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Synthesis of carbon fibers support graphitic carbon nitride immobilize ZnBr2 catalyst in the catalytic reaction between styrene oxide and CO2
Journal of CO₂ Utilization 34 (2019) 716–724
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Synthesis of carbon fibers support graphitic carbon nitride immobilize ZnBr2 catalyst in the catalytic reaction between styrene oxide and CO2 Dongdong Zhanga,b, Tong Xua,b, Chunping Lia,b, Wei Xua, Junzhong Wanga,b, Jie Baia,b, a b
T
⁎
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, 010051, People’s Republic of China Inner Mongolia Key Laboratory of Industrial Catalysis, Hohhot, 010051, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cycloaddition of CO2 Heterogeneous catalysts Carbon fibers Composite catalyst
Chemically fixing CO2 into the fine chemicals is a sustainable route to the utilization of CO2 and meets the requirements of green chemistry. Generally, an important intermediate, high-valued cyclic carbonates, is produced by catalyzed cycloaddition of CO2 with epoxides during the CO2 fixation. However, most of the catalysts for cycloaddition can hardly meet the standards of the green chemistry, including highly efficient, low cost, easily recoverable and recyclable etc. Therefore, in this work, an environmentally friendly composite Zn-CN/C catalyst for the cycloaddition of styrene oxide and CO2 was successfully prepared by a facile method. To this end, zinc bromide was immobilized by graphitic carbon nitride (gCN) and then supported on carbon fibers. Meanwhile, the crystal structure, types of chemical bonding, surface morphologies, and electronic transmission of the prepared catalyst were also investigated. Furthermore, the catalytic performance was evaluated by the cycloaddition reactions of styrene oxide and CO2, a satisfactory result of the moderate yield of 50–90% with a good selectivity (> 99%) was obtained. Finally, a credible mechanism of the reaction was proposed.
1. Introduction Nowadays, as an abundant C1 feedstock in the synthesis of fine chemicals, CO2 is getting more and more attentions due to its nontoxicity, renewability, low cost and easy preparation [1,2]. A prospective path for CO2 utilization is reacting CO2 with epoxides to synthesize cyclic carbonates [3–8]. Products including ethylene carbonate, propylene carbonate and styrene carbonate (SC) can be obtained from this route, which have widely been used in many chemistry and chemical industry fields, such as aprotic polar solvents [9,10], active intermediates for the production of lubricants [11] and electrolytes for lithium batteries [12]. However, for the cyclic reaction of carbon dioxide mentioned, the thermodynamic stability and kinetic inertness in the molecule dimension is still a major concern [13]. To overcome the low reactivity of CO2, the synergistic effects of Lewis acid and Lewis base were introduced to make the cycloaddition with epoxides easily [14]. Briefly, the cycloaddition reaction usually starts with three steps: (i) ring-opening of epoxides by nucleophiles attack, (ii) insertion of CO2 into the halogenated alkoxide and (iii) ringclosing, intramolecular SN2 reaction and release the anion [15,16]. Various homogeneous and heterogeneous catalysts have been applied, such as, metal-organic frameworks (MOFs) [17–19], salen complexes [20], porous organic polymers [21], metal nanoparticles [22], ionic ⁎
liquids [23], metalloporphyrins [24], zeolites [25], smectite [26], mesoporous silica [27], poly-oxometalates (POMs) [28] and metallic oxides [29]. Comparing with the homogeneous catalysts, the heterogeneous catalysts can offer more advantages, ranging from the good stability to the minimal cost for the product separation. In recent years, there is a tendency to loading the homogeneous actives on heterogeneous materials to composite the high catalytic efficiency with the good recovery and reusability. For example, Kim et al. [30] found that imidazolyl ionic liquid species displayed a high catalytic efficiency in the cycloaddition reaction, and fixed the species on polystyrene without any activity lose. Pankaj et al. [6] loaded cobalt on the modified titanium dioxide for the synthesis of cyclic carbonates under the illumination of incandescent lamps. Zhang et al. [31] used graphene oxides (GOs) as the fast catalysts for the coupling reaction of styrene oxide with CO2, and the styrene carbonate was obtained at a low pressure and a high temperature. However, the catalytic efficiency of the activity species on these heterogeneous catalysts are still too low to meet the requirements of the reaction even under a harsh reaction condition. Besides, the major nucleophiles are not qualified in the cycloaddition reaction. Therefore, developing a catalyst consisting of a perfect pair of Lewis acid and base is critical for the cycloaddition. In this regard, transition metal element is the best candidate for high efficient Lewis acid due to
Corresponding author at: Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, 010051, People’s Republic of China. E-mail address: [email protected] (J. Bai).
https://doi.org/10.1016/j.jcou.2019.09.005 Received 9 April 2019; Received in revised form 3 September 2019; Accepted 9 September 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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deposition furnace. The crucible was heated to 520 °C at the rate of 5 °C /min and kept the temperature at 350 °C and 400 °C for 1 h and 520 °C for 2 h, respectively. Finally, the sample was taken from the furnace after cooled to the room temperature and grinded into a powder. The final sample was denominated as Zn-CN/C. The pure gCN was prepared directly by the pyrolysis of urea in the absence of ZnBr2.
the special properties and advantages in the term of the environment and energy. Fan et al. [32] proved that Cu(acac)2 played a key role in the coupling reaction of epoxides with CO2, which showed an excellent catalytic performance when supported by the Fe3O4@MCM-41. Buonerba et al. [33] prepared an iron(III) catalyst with a good selectivity (＞99%) for the coupling reaction. Moreover, several organic frameworks including metal-corn of Cu, Cr, Co, Zn and Ni have also been reported to facilitate the cycloaddition reaction [34–36]. Among them, zinc-containing materials have attracted a great attention due to their excellent performance. Sun et al. [37] developed a direct synthetic path of styrene carbonate from the styrene. In this path, zinc bromide was added to the catalyst system as an active ingredient. Han et al. [38] prepared a polyoxometalate-based homochiral metal-organic framework involving with zinc, and it possessed a high catalytic activity for the one-pot synthesis of cyclic carbonates from olefins. Due to the good dispersion and large surface area, all zinc-containing MOFs showed the outstanding conversion of epoxides, high selectivity and yield of the corresponding cyclic carbonates at a mild temperature and atmospheric pressure. Nevertheless, these homogeneous MOF catalysts would not get a satisfied result without quaternary ammonium halides. So, the development of bifunctional heterogeneous catalysts will be demanded for the synthesis of cyclic carbonates [39,40]. Graphitic carbon nitride (gCN), as a charming functional material, has gained many concerns in the fields involving antibacterial materials and photocatalysts [41]. Lately, gCN has been one of the most significant components for the catalysts [42,43]. The tri-s-triazine group endows gCN the potentiality and extensive adaptability for various reactions. For example, Antonietti et al. [44] demonstrated that gCN can absorb and activate the carbon dioxide molecules due to the existence of uncondensed amines. Obviously, gCN-containing catalytic system could be used for catalyzing the coupling of epoxides with CO2. Nevertheless, in the most cases, the metal cations acted as Lewis acid sites only when the heterogeneous gCN was applied in the cycloaddition reaction. In this work, the easily obtainable urea was used as a precursor for the preparation of gCN and carbon fibers. These carbon-based materials as the carriers of the catalytic system were prepared by electrospinning and the subsequent high temperature carbonization. Moreover, the addition of ZnBr2 into gCN can not only provide the Lewis acid and base sites but also activate the epoxides through the coordination with the oxygen atoms [45]. The bonding state of Zn2+ with halogen ion and gCN were also investigated in detail. It is proved that the better dispersion of catalytically active component is desired. At last, a possible catalytic mechanism was proposed according to the previous literatures and the experimental data in this study.
