Construction of Nitrogen-Containing Hierarchical Porous Polymers

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Materials and Interfaces

Construction of Nitrogen-containing Hierarchical Porous Polymers and Its Application on Carbon Dioxide Capturing Cheng Duan, Zhongjie Du, Wei Zou, Hangquan Li, and Chen Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Industrial & Engineering Chemistry Research

Construction of Nitrogen-containing Hierarchical Porous Polymers and Its Application on Carbon Dioxide Capturing Cheng Duan, Zhongjie Du*, Wei Zou, Hangquan Li, and Chen Zhang*

Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of chemical Technology), Ministry of Education; College of Materials Science and Engineering, Beijing University of chemical Technology, Beijing 100029, PR China.

ABSTRACT: The nitrogen-containing hierarchical porous polymers (NHPPs), which possessed micropores, mesopores, and marcopores simultaneously, were constructed by introducing micropores and mesopores into polystyrene foam by post-crosslinking of the skeleton via Friedel-Crafts reaction. Firstly, polystyrene foams with abundant macropores (voids and windows) were obtained by concentrated emulsion polymerization. Subsequently, micropores and mesopores were generated by post-crosslinking cyanuric chloride into the skeleton of polystyrene foam. Cyanuric chloride was used as external crosslinker because of its rich nitrogen content. Finally, NHPPs with high specific surface areas (up to 1226 m2/g) were obtained. The effects of 1

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volume fraction of the dispersed phase of concentrated emulsion, DVB in polystyrene matrix, external crosslinker, and solvent on the textural properties of NHPPs were investigated. Then NHPPs were applied as CO2 absorbent, and the maximum CO2 capacity of 141 mg/g at 273 K, 1 bar and the good reproducibility were achieved. Furthermore, the tentative adsorption mechanism of triazine-ring in NHPPs to CO2 was proposed. Most importantly, the relationship between the structures of NHPPs and their CO2 adsorption capacities was analyzed. The results indicated that the CO2 capacities mostly depended on the microporous volume, and the nitrogen content also had impact on the CO2 capacities. KEYWORDS: polystyrene foam, Friedel-Crafts reaction, concentrated emulsion, hierarchical porous polymers, CO2 capture

1 . INTRODUCTION Increasing CO2 level in atmosphere becomes a great issue because it leads to global warming, and the CO2-capture technology based on absorbent is considered as an effect strategy1, 2. Solid amine-containing sorbents3, 4, porous carbons5, 6, and metal-organic frameworks (MOFs)7, 8, etc., are developed and applied for CO2 capture, and the different features and advantages are exhibited. Porous organic polymers (POPs), such as covalent organic frameworks 2


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(COFs)9-11, conjugated microporous polymers (CMP)12-14, hyper-crosslinked polymers (HCPs)15-17, have been focused recently because of their large specific surface areas, high pore volumes, and abundant functional groups. When POPs are applied as CO2 absorbent, they show high CO2 capacity, excellent environment stability, good recyclability, and then exhibit great potential in industrial applications18-23. Hierarchical porous polymers, marked by containing the pores with series dimensional scale, have been investigated for gas absorbers, catalysts, photo sensitizers and cathode materials24-28. Besides the high efficient contacting with the target object provided by micropores, the fast mass transport supported by the interconnect marcopores and mesopores has an advantage on the practical applications29-31. In recent report, some hierarchical porous polymers could be obtained by solvothermal synthesis method20,

27

. In this work, the typical

hierarchical porous polymer was achieved by constructing micropores in macroporous polymer foam. The polymer foam matrix was firstly prepared by concentrated emulsion polymerization. Concentrated emulsion (also known as “high internal phase emulsion”) refers to the emulsion that the volume fraction of dispersed phase is higher than 74 %, which means that the dispersed phase droplets compacts each other and separated by a thin continuous phase layer. 3



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After the polymerization of continuous phase and the removal of dispersed phase, there would be voids reproducing from the dispersed phase droplet, and windows coming from the instability of the concentrated emulsion. As the result, the monoliths with highly-interconnected-open-cell porous structure could be obtained32-34. Subsequently, the micropores were generated by crosslinking the swollen skeleton of the polymer foam in dichloromethane (DCM) via Friedel-Crafts reaction, which could be carried out by cheap catalyst under mild condition35,

36

, as illustrated in Fig. 1 (a). Moreover, the polymers containing

nitrogen species (amine, triazine ring, imidazole, etc.) could bring better CO2 adsorption capacity of porous polymer37-39, then cyanuric chloride was chosen as the external crosslinker to introduce trazine ring into the network21, 38, 40. (Fig.1 (b)).

