Catalysis Letters https://doi.org/10.1007/s10562-018-02650-1
Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost Catalyst for the Cycloaddition of CO2 with Epoxides Weili Dai1 · Jie Mao1 · Ying Liu1 · Pei Mao1 · Xubiao Luo1 · Jianping Zou1 Received: 18 November 2018 / Accepted: 27 December 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract Development of efficient, cheap and recyclable catalysts for the synthesis of cyclic carbonates from C O2 and epoxides is still a very attractive topic. Herein, the polymer grafted TBD-based ionic liquids (ILs) were fabricated from commercially available polystyrene (PS) and ingredients of the ILs, and used as heterogeneous catalysts for the conversion of C O2 into cyclic carbonates in the absence of solvent and co-catalyst. To improve the catalytic performance, various substitutes (such as –COOH, –OH and –NH2) were functionalized on the TBD cation. Among the as-obtained catalysts, carboxyl-containing catalyst (PS–[CETBD]Br) showed superior activity than others, which may be attributed to the stronger polarization capability of –COOH on the C–O bond of epoxide through the formed hydrogen bonding. Additionally, the combined synergistic effect of the nucleophilic attack by the halide anions also account for the facile ring-opening of epoxide. It is noted that CO2 can be activated by the formation of carbamate between C O2 and alkaline nitrogen of the TBD cations, thus facilitating the formation of cyclic carbonates. Moreover, the catalyst shows good chemical stability and catalytic reusability, which is very important for the practical conversion of CO2 in chemical industry. Graphical Abstract
Keywords TBD-based ionic liquids · Heterogeneous catalysis · Carbon dioxide · Cyclic carbonate · Epoxide
1 Introduction
* Weili Dai
[email protected];
[email protected] 1
Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, Jiangxi 330063, China
Nowadays, CO2 is regarded as the most significant greenhouse gas, which is causing global warming and climate change [1, 2]. Nevertheless, CO2 itself is an abundant, nontoxic, nonflammable, easily available, and renewable carbon resource [3]. Therefore, the conversion of CO2 into valuable chemicals has attracted great attention of organic chemists, which facilitates the carbon cycle in nature. One of the most
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promising ways for effective utilization of CO2 is cycloaddition of CO2 to epoxides for cyclic carbonates synthesis, which possesses an atom utilization ratio of 100%, and conforms to the concept of “green chemistry” and “sustainable society” [4]. The cyclic carbonates have interesting applications as electrolytes in secondary batteries, valuable monomers of polycarbonates and polyurethanes, excellent aprotic polar solvents, and intermediates for fine chemicals [5]. At present, numerous homogeneous catalysts like alkali metal salts [6], organic bases [7], organometallic complexes [8], and ionic liquids (ILs) [9] have been proposed for the cycloaddition reaction. But the complicated product separation from the catalysts severely limits their large-scale applications. To overcome this problem, heterogeneous catalysts have been explored, such as metal oxide [10], modified molecular sieve [11], metal–organic frameworks (MOFs) [12], and active species supported materials [13]. Though they have contributed more or less to the catalytic performance, some disadvantages including low activity, low stability, need of co-catalyst, and/or high cost still perplex the academia and industry. Hence, it remains a challenge for the development of low cost, stable, and efficient single component catalyst for the conversion of CO2 to cyclic carbonate. Since 2000s, the use of ILs as catalysts for the cycloaddition reaction is studied extensively and intensely, due to their specific features such as high thermal stability, diverse structure and property modulation, and acid–basic property [14]. In recent years, the ILs-based heterogeneous catalysts prepared by supporting ILs as active moiety onto suitable materials have attracted great attention, because they permit mutual advantages of both homogeneous ILs and heterogeneous catalysts. To date, however, most of the ILs immobilized on supports are quaternary ammonium-, phosphonium-, imidazolium-based ILs, still suffering from low catalyst activity and/or high cost [15]. 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) is a commercially available organic superbase, which has been widely utilized as catalyst in many base-catalytic reactions [16]. CO2 is a typical Lewis acid molecule. Hence, the TBD has the potential to activate CO2 through forming the carbamate species between the tertiary nitrogen and CO2. While using TBD as cycloaddition catalyst [17], however, it exhibited very low catalytic activity. Nevertheless, high catalytic activity becomes possible when TBD is converted into an ionic liquid composing of TBD cation and halide anion. This is because the halide could nucleophilically attack the less hindered carbon atom of epoxide, which promotes the ring-opening of epoxide, thus facilitating the cycloaddition reaction [18]. Nevertheless, there are few studies about the TBD-based ILs used as catalysts for the cyclic carbonates synthesis from C O2 and epoxides. He group used [HTBD]Cl (1,5,7-triazabicyclo[4.4.0]dec-5-enium chloride) to catalyze the cycloaddition reaction [19]. Although it showed much higher catalytic
