Covalent Triazine-Based Frameworks with Ultramicropores and High

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Covalent Triazine-Based Frameworks with Ultramicropores and High Nitrogen Contents for Highly Selective CO2 Capture Keke Wang, Hongliang Huang, Dahuan Liu, Chang Wang, Jinping Li, and Chongli Zhong Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00425 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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Environmental Science & Technology

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Covalent Triazine-Based Frameworks with

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Ultramicropores and High Nitrogen Contents for

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Highly Selective CO2 Capture

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Keke Wang,† Hongliang Huang,*,† Dahuan Liu,† Chang Wang,‡ Jinping Li,‡ and

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Chongli Zhong*,† †

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State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

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Technology, Beijing 100029, China ‡

Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024,

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Shanxi, China

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*Corresponding Author: Phone: +86-10-64431705; E-mail: [email protected]; Address:

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Beijing city Chaoyang District North Third Ring Road 15 (H.H.). Phone/Fax: +86-10-64419862;

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E-mail: [email protected]; Address: Beijing city Chaoyang District North Third Ring

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Road 15 (C.Z.).

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ABSTRACT: Porous organic frameworks (POFs) are a class of porous materials composed of

15

organic precursors linked by covalent bonds. The objective of this work is to develop POFs with

16

both ultramicropores and high nitrogen contents for CO2 capture. Specifically, two covalent

17

triazine-based frameworks (CTFs) with ultramicropores (pores of width < 7 Å) based on short

ACS Paragon Plus Environment

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(fumaronitrile, FUM) and wide monomers (1,4-dicyanonaphthalene, DCN) were synthesized.

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The obtained CTF-FUM and CTF-DCN possess excellent chemical and thermal stability with

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ultramicropores of 5.2 and 5.4 Å, respectively. In addition, they exhibit excellent ability to

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selectively capture CO2 due to ultramicroporous nature. Especially, CTF-FUM-350 has the

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highest nitrogen content (27.64%) and thus the highest CO2 adsorption capacity (57.2 cc/g at 298

23

K) and selectivities for CO2 over N2 and CH4 (102.4 and 20.5 at 298 K, respectively) among all

24

CTF-FUM and CTF-DCN. More impressively, as far as we know, the CO2/CH4 selectivity is

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larger than that of all reported CTFs and ranks in top ten among all reported POFs. Dynamic

26

breakthrough curves indicate that both CTFs could indeed separate gas mixtures of CO2/N2 and

27

CO2/CH4 completely.

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1. INTRODUCTION

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The escalating level of atmospheric CO2 contributed to global warming significantly, is one of

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the greatest environmental concerns. The anthropogenic CO2 emission stems predominantly

31

from the flue gas (mainly consisting of CO2 and N2) from coal- or natural gas-fired power

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plants.1 Moreover, in natural gas as well as landfill gas, CO2 is a major contaminant, which

33

reduces the heating capacity of natural gas and also causes corrosion of the pipeline and

34

equipment due to its acidic nature.2 Therefore, the selective removal of CO2 from both flue gas

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and natural gas is of great importance from both environmental and economic points of view.

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Several approaches have been proposed and developed for this purpose: absorption with

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solvents, adsorption by porous materials, membrane separation, cryogenic separation, etc.

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Currently, the most widely adopted approach is “wet scrubbing” involving the chemical

39

absorption of CO2 by aqueous alkanolamine solutions such as monoethanolamine (MEA).3


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Although possessing high capacity and selectivity for CO2, this method has several significant

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problems including relative unstability towards heating related to regeneration, severe corrosion

42

of the equipment, and high regeneration cost. To avoid these limitations, adsorption by porous

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materials including metal-organic frameworks (MOFs) and porous organic frameworks (POFs)

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as an alternative method has been intensively investigated.4-7 Due to their high surface areas,

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adjustable pore sizes, and controllable pore surface property, MOFs have an excellent CO2

46

capture ability; however, the majority of them have poor chemical stability because of the weak

47

coordination bonds, which seriously limits their practical applications.8 In contrast, POFs, as a

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class of porous materials composed of organic precursors linked by covalent bonds, have

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relatively great chemical stability.9,10 In addition, POFs possess large surface areas, low

50

framework densities, and flexibility for rational design, thus attracting great attentions in various

51

fields including gas storage, gas separation, catalysis, and photoelectricity.11 In the past few

52

years, many classes of POFs have been synthesized by various routes, and notable examples

