Zeolitic imidazolate framework-68 as an efficient heterogeneous catalyst for chemical fixation of carbon dioxide

Accepted Manuscript Title: Zeolitic Imidazolate Framework-68 as an Efficient Heterogeneous Catalyst for Chemical Fixation of Carbon Dioxide Author: Lili Yang Lin Yu Guiqiang Diao Ming Sun Gao Cheng Suyi Chen PII: DOI: Reference:

S1381-1169(14)00243-X http://dx.doi.org/doi:10.1016/j.molcata.2014.05.033 MOLCAA 9133

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

11-3-2014 29-5-2014 29-5-2014

Please cite this article as: L. Yang, L. Yu, G. Diao, M. Sun, G. Cheng, S. Chen, Zeolitic Imidazolate Framework-68 as an Efficient Heterogeneous Catalyst for Chemical Fixation of Carbon Dioxide, Journal of Molecular Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.05.033 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.

*Graphical Abstract (for review)

Relative Intensity(a.u.)

Graphical Abstract

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Figure 1. Powder X-ray diffraction patterns of ZIF-68 catalyst: (a) fresh; (b) the first recycle; (c) the

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second recycle; and (d) the third recycle.

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*Highlights (for review)

Research highlights ▶ ZIF-68 with exceptional stability was firstly synthesized by hydrothermal method.

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▶ ZIF-68 was firstly used to catalyze the cyclic addition reaction of CO2.

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▶ High yield of cyclic carbonate was obtained under mild conditions.

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▶ The presence of acid-base site in ZIF-68 plays a critical role in the reaction.

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Zeolitic Imidazolate Framework-68 as an Efficient Heterogeneous Catalyst for Chemical Fixation of Carbon Dioxide Lili Yang, Lin Yu*, Guiqiang Diao, Ming Sun, Gao Cheng, and Suyi Chen

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School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China.

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*Corresponding author. Tel: +86 20 39322202; Fax: +86 20 39322231; E-mail: [email protected]

Abstract

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An efficient heterogeneous catalyst, namely, zeolitic imidazolate framework-68 (ZIF-68), was

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developed for the synthesis of cyclic carbonate from CO2 and styrene oxide in the absence of any solvents or co-catalysts under mild reaction conditions (120 °C and 1.00 MPa). The textural properties

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of the ZIF-68 catalyst were determined by the powder X-ray diffraction, N2 adsorption-desorption and thermal analysis. The acid-base property of ZIF-68 catalyst was investigated by NH3 and CO2

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temperature-programmed desorption methods. The results indicate that the yield of cyclic carbonate reached up to 93.3 % after 12 h in the mild reaction conditions. Moreover, the ZIF-68 catalyst could be

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successfully reused for three times without any significant loss in catalytic activity. Keywords: zeolitic imidazolate framework; carbon dioxide fixation; cyclic carbonate; heterogeneous catalyst.

1. Introduction Carbon dioxide (CO2) is one of the greenhouse gases that have posed great threat to environment. Recently, the chemical fixation of CO2 into useful chemicals has attracted great interest [1-3]. One of

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the most effective approaches to transform CO2 into useful chemicals is to utilize CO2 and styrene oxide to produce five-membered cyclic carbonates (Scheme 1). The produced cyclic carbonate can be used as polar aprotic solvents, electrolytes, and intermediates in a wide range of chemical reactions [4].

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Various homogeneous catalysts, such as transition metal complexes [5], ionic liquid catalysts [6], and quaternary ammonium salts [7], have been studied in the cyclic addition of CO2 and styrene oxide.

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Considering that the homogeneous catalyst is difficult to separate and recover, several heterogeneous

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catalysts have been suggested for the cyclic addition reactions. Most of these catalysts, however, suffer from low activities or harsh reaction conditions such as, elevated temperature, high pressure of CO2,

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and the necessity of organic solvent or co-catalyst [8-10]. To date, investigating an environmentally

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benign heterogeneous catalyst that is highly effective under mild conditions still remains a challenge

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[11].

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Scheme 1. Synthesis of cyclic carbonate from CO2 and styrene oxide.

