Mn-based MOFs as efficient catalysts for catalytic conversion of carbon dioxide into cyclic carbonates and DFT studies

Chemical Engineering Science 201 (2019) 288–297

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Mn-based MOFs as efficient catalysts for catalytic conversion of carbon dioxide into cyclic carbonates and DFT studies Yuanfeng Wu, Xianghai Song, Jiahui Zhang, Siquan Xu, Lijing Gao, Jin Zhang, Guomin Xiao ⇑ School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Mn-based MOF including imidazole

Synthesis of cyclic carbonates from CO2 and epoxides.

linker was observed with the highest catalytic activity.
Carbon dioxide can be inserted into various epoxides in the presence of Im-MnF sample.
94.99% yield of allyl-glycidyl carbonate was obtained under the optimal conditions.
A proposed mechanism was in line with the DFT calculation.

a r t i c l e

i n f o

Article history: Received 19 November 2018 Received in revised form 17 February 2019 Accepted 19 February 2019 Available online 6 March 2019 Keywords: CO2 Cycloaddition Allyl-glycidyl ether Epoxides DFT

a b s t r a c t The present work is aimed to investigate the catalytic activities of various Mn-based MOFs for carbon dioxide coupling with epoxides. The metal organic frameworks, [(CH3NH3][Mn(COOH)3] (MA-MnF), [(CH3CH2NH3][Mn(COOH)3] (EA-MnF), and [C3H5N2][Mn(COOH)3] (Im-MnF) were studied via several characterizations such as XRD, FT-IR, XPS, N2-adsorption, TG-DSC, CO2-adsorption and NH3-TPD. Interestingly, Im-MnF compound was observed to possess the highest catalytic activity among the studied compounds, which is associated not only with the amounts of basic sites, but also related to the nitrogen-containing species. 97.27% conversion of allyl-glycidyl ether (AGE, TOF: 36.78 h1) and 97.66% selectivity to allyl-glycidyl carbonate (AGC) were obtained under the explored optimized conditions (100 °C, 15 bar, 6 h, 1.0 wt% of AGE). In addition, only a slight downward in catalytic activity was found when the sample was reused twice. Furthermore, coupling reactions of CO2 with various epoxides were also performed, of which, the yield of the cyclic carbonates followed the order: Epichlorohydrin > Allyl glycidyl ether > Styrene oxide > Cyclohexene oxide > Propylene oxide. Finally, a mechanism was proposed, which is in good agreement with the DFT calculation. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Metal organic frameworks (MOFs) have become an interesting focus in the past decade because of their simultaneous assembly of metallic ions and organic linkers within the crystal structures, ⇑ Corresponding author. Tel./fax: +86-25-52090612. E-mail address: [email protected] (G. Xiao). https://doi.org/10.1016/j.ces.2019.02.032 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

thereby efficiently making the compounds equipped with both organic and inorganic properties. In addition, the special structures functioned with various groups and metal nodes have been investigated with myriad potentials in many fields such as sensing (Lee et al., 2009), optics (Liu et al., 2014), magnetism (Maspoch et al., 2007), gases storage/separation (Li and Li, 2018; Morris and Wheatley, 2008; Rodenas et al., 2015; Szczesniak et al., 2018) and catalysis (Jeong et al., 2018; Opelt et al., 2012; Sun et al.,

