Nitrogen-Doped Coal Tar Pitch Based Microporous Carbons with

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Article

Nitrogen-doped coal tar pitch based microporous carbons with superior CO2 capture performance Dai Yu, Jun Hu, Lihui Zhou, Jinxia Li, Jing Tang, Changjun Peng, and Honglai Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00125 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 15 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

Energy & Fuels

Nitrogen-doped coal tar pitch based microporous carbons with superior CO2 capture performance Dai Yu, Jun Hu, Lihui Zhou*, Jinxia Li, Jing Tang, Changjun Peng, and Honglai Liu School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China

Abstract Coal tar pitch (CTP) based nitrogen-doped porous carbons were developed by FeCl3 activation for enhanced CO2 capture. The carbon materials, synthesized with different FeCl3/carbon precursor mass ratios and activation temperatures, exhibit high porosity, especially microporosity and nitrogen species contents, leading to superior CO2 adsorption capacities in the range of 3.71–4.58 mmol g-1 and 5.68–7.18 mmol g-1 at 25 oC and 0 oC under 1 bar, respectively. In particular, N-AC-600-2 and N-AC-600-3 materials show the highest uptakes of CO2 at 25 oC and 0 oC, respectively, which rank among the best porous carbon sorbents. The nitrogen-doping and porosity of the fabricated carbons have combined impact on CO2 adsorption. In addition, both excellent regenerability and high CO2/N2 selectivity were achieved, making these carbon materials promising candidates for large scale CO2 adsorption.

Key words: Ferric chloride activation, Nitrogen-doped microporous carbons, Coal tar pitch, CO2 capture

ACS Paragon Plus Environment

Energy & Fuels 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

Page 2 of 15

1. INTRODUCTION The ever increasing greenhouse gases emission relevant to the ongoing climate change has caused worldwide concern.1 CO2, the major greenhouse gas mainly produced from fossil fuel combustion, is recognized as the primary contributor to global warming.1–3 Both the worrying fact that the atmospheric concentration of CO2 has dramatically increased and surpassed 400 ppm1,4 and the target that governments have pledged not to exceed 2 oC for the global temperature rise1,2 have triggered extensive research to capture CO2 from anthropogenic sources such as power plants and other industries.3 Adsorption of CO2 via solid sorbents such as zeolites,5 metal organic frameworks (MOFs),6,7 porous organic polymers (POPs),8 and carbons9,10 has emerged as one of the most promising technologies. Among these sorbents, porous carbonaceous materials have gained special attention due to their great advantages such as unlimited availability of raw materials, ease of preparation and surface modification, low cost, low

regeneration

hydrophobicity.

11–15

energy,

ideal

thermal

and

chemical

stabilities,

and

To achieve large scale production of porous carbons for practical

application in CO2 adsorption, both long-term sustainability and high CO2 adsorption capacity are strictly required. Recently, Bai et al.12 utilized inexpensive urea and petroleum coke to develop a nanoporous nitrogen-doped carbonaceous CO2 sorbent, showing a maximum CO2 adsorption capacity of 4.40 mmol g-1 and 6.75 mmol g-1 at 25 oC and 0 oC under atmospheric pressure (1 bar). As cost-effective and naturally renewable carbon sources, sucrose15 and coconut shell16 were also used respectively by Sivadas et al. and Chen et al. to prepare nitrogen-doped porous carbons, both achieving a maximum CO2 adsorption capacity of 7.0 mmol g-1 at 0 oC and 1 bar. Above studies all emphasized that the high CO2 capture capacity of these sorbents is ascribed to their high microporosity and nitrogen content.12,15,16 Micropores, especially those with pore diameter less than 1 nm, are favorable for CO2 capture as the dynamic diameter of CO2 is 3.30 Å.12,17 Nitrogen species are responsible for increasing the surface polarity and basicity of carbon, which is beneficial to the adsorption of CO2.12,15 Coal tar pitch (CTP), an abundant and cheap byproduct obtained from the distillation of coal tar, has long been remained to be fully exploited.18 Gao et al.18,19 synthesized hyper-cross-linked polymers (HCPs) from CTP but even the highest uptake capacity for CO2, with a value of 15.28 wt% at 273 K and 8.74 wt% at 298 K (equivalent to 4.10 mmol g-1 and 2.18 mmol g-1, respectively) at 1 bar, was still far


