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Synthesis of styrene carbonate from styrene and CO2 catalyzed by walnut-like zeolite LZ-276
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Synthesis of styrene carbonate from styrene and CO2 catalyzed by walnut-like zeolite LZ-276 Beibei Gu a, b, Tong Xu a, b, Guangran Xu a, b, Jie Bai a, b, *, Chunping Li a, b a b
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 L E I N F O
A B S T R A C T
Keywords: Zeolite LZ-276 CO2 Catalyst One-pot reaction Styrene carbonate
In this work, walnut-like zeolite LZ-276 was synthesized by hydrothermal method without any template and evaluated by the oxidative carboxylation of styrene. The structural features and textural properties of the ob tained catalyst sample were characterized by a series of characterization methods. The evaluation reaction, oxidative carboxylation of styrene, exhibited good catalytic efficiency under the synergistic effect of the walnutlike zeolite and KI. After exploring different reaction conditions, an ideal yield of styrene carbonate (77%) is obtained without preliminary synthesis and isolation of the intermediate styrene oxide. What is more consid erable is that the catalytic activity such as the yield of styrene carbonate did not show significant reduction in several cycles of one-pot reaction.
1. Introduction Cyclic carbonate is an aprotic polar solvent that can be widely used in the production of polycarbonate, the preparation of pharmaceutical and fine chemical intermediates, electrolytes in lithium battery devices, and petroleum additives [1–5]. One of the most promising strategies is the production of five membered cyclic carbonates (Scheme 1) [6–10], which can be obtained through coupling reaction between epoxide and CO2 [11]. However, since epoxides are expensive and unstable, the synthesis cost of the cyclic carbonates is high. If cyclic carbonates could synthesized directly from cheap and readily available olefins, it will have broad industrial application prospects. The catalysts of the reaction system can be divided into two types, one is homogeneous catalyst, such as salen-complexes, quaternary ammonium salts and ionic liquids [12–15], and the other is heterogeneous catalyst, which mainly includes metal oxide, zeolites, and metal-organic frameworks (MOFs) [16–18]. To comprehensively analyze the advantages and disadvantages of these catalysts and based on the possibility of larger-scale reaction, we chose zeolite to catalyze this reaction. The crystal structure of zeolite framework CHA, hexagonal cha cages, which are interconnected with each other through AABBCC stacking faults sequence of six-membered rings of (Si,Al)O4 tetrahedra [19]. Tiny changes of batch sizes, the ratio and mixing sequence of initial materials, and hydrothermal conditions can lead to zeolites of other structures,
such as PHI, SOD and FAU [20]. Many researchers used tetraethy lammonium hydroxide (TEAOH) as an organic template agent to syn thesize zeolite Phi [21–24]. Most of the crystallization times reported were too long, while Lillerud and co-workers synthesized a Phi-type zeolite with the crystallization time of only three days [25]. The mate rial showed similar structural features to the zeolite Phi, which has faults in the double 6 rings stacking of chabazite [23,25]. Among these zeolite frameworks, the walnut-like zeolite LZ-276 possesses the same topo logical structure as zeolite Phi [26]. In this work, we explored a simple procedure for the preparation of the walnut-like zeolite LZ-276 in the absence of organic templating agent. The zeolite catalyst was used for the efficient synthesis of styrene carbonate (SC) through one-pot styrene oxidation carboxylation reac tion. The main significance of this study is to optimize the experimental conditions and ensure high yield of the SC. 2. Experiment sections 2.1. Chemicals Hydrated silica (SiO2⋅nH2O) was obtained from Xiya Reagent. So dium aluminate (NaAlO2, CP), ethyl acetate (C4H8O2, AR) and aceto nitrile (CH3CN, AR) were purchased from Sinopharm Chemical Reagent (China). Sodium hydroxide (NaOH, AR) was provided by Tianjin
* Corresponding author. 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.micromeso.2019.109779 Received 1 February 2019; Received in revised form 5 September 2019; Accepted 30 September 2019 Available online 2 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Beibei Gu, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109779
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of Al2O3: 32SiO2: 7Na2O: 603H2O was aged at room temperature for 25 h. After this, 0.2254 g aluminum sulfate and 0.1083 g sodium aluminate were added to the gel, and the mixture was further stirred and crystallized at 100 � C for 26–28 h. In order to quantify the alkalinity, the amount of the second supplemental sodium aluminate (NaAlO2-2) was changed. The crystallization process is performed in a box atmosphere furnace at a heating rate of 1 � C/min. Zeolite LZ-276 was collected by filtration, washed with deionized water, dried overnight at 100 � C and calcined for 2 h at 550 � C.
Scheme 1. Cycloaddition of CO2 to epoxides.
Fengchuan Chemical Reagent Technologies Co., Ltd. Distilled water was produced by the reverse osmosis membrane separation pure water de vice provided by Tianjin University Chemical Basic Experiment Center. Styrene (C8H8, AR, 99%) was provided by Acros Organics USA. Tertbutyl hydroperoxide (TBHP, 70%) was supplied from Alfa Aesar (Tian jin) Chemical Co., Ltd. Potassium iodide (KI, AR) was purchased from Tianjin Yongsheng Fine Chemical Co., Ltd. All of the reagents can be used without purification.
