Nitrogen-Doped Mesoporous Carbon for Carbon Capture – A

Article pubs.acs.org/JPCC

Nitrogen-Doped Mesoporous Carbon for Carbon Capture − A Molecular Simulation Study Ravichandar Babarao,† Sheng Dai,†,‡ and De-en Jiang*,† †

Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37966, United States

‡

S Supporting Information *

ABSTRACT: Using molecular simulation, we investigate the effect of nitrogen doping on adsorption capacity and selectivity of CO2 versus N2 in model mesoporous carbon. We show that nitrogen doping greatly enhances CO2 adsorption capacity; with a 7 wt % dopant concentration, the adsorption capacity at 1 bar and 298 K increases from 3 to 12 mmol/g (or 48% uptake by weight). This great enhancement is due to the preferred interaction between CO2 and the electronegative nitrogen. The nitrogen doping coupled with the mesoporosity also leads to a much higher working capacity for adsorption of the CO2/N2 mixture in nitrogen-doped mesoporous carbon. In addition, the CO2/N2 selectivity is almost 5 times greater than in nondoped carbon at ambient conditions. This work indicates that nitrogen doping is a promising strategy to create mesoporous carbons for high-capacity, selective carbon capture.

1. INTRODUCTION Separation of carbon dioxide from nitrogen is an important solution to address the issue of global warming. To that end, porous adsorbents have been extensively explored, including porous carbons, zeolites, metal−organic frameworks, zeolitic− imidazolate frameworks, and porous aromatic frameworks.1−5 Functionalizing porous solids with polar group can enhance CO2 adsorption capacity and also increase its selectivity over other gases.6,7 Nanoporous carbons have been extensively studied for CO2 separation by varying the pore morphology of nanoporous carbon from slit-, cylindrical-shaped pores to more complex geometry.8−11 Recently, several nitrogen-doped porous carbon materials have been synthesized using different techniques and tested for CO2 adsorption capacity. High surface area, microporous Ndoped carbon materials synthesized from chemical vapor deposition exhibit the highest CO2 uptake capacity recorded to date, up to 6.9 mmol/g at 273 K/ambient pressure and 4.4 mmol/g at 298 K/ambient pressure.12 Pyrolysis of polymer monolith results in a nitrogen-doped microporous carbon exhibiting CO2 adsorption capacity of ∼3.1 mmol/g at 298 K/ ambient pressure.13 Highly porous N-doped carbons were prepared using KOH as activating agent and polypyrrole as carbon precursor, and a high CO2 uptake of 6.2 mmol/g was achieved for porous carbons with pore size ∼1 nm and 10.1 wt % N.14 These studies revealed that high nitrogen content, surface area, and micropore volume influence the CO2 adsorption capacity and selectivity. But there is a lack of understanding of the molecular-level mechanism responsible for the enhanced capacity. Molecular simulations have been used extensively to explore the potential of nanoporous carbon for CO2 separation. Most © 2012 American Chemical Society

of the studies considered simple geometry such as slit and cylindrical pores.4,15−17 To account for complex pore topology, C168 schwarzite is employed previously to mimic nanoporous carbon.18,19 But this is still in the microporous regime. To unleash the great potential of mesoporous carbons for carbon capture, in this study we aim to understand the role of the nitrogen dopant in selective adsorption of CO2 by simulating adsorption of CO2 and N2 in nitrogen-doped mesoporous carbon of complex topology. The models for the adsorbent and adsorbate are discussed in section 2, followed by simulation methodology in section 3. Finally, the results and discussion are presented in section 4, followed by concluding remarks in section 5.

2. MODELS 2.1. Adsorbent. Based on the primitive cell of C168 schwarzite, a hypothetical mesoporous carbon model consisting of ∼30 Å pore diameter was constructed with similar pore morphology. First, the unit cell of C168 schwarzite is converted into primitive cell, and then the cell parameters are increased so that the pore size is within the mesoporous range (greater than 2 nm but less than 50 nm). Then the C−C bond is replaced with the C6 ring, and the final structure is relaxed using the Forcite module with the COMPASS force field.20 Then in the mesoporous carbon model, a pore size of ∼10 Å is cleaved on the surface of the structure, and the edges are terminated with nitrogen at a concentration of ∼7 wt % and the rest with hydrogen atoms. A schematic representation of a unit cell of Received: February 13, 2012 Revised: February 28, 2012 Published: February 29, 2012 7106

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Figure 1. Schematic representation of a unit cell of the nondoped (left) and nitrogen-doped (right) mesoporous carbons. C, gray; N, blue; H, white.

