Quaternary Ammonium Salt Integrated Heterogeneous

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Kinetics, Catalysis, and Reaction Engineering

Ionic Liquid/Quaternary Ammonium Salt Integrated Heterogeneous Catalytic System for Efficient Coupling of CO with Epoxides 2

Lijuan Shi, Shaobo Xu, Qiri Zhang, Tingting Liu, Bohui Wei, Yanfei Zhao, Lixin Meng, and Jun Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04108 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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Industrial & Engineering Chemistry Research

Ionic

Liquid/Quaternary

Ammonium

Salt

Integrated Heterogeneous Catalytic System for Efficient Coupling of CO2 with Epoxides Lijuan Shi,*,† Shaobo Xu,† Qiri Zhang,† Tingting Liu,† Bohui Wei,‡ Yanfei Zhao,‡ Lixin Meng,‡ and Jun Li*,‡ †Key

Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province,

Taiyuan University of Technology, Taiyuan 030024, P. R. China. E-mail: [email protected]. ‡Department

of Applied Chemistry, Yuncheng University, Yuncheng 044000, P. R. China. E-

mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: A heterogeneous catalytic system featuring multifunctional sites has been constructed through grafting amino-functional imidazolium ionic liquid (Si-IM-NH2) and quaternary ammonium salt (Si-TBAI) on mesoporous cage-like silica SBA-16 for the coupling of CO2 with epoxides. The resultant composites SBA-16@IM-NH2 and SBA-16@TBAI, even with much lower BET surface areas than SBA-16, exhibit highly increased CO2 adsorption capacities (i.e., up to 3.13 mmol·g-1 at 80 °C, 3.5 times that of SBA-16). Under the synergistic effect of the good CO2 enrichment ability, multiple active sites (I–, Cl–, –NH2 group and imidazolium ring), and open mesoporous channels, the catalytic system with Si-TBAI/Si-IM-NH2 molar ratio of 3 : 1 can catalyze CO2 into several kinds of cyclic carbonates with excellent yield and selectivity at low temperature and pressure (50 °C, 0.5 MPa) in the absence of any co-catalyst. This catalytic system can be easily recovered and recycled several times without loss of activity, making it a competitive catalyst for CO2 conversion. KEYWORDS: Ionic liquid, CO2 conversion, cyclic carbonates, mild condition, co-catalyst-free


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1. INTRODUCTION Excessive carbon dioxide (CO2) emission has been considered as one of the dominant factors to cause global warming over the past decades.1,2 On the other hand, as an abundant, inexpensive, and renewable C1 building block, CO2 can be converted into high value-added fine chemicals, such as methanol, formic acid, cyclic carbonates, dimethyl carbonate, and urea derivatives.3−7 In particular, cyclic carbonates derived from the 100% atom-economical coupling of CO2 and epoxides are considered as promising target molecules, which have been widely used as synthetic organic intermediates, electrolytes, precursors for biomedical applications, raw materials for engineering plastics, etc.7,8 Despite numerous catalytic systems including homogeneous and heterogeneous ones have been developed,9-18 the coupling of CO2 and epoxides under mild conditions is still a great challenge in terms of the high thermodynamic and kinetic stability of CO2. To afford high yields of cyclic carbonates under mild conditions, organic ammonium salts such as tetrabutyl ammonium iodide (TBAI) acting as homogeneous co-catalysts are usually required.13−18 However, the drawbacks of homogeneous co-catalysts like increased cost and cumbersome purification/regeneration steps limit the wide applications of the relevant catalytic systems.19−21 The rational integration of catalyst and co-catalyst into one heterogeneous system is an alternative strategy to circumvent this drawback, which is also a promising approach to achieve excellent activity and good recyclability in the coupling of CO2 with epoxides under mild conditions.22−25 Ionic liquids (ILs) have attracted increasing attentions in the field of CO2 capture and conversion because of their unique properties, including negligible vapor pressures, high thermal stabilities, and most importantly, tunable functionalizations.26−28 N-heterocyclic carbene (NHC)


