Cationic ZnâPorphyrin Polymer Coated onto CNTs as a Cooperative

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Cationic Zn-porphyrin Polymer Coated onto CNTs as Cooperative Catalyst for Synthesis of Cyclic Carbonates Sanjeevi Jayakumar, He Li, Jian Chen, and Qihua Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16045 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Applied Materials & Interfaces

Cationic Zn-porphyrin Polymer Coated onto CNTs as Cooperative Catalyst for Synthesis of Cyclic Carbonates Sanjeevi Jayakumar, a, b He Li, a Jian Chen, a, c and Qihua Yang * a a

State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b

International College, University of Chinese Academy of Sciences, Beijing 100049, China

c

University of Chinese Academy of Sciences, Beijing 100049, China

Corresponding author: Prof. Qihua Yang, E-mail: [email protected]. KEYWORDS: Cooperative activation, bifunctional catalyst, cationic porphyrin polymer, π-π interaction, CO2 cycloaddition reaction.

ACS Paragon Plus Environment

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ABSTRACT The development of solid catalysts containing multiple active sites that work in a cooperation mode is very attractive for biomimetic catalysis. Herein, we report the synthesis of bifunctional catalysts by supporting cationic porphyrin-based polymers on carbon nanotubes (CNTs) using direct reaction of 5,10,15,20-tetrakis(4-pyridyl)porphyrin zinc(II), di(1H-imidazol-1-yl)methane, 1,4-bis(bromomethyl)benzene in the presence of CNTs. The bifunctional catalysts could efficiently catalyze the cycloaddition reaction of epoxides and CO2 under solvent free condition with porphyrin zinc(II) as Lewis acid site and bromine anion as a nucleophilic agent working in a cooperative way. Furthermore, relative amount of porphyrin zinc(II) and quaternary ammonium bromide could be facilely adjusted for facilitating cooperative behavior. The bifunctional catalyst with TOF up to 2602 h-1 is much more active than corresponding homogeneous counterpart and among the highest heterogeneous catalysts ever reported under co-catalyst free conditions. The high activity is mainly attributed to the enhanced cooperation effect of bifunctional catalyst. With wide substrate scope, the bifunctional catalyst could be stably recycled. This work demonstrates a new approach for the generation of cooperative activation effect for solid catalysts.


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Introduction The conversion of carbon dioxide, the well-known C1 resource in chemical building blocks, into valuable chemicals has attracted much research attention nowadays.1-2 Addition of CO2 with epoxide into cyclic carbonates represents one of the important reactions relating with CO2 transformation.3-8 Various homogeneous organocatalysts9-11 and metal complexes12-20 were developed for catalytic cycloaddition of CO2 and epoxides. Generally, metal complexes and nucleophilic co-catalysts work in a cooperative activation pathway for catalytic cycloaddition reaction. For facilitating of this cooperative activation effect, metalloporphyrin-based cationic bifunctional complexes with enhanced catalytic activity towards cycloaddition reaction have been reported.21-27 However, homogeneous catalysts are difficult to be separated and reused. Furthermore, the purification of cyclic carbonates from homogenous reaction system is tedious and energy consuming. Heterogeneous catalysts could solve the above problems, and hence solid catalysts for CO2 cycloaddition

reactions

have

been

developed,

including

immobilized

metal

complexes/organocatalysts,28-30 porous polymers with integrated active sites (ILs or metal complexes etc.) in the network,31-35 carbon based materials36-38 and so on. Most of the solid catalysts still needed co-catalysts for the generation of cooperative activation during the catalytic process.14-18 Up to dates, several groups reported the synthesis of bifunctional solid catalysts towards co-catalyst free cycloaddition reaction. Ding and co-workers reported that zinc bromide immobilized on hierarchical porous polymer was highly efficient for the catalytic cycloaddition reaction under co-catalyst free condition.39-40 Ma and co-workers reported that effective cooperative activation could be generated by confining liner polymeric phosphonium salts in the nanopore of TpBpy-Cu-based COFs in cycloaddition reaction.41 The ionic imidazole


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functionalized metal–organic framework42-43 and salen-based cationic polymers12,44-45 have been synthesized and employed as bifunctionalized catalyst towards co-catalyst free cycloaddition reaction. The generation of porous structure for solid catalysts is very important for obtaining high catalytic activity via increasing accessibility of active sites to reactants. However, synthesis of cationic polymers with porous structure is still very difficult, though hard or soft template methods have been used.46 In addition to the synthesis of porous cationic polymer via template method, supporting nonporous materials on other porous support represents one of the efficient methods to increase the accessibility of active sites. Takada and co-workers reported the synthesis of BIO-IC by immobilization of metal-porphyrin complex on biogenous iron oxide.47 Precise position of different types of active sites on solid catalyst is necessary to fulfill their cooperation during catalytic process, which needs designed synthesis at molecular level to achieve this goal. In this work, we judiciously use 5,10,15,20-tetrakis(4-pyridyl)porphyrin zinc(II), di(1H-imidazol-1-yl)methane and 1,4-bis(bromomethyl)benzene as monomer and carbon nanotubes (CNTs) as support for the synthesis of CNTs supported cationic porphyrinbased polymers. By this approach, pyridyl/imidazole quaternary ammonium bromide and porphyrin zinc(II) are cross-linked very closely, which could facilitate their cooperation towards cycloaddition reaction. Via controlling the monomer ratio, the relative ratio of porphyrin zinc(II) and quaternary ammonium bromide could be facilely adjusted for optimizing the cooperative behavior of two different types of active sites. The usage of CNTs as a support will increase the exposure degree of cationic polymer active sites. The cationic polymer/CNTs hybrid materials showed high catalytic activity in cycloaddition of CO2 with epoxides under co-catalyst free conditons, implying that porphyrin zinc(II) and quaternary ammonium bromide integrated on