3. SIMULATION METHODOLOGY The adsorption of pure CO2, N2, and CO2/N2 mixture with bulk composition of 15:85 was simulated using the grand canonical Monte Carlo (GCMC) method.30 Because the chemical potentials of adsorbate in adsorbed and bulk phases are identical at thermodynamic equilibrium, GCMC simulation allows one to relate the chemical potentials of adsorbate in both phases and has been widely used for the simulation of adsorption. The framework atoms are kept frozen during simulation. This is because adsorption involves low-energy equilibrium configurations and the flexibility of framework has a marginal effect, particularly on the adsorption of small gases. The LJ interactions were evaluated with a spherical cutoff equal to half of the simulation box with long-range corrections added; the Coulombic interactions were calculated using the Ewald sum method. The number of trial moves in a typical GCMC simulation was 2 × 107, though additional trial moves were used at high loadings. The first 107 moves were used for equilibration and the subsequent 107 moves for ensemble averages. Six types of trial moves were attempted in GCMC simulation, namely, displacement, rotation, and partial regrowth at a neighboring position, entire regrowth at a new position, swap with reservoir, and exchange of molecular identity. Unless otherwise mentioned, the uncertainties are smaller than the symbol sizes presented in the figures. Canonical ensemble (NVT) simulation is performed to estimate the isosteric heat of adsorption at infinite dilution. A single adsorbate molecule is subjected to three types of trial moves employed in the NVT simulation, namely, translation, rotation, and regrowth. The isosteric heat at infinite dilution is calculated from

nondoped and nitrogen-doped mesoporous carbons is shown in Figure 1. The LJ potential parameters of the carbon atoms in pure carbon structures are adopted from the Steele potential.21 To take into account the curvature effect, a scaling factor is employed for the gas−adsorbent well-depth parameter. A similar scaling factor is used recently in the simulation study of CO2 adsorption in zeolite template carbon to match the experimental results.22 In nitrogen-doped carbon structures, the LJ potential parameters of the carbon atom is adopted from the Steele potential for the majority of the carbon atoms, whereas LJ potential parameters from the OPLS-FF23 are used for nitrogen, hydrogen, and the carbon atoms connected to nitrogen and hydrogen. To take into account the electrostatic effect on gas adsorption in nitrogen-doped carbon structures, atomic partial charges are assigned to the framework atom. The atomic partial charges are calculated using B3LYP density functional theory (DFT) on a cleaved cluster (Figure S1). In this work, the CHELPG method is used to estimate the atomic charges which are widely used to predict gas adsorption in other porous materials.24−26 Fitting of the electrostatic charges on the atoms was performed at the B3LYP level of theory, and the 6-31+G(d,p) basis set is used for all atoms. All DFT computations are carried out with the Gaussian 03 suite of programs.27 2.2. Adsorbate. The CO2 adsorbates were mimicked as three-site model to account for the quadrupole moment. The C−O bond length in CO2 was 1.18 Å, and the bond angle ∠OCO was 180°. The charges on C and O atoms were +0.576e and −0.288e (e = 1.6022 × 10−19 C the elementary charge), resulting in a quadrupole moment of −1.29 × 10−39 C·m2. The model reproduced the isosteric heat and isotherm of CO2 adsorption in slilicate.28 The CO2−CO2 interaction was modeled as a combination of LJ and Coulombic potentials. N2 was considered as a two-site model, with the LJ potential parameters fitted to the experimental bulk properties.29 The electrostatic interaction between N2 molecules was not considered, as it was found in previous study that the incorporation of the quadrupole moment has an insignificant effect on N2 adsorption.18 The interactions of gas−adsorbent and gas−gas were modeled as a combination of pairwise site− site Lennard-Jones (LJ) and Coulombic potentials. The crossLJ parameters were evaluated by the Lorentz−Berthelot combining rules.

0 0 qst0 = RT − (Utotal − Uintra )

(1)

0 where Utotal is the total adsorption energy of a single molecule 0 with adsorbent and Uintra is the intramolecular interaction of a single gas molecule in bulk phase. Vfree is the free volume in adsorbent available for adsorption and is estimated from

Vfree =

∫v exp[−uadHe(r)/kBT ] dr

(2)

He uad

where is the interaction between helium and adsorbent, in which σHe = 2.58 Å and εHe/kB = 10.22 K.31 Note that the free volume detected by helium is temperature dependent, and usually the room temperature is chosen. The ratio of free 7107

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volume Vfree to the occupied volume Vtotal gives the porosity ϕ of adsorbent.