2.3. Characterization The morphologies and surface states of the samples were obtained by scanning electron microscope (SEM) using a Phenom machine (Phenom Pro, Netherlands). The ultrastructure of samples was observed by a transmission electron microscope (TEM, Jeol, JEM-2010, Tokyo, Japan). The X-ray diffraction (XRD) patterns were presented by an X-ray diffractometer (D/max 2500 PC, Rigaku, Japan) to get the information about the crystalline structure (crystallinity) of composite catalyst. An X-ray photoelectron spectroscopy instrument (XPS, EscaLab 250, Thermo Fisher Scientific, Massachusetts, USA) was used to de-irradiate the samples, so that their inner electrons or valence electrons were excited and emitted, thereby the composition was obtained. The chemical groups of samples were determined by the Fourier transform infrared spectroscopy (FTIR) using a Nicolet iS50 instrument (Thermo Fisher Scientific, USA). The contents of zinc loadings were detected by inductively coupled plasma-optical emission spectroscopy (ICP-OES730, Agilent, USA). The pore size distribution and the BET specific surface area were determined by the BJH (Barrett-JoynerHalenda) and BET (Brunauer, Emmett, and Teller) methods, respectively. The temperature program of degassing procedure consisted with the following three steps: (1) Pretreating at 70 °C for 15 min, the heating speed was 10 °C /min. (2) Heating the heating mantles to 120 °C at the same speed as step (1) and keeping the temperature for 15 min. (3) Keeping the temperature at 300 °C for 3 h. The product analysis was determined by using a gas chromatography (GC) (Shimadzu 2010 Plus, RTX-5 capillary column, flame ionization detector), and a GC–MS (Agilent 5975C). The GC programme was performed at a temperature ramp of 10 °C / min up from 60 °C to 280 °C and kept for 2 min.
2.4. Catalytic performance The synthesis of styrene carbonate (SC) took place in a 50 mL steel kettle with a pressure gauge filled with styrene oxide (SO) and CO2. At first, SO (4 mL, 35 mmol), N,N-dimethyl formamide (DMF, 1 mL) and 0.1 g of catalyst were added into the kettle with a magnetic stirrer. Then the container was sealed and purged with CO2 three times and the pressure was eventually raised to 2 MPa. Thereafter, the reaction was run at 140 °C for 8 h. At last, the reaction system was cooled to the room temperature in the cold-water bath and the gas was slowly released. The catalyst was separated from the system by a centrifuge and washed by ethanol for the next run. The reaction products from the supernatant were extracted by a mixed solution of ethyl acetate and water, and qualitatively detected by the GC. The conversion (Conv.) of styrene and the selectivity (Sel.) of the target product were determined by the integral areas of the GC peaks, and calculated as the following equations:
2. Experimental section 2.1. Materials Polyacrylonitrile (PAN, Mw = 80,000) was purchased from Kunshan Hongyu Plastic (Kunshan China). N, N-dimethyl formamide (DMF, A.R.) was purchased from Fengchuan Chemical Reagent Technologies Co., Ltd (Tianjin China). Urea was purchased from Tianjin Beilian Fine Chemical Research Institute (Tianjin China). Deionized water was obtained from Wastons. Zinc bromide and other chemicals were purchased from Innochem. All chemical reagents were directly used without any further purification.
Conv. = [(areas of reactant converted) × 100]/areas of reactant used
2.2. Catalyst preparation Sel. = [(areas of product formed) × 100]/areas of reactant converted Firstly, 0.2 g of ZnBr2 and 1 g of urea were dissolved into 20 mL deionized water with stirring at 70 °C for 20 min. Thereafter 0.5 g of prepared carbon fibers was added and the mixture was obtained after all of the water was evaporated. Then, the black solid products were taken out and then placed in the crucible to calcined in a vapor
The preparation and application of the catalysts were shown in Scheme 1.
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Scheme 1. Catalyst preparation and experimental flow.
Fig. 1. XRD patterns of carbon (A), gCN (B), Zn-CN (C) and Zn-CN/C (D).
3. Results and discussion
drum peak in Fig. 1A at ca. 2θ = 24.2° corresponds to the (002) plane of amorphous carbon and the peak at 43.5° is assigned to its (100) plane [46]. Two peaks with the clear identification of gCN are displayed in the Fig. 1B. A relative weak peak has been found at ca. 2θ = 12.9°,
To investigate the crystal structure of the samples, XRD patterns of the raw materials and catalysts are obtained, as shown in Fig. 1. The
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Fig. 2. FT - IR spectra of Zn-CN (a), Zn-CN/C (b), carbon (c) and gCN (d).
Fig. 4. N2 adsorption-desorption isotherm of Zn-CN/C composite catalyst.
which is attributed to the in-plane repeated accumulation, i.e. the (100) plane, and another strong diffraction peak at ca. 2θ = 27.7° is the unique interplanar stacking of gCN, i.e. the (002) plane [47]. In comparation with the pattern of the gCN, the intensities of both (002) and (100) plane in Fig. 1C&D are decreased, indicating that the original ordered internal structure of gCN might be destroyed. In addition, no obvious shift of peaks is found in Fig. 1C&D. However, the diffraction peak of (002) planefor gCN is quite dispersed with relatively low strength. The relatively weak peak is due to the incorporation of metal ions into gCN, which is consistent with the previous literature involving the K, Zn and Fe doped gCN [48]. Many characteristic diffraction peaks of zinc bromide are also clearly distinguished and listed in the Fig. 1C, indicating the fixation of zinc bromide in the essential active species. The diffractions peaks appeared at 17.5° and 21.06° correspond to the (211) and (213) plane of zinc bromide. Peaks located at 28.4°, 47.4° and 51.7° in Fig. 1D are assigned to the (321), (329) and (527) planes of zinc bromide as well (PDF36-0756). Simultaneously, many peaks of zinc oxide such as the (100) plane (ca. 2θ = 31.6°) and (110) plane (ca. 2θ = 56.6°) [49] can be observed in Fig. 1C&D, which may be caused by the excessive addition of zinc bromide. Furthermore, according to Debye-Scherrer formula, the average crystallite size of Zn-CN is 8.6 ± 1 nm.
