Adsorption of CO2, CH4, and N2 in Micro-Mesoporous Nanographene

Article pubs.acs.org/jced

Adsorption of CO2, CH4, and N2 in Micro-Mesoporous Nanographene: A Comparative Study Dipendu Saha,*,† Karl Nelson,† Jihua Chen,‡ Yuan Lu,§ and Soydan Ozcan§ †

Department of Chemical Engineering, Widener University, One University Place, Chester, Pennsylvania 19013, United States Center for Nanophase Materials Sciences, and §Carbon and Composites Group, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡

ABSTRACT: In this work, we have measured the adsorption isotherms and calculated the equilibrium selectivity for CO2, CH4, and N2 on micro-mesoporous nanographene at three temperatures of 298 K, 278 K, and 263 K and pressures up to 760 Torr. The nanographene sample possesses a particle size range of 50 nm to 250 nm along with a Brunauer−Emmett−Teller (BET) specific surface area of 514 m2/g and total pore volume of 3 cm3/g. The pore widths varied from 3.5 Å to 8 Å in the microporous region and very large distributed widths within 45 Å to 250 Å in the region of mesoporosity. The calculated equilibrium selectivity of gas separation at 298 K by pressure swing adsorption for CO2/N2, CO2/CH4, and CH4/N2 are 55, 8.2, and 6.5, respectively, whereas the adsorption selection parameters for same pair of gases are 540, 101, and 117, respectively. To compare the equilibrium selectivity values with other adsorbents, we have measured the gas adsorption isotherms for CO2, CH4, and N2 on Maxsorb (a commercial activated carbon with BET surface area 3200 m2/g) and calculated the selectivity values for several adsorbents based on their adsorption isotherms reported in the literature. We have found that equilibrium selectivity for all the gas pairs are higher for graphene compared to Maxsorb. We also found that the equilibrium selectivity for CO2/N2 for graphene is higher than all the carbon-based materials reported so far. The equilibrium selectivity for CO2/CH4 and CH4/N2 in graphene is also higher than the majority of the adsorbents reported in the literature. Our findings suggest that graphene can serve as a potential adsorbent for gas separation purposes.

1. INTRODUCTION

from N2 is required to minimize the emission of CH4 to the atmosphere. The current technique for CO2 scrubbing utilizes chemisorption-based carbamate technology that requires high-energy input, expensive recycling, and other serious corrosion problems.10,11 Isolation of CO2 from N2 by membrane-based separation may also provide a possible solution, but such strategy needs to overcome the trade-off between selectivity and permeability.11 Separation of CH4 from CO2 is usually done in cryogenic distillation, which is an energy intensive and costly separation process, especially in the case of small to medium-scale separation needs.12,13,5,14 Compared to these operations, adsorption-based technology in the form of pressure swing adsorption (PSA) provides a unique strategy for gas separations in terms of an easy and energy efficient process, low operating cost, and high regenerability. However, to provide a successful operation, the adsorbent materials must provide high selectivity, moderate adsorption capacity, and high chemical and thermal stability. Traditionally, the porous carbon-based materials were the most ubiquitously employed adsorption-based separation technologies owing to their high

Separation needs for CO2 from N2 and hydrocarbons, mostly CH4, can be attributed to the minimization of global warming and enhancement of fuel value of methane through biogas upgrading. The chronic increase in global warming as a result of accumulation of CO21 in the atmosphere is mostly caused by the effluent gases from coal power plants worldwide that bear an average of 15% CO2 accompanied by 70% N2.2 According to the Environmental Protection Agency (EPA), around 6.1 billion metric tons of CO2 was released by the USA alone in 2007.3 Therefore, separation of CO2 from N2 definitely claims to be the major separation need. On the other side, methane, the key component of natural gas, is one of the main sources of “cleaner” energy for both industrial and household purposes that can be harvested from nonrenewable and renewable sources. The biogas and landfill gas contain 50−85% methane, the remainder being rich with CO2 (20−35%) along with N2 and other gaseous entities.4 Therefore, isolating CH4 from CO2 and N2 enhances its fuel value. Additionally, separation of CO2 from CH4 helps to minimize the corrosion in pipelines caused by the acidic CO2 gas mixed with methane.5−7 Besides these reasons, CH4 has also been proven to be a greenhouse gas, and its global warning potential (GWP) is even higher than CO2.8,9 During the processing of landfill gas (LFG), separation of CH4 © XXXX American Chemical Society