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2008; Wu et al., 2018). MOFs endowed with some special properties can be designedly obtained via selecting the synthesis methods, organic ligands and metal ions. Therefore, the required functional groups for the catalytic conversion of carbon dioxide into fine chemicals can be easily realized via employing proper components which could serve as active sites (Lewis/Brønsted base/acid sites) in the catalytic system (Chizallet et al., 2010; Chang et al., 2015). On the other hand, the abundant combustion of fossil fuel makes an obvious effect on the increase of carbon dioxide concentration in air (Malik et al., 2016; Zeng et al., 2018), which has resulted in some serious changes especially for climates (Chiang et al., 2018; Jayakumar et al., 2017; Zhou et al., 2016). Fortunately, the use of CO2 as the starting raw material for the synthesis of value-added chemicals has been studied in the recent years, which can be further developed to be the efficient path for carbon dioxide utilization in industry. For instance, the use of aniline as the substrate for efficient fixation of carbon dioxide has been studied by Fan et al (Fan et al., 2015), while the synthesis of diphenyl carbonate was carried out when carbon dioxide was employed as the coupling reagent (Fan et al., 2011). In addition, methanol coupling with carbon dioxide for dimethyl carbonate synthesis was also explored in the presence of ceria-calcium bimetal oxides (Kumar et al., 2017). Moreover, the conversion of aminoalcohols into the corresponding cyclic carbamates can be efficiently promoted when acetonitrile is applied in the catalytic system (Tamura et al., 2013). More importantly, the use of epoxides as the coupling reagents for the chemical fixation of CO2 has attracted a lot of attention because the products of five-numbered cyclic carbonates have wide industrial applications such as polar solvents, electrolytes in lithium secondary batteries, sources for the production of fine chemicals and pharmaceutically important compounds and polymers (Milani et al., 2018; Song et al., 2017; Tian et al., 2012). Moreover, AGC can be further converted into glycerol carbonate and allyl alcohol, with both of them being easily separated and purified due to a large difference in boiling point (Jia et al., 2018; Kovvali and Sirkar, 2002). Recently, some MOFs have been exploited as the efficient catalysts for enhancing carbon dioxide conversion into cyclic carbonates. Nonetheless, some components such as additional solvent or/and co-catalyst are also required for achieving higher catalytic activity for carbon dioxide conversion. Moreover, the process for the chemical fixation of carbon dioxide can be obviously promoted in the presence of Lewis/Brønsted base-acid sites (Llabrés i Xamena et al., 2012). For instances, Zirconium-based isoreticular MOFs (MIL-140A) (Jeong et al., 2017), NH2-MIL-101(Al) (Senthilkumar et al., 2018) and MOF-5(Song et al., 2009) became more active, when TBAB was simultaneously added into the cycloaddition system. Besides, UTSA-16 was observed with higher activity for carbon dioxide coupling with propylene oxide after the partial cations were exchanged by alkali metal ions (Li > Na > K > Rb > Cs) (Zhang et al., 2018b). Additionally, after ZIF-67 was doped with Zn, an obvious elevation for CO2 insertion into epichlorohydrin was observed (Zanon et al., 2017). It is worth mentioning that the process including opening ring of epoxide and the activation of carbon dioxide can be synchronously accelerated by these MOFs equipped with base-acid sites. Significantly, the crystal materials of [(CH3NH3][Mn(COOH)3] (MA-MnF), [(CH3CH2NH3][Mn(COOH)3] (EA-MnF), and [C3H5N2] [Mn(COOH)3] (Im-MnF) were observed with Mn2+ ions and nitrogen-containing groups which can function as Lewis acid and basic sites in the cycloaddition of carbon dioxide with epoxides. To the best of our knowledge, these Mn-based compounds have been studied in some areas such as dielectric (Wang et al., 2015) and ferromagnetism (Wang et al., 2004), however there are few reports on the catalytic properties, especially for carbon dioxide