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based ILs, especially the functionalized ones containing both Lewis acid and base sites, could activate CO2 molecules and simultaneously interact with other substrates, thus showing attractive activity for CO2 capture and/or conversion.23,29−32 However, the inherent limitations of homogeneous ILs for industrial scale up like high viscosity, high economic cost, and intricate separation, led to great efforts to immobilize ILs into porous solid supports.33−41 Despite various supports, like silica,33−35 metal-organic frameworks (MOFs),36−38 and polymers39−41 have been developed, it remains challenging to develop IL-based heterogeneous catalysts for the CO2 cycloaddition reaction in the absence of co-catalysts at low temperature and pressure.38 Herein, a heterogeneous catalyst/co-catalyst integrated catalytic system has been successfully constructed through grafting amino-functional imidazolium IL (Si-IM-NH2) and quaternary ammonium salt (Si-TBAI) on mesoporous cage-like material SBA-16. SBA-16, as a typical mesoporous silica, has been considered as a competitive support due to its inherent advantageous properties, such as regular mesoporous structures, three-dimensionally interconnected pores, high thermal stabilities, easily modified surfaces as well as abundant Si–OH groups that might function as a hydrogen donor facilitating epoxide activation.33−35,42 Of great interest, the resultant composites (SBA-16@IM-NH2 and SBA-16@TBAI), even with much lower BET surface areas than SBA-16, exhibit highly increased CO2 adsorption capacities, which is beneficial for CO2 activation. Under the synergistic effect of the good CO2 enrichment ability, multiple active sites (I–, Cl–, –NH2 group and imidazolium ring) and open mesoporous channels, the catalytic system exhibits excellent catalytic activity and recycling efficiency for the CO2 coupling reaction under mild conditions in the absence of any co-catalyst.

2. EXPERIMENTAL SECTION


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2.1.

Materials.

Pluronic

P123

(EO20PO70EO20),

Pluronic

F127

(EO106PO70EO106),

epichlorohydrin (ECH, 99%), 1,2-epoxyoctane (97%), 1,2-epoxydecane (97%), cyclohexene oxide (98%), styrene oxide (97.5%), (R)-glycidyl phenyl ether (95%), tetraethyl orthosilicate (TEOS,

99%),

N-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole

aminopropyltriethoxysilane

(98%)

were

purchased

from

J&K

(99%) Scientific

and Ltd.

33-

Chloropropylamine hydrochloride (98%), hydrochloric acid (36%) and 1-butyl iodide (99%) were the products of Shanghai Jingchun Industry Co. CO2 (99.99%) and N2 (99.99%) were purchased from Taiyuan Steel Co. 2.2. Synthesis of SBA-16.42 F127 (3.71 g) and P123 (0.6 g) were dissolved in a solution that contains 150 mL of deionized water and 26.25 g concentrated hydrochloric acid (36%), and maintained at 35 °C for 4 h with stirring. Then, TEOS (14 mL) was dropwise added into the solution. After being stirred for 40 min, the resultant suspension was placed in a Teflon-lined autoclave and kept at 35 °C for 24 h. Then, the temperature was raised up to 100 °C and held for 32 h. After the hydrothermal treatment, the solid was isolated by filtration and then thoroughly washed with deionized water and ethanol. The obtained solid was dried at 100 °C for 24 h, following by calcination at 552 °C for 10 h. 2.3.

Synthesis

of

N-3-(3-Trimethoxysilylpropyl)-3-Aminopropyl

Hydrochloride

Imidazolium Chloride (Si-IM-NH2∙HCl).43 3-Chloropropylamine hydrochloride (3.6 mmol, 0.47 g) and N-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole (3 mmol, 0.82 g) were mixed in 25 mL of dry acetonitrile, and the mixture was refluxed at 78 °C for 48 h under N2 atmosphere. The mixture was cooled down to room temperature, evaporated to remove acetonitrile, and then washed with n-pentane three times. After being dried under vacuum for 24 h at 60 °C, a yellow viscous liquid was obtained and denoted as Si-IM-NH2∙HCl (see Scheme 1a). 1H NMR (400


5


MHz,

DMSO-d6,

δ,

ppm):