Received: March 27, 2015 Accepted: August 3, 2015

A

DOI: 10.1021/acs.jced.5b00291 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Nitrogen adsorption−desorption plot of graphene at 77 K.

Figure 2. Pore size distribution of nanographene calculated by nonlocal density functional theory (NLDFT) and based on N2 and CO2 adsorption.

to ambient temperature and calculated the selectivity of separation through the adsorption process.

surface area and pore volume, chemical and thermal stability, and affinity toward unsaturated molecules because of the π−π interactions with the sp2 carbons. Among the carbon-based materials, graphene is an important allotrope that has a high theoretical surface area of 2630 m2/g15. Although there are few computational reports on gas adsorption on graphene material15−19 the experimental work on such a topic is very rare.20 In this work, we have reported the adsorption isotherms of three gases, CO2, N2, and CH4 at three different temperatures close

2. MATERIALS AND METHODS Single layer nanographene samples were obtained from ACS Material and used as-received. Pore textural properties and pure gas isotherms were measured in Quantachrome’s Autosorb iQ instrument. The pore textural properties were obtained by N2 and CO2 adsorption−desorption at 77 K and 273 K, respectively, and at 1 bar. Brunauer−Emmett−Teller (BET) B



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analysis and nonlocal density functional theory (NLDFT) were employed to calculate the surface area and pore size distribution analysis. The isotherms of CO2, N2, and CH4 at 298 K, 278 K, and 263 K were measured with ultrahigh purity (UHP) grade of each type of gas. The graphene samples were outgassed at 300 °C for 3 h before the starting the experiment. Except for 77 K, which was maintained by liquid nitrogen, all the temperatures were maintained by Julabo’s temperature controlled unit with 1:1 mixture of water and propylene glycol as chilling liquid. The kinetic runs of gas adsorptions were obtained at the vector mode of the instrument control software. Transmission electron microscopy (TEM) and electron energy loss spectra (EELS) were obtained on a Carl Zeiss Libra 120 transmission electron microscope operating at 120 kV. X-ray photoelectron spectra (XPS) were obtained on a Thermo Scientific K-Alpha X-ray photoelectron spectrometer equipped with a conventional electron energy analyzer. For incident radiation, a monochromatic AlKα X-ray source was employed.

Table 1. Quantification of Functional Composition of Graphene by X-ray Photoelectron Spectroscopy (XPS) species 2

sp sp3 C−O O−CO

percent 53.84 19.00 12.58 14.58

O−CO present in the amount of 12.5 % and 14.5 %, respectively. Pertinent XPS analyses of these graphene-based materials are revealed in details elsewhere.21 TEM images of graphene are shown in Figure 4a,b. The dimension of the nanographene sample is within 50 nm to 200

3. RESULTS AND DISCUSSION 3.1. Materials Characterizations. The nitrogen adsorption−desorption plots at 77 K (Figure 1) demonstrate an initial flat plateau and then a very large hysteresis loop that supports the coexistence of both micro- and mesoporosity. BET specific surface area and total pore volume of the graphene sample are 514 m2/g and 3.0 cm3/g, respectively. The pore size distribution (PSD) plot (Figure 2) was made by combining the NLDFT results obtained from N2 and CO2 adsorption at 77 K and 273 K, respectively. The graphene sample possesses the micropore distribution within 3.5 Å to 8 Å and very large distributed mesopores within 45 Å to 250 Å. Most likely, the unusually high total pore volume (3.0 cm3/g) for a relatively lower surface area (514 m2/g) is caused by large distributed mesopores. X-ray photoelectron spectroscopy (XPS) was employed to quantify the sp2/sp3 character and oxygen containing functional groups. Figure 3 shows the C1S peak and the curve fitting to identify the sp2, sp3 character of carbon and oxygen containing functional groups. According to Table 1, the graphene possesses around 53.84 % sp2 and 19 % sp3 character. Two prominent oxygen containing functional groups of C−O and

