Synthesis of Porous Polymeric Catalysts for the Conversion of Carbon

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Synthesis of Porous Polymeric Catalysts for the Conversion of Carbon Dioxide Kuan Huang, Jia-Yin Zhang, Fujian Liu, and Sheng Dai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02151 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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ACS Catalysis

Synthesis of Porous Polymeric Catalysts for the Conversion of Carbon Dioxide Kuan Huang,†* Jia-Yin Zhang,† Fujian Liu,‡* and Sheng Dai§ǁ

†Key Laboratory of Poyang Lake Environment and Resource Utilization of Ministry of Education, School of Resources Environmental and Chemical Engineering, Nanchang University, Nanchang, Jiangxi 330031, China; E-mail: [email protected] ‡National Engineering Research Center for Chemical Fertilizer Catalyst (NERC-CFC), School of Chemical Engineering, Fuzhou University, Fuzhou, Fujian 350116, China; E-mail: [email protected] §Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA. ǁChemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

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ABSTRACT

As the main greenhouse gas, CO2 capture and its catalytic conversion has been thought to be a significant issue to be solved in these years. Due to the thermodynamically stable and inert nature of CO2, it is highly desirable to develop advanced catalysts to facilitate the transformation of CO2 to other high value-added chemicals under mild conditions. Within this regard, porous organic polymers (POPs), featuring large surface areas, high thermal stabilities, diverse building blocks, and tunable porous structures, are an ideal platform for the construction of heterogeneous catalysts for CO2 conversion. Incorporating active sites that are capable of activating CO2 and/or substrates into the frameworks of POPs can facilitate CO2 conversion. In this review, the most recent advances in the design and synthesis of POP-based heterogeneous catalysts for the conversion of CO2 are summarized. We mainly focus on the synthetic strategies researchers have used for incorporating active sites into POP frameworks to prepare heterogeneous catalysts for CO2 conversion, including N-doping, metalation, and ionic functionalization. Problems remaining to be addressed in this field are analyzed, and future directions are outlined. KEYWORDS: Porous Organic Polymers; Heterogeneous Catalysis; CO2 Conversion; Greenhouse gas; Reusability.


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1. INTRODUCTION The continual consumption of fossil fuels since the Industrial Revolution has caused the atmospheric concentration of CO2 to increase by 25 % over the past century.1 CO2 is one of the most important greenhouse gases; and its accumulation in the atmosphere may cause multiple disastrous climatic and environmental effects, for example, global warming and sea level rise. There is high demand for controlling CO2 emissions resulting from the combustion of fossil fuels in industrial activities. In this context, there are proposals for CO2 capture, which typically is realized by absorption in liquids or adsorption on solids.2-4 The captured CO2 is then stripped out by a temperature and/or pressure swing, followed by compression and storage. However, the application of CO2 capture technologies in industry is quite limited because of the high costs of capture. It is estimated that $59.1 is spent for each ton of CO2 captured by the amine-scrubbing process in a typical coal-fired power plant.5 The high cost of CO2 capture arises mainly from the intensive energy required for CO2 desorption and compression.

Figure 1. Representative pathways for CO2 conversion.


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On the other hand, CO2 is a cheap, renewable, nontoxic C1 feedstock for the production of a wide range of high–value-added chemicals, such as organic carbonates,6,7 formamides,8-10 carboxylic acids,11,12 urea-derivates,13-15 alkylamines,16-18 and alcohols.19-21 Figure 1 shows six representative pathways for CO2 conversion. Therefore, the integration of CO2 capture with CO2 conversion, by which energy-intensive desorption and compression of CO2 are avoided and high–value-added chemicals are obtained, is very promising from the viewpoints of modern chemistry and economics. However, it remains a great challenge to transform CO2 to other chemicals under mild conditions because of the thermodynamically stable and inert nature of CO2. To overcome this problem, various advanced catalysts, both homogeneous and heterogeneous, have been developed in the past decade to facilitate CO2 conversion. Homogeneous catalysts include metal complexes,22-25 ionic liquids (ILs),26-29 N-heterocyclic carbenes (NHCs),30-33 superbases,34-36 and frustrated Lewis pairs (FLPs).37-40 Heterogeneous catalysts include porous carbons,41-43 porous silicas,44-47 zeolites,48,49 metal-organic frameworks (MOFs),50-53 and porous organic polymers (POPs).54-56 Compared with homogeneous catalysts, heterogeneous catalysts show obvious superiority in catalyst recycling and thus have become a frontier field in modern catalysis research. Among the heterogeneous catalysts mentioned, POPs are particularly attractive owing to their unique features such as large surface areas, high thermal stabilities, diverse building blocks, and tunable porous structures.57-59 They are regarded as an ideal platform for the construction of heterogeneous catalysts for CO2 conversion. To endow POPs with the ability to catalyze CO2 conversion, active sites that are capable of activating CO2 and/or substrates should be incorporated into their frameworks. For example, nitrogen species with Lewis basicity can help densify CO2 in the inner pores of POPs, activating it for subsequent conversion; metal ions with 4 Environment ACS Paragon Plus

