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Research Article pubs.acs.org/acscatalysis

Ionic Liquid/Zn-PPh3 Integrated Porous Organic Polymers Featuring Multifunctional Sites: Highly Active Heterogeneous Catalyst for Cooperative Conversion of CO2 to Cyclic Carbonates Wenlong Wang,†,‡ Cunyao Li,†,∥,‡ Li Yan,*,† Yuqing Wang,†,∥ Miao Jiang,† and Yunjie Ding*,†,§ †

Division of Fossil Energy Conversion, Dalian National Laboratory for Clean Energy, Dalian 116023, P. R. China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China ∥ University of Chinese Academy of Sciences, Beijing 100039, P.R. China §

S Supporting Information *

ABSTRACT: A series of ionic liquid (IL), zinc halide (ZnX2), and triphenylphosphine (PPh3) integrated porous organic polymers (POPs) featuring multifunctional sites were afforded through solvothermal synthesis for cyclic carbonate synthesis which utilizes epoxides and carbon dioxide (CO2). Owing to the cooperative effect of ionic liquid and homogeneously distributed Zn-PPh3 specie, which is probably strengthened by the confined microporous structure and flexible frameworks, these POPs catalysts exhibited high CO2 capture and conversion performance and provided the highest activity (initial turnover frequencies up to 5200 h−1) of heterogeneous catalysts reported to date within the context of cyclic carbonate formation. Even more surprising, very favorable turnover numbers (TONs) of 2120 and 720 were attained at 40 and 25 °C, respectively. The effect of reaction parameters (reaction time, temperature, CO2 pressure) on the catalytic performance as well as other epoxide substrates were also investigated in detail. Furthermore, the catalyst can be easily recovered and reused five times without a significant loss of activity. These ionic liquid and Zn-PPh3 constructed porous polymers may provide an industrial opportunity for cyclic carbonate products. KEYWORDS: porous organic polymers, heterogeneous catalysis, carbon dioxide, cyclic carbonates, multifunctional catalyst tems.24−30 Through careful analysis of the above homogeneous catalytic systems, a general character of dual activation mode which include a Lewis acid and a nucleophile group could be found to explain the high activity (Figure 1). Although these

1. INTRODUCTION Carbon dioxide is an abundant, economical, and renewable source of carbon on earth and therefore an attractive C1 building block for the production of fine chemicals.1−4 In particular, cyclic carbonates, which have been widely used as aprotic solvents, electrolytes in lithium-ion batteries, and starting materials in polycarbonates and polyurethane syntheses, are formed by coupling CO 2 to epoxides. 5−8 Furthermore, cyclic carbonates can be directly hydrogenated to form methanol and diols under mild conditions, which is expected to be developed into a new pathway from CO2 to methanol.2,4,9−11 Currently, the production capacity of cyclic carbonate is approximately 100 kt per year, and new industrial plants are being constructed to meet the increasing demand.12 It is not widespread for all the applications presently only because the current cost and scale of its production could not meet the demand. However, owing to the inherent thermodynamic stability and kinetic inertness of CO2, a catalyst is essential for CO2-related transformations. Numerous catalytic systems including homogeneous and heterogeneous ones have been developed for the cycloaddition of CO2 to epoxides,13,14 and some homogeneous catalytic systems exhibit high activity, such as the porphyrin complexes,12,15−17 amino-phenolate coordinated complexes,18 salen complexes,19−23 and Lewis acid/ionic liquids sys© 2016 American Chemical Society

Figure 1. Homogeneous dual activation mode of epoxide.

homogeneous catalytic systems are efficient toward coupling CO2 to epoxides, their complicated synthesis, intricate recycling problems, and thus the metal contamination of the prouducts severely impede their industrial process. So far, a number of heterogeneous catalytic systems which have either an immobilized oxophilic part31−36 (metal ions, − OH, − COOH, etc) or an immobilized nucleophilic part37−42 (such as ionic liquids) have been developed to realize the catalyst recycling (Figure 2a,b). The third method is to Received: April 22, 2016 Revised: July 21, 2016 Published: August 1, 2016 6091

DOI: 10.1021/acscatal.6b01142 ACS Catal. 2016, 6, 6091−6100

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ACS Catalysis

Scheme 1. Preparation of PPh3-ILX@POPs via Solvothermal Synthesis and PPh3-ILX-ZnX2@POPs through the Process of Impregnation

