effect (Fig.7b). Therefore, the best result was observed with 98.93 % of ECH conversion and 98.32 % of CPC selectivity under the exploringly optimum conditions. 3.3 Coupling reaction with epoxides To extend the scope of the coupling reaction, various epoxides such as Propylene oxide, Allyl glycidyl ether, 1, 2-epoxybutane were respectively employed as substrates

for investigation of the catalytic activity of Zn-BTC-2MeIm and the result was listed in Table 1. Interestingly, Zn-BTC-2MeIm can effectively promote all the studied coupling reactions under the optimized reaction conditions, of which ECH, Allyl glycidyl ether (AGE) and 1, 2-epoxybutane (1, 2EB) were converted to the corresponding cyclic carbonate with more than 80% yield. When propylene oxide (PO) was applied as a substrate for CO2 fixation, the lowest yield was achieved, which was mainly caused by the low reactivity of PO molecular and the opening ring difficultly occurred during the coupling process (Miao et al., 2008). Besides, a relatively high catalytic activity of ZnBTC-2MeIm was perfectly clarified compared with the reported catalysts (Table 1).

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According to the literature(Mousavi et al., 2016), nitrogen atoms of 2MeIm have been found with the ability of serving as basic sites. Therefore, Zn-BTC-2MeIm can be confirmed with the catalytic activity for promoting conversion of CO2 with the epoxide as coupling reagent. According to the mentioned above, a mechanism for carbon dioxide coupling with epoxide was proposed in Scheme 3. Firstly, the epoxide linked to the unsaturated Zinc nodes and CO2 was activated by the 2MeIm ligand, forming the carbonate species. Then the carbonate species attracted the less sterically hindered carbon atom of the coordinated epoxide, subsequently producing an intermediate which was further converted into the cyclic carbonate after the ring-close reaction occurred. Finally, the regenerated catalyst participates in the next cycle.

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3.4 Recyclability of the Zn-BTC-2MeIm The recyclability of Zn-BTC-2MeIm on promotion of CO2 coupling with ECH was studied under the optimal reaction conditions (Fig.8). It can be observed that the selectivity to CPC firstly increases from 97.86% to 99.15% and then shows a slight fluctuant, which was attributed that the adsorbed water molecular has reacted with ECH in the first catalytic reaction. The conversion of ECH only exhibited a slight decrease when the sample was reused for three times, which can be interpreted by the following reasons that organic components was gradually accumulated and adsorbed on the surface of the Zn-BTC-2MeIm and the increasing difficult in contact between the active site and the reactants occurred during the catalytic reaction, which can be further confirmed by CO2-adsorption (Fig.S1) and N2-adsorption (Fig.S2). To further confirm the viewpoint, the XRD pattern of the recovered crystal material washed with ethanol three times was measured as presented in Fig.9. Interestingly, the XRD pattern of the recovered sample was in good agreement with the simulated pattern, which means the decreased catalytic activity was mainly due to the adsorbed organic components instead the structure collapse. Additionally, the stable crystal structure of the recovered sample was also validated by FT-IR (Fig.2).

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3.5 kinetic studies For carbon dioxide coupling with ECH, the reaction kinetic equations were deduced as follows

Where, A, B, C represent the ECH, carbon dioxide, chloropropene carbonate, respectively. The starting raw of carbon dioxide was existed in the form of gas. Therefore, the general

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rate equation for the Eqs.(1) is: dc  A  kcAn (2) dt Where k is the rate constant, cA is the concentration of ECH after time t, n is the order of reactant ECH. In addition: cA  cA 0 1  x  (3) Where c A0 and x represent the initial concentration and conversion of ECH, respectively. Combined Eqs.(2) with Eqs.(3), and the equation can be further transformed as: dx n 1 (4)  1  x n  kcA0  dt

 1  1  1  kcAn01t (n  1)  n 1 n  1  1  x  

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Then the definite integrals of Eqs.(4) was performed in the integral interval of (0, 0) to (x, t), and the equation was shown as follow:

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When n=0, Eqs.(5) was transformed as follow: x  kt (6) cA0

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When n=1, Eqs.(4) was applied to deduce the simplified model as follow:  1  ln  (7)   kt  1 x  When n=2, Eqs.(5) was transformed as follow: x  kcA0 t (8) 1 x The theoretical rate of CO2 conversion was fitted based on Eqs. (6), (7), (8) with the use of the measured data (Fig.S3). The results were shown in Table S1, Table 2 and Fig.10a. Compared with other equations, the Eqs.(7) achieved the highest determining coefficient for all data. Therefore, the law of carbon dioxide coupling with ECH was very close to first order kinetic.

