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Enhanced CO2 adsorption on nitrogen-doped porous carbons derived from commercial phenolic resin Limin Yue, Linli Rao, Linlin Wang, Yan Sun, Zhenzhen Wu, Herbert DaCosta, and Xin Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03646 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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Enhanced CO2 adsorption on nitrogen-doped porous carbons derived from commercial phenolic resin Limin Yuea, Linli Raoa, Linlin Wangb, Yan Suna, Zhenzhen Wua, Herbert DaCostac, Xin Hu*,a a

College of Chemistry and Life Sciences, Zhejiang Normal University, 688 Yingbin

Ave. Jinhua 321004, PR China b

College of Engineering, Zhejiang Normal University, 688 Yingbin Ave. Jinhua

321004, PR China c

Math, Science, and Engineering Division, Illinois Central College, 1 College Drive

East Peoria, IL 61635, USA *

Corresponding author’s e-mail: [email protected]; phone: 86-151-0579-0257; fax:

86-579-8228-8269

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Abstract The CO2-capture potential of porous carbons that have been derived from phenolic resin and doped with nitrogen was assessed in this work. Using carbonized commercial phenolic resin as carbon precursors, a series of carbons have been synthesized using urea modification and KOH activation under different conditions. The activation temperature and mass ratio of KOH to precursor affected the CO2 uptake capacity. These phenolic resin-derived carbons show high CO2 capture capacity, up to 4.61 mmol/g at 25ºC and 7.13 mmol/g at 0ºC under atmospheric pressure. The sample prepared under relatively mild conditions, i.e. activation temperature of 600ºC and mass KOH/precursor of 3, demonstrated the maximum CO2 uptake capacity under ambient conditions. A systematic study shows that the synergetic effects of narrow microporosity and nitrogen content determine the sorbents’ capability to capture CO2. In addition, the pore size and the narrow micropores’ distribution affect the CO2 adsorption capacity of this series of porous carbons. Moreover, these resin derived carbons show other superior CO2 capture properties such as fast sorption kinetics, high CO2/N2 selectivity, moderate heat of adsorption, stable recyclability and high dynamic CO2 capture capacity.

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1. Introduction CO2 capture from flue gas is currently an essential approach of reducing CO2 emissions.1 Among different CO2 capture strategies, adsorption via solid sorbents has shown great promise and has attracted a lot of research interests.2-5 In this respect, different porous materials such as zeolites,6 silicas,7,

8

carbons,9-12 metal-organic

frameworks13 (MOF) and porous polymers14-16 have been extensively studied as CO2 adsorbents. Among various porous materials, porous carbons have merits of minimal costs, tunable porosity and surface functionalization, reversibility, inertness to chemicals, hydrophobicity, high thermal stability and low energy requirements for regeneration.9 Porous carbons have also shown very high CO2 sorption capacity under ambient conditions. For example, pine nut shell-derived porous carbons showed a maximum CO2 uptake of 5.0 mmol/g at 25ºC and ambient pressure.17 A sawdust-derived porous carbon developed by Fuertes and co-workers had 4.8 mmol/g CO2 adsorption capacity at 25ºC and 1 bar.18 In another work, Wahby and collaborators demonstrated a CO2 uptake of 4.7 mmol/g for a petroleum pitch derived porous carbon at 25ºC and ambient pressure.19 The high CO2 uptake of these porous carbons is attributed to their high amount of narrow micropores (< 1nm).18, 20-27 One effective way to further enhance the CO2 uptake of carbonaceous sorbents is to incorporate basic nitrogen-containing groups into the carbon structure. For example, a nitrogen-doped activated carbon synthesized by Sayari et al. had CO2 uptake as high as 5.4 mmol/g at 25ºC and 1 bar.28 Mokaya synthesized nitrogen-doped porous carbon from polypyrrole which showed a maximum of 5.5 mmol/g for CO2 adsorption capacity at 25ºC and 1 bar.29 Recently, phenolic-resin based porous carbons have attracted a lot research interests and shown great promise as CO2 adsorbent. High CO2 adsorption capacity up to 4.4 mmol/g at 25ºC,30 and 8.9 mmol/g at 0ºC31 and ambient 3

