Carbon Dioxide Capture and Gas Separation on B80 Fullerene - The

Jan 9, 2014 - Weihua Wang , Xiaoxiao Zhang , Ping Li , Qiao Sun , Zhen Li , Cong Ren .... Guoping Gao , Fengxian Ma , Yalong Jiao , Qiao Sun , Yan Jia...
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Carbon Dioxide Capture and Gas Separation on B80 Fullerene Qiao Sun,*,† Meng Wang,†,‡ Zhen Li,*,∥ Aijun Du,§ and Debra J. Searles*,†,⊥ †

Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology and School of Chemistry and Molecular Biosciences, The University of Queensland, QLD 4072, Brisbane, Australia ‡ Center for Bioengineering and Biotechnology, China University of Petroleum (East China), Qingdao 266555, China ∥ Institute of Superconducting & Electronic Materials, The University of Wollongong, NSW 2500, Australia § School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia ⊥

S Supporting Information *

ABSTRACT: Exploring advanced materials for efficient capture and separation of CO2 is important for CO2 reduction and fuel purification. In this study, we have carried out first-principles density functional theory calculations to investigate CO2, N2, CH4, and H2 adsorption on the amphoteric regioselective B80 fullerene. Based on our calculations, we find that CO2 molecules form strong interactions with the basic sites of the B80 by Lewis acid−base interactions, while there are only weak bindings between the other three gases (N2, CH4, and H2) and the B80 adsorbent. The study also provides insight into the reaction mechanism of capture and separation of CO2 using the electron deficient B80 fullerene.

1. INTRODUCTION Carbon dioxide (CO2) is considered a predominate contributor to the greenhouse effect, which is believed to have a significant effect on global warming and a worsening climatic situation.1−3 It has been found that the CO2 concentration in the atmosphere has increased from 280 ppm at the beginning of the industrial age in ca. 1750 to 390 ppm in 2010.4 The combustion of fossil fuels is responsible for most of the greenhouse gas emissions due to humans (estimated at 86%5), and therefore capture of CO2 from emissions, and specifically the separation of N2 and CO2, is vital to reducing greenhouse gases in the atmosphere. An alternative way to reduce CO2 emissions is to use a cleaner fossil fuel. Natural gas contains methane and therefore has lower CO2 emission than other fossil fuels, and is becoming more and more commonly used. It can be obtained from various sources, and the natural gas extracted has various levels of impurities including CO2. The presence of CO2 is problematic, as it degrades the fuel and can lead to corrosion. Landfill is a source of natural gas that is cheap and readily available, but can have large amounts of CO2. Therefore separation of CO2 from methane is an important issue. Hydrogen is a clean fuel which does not produce any CO2 on combustion, and separation of CO2 from H2 is also of practical importance. In this paper, we will consider the utility of B80 as a material for capture of CO2. We will also consider its relative ability to adsorb N2, CH4, and H2, since the ability to separate CO2 from mixtures of these gases is necessary to address the problems outlined above.6,7 Separation of CO2 from other light gases is normally carried out using cryogenic distillation, separation using membranes, absorption using liquids (including amines © 2014 American Chemical Society

and ionic liquids), and adsorption on solids. The use of solid materials is attractive for CO2 capture and has recently attracted much attention.8 We propose that B80 might perform well as a material for CO2 capture and gas separation because it has a large surface area, strong adsorption sites, and high CO2 selectivity, and in this manuscript, we use a computational approach to demonstrate its ability to adsorb CO2 and separate it from H2, N2, and CH4. Boron is an element with unique structures, bonding, and physical properties.9−14 There is growing interest in exploring pure boron clusters and boron containing molecules because of their light weight, high strength, high hardness, high melting point, high chemical resistance, and conductivity.12,15,16 Boron (1s2 2s2 2p1) has one less valence electron than carbon (1s2 2s2 2p2), and therefore the covalent bonds between boron atoms differ from those between carbon atoms. Grimes notes that a wide variety of boron clusters exist, including boranes, polyhedral boranes, and their derivatives, and points out that occurrence of unusual or unexpected structures is important; as they challenge preconceived ideas of bonding.17 Boron clusters have been found to comprise triangulated polyhedra (deltahedra) joined by two-center, two-electron bonds or three-center, two-electron bonds. Among the boron clusters, the fullerenelike hollow cage B80 proposed by Szwacki et al.13,14 has drawn much attention. B80 fullerene consists of 60 frame atoms which are made up of 12 pentagons. The 12 pentagons are surrounded by 20 hexagons in which an extra boron atom sits at the center of each hexagon. The extra 20 atoms are Received: August 8, 2013 Revised: December 16, 2013 Published: January 9, 2014 2170