Metal organic frameworks (MOF) has emerged as a novel kind of porous crystalline materials with excellent properties, such as uniform microspores, accessible pore volumes, and large surface areas [12], making them potentially interesting candidates for gas storage [13], gas separation [14], magnetism [15], and especially as catalysts [16-19]. MOF materials, including Mg-MOF-74 [20], Co-MOF-74 [8], and MOF-5 [21], have been well-studied in the CO2 fixation reaction. However, these

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MOF catalysts are sensitive to water and air, limiting their further applications. Zeolitic imidazolate frameworks (ZIF), a subclass of MOF, are one of the potentially interesting catalyst candidates because of their exceptional chemical and thermal stabilities [22]. Miralda and coworkers [23] studied the

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activity of ZIF-8 in the cyclic addition of CO2 and epichlorohydrin, and the conversion of epichlorohydrin was 65.5 % while the chloropropene carbonate selectivity was 63.4 %. In addition,

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Carreon et al. [24] also researched the catalytic performance of ZIF-8 in the synthesis of cyclic

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carbonate from CO2 and styrene oxide, and showed that the yield to styrene carbonate was only 53.0 % at 100 °C. Besides, the ZIF-8 catalyst lost its distinctive crystalline nature and catalytic performance

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after recycling.

Scheme 2. Synthesis and topology structure of ZIF-68: the largest cage is shown with ZnN4 tetrahedra

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in blue, the yellow ball is placed in the structure for clarity and to indicate space in the cage.(C, black; N, green; O, red;)

Thus, finding other alternatives remains a challenge when ZIF-8 still cannot conquer the stability problem for the synthesis of cyclic carbonate. Recently, Tom K. Woo et al. [25] demonstrated that ZIF-68 (Scheme 2) has extraordinary adsorption capacity of CO 2 by Grand-Canonical Monte Carlo and Molecular-Dynamics Simulations. ZIF-68 with GME topology is composed of one 2-nitroimidazole

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and one benzimidazole per Zn in an asymmetric unit, displaying large pores with diameter of 10.3 Å and accessible through apertures of 7.5 Å as well as exceptional chemical and thermal stabilities [26]. These materials with higher CO2 adsorption capacity may possess better catalytic activities for CO2

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cyclic addition reaction [24]. Unfortunately, research on ZIF-68 materials for CO2 fixation is scare. Herein, we first synthesized ZIF-68 through hydrothermal method, and used it to catalyze the cyclic

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addition of CO2 and styrene oxide. Interestingly, the ZIF-68 materials showed excellent catalytic

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performance and recycle ability.

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2. Experimental Section

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2.1. General Material and Characterization

All the chemicals were purchased from Aldrich Chemical Co. and were used without further

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purification, unless otherwise stated. Powder X-ray diffraction (XRD) was carried out on a MSAL-XD2 X-ray diffractometer operating at a voltage of 40 kV and a current of 25 mA, with Cu Ka

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radiation. The data were collected at room temperature with a 5° step size in 2θ, 2θ from 5° to 50°. N2 adsorption-desorption isotherm and pore size distribution of the materials were measured with an

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ASAP 2020 sorptometer analyzer (Micrometitics Company) at 77 K using liquid nitrogen as coolant. The catalyst was degassed at 150 °C for 3 h before analysis. Thermal gravimetric analysis-differential scanning calorimetry (TGA-DSC) was recorded on a Netzsch STA 409 thermoanalyzer. Approximately 5 mg catalyst was filled into an alumina crucible and heated in a continuous-flow of nitrogen gas with a ramp rate of 10 °C/min from 40 °C up to 800 °C. The acid-base property of ZIF-68 was measured by NH3-TPD and CO2-TPD using the AutoChem II 2920 instrument, with helium as a carrier gas. After pretreatment at 150 °C for 3 h, the samples were cooled down to room temperature and saturated with

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CO2 (or NH3). Subsequently, the physically absorbed CO2 or NH3 were removed by flowing helium. Finally, the desorption process was started from 50 °C to 400 °C at a heating rate of 5 °C/min, and monitored by a thermal conductivity detector.