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include porous aromatic frameworks (PAFs),12 porous polymer networks (PPNs),13 covalent

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organic frameworks (COFs),14 polymers of intrinsic microporosity (PIMs),15 porous

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benzimidazole-linked polymers (BILPs),16 and covalent triazine-based frameworks (CTFs).17,18

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In contrast to other synthetic reactions including Sonogashira coupling,19 Suzuki coupling,20

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Yamamoto coupling,21 and trimerization of alkynes,22 which require expensive catalysts and

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harsh reaction conditions, trimerization of nitrile under ionothermal conditions in the preparation

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of CTFs only requires cheap catalyst like ZnCl2 and there is no need of organic solvent, which

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reduces economic cost and environmental impact.23 Bearing these in mind, CTFs therefore have

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great promise for practical applications.


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From the view of quantitative structure-property relationship, structure and composition of

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porous materials have great effect on CO2 capture. From the aspect of pore structure,

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ultramicropores (pores of width < 7 Å)24 are generally beneficial for CO2 capture since small

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pore size could lead to deep overlap of potential and thus strong interaction.25,26 However,

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construction of ultramicropores in porous materials such as MOFs and POFs still remains

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challenging. On the other hand, from the aspect of the chemical composition, introduction of

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basic nitrogen functionalities into porous materials also has been regarded as a useful means to

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enhance the uptake of CO2 owing to Lewis acid-Lewis base electrostatic interactions of the

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nitrogen atoms with the carbon atoms of the CO2 molecules.

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In this work, we proposed a strategy of designing porous materials with ultramicropores and

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high nitrogen contents to capture CO2. As a proof-of-concept demonstration, based on

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microporous CTF-1 as a platform, short monomer fumaronitrile (FUM) and wide monomer 1,4-

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dicyanonaphthalene (DCN) were chosen to afford two CTFs (CTF-FUM and CTF-DCN,

75

respectively) with ultramicropores. Both CTFs are isoreticular to CTF-1 produced from 1,4-

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dicyanobenzene (DCB), which is the shortest among all previously reported linear dinitrile

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monomer for the CTF synthesis.17 Compared with DCB, FUM and DCN are shorter and wider,

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respectively. As a result, the CTF-FUM and CTF-DCN possess high adsorption capacities and

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selectivities for CO2 over N2 and CH4. In addition, compared with CTF-DCN, CTF-FUM has

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higher separation performance due to its rich nitrogen content. More importantly, the strategy is

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facile and inexpensive by using commercially available monomers, which provides a guideline

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for the design of new porous materials towards CO2 capture.

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2. EXPERIMENTAL SECTION


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84 85


2.1. Materials and methods. Sources of chemicals and reagents, characterization methods, and the details of breakthrough measurement are provided in the Supporting Information.

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2.2. Synthesis of CTFs. 2.2.1. Synthesis of CTF-FUM. FUM (0.6 g, 7.7 mmol) and ZnCl2 (2.1

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g, 15.4 mmol) were transferred into a quartz ampoule. The ampoule was evacuated, sealed,

88

heated to the desired temperature (350, 400, or 500 °C) in 1 h, and held the temperature for 40 h.

89

Then the ampoule was cooled down to room temperature and opened. The reaction mixture was

90

subsequently ground and then washed thoroughly with water to remove most of the ZnCl2.

91

Further stirring in 0.5 M HCl for 24 h was carried out to remove the residual salt. Then the

92

resulting black powder was filtered, washed with water and methanol, and dried in vacuum at

93

120 °C. The yield is about 90% (about 0.54 g).

94

2.2.2. Synthesis of CTF-DCN. DCN (0.6 g, 3 mmol) and ZnCl2 (0.816 g, 6 mmol) were

95

transferred into a quartz ampoule. The ampoule was evacuated, sealed, heated to the desired

96

temperature (400 or 500 °C) in 1 h, and held the temperature for 40 h. Then the ampoule was

97

cooled down to room temperature and opened. The reaction mixture was activated by following

98

the same procedure as given for CTF-FUM. The yield is about 80% (about 0.48 g).