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CNTs could work smoothly in a cooperative way for ring opening of epoxides and subsquent CO2 insertion. Results and discussion Synthesis and characterization

A

B

N Br

Br N

N

N Zn N N

N

N

ZnTPy/CNTs-m

Zn-TPy

Br

+ BBMBr

Br

+

ZnTPy-CP

MBIM CNTs

ZnTPy-BIMn/CNTs-3

Scheme 1. Schematic illustration for the synthesis of (A) porphyrin based cationic polymer ZnTPy-CP and (B) cationic polymers coated CNTs, ZnTPy/CNTs-m and ZnTPy-BIMn/CNTs-3, where m refers to weight percent of Zn-TPy unit on CNTs and n refers to molar ratio of MBIM to Zn-TPy used in initial synthesis mixture. Different synthetic methods have been used for the synthesis of porphyrin based polymers such as various coupling reactions, Friedel-Crafts alkylation reactions15, 18 and so on. However, the co-incorporation of different type of active sites with precisely controlled proximity is difficult via above methods. In this work, CNTs supported cationic polymers were constructed by direct reaction of 5,10,15,20-tetrakis(4-pyridyl)porphyrin zinc(II) (Zn-TPy), di(1H-imidazol1-yl)methane (MBIM), 1,4-bis(bromomethyl)benzene (BBMBr) and CNTs (Scheme 1). For


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comparison, pure cationic polymer, ZnTPy-CP was also synthesized. The polymerization of cationic Zn-TPy on CNTs results in the formation of bifunctional materials. The loading amount of cationic polymer on CNTs and the ratio of Zn-TPy to MBIM were facilely varied during the one-pot synthesis process for optimizing the catalytic performance of bifunctional materials. ZnTPy-BIM4/CNTs-3

C

N

Br

Zn

250 nm

Figure 1. SEM image (left) and corresponding EDX elemental (C, N, Br, Zn) mapping images of ZnTPy-BIM4/CNTs-3. The SEM images showed that all ZnTPy-BIMn/CNTs-m samples with polymer loading from 5 to 50 wt% had tubular morphology, identical to parent CNTs (Figure 1 and S1). The SEM images of ZnTPy-BIMn/CNTs-m samples were quite different from that of ZnTPy-CP composed of aggregated nanoparticles with size ranging from several hundred nanometers to micrometers (Figure S1). The EDX mapping of ZnTPy/CNTs-28 gave signals of C, N, Br and Zn, suggesting that cationic polymer was loaded on CNTs (Figure 1 and S1). The TEM image of ZnTPyBIM4/CNTs-3 clearly confirmed that the polymer layer adhered closely on CNTs (Figure 2). With polymer loading increasing, the thickness of polymer layer on CNTs also increased as evidenced by the increment in the diameter of nanotubes. The TEM and SEM characterizations suggested that cationic polymer was uniformly supported on CNTs. The strong π-π interaction


6

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among Zn-porphyrin precursor and CNTs was the main reason for successful supporting of cationic polymer on CNTs.48-50

Figure 2. TEM images of (a) CNTs, (b) ZnTPy/CNTs-28, (c) ZnTPy/CNTs-19, (d) ZnTPy/CNTs-10 and (e-f) ZnTPy-BIM4/CNTs-3 (scale bar is 25 nm). The porous structure of cationic polymer and cationic polymer coated CNTs was investigated and the textural data are summarized in Table 1. ZnTPy-CP only had BET surface area of 16 m2 g-1, implying that cationic polymer did not have porosity. CNTs used as a support in this work had BET surface area of 202 m2 g−1 and pore volume of 0.80 cm3 g−1. All polymer/CNTs hybrid samples had larger BET surface area and pore volume than ZnTPy-CP, verifying that supporting nonporous polymer on CNTs was efficient for increasing both BET surface area and pore volume. As the polymer content on CNTs increased, the BET surface area and pore volume gradually decreased because cationic polymer had no porosity.