2.3. Characterization methods The desktop scanning electron microscopy (SEM, Phenom Pro, Netherland) was used to characterize the morphology of the zeolite sample. The crystal structure of the zeolite was tested by X-ray diffrac tometer (XRD, SmartLab 9 kW, Rigaku, Japan) in the range of 2θ from 10� to 90� . Fourier transform infrared spectrum (FT-IR) was recorded in Nicolet Nexus 670 spectrophotometer (Thermo Fisher Scientific, USA) with the range of 4000–400 cm 1. Elemental composition and valence information of zeolite were tested by X-ray photoelectron spectroscopy (XPS, EscaLab 250, Thermo Fisher Scientific, USA). N2 adsorptiondesorption operation was performed at 196 � C by Autosorb-iQ (Quantachrome Instruments, USA) after vacuum high temperature degassing pretreatment. The product of the catalytic reaction was monitored by gas chromatography (GC-2010 Plus, Shimadzu, Japan). Temperature programmed desorption of CO2 (CO2-TPD) and NH3 (NH3-
2.2. Synthesis of LZ-276 First, a starting aluminosilicate gel was prepared with the molar ratio of Al2O3: 45SiO2: 8Na2O: 840H2O. In a typical preparation, 2.3296 g SiO2⋅nH2O, 0.1409 g NaAlO2 and 0.51 g NaOH were mixed with 13 mL of deionized water in a Teflon-coated reactor, followed by stirring for 1–2 h to form aluminosilicate gel. The resultant gel with the molar ratio
Fig. 1. SEM image of (A) sample 1 (m(NaAlO2-2) ¼ 0.2254 g, the final gel molar ratio is Al2O3:15SiO2:3.3Na2O:280.7H2O); (B) sample 2 (m(NaAlO2-2) ¼ 0.1669 g, the final gel molar ratio is Al2O3:17.5SiO2:3.7Na2O:326H2O); (C) sample 3-zeolite LZ-276 (m(NaAlO2-2) ¼ 0.1083 g, the final gel molar ratio is Al2O3:32SiO2:7 Na2O:603H2O); (D) SEM image of the catalyst (after reaction). 2
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Fig. 2. XRD pattern of zeolite LZ-276.
Fig. 3. FT-IR spectrum of zeolite LZ-276.
TPD) study were performed using a ChemBET TPR/TPD instrument (Quantachrome Instruments, USA). The conversion of styrene and the content of each product were calculated by peak area ratio. The area normalization method was used to calculate the content of each component. The quantitative analysis was determined by gas chroma tography (GC, Shimadzu 2010 Plus, RTX-5 capillary column, flame ionization detector, Japan), and the products were identified by a gas chromatography mass spectrometer (GC-MS, Agilent 5975C, USA). A temperature ramp was applied to the product analysis, the column temperature was started at 60 � C followed by the heating rate of 10 � C/ min until 280 � C. The temperature was maintained for 2 min before the column oven was cooled to 60 � C for next analysis.
pattern of the sample and the theoretical simulation of the ideal CHA topology in the rhombohedral space group R-3m [27]. The FTIR spectrum in the region of 4000–500 cm 1 for the zeolite LZ276 was showed in Fig. 3. The broad band at 3432 cm 1 and band at 1652 cm 1 are attributed to zeolitic water [28]. The result shows that the band at 1032 cm 1 is the Si–O stretching vibration [22]. The ab sorption bands at 618 cm 1 and 700 cm 1 are corresponding to sym metrical T–O–T stretching mode (T ¼ Si, Al) [29,30]. For qualitative and quantitative measurement of elemental compo sition of the zeolite LZ-276 surface, the XPS survey scan shows the Si2p, Al2p, O1s and Na1s photoelectron line (see Fig. 4A). Fig. 4B, C, D and E clearly display that the binding energy of each element of the zeolite LZ276. The binding energies, contents of each element on the zeolite surface and Si/Al atomic ratio are shown in Table 1 [31]. The surface area and pore size distribution were analyzed using Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) mode, respectively. The N2 adsorption-desorption isotherm of the samples were obtained in Fig. 5, which exhibited a hysteresis loop at relative pressure P/P0 between 0.4 and 0.9. The BET surface area of zeolite LZ-276 is 201 m2 g 1, in which contains micropore area of 155 m2 g 1. Its average pore diameter is 3.8 nm (Fig. 5, inset). Large pore size and surface area provided sufficient space for adsorption of reactants and desorption of products. The acidity of zeolite LZ-276 can be investigated by NH3-TPD. The acid strength will be proportional to the temperature required to release the probe gas molecules [32]. NH3-TPD curves are shown in Fig. 6A. The peaks at 251 � C and 610 � C can be attributed to the weak acid sites and strong acid sites, respectively. The alkalinity is estimated as the total amount of CO2 released by TPD of the sample, and the results are shown in Fig. 6B. The CO2-TPD profile demonstrates the presence of basic sites in zeolite LZ-276. The Lewis base sites provided by Si atoms in the zeolite framework contribute to the activation of CO2 and promote the catalytic reaction [33]. Combining the results of NH3-TPD and CO2-TPD, walnut-like zeolite LZ-276 with both acidic and basic sites will have good catalytic activity.