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pressure were reported. Despite the exceptionally high CO2 adsorption capacity, the above mentioned phenolic-resin based porous carbons have been achieved by costly or complex approaches in synthesizing resin precursors. Considering the large-scale application and manufacturing cost of porous carbons, cheap carbon precursors and simple synthetic process should be adopted. In this work, using commercial phenolic-resin as carbon precursors, we show an easy and cost-effective method to produce N-doped porous carbons with excellent CO2 capture properties. The commercial resins were firstly carbonized then further treated by modification with urea and activation with KOH under different conditions to tailor the porosity as well as the surface functionalization. The effects of both activation temperature and mass KOH/precursor ratio on the CO2 uptake capacity were investigated. The goal of this work is to evaluate the potential of nitrogen-doped porous carbons derived from commercial phenolic-resin for CO2 capture. In addition, this work will provide experimental proof for the importance of narrow microporosity and the content of nitrogen in determining CO2 adsorption capacity at ambient conditions for nitrogen-doped porous carbons. 2. Experimental 2.1 Synthesis of adsorbents Commercial phenolic resins (2123-type) obtained from Xinxiang Bomafengfan Industrial Co., Ltd, China were used as the carbon precursor for preparing nitrogen-doped porous carbons. The phenolic resin was carbonized using a horizontal quartz tubular reactor under a N2 flow of 200 mL/min. The carbonization temperature is set to 500°C with a heat rate of 5°C/min, and the duration of carbonization is 2 h. To simplify, carbonized phenolic resins was denoted as R. The detailed urea modification and KOH activation procedure can refer to our previous reports32, 33 and 4

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be found in the supporting information. For convenience of the following discussion, after R was modified by urea, the resultant sample was denoted as RU. The N-doped carbonaceous sorbents synthesized using further activation with KOH under different conditions were designated as RUK-X-Y, with X being the activation temperature and Y means the KOH/RU mass ratio. The temperatures of 600, 650 or 700ºC and KOH/precursor mass ratios of 2, 3 or 4 were chosen in this study. Thus, the samples prepared in this study were denoted from RUK-600-2 to RUK-700-4. The yield of this process varies from 58% to 78% depending on the activation conditions. For comparison, a control sample RK-600-3 was synthesized using R as the precursor under 600°C and KOH/R mass ratio of 3. 2.2 Characterization A PHILIPS PW3040/60 diffractometer was used to obtain X-ray diffraction (XRD) pattern using CuKα radiation (λ =0.15406nm). The morphology of the carbon sample was observed by scanning electron microscopy (SEM, Hitachi S-4800) and a JEOL-2100F transmission electron microscopy (TEM) at 200 KV. The CHN elements were analyzed using a VarioEL III Elemental Analyzer. X-ray photoelectron spectroscopy (XPS) measurements were carried out using an AXIS Nova spectrometer (Kratos Inc., NY, USA) equipped with an Al Kα source. Surveyscans were recorded with a pass energy equal to 160 eV. A non-linear least squares curve fitting program (Peak-Fit version 4) with a Gaussian-Lorentzian mix function and Shirley background subtraction was used to deconvolve the XPS subpeaks. Isotherm measurements of nitrogen adsorption/desorption were determined by physisorption at -196 °C in a Beishide 3H-2000PS2 sorption analyzer. Ultrahigh-purity N2 (99.999%, Shanghai Pujiang Gas Co., Ltd) was used for measurement. Using the multipoint Brunauer-Emmett-Teller (BET) method, the specific surface area (SBET) was 5