Fig. 5. Pore size distribution of Zn-CN/C composite catalyst.
To determine the chemical structures of the samples, the FTIR spectra are conducted. As shown in Fig. 2, the samples supported by gCN demonstrate a similar spectra compared to the original gCN, the bands between 1200 and 1700 cm−1 are assigned to the stretching
Fig. 3. XPS spectra of samples. 719
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vibrations of CeN and CN] in the aromatic nitrogen-carbon heterocyclic structure. The sharp peak at 804 cm−1 corresponds to the stretching vibration mode of the s-triazine unit, which is regarded as the elementary building block of gCN [50]. The spectrum of Zn-C samples also present the characteristic peaks of gCN (line a and b), demonstrate an undamaged gCN structure in Zn-C samples. The wide absorption peak at 3500˜3000 cm−1 is attributed to the NeH of incompletely polycondensated amino groups and OeH in absorption water on the sample surface. Interestingly, a low-intensity band at 2168 cm−1 is identified in the spectra of Zn-CN, but not found in the bulk gCN (line d), indicating the unique signal of the azide group appears due to the presence of Zn-N bond. The result irrefutably confirms the organic combination of gCN with zinc bromide. Thus, the loading of zinc bromide would not change the main chemical functionalities of gCN, and the Zn species could be well inserted into the gCN skeleton [45]. The valence information of the elements is determined by XPS spectra as Fig. 3 shows. The results of XPS analysis confirm the presence of C, N, Zn and Br in the Zn-CN and Zn-CN/C samples, and their bond combination is further analyzed. The C1s peak of the samples can be deconvoluted into three components, peaks at 284.4 eV, 286.4 eV and 284.7 eV corresponding to the sp2-bonded carbon in the Zn-CN network (N]CN), COee bond of the samples and CeC bond of gCN, respectively [44]. By contrast, the ratio of gCN in Zn-CN/C was higher than that in Zn-CN, which indicates the dominant graphitic carbon in ZnCN/C and thus high basicity of Zn-CN [51]. The Zn2p peaks both in ZnCN and Zn-CN/C can be divided into the Zn 2p1/2 and 2p3/2 peak, indicating two chemical environments of Zn element. As can be seen from the Zn2p spectra of Zn-CN sample, peaks at 1046 eV and 1023 eV are assigned to zinc in the metal ion state, and another one at 1020 eV is corresponding to the metallic Zn. Meanwhile, it is worth noticed that the binding energies of the intensity in Zn 2p1/2 and 2p3/2 peaks are weaker than the ionic state Zn (II) but higher than the metallic state (0). This accounts for the existing of an intermediate valence state of the zinc cation in gCN, which may be caused by the interaction between Zn and the nitride of gCN. The similar result can also been observed in the Zn-CN/C samples [52]. The above results revealed that zinc existed in the samples as ions state instead of metallic state, which is similar to zinc halide loaded on the other materials [45]. The N1s spectra of both Zn-CN and Zn-CN/C can be fitted into three main peaks. The peaks at 400.86 and 401.3 eV are assigned to the uncondensed amino group (–NH2), the peaks at 399.85 and 399.98 eV are corresponding to the partially condensed tertiary groups [CeN (C/H)Cee] and the peaks at 398.28 and 398.88 eV are attributed to the tris-s-triazine (C]NCe) [53]. The Br3d spectra are also fitted as shown in Fig. 3. The binding energies of Br3d5/2 and Br3d3/2 peak are 68.19 eV and 69.28 eV [54], respectively, indicating an ionic state of Br. The ionic bromine presented in the catalyst as Lewis base plays a considerable role in the cycloaddition reaction. The surface adsorption properties of the catalysts are determined by the N2 adsorption-desorption measurements. The BET pore volume and surface area are studied by N2 adsorption/desorption isotherm at -196 °C as shown in Fig. 4. It can be seen that the N2 adsorption/desorption isotherm of the catalyst presents a type II isotherm. According to the results of BJH analysis, the average pore size of ZN-CN/C is 0.995 nm. The micropores, mesopores together with macropores coexist in the catalyst simultaneously (Fig. 5). The surface area of Zn-CN/C is 19 m2 /g. Raman spectroscopic results are conducted to detect the electron transmission of the catalyst system. As shown in Fig. 8, no D and G peaks are found in gCN and Zn-CN, whereas they appear in C and ZnCN/C, demonstrating the carbon fibers act as a main carrier for the electron transport. The microstructures and morphologies of the catalysts are observed by FE-SEM. From Fig. 6A and B that the smooth surfaces of both carbon fibers and Zn-CN/C can be clearly observed. Many granular substances
Fig. 6. FE-SEM images of C fibers (A) and Zn-CN/C composite catalyst (B and C).
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Fig. 7. TEM images of Zn-CN/C composite catalyst.
performance of zinc oxide is also investigated under the same conditions. Although the selectivity is high, the conversion is quite low (11.36%) when using pure ZnO as the catalyst, (Table 1, entry 13). Moreover, the catalyst Zn-CN/C shows an excellent selectivity of the target products in the subsequent tests. By comparing the data of the entry 4, 7 and 8 in Table 1, it can be concluded that the conversion of reactants increases with the rising reaction time, and keep the same at temperatures under 160 °C (Table 1, entry 4–6). As the temperature raises to 160 °C, the selectivity of reaction decreases because the high temperature leads to further decomposition of the products into phenylethylene glycol [31]. Additionally, the effect of solvents on the cycloaddition reaction is investigated under the same conditions. The polar solvents, acetonitrile, DMF and methanol are used for the reactions, of which the polarity order is acetonitrile (6.2) < DMF (6.4) < methanol (6.6). By comparing the results in entry 7 and 9 of Table 1, the conversion of styrene increases with the polarity of solvent. However, among these solvents, methanol is a unique one that it can produce H bonds as a protic solvent [55]. Thus, the conversion of the reaction using methanol as a solvent is lower than DMF, but higher than acetonitrile (Table 1, entry 4, 9 and 10). It can be seen from entry 4, 11 and 12 of Table 1, the reaction yield increases with the increased catalyst amount. Interestingly, the growth speed of the yields in the low catalyst mass range (0.01 g ˜ 0.05 g) is faster than that in the large one (0.1 g ˜ 0.5 g) (Table 1, entry 4, 11 and 12). The low mass of catalyst is beneficial to the fast mass transmission. Moreover, the various epoxides used for cycloaddition reaction with our catalyst is also investigated as shown in Table 2. All the catalyst can catalyze the reaction of epoxides with the high selectivity and the expected conversion. As an important issue, the recyclability is performed to evaluate the performance of the catalysts. The results of the repeatability tests are shown in Fig. 9, after five consecutive cycles, the epichlorohydrin can still completely be converted in four hours with a high selectivity. Moreover, it can be found that the structure of catalyst is not destroyed. after five cycles from the FTIR spectra (Fig. 10). The Comparison of the Zn-CN/C catalysts and current state-of-art
Fig. 8. Raman spectra of C, gCN, Zn-CN and Zn-CN/C.