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activity than pure TBD, the selectivity was not satisfactory. Subsequently, they synthesized polyethylene glycol-functionalized [TBD]Br with elevated catalytic activity [14]. However, the inherent homogeneous property of those catalysts made them difficult to separate from the products. To the best of our knowledge, there is no reported use of heterogenized TBD-based ILs as catalyst for the cycloaddition reaction. In previous reports, various materials such as SiO2 [20], SBA-15 [21], MCM-41 [22, 23], chitosan [24], MOFs [25] and polymers [26] were used to support ILs. Although some efficient and stable heterogeneous catalysts can be obtained, the problem one should not overlook is that the preparation of supports and the immobilization of ILs are complex and costly. Chloromethyl polystyrene (PS–Cl) is a commercially available and low-cost polymer support with good chemical and thermal stability. Its active –Cl group and porous structure are benefit for the immobilization of active specie moieties, and accessibility of reactants to catalytic sites, respectively. Hence, immobilization of TBD-based ILs onto PS surface could be an efficient strategy to develop efficient and low-cost heterogeneous catalyst. Moreover, it was demonstrated that introducing hydroxyl or carboxyl groups into the cation of traditional ILs can effectively improve their catalytic activity towards the cycloaddition reaction, attributing to the formation of hydrogen bonding between the functional groups and epoxides [27]. In this study, a series of TBD-based ILs that are functionalized with hydroxyl, carboxyl and amino groups are grafted onto PS surface by a simple method. Indeed, the asfabricated catalysts display superior catalytic performance for the cycloaddition reaction compared with other polymersupported ILs. Systematic investigation on the effects of ILs structure (i.e. length of alkyl chain, species of functional group and halide anion) and reaction parameters (i.e. catalyst loading amount, temperature, CO2 pressure, and reaction time) on the target reaction were also conducted. Additionally, a plausible reaction mechanism is proposed based on the experimental results.
2 Experimental 2.1 Materials All chemicals were purchased from commercial sources and used without further treatment. The PS–Cl, 2-bromoethanol, 3-chloropropionic acid, 3-iodopropionic acid and propylene oxide were purchased from Macklin Biochemical Co., Ltd. The 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) and other epoxides were obtained from Aladdin Co. The CO2 (99.9% purity) purchased from Nanchang Guoteng Gas Co.
Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost…
2.2 Characterization The X-ray diffraction (XRD) patterns of samples were collected by a Bruker AXS-D 8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5408 Å) in the 2θ range of 5–50°. Scanning electron microscopy (SEM) observations were carried out over a Nova NanoSEM 450 microscope. Energy dispersive X-ray spectroscopy was performed using the accessory INCA 250 of a Nova NanoSEM 450. The Fourier transformed infrared (FTIR) spectra were recorded on a Bruker vertex 70 FT-IR spectrophotometer. Solid-state 13C nuclear magnetic resonance (13C CP/MAS NMR) experiment was performed on a Bruker AVANCE III 600 spectrometer with a resonance frequency of 150.9 MHz. 13C CP/MAS NMR spectra were recorded using a 4 mm MAS probe and a spinning rate of 12 kHz. A contact time of 3 ms and a recycle delay of 3 s were used for the 13C CP/MAS measurement. The chemical shifts of 13C were externally referenced to TMS. X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Ultra DLD (delay line detector) spectrometer with an Al Kα radiation line source. All the binding energies for the high-resolution spectra were calibrated by C
1 s to 284.6 eV. Thermogravimetric analysis (TGA) was performed on a SDT Q600 (TA Instruments-Waters LLC) at a heating rate of 15 °C/min in a nitrogen flow.
2.3 Preparation of Polystyrene Grafted TBD‑Based ILs The procedure for the preparation of polystyrene microsphere grafted TBD-based ILs is shown in Scheme 1. 2.3.1 Synthesis of Polystyrene Grafted TBD (Scheme 1a) In a typical reaction, a mixture of PS–Cl (2 g) and TBD (5.96 mmol) in toluene (20 mL) were added into a 100 mL flask. The mixture was refluxed for 72 h under a nitrogen atmosphere. After reaction, the solid residue was collected by filtration and washed with ethyl acetate and dichloromethane for three times, respectively. Then solid was dried at 60 °C under vacuum for 12 h to give polystyrene grafted TBD (denoted as PS–TBD) as a slightly yellow sample.