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The reaction constants (k) obtained under different temperature were employed for calculating activation energy (Ea) according to Arrhenius equation (9).  E  k  kr exp   a  (9)  RT  Where kr is the pre-exponential factor. Ea is the activation energy of formation of CPC from CO2 and ECH. Besides, Eqs.(9) can be transformed to form a linear equation, as follows:

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k Ea  1 1       i  1, 2,3 ki R  Ti T 

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Besides, activation energy (Ea) can be calculated according to the slope of Eqs.(10) and the curve was plotted in Fig.10b. From the curve, the value of Ea was calculated to be

113.38kJ/mol. (All the calculation was carried out on MATLAB 16a)

Conclusions

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Zn2(C9H3O6)(C4H5N2)(C4H6N2)3 (Zn-BTC-2MeIm), as an air stable, cheap and heterogeneous catalyst was synthesized under hydrothermal conditions and applied as the highly efficient catalyst for CO2 conversion. 98.93% conversion of ECH and 98.32% selectivity to chloropropene carbonate was achieved under the optimum catalytic conditions (100 oC, 120 mesh, 1000 rpm, 3.0 MPa, 6 h, 0.75 wt.% of ECH). Besides, the recyclability and regeneration results exhibited Zn-BTC-2MeIm crystal material can be reused no less than three times with a slight decrease in its catalytic ability. Moreover, CO2 coupling with other epoxides were also accomplished under the same catalytic conditions. Finally, the investigation of the kinetic exhibited the law of CO2 coupling with ECH was coincident with the first order kinetic, and the activation energy (Ea) was 113.38kJ/mol.

Acknowledgments

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The financial supports from the National Natural Science Foundation of china (Nos. 21276050, 21676054, 21406034), Natural Science foundation of Jiangsu (No. BK20161415), Fundamental Research Funds for the central Universities (No. 3207045414, 3207045101, 3207045426), Key Laboratory Open Fund of Jiangsu Province (JSBEM201409) are gratefully acknowledged.

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Chemical Engineers 61, 351-355.

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Fig.1. Simulated and experimental X-ray diffraction patterns of Zn-BTC-2MeIm

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Fig.2. FT-IR spectra of the as prepared Zn-BTC-2MeIm compound. (a) Fresh sample, (b)

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Recovered sample after three cycles.

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Fig.3. XPS spectra of the Zn-BTC-2MeIm. (a) Full spectrum, (b) C 1s, (c) N 1s, (d) Zn 2p.

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Fig.4. TG-DTG profiles of the as-prepared Zn-BTC-2MeIm compound.

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Fig.5 TPD profiles of the Zn-BTC-2MeIm sample. (a) NH3–TPD, (b) CO2–TPD.

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Fig.6 (a) Effects of particle size: ECH 20.0 g, 3.0MPa, 100 oC, stirring speed 700 rpm, 1.0 wt.% catalyst of ECH. (b) Effects of stirring speed: ECH 20.0 g, 3.0Mpa, 100 oC, particle size 120 mesh, 1.0 wt.% catalyst of ECH.

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Fig.7 (a) Effects of reaction temperature: ECH 20.0 g, initial CO2 pressure 3.0MPa, reaction time 6 h, 1.0 wt.% catalyst of ECH. (b) Effects of catalyst amount: ECH 20.0 g, initial CO2 pressure 3.0MPa, reaction temperature 100 oC, reaction time 6 h. (c) Effects of initial pressure: ECH 20.0 g, reaction temperature 100 oC, reaction time 6 h, 0.75 wt.% catalyst of ECH. (d) Effects of reaction time: ECH 20.0 g, reaction temperature 100 oC, 0.75 wt.% catalyst of ECH.

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Fig.8 The recyclability of the Zn-BTC-2MeIm catalyst: ECH 20.0 g, 100 oC, 3.0MPa, 8 h, 1.0 wt.% catalyst of ECH. ( : Conversion, : Selectivity).

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Fig.9 XRD pattern of recovered Zn-BTC-2MeIm after four cycles.

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Fig.10 (a) Experimental and theoretical curves and (b) kinetic model parameter ln k vs.1/T

dioxide

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carbon

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Scheme1. One-pot conversion Zn2(C9H3O6)(C4H5N2)(C4H6N2)3.

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Scheme 2. Basic structural unit of the Zn-BTC-2MeIm (hydrogen atoms are not shown for clarity).

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Scheme 3. A proposed mechanism for CO2 coupling with epoxide in the presence of Zn-BTC-2MeIm.

Table 1. Carbon dioxide cycloaddition with different epoxides*1 Sel. (%)

Yie. (%)

Tem. (oC)

Time(h)

1

98.93

98.32

97.27

100

6

2

69.96

99.34

69.50

100

6

3

85.86

97.48

83.70

100

6

4

91.58

95.88

87.81

100

5a

—

—

>99.9

130

6b

71.2

99.0

—

7c

—

90.0

75.0

8d

99

—

9d

25.3

—

6

100

2

>99

120

6

120

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Epoxides

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Con. (%)

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*1 Epoxides 20.0 g, initial CO2 pressure 3.0MPa, 0.75wt.% catalyst of the epoxide.

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a: Epoxide 214mmol, 3.0MPa, catalyst 44.7mg, Ni(PPh3)2(Naphthyl)Cl/PPh3/n-Bu4NBr/Zn/ Epoxide = 1: 2: 4: 20: 3600 (Peng et al., 2015);

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b: Epoxide 105 mmol, 2.5 MPa, ZnBr2 0.015 mmol, Ph4PI 0.09 mmol(Wu et al., 2008); c: Epoxide 45mmol, 1.6MPa, DMAP 0.09 mmol, Cat. 0.045 mmol (Kilic et al., 2014);

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d: Epoxide 3.5mL,1.5MPa, catalyst 60 mg (Zhang et al., 2018).

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Table 2. Kinetic parameter (k) and the determining coefficient (R2) i Temperature (k) k R2 1 363 0.3747 0.997 2 368 0.5480 0.975 3 373 1.0270 0.985