2.4. Catalytic cycloaddition of CO2 to styrene Based on the original work of the optimal conditions for styrene oxidation, the experimental scheme for the one-pot synthesis of styrene cyclic carbonate was designed (Scheme 1). The catalytic experiments were carried out in a 50 mL high pressure stainless-steel autoclave. In a typical run, 1 mL styrene, 1 mL TBHP, 5 mL acetonitrile and 0.05 g catalyst (0.02 g LZ-276 and 0.03 g KI) were firstly added to the auto clave. Then, the high-pressure reactor was filled with a certain pressure of CO2 (99.999% purity). The reaction mixture was stirred by magnetic force at 300 r/min and heated to a certain temperature for a period of time. After the reaction was completed, the reactor was quickly cooled to room temperature in cold water, followed by the discharge of unreacted CO2. The reaction solution was extracted with deionized water and ethyl acetate, and the supernatant was detected by GC-MS. 3. Results and discussion 3.1. Catalyst characterization Fig. 1A, B and C show SEM images of second different amount of sodium aluminate (0.1083 g–0.2254 g). Fig. 1D shows the micromor phology of zeolite LZ-276 after reaction. It can be seen that both of the catalyst have a walnut-like structure with uniform size. This not only proves that zeolite LZ-276 has stable structure and chemical stability, but also explains excellent cycle performance. The crystal structures of carrier zeolite LZ-276 is characterized by XRD (Fig. 2). All the samples were collected in the range of 5–90� with the scanning speed was 2� /min. For the sample, all diffraction peaks are perfectly matched to the zeolite LZ-276 standard card (PDF-49-0921). In other words, Fig. 2 shows a good agreement between the powder XRD
3.2. Catalytic test The effects of substrates amount, reaction temperature, the pressure of CO2 and reaction time on the synthesis of SC were investigated in details. As shown in Fig. 7, the volume ratio of ST and TBHP had a significant impact on the SC yield. Volume ratio V (ST: TBHP) ¼ 3:1, 2:1, 1:1 and 2:1 corresponding to the molar ratio n (ST: TBHP) ¼ 0.4, 0.6, 3
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Fig. 4. (A) The fully scanned spectra, (B) Si 2p, (C) Al 2p, (D) O 1s, and (E) Na 1s XPS spectra of zeolite LZ-276.
1.12 and 2.4. Since ST can be almost completely converted when the volume ratio of ST: TBHP was 1: 1, this ratio was selected for the following exploration experiment. After comparing the data in Table 2 (entries 1–3), it was found that the yield of styrene carbonate was highest at 140 � C (YSC ¼ 73%). Therefore, we believe that 140 � C was the optimal temperature for one-pot reaction. A reasonable explanation is that an appropriate temperature can provide sufficient energy for epoxide ring opening and CO2 activation [34]. Too low or too high pressure is unfavorable for the synthesis of SC. After the investigation, it was found that 0.5 MPa is a suitable carbon dioxide pressure (entry 2, 4
Table 1 Binding energy, atomic proportions of elements and Si/Al atomic ratio by XPS survey scan analysis. Element
Binding energy (eV)
Atomic %
Si/Al ratio
Al 2p Si 2p C 1s O 1s Na 1s
73 101 284 531 1071
5.79 9.84 46.09 33.99 4.28
1.70
4
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Fig. 7. Effect of substrate volume ratio (ST: TBHP) on the SC yield (reaction conditions: temperature 150 � C, CO2 pressure 1.2 MPa).
Fig. 5. N2 adsorption-desorption isotherm of zeolite LZ-276 (inset shows pore size distribution).
Table 2 Effects of reaction parameters on the SC synthesis over the walnut-like zeolite LZ-276.a. Entry
Temperature (� C)
Pressure (MPa)
Time (h)
YSC (%)b,e
1 2 3 4 5 6 7 8 9b 10c 11d
130 140 150 140 140 140 140 140 140 140 140
1.2 1.2 1.2 0.5 2.0 0.5 0.5 0.5 0.5 0.5 0.5
9 9 9 9 9 6 8 10 10 10 10
44 73 53 74 50 44 58 77 74 52 28
e
Determined by GC. a Reaction conditions: styrene 1 mL, TBHP 1 mL, catalyst (0.02 g LZ-276 and 0.03 g KI), CH3CN 5 mL. b Triple the substrate: styrene 3 mL, TBHP 3 mL. c Six times the substrate: styrene 6 mL, TBHP 6 mL. d Ten times the substrate: styrene 10 mL, TBHP 10 mL. e Determined by GC.
and 5). The cycloaddition of CO2 to ST proceeded further with time and (entry 4, 6–8) the reaction reached a satisfactory yield at 10 h (entry 8). In order to explore the maximum catalytic capacity of the zeolite cata lyst, we assessed a larger scale experiment (entry 9 and 10). Under conditions where the catalyst and solvent were not enlarged (0.02 g LZ-276 þ 0.03 g KI, 5 mL CH3CN), the triple increase of substrate had little effect on SC yield. This meant that the catalyst can catalyze three times the amount of substrate. But when the substrate was expanded six times, the yield of SC was reduced to 52%. It can be seen that increasing the concentration had effect on the yield of SC, that is to say, the oxidative carboxylation reaction of styrene is not a zero-order reaction. Table 3 Blank control experiment for the synthesis of SC.a.
Fig. 6. NH3-TPD (A) and CO2-TPD (B) profiles of zeolite LZ-276.
Entry
Catalyst
Co-catalyst
Time (h)
Pressure (MPa)
YSC (%)
1 2 3
LZ-276 – LZ-276
– KI KI
10 10 10
0.5 0.5 0.5
Trace 52 77
a Reaction condition: ST-1 mL, TBHP-1 mL, catalyst-0.02 g, KI-0.03 g, T ¼ 140 � C.
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recovered by simple centrifugal separation and each cycle experiment only needs to be added with fresh KI. As shown in Fig. 8, styrene was almost completely converted in the four cycles and the yield of SC was started to decrease slightly after four cycles. This result can prove that the zeolite catalyst has good long term stability. 3.2.2. Comparison of catalytic performance of various catalytic systems The comparison of catalytic performance among LZ-276/KI and other catalytic systems reported in the literatures was given in Table 4. The results in Table 4 reveal that the walnut-like zeolite catalyst in this work exhibits excellent catalytic activity. Comparing these five reaction systems, it is found that preparation process of zeolite LZ-276 is simple and the pressure used in this work is the lowest. The low reaction pressure makes the equipment standard easy to meet, and most impor tantly, it meets the requirements of chemical safety production. The operation step of the LZ-276/KI catalytic system is simple. For example, LZ-276 is a heterogeneous catalyst that is easy to recycle. Therefore, we can consider that the walnut-like zeolite catalyst exhibited good cata lytic performance in the synthesis of styrene carbonate by one-pot process.