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transformation, while there are few studies of allyl-glycidyl ether (AGE) being utilized as the substrate for fixation of carbon dioxide. Herein, the present work is aimed at using these compounds as catalysts for chemical fixation of carbon dioxide with AGE as probe, in addition to investigating the difference in catalytic activity of these Mn-based materials. Furthermore, the synthesis of some other cyclic carbonates from the corresponding substrates (Propylene oxide, Epichlorohydrin, Cyclohexene oxide, Styrene oxide) have been comparatively accomplished over the optimal sample (Im-MnF) While the proposed mechanism is in line with DFT calculation. This work made a significant improvement in the exploration of the new materials for carbon dioxide conversion and theoretical confirmation. 2. Experimental 2.1. Synthesis of Mn-based MOFs Methylamine (4 mol) and formic acid (6 mol) were firstly dissolved into 20 mL methanol solution to form a mixed solution which was carefully layered with 4 mL methanol, then 20 mL methanol solution including 1 mmol MnCl24H2O was slow added into the mixed solution. Thereafter, glass reactor was sealed and maintained undisturbed. After several hours, the colorless crystals became visible which was named MA-MnF. The process for the synthesis of EA-MnF was similar with MA-MnF, only that methylamine was replaced with ethylamine. The process for the synthesis of Im-MnF was also similar with MA-MnF. Briefly, a mixed solution including 6 mmol imidazole, 15 mmol formic acid and 20 mL methanol was firstly charged into a glass reactor, followed by the addition of 4 mL of methanol as well as 10 mL of methanol solution containing 4 mmol MnCl24H2O, with the described step of MA-MnF synthesis. The colorless crystals were gained when the glass reactor was undisturbedly maintained for two days under a sealed condition. The as-obtained samples were then dried (60 °C, 12 h) and grinded into 120 mesh particles. 2.2. Catalyst characterization Rigaku D/max-A instrument equipped with Cu ka irradiation was employed for measuring the X-ray diffraction (XRD) patterns of Mn-based materials, with the data recorded from 5 to 60° at a scan speed of 10°/min. X-ray photoelectron spectroscopy (XPS) measurements of the synthesized Mn-based frameworks were conducted on an ESCALAB-250 spectrometer equipped with Al Ka (1486.6 eV) irradiation. The adventitious carbon deposit C (1 s) peak (284.8 eV) was used as standard for internal calibration of the binding energies (B.E.). Nicolet 5700 spectrometer was used for investigating the existing functional groups, with Fourier transform infrared (FT-IR) spectra recorded from 400 to 4000 cm1 (2 cm1 resolution) and potassium bromide as fixative. N2 adsorption-desorption-isotherms and pore characteristics of the Mn-based materials were determined over Micromeritics Instrument (TriStar II 3020) under liquid nitrogen atmosphere after the sample was outgassed at 120 °C for 12 h. CO2-adsortpion profiles of the Mn-based compounds were detected by CO2 physisorption at 196 °C on a Beishide 3H-2000 analyzer after the samples were treated at 120 °C for 12 h. Thermo gravimetric analysis (TGA) profiles of these crystal materials were determined on a TG209 F3 instrument under nitrogen and air atmosphere, with about 5 mg sample linearly heated from 30 to 800 °C (10°/min).

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Scheme 1. Chemical fixation of carbon dioxide into cyclic carbonates in the presence of Mn-based MOFs.

The acid distribution associated with the crystal framework was measured by temperature-programmed desorption of NH3 (NH3TPD) using the TP-5056 instrument. Before the analysis was carried out, the sample was pretreated at 150 °C for 1 h under helium atmosphere. The distribution profile was obtained via heating the sample from 30 to 400 °C (10 °C/min). The geometry optimization for the reactants, products and transition state included in the synthesis of cyclic carbonates was performed on Gaussian 09 program equipped with the M06 method, and the mixed basis sets consisting of ECP-based LANL2DZ (for Mn atom) and 6-31G(d, p) (for other atoms) were employed for searching optimal structure. 2.3. Catalytic reaction The catalytic activity of synthesized Mn-based was evaluated in the promotion conversion of carbon dioxide into cyclic carbonates, which was carried out in a stainless autoclave (Scheme 1). In the process, epoxide and Mn-based material were firstly put into the autoclave, after which the internal gas was replaced with CO2 three times, and subsequently heating the system to the setting value (1000 rpm). At the end of the coupling reaction, the reactor was rapidly cooled down to room temperature. After which the mixed solution was separated to achieve catalyst-free liquid and used catalyst. When propylene oxide was used as the probe, the cooled process was conducted in an ice bath. Because the existence of water molecules within the solid cannot be completely excluded during the preparation, which can react with the epoxide to form the corresponding by-product. In addition, the by-product may be more than one specie when AGE was employed. Therefore, GC–MS and 1H NMR were adopted to determine the components of the mixture after the reaction of AGE with carbon dioxide (Figs. S1 and S2). Epoxide conversion and cyclic carbonate (CLC) selectivity were calculated according to Eqs. (1) and (2) with ethylene glycol monobutyl ether as the internal standard performed on GC-6890 instrument equipped with SE-54 capillary column (Column temperature: 170 °C, injector temperature: 270 °C and detector temperature: 280 °C). Epoxide conversion ¼