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calculated between 0.005 and 0.05 P/P0. The total micropore volume (V0) was calculated from the N2 adsorption data by the t-plot method, and the total pore volume (Vt) was estimated from the adsorbed amount of liquid nitrogen at a relative pressure of 0.99. Density functional theory (DFT) method was employed for calculating pore size distributions (PSD). The CO2 adsorption isotherms were measured using the Beishide 3H-2000PS2 sorption analyzer at 0oC and 25oC, respectively. Pure CO2 (99.99%, Shanghai Pujiang Gas Co., Ltd) was used for adsorption. The samples were degassed at 200oC for at least 6h before performing adsorption measurements. The measurements were repeated for each sample, with the results falling within ±1% of each other. The narrow micropore volume (Vn, micropores N-6>N-Q for all the samples. 3.3 Porous Properties Figure 3 shows the N2 sorption isotherms at -196 °C for as-synthesized nitrogen-doped porous carbons obtained under various conditions. Table 1 summarizes all the porosity characterization results. For all the phenolic resin-derived nitrogen-doped porous carbons, the isotherms present type I curves with most N2 adsorption capacity at a very low relative pressure (P/P0 0.02, the isotherm displays an almost horizontal sorption feature. No hysteresis loops in the isotherms in the whole P/P0 range were found for this series of samples, which suggests the lack of mesoporosity in the prepared carbons. The value of each porosity characteristic increases with the increase of KOH/precursor ratio or activation temperature as shown in Table 1. The BET surface area (SBET), total pore volume (V0) and micropore pore volume (Vt) of this series of carbons range from 1040 to 2172 m2/g, 0.44 to 1.00 cm3/g and 0.40 to 0.90 cm3/g, respectively. The narrow microporosity of the 8

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as-synthesized sorbents was calculated by the CO2 adsorption data measured at 0 °C using the Dubinin−Radushkevich (D−R) equation. The value of narrow micropore volume (Vn) is in the range of 0.64-0.97 cm3/g for these carbons. For all the samples, Vn is higher than Vt, indicating that the combination of urea treatment and KOH activation are very favorable to produce narrow micropores. Comparing RUK-600-3 with the reference sample RK-600-3, the former has elevated value in every porous characteristic (SBET, V0, Vt, Vn). On one hand, the urea modification process can additionally activate the carbon precursor to develop the porous properties of the resulting carbons. On the other hand, a certain amount of nitrogen species introduced upon urea modification will react with KOH during the activation process, which facilitates the penetration of activator into the deeper layers of the carbon structure. This activation process can be ascribed to the depth activation,37 which can increase the degree of narrow microporosity. 3.4 CO2 Adsorption features CO2 adsorption capacities of these nitrogen-doped carbons are tested at 25 °C and 0 °C under atmospheric pressure, respectively, with the isotherms shown in Figure 4. These carbonaceous sorbents exhibit superior CO2 adsorption capacities, up to 4.61 mmol/g at 25 °C and 7.13mmol/g at 0 °C under 1 bar. These values are among the best of the known porous carbons21, 38, 39 and higher than many other well-known microporous sorbents such as zeolites,6 MOFs,13 COFs,40 PAFs41 and porous polymers.15 However, there is quite a gap between this value and 5.8 mmol/g reported by Mokaya and co-workers for a biomass-derived porous carbon under the same test condition.42 Correlation between CO2 adsorption capacity and N content as well as each porous properties (i.e. SBET, V0, Vt, Vn) are shown in Figure S4. Observably, no linear correlations of CO2 adsorption capacity and SBET, V0, Vt , Vn or the nitrogen 9

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content were found for these samples. RUK-600-3 has the maximum CO2 adsorption capacity of 4.61 mmol/g at 25 °C, which possesses a medium value of Vn and the second-highest N content of all sorbents. The second-highest CO2 uptake was found in RUK-600-4, which also shows only a medium value of Vn and N content. On the other hand, RUK-600-2 and RUK-650-4 with the highest N content and Vn, respectively, have lower CO2 uptake than RUK-600-3 or RUK-600-4. These results indicate the synergetic effect of narrow micropores and nitrogen content determines the CO2 adsorption capacity of these sorbents at ambient conditions. Another thing worth discussing is that RUK-700-4 has a very low CO2 uptake of only 3.3 mmol/g, although it possesses a certain amount of N content and Vn. This result can be attributed to the larger micropore size and wider micropore size distribution of RUK-700-4 than RUK-600-3 or RUK-600-4 shown as Figure 5. This result hints that in addition to the narrow micropores and nitrogen content, the pore size and narrow micropores’ distribution also play major roles in determining the CO2 adsorption capacity. When comparing RUK-600-3 with the control sample RK-600-3, the latter exhibits a much lower CO2 uptake than the former, i.e. 3.84 mmol/g vs 4.61mmol/g, which is attributed to the much higher Vn and N content of RUK-600-3 than those of RK-600-3. In addition to high CO2 adsorption capacity, other features, such as suitable isosteric heat of adsorption (Qst), stable recycling adsorption ability, quick CO2 adsorption kinetics, high CO2/N2 selectivity and dynamic CO2 capture ability from CO2/N2 gas mixtures are also needed for potential sorbents used in practical applications. These CO2 adsorption properties for the sorbents in this work were investigated and are discussed below. Clausius–Clapeyron equation was employed to calculate the isosteric heat of 10

