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Metalloporphyrin Polymers with Intercalated Ionic Liquids for Synergistic CO2 Fixation via Cyclic Carbonate Production Yaju Chen, Rongchang Luo, Qihang Xu, Jun Jiang, Xian-Tai Zhou, and Hongbing Ji ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03371 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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ACS Sustainable Chemistry & Engineering

Metalloporphyrin Polymers with Intercalated Ionic Liquids for Synergistic CO2 Fixation via Cyclic Carbonate Production Yaju Chen,† Rongchang Luo,*† Qihang Xu,† Jun Jiang,† Xiantai Zhou,‡ and Hongbing Ji*† †

Fine Chemical Industry Research Institute, Key Laboratory of Low-Carbon Chemistry &

Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510275, P.R. China ‡

School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai, Guangdong

519082, P.R. China *

Corresponding authors: [email protected] (Rongchang Luo); [email protected]

(Hongbing Ji)

ACS Paragon Plus Environment

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ABSTRACT:

Multifunctionalization of organic polymers for acting synergistically on substrate is of wide interest in the field of modern catalysis, but it is still a significant challenge. Herein, two novel bifunctional polymers were firstly designed and synthesized by combining ionic liquids (ILs) with zinc(II) porphyrin through simple and reversible Schiff base reactions. The fabricated polymers with flexible structures and nitrogen-rich environments presented high affinity toward CO2 molecules at ambient conditions. Owing to the cooperative nature of intercalated ILs and Lewis acidic metal sites, these materials could serve as efficient heterogeneous catalysts for the insertion of CO2 into epoxides to produce cyclic carbonates. As expected, these polymers exhibited good catalytic performance, robust constancy, excellent recyclability, and good substrate expansibility for this reaction in the absence of co-catalyst under mild or even ambient conditions. Notably, the selected catalyst SYSU-Zn@IL2 could directly convert diluted CO2 (15% CO2 in N2) into cyclic carbonate at 80 oC and 3.0 MPa, further offering the great application potential for recycling real-world carbon resource.

KEYWORDS: Synergistic catalysis, Carbon dioxide, Cyclic carbonates, Ionic liquids, Porphyrin-based polymers


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INTRODUCTION Chemical fixation of CO2 into available chemicals and fuels has recently turned into one of the most fertile arenas in the field of industry and energy, which is in line with the view of sustainable development and environmental protection. 1-4 To date, more than 20 reactions involving CO2 as a C1 feedstock have been intensively developed over the past decades. 5-9 In particular, the insertion of CO2 into epoxides for affording cyclic carbonates has been proposed as a promising strategy for recycling carbon resource, not only due to its 100 % atom economy but also on account of great industrial potential of cyclic carbonates.10-12 So far, plenty of effort has been devoted to the development of efficient catalytic systems for this conversion, especially several metal-based systems with extremely high reactivity and selectivity such as metallosalen complexes13-16 and metalloporphyrins17-19. On closer inspection of these catalytic systems have been reported so far, the dual electrophilic/nucleophilic behavior toward the activation of an epoxide has been widely accepted in this transformation (Scheme 1A). Based on this, the metalbased catalysts often need to combine with additional co-catalyst as a nucleophile to ensure their efficiency. However, a large number of these cooperative catalytic processes for this reaction are still confined to fundamental research, mainly due to the common problem of homogeneous system like intricacy of separating and purifying operation. One of promising and effective solution to these drawbacks is to construct heterogeneous polymer catalysts from homogeneous analogue with functionality that can be readily separated and then, potentially, be reused. From this point of view, functional organic polymers, if they can be fabricated, will be in line with these requirements and provide high-density multiple active sites and flexible structural features for the reaction.


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Scheme 1. The synthesis of cyclic carbonates from CO2 and epoxides on the basis of synergistic catalysis

Gratefully, plentiful efforts have been devoted to the development of functional organic polymers over the past decade, thus yielding numerous heterogeneous catalysts for CO2 capture and conversion.20-24 Through careful analysis of these polymers, they were mainly constructed through two methodologies, namely, post-synthetic approach and pre-synthetic approach.25-27 The post-synthetic strategy is to modify functional moieties on the initially prepared organic polymers; nevertheless, this strategy can not ensure that all functional groups incorporated in the networks.28 For instance, Leng and coworks developed an ionic polymeric catalyst by introducing CoIII–Salen moieties onto halogen anion based ionic polymer.20 Unsatisfactorily, limited active centers could be loaded onto polymeric support, thereby resulting in the lower catalytic activity for CO2 coupling reaction. Unlike the previous method, the pre-synthetic strategy is to fabricate functional organic polymers by polymerization of chemically well-defined and function-oriented building blocks (monomers). These polymers could be used directly as functional heterogeneous catalysts for CO2 transformation. Recently, our group has developed bifunctional organic polymers bearing ionic liquids and metalloporphyrins via YamamotoUllmann couplings reactions, which were successfully employed as cooperative catalysts in the synthesis of cyclic carbonates.29 Despite these advancements, developing advanced functional polymers as heterogeneous catalysts for this reaction is a long-term goal that may require the consideration of four main aspects as follows: (1) employing a facile and eco-friendly synthetic