are also found on the Zn-CN/C, which indicates the Zn-CN particles are loaded on the carbon fibers. An amplified SEM image is also obtained to observe the surface details of the fibers. As shown in Fig. 6C, the carbon fibers are completely surrounded by Zn-CN. Furthermore, the detailed morphologies of Zn-CN/C are investigated by TEM as shown in Fig. 7. The activity substances are obviously attached to the surface of support, confirming that Zn-CN is loaded on the carbon fibers. The results mentioned above reveal that the expected Zn-CN/C catalyst is successfully prepared, and the zinc content of this catalyst is 7.49% according to the ICP-OES result. To evaluate the catalytic performance, CNFs, gCN, Zn-CN and ZnCN/C are used for the cycloaddition reaction of styrene oxide and CO2 without any co-catalyst. The results of reactions are shown in Table 1. The Zn-CN/C exhibited an excellent catalytic performance with 54.36% of conversion and a high selectivity. By contrast, the catalytic 721
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Table 1 Catalytic results of catalysts prepared at various conditions. Entry
Catalyst
Temperature (℃)
Time (h)
Solvent
Conversiond (%)
Selectivityd (%)
1 2 3 4 5 6 7 8 9 10 11b 12c 13
CNFs gCN Zn-CN Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C ZnO
140 140 140 140 120 160 140 140 140 140 140 140 140
8 8 8 8 8 8 6 10 8 8 8 8 8
DMF DMF DMF DMF DMF DMF DMF DMF CH3OH MeCN DMF DMF DMF
3.53 11.30 35.59 54.36 43.18 91.03 43.92 69.45 13.62 4.72 48.47 21.29 11.36
＞99 72 92 ＞99 ＞99 98.55 ＞99 ＞99 ＞99 ＞99 ＞99 ＞99 98.75
a Reaction conditions: 4 mL styrene oxide, 1 mL DMF, p(CO2) =2.0 MPa, T = 140 ℃, t =8 h, 0.1 g catalyst. b 0.05 g Zn-CN/C. c 0.01 g Zn-CN/C. d Determined by GC analysis.
Table 2 Catalytic results for the cycloaddition reactions using various epoxides. Entry
Substrates
Products
Conv.
a
(%)
Sel.
a
1
91.59
> 99
2
74.34
96.78
3
99
93.13
(%)
Reaction conditions: 4 mL epoxide, 1 mL DMF, p(CO2) =2.0 MPa, T = 140 ℃, t =8 h, 0.1 g catalyst. a Determined by GC analysis.
Fig. 10. FT-IR spectra of fresh and reused Zn-CN/C. Table 3 The comparison of the current state-of-art catalysts with Zn-CN/C for the cycloaddition reaction of epichlorohydrin with CO2. Entry
Catalyst
Co-catalyst
Yield (%)
Source
1 2a 3b 4c
Zn-CN/C J500 PS-(IM)2ZnI2 FJI-H14
/ / / TBAB
92.2 92 57.8 95
This work Ref. 56 Ref. 30 Ref. 57
Reaction conditions: a. 100 mg catalyst, 20 mmol epichlorohydrin, 150 ℃, 15 bar, 15 h, 400 rpm; b. 0.2 g PS-(IM)2ZnI2, 50 mmol epichlorohydrin, 60 ℃, 10 bar, 4 h; c. 0.48 mol% FJI-H14, 2.5 mol% TBAB, 20 mmol epichlorohydrin, 80 ℃, 1 atm flue (0.15 bar CO2,0.85 bar N2), 4 h.
in the Scheme 2. First of all, carbon dioxide is captured by the catalyst and activated by the nucleophilic attack of carbon nitride. Then, epoxy ring opens by the synergistic action of zinc and bromide ions, thereafter the bromide ions are linked to the opened skeleton. Next, the activated carbon dioxide is linked into this skeleton, forming a unique structure as illustrated in the right side of Scheme 2. Finally, the intramolecular SN2 reaction happens and the anion releases, meanwhile forming the five-membered cyclic carbonates, which are the ultimate products in this reaction.
Fig. 9. Recycling tests of cycloaddition of CO2 with epichlorohydrin catalyzed by Zn-CN/C. Reaction conditions: 0.1 g Zn-CN/C, 4 mL epichlorohydrin, 1 mL DMF, p(CO2) =2 MPa, T = 140 ℃, t =4 h.
catalysts for the cycloaddition of epichlorohydrin with CO2 is presented in Table 3 [30,56,57]. As can be seen from the table, both the Zn-CN/C, J500 and FJI-H14 exhibit the excellent catalytic performance. However, when using J500 and FJI-H14 as catalysts, the reactions took a long time. Besides, FJI-H14 can only work in the presence of TBAB. PS(IM)2ZnI2 is an ionic liquid, which is usually difficult to recycle and reuse. Based on the previous literatures and experimental data of this study, a hypothetical mechanism of the reaction is presented, as shown
4. Conclusion In summary, an environmentally friendly composite catalyst Zn-CN/ C was fabricated by a facile method. Zinc ions were immobilized by the graphitic carbon nitride and then supported on the carbon fibers. In this 722
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Scheme 2. A hypothetical mechanism of cycloaddition of CO2 and styrene oxide catalyzed by the Zn-CN/C composite.
catalyst, zinc and bromide play a key role as Lewis acid-based pair of the styrene oxide activating, and gCN acts as a solid base for activating CO2 and a support for zinc bromide immobilization. The catalyst showed an excellent catalytic performance with a high selectivity and yield in the cycloaddition reaction of styrene oxide and CO2. In addition, a proposed reaction mechanism of was presented, which would provide a theoretical support for virous cycloaddition reactions catalyzed by this kind of material. This work will offer a new idea for the development of novel heterogeneous zinc containing catalysts, and a highly efficient and green route for the utilization of CO2.
