Nitrogen-Rich Porous Polymers for Carbon Dioxide and Iodine

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Nitrogen-Rich Porous Polymers for Carbon Dioxide and Iodine Sequestration for Environmental Remediation Yomna H Abdelmoaty, Tsemre-Dingel Tessema, Fatema Akthar Choudhury, Oussama M. El-Kadri, and Hani M. El-Kaderi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03772 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Nitrogen-Rich Porous Polymers for Carbon Dioxide and Iodine Sequestration for Environmental Remediation

Yomna H. Abdelmoaty†‡, Tsemre-Dingel Tessema§, Fatema Akthar Choudhury, § Oussama M. ElKadri⊥*, and Hani M. El-Kaderi§* †

Department of Mechanical and Nuclear Engineering, Virginia Commonwealth University, 401 West Main Street, Richmond, Virginia 23284-2006, United States ‡

Department of Nuclear and Radiation Engineering, Alexandria University, Alexandria 21544, Egypt

§

Department of Chemistry, Virginia Commonwealth University, 1001 W. Main St., Richmond, Virginia 23284-2006, United States ⊥

Department of Biology, Chemistry, and Environmental Sciences, American University of Sharjah, PO Box 26666, Sharjah, United Arab Emirates

Keywords: conjugated porous polymers, CO2 capture, gas separation, iodine uptake, nuclear waste management

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ABSTRACT The use of fossil fuels for energy production is accompanied by carbon dioxide release into the environment causing catastrophic climate changes. Meanwhile, replacing fossil fuels with carbon-free nuclear energy has the potential to release radioactive iodine during nuclear waste processing and in case of a nuclear accident. Therefore, developing efficient adsorbents for carbon dioxide and iodine capture is of great importance. Two nitrogen-rich porous polymers (NRPPs) derived from 4-bis-(2,4-diamino-1,3,5,-triazine)-benzene building block were prepared and tested for use in CO2 and I2 capture. Copolymerization of 1,4-bis-(2,4-diamino-1,3,5,triazine)-benzene with terephthalaldehyde and 1,3,5-tris(4-formylphenyl)benzene in DMSO at 180 oC afforded highly porous NRPP-1 (SABET = 1579 m2 g-1) and NRPP-2 (SABET = 1028 m2 g-1), respectively. The combination of high nitrogen content, π-electron conjugated structure, and microporosity makes NRPPs very effective in CO2 uptake and I2 capture. NRPPs exhibit high CO2 uptakes (NRPP-1, 6.1 mmol g-1) and (NRPP-2, 7.06 mmol g-1) at 273 K and 1.0 bar. The 7.06 mmol g-1 CO2 uptake by NRPP-2 is the second highest value reported to date for porous organic polymers. According to vapor iodine uptake studies, the polymers display high capacity and rapid reversible uptake-release for I2 (NRPP-1, 192 wt. %) and (NRPP-2: 222 wt. %). Our studies show that the green nature (metal-free) of NRPPs and their effective capture of CO2 and I2 make this class of porous materials promising for environmental remediation.


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INTRODUCTION The excessive emission of carbon dioxide (CO2) gas into the atmosphere, mainly believed due to the burning of fossil fuels, contributes significantly to the increase in temperature of the earth’s surface and climate change.1-3 Possible solutions include CO2 capture and storage before its release into the atmosphere from coal-fired power plants and replacement of fossil fuels with more efficient, safe, and environmentally friendly renewable energy sources. The most prominent method for the capture and separation of CO2 from post-combustion of fossil fuels is wetscrubbing by the use of amine-based solutions such as monoethanolamine.4 Such process has many shortcomings that include solvent decomposition and evaporation, toxicity, high energy consumption for the regeneration of amine and recovery of the CO2.5 Nuclear energy is considered to be one of the best alternatives to fossil fuels given its high energy density and low carbon footprint. Nevertheless, exhaust fumes of nuclear energy power plants contain significant radioactive materials such as