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Table 1. Physiochemical parameters of cationic polymer and cationic polymer coated CNTs materials. Sample

SBET (m2/g)

Vp

N[a]

Zn[b]

(cm3/g)

(mmol/g)

(mmol/g)

Zn/Br ratio[c]

ZnTPy-CP

16

-

7.32

0.90

1:4

CNTs

202

0.86

-

-

-

ZnTPy/CNTs-28

84

0.40

3.34

0.41

1:4 (1:3.2)

ZnTPy/CNTs-19

100

0.37

2.37

0.29

1:4

ZnTPy/CNTs-10

125

0.48

1.19

0.15

1:4

ZnTPy/CNTs-3

122

0.73

0.39

0.05

1:4 (1:3.7)

ZnTPy-BIM4/CNTs-3

101

0.58

0.96

0.05

1:10 (1:9.3)

ZnTPy-BIM12/CNTs-3

93

0.53

1.95

0.05

1:20 (1:19)

a

Calculated based on C, H, N elemental analysis. b Calculated based on ICP analysis. c For

MBIM functionalized samples, the Br content was calculated by using the formula N(Br) = ; data in the parenthesis refers to Zn/Br ratio obtained with XPS analysis. Zn and N content of all samples were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and C, H, N elemental analysis, respectively (Table 1). ZnTPy-CP had Zn content of 0.90 mmol/g and N content of 7.32 mmol/g. The Zn content of CNTs supported samples varied in the range of 0.05 to 0.41 mmol/g. As the monomer content in initial mixture increased, the Zn content increased monotonously. The molar ratio of N to Zn was ca. 8 for ZnTPy/CNTs-m samples. For ZnTPy-BIMn/CNTs-m samples, N to Zn molar ratio was higher than 8 due to the introduction of MBIM. Based on Zn and N content, the Br to Zn molar


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ratio is calculated (Table 1, see note). For ZnTPy-CP and ZnTPy/CNTs-m, the molar ratio of Br to Zn is ca. 4, suggesting the stoichiometric reaction of Zn-TPy and BBMBr. The molar ratio of Br to Zn has large influence on the catalytic activity in CO2 cycloaddition reaction.23 For adjusting of Br to Zn ratio, MBIM was used together with Zn-TPy in the synthesis of ZnTPyBIMn/CNTs-m. As per the expection, Br to Zn molar ratios of ZnTPy-BIM4/CNTs-3 and ZnTPy-BIM12/CNTs-3 were increased to ca. 10 and 20, respectively. These results were close to the theoretical values, indicating that relative content of two types of active sites, an important factor for cooperative catalysis, could be facilely tuned by controlling the monomer content in the initial mixture. The chemical composition of ZnTPy-CP and ZnTPy/CNTs-28 was characterized with FT-IR spectroscopy (Figure 3A). Both samples showed the characteristic vibration peak of pyrrole rings from Zn-porphyrins at 993 cm-1 in the FT-IR spectra, demonstrating the existence of Znporphyrin moieties.51-53 Comparing to Zn-TPy monomer, a new peak at 1156 cm-1 assigned to alkyl C-N stretching and a red shift of C=N bond of pyridine moieties from 1593 to 1629 cm-1 were observed in the FT-IR spectrum of ZnTPy-CP, which indicated the formation of pyridine iminium ion (-C=N+-). Besides, a new band at 1452 cm-1 in the FT-IR spectrum of ZnTPy-CP confirmed the existence of methylene groups. The FT-IR spectrum of ZnTPy/CNTs-28 was almost identical to that of ZnTPy-CP.


9


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A

a

B

f d b

c e b c

g

* *

a* *

(a) (b)

3000

1500

1000

Wavenumber (cm-1)

250 500

200

150

100

50

0

-50

Chemical shift (ppm)

Figure 3. (A) FT-IR spectra of (a) ZnTPy (b) ZnTPy-CP and (c) ZnTPy/CNTs-28 and (B) 13C CP-MAS NMR spectra of (a) ZnTPy-CP and (b) ZnTPy/CNTs-28. The solid-state 13C CP-MAS NMR spectra of ZnTPy-CP and ZnTPy/CNTs-28 are shown in Figure 3B. The signal at 63 ppm (a) was from the methylene carbon attached with pyridyl quaternary ammonium linkage. The peak at 116 ppm (b) could be assigned to the carbon of porphyrin macro cycle attached with pyridyl ring. The peak for carbon of pyridyl ring attached with porphyrin macro cycle was found at 157 ppm (g). The signals at 124 (c) and 139 ppm (e) were corresponding to the carbons of pyrrole ring in porphyrin. The resonance peaks at 129 (d) and 148 ppm (f) could be assigned to the carbons of benzene ring and carbon adjacent to pyridylN respectively. The

13

C CP−MAS NMR spectrum of ZnTPy/CNTs-28 was almost identical to

that of ZnTPy-CP, indicating successful loading of cationic polymer on CNTs via a one-pot synthesis method. The compositions of ZnTPy-CP and ZnTPy-BIMn/CNTs-m were further characterized by Raman spectroscopy (Figure 4). The Raman spectrum of ZnTPy-CP displayed three strong peaks at 1345 cm-1, 1492 cm-1 and 1548 cm-1, which were characteristic signals of Cα–N and Cβ–Cβ stretching/bending for porphyrin skeleton. Similar results were found for CNTs-supported


