Luffa sponge supported dendritic imidazolium ILs with high-density active sites as highly efficient and environmentally friendly catalysts for CO2 chemical fixation

Journal of CO₂ Utilization 38 (2020) 148–157

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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Luffa sponge supported dendritic imidazolium ILs with high-density active sites as highly efficient and environmentally friendly catalysts for CO2 chemical fixation

T

Shilin Lai, Jinbing Gao, Hui Zhang, Lin Cheng, Xingquan Xiong* College of Materials Science and Engineering, University of Huaqiao, Xiamen 361021, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Luffa sponge fibers CO2 Dendritic imidazolium ILs Cyclic carbonates Green synthesis

Efficient using of green heterogeneous catalysts from renewable natural sources through chemical modification for the cycloaddition of terminal epoxides with CO2 is attractive but still challenging. Luffa sponge is a cheap, biodegradable, abundant and renewable source with netting-like three-dimensional porous structure. Herein, we report a simple and facile approach to the preparation of a series of novel luffa sponge-supported dendritic imidazolium ILs heterogeneous catalysts LS-DIL-(G1-G3) with high ionic density from natural luffa sponge fibers. LS-DIL-(G1-G3) were properly characterized by FT-IR spectroscopy, TGA, elemental analysis, and SEM. Among them, LS-DIL-G3 was found to exhibit the highest catalytic activity for the synthesis of cyclic carbonates under metal-free and solvent-free conditions. It was clearly observed that there was a positive dendritic effect on the yields of cyclic carbonates. Furthermore, a wide range of terminal epoxides was converted smoothly to the corresponding cyclic carbonates with high yields (84–99 %). LS-DIL-G3 could be reused for up to six consecutive runs without any obvious decline in catalytic activity. Importantly, the desired cyclic carbonates could be prepared on a multigram scale by using LS-DIL-G3 as a green heterogeneous catalyst.

1. Introduction CO2 is the most important carbon resource in the C1 family, but excessive CO2 in the atmosphere can seriously threaten the ecological environment [1]. As a renewable C1 resource, CO2 is abundant in resources, non-toxic and cheap [2,3]. Therefore, effective conversion and utilization of CO2 has attracted increasing attention of the international community. Up to now, cycloaddition of CO2 with terminal epoxides into value-added fine chemicals–cyclic carbonates under green and simple conditions is one of the most promising routes for effective CO2 utilization in a large scale, which is a perfect example of 100 % atom economy in green and sustainable chemistry [4–9]. Because of its excellent solubility, high boiling point and biodegradability, cyclic carbonates have been widely used as green polar solvents, cosmetic additives, electrolytes, and metal extractants, etc [10,11]. Furthermore, cyclic carbonate is an important chemical product that can be used as versatile building blocks in organic synthesis to produce phenolic resins, thermosetting resins, polycarbonates and thermal recording materials [12–14]. Generally, there are three conventional strategies to prepare cyclic carbonates through reaction of diols with phosgene [15], diols with

⁎

diphenyl carbonate [16], and CO2 with terminal epoxides [17], respectively (Scheme 1). Unfortunately, for the first two strategies, expensive diols and highly toxic phosgene were used. At the same time, these synthetic methods had poor atom economy. Compared with the methods (I) and (II), the synthesis of cyclic carbonates via cycloaddition reaction of terminal epoxides and CO2 has the following advantages, such as cheap raw materials, less by-products and 100 % atom economy, which meets the strategic requirements of sustainable development. Therefore, chemical cycloaddition of CO2 with terminal epoxides is very attractive because it represents a greener and safer alternative to the conventional synthesis of cyclic carbonates from diols/phosgene, and diols/diphenyl carbonate [18]. However, CO2 molecule is linear and it is composed of C]O bond. The bond length of C]O in CO2 is 116 pm, which is between C]O double bond (C]O bond length in acetaldehyde is 124 pm) and C^O triple bond (C^O bond length in CO molecule is 112.8 pm), indicating that it has a certain degree of triple bond characteristics. Therefore, CO2 has thermodynamic and kinetic stability and is difficult to be activated [19]. So far, the key issue for CO2 chemical transformation still is its activation. The cycloaddition reaction of CO2 with terminal epoxides is often

Corresponding author. E-mail address: [email protected] (X. Xiong).

https://doi.org/10.1016/j.jcou.2020.01.004 Received 17 October 2019; Received in revised form 26 December 2019; Accepted 3 January 2020 2212-9820/ © 2020 Elsevier Ltd. All rights reserved.