129

I and

131

I isotopes which have half-lives of

approximately 8 days and 15.7 years, respectively, impose major environmental and public concerns.6-8 Moreover, fast removal of iodine in case of nuclear accidents is of particular importance since it can accumulate in living organisms at high rates. Solid inorganic adsorbents such as silver-doped zeolites and metal organic frameworks (MOFs) have been used in CO2 and iodine capture and sequestration (CCS).9-10 The former have found to have low CO2 and iodine uptakes due to their limited accessible surface area.11-12 Furthermore, the high cost of silver-based materials and their environmental impact impede their use at an industrial scale. MOFs show higher CO2 and iodine uptake capacities compared to zeolites, however, their poor stability in water or humid conditions make them impractical


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for such applications since exhaust fumes from nuclear and fossil fuel reactors contain high level water vapor content. 13-15 Porous organic frameworks (POPs) are made of covalently bonded light atoms such as H, B, C, N, and O, have superb thermal and chemical stability, high surface area, tunable pore size and functionality, and most importantly stable in wet condition which make them ideal for use in CO2 and iodine capture from flue gases and nuclear power plant exhaust fumes, respectively.16-20 As such, there has been a great interest in the use of POPs as adsorbents for CO2 and iodine. In particular, nitrogen-rich POPs have been found to increase the CO2 uptake and selectivity over other gases such as nitrogen and methane. It has been shown that the incorporation of nitrogen atoms into the framework increases the number of basic sites leading to more CO2 binding through specific dipole-quadruple interactions.21-24 Up to date, several POPs have been developed to capture and store volatile iodine.25-33 The performance of these POPs depends mainly on the surface area, pore volume, and active sites such as the presence of heteroatoms (N, S, etc.) which leads to enhanced iodine affinity that originate from the lone pairs on the heteroatoms and the iodine molecules interactions. With these considerations in mind, we report in this study the synthesis of two NRPPs by reacting 1,4-bis-(2,4-diamino-1,3,5-triazine)-benzene with terephthalaldeyde and 1,3,5tris(4-formylphenyl)benzene in DMSO. The resulting polymers have high surface area and exhibit high CO2 uptake and selectivity as well as high capacity for iodine adsorption from both vapor and liquid phases.

EXPERMENTAL SECTION Preparation of polymers


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Terephthalaldeyde, anhydrous dimethyl sulfoxide (DMSO), tetrahydrofuran, acetone, and dichloromethane were purchased from commercial suppliers (Sigma-Aldrich, Acros Organics, TCA America, and Frontier Scientific) and used without further purification, unless otherwise

noted.

1,4-bis-(2,4-diamino-1,3,5,-triazine)-benzene34

and

1,3,5-tris(4-

formylphenyl)benzene 35 were prepared according to previously reported literature procedures. The time and temperature are two important conditions in the synthesis of the NRPPs. The time and temperature are two important parameters in the synthesis of the NRPPs. Shorter reaction time and/or lower temperature have significant drawbacks such as low yield, degree of polymerization, and surface area presumably due to incomplete reaction. Such drawbacks will have detrimental effect on the performance of NRPPs. However, these factors are expected to have minor effects on nitrogen content which is mainly controlled by the chemical nature of the used N-rich monomers Synthesis of NRPP-1. A 50 mL round-bottom flask equipped with a condenser and a stir bar was charged with 1,4-bis-(2,4-diamino-1,3,5,-triazine)-benzene (552 mg, 1.86 mmol), terephthalaldehyde (500 mg, 3.73 mmol) and DMSO (15 ml). The resultant suspension was degassed with argon for 15 min then heated to 180 °C for 72 hr under inert atmosphere. The reaction flask was then cooled to room temperature and the resultant material was filtered, washed with acetone, tetrahydrofuran, dichloromethane, and dried under vacuum at 120 °C to afford NRPP-1 as a white powder in 65% yield. Anal. Calcd for C28H16N10: C, 68.29%, H, 3.27%; N, 28.44 %. Found: C, 41.06%; H, 4.68%; N, 27.56%.