10

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materials.54-55 Besides, for CNTs supported samples, the peaks located at 1360 cm-1 and 1595 cm-1 were derived from D and G bands of CNTs, respectively. The combined results of FT-IR, Raman and NMR spectroscopy confirmed the formation of cationic polymer and the successful loading of cationic polymer on CNTs.

d c

b a 2000 1800 1600 1400 1200 1000

800

600

wavenumber (cm-1)

Figure 4. Raman spectra of (a) ZnTPy-CP, (b) ZnTPy/CNTs-3, (c) ZnTPy-BIM4/CNTs-3, (d) CNTs. The characteristic samples, ZnTPy/CNTs-28, ZnTPy/CNTs-3, ZnTPy-BIM4/CNTs-3 and ZnTPy-BIM12/CNTs-3 were characterized with X-ray photoelectron spectroscopy (XPS) (Figure 5, S2-S4). The binding energy of Zn 2p3/2 for all samples appeared at 1021.9 eV similar to that of porphyrin zinc(II).56-57 The C 1s spectra of all samples could be classified into sp2 carbon at 284.4 eV, and sp3 carbon at 285.4 eV. In addition, the peak at 290.3 eV could be attributed to π-π interactions (Figure 5).50 The N 1s spectra for all samples gave binding energies for pyridinic (-C=N-) and pyrrolic nitrogen of the porphyrin ring at 398.7 eV and 399.9 eV, respectively. The peak appeared at 401.8 eV could be assigned to the pyridinium and imidazolinium cationic (N+) species.57-59 The binding energies of Br- 3d level appeared at 67.8 eV (3d5/2) and 68.9 eV (3d3/2) for all samples. 57, 60-61 The peaks for Br (C-Br) 3d level at 70.6 eV


11


(3d5/2) and 71.8 eV (3d3/2) were very weak. The above results verified that most of the Br was in the state of Br- for the hybrid materials. Based on XPS data, the molar ratios of Zn/Br were 1: 3.2, 1: 3.7, 1: 9.3 and 1: 19.1 respectively for ZnTPy/CNTs-28, ZnTPy/CNTs-3, ZnTPyBIM4/CNTs-3 and ZnTPy-BIM12/CNTs-3, which were slightly lower than the calculated values (Table 1).

A

B

Raw Sum Pyridinic Pyrrolic Quaternary

Raw Sum

Intensity (a.u.)

Intensity (a.u.)

C(sp2) C(sp3) ∗

π−π

294

292

290

288

286

284

282

408 406 404 402 400 398 396 394 392

Binding Energy (eV)

Binding Energy (eV) Raw Sum Br- 3d

C

D

1021.9 Zn(2p3/2)

Br- 3d3/2 C-Br 3d5/2 C-Br 3d3/2

74

72

70

68

66

Binding Energy (eV)

64

Intensity (a.u.)

5/2

Intensity (a.u.)

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

Page 12 of 34

1045.0 Zn(2p1/2)

1050 1045 1040 1035 1030 1025 1020 1015

Binding Energy (eV)

Figure 5. (A) C 1s, (B) N 1s, (C) Br 3d, and (D) Zn 2p3/2 XPS spectra of ZnTPy-BIM4/CNTs-3. The thermal stability of characteristic ZnTPy-CP, ZnTPy/CNTs-3 and ZnTPy-BIM4/CNTs3 was tested using thermal gravimetric analysis (Figure 6). All samples afforded two obvious weight loss steps at temperature higher than 300 °C in air atmosphere. It suggested that supported and unsupported cationic polymer had enough thermal stability for catalytic CO2


12

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cycloaddition reaction. As compared with ZnTPy-CP, the temperature of second weight loss step for ZnTPy/CNTs-3 and ZnTPy-BIM4/CNTs-3 shifted from 350 to 450 °C. It confirmed that cationic polymer supported on CNTs had higher thermal stability than pure polymer, possibly due to the π-π interaction of cationic polymer with CNTs and also, the different polymerization degree of these materials.

100

Weight loss (%)

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


80 60 40 20

a b c

0 200

400

600

800

o

Temperature( C)

Figure 6. TG curves of (a) ZnTPy-CP, (b) ZnTPy/CNTs-3, (c) ZnTPy-BIM4/CNTs-3 in air atmosphere. Catalytic performances of supported and unsupported cationic polymers in CO2 cycloaddition reaction under co-catalysts free conditions The catalytic performances of cationic polymer and supported cationic polymer were investigated in CO2 cycloaddition reaction with propylene oxide (PO) as model substrate under co-catalyst free conditions (Table 2). All supported and unsupported cationic polymer smoothly catalyzed the cycloaddition reaction to afford 64 to 99% yield with 99% selectivity to propylene carbonate (PC) under co-catalyst free conditions. With ZnTPy/CNTs-3 as a model catalyst, the yield of PC reached to 99% within 2.5 h (Figure 7).