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ACS Catalysis

strong coordination ability and halide anions with high nucleophilicity can help activate substrates for complexing with CO2. Based on these principles, different POPs embedded with one or more kinds of active sites can be rationally designed and synthesized for catalytic CO2 conversion. To date, significant progress has been achieved in this field, and thus a timely and comprehensive review is needed. In this review, the most recent advances in the design and synthesis of POP-based heterogeneous catalysts for CO2 conversion are summarized. We mainly focus on the synthetic strategies researchers use to incorporate active sites into POP frameworks to prepare heterogeneous catalysts for CO2 conversion. In the first part, we introduce the incorporation of active sites that are capable of activating CO2, mainly focusing on the synthesis of N-doped POPs. In the second part, we introduce the incorporation of active sites that are capable of activating substrates, mainly focusing on the synthesis of metalated and ionic-functionalized POPs. Finally, we summarize the state-of-the-art routes such as electrochemical and photocatalytic CO2 conversion. We also analyze the problems remaining to be addressed in this field and discuss future directions of research. 2. SYNTHESIS OF POPs AND THEIR APPLICATIONS IN CO2 CONVERSIONS 2.1 POPs activating CO2 Considering the Lewis acidic nature of the CO2 molecule, it is within expectations that basic sites are capable of activating CO2. Actually, the activation of CO2 by most homogeneous catalysts, such as NHCs and superbases, is realized through the acid-base interaction between CO2 and basic sites.30-36 Basic sites play two roles in activating CO2: (1) making the CO2 molecules much denser around them than in the gas phase and (2) changing the electron


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distribution in CO2 molecules. Therefore, the incorporation of basic sites into the frameworks of POPs is believed to be a promising strategy to prepare POPs that are capable of activating CO2.

Figure 2. Chemical structures of CTF-1 (left) and CTF-P (right).

Roeser et al. 55 reported the use of covalent triazine frameworks (CTFs) as catalysts for the conversion of CO2 under solvent- and metal-free conditions. Figure 2 shows the idealized chemical structures of two CTFs (CTF-1 and CTF-P), which were synthesized by trimerization of corresponding dicyano-compounds under ionothermal conditions. To obtain the amorphous high-surface-area analogues (CTF-1-HSA or CTF-P-HSA), they were prepared from the same monomer but at a higher reaction temperature. Triazine and pyridine units, which could selectively bind with CO2 through Lewis base-acid interaction, were the active sites in CTFs for activating CO2. CTF-1-HSA and CTF-P-HSA showed comparably high activity for catalyzing the cycloaddition of CO2 to epichlorohydrin at 130 °C and 6.9 bar, with an epichlorohydrin conversion rate of up to 100% after 4 h. However, CTF-P-HSA showed much higher activity than CTF-1-HSA if the cycloaddition reaction was performed at lower temperatures (99 % at 25 °C and 0.1 MPa after 48 h. Another factor contributing to the excellent activity of the Zn/HAzo-POPs was the presence of abundant hydroxyl groups, which formed hydrogen bonds with the oxygen in epoxides and promoted the opening of the three-member ring to complex with CO2. Therefore, a porous phenolic resin-type polymer (PRP-1) was also demonstrated to be an effective catalyst for the cycloaddition of CO2 to epoxides.90 More recently, our group91 developed a solvent-free technology for the fast, scalable synthesis of nitrogen-doped ordered mesoporous polymers (N-OMPs) via self-assembly of a phenolic resin and block copolymer template (Figure 17). The key was the thermally induced decomposition of


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hexamethylenetetramine (HMTA) into ammonia and formaldehyde, which acted as in situ Ndoping and cross-linking agents, respectively. No solvents or catalysts were used in the synthetic route, making it economically feasible and environmentally favorable. The synthesized N-OMPs were almost free of micropores, so most of the porosity was contributed by mesopores. Metallated N-OMPs (Co@N-OMPs and Zn@N-OMPs) exhibited competitive activity for catalyzing the cycloaddition of CO2 to propylene oxide in the presence of TBAB as a cocatalyst, with TOFs of up to 168.6 h-1 at 100 °C and 1.0 MPa and 4.7 h-1 at 25 °C and 1.0 MPa.