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Synthesis of carbon fibers support graphitic carbon nitride immobilize ZnBr2 catalyst in the catalytic reaction between styrene oxide and CO2 Dongdong Zhanga,b, Tong Xua,b, Chunping Lia,b, Wei Xua, Junzhong Wanga,b, Jie Baia,b, a b
T
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Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, 010051, People’s Republic of China Inner Mongolia Key Laboratory of Industrial Catalysis, Hohhot, 010051, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cycloaddition of CO2 Heterogeneous catalysts Carbon fibers Composite catalyst
Chemically fixing CO2 into the fine chemicals is a sustainable route to the utilization of CO2 and meets the requirements of green chemistry. Generally, an important intermediate, high-valued cyclic carbonates, is produced by catalyzed cycloaddition of CO2 with epoxides during the CO2 fixation. However, most of the catalysts for cycloaddition can hardly meet the standards of the green chemistry, including highly efficient, low cost, easily recoverable and recyclable etc. Therefore, in this work, an environmentally friendly composite Zn-CN/C catalyst for the cycloaddition of styrene oxide and CO2 was successfully prepared by a facile method. To this end, zinc bromide was immobilized by graphitic carbon nitride (gCN) and then supported on carbon fibers. Meanwhile, the crystal structure, types of chemical bonding, surface morphologies, and electronic transmission of the prepared catalyst were also investigated. Furthermore, the catalytic performance was evaluated by the cycloaddition reactions of styrene oxide and CO2, a satisfactory result of the moderate yield of 50–90% with a good selectivity (> 99%) was obtained. Finally, a credible mechanism of the reaction was proposed.
1. Introduction Nowadays, as an abundant C1 feedstock in the synthesis of fine chemicals, CO2 is getting more and more attentions due to its nontoxicity, renewability, low cost and easy preparation [1,2]. A prospective path for CO2 utilization is reacting CO2 with epoxides to synthesize cyclic carbonates [3–8]. Products including ethylene carbonate, propylene carbonate and styrene carbonate (SC) can be obtained from this route, which have widely been used in many chemistry and chemical industry fields, such as aprotic polar solvents [9,10], active intermediates for the production of lubricants [11] and electrolytes for lithium batteries [12]. However, for the cyclic reaction of carbon dioxide mentioned, the thermodynamic stability and kinetic inertness in the molecule dimension is still a major concern [13]. To overcome the low reactivity of CO2, the synergistic effects of Lewis acid and Lewis base were introduced to make the cycloaddition with epoxides easily [14]. Briefly, the cycloaddition reaction usually starts with three steps: (i) ring-opening of epoxides by nucleophiles attack, (ii) insertion of CO2 into the halogenated alkoxide and (iii) ringclosing, intramolecular SN2 reaction and release the anion [15,16]. Various homogeneous and heterogeneous catalysts have been applied, such as, metal-organic frameworks (MOFs) [17–19], salen complexes [20], porous organic polymers [21], metal nanoparticles [22], ionic ⁎
liquids [23], metalloporphyrins [24], zeolites [25], smectite [26], mesoporous silica [27], poly-oxometalates (POMs) [28] and metallic oxides [29]. Comparing with the homogeneous catalysts, the heterogeneous catalysts can offer more advantages, ranging from the good stability to the minimal cost for the product separation. In recent years, there is a tendency to loading the homogeneous actives on heterogeneous materials to composite the high catalytic efficiency with the good recovery and reusability. For example, Kim et al. [30] found that imidazolyl ionic liquid species displayed a high catalytic efficiency in the cycloaddition reaction, and fixed the species on polystyrene without any activity lose. Pankaj et al. [6] loaded cobalt on the modified titanium dioxide for the synthesis of cyclic carbonates under the illumination of incandescent lamps. Zhang et al. [31] used graphene oxides (GOs) as the fast catalysts for the coupling reaction of styrene oxide with CO2, and the styrene carbonate was obtained at a low pressure and a high temperature. However, the catalytic efficiency of the activity species on these heterogeneous catalysts are still too low to meet the requirements of the reaction even under a harsh reaction condition. Besides, the major nucleophiles are not qualified in the cycloaddition reaction. Therefore, developing a catalyst consisting of a perfect pair of Lewis acid and base is critical for the cycloaddition. In this regard, transition metal element is the best candidate for high efficient Lewis acid due to
Corresponding author at: Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, 010051, People’s Republic of China. E-mail address: [email protected] (J. Bai).
https://doi.org/10.1016/j.jcou.2019.09.005 Received 9 April 2019; Received in revised form 3 September 2019; Accepted 9 September 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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deposition furnace. The crucible was heated to 520 °C at the rate of 5 °C /min and kept the temperature at 350 °C and 400 °C for 1 h and 520 °C for 2 h, respectively. Finally, the sample was taken from the furnace after cooled to the room temperature and grinded into a powder. The final sample was denominated as Zn-CN/C. The pure gCN was prepared directly by the pyrolysis of urea in the absence of ZnBr2.
the special properties and advantages in the term of the environment and energy. Fan et al. [32] proved that Cu(acac)2 played a key role in the coupling reaction of epoxides with CO2, which showed an excellent catalytic performance when supported by the Fe3O4@MCM-41. Buonerba et al. [33] prepared an iron(III) catalyst with a good selectivity (＞99%) for the coupling reaction. Moreover, several organic frameworks including metal-corn of Cu, Cr, Co, Zn and Ni have also been reported to facilitate the cycloaddition reaction [34–36]. Among them, zinc-containing materials have attracted a great attention due to their excellent performance. Sun et al. [37] developed a direct synthetic path of styrene carbonate from the styrene. In this path, zinc bromide was added to the catalyst system as an active ingredient. Han et al. [38] prepared a polyoxometalate-based homochiral metal-organic framework involving with zinc, and it possessed a high catalytic activity for the one-pot synthesis of cyclic carbonates from olefins. Due to the good dispersion and large surface area, all zinc-containing MOFs showed the outstanding conversion of epoxides, high selectivity and yield of the corresponding cyclic carbonates at a mild temperature and atmospheric pressure. Nevertheless, these homogeneous MOF catalysts would not get a satisfied result without quaternary ammonium halides. So, the development of bifunctional heterogeneous catalysts will be demanded for the synthesis of cyclic carbonates [39,40]. Graphitic carbon nitride (gCN), as a charming functional material, has gained many concerns in the fields involving antibacterial materials and photocatalysts [41]. Lately, gCN has been one of the most significant components for the catalysts [42,43]. The tri-s-triazine group endows gCN the potentiality and extensive adaptability for various reactions. For example, Antonietti et al. [44] demonstrated that gCN can absorb and activate the carbon dioxide molecules due to the existence of uncondensed amines. Obviously, gCN-containing catalytic system could be used for catalyzing the coupling of epoxides with CO2. Nevertheless, in the most cases, the metal cations acted as Lewis acid sites only when the heterogeneous gCN was applied in the cycloaddition reaction. In this work, the easily obtainable urea was used as a precursor for the preparation of gCN and carbon fibers. These carbon-based materials as the carriers of the catalytic system were prepared by electrospinning and the subsequent high temperature carbonization. Moreover, the addition of ZnBr2 into gCN can not only provide the Lewis acid and base sites but also activate the epoxides through the coordination with the oxygen atoms [45]. The bonding state of Zn2+ with halogen ion and gCN were also investigated in detail. It is proved that the better dispersion of catalytically active component is desired. At last, a possible catalytic mechanism was proposed according to the previous literatures and the experimental data in this study.