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3. RESULTS AND DISCUSSION

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3.1. Synthesis and characterization of CTFs. Porous CTFs were mainly prepared by an

101

ionothermal reaction between nitrile monomers in molten ZnCl2, which functions as both a

102

Lewis acid catalyst and a solvent.17 From the trimerization of nitrile monomers, triazine rings can

103

be conveniently obtained, leading to robust structures with permanent porosity. Several previous

104

reports indicate that the reaction temperature has great effect on porous properties of resulting

105

CTFs.27,28 Therefore, CTF-FUM and CTF-DCN were obtained by the synthetic procedure at

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different temperatures as shown in Scheme 1. Trimerization of FUM at 350, 400, and 500 °C


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yielded CTF-FUM-350, CTF-FUM-400, and CTF-FUM-500, respectively. For DCN, the

108

synthesis at 400 and 500 °C yielded CTF-DCN-400 and CTF-DCN-500, respectively. The

109

reaction times were 40 h for all samples. N N

CN

ZnCl2 (a)

N

N

N

N

N N

N N

N

350-500 ºC N

NC N

N

N

CN

FUM

N

N N

CTF-FUM

CN

N

N

N

N

N

N

CN

ZnCl2

(b)

400-500 ºC

N

N N

N

N

N

CN N

N N

N N

N

DCN

110

CTF-DCN

111

Scheme 1. Schematic illustration of the synthesis of CTF-FUM and CTF-DCN with

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ultramicropores.

113

The successful formation of the triazine rings can be indicated by FT-IR analysis. As shown in

114

Figure S2 in the Supporting Information, the characteristic carbonitrile stretching band of FUM

115

(DCN) around 2240 cm-1 (2229 cm-1) almost disappears after reaction. Meanwhile, the

116

characteristic bands for the triazine units at 1560 and 1384 cm-1 appear for all the synthesized

117

frameworks although the bands are not obvious, which may be attributed to the black surface

118

coming from partial carbonization of the samples.29,30 In addition, CTF-FUM and CTF-DCN

119

were characterized by 13C CP-MAS spectra (Figures S3-S7). For CTF-FUM, the peak at about

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175 ppm confirms the presence of sp2 carbon from the triazine unit. However, the spectra of

121

CTF-DCN only have an obvious peak at about 127 ppm corresponding to aromatic carbons. The

122

characteristic peak of carbon of triazine is so weak that it could not be distinguished due to the

123

partial graphitization of samples, which has been observed previously.31,32 The formed triazine

124

rings are further confirmed by XPS. The N1s XPS spectra for CTF-FUM and CTF-DCN contain


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two contributions with binding energies at 398.6 (or 398.9) and 400.6 eV (Figures S8-S12). The

126

former corresponds to the N in the triazine rings. The signal at 400.6 eV may be assigned to

127

pyrrolic nitrogen (-C-NH-C-) coming from partial decomposition of samples.33,34

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Then, the porosity properties of CTF-FUM and CTF-DCN prepared at different temperatures

129

were investigated by N2 adsorption measurements at 77 K. As shown in Figures 1a and 1c, CTF-

130

FUM-350, CTF-FUM-400, and CTF-FUM-500 all exhibit type I isotherms, indicating that they

131

are microporous materials. For CTF-DCN-400 and CTF-DCN-500, the isotherms exhibit a sharp

132

uptake in the low pressure region (P/P0 < 0.01), implying a significant microporisity, whereas

133

the isotherms also exist hysteresis loops, indicating the presence of mesopores. The pore

134

characteristics of all samples are summarized in Table 1. With the increase of temperature, the

135

BET surface area and total pore volume of CTF-FUM increase from 230 m2/g and 0.12 cm3/g to

136

603 m2/g and 0.28 cm3/g, respectively. It may be attributed to that higher temperature may lead

137

to more defects in the framework.27 While for CTF-DCN, the BET surface area and total pore

138

volume are relatively independent of temperature (690 m2/g and 0.58 cm3/g for CTF-DCN-400,

139

735 m2/g and 0.59 cm3/g for CTF-DCN-500). These values are lower than those of CTF-1 (920-

140

1600 m2/g and 0.47-1.00 cm3/g)23 and other most CTFs, attributed to that FUM is shorter and

141

DCN wider than DCB. Pore size distributions of materials were derived by the nonlocal density

142

functional theory method. As shown in Figures 1b and 1d, CTF-FUM mainly has pore size of 5.2

143

Å. CTF-DCN has two kinds of micropores (5.4 Å and 14.1 Å) in addition to mesopores

144

distributed between 20 and 60 Å. In contrast to CTF-1 with pore sizes larger than 10 Å,23,35 CTF-

145

FUM and CTF-DCN possess ultramicropores of 5.2 and 5.4 Å, respectively. The PXRD patterns