Figure 2. Three modes of traditional immobilization methods. (a) Immobilized oxophilic part mode. (b) Immobilized nucleophile part mode. (c) Immobilized both oxophilic and nucleophile part mode.

immobilize both of the oxophilic and nucleophilic part (Figure 2c),43−51 for instance, the classical anchoring method, in which the catalyst is anchored to various kinds of supports, such as silica-based materials and styrene-based materials. However, the catalytic moieties are inhomogeneously distributed and pendant into the pore volume, which may cause pore blocking and eventually deactive the catalytic system. And the catalytic efficiency (TOFs) of these immobilized catalysts are not satisfactory compared to their homogeneous counterparts. Porous organic polymers (POPs), which have emerged in recent years, are of great interest because of their porous structures containing organic functionalities that, in principle, allow significant synthetic diversification. The direct assembly of several organic building blocks into a microporous network using stable covalent organic bonds can provide chemically and thermally stable materials with both high surface areas and a wide range of chemical functionalities.52−55 A variety of POPs have been developed for applications in areas such as gas sorption,56,57 catalysis,58−61 molecular separation,62 and electronics.63 More recently, several demonstrations of the use of metal-functionalized POPs that combine valuable physical and chemical properties, such as both gas sorption and catalytic activities, have been reported. For example, Jiang’s group reported a metalloporphyrin-derived POPs with excellent catalytic activity for the oxidation of thiols by oxygen.60 Deng et al. developed a Li atom-doped POPs that displayed amazing H2 sorption due to the interaction between Li sites and molecular hydrogen.64 Highly efficient single atom size distributed Rh/PPh3 POPs for the reaction of hydroformylation have been reported by our and Xiao’s group.65−69 These materials combine the physical properties and chemical properties of the metal−organic moieties which are incorporated in the porous structure of the POPs. Similar incorporation of metal−organic moieties and CO2-philic group in POPs may lead to materials capable of both CO2 capture and its simultaneous conversion. In this study, we demonstrate a class of ionic liquid, zinc halide, and triphenylphosphine integrated POPs featuring multifunctional sites prepared through solvothermal synthesis (free radical polymerization of vinyl-functionalized PPh3 and vinyl-functionalized ionic liquid under solvothermal conditions, and subsequently impregnated by ZnX2 salts, Scheme 1). These

new POPs (hereafter referred to as PPh3-ILX-ZnX2@POPs, X = Cl, Br, I) were capable of capturing and converting CO2 with very high turnover frequency values (initial TOFS up to 5200 h−1, which is the highest heterogeneous catalyst reported to date). Even more important, very encouraging turnover numbers (TONs) of 2120 and 720 were achieved at 40 and 25 °C, respectively. The ionic liquid/ZnX2 catalytic system has previously been identified as an efficient homogeneous catalyst in the formation of cyclic carbonate from CO2 and epoxides, and the POPs materials, especially for the P atom-doped POPs, have been proven as appropriate candidates for the capture of CO2 as well as a ligand for stabilizing metal elements.70,71 Our combination of these three components led to PPh3-ILXZnX2@POPs that satisfy the requirements of efficient conversion of CO2 to useful cyclic carbonates.

2. EXPERIMENTAL SECTION 2.1. General Information. Vinyl-functionalized PPh372 and vinyl-functionalized ionic liquids73 were prepared according to literature methods. DMF was dried with molecular sieves and distilled before use. Other chemicals were commercially available and used without further purification. 1H and 13C NMR spectra were recorded on a Bruker AVANCE III NMR spectromer at 400 and 100 MHz, respectively, using trimethylsilane (TMS) as an internal standard. Solid-state 31P and 13C CP/MS NMR was performed on a VARIAN Infinityplus spectrometer. In situ diffuse reflectance infrared FT-IR spectra was performed on a Thermo Scientific Nicolet iS50 FT-IR instrument. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Quantachrome Autosorb-1 system, and the samples were degassed at 100 °C for 10 h before the measurements. Surface areas were calculated from the adsorption data using Brunauer−Emmett−Teller (BET) method. The pore size distribution curves were obtained from the adsorption branches using nonlocal density functional theory (NLDFT) method. Single-component gas sorption were carried out on IGA-100A Hiden Analytical. Thermogravimetric analysis (TGA) was carried out on NETZSCH STA 449F3 by heating samples from room 6092