Page 3 of 15 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

Energy & Fuels

from expectation. The addition of CTP to CNTs was studied to prepare activated carbon discs very recently, achieving some CO2 adsorption capacity.20 However, the involvement of other precursors such as CNTs makes the synthesis complicated. In this context, the idea to prepare CTP-based porous carbon with excellent CO2 capture capacity can be viable and of great significance to the coal industry and environment protection, which has rarely been reported. In our previous work, FeCl3 activation method was successfully used to synthesize CTP-based carbon as an efficient oxygen-reduction catalyst.21 Herein, bearing in mind the above factors and challenges, CTP-based nitrogen-doped microporous carbons were prepared using the previously-reported method with some modifications for CO2 adsorption. To our delight, at 25 oC and 0 oC under 1 bar, the maximum CO2 capture capacity was evaluated to be 4.58 mmol g-1 and 7.18 mmol g-1, respectively, which is one of the highest CO2 uptake among all reported carbon-based sorbents to date. The influence of nitrogen-doping and porosity of the fabricated carbons on the CO2 adsorption has been analyzed in detail. Besides, both good regenerability and high CO2/N2 selectivity were achieved in this study, showing the potential application of CTP-based nitrogen-doped porous carbons in large-scale CO2 capture.

2. EXPERIMENTAL SECTION 2.1. Synthesis 2.1.1. Synthesis of nitrogen-doped carbon precursor CTP with the softening point of 105

o

C was provided by Jining Lu

Coal-Chemical Co., Ltd. A two-step method was applied to prepare nitrogen-doped carbon precursor (N-CP). Firstly, CTP was ground to 200-mesh powder and then the powder was extracted by n-hexane at 105 oC for 24 h in a Soxhlet extractor. Secondly, extracts (1.8 g) were added into 50 mL of mixed acid (VHNO3:VH2SO4=1:4) and stirred for 1 h at room temperature, after which the suspension was poured into 500 mL of deionized water and then filtrated. The resulting solids were washed with deionized water and dried at 80 oC overnight to obtain N-CP. 2.1.2. Synthesis of nitrogen-doped microporous carbon The nitrogen-doped microporous carbon was synthesized by a simple activation process. N-CP (0.2 g) was dispersed in 6.7 mL of ethanol solution of FeCl3 with a FeCl3/N-CP mass ratio of 1:1, 2:1, and 3:1 by stirring for at least 2 h. Then the mixed