Figure 17. Illustration for the synthesis of N-OMPs. Reprinted with permission from Ref. 91. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

In comparison with the pre-polymerization incorporation of metal ions, post-polymerization incorporation can result in metalated POPs with much higher metal ion contents. However, the dispersion of metal ions in the inner pores of POPs is an issue. The aggregation of metal salts to form clusters is inevitable, making the utilization of metal ions insufficient. Therefore, it is important to optimize the porous structures and distribution of coordination sites in presynthesized POPs.


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2.2.2 Ionic-functionalized POPs ILs have attracted widespread attentions owing to their unique properties such as extremely low volatility, high thermal stability, and structural designability.92,93 They have shown great potential for applications such as homogeneous catalysts for CO2 conversion.26-29 The catalytic activity of ILs mostly originates from the nucleophilic anions, especially halide anions. To make the catalytic process more environmental benign, IL moieties have been incorporated into POP frameworks to result in ionic-functionalized POPs as heterogeneous catalysts. IL-functionalized POPs are also referred as poly ionic liquids (PILs) or porous ionic polymers (PIPs).

Figure 18. Synthesis of a cross-linked-polymer-supported IL.

Han et al.55 reported the first example of using ionic-functionalized POPs to catalyze CO2 conversion. The polymer-supported IL (PSIL) was synthesized by free radical copolymerization of an IL monomer containing a vinyl group with divinylbenzene (DVB) (Figure 18). PSIL catalyzed the cycloaddition of CO2 to epoxides at 110 °C and 6.0 MPa with carbonate yields of up to 97.4% after 7 h. Similarly, Dyson et al.94 reported the synthesis of cross-linked ionic polymers by free radical self-polymerization of IL monomers containing two vinyl groups. The cross-linked ionic polymers catalyzed the cycloaddition of CO2 to epoxides at 140 °C and 5.0 MPa with carbonate yields of up to 98% after 3 h. However, the reaction conditions required were somewhat harsh, which were not favorable for their practical applications.


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Figure 19. Synthesis of polymer-supported ionic liquids (PSILs). Reprinted with permission from Ref. 95. Copyright 2009 Elsevier.

To improve the catalytic activity of ionic-functionalized POPs for CO2 conversion, hydroxyl and carboxyl groups—which can form hydrogen bonds with the oxygen in epoxides and promote the opening of three-member rings to complex with CO2—have been tethered to the IL moieties in ionic-functionalized POPs. This is easily realized by taking advantage of the structural designability of ILs. For example, Zhang et al.95 synthesized PSILs tethered with hydroxyl groups (PS-HEIMX, X = Br, Cl) by quaternization of a polymer-supported imidazole (PS-IM) with halogen-substituted alcohols (Figure 19). PS-HEIMX effectively catalyzed the cycloaddition of CO2 to epoxides at 120 °C and 2.5 MPa with carbonate yields of up to 98% after 4 h. Using a different approach, Cheng et al.96 synthesized PILs tethered with hydroxyl groups by free radical copolymerization of hydroxyl-tethered IL monomers containing vinyl groups with DVB. By replacing the halogen-substituted alcohols with halogen-substituted carboxylic acids, ionic-functionalized POPs tethered with carboxyl groups were also obtained.97 The synergistic functions of the nucleophilic anions and hydroxyl/carboxyl groups significantly reduced the reaction temperature and pressure required for CO2 conversion catalyzed by ionicfunctionalized POPs. Figure 20 shows a possible mechanism for the cycloaddition of CO2 to epoxides catalyzed by ionic-functionalized POPs tethered with hydroxyl groups. There have been numerous reviews on this topic.98-100


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ACS Catalysis

Figure 20. A possible mechanism for the use of ionic-functionalized POPs tethered with hydroxyl groups. Reprinted with permission from Ref. 95. Copyright 2009 Elsevier.

Figure 21. Preparation of porous organic networks bearing imidazolium salts.

Another method of improving the catalytic activity of ionic-functionalized POPs for CO2 conversion is to enrich their porosity to increase their surface areas and the accessibility of active sites to reactants. Son et al.101 synthesized a novel microporous organic network bearing imidazolium

salts

(T-IM)

by

the

Sonogashira

coupling

reaction

of

tetrakis(4-

ethynylphenyl)methane and diiodoimidazolium salts, as shown in Figure 21. The BET surface area of T-IM was determined to be 620 m2/g. T-IM was able to catalyze the cycloaddition of CO2 to epoxides at 150 °C and 1.0 MPa, with carbonate yields of up to 87% after 10 h. A similar


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synthetic strategy was adopted by Coskun et al.102 to synthesize porous cationic polymers (PCPs) for catalyzing the cycloaddition of CO2 to epoxides. The BET surface areas of PCPs were measured to be 433~755 m2/g. However, the synthesis of T-IM and PCPs by the Sonogashira coupling reaction required complex monomers and expensive catalysts.