3.13−3.63

(8H,

m,

NCH2CH2N,

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SiCH2CH2CH2N,

NCH2CH2CH2NH2·HCl), 3.78 (6H, q, CH3CH2OSi), 2.83 (2H, m, NCH2CH2CH2NH2·HCl), 1.59−1.96 (4H, m, SiCH2CH2CH2N, NCH2CH2CH2NH2·HCl), 1.17 (9H, t, CH3CH2OSi), 0.62 (2H, m, SiCH2CH2). 2.4. Synthesis of N,N,N-Tributyl-3-(Triethoxysilyl)Propan-1-Ammonium Iodide (SiTBAI).44 The reaction of 3-aminopropyltriethoxysilane (3 mmol, 0.66 g) with 1-butyl iodide (15 mmol, 2.76 g) was performed at 78 °C for 48 h under N2 atmosphere in dry acetonitrile (25 mL) in the presence of potassium carbonate (15 mmol, 2.07 g). After being cooled down to room temperature, dichloromethane was added to the reaction mixture, which was separated from potassium carbonate via filtration under a N2 stream. After evaporation of dichloromethane and acetonitrile under vacuum for 48 h at 50 °C, a kind of yellowish sticky liquid was obtained and denoted as Si-TBAI (see Scheme 1b). 1H NMR (CDCl3, δ, ppm): 0.73−0.75 (m, 2H, CH2Si), 0.94−0.99 (m, 9H, CH3(CH2)3N) 1.18−1.23 (m, 2H, NCH2CH2CH2Si), 1.33−1.41 (m, 9H, CH3CH2OSi), 1.60−1.68 (m, 6H, CH3CH2(CH2)2N), 1.69−1.79 (m, 6H, CH3CH2CH2CH2N), 3.26−3.40 (m, 8H, CH2N), 3.49−3.53 (m, 6H, CH3CH2OSi). 2.5. Immobilization of Si-TBAI/Si-IM-NH2∙HCl on SBA-16. SBA-16 (1 g, dried under vacuum at 70 °C for 12 h) was uniformly dispersed into 50 mL of dry acetonitrile, and then the acetonitrile solution of Si-TBAI or Si-IM-NH2∙HCl (3 mmol) was added. The resulting mixture was refluxed at 80 °C for 48 h under nitrogen atmosphere. After being cooled down to room temperature, the solid material was isolated by filtration and repeatedly washed with ethanol. Then, Si-TBAI grafted SBA-16 was dried under vacuum for 12 h at 70 °C and denoted as SBA-16@TBAI (see Scheme 1c).

13C

CP-MAS NMR (solid state, δ, ppm): 8 (a:

SiCH2CH2CH2N), 12 (b: CH3CH2OSi), 13 (c: N(CH2)3CH3), 16 (d: SiCH2CH2CH2N), 19 (e:


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NCH2CH2CH2CH3), 24 (f: NCH2CH2CH2CH3), 58 (g: NCH2CH2CH2CH3, SiCH2CH2CH2N, CH3CH2OSi).

29Si

CP-MAS NMR (solid state, ppm): Qm = Si(OSi)m(OH)4-m, m = 2−4; –101

(Q3), –110 (Q4); Tn = RSi(OSi)n(OH)3-n, n = 2,3; –57 (T2), –68 (T3). Si-IM-NH2∙HCl grafted SBA-16 was dispersed into 30 mL of ethanol at room temperature, and the pH value of the mixture was adjusted to ~ 8 by the addition of KHCO3 in order to move HCl. The solid sample was isolated by filtration and washed with ethanol and water in sequence. After being dried under vacuum for 12 h at 70 °C, IL immobilized SBA-16 was obtained (denoted as SBA-16@IM-NH2, Scheme 1c).

13C

CP-MAS NMR (solid state, δ, ppm): 10 (a:

SiCH2CH2), 19 (b: CH3CH2OSi), 21 (c: SiCH2CH2CH2N, NCH2CH2CH2NH2), 38 (d: NCH2CH2CH2NH2), 49 (e: NCH2CH2N, SiCH2CH2CH2N, NCH2CH2CH2NH2), 60 (f: CH3CH2OSi), 165 (g: NCH2N).

29Si

CP-MAS NMR (solid state): Qm = Si(OSi)m(OH)4-m, m =

2−4; –102 (Q3), –110 (Q4); Tn = RSi(OSi)n(OH)3-n, n = 2,3; –57 (T2), –65 (T3).

Scheme 1. Synthetic routes of Si-IM-NH2∙HCl (a), Si-TBAI (b), and SBA-16@IM-NH2 and SBA-16@TBAI (c).