Figure 4. TEM image of nanographene (a) and its magnified view (b).

nm. The magnified view of graphene reveals the wrinkles and the possible presence of multiple layers in the darker regions. Electron energy loss spectra (EELS) of graphene is shown in Figure 5a,b for plasmon and K-edge loss, respectively. In plasmon, the broad peak at around 25 eV belongs to the collective contribution from planar electrons forming σ bond and π electron. In this region, we could not locate a weaker peak at around 5 eV to 6 eV unlike the report by Biener et al. for nanographene.22 In the K-edge loss region, the very weak peak at around 285 eV to 290 eV is caused by the electron transition from the 1s to π* antibonding orbital, whereas the larger peak at 300 eV is originated by the electron transition from the 1s to σ* antibonding orbital. Detailed analysis and

Figure 3. Peak fitting of the X-ray photoelectron spectroscopy (XPS) of nanographene. C



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Figure 5. Electron energy loss spectra (EELS) of nanographene in plasmon (a) and K-edge loss region (b).

Figure 6. CO2, CH4, and N2 adsorption isotherms on nanographene at 298 K.

relevant causes of EELS peaks of sp2 carbon materials can be obtained in the literature.23,24 3.2. Gas Adsorption Studies. Adsorption and desorption of CO2, N2, and CH4 were performed at the three temperatures of 298 K, 278 K, and 263 K up to ambient pressure. The gas isotherms at the three temperatures are shown in Figures 6, 7,

and 8, respectively. At ambient temperature, CO2 adsorption was as high as 0.7 mmol/g followed by 0.15 mmol/g of CH4 and 0.02 mmol/g for N2 at atmospheric pressure. It is clear that at all temperatures, CO2 adsorption was the highest, followed by CH4 and N2. It is also noticeable that the isotherms of CH4 and N2 are almost linear, unlike CO2, representing weak D



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three times higher than that of N225 and that helps it to be preferentially adsorbed on the edge sites of graphene compared to the basal plane through strong Coulombic interactions. On the other hand, N2 is more favorably adsorbed on the basal plane through dispersion energy where there is no charge concentration present. It is suggested that a CH4 molecule, with a low octapole moment, is also preferentially adsorbed on the basal plane graphene through relatively stronger dispersive forces. Since the Coulombic interactions are stronger than dispersive forces and both BET and geometrical surface areas of

interactions between graphene and those two gases. The differences in adsorption amounts of different gases on nanographene can be explained with the help of computation studies.15 It is suggested the edge sites of nanographene particles contain more concentrated charges than the basal plane and hence these sites interact differently with the molecules that contain partial charges. N2 and CO2 molecules have quadruple moments, but a CH4 molecule possesses an octapole moment. Although both N2 and CO2 molecules have quadruple moment, the quadruple moment of CO2 is about E



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Table 2. Numerical Values for Isotherm Model Fitting Parameters Langmuir T/K

value

Freundlich ARE

value

Sips ARE

Henry’s Law

value

ARE

value

ARE

a′m = 37.37 b′ = 0.0001157 n′ = 1.3013 am′ = 76.55 b′ = 0.0001274 n′ = 1.396 am′ = 108.5 b′ = 0.0001074 n′ = 1.367

0.0029

k = 0.0009908

0.034

0.0041

k = 0.001617

0.070

0.0046

k = 0.002126

0.086

am′ = 7.211 b′ = 2.177·10−5 n′ = 0.9643 am′ = 2.6189 b′ = 0.0002378 n′ = 1.0467 a′m = 22.08 b′ = 7.222·10−5 n′ = 0.9947