Figure 22. The synthesis of hypercross-linked phosphonium polymers. Reprinted with permission from Ref. 103. Copyright 2015 Royal Society of Chemistry.

Figure 23. Synthesis of 2-phenylimidazolinium–based HIPs in a one-pot route involving simultaneous quaternization and Friedel–Crafts alkylation between 2-phenylimidazoline and benzyl halide. Reprinted with permission from Ref. 104. Copyright 2017 Royal Society of Chemistry.


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Figure 24. Synthesis of [Et4NBr]X%-Py-COFs with the ionization of channel walls through the Williamson ether reaction of [HO]X%-Py-COFs with (2-bromoethyl)triethylammonium bromide. The [HO]X%-Py-COFs were synthesized via the condensation of PyTTA with DHPA and PA at various molar ratios. Reprinted with permission from Ref. 105. Copyright 2016 Royal Society of Chemistry.

To simplify the preparation of POP-based heterogeneous catalysts for CO2 conversion, some other synthetic strategies have been used to prepare ionic-functionalized POPs with enriched


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porosity. For example, Zhang et al.103 reported the synthesis of hypercross-linked porous polymers incorporated with phosphonium salts by the Friedel-Crafts alkylation reaction of phosphonium salts and benzene with formaldehyde dimethyl acetal (FDA) (Figure 22). Wang et al.104 reported the synthesis of imidazolinium-based porous hypercross-linked ionic polymers (HIPs) by simultaneous quaternization and Friedel-Crafts alkylation reaction of 2phenylimidazoline with α,α′-dichloro-p-xylene (DCX) or α,α′-dibromo-p-xylene (DBX) (Figure 23). Gao et al.105 described the synthesis of covalent organic frameworks (COFs) immobilized with ILs by the polycondensation reaction of 4,4’,4’’,4’’’-(pyrene-1,3,6,8-tetrayl) tetraaniline (PyTTA) with 2,5-dihydroxyterephthalaldehyde (DHPA) and 1,4-phthalaldehyde (PA), followed by the Williamson ether reaction of the phenol groups with (2-bromoethyl)triethylammonium bromide, as shown in Figure 24. Coskun et al.106 described the synthesis of charged covalent triazine frameworks (cCTFs) by ionothermal trimerization of cyanophenyl-substituted viologen dication as a monomer (Figure 25). However, these ionic-functionalized POPs did not show an obvious improvement in catalytic activity for CO2 conversion in comparison with T-IM and PCPs because of their predominant micropores (0~2 nm) or small mesopores (2~5 nm). These were not beneficial for the diffusion of reactants in the inner pores.


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ACS Catalysis

Figure 25. Synthetic route for the preparation of charged covalent triazine frameworks (cCTFs). Reprinted with permission from Ref. 106. Copyright 2017 American Chemical Society.

Free radical polymerization under solvothermal conditions has proved to be an advanced technique to synthesize ionic-functionalized POPs with large mesopores without the assistance of any template. Park et al.107 and Bordiga et al.108 prepared non-ionic POPs with mesoporous structures by first copolymerizing 1-vinylimidazole (1-VIm) and DVB and then quaternizing the non-ionic POPs containing imidazole units to produce ionic-functionalized POPs with average pore sizes of 5~8 nm. The synthesized ionic-functionalized POPs effectively catalyzed the cycloaddition of CO2 to epoxides at 110 °C and 1.34 MPa, with epoxide conversions of up to 92% after 6 h. Ma et al.109 prepared PIPs by self-polymerization of quaternized phosphonium salts tethered with three vinyl groups. The synthesized PIPs exhibited excellent activity for catalyzing the cycloaddition of atmospheric CO2 to epoxides, with carbonate yields of up to 99.0% after 3 h


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at 100 °C and 0.1 MPa. To eliminate the use of volatile organic solvents for solvothermal polymerization, Wang et al.110 reported the synthesis of hierarchically meso-macroporous PILs by self-polymerization of IL monomers tethered with two vinyl groups in IL solvents. The synthesized PILs were found to have average pore sizes of 7~23 nm. As a result, they showed excellent activity for catalyzing the cycloaddition of atmospheric CO2 to epoxides, especially those epoxides with bulky substitutions; and epoxide conversions of >90.0% were achieved after 48 h at 70 °C and 0.1 MPa. Obviously, the catalytic activities of ionic-functionalized POPs with abundant large mesopores for CO2 conversion were superior to those with only micropores or small mesopores.

Figure 26. Schematic illustration for preparing a series of FDU-15 mesoporous polymer–supported imidazolium-based ionic liquids: (a) FDU-HEIMBr, (b) FDU-CMIMBr, (c) FDU-DHPIMBr, and (d) FDUEIMB. Reprinted with permission from Ref. 111. Copyright 2014 Royal Society of Chemistry.