Energy & Fuels 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

suspension was heated to evaporate solvent under stirring until a viscous liquid was obtained, and dried at 100 oC overnight. The resulting dark samples were transferred to a tubular furnace and heated at 500, 600, or 700 oC for 3 h under ultra-high pure N2 flow atmosphere. The products were then ground and the powder was added into 2 mol L-1 HCl solution with stirring at 80 oC for 6 h to remove any inorganic species. After filtering, the filtration residue was washed with deionized water several times until a neutral pH was reached. Finally the samples were dried at 100 oC overnight to get the nitrogen-doped activated carbon designated as N-AC-x-y, where N-AC refers to nitrogen-doped activated carbon, x represents the activation temperature and y stands for the mass ratio of FeCl3 to N-CP. 2.2. Characterization The morphology of the samples was characterized by transmission electron microscopy (TEM, JEOL JEM-2100). The porosity of the as-synthesized materials were measured by nitrogen adsorption/desorption at 77 K using Tristar Ⅱ 3020 apparatus. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method applied to nitrogen adsorption data in the relative pressure (P/Po) range of 0.005–0.05. The total pore volume was obtained from the amount of nitrogen adsorbed at P/P0=0.99. The pore size distribution (PSD) was determined by non-local density functional theory (NLDFT) using nitrogen adsorption data. The volume of total micropores Vmi and narrow micropores Vn with diameter less than 1 nm were calculated by t-plot method and cumulative NLDFT pore volume, respectively. Elemental analysis (EA) of C, H, and N element was carried out on a Vario EL Ⅲ instrument. The nature and content of nitrogen species in the material were evaluated by X-ray photoelectron spectroscopy (XPS) conducted on an ESCALAB 250Xi spectrometer (Thermo Scientific) with Al Kα X-ray source for excitation. C 1s peak with binding energy of 284.6 eV was used as reference. 2.3. CO2 Adsorption Measurements CO2 adsorption was performed on a Tristar Ⅱ 3020 static volumetric analyzer at different temperatures (0 oC and 25 oC) measured up to 1 bar. Prior to the adsorption measurement, the samples were degassed at 120 oC for 12 h. The desorption process initiated right after the adsorption measurement terminated. Different temperatures were applied to measure the adsorption performance consecutively without redegassing. The adsorption capacity, quantified as the amount


Page 4 of 15

Page 5 of 15 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

Energy & Fuels

of gas adsorbed per gram of adsorbent and the isosteric heat of adsorption were calculated based on the adsorption isotherms recorded at 0 oC and 25 oC. The regeneration performance was measured by five cycles of CO2 adsorption/desorption via the pressure-swing process and the sample was regenerated purely through the pressure-swing desorption process and without any heat treatment.

3. RESULTS AND DISCUSSION In the common syntheses of activated carbons for CO2 capture, KOH is used mostly to create harsh activation conditions but it is an extremely corrosive and toxic agent. Accordingly, FeCl3, a mild activator, which has been reported to be an ideal alternative,21,22 is used in this study. As aromatic precursors facilitate the incorporation of nitrogen-containing groups into the carbon matrix in the presence of iron species during heat treatment, Soxhlet extraction was applied to eliminate aliphatic fractions.21,23 Then nitrogen and oxygen-containing groups can be introduced into the carbon matrix by mixed acid treatment. During pyrolysis, gasification (hydrogen and CO evolution) is enhanced by FeCl3 addition,24 which leaves numerous pores in the products. After the pyrolysis at 500–700 oC in a N2 flow and HCl wash, the as-synthesized nitrogen-doped carbons (N-AC-x-y) are obtained in high yields (47%-60%) as shown in Table S1 (Supporting Information), and the yield decreases as activation temperature increases. 3.1. Morphology and Microstructure As shown in Figure S1 (Supporting Information), CTP has thick and small particle structure while N-CP exhibits thinner and even smaller particles that disperse well. The morphologies of N-AC-x-y samples were characterized by TEM shown in Figure 1 and Figure S2. N-AC-x-y samples exhibit flake structures with pores and wrinkles on the surface. With the increase of carbonization temperature, the nanosheets of N-AC-x-y samples (Figure S2) become thinner and more transparent. When the FeCl3/N-CP mass ratio increases from 1 to 3, the same phenomenon that the nanosheets get thinner and more transparent is observed (Figure 1).


Energy & Fuels 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

Figure 1. TEM images of N-AC-600-1 (a–b), N-AC-600-2 (c–d), and N-AC-600-3 (e–f).