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Synthesis of NRPP-2. In a similar fashion to the preparation of NRPP-1, a 50 mL roundbottom flask equipped with a condenser and a stir bar was charged with 1,4-bis-(2,4-diamino1,3,5,-triazine)-benzene (198 mg, 0.668 mmol), 1,3,5-tris(4-formylphenyl)benzene (350 mg, 0.896 mmol) and DMSO (5 ml) to afford NRPP-2 as a white powder in 58% yield. Anal. Calcd for C144H84N30: C, 77.40.99; H, 3.79%; N, 18.81%. Found: C, 31.63%; H, 3.38%; N, 23.26. Note: nitrogen-rich aminal-linked polymers typically give lowered carbon values in elemental microanalysis.

Gas Adsorption Studies. Autosorb-IQ2 volumetric adsorption analyzer (Quantachrome instruments) was used to collect CO2 adsorption isotherms. Samples were evacuated at 100 oC for 20 hr prior to collecting the isotherms. Specific surface area was measured using N2 at 77 K employing Brunauer-Emmett-Teller (BET) approach. CO2 adsorption isotherms were collected at 298 and 273 K. Pore size distribution (PSD) was produced through N2 and CO2 adsorption data. By performing Quench Solid Density Functional Theory (QSDFT) for N2 isotherm at 77 K and Non-Local Density Functional Theory (NLDFT) for CO2 isotherm at 273 K, assuming slit-like geometry for the carbon material. Iodine Adsorption and Release. Iodine uptake studies were investigated for iodine vapor and iodine dissolved in cyclohexane. For iodine vapor, the gravimetric uptake of iodine vapor was collected by placing 50 mg of NRPPs into open glass vial, which was placed in anther sealed container containing I2 powder (Figure S1). The whole system was heated at 80 oC under ambient pressure. After specified time intervals, the samples were cooled down to room


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temperature then weighed to determine iodine uptake. The weight percentage of captured Iodine was calculated using the following equation:

∗ 100 wt%, where m2 and m1 are

masses of polymer powder after and before iodine uptake, respectively. The reversibility of iodine sorption was evaluated using ethanol as an extraction solvent. 15 ml of ethanol was added to 5 mg of iodine loaded polymers and the amount of iodine released was detected via UV-Vis spectroscopy at selected time intervals. For the adsorption capacity of dissolved iodine in cyclohexane, adsorption isotherms were collected by preparing different concentrations of iodine-cyclohexane (0.03, 0.05, 0.1, 0.2, 0.3 and 0.4 mg/ml) solutions. 5 mg of fresh NRPPs samples were soaked in 5ml for each concentration for 72hr until reaching equilibrium. The samples were filtered and the filtrates were used for measuring the amount of iodine by UV−Vis spectrophotometer. Physical Characterization. Scanning electron microscopy (SEM) images were collected using a Hitachi SU-70 scanning electron microscope after coating samples with Pt film. Energydispersive X-ray spectroscopy (EDX) was performed to identify elements present in the samples. Powder X-ray diffraction (PXRD) was used to study the crystallinity of the samples. The Fourier transform infrared (FT-IR) spectra were recorded from 400-4000 cm-1 on Nicolet–Nexus 670 FTIR. UV/Vis spectrum was collected on HP (Aglient) 8453 UV-Vis.

13

C MAS NMR experiments

were performed on a 17.6 T (750 MHz) wide bore (Bruker, Billerica, MA) spectrometer using a Bruker BL4 HXY 4 mm MAS probe (tuned to

13

C). Spinning was regulated at 10.0 kHz ± 5 Hz

using a Bruker MAS II pneumatic MAS controller. All experiments were performed at 25 °C using a Bruker BVT-3000 temperature controller. Thermal gravimetric analysis (TGA) was carried out on a Perkin thermo-gravimetric analyzer apparatus with heating rate of 5 °C/min under N2 low.