Based on the results discussed above, ionic-functionalized POPs with simultaneous hydroxyl/carboxyl groups and enriched porosity should be a fascinating option for the catalytic conversion of CO2. He et al.111 synthesized highly ordered mesoporous polymer–supported imidazolium-based ILs by chloromethylation of the FDU-15 mesoporous polymer, followed by functionalization with imidazole and quaternization with halogen-substituted alcohols or


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ACS Catalysis

carboxylic acids (Figure 26). The same group also demonstrated the direct synthesis of ordered imidazolyl-functionalized mesoporous polymers (IM-MPs) sing an imidazolyl-functionalized resol for cross-linking with formaldehyde. The pre-synthesized IM-MPs were then quaternized with bromoethane to give ionic-functionalized POPs.112 Using a different route, Zhang et al.113 synthesized imidazolium salt-modified porous hypercrosslinked polymers (POM-IMs) by the Friedel-Crafts alkylation reaction of benzyl halide monomers containing hydroxyl groups with dichloroethane, followed by quaternization with 1-methylimidazole (Figure 27). As an alternative, Yang et al.114 used free radical polymerization under solvothermal conditions to synthesize ionic-functionalized POPs. Taking the synthesis of IL-based porous polymers functionalized with intermolecular hydroxyl groups as an example (Figure 28), the solvothermal copolymerization of 1-butyl-3-vinylimidazolium bromide, (4-vinylphenyl)methanol, and divinylbenzene (DVB) resulted in the targeted ionic-functionalized POPs. When SBA-15 was used as the support for copolymerization, ionic-functionalized POPs with higher hydroxyl contents and BET surface areas were obtained. The catalytic activity of these ionicfunctionalized POPs for CO2 conversion was determined by the content of the hydroxyl groups and the porosity. More recently, the combination of polyILs with porous supports has been further utilized to design highly efficient catalysts for CO2 conversion to useful chemicals.115-118 For instance, Jiang and coworkers incorporated imidazolium-based polyILs into a MOF, and the resultant composite catalysts showed excellent activities for the selective capture and conversion of CO2. Their excellent activities should be attributed to the synergistic effect among the good CO2 enrichment capacity, the Lewis acid sites in MOF, as well as the Lewis base sites in polyILs.115 Yang and


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coworkers reported catalysts based on porphyrin-based porous polymers and thermal-responsive ILs, which exhibited excellent activities for catalyzing CO2 cycloaddition reaction.116 For clearly comparison, the sample information, reaction conditions, and activities of some typical POP-based catalysts designed for CO2 conversion are summarized in Table 1.

Figure 27. The synthesis of supported imidazolium salts and the typical structure of POM-IM. Reprinted with permission from Ref. 113. Copyright 2015 Royal Society of Chemistry.


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Figure 28. Illustration of the synthesis of (A) porous polymers functionalized with both ionic liquid and intermolecular hydroxyl group and (B) hybrid SBA-[VxOHy]R materials. Reprinted with permission from Ref. 114. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Table 1. Activities of different catalysts for CO2 conversion Catalysts

Reactions

Cocatalysts

CO2

Temperature

Time

Yield (%)

(MPa)

(°C)

(h)

or TOF

Refs.