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energy and electron density. The direct inversion of the iterative subspace technique developed by Pulay was used with a subspace size 6 to speed up SCF convergence on these large systems. Geometry optimizations were performed with a convergence threshold of 0.002 a.u./Å on the gradient, 0.005 Å on displacements, and 10−5 a.u. on the energy. The real-space global cutoff radius was set to be 4.10 Å. The B80 fullerene was placed in a sufficiently large supercell (30 Å) to avoid interactions with its periodic images. The adsorption energy of CO2, CH4, and H2 on B80 are calculated from eq 1:

necessary to reinforce the unstable B60 cage and to satisfy the Aufbau principle for boron cluster construction.18 Reports based on density functional theory calculations indicate that boron fullerenes can be constructed from boron sheets19−23 and the hollow B80 fullerene has the highest deformation temperature among boron clusters ranging from B10 to B100.13,24 The hollow B80 fullerene is believed to be the most stable configuration among the B80 clusters13,14,22,25,26 and it is generally accepted that the stability of B80 fullerene is again due to its unusual structure, in which the six B30 double-rings are interwoven to form a round hollow cage.24 Interestingly, some double-rings have been synthesized experimentally.27 Since theoretical calculations predict that B80 fullerene possesses a large cohesive energy around 5.76 eV/atom, high deformation temperature, and high stability,13,21,24,25,28 it is expected to be able to produce experimentally. However this has not been achieved, and recently it has been shown that the stability of structures with sp3 bonds may explain why it has been difficult to synthesize B80.29 A route to its synthesis that avoids formation of these structures has therefore been proposed.29 In anticipation that it will be able to be synthesized, comprehensive computational studies have been carried out to predicate various properties of B80 fullerene, which include its electronic structure,30 stability,13,24 reactivity,31,32 optical and magnetic properties,23,33 and electrical conductivity.34 In addition, research into the design of B80-based nanomaterials is being carried out, especially for use as hydrogen storage materials.13,25,31,33,35−41 These studies indicate that there is an exciting future for boron chemistry. Recently, Muya et al. showed that the boron fullerene B80 (Th symmetry) is a hard amphoteric molecule.31 That is, B80 is an amphoteric molecule containing acidic and basic sites, with the cap atoms being hard acids and the frame atoms hard bases. Moreover, they investigated the mechanisms of the interactions between B80 and NH3, PH3, AsH3, P4N4, due to the amphoteric regioselectivity of B80.31,42 It is well-known that CO2 is a Lewis acid and it prefers to accept electrons during reactions. Due to the electron deficiency of boron materials, such as B80, they might initially be presumed to be poor adsorbents of CO2. However, the amphoteric regioselectivity of B80 might play an important role in making it a suitable material for CO2 capture if the CO2 molecule can be adsorbed on the frame atoms of basic sites of B80 due to Lewis acid−base interactions. In this study, we will reveal the reaction mechanism and how the electron deficient B80 fullerene can effectively capture CO2 (a Lewis acid) and separate the mixture gases (CO2/N2, CO2/ CH4, CO2/H2).