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Elemental analysis was performed for carbon, nitrogen, and hydrogen on EuroEA3000 Series CHN Elemental Analyzer (Eurovector Instruments). RESULTS AND DISCUSSION Synthesis of Polymers The synthesis of NRPPs was carried out by copolymerization of 1,4-bis-(2,4-diamino-1,3,5triazine)-benzene with terephthalaldeyde and 1,3,5-tris(4-formylphenyl)benzene in DMSO at 180 °C for three days under an inert atmosphere as depicted in Scheme 1.36 This copolymerization route has been employed before and the reaction proceeds to link the building blocks through the formation of aminal linkages.

36 37-38

The resulting NRPPs are

chemically stable which enabled their isolation and use under ambient conditions. Due to their highly cross-linked nature, NRPPs are insoluble in common organic solvents such as THF, toluene, dichloromethane and acetone.


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H2N

NH2

N N

OHC

N

N

N

N

N

CHO

N

N

NH DMSO

+ N H2N

NH

180 oC, 3 Days - H2O

(NRPP-1)

N N

CHO

NH2

N

N

N

N

N

N

NH NH OHC

CHO (NRPP-2)

Scheme 1. Synthesis of nitrogen-rich porous polymers. Structure analysis.

The chemical connectivity between the building blocks was first

investigated by collecting Fourier transform infrared (FT-IR) spectra for both NRPPs (Figure 1a). The FT-IR spectra show a broad band at approximately 3410 cm-1 corresponding to N-H aminal stretching vibrations of a secondary amine and a distinct triazine quadrant stretch at 1520 cm-1. Moreover, peaks for the triazine -NH2 stretching (3470 and 3420 cm-1) or deformation (1607 cm-1) and those of the aldehyde’s C=O (1690 cm-1) and C-H (2870 cm-1) stretching are either absent or strongly reduced. These observation supports the successful conversion of the functional groups initially present in the building blocks into aminal linkages and thus, supports the formation of the polymers. Solid-state 13C cross polarization magic-angle spinning NMR (CP-MAS) spectra of NRPPs were collected (Figure 1b) and show carbon resonances at 166 ppm (C-triazine ring), 128 and 139 ppm (C-aryl), and 55 ppm (C-aminal functionality).

36

PXRD spectra show no peaks for NRPPs


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polymers (Figure S2), consistent with their expected amorphous nature while the SEM images for NRPPs samples show aggregation of nanoparticles having a dimeter of ~ 100 nm and 250 nm for NRPP-1 and NRPP-2 (Figure 2), respectively.

(a)

NRPP-1

(b)

NRPP-1

NRPP-2

NRPP-2

1380

**

1530

*

4000 3500 3000 2500 2000 1500 1000

*

*

*

*

500 200 180 160 140 120 100 80

Wavenumber (cm-1)

*

60

40

20

0

ppm

Figure 1. (a) Fourier transform infrared (FTIR) spectra of NRPPs and (b) Solid-state 13C CP-MAS of NRPPs, * sidebands and ** grease contamination.


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(a)

(b)

100µ µm

500µ µm

Figure 2. Scanning electron microscope (SEM) of as prepared NRPP-1(a) and NRPP-2 (b).

Porosity Measurements and Textural Properties Porosity Measurements and Textural Properties. The porosity of NRPP-1 and NRPP-2 was evaluated via N2 (at 77 K) adsorption-desorption isotherms presented in Figure 3-a. Both polymers display a sharp N2 uptake at very low pressure (P/Po< 0.01) followed by a plateau P/Po which signifies the permanent microporous nature of the polymers.39 The sharp uptake at P/Po > 0.9 is caused by adsorbate condensation within interparticle voids. The Brunauer-EmmettTeller (BET) method was applied to calculate the specific surface areas of the polymers using the adsorption branch of N2 isotherms (Table 1). The polymers possess high porosity with surface areas of 1579 m2 g-1 and 1028 m2 g-1 for NRPP-1 and NRPP-2, respectively. The pore size distributions (PSD) of NRPP-1 and NRPP-2 were calculated from NLDFT fittings of the adsorption and desorption branches of N2 (77 K) isotherm with carbon as adsorbent and slit/cylindrical pores (Figure 3-b). NRPP-1 and NRPP-2 have dominant pores of around 0.68 and 0.70 nm respectively. The total pore volumes determined from single point adsorption values at P/P0 = 0.90 were found to be 0.91 and 0.81 cm3 g-1 for NRPP-1 and NRPP-2 respectively. The


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micropore volume analysis from the quenched solid density functional theory (QSDFT) fitting showed that NRPP-2 had a higher percentage of micropores (53.5%) than NRPP-1 (33.6%). The higher micropore volume in NRPP-1 is very significant because it can influence gas sorption and selectivity of porous polymers.