3.2. Porosity Properties The porosity properties of all samples were characterized by nitrogen sorption at 77 K. Figure 2 shows the nitrogen adsorption/desorption isotherms and pore size distributions of the prepared porous carbons and the porous property results are summarized in Table 1. It turns out that CTP and N-CP have almost no pores. However, with the increase of activation temperature or FeCl3/N-CP mass ratio, both BET surface area and total pore volume of carbons increase evidently, with values in the range of 1152.0–1702.2 m2 g-1 and 0.52–0.79 cm3 g-1, respectively. Therefore, it proves that FeCl3 is an effective porogen. The sorption isotherms (Figure 2a–b) of all carbons exhibit a sharp increase in N2 uptake at a relative pressure of P/P0 < 0.1, and almost flat sorption for P/P0 > 0.1, which is characteristic of type I according to the International Union of Pure and Applied Chemistry (IUPAC) classification. What’s more, no hysteresis loops are observed in the whole P/P0 range. The results indicate


Page 6 of 15

Page 7 of 15 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

Energy & Fuels

that micropores are dominant in these samples, as Vmi shown in Table 1. Particularly the volume of narrow micropores, which account for the enhanced CO2 uptake at ambient temperature, is calculated by NLDFT method. As a result, the samples (N-AC-600-2, N-AC-600-3, and N-AC-700-2) obtained with higher FeCl3/N-CP mass ratio and activation temperature own larger narrow micropores volume (0.58–0.61 cm3 g-1), which is confirmed by the PSD (Figure 2c–d) calculated using NLDFT. The PSD also shows that the diameters of the samples’ pores are mainly located at the range of 0.5–1.0 nm (Figure S3), which is able to enhance CO2 adsorption.12,17

Figure 2. N2 adsorption/desorption isotherms (a–b) and NLDFT pore size distributions (c–d) of the samples (N-AC-x-y) prepared under different conditions.


Energy & Fuels 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

Page 8 of 15

Table 1. Porosity Properties, Elemental Contents (N, C, and H element), and CO2 Uptakes of Carbons Derived from CTP. SBETa (m2 g-1)

Smib (m2 g-1)

Vtc (cm3 g-1)

Vmib (cm3 g-1)

Vnd (cm3 g-1)

N (wt%)

C (wt%)

H (wt%)

CTP

-e

-e

-e

-e

-e

0.9

92.3

N-CP

38.5

16.7

0.18

0.01

-e

8.9

N-AC-500-2

1152.0

992.2

0.55

0.38

0.46

N-AC-600-1

1173.8

1036.8

0.52

0.40

N-AC-600-2

1504.8

1335.7

0.68

N-AC-600-3

1684.4

1469.1

N-AC-700-2

1702.2

1471.9

Sample

CO2 uptake

(mmol g-1) 25 oC

0 oC

4.4

0

0.07

48.6

2.0

0.34

0.62

9.1

63.5

3.1

3.90

5.87

0.42

7.2

60.0

3.2

3.71

5.68

0.51

0.61

7.7

55.0

3.8

4.58

7.10

0.78

0.57

0.58

7.4

54.0

4.1

4.34

7.18

0.79

0.58

0.59

5.6

54.5

3.1

4.14

6.81

a

Surface area is calculated using the BET method at P/P0=0.005–0.05.

b

Micropore volume and micropore surface area are evaluated by the t-plot method.

c

Total pore volume at P/P0=0.99.

d

Cumulative NLDFT pore volume of narrow micropores with diameter less than 1 nm.

e

No porosity.

3.3. Elemental Analysis and Surface Chemical Composition Table 1 shows the N, C, and H element contents of all carbons measured by EA. It is noticeable that N-CP has more nitrogen content (8.9 wt%) than CTP (0.9 wt%), indicating that certain nitrogen has been successfully introduced into the carbon matrix of CTP. The N contents of N-AC-x-y samples derived from N-CP are kept well during the synthesis and vary in the range of 5.6–9.1 wt%, which are favorable for CO2 adsorption. An interesting result can be found that the N content decreases as the activation temperature increases but it remains almost the same when the FeCl3/N-CP mass ratio varies. The surface chemical composition and the nature of nitrogen species are further evaluated by XPS (Figure 3). The presence of C 1s, O 1s, and N 1s signal reveals the existence of C, O, and N species in all carbons (Figure 3a). The slight signals at around 711 eV (Fe 2p3/2) and 724 eV (Fe 2p1/2) show there is still trace Fe residue in the products (Figure S4). The high resolution N 1s spectra were investigated by reason that the nitrogen species embedded in the carbon framework play an important role in CO2 adsorption.12,15 The XPS N 1s spectrum of N-AC-500-2 can be deconvoluted into two peaks centered at 398.3 eV and 400.1 eV (Figure 3b), and N-AC-600-y (y:1–3) samples have a third peak at 401.5 eV (Figure 3c–e) whereas