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2.2. Preparation of ZIF-68 Catalyst ZIF-68 was prepared by hydrothermal method. A solid mixture of zinc nitrate hexahydrate

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([Zn(NO3)2]·6H2O, 3 mmol), benzimidazole (C7H6N2, 4 mmol), and 2-nitroimidazole (C3H3N3O2, 7

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mmol) was dissolved in N,N-Dimethylformamide (DMF, 50 mL), then sealed in a 100 mL Teflon container. The container was maintained in the oven at 100 °C for 72 h. After the reaction, the sample

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was cooled down to room temperature. The product was washed with DMF (10 mL × 3) and ethanol

(yield: 83.8 % based on benzimidazole).

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(95 %, 10 mL × 3), and the obtained yellow polyhedral crystals were finally dried overnight at 150 °C

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2.3. General Procedure for the Cyclic Addition Reaction of CO2 and Styrene Oxide The cyclic addition reaction was carried out in a 100 mL stainless steel autoclave equipped with a

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magnetic stirrer. The catalyst (0.100 g, pretreated for 3 h at 150 °C), biphenyl (0.222 g) as internal standard, and styrene oxide (0.848 g) were first placed in the autoclave. Subsequently, CO2 was

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introduced into the reactor until reaching the desired pressure. The reactor was then heated to a desired temperature for several hours under vigorous stirring. After the reaction, the autoclave was cooled down to room temperature, and the un-reacted CO2 was slowly released. The product was then diluted with DMF and filtered before GC analysis (Agilent 7890A, HP-5 column: 30 m, 0.320 mm, 0.250 μm) using an internal standard technique. In the catalyst recycling process, ZIF-68 was separated by centrifugation, washed using ethanol (95 %, 10 mL × 3), then dried in air at 150 °C.

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3. Results and discussion

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Relative Intensity(a.u.)

3.1. Characterization of ZIF-68 Catalyst

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Figure 1. Powder X-ray diffraction patterns of ZIF-68 catalyst: (a) fresh; (b) the first recycle; (c) the

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second recycle; and (d) the third recycle.

400

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adsorption

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3

Volume Absorbed(cm /g STP)

desorption

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200

1.5

2

dV/dD (cm /g)

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100

1.0

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0.0 0.2

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0.6 0.8 Pore Diameter (nm)

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0 0.0

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0.4

0.6

0.8

1.0

(P/Po)

Figure 2. N2 adsorption-desorption isotherm and the pore size distribution ( insert) of ZIF-68 catalyst.

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100 ZIF-68-TGA

6

ZIF-68-DSC 4

Mass/%

2

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DSC/(mW/mg)

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60

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40 200

400 O Temperature ( C)

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Figure 3. Thermal gravimetric analysis-differential scanning calorimetry (TGA-DSC) curves of ZIF-68

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TCD Signal (a.u.)

catalyst.

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200 300 o Temperature / C

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(a)

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TCD Signal (a.u.)

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200 300 o Temperature / C

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(b)

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Figure 4. (a) CO2 temperature -programmed desorption (CO2-TPD); and (b) NH3

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temperature-programmed desorption (NH3-TPD) analysis of ZIF-68 catalyst.

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The textural properties of the ZIF-68 catalyst were determined by the powder X-ray diffraction, N2 adsorption-desorption and thermal analysis. The acid-base property of ZIF-68 catalyst was investigated

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by NH3 and CO2 temperature-programmed desorption methods. The powder XRD pattern of the fresh ZIF-68 (Fig 1a) is in good agreement with the pattern reported by Park, K.S et al. [27]. Moreover, the

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XRD patterns of the reused catalysts (Fig 1b-d) showed no significant change in the crystalline structure compared with that of the fresh one. These results indicate that ZIF-68 showed good stability in the cyclic addition reactions. We can see that ZIF-68 exhibits a Ι type adsorption-desorption isotherm without hysteresis loop in Figure 2, suggesting the microporous nature of ZIF-68 catalyst. The Langmuir surface area of ZIF-68 was up to 1295.95 cm2/g, and the pore size distribution (Fig 2 insert) calculated by Horvath-Kawazoe (HK) method provided two types of microspores located at 5.5 Å and 8.5 Å, which was consistent with the reported literature [27]. Figure 3 displays the result of TGA-DSC