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ACS Catalysis temperature to 850 °C in a dynamic nitrogen atmosphere with a heating rate of 10 °C·min−1. Inductively coupled plasma spectroscopy (ICP) was measured on a PerkinElmer apparatus Optima 7300 DV. Field-emission scanning electron microscopy (SEM) was performed on a JSM-7800F operated at an accelerating voltage of 3.0 kV. Transmission electron microscope (TEM) images were obtained on a JEM-2100 with an accelerating voltage of 200 kV. Gas chromatography (GC) was performed on an Agilent 7890A equipped with a capillary column (HP-5 column, 30 m × 0.32 μm diameter) using a flame ionization detector. 2.2. Synthesis of PPh3-ILX@POPs via Solvothermal Synthesis. The synthetic scheme is shown in Scheme 1. Vinylfunctionalized PPh3 (1, 4 mmol), vinyl-functionalized ionic liquids (2, 0.8 mmol), and AIBN (2.5 wt %) were dissolved in 16 mL of DMF. The mixture was transferred into an autoclave and maintained at 100 °C for 24 h. After evaporation of solvent, a white solid with quantitative yield was obtained and denoted as PPh3-ILX@POPs (X = Cl, Br, I). 2.3. Synthesis of PPh3-ILBr-ZnX2@POPs via Impregnation Method. As a typical synthesis recipe, 1546 mg of the PPh3-ILBr@POPs polymer was swollen in 60 mL of THF for 30 min, followed by adding 450 mg of ZnBr2 (NP/Zn = 2). After stirring at room temperature for 24 h, the mixture was filtrated and washed with an excess of THF, and lastly dried at 50 °C under vacuum. The obtained white solid was denoted as PPh3ILBr-ZnBr2@POPs with a theoretical Zn loading of 6.4 wt %, and the ICP analysis showed a real loading amount of 4.6 wt %. 2.4. Typical Procedures for Propylene Carbonate (PC) Synthesis from Propylene Oxide (PO) and CO2. In a typical procedure, PPh3-ILBr-ZnBr2@POPs (65 mg, equal to 0.028 mmol ionic liquid ILBr), PO (13.0 g, 224 mmol) were added into a stainless steel autoclave (25 mL inner volume), substrate/catalyst (S/C) = 8000. After sealing and purging with CO2 for 3 times, the pressure was adjusted to 3 MPa as an initial pressure. Then the autoclave was put into a preheated oil bath and stirring at 120 °C for 1 h. After the reaction was completed, the autoclave was cooled to ambient temperature, and the excess CO2 was vented. The catalyst was separated by centrifugation, and the supernatant was analyzed by gas chromatography to determine the product yield. 2.5. Recyclability Test for PPh3-ILBr-ZnX2@POPs. After the cycloaddition reaction of epoxides with CO2 was completed, the recycled catalyst was separated by centrifugation, washed with degassed THF, and used directly for the next run.

porous organic vinyl polymers synthesized under solvothermal conditions exhibit an outstanding swelling property (Figure S1); the swollen polymers can be characterized as a solution to a certain degree, although they are elastic solids rather than liquids.72 Even more importantly, the swollen polymeric catalysts endow the organic framework with excellently high flexibility which could enhance the catalytic performance. ZnX2 (X = Cl, Br, I) species supported PPh3-ILBr-ZnX2@POPs have been readily obtained by treating PPh3-ILBr@POPs with ZnX2 in the THF solvent. PPh3-ILX@POPs and PPh3-ILBr-ZnX2@POPs (X = Cl, Br, I) are stable in an air atmosphere, and their structures and compositions were defined by solid-state 13C, 31P NMR, and ICP analysis. In the solid-state 13C NMR spectrum, the signals ranging from 120 to 145 ppm (Figure 3A) are assignable to the