0.00099

k = 0.0001954

0.0011

0.0010

k = 0.0004264

0.0066

0.00091

k = 0.001582

0.0058

a′m = 2.294 b′ = 7.001·10−6 n′ = 0.9089 a′m = 1.705 b′ = 8.370·10−5 n′ = 0.9644 am′ = 1.550 b′ = 0.0001090 n′ = 0.9553

0.0010

k = 3.002·10−5

0.0011

0.00076

k = 0.0001698

0.00078

0.00067

k = 0.0002094

0.00084

CO2 298

am = 1.139

0.024

K = 0.004923

0.0033

278

b = 0.001539 am = 1.6689

0.038

n = 1.342 K = 0.01116

0.0055

263

b = 0.001898 am = 2.278

0.050

n = 1.444 K = 0.01348

0.0079

b = 0.001761

n = 1.417

298

am = 0.3346

0.023

K = 0.0001738

CH4 0.00099

278

b = 0.0005048 am = 0.9082

0.0068

n = 0.9815 K = 0.0007382

0.0020

263

b = 0.0006003 am = 11.60

0.011

n = 1.0940 K = 0.001795

0.0018

b = 0.0001448

n = 1.0202 N2

298

am = 0.008682

0.0086

K = 4.529·10−6

0.0012

278

b = 0.001475 am = 2.930

0.0029

n = 7663 K = 0.0001670

0.00078

263

b = 5.707·10−5 am = 0.8917

0.013

n = 0.9970 K = 0.0001762

0.0015

b = 0.0002196

n

= 0.9715

edge sites are greater than that of the basal plane of graphene,15 the adsorption amount of CO2 is much higher than that of the other two gases. We have model fit the isotherms by four types of well-known isotherm models. For all the isotherms, a and P are adsorption amount and pressure, respectively.

where qexp is the experimental adsorption point, qmod is the modeling point, and N is the number of points in the isotherm. The model fitting parameters are given in Table 2. It is found that Henry’s law and the Sips model fit the best in most of the isotherms, whereas the Langmuir model quite rarely fits in. As representatives, we have shown the model fitting plots in Figure 9 panels a, b, and c for the adsorption of CO2, CH4, and N2 at 263 K. The kinetic data for CO2, CH4, and N2 are given in Figure 10. All the kinetic data were measured in vector dose mode in Autosorb-iQ at the temperature of 298 K and at the very low pressure of 10 Torr. The plot shows that CO2 adsorption is slightly sluggish compared to rest of the gases, but there was no significant visual difference in the plot of CH4 and N2. The heat of adsorption (ΔH) was calculated from the three isotherms at 298 K, 278 K and 263 K for all the gases. Heat of adsorption was calculated from the Van’t Hoff equation given by

Langmuir model: a=

ambp 1 + bp

(1)

Freundlich model: a = KP1/ n

(2)

Sips Langmuir−Freundlich model: a=

am′b′P1/ n ′ 1 + b′P1/ n ′

(3)

⎛ ∂ ln P ⎞ ΔH ⎟ = −⎜ 2 ⎝ ∂T ⎠a RT

Henry’s Law: (4)

a = kP

or, upon integration,

In these models, am, b, K, n,am′ , b′, n′, and k are constants. Absolute relative error (ARE) of the model fitting is given by

ΔH = ln P + C RT

N

ARE % =

∑n = 1 |qexp − qmod| N

100

(6)

(7)

where C is the constant of integration. A series of isosteres are drawn at preselected adsorption amounts and from the slope of

(5) F



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Figure 10. Kinetics of CO2, CH4, and N2 adsorption plot on nanographene at 298 K.

