Effective Approach for Increasing the Heteroatom Doping Levels of

Subscriber access provided by LONDON METROPOLITAN UNIV

Article

Effective Approach for Increasing the Heteroatom Doping Levels of Porous Carbons for Superior CO2 Capture and Separation Performance Yomna H. Abdelmoaty, Tsemre-Dingel Tessema, Nazgol Norouzi, Oussama M. El-Kadri, Joseph B McGee Turner, and Hani M. El-Kaderi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09989 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 25

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

ACS Applied Materials & Interfaces

Effective Approach for Increasing the Heteroatom Doping Levels of Porous Carbons for Superior CO2 Capture and Separation Performance

Yomna H. Abdelmoaty†‡, Tsemre-Dingel Tessema§, Nazgol Norouzi§, Oussama M. El-Kadri⊥, Joseph B McGee Turner§ 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, 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: Porous carbon, CO2 capture, Gas separation, IAST, flue gas, landfill gas, Nitrogendoped carbon

1 ACS Paragon Plus Environment


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 25

Abstract Development of efficient sorbents for carbon dioxide (CO2) capture from flue gas or its removal from natural gas and landfill gas is very important for environmental protection. A new series of heteroatom doped porous carbon was synthesized directly from pyrazole/KOH by thermolysis. The resulting pyrazole-derived carbons (PYDCs) are highly doped with nitrogen (14.9-15.5 wt. %) as a result of the high nitrogen to carbon ratio in pyrazole (43 wt. %) and also have high oxygen content (16.4-18.4 wt. %). PYDCs have high surface area (SABET = 1266–2013 m2 g−1), high CO2 Qst (33.2-37.1 kJ mol−1), and a combination of mesoporous and microporous pores. PYDCs exhibit significantly high CO2 uptakes that reach 2.15 mmol g−1 and 6.06 mmol g−1 at 0.15 and 1 bar, respectively, at 298 K. At 273 K, the CO2 uptake improves to 3.7 mmol g−1 and 8.59 mmol g−1 at 0.15 and 1 bar, respectively. The reported porous carbons also show significantly high adsorption selectivity for CO2/N2 (128) and CO2/CH4 (13.4) according to Ideal Adsorbed Solution Theory calculations at 298 K. Gas breakthrough studies of CO2/N2 (10:90) at 298 K showed that PYDCs display excellent separation properties. The ability to tailor the physical properties of PYDCs as well as their chemical composition provides an effective strategy for designing efficient CO2 sorbents.


Page 3 of 25

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


Introduction The burning of fossil fuels to produce electricity or heat results in the release of enormous amounts of carbon dioxide to the environment, which contributes greatly to climate change and ocean acidification.1 Because of their affordable price and wide accessibility, fossil fuels are expected to remain as the main source of energy for many years to come. To mitigate CO2 release to the environment, carbon dioxide capture and sequestration (CCS) strategies have been intensively examined.2-3 Currently, aqueous amine solutions are used to treat flue gas (mainly consisting of N2 and CO2) however, these solutions suffer from major drawbacks like volatility, toxicity, and energy-intensive regeneration processes.4 Although amine solutions react selectively with CO2 to form carbamate, they require heating to boil off CO2 for regeneration. As such, there has been great interest in developing new sorbents that combine selectivity towards CO2, chemical and physical stability, and moderate binding affinity for CO2 to make adsorbents regeneration effective and affordable. In this regard, many porous adsorbents like metal-organic frameworks (MOFs)5-7, porous organic polymers (POPs),8-10 and porous carbons11-13 have been explored due to their tailor-made properties that favor high and selective CO2 capture. Porous carbon materials are very promising CO2 adsorbents because they are green (metal-free) and can be made from cheap and renewable sources. Among the most challenging aspects of using porous adsorbents is attaining concurrent high CO2 uptake and selectivity at the partial pressure of CO2 in flue gas (~0.15 bar). These desirable properties can be targeted by creating high surface area materials with CO2-philic sites like open metal sites or functional groups (-NH2, -OH, etc.) in MOFs or POPs. Furthermore, we have demonstrated that heteroatom-doped porous carbons containing a variety of oxygen and nitrogen functionalities exhibit very high CO2 capture capacity at both high and low pressure range complemented by 3 ACS Paragon Plus Environment


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 4 of 25

moderate CO2 selectivity over nitrogen and methane. There have been differing accounts as to the extent to which heteroatom doping affect the CO2 uptake of porous carbons. The positive influence of nitrogen and oxygen moieties has been demonstrated12,

14-20

while others have

argued that parameters like pore size and pore volume play a more significant role.