146

of CTF-FUM and CTF-DCN indicate that all samples are amorphous (Figures S13 and S14). The

147

broad diffraction peak at 24.7º (25.9º) for CTF-FUM (CTF-DCN) suggests the potential stacking


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148

of triazine linkage (001) plane.35 Therefore, the pore sizes in CTFs could not be exactly reflected

149

by structures in Scheme 1. However, the successful construction of ultramicropores still indicates

150

that the strategy is feasible and effective by shortening or widening the size of monomer. (b) 5.2 Å

150 100

CTF-FUM-350 CTF-FUM-400 CTF-FUM-500

50 0 0.0

0.2

0.4

0.6

0.8

N2 uptake at 77 K (cc/g)

10

1.0

P/P0

(c)

dV(d) (cc/Å/g)

CTF-DCN-400 CTF-DCN-500

200

151

0 0.0

0.2

0.4

0.6

20

0.8

30

40

50

60

70

80

Pore width (Å)

(d)

600 400

CTF-FUM-350 CTF-FUM-400 CTF-FUM-500

dV(d) (cc/Å/g)

N2 uptake at 77 K (cc/g)

(a)200

5.4 Å

10

1.0

CTF-DCN-400 CTF-DCN-500

14.1 Å

20

30

40

50

60

70

80

Pore width (Å)

P/P0

152

Figure 1. (a, c) N2 adsoption-desorption isotherms at 77 K and (b, d) pore size distributions of

153

CTF-FUM and CTF-DCN.

154

Table 1. Pore characteristics and elemental analysis of CTF-FUM and CTF-DCN. Temperature

SBETa

Compound

%Found VMicrob

Vtc

(°C)

(m2/g)

CTF-FUM-350

350

230

0.09

0.12

CTF-FUM-400

400

480

0.17

CTF-FUM-500

500

603

CTF-DCN-400

400

CTF-DCN-500

500

%Theory

VMicro/Vt C

H

N

C

H

N

0.75

62.47

4.41

27.64

61.54

2.56

35.9

0.24

0.71

60.28

4.38

26.93

61.54

2.56

35.9

0.22

0.28

0.79

60.79

4.06

20.17

61.54

2.56

35.9

690

0.13

0.67

0.19

77.06

2.76

8.41

80.90

3.37

15.73

735

0.14

0.60

0.30

79.68

2.87

8.26

80.90

3.37

15.73

155 156 157

a

SBET is the BET specific surface area. bVMicro is the pore volume determined by N2 adsorption isotherm using t-plot method. cVt is the total pore volume determined by using the adsorption branch of N2 isotherm at P/P0 = 0.95.

158

Elemental analysis of CTF-FUM and CTF-DCN indicates that nitrogen contents decrease

159

significantly compared with the theoretical values due to CN group elimination in the

160

frameworks as well as decomposition of triazine rings during the polymerization reaction.23 The


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decomposition of triazine rings in the frameworks may produce NH3 and ring condensation

162

networks.34 Therefore, the measured C/N ratios of materials are higher than theoretical values

163

(Table S1 in the Supporting Information). In addition, the overall carbon, nitrogen, and hydrogen

164

contents of the samples are lower than theoretical values due to residual water adsorbed and

165

metal salts.29 As a result, the nitrogen contents significantly decrease, while the carbon contents

166

are close to the theoretical values. The similar phenomenon has been observed previously.25,33,36

167

With the increase of reaction temperature from 350 to 500 °C, nitrogen content of CTF-FUM

168

decreases from 27.64% to 20.17%, while for CTF-DCN the contents are almost invariable.

169

Although nitrogen elimination occurred, CTF-FUM-350 still has nitrogen content of 27.64%,

170

which is larger than that in most of POFs.23,37,38 Inductively Coupled Plasma analysis indicates

171

0.63 wt%, 0.61 wt%, 0.56 wt%, 0.67 wt%, and 0.48 wt% Zn contents for CTF-FUM-350, CTF-

172

FUM-400, CTF-FUM-500, CTF-DCN-400, and CTF-500, respectively. The Zn contents are low

173

and reasonable.32 Meanwhile, the morphology study of CTF-FUM and CTF-DCN by SEM

174

demonstrates the aggregations of uniform particles (Figures S15-S19). In addition, TEM pictures

175

of the samples demonstrate the layered structures of CTF-FUM and CTF-DCN (Figures S20-

176

S24).