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2.6. CO2 Adsorption Performance. The CO2 adsorption performance was carried out in a fixed-bed flow system shown in Figure S1. In a typical adsorption process, 0.3 g sample was placed in the adsorption column, and the column was heated to 140 °C for 1 h in N2 at a flow rate of 85 mL·min-1. A mixture of CO2 and N2 (ϕCO2 = 15%, v/v) at a flow rate of 118 mL·min-1 flowed through the analytical system, and the initial concentration of CO2 was measured by the gas analyzer. Then, the column was cooled to the test temperature and the gas mixture above was introduced into the column. When the effluent CO2 concentration reached the initial concentration of CO2, the adsorption process was terminated. After adsorption, the column was heated to 140 °C for 1 h in N2 at a flow rate of 85 mL·min-1 to implement desorption. The CO2 adsorption capacity, q (mmol·g-1 SBA-16) was calculated using the equation (Equation S1). To ensure the reliability of CO2 adsorption capacities, the adsorption experiments were repeated five times for each sample. 2.7. Coupling Reactions of CO2 and Epoxides. The coupling reactions of CO2 and epoxides were carried out in a 30 mL of high-pressure stainless-steel autoclave equipped with pressure gauge and temperature sensor. Epoxides (25 mmol) and an appropriate amount of catalysts were put into the reactor. The reactor was purged with a CO2 flow to remove air and then pressurized with CO2 up to a certain pressure. Then the autoclave was heated to a selected temperature and stirred for a desired reaction time. After the reaction, the autoclave was cooled down to room temperature, and the unreacted CO2 was vented out. The product was separated by centrifugation and analyzed by

1H

NMR. After each catalytic cycle, the catalyst was recovered by

centrifugation, washed with methanol, and dried in vacuum at 70 °C for the next run. 2.8. Characterizations. 1H NMR spectra were recorded on a Advance plus 400 MHz spectrometer (Bruker, Germany). Solid state NMR spectra (13C CP-MAS NMR and


29Si

MAS

8

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NMR) were obtained on a Advance plus 600 MHz spectrometer (Bruker, Germany). Scanning electron microscopy (SEM) was performed on a JSM-7100F (JEOL, Japan) operated at an acceleration voltage of 10.0 kV. Transmission electron microscopy (TEM) was performed on a JEM-2010 (JEOL, Japan). Energy-dispersive X-ray spectroscopy (EDX) was performed on a Inca X-Max 50 spectrometer (Oxford, England). Thermogravimetric analysis (TGA) was performed on a STA449F5 thermal analyzer (Netzsch, Germany) under a flowing N2 stream at a heating rate of 10 °C /min from 100 to 900 °C. N2 adsorption/desorption measurements were performed at 77 K on an ASAP2020C volumetric adsorption analyzer (Micromeritics, USA). The samples were degassed at 120 °C for 6 h before measurement.

Figure 1. SEM images of SBA-16@IM-NH2 (a) and SBA-16@TBAI (e); EDX mappings of silicon (b) and chlorinum (c) for SBA-16@IM-NH2; EDX mappings of silicon (f) and iodine (g) for SBA-16@TBAI; TEM images of SBA-16@IM-NH2 (d) and SBA-16@TBAI (h).

3. RESULTS AND DISCUSSION The microstructure and elemental constituent of the functionalized SBA-16 materials were firstly observed with SEM, TEM and EDX mapping. Both the resultant SBA-16@IM-NH2 and SBA-


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16@TBAI samples show regular and large-sized spheres (Figure 1a and e), which are highly consistent with the morphology of SBA-16.45 The EDX mappings of Cl– and I– that are derived from Si-IM-NH2 and Si-TBAI verify that Si-IM-NH2 and Si-TBAI are dispersed evenly throughout the whole spheres (Figure 1c and g). It is notable that the mesoporous channels with highly uniform arrays of SBA-16 (see Figure S2) are well retained after the introduction of SiIM-NH2 and Si-TBAI (Figure 1d and h), which is beneficial for the mass transfer in the coupling reaction of CO2 with epoxides.

Figure 2. 13C CP-MAS (a, c) and 29Si CP-MAS (b, d) NMR spectra of SBA-16@IM-NH2 (a, b) and SBA-16@TBAI (c, d). To further confirm the successful graft of Si-IM-NH2 and Si-TBAI on SBA-16, the solid state 13C

CP-MAS NMR and 29Si MAS NMR studies were performed (Figure 2). As shown in Figure

2a and c, the signals on the 13C CP-MAS NMR spectra are in good agreement with the chemical shifts of carbon atoms on both the Si-IM-NH2 and Si-TBAI.43,44 Meanwhile, two peaks centered at –102 and –110 ppm can be clearly observed in both the

29Si

MAS NMR spectra of SBA-

16@IM-NH2 and SBA-16@TBAI (Figure 2b and d), which are assigned to the silicon atoms [Q3:


10

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Si(OSi)3(OH); Q4: Si(OSi)4]. It is noted that two peaks assigned to organosiloxane [Tn = RSi(OSi)n(OH)3-n, n = 2,3; T3 at –65 ppm, T2 at –57 ppm] can be observed, indicating that SiIM-NH2 and Si-TBAI have been immobilized on SBA-16 through the condensation reactions of the surface silanols and trimethoxysilyl derivatives.