Scheme 1 Preparation of polymer supported TBD-based ILs
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2.3.2 Synthesis of Polystyrene Grafted TBD‑Based ILs (Scheme 1b, c) In a typical procedure, a mixture of PS–TBD (0.4 g) and 20 mL toluene were added into 100 mL flask, then 3-bromopropionic acid (0.6 mmol) was added slowly, and the mixture was stirred at 70 °C for 24 h under nitrogen atmosphere. After the reaction, the solid residue was isolated by filtration, and washed with ethyl acetate and dichloromethane for three times, respectively. Then the solid was dried at 60 °C for 12 h under vacuum to give product PS–[CETBD]Br as a slightly yellow sample. According to the above similar procedure, a series of other polymer grafted TBD-based ILs (PS–[CMTBD] Br, PS–[CPTBD]Br, PS–[CBTBD]Br, PS–[CETBD]Cl, PS–[CETBD]I, PS–[HETBD]Br, and PS–[AETBD]Br) were prepared.
2.4 Cycloaddition Reaction of CO2 and Epoxide The cycloaddition reactions were carried out in a 50 mL high-pressure stainless-steel autoclave equipped with a magnetic stirring bar. In a typical procedure, the reactor was charged with epoxide (35.7 mmol), catalyst (0.24 mol%, calculated based on the amount of IL), and appropriate amount of biphenyl (as the internal standard for GC analysis). After the reactor was fed with CO2 to a desired pressure, the autoclave with its contents was heated to a designated temperature and stirred for a designated period of time. After the reaction was completed, the reactor was cooled to ambient temperature, and the excess C O2 was released. The resulting mixture was quantitatively analyzed using on a gas chromatograph (GC, Agilent 7890A) with a TCD detector and a DB-wax capillary column (30 m × 0.53 mm × 1.0 µm), and further identified using a GC-mass spectrometry.
3 Results and Discussion 3.1 Catalyst Characterization To make sure grafting the TBD-based ILs on PS successfully, the chemical structures of the as-synthesized heterogeneous samples were verified by FT-IR spectra, solid-state 13 C NMR spectra, and XPS spectra. The presence of TBDbased ILs on polystyrene support was confirmed by FT-IR spectra (Fig. 1). For PS–Cl (Fig. 1a), the distinguishing band at 1259 cm−1 is owing to the stretching frequency of the functional group CH2Cl [28]. When the TBD was grafted on the PS (PS–TBD), the peak was disappeared and the characteristic peaks of TBD around 1600 and 1200 cm−1 band could be clearly observed, which were ascribed to C = C and C–N stretching vibrations, respectively (Fig. 1b) [29, 30]. It
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Fig. 1 FT-IR spectra of (a) PS–Cl, (b) PS–TBD, (c) PS–[CETBD]Br, (d) PS–[HETBD]Br, and (e) PS–[AETBD]Br
is noted that the weak band at around 1600 cm−1 observed at the same frequency for PS–Cl spectrum is due to benzene skeleton. The results indicated that TBD was successfully grafted on the PS (Scheme 1a). Further converting the grafted TBD to ILs (Scheme 1b, c), there are newly appeared characteristic peaks of the functional groups of TBD-based ILs (Fig. 1c–e) in comparison with PS–TBD (Fig. 1b). As shown in Fig. 1c, catalyst PS–[CETBD]Br exhibits the characteristic bands of C = O (1741 cm−1), and C–O (1233 cm−1) and O–H (3404 cm−1) stretching vibrations of carboxylic acid groups [31]. The characteristic bands of hydroxyl group at 3419 cm−1 corresponding to the H–O [32] are detected with catalyst PS–[HETBD]Br (Fig. 1d). Over catalyst PS–[AETBD]Br (Fig. 1e), the bands of primary amine (–NH2) stretching (3464 and 3377 cm−1) vibrations are observed. All of the above mentioned IR bands were found in the process for the synthesis of grafted TBD-based ILs, confirming the immobilization of the ILs on the PS surface. A comparative study of solid-state 13C NMR spectra among PS–TBD and PS–[CETBD]Br further proves the successful grafting of the TBD-based ILs on the PS surface. In comparison with PS–Cl, a series of new peaks in the NMR spectrum of PS–[CETBD]Br were observed (Fig. 2), which were ascribed to the carbon atoms of carboxyl-functionalized TBD-based ILs. The strong over-lapping peaks at around 49 ppm are ascribed to the methylene units (C–N) of TBD ring (C2, C4, C5, C7), methylene (C1) and ethylene chain (C9) linking the TBD ring. In addition, the distinct peak at ca. 24 ppm corresponds to the methylene units (C–C) of TBD ring (C3, C6), and ethylene chain (C10) connected to the TBD ring [33]. The chemical shift at 151 and 170 ppm are attributed to the C8 carbon (C=N) of TBD ring and C11 carbon of carboxyl group, respectively [30, 34]. These results indicate the presence of TBD-based ILs
Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost…
Fig. 2 Solid 13C NMR spectra of PS–Cl and PS–[CETBD]Br
moieties as part of the catalyst PS–[CETBD]Br, which was coincide with the results of FT-IR. The chemical composition and states of the catalysts were further investigated by XPS and the results are shown in Fig. 3. The survey spectra shown in Fig. 3a show that C, Cl and O elements exit in PS–Cl, while C, O and N, as well as C, O, N and Br elements can be observed in PS–TBD and PS–[CETBD]Br, respectively. The results are consistent with the chemical structure of the as-fabricated catalysts illustrated in Scheme 1. Moreover, high-resolution N 1 s and Br 3d spectra were also recorded, and are shown in Fig. 3b, c, respectively. The N 1s spectra of PS–TBD can be deconvoluted into two main components peaks at binding energy of 400.0 and 397.9 eV, which are ascribed to tertiary nitrogen and pyridinic nitrogen in TBD ring, respectively [35]. For PS–[CETBD]Br, the N 1s peak can be fitted into two peaks at 400.3 and 401.2 eV, assigning to tertiary nitrogen and quaternary nitrogen, respectively. Notably, the pyridinic
Fig. 3 a XPS survery spectra of PS–Cl, PS–TBD and PS–[CETBD]Br; b N 1s XPS spectra of PS–TBD and PS–[CETBD]Br; c Br 3d XPS spectra of PS–[CETBD]Br
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nitrogen of PS–TBD transforms to quaternary nitrogen of PS–[CETBD]Br, which indicates the successful conversion of the grafted TBD into TBD-based ILs on PS surface. Additionally, the slightly increment of binding energy of tertiary nitrogen could result from the strong electron withdrawing ability of quaternary nitrogen. Figure 3c shows the
Fig. 4 XRD patterns of (a) PS–Cl and (b) PS–[CETBD]Br
Fig. 5 SEM images of a, b PS–Cl and c, d PS–[CETBD]Br
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high resolution spectra of Br 3d of PS–[CETBD]Br. It can be seen that the main peak is fitted with two peaks at 68.5 and 67.5 eV, which are attributing to Br 3d 3/2 and Br 3d 5/2 of ionic bromine, respectively [36]. The results are well agreement with the structures of the target product as shown in Scheme 1. The crystallinity and morphology of PS–Cl and PS–[CETBD]Br samples were observed by XRD and SEM, respectively. As shown in Fig. 4a, the XRD pattern of PS–Cl exhibits a peak at 2θ = 19.6° ascribe to diffraction at the crystalline region of the polymer structure. When PS surface was modified with TBD-based IL, the obtained PS–[CETBD]Br shows a broader peak (Fig. 4b), suggesting an amorphous structure. This phenomenon possibly attributes to the collapse of symmetry and regularity of the polystyrene support [37]. The morphology study also validates this point (Fig. 5). It can be seen that the PS–Cl consists of uniformly-sized microspheres with diameters ca. 130 µm, and the surface is smooth (Fig. 5a, b). With the grafting of TBD-based ILs on the PS surface, the PS–[CETBD]Br has similar morphology and size, but roughening surface (Fig. 5c, d), which is ascribed to the loss of crystallinity. The results also suggested the TBD-based IL was grafted on the PS surface. Moreover, the thermal properties of PS–Cl and PS–[CETBD]Br were examined using TGA under nitrogen
Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost… Table 1 Catalytic performance of catalystsa Entry
Fig. 6 TGA curves of (a) PS–Cl and (b) PS–[CETBD]Br
atmosphere, and the curves are shown in Fig. 6. It can be seen that the weight loss of PS–Cl is less than 1% up to 230 °C. And then, polymer matrix decomposition goes through two stages, at 230 and 380 °C, respectively (Fig. 6a) [38]. However, there is about 7% weight loss over PS–[CETBD]Br below 200 °C, which may be due to the desorption of water trapped by the hydrophilic IL on the surface of PS–[CETBD]Br [39]. The further decomposition begins at 320 °C, which is ascribable to the co-decomposition of IL and polymer (Fig. 6b). It can be drawn that the thermal stability of PS–Cl was enhanced due to the graft of TBD-based IL. It is apparent that the as-prepared catalysts can be operated in the cycloaddition reaction that occurs below 150 °C.