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adsorption (Qst) on different carbon materials using the CO2 adsorption data at 25 °C and 0 °C.43, 44 The Qst value in the initial adsorption stage (low CO2 loading) for the selected carbons is found to be between 29-36 kJ mol-1 (Figure 6), which is higher than the values presented for some typical carbonaceous adsorbents.38, 39, 45However, these values are still in the range of physisorption processes. Generally, a higher Qst value means the stronger interaction between the sorbent and the CO2 molecules. The higher Qst at the lower CO2 loading is beneficial for flue gas separation, because a high CO2 adsorption capacity at low pressures is needed. But a too high Qst would lead to difficulty in the regeneration of sorbents. Thus, the moderate Qst of these carbons is favorable from the viewpoint of potential applications. In addition, as a result of the surface heterogeneity, the Qst value decreases with the increase in the amount of the CO2 adsorbed. To determine CO2/N2 selectivity, the N2 adsorption capacity of RUK-600-3 is also measured under the same experimental conditions (Figure S5). CO2 isotherm data was fit to the Langmuir-Freundlich model, and N2 adsorption isotherm data was fit to a linear equation. The selectivity of CO2 over N2 was predicted for CO2-N2 binary mixtures with N2 molar fraction in the range of 70%-99%, which is a characteristic composition range of flue gases, by the ideal adsorption solution theory (IAST)46. As shown in Figure 7a, the obtained CO2/N2 selectivities increase as the N2 molar fraction increases. At a total pressure of 1 bar and CO2 molar fraction of 10%, a selectivity of 12 is obtained for RUK-600-3, which is comparable to or better than those of carbonaceous sorbents reported previously32, 47-52. The CO2/N2 selectivity of RUK-600-3 predicted by IAST theory using a binary mixture of 10 : 90 CO2/N2 at 25 °C and up to 1 bar is illustrated in Figure 7b. It can be found that CO2/N2 selectivity can be as high as 48 indicating the high potential of these resin-based 11

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nitrogen-doped porous carbons in post-combustion CO2 capture. The adsorption/desorption process for the RUK-600-3 sample was repeated five times to assess its cyclic capacity and stability. As shown in Figure 8, the CO2 adsorbed amount at the fifth cycle was about 96% of the value in the first cycle, suggesting its excellent regenerability and recyclability. CO2 adsorption kinetics for RUK-600-3 at 25°C was shown at Figure 9a. Once the sample was exposed to the CO2 stream, the weight quickly increased and reached 90% saturation levels in around 2.5 mins, indicating fast rate of adsorption. Furthermore, a classical Fick’s diffusion model was applied to correlate the adsorption kinetics data and to calculate the diffusion time constant for CO2 of RUK-600-3.53, 54 Fick’s diffusion model is expressed as 1−

 − π 2 Dc t  mt 6  = 2 exp 2 mmax π  rc 

where mt/mmax is the fractional adsorption uptake and the ratio Dc/rc2 (min-1) is known as the diffusion time constant. Only data points with (mt/mmax) greater than 70% were used for estimating the diffusion time constant. As seen in Figure 9b, Fick’s diffusion model fits the experimental data well with the squared correlation coefficient (R2) for regressions of around 0.95. The value of the diffusion time constant is found to be 0.068 min-1, which is higher than those of previously reported carbonaceous adsorbents24, 26. This relatively high diffusion time constant confirms the high CO2 diffusion rate in this sorbent indicating that the sorbent is beneficial to reduce the adsorption cycle time in practical applications. Finally, to evaluate the potential of RUK-600-3 for real-world applications, continuous-flow breakthrough experiment was performed to determine its dynamic CO2 capture capacity from a CO2/N2 (10:90 v/v) gas mixture. From Figure 10, the 12