CLC selectivity ¼

mol of epoxideinitial  mol of epoxideresidual  100% mol of epoxideinitial ð1Þ

mol of CLCformed  100% mol of CLCformed þ mol of by - products

ð2Þ

3. Results and discussion 3.1. Characterization of the compound XRD patterns of Mn-based compounds were displayed in Fig. 1. Interestingly, the detectably-obtained datum were in well correspondence with the simulated patterns, and also reported by the

Fig. 1. X-ray diffraction (XRD) patterns of MA-MnF, EA-MnF, and Im-MnF.

previous literatures (Wang et al., 2015; Wang et al., 2004), suggesting that the three kinds of Mn-based materials were successfully synthesized in the present work. Because the reversible phase change for Im-MnF occurred when the temperature was changed from 20 to 180 °C (Wang et al., 2015). Therefore, the basic structural units of the synthesized Mn-based materials under room temperature were shown in Scheme 2. Interestingly, for each compound, each Mn ion is coordinated with six oxygen atoms, forming a MnO6 octahedron which was further bridged to six neighboring octahedra by formate ligands to form a 3-dimensional framework structure. Besides, the formate oxygen atom was further bridged with amine (EA, MA, Im) via hydrogen bond. Significantly, in the framework, Lewis acid/basic sites can be provided simultaneously due to the existing Mn ions and amine groups (Pato-Doldan et al., 2012). Therefore, according to the analysis, we can speculate these three kinds of Mn-based crystal materials can promote carbon dioxide coupling with epoxides (He et al., 2016; Gao et al., 2016). Functionalized groups of the synthesized Mn-based materials were determined by Nicolet 5700 spectrometer (Fig. 2). The band at 800 cm1 located in each spectrum, which is related to deformation vibrations of CAH bonds (ACOOH groups) (Szymborska-Malek et al., 2016). Besides, the presence of water molecules is also perfectly confirmed by the broad band located at 3600–3000 cm1 (Lyszczek, 2008; Sun et al., 2008). Additionally, the functional groups (ACOOH) was not detected in the characteristic region (1800–1680 cm1), instead, 1645–1550 cm1 and 1470– 1390 cm1 bands, which implied that the formic ligands have completely coordinated with the Mn ions (Sun et al., 2017; Zeng et al., 2016) and other initially-added formic acid has been removed during the preparation process. The chemical state of the Mn-based compounds was further analyzed according to the measured XPS data. The full spectra of the Mn-based compound were shown in Fig. S3a. the binding peaks associated with C 1s, N 1s, O 1s and Mn 2p were observed respectively in the corresponding region, which indicated all other initially-added components have been removed during the preparation process. C1s spectrum (Fig. 3a) of these compounds were deconvoluted with three symmetrical peaks that were associated with the CAC, CAN/C@N, and ACOOA groups respectively (Tu et al., 2017; Wang et al., 2017), while B.E. of CAN was observed to be in the order of: 286.5 eV (MA-MnF) > 286.3 eV (EA-MnF) > 285.9 eV (Im-MnF). It is likely that such results were related to molecular activity of nitrogen-containing groups. N 1s spectra of Mn-based samples were fitted with several symmetrical peaks,

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Scheme 2. Basic structural units of the synthesized Mn-based materials. (A) MAMnF, (B) EA-MnF, (C) Im-MnF.

Fig. 2. FT-IR spectrum of the Mn-based MOFs.