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2. Experimental section 2.1. Chemicals and reagents Loofa sponge was purchased from Xiamen agricultural trade market. 3-Aminopropyltrimethoxysilane (APTS), cyanuric chloride, 1-methylimidazole, ethylenediamine, diisopropylethylamine (DIPEA), 1,2-epoxy5-hexene, cyclohexene oxide, propylene oxide, styrene oxide, and the other epoxides were obtained from Aladdin Reagent Company. NaOH, dichloromethane, ethyl acetate, and tetrahydrofuran were obtained from Xiamen Lvyin Reagent Company. All chemicals were obtained commercially and used as received, without further purification. CO2 (99.999 %) was purchased from Xiamen Kongfente Gas Co., Ltd.

Scheme 1. Conventional methods for synthesis of cyclic carbonates.

2.2. Instruments

assisted by homogenous and heterogeneous catalysts, such as functional ILs [20–25], metal oxides [26,27], transition metal–salen complexes [28–31], metal organic frameworks [32–38], porous hypercrosslinked polymers [39–45], covalent organic frameworks [46–50], succinimideKI [51], N/B-doped carbon materials [52,53], supported polymeric IL [54], etc. However, the utilization of most of these catalysts suffers from one or more drawbacks such as low catalyst stability, harsh preparation conditions, use of expensive reagents, tedious preparation processes, and excessive use of fossil resource. Therefore, highly efficient, renewable, low-cost, biodegradable and easily prepared catalysts for cycloaddition reaction of CO2 with terminal epoxides are still highly desirable. Dendrimers have been used widely in the field of catalysis because of highly branched tridimensional structures, unique dendritic effects, and abundant peripheral groups [55,56]. Immobilization of dendrimer catalyst on support materials has been an emerging area, attracting more and more attention from chemists because of their easy and simple reusability and recoverability [57–59]. ILs have been widely used in green chemistry due to their unique properties like low vapor pressure, low flammability, good stability, etc. The adjustability of structure and properties is the most important characteristic of ILs. The physical and chemical properties of ILs can be adjusted by introducing different functional groups into anions or cations with different structures. Dendritic ILs (DILs), combining the properties of dendrimers and ILs, have been widely used in the fields of catalysis, transporters, adsorption of heavy metal cations [60,61]. Luffa sponges are the dried fruits of luffa cylindrica. As a cheap, biodegradable, abundant and renewable natural resource, luffa sponges, consisting of cellulose (60 %), hemicellulose (30 %), and lignin (10 %), with continuous, interconnected netting-like three-dimensional porous structure, are mainly used as adsorbent for the removal of organic pollutants and functional materials [62–65]. Herein, a series of novel luffa sponge-supported dendritic imidazolium ILs heterogeneous catalysts with high-density active sites LS-DIL-G1, LS-DILG2, and LS-DIL-G3 from natural luffa sponge fibers have been prepared and then employed as highly efficient and recyclable heterogeneous catalysts for the cycloaddition reaction of CO2 with terminal epoxides in solvent-free condition (Scheme 2). By combining with tetrabutylammonium bromide (TBABr), which has been served as a co-catalyst in chemical industry for cycloaddition of CO2 with terminal epoxides, the catalysis activity of luffa sponges supported DILs improved significantly. The desired cyclic carbonates could be obtained in high yields, and the heterogeneous catalytic system was stable and reusable for the cycloaddition reaction. Due to the presence of the high-density peripheral imidazolium functional groups on the surface of the biobased LS-DIL-G3 catalyst, LS-DIL-G3 exhibited the highest catalytic activity, and a positive dendritic effect on the yields of the cycloaddition of CO2 with terminal epoxides was observed.