21

Thus, it

was important to study the influence of heteroatoms content on PYDCs sample uptake and selectivity performance. Diverse synthetic approaches were devised to advance the function of porous carbons, which include their template-free synthesis by thermolysis and chemical activation from porous polymers or simple monomeric precursors. The latter approach is very effective because it eliminates the use of solvents or the need for additional N-sources like ammonia or amines.11 In this study, a series of oxygen and nitrogen codoped porous carbon was prepared from a single monomer, pyrazole (C3N2H4), by thermolysis and chemical activation with KOH. The pyrazole-derived carbons have very high nitrogen content, which leads to improved CO2 uptake and impressive selectivity. The high nitrogen content in pyrazole (43 wt. %) and its effective chemical and thermal activation at relatively low temperatures make pyrazole very convenient for porous carbon preparation.

Experimental Synthesis PYDCs were prepared by thermolysis of pyrazole/KOH mixtures. Initially, pyrazole (98% from Acros Organic) was sublimed before use to remove moisture and KOH (potassium hydroxide, Fisher Scientific) was crushed and degassed for 18 hr under vacuum. Pyrazole/KOH mixtures were prepared inside a glovebox under Ar by mixing of pyrazole and KOH in 2:1 weight ratio 4 ACS Paragon Plus Environment

Page 5 of 25

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


(KOH:pyrazole). The inert atmosphere was used to minimize KOH and pyrazole exposure to moisture due to their highly hygroscopic nature. The resulting mixtures were loaded to a porcelain boat (100x20x13mm) and inserted in a temperature-programmed furnace (CARBOLITE) under Ar flow for thermal activation. Many samples were prepared and tested under different conditions (activation temperature, time, and weight ratio) to optimize the porosity and CO2 uptake. Among these samples, four samples were found to have optimal surface area and gas uptake. The temperature was increased gradually at rate of 5 °C/min to the required value. Samples PYDC-550-1, PYDC-550-2 and PYDC-550-3 were activated at 550 °C for 1, 2 and 3 hr respectively. For PYDC-600-2, the sample was activated at 600 °C for 2 hr. The samples were cooled down to room temperature under Argon flow. Obtained carbons were soaked in HCl (0.1 M) to remove unreacted KOH. The resulting PYDCs were then rinsed with de-ionized water (2 L) followed by excess ethanol to yield fine black powders. The resulting carbons were outgassed at 200 °C for 18 hr at 1x10-5 torr before porosity and gas uptake studies. Characterization Surface topography and composition were examined by scanning electron microscopy (SEM) using a Hitachi SU-70 microscope. Silver paste was applied to stick the samples instead of carbon tape to avoid conflict between carbon in samples and the tape. Elemental composition was examined using Energy-dispersive X-ray spectroscopy (EDX) to identify elements in the samples. Elemental composition was measured by X-ray photoelectron spectroscopy (XPS) technique using thermo fisher scientific ESCALAB 250 spectrometer. Elemental analysis was performed for carbon, nitrogen and hydrogen, on EuroEA3000 Series CHN Elemental Analyzer (Eurovector Instruments).



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 6 of 25

Gas Uptake Measurements Autosorb-IQ2 volumetric adsorption analyzer (Quantachrome instruments) was used to assess porosity and gas uptakes by measuring isotherms for N2, CO2 and CH4 after evacuating the samples at 200 °C for 20 hr. Specific surface area was measured using N2 (77 K) employing Brunauer-Emmett-Teller (BET) approach. CO2, CH4, N2 gas uptakes were collected at 298 and 273 K for each of the samples. Pore size distribution (PSD) was calculated from N2 and CO2 adsorption isotherm 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 model for carbon. In the cumulative pore size distribution curve, microporous volume (Vmic) was generated using the volume of pores having a diameter < 2 nm. In a typical breakthrough experiment, the sample column was loaded with 150-200 mg of PYDCs-carbon. The sample was activated at 150 °C and 5ml/min He flow. Breakthrough experiments were conducted at 298 K / 1.0 bar. For flue gas settings, a CO2/N2 : 0.5/4.5 ml min-1 (10:90) mixture is fed through the column and the breakthrough time for each gas is assessed by recording P vs. T curves. The complete breakthrough time is denoted as the time it takes for downstream gas to reach the total signal of the feed gas. The selectivity (S) of CO2 over N2 was determined by applying the following equation22:

SCO2/N2=

(Uptake CO2/Uptake CO2 and N2) / (Uptake N2/Uptake CO2 and N2) Composition of CO2 / Composition of N2

Result and Discussion


Page 7 of 25

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


Structural Characterization Scanning Electron Microscopy (SEM) was performed to show the morphology of PYDCs, which showed large pieces (~500 µm) with rough surface. EDX studies indicate that PYDCs contain uniformly distributed N and O content (Figure 1). XPS was used to identify the functionalities of nitrogen and oxygen in the activated carbon. Three major peaks were centered at 286.33, 400.77 and 533.23 eV corresponding to C1s, N1s and O1s, respectively (Figure S 1A). The presence of oxygen in all samples is due to the use of KOH as the activating agent while nitrogen originates from the high nitrogen content in pyrazole. The surface percentage of nitrogen and oxygen within the activated carbon calculated from XPS is very close to the values obtained from micro elemental analysis (Table S1). The chemical nature of C, O, and N was examined by resolving the spectra of their 1s core level. The O1s spectra indicate the existence of three peaks (Figure S2) that correspond to (O-I) quinone type C=O, (O-II) phenol type C=OH/C-O-C and (O-III) carboxylic oxygen group/water (O-1) and (O-II) peaks appeared in all samples with binding energies 530.5-531 eV and 532.1-532.9 eV respectively23. (O-III) was observed for PYDC-660-2 and PYDC-550-2 with corresponding binding energies of 534.9 and 235.3 eV, respectively. The N1s functionalities indicate the presence of different species (Figure S2). There are three major peaks; peak (I) center is in the 400-400.7 eV region for all samples. Peak (II) and Peak (III) are positioned at 401.9 and 403.1-406.1 eV region, respectively. Peak (I) indicates existence of N-5 pyrrolic/pyridonic nitrogen,25 which is expected due to the N-heterocyclic structure of pyrazole. For peak (II), the peak at 401.9 eV could result from ammonium end group,26-29 quaternary nitrogen24, 30-33 or oxidized N groups.34-36 Other N functionalities include quaternary atoms that appear in the range of 401.3 to 401.9 eV which could include protonated pyridine or graphitic nitrogen.25 Furthermore, peak (III) corresponds to the presence of pyridine-N-oxide.37 7 ACS Paragon Plus Environment


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 25

Noteworthy, pyridinic nitrogen was not detected in XPS spectra of PYDCs, which is expected because the rearrangement of pyrollic structures to pyridine would need a higher temperature than 550 °C.25 Also, the presence of oxygen due to KOH use for activation promotes the formation of N-oxides of pyridinic nitrogen and thus gives rise to N-5 and N-X contribution.25 Schematic illustration of various nitrogen and oxygen functionalities on a typical porous carbon is shown in Figure S1B. All PYDCs are non-crystalline as shown by their PXRD studies (Figure S3).

Figure 1. SEM image of (a) PYDC-550-2, (b) PYDC-550-1, (c) PYDC-550-3, (d) PYDC-550-2, and (e-g) the corresponding EDS elemental (C, N, O) mapping of PYDC-550-2.