Figure 3. TGA mass loss plots of SBA-16, SBA-16@IM-NH2 and SBA-16@TBAI; Insert: DTG curves of SBA-16, SBA-16@IM-NH2, and SBA-16@TBAI. The loading capacities of Si-IM-NH2 and Si-TBAI on SBA-16 were investigated by TGA (Figure 3). SBA-16 exhibits a weight loss of about 2.6 wt% in the temperature range of 125 – 300 °C, which is probably due to the decomposition of the residual templating agent. The weight losses of SBA-16@IM-NH2 and SBA-16@TBAI are about 15.9% and 20.3% respectively when temperature is increased to 900 °C, which is assigned to the decomposition of Si-IM-NH2/SiTBAI and the residual templating agents. The contents of Si-IM-NH2 and Si-TBAI were then calculated to be about 0.57 mmol∙g-1 and 0.46 mmol∙g-1, respectively. The lower content of SiTBAI than that of Si-IM-NH2 is probably ascribed to the larger steric effect of quaternary ammonium cations (TBA+). Stability of the functionalized SBA-16 materials is one of the key factors for their practical applications. The maximum decomposition temperatures of SBA-


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16@IM-NH2 and SAB-16@TBAI are 313 and 249 °C respectively, far higher than the catalytic temperature.

Figure 4. N2 adsorption-desorption isotherms (a) and pore-size distributions (b) of SBA-16 and IL-functionalized SBA-16. To evaluate the pore structures of the functionalized SBA-16 materials, N2 adsorptiondesorption performances were investigated (Figure 4). SBA-16 shows a type IV isotherm with an H2 hysteresis loop at relatively high P/P0 pressures (0.4 – 0.8), which is the feature of mesoporous structures.42,46 Similar isotherms are exhibited by SBA-16@IM-NH2 and SBA16@TBAI, suggesting the mesoporous cage-like structure of SBA-16 is maintained well, which is beneficial for mass transport. Obvious decreases of Brunauer−Emmett−Teller (BET) surface area, pore volume and pore size are exhibited after grafting Si-IM-NH2 and Si-TBAI (see Table


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1), indicating that both the Si-IM-NH2 and Si-TBAI have successfully entered the mesoporous channels. Table 1. Textural Parameters of SBA-16 and Functionalized SBA-16. Samples

SBET (m2·g-1)

V (cm3·g-1)

Pore size (nm)

SBA-16 694 0.76 SBA-16@IM-NH2 478 0.47 SBA-16@TBAI 329 0.29 aThe content of Si-IM-NH and Si-TBAI was determined by TGA. 2

5.5 4.7 3.7

Si-IM-NH2/Si-TBAI content (mmol·g-1)a 0 0.57 0.46

To estimate the CO2 activation abilities of the functionalized SBA-16 materials, CO2 adsorption performances from a mixture of CO2 and N2 (ϕCO2 = 15%, v/v) were measured (Figure 5). Of great surprising, both the SBA-16@IM-NH2 and SBA-16@TBAI, even with much lower BET surface areas than SBA-16, exhibit highly increased CO2 adsorption capacities compared to SBA-16. At 35 °C, for instance, the CO2 adsorption capacities of SBA-16@IM-NH2 and SBA16@TBAI are 1.68 mmol·g-1 and 2.16 mmol·g-1 respectively, much higher than that of SBA-16 (1.08 mmol·g-1 at 35 °C), which should be ascribed to the chemical adsorption ability of Si-IMNH2 and Si-TBAI. For SBA-16@IM-NH2, the enhanced CO2 uptake should be mainly attributed to the CO2-philic ability of –NH2 group. Very recently, we have proved by 13C NMR that the – NH2 group in imidazolium IL can interact with CO2 to generate an ammonium carbamate,47 which should be beneficial for CO2 activation.30,48 For SBA-16@TBAI, the outstanding CO2 adsorption capacity is probably due to the binding affinity between TABI and CO2. It has been widely proved that in the coupling reaction of CO2 with epoxides, I– in TBAI prefers to induce a nucleophilic attack on the nonsubstituted carbon of the epoxide ring, thus promoting the ringopening.49−51 The high CO2 adsorption capacity of SBA-16@TBAI in this work further verifies that I– can also interact with the charge-positive carbon atom of CO2.52 To validate this assumption, the temperature dependence of the CO2 adsorption performance was then detected.