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strategy; (2) incorporating task-specific functionalities (electrophilic and nucleophilic sites); (3) enhancing the CO2-philicity of polymer skeleton; (4) possessing some level of hierarchical porous structures. Herein, based on the above considerations and our previous works, we have synthesized a series of novel zinc(II) porphyrin-based ionic polymers (SYSU-Zn@ILs) by using zinc(II) tetrakis(4-aminophenyl)porphyrin (Zn-TAPP) and various

IL-functionalized dialdehyde

compounds (M1 and M2) as raw materials (Scheme 2). To the best of our knowledge, it is unprecedented that the synthesis of bifunctional heterogeneous catalysts consisting of metalloporphyrin and imidazolium-based ILs from the one-step and catalyst-free imine condensation reaction. These bifunctional materials could offer the advantage of homogenous catalysts (higher activity) and heterogeneous catalysts (ease of separation/reusing) (Scheme 1B), and then realize the transformation of CO2 at mild conditions. Then, the obtained materials are characterized by solid-state

13C

NMR spectroscopy, elemental analysis (EA), fourier transform

infrared spectroscopy (FTIR), inductively coupled plasma optical emission spectroscopy (ICPOES), thermogravimetric analysis (TGA), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and CO 2/N2 adsorption-desorption measurements. As a result, these functional polymers possess flexible polar skeleton and nitrogen-rich environments with hierarchical porous structure and CO2/N2 adsorptive selectivity. Due to the synergistic effect and heterogeneous nature of these bifunctional catalysts, they were found to be efficient and selective in the CO2/epoxide coupling reaction with good recyclability under solvent- and additive-free conditions. Therefore, our simple and green synthetic strategy will inspire the further development of bifunctional even multifunctional polymeric materials for application in CO2 catalytic transformation.


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Scheme 2. Schematic description of synthesis route to SYSU-Zn, SYSU-Zn@IL1 and SYSU-Zn@IL2

EXPERIMENTAL SECTION Materials Unless otherwise stated, all solvents and chemicals were available from local suppliers with the highest purity and used as received. Deionized water was commonly used in our laboratory. 4,4Bipyridine, propylene carbonate (PC), tetrabutylammonium bromide (TBAB) and

various

epoxides were purchased from J&K Scientific Ltd. N,N-dimethylglycine, copper iodide, 4,4’biphenyldicarboxaldehyde,

1,4-dibromobenzene,

4-nitrobenzaldehyde

and

4-

(bromomethyl)benzaldehyde were was obtained from Energy Chemical or Beijing HWRK CHEM. Pyrrole was obtained from was purchased from Aladdin and distilled under a N 2 atmosphere

before

use.

Zinc(II)

tetrakis(4-aminophenyl)porphyrin

and

1,4-bis(1-

imidazolyl)benzene were prepared following the reported literature (see Section 2 in the Supporting Information).


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Synthesis of SYSU-Zn@IL1, SYSU-Zn@IL2, SYSU-Zn and SYSU- IL In a typical synthesis of SYSU-Zn@IL1, to a solution of Zn-TAPP (369 mg, 0.5 mmol) in DMF (10 mL) was added M1 (554 mg, 1.0mmol). Then the reaction mixture was stirred under nitrogen at 140 oC for 24 h yielding a dark green precipitation. After cooling to room temperature, the solid was isolated by filtration, and washed with DMF, CH 3OH, CH2Cl2 and acetone. The wet sample was then transferred to a Soxhlet extractor and thoroughly washed with CH3OH for 24 hours. Finally, the resulting polymer was dried under vacuum at 80 oC overnight. The as-prepared catalyst was denoted as SYSU-Zn@IL1. Additionally, the synthesis of SYSUZn@IL2 was carried out following the same protocol as for SYSU-Zn@IL1, by replacing the M1 with M2 (608 mg, 1.0 mmol). For comparison, the IL-free polymeric analogue SYSU-Zn and metal-free polymeric analogue SYSU@IL were also prepared according to the similar procedure by the imine condensation of Zn-TAPP (369 mg, 0.5 mmol) with M0 (210 mg, 1.0 mmol) and 5,10,15,20-tetra(4aminophenyl)porphyrin (TAPP, 337 mg, 0.5 mmol) with M2 (608 mg, 1.0 mmol), respectively. Typical Procedures for Synthesis of cyclic carbonates from propylene oxide and CO2 Taking propylene oxide (PO) as an example, the cycloaddition reaction was carried out in a 10 mL stainless autoclave at specified temperature and pressure. PO and catalyst were in turn added to the reactor, and the reaction system was sealed and purged with CO 2 for three times, followed by adjusted to 1.0 MPa as an initial pressure. Then the autoclave was put into a preheated oil bath and the mixture was stirred at the needed temperature. After the appropriate time, the autoclave was cooled to 0 oC, and the excess CO2 was vented slowly. The reaction mixture was diluted with ethyl acetate, and the catalyst was removed by filtration. Subsequently, the filtrate was analyzed by gas chromatography to determine the product (propylene carbonate, PC) yield.