The chemical cycloaddition progress of CO2 with terminal epoxides was monitored by TLC method. Fourier transform infrared (FT-IR) spectra (4000–400 cm−1) were registered on a Nicolet Nexus FT-IR instrument at 4 cm−1 resolution and 32 scans. Samples for FT-IR analysis were prepared using the KBr press disc method. 1H nuclear magnetic resonance (1H NMR) spectra were obtained on a Bruker AVANCE III-500 spectrometer in a magnetic field strength of 11.74 T. Derivative thermogravimetry (DTG) was run on TG instruments DTG-60H, samples were heated from 25 to 800 °C at a heating rate of 20 °C min-1 under an Ar atmosphere. SEM images were obtained using a SU8020 (Hitachi, Japan) scanning electron microscope. Nitrogen adsorption and desorption isotherms were measured at 77 K using a 3FLEX Gas Adsorption System, and the samples were degassed at 180 °C for 10 h before the measurements. Surface areas were calculated from the adsorption data using Brunauer − Emmett − Teller (BET) method. 2.3. Preparation of amino-modified luffa sponges (LS@APTS) For the preparation of LS@APTS, first, the luffa sponge fibers need to be activated. The luffa sponge fibers (5.0 g) were added to 100 mL of NaOH solution (1.0 M), and the mixture was refluxed at 100 °C for 2 h to improve its hydrophilicity. The obtained fibers were then washed many times with H2O until the pH of the wash water was neutral. The activated fibers were then dried in a vacuum oven at 60 °C for 8 h. Then, the activated luffa sponge fibers were functionalized with 3-aminopropyltrimethoxysilane (APTS) to prepare amino-modified LS@APTS. In detail, a mixture of activated luffa sponge fibers (4.0 g) and 3-aminopropyltrimethoxysilane (APTS, 8.0 mL) in 80 mL anhydrous EtOH was magnetically stirred at 80 °C for 24 h. Then, LS@APTS was filtered and washed with ethanol for two times and dried at 60 °C for 12 h. 2.4. Preparation of the first generation of bio-based heterogeneous catalyst LS-DIL-G1 A mixture of LS@APTS fibers (3.0 g), cyanuric chloride (CC) (3.0 g, 16.3 mmol) and diisopropylethylamine (DIPEA) (2.8 mL, 16.3 mmol) in THF (60.0 mL) was stirred at 25 °C for 12 h. The mixture was filtered and the resulting fibers LS@APTS/CC-1 were washed with EtOAc (10 mL × 3) and dried for 12 h. The LS@APTS/CC-1 (3.0 g) was suspended in THF (60.0 mL), 1-methylimidazole (2.7 g, 33.2 mmol) were added, and the mixture was magnetically stirred at 65 °C for 24 h. Then, LS-DIL-G1 was obtained and washed with EtOAc (10 mL × 3), and dried in in vacuum oven at 50 °C for 12 h. 2.5. Preparation of the second and third generation of bio-based heterogeneous catalysts LS-DIL-G2 and LS-DIL-G3 LS@APTS/CC-1 (3.0 g) in THF (60.0 mL) was added ethylenediamine (3.3 mL, 50.0 mmol) and DIPEA (8.38 mL, 50.0 mmol). The reaction mixture was magnetically stirred at 85 °C for 24 h. The 149

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Scheme 2. Synthesis of the LS-DIL-G1, LS-DIL-G2 and LS-DIL-G3.

ethylenediamine-modified LS@APTS/CC/EA-2 was filtered, washed with EtOAc (10 mL × 3), and dried in in vacuum oven at 50 °C for 12 h. A mixture of ethylenediamine-modified LS@APTS/CC/EA-2 (3.0 g), cyanuric chloride (CC) (6.0 g, 32.6 mmol) and diisopropylethylamine (DIPEA) (8.38 mL, 50.0 mmol) in THF (60 mL) was magnetically stirred at 25 °C for 12 h. Then, the cyanuric chloride-modified fibers (LS@APTS/CC-2) could be easily separated by filtration, and LS@APTS/CC-2 was washed with EtOAc (10 mL × 3) and dried in vacuo for 12 h. The LS@APTS/CC-2 (3.0 g) was suspended in THF (60 mL), 1-methylimidazole (2.7 g, 73.8 mmol) and DIPEA (16.8 mL, 100.0 mmol) were added, and the mixture was magnetically stirred at 90 °C for 24 h. The second generation of bio-based heterogeneous catalyst LS-DIL-G2 was washed with EtOAc (10 mL × 3), and dried in in vacuum oven at 50 °C for 12 h. The third-generation catalyst LS-DIL-G3 could be synthesized by the similar procedure mentioned above.