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curve of ZIF-68 catalyst. The weight loss steps were observed from 40 °C to 800 °C. The first weight loss with approximately 10 % occurred before 400 °C, resulting from the desorption of H2O contained in the ZIF-68 frameworks (1 H2O: 9.6 wt %) [27]. An endothermic peak at about 400 °C was observed

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in the DSC curve, indicating the decomposition of ZIF-68. Therefore, ZIF-68 is stable up to 400 °C. In order to confirm the presence of acid sites and basic sites in the ZIF-68 catalyst, CO2-TPD and

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NH3-TPD analysis were carried out between 50 °C and 400 °C. These results in Figure 4 clearly

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indicate that the acid and the basic sites coexist in the ZIF-68 structure, probably due to the presence of unsaturated coordinative Zn atoms and nitrogen atoms of benzimidazole and 2-nitroimidazole ligand

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(Scheme 2) [28].

100

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catalyst amount=0.1 g reaction time=12 h CO2 pressure =1.00 MPa

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catalyst amount=0.1 g reaction time=12 h reaction temperature =120 ° C

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catalyst amount=0.1 g o reaction temperature=120C CO2 pressure =1.00 MPa

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0.10 0.15 Catalyst amount / g

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reaction temperature=120 C reaction time=12 h CO2 pressure =1.00 MPa

1.5 2.0 Pressure / MPa

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3.2. Effect of Reaction Conditions on the Yield and Selectivity of Cyclic Carbonate

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10 15 Reaction time / h

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Figure 5. Effect of different reaction conditions on the yields and selectivities of cyclic carbonate: (a) Reaction temperature; (b) Initial CO2 pressure; (c) Catalyst amount; and (d) Reaction time.

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The catalytic performance of ZIF-68 was evaluated in the cycloaddition reaction of CO2 and styrene oxide. The effect of different reaction conditions on the yield and selectivity of cyclic carbonate was

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shown in Figure 5. The yields of cyclic carbonate increased from 43.4 % to 99.1 % with the increase in temperature from 80 °C to 130 °C (Fig 5a). The selectivity of cyclic carbonate was almost 100.0 % at

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the temperature range lower than 120 °C, but decreased to 78.6 % when the temperature was further

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increased to 130 °C. Some side products of cyclic carbonate, such as diols and dimmers, were formed at high temperature, which was in agreement with that reported by Miralda [23].

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It has been reported that the cyclic addition reaction mainly occurs in the liquid phase because the

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catalysts are dispersed in this phase [29]. Figure 5b exhibits the dependence of the yields on the reaction pressure between 0.2 and 3.0 MPa. The yields increased from 75.2 % to 93.3 % in the pressure

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range of 0.2 MPa to 1.0 MPa, resulting from the fact that higher pressure enhances higher CO2 concentration in the liquid phase. Further increasing the pressure from 1.5 MPa to 3.0 MPa, however,

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decreased both the yield and the selectivity, probably due to the fact that the amount of styrene oxide in the liquid becomes smaller as the pressure rises.

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The effect of catalyst amount on the yield of cyclic carbonate was displayed in Fig 5c. The yields were enhanced almost in linear with the increase of ZIF-68 catalysts amount from 0.01 g until to 0.1 g, and then remained constant with further addition of ZIF-68 to 0.2 g. Moreover, as shown in the selectivity curve, the amount of catalyst barely affected the selectivity. Fig 5d shows the formation of cyclic carbonate as function of reaction time. In the presence of ZIF-68 catalyst, at 120 °C and 1.0 MPa CO2 initial pressure, the yields of the product carbonate increased from about 24.0 % to 93.0 % as the reaction time elapsed from 1 h to 12 h. Prolong the time to 24 h, the yield rised slowly to approach

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94.0 %. The selectivity of cyclic carbonate is almost 100.0 % during the evaluated reaction time. Therefore, the cyclic addition reaction of CO2 and styrene oxide was carried out at the optimum reaction conditions: at 1.0 MPa of CO2 initial pressure and 120 °C for 12 h in the presence of 0.100 g

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ZIF-68 catalyst.