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The PC was further assessed using 1H NMR and

13C

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NMR spectroscopy. Additionally, this

reaction was also operated under low CO 2 concentration (15% CO2 and 85% N2, v/v). The recycled catalyst was collected by filtration, washed with methanol, dried in the vacuum and then directly charged into the next run.

RESULTS AND DISCUSSION Synthesis of catalysts Benefiting from the diversity of the organic reactions, there were plenty of available strategies for the construction of organic polymers.30-34 Most of these reactions usually performed in the presence of metal catalysts or initiators under harsh conditions, which increased the cost and posed separation problems, even disturbed the intrinsic performances of polymers.35,36 Moreover, the number of bifunctional organic polymers was still very small compared with the large family of polymeric materials. As a consequence, it is highly desirable to develop a sustainable, facile, and applicable strategy for the synthesis of advanced bifunctional organic polymers. Fortunately, the solvothermal synthetic method, based on Schiff-base reaction, provides a practical strategy to construct the polymeric materials under the catalyst- and initiator-free conditions. We firstly utilize this pre-synthetic method to prepare efficient heterogeneous bifunctional polymeric catalysts, and employ them into the CO2-involved organic reactions. The synthetic procedure routes of catalysts were outlined in Scheme 2. Characterization of catalysts Firstly, the resultant polymers are insoluble in any common solvents such as DMSO, DMF, THF, MeOH, CHCl3 and H2O, suggesting their heterogeneous nature as hyper-crosslinked structures. Taking SYSU-Zn@IL2 as an example, it shows good dispersity in apolar and polar organic


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solvents, such as n-hexane, toluene, methanol, DMF and PO (see Figure S1 in the Supporting Information). Then, their chemical structures and compositions were defined by solid-state

13C

NMR, FTIR, EA, ICP, and XPS analysis. In the solid-state 13C NMR spectrum, the characteristic signals in the range of δ=115-155 ppm (Figure 1A) correspond to the aromatic carbons atoms in the polymer network,29,37 and the signal at 160.6 ppm is attributed to the carbons atoms of bridge bond (C=N), directly confirms the formation of imine bonds.28,38,39 The FTIR spectra demonstrate the formation of aminal linkages by the appearance of peaks belonging to the C=N stretching frequency (1600 cm-1 for SYSU-Zn, 1604 cm-1 for SYSU-Zn@IL1, and 1608 cm-1 for SYSU-Zn@IL2),28 which confirms the formation of polymers (Figure S2). Besides, the skeletal stretching vibration of imidazole ring is observed at 1656 cm-1, which proves the existence of ILs moieties in skeleton of [email protected] (B)

(A) 160.6 *

Intensity

b

N 1s

*

399.5

398.4

401.7

a

399.5

b

399.7 c

240

(C)

c

*

200 160 120 80 Chemical shift (ppm)

40

0

411 408 405 402 399 396 393 Binding Energy (eV)

(D) Zn 2p3/2

1022.7

a 1022.2 b

Intensity

Intensity

a

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Br 3d

68.5

a 68.8

1022.3

b

c 1036 1032 1028 1024 1020 1016 1012

Binding energy (eV)

78 76 74 72 70 68 66 64 62 Binding energy (eV)

Figure 1. (A) Solid-state 13C NMR spectra of SYSU-Zn@IL2(a), SYSU-Zn@IL1(b) and SYSU-Zn(c). (B) N


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1s spectra of SYSU-Zn@IL2(a), SYSU-Zn(b) and Zn-TAPP(c). (C) Zn 2p3/2 XPS spectra of SYSUZn@IL2(a), SYSU-Zn@IL1(b) and Zn-TAPP(c). (D) Br 3d XPS spectra of SYSU-Zn@IL2(a) and SYSUZn@IL1(b).