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For SBA-16, the CO2 adsorption capacity is decreased with increasing temperature from 35 to 100 °C, which is the typical performance of physical adsorption.53,54 As a sharp contrast, the CO2 adsorption capacity of SBA-16@IM-NH2 increases significantly when the temperature increases from 35 to 80 °C, and then decrease slightly with further increasing temperature, which is probably due to the competitive effect of CO2 diffusion and chemisorptions.53-56 It is known from the N2 adsorption/desorption results that Si-IM-NH2 and Si-TBAI are not only loaded on the external surface of SBA-16 but also dispersed in the pores and internal surface. The initial increase in temperature facilitates the CO2 diffusion into the interior of SBA-16 by overcoming the kinetic barrier,55 thus leading to a significant enhancement of the total number of the accessible sorption sites at 80 °C. However, the reaction of the amino group on IL and CO2 is exothermic,56 so an increase in temperature above 80 °C causes a converse shift in the equilibrium, and thus the adsorption capacity decreases. Similar temperature-dependence has been exhibited by SBA-16@TBAI, further verifying the strong binding interaction between TBAI and CO2, which should highly facilitate the activation of CO2 in the coupling reaction of CO2 with epoxides.


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Figure 5. CO2 adsorption isotherms (a) and capacities (b) of SBA-16, SBA-16@IM-NH2 and SBA-16@TBAI from a gas mixture with N2 (ϕCO2 = 15%). Temperature in (a) is 35 °C. The excellent cyclic stability of an adsorbent is crucial for its practical application. The cyclic performances of SBA-16@IM-NH2 and SBA-16@TBAI were investigated by performing ten cycles of CO2 adsorption at 35 oC and the results are listed in Figure S3. After adsorbed CO2 from the gas mixture with N2, the adsorbents were regenerated by heating up to 140 oC in N2 flow for the next cyclic experiment. It can be seen that similar adsorption process and slight variation in adsorption capacity are present in each cycle. After ten cycles, more than 91% and 93% of the initial CO2 uptakes can be retained by SBA-16@IM-NH2 and SBA-16@TBAI, respectively, implying the good cyclic stability of the IL-functionalized SBA-16 materials.


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Given the mesoporous cage-like structure and good CO2 enrichment ability, the catalytic activities of SBA-16@IM-NH2 and SBA-16@TBAI in the coupling reaction of CO2 with epoxides were then verified. For the coupling reaction of CO2 with epichlorohydrin (ECH), SBA-16@IM-NH2 and SBA-16@TBAI afford 3.2% and 68.0% yields of chloropropene carbonate (CPC) under a mild reaction condition (temperature: 50 °C; CO2 pressure: 0.5 MPa), and as a contrast, SBA-16 is failed to afford any product (see Figure 6). Expectedly, SBA16@TBAI exhibits highly increased catalytic activity compared to SBA-16, which is consistent with its good CO2 adsorption performance.

Figure 6. CPC yields catalyzed by various catalytic systems; Reaction condition: reaction temperature 50 °C, CO2 pressure 0.5 MPa, reaction time 48 h. All the selectivities of CPC are ≥ 99%. To clarify the synergistic effect of SBA-16@TBAI and SBA-16@IM-NH2 in the coupling reaction of CO2 with epoxides, catalytic systems with different SBA-16@TBAI/SBA-16@IMNH2 molar ratios were constructed. The CPC yield is highly enhanced with an increasing molar ratio of Si-TBAI/Si-IM-NH2, and an excellent CPC yield of 98.5% is achieved when the molar ACS Paragon Plus Environment