Page 9 of 15 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

Energy & Fuels

N-AC-700-2 has even one more peak at 403.2 eV (Figure 3f). These four peaks are assigned to pyridinic-N (N-6), pyrrolic-/pyridonic-N (N-5), quaternary-N (N-Q), and oxidized-N, respectively.12,17 Given the condition of high temperatures during the activation process, the peak at 400.1 eV is probably attributed to pyridonic-N.12,25 The relative contents of these N species are given in Table S1. It’s noticed that with the increase of activation temperature, the pyridinic-N (N-6) content decreases gradually while the contents of quaternary-N (N-Q) and oxidized-N increase gradually, probably by reason that during high temperature activation, pyridinic-N (N-6) was converted to other species, particularly N-Q and N-oxidized.26 The N species in all N-AC-x-y samples mainly consist of N-6 and N-5, and N-5 accounts for more than 50% of N contents in all carbons. Previous researches have pointed out that N-5 plays a more important role in CO2 capture than N-6 and N-Q.25,27 Therefore, these N-AC-x-y samples have great potential to adsorb CO2.

Figure 3. XPS survey of N-AC-x-y samples (a) and corresponding high resolution N 1s XPS spectra of (b) N-AC-500-2, (c) N-AC-600-1, (d) N-AC-600-2, (e) N-AC-600-3, and (f) N-AC-700-2.

3.4. CO2 Adsorption Capacities CO2 adsorption on synthesized carbons was carried out at 25 oC and 0 oC up to atmospheric pressure (1 bar). Figure 4a–b show the CO2 adsorption isotherms at different temperatures and the CO2 adsorption capacities are summarized in Table 1. Although CTP and N-CP show almost no adsorption performance of CO2, all of the N-AC-x-y materials exhibit excellent CO2 adsorption capacities more than 3.71 mmol


Energy & Fuels 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

g-1 and 5.68 mmol g-1 at 25 oC and 0 oC (1 bar), respectively. As the activation temperature increases from 500 oC to 700 oC at the FeCl3/N-CP mass ratio of 2, the CO2 adsorption capacities increase first and then decrease. Under the same activation temperature of 600

o

C, the CO2 adsorption capacities of N-AC-600-2 and

N-AC-600-3 with higher FeCl3/N-CP mass ratio are higher than that of N-AC-600-1. It is worth noting that N-AC-600-2 possesses the highest CO2 adsorption capacity of 4.58 mmol g-1 at 25 oC when N-AC-600-3 demonstrates the highest of 7.18 mmol g-1 at 0 oC (1 bar), both of which are among the best of the reported porous carbons (Table S2).12–17,26,28–34 Significantly, the CO2 uptake at 25 oC/1 bar is quite close to the highest CO2 uptake of the petroleum pitch based carbon (207 mg g-1, 4.70 mmol g-1).35 In essence, the relationship between CO2 adsorption capacities and activation temperature or FeCl3/N-CP mass ratio is reflected in the impact of porous textures or N-functional groups on CO2 uptake. On one hand, lower activation temperature results in relatively low BET surface area and Vn of the sample N-AC-500-2 whereas higher activation temperature leads to fewer N contents contained in the sample N-AC-700-2, from which the CO2 adsorption capacities will suffer. On the other hand, when FeCl3/N-CP mass ratio varies at the same activation temperature, N contents are at approximately the same level (7.2–7.7 wt%), but lower FeCl3/N-CP mass ratio causes lower BET surface area and Vn of the sample N-AC-600-1, which also has a negative effect on CO2 adsorption. Comparatively, N-AC-600-2 and N-AC-600-3, which have both relatively high porosity (Vn) and N contents, exhibit excellent CO2 adsorption capacity. Therefore, a conclusion is drawn that porosity, especially narrow micropores and N content have a joint influence on CO2 adsorption capacity. Moreover, the regeneration performance of the best sorbents N-AC-600-2 and N-AC-600-3 at 25 oC was further tested by CO2 adsorption/desorption for five cycles via the pressure-swing process. As a result, negligible loss of CO2 adsorption capacity is observed in Figure 4c and Figure S5, indicating their ideal regenerability.