Figure 3. (A) Solid-state 13C NMR spectrum of PPh3-ILBr@POPs, * refers to sidebands. (B) Solid-state 31P NMR spectra of PPh3-ILBr@ POPs (black), PPh3-ILBr-ZnBr2@POPs (red) ,and reused PPh3-ILBrZnBr2@POPs after 4 times (blue).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. Following the synthetic route outlined in Scheme 1, PPh3-ILX@POPs (X = Cl, Br, I) were readily prepared through free-radical polymerization of vinyl-PPh3 and vinyl-ionic liquids in DMF at 100 °C for 24 h. The resultant solid was washed with THF and then dried under vacuum. The white solid product was finally obtained in nearly quantitative yield. This solvothermal synthetic method, which was inspired by molecular sieve synthesis, offers several distinct benefits: (1) DMF in this case was used not only as a good solvent to dissolve polymeric monomers but also as a very important template for the micropore formation. (2) The free-radical polymerization process is very efficient once triggered, and this method is one of the most efficient polymerization processes that has been extensively applied in the plastics industry. (3) The

aromatic carbons of PPh3-ILBr@POPs, the signal at around 50 ppm is attributed to the CH2 group directly linked to the imidazole ring, and the peaks at around 25 ppm is ascribed to the “−CH2CH2−” linker. Additionally, representative solidstate 31P NMR spectrum of PPh3-ILBr@POPs shows just one signal at −5.5 ppm (Figure 3B), which is at the same position as the ligand. After ZnBr2 was loaded on PPh3-ILBr@POPs, an obvious peak at 40.4 ppm was observed, which is quite different from the signal of PO bond around 30 ppm74 and is attributed to the P−Zn coordination bond of PPh3-ILBrZnBr2@POPs. These observations indicate that the P species was stable during the solvothermal polymerization process and 6093


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ACS Catalysis the easy coordination between PPh3 and ZnX2 (X = Cl, Br, I). Representative TG analysis demonstrates that the weight loss of the PPh3-ILBr-ZnBr2@POPs starts over 400 °C (Figure S2), which indicates that the POPs catalysts are thermally stable below 400 °C. The porosities of PPh3-ILX@POPs and PPh3-ILBr-ZnX2@ POPs (X = Cl, Br, I) were investigated by physisorption of nitrogen at 77 K. The representative curve of PPh3-ILBr@ POPs exhibits combined features of type I and type IV with two obviously steep steps in the P/P0 < 0.01 and 0.70 < P/P0 < 1.0 regions (Figure 4A), which suggests that there are micro- and

Figure 5. (A) SEM image and (B) TEM image of PPh3-ILBr@POPs.

Elemental distribution in catalyst PPh3-ILBr-ZnBr2@POPs was determined by the energy-dispersive X-ray spectroscopy (EDX) mapping technique in scanning electron microscopy (Figure 6). Moreover, the highly dispersed character of all

Figure 6. Elemental distribution in PPh3-ILBr-ZnBr2@POPs determined by SEM-EDX: (A) carbon, (B) phosphorus, (C) bromine, and (D) zinc.

functional elements (C, P, Zn, and Br) demonstrates the excellent integration of homogeneously distributed active functional sites. This kind of integration in micropore environments creates a very favorable cooperative condition for the CO2 transformation process which will be discussed later. In addition, these materials (PPh3-ILX@POPs, X = Cl, Br, and I) contained microporosity, triphenylphosphine, and ionic liquid functionalities, which have been identified as important characteristics for CO2 adsorption. In addition, for postcombustion CO2 adsorption, CO2 should be adsorbed preferentially over N2.70,75−77 To test the separation capacity of these polymers, single-component gas sorption was carried out at 298 K and up to 1.1 bar. Encouragingly, the polymer PPh3-ILBr@POPs shows very good CO2 adsorption selectivity over N2 at 298 K (Figure 7A), whereas DVB-ILBr@POPs (DVB = divinylbenzene, preparing details see Figure S3) exhibits an inferior selectivity (Figure 7B), which strongly indicates that PPh3 building blocks that homogeneously embedded in POPs materials play a key role in the adsorption and subsequent activation of CO2. 3.2. Evaluation of Catalytic Performance. First, the catalytic activities of PPh3-ILX@POPs (X = Cl, Br, and I) were evaluated for the cycloaddition reaction of CO2 and propylene oxide. The order of the activity of POPs with different ionic

Figure 4. (A) N2 sorption isotherms measured at 77 K. (B) Pore-size distribution curves calculated by NLDFT method.