Table 1: Textural Properties of NRPP-1 and NRPP-2. a

SA(BET) 2

-1

m g

b

PSD nm

c

Pore Vol.(Tot.) 3 -1

cm g

d

Pore Vol.(Mic.)

cm3 g-1

NRPP-1 1579 0.68 0.91 0.487(53.5) NRPP-2 1028 0.70 0.81 0.272(33.6) a Brunauer–Emmett–Teller (BET) surface area. bDominant pore size determined by QSDFT fittings of N2 isotherms at 77 K.

c

Total pore volume at P/ Po = 0.90. dMicropore volume determined by DFT (the

values in parenthesis are the percentage of micropore volume relative to total pore volume)


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(a )

(b )

N R P P -1 N R P P -2

2500

N R P P -1 N R P P -2

0 .8

2000

dV(d) (cc/nm/g)

0 .6

1500

Uptake (cc/g)

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


1000

0 .4

0 .2 500

0 .0

0

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

0 .0

0 .5

1 .0

P /P 0

1 .5

2 .0

2 .5

3 .0

3 .5

4 .0

P o r e W id t h ( n m )

Figure 3. (a) N2 isotherms of NRPP-1 and NRPP-2 at 77 and (b) Pore size distribution of NRPP-1 and NRPP-2 from N2 isotherms at 77 K.

Gas Uptake and Selectivity Studies Microporous materials possessing pores pre-dominantly below 0.1 nm have been known to exhibit desirable CO2 capture and separation properties.40 To further elucidate such properties of NRPP-1 and NRPP-2, single CO2 gas isotherms were measured at 273 and 298 K (Figure 4). The CO2 isotherms display minimal hysteresis indicating regenerability of the polymers by pressure reduction at ambient pressure or applying vacuum. This is due to the Lewis basic nature of imidazole moieties which display a strong affinity for acidic and polarizable CO2 molecules.35, 41-42 In comparison, NRPP-2 and NRPP-1 display relatively high CO2 uptake (7.06 and 6.10 mmol g-1 at 273K, respectively) and (2.22 and 3.71 mmol g-1 at 298 K, respectively) at 1 bar as reported in Table 2 compared to published POPs (Table S1). The binding affinity of CO2 to the polymers’ pore

walls was evaluated via their heat of adsorption, Qst values, calculated using the Virial equation.


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Consistent with the trend observed for the trend in uptake NRPP-1 and NRPP-2 displayed Qst values of 28 and 29.1 kJ mol-1 at zero coverage respectively (Figure 5-a). The binding affinities gradually decrease with increasing CO2 loading due to saturation of binding sites. The Qst values of the NRPPs are within close proximity of the optimum value for CO2 capture from flue gas (~33 kJ mol-1) as previously postulated in literature. The CO2 uptakes of NRPP-1 and NRPP-2 are amongst the highest of other nitrogen rich polymers including BILPs,35, 42-44 ALPs,41, 45 azoCOPs46 and PECONFs.47 The CO2 uptake at 1 bar is not a conclusive depiction of a porous material’s performance to capture CO2 from gas mixtures as the partial pressure of CO2 varies depending of the ratio of gases present in the mixture. Additionally, we were encouraged by some key properties including respectable surface area, Qst, CO2 uptake as well as narrow pores which make them promising candidates for selective capture of CO2 in predominantly CO2/N2 and CO2/CH4 gas mixtures. As such, we have conducted selectivity studies for flue gas, CO2/N2, and landfill gas, CO2/CH4, compositions using two well established methods: initial slope and Ideal Adsorbed Solutions Theory (IAST). For flue gas mixtures, the initial slope method yielded CO2/N2 selectivities of 21 and 24 for NRPP-1 and NRPP-2 respectively. This trend was in agreement with values for IAST selectivities (CO2:N2 = 10:90) of 21 and 36 for NRPP-1 and NRPP-2 (Figure 6-a) respectively. In this case the high CO2/N2 selectivity of NRPP-2 can be attributed to the presence of the larger ratio of nitrogen moieties induced from the 1,3,5-tris(4-formylphenyl)benzene building block used in the synthesis of the polymers. This leads to a higher CO2 uptake in the low pressure region relevant to the partial pressure of CO2 in flue gas compositions. For landfill gas composition, the initial slope method displayed a CO2/CH4 selectivity of 5 for both NRPP-1