The compositions of the samples were further verified by XPS analysis (Figure S3). Specifically, as displayed in Figure 1B, the N 1s spectra show the values at 399.5 and 398.4 eV for both SYSU-Zn@IL2 and SYSU-Zn, which are related to the contribution of iminic nitrogen in condensed bridge groups (C=N) and pyrrolic nitrogen in the porphyrin ring, respectively. 41 The above two peaks at 399.5 and 398.4 eV are well consistent with those in the Zn-TAPP monomer, giving the values at 399.7 eV (amino N, -NH2) and 398.4 eV (pyrrolic N).42 Notably, the appearance of the peak at 401.7 eV, responding to N 1s of imidazolinium cations,43 suggests that the imidazole species have been successfully implanted into the skeleton of SYSU-Zn@IL2 by one step imine condensation reaction. Additionally, the Zn 2p3/2 spectra of SYSU-Zn@IL2 and SYSU-Zn@IL1 show the peaks at binding energies (BEs) of 1022.7 and 1022.2 eV respectively, in accordance with the spectra of Zn-TAPP, which show the related value at 1022.3 eV (Figure 1C).44 This result demonstrates that the bifunctional catalysts have identical active metal centers with similar coordination environment to homogeneous counterpart. Especially noteworthy is that the BE value of Zn 2p3/2 for SYSU-Zn@IL2 is slightly higher than SYSUZn@IL1, indicating that the electron density of zinc atom in SYSU-Zn@IL2 is lower than that of SYSU-Zn@IL1. This result leads to the relatively strong eletrophilicity of zinc atom in SYSU-Zn@IL2. Meanwhile, as seen in the Figure 1D, the existence of the Br 3d signals in the XPS spectra of SYSU-Zn@IL2 and SYSU-Zn@IL1 (68.5 and 68.8 eV, respectively) also indicates that ionic liquids fragments have been intercalated into the networks of porphyrinbased polymers. Contrary to the previous analysis for the BE value of Zn 2p3/2, the lower BE value of Br 3d demonstrates the higher electron density of bromide ion in SYSU-Zn@IL2,


10

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thereby rusulting in its strong nucleophilicity. Therefore, from the point of view of the reaction mechanism, the catalyst SYSU-Zn@IL2 (imidazole ILs group) makes the dual activation of epoxide more favorable, thereby exhibits higher catalytic activity than SYSU-Zn@IL1 (bearing pyridiniumium ILs group). Moreover, these polymers exhibit high thermal stability (up to 320 oC)

as evidenced by thermogravimetric analysis in air flow (Figure S4). Table S1 shows the ICP-

OES and CHN elemental analysis results of the obtained polymers, and ICP-OES results reveal that the Zn loading amount in the SYSU-Zn, SYSU-Zn@IL1 and SYSU-Zn@IL2 is 0.69, 0.52 and 0.40 mmol·g-1, respectively. The porous characters of prepared polymers were obtained by N2 adsorption-desorption analysis at 77 K. As shown in Figure S5 and Table S2, the Brunauer-Emmett-Teller (BET) specific surface area of SYSU-Zn@IL1 and SYSU-Zn@IL2 are calculated as 38 m2·g-1 and 21 m2·g-1, respectively, whereas that of IL-free SYSU-Zn was up to 136 m2·g-1. This phenomenon, which has been observed in previously reported work, possibly can be explained as follows: in the presence of polar IL component within the networks of bifunctional samples, strong interaction led to the decrease of the surface area in the formation of flexible polymers; the abundant high atomic mass of Br - in the nanopore structure directly result in a decreased BET surface area.29,45 Additionally, powder XRD patterns of these materials suggest their amorphous state (Figure S6). Furthermore, SEM and TEM analysis were used to investigate the morphology of the polymers. SEM image (Figure 2A) indicates that SYSU-Zn@IL2 displays essentially a rather rough morphology, and TEM image shows its randomly oriented nanopores (Figure 2B). Moreover, the homogeneity of the element distribution was also determined by energy dispersive X-ray (EDX) spectroscopy in TEM (Figure 2C), which revealed the homogeneous intercalation of Br-1 and Zn sites into the networks of SYSU-Zn@ILs within the instrumental resolution.