Page 10 of 15

Page 11 of 15 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

Energy & Fuels

Figure 4. CO2 adsorption isotherms of N-CP and N-AC-x-y samples measured at (a) 298 K and (b) 273 K. (c) Five cycles of CO2 adsorption on N-AC-600-2 at 298 K without any heat treatment. (d) Isosteric heat of CO2 adsorption on N-AC-x-y samples calculated based on CO2 adsorption isotherms at 298 K and 273 K.

3.5. Isosteric Heat of Adsorption (Qst) and CO2/N2 Selectivity Isosteric heats of adsorption (Qst), which show the strength of the interactions between CO2 and the sorbents as well as the energy required for regeneration, are calculated by fitting the CO2 adsorption isotherms measured at 25 oC and 0 oC and applying the Clausius-Clapeyron equation (Figure 4d). The initial Qst for all samples is in the range of 37–47 kJ mol-1, which indicates stronger interaction between N-doped carbons and CO2 than that of previously reported studies,12,17,36 leading to a greater selectivity for the CO2 adsorption.27 The initial Qst of N-AC-500-2 with higher N content is relatively higher, proving the important role of nitrogen functional groups in CO2 uptake. What’s more, the Qst varies from 26 to 47 kJ mol-1 in the whole coverage of CO2 uptake, which is typical of physisorption process3 and propitious for an excellent regenerability. And Qst decreases as adsorbed CO2 amount increases until reaching a plateau, arising from the heterogeneity of binding energies of CO2 on N-AC-x-y samples. In addition, N2 adsorption of these samples was also performed


Energy & Fuels 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

under the same conditions as CO2 adsorption in order to calculate CO2/N2 selectivity, which is another key factor for CO2 capture from flue gas. Figure S6a–b show the CO2 and N2 isotherms of N-AC-600-2 and N-AC-600-3 at 25 oC. CO2/N2 selectivity is calculated by initial slope method and ideal adsorbed solution theory (IAST) approach. According to the ratio of initial slopes of CO2 and N2 adsorption isotherms (Figure S6c–d), the CO2/N2 selectivities of N-AC-600-2 and N-AC-600-3 are estimated to be 23.6 and 23.8, respectively, comparable to or higher than that of those reported carbons.12,37,38 N-AC-500-2 with the highest N content exhibits the highest CO2/N2 selectivity (Table S1), indicating nitrogen groups have a great influence on selective CO2 adsorption. The CO2/N2 selectivity is also calculated by fitting CO2 isotherms with dual-site Langmuir model and N2 isotherms with single-site Langmuir model and then using IAST (Figure S7). It turns out that the CO2/N2 selectivities of N-AC-600-2 and N-AC-600-3 are 23.6 and 24.6, respectively, quite consistent with the initial slope method results, suggesting that N-AC-600-2 and N-AC-600-3 possess high CO2/N2 selectivity under ambient conditions.