with the results illustrated in Fig. 3b. For MA-MnF and EA-MnF, the B.E. centered at 401.9 eV and 401.8 eV were assigned to N-C of CH3NH+3 and CH3CH2NH+3 groups. While, N 1 s of Im-MnF was deconvoluted and completely replaced with two symmetrical peaks which were attributed to C-N and C = N of imidazole ring, this further implied that the bond between C and N atom was more stable within the Im-MnF compound. Besides, Mn 2p spectrum of these Mn-based materials exhibited in Fig. S3b suggested the existence of identical valence of Mn atoms in each sample. The thermal behaviors of MA-MnF, EA-MnF and Im-MnF were recorded under the nitrogen atmosphere (Fig. 4). The similar process in weight loss was observed for MA-MnF and EA-MnF (Wang et al., 2004). The first step was related to simultaneous loss of the one amine and one formic acid and the generation of [Mn (HCOO)2]. Subsequently, manganese formate was continuously decomposed to become binary oxide (Nagabhushana et al., 2015). However, for Im-MnF, the weight loss was generally identified into three stages according to the DTG curve (Fig. S4). 25.58% of weight loss was detected in the first stage (30–204 °C), and it is associated with loss of imidazolium per formula unit (calculated 26.64%). The second (204–245 °C) was related to loss of one formic acid and formation of [Mn(HCOO)2]. Thereafter the residual component underwent decomposition process leading to the generation of MnO (found 28.44%, calculated 28.57%), which is in good agreement with the previous report (Wang et al., 2015). As a result, the structure of Im-MnF was stable when the temperature is below150 °C. NH3-TPD profiles of these samples were exhibited in Fig. 5a. For each of the sample, the area associated with NH3 desorption appeared in the range of 30–190 °C, corresponding to weak Lewis acid sites (carboxylic groups were not detected in Fig. 2). However, another desorption peak was also detected for Im-MnF sample when the temperature was no more than 218 °C, suggesting the

Fig. 3. XPS spectra of MA-MnF, EA-MnF and Im-MnF. (a) C1s, (b) N1s.

Fig. 4. TGA profiles of the Mn-based MOFs.

existence of moderate acid sites within the crystal structure. Besides, the area appearing in range of 220–400 °C is attributed to the decomposition of the studied compounds, which is in well correspondence with the TGA profiles (Fig. 4). In addition, the existence of Lewis basic sites caused by nitrogen-containing functional groups were also validated by the appearance of carbon dioxide uptake (Fig. 5b). However, the amount of CO2 sorption associated with basic sites was measured with the result in the order of

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MA-MnF > EA-MnF > Im-MnF, which is in coincidence with the content of one nitrogen atom per formula weight (Table S1). This result implies that only one nitrogen atom of imidazole molecule can function as the Lewis basic site. Therefore, these three kinds of Mn-based materials can be used as bifunctional catalysts for basic-acid promoting system. Furthermore, N2-adsorptiondesorption isotherms of the Mn-based MOFs validated the presence of mesoporos within the solid (Fig. S5a), with the pore size distribution mainly in the range of 2–8 nm (Fig. S5b, Table S2). Besides, SBET of these synthesized samples follows the order: MAMnF (101.03 m2/g) > EA-MnF (98.87 m2/g) > Im-MnF (81.57 m2/g), which is positively related to the manganese content per unit (Table S1). It is likely that the Mn ion as the framework node may facilitate the formation of porestructure efficiently. 3.2. Catalytic activity of Mn-based MOFs The catalytic activity of synthesized Mn-based MOFs was investigated with the AGE as probe which was employed as the substrate for exporting the optimal reaction conditions. Catalytic activity of Mn-based MOFs (MA-MnF, EA-MnF, ImMnF) for CO2 fixation was investigated with AGE as the substrate and the results were exhibited in Fig. 6. The highest catalytic activity among the studied materials was observed with 97.33% AGE conversion and 97.14% AGC selectivity over Im-MnF, while EA-MnF was detected with the lowest activity for carbon dioxide conversion (35.26% conversion and 99.51% selectivity). On the contrary, sorption amount of CO2 follows the order: MA-MnF

Fig. 5. (a) NH3-TPD and (b) CO2 adsorption profiles of Mn-based samples.