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(A)

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(C) Zn

(B) N

Br

Figure 2. (A) SEM, (B) TEM and (C) EDX Elemental-Mapping images of SYSU-Zn@IL2

It have been reported that the abundant polar ILs site, nitrogen atoms of porphyrin component, as well as nanoporous structures can enhance the interaction between CO 2 and the skeleton of materials. This promoted us to further investigate their CO 2 uptake performance at 273 and 298 K (Figure 3A). The CO2 adsorption isotherms of samples reveal that the adsorption capacity of CO2 continually increased with the pressure, implying that the adsorptions have not reached their equilibrium or saturated state within the pressure range of 0-1.0 bar.46,47 The lack of hysteresis loops of these isotherms confirms the physisorption reversibility.48 The CO2 capture capacity of SYSU-Zn@IL1 is found to be up to 1.55 mmol·g-1 (1 bar and 273 K) and 0.91 mmol·g-1 (1 bar and 298 K), which is a little higher than that of SYSU-Zn@IL2 (1.36 mmol·g-1 at 1 bar and 273 K and 0.87 mmol·g-1 at 1 bar and 298 K). This is mainly because of the higher surface area for SYSU-Zn@IL1 than SYSU-Zn@IL2. In addition, the corresponding value for the IL-free analogue (SYSU-Zn) is slightly lower of 1.06 mmol·g -1 at 273 K. In spite of the dominant high BET surface area of SYSU-Zn, its CO2 capture capacity is obviously inferior to that of both SYSU-Zn@IL1 and SYSU-Zn@IL2, which demonstrates the important role of intercalated ILs functional groups and N-rich environment in CO2 capture. To gain further insights from the interaction between materials and CO2 molecules, the CO2 isosteric enthalpies of adsorption


12

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(Qst) was obtained with the Clausius–Clapeyron equation from the sorption isotherms recorded at 273 K and 298 K. As we can see from Figure 3B, both ILs intercalated polymers exhibit moderate Qst for CO2 (27.2 KJ·mol-1 for SYSU-Zn@IL1 and 28.3 kJ·mol-1 for SYSUZn@IL2) at zero coverage. This difference of Qst value for the two polymers shows that the interaction of the CO2 molecules with the skeleton of SYSU-Zn@IL2 is a little stronger than with SYSU-Zn@IL1, thereby revealing the relatively stronger enrichment ability of CO2 for SYSU-Zn@IL2. It is also worth mentioning that the moderate initial CO 2 Qst value of them indicates the stronger host-guest interaction of the adsorbent networks with CO 2, as well as implies the effortless regeneration of adsorbent at a low energy penalty. 49,34 Additionally, to assess the potential in separation CO2 from flue gas (>70% N2), some accounts can be taken by considering CO2/N2 selectivity. The selectivity ability of samples were calculated using Henry’s Law constants by the ratios of the initial slopes from two gases adsorption isotherms measured at 273 K according to many other studies.48,50,51 As mentioned above, the CO2 uptake capacity of SYSU-Zn@IL2 obviously increases with the rising pressure, whereas the N 2 uptake capacity shows almost no change (Figure 3A). As a result, a higher CO 2/N2 selectivity of SYSU-Zn@IL2 is found to be 33.5 (32.3 for SYSU-Zn@IL1) (Figure S7), which is higher than those reported for many other polymeric materials, such as polycarbazole (CPOP-1, 25).50 Therefore, these functional polymers shows their great potential for CO2 capture and separation, which is benefit for its subsequent transformation, especially if industrial emission gas is employed as a C1 building block.


13

70

CO2 (273K) SYSU-Zn CO2 (273K) SYSU-Zn@IL1 CO2 (298K) SYSU-Zn@IL1 CO2 (273K) SYSU-Zn@IL2 CO2 (298K) SYSU-Zn@IL2 N2 (273K) SYSU-Zn@IL2

(A)

60

3

Gas Adsorbed (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Heat of Adsorption (kJ/mol)


50 40 30 20 10 0 0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

40

(B)

Page 14 of 29

SYSU-Zn@IL1 SYSU-Zn@IL2

30 20 10 0

4

6

8 10 12 14 16 3 CO2 Adsorbed (cm /g)

18

Figure 3. (A) Gas adsorption isotherms and (B) Isosteric heat of CO2 adsorption of samples