13

100

100

90

90

80

80

70

70

60

60

Selectivity (%)

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

Page 14 of 34

50

50

Yield Selectivity

40

Yield (%)


40 30

30 0.5

1.0

1.5

2.0

2.5

3.0

3.5

Time (h)

Figure 7. Kinetic curves for PO cycloaddition reactions using ZnTPy/CNTs-3 as the catalyst. Table 2. Catalytic performance of supported, unsupported cationic polymer and other solid catalysts in the synthesis of cyclic carbonate with PO and CO2 under co-catalyst free conditions. Catal.

T

P

Time

Yield

Sel.

TOF

(°C)

(MPa)

(h)

(%)

(%)

(h-1)

S/C

Zn-TPy + TBAB[a]

1450

120

1.5

2.5

54

>99

299

Zn-TPy + TBAB[a]

7100

120

1.5

2.5

33

>99

468

ZnTPy-CP

1450

120

1.5

2.5

64

>99

471

ZnTPy/CNTs-28

1450

120

1.5

2.5

73

>99

602

ZnTPy/CNTs-19

1450

120

1.5

2.5

79

>99

706

ZnTPy/CNTs-10

1450

120

1.5

2.5

89

>99

945

ZnTPy/CNTs-3

1450

120

1.5

2.5

99

>99

1065

ZnTPy/CNTs-3

7100

120

1.5

2.5

76

>99

1896

ZnTPy-BIM4/CNTs-3

1450

120

1.5

2.5

98

>99

1650

ZnTPy-BIM12/CNTs-3

1450

120

1.5

2.5

94

>99

1216

ZnTPy-BIM4/CNTs-3

7100

120

1.5

6

95

>99

2602

Mg-por/pho@POP14

20000

140

3

1

78

>98

15600

SYSU-Zn@IL257

625

100

0.1

10

99

>99

-


14

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a

1P+Br-&ZnCl2-1PPh3@POPs39

8000

120

3

1

22

-

1765

NH3I-Zn/SBA-1528

-

150

3

12

99

99

326

T-IM62

1540

150

1

10

87

-

134

ZIF-2263

500

120

1.2

2

99

99

237

Zn-MOF64

715

110

3

6

92

99

110

BIO-1C47

1000

120

1.7

6

90

-

-

Molar ratio of TBAB/Zn-TPy is 4. The cationic polymer, ZnTPy-CP catalyzed the reaction to afforded 64% conversion of PO.

Under similar conditions, homogeneous catalyst Zn-TPy gave PO conversion of 54% using TBAB as co-catalyst (the molar ratio of Br to Zn is ca. 4, similar to that of heterogeneous system). The TOF of ZnTPy-CP (471 h-1) was over 1.5 fold that of Zn-TPy, indicating that close contact of porphyrin Zn(II) and bromide anion in cationic polymer could enhance the cooperative activation of epoxide and CO2 during catalytic process. The BET surface area of ZnTPy-CP was only 16 m2/g, therefore, most of active sites were buried in the matrix of cationic polymer and could not be accessed by reactants. The diffusion barrier of reactants and products is also a big issue for nonporous materials. For increasing the BET surface area and decreasing diffusion resistance, cationic polymers were supported on CNTs. ZnTPy/CNTs-28 showed higher PC yield (73% versus 64%) and TOF (602 h-1 versus 471 h-1) than those of pure cationic polymer. CNTs had no activity in cycloaddition reaction. This indicated that the higher activity of supported cationic polymer was mainly attributed to the increment in BET surface area. Because cationic polymer had no porosity, only outer surface of ZnTPy/CNTs-28 contributed to the catalytic activity. Decreasing the thickness of polymer layer on CNTs favored the increase in the dispersion degree of active sites. Thus, the loading content of Zn-TPy on CNTs was tuned from 28 wt% to 3 wt%. Based on TEM characterizations, the polymer thickness on CNTs decreased with decreasing of polymer loading. Overall, the PC yield increased monotonically


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Page 16 of 34

from 73% to 99% as Zn-TPy content decreasing from 28 wt% to 3 wt%. The similar tendency was also observed for TOF (from 602 h-1 to 1065 h-1). This was mainly due to high dispersion degree of active sites and fast diffusion rate of reactants and products. Supported and unsupported cationic polymer could efficiently catalyze cycloaddition reaction, indicating that the cationic polymer with bifunctional active sites (porphyrin Zn(II) and Br anion) acted as cooperative catalyst. According to previous reports21-24 and present experimental observations, the cooperative activation mechanism of cationic polymer for cycloaddition reaction is summarized in Scheme 2. Here, Porphyrin Zn(II) acts as a Lewis acid, and the Br– serves as the nucleophilic agent. Initially, the oxygen atom of epoxide was bonded with Lewis acidic Zn to form a Zn-O coordination bond for activation of epoxide. Then, Br– anion shifted from pyridine cation to attack less sterically hindered β-carbon of epoxide for the formation of Zn-coordinated Br-alkoxide intermediate. Subsequently, CO2 inserted into Zn-O bond, leading to the formation of Zn-carbonate intermediate. Finally, the corresponding cyclic carbonate was produced after an intramolecular ring-closure step, and the catalyst was regenerated.