mesopores in the polymer. Based upon the calculations of the nonlocal density functional theory method (NLDFT), the pore sizes of two types are distributed around 0.7−1.5 and 3−12 nm (Figure 4B), respectively. The Brunauer−Emmett−Teller (BET) surface area of PPh3-ILBr@POPs is estimated at 591 m2/g with a total pore volume of 1.03 cm3/g. Furthermore, compared with PPh3-ILBr@POPs, the BET surface area of PPh3-ILBr-ZnBr2@POPs is 482 m2/g, which is lower due to the additional weight of ZnBr2 salt. Representative scanning electron micrograph (SEM, Figure 5A) and transmission electron micrograph (TEM, Figure 5B) images of PPh3ILBr@POPs further confirm the rough surfaces and the existence of hierarchical porosities, and this porous structural property is very favorable for the diffusion of reactants and products, especially for gas-involved transformations. 6094


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Table 1. Catalytic Activities of Various Catalysts in the Cycloaddition Reactions of CO2 and Propylene Oxidea

entry

catalyst

yield (%)

TOF (h−1)

1 2 3 4 5 6 7 8 9 10 11 12 13b 14

PPh3-ILCl@POPs PPh3-ILBr@POPs PPh3-ILI@POPs PPh3-ILBr-ZnCl2@POPs PPh3-ILBr-ZnBr2@POPs PPh3-ILBr-ZnI2@POPs PPh3-ZnBr2@POPs DVB-ILBr@POPs DVB-ILBr-ZnBr2@POPs ILs2b ILs2b/ZnBr2 (1:2.5) PPh3/ILs2b/ZnBr2 (5:1:2.5) PPh3/ZnBr2 (2:1) PPh3

2.7 8.8 3.5 17 44 19 trace 5.9 33 7.1 36 50 9.3 trace

216 704 280 1360 3520 1520 − 472 2640 568 2880 4000 744 −

a

Reaction conditions: propylene oxide (13.0 g, 224 mmol), an initial pressure of 3 MPa CO2, substrate/catalyst = 8000 (catalyst amount equal to the amount of ionic liquids), 120 °C, 1 h. The selectivities of all the results are >99%. All the results are averaged by two runs. bS/C = 8000 (catalyst amount equal to the amount of ZnBr2).

PPh3 building block in the POPs, DVB-ILBr@POPs, a POPs which do not contain P elements, was prepared as a control material (Figure S3), and DVB-ILBr@POPs gave a TOFs value of 472 h−1 (entry 8), which is lower than that of 704 h−1 of PPh3-ILBr@POPs. Similarly, DVB-ILBr-ZnBr2@POPs (Figure S5) provides a lower TOFs value of 2640 h−1 (entry 9) compared with 3520 h−1 of PPh3-ILBr-ZnBr2@POPs. The superior catalytic performance of PPh3-related materials could be attributed to the combination of excellent CO2 sorption ability (Figure 7) and the coordination effect between PPh3 and ZnX2 salts, which was confirmed by 31P solid-state NMR spectra (Figure 3).71 ZnX2 was stabilized after coordination with PPh3 and further increased the integrity in the confined microporous structures as well as the cooperative catalytic activity (Scheme 2). To further study the role of ionic liquid, PPh3 and ZnX2 salts in the multifunctional catalytic system, four homogeneous catalytic systems (entries 10−13) were carried out to investigate the synergistic effect. When ionic liquid 2b was used alone, a TOFs value of 568 h−1 was obtained (entry 10), and a better result (TOF = 2880 h−1) was achieved by using a homogeneous ILs2b/ZnBr2 (1:2.5) system (entry 11). It is worthwhile to notice that when PPh3 was added, the homogeneous PPh3/ILs2b/ZnBr2 (5:1:2.5) system (entry 12) exhibited a high TOFs value of 4000 h−1, which clearly manifests that PPh3 plays a crucial role in the synergistic system. The homogeneous PPh3/ZnBr2 (2:1) catalyst system without ionic liquid gave a TOFs value of 744 h−1 (entry 13), which is quite different from the analogue heterogeneous system (entry 7) due to the possible formation of trimeric zinc complex in totally free solvent conditions.71 Additionally, comparing entry 2 with entry 10, it could be found out that PPh3-ILBr@POPs, an insoluble heterogeneous POPs catalyst, is superior to its homogeneous analogues ILs2b. Furthermore, the catalytic activity of PPh3-ILBr-ZnBr2@POPs (entry 5,