(h-1) CTF-1-HSA

Cycloaddition

-

0.69

130

4

100

56

NP-NHC

Cycloaddition

-

0.1

120

24

98

66

Zn-CMP

Cycloaddition

TBAB

3.0

120

/

11600

71

Co-MON

Cycloaddition

TBAC

3.0

60

/

155

73

Zn@SBMMP

Cycloaddition

TBAB

2.0

60

4

97

74

Al-CMP

Cycloaddition

PPNCl

3.0

100

/

364

74

Bp-Zn@MA

Cycloaddition

TBAB

1.0

100

/

1580

76

M(Por)OP

Cycloaddition

KI

1.0

120

/

2124

78

M/POP-TPP

Cycloaddition

TBAB

0.1

25

24

95.6

80

HUST-1-Co

Cycloaddition

TBAB

0.1

25

30

>94.0

81

TB-MOP-Ru

Hydrogenation

Ph3P

12

40

12

2254

88

(TON) Zn/HAzo-POPs

Cycloaddition

TBAB

3.0

100

/

3330

89

Zn/HAzo-POPs

Cycloaddition

TBAB

0.1

25

48

>99

89

Co@N-OMPs

Cycloaddition

TBAB

1.0

100

/

168.6

91

Cross-linked ionic

Cycloaddition

-

5.0

140

3

98

94

Cycloaddition

TBAB

2.5

120

4

98

95

polymers PS-HEIMX


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T-IM

Cycloaddition

-

1.0

150

10

87

101

Ionic-functionalized

Cycloaddition

-

1.34

110

6

92

108

PIPs

Cycloaddition

-

0.1

100

3

99

109

PILs

Cycloaddition

-

0.1

70

48

>90

110

ZnX2–PPh3

Cycloaddition

-

3.0

120

/

3985

119

The latter samples

Cycloaddition

-

3.0

120

24

15600

121

Al-POP

Cycloaddition

-

0.1

120

24

99

122

DVB@ISA

Cycloaddition

-

1.0

60

24

99

123

Al-iPOPs

Cycloaddition

-

1.0

25

8

99

124

Zn@N-MMOPs

Cycloaddition

TBAB

1.0

25

48

>97.5

125

PPS⊂COF-TpBpy-

Cycloaddition

-

0.1

40

24

95

126

POPs

Cu

2.2.3 Multi-functionalized POPs In the conversion of CO2 catalyzed by metalated POPs, cocatalysts such as halide salts are commonly used. However, the recycling of cocatalysts is an important issue. It has been demonstrated that both metal ions and halide anions can be incorporated into POP frameworks by taking advantage of the diverse building blocks and synthetic methods of POPs. Based on these achievements, multi-functionalized POPs in which metal ions and halide anions are integrated into the same polymeric chain are proposed as heterogeneous catalysts for CO2 conversion. The cooperative functions of metal ions and halide anions underlie the powerful ability of multi-functionalized POPs to catalyze the CO2 conversion without the assistance of any other cocatalysts.


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Figure 29. Preparation procedure for 1P+X−&1PPh3@POPs and the corresponding 1P+X−&ZnX2–1PPh3@POP. Reprinted with permission from Ref. 119. Copyright 2016 Royal Society of Chemistry.

Ding et al.119 reported the first example of using multi-functionalized POPs for catalyzing CO2 conversion. As shown in Figure 29, phosphonium salt and ZnX2–PPh3 integrated hierarchical POPs were synthesized by free radical copolymerization of phosphonium salts tethered with three vinyl groups and tertiary phosphine tethered with three vinyl groups under solvothermal conditions. This was followed by metalation with metal ions. The ratios of phosphonium salt to ZnX2–PPh3 were tuned by adjusting the ratios of monomers. The catalytic activity of synthesized multi-functionalized POPs for cycloaddition of CO2 to epoxides was among the highest of the heterogeneous catalysts—initial TOFs of up to 3985 h-1 at 120 °C and 3.0 MPa. The same group also synthesized analogous multi-functionalized POPs by free radical copolymerization of imidazolium salts tethered with two vinyl groups and tertiary phosphine tethered with three vinyl groups under solvothermal conditions. This was followed by metalation with metal ions,120 or


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free radical copolymerization of phosphonium salts tethered with three vinyl groups and metalated porphyrins tethered with four vinyl groups under solvothermal conditions.121 The latter samples offered extraordinary initial TOFs of up to 15600 h-1 at 120 °C and 3.0 MPa and TONs of up to 2400 after 24 h at 25 °C and 0.1 MPa for the cycloaddition of CO2 to epoxides. Inspired by this work, other multi-functionalized POPs were also designed and synthesized. Cao et al.122 synthesized a bifunctional cationic POP (Al-CPOP) by quaternization-induced hypercross-linking of metalated salen containing chloromethyl groups with 2,4,6-tris(imidazol-1yl)-1,3,5-s-triazine (TIST), as shown in Figure 30. Al-POP effectively catalyzed the cycloaddition of CO2 to epoxides at 120 °C and 0.1 MPa with epoxide conversions of up to 99 % after 24 h. Ji et al.123 synthesized metallosalen-based ionic porous polymers (DVB@ISA and DVB@ISZ) by free radical copolymerization of metalated salens containing vinylimidazoliumbased ILs with DVB under solvothermal conditions (Figure 31). DVB@ISA and DVB@ISZ were able to effectively catalyze the cycloaddition of CO2 to epoxides at 60 °C and 1.0 MPa, with epoxide conversions of up to 99 % after 24 h. The same group synthesized charged metalloporphyrin polymers (Al-iPOPs) by the Yamamoto-Ullmann coupling reaction of 5,10,15,20-tetrakis(4-bromophenyl)porphyrin-aluminum(III)

chloride

(Al-TBPP)

with

brominated ILs, as shown in Figure 32.124 Al-iPOPs effectively catalyzed the cycloaddition of CO2 to epoxides at 25 °C and 1.0 MPa, with epoxide conversions of up to 99 % after 8 h.


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Figure 30. Synthesis of Al-CPOP.