2.6. Representative procedure for cyclic carbonates synthesis catalysed by dendritic imidazolium IL catalysts All the cycloaddition reactions of CO2 with terminal epoxides were carried out in a Teflon-lined stainless-steel reactor (20.0 mL) equipped with a pressure gauge and a magnetic stirrer. In the typical procedure, desired amounts of terminal epoxide (10.0 mmol), dendritic imidazolium ILs catalyst, and the co-catalyst TBABr were placed into the reactor. Then the reactor was purged three times with CO2. Subsequently, the reactor was heated to the desired temperature and pressure. After completion of the reaction, the reactor was placed into iced water and then the excess CO2 was released slowly. EtOH (10 mL) was added into the reaction mixture, and then the dendritic imidazolium ILs catalyst was recovered by filtration. Subsequently, the obtained catalyst was washed two times with EtOH and dried under vacuum.

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Fig. 1. FT-IR spectra of LS (a), LS@APTS (b), LS@APTS/CC-1 (c), LS@APTS/CC/EA-2 (d), LS-DIL-G1 (e), LS-DIL-G2 (f), LS-DIL-G3 (g).

3. Results and discussion

successfully. With the increase of generation of dendritic molecules, the nitrogen contents of these dendritic imidazolium ILs catalysts increased obviously. Based on the elemental analysis results, it could be found that the densities of the active catalytic sites of LS-DIL-(G1-G3) were 1.21 mmol/g, 1.92 mmol/g, and 3.78 mmol/g, respectively. The SEM images of the luffa sponge and the dendritic imidazolium ILs catalysts LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3 were recorded to observe the morphological changes occurring on the surface of the luffa sponge fibers. As shown in Fig. 2a, the SEM image displayed the highly interconnected and unique 3D porous structure of natural loofa sponge. The SEM image of Fig. S1 in the supplementary material exhibited some scratches caused by the removal of the lignin and hemicellulose after NaOH treatment. After a multistep surface modification procedure, the surface morphology of dendritic imidazolium ILs catalysts changed obviously. As shown in Fig. 2b, a rough surface with irregular layered structure was obtained from LS-DIL-G1. More importantly, with the increase of generation of dendritic imidazolium ILs catalysts, the luffa sponge surface becomes more and more rough. As shown in Fig. 2c and d, a significantly rougher surface with irregular layered structure containing many small pores was obtained from LS-DIL-G2 and LS-DIL-G3. The surface porosity parameters of LS-DIL-G1, LS-DIL-G2, and LS-DILG3 are investigated by N2 adsorption analysis at 77 K. As shown in Fig. 3, the isotherms of the polymers exhibited type I character with high nitrogen gas uptake at low P/P0 values and large microporous structures. With the increase of generation of LS-DIL-(G1-G3), BET surface area also increases. Compared with LS-DIL-G1 (327 m2/g) and LS-DIL-G2 (354 m2/g), LS-DIL-G3 has the highest BET surface area (370 m2/g) at a low relative pressure (P/P0 < 0.001), which indicates the occurrence of further chemical modification on the surface of the luffa sponge fiber matrix, resulting in an increase in the surface area. The rougher surface could enhance catalytic activity for the cycloaddition of CO2 with epoxides. The decomposition behavior of the natural luffa sponge fibers and dendritic imidazolium ILs catalysts LS-DIL-G1, LS-DIL-G2 and LS-DILG3 has been compared in order to understand the effects of the grafted dendritic imidazolium molecules (Fig. 4). Fig. 4 shows the TGA of natural luffa sponge fibers (a), LS-DIL-G1 (b), LS-DIL-G2 (c) and LS-DILG3 (d). It was clear that the above four samples showed similar thermal stability. All the TGA curves contained two processes of weight loss. The first process between 25 and 100 °C displayed small weight loss of 6.5 %, which was attributed to the evaporation of physical adsorbed water from the samples. The next process between 230 and 367 °C revealed obvious decreases with large weight loss of about 63.1 % which could be attributed to the degradation of lignin, hemicellulose, etc.