177

3.2. Stability studies. Generally, the environments in flue gas and landfill gas are harsh where

178

several kinds of impurity exist including acid gases (HCl, NOx, SOx, and H2S) and H2O, which

179

calls for adsorbents with excellent stability. Therefore, the stability of CTF-FUM and CTF-DCN

180

was investigated. Immersed in common organic solvents such as N,N-dimetylformamide, N-

181

methylpyrrolidone, methanol, and ethanol, CTF-FUM and CTF-DCN were insoluble. Especially,

182

CTF-FUM-400 and CTF-DCN-400 were immersed in boiling water, 4 M HCl solution, and 0.1

183

M NaOH solution for 24 h as the representatives of CTF-FUM and CTF-DCN, respectively. As


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184

shown in Figure S25, the N2 adsorption isotherms of these samples remain almost the same after

185

these treatments, indicating both frameworks remain intact in such a wide pH range. Their

186

excellent chemical stability may be attributed to the strong C-N bands in triazine rings and the

187

carbonization of surfaces of materials.39 Compared with many POFs including boronate based

188

COFs40,41 and Schiff base polymers,42 CTF-FUM and CTF-DCN show very high stability. In

189

addition, CTF-FUM and CTF-NDC were characterized by TGA under an air atmosphere. As

190

shown in Figures S26-30, they are thermally stable up to 450 °C.

CH4 298 K

CH4 273 K

80

60 40 20

CO2 273 K N2 273 K CTF-FUM-400

CH4 298 K

CH4 273 K

40 20

60

0.6

0.8

Pressure (bar)

(d) CO2 298 K N2 298 K CH4 298 K

80 60

0.2

CH4 273 K

40 20 0

0.4

0.6

0.8

40 20

1.0

Pressure (bar)

(e)

CO2 273 K N2 273 K CTF-DCN-400

CO2 298 K N2 298 K CH4 298 K

80 60

0.0

0.2

0.4

0.6

Pressure (bar)

0.8

1.0

0.2

CO2 273 K N2 273 K CTF-DCN-500 CH4 273 K

0.4

0.6

1.0

CTF-FUM-350 CTF-FUM-400 CTF-FUM-500 CTF-DCN-400 CTF-DCN-500

60

40

40

0.8

Pressure (bar)

(f)

20

20 0

0.0

CH4 273 K

0 0.0

1.0

CO2 273 K N2 273 K CTF-FUM-500

Qst (kJ/mol)

0.4

Gas uptake (cc/g)

0.2

CO2 298 K N2 298 K CH4 298 K

80

0 0.0

Gas uptake (cc/g)

60

CO2 298 K N2 298 K

Gas uptake (cc/g)

80

CO2 273 K N2 273 K CTF-FUM-350

0

191

(c)

(b) CO2 298 K N2 298 K

Gas uptake (cc/g)

Gas uptake (cc/g)

(a)

0.0

0.2

0.4

0.6

0.8

1.0

Pressure (bar)

0

0

10

20

30

40

50

60

CO2 uptake (cc/g)

192

Figure 2. (a-e) Adsorption isotherms of CO2, N2, and CH4 in CTF-FUM and CTF-DCN at 298

193

and 273 K; (f) the calculated isosteric heat values of CO2 adsorption of CTF-FUM and CTF-

194

DCN.

195

3.3. Gas uptake studies. The ultramicroporous nature and the high nitrogen contents of CTF-

196

FUM and CTF-DCN prompted us to explore their CO2 capture properties. As shown in Figure 2,

197

the CO2 adsorption isotherms of CTF-FUM and CTF-DCN exhibit a steep rise at low pressure.

198

CTF-FUM-350 has the highest CO2 uptake of 57.2 cc/g at 298 K (Table 2) among all CTF-FUM

199

and CTF-DCN. A comparison of CO2 uptake by CTF-FUM-350 with other POFs is shown in


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200

Table S2. The uptake capacity of CTF-FUM-350 is higher than that of numerous of POFs such

201

as CTF-1 (31.6-50.4 cc/g),43 3D PCTF-1-3 (21.4-50.4 cc/g),31 PPN-6 (25.9 cc/g),44 MPN-P1-P3

202

(19.3-32.1 cc/g),45 HMP (31.5-38.1 cc/g),46 POP-diimide-Li6 (33.6 cc/g),47 TPI-1-7 (9.6-28

203

cc/g),48 and functionalized CMP-1 (20.9-26.4 cc/g).49 In addition, CTF-FUM-350 also