(7.69 mL/g) > EA-MnF (7.06 mL/g) > Im-MnF (6.71 mL/g), which represents the amounts of basic sites per unit weight (Table S1). It is possible that the difference in catalytic activity was associated with nitrogen-containing functionalized groups which can efficiently activate carbon dioxide during the coupling process. Additionally, the catalytic activity of these compounds is not only associated with the amounts of basic sites, but also related to the species as the imidazole ligand possessed the stronger basicity among the nitrogen-containing groups. Therefore, Im-MnF was employed to explore the optimal parameters in the fixed system. Fig. 7a exhibited the effects of reaction temperature on the AGE conversion and AGC selectivity. With the increase in reaction temperature, the conversion of AGE was significantly enhanced from 16.78% (90 °C) to 97.33% (100 °C), followed by a slight increase (97.99%, 110 °C), while the AGC selectivity exhibited only a slight decrease after the temperature goes 100 °C. This may be due to the increasing generation of 3-allyloxy-1,2-propanediol (Fig. S1, GC–MS) from AGE hydrolysis with the increasing temperature. Besides, the low temperature cannot offer enough active molecules to be used for AGE conversion in the fixed reaction time. Thus, to obtain a high yield of AGC with less energy consumption, 100 °C was adopted for investigating other parameters. The effects of initial pressure on the AGE conversion and AGC yield were displayed in Fig. 7b. It can be observed that the conversion of AGE was visibly facilitated from 73.26% (10 bar) to 97.27% (15 bar), then displayed an obvious decrease when initiallypressurized carbon dioxide was above 25 bar (93.87%). Such results arising from pressure change suggested a relative high concentration of carbon dioxide easily supported AGC synthesis (Wu et al., 2008) and the conversion of AGE could also be inhibited when the reactant was unlimitedly diluted with plenty of carbon dioxide (Han et al., 2011). Hence, the fixed system charged with 15 bar of carbon dioxide was more appropriate for sufficient promotion of AGE conversion. The effects of catalyst concentration were investigated via altering the initially-added amount of Im-MnF compound (Fig. 7c). The conversion of AGE was efficiently promoted with the increase in catalyst amount from 0.5 wt% (53.78%) to 1.0 wt% (97.27%), after which only a slight fluctuation was observed. The increasing conversion of AGE suggested appropriate concentration of catalyst in the reaction system can efficiently enhance the transformation of AGE into glycidyl ether carbonate. Besides, the excessive catalyst only accelerated the progress of the coupling reaction with less

Fig. 6. Catalytic activity of Mn-based MOFs in acceleration of CO2 coupling reaction. AGE 10 g, INP 20 bar, RTP 100 °C, RTM 6 h, Cat. 0.1 g (Reaction temperature: RTP, Reaction time: RTM, Initial pressure: INP).

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Fig. 7. (a) Effects of reaction temperature: AGE 10 g, INP 20 bar, RTM 6 h, Cat. 0.1 g; (b) Effects of initial pressure: AGE 10 g, RTP 100 °C, RTM 6 h, Cat. 0.1 g; (b) Effects of catalyst concentration: AGE 10 g, INP 15 bar, RTP 100 °C, RTM 6 h; (d) Effects of reaction time: AGE 10 g, INP 15 bar, RTP 100 °C, Cat. 0.1 g.

time but the equilibrium remains unchanged. Therefore, 1.0 wt% catalyst of AGE employed in this system is considered as the optimal concentration for AGC synthesis. Reaction time, as the important factor for studying the progress of coupling reaction, is usually required to intermittently detect the component of the reaction system. The effects of reaction time on AGE yield is illustrated in Fig. 7d. The conversion of AGE was obviously accelerated from 9.83% to 88.12% in the first 4 h, followed by slow maximization to 97.27% (6 h) and thereafter no obvious change was observed with time, which is associated with the rapid increase of AGC and decrease of AGE in amount during the catalytic process (Bai et al., 2012). Interestingly, the selectivity to AGC almost remained constant during the catalytic process. It is likely that the adsorbed water molecules existing on the surface of Im-MnF have been abundantly removed. Therefore, 6 h is considered as the optimal time for converting carbon dioxide into glycidyl ether carbonate. After investigating all the various parameters, the best results with 97.27% AGE conversion (TOF: 36.78 h1) and 97.66% AGC selectivity were observed under the optimal conditions (100 °C, 15 bar, 6 h, 1.0 wt% catalyst of AGE).

Fig. 8. Recyclable results of the Im-MnF in promotion of CO2 conversion with AGE as probe. AGE 10.0 g, 100 °C, 15 bar, 6 h, Cat. 0.1 g.