Eads = E B80_gas − (E B80 + Egas)

(1)

where EB80_gas is the total energy of B80 with adsorbed gas, EB80 is the energy of the isolated B80, and Egas is the energy of the isolated gas molecules, CO2, N2, CH4, and H2. Atoms in molecules (AIM) theory has been used to better understand the adsorption and the nature of the interaction of the four gases on B80. Based on the optimized structures at the DFT-D level, we have calculated the wave functions at the B3LYP/6-31+G(d) level of theory,45 then the AIM theory is used. This approach has been used to successfully determine intermolecular interactions for a range of systems.47,51,52 In the AIM analyses,53 existence of an interaction is indicated by the presence of a bond critical point (BCP) and the strength of the bond is estimated from the magnitude of the electron density (ρbcp) at the BCP. Similarly, ring or cage structures are characterized by the existence of a ring critical point (RCP) or cage critical point (CCP). The topological analysis of the system of four gas adsorption on B80 has been carried out via the AIMALL program.53 We have used the complete LST (linear synchronous transit)/QST (quadratic synchronous transit) method54 implemented in the DMol3 code to find the transition states between chemisorbed and physisorbed configurations of CO2 on B80. The electron distribution and transfer mechanism are determined using the Mulliken method.55

3. RESULTS AND DISCUSSION 3.1. CO2 Adsorption on B80. In order to understand the mechanism of CO2 (Lewis acid) capture on the electron deficient boron material, B80 (Th symmetry), we have first analyzed the structural properties and the Mulliken atomic charge distributions of B80. Three types of boron atoms are present: the endohedral (endo) and exohedral (exo) caps in the center of the hexagons at a radial distance from the center which is smaller and larger, respectively, than the average radius of the cage (labeled ‘1’ (exocap) and ‘8’ (endocap) on the isolated B80 in Figure 1); and the frame atoms located on the truncated icosahedral frame (labeled 2−7 and 9−14 in Figure 1). We start with the discussion of the regioselectivity of the B80. The Mulliken atomic charge distributions of B80 are listed in Table 1. From Figure 1 and Table 1 we can see that the Mulliken atomic charges of the exocap (0.392 e) and endocap (0.429 e) boron atoms in B80 are positive and the frame boron atoms have negative charges (in the range of −0.1 e to −0.16 e). This means that there is electron transfer from the cap to the frame boron atoms and the calculated results show that there are ∼8 electrons transferred from the cap atoms to the frame atoms in one B80 molecule. The computational results are consistent with the report by Muya et al.31 that B80 is an amphoteric molecule containing acidic and basic sites, and the

2. COMPUTATIONAL METHODS Calculations using density-functional theory with dispersion (DFT-D)43,44 have been performed using the DMol3 module in Materials Studio. The boron B80 fullerene and the complexes formed by B80 and the gases (CO2, N2, CH4, and H2) were fully optimized using a generalized gradient approximation45 treated with the Perdew-Burke-Ernzerhof exchange-correlation potential and with long range dispersion correction made using the Grimme’s scheme.46 An all-electron double-numeric atomic orbital basis set augmented by d-polarization functions (DNP) was used. This level of theory has been used to successfully determine the geometrical, energetic, and electronic structural properties of boron cluster, boron phase, and boron containing materials.13,22,47−50 The self-consistent field (SCF) procedure was used with a convergence threshold of 10−6 a.u. on the 2171

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energies than the other four due to the charge distributions, and calculations confirmed this. The data for these two configurations is included in the Supporting Information for completeness (see Figure S1 and Table S1). The configurations of types 1−4 when the CO2 is chemisorbed are shown in Figure 1 and the calculated geometrical parameters and adsorption energies of CO2 adsorbed on B80 are summarized in Table 2. Table 2. Adsorption Energy (Ead) in kcal/mol, Bond Distance (r) in Å, and Bond Angle (α) in deg of CO2 Adsorbed on B80 at Sites of Type 1, 2, 3, and 4, and Charge Transfer (CT) from B80 to CO2 of These Configurations adsorption sites Type 1

Figure 1. The first B80 cluster shows the boron atom labels used in this paper. All other clusters show the optimized chemisorbed configurations of CO2 captured on B80. Atom color code: pink, boron; gray, carbon; red, oxygen.