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and NRPP-2 whereas IAST yielded values of 5.3 and 7.4 for NRPP-1 and NRPP-2 (Figure 6-b) respectively. The CO2/CH4 selectivity is calculated at higher partial gas pressure of both gases therefore the uptake at these pressures has a direct correlation with selectivity. Previous reports have established that, CO2 uptake is heavily dependent on pore functionality whereas CH4 uptake is more influenced by surface area.48 In agreement, NRPP-1 is observed to have a higher CH4 than NRPP-2 (1.12 versus 1.07 mmol/g at 298 K and 1 bar). This is translated in the higher CO2/CH4 selectivity values for NRPP-2 which has lower surface area than NRPP-1.

Table 2: Gas uptakes, heat of adsorption and selectivity of NRPPs a

CO2 uptake at 1 bar

a

CH4 uptake at 1 bar

a

N2 uptake at 1 bar

b

Selectivity

Polymer

273K

298K

Qst

273K

298K

Qst

273K

298K

CO2/N2

CO2/CH4

NRPP-1

6.10

3.71

28

1.94

1.12

20.8

0.55

0.31

21 (21)

5.3 (5)

NRPP-2

7.06

2.22

29.1

1.33

1.07

20.4

0.67

0.24

36 (24)

7.4 (5)

a

Gas uptake in mmol g-1, and isosteric heat of adsorption (Qst) at zero coverage in kJ mol-1. bSelectivity

(mol mol-1, at 1.0 bar) calculated by IAST method at mole ratio of 10:90 for CO2/N2 and mole ratio of 50:50 for CO2/CH4 at 298 K and values in parenthesis are calculated from by the initial slope method.


15


7 CO2 CH4 N2

CH4

4 3 2

4 3 2 1

0

0

0.2

0.4 P/Po 0.6

0.8

N2

5

1

0.0

CO2

6

Uptake (mmol/g)

Uptake (mmol/g)

5

NRPP-2 Gas Uptake at 273K

7

NRPP-1 Gas Uptakes at 273K 6

0.0

1.0

0.2

0.4

P/Po 0.6

0.8

1.0

0.8

1.0

4.0 4.5


3.5


4.0

CO2 CH4

2.5

CO2

3.5

Uptake (mmol/g)

3.0

Uptake (mmol/g)

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

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N2

2.0 1.5 1.0

CH4 3.0

N2

2.5 2.0 1.5 1.0

0.5

0.5

0.0

0.0

0.0

0.2

0.4

P/P0

0.6

0.8

1.0

0.0

0.2

0.4

0.6

P/P0

Figure 4. CO2, CH4 and N2 uptake isotherms of NRPP-1 and NRPP-2 at 273 and 298 K.


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22

32

(b)

(a)

30

21 28

20

Qst (kJ/mol)

Qst (kJ/mol)

26

24

19

22

18

20

18

17 16

NRPP-1 NRPP-2

NRPP-1 NRPP-2

16

14 0

1

2

3

0.0

4

0.2

0.4

0.6

0.8

1.0

1.2

CH 4 Uptake (mmol/g )

CO 2 Uptake (mmol/g)

Figure 5. Isosteric heat of adsorption CH4 for NRPP-1 and NRPP-2 determined from virial equation of a)CO2 and b)CH4.