4. CONCLUSION In summary, N-doped porous carbons with high microporosity and N contents were prepared from cheap and abundant coal tar pitch (CTP) by a facile FeCl3 activation method. These carbons have BET surface areas varying from 1152.0 m2 g-1 to 1702.2 m2 g-1 and N contents in the range of 5.6–9.1 wt%, depending on the preparation conditions. Due to the synergy of the carbons’ high microporosity and nitrogen contents, these samples exhibit excellent CO2 uptakes of 3.71–4.58 mmol g-1 at 25 oC and 5.68–7.18 mmol g-1 at 0 oC (1 bar), respectively, as well as moderate CO2 adsorption heat, CO2/N2 selectivity, and good regenerability. These merits enable these sorbents to be the most promising candidates for CO2 capture. This study may also shed light on the exploitation of other cheap carbonaceous precursors to prepare efficient sorbents for large-scale CO2 adsorption.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Project 21676080).


Page 12 of 15

Page 13 of 15 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

Energy & Fuels

REFERENCES (1) Monastersky, R. Nature 2013, 497 (7447), 13–14. (2) Haszeldine, R. S. Science 2009, 325 (5948), 1647−1652. (3) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Ind. Eng. Chem. Res. 2012, 51 (4),1438−1463. (4) https://www.esrl.noaa.gov/gmd/ccgg/trends/. Trends in Atmospheric Carbon Dioxide. (5) Su, F.; Lu, C.; Kuo, S. C.; Zeng, W. Energy Fuels 2010, 24 (2), 1441–1448. (6) Zhang, Z.; Yao, Z. Z.; Xiang, S.; Chen, B. Energy Environ. Sci. 2014, 7 (9), 2868−2899. (7) Chaemchuen, S.; Kabir, N. A.; Zhou, K.; Verpoort, F. Chem. Soc. Rev. 2013, 42 (24), 9304−9332. (8) Jin, T.; Xiong, Y.; Zhu, X.; Tian, Z.; Tao, D. J.; Hu, J.; Jiang, D.; Wang, H.; Liu, H.; et al. Chem. Commun. 2016, 52 (24), 4454−4457. (9) Sevilla, M.; Fuertes, A. B. Energy Environ. Sci. 2011, 4 (5), 1765−1771. (10) To, J. W. F.; He, J.; Mei, J.; Haghpanah, R.; Chen, Z.; Kurosawa, T.; Chen, S.; Bae, W. G.; Pan, L.; et al. J. Am. Chem. Soc. 2016, 138 (3), 1001−1009. (11) Nandi, M.; Okada, K.; Dutta, A.; Bhaumik, A.; Maruyama, J.; Derks, D.; Uyama, H. Chem. Commun. 2012, 48 (83), 10283−10285. (12) Bai, R.; Yang, M.; Hu, G.; Xu, L.; Hu, X.; Li, Z.; Wang, S.; Dai, W.; Fan, M. Carbon 2015, 81, 465−473. (13) Ma, X.; Cao, M.; Hu, C. J. Mater. Chem. A 2013, 1 (3), 913−918. (14) Fujiki, J.; Yogo, K. Chem. Commun. 2016, 52 (1), 186−189. (15) Sivadas, D. L.; Vijayan, S.; Rajeev, R.; Ninan, K. N.; Prabhakaran, K. Carbon 2016, 109, 7−18. (16) Chen, J.; Yang, J.; Hu, G.; Hu, X.; Li, Z.; Shen, S.; Radosz, M.; Fan, M. ACS Sustainable Chem. Eng. 2016, 4 (3), 1439−1445. (17) Ren, X.; Li, H.; Chen, J.; Wei, L.; Modak, A.; Yang, H.; Yang, Q. Carbon 2017, 114, 473−481. (18) Gao, H.; Ding, L.; Bai, H.; Liu, A.; Li, S.; Li, L. J. Mater. Chem. A 2016, 4 (42), 16490−16498. (19) Li, W.; Zhang, A.; Gao, H.; Chen, M.; Liu, A.; Bai, H.; Li, L. Chem. Commun. 2016, 52 (13), 2780−2783. (20) Gao, S.; Villacorta, B. S.; Ge, L.; Rufford, T. E.; Zhu, Z. Carbon 2017, 124, 142−151.