Type 2

Table 1. Mulliken Atomic Charges of Exo-Cap, Endo-Cap, and Frame Atoms of B80 as Shown in Figure 1 atom label

charge/e

atom label

charge/e

B1_exo-cap B2_frame B3_frame B4_frame B5_frame B6_frame B7_frame

0.392 −0.127 −0.128 −0.118 −0.121 −0.122 −0.120

B8_endo-cap B9_frame B10_frame B11_frame B12_frame B13_frame B14_frame

0.429 −0.160 −0.145 −0.151 −0.157 −0.128 −0.140

Type 3

Type 4

cap atoms are acids and the frame atoms are bases. We propose that CO2 molecules (Lewis acid) should therefore have strong interactions with the basic sites (frame atoms) of B80 through Lewis acid−base interactions. In the following section, we will discuss the possible configurations of CO2 adsorbed on B80. We have found six types of configuration where CO2 is chemisorbed on different sites of B80, and label them type 1−6. The adsorption sites can be characterized as follows: types 1 and 2 involve interaction of the carbon atom and one oxygen atom of the CO2 molecule with two frame atoms of B80, with the two frame boron atoms being from the 6-membered ring and 6-membered ring (type 1) and 5-membered ring and 6-membered ring (type 2); type 3 involves interaction of the carbon atom and one oxygen atom of the CO2 molecule with one frame atom and one exo-cap atom of B80, respectively; type 4 involves interaction of the carbon atom and one oxygen atom of CO2 molecule with one frame atom and one endo-cap atom of B80, respectively; type 5 involves interaction of the carbon atom and one oxygen atom of CO2 molecule with one exo-cap atom and one frame atom of B80, respectively; and type 6 involves interaction of the carbon atom and one oxygen atom of CO2 molecule with one frame atom and one endo-cap atom of B80, respectively. Two of these (types 5 and 6) were expected to have lower interaction

Ead r(B7−O1) r(B1−C) r(C−O1) r(C−O2) r(B7−B2) α(O−C−O) CT Ead r(B2−O1) r(B2−C) r(C−O1) r(C−O2) r(B1−B2) α(O−C−O) CT Ead r(B1−O1) r(B2−C) r(C−O1) r(C−O2) r(B1−B2) α(O−C−O) CT Ead r(B9−O1) r(B8−C) r(C−O1) r(C−O2) r(B9−B8) α(O−C−O) CT

physisorption

transition state

chemisorption

−6.45 3.273 3.318 1.177 1.177 1.686 179.9 0.001 −6.47 3.273 3.227 1.776 1.776 1.717 179.5 0.0 −6.49 3.427 3.124 1.176 1.177 1.711 179.3 0.001 −7.40 3.373 3.276 1.176 1.176 1.711 179.1 −0.004

13.97 2.066 2.252 1.233 1.190 1.739 150.0 −0.123 12.95 2.059 2.147 1.228 1.191 1.783 148.6 −0.150 10.72 2.039 2.226 1.233 1.188 1.823 151.8 −0.102 9.15 1.901 2.371 1.231 1.179 1.823 155.5 −0.030

−18.21 1.494 1.650 1.382 1.206 1.879 124.7 −0.425 −17.49 1.468 1.622 1.384 1.207 2.058 124.0 −0.417 −19.13 1.439 1.637 1.424 1.201 2.060 122.4 −0.410 −17.19 1.443 1.640 1.420 1.203 2.012 122.6 −0.410

All six types of configuration result in structures of similar geometry, with the carbon and one oxygen atom of CO2 interacting with two boron atoms of B80. All chemisorbed species had a corresponding stable physisorbed configuration with adsorption energies in the range of 4.57−7.40 kcal/mol, and the physisorbed CO2 molecule had little structural change compared with the geometry of gas phase CO2. In these six physisorbed configurations, the linear CO2 molecules are attached to B80 parallel to the B−B bonds. The O−C−O angles are almost 180° and the B−C or B−O distance is in the range of 3.1−3.4 Å, which indicates the interactions in the physisorbed configuration are mainly due to the van der Waals interaction between CO2 and B80. Due to the weak interaction, the charge transfer between the B80 and CO2 molecule is negligible with the values in the range of 0.004 e to −0.004 e. 2172