Fig. 8. Reusability of zeolite LZ-276 catalyst. Table 4 Comparison of catalytic performance among various catalyst systems for one-pot synthesis of styrene carbonate from styrene and CO2. Catalyst
Temperature (� C)
Time (h)
Pressure (MPa)
YSC (%)
Ref.
Au/SiO2 Na2H5P (W2O7)6 Cr-MIL-101 K2S2O8/ NaBr LZ-276/KI
80 50
4 4
8 2.4
35 68
[35] [36]
80 60
7 10
10 3
44 79
[37] [38]
140
10
0.5
77
This work
3.3. Possible mechanism Based on the present experimental results and pioneering literature [33,38–41], the plausible mechanism for the oxidative carboxylation of styrene over LZ-276/KI catalyst was postulated in Scheme 2. The one-pot reaction can be regarded as two-step processes which should consist of the epoxidation catalyzed by LZ-276/TBHP-KI and the sub sequent cycloaddition catalyzed by LZ-276/KI. Firstly, catalytic oxida tion of I with TBHP leaded to the formation of hypoiodite OI (Step 1). Then iodohydrin was formed by iodination of styrene in the presence of a small amount of H2O (Step 2). Thereafter, iodohydrin was dehy droiodinated to give important intermediate styrene oxide (SO) under alkaline condition (Step 3). Subsequently, nucleophile I anions and the aluminum atoms in the zeolite framework synergistically attacked the less sterically hindered β-carbon atom of SO, which contribute to the ring-opening and form oxyanion intermediate (Step 4). Generally speaking, step 4 was considered to be the rate-determining step for the CO2 cycloaddition reaction [42,43]. Next, the CO2 molecule activated by the Si atom in the zeolite framework was inserted into the oxyanion intermediate to produce a new alkyl carbonate compound. Finally, I was taken off from the alkyl carbonate and closed to form the target product SC (Step 5).
To assess the application prospects of this reaction, a ten-fold substrate amplification experiment was performed and the result showed that the yield of SC was 28%. In addition, a blank control experiment was conducted to demon strate the catalytic activity of the walnut-like zeolite catalyst. The cat alytic results are shown in Table 3. After comparing these three groups of data, it was found that no product was detached when only LZ-276 was used as catalyst (entry 1). While the co-catalyst KI was used alone, the catalytic result was not satisfactory (entry 2). When both LZ276 and KI were used, the yield of SC was as high as 77% under the same condition. Combining analysis of GC and GC-MS spectra, the selectivity and yield of the target product SC was determined (Figs. S1 and S2). 3.2.1. Catalyst recycling The recyclability of catalyst plays a significant role in industrial production and practical application. The zeolite catalyst could be
Scheme 2. A proposed mechanism for the one-pot synthesis of styrene carbonate from styrene and CO2 catalyzed by the LZ-276/KI system. 6
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4. Conclusions
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In summary, a simple and highly-efficient synthesis of styrene car bonate from styrene and CO2 has been achieved by the walnut-like zeolite LZ-276/KI system. The yield of SC could be as high as 77% under the synergistic effect of zeolite catalyst and KI. No significant loss of activity was observed in the initial four cycles. Future work will focus on tightly combining KI with zeolite with excellent catalytic and cycling performance. Acknowledgements The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 51772158). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109779. References [1] V. Laserna, G. Fiorani, C.J. Whiteoak, E. Martin, E. Escudero-Ad� an, A.W. Kleij, Angew. Chem. Int. Ed. 53 (2014) 10416–10419. [2] R.-S. Kühnel, N. B€ ockenfeld, S. Passerini, M. Winter, A. Balducci, Electrochim. Acta 56 (2011) 4092–4099. [3] Z.-M. Cui, Z. Chen, C.-Y. Cao, W.-G. Song, L. Jiang, Chem. Commun. 49 (2013) 6093–6095. [4] B. Nohra, L. Candy, J.-F. Blanco, C. Guerin, Y. Raoul, Z. Mouloungui, Macromolecules 46 (2013), 3771–379. [5] Q.W. Song, Z.H. Zhou, L.N. He, Green Chem. 19 (2017) 3707–3728. [6] M. North, R. Pasquale, C. Young, Green Chem. 12 (2010) 1514–1539. [7] S. Kumar, N. Singhal, R.K. Singh, P. Gupta, R. Singh, S.L. Jain, Dalton Trans. 44 (2015) 11860–11866. [8] Z. Zhang, F. Fan, H. Xing, Q. Yang, Z. Bao, Q. Ren, ACS Sustain. Chem. Eng. 5 (2017) 2841–2846. [9] Q. Han, B. Qi, W. Ren, C. He, J. Niu, C. Duan, Nat. Commun. 6 (2015) 10007. [10] P.T.K. Nguyen, H.T.D. Nguyen, H.N. Nguyen, C.A. Trickett, Q.T. Ton, E. Guti�errezPuebla, M.A. Monge, K.E. Cordova, F. G� andara, ACS Appl. Mater. Interfaces 10 (2018) 733–744. [11] M.S. Liu, B. Liu, L. Liang, F.X. Wang, L. Shi, J.M. Sun, J. Mol. Catal. A Chem. 418–419 (2016) 78–85. [12] M. Taherimehr, A. Decortes, S.M. Al-Amsyar, W. Lueangchaichaweng, C. J. Whiteoak, E.C. Escudero-Ad� an, A.W. Kleij, P.P. Pescarmona, Catal. Sci. Technol. 2 (2012) 2231–2237.