3.2 Catalytic Performance The catalytic performance of the as-fabricated PS-grafted TBD-based ILs was evaluated via the synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO) under solvent-free and metal-free conditions, and the results are summarized in Table 1. It can be seen that the PS–Cl as support precursor shows almost no catalytic activity (entry 1) for the cycloaddition reaction. Additionally, after grafting TBD on the surface, the reaction almost did not proceed either (3.8% of yield, entry 2), although TBD can directly catalyze the cycloaddition reaction [17]. To out delight, when the TBD was transformed to ILs, the obtained PS-grafted TBD-based ILs exhibited good catalytic performance. While using PS–[CETBD]Br as catalyst, a PC yield of 96.3% and selectivity of 100% can be achieved at a low catalyst loading amount (0.24 mol%, entry 3). It was reported that the carboxyl, hydroxyl and amino groups can accelerated the ringopening of epoxides by forming hydrogen bonds [22, 28, 40]. Hence, the hydroxyl- and amino-functionalized TBDbased ILs grafted on PS were also prepared and used as
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Catalyst
PS–Cl PS–TBD PS–[CETBD]Br PS–[HETBD]Br PS–[AETBD]Br PS–[ETBD]Br CH3COOH PS–[ETBD]Br/CH3COOHb PS–[CETBD]Cl PS–[CETBD]I PS–[CMTBD]Br PS–[CPTBD]Br PS–[CBTBD]Br P–[CEIm]Brc P–[HEIm]Brc
Catalytic results Yield (%)
Selectivity (%)
0.6 3.8 96.3 92.3 87.9 73.5 2.5 85.6 81.6 97.7 95.7 91.3 90.1 79.3 76.5
87.6 89.5 100 100 100 100 93.4 99.5 99.9 100 99.9 100 99.8 99.8 99.6
a Reaction conditions: PO 35.7 mmol, catalyst 0.24 mol%, CO2 pressure 2 MPa, temperature 140 °C, time 2 h b c
Equal catalyst amount (0.086 mmol)
[P–HEIm]Br and [P–CEIm]Br was prepared according to references [42, 43]
catalysts for the cycloaddition reaction. As for PS–[HETBD] Br and PS–[AETBD]Br (entries 4 and 5), both show slightly lower activity than that of PS–[CETBD]Br, but the catalytic performance can still be considered as good. This is because the polarization capability of hydroxyl and amino groups is not strong enough for the opening of epoxide ring [41]. This point was further proven by the results obtained over the catalyst without functional group in IL (PS–[ETBD]Br). In comparison with PS–[CETBD]Br, PS–[HETBD]Br and PS–[AETBD]Br, PS–[ETBD]Br is much lower in catalytic activity (entry 6). Nevertheless, in the presence of acetic acid that are almost inactivity by itself (entry 7), there is obvious enhancement of PS–[ETBD]Br activity (entry 8). The results indicate that the carboxyl group plays an important role in the promotion of the reaction. In order to elucidate the effect of the structures of TBDbased ILs, the functions of carboxylic acid and halide anion on the catalytic activity was further investigated. As previously reported [18, 44, 45], the nucleophilic attack on the β-carbon atom of the epoxide by the halide anion promotes the opening of the epoxide ring. At the same time, the leaving ability of the halide anion is also important for the formation of cyclic carbonate. In this study, various PS-grafted TBD-based ILs with different counter halide anions (Cl−, Br− and I−) were prepared and evaluated. It was observed that the activities of the as-prepared catalysts decreased in the order of PS–[CETBD]I (entry 10) > PS–[CETBD]Br
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(entry 3) > PS–[CETBD]Cl (entry 9), which is coincides with the order of nucleophilicity and leaving ability of the halides anions: I− > Br− > Cl−. Moreover, the structure of the TBD-based ILs cations also has a strong influence on the catalytic activity. With B r − as anion and under equal reaction conditions, the influence of varying the molecular weight of TBD cations (PS–[CCnH2nTBD]Br, n = 1–4) was investigated. It is found that the catalytic activity increases of the methylene chain length (n = 1 and 2; entries 11 and 3). A further increase of methylene chain length (n = 3 and 4) results a decrease in the catalytic activity (entries 12 and 13). This phenomena may be due to two factors: (i) An increase of methylene chain length weakens the electrostatic interaction between halide anion and TBD cation, enhancing the nucleophilic attack ability of halide anion, and facilitating the cycloaddition reaction as a result [46]; (ii) The acidity of carboxylic acid with a shorter methylene chain is stronger than that with a longer methylene chain, thus the cation with shorter methylene chain has higher ring-opening capability, resulting in rise of PC yield [37]. Therefore, the competition between the two opposite factors results in the carboxylfunctionalized TBD-based ILs with