Table 1, the qst0 for CO2 in nitrogen-doped mesoporous carbon is almost twice that in the nondoped one, due to the stronger

4. RESULTS AND DISCUSSION 4.1. Characterization of Adsorbent. We first build a model for the mesoporous carbon and then add dopants around the rim of the pore opening. The doping concentration is around 7 wt %, comparable to typical experimental values such as 6.5−7.6 wt %.12 The nitrogen dopants exist mainly as the pyridinic type. The diameter of the channel is ∼30 Å in both nondoped and nitrogen-doped carbon, and the pore window connecting the channels is around 24 and 22 Å in nondoped and nitrogen-doped carbon, respectively (Figure 2).

Table 1. Isosteric Heat qst0 and Henry’s Constant KH of Pure CO2 Adsorption in Nondoped and Nitrogen-Doped Mesoporous Carbon qst0 (kJ/mol) KH (mmol/g/kPa)

nondoped

nitrogen-doped

18.39 0.034

33.57 0.435

interaction of CO2 with the nitrogen atom. The estimated qst0 of CO2 in nitrogen-doped carbon is similar to those predicted earlier in ether functionalized porous aromatic frameworks.6 4.2. Adsorption of CO2. We then simulate the adsorption isotherms of pure CO2 in nondoped and nitrogen-doped mesoporous carbons. As shown in Figure 4, the amount of CO2

Figure 2. Diameter of the channel along the XY plane in mesoporous carbon with (dashed line) and without (solid) nitrogen doping.

We next characterize adsorption isotherms of N2 at 77 K, pore volume, and surface area of the carbon structures used in this study. The predicted isotherms follows type IV isotherm which is characteristic of mesopores (Figure 3). We then

Figure 4. Adsorption isotherms for CO2 at 298 K in nondoped and nitrogen-doped mesoporous carbon at low pressure. The open circles are the isotherm of nitrogen-doped mesoporous carbon in the absence of electrostatic interaction.

adsorption in nondoped mesoporous carbon is ∼3.2 mmol/at 1 bar and 298 K. In nitrogen-doped mesoporous carbon the adsorption capacity increases by almost 3 times when compared to its nondoped counterpart to 12 mmol/g at 1 bar and 298 K. The predicted CO2 capacity in nitrogen-doped mesoporous carbon is higher than those reported earlier in other porous materials such as Mg(dobdc) (8.08 mmol/g),33 ZIF-78 (2.23 mmol/g),34 COF-10 (6.99 mmol/g),35 and N-doped microporous carbon (4.4 mmol/g).12 We attribute this tremendous increase in CO2 capacity to the strong electrostatic interaction between CO2 and negatively charged nitrogen groups present on the surface of the structure. To corroborate this conclusion, we switched off the framework charges in nitrogen-doped mesoporous carbon. We observed that the simulated isotherm in the absence of framework charges matches closely with the isotherm predicted in nondoped mesoporous carbon (Figure 4), indicating the importance of electrostatic interaction in nitrogen-doped mesoporous carbon. This importance is persistent up to quite high pressure (∼20 bar; Figure S2). To explore the structural information for CO2 in nitrogendoped mesoporous carbon, the radial distribution functions g(r) between CO2 and nitrogen atom at 1 kPa is shown in Figure 5a. A pronounced peak in g(r) is observed at r = 3.1 Å at 1 kPa in nitrogen-doped mesoporous carbon, and this confirms that CO2 interacts with nitrogen atom more strongly (Figure

Figure 3. Adsorption isotherms for N2 at 77 K in nondoped and nitrogen-doped mesoporous carbon.

estimated the accessible surface area, with the method used previously,32 in nondoped and nitrogen-doped carbon structures using a probe molecule with a diameter equal to 3.681 Å. The calculated values are 2538 m2/g for nondoped and 3106 m2/g for nitrogen-doped carbon. In addition, the available pore volumes for adsorption are calculated based on helium adsorption at room temperature; the estimated values are 2.56 and 3.09 cm3/g for nondoped and nitrogen-doped carbon, respectively. The strength of interaction between adsorbate and adsorbent is directly reflected by the isosteric heat qst0 and Henry constant KH at infinite dilution. As listed in 7108

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Figure 5. (a) Radial distribution functions between CO2 and N atom in nitrogen-doped mesoporous carbon at 1 kPa. (b) Density contours for CO2 at 1 kPa. The density has a unit of 1/Å3 (scale: 1 × 10−4). Color code: C, ash; H, white; N, blue.