Figure 7. Sorption isotherms comparison of CO2 and N2: (A) PPh3ILBr@POPs, (B) DVB-ILBr@POPs.

liquids was found to be ILBr > ILI > ILCl (entries 1−3, Table 1), and this order might be ascribed to the balance of nucleophilicity and leaving ability of halide anions (Cl−, Br−, and I−) which play a critical role in the ring-opening and closure steps, respectively. The TOFs values ranging from 216 to 704 h−1 are much higher than ever reported for ionic-liquidembedded POPs materials,37−40 and this advantage of catalytic performance could be attributed to the superior CO 2 adsorption ability together with their flexible and freely movable frameworks endowed by vinyl-polymerization process compared with other type of relatively rigid POPs materials. With the selected best ionic-liquid incorporated POPs (i.e., PPh3-ILBr@POPs) in hand, we then investigated the catalytic performance of Zn loaded PPh3-ILBr-ZnX2@POPs (X = Cl, Br, and I) in the model reaction of CO2 and propylene oxide. Very encouragingly, three excellent results with TOFs of 1360, 3520, and 1520 h−1 were obtained (entries 4−6) ,and the order of the activity of POPs with different zinc halide salts was found to be ZnBr2 > ZnI2 > ZnCl2, which indicates the zinc halide salts as well as halide anions plays an important role in the cooperative catalytic process. These results, especially for 3520 h−1 of PPh3ILBr-ZnBr2@POPs, are the highest heterogeneous catalysts ever reported, to the best of our knowledge. To understand the outstanding performance of PPh3-ILBr-ZnBr2@POPs, a series of control experiments have been carried out. When the PPh3ZnBr2@POPs (Figure S4) without copolymerizing with ionic liquids was used as a catalyst, only a trace amount of propylene carbonate was obtained (entry 7), which shows that ionic liquid is essential for the catalytic system. To study the role of the 6095


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ACS Catalysis Scheme 2. Proposed Dual-Activation, Cooperative Reaction Mechanism

TOFs = 3520 h−1) is also very close to its homogeneous analogue (entry 12, TOFs = 4000 h−1). All these results obtained suggest that our synthesized ionic liquid/Zn-PPh3 integrated porous organic polymers featuring multifunctional sites displayed quite high efficiency in cooperative catalytic conversion of CO2 to cyclic carbonates. On the basis of our observations and in accord with literature reports, a dual cooperative activation mechanism was proposed for the catalytic conversion of CO2 to cyclic carbonates (Scheme 2). Previously, an epoxide could be activated by the Lewis acid electrostatic interactions of ZnBr2−PPh3 part in the POPs’ confined micropores, and then a nucleophilic substitution reaction at the less hindered carbon atom of the bromide forms an alcoholate; subsequent to another nucleophilic attack from the alcoholate toward CO2, an acyclic carbonate can be formed, which then undergoes an intramolecular substitution reaction, producing cyclic carbonate with liberation of the integral PPh3-ILBr-ZnBr2@POPs catalyst. Owning to the extensively and homogeneously built-in PPh3 species, ZnBr2 salts could be stabilized and confined in the micropore environment of the flexible POPs frameworks, which probably enhanced the cooperative catalytic efficiency with a dual activation mode. In addition, the outstanding CO2 enrichment capacity of PPh3-ILBr-ZnBr2@POPs (Figure 7) together with the known knowledge that POPs materials can sequester and confine the reaction substrates in the nanometersized pore and then increase their concentration around the catalytic sites beyond that in solutions fully explained the excellent activity of this highly integrated PPh3-ILBr-ZnBr2@ POPs catalyst.78,79 With regard to the investigated reaction parameters, temperature had the most decisive effect on the model reaction (Figure 8). As mentioned above, a TOFs value of 3520 h−1 was obtained at 120 °C, which is already the highest heterogeneous catalyst ever reported. When the temperature was further increased up to 140 °C, an even excellent TOFs value of 4800