Figure 31. Synthesis of metallosalen-based ionic POPs. Reprinted with permission from Ref. 123. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


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Figure 32. Synthesis of Al-POP, Al-iPOP-1, and Al-iPOP-2. [Ni(cod)2]=bis(1,5-cyclooctadiene)-nickel(0).

In general, it is highly desirable to use nontoxic solvents for the synthesis of catalysts to eliminate the potential impacts of solvents on the environment. To this end, our group recently developed a hydrothermal technology for the synthesis of ionic-functionalized POPs.125 As shown in Figure 33, the free radical self-polymerization of 1-(4-vinylbenzyl)-1,3,5,7tetraazaadamantan-1-ium

chloride—a

monomer

prepared

by

quaternization

of

hexamethylenetetramine (HMTA) with 4-vinylbenzyl chloride (VBC)—in aqueous solution under high temperature induced the decomposition of the HMTA unit into ammonia and formaldehyde. The ammonia and formaldehyde produced then induced the cross-linking of benzene rings via "resol chemistry" to produce N-doped meso-macroporous organic polymers (N-MMOPs) containing IL moieties. The pre-synthesized N-MMOPs were further metalated with metal ions to give multi-functionalized POPs (Zn@N-MMOPs and Co@N-MMOPs). The Zn@N-MMOPs and Co@N-MMOPs showed competitive activity for catalyzing the cycloaddition of CO2 to propylene oxide at 25 °C and 1.0 MPa in the presence of TBAB as a cocatalyst, with carbonate yields of up to 97.5% after 48 h. Although cocatalyst was required in the conversion of CO2 catalyzed by Zn@N-MMOPs and Co@N-MMOPs because of their limited number of ionic sites, this approach did represent a significant advance in the facile and


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green synthesis of multi-functionalized POPs with promising applications in catalytic CO2 conversion.

Figure 33. Synthetic route for nitrogen-functionalized meso-macroporous organic polymers (N-MMOPs). Reprinted with permission from Ref. 125. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Another significant advance in the synthesis of multi-functionalized POPs for catalyzing CO2 conversion was achieved by Ma et al.126 In the above-mentioned multi-functionalized POPs, the two kinds of active sites (metal ions and halide anions) that are capable of activating substrates are spatially separated by the rigid frameworks. It is actually difficult for the metal ions and halide anions to cooperate with each other in catalyzing CO2 conversion, thus limiting the catalytic activity of those multi-functionalized POPs. Ma et al. proposed the concept of heterogeneous concerted catalysis between active sites on the porous materials and the use of highly flexible linear polymers to address this issue. As shown in Figure 34, the metal ions and


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halide anions were not appended to the same polymeric chain. Instead, the metal ions were anchored on the rigid framework of a COF, and the halide anions were anchored on the chains of a linear ionic polymer with high flexibility. The linear ionic polymer was formed in the pores of a COF bearing 2,2’-bipyridine units by in situ free radical polymerization of phosphonium salt tethered with a vinyl group. The resultant ionic polymer-COF composite (PPS⊂COF-TpBpy) was then metalated with Cu2+ by the coordination reaction of 2,2’-bipyridine units with Cu2+ to give multi-functionalized POPs (PPS⊂COF-TpBpy-Cu). PPS⊂COF-TpBpy-Cu retained the crystallinity and nanoporosity of the COF, exhibiting high thermal stability and accessibility of reactants to active sites. It was found that PPS⊂COF-TpBpy-Cu showed excellent activity for catalyzing the cycloaddition of CO2 to epoxides at 40 °C and 0.1 MPa, with carbonate yields of up to 95 % after 24 h.

Figure 34. (a) The concept of heterogeneous concerted catalysis between active sites on porous materials with the use of highly flexible linear polymers and (b) schematic of PPS⊂COF-TpBpy-Cu synthesis and structures of COF-TpBpy and PPS⊂COF-TpBpy-Cu. Reprinted with permission from Ref. 126. Copyright 2016 American Chemical Society.