moment of CO2 helps it to be strongly adsorbed on the edge sites through Coulombic attraction. It can also be inferred that highly polar CO bonds of the CO2 molecule can interact with the surface functionalities of −C−O or CO on the graphene surface and can form attachment with those functionalities. Such strong affinity of CO2 with the graphene surface may help to form a multilayer of adsorbed molecules, especially in the mesopores, as suggested in the report of Mishra et al.20 and that is also supportive of the higher adsorption amounts of CO2 compared to other gases. Although surface interactions between the first layer of CO2 and the graphene surface should have the highest potential, the energy of interaction will decrease progressively with the increase in number of layers. While calculating the heat of adsorption for overall adsorption, quite expectedly, all the interactions will average out and the resultant interactions will ultimately appear to be smaller compared to the other two gases. The exact identity of multilayer requires rigorous computation studies that are beyond the scope of this work. Although there are few scientific reports that demonstrated the mixed gas adsorption data for CO2, CH4 and N2,26−28 generally mixed adsorption is very difficult owing to the complex experimental setup and data validation protocols. Because of this difficulty, it is a common practice to model the equilibrium selectivity from pure component isotherms. For adsorptive separation of two components through pressure swing adsorption (PSA), equilibrium selectivity (α1−2) of component 1 (stronger adsorbate) over component 2 (weaker adsorbate) is defined as29

Figure 9. Representative isotherm model fitting results on CO2, CH4, and N2 at 263 K.

the isosteres, the heats of adsorption values were calculated as a function of adsorption amount (a). Heat of adsorption is shown in Figure 11 for CO2, CH4, and N2. Unlike most other carbon species, we found CH4 possesses the highest heat of adsorption, 42 kJ/mol to 38 kJ/mol followed by N2, 36 kJ/mol to 26 kJ/mol. According to our calculation, CO2 demonstrated the lowest heat of adsorption, within 23 kJ/mol to 20 kJ/mol. Heat of adsorption of all the gases demonstrated a common trend in decrease in values with an adsorption amount that correlates to the heterogeneous surface of graphene. It is noticeable in the plot that the heats of adsorption values do not correspond to the adsorption amounts. The higher heat of adsorption CH4 compared to N2 can be related to the higher adsorption of CH4, but the lowest heat of adsorption of CO2 cannot be directly related to its highest uptake compared to that of the other two gases and hence it needs further explanation. As mentioned earlier, the higher quadruple

α1−2 =

x1/y1 x 2/y2

(8)

where x and y are the mole fractions in adsorbed phase and bulk gas phase, respectively. For simplicity, equilibrium selectivity can be further correlated from Henry’s law and Langmuir isotherms as30 α1−2 =

x1/y1 x 2/y2

≈

a b k1 ≈ m1 1 k2 am2b2

(9)

where, k1 and k2 are Henry’s constants for component 1 and 2 and am1, am2, b1, and b2 are Langmuir constants for components G



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Figure 11. Heat of adsorption of CO2, CH4, and N2 on graphene.

Figure 12. Equilibrium selectivity of αCO2−N2, αCO2−CH4 and αCH4−N2 on nanographene at the three temperatures of 298 K, 278 K, and 263 K.

the adsorption and desorption capacity at 1 and 0.1 bar, respectively. The adsorption selection parameters, SCO2−N2, SCO2−CH4 and SCH4−N2 are 540, 101 and 117, respectively, under the same conditions. To compare the selectivity values with other carbonaceous and noncarbonaceous adsorbents reported in the literature, we have calculated the selectivity from the given isotherms and plotted in Figure 13 panels a, b, and c. All the values were calculated for 298 K or otherwise mentioned in the plot. To the best of our knowledge, the CO2/N2 selectivity of graphene is highest in the carbon-based materials. In the comparison plot, the CO2/N2 selectivity of graphene is higher than Maxsorb, nitrogen-doped graphene,32 ordered mesoporous carbon (OMC),14 and activated carbon33 (at 288 K) among pure carbon-based materials. Besides that, the selectivity is also higher than several zeolite-based materials (13X,33 H+ Mordenite34) and nitrogen doped carbon (RFL-50035). The selectivity is also quite higher than few MOF based materials, like MOF-5,30 MOF-177,30 and a copper-based MOF.36 Selectivity of CO2/CH4 is also higher than the carbon-based materials (nitrogen-doped graphene,32 Maxsorb, activated carbon, 33 OMC,14 nitrogen-doped carbon (RFL-50035) MOFs30,36 (MOF-5, MOF-177, and Cu-MOF) and zeolite (13X33). Only H+ Mordenite34 has higher CO2/CH4 selectivity than graphene (H+ Mordenite, 11.5). The selectivity of CH4/ N2 on graphene is also higher than other carbon-based materials (Maxsorb, BDH-treated activated carbon,37 OMC,14 other activated carbon33), H+ Mordenite,34 and MOF-5.30 MOF-17730 and the Cu-MOF36 possessed slightly higher selectivity than graphene (Cu-MOF, 6.9; MOF-177, 6.98; graphene, 6.5).