Page 9 of 25

PY D C -550-1

N 1S

PYD C -550-2

(A)

N1S

(B ) P ea k (II)

Pe a k (I) Pe a k (III)

Intensity (a.u)

Intensity (a.u)

P ea k (II)

Pe a k (I)

P e a k (III)

412

410

408

406

404

402

400

398

410

408

406

PY D C -550-3

404

402

400

398

Bind in g En ergy(eV)

Binding E nerg y (eV )

N1S

P YD C -600-2

(C )

(D )

N1S

P e a k (II)

Pe a k (I)

Intensity (a.u)

P e ak (II) Intensity (a.u)

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


P ea k (I)

P e a k (III) Pe a k (III)

410

408

406

404

402

400

398

410

408

406

Bin ding E nergy(eV)

404

402

400

398

Binding E nerg y(eV )

Figure 2. Deconvoluted N1S spectra of PYDC-550-1 (A), PYDC-550-2 (B), PYDC-550-3 (C), PYDC-600-2 (D).



1000

(a)

P YD C -550 -1 P YD C -550 -2 P YD C -55 0-3 P YD C -600 -2

800

(b)

N 2 at 77K

PYDC-550-1 PYDC-550-2 PYDC-550-3 PYDC-600-2

0.20

0.15

dV(d) (cc/A/g)

Volume adsorbed @STP (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

Page 10 of 25

600

0.10

400

0.05

200

0

0.00

0.0

0.2

0.4

0.6

0.8

1.0

4

Relative Pressure ( P/ P o )

6

8

10

12

P ore S ize ( Å )

Figure 3. (a) N2 adsorption-desorption isotherm of PYDCs (77 K). (b) PSD from QSDFT using nitrogen adsorption isotherms.

Textural properties The porous nature of PYDCs was investigated by N2 adsorption/desorption isotherm at 77 K, the Brunauer-Emmett-Teller (BET) surface area ranged from 1266 to 2013 m2 g-1 (Table 1). All isotherms (Figure 3) show steep increase in nitrogen uptake at low pressure then the isotherms show a gradual rise in N2 uptake as the pressure increase to reach 1 bar. A sharp uptake increase near P/Po = 1 in the N2 isotherm is due to nitrogen condensation within interparticulate voids. The pore size distribution (PSD) was obtained from N2 isotherm at 77 K based on Quenched Solid State Functional Theory (QSDFT) assuming slit-like pores (Figure 3B and Figure S4).


Page 11 of 25

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


Furthermore, PSD was investigated using CO2 isotherm at 273 K and nonlocal density functional theory (NLDFT) model to examine the presence of fine micropores (Figure S5).11 The combination of these two isotherms provides expressive pore size distribution spectra for the whole range. Total pore volume and micro pore volume were extracted from cumulative pore volume curve using N2 isotherm at 77 K (Figure S4). Accordingly, the ratio of micropore volume to total pore volume was calculated and revealed a 60-70% microporous nature for all PYDCs (Table 1). Pore width maxima were located in the range of 4.7-10.96 Å (Figure 3B). Additional broad mesopores around 30 nm were observed for all samples activated at 550 °C. The total pore volume to micropore volume (Vtot/Vmic) ratio is listed in Table 1. The highest total pore volume is reported for PYDC-550-2 (1.173 cm3 g-1), while PYDC-550-1 has the lowest pore volume (0.69 cm3 g-1). PYDC-550-3 and PYDC-600-2 have pore size volumes of 0.78 and 0.88 cm3 g-1 respectively. Table 1.Textural properties and elemental analysis of PYDCs.

SBETa

VTOTb

VMicc

Vmesoc

V0d

C

O

N

(m2 g-1 )

(cm3 g-1 )

(cm3 g-1 )

(cm3 g-1 )

(cm3 g-1 )

(wt. %)

(wt. %)

(wt. %)

PYDC-550-1

1266

0.69

0.47(68)

0.22

0.058

66.6

18.4

14.9

PYDC-550-2

2013

1.173

0.75(63)

0.42

0.065

68

16.9

15.1

PYDC-550-3

1400

0.78

0.53 (68)

0.25

0.046

68.1

16.4

15.5

PYDC-600-2

1398

0.88

0.61 (69)

0.27

0.069

68.2

16.5

15.3

Sample

a

Brunauer–Emmett–Teller (BET) surface area. bTotal pore volume at P/Po = 0.95. cCalculated from PSD

and QSDFT model using N2 adsorption isotherm at 77 K (micropore volume/total pore volume percentage). dPore volume of ultramicropores (

Effective Approach for Increasing the Heteroatom Doping Levels of

Recommend Documents