moderate methylene chain length (n = 2) having the best catalytic activity. Moreover, it is noted that almost 100% of PC selectivity can be achieved while using the as-fabricated catalysts. There was only a minute amount of 1,2-propanediol generated due to PO hydrolysis detected as by-product in the cycloaddition reaction, when the selectivity was below 100%. To identify the potential of PS–[CETBD]Br as heterogeneous catalyst for the synthesis of cyclic carbonates, we deemed it worthwhile to compare its activity to that of early reported similar polymer-supported ILs catalysts. It was reported that the polydivinylbenzene grafted with carboxyl- and hydroxyl-functionalized imidazolium-based ILs (denoted as P–[CEIm]Br and P–[HEIm]Br, respectively) showed good catalytic performance for the cycloaddition reaction. In order to make a direct comparison, we evaluated P–[CEIm]Br and P–[HEIm]Br under the conditions adopted in this study, and the PC yields are 79.3 and 76.5%, respectively (entries 14 and 15), much lower than that obtained over PS–[CETBD]Br. It is apparent that the catalyst PS–[CETBD]Br has great potential for industrial application as a heterogeneous catalyst for the cycloaddition of CO2 to epoxides due to its superior catalytic activity as well as lowcost and simple preparation approach.
3.3 Effects of Reaction Parameters on Catalytic Activity of PS–[CETBD]Br The PC synthesis over PS–[CETBD]Br was studied at different catalyst amounts, temperatures, CO2 pressure and reaction time to optimize these reaction variables (Fig. 7).
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Figure 7a depicts the relationship between catalyst loading and PC yield as well as selectivity. It can be seen that the PC yield sharply rised from 79.6 to 96.3% with the catalyst concentration increasing from 0.08 to 0.24 mol%, which possibly attributes to the increase of active sites introduced into the reaction. Further increase of the catalyst amount from 0.24 to 0.40 mol% did not cause significant enhancement of PC yield (to 97.6% only), indicating an optimal catalyst loading of 0.24 mol%. Obviously, PS–[CETBD]Br could exhibit high activity at a low catalyst loading level. Furthermore, the PC selectivity always keeps at almost 100%, which is independent of the catalyst amount. The dependence of PC yield and selectivity on the reaction temperature is illustrated in Fig. 7b. It is obvious that the temperature has a pronounced positive effect on the cycloaddition reaction. At lower temperature (110 °C), the PC yield was obtained 63.2% due to the low activity of the catalyst. While increasing the temperature from 110 to 140 °C, PC yield rapidly increased to 96.3%. Nevertheless, further increase in temperature results in only slight increase of conversion. In many studies, high reaction temperature (> 130 or 140 °C) could cause a distinct decline of PC selectivity while using ILs as catalysts, due to the acceleration of side reactions such as isomerization of acetone, hydrolysis to diol, and/or thermal polymerization [47, 48]. To our delight, the PC selectivity stays almost 100% even in high reaction temperature. This indicates that PS–[CETBD]Br has steadily catalytic activity for the cycloaddition reaction. In general, the CO2 pressure has a significant influence on the conversion of PO. As shown in Fig. 7c, with rise of CO2 pressure from 0.5 to 2.0 MPa, the PC yield rapidly increases from 52.9 to 96.3%. Nevertheless, further rise from 2.0 to 3.0 MPa results in a moderate decrease of the PC yield. Such influence of C O2 pressure on the conversion of PO was also observed in other catalytic systems [49, 50]. This phenomenon possibly attributes to the phase behavior involving CO2-rich gas phase and PO-rich liquid phase in the reaction system. The initial increase of CO2 pressure results in the rapid enhancement of C O2 concentration in the liquid phase, which facilitates the reaction. Whereas further increasing CO2 pressure beyond 2.0 MPa causes a decrease of PC yield, possibly because higher pressure extracted more PO into the gas phase, leading to the reduction of PO contention in the vicinity of the catalysts in the liquid phase. Accordingly, the appropriate CO2 pressure would be 2.0 MPa. Moreover, there were no obvious changes in PC selectivity. The influence of reaction time on PC synthesis is depicted in Fig. 7d. One can see that the cycloaddition reaction proceeds rapidly in the initial 2 h, reaching a PC yield of 96.3%. After that, the increase of PC yield becomes gently. Hence, 2 h was chosen as the reaction time in this study. Furthermore, the selectivity to PC stays above 99.8% throughout.
Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost…
Fig. 7 Influence of reaction parameters (reaction conditions: PO 35.7 mmol, a temperature 140 °C, CO2 pressure 2.0 MPa, time 2 h; b catalyst 0.24 mol%, C O2 pressure 2.0 MPa, time 2 h; c catalyst
0.24 mol%, temperature 140 °C, time 2 h; d catalyst 0.24 mol%, temperature 140 °C, CO2 pressure 2.0 MPa)
3.4 Recycling Studies In order to craft the greener and economical aspect of the developed catalytic system, the reusability and stability study were carried out for PC synthesis. To test the catalyst reusability, a series of cycles were run over catalyst PS–[CETBD]Br under the same reaction conditions. In each cycle, the catalyst was recovered by simple filtration and subjected to washing and drying before directly used for the next run. As displayed in Fig. 8, it can be observed that the catalyst performs well across the five consecutive cycles without any significant loss of activity, affirming good reusability of PS–[CETBD]Br. In addition, the chemical and thermal stability of recycled PS–[CETBD]Br were studied by FT-IR, SEM, and TGA analysis. In comparison with the spectra of fresh one, the recycled PS–[CETBD]Br remains all the characteristic peaks (Fig. 9). From the SEM images shown in Fig. 10, it is clear that the particle size and surface morphology have almost no change after the recycled runs. Furthermore, TGA curves (Fig. 11) show that the decomposition temperature
Fig. 8 Catalytic activity of recycled PS–[CETBD]Br (reaction conditions: PO 35.7 mmol, catalyst 0.24 mol%, temperature 140 °C, CO2 pressure 2.0 MPa, time 2 h)
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demonstrates that the catalyst is thermally as well as chemically stable in the reaction, which is beneficial for the industrial application.
3.5 Cycloaddition of CO2 to Various Epoxides
Fig. 9 FT-IR spectra of PS–[CETBD]Br: (a) fresh, and (b) recovered
of used catalyst stayed high ca. 320 °C, consisting with that of the fresh one. Notably, the weight loss (ca. 5%) over the recovered PS–[CETBD]Br at the range of 160–320 °C attributes to the desorption of water adsorbed on the catalyst surface, which is also similar to that of fresh one. This
Fig. 10 SEM images of PS–[CETBD]Br: a, c fresh, and b, d recovered
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In order to study the efficiency and general applicability of the developed catalytic system, the substrate scope was investigated under the optimized reaction conditions, and the results are listed in Table 2. It can be observed that PS–[CETBD]Br is efficient for the terminal epoxides, and all reactions run smoothly, giving appreciable corresponding product yields in 2 h (entries 1–5). However, while using cyclohexene oxide as the substrate, much longer reaction time (24 h) was needed to reach 56.6% of yield (entry 6), which is ascribe to the higher hindrance originated from the two rings. The DFT studies reported by Wang et al. testify this result [51]. It was found that the barrier height of ringopening of epoxide using cyclohexene oxide as substrate (29.2 kcal/mol) is much higher than that using PO as substrate (19.4 kcal/mol). This indicates that the bulky steric hindrance of cyclohexene oxide weakens the nucleophilicity of Br− anion, and restricts the ring-opening of epoxide, consequently lowers the reactivity. Moreover, it is reported that the epoxides with an electron-withdrawing group are able
Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost…
Fig. 11 TGA curves of PS–[CETBD]Br: (a) fresh, and (b) recovered
to stabilize the intermediate formed after ring-opening of epoxides, thus enhancing the cycloaddition reactivity [52]. Hence, it can be seen that epichlorohydrin is the most reactive substrate due to the strong electron-withdrawing groups, and almost 100% yield is achieved in 2 h (entry 2). Additionally, the activity of other epoxides with electron-donating groups decreased with the substitute hindrance increasing: PO (entry 1) > 1,2-butylene oxide (entry 3) > 1,2-hexene oxide (entry 4) > styrene oxide (entry 5). The extension to various epoxides reflects the outstanding efficiency of the PS–[CETBD]Br catalyst.