10

45 (a)

(b)

40

8

CO /CH Selectivity 2 4

35 CO /N Selectivity 2 2

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


6

30

25

4

20

2 15

NRPP-1 NRPP-2

NRPP-1 NRPP-2

10

0 0.0

0.2

0.4 0.6 Pressure (Bar)

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Pressure (Bar)

Figure 6. (a) IAST CO2/N2 and (b) CO2/CH4 selectivity at 298 K of NRPP-1 and NRPP-2


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Capture and reversible storage of iodine Iodine uptake studies were carried out by exposing activated NRPPs to iodine vapor inside a closed vessel. It was observed that exposure lead to a color change in the sample from offwhite to dark brown (Figure S3). The maximum iodine uptake capacity of NRPP-1 and NRPP-2 are 192 wt. % and 222 wt. % (Figure 7), respectively, these values are among the highest reported for porous materials ( Table S2).18, 49-52 The iodine sorption rate is fast when compared to other porous sorbents and the uptake reaches equilibrium after 1.0 hr and 1.5 hr for NRPP-1 and NRPP-2, respectively. The high iodine capture capacity of NRPPs can be attributed to a combination of several factors such as high porosity, high N-doping53 (Table S3), and π-electron conjugated structures18, 54 of the polymers. The thermal stability of iodine-loaded NRPPs samples was tested by thermogravimetric analysis (TGA) under nitrogen (Figure S4). According to TGA studies, the as-prepared polymers show steady weight loss presumably due to solvent evaporation and sublimation of unreacted monomers trapped inside the pores, before thermal decomposition at ~ 400 °C. For iodine loaded NRPPs samples (NRPPs were thermally activated and degassed prior to iodine loading), the weight loss was about 60-70 wt% at 300 °C. The sublimed iodine mass is in agreement with the gravimetric uptake of iodine by NRPPs. Reversible iodine uptake by porous adsorbent is vital for their effective use and therefore, NRPPs were subjected to such studies. As stated above, iodine release from iodineloaded NRPPs upon thermal heating leads to iodine sublimation and thus adsorbent (NRPPs) regeneration as evidenced by TGA studies. Furthermore, iodine release in ethanol under ambient conditions was monitored by UV-Vis. The gradual iodine release into solution is accompanied by color change from light to dark yellow as a function of time (Figure S5, S6). The


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UV-vis spectra of the filtrates show two absorbance maxima at 291 and 360 nm (Figure S7, S8) that support the presence of polyiodide anions.55 The released iodine amount increased with time up to 24 hr (Figure 8), which is different from vapor adsorption time required to reach equilibrium. The linear release of iodine from NRPPs during the first 20 minutes indicates that host−guest interacƟons releasing mechanism (Figure 8).18, 56-57 The majority of iodine release took place in the first 50 min of soaking. Interestingly, both NRPPs show excellent ability to release adsorbed iodine (~ 98%), which is highly desirable for adsorbent regeneration and reuse.

100 90

Iodine Release Percentage(%)

200

Quantity Adsorbed(wt%)

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


150

100

NRPP-1 NRPP-2

50

0

80 70

NRPP-1

60 50

NRPP-2 40 30

0

1

2

3

4

5

6

0

10

20

30

40

50

60

70

80

Time (min)

Time/hr

Figure 7. Gravimetric uptake of iodine at 80 Figure 8. Iodine release from NRPPs into C for NRPPs and their color change upon ethanol and solution color change (inset). iodine adsorption (inset). o

Removal of dissolved iodine by NRPPs. The ability of adsorbents to rapidly remove dissolved iodine is very important for nuclear waste management. We have tested the applicability of NRPPs by immersing the polymers in cyclohexane solutions with variable concentrations. The