3.1. Synthesis and characterization of luffa sponge supported dendritic imidazolium ILs catalysts For the synthesis of luffa sponge supported dendritic imidazolium ILs catalysts LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3, activated luffa sponge fibers were reacted with amino functionalized silane coupling agent APTS to produce the LS@APTS that needed for growing dendritic molecules. Reaction of amino functionalized LS@APTS with cyanuric chloride (CC) was carried out at room temperature to afford LS@APTS/ CC-1, which upon reaction with 1-methylimidazole prepared the firstgeneration dendritic imidazolium IL catalyst LS-DIL-G1. As shown in the Scheme 1, the similar procedure mentioned above was applied to increase the generation of dendritic molecules of LS@APTS/CC-2 on the surface of luffa sponge fibers for preparation of LS-DIL-G2 and LS-DILG3 (Scheme 1). These chemical modification processes were confirmed and monitored by FT-IR, elemental analysis, SEM, and TGA. The FT-IR spectra of luffa sponge, LS@APTS, LS@APTS/CC-1, LS@APTS/CC/EA-2, LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3 are shown in Fig. 1. The FT-IR spectra of luffa sponge, LS@APTS, LS@APTS/CC-1, and LS@APTS/CC/EA-2 are shown in Fig. 1A. Compared to the FT-IR spectrum of luffa sponge (Fig. 1a), some new absorption peaks at 1596 cm−1, 1059 cm−1, and 897 cm−1 appears (Fig. 1b), which belong to the bending vibrations of CeN bond and the stretching vibration of SieO bond on the surface of LS@APTS, respectively. It was proved that amino functionalized silane coupling agent APTS had been grafted onto the surface of activated luffa sponge fibers successfully. In the FT-IR spectrum of LS@APTS/CC-1 (Fig. 1c), two new absorption peaks at 1607 cm−1 and 1262 cm−1 appears, which belong to the bending vibrations of C]N bond and the stretching vibration of C-Cl bond of cyanuric chloride (CC) molecule. It was an obvious indication for the presence of cyanuric chloride fragment on the LS@APTS/CC-1. After replacing Cl with 1-methylimidazole, the characteristic absorption bands of the imidazolium ring C]N (1424, 1059, and 619 cm−1) appeared in the spectrums of LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3 (Fig. 1B e–g). The C, H and N contents in the luffa sponge-supported dendritic imidazolium IL catalysts LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3 were determined by elemental analysis. As shown in Table S1 in the supplementary material, the nitrogen contents of LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3 were found to be 1.699 wt %, 2.682 wt %, and 5.289 wt %, respectively. It can be further confirmed that these luffa spongesupported dendritic imidazolium ILs catalysts have been prepared 151

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Fig. 2. The SEM images of natural LS (a), LS-DIL-G1 (b), LS-DIL-G2 (c), and LS-DIL-G3 (d).

These obtained TGA results indicated that the dendritic imidazolium ILs catalysts LS-DIL-G1, LS-DIL-G2 and LS-DIL-G3 were thermally stable up to 230 °C, which could meet the application requirements in heterogeneous catalysis.

3.2. Catalytic performance of LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3 We then examined the catalytic activity of the various generations of dendritic imidazolium ILs catalysts LS-DIL-G1, LS-DIL-G2, and LSDIL-G3 by performing the cycloaddition reaction of CO2 with styrene epoxide (Table 1). First, we employed styrene oxide as a representative substrate to produce styrene carbonate with CO2 (1.0 MPa) at 90 °C in the presence of dendritic imidazolium IL catalyst LS-DIL-(G1, G2, G3) (200 mg), as shown in Table 1. Very low styrene carbonate yields were observed when the cycloaddition reaction was performed by using either dendritic imidazolium IL catalyst LS-DIL-(G1, G2, G3) or TBABr as the sole catalyst for the cycloaddition reaction with styrene epoxide (entries 1–4, Table 1). However, the cycloaddition reaction proceeded

Fig. 4. TGA of LS (a), LS-DIL-G1 (b), LS-DIL-G2 (c), and LS-DIL-G3 (d).