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ratio of Si-TBAI/Si-IM-NH2 reaches 3 : 1 (see Figure 6), indicating the role of TBAI is critical in the CO2 cycloaddition reaction. It is noted that the catalytic system with Si-TBAI/Si-IM-NH2 molar ratio of 3 : 1 can still afford a desirable yield even at room temperature (52.9%, Figure S4) or at a lower CO2 pressure (89.8% at 0.3 MPa), further verifying the excellent activity of the catalytic system. To assess the reusability and stability of the catalytic system, a cycling experiment of the coupling reaction of CO2 and ECH was conducted. After five cycles, as shown in Figure 7, the catalyst is still highly active with outstanding CPC yield (96.8%) and selectivity (≥ 99%). SEM and TEM observations further verified that the spherical structure and mesoporous channel of the catalyst are well retained after five cycles (Figure S5). Moreover, no obvious mass loss was observed in each cycle and 98.6 wt% of the initial catalyst was recovered after five cycles, verifying the good stability of the IL-functionalized SBA-16 materials. The high cyclic stability and easy recyclability make the novel composite a competitive industrial catalyst for CO2 conversion.

Figure 7. Recycling of the catalyst in the coupling reaction of CO2 and ECH. Reaction condition: CO2 pressure 0.5 MPa, reaction temperature 50 °C, reaction time 48 h. All the selectivities of CPC are ≥ 99 %.


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The catalytic performances of the IL-based catalytic system in the coupling reactions of CO2 with various epoxides were further investigated (Table 2). Most of the examined substrates are converted to the corresponding cyclic carbonates with good to excellent yields. Even cyclohexene oxide (entry 5), which is known as a more difficult substrate for the coupling reaction with CO2,22 is converted to the corresponding carbonate in 35.0% yield. Given the CO2 adsorption and catalytic results above, the active sites in the SBA16@TBAI/SBA-16@IM-NH2 catalytic system can be deduced. As a critical active site, I– can nucleophilically attack the less sterically hindered carbon atom of epoxide ring as well as the charge-positive carbon atom of CO2, which facilitates the ring-opening of epoxide and the subsequent CO2 insertion.49−52 Simultaneously, the –NH2 group can react reversibly with CO2 to generate an ammonium carbamate that can achieve CO2 activation.47,57 Meanwhile, CI– and imidazolium ring can also allow the activation of epoxide via nucleophilic attack and hydrogen bonding interaction, respectively.51 Under the synergistic effect of the active sites above, the mechanism probably involves the following four steps (Scheme 2): (1) As a the rate-determining step, I– and CI– nucleophilically attack the less sterically hindered carbon atom of the epoxide ring to form the ring-opened intermediate, and simultaneously, the epoxide ring can also be activated by the coordination of the protons in the –NH2 group and imidazolium ring through hydrogen bonding; (2) the –NH2 group reacts reversibly with CO2 to generate the ammonium carbamate, and meanwhile, I– and CI– nucleophilically attack the carbon atom of CO2 to facilitate CO2 activation; (3) the activated CO2 is then inserted into the C–I and/or C–Cl bonds of the ringopened intermediate, producing a new intermediate; (4) cyclic carbonate is then generated via the ring-closing of the intermediate, and meanwhile, the catalyst is regenerated.


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Scheme 2. Possible reaction mechanism for the coupling reaction of CO2 and epoxides catalyzed by SBA-16@TBAI/SBA-16@IM-NH2.

To further demonstrate the superiority of the SBA-16@TBAI/SBA-16@IM-NH2 catalyst, we compared the its catalytic performance with a series of reported catalysts (see Table S1). It is known that numerous catalytic systems including homogeneous and heterogeneous ones, such as ILs (entries 1-4 in Table S1),23,30-32 supported ILs (entries 6-10),33,35,36,38,39 metal halides (entries 11 andand 12),18,19 metal-porphyrin complexes (entry 13),15 metal(salen) complexes (entry 14),2022

conjugated microporous polymers (CMP) (entry 16),13 metal−organic frameworks (MOFs,

entries 17 and 18),16,18 and covalent organic frameworks (COFs, entry 19 and 20),17,24 have been developed for the coupling of CO2 and epoxides. It can be seen that relatively high temperature (≥ 100 °C) and pressure (≥ 1.5 MPa) or organic ammonium salts such as TBAB acting as homogeneous co-catalysts are generally required to afford high yields of cyclic carbonates, which would cause increased cost and/or cumbersome purification/regeneration steps. Just few kinds of catalysts bearing multifunctional sites exhibit excellent catalytic activity under mild