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Synthesis of styrene carbonate from styrene and CO2 catalyzed by walnut-like zeolite LZ-276 Beibei Gu a, b, Tong Xu a, b, Guangran Xu a, b, Jie Bai a, b, *, Chunping Li a, b a b
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 L E I N F O
A B S T R A C T
Keywords: Zeolite LZ-276 CO2 Catalyst One-pot reaction Styrene carbonate
In this work, walnut-like zeolite LZ-276 was synthesized by hydrothermal method without any template and evaluated by the oxidative carboxylation of styrene. The structural features and textural properties of the ob tained catalyst sample were characterized by a series of characterization methods. The evaluation reaction, oxidative carboxylation of styrene, exhibited good catalytic efficiency under the synergistic effect of the walnutlike zeolite and KI. After exploring different reaction conditions, an ideal yield of styrene carbonate (77%) is obtained without preliminary synthesis and isolation of the intermediate styrene oxide. What is more consid erable is that the catalytic activity such as the yield of styrene carbonate did not show significant reduction in several cycles of one-pot reaction.
1. Introduction Cyclic carbonate is an aprotic polar solvent that can be widely used in the production of polycarbonate, the preparation of pharmaceutical and fine chemical intermediates, electrolytes in lithium battery devices, and petroleum additives [1–5]. One of the most promising strategies is the production of five membered cyclic carbonates (Scheme 1) [6–10], which can be obtained through coupling reaction between epoxide and CO2 [11]. However, since epoxides are expensive and unstable, the synthesis cost of the cyclic carbonates is high. If cyclic carbonates could synthesized directly from cheap and readily available olefins, it will have broad industrial application prospects. The catalysts of the reaction system can be divided into two types, one is homogeneous catalyst, such as salen-complexes, quaternary ammonium salts and ionic liquids [12–15], and the other is heterogeneous catalyst, which mainly includes metal oxide, zeolites, and metal-organic frameworks (MOFs) [16–18]. To comprehensively analyze the advantages and disadvantages of these catalysts and based on the possibility of larger-scale reaction, we chose zeolite to catalyze this reaction. The crystal structure of zeolite framework CHA, hexagonal cha cages, which are interconnected with each other through AABBCC stacking faults sequence of six-membered rings of (Si,Al)O4 tetrahedra [19]. Tiny changes of batch sizes, the ratio and mixing sequence of initial materials, and hydrothermal conditions can lead to zeolites of other structures,
such as PHI, SOD and FAU [20]. Many researchers used tetraethy lammonium hydroxide (TEAOH) as an organic template agent to syn thesize zeolite Phi [21–24]. Most of the crystallization times reported were too long, while Lillerud and co-workers synthesized a Phi-type zeolite with the crystallization time of only three days [25]. The mate rial showed similar structural features to the zeolite Phi, which has faults in the double 6 rings stacking of chabazite [23,25]. Among these zeolite frameworks, the walnut-like zeolite LZ-276 possesses the same topo logical structure as zeolite Phi [26]. In this work, we explored a simple procedure for the preparation of the walnut-like zeolite LZ-276 in the absence of organic templating agent. The zeolite catalyst was used for the efficient synthesis of styrene carbonate (SC) through one-pot styrene oxidation carboxylation reac tion. The main significance of this study is to optimize the experimental conditions and ensure high yield of the SC. 2. Experiment sections 2.1. Chemicals Hydrated silica (SiO2⋅nH2O) was obtained from Xiya Reagent. So dium aluminate (NaAlO2, CP), ethyl acetate (C4H8O2, AR) and aceto nitrile (CH3CN, AR) were purchased from Sinopharm Chemical Reagent (China). Sodium hydroxide (NaOH, AR) was provided by Tianjin
* Corresponding author. 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.micromeso.2019.109779 Received 1 February 2019; Received in revised form 5 September 2019; Accepted 30 September 2019 Available online 2 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Beibei Gu, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109779
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of Al2O3: 32SiO2: 7Na2O: 603H2O was aged at room temperature for 25 h. After this, 0.2254 g aluminum sulfate and 0.1083 g sodium aluminate were added to the gel, and the mixture was further stirred and crystallized at 100 � C for 26–28 h. In order to quantify the alkalinity, the amount of the second supplemental sodium aluminate (NaAlO2-2) was changed. The crystallization process is performed in a box atmosphere furnace at a heating rate of 1 � C/min. Zeolite LZ-276 was collected by filtration, washed with deionized water, dried overnight at 100 � C and calcined for 2 h at 550 � C.
Scheme 1. Cycloaddition of CO2 to epoxides.
Fengchuan Chemical Reagent Technologies Co., Ltd. Distilled water was produced by the reverse osmosis membrane separation pure water de vice provided by Tianjin University Chemical Basic Experiment Center. Styrene (C8H8, AR, 99%) was provided by Acros Organics USA. Tertbutyl hydroperoxide (TBHP, 70%) was supplied from Alfa Aesar (Tian jin) Chemical Co., Ltd. Potassium iodide (KI, AR) was purchased from Tianjin Yongsheng Fine Chemical Co., Ltd. All of the reagents can be used without purification.