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Scheme 2. Proposed mechanism of bifunctional catalyst for the cycloaddition reaction. The above reaction mechanism suggested that Br to Zn ratio and their distant may have big influence on catalytic performance of cationic polymer. Thus, ZnTPy-BIM4/CNTs-3 and ZnTPy-BIM12/CNTs-3 with Br to Zn ratio respectively of 10 and 20 were synthesized by adjusting initial ratio of MBIM to Zn-TPy. The TOF followed the order of ZnTPy-BIM4/CNTs-3 (1650 h-1)> ZnTPy-BIM12/CNTs-3 (1216 h-1)> ZnTPy/CNTs-3 (1065 h-1). The activity of cationic polymer increased with increasing of Br to Zn ratio and reached the maximum at Br to Zn ratio of 10. The decrease in activity was observed by further increasing of Br to Zn ratio. For heterogeneous cycloaddition reaction with small molecules as co-catalysts, higher amounts of co-catalysts may benefit higher activity by facilitating cooperation of different types of active sites.23 But, for cationic polymer with porphyrin Zn(II) and Br anion both integrated in polymer network, the higher amounts of Br and BIM units increased the distance between porphyrin Zn(II) and Br and also, hindered the accessibility of active sites to substrate. These were not


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advantageous for the cooperation during catalytic process. Thus, decrease in activity was observed for ZnTPy-BIM12/CNTs-3. Even with S/C ratio as high as 7100, ZnTPy-BIM4/CNTs-3 still could smoothly catalyze the cycloaddition reaction to obtain 95% PC yield with TOF of 2602 h-1. ZnTPy/CNTs-3 afforded only 76% PC yield with TOF of 1896 h-1. This confirmed that ZnTPy-BIM4/CNTs-3 was more active than ZnTPy/CNTs-3. Under similar conditions, homogeneous Zn-TPy only gave TOF of 468 h-1. The low activity of ZnTPy at high S/C ratio is mainly due to the low concentration of active sites, which could not efficiently generate the cooperation. The high activity of solid catalyst at high S/C ratio suggested the enhanced cooperation in cationic polymer. At high S/C ratio, ZnTPy and Br- integrated in polymer network still worked smoothly in a cooperative mode because their local concertrations in cationic polymer did not change with the variation in S/C ratio. Under co-catalyst free conditions, the catalytic activity of ZnTPy-BIM4/CNTs-3 outperformed those of reported bifunctional catalysts such as metal porphyrin polymers,57 immobilized catalysts,28,47 porous organic polymers,39-40 zeolitic imidazolate framework,63 metalorganic frameworks64 and zinc based ionic liquids65 (Table 2). The catalytic performance of cationic polymer was also tested at ambient conditions with ZnTPy-BIM4/CNTs-3 as model catalyst and styrene oxide (SO) as substrate (Figure 8). SO was almost completely converted into corresponding cyclic carbonate with selectivity of 99% at 25 o

C and 1 bar of CO2, indicating that ZnTPy-BIM4/CNTs-3 was an efficient bifuncional catalyst

for cycloaddition reaction under ambient conditions.


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100

Conversion & Selectivity (%)

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


Conversion (%) Selectivity (%)

80 60 40 20 0 0

20

40

60

80

100

120

Time (h)

Figure 8. Kinetic curves for SO cycloaddition reaction using ZnTPy-BIM4/CNTs-3 as a catalyst under 25 oC and 1 bar of CO2. The substrate scope of ZnTPy-BIM4/CNTs-3 was investigated and results are summarized in Scheme 3. ZnTPy-BIM4/CNTs-3 could efficiently convert a wide range of terminal epoxides into corresponding cyclic carbonates with high conversion and selectivity. For epichlorohydrin (EPH), SO and 1,2-epoxyhexane, ZnTPy-BIM4/CNTs-3 showed high activity with TOF varying in the range of 1842 to 1205 h-1. Butyl glycidyl ether with a long alkyl chain length could be also converted into corresponding cyclic carbonate with 85% conversion and TOF of 663 h-1. This suggested that the active sites of ZnTPy-BIM4/CNTs-3 could be accessed by substrate with large size due to high dispersion of cationic polymer on CNTs. ZnTPy-BIM4/CNTs-3 also catalyzed a very challenging epoxide, cyclohexene oxide (CHO) into corresponding cyclic carbonate with conversion of 51%. The above results showed good substrate universality of bifunctional catalysts.


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Scheme 3. The catalytic performance of ZnTPy-BIM4/CNTs-3 for cycloaddition of different types of epoxides with CO2.