204

outperform most of MOFs such as ZIF-8 (16.9 cc/g),50 MIL-47 (41.2 cc/g),51 and MIL-53(Cr)

205

(43.3 cc/g).52 It is worth noting that with the decrease of temperature, CO2 uptake of CTF-FUM

206

increases attributed to that low reaction temperature leads to CTF-FUM with high nitrogen

207

content in spite of low surface area, which indicates that CO2 uptakes are not proportional to the

208

surface areas. Therefore, only increasing surface area may not be an effective strategy to enhance

209

CO2 capture at this condition. Considering that the recyclability is vital to the practical

210

applications, reproducibility of two CTFs was investigated by performing the adsorption-

211

desorption cycle at 298 °C. As shown in Figures S33-S36, the adsorption is reversible and no

212

obvious loss of activity is observed even after ten cycles, indicating the excellent recyclability of

213

two CTFs without any other heat energy input. In practical conditions, flue gas and natural gas

214

inevitably contain water vapor. Thus, it is important to investigate the CO2 capture performance

215

of the samples in the presence of water vapor. As reported in the literature,31,53 CTF-FUM-350

216

and CTF-DCN-500, as representatives for the best-performing samples in CTF-FUM and CTF-

217

DCN respectively, were exposed to the atmosphere (about 70% relative humidity conditions) for

218

24 h to equilibrate with moisture, CO2, and N2. Then, the treated samples were remeasured for

219

CO2 adsorption at 298 K without degassing. As shown in Figures S37-S38, the CO2 uptakes of

220

the treated CTF-FUM-350 (36.8 cc/g) and CTF-DCN-500 (29.0 cc/g) show 35% and 25% drops

221

relative to their outgassed samples, respectively. Large drops in CO2 capacity have been


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222

observed for POFs,31,53 MOFs,54 and activated carbon.53 Meanwhile, the adsorption behaviors of

223

N2 and CH4 in CTF-FUM and CTF-DCN were studied.

224

Table 2. CO2 capture capacities, selectivities, and heats of adsorption of CTF-FUM and CTF-

225

DCN.

Compound

CO2 uptake (cc/g)

Qsta for CO2 (kJ/mol)

CO2/N2 selectivityb

CO2/CH4 selectivityb

298 K

273 K

CTF-FUM-350

57.2

78.2

58.1

102.4

20.5

CTF-FUM-400

49.1

64.3

55.0

96.3

20.3

CTF-FUM-500

37.1

53.7

50.3

67.3

13.7

CTF-DCN-400

29.2

47.5

30.6

33.7

8.1

CTF-DCN-500

38.4

60.4

34.5

37.0

9.7

226 227

a

Heat of adsorption calculated using the Clausius-Clapeyron equation. bSelectivity was calculated from the ratio of initial slopes of CO2 and N2 or CH4 adsorption isotherms at 298 K.

228

To determine the binding affinity of CTF-FUM and CTF-DCN for CO2, the isosteric heats of

229

adsorption were calculated with the Clausius-Clapeyron equation from CO2 adsorption isotherms

230

at 298 and 273 K. As shown in Figure 2f, all materials show a high Qst (above 30 kJ/mol) at low

231

coverage. In addition, the Qst at zero coverage for CTF-FUM-350, CTF-FUM-400, and CTF-

232

FUM-500 are 58.1, 55.0, and 50.3 kJ/mol, respectively. These values are higher than those for

233

CTF-DCN (30.6 and 34.5 kJ/mol for CTF-DCN-400 and CTF-DCN-500, respectively), which

234

may be attributed to that CTF-FUM has higher nitrogen contents than CTF-DCN. Especially, the

235

value of CTF-FUM-350 is about 110% higher than that of CTF-1 (27.5 kJ/mol).43 In addition,

236

these Qst values for CTF-FUM and CTF-DCN are also higher than values reported for most of

237

POFs including BILP-1-7 (26.5-28.8 kJ/mol),55-57 HCP-1-4 (21.2-23.5 kJ/mol),58 and MCTF-

238

300-500 (24.6-26.3 kJ/mol).59 Although fast decay in the Qst plot of CTFs was observed due to

239

the fact that the numbers of available high-affinity N sites decrease with the increase of CO2


12

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240

loading in the frameworks, CTFs still possess high Qst at the whole pressure region. The high

241

CO2 uptakes and binding affinities by CTF-FUM and CTF-DCN can be attributed to the

242

presence of ultramicropore and high nitrogen contents. In addition, as shown in Figures S39-S43,

243

Qst values for N2 and CH4 of CTF-FUM (CTF-DCN) are about 9 (8) and 25 kJ/mol (23 kJ/mol),

244

respectively. These values are lower than Qst for CO2, which means stronger interactions

245

between CO2 and CTFs.