3.3. Recyclability of Im-MnF The recyclable results of the Im-MnF in catalytic conversion of carbon dioxide were studied with AGE as substrate (Fig. 8). For

the recyclable investigation, the sample was only leached from the mixture and recused in the next cycle. Interestingly, the conversion of AGE only showed a slight drop from 97.27% (fresh) to

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Table 1 Carbon dioxide coupling with various epoxide.* Entry

Epoxides

Con. (%)

Sel. (%)

Yie. (%)

Tem. (oC)

Time(h)

1

95.81

99.37

95.21

100

6

2

97.27

97.66

94.55

100

6

3

85.86

97.48

83.70

120

6

4

58.29

99.35

57.91

100

6

5

88.67

99.79

88.48

100

6

6a

57.7

96.6

—

140

6

7c

35

>99

—

80

8

8d

15

99

15

120

6

a: SO 5 mmol, 1.0 bar, DMF 4 mL, catalyst 2.5 mg(Zhang et al., 2018a); b: ECH 25 mmol, 1.2 MPa, Cat.(Ni-Co MOF(M)) 0.6 mol%(Kurisingal et al., 2018); c: CHO 105 mmol,18 bar, NH2-MIL-101(Al) 0.17 mol, %, TBAB 0.14 mol% (Senthilkumar et al., 2018). * Epoxide 10.0 g, initial CO2 pressure 15 bar, Cat. 0.1 g.

91.76% (2nd) when the Im-MnF was reused twice, which may be attributed to the slow leaching of the active component. To confirm this viewpoint, a leach experiment was carried out through filtering the catalyst and the filtrate was continuously reheated under the same conditions (Fig. S6). Intriguing, only a slight increase in AGE conversion was observed when the filtrate was treated at the optimal conditions, indicating that the slow leaching of the active component is not the only one factor for sample deactivation. Structural collapse or pore blockage can also account for the loss of the catalytic activity.

3.4. CO2 coupling reaction and DFT studies Various epoxides such as ECH, SO, PO, CHO were respectively employed as the substrates for investigating the application scope of Im-MnF in the promotion of carbon dioxide conversion and the results were listed in Table 1. Interestingly, all the studied epoxides can be efficiently transformed into the corresponding carbonates, of which, the yields of ECH and AGE were more than 90%, while PO was observed to have the lowest yield. The former could be due to the high activity of epoxide, with the CO2 insertion into

Scheme 3. carbon dioxide coupling with epoxide in the presence of Im-MnF compound.

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ECH easily occurred during the coupling process. Also, apart from the low reactivity of PO molecules, which could be a factor, the low conversion of PO was ascribed to the limited concentration of carbon dioxide which cannot completely promote all the PO conversion. Interestingly, Im-MnF was found with higher catalytic activity for the coupling reaction when compared with previously reported catalysts (Table 1). NH3-TPD profiles (Fig. 5a) have confirmed the presence of Lewis acid sites within crystal structure. Besides, nitrogen-containing group cations as the part of the Mn-based materials could also functionalize as the Lewis basic sites, further validated by CO2adsorption (Fig. 5b). According to the literature (Song et al., 2017), a mechanism for carbon dioxide conversion was drawn with Im-MnF as catalyst (Scheme 3). The epoxide and CO2 were activated by the Lewis acid-basic sites. Then b-carbon of the activated epoxide was attracted by O atom of the activated carbon dioxide, forming a transition state species which eventually cyclized to

295

form cyclic carbonate. Meanwhile, manganese atom of the regenerated Im-MnF will be re-coordinated with epoxide in next cycle. To confirm the validity and rationality of the proposed mechanism, Density Functional Theory (DFT) calculation was exploited for the coupling progress (Scheme 4 and Fig. 9). For the transient state I, the total energy obviously increased compared with the initial state (Fig. 9 I), suggesting that the process for activating AGE is key step during the coupling reaction. Moreover, the relative system energy decreased from 17.982 to 22.662 Kcal/mol after carbon dioxide insertion into the epoxide (Fig. 9IV), which indicated more stable intermediate was formed once in the opening-ring as AGE was inserted with carbon dioxide (Fig. 9IV–V). This calculation well revealed that the opening ring of epoxide also easily occurred without the addition of other nucleophile and AGE activation was the key step for the coupling progress (All the atomic coordinates including the reactants, transition states, and products were in supporting information).