Figure 8. Effect of the reaction temperature on the initial 1 h TOFs value of PPh3-ILBr-ZnBr2@POPs. Reaction conditions: propylene oxide (13.0 g, 224 mmol), an initial pressure of 3 MPa CO2, 65 mg of catalyst, substrate/catalyst = 8000 (catalyst amount equal to the amount of ionic liquids), 1 h. The selectivities of all the results are >99%. All the results are averaged by two runs.

h−1 was attained; however, further increasing the temperature to 160 °C, a TOFs value of 5200 h−1 was reached, which only increased a little compared with 140 °C and stepped into a plateau region. At 60 °C, though it dropped dramatically, a TOFs value of 312 h−1 was still obtained, and this represents a very hard-won result at such a mild temperature. Although our POPs catalyst exhibits extremely high efficiency in CO2 conversion to cyclic carbonates at elevated temperatures, it is attractive that the catalytic materials used in the process be able to capture and convert CO2 at room temperature with only heat from the surrounding environment to avoid the generation of new CO2. Unfortunately, thus far, few satisfactory results have been reported. In 2003, Deng and 6096


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ACS Catalysis coworkers have reported a turnover number (TON) result around 200 at 25 °C regarding this reaction.80 Herein, we tested our selected PPh3-ILBr-ZnBr2@POPs catalyst in this transformation at 40 and 25 °C, respectively (Table 2). A high Table 2. Catalytic Activities of PPh3-ILBr-ZnBr2@POPs in Capture and Conversion of CO2 to Cyclic Carbonates at 40 and 25 °Ca

entry

temperature (°C)

yield (%)

TON

1 2

40 25

53 18

2120 720

a

Reaction conditions: propylene oxide (6.5 g, 112 mmol), an initial pressure of 3 MPa CO2, 65 mg of catalyst, substrate/catalyst = 4000 (catalyst amount equal to the amount of ionic liquids), 48 h. The selectivities of all the results are >99%. All the results are averaged by two runs.

Figure 10. Effect of the reaction time on the yield of propene carbonate in the presence of PPh3-ILBr-ZnBr2@POPs. Reaction conditions: propylene oxide (13.0 g, 224 mmol), 120 °C, 65 mg of catalyst, substrate/catalyst = 8000 (catalyst amount equal to the amount of ionic liquids). The selectivities of all the results are >99%. All the results are averaged by two runs.

TON of 2120 was obtained when the reaction was carried out at a very mild temperature of 40 °C, and a TON of 720 was acquired at an even mild temperature of 25 °C. These investigations clearly indicate that PPh3-ILBr-ZnBr2@POPs could be an energy-saving and real CO2 mitigating catalyst. Furthermore, the carbon dioxide pressure was investigated between 0.5 and 4.0 MPa at 120 °C (Figure 9). In the model

64% of propene carbonate was detected with a TON value of 5120. A reaction of 3 h yielded 79% of cyclic carbonate with a TON value of 6320. However, for ≥90% conversion (TON ≥ 7200), a reaction time of 5 h was necessary. To further investigate the reaction process synchronously, in situ diffuse reflectance FT-IR spectra was applied to monitor the formation of propene carbonate with the decrease of CO2 (Figure 11). The absorption intensity of the carbonyl group (ν (CO)) centered at 1811 cm−1 quickly increased with the reaction time, indicating the formation of propene carbonate. The peaks at 667 and 2349 cm−1 which are assigned to the CO2 bending vibration and stretching vibration respectively gradually

Figure 9. Effect of the initial CO2 pressure on the initial 1 h TOFs value of PPh3-ILBr-ZnBr2@POPs. Reaction conditions: propylene oxide (13.0 g, 224 mmol), 120 °C, 65 mg of catalyst, substrate/catalyst = 8000 (catalyst amount equal to the amount of ionic liquids), 1 h. The selectivities of all the results are >99%. All the results are averaged by two runs.

reaction, propene carbonate was obtained in excellent yields for pressure of ≥2.0 MPa. At relatively low pressure of 1.0 and 0.5 MPa, the first 1 h TOFs value decreased to a certain extent. By considering the efficiency and safety production, a CO2 pressure of 3.0 MPa was chosen for further investigations. The effect of reaction time was examined between 1 and 6 h at 120 °C and a CO2 pressure of 3.0 MPa (Figure 10). In the model reaction, the conversion of propene epoxide proceeded quickly within the first 1 h (44% yield of propene carbonate was obtained with a TON = 3520). Remarkably, after only 2 h, more than half of the substrates were converted and a yield of