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type_3_chem with an adsorption energy of 19.13 kcal/mol is the most favorable one. The adsorption of more than one CO2 molecule on the B80 fullerene through a type 3 configuration has also been investigated. The calculated structural properties and average adsorption energy (kcal/mol) of two to ten CO2 molecules adsorbed on a B80 have been included in Figure 3 and Figure S2

Of the six physisorbed configurations of CO2 on B80, type 4, in which the C atom (0.571 e) of CO2 interacts with the frame boron atom (−0.126 e) and the O atom (−0.288 e) of CO2 interacts with the endo-cap boron atom (0.435 e) of B80 is the most stable, with an adsorption energy of 7.40 kcal/mol. The optimized chemisorbed configurations and the potential energy diagram for CO2 adsorption on B80 in the four types of configuration are shown in Figures 1 and 2. The structural

Figure 3. Absolute value of average adsorption energies (kcal/mol) with the number of CO2 molecules adsorbed on a B80 with a type 3 configuration.

Figure 2. Potential energy diagram for CO2 adsorption on B80 with the four types of configuration.

in Supporting Information. The average adsorption energies of CO2 molecule adsorption on B80 are defined as

properties of the four chemisorbed configurations and their transition states are listed in Table 2. Figure 1 shows that the four configurations where CO2 is chemisorbed on B80 have similar geometric structures. From Table 2 we can see that the adsorption energies of the four chemisorbed configurations are in the range of 17.19−19.13 kcal/mol. In the following part, we will mainly discuss the adsorption mechanism of CO2 on B80 in configurations of type 1, 2, 3, and 4, because they involve the stronger interactions between CO2 and B80. The adsorption energies of configurations of type 1, 2, 3, and 4 are calculated to be 18.21, 17.49, 19.13, and 17.19 kcal/mol (75.03, 72.06, 78.82, and 70.82 kJ/mol), respectively. Ideally, the adsorption energy of CO2 on a high-performance adsorbent should be in the range of 40−80 kJ/mol.56 According to this criterion, B80 renders a good sorbent for CO2 capture. In the four chemisorbed configurations, the molecules undergo structural distortion to bent geometries and a CO double bond of the CO2 molecule becomes a single bond. The O−C−O bond angles are 124.7°, 124.0°, 122.4°, and 122.6°, and the C−O bonds are significantly elongated to 1.382, 1.384, 1.424, and 1.420 Å on top of the B atoms, for type 1, 2, 3, and 4, configurations, respectively. The B−B sites are also pulled away from the B80 and the bonds are elongated by around 0.20−0.3 Å. The B−C and B−O distances are 1.650, 1.622, 1.637, 1.640 Å and 1.494, 1.468, 1.439, 1.443 Å for type 1, 2, 3, and 4 configurations, respectively. A Mulliken charge population analysis shows there are charge transfers of −0.425 e, −0.417 e, −0.410 e, and −0.410 e, from B80 to the CO2 molecules for the configurations of type_1_chem, type_2_chem, type_3_chem, and type_4_chem, respectively. This also provides evidence of the strong interactions between them. Moreover, we have also performed LST/QST calculations to identify the transition states from the physisorbed to chemisorbed configurations for each type. For the six chemisorbed configurations, the

Eads = (E(B80_nCO2) − nECO2 − E(B80))/n

(2)