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dynamic CO2 capture capacity of RUK-600-3 was calculated to be 0.98 mmol/g at 25°C, suggesting its great potential for the capture of CO2 from flue gas. It should be noted that moisture is one of the key components of flue gas. With the existence of water, the CO2 uptake of the carbonaceous sorbents will decrease, which is due to the competitive adsorption between CO2 and H2O. 4. Conclusion In summary, N-doped porous carbons were prepared by combining urea treatment and KOH activation of carbonized commercial phenolic resin. The obtained samples show very high CO2 uptake under atmospheric pressure, up to 4.61 and 7.13 mmol/g at 25°C and 0°C, respectively. Further investigation shows that the synergetic effect of narrow microporosity and nitrogen content determines the CO2 uptake capacity of these carbonaceous sorbents. Additionally, the pore size and PSD of narrow micropores also play important roles in determining the CO2 uptake capacity of the carbonaceous sorbents. These phenolic resin-derived porous carbons have other merits such as excellent recyclability and stability, medium heat of adsorption, high CO2 over N2 selectivity, and excellent dynamic CO2 capture capacity. The simple preparation procedure and the cost-effective precursor make these phenolic resin-derived porous carbons promising candidates for CO2 capture. Acknowledgments Financial support was provided by the NSF of China (21106136), Zhejiang Provincial Natural Science Foundation (LY17E010002), and National Undergraduate Training Program for Innovation and Entrepreneurship of China (201710345010). The authors would like to thank Dr. Bryce Dutcher for his help in editing this work for English usage. 13

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Supporting Information: A schematic diagram of the CO2 capture system; XPS survey spectra of selected samples; the XRD pattern, CO2 and N2 adsorption isotherms at 25ºC, and CO2 breakthrough curve under simulated flue gas for RUK-600-3; and the plots of each porous properties characteristics and nitrogen content versus CO2 uptake is available free of charge via the internet at http://pubs.acs.org. References 1.

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Nitrogen-Doped

Porous

Coconut

Shell-Based

CO2

Sorbent

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microporous materials at low pressures and temperatures. Chem. Eng. J. 2016, 302, 278-286.

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Table 1. Porous properties, elemental compositions, and CO2 uptakes of sorbents derived from phenolic resin under different conditions

Sample

SBETa

V0 b

Vt c

Vn d

(m2/g) (cm3/g) (cm3/g) (cm3/g)

N (wt%) C (wt%) H(wt%)

CO2 uptake (mmol/g) 25°C

0°C

R

536

0.20

0.20

0.32

1.05

91.25

1.64

2.82

3.71

RU

-

-

-

0.28

6.51

83.78

1.67

2.62

3.35

RUK-600-2 1040

0.44

0.40

0.64

0.53

0.78

1.40 1.50

6.09

0.58

85.11 86.32

4.27

RUK-600-3 1404

2.33 2.13

4.61

6.95

RUK-600-4 1621

0.67

0.63

0.82

1.95

90.89

1.31

4.58

6.86

RUK-650-2 1556

0.63

0.59

0.94

1.88

85.69

1.00

4.39

7.13

RUK-650-3 1844

0.77

0.71

0.82

1.56

82.65

1.12

4.34

6.68

RUK-650-4 1955

0.83

0.76

0.97

1.38

80.87

0.94

4.32

7.01

RUK-700-2 1914

0.85

0.78

0.89

1.62

93.93

0.91

4.10

6.28

RUK-700-3 2172

1.00

0.90

0.95

1.45

90.65

0.95

3.92

6.28

RUK-700-4 2015

0.95

0.84

0.84

1.26

88.44

0.84

3.30

5.32

1.01 87.77 1.44 RK-600-3 984 0.39 0.36 0.53 3.84 a Surface area was calculated using the BET method at P/P0=0.005-0.05. b Total pore volume at P/P0= 0.99. c Evaluated by the t-plot method. d Pore volume of narrow micropores (