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conditions without the help of co-catalysts (entries 4, 10 and 19).23,24,38 Under the synergistic effect of the good CO2 enrichment ability, multiple active sites (I–, Cl–, –NH2 group and imidazolium ring), and open mesoporous channels, the catalytic system prepared in this work can catalyze CO2 into several kinds of cyclic carbonates with excellent yield and selectivity at relatively low temperature and pressure (50 °C, 0.5 MPa) in the absence of any co-catalyst. Considering the co-catalyst- and metal-free nature, low IL consumption, mild catalytic conditions and efficient recyclability comprehensively, one can see that the SBA16@TBAI/SBA-16@IM-NH2 catalyst can act as a competitive candidate for CO2 conversion. Table 2. Coupling Reactions of CO2 with Epoxides Substituted with Different Functional Groups Catalyzed by SBA-16@TBAI/[email protected]

Entry

Substrates

Products

Yieldb (%)

1

92.5

2

90.3

3

95.0

4

85.6

5

35.0

aReaction

conditions: 25 mmol epoxides, CO2 pressure 0.5 MPa, reaction temperature 50 °C, reaction time 48 h, molar ratio of Si-TBAI/Si-IM-NH2 is 3 : 1; bYields were determined by 1H NMR. All the selectivities of cyclic carbonates were ≥ 99 %.

4. CONCLUSIONS In summary, a heterogeneous catalyst/co-catalyst integrated catalytic system has been constructed through grafting amino-functional imidazolium IL (Si-IM-NH2) and quaternary ammonium salt (Si-TBAI) on SBA-16 for the coupling of CO2 with epoxides under mild and co-


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catalyst-free conditions. Due to the CO2-philic ability of I– and –NH2 group, the resultant composites SBA-16@TBAI and SBA-16@IM-NH2, exhibit outstanding CO2 adsorption capacities. Especially, the adsorption capacity of SBA-16@TBAI can achieve 3.13 mmol·g-1 at 80 °C, which is 3.5 times that of SBA-16. Under the synergistic effect of the good CO2 enrichment ability, multiple active sites (I–, Cl–, –NH2 group and imidazolium ring) and open mesoporous channels, the composite with Si-TBAI/Si-IM-NH2 molar ratio of 3 : 1 can catalyze CO2 into several kinds of cyclic carbonates with excellent yield and selectivity at 50 °C and 0.5 MPa. The catalytic system featuring solvent- and co-catalyst-free nature, low IL consumption, high catalytic activity and efficient recyclability has great potential for CO2 utilization. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03404. Experimental details for CO2 adsorption performance and supplemental characterization and catalytic results. (PDF) AUTHOR INFORMATION Corresponding Author E-mail: [email protected] ORCID Lijuan Shi: 0000-0001-5829-0766 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21403151, No. 21576230), the Natural Science Foundation for Young Scientists of Shanxi Province (No. 201601D021045), and the Personnel Training Project for Joint Postgraduate Training Base of Shanxi Province (No. 2017JD17). J. Li gratefully acknowledges the financial support from the Doctoral Research Project of Yuncheng University (No. YQ-2017008). ABBREVIATIONS CO2

carbon dioxide

Cl

chlorinum

CPC

chloropropene carbonate

CP-MAS NMR

cross polarization/magic angle spinning nuclear magnetic resonance

ECH

epichlorohydrin

EDX

Energy-dispersive X-ray spectroscopy

F127

EO106PO70EO106

I

iodine

IL

ionic liquid

MOFs

metal-organic frameworks

MAS NMR

magic angle spinning nuclear magnetic resonance

NHC

N-heterocyclic carbene

NH2

amino


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P123

EO20PO70EO20

SBET

Brunauer−Emmett−Teller surface area

SEM

scanning electron microscopy

Si

silicon

Si-IM-NH2

N-3-(3-trimethoxysilylpropyl)-3-aminopropyl imidazolium chloride

Si-TBAI

N,N,N-tributyl-3-(triethoxysilyl)propan-1-ammonium iodide

TEM

transmission electron microscopy

TEOS

tetraethyl orthosilicate

TGA

thermogravimetric analysis

v

volume

ϕCO2

volume fraction of CO2 in the mixture of CO2 and N2

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Synopsis

A heterogeneous ionic liquid/quaternary ammonium salt integrated catalytic system has been constructed for the coupling of CO2 with epoxides under mild and co-catalyst-free conditions.


31

Quaternary Ammonium Salt Integrated Heterogeneous

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