2.3. Characterization The morphologies and surface states of the samples were obtained by scanning electron microscope (SEM) using a Phenom machine (Phenom Pro, Netherlands). The ultrastructure of samples was observed by a transmission electron microscope (TEM, Jeol, JEM-2010, Tokyo, Japan). The X-ray diffraction (XRD) patterns were presented by an X-ray diffractometer (D/max 2500 PC, Rigaku, Japan) to get the information about the crystalline structure (crystallinity) of composite catalyst. An X-ray photoelectron spectroscopy instrument (XPS, EscaLab 250, Thermo Fisher Scientific, Massachusetts, USA) was used to de-irradiate the samples, so that their inner electrons or valence electrons were excited and emitted, thereby the composition was obtained. The chemical groups of samples were determined by the Fourier transform infrared spectroscopy (FTIR) using a Nicolet iS50 instrument (Thermo Fisher Scientific, USA). The contents of zinc loadings were detected by inductively coupled plasma-optical emission spectroscopy (ICP-OES730, Agilent, USA). The pore size distribution and the BET specific surface area were determined by the BJH (Barrett-JoynerHalenda) and BET (Brunauer, Emmett, and Teller) methods, respectively. The temperature program of degassing procedure consisted with the following three steps: (1) Pretreating at 70 °C for 15 min, the heating speed was 10 °C /min. (2) Heating the heating mantles to 120 °C at the same speed as step (1) and keeping the temperature for 15 min. (3) Keeping the temperature at 300 °C for 3 h. The product analysis was determined by using a gas chromatography (GC) (Shimadzu 2010 Plus, RTX-5 capillary column, flame ionization detector), and a GC–MS (Agilent 5975C). The GC programme was performed at a temperature ramp of 10 °C / min up from 60 °C to 280 °C and kept for 2 min.
2.4. Catalytic performance The synthesis of styrene carbonate (SC) took place in a 50 mL steel kettle with a pressure gauge filled with styrene oxide (SO) and CO2. At first, SO (4 mL, 35 mmol), N,N-dimethyl formamide (DMF, 1 mL) and 0.1 g of catalyst were added into the kettle with a magnetic stirrer. Then the container was sealed and purged with CO2 three times and the pressure was eventually raised to 2 MPa. Thereafter, the reaction was run at 140 °C for 8 h. At last, the reaction system was cooled to the room temperature in the cold-water bath and the gas was slowly released. The catalyst was separated from the system by a centrifuge and washed by ethanol for the next run. The reaction products from the supernatant were extracted by a mixed solution of ethyl acetate and water, and qualitatively detected by the GC. The conversion (Conv.) of styrene and the selectivity (Sel.) of the target product were determined by the integral areas of the GC peaks, and calculated as the following equations:
2. Experimental section 2.1. Materials Polyacrylonitrile (PAN, Mw = 80,000) was purchased from Kunshan Hongyu Plastic (Kunshan China). N, N-dimethyl formamide (DMF, A.R.) was purchased from Fengchuan Chemical Reagent Technologies Co., Ltd (Tianjin China). Urea was purchased from Tianjin Beilian Fine Chemical Research Institute (Tianjin China). Deionized water was obtained from Wastons. Zinc bromide and other chemicals were purchased from Innochem. All chemical reagents were directly used without any further purification.
Conv. = [(areas of reactant converted) × 100]/areas of reactant used
2.2. Catalyst preparation Sel. = [(areas of product formed) × 100]/areas of reactant converted Firstly, 0.2 g of ZnBr2 and 1 g of urea were dissolved into 20 mL deionized water with stirring at 70 °C for 20 min. Thereafter 0.5 g of prepared carbon fibers was added and the mixture was obtained after all of the water was evaporated. Then, the black solid products were taken out and then placed in the crucible to calcined in a vapor
The preparation and application of the catalysts were shown in Scheme 1.
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Scheme 1. Catalyst preparation and experimental flow.
Fig. 1. XRD patterns of carbon (A), gCN (B), Zn-CN (C) and Zn-CN/C (D).
3. Results and discussion
drum peak in Fig. 1A at ca. 2θ = 24.2° corresponds to the (002) plane of amorphous carbon and the peak at 43.5° is assigned to its (100) plane [46]. Two peaks with the clear identification of gCN are displayed in the Fig. 1B. A relative weak peak has been found at ca. 2θ = 12.9°,
To investigate the crystal structure of the samples, XRD patterns of the raw materials and catalysts are obtained, as shown in Fig. 1. The
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Fig. 2. FT - IR spectra of Zn-CN (a), Zn-CN/C (b), carbon (c) and gCN (d).
Fig. 4. N2 adsorption-desorption isotherm of Zn-CN/C composite catalyst.
which is attributed to the in-plane repeated accumulation, i.e. the (100) plane, and another strong diffraction peak at ca. 2θ = 27.7° is the unique interplanar stacking of gCN, i.e. the (002) plane [47]. In comparation with the pattern of the gCN, the intensities of both (002) and (100) plane in Fig. 1C&D are decreased, indicating that the original ordered internal structure of gCN might be destroyed. In addition, no obvious shift of peaks is found in Fig. 1C&D. However, the diffraction peak of (002) planefor gCN is quite dispersed with relatively low strength. The relatively weak peak is due to the incorporation of metal ions into gCN, which is consistent with the previous literature involving the K, Zn and Fe doped gCN [48]. Many characteristic diffraction peaks of zinc bromide are also clearly distinguished and listed in the Fig. 1C, indicating the fixation of zinc bromide in the essential active species. The diffractions peaks appeared at 17.5° and 21.06° correspond to the (211) and (213) plane of zinc bromide. Peaks located at 28.4°, 47.4° and 51.7° in Fig. 1D are assigned to the (321), (329) and (527) planes of zinc bromide as well (PDF36-0756). Simultaneously, many peaks of zinc oxide such as the (100) plane (ca. 2θ = 31.6°) and (110) plane (ca. 2θ = 56.6°) [49] can be observed in Fig. 1C&D, which may be caused by the excessive addition of zinc bromide. Furthermore, according to Debye-Scherrer formula, the average crystallite size of Zn-CN is 8.6 ± 1 nm.