3.6 Proposed Mechanism The theoretical studies on the mechanism of the cycloaddition reaction have been performed by DFT method [53–55]. The calculations show that the production of cyclic carbonate proceeds in three steps, containing ring-opening of epoxide, insertion of CO2, and intermolecular ring closure forming cyclic carbonate. Thereinto, the ring-opening of epoxide is identified as the rate-determining step. Hence, the components of the catalyst that are benefit to activate the epoxy-ring should play most important role in the cycloaddition reaction. It is well known that the carboxyl or hydroxyl group could polarize the C–O bond through the formed hydrogen bonding between the –OH group and O atom of epoxide, which combines with the nucleophilic attack of Br− on the β-carbon atom of PO, making the break of C–O bonds occur much easier. Hence, the synergistic effect of carboxyl group and halide plays important role for the excellent catalytic performance of the catalysts PS–[CETBD]Br.
Moreover, in order to gain more insight into reaction mechanism over the catalysts, FT-IR spectra were employed to verify the C O2 activation by the tertiary nitrogen atom in the TBD ring. As shown in Fig. 12, there appeared a new band centered at 1740 cm− 1, which corresponded to the new asymmetric C=O vibration of carbamate salt, implying the activation of C O2 by the alkaline nitrogen from TBD-ILs. The result is consistent with the previously reports [56–58]. Hence, based on our results and those of previous reports, a possible mechanism for the cycloaddition of CO2 to PO over PS–[CETBD]Br is suggested in Scheme 2. First, the coordination of –COOH with the O atom of PO occurs through hydrogen bonding, resulting in polarization of C–O bond of epoxide. Simultaneously, the B r− nucleohilically attacks the less sterically hindered β-carbon atom of PO, which facilitates the ring opening of PO and generation of oxy anion intermediate. In parallel, the tertiary N atom of the TBD cation reacts reversibly with CO2 to form the carbamate salt as the activated species of CO2. Then, the intermediate alkoxide makes a nucleophilic attack on the carbamate salt to produce the alkyl carbonate, which eventually affords the cyclic carbonate and regenerates the catalyst by the subsequent intramolecular ring-closure.
4 Conclusions In summary, a series of PS grafted TBD-based ILs were synthesized from low-cost materials by simple method. The as-synthesized samples can be used as efficient and reusable heterogeneous catalysts for the synthesis of cyclic carbonates from C O2 and various epoxides in the absence of any solvent and co-solvent. While functionalizing with variety functional groups on the TBD cation, the obtained catalysts show enhanced reactivity. Among them, the carboxyl-functionalized one (PS–[CETBD]Br) shows superior activity than that with hydroxyl and amino group, as well as without any functional group. The hydrogen bonding formed between the –COOH and epoxide combining a synergistic role with the nucleophilic attack by the halide anions account for the facile ring-opening of epoxide. Additionally, it is worth to note that the tertiary nitrogen atoms of the TBD cations coordinates reversibly with CO2 to afford the carbamate salts, and lead to an activated form of CO2, thus facilitating the cycloaddition reaction. Moreover, the catalyst shows good chemical and thermal stability, and can be facilely separated and reused with very steady activity. In terms of simplicity, low-cost, stability, and reusability, the catalyst is meaningful for the industrial production of cyclic carbonates from CO2 and epoxides.
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Table 2 Cycloaddition of CO2 with various epoxides
Reaction conditions: PO 35.7 mmol, catalyst 0.24 mol%, C O2 pressure 2.0 MPa, temperature 140 °C, time 2 h
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Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost…
Fig. 12 FT-IR spectra of PS–[ETBD]Br before and after reaction with CO2 (at 2.0 MPa, 140 oC)
Scheme 2 Plausible mechanism for the cycloaddition of C O2 with epoxides catalyzed by PS–[CETBD]Br
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51572119, and 51662031), and Distinguished Young Scientists program of Jiangxi Province (Grant No. 20162BCB23040).
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