Figure 6. (a) Adsorption isotherm for CO2 in CO2/N2 mixture and (b) selectivity for CO2/N2 in nondoped (closed triangle) and nitrogen-doped (closed circle) mesoporous carbon.

pressure, followed by less favorable sites with increasing pressure. Delta loading or working capacity is a very important parameter for adsorbent selection, which is defined as the difference of adsorbed amounts of one component in a mixture between high (production step) and low (regeneration step) pressure in a pressure-swing adsorption process. The delta loadings of CO2 between 1.0 and 0.1 MPa for CO2/N2 (15:85) mixture are 3.67 and 11.23 mmol/g for nondoped and nitrogen-doped mesoporous carbon, respectively. The predicted delta loading of CO2 in mesoporous carbon is much higher than those in microporous carbons such as C168 schwarzite (0.9 mmol/g),18 zeolitic imidazolate frameworks such as ZIF-68 (1.46 mmol/g) and ZIF-69 (1.48 mmol/g),6 or traditional zeolite such as NaY (∼4 mmol/g)36 at the same condition. This high working capacity of N-doped mesoporous carbon shows the synergistic effect of combining mesoporosity and N doping. Nanoporous carbons are promising adsorbents for postcombustion carbon capture (that is, separating CO2 from the flue gas of power plants), which is necessary in mitigating the CO2 level in the atmosphere due to man-made emissions. Previous work has focused mainly on microporous carbons which tend to have limited working capacity for CO 2 separation. As a first step to unleash the potential of mesoporous carbons for carbon capture, in this study we showed the effect of nitrogen dopant in a mesoporous carbon on CO2 adsorption and selectivity. We observed 3× increase in

5a). The density distribution contour for CO2 adsorption in nitrogen-doped mesoporous carbon is shown in Figure 5b at 1 kPa. At this low pressure, CO2 is mainly located near the pore opening present on the surface of the structure, due to the strong attraction of CO2 with the nitrogen atom. With increase in pressure, the pores are saturated with more CO2 molecules and then start to adsorb in other regions, particularly on the walls of the structure forming a monolayer (Figure S3). 4.3. Adsorption of CO2/N2 Mixture. Figure 6 shows the adsorption isotherms and selectivity for CO2/N2 mixture with a bulk molar composition of 15:85 in nondoped and nitrogendoped mesoporous carbon. Over the entire range of pressure, CO2 is more strongly adsorbed than N2 in both the structures (N2 isotherm not shown in Figure 6a). Adsorption of CO2 is much higher in nitrogen-doped mesoporous carbon than in its nondoped counterpart, similar to those observed in pure component isotherm, due to the enhanced electrostatic interactions between CO2 and nitrogen atom in the former. Figure 6b shows the adsorption selectivity of CO2/N2 in both nondoped and nitrogen-doped mesoporous carbon. Adsorption selectivity in nondoped mesoporous carbon remains almost constant (∼14) over the entire pressure range, indicating homogeneous characteristic of the adsorbent. The adsorption selectivity in nitrogen-doped mesoporous carbon is much greater than in the nondoped carbon, from 74 at low pressure to 46 at 1 bar and 32 at 10 bar. This decrease in selectivity is because the more favorable sites are adsorbed first at low 7109