Energy & Fuels 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

(21) Yu, D.; Zhou, L.; Tang, J.; Li, J.; Hu, J.; Peng, C.; Liu, H. Ind. Eng. Chem. Res. 2017, 56 (31), 8880−8887. (22) Sun, G.; Zhou, L.; Li, J.; Tang, J.; Wang, Y. RSC Adv. 2015, 5 (88), 71980−71987. (23) Li, Q.; Cao, R.; Cho, J.; Wu, G. Adv. Energy Mater. 2014, 4 (6), 1301415. (24) Boudou, J. P.; Bégin, D.; Alain, E.; Furdin, G.; Marêché, J. F.; Albiniak, A. Fuel 1998, 77 (6), 601-606. (25) Sevilla, M.; Valle-Vigón, P.; Fuertes, A. B. Adv. Funct. Mater. 2011, 21 (14), 2781–2787. (26) Sethia, G.; Sayari, A. Carbon 2015, 93, 68–80. (27) Sanchez-Sanchez, A.; Suarez-Garcia, F.; Martinez-Alonso, A.; Tascon, J. M. D. ACS Appl. Mater. Interfaces 2014, 6 (23), 21237−21247. (28) Sun, H.; La, P.; Yang, R.; Zhu, Z.; Liang, W.; Yang, B.; Li, A.; Deng, W. J. Hazard. Mater. 2017, 321, 210–217. (29) Ashourirad, B.; Sekizkardes, A. K.; Altarawneh, S.; El-Kaderi, H. M. Chem. Mater. 2015, 27 (4), 1349−1358. (30) Yu, J.; Guo, M.; Muhammad, F.; Wang, A.; Zhang, F.; Li, Q.; Zhu, G. Carbon 2014, 69, 502–514. (31) Fan, X.; Zhang, L.; Zhang, G.; Shu, Z.; Shi, J. Carbon 2013, 61, 423–430. (32) Chandra, V.; Yu, S. U.; Kim, S. H.; Yoon, Y. S.; Kim, D. Y.; Kwon, A. H.; Meyyappan, M.; Kim, K. S. Chem. Commun. 2012, 48 (5), 735–737. (33) Yang, J.; Yue, L.; Lin, B.; Wang, L.; Zhao, Y.; Lin, Y.; Chang, K.; DaCosta, H.; Hu, X. Energy Fuels 2017, 31 (10), 11060–11068. (34) Yue, L.; Rao, L.; Wang, L.; Wang, L.; Wu, J.; Hu, X.; DaCosta, H.; Yang, J.; Fan, M. Ind. Eng. Chem. Res. 2017, 56 (47), 14115–14122. (35) Wahby, A.; Ramos-Fernández, J. M.; Martínez-Escandell, M.; SepúlvedaEscribano, A.; Silvestre-Albero, J.; Rodríguez-Reinoso, F. ChemSusChem 2010, 3 (8), 974–981. (36) Li, D.; Chen, Y.; Zheng, M.; Zhao, H.; Zhao, Y.; Sun, Z. ACS Sustainable Chem. Eng. 2016, 4 (1), 298−304. (37) Mahurin, S. M.; Górka, J.; Nelson, K. M.; Mayes, R. T.; Dai, S. Carbon 2014, 67, 457–464. (38) Wang, J.; Heerwig, A.; Lohe, M. R.; Oschatz, M.; Borchardt, L.; Kaskel, S. J. Mater. Chem. 2012, 22 (28), 13911–13913.


Page 14 of 15

Page 15 of 15 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

Energy & Fuels

TOC Graphic


Nitrogen-Doped Coal Tar Pitch Based Microporous Carbons with

Recommend Documents