and N2 adsorption isotherms at 298 K on Maxsorb, which is a commercially available high surface area (BET: 3200 m2/g) activated carbon. The selectivity values of αCO2−N2, αCO2−CH4 and αCH4−N2 on Maxsorb are 9.6, 2.54, and 3.76, respectively, which are quite low compared to pure graphene. To calculate the adsorption selection parameter (S1−2), we have employed

4. CONCLUSIONS We have performed the adsorption and desorption of CO2, N2, and CH4 in micro-mesoporous nanographene at the three temperatures of 298 K, 278 K, and 263 K at pressures up to 760 Torr. The nanographene samples are within 50 nm to 200 nm in particle size and possess the BET specific surface area of 514 m2/g with total pore volume of 3.0 cm3/g. It has micropore

1 and 2, respectively. For pressure swing adsorption, it is also useful to calculate adsorption selection parameter (S1−2) given by31 S1−2 =

Δq1 Δq2

α1 − 2

(10)

where Δq1 and Δq2 are the differences in adsorption capacity (working capacity) at adsorption pressure and desorption pressure for component 1 and 2, respectively. The equilibrium selectivity of αCO2−N2, αCO2−CH4 and αCH4−N2 on graphene at 298 K are 55, 8.58, and 6.5, respectively. The selectivity values in two other temperatures are show in Figure 12. For comparison purposes, we have measured the CO2, CH4,

H



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confirmed that graphene demonstrated better selectivity compared to most of the adsorbents. On the basis of this result, it can be argued that graphene can serve as a potential adsorbent for gas separation purposes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 610 499 4056. Fax: 610 499 4059. Funding

D.S. acknowledges the faculty development award (2014-2015) and provost grant (2014-2015) from School of Engineering of Widener University. Part of the work is supported by American Chemical Society sponsored Petroleum Research Fund (54205UNI10). TEM and EELS (J.C.) experiments were conducted at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, which is a DOE Office of Science User Facility. X-ray photoelectron spectroscopy measurements (Y.L.) were conducted at ORNL, research was partially sponsored by the Laboratory Directed Research and Development Program of ORNL, managed by UT-Battelle, LLC, for the U.S. Department of Energy. Part of this manuscript has been authored by UT-Battelle, LLC under Contract No. DEAC05-00OR22725 with the U.S. Department of Energy. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 13. Comparison of selectivity of (a) CO2/N2, (b) CO2/CH4, (c) CH4/N2 between nanographene and other adsorbents published in literature. The data for Graphene and Maxsorb are obtained in this work. For other adsorbents, the reference number is provided in the parentheses. Except 13X and activated carbon (AC), all the selectivity values are at 298 K.

distribution up to 10 Å and a wide mesopore distribution of 45 Å to 250 Å. For all the temperatures, the highest adsorption is for CO2 followed by CH4 and N2. The kinetic measurements suggested that CO2 adsorption is slightly sluggish than that of the other gases. The selectivity values of CO2/N2, CO2/CH4, and CH4/N2 at 298 K are 55.8, 8.58, and 6.5, respectively. To compare, we performed the adsorption of three gases on Maxsorb, a well-known and commercial high-surface-area activated carbon and the results suggested that all selectivity values for graphene are superior compared to those for Maxsorb. We also compared the selectivity of these gases with that of other adsorbents reported in the literature and I



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Adsorption of CO2, CH4, and N2 in Micro-Mesoporous Nanographene

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