Table 1 Comparison with other catalysts in the cyclic addition of CO2 and styrene oxide a

none

-

2

[Zn(NO 3 ) 2 ]·6H 2 O

3.5

3

benzimidazole

53.4

4

2-nitroimidazole

22.1

MgO

d

6

ZIF-8

7

ZIF-68

9 10

ZIF-68(recycle 1 ) nd

ZIF-68(recycle 2 ) rd

ZIF-68(recycle 3 )

3.6 55. 4 32. 7 74.6

52.5

73.6

93.3 (85.2 e)

99.0

88.3

84.0

80.9

79.8

66.4

78.8

Reaction conditions: catalyst (0.100 g, 2 wt%); initial CO2 pressure (1.00 MPa); temperature

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a

st

-

37.5

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8

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5

c

Selectivity (%)b

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1

Yield (%)b

Catalyst

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Entry

(120 °C); reaction time (12 h); b Determined by GC using an internal standard technique; c MgO was purchased from Aldrich Chemical Co. and was used without further purification; d ZIF-8 was prepared

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according to the procedure described by Moises A. Carreon et al [24];e the isolated yield of the product, which was confirmed with standard sample and analyzed by thin-layer chromatography, then purified by silica gel column chromatography.

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To confirm the catalytic activity of ZIF-68, several experiments over other catalysts and blank test for the cyclic addition of CO2 to styrene oxide were also performed, and the results are shown in Table 1. No detectable product was observed in the blank test without catalyst. In another set of controlled experiments, the synthetic precursors of ZIF-68 have generally low activities compared with ZIF-68 catalyst in the cyclic addition reactions (3.5 %, 53.4 %, 22.1 %, and 93.3 % of yield for [Zn(NO3)2]·6H2O, benzimidazole, 2-nitroimidazole, and ZIF-68, respectively). In the past few decades, the synthesis of organic compounds using metal oxides as catalysts has gained considerable interest. 11

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For example, magnesium oxide has been used to catalyze the fixation of CO2 [30]. However, magnesium oxide shows a relatively poor activity with 37.5 % yield and 74.6 % selectivity under our reaction conditions. The first ZIF material used in the preparation of cyclic carbonate from CO 2 and

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styrene oxide is ZIF-8 [24], thus we also investigated the catalytic performance of ZIF-8 under our conditions. Obviously, the yield and selectivity over ZIF-8 are much lower than that of ZIF-68 (Entry 6

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- Entry 7). Further, no obvious loss of catalytic activity for ZIF-68 in three successive catalytic runs

showed that the crystalline was retained as the fresh one.

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3.3. Cyclic Addition Reaction Mechanism

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was observed (Entry 8 - Entry 10), and the XRD patterns (Fig 1) of the recycled ZIF-68 catalyst

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Many researchers [29-31] have reported that the combination of Lewis acid and Lewis base paly an important role in catalyzing the cyclic addition reaction of CO2 and styrene oxide. The ZIF-68 used in

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this study possesses both Lewis acid and Lewis basic sites as proven by CO2-TPD and NH3-TPD technologies (Fig 4). Amarajothi et al. [28] summarized the possible existence of active sites in the

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MOF materials, which include metal nodes with free coordination sites, functional linkers, and structural defects. For the ZIF-68 synthesized in this paper, the presence of unsaturated coordinative Zn

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probably resulted in the Lewis acid sites. Simultaneously, the N at the extremity of mono-coordinated imidazolate ligand, as well as the NH groups, can participate as basic sites. Structural defects not expected in ideal frameworks also ought to be recognized as potential active sites in the catalysis. Furthermore, previous reports [32] suggested that a strong Lewis acid-base interaction exists between CO2 molecules and nitro groups of 2-nitroimidazole linkers, thereby preventing the entry of gas molecules into the small pores (5.5 Å) of ZIF-68. The presence of electron withdrawing groups in the phenylimidazole linker of ZIF-68 increases the interaction between the oxygen atoms of CO2 and

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hydrogen atoms of the phenyl rings in its large pores (8.5 Å). Therefore, cyclic addition reaction of CO2 and styrene oxide might occur on the surface of the catalysts or in the large pores, instead of the

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small pores of frameworks.

Scheme 3. Proposed mechanism for the cyclic addition reaction of CO2 and styrene oxide in the

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presence of ZIF-68.