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(8) Barata-Rodrigues, P. M.; Mays, T. J.; Moggridge, G. D. Carbon 2003, 41, 2231. (9) Iijima, S. Nature 1991, 354, 56. (10) Kaneko, K.; Cracknell, R. F.; Nicholson, D. Langmuir 1994, 10, 4606. (11) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (12) Xia, Y. D.; Mokaya, R.; Walker, G. S.; Zhu, Y. Q. Adv. Energy Mater. 2011, 1, 678. (13) Hao, G. P.; Li, W. C.; Qian, D.; Lu, A. H. Adv. Mater. 2010, 22, 853. (14) Sevilla, M.; Valle-Vigon, P.; Fuertes, A. B. Adv. Funct. Mater. 2011, 21, 2781. (15) Cracknell, R. F.; Nicholson, D.; Tennison, S. R.; Bromhead, J. Adsorption 1996, 2, 193. (16) Nicholson, D.; Gubbins, K. E. J. Chem. Phys. 1996, 104, 8126. (17) Huang, L. L.; Zhang, L. Z.; Shao, Q.; Lu, L. H.; Lu, X. H.; Jiang, S. Y.; Shen, W. F. J. Phys. Chem. C 2007, 111, 11912. (18) Jiang, J. W.; Sandler, S. I. J. Am. Chem. Soc. 2005, 127, 11989. (19) Anderson, C. J.; Tao, W. D.; Jiang, J. W.; Sandler, S. I.; Stevens, G. W.; Kentish, S. E. Carbon 2011, 49, 117. (20) Materials Studio, 4.3 ed.; Accelrys, San Diego, 2008. (21) Steele, W. Chem. Rev. 1993, 93, 2355. (22) Builes, S.; Roussel, T.; Ghimbeu, C. M.; Parmentier, J.; Gadiou, R.; Vix-Guterl, C.; Vega, L. F. Phys. Chem. Chem. Phys. 2011, 13, 16063. (23) Jorgensen, W. L.; Maxwell, D. S.; TiradoRives, J. J. Am. Chem. Soc. 1996, 118, 11225. (24) Yang, Q. Y.; Zhong, C. L. J. Phys. Chem. B 2006, 110, 17776. (25) Yang, Q. Y.; Zhong, C. L. ChemPhysChem 2006, 7, 1417. (26) Yang, Q. Y.; Xue, C. Y.; Zhong, C. L.; Chen, J. F. AIChE J. 2007, 53, 2832. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03, Revision D.01 ed.; Gaussian, Inc.: Wallingford, CT, 2004. (28) Hirotani, A.; Mizukami, K.; Miura, R.; Takaba, H.; Miya, T.; Fahmi, A.; Stirling, A.; Kubo, M.; Miyamoto, A. Appl. Surf. Sci. 1997, 120, 81. (29) Murthy, C. S.; Singer, K.; Klein, M. L.; McDonald, I. R. Mol. Phys. 1980, 41, 1387. (30) Frenkel, D.; Smit, B. Understanding Molecular Simulations: From Algorithms to Applications, 2nd ed.; Academic Press: San Diego, 2002. (31) Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; John Wiley: New York, 1964. (32) Babarao, R.; Dai, S.; Jiang, D. E. J. Phys. Chem. C 2011, 115, 8126. (33) Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2008, 130, 10870. (34) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 3875. (35) Babarao, R.; Jiang, J. W. Energy Environ. Sci 2008, 1, 139. (36) Krishna, R.; van Baten, J. M. J. Membr. Sci. 2010, 360, 323.

CO2 adsorption capacity and 5× increase in CO2/N2 selectivity in nitrogen-doped mesoporous carbon when compared to its nondoped counterpart at ambient condition. The simulated working capacity of the nitrogen-doped mesoporous carbon for CO2/N2 separation is among the highest reported. Our work indicates that nitrogen doping could be an important strategy to create mesoporous materials for high-capacity carbon capture. Our group is currently pursuing this idea experimentally.

5. CONCLUSIONS In this work, we investigated the effect of nitrogen doping on CO2 adsorption and selectivity in nondoped and nitrogendoped mesoporous carbon from molecular simulation. We found a tremendous increase in adsorption capacity of CO2 at ambient conditions in the case of nitrogen-doped mesoporous carbon (∼12 mol per kilogram of adsorbent at 1 bar and 298 K). This is due to the presence of enhanced electrostatic interactions between CO2 and the dopant of pyridinic nitrogen, in addition to the mesoporosity. A great working capacity with a reasonably high CO2/N2 selectivity is observed in nitrogendoped mesoporous carbon. This work suggests that introducing the N dopant to the mesoporous carbon is a promising approach for high-capacity and selective separation of CO2 from N2.

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ASSOCIATED CONTENT

S Supporting Information *

Atomic partial charges based on the fragmental cluster calculated using density functional theory, adsorption isotherm, and density contours. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract DE-AC02-05CH11231. R.B. thanks the World Future Foundation (WFF), Singapore, for giving him the award in Environmental and Sustainability Research (2011) for his Ph.D. thesis.

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REFERENCES

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