Fig. 3. N2 adsorption/desorption isotherms of LS-DIL-G1 (A), LS-DIL-G2 (B), and LS-DIL-G3 (C). 152

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from natural luffa sponge fibers (Fig. S2). The obtained results showed that H atoms on the imidazole rings played a key role in the chemical cyclization. When using LS-DIL-(G1-G3) as catalysts, the yields of chemical cyclization were 82 %, 90 % and 94 %, respectively. However, when using LS-BA-(G1-G3) as catalysts, the yields were 59 %, 66 % and 70 %, respectively.

Table 1 Cycloaddition of CO2 to styrene carbonate over dendritic imidazolium ILs catalysts LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3. Entry

Catalyst

Cat. (mg)

Time (h)

Temp (oC)

Isolated yield (%)a

1 2 3 4 5 6 7 8 9 10

None LS-DIL-G1 LS-DIL-G2 LS-DIL-G3 LS-DIL-G1 LS-DIL-G2 LS-DIL-G3 LS-DIL-G3 LS-DIL-G3 LS-DIL-G3

None 200 200 200 200 200 200 65 80 100

5 5 5 5 5 5 5 5 5 5

90 90 90 90 90 90 90 90 90 90

27 < 5b < 5b < 5b 82 90 94 77 85 94

3.3. Effects of reaction conditions The above results demonstrate that the catalyst LS-DIL-G3 with TBABr showed much higher catalytic activity than other tested catalysts. Thereafter, the catalyst LS-DIL-G3 was further investigated to get the optimized reaction conditions in subsequent reactions. The influence of temperature, amount of LS-DIL-G3 catalyst, reaction time, and solvent on the cycloaddition reaction was then examined more closely, and the results are shown in Fig. 5. As shown in Fig. 4a, the reaction temperature had a significant impact on the yield of styrene carbonate. The yield of styrene carbonate increased with the increasing reaction temperature in the range of 70–90 °C (94 %, 90 °C) and levels off upon further increasing temperature to 120 °C. As shown in Fig. 5(b), increasing the amount of catalyst from 50 to 100, 150, 200, and 250 mg increased the cycloaddition yield from 52 to 94, 93, 94 and 94, respectively. Obviously, the yield of styrene carbonate rose with increasing catalyst amount and reached 94 % in the presence of 100 mg of LS-DIL-G3. However, further increase in catalyst amount did not enhance the yield of styrene carbonate apparently. The impact of reaction time under the condition of 90 °C and 1.0 MPa of CO2 could also be obtained in Fig. 5c. The yield of styrene carbonate had a gradual increasing from 1 to 5 h (94 %, 5 h) and then remained almost unchanged all time. Clearly, styrene epoxide was almost completely converted into the desired carbonate within 5 h. In the

a Reaction conditions: styrene oxide (10.0 mmol), CO2 (1.0 Mpa), TBABr (0.9 mmol, 9.0 mol%); b In the absence of TBABr.

smoothly in all generations when using TBABr as co-catalyst. Styrene carbonate yields of 82–94 % were obtained by the catalyst LS-DIL-G1, LS-DIL-G2, or LS-DIL-G3 (200 mg) after 5 h by using TBABr as co-catalyst. The result showed that the catalytic activity increased with increasing generation of the dendritic imidazolium ILs. Therefore, a positive dendritic effect on the yields of the cycloaddition reaction of CO2 with terminal epoxides was observed. More importantly, the yield of the desired product could still reach 94 % when the amount of catalyst was reduced from 200 mg to 100 mg because of its high catalytic activity of LS-DIL-G3. Under the same experimental conditions, when the amounts of catalyst were reduced to 65 mg and 80 mg, the yields were 77 % and 85 %, respectively. In order to compare the catalytic activity of H atoms on secondary amines (NH) with that of H atoms on the imidazole rings for chemical cyclization, we synthesized the benzylamine (BA)-modified LS-based heterogeneous catalysts LS-BA-(G1-G3)

Fig. 5. Effect of reaction parameters on the CO2 cycloaddition reaction over LS-DIL-G3: (a) temperature on yield, (b) amount of catalyst on yield, (c) reaction time on yield, and (d) solvent on yield. 153

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Table 2 Cycloaddition of CO2 with various epoxides.a

Entry

Selectivity (%)

Isolated Yield (%)