2.3 Other advanced routes for the conversion of CO2


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Besides the chemical conversion of CO2, other routes such as electrochemical and photocatalytic conversion of CO2 are also hot topics, having received considerable attentions in these years. Compared with chemical conversion of CO2, electrochemical and photocatalytic conversion of CO2 could be performed under rather mild condition, and valuable products such as formic acid, methanol, ethanol, CH4, CO and aldehydes could be obtained by using these state-of-the-art routes. However, the electrochemical and photocatalytic conversion of CO2 usually employ non-polymeric nanocatalysts. For instance, Xie and coworkers successfully developed partially oxidized atomic cobalt layer electrocatalysts, which exhibit excellent activities for the electroreduction of CO2 to liquid fuels such as formic acid.127 Fan and coworkers successfully synthesized Z-scheme Ag3PO4/g-C3N4 composited photocatalysts, which could catalyze the conversion of CO2 into useful chemicals such as methanol, CH4 and CO.128 The excellent activities of Ag3PO4/g-C3N4 composites should be attributed to the efficient separation of photo-excited electron holes in the composites. Besides the above-mentioned results, there are many other pioneering works focusing on the electrochemical and photocatalytic conversion of CO2, and many important progresses have been achieved.129-132 3. OUTLOOK Although significant progress has been achieved in the design and synthesis of POPs for catalytic CO2 conversion, two important issues still must be addressed in this field. (1) The major source of CO2 emissions is flue gas, the combustion product of fossil fuels. In flue gas, CO2 concentrations are low (i.e., 10~15 v/v%).133 Considerable research works have been concerned for the treating of diluted CO2 streams in these years. How to catalyze the conversion of CO2 from a dilute source especially for the CO2 derived from flue gas and air, is a significant issue in the field. Most POP-based heterogeneous catalysts reported to date are


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effective only under elevated pressure (0.1~6.0 MPa) because these POPs are only capable of effectively activating either CO2 or substrates. For example, NHC-functionalized POPs are capable of effectively activating CO2 but cannot effectively activate substrates; metalated and ionic-functionalized POPs can effectively activate substrates, but not CO2. Therefore, a future direction is to design and synthesize multi-functionalized POPs that are capable of activating CO2 and substrates simultaneously. On the other hand, there exist many other components in flue gas, such as steam and oxygen,133 which may influence the stabilities and catalytic activities of POPs. In fact, the rational adjusting surface wettability, controlling compositions and enhancing crosslinking degree of the networks of POPs could effectively improve their hydrothermal stabilities and water-tolerant properties under harsh reaction conditions134. It is urgent to thoroughly investigate the stabilities and catalytic activities of POPs in the presence of steam and oxygen at high adsorption and reaction temperatures. In particular, the underlying mechanisms for the degradation of POPs should be investigated, with the assistance of experimental and theoretical tools, to provide guidance for the design and synthesis of POP-based heterogeneous catalysts with enhanced stabilities under industrial conditions. (2) The development of POP-based heterogeneous catalysts relies greatly on the versatile building blocks and synthetic strategies of POPs. However, the construction of most POPs requires expensive monomers and/or complex synthetic routes, making it unfavorable for industrial applications of POPs. Solvent-free91 and hydrothermal89,122 technologies represent significant advances in the facile and green synthesis of POPs. However, simpler methods are still greatly needed. Mechanochemical synthesis is an advanced technology developed in recent years.135 In mechanochemical synthesis, chemical reactions are accelerated by mechanical force and/or instantaneous frictional heat. Therefore, chemical reactions proceed in a solid state,


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avoiding the use of large amounts of solvents. Thus, the solubility of monomers in solvents is no longer a limitation for the design and synthesis of POPs. Mechanochemical synthesis has been widely applied in the preparation of a wide range of porous materials and has proved to be a powerful tool for fast, scalable production.136 Therefore, it is suggested that mechanochemical synthesis be extended to the preparation of POP-based heterogeneous catalysts for CO2 conversion. 4. CONCLUSION The most recent advances in the design and synthesis of POP-based heterogeneous catalysts for CO2 conversion are summarized in this review. The development of N-doped, metalated, and ionic-functionalized POPs for catalytic conversion of CO2 is examined in detail, with an emphasis on synthetic strategies researchers have used to incorporate active sites into POP frameworks. POPs have been demonstrated to be an ideal platform for the construction of heterogeneous catalysts for CO2 conversion owing to their tunable structures and high activity. Future work in this field should pay special attention to the design and synthesis of POPs capable of effectively catalyzing the conversion of CO2 from dilute sources, with enhanced stabilities under industrial conditions, which are easily synthesized from low-cost reagents by simple methods. The establishment of structure-property relationships for the catalytic activity of POPs is also an urgent task to provide guidance for the rational design and synthesis of catalysts. Overall, the application of POP-based heterogeneous catalysts for CO2 conversion is still in its infancy, and extensive work is required to promote the industrial application of POPs for catalytic CO2 conversion. AUTHOR INFORMATION Corresponding Author


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[email protected] (K. H.); [email protected] (F. L.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no conflicting interest. ACKNOWLEDGMENT F. L. acknowledges the financial support of the National Natural Science Foundation of China (Nos. 21573150 and 21203122), National Natural Science Foundation of Zhejiang Province (LY15B030002). K. H. acknowledges sponsorship from the Natural Science Foundation of Jiangxi Province (No. 20171BAB203019) and Nanchang University. REFERENCES (1)

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ACS Catalysis

Graphic Abstract


Synthesis of Porous Polymeric Catalysts for the Conversion of Carbon

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