2.3. Characterization methods The desktop scanning electron microscopy (SEM, Phenom Pro, Netherland) was used to characterize the morphology of the zeolite sample. The crystal structure of the zeolite was tested by X-ray diffrac tometer (XRD, SmartLab 9 kW, Rigaku, Japan) in the range of 2θ from 10� to 90� . Fourier transform infrared spectrum (FT-IR) was recorded in Nicolet Nexus 670 spectrophotometer (Thermo Fisher Scientific, USA) with the range of 4000–400 cm 1. Elemental composition and valence information of zeolite were tested by X-ray photoelectron spectroscopy (XPS, EscaLab 250, Thermo Fisher Scientific, USA). N2 adsorptiondesorption operation was performed at 196 � C by Autosorb-iQ (Quantachrome Instruments, USA) after vacuum high temperature degassing pretreatment. The product of the catalytic reaction was monitored by gas chromatography (GC-2010 Plus, Shimadzu, Japan). Temperature programmed desorption of CO2 (CO2-TPD) and NH3 (NH3-
2.2. Synthesis of LZ-276 First, a starting aluminosilicate gel was prepared with the molar ratio of Al2O3: 45SiO2: 8Na2O: 840H2O. In a typical preparation, 2.3296 g SiO2⋅nH2O, 0.1409 g NaAlO2 and 0.51 g NaOH were mixed with 13 mL of deionized water in a Teflon-coated reactor, followed by stirring for 1–2 h to form aluminosilicate gel. The resultant gel with the molar ratio
Fig. 1. SEM image of (A) sample 1 (m(NaAlO2-2) ¼ 0.2254 g, the final gel molar ratio is Al2O3:15SiO2:3.3Na2O:280.7H2O); (B) sample 2 (m(NaAlO2-2) ¼ 0.1669 g, the final gel molar ratio is Al2O3:17.5SiO2:3.7Na2O:326H2O); (C) sample 3-zeolite LZ-276 (m(NaAlO2-2) ¼ 0.1083 g, the final gel molar ratio is Al2O3:32SiO2:7 Na2O:603H2O); (D) SEM image of the catalyst (after reaction). 2
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Fig. 2. XRD pattern of zeolite LZ-276.
Fig. 3. FT-IR spectrum of zeolite LZ-276.
TPD) study were performed using a ChemBET TPR/TPD instrument (Quantachrome Instruments, USA). The conversion of styrene and the content of each product were calculated by peak area ratio. The area normalization method was used to calculate the content of each component. The quantitative analysis was determined by gas chroma tography (GC, Shimadzu 2010 Plus, RTX-5 capillary column, flame ionization detector, Japan), and the products were identified by a gas chromatography mass spectrometer (GC-MS, Agilent 5975C, USA). A temperature ramp was applied to the product analysis, the column temperature was started at 60 � C followed by the heating rate of 10 � C/ min until 280 � C. The temperature was maintained for 2 min before the column oven was cooled to 60 � C for next analysis.
pattern of the sample and the theoretical simulation of the ideal CHA topology in the rhombohedral space group R-3m [27]. The FTIR spectrum in the region of 4000–500 cm 1 for the zeolite LZ276 was showed in Fig. 3. The broad band at 3432 cm 1 and band at 1652 cm 1 are attributed to zeolitic water [28]. The result shows that the band at 1032 cm 1 is the Si–O stretching vibration [22]. The ab sorption bands at 618 cm 1 and 700 cm 1 are corresponding to sym metrical T–O–T stretching mode (T ¼ Si, Al) [29,30]. For qualitative and quantitative measurement of elemental compo sition of the zeolite LZ-276 surface, the XPS survey scan shows the Si2p, Al2p, O1s and Na1s photoelectron line (see Fig. 4A). Fig. 4B, C, D and E clearly display that the binding energy of each element of the zeolite LZ276. The binding energies, contents of each element on the zeolite surface and Si/Al atomic ratio are shown in Table 1 [31]. The surface area and pore size distribution were analyzed using Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) mode, respectively. The N2 adsorption-desorption isotherm of the samples were obtained in Fig. 5, which exhibited a hysteresis loop at relative pressure P/P0 between 0.4 and 0.9. The BET surface area of zeolite LZ-276 is 201 m2 g 1, in which contains micropore area of 155 m2 g 1. Its average pore diameter is 3.8 nm (Fig. 5, inset). Large pore size and surface area provided sufficient space for adsorption of reactants and desorption of products. The acidity of zeolite LZ-276 can be investigated by NH3-TPD. The acid strength will be proportional to the temperature required to release the probe gas molecules [32]. NH3-TPD curves are shown in Fig. 6A. The peaks at 251 � C and 610 � C can be attributed to the weak acid sites and strong acid sites, respectively. The alkalinity is estimated as the total amount of CO2 released by TPD of the sample, and the results are shown in Fig. 6B. The CO2-TPD profile demonstrates the presence of basic sites in zeolite LZ-276. The Lewis base sites provided by Si atoms in the zeolite framework contribute to the activation of CO2 and promote the catalytic reaction [33]. Combining the results of NH3-TPD and CO2-TPD, walnut-like zeolite LZ-276 with both acidic and basic sites will have good catalytic activity.