Scheme 4. Calculated state of CO2 coupling with AGE over Im-MnF compound. Structure optimization including AGE, carbon dioxide, transition states, intermediates and AGC were performed on M06 equipped with mixed basis sets. ECP-based LANL2DZ for Mn atom and 6-31G(d, p) for C, H, O, N atoms.

Fig. 9. Theoretical calculation of carbon dioxide coupling with AGE in the presence of the Im-MnF.

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4. Conclusions In this work, three kinds of Mn-based materials were successfully synthesized and further applied as the catalysts for the chemical fixation of carbon dioxide with the epoxides as substrates. When the coupling reaction was performed with AGE as probe under 15 bar, 100 °C (6 h) as well as 1.0 wt% catalyst, the optimal achievement with 97.27% AGE conversion and 97.66% AGC selectivity was observed. Besides, only a slight decrease in catalytic activity was detected after Im-MnF was reused for twice, while, carbon dioxide coupling with different epoxides was also accomplished. In addition, a proposed mechanism for carbon dioxide conversion is in good agreement with DTF calculation. Declaration of interest statement The present work is aimed to investigate the catalytic activities of various Mn-based MOFs for carbon dioxide coupling with epoxides. The metal organic frameworks, [(CH3NH3][Mn(COOH)3] (MAMnF), [(CH3CH2NH3][Mn(COOH)3] (EA-MnF), and [C3H5N2][Mn (COOH)3] (Im-MnF) were studied via several characterizations such as XRD, FT-IR, XPS, N2-adsorption, TG-DSC, CO2-adsorption and NH3-TPD. Interestingly, Im-MnF compound was observed to possess the highest catalytic activity among the studied compounds, which is associated not only with the amounts of basic sites, but also related to the nitrogen-containing species. 97.27% conversion of allyl-glycidyl ether (AGE, TOF: 36.78 h1) and 97.66% selectivity to allyl-glycidyl carbonate (AGC) were obtained under the explored optimized conditions (100 °C, 15 bar, 6 h, 1.0 wt% of AGE). In addition, only a slight downward in catalytic activity was found when the sample was reused twice. Furthermore, coupling reactions of CO2 with various epoxides were also performed, of which, the yield of the cyclic carbonates followed the order: Epichlorohydrin > Allyl glycidyl ether > Styrene oxide > Cyclohexene oxide > Propylene oxide. Finally, a mechanism was proposed, which is in good agreement with the DFT calculation. Acknowledgments This work was financially supported by National Natural Science Foundation of China (Nos. 21676054, 21406034), Natural Science foundation of Jiangsu (No. BK20161415), Fundamental Research Funds for the Central Universities (No. 2242018K40041). Also, we are very grateful to Dr. Qi for providing DFT calculation. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ces.2019.02.032. References Bai, D.S., Jing, H.W., Wang, G.J., 2012. Cyclic carbonate synthesis from epoxides and CO2 over cyanocobalamin/n-Bu4NI. Appl. Organomet. Chem. 26, 600–603. Chizallet, C., Lazare, S., Bazer-Bachi, D., Bonnier, F., Lecocq, V., Soyer, E., Quoineaud, A.A., Bats, N., 2010. Catalysis of transesterification by a nonfunctionalized metal-organic framework: acido-basicity at the external surface of ZIF-8 probed by FTIR and ab initio calculations. J. Am. Chem. Soc. 132, 12365–12377. Chang, T., Gao, X.R., Bian, L., Fu, X.Y., Yuan, M.X., Jing, H.W., 2015. Coupling of epoxides and carbon dioxide catalyzed by Brönsted acid ionic liquids. Chin. J. Catal. 36, 408–413. Chiang, C.L., Lin, K.S., Chuang, H.W., 2018. Direct synthesis of formic acid via CO 2 hydrogenation over Cu/ZnO/Al 2 O 3 catalyst. J. Cleaner Prod. 172, 1957–1977. Fan, G.Z., Luo, S.S., Fang, T., Wu, Q., Song, G.S., Li, J.F., 2015. Cerium dioxide catalyzed synthesis of methyl N-phenylcarbamate from carbon dioxide, aniline and

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