Catalytic evaluation First of all, we focused on the cycloaddition of CO2 with PO under solvent-free condition to investigate the catalytic performance of various catalysts and the results were listed in the Table 1. As expected, almost no conversion of PO was observed by the control experiment in the absence of any catalyst at 80 °C and an initial CO 2 pressure of 1.0 MPa (entry 1, Table 1). The compared catalysts SYSU-Zn (IL-free polymeric analogue) and SYSU@IL (Zn-free polymeric analogue) themselves proved to be rarely productive and gave the PC yield of 8% and 20% (entries 2 and 3), respectively. Whereas once a nucleophile (TBAB) was added into the SYSUZn system, all of PO was converted into PC with a high selectivity of >99% under the same conditions (entry 4, Table 1). The above control experiments indicate that both metalloporphyrin and TBAB play critical roles, and are indispensable to this coupling reaction under the current conditions, which owing to that the synergistic effect of Lewis acid Zn site and a nucleophile Brenhances the catalytic performance. As a consequence, at a catalyst loading of 0.16 mol%, both bifunctional catalysts (SYSU-Zn@IL1, SYSU-Zn@IL2) provided improved results without any additives. Particularly, for SYSU-Zn@IL2 (containing the imidazole ionic liquids component), nearly quantitative conversion of PO into PC was achieved (entry 6, Table 1), and the catalytic activity of it is comparable to that of most of the reported examples (Table S3). Then, the


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dependence of the PC yield on reaction time was depicted in Figure 4A. It illustrated that an increase in time led to a gradual rise in PC yield within first 10 h. When the reaction time was further extended from 10 to 14 h, the yield improved at a relatively slow rate (from 92% to 99%), predicting the approaching of thermodynamic equilibrium. 52 Nevertheless, SYSUZn@IL1 bearing the pyridiniumium ionic liquids component showed much lower activity with the yield of 62% (entry 5, Table 1), which might account for the poor nucleophilicity of the bromide ion of the tight ion pair17 and the poor eletrophilicity of zinc atom. This result is supported by XPS analysis of the BE values of Br 3d and Zn 2p3/2 for SYSU-Zn@IL1 and SYSU-Zn@IL2. Table 1. Results of the cycloaddition reaction of CO2 with PO over various catalysts. a

a

Entry

Catalyst

T [oC]

t [h]

Yield b [%]

Selectivity b [%]

1

Blank

80

12

99

4c

SYSU-Zn/TBAB

80

12

99

>99

5

SYSU-Zn@IL1

80

12

62

>99

6

SYSU-Zn@IL2

80

12

99

>99

7d

SYSU-Zn@IL2

80

12

70

>99

8

SYSU-Zn@IL2

100

10

99

>99

9

SYSU-Zn@IL2

60

12

74

>99

10e

SYSU-Zn@IL2

80

12

82

>99

11f

SYSU-Zn@IL2

25

120

80

98

12g

SYSU-Zn@IL2

80

16

99

>99

Reaction conditions: PO (3.0 mmol), catalyst (0.16 mol%), initial CO2 pressure 1.0 MPa

Determined by GC using biphenyl as an internal standard. mol%).

e

c

TBAB (0.4 mol%).

f

d

Keeping CO2 pressure at 0.5 MPa. Keeping CO2 pressure at 0.1 MPa.

b

Catalyst (0.08 g

Diluted CO2

(15 % CO2 in N2, v/v), initial CO2 pressure 3.0 MPa.


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After witnessing the successful applications for SYSU-Zn@IL2, the influence of reaction parameters, including catalyst loading, temperature and CO2 pressure, was further investigated. Unfortunately, an obviously decreased conversion of this substrate (yield of 70%) was obtained as the catalyst loading reduced from 0.16 to 0.08 mol % (entry 7, Table 1). Then, the effect of temperature on the catalytic performance was shown in Table 1 (entry 6, and 8-9). As expected, the yield of PC was strongly affected by the reaction temperature, which increased with temperature varying from 60 to 100 oC. When the temperature was increased to 100 oC, almost of PO was converted into PC within 10 h. It was worth mentioning that the selectivity remained the same even at enhanced temperatures. Finally, the reaction profile of SYSU-Zn@IL2 with CO2 pressure was also shown in this table and Figure S8. Notably, we were able to achieve a satisfactory yield of this substrate (yield of 82%) as the CO2 pressure reduced to 0.5 MPa (entry 10, Table 1). The PO yield increased gradually with the rise of reaction pressure in the low pressure range (0.1-1.0 MPa), but changed only slightly with further increase of pressure (1.0-2.0 MPa). This phenomenon can be explained by the fact that the concentration of CO 2 in the liquid phase increased with the rise of pressure at a low range, thereby enhancing gas–liquid diffusion.52,53 Moreover, with the moderate adsorption capacity of CO2 for SYSU-Zn@IL2 at ambient temperature (25 oC) and atmospheric CO2 pressure (0.1 MPa) understood, we decided to use this catalyst for further study. We were pleased to find that SYSU-Zn@IL2 still provided promising yield of PC by prolonging the reaction time to 120 h (80%, entry 11 in Table 1). It is particularly worth mentioning that almost no by-products (e.g. hydrolyzate: propanediol) was observed in this reaction for such a long time. This is probably resulted from the high concentration of CO2 around the active centers of this polymer54, the hydrophobicity of the abundant aromatic