2.4. Catalytic cycloaddition of CO2 to styrene Based on the original work of the optimal conditions for styrene oxidation, the experimental scheme for the one-pot synthesis of styrene cyclic carbonate was designed (Scheme 1). The catalytic experiments were carried out in a 50 mL high pressure stainless-steel autoclave. In a typical run, 1 mL styrene, 1 mL TBHP, 5 mL acetonitrile and 0.05 g catalyst (0.02 g LZ-276 and 0.03 g KI) were firstly added to the auto clave. Then, the high-pressure reactor was filled with a certain pressure of CO2 (99.999% purity). The reaction mixture was stirred by magnetic force at 300 r/min and heated to a certain temperature for a period of time. After the reaction was completed, the reactor was quickly cooled to room temperature in cold water, followed by the discharge of unreacted CO2. The reaction solution was extracted with deionized water and ethyl acetate, and the supernatant was detected by GC-MS. 3. Results and discussion 3.1. Catalyst characterization Fig. 1A, B and C show SEM images of second different amount of sodium aluminate (0.1083 g–0.2254 g). Fig. 1D shows the micromor phology of zeolite LZ-276 after reaction. It can be seen that both of the catalyst have a walnut-like structure with uniform size. This not only proves that zeolite LZ-276 has stable structure and chemical stability, but also explains excellent cycle performance. The crystal structures of carrier zeolite LZ-276 is characterized by XRD (Fig. 2). All the samples were collected in the range of 5–90� with the scanning speed was 2� /min. For the sample, all diffraction peaks are perfectly matched to the zeolite LZ-276 standard card (PDF-49-0921). In other words, Fig. 2 shows a good agreement between the powder XRD
3.2. Catalytic test The effects of substrates amount, reaction temperature, the pressure of CO2 and reaction time on the synthesis of SC were investigated in details. As shown in Fig. 7, the volume ratio of ST and TBHP had a significant impact on the SC yield. Volume ratio V (ST: TBHP) ¼ 3:1, 2:1, 1:1 and 2:1 corresponding to the molar ratio n (ST: TBHP) ¼ 0.4, 0.6, 3
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Fig. 4. (A) The fully scanned spectra, (B) Si 2p, (C) Al 2p, (D) O 1s, and (E) Na 1s XPS spectra of zeolite LZ-276.
1.12 and 2.4. Since ST can be almost completely converted when the volume ratio of ST: TBHP was 1: 1, this ratio was selected for the following exploration experiment. After comparing the data in Table 2 (entries 1–3), it was found that the yield of styrene carbonate was highest at 140 � C (YSC ¼ 73%). Therefore, we believe that 140 � C was the optimal temperature for one-pot reaction. A reasonable explanation is that an appropriate temperature can provide sufficient energy for epoxide ring opening and CO2 activation [34]. Too low or too high pressure is unfavorable for the synthesis of SC. After the investigation, it was found that 0.5 MPa is a suitable carbon dioxide pressure (entry 2, 4
Table 1 Binding energy, atomic proportions of elements and Si/Al atomic ratio by XPS survey scan analysis. Element
Binding energy (eV)
Atomic %
Si/Al ratio
Al 2p Si 2p C 1s O 1s Na 1s
73 101 284 531 1071
5.79 9.84 46.09 33.99 4.28
1.70
4
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Fig. 7. Effect of substrate volume ratio (ST: TBHP) on the SC yield (reaction conditions: temperature 150 � C, CO2 pressure 1.2 MPa).
Fig. 5. N2 adsorption-desorption isotherm of zeolite LZ-276 (inset shows pore size distribution).
Table 2 Effects of reaction parameters on the SC synthesis over the walnut-like zeolite LZ-276.a. Entry
Temperature (� C)
Pressure (MPa)
Time (h)
YSC (%)b,e
1 2 3 4 5 6 7 8 9b 10c 11d
130 140 150 140 140 140 140 140 140 140 140
1.2 1.2 1.2 0.5 2.0 0.5 0.5 0.5 0.5 0.5 0.5
9 9 9 9 9 6 8 10 10 10 10
44 73 53 74 50 44 58 77 74 52 28
e
Determined by GC. a Reaction conditions: styrene 1 mL, TBHP 1 mL, catalyst (0.02 g LZ-276 and 0.03 g KI), CH3CN 5 mL. b Triple the substrate: styrene 3 mL, TBHP 3 mL. c Six times the substrate: styrene 6 mL, TBHP 6 mL. d Ten times the substrate: styrene 10 mL, TBHP 10 mL. e Determined by GC.
and 5). The cycloaddition of CO2 to ST proceeded further with time and (entry 4, 6–8) the reaction reached a satisfactory yield at 10 h (entry 8). In order to explore the maximum catalytic capacity of the zeolite cata lyst, we assessed a larger scale experiment (entry 9 and 10). Under conditions where the catalyst and solvent were not enlarged (0.02 g LZ-276 þ 0.03 g KI, 5 mL CH3CN), the triple increase of substrate had little effect on SC yield. This meant that the catalyst can catalyze three times the amount of substrate. But when the substrate was expanded six times, the yield of SC was reduced to 52%. It can be seen that increasing the concentration had effect on the yield of SC, that is to say, the oxidative carboxylation reaction of styrene is not a zero-order reaction. Table 3 Blank control experiment for the synthesis of SC.a.
Fig. 6. NH3-TPD (A) and CO2-TPD (B) profiles of zeolite LZ-276.
Entry
Catalyst
Co-catalyst
Time (h)
Pressure (MPa)
YSC (%)
1 2 3
LZ-276 – LZ-276
– KI KI
10 10 10
0.5 0.5 0.5
Trace 52 77
a Reaction condition: ST-1 mL, TBHP-1 mL, catalyst-0.02 g, KI-0.03 g, T ¼ 140 � C.
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recovered by simple centrifugal separation and each cycle experiment only needs to be added with fresh KI. As shown in Fig. 8, styrene was almost completely converted in the four cycles and the yield of SC was started to decrease slightly after four cycles. This result can prove that the zeolite catalyst has good long term stability. 3.2.2. Comparison of catalytic performance of various catalytic systems The comparison of catalytic performance among LZ-276/KI and other catalytic systems reported in the literatures was given in Table 4. The results in Table 4 reveal that the walnut-like zeolite catalyst in this work exhibits excellent catalytic activity. Comparing these five reaction systems, it is found that preparation process of zeolite LZ-276 is simple and the pressure used in this work is the lowest. The low reaction pressure makes the equipment standard easy to meet, and most impor tantly, it meets the requirements of chemical safety production. The operation step of the LZ-276/KI catalytic system is simple. For example, LZ-276 is a heterogeneous catalyst that is easy to recycle. Therefore, we can consider that the walnut-like zeolite catalyst exhibited good cata lytic performance in the synthesis of styrene carbonate by one-pot process.