Figure 11. In situ diffuse reflectance FT-IR spectra of the reaction system. Reaction conditions: PO (5 mL), 120 °C, 0.3 MPa CO2, PPh3ILBr-ZnBr2@POPs (5 mg). IR absorption of catalyst is subtracted. 6097


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(phenoxymethyl)oxirane (entry 4), styrene oxide (entry 5), and cyclohexene epoxide (entry 6) with CO2 over the PPh3-ILBrZnBr2@POPs catalyst. This catalyst is still active for the conversion of these epoxides, but the activities are strongly dependent on their structures. Clearly, size-dependent catalysis is observed when using the substrates with large dimensions give rise to lower activities. In addition, the electron-donation effect of alkane and phenyl group may enhance the electronegativity of the carbon center which Br− attacks and thus deactivates the process compared with propene oxide.

decreased with the reaction time. The steep slopes at 667 and 1810 cm−1 at the initial step are accord with the reaction profile of time versus yield in Figure 10. In addition, the catalyst of PPh3-ILBr-ZnBr2@POPs could be easily recycled by simple centrifugation or filtration due to its insolubility property in organic solvents. Thus, we investigated the reusability of the POPs catalyst, and the results showed that recycling the catalyst PPh3-ILBr-ZnBr2@POPs over four runs did not lead to a significant decline in propene carbonate yields (Figure 12). Analysis of the aqueous reaction solution after the

4. CONCLUSION In conclusion, a series of ionic liquid, ZnX2, and PPh3 integrated porous organic polymers (POPs) featuring multifunctional sites were prepared through solvothermal synthesis and postmetalation procedure. As a selected catalyst, PPh3ILBr-ZnBr2@POPs has been thoroughly studied in the cycloaddition of CO2 with epoxides. The catalyst demonstrated excellent catalytic performance which represents the highest TOFs ever reported at ≥120 °C. Furthermore, high TON values of 2120 and 720 were obtained at 40 and 25 °C, respectively, which is also among the best results under mild temperatures. Through multiple comparisons of experimental details, it is reasonable to suggest that the wonderful catalytic performance of these porous polymers should be attributed to the cooperative effect of ionic liquid and homogeneously distributed Zn-PPh3 specie, which is probably strengthened by the confined microporous structure and the excellent mobility of functional groups in the framework in the swollen state. These highly integrated porous organic polymers exhibit “quasihomogeneous” catalytic behavior; like enzyme catalysis, these catalysts realize the efficiency of homogeneous catalysis under a heterogeneous medium system. In addition, as insoluble solids in nature, they are also easily separated and recycled from the reaction system without losing any activity and selectivity. Therefore, these porous polymers provide new perspectives for preparing heterogeneous catalysts with (or even beyond) homogeneous activity.

Figure 12. Recyclability test of PPh3-ILBr-ZnBr2@POPs. Reaction conditions: propylene oxide (13.0 g, 224 mmol), 120 °C, 65 mg of catalyst, substrate/catalyst = 8000 (catalyst amount equal to the amount of ionic liquids), 1 h. The selectivities of all the results are >99%.

fifth run by ICP showed a Zn element leaching of 5 ppm, which suggested the high stability of the catalyst. Moreover, the SEM image of the recycled catalyst after the fifth run indicated the well-preserved porous structure (Figure S6). Table 3 presents catalytic data in the cycloaddition of epichlorohydrin (entry 1), 1,2-epoxyhexane (entry 2), allyl glycidyl ether (entry 3), 2-

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ASSOCIATED CONTENT

S Supporting Information *

Table 3. Cycloaddition of CO2 with Various Substrates over PPh3-ILBr-ZnBr2@POPs Catalysta

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01142.

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Experimental procedures and spectra data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ‡

These authors contributed equally (W.W. and C.L.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate the financial support by the young scientist fund of NSFC (21503218) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020400), National Natural Science Foundation of China (21273227), and China Postdoctoral Science Fundation.

a

Reaction conditions: substrate (56 mmol), 16.3 mg of catalyst, substrate/catalyst = 8000 (catalyst amount equal to the amount of ionic liquids). The selectivities of all the results are >99%. All the results are averaged by two runs. 6098


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