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Thus, a reasonable mechanism for the cyclic addition reaction of CO 2 and styrene oxide in the presence of ZIF-68 was proposed (Scheme 3), similar to that of the homogenous catalytic system with dimeric zinc complex bridged pyridinium alkoxy ion [33]. Firstly, ZIF-68 catalyst loses the coordinated

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H2O molecule after pretreating for 3 h at 150 °C to form unsaturated coordinative Zn atoms. Secondly, the addition reaction is initiated by adsorption of CO2 on the Lewis basic sites (N and NH groups) of ZIF-68, and simultaneously, styrene oxide is activated by the acid sites of ZIF-68, such as unsaturated coordinative Zn and structural defects. Thirdly, the activated CO2 attack the less sterically hindered carbon atom of styrene oxide, and then the three-numbered ring of styrene oxide is opened, which leads to an oxy anion species yielding the corresponding cyclic carbonate as a product. Meanwhile, the ZIF-68 catalyst is regenerated. 13

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4. Conclusions ZIF-68, a highly efficient and environmentally benign heterogeneous catalyst, was synthesized by hydrothermal method. The presence of both acid and base sites in ZIF-68 promotes the adsorption of

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CO2 in the structure and its further conversion to the cyclic carbonate. ZIF-68 has significant advantages over other catalysts in three respects: first, ZIF-68 exhibits exceptional chemical and

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thermal stability, which is vital for industrial application; second, co-catalysts or solvents are not need,

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which is significant to environmental protection; and third, a high yield of 93.3 % in cyclic carbonate was obtained under relatively mild reaction conditions (at 1.00 MPa of initial CO2 pressure and 120 °C).

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Furthermore, the ZIF-68 catalyst was stable during the reaction and reusable without a significant loss

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in activity. Acknowledgments

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This work was financially supported by the Natural Science Foundation of Guangdong Province (10251009001000003), Scientific Program of Guangdong Province (2012A030600006), Fund of

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Higher Education of Guangdong Province (cgzhzd1104). We would like to thank Dr. Francois CHAU

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and Dr. Jean-Yves Piquemal (University Paris Diderot, France) for their kind assistant.

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Ac

[24] M. Zhu, D. Srinivas, S. Bhogeswararao, P. Ratnasamy, M.A. Carreon, Catal. Commun., 32 (2013) 36-40.

[25] S.A. Andrew Sirjoosingh, and Tom K. Woo, J. Phys. Chem. C, 114 (2010) 2171-2178. [26] H.F. Rahul Banerjee, David Britt, Carolyn Knobler, Michael O’Keeffe, and Omar M. Yaghi, J. Am. Chem. Soc., 131 (2009) 3875-3877. [27] T.D.B. Jin Chong Tan, and Anthony K. Cheetham, PNAS, 107 (2010) 9938-9947. [28] A. Dhakshinamoorthy, M. Opanasenko, J. Čejka, H. Garcia, Catal. Sci. Technol., 3 (2013) 2509-2597. [29] L.H. Jian Sun, Weiguo Cheng, Jinquan Wang, Xiangping Zhang, and, S. Zhang, ChemSusChem., 4 (2011) 502-507. [30] H.M. Takashi Yano, Takahiro Koike, Hiroyasu Ishiguro, Hisashi Fujihara, Masakuni Yoshihara and Toshihisa Maeshima, Chem. Commun., (1997) 1129-1130.

15

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[31] L. Jian Sun, Suojiang Zhang, Zengxi Li, Xiangping Zhang,Wenbin Dai, Ryohei Mori, J. Mol. Catal. A: Chem., 256 (2006) 295-300. [32] X.-J.H.a.H. Li, J. Phys. Chem. C, 114 (2010) 13501-13508. [33] J.J.K. Hoon Sik Kim, B yung Gwon Lee, Ok Sang Jung, Ho Gyeom Jang, Sang Ook Kang, Angew.

Ac

ce pt

ed

M

an

us

cr

ip t

Chem. Int. Ed., 39 (2000) 4096-4098.