1

> 99

99

2

> 99

96

3

> 99

98

4

> 99

99

5

> 99

98

6

> 99

84

7

> 99

99

8

> 99

99

9

> 99

98

10

> 99

94

a

Substrate

Product

Reaction conditions: epoxide (10.0 mmol), CO2 (1.0 Mpa), TBABr (0.9 mmol, 9.0 mol%), LS-DIL-G3 (100 mg).

and full chemoselectivity. Epichlorohydrin and glycidol could give excellent yields (99 % and 96 %) at 90 °C and 5 h (entries 1 and 2, Table 2). Furthermore, even when the relatively less reactive alkyl epoxides with long carbon chains and oxymethylene moiety were used, good to excellent yields were still obtained under mild conditions (84–99 %, entries 3–9, Table 2). The oxymethylene moiety facilitated nucleophilic attack of carbon atoms at the epoxide ring. In addition, even the inert terminal epoxides containing phenyl groups afforded high yield (94 %, entry 10, Table 2).

3.5. Reuse performance of the bio-based catalyst LS-DIL-G3 Fig. 6. The reusability results of the LS-DIL-G3.

Based on economic and practical consideration, the reusability of the heterogeneous catalyst is a critical essential to evaluate catalyst performance. To check the recovery capability of the dendritic imidazolium ILs catalyst LS-DIL-G3, the reaction of CO2 with styrene epoxide under the optimized reaction conditions was studied again. After completion of the cycloaddition reaction, EtOAc was added to the autoclave to dissolve the product, and then the heterogeneous catalyst LSDIL-G3 was separated by filtration. The separated catalyst was dried under vacuum before directly used for the next run. As shown in Fig. 6, this chemical process was repeated more than six times and all the cycloaddition reactions were well done without a very significant decrease in the reaction yields, which clearly showed the practical recyclability of the bio-based catalyst LS-DIL-G3. The stability is one of the main indicators of excellent performance of the immobilized ILs catalysts for CO2 chemical fixation [9]. The activity stability of catalyst is closely related to its recycling performance. Good cycle stability of the immobilized ILs catalysts can effectively reduce the cost and meet the production needs. However, some immobilized ILs catalysts could be recycled only 4 times due to their

next step, the solvent effect was investigated. As shown in Fig. 5(d), moderate to excellent yields were obtained under different solvent conditions such as glycerol, water, PEG-400, deep eutectic solvent (DES, choline chloride/urea = 1/2, mol/mol), and under solvent-free conditions. Fortunately, it was found that the solvent-free condition was more efficient than using organic solvents, with respect to the yield of the desired styrene carbonate. 3.4. Synthesis of different cyclic carbonates by using LS-DIL-G3 as catalyst Encouraged by the above promising results, we further evaluated the catalytic activity of the dendritic imidazolium ILs catalyst LS-DILG3 in heterogeneous cycloaddition of CO2 to other epoxides under the optimized conditions, and the obtained results are summarized in Table 2. As summarized in Table 2, all the electron-deficient and electron-rich terminal epoxides tested could be smoothly converted into the corresponding cyclic carbonates with moderate to excellent yields 154

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hemicellulose, etc. in luffa sponge fibers during recycling (Fig. 8). Fortunately, it did not affect its practicability when used at 90 °C. 3.6. Multigram-scale test At last, to prove the practical application value of the dendritic imidazolium ILs catalyst LS-DIL-G3, multigram-scale experiments on the cycloaddition of CO2 with styrene epoxide (10−80 mmol) were also conducted. As summarized in Table 3, the results showed that the cycloaddition reactions proceeded successfully without any extension of reaction time, to afford stationary product yields (78–95 %). From a combination of the above experimental results and the superiority of the dendritic imidazolium ILs catalyst LS-DIL-G3, it was obvious that the synthetic method was practically viable and the bio-based LS-DILG3 catalyst was very attractive to the chemical industry. 3.7. Plausible reaction mechanism Based on the experimental results we have obtained and the previous literature [67,68], a plausible reaction mechanism for this chemical cycloaddition reaction of CO2 to cyclic carbonates by using LSDIL-G3/TBABr as catalysts could be proposed (Fig. 9). It was found that the presence of the acidic proton of imidazolium ring was beneficial to the activation of epoxides, which resulted in the polarization of CeO bond and made the epoxide ring opening easier (step I). Subsequently, the ring of the epoxide opened via nucleophilic attack by Br− from the less hindered β-carbon atom of the epoxide (step II), and then the O− anion intermingled with C atom of the carbon dioxide to generate the new intermediate (step III). Lastly, cyclization via an intermolecular nucleophilic attack produced the desired product and led to the regeneration of the LS-DIL-G3 catalyst (step IV).