Fig. 8. Reusability of zeolite LZ-276 catalyst. Table 4 Comparison of catalytic performance among various catalyst systems for one-pot synthesis of styrene carbonate from styrene and CO2. Catalyst
Temperature (� C)
Time (h)
Pressure (MPa)
YSC (%)
Ref.
Au/SiO2 Na2H5P (W2O7)6 Cr-MIL-101 K2S2O8/ NaBr LZ-276/KI
80 50
4 4
8 2.4
35 68
[35] [36]
80 60
7 10
10 3
44 79
[37] [38]
140
10
0.5
77
This work
3.3. Possible mechanism Based on the present experimental results and pioneering literature [33,38–41], the plausible mechanism for the oxidative carboxylation of styrene over LZ-276/KI catalyst was postulated in Scheme 2. The one-pot reaction can be regarded as two-step processes which should consist of the epoxidation catalyzed by LZ-276/TBHP-KI and the sub sequent cycloaddition catalyzed by LZ-276/KI. Firstly, catalytic oxida tion of I with TBHP leaded to the formation of hypoiodite OI (Step 1). Then iodohydrin was formed by iodination of styrene in the presence of a small amount of H2O (Step 2). Thereafter, iodohydrin was dehy droiodinated to give important intermediate styrene oxide (SO) under alkaline condition (Step 3). Subsequently, nucleophile I anions and the aluminum atoms in the zeolite framework synergistically attacked the less sterically hindered β-carbon atom of SO, which contribute to the ring-opening and form oxyanion intermediate (Step 4). Generally speaking, step 4 was considered to be the rate-determining step for the CO2 cycloaddition reaction [42,43]. Next, the CO2 molecule activated by the Si atom in the zeolite framework was inserted into the oxyanion intermediate to produce a new alkyl carbonate compound. Finally, I was taken off from the alkyl carbonate and closed to form the target product SC (Step 5).
To assess the application prospects of this reaction, a ten-fold substrate amplification experiment was performed and the result showed that the yield of SC was 28%. In addition, a blank control experiment was conducted to demon strate the catalytic activity of the walnut-like zeolite catalyst. The cat alytic results are shown in Table 3. After comparing these three groups of data, it was found that no product was detached when only LZ-276 was used as catalyst (entry 1). While the co-catalyst KI was used alone, the catalytic result was not satisfactory (entry 2). When both LZ276 and KI were used, the yield of SC was as high as 77% under the same condition. Combining analysis of GC and GC-MS spectra, the selectivity and yield of the target product SC was determined (Figs. S1 and S2). 3.2.1. Catalyst recycling The recyclability of catalyst plays a significant role in industrial production and practical application. The zeolite catalyst could be
Scheme 2. A proposed mechanism for the one-pot synthesis of styrene carbonate from styrene and CO2 catalyzed by the LZ-276/KI system. 6
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4. Conclusions
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In summary, a simple and highly-efficient synthesis of styrene car bonate from styrene and CO2 has been achieved by the walnut-like zeolite LZ-276/KI system. The yield of SC could be as high as 77% under the synergistic effect of zeolite catalyst and KI. No significant loss of activity was observed in the initial four cycles. Future work will focus on tightly combining KI with zeolite with excellent catalytic and cycling performance. Acknowledgements The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 51772158). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109779. References [1] V. Laserna, G. Fiorani, C.J. Whiteoak, E. Martin, E. Escudero-Ad� an, A.W. Kleij, Angew. Chem. Int. Ed. 53 (2014) 10416–10419. [2] R.-S. Kühnel, N. B€ ockenfeld, S. Passerini, M. Winter, A. Balducci, Electrochim. Acta 56 (2011) 4092–4099. [3] Z.-M. Cui, Z. Chen, C.-Y. Cao, W.-G. Song, L. Jiang, Chem. Commun. 49 (2013) 6093–6095. [4] B. Nohra, L. Candy, J.-F. Blanco, C. Guerin, Y. Raoul, Z. Mouloungui, Macromolecules 46 (2013), 3771–379. [5] Q.W. Song, Z.H. Zhou, L.N. He, Green Chem. 19 (2017) 3707–3728. [6] M. North, R. Pasquale, C. Young, Green Chem. 12 (2010) 1514–1539. [7] S. Kumar, N. Singhal, R.K. Singh, P. Gupta, R. Singh, S.L. Jain, Dalton Trans. 44 (2015) 11860–11866. [8] Z. Zhang, F. Fan, H. Xing, Q. Yang, Z. Bao, Q. Ren, ACS Sustain. Chem. Eng. 5 (2017) 2841–2846. [9] Q. Han, B. Qi, W. Ren, C. He, J. Niu, C. Duan, Nat. Commun. 6 (2015) 10007. [10] P.T.K. Nguyen, H.T.D. Nguyen, H.N. Nguyen, C.A. Trickett, Q.T. Ton, E. Guti�errezPuebla, M.A. Monge, K.E. Cordova, F. G� andara, ACS Appl. Mater. Interfaces 10 (2018) 733–744. [11] M.S. Liu, B. Liu, L. Liang, F.X. Wang, L. Shi, J.M. Sun, J. Mol. Catal. A Chem. 418–419 (2016) 78–85. [12] M. Taherimehr, A. Decortes, S.M. Al-Amsyar, W. Lueangchaichaweng, C. J. Whiteoak, E.C. Escudero-Ad� an, A.W. Kleij, P.P. Pescarmona, Catal. Sci. Technol. 2 (2012) 2231–2237.
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