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skeleton22, as well as the benefit of relative low porosity for completely removing water residues in catalyst. Finally, the above encouraging results prompted us to make great efforts for conversion of real-world CO2. For this purpose, with the CO2/N2 selective adsorption materials in hand, simulating flue gas (15% CO2 in N2) was used as a CO2 resource for this reaction, which was in line with the concept of energy saving and economic efficiency. To our delight, the SYSU-Zn@IL2 also displayed significant catalytic activity with low concentration of CO 2 to afford 99% yield of PC after 16 h at 80 oC and 3.0 MPa (entry 12, Table 1). 100

(A)

100 PC Yield (%)

80 PC Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0

80 60 40 20

0

2

4

6 8 Time (h)

10

12

14

0

1

2

3 4 Cycles

5

6

Figure 4. (A) Kinetic curve for the cycloaddition reaction between CO2 and PO catalyzed by SYSU-Zn@IL2 (PO 3.0 mmol, 0.16 mol% SYSU-Zn@IL2, 80 oC). (B) Recycle stability of SYSU-Zn@IL2 for the cycloaddition reaction (PO 3.0 mmol, 0.16 mol% SYSU-Zn@IL2, 80 oC, 12 h).

In consideration of the heterogeneous nature of our polymeric catalysts, recyclability and reusability of the selected SYSU-Zn@IL2 was assessed by performing PO/CO2 cycloaddition under the optimal reaction conditions. After each run, the insoluble catalyst can be easily recovered by centrifugation and reused directly for the next cycle after simply washing and drying. Figure 4B shows the results of reusing test, indicating that SYSU-Zn@IL2 can be recycled for at least six successive times with a slight decrease in catalytic activity, and the selectivity of product remains almost constant (>98%) during each reused process. The SEM image and FTIR spectrum of the reused SYSU-Zn@IL2 were confirmed, and the morphology


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and structure of the catalyst showed no obvious change (Figure S9 in the Supporting Information). Moreover, ICP analysis shows that recovered SYSU-Zn@IL2 has Zn content of 0.38 mmol·g-1, which is comparable to that of the fresh sample (0.40 mmol·g-1). This clearly verifies that it is a robust catalyst candidate for this reaction.

Substrate scope Next, motivated by the above encouraging results, we investigated the scope of substrates (1a– 1h) in the synthesis of a wide variety of cyclic carbonates (2a-2h) at 80 oC and 1.0 MPa using the selected catalyst SYSU-Zn@IL2. Fortunately, as shown in Figure 5, most of epoxides substituted with different functional groups could efficiently convert into desired products (2a2g) with appreciable isolated yields and high selectivity at only 0.16 mol% catalyst loading. The selectivities of all cyclic carbonate products remains almost constant (>98%), and almost no byproducts was observed in this reaction. Notably, small epoxides as substrates (2a-2c) were particularly suitable substrate partners and nearly quantitative yields were achieved within relatively short reaction time. With the increase of the substituted alkyl chains of epoxides, a clear yet steady decrease in the yield of cyclic carbonates was observed, as indicated by the 78 % formation of 2d and 47% formation of 2e from 1d and 1e, respectively. These observations could potentially be attributed to the diffusion limitation of large substrate molecules into the amorphous nanopore structures of SYSU-Zn@IL2. Moreover, the developed catalytic system had a certain degree of tolerance to various functional groups of epoxides, including alkenyl, chloromethyl, phenyl, glycidyl ether group, etc. The combined results distinctly demonstrated the high catalytic potential of our bifunctional solid catalysts for CO 2 conversion. However, we screened the transformation of a challenging internal epoxide 1h, and this afforded 2h with only 5 % yield under similar conditions, presumably owing to the steric-hindrance.55,


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Figure 5. Substrate scope in the synthesis of various cyclic carbonates 2a-2h from epoxides 1a–1h using SYSU-Zn@IL2 as catalyst (epoxide 3.0 mmol, 0.16 mol% SYSU-Zn@IL2; GC-yields of products are shown)