246

3.4. Selective CO2 capture over N2 and CH4. In addition to the high stability, CO2 uptake

247

capacity, and isosteric heat of adsorption of CO2, high selectivities of CO2 over N2 and CH4 are

248

also vital for a CO2 adsorbent. The selectivity for CTF-FUM and CTF-DCN was estimated using

249

the ratio of the initial slopes in the Henry region of the adsorption isotherms. The initial slopes of

250

adsorption isotherms were obtained by a linear fit, as shown in Figures S44-S48. This method is

251

one of the most common methods to estimate gas separation performance and has been widely

252

applied for porous materials including MOFs, POFs, and cage molecules. As shown in Table 2,

253

the calculated CO2/N2 and CO2/CH4 selectivites of CTF-FUM-350 reach values of 102.4 and

254

20.5 at 298 K, respectively. These values are slightly higher than those of CTF-FUM-400 and

255

CTF-FUM-500, and are much higher than those of CTF-DCN-400 and CTF-DCN-500. For CTF-

256

FUM, higher reaction temperature may lead to more defects in the framework and lower nitrogen

257

content, and thus decreases the interactions between CTF-FUM and CO2. Therefore, CO2/N2 and

258

CO2/CH4 selectivities as well as the Qst for CO2 decrease with the increase of reaction

259

temperature. However, for CTF-DCN, the higher reaction temperature does not greatly affect

260

porosity and nitrogen content. Therefore, CO2 selectivities for CTF-DCN are relatively

261

independent of temperature. These much different phenomenons have been observed

262

previously.43,60 The higher selectivities of CTF-FUM compared with CTF-DCN can be attributed


13


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263

to its higher nitrogen content. Especially, CO2/N2 selectivity of CTF-FUM-350 is about five

264

times of that of CTF-1 (20).43 In addition, the CO2/N2 selectivity of CTF-FUM-350 is higher

265

than that of most POFs60-62 (Table S2) and carbon materials.63-65 Especially, as shown in Table

266

S2, the selectivity of CO2 over CH4 for CTF-FUM-350 ranks in top ten among all reported POFs.

267

It far outperforms that of all reported CTFs and most of POFs including BILP-2-7 (5.3-12, 298

268

K),55,57 functionalized NPOF (8-10, 298 K),66 DBMOP-6 (12, 298 K),67 OMPM-1 (14.8, 298

269

K),68 PCTF-1-7 (4-5, 298 K),25,34 PPF-1-4 (8.6-11.0, 273 K),69 TPI-1-3@IC (2-11, 298 K),32, and

270

PCTF-1-4,31 (11-20, 273K), but just are smaller than those of PAF-30 (30.2, 298 K),70 POP-

271

diimide-Li6 (46, 298 K),47 POP(2) (60, 298 K),71 PPN-80 (90, 295 K),72 PPN-81 (250, 295 K),72

272

MPN-P1 (400, 298 K),45 and PPN-6-SO3NH4 (40, 295 K).73 However, the former six materials

273

have much low CO2 uptakes; and PPN-6-SO3NH4 needs not only more complex monomer but

274

also expensive catalyst, which significantly limits the possibility of scaling-up.

275

Because value calculated from Henry constant ratio represent selectivity close to zero pressure,

276

the ideal adsorption solution theory (IAST) model was additionally used to predict CO2

277

separation performance of CTFs at the whole pressure region. As shown in Figure S49, CO2/N2

278

and CO2/CH4 selectivities of CTF-FUM are higher than those of CTF-DCN at the whole pressure

279

range. In addition, there is no dramatic decrease in selectivities observed with the increase of

280

pressure. To confirm that CTF-FUM and CTF-DCN could completely separate gas mixtures in

281

practical applications, breakthrough experiments were carried out. Two binary gas mixtures,

282

namely CO2/N2 (15%/85%) and CO2/CH4 (50%/50%), were used for column breakthrough

283

experiments. As shown in Figure 3, for both CTFs, the breakthrough of CO2 is obviously later

284

than those for N2 and CH4, indicating the highly selective adsorption of CO2 in CTF-FUM and