Fig. 5. Pore size distribution of Zn-CN/C composite catalyst.
To determine the chemical structures of the samples, the FTIR spectra are conducted. As shown in Fig. 2, the samples supported by gCN demonstrate a similar spectra compared to the original gCN, the bands between 1200 and 1700 cm−1 are assigned to the stretching
Fig. 3. XPS spectra of samples. 719
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vibrations of CeN and CN] in the aromatic nitrogen-carbon heterocyclic structure. The sharp peak at 804 cm−1 corresponds to the stretching vibration mode of the s-triazine unit, which is regarded as the elementary building block of gCN [50]. The spectrum of Zn-C samples also present the characteristic peaks of gCN (line a and b), demonstrate an undamaged gCN structure in Zn-C samples. The wide absorption peak at 3500˜3000 cm−1 is attributed to the NeH of incompletely polycondensated amino groups and OeH in absorption water on the sample surface. Interestingly, a low-intensity band at 2168 cm−1 is identified in the spectra of Zn-CN, but not found in the bulk gCN (line d), indicating the unique signal of the azide group appears due to the presence of Zn-N bond. The result irrefutably confirms the organic combination of gCN with zinc bromide. Thus, the loading of zinc bromide would not change the main chemical functionalities of gCN, and the Zn species could be well inserted into the gCN skeleton [45]. The valence information of the elements is determined by XPS spectra as Fig. 3 shows. The results of XPS analysis confirm the presence of C, N, Zn and Br in the Zn-CN and Zn-CN/C samples, and their bond combination is further analyzed. The C1s peak of the samples can be deconvoluted into three components, peaks at 284.4 eV, 286.4 eV and 284.7 eV corresponding to the sp2-bonded carbon in the Zn-CN network (N]CN), COee bond of the samples and CeC bond of gCN, respectively [44]. By contrast, the ratio of gCN in Zn-CN/C was higher than that in Zn-CN, which indicates the dominant graphitic carbon in ZnCN/C and thus high basicity of Zn-CN [51]. The Zn2p peaks both in ZnCN and Zn-CN/C can be divided into the Zn 2p1/2 and 2p3/2 peak, indicating two chemical environments of Zn element. As can be seen from the Zn2p spectra of Zn-CN sample, peaks at 1046 eV and 1023 eV are assigned to zinc in the metal ion state, and another one at 1020 eV is corresponding to the metallic Zn. Meanwhile, it is worth noticed that the binding energies of the intensity in Zn 2p1/2 and 2p3/2 peaks are weaker than the ionic state Zn (II) but higher than the metallic state (0). This accounts for the existing of an intermediate valence state of the zinc cation in gCN, which may be caused by the interaction between Zn and the nitride of gCN. The similar result can also been observed in the Zn-CN/C samples [52]. The above results revealed that zinc existed in the samples as ions state instead of metallic state, which is similar to zinc halide loaded on the other materials [45]. The N1s spectra of both Zn-CN and Zn-CN/C can be fitted into three main peaks. The peaks at 400.86 and 401.3 eV are assigned to the uncondensed amino group (–NH2), the peaks at 399.85 and 399.98 eV are corresponding to the partially condensed tertiary groups [CeN (C/H)Cee] and the peaks at 398.28 and 398.88 eV are attributed to the tris-s-triazine (C]NCe) [53]. The Br3d spectra are also fitted as shown in Fig. 3. The binding energies of Br3d5/2 and Br3d3/2 peak are 68.19 eV and 69.28 eV [54], respectively, indicating an ionic state of Br. The ionic bromine presented in the catalyst as Lewis base plays a considerable role in the cycloaddition reaction. The surface adsorption properties of the catalysts are determined by the N2 adsorption-desorption measurements. The BET pore volume and surface area are studied by N2 adsorption/desorption isotherm at -196 °C as shown in Fig. 4. It can be seen that the N2 adsorption/desorption isotherm of the catalyst presents a type II isotherm. According to the results of BJH analysis, the average pore size of ZN-CN/C is 0.995 nm. The micropores, mesopores together with macropores coexist in the catalyst simultaneously (Fig. 5). The surface area of Zn-CN/C is 19 m2 /g. Raman spectroscopic results are conducted to detect the electron transmission of the catalyst system. As shown in Fig. 8, no D and G peaks are found in gCN and Zn-CN, whereas they appear in C and ZnCN/C, demonstrating the carbon fibers act as a main carrier for the electron transport. The microstructures and morphologies of the catalysts are observed by FE-SEM. From Fig. 6A and B that the smooth surfaces of both carbon fibers and Zn-CN/C can be clearly observed. Many granular substances
Fig. 6. FE-SEM images of C fibers (A) and Zn-CN/C composite catalyst (B and C).
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Fig. 7. TEM images of Zn-CN/C composite catalyst.
performance of zinc oxide is also investigated under the same conditions. Although the selectivity is high, the conversion is quite low (11.36%) when using pure ZnO as the catalyst, (Table 1, entry 13). Moreover, the catalyst Zn-CN/C shows an excellent selectivity of the target products in the subsequent tests. By comparing the data of the entry 4, 7 and 8 in Table 1, it can be concluded that the conversion of reactants increases with the rising reaction time, and keep the same at temperatures under 160 °C (Table 1, entry 4–6). As the temperature raises to 160 °C, the selectivity of reaction decreases because the high temperature leads to further decomposition of the products into phenylethylene glycol [31]. Additionally, the effect of solvents on the cycloaddition reaction is investigated under the same conditions. The polar solvents, acetonitrile, DMF and methanol are used for the reactions, of which the polarity order is acetonitrile (6.2) < DMF (6.4) < methanol (6.6). By comparing the results in entry 7 and 9 of Table 1, the conversion of styrene increases with the polarity of solvent. However, among these solvents, methanol is a unique one that it can produce H bonds as a protic solvent [55]. Thus, the conversion of the reaction using methanol as a solvent is lower than DMF, but higher than acetonitrile (Table 1, entry 4, 9 and 10). It can be seen from entry 4, 11 and 12 of Table 1, the reaction yield increases with the increased catalyst amount. Interestingly, the growth speed of the yields in the low catalyst mass range (0.01 g ˜ 0.05 g) is faster than that in the large one (0.1 g ˜ 0.5 g) (Table 1, entry 4, 11 and 12). The low mass of catalyst is beneficial to the fast mass transmission. Moreover, the various epoxides used for cycloaddition reaction with our catalyst is also investigated as shown in Table 2. All the catalyst can catalyze the reaction of epoxides with the high selectivity and the expected conversion. As an important issue, the recyclability is performed to evaluate the performance of the catalysts. The results of the repeatability tests are shown in Fig. 9, after five consecutive cycles, the epichlorohydrin can still completely be converted in four hours with a high selectivity. Moreover, it can be found that the structure of catalyst is not destroyed. after five cycles from the FTIR spectra (Fig. 10). The Comparison of the Zn-CN/C catalysts and current state-of-art
Fig. 8. Raman spectra of C, gCN, Zn-CN and Zn-CN/C.