where n is the number of CO2 molecules adsorbed on B80 and EB80−nCO2 is the total energy of B80 with the adsorbed gases. We find that the average adsorption energies of CO2 adsorption on B80 decrease from 19.21 to 8.91 kcal/mol (79.15 to 36.71 kJ/mol) with the increase of CO2 molecules from 1 to 10. The adsorption energy of CO2 on a highperformance adsorbent should be approximately in the range of 40−80 kJ/mol;56 therefore, our calculations indicate that one B80 can capture up to 10 CO2 molecules effectively. We have also plotted the frontier orbitals, the highest occupied molecular orbitals (HOMO), and lowest unoccupied molecular orbitals (LUMO) of CO2, B80, and the configuration of one CO2 chemisorbed on B80 with interaction of type 3. Figure 4 of the LUMO of the chemisorbed configuration indicates that there is effective orbital overlap between B80 and CO2 through the B−B local part of the LUMO of B80 and the HOMO of the CO2. The effective orbital overlap between B80 and CO2 supports the strong interaction in this configuration. The electron densities of the physisorbed configuration (Figure 7a), transition state (Figure 7b), and chemisorbed configuration (Figure 7c) at the BCPs for type 3 configurations have also been listed in Figure 7 and Table S2 in order to verify the reaction procedes from physisorbed to chemisorbed configurations. Figure 7a indicates that the interaction between CO2 and B80 of type_3_phy can be confirmed by the existence of the bond critical point (BCP) of the C−B contact. Obviously, the electron densities of the physisorbed configuration at the BCPs of the bond between CO2 and B80 are small (Table S2), which also indicates that interactions are weak. For the configurations of the transition state (type_3_TS) and chemisorbed configuration (type_3_chem), the presence of the so-called 2173

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Figure 5. Variation of thermodynamic properties with temperature (K) for CO2 capture on B80 to form chemisorbed configurations of different types. Squares, triangles, and circles correspond to the change in Gibbs free energy (kcal/mol), change in entropy (cal/(mol K)), and change in enthalpy (kcal/mol), respectively.

Figure 4. HOMO (a) and LUMO (b) of CO2; HOMO (c) and LUMO (d) of B80; HOMO (e) and LUMO (f) of chemisorbed CO2 on B80 in a type 3 configuration. These orbitals are drawn with an isosurface value of 0.03 e/Å3. Atom color code: pink, boron; gray, carbon; red, oxygen.

control CO2 adsorption and release on B80 at temperatures close to 300 K. We have also investigated the other two types of site (5 and 6 in Figure S1 in Supporting Information) where CO2 might adsorb on B80, and we found that their configurations are very similar to the 1, 2, 3, and 4 configurations. However, the adsorption energies of the two chemisorbed configurations, 7.64 and 9.00 kcal/mol for type 5 and 6 sites, respectively, are much weaker than those of the type 1, 2, 3, and 4 sites due to the repulsive effect of the electronic charge between the two boron atoms of B80 and the carbon and oxygen atom of CO2. The barriers between the physisorbed and chemisorbed configurations for the type 5 and 6 sites have values of 30.13 and 30.45 kcal/mol, respectively, which are much higher than the barriers between physisorbed to chemisorbed configurations for types 1, 2, 3, and 9. The results indicate that CO2 adsorption on B80 for these two types of site is not kinetically favorable. Moreover, the change in Gibbs free energy (kcal/ mol), change in entropy (cal/(mol K)), and change in enthalpy with temperature (K) have been calculated (Figure S3 in Supporting Information) for the reactions of CO2 adsorption on B80 to form chemisorbed configurations with the type 5 and 6 sites. Figure S3 shows that ΔG of the reactions for CO2 adsorption to form chemisorbed configurations with the two types are positive over the temperature range considered, which mean that reactions via the two types are not spontaneous. The above computational results indicate that CO2 adsorption on B80 with the type 5 and 6 sites are neither kinetically nor thermodynamically favorable. Overall, our study demonstrates that CO2 adsorption on B80 on type 1, 2, 3, and 4 configurations are thermodynamically and kinetically favorable, so B80 could be a useful material for CO2 capture. 3.2. N2, CH4, and H2 Adsorption on B80. In order to demonstrate the high selectivity of B80 for CO2 capture, the adsorption energies of N2, CH4, and H2 on B80 are calculated and compared with those of CO2. For N2, CH4, and H2 adsorption on the B80 fullerene, we have considered three types of binding site: type 1, type 3, and type 4, in which N2, CH4, and H2 interact with the frame, exo-cap, and endo-cap atoms of B80, respectively. The optimized configurations of