285

CTF-DCN. The CO2 storage capacities of the CTFs have been calculated from breakthrough


14

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286

experiments. The results indicate that storage capacities of CTF-FUM-350 (CTF-DCN-500) are

287

11.4 (8.3) and 30.1 cc/g (25.4 cc/g) in the flows of CO2/N2 and CO2/CH4, respectively. These

288

values are lower than adsorption amounts obtained in the pure component adsorption at their

289

partial pressures (for CTF-FUM-350, 25.5 and 43.7 cc/g at 0.15 and 0.5 bar respectively; for

290

CTF-DCN-500, 13.0 and 26.6 cc/g at 0.15 and 0.5 bar respectively), which may be attributed to

291

the dynamic conditions.74 The similar phenomenon has been observed previously.75-77 In

292

addition, although the CO2 adsorption capacities of CTF-FUM-350 and CTF-DCN-500 decrease,

293

they still could completely separate gas mixtures. These results demonstrate their great promise

294

for industrial applications. In addition, further studies on the effect of different polymerizing

295

methods (catalyzed by ZnCl2 and trifluoromethanesulfonic acid) on the porous properties and

296

CO2 separation performances of CTF-FUM and CTF-DCN are ongoing. (a)

(b) 100

Volume fraction (%)

Volume fraction (%)

100 80 CTF-FUM-350

60

N2 CO2

40 20 0

0

2

4

6

8

10

12

14

20 0

2

4

6

8

10

12

14

16

Time (min)

100

Volume fraction (%)

Volume fraction (%)

297

40

(d)

100 80 CTF-DCN-500 N2

60 40

CO2

20 0

CH4 CO2

60

0

16

Time (min)

(c)

CTF-FUM-350

80

0

2

4

6

8

10

Time (min)

12

14

16

CTF-DCN-500 CH4

80

CO2

60 40 20 0

0

2

4

6

8

10

12

14

16

Time (min)

298

Figure 3. Dynamic breakthrough curves of CO2/N2 and CO2/CH4 at 298 K over (a, b) CTF-

299

FUM-350 and (c, d) CTF-DCN-500.

300

In summary, we successfully synthesized two CTFs, CTF-FUM and CTF-DCN, with

301

ultramicropores by using short and wide monomers, which are simple and readily available.

302

CTF-FUM and CTF-DCN possess ultramicropores of 5.2 and 5.4 Å, respectively. They also


15


Page 16 of 28

303

have high chemical and thermal stability. Due to the ultramicroporous nature, CTF-FUM and

304

CTF-DCN exhibit excellent adsorption capacities and selectivities for CO2 over N2 and CH4. In

305

addition, owing to higher nitrogen contents, CTF-FUM has higher isosteric heats of adsorption

306

of CO2 and CO2 selectivity over N2 and CH4. Especially, among all CTF-FUM and CTF-DCN,

307

CTF-FUM-350 has the highest nitrogen content (27.64%) and thus the highest CO2 adsorption

308

capacity (57.2 cc/g at 298 K) and selectivities for CO2 over N2 and CH4 (102.4 and 20.5 at 298

309

K, respectively). As far as we know, the CO2/CH4 selectivity ranks in top ten among all reported

310

POFs. Dynamic breakthrough curves indicate that both CTFs completely could separate gas

311

mixtures of CO2/N2 and CO2/CH4. Furthermore, CTF-FUM and CTF-DCN exhibit great

312

recyclability without evident loss of the CO2 adsorption capacity after ten cycles. These results

313

show that designing porous materials with both ultramicropores and high nitrogen contents is a

314

useful strategy for CO2 capture.

315

ASSOCIATED CONTENT

316

Supporting Information. Materials and methods, breakthrough measurement, characterization

317

results, recycle adsorption of CO2, calculation of isosteric heat of adsorption, IAST selectivity

318

calculation, initial slope selectivity studies, CO2 uptake in the humidity condition, IAST

319

selectivities, and comparison of the CO2 adsorption performance. This material is available free

320

of charge via the Internet at http://pubs.acs.org.

321

Notes

322

The authors declare no competing financial interest.

323

ACKNOWLEDGMENT


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324

Financial support by the National Key Basic Research Program of China (“973”)

325

(2013CB733503), the Natural Science Foundation of China (Nos. 21136001 and 21536001), and

326

the Fundamental Research Funds for the Central Universities (No. ZY1509) are greatly

327

appreciated.

328

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