Accordingly, we reported a strategy of preparing the hierarchical porous polymers containing triazine ring in the framework (which were named as "NHPPs"), and the porous structure covered macropores, mesopores and micropores. The macroporous morphologies of NHPPs were observed by scanning electron microscopy (SEM), and the mesopores and micropores in NHPPs were analyzed by the N2 adsorption–desorption isotherm at 77 K. Moreover, the influences of the volume fraction of dispersed phase of 4


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concentrated emulsions,

divinyl benzene ratio in polystyrene, usage of CC, and

DCM volume on the macroporous morphologies, textural properties and nitrogen content of NHPPs were investigated in detail. Furthermore, the CO2 capacities of NHPPs with different pore structures and nitrogen contents were measured. The influences of textural properties and nitrogen content of NHPPs on the CO2 adsorption capacity were discussed by applying the linear fitting. Finally, the adsorption mechanism of triazine-ring on CO2 was explained.

Figure 1. Scheme of the formation of NHPPs. (a) porous structure; (b) chemical structure

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2. EXPERIMENTAL 2.1. Reagents. Styrene (St), divinylbenzene (DVB), sorbitan oleate (Span 80), peroxydisulfate (K2S2O8), and anhydrous aluminium chloride (AlCl3) were purchased from Tianjin Guangfu Fine Chemical Research Institute, China. Cyanuric chloride (CC) was obtained from Tokyo Chemical Industry Co., Ltd. Ethanol, methanol, dichloromethane (DCM) and hydrochloric acid were supplied by Beijing Chemical Works,

China. St

and

DVB

were

washed by

sodium hydroxide solution to remove the inhibitor. Other reagents were used as received. 2.2. Preparation of Polystyrene Foam. St (4.576 g), DVB and Span 80 (1.12 g) were mixed in a round bottomed flask as the organic phase, and then certain volume of aqueous solution containing K2S2O8 (0.20 g) as disperse phase was added drop by drop under mechanical stirring with a D-shaped paddle. Then the milky-white, paste-like concentrated emulsion was poured into a mould and polymerized for 24 h at 65 ℃ in an air-circulating oven. Finally, the obtained white monolith was washed with ethanol in a Soxhlet for 24 h and then dried in vacuum oven. 2.3. Preparation of NHPPs. The polystyrene foam (1.00 g), DCM, CC and anhydrous AlCl3 (CC/AlCl3 mole ratio was fixed at 1:4) were added into a 250 ml 6


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flask and mixed for 10 h by magnetic stirring in ice bath. The resultant mixture was fitted with a condenser and heated to 70 ℃ for 16 h under reflux. Finally, the orange monolith was separated from the solvent by filtering. The monolith was washed by DCM and acetone, and then soaked in hydrochloric acid solution. The final product was deeply purified by Soxhlet extractor in methanol for 24 h and then dried at 110 ℃ in a vacuum oven. 2.4. Characterization. The Fourier transform infrared spectra (FTIR) were obtained by Nicolet-Nexus 670 Spectrometer (Thermo). The contents of C, H, and N in the sample were determined by VarioEL cube (Elementar). The solid state

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C cross-polarization magic angle spinning (CP/MAS) NMR spectrum was

obtained by Avance II 300 WB 300 MHz spectrometer (Burker) equipped with a 4 mm double-resonance MAS probe with the spinning frequency of 12 kHz. The morphologies of monolith were characterized by scanning electron microscopy (SEM, S-4700 JEOL Ltd.) operating at an accelerating voltage of 20 kV. The average macropore size and macropore size distribution of the foams were measured by counting more than 100 pores in the SEM images with Nano Measurer 1.2.0 software. The N2 adsorption–desorption isotherms were measured by Micromeritics QUDRASORB SI sorptometer (Quanta chrome) at 77 K. The specific surface areas of samples were determined by using BET method and pore 7



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size distributions of samples were calculated by using the NL-DFT model with carbon slit pore geometry. The CO2 adsorption capacity at different temperatures and the adsorption isotherm were measured by JW-BK122W gas sorption analyzer (JWGB Sci. & TECH).