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iodine removal was monitored by UV-Vis spectroscopy. Iodine removal was very visible as the dark pink color of the iodine solutions became lighter with time (Figure 9-a, S9). It was noticed that the iodine uptake by NRPPs increased quickly during the first 15 minutes followed by slow increase until equilibrium was established. After 72 hours of soaking, the polymers were removed by filtration and the iodine concentration in the filtrates was fitted using calibration curves to determine the maximal amount adsorbed iodine (Figure S10, S11). To gain insight into the iodine adsorption mechanism, we fitted the adsorption isotherms using the Langmuir model (Figure 9b). Langmuir isotherms were generated by plotting 1/Ce vs 1/qe; where Ce is the equilibrium concentration of iodine (mg/L) and qe is the amount of iodine adsorbed per gram of NRPPs at equilibrium (mg/g). Langmuir model assumes monolayer coverage on a homogeneous surface with identical adsorption sites and equal adsorption activation energy. The results show that the correlation coefficients (R2) of linear fitted Langmuir isotherm models are high indicating that the adsorption isotherms could be well fitted by the Langmuir model. Furthermore, we tested the Freundlich model by plotting Ln(Qe) vs Ln(Ce) to better understand the iodine adsorption processes (Figure 9c). In contrast to Langmuir model, Freundlich model assumes heterogenic multilayer interaction and reversible adsorption. According to our results summarized in Figure 9 and Table S4, the correlation coefficients of the linear fitted Freundlich isotherms are higher than those obtained for Langmuir model fittings. However, both models seem to adequately describe iodine adsorption isotherm with relatively high correlation coefficients. It should be noted that the enhanced fitting by the Freundlich model is most likely because NRPPs have three potential adsorption sites for iodine; phenyl rings, triazine moieties, and aminal N-H sites.


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(a)

0.040

(b)

4.5 (c)

NRPP-1, R2=0.9623 NRPP-2, R2=0.9954

0.035

NRPP-1, R2=0.99972 NRPP-2, R2=0.99822

4.0 0.030

0.025

Ln(q e )

3.5

1/q e(g/mg)

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


0.020

3.0

0.015

0.010

2.5

0.005

2.0 0.000 0.00

0.02

0.04

0.06

1/Ce(L/mg)

0.08

0.10

3.5

4.0

4.5

5.0

5.5

Ln(Ce)

6.0

6.5

7.0

Figure 9. (a) Equilibrium concentration for NRPP-2 after 72 hr for different concentrations, experimental iodine adsorption isotherms at fitted to Langmuir model (b) and Freundlich model (c). Ce: iodine concentration at equilibrium and Qe: iodine adsorbed quantity. CONCLUSIONS In summary, two new nitrogen rich, highly porous polymers were synthesized employing Schiff base reaction. The high surface area, nitrogen decorated pores, electron-rich aromatic structures, lead to an exceptional CO2 uptake of 7.06 mmol g-1 which represents one of the highest values reported to date for POPs. In addition, these polymers can capture up to 222 wt. % of volatile iodine and release over 98% of the loaded iodine when immersed in ethanol solution at room


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temperature. The adsorption isotherms can be described by either the Langmuir or the Freundlich model, with the latter having a more enhanced fitting. However, the presence of competing species such as water may make the polymers less efficient toward CO2 and iodine capture. The good performance of the NRPPs in both CO2 and iodine uptake suggests that POPs with high nitrogen content and electron-rich aromatic structures, along with high porosity offer a promising solution for the ever-increasing environmental concerns posed by fossil fuel and nuclear energy. ASSOCIATED CONTENT Supporting Information Iodine uptake and release, powder X-ray, TGA, and IAST selectivity studies of NRPPs. This material is available free of charge on the Publications website. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Fax +971 6 515-2450, Tel +971 6 515-2787 * E-mail: [email protected], Fax (804) 828-8599. Tel (804) 828-7505 Note: YHA and TDT contributed equally. ACKNOWLEDGMENT TDT thanks Altria for graduate assistantship. OME thanks the American University of Sharjah, grant # FRG17-R-12, for supporting this research project. References


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TOC

Designed synthesis of highly porous organic polymers featuring nitrogen-rich building blocks linked by aminal linkages leads to one of the highest carbon dioxide and iodine uptake capacity by porous organic polymers which make the polymers promising for use in environmental remediation


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Nitrogen-Rich Porous Polymers for Carbon Dioxide and Iodine

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