Conv. (%)

98

95

85

96

51

Time (h)

2.5

4

4

4

24

TOF (h-1)

1842

1205

663

1292

484

Reaction conditions: S/C = 1450; CO2, 1.5 Mpa; 120 ºC. TOF was calculated at less than 35% conversion and S/C = 7100. The stability of a heterogeneous catalyst is an important factor for its practical application. Therefore, the recycle experiment of ZnTPy-BIM4/CNTs-3 was carried out with EPH as substrate and results are depicted in Figure 9. During recycle process, the yield of 4(chloromethyl)-1,3-dioxolan-2-one was kept at ca. 60% for evaluating the stability of cationic polymer. The catalyst was recovered by a simple centrifugation followed by washing with dichloromethane and dried under vacuum at 70 oC for 3 h. ZnTPy-BIM4/CNTs-3 was very stable and could be recycled at least for seven times without obvious loss in the catalytic activity and selectivity. The high stability of ZnTPy-BIM4/CNTs-3 was possibly due to the strong π-π interaction between porphyrin and CNTs.48-49 C, H, N and ICP-AES analysis of reused samples showed that the Zn (0.05 mmol/g) and N (0.96 mmol/g) content were almost identical to the fresh one. All above results confirmed that CNTs supported cationic Zn-porphyrin polymer was stable during the catalytic process.


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Yield (%)

100

Yield & selectivity (%)

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


Selectivity (%)

80 60 40 20 0 1

2

3

4

5

6

7

Run

Figure 9. Recycling stability of ZnTPy-BIM4/CNTs-3 for cycloaddition of CO2 and EPH. Reaction conditions: S/C = 1450, 1.5 MPa of CO2, 120 ºC, 1 h. CONCLUSION In summary, cationic polymer and CNTs supported cationic polymer were facilely prepared by a one-pot reaction approach, in which the relative amount of porphyrin zinc(II) and quaternary ammonium bromide could be facilely adjusted by controlling the monomer ratio in initial synthesis. Under co-catalysts free conditions, these bifunctional catalysts were efficiently catalyzed for the cycloaddition reaction of CO2 and epoxides via a cooperative reaction pathway with porphyrin zinc(II) as Lewis acid site and bromine anion as a nucleophilic agent. The catalytic activity of cationic polymer was increased greatly by supporting on CNTs, mainly attributed to the increase of BET surface area. Under similar reaction conditions, CNTs supported cationic polymer exhibited much higher activity than homogeneous counterpart due to the enhancement of cooperative activation effect on solid catalyst. By adjusting polymer content on CNTs and molar ratio of porphyrin zinc(II) to quaternary ammonium bromide, the optimized


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solid catalyst afforded TOF up to 2602 h-1 in cycloaddition reaction of propylene oxide with CO2, which is among the highest ever reported for heterogeneous cycloaddition reaction under cocatalysts free conditions. The high efficiency of bifunctional catalysts reported in this work was mainly attributed to enhanced cooperative behaviour of porphyrin zinc(II) and Br integrated in polymer network. Integration of different types of active sites in solid network may provide the possibility for the synthesis of solid catalysts with cooperative activation effect. Experimental section Chemicals and reagents All reagents were analytical grade and used as received unless otherwise stated. 1,4bis(bromomethyl)benzene (BBMBr) was purchased from Heowns Biochem LLC (China). Carbon nanotubes with a length of 0.5-2 µm and an OD (Outer Diameter) of 10-20 nm were purchased from Timesnano Inc. 5,10,15,20-Tetrakis(4-pyridyl)-21H,23H-porphine,66 5,10,15,20Tetrakis(4-pyridyl)porphyrin zinc(II) (Zn-TPy),67 oxidized CNTs68 and di(1H-imidazol-1yl)methane69 were prepared according to the previous literature methods. Characterization The C, H, N elemental analysis was performed with an Oxygen/Nitrogen/Hydrogen Analyzer (EMGA-930, HORIBA, Japan) and a Carbon/Sulfur Analyzer (EMIA-8100, HORIBA, Japan). The N2 sorption isotherm measurement was carried out on a Micromeritics ASAP2020 volumetric adsorption analyzer. Before the sorption measurements, samples were out-gassed at 373 K for 6 h. The BET surface area was calculated from the data in the relative pressure range P/P0 of 0.04 to 0.20. The total pore volume was estimated from the amount adsorbed at P/P0 of 0.99. The pore diameter was determined from the adsorption branch by the BJH method. X-ray photoelectron spectroscopy (XPS) was recorded on a VG ESCALAB MK2 apparatus by using


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AlKα (hλ = 1486.6 eV) as the excitation light source. Scanning electron microscopy (SEM) was undertaken using a FEI Quanta 200F scanning electron microscope operating at an acceleration voltage of 2-30 kV. TEM characterization was performed by using a HITACHI HT7700 microscope at an acceleration voltage of 100 kV. The TGA analysis was performed by using a NETZSCH STA 449F3 analyzer from 30 to 900 oC with the heating rate of 10 oC min-1 under air atmosphere. FT-IR spectra were recorded on a Nicolet Nexus 470 IR spectrometer (KBr pellets were prepared). RAMAN spectra were recorded at room temperature on a Jobin Yvon Lab RAM HR 800 instrument with 532 nm excitation laser at the power of around 0.2 mW. The solid-state 13

C CP-MAS NMR spectra were obtained with a Bruker 500 MHz spectrometer equipped with a

magic-angle spin probe using a 4 mm ZrO2 rotor.