3.RESULTS and DISCUSSION 3.1. Synthesis of the NHPPs. A series of light-brown NHPPs were prepared by post-crosslinking the white polystyrene foams via Friedel-Crafts reaction. The polystyrene foams were prepared by using concentrated emulsion as template, and the samples of NHPP-1 to NHPP-5 were prepared by adjusting the volume fraction of dispersed phase and DVB content in continuous phase of concentrated emulsion. Moreover, NHPP-6 to NHPP-9 were prepared using different amount of CC or DCM in the post-crosslinking process based on the same polystyrene foam as precursor. The formulas of NHPPs were presented in Tab. 1. Table 1. The formulas of NHPPs

DVB/St (mol %)

NHPP

NHPP

NHPP

NHPP

NHPP

NHPP

NHPP

NHPP

NHPP

-1

-2

-3

-4

-5

-6

-7

-8

-9

2

2

2

6

10

2

2

2

2

8


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volume fraction of Dispersed

80

85

90

85

85

85

85

85

85

100

100

100

100

100

100

100

75

50

2.8

2.8

2.8

2.8

2.8

5.6

8.4

5.6

5.6

phase %

DCM (ml)

CC/ polystyrene foam weight ratio

3.2. Chemical structure of NHPP. As a typical sample, NHPP-2 was examined by FTIR and

13

C CP/MAS NMR for figuring out the chemical structure. As

shown in the FTIR spectrum of polystyrene foam (Fig. 2 (a)),

the peaks in the

region 3080-3020 cm-1 were attributed to C-H stretching vibration of benzene ring, and the peaks at 2853 and 2924 cm-1 were due to aliphatic C–H stretching vibration.41 Moreover, the peaks at 697, 757, and 900 cm-1 were due to aromatic =C-H out-of-plane deformation vibration of mono substituted benzene, and the four peaks in the region 2000-1700 cm-1 also indicated that benzene ring were mono-substituted. The emerging of these characteristic peaks was consistent with the chemical structure of polystyrene. In contrast, only single peak at 1719 cm-1 9



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in the region of 2000-1700 cm-1 being found and the peaks of aromatic =C-H out-of-plane deformation vibration shifting to 730, 814, and 896 cm-1 in the spectrum of NHPP-2 suggested that the benzene ring in NHPP was multi-substituted,42 which inferred to the successful post-crosslinking reaction (Fig. 2 (a)). Moreover, in the 13C CP/MAS NMR spectrum of NHPP-2 (Fig. 2 (b)), the peaks of methylene carbon (40 ppm), non-substituted aromatic carbon (127 ppm), substituted aromatic carbon (137 ppm), and particularly the carbon in triazine ring (165 ppm) were found, which also constituted the evidence for the post crosslinking by CC.43

Figure 2. FTIR spectra of polystyrene foam and NHPP (a) and

13

C CP/MAS

NMR spectrum of NHPP (b)

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3.3. Porous structure of polystyrene foam and NHPP-2. The typical macroporous morphologies of polystyrene foam and the resultant NHPP-2 were observed by SEM (Fig.3). In Fig. 3 (a), it could be found there were many cavities (which were always called as “voids”) interconnected with the adjacent ones by small pores (which were always called as “windows”) in polystyrene foam prepared by concentrated emulsion polymerization. The voids were derived from the dispersed phase of concentrated emulsion, and the windows were formed by the volume contraction of monomer converting into polymer.43 Such macropores could provide the permeating channel for the external crosslinker and solvent, which were benefit for the following post-crosslinking process. Moreover, it would also be the channel for delivering materials in the further application, such as adsorption, separation, or catalyst. Compared with the SEM image of polystyrene foam, there was no obvious difference found in the shape of macropores of NHPP-2, which illustrated that NHPP-2 could retain the macroporous morphology of its precursor after the post-crosslinking process (Fig. 3 (b)). However, the void sizes of NHPP-2 were greater than those of polystyrene foam (insert bar graphs in Fig. 3 (a) and (b)) and the windows number of NHPP-2 seemed more than that of the foam. Moreover, the surface of NHPP-2 was rougher than that of the foam. All of these differences might be due to the 11



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swollen and post-crosslinking process and was considered a less effect on the further application. As conclusion, it could be expected that the marcoporous morphologies of NHPPs were tunable as the polystyrene foam by adjusting the formulation of the concentrated emulsion, which would be discussed in the following.

The N2 sorption isotherm of polystyrene foam and NHPP-2 were shown in Fig. 2 (c). It could be seen that polystyrene foam showed quite a low gas uptake, but NHPP-2 showed a steep gas uptake at relatively low pressure (P/Po

Construction of Nitrogen-Containing Hierarchical Porous Polymers

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