bond critical points (BCP) in the AIM analysis indicates the formation of B1−O1 and B2−C and four atoms rings, B1− O1−C−B2, are characterized by the existence of ring critical points (RCP). As shown in Table S2, for the type 3 configuration, the electron densities at the BCPs for the C− B2 bonds of physisorption, transition state, and chemisorption increase gradually, which is consistent with the adsorption process transforming from a weak to a strong interaction. The C−B bond distances decrease with values of 3.124 Å, 2.226 Å, and 1.637 Å and the O1−B bond distances decrease with the values of 3.427 Å, 2.039 Å, and 1.439 Å, for the physisorbed, transition, and chemisorbed states, respectively. For type 1, 2, 3, and 4 configurations, the computational results show that the chemisorption processes have barriers of 20.42, 19.42, 17.21, and 16.55 kcal/mol between the physisorbed configurations at sites of type 1, 2, 3, and 4, respectively. The one imaginary frequency for the transition states of the reactions from physisorbed to chemisorbed configurations at sites of type 1, 2, 3, and 4 are 572.1i, 490.1i, 466.8i, and 465.8i cm−1, respectively. In all cases, the strong interaction of the chemisorbed species and low barriers indicate that CO2 adsorption on type 1, 2, 3, and 4 configurations is favorable. In addition, the variations with temperature of thermodynamic properties for the adsorption of an isolated CO2 molecule onto an isolated B80 molecule have been calculated. Figure 5 gives the changes in Gibbs free energy (ΔG, kcal/ mol), enthalpy (ΔH, kcal/mol), and entropy (ΔS, cal/(mol K)) in order to show the entropic and temperature effects on CO2 adsorption for configruations of type 1, 2, 3, and 4. It is found that, for the type 1 configuration, ΔS is almost constant when the temperatures are above 200 K. For the type 2, 3, and 4 sites, ΔS linearly increases over the temperature range considered. For the four types of configuration, both ΔH and ΔG monotonically increase over the temperature range considered. Moreover, ΔG is close to zero for all four types of configuration at temperatures close to 300 K. These results indicate that the temperature and pressure could be tuned to 2174

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CH4, N2, and H2 adsorption on the endo-cap atom of B80 are listed in Figure 6. Tables 3, 4, and Figure 8 display important

parameters associated with these systems, such as bond distances, adsorption energies, and charge transfers from B80 to the gases for the three types of binding site. The calculated results indicate that the adsorptions of N2, CH4, and H2 on B80 with all types considered are physical rather than chemical. We can see that the N···B distances for N2 adsorbed on B80 are between 3.1−3.2 Å, the C···B distances for CH4 adsorption are in the range of 3.5−3.6 Å, and H···B distances for H2 absorption are in a range of 2.7−3.0 Å. The charge transfer between B80 and the three gases with the three types are in the range of 0.008 e to −0.019e. The adsorption energies of the three types are 1.99−3.13, 2.82−3.36, and 1.40−1.98 kcal/mol for N2, CH4, and H2 adsorption on B80, respectively, which are consistent with the small values of electron density (Table S2) at the BCPs of the bonds between N2, CH4, H2, and B80. In contrast, the absorptions of CO2 on B80 are much stronger with the adsorption energies in the range of 17.19−19.13 kcal/mol for the three types, which are also supported by the large electron densities at the BCPs for the O1−B and C−B bonds (Table S2) of the configuration of CO2 adsorption on B80 in Figure 7c (type_3_chem). Figure S4 in the Supporting

Figure 6. Configurations of CH4, N2, and H2 adsorbed on the endocap atom of B80. Atom color code: pink, boron; gray, carbon; blue, nitrogen; light gray, hydrogen.