Reaction mechanism Based on our current observations and in line with the previous literatures,13,17,29,44 we proposed a tentative synergistic catalysis mechanism for the cycloaddition of CO2 with PO in the presence of bifunctional polymeric catalyst (Scheme 3). The catalytic cycle starts from a transition state of intramolecularly cooperative activation of epoxide. More specifically, epoxide binds with the Lewis acidic zinc site in the networks of SYSU-Zn@ILs by formation of Zn-O coordination bond. At the same time, a nucleophilic attack of bromide ion on the less-hindered side of epoxide give rise to the ring-opening of the activated epoxide, affording a zinc-bound bromo-alkoxide. Subsequently, the enriched CO2 around the zinc center inserted quickly into the Zn-O bond of opened epoxy. After completing the insertion of CO2, the intramolecular cyclization results in termination for that catalytic cycle to produce corresponding cyclic carbonate with regeneration of the integral catalyst. On account of the intramolecularly synergistic effect between an electrophilic zinc unit and a nucleophilic bromide, the developed bifunctional


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catalysts exhibited good catalytic activity for this reaction. In addition, the flexibility of skeleton for polymers, enriched zinc porphyrins and exposed ionic liquids might enhance the cooperative activation behavior. Moreover, the nanoporous structure with CO 2-philic nature of SYSUZn@ILs resulted in a higher CO2 concentration near abundant catalytic sites and made the catalytic reaction more efficient. The above-mentioned points could possibly be responsible for the efficient catalytic performance of the SYSU-Zn@ILs catalysts for CO2 conversion.

Scheme 3. Tentative mechanism for the coupling of epoxide and CO2 over SYSU-Zn@ILs

CONCLUSIONS

We developed the facile and green synthesis of two versatile bifunctional organic polymers containing metalloporphyrin and ionic liquid moieties via a one-pot solvothermal method. By using structural characterization, the resultant polymers demonstrated to have N-rich environments, hierarchical porous structure, as well as satisfactory CO2 uptake capacities and CO2/N2 selectivity. As a consequence, these promising bifunctional polymeric materials exhibited excellent catalytic activity for the cycloaddition of epoxides with CO2 to produce valuable cyclic carbonates in the absence of additives under mild conditions. Owing to the


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intramolecular cooperative behavior between Lewis acidic metal center and nucleophilic bromide ion as well as the polar CO2-philic nature, encouraging yield (80%) was achieved at ambient temperature (25 oC) and atmospheric CO2 pressure (0.1 MPa). Moreover, the selected catalyst SYSU-Zn@IL2 offered high yields (99%) at 80 oC and 3.0 MPa in catalyzing the coupling of CO2 with PO by using diluted CO2 (15% CO2 in N2, v/v) as a raw material. Additionally, it had good tolerance to a series of groups of terminal epoxides, and could be readily reused for six cycles without significant loss of catalytic activity and selectivity. Therefore, we expect that our effort will push forward the development of bifunctional polymeric materials and the applications of them as heterogeneous cooperative catalysts for CO2 transformation. ASSOCIATED CONTENT Supporting Information The following supporting information is available free of charge via the Internet at http://pubs.acs.org. Synthesis of ionic liquids-functionalized dialdehyde compounds (M1 and M2); synthesis of Zn-TAPP; dispersity and XPS of SYSU-Zn@IL2; adsorption selectivity of CO2 over N2 for SYSU-Zn@IL2 and SYSU-Zn@IL1; FTIR, TGA, sorption isotherms and pore size distributions of SYSU-Zn, SYSU-Zn@IL1 and SYSU-Zn@IL2; CHN and ICP-OES of SYSUZn, SYSU@IL, SYSU-Zn@IL1, SYSU-Zn@IL2 and recovered SYSU-Zn@IL2; NMR of cyclic carbonates. AUTHOR INFORMATION Corresponding Author


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*

Rongchang Luo. E-mail: [email protected]

*

Hongbing Ji. E-mail: [email protected]

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21676306 and 21425627), the Natural Science Foundation of Guangdong Province (2016A030310211), the National Key Research and Development Program of China (2016YFA0602900), and the Characteristic Innovation Project (Natural Science) of Guangdong Colleges and Universities (2016KTSCX004). REFERENCES (1)

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Multifunctional Heterogeneous POP Catalyst for Cooperative Transformation of CO 2 to Cyclic


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Carbonates.

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SYNOPSIS

BRIEFS Readily and greenly available bifunctional organic polymers have been successfully synthesized and employed as synergistic catalysts for the solvent-free production of cyclic carbonates from CO2 and epoxides with high activity and selectivity, good recyclability and wide substrate scope under mild conditions.


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Metalloporphyrin Polymers with Intercalated Ionic ... - ACS Publications

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