13

C signals were referenced to glycine

(C2H5NO2). The experimental parameters were as followings. 8 kHz spin rate, 5 s pulse delay, and 2500 scans. Zinc content in the samples was determined by PLASAM-SPEC-II inductively coupled plasma atomic emission spectrometry (ICP-AES). Synthesis of porphyrin based cationic polymer ZnTPy-CP To a suspension of Zn-TPy (341 mg, 0.5 mmol) in 25 mL THF, 1,4bis(bromomethyl)benzene (BBMBr) (264 mg, 1 mmol) was added and the mixture was refluxed for 90 h under nitrogen atmosphere. The resulting black precipitates were washed 3 times with chloroform. The product was extract with chloroform in a Soxhlet appartus for 48 h, and dried at 80 ºC under vacuum for 12 h. Synthesis of CNTs-supported cationic polymers ZnTPy/CNTs-m To a 100 mL flask, desired amount of oxidized CNTs (pore diameter of 10-20 nm) and 15 mL THF were added and the suspension was dispersed under sonification for 30 minutes. Then, Zn-TPy (110 mg, 1.6×10-4 mmol) and BBMBr (89 mg, 3.3×10-4 mmol) were added and the


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system was refluxed for 90 h under nitrogen atmosphere. After cooling down to room temparature, the product was collected by filtration. The resulting materials were washed 3 times with THF and chloroform, then dried at 70°C for 6 h under vacuum. The sample was donated as ZnTPy/CNTs-m, where m refers to the weight percent of Zn-TPy on CNTs. Synthesis of CNTs-supported cationic polymers ZnTPy-BIMn/CNTs-3 (n = 4 or 12) To a 100 mL flask, 1 g of oxidized CNTs (pore diameter of 10-20 nm) was dispersed in 40 mL of THF and the mixture was treated under sonication for 30 min, followed by addition of ZnTPy (35.5 mg, 5.2×10-5 mmol), BBMBr (82 or 164 mg, 3.12×10-4 or 6.24×10-4 mmol) and di(1Himidazol-1-yl)methane (MBIM) (30.7 or 61.4 mg, 2.08×10-4 or 4.15×10-4 mmol). After sonication for one min, the mixture was heated at 70 °C for 90 h under nitrogen atmosphere. The solid was filtered, washed with THF and chloroform and then, dried at 60°C for 3 h under vacuum. The sample was donated as ZnTPy-BIMn/CNTs-3, where n refers to the molar ratio of MBIM to ZnTPy. General procedure for the cycloaddition reaction of CO2 and epoxides. The reactions were carried out in a 15 mL stainless steel reactor equipped with a magnetic stirrer. A typical reaction was carried out as following: The propylene oxide (PO) (770 mg, 13.27 mmol) and catalyst (0.069 mol% based on Zn) were placed into the reactor at room temperature and purged with CO2 for 3 times to remove air. Then, the pressure was gradually increased to 1.5 MPa and the autoclave was kept in the pre-heated oil bath. After stirring with 540 rpm at 120 °C for 2.5 h, the reactor was cooled in ice water. After slowly releasing CO2, appropriate amount of n-butyl acetate (as external standard for GC analysis) was added. The liquid products were analysed on a gas chromatograph (Agilent 7890 GC) equipped with a PEGcolumn (30 m×0.25 mm×0.25 mm).


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For other epoxides such as epichlorohydrin, 1,2-epoxyhexane, butyl glycidyl ether, styrene oxide and cyclohexene oxide, similar procedure was used instead of propylene oxide, with the exception that 6.63 mmol of epoxide was added. The procedure for the room temperature reaction was conducted as following: styrene oxide (90 mg, 0.75 mmol), catalyst (0.4 mol% based on Zn) and the internal standard biphenyl were fixed in a Schlenk tube equipped with a magnetic stirrer. Then, the tube was purged with a balloon filled with CO2 at 0.1 MPa, and stirred at a speed of 350 rpm at 25 ºC for 120 h. For catalyst recycling, the solid catalyst was recovered by a simple centrifugation. After washing carefully with dichloromethane, and dried under vacuum at 60 oC for 3 h, the recovered catalyst was used directly for the next run. ASSOCIATED CONTENT Supporting Information Additional SEM images, EDX images and XPS spectra. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; ORCID ID: 0000-0002-1118-3397. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (No. 2017YFB0702800), the NSFC (No. 21232008, 21621063, 21733009) and the Strategic Priority


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Research Program of the Chinese Academy of Sciences Grant (No. XDB17020200). J.S. thanks the International College, University of Chinese Academy of Sciences (UCAS), and The World Academy of Sciences (TWAS) for the award of CAS-TWAS President’s Research Fellowship.

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Table of Contents

TOF= 2602 h-1 at 120 ºC

Br N N N N Br

CO2


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Cationic ZnâPorphyrin Polymer Coated onto CNTs as a Cooperative

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