are also found on the Zn-CN/C, which indicates the Zn-CN particles are loaded on the carbon fibers. An amplified SEM image is also obtained to observe the surface details of the fibers. As shown in Fig. 6C, the carbon fibers are completely surrounded by Zn-CN. Furthermore, the detailed morphologies of Zn-CN/C are investigated by TEM as shown in Fig. 7. The activity substances are obviously attached to the surface of support, confirming that Zn-CN is loaded on the carbon fibers. The results mentioned above reveal that the expected Zn-CN/C catalyst is successfully prepared, and the zinc content of this catalyst is 7.49% according to the ICP-OES result. To evaluate the catalytic performance, CNFs, gCN, Zn-CN and ZnCN/C are used for the cycloaddition reaction of styrene oxide and CO2 without any co-catalyst. The results of reactions are shown in Table 1. The Zn-CN/C exhibited an excellent catalytic performance with 54.36% of conversion and a high selectivity. By contrast, the catalytic 721
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Table 1 Catalytic results of catalysts prepared at various conditions. Entry
Catalyst
Temperature (℃)
Time (h)
Solvent
Conversiond (%)
Selectivityd (%)
1 2 3 4 5 6 7 8 9 10 11b 12c 13
CNFs gCN Zn-CN Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C Zn-CN/C ZnO
140 140 140 140 120 160 140 140 140 140 140 140 140
8 8 8 8 8 8 6 10 8 8 8 8 8
DMF DMF DMF DMF DMF DMF DMF DMF CH3OH MeCN DMF DMF DMF
3.53 11.30 35.59 54.36 43.18 91.03 43.92 69.45 13.62 4.72 48.47 21.29 11.36
＞99 72 92 ＞99 ＞99 98.55 ＞99 ＞99 ＞99 ＞99 ＞99 ＞99 98.75
a Reaction conditions: 4 mL styrene oxide, 1 mL DMF, p(CO2) =2.0 MPa, T = 140 ℃, t =8 h, 0.1 g catalyst. b 0.05 g Zn-CN/C. c 0.01 g Zn-CN/C. d Determined by GC analysis.
Table 2 Catalytic results for the cycloaddition reactions using various epoxides. Entry
Substrates
Products
Conv.
a
(%)
Sel.
a
1
91.59
> 99
2
74.34
96.78
3
99
93.13
(%)
Reaction conditions: 4 mL epoxide, 1 mL DMF, p(CO2) =2.0 MPa, T = 140 ℃, t =8 h, 0.1 g catalyst. a Determined by GC analysis.
Fig. 10. FT-IR spectra of fresh and reused Zn-CN/C. Table 3 The comparison of the current state-of-art catalysts with Zn-CN/C for the cycloaddition reaction of epichlorohydrin with CO2. Entry
Catalyst
Co-catalyst
Yield (%)
Source
1 2a 3b 4c
Zn-CN/C J500 PS-(IM)2ZnI2 FJI-H14
/ / / TBAB
92.2 92 57.8 95
This work Ref. 56 Ref. 30 Ref. 57
Reaction conditions: a. 100 mg catalyst, 20 mmol epichlorohydrin, 150 ℃, 15 bar, 15 h, 400 rpm; b. 0.2 g PS-(IM)2ZnI2, 50 mmol epichlorohydrin, 60 ℃, 10 bar, 4 h; c. 0.48 mol% FJI-H14, 2.5 mol% TBAB, 20 mmol epichlorohydrin, 80 ℃, 1 atm flue (0.15 bar CO2,0.85 bar N2), 4 h.
in the Scheme 2. First of all, carbon dioxide is captured by the catalyst and activated by the nucleophilic attack of carbon nitride. Then, epoxy ring opens by the synergistic action of zinc and bromide ions, thereafter the bromide ions are linked to the opened skeleton. Next, the activated carbon dioxide is linked into this skeleton, forming a unique structure as illustrated in the right side of Scheme 2. Finally, the intramolecular SN2 reaction happens and the anion releases, meanwhile forming the five-membered cyclic carbonates, which are the ultimate products in this reaction.
Fig. 9. Recycling tests of cycloaddition of CO2 with epichlorohydrin catalyzed by Zn-CN/C. Reaction conditions: 0.1 g Zn-CN/C, 4 mL epichlorohydrin, 1 mL DMF, p(CO2) =2 MPa, T = 140 ℃, t =4 h.
catalysts for the cycloaddition of epichlorohydrin with CO2 is presented in Table 3 [30,56,57]. As can be seen from the table, both the Zn-CN/C, J500 and FJI-H14 exhibit the excellent catalytic performance. However, when using J500 and FJI-H14 as catalysts, the reactions took a long time. Besides, FJI-H14 can only work in the presence of TBAB. PS(IM)2ZnI2 is an ionic liquid, which is usually difficult to recycle and reuse. Based on the previous literatures and experimental data of this study, a hypothetical mechanism of the reaction is presented, as shown
4. Conclusion In summary, an environmentally friendly composite catalyst Zn-CN/ C was fabricated by a facile method. Zinc ions were immobilized by the graphitic carbon nitride and then supported on the carbon fibers. In this 722
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Scheme 2. A hypothetical mechanism of cycloaddition of CO2 and styrene oxide catalyzed by the Zn-CN/C composite.
catalyst, zinc and bromide play a key role as Lewis acid-based pair of the styrene oxide activating, and gCN acts as a solid base for activating CO2 and a support for zinc bromide immobilization. The catalyst showed an excellent catalytic performance with a high selectivity and yield in the cycloaddition reaction of styrene oxide and CO2. In addition, a proposed reaction mechanism of was presented, which would provide a theoretical support for virous cycloaddition reactions catalyzed by this kind of material. This work will offer a new idea for the development of novel heterogeneous zinc containing catalysts, and a highly efficient and green route for the utilization of CO2.
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