Fig. 7. FT-IR spectra of the fresh LS-DIL-G3 catalyst (a) and the reused LS-DILG3 catalyst after 6 times recycling (b).

3.8. Comparison of catalytic activity In order to examine the catalytic activity of the present bio-based catalyst LS-DIL-G3, we compared the catalytic activities of LS-DIL-G3 in the CO2 chemical fixation with other reported catalyst systems (Table 4). Obviously, LS-DIL-G3 presents promising features in terms of high activity, excellent yield, and slight mild reaction conditions, which maybe owing to the positive dendritic effect of the LS-DIL-G3 catalyst. Based on the obtained results, we are able to establish that LS-DIL-G3 is a type of practical and suitable bio-based heterogeneous catalyst for the CO2 chemical fixation.

Fig. 8. TGA of the fresh LS-DIL-G3 catalyst (a) and the reused LS-DIL-G3 catalyst after 6 times recycling (b).

4. Conclusions

Table 3 The results of scaled-up experiment.a Entrya

Scale (mmol)

Catalyst (g)

Isolated Yield (%)

Product (g)

1 2 3 4 5 6

10 20 30 40 60 80

0.1 0.2 0.3 0.4 0.6 0.8

94 92 95 88 87 78

0.9 3.0 4.7 5.8 8.6 10.2

In this paper, we report for the first time the preparation of luffa sponges supported dendritic ILs catalysts LS-DIL-G1, LS-DIL-G2, and LSDIL-G3 with high ionic density and different generations. It is clearly observed that there is a positive dendritic effect of the dendritic ILs catalysts LS-DIL-G1, LS-DIL-G2, and LS-DIL-G3 on the yields of the cycloaddition reaction of CO2 with terminal epoxides. Among them, LSDIL-G3 together with TBABr as a co-catalyst proved to be the optimal heterogeneous catalytic system with excellent catalytic activity for chemical fixation of CO2 into cyclic carbonates, giving good to excellent yields at 90 °C and 1.0 atm CO2. LS-DIL-G3 could be recycled up to six consecutive runs without any obvious decline in catalytic activity. Compared with the reported heterogeneous catalysts, LS-DIL-G3 showed several advantages, such as low cost, renewable nature, easy preparation, and higher catalytic potential for the cycloaddition reaction, which made it a viable candidate for industrial applications.

a Reaction conditions: CO2 (1.0 MPa), temperature 90 °C, 5 h, TBABr (9.0 mol%).

poor stability, which led to obvious waste of catalysts [66]. To evaluate the stability of the LS-DIL-G3 catalyst after 6 times recycling, the reused catalyst has been characterized by FT-IR and TGA. Compared with the FT-IR spectrum of fresh catalyst, there were no obvious changes in the infrared absorption peaks of the recycled catalysts (the characteristic absorption bands of the imidazolium ring C]N, 1424, 1059, and 619 cm−1) (Fig. 7). In addition, the thermal stability of the reused LSDIL-G3 catalyst after 6 times recycling was not as good as that of the fresh catalyst, which might be caused by the loss of lignin,

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to 155

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Fig. 9. Plausible reaction mechanism of LS-DIL-G3/TBABr catalyzed cycloaddition reaction of carbon dioxide with epoxide. Table 4 Comparison of catalytic activity of LS-DIL-G3 with several known heterogeneous catalysts. Entry

Catalyst

Condition

Recycle

Yield (%)

1 2 3 4 5 6

MIL-101-IMBr-6 IL-ZIF-90 DMAP-Zr-BDC-MOF Polymer supported ILs Ch-ILBr LS-DIL-G3

80 °C/4 h/0.8 MPa 120 °C/3 h/1.0 MPa 100 °C/12 h/1.0 MPa 100 °C/8 h/0.8MPa 120 °C/5 h/2.0MPa 90°C/5 h/1.0MPa

5 4 5 6 5 6

9369 9566 8470 8871 9672 94

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