Table 3. Adsorption Energy in kcal/mol, Bond Distance (r) in Å, and Bond Angle (α) in deg of N2, CH4, and H2 Adsorbed on B80 at Sites of Type 1, 3, and 4 N2

CH4

H2

adsorption energy r(B···N1) r(N1−N2) α(N2−N1−B) adsorption energy r(B···C) r(B···H) adsorption energy r(B···H1) r(H1−H2) α(H2−H1−B)

type 1

type 3

type 4

−1.99 3.233 1.109 174.1 −2.82 3.547 2.783 −1.40 2.966 0.754 168.6

−2.80 3.092 1.108 179.4 −3.33 3.595 2.817 −1.45 2.916 0.752 178.3

−3.13 3.246 1.109 174.4 −3.36 3.566 2.837 −1.98 2.811 0.755 177.6

Table 4. Mulliken Charge Distribution of the Configurations of CO2, N2, CH4, and H2 Adsorbed on B80 and Charge Transfer (CT) from B80 to the Gases of These Configurations gases CO2

physisorption

chemisorption

N2

physisorption

CH4

physisorption

H2

physisorption

atoms

type 1

type 3

type 4

C O1 O2 CT C O1 O2 CT N1 N2 CT C H1 H2 H3 H4 CT H1 H2 CT

0.578 −0.287 −0.287 0.004 0.331 −0.414 −0.342 −0.425 −0.023 0.031 0.008 −0.876 0.230 0.212 0.210 0.233 0.009 0.005 −0.013 −0.008

0.573 −0.287 −0.285 0.001 0.331 −0.411 −0.330 −0.410 −0.027 0.027 0.0 −0.882 0.235 0.213 0.214 0.221 0.001 0.014 −0.022 −0.008

0.571 −0.288 −0.287 −0.004 0.331 −0.409 −0.332 −0.410 −0.035 0.022 −0.013 −0.894 0.236 0.216 0.211 0.229 −0.002 0.010 −0.029 −0.019

Figure 7. Molecular graphs of the intermediates and transition states of CO2, CH4, N2, and H2 for adsorption on B80, where the bond critical points (BCPs), ring critical points (RCPs), and cage critical point (CCP) are denoted as small green, red, and blue dots, respectively.

Information lists the HOMO and LUMO of the configurations of CH4, N2, and H2 adsorbed on the endo-cap atom of B80. From Figure S4 we cannot see any effective orbital overlap between the three gases and B80 of the frontier orbitals, while there is effective overlap between CO2 and B80 (Figure 4), which also supports the proposal that CO2 can strongly interact with B80 and the three gases can only interact with B80 weakly. From Figure 8 we can clearly see that there are large differences in the adsorption energies of CO2 and N2, CH4, H2, on B80 with the three types. In summary, the above comparisons demonstrate that B80 has very high selectivity for capturing CO2 from N2, CH4, and H2 mixtures, and B80 can serve as a 2175

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appreciates financial support of the Australian Research Council QEII Fellowship. We also acknowledge generous grants of high performance computer time from both The University of Queensland and the National Computational Infrastructure (NCI).

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Figure 8. Adsorption energies (kcal/mol) for CO2, N2, CH4, and H2 adsorption on frame, exo-cap, and endo-cap atoms of B80.

good adsorbent for separation of these gases. Here we need to point out that B80 has not been synthesized yet; however, several theoretical calculations indicate that B80 is very stable and it could be synthesized in principle.13,21,25,28 The purpose of this study is to stimulate research to perform experimental studies of B80, and to further verify our theoretical predication of CO2 capture and the potential for gas separation using this boron material.

4. CONCLUSIONS In the present study, we have investigated the adsorption of CO2, N2, CH4, and H2 on B80 fullerene. Our calculations indicate that the electron deficient B80 can efficiently capture CO2 (Lewis acid) on its basic sites because the B80 molecule is amphoteric. The computational results indicate that CO2 can form strong interactions with B80, and the formation of chemisorbed configurations is kinetically and thermodynamically favorable; however, N2, CH4, and H2 can only form weak interactions with B80. Our results indicate that B80 can act as a sorbent for CO2 and it can be used to separate CO2 from gas mixtures, in processes such as postcombustion capture (CO2/ N2), natural gas sweetening (CO2/CH4), and precombustion (CO2/H2) capture, and this should stimulate some further experiments to verify our prediction.

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ASSOCIATED CONTENT

* Supporting Information S

Additional configurations, HOMO and LUMO, variation of thermodynamic properties, and topological parameters. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.S. acknowledges financial support from Early Career Researcher grant of The University of Queensland. A.D. greatly 2176

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