16

Page 18 of 24

Table

Table 1 Comparison with other catalysts in the cyclic addition of CO2 and styrene oxide a Entry 1

Yield (%)b

Catalyst none

Selectivity (%)b

-

-

[Zn(NO 3 ) 2 ]·6H 2 O

3.5

3.6

3

benzimidazole

53.4

55. 4

4

2-nitroimidazole

22.1

32. 7

37.5

74.6

52.5

73.6

MgO

d

6

ZIF-8

7

ZIF-68

8 9 10

st

88.3

nd

80.9

rd

66.4

ZIF-68(recycle 1 ) ZIF-68(recycle 2 ) ZIF-68(recycle 3 )

99.0

84.0 79.8

78.8

Reaction conditions: catalyst (0.100 g, 2 wt%); initial CO2 pressure (1.00 MPa); temperature

us

a

93.3 (85.2 e)

cr

5

c

ip t

2

(120 °C); reaction time (12 h); b Determined by GC using an internal standard technique; c MgO was purchased from Aldrich Chemical Co. and was used without further purification; d ZIF-8 was prepared

an

according to the procedure described by Moises A. Carreon et al [24];e the isolated yield of the product, which was confirmed with standard sample and analyzed by thin-layer chromatography, then purified

Ac

ce pt

ed

M

by silica gel column chromatography.

Page 19 of 24

Relative Intensity(a.u.)

Figure 1

ip t

d

c

cr

b

10

20

30

40

50

an

2 Theta Degree

us

a

Figure 1. Powder X-ray diffraction patterns of ZIF-68 catalyst: (a) fresh; (b) the first recycle; (c) the

Ac

ce pt

ed

M

second recycle; and (d) the third recycle.

Page 20 of 24

Figure 2

400 adsorption

300

3

Volume Absorbed(cm /g STP)

desorption

2.5

200

1.0

0.5

0.0 0.2

0.4

0.6 0.8 Pore Diameter (nm)

1.0

0 0.2

0.4

0.6

0.8

1.0

us

0.0

cr

100

ip t

1.5

2

dV/dD (cm /g)

2.0

Ac

ce pt

ed

M

an

Figure 2. N2 adsorption-desorption isotherm and the pore size distribution ( insert) of ZIF-68 catalyst.

Page 21 of 24

Figure 3

100 ZIF-68-TGA

6

ZIF-68-DSC 4

Mass/%

2

ip t

0

DSC/(mW/mg)

80

60

cr

-2

-4

40 200

400 O Temperature ( C)

600

800

us

0

an

Figure 3. Thermal gravimetric analysis-differential scanning calorimetry (TGA-DSC) curves of ZIF-68

Ac

ce pt

ed

M

catalyst.

Page 22 of 24

200 300 o Temperature / C

M ed

ce pt

TCD Signal (a.u.)

an

(a)

400

us

100

cr

ip t

TCD Signal (a.u.)

Figure 4

Ac

100

200 300 o Temperature / C

400

(b)

Figure 4. (a) CO2 temperature -programmed desorption (CO2-TPD); and (b) NH3 temperature-programmed desorption (NH3-TPD) analysis of ZIF-68 catalyst.

Page 23 of 24

60

60

40

40

catalyst amount=0.1 g reaction time=12 h CO2 pressure =1.00 MPa

20

0 100 120 o Temperature / C

60

60

40

40

catalyst amount=0.1 g reaction time=12 h reaction temperature =120 ° C

20

0 0.0

0

80

80

140

0

0.5

1.0

60

40

40

3.0

0.10 0.15 Catalyst amount / g

0.20

80

60

40

20

0

0

0.25

100

60

20

M

reaction temperature=120 ° C reaction time=12 h CO2 pressure =1.00 MPa 0.05

2.5

us

60

80

an

80

Yield / %

80

100

Selectivity/ %

100

Yield / %

100

0 0.00

1.5 2.0 Pressure / MPa

(b)

(a)

20

20

40

catalyst amount=0.1 g o reaction temperature=120C CO2 pressure =1.00 MPa

Selectivity/ %

20

80

ip t

80

100

cr

80

100

Yield / %

100

Selectivity/ %

Yield / %

100

Selectivity/ %

Figure 5

20

0

0

5

10 15 Reaction time / h

20

25

(d)

ed

(c)

Figure 5. Effect of different reaction conditions on the yields and selectivities of cyclic carbonate: (a)

Ac

ce pt

Reaction temperature; (b) Initial CO2 pressure; (c) Catalyst amount; and (d) Reaction time.

Page 24 of 24