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Porous Carbon Materials Based on Graphdiyne Basis Units by the Incorporation of the Functional Groups and Li Atoms for Superior CO2 Capture and Sequestration Yong Dang, Wenyue Guo,* Lianming Zhao, and Houyu Zhu College of Science, China University of Petroleum, Qingdao, Shandong 266580, PR China S Supporting Information *

ABSTRACT: The graphdiyne family has attracted a high degree of concern because of its intriguing and promising properties. However, graphdiyne materials reported to date represent only a tiny fraction of the possible combinations. In this work, we demonstrate a computational approach to generate a series of conceivable graphdiyne-based frameworks (GDY-Rs and Li@GDY-Rs) by introducing a variety of functional groups (R = −NH2, −OH, −COOH, and −F) and doping metal (Li) in the molecular building blocks of graphdiyne without restriction of experimental conditions and rapidly screen the best candidates for the application of CO2 capture and sequestration (CCS). The pore topology and morphology and CO2 adsorption and separation properties of these frameworks are systematically investigated by combining density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations. On the basis of our computer simulations, combining Lidoping and hydroxyl groups strategies offer an unexpected synergistic effect for efficient CO2 capture with an extremely CO2 uptake of 4.83 mmol/g at 298 K and 1 bar. Combined with its superior selectivity (13 at 298 K and 1 bar) for CO2 over CH4, Li@GDY-OH is verified to be one of the most promising materials for CO2 capture and separation. KEYWORDS: graphdiyne, porous carbon materials, CO2 adsorption, functional groups, Li doping basicity of carbon, which is beneficial for CO2 adsorption.16 Sumida et al. suggested that strongly polarizing groups (hydroxy, nitro, cyano, thio, and halide groups) influence the CO2 adsorption favorably.17 Indeed, a variety of functional groups, e.g., −NO2, −NH2, −OH, −COOH, and halide, has been introduced to porous materials to improve the selectivity for CCS.17−20 Furthermore, incorporation of metal atoms (Li, Na, and K, etc.) which can significantly enhance the gas adsorption capacity is one of the most promising strategies to improve gas storage performance of materials. Recently, Lan et al. studied the effect of metal dopants on CO2 capture by doping a series of metals (Li, Na, K, Be, Mg, Ca, Sc, and Ti) into COFs and found that among all the studied metals Li is the best surface modifier for CO2 capture.21 In fact, Li doping has been widely used in porous materials, such as carbon-based materials, MOFs, and COFs, to enhance the gas adsorption capacities.18,22−26 As a quintessential material in the porous carbon materials family, graphdiyne has attracted a great deal of attention from many structural, theoretical, and synthetic scientists due to its unique structure and intriguing characteristics.27−32 Graphdiyne

1. INTRODUCTION The rapid increase of CO2 concentration in the atmosphere produced by combustion of fossil fuels is playing a critical role in a series of environmental issues, such as global warming and sea level rising.1 Carbon dioxide (CO 2 ) capture and sequestration (CCS) have aroused a great deal of attention and interest in technologies to mitigate the CO2 emission. As an alternative to the liquid-phase absorption process, a promising strategy for CCS is the use of solid adsorbents, e.g., porous carbon materials,2−6 zeolites,7,8 mesoporous silicas,9 metal−organic-frameworks (MOFs),10,11 porous organic polymers (POPs),12,13 and covalent-organic frameworks (COFs).14,15 Among them, porous carbon materials are one of the most versatile candidates because of the specific properties of higher surface area, more excellent thermal and chemical stability, and less humidity sensitivity. However, the generally weak adsorption potential, which often leads to a low adsorption capacity with poor selectivity and large temperature sensitivity, makes carbon and other porous materials trap CO2 inefficiently. In order to remedy the limitation, great efforts have been concentrated on strengthening the adsorbate− adsorbent interactions via improving the internal surface area, regulating the pore size, and functionalizing the adsorbing surfaces. Bai et al. reported that nitrogen-containing functionalities are responsible for increasing the surface polarity and © 2017 American Chemical Society

Received: July 23, 2017 Accepted: August 15, 2017 Published: August 15, 2017 30002

DOI: 10.1021/acsami.7b10836 ACS Appl. Mater. Interfaces 2017, 9, 30002−30013

Research Article

ACS Applied Materials & Interfaces

morphology, (iii) verify the impact of intrinsic properties (especially alkyne groups) of graphdiyne on CO2 adsorption, (iv) elucidate the influence mechanism of functional groups on CO2 adsorption properties, and (v) assess the effect of metaldoping strategy on CO2 capture. This work is conducted to illuminate the intrinsic relationships between the functional groups/Li atom/alkyne groups and performance for CO2 capture and to provide an effective and superior alternative approach in the design and screening of porous materials for CO2 capture and separation. Importantly, the obtained GDY modified with hydroxyl group and Li (Li@GDY-OH) is found to exhibit remarkably high CO2 uptake at room temperature and ambient pressure, ranking among the highest reported CO2 uptakes in carbon and other porous materials, which means that Li@GDY-OH may be an excellent candidate of the next generation of CO2 absorbents.

contains sp hybridized acetylenic linkages between neighboring benzoic rings, the triple carbon−carbon bond is a rather useful connecting unit because of the structural linearity that does not suffer from fluctuation arising from cis−trans isomerization.28 The flat (sp2- and sp-hybridized) carbon networks endow the graphdiyne family with excellent mechanical flexibility, uniformly distributed pores, and extremely high specific surface area.27,29 Furthermore, as a result of the specific structure of 6fold coordinated benzene rings by alkyne ligands, the pore size of the graphdiyne network is greatly enlarged compared to that of graphene, which facilitates the transportation of gas molecules. Interestingly, Hupp et al. suggested that the triple bond spacers are more efficient than phenyls for boosting the molecule-accessible surface area of MOFs.33 Cao et al. reported that a porous diamond-like carbon framework with − CC− units shows extremely high gas adsorption.34 Liu et al. demonstrated that graphdiyne shows high adsorption capacity for H2S and can act as a delightful separation material.35 However, the research on the CO2/CH4 adsorption and separation in graphdiyne materials, which is beneficial for design of new graphdiyne-based frameworks as the CCS materials, is seriously scarce. Computational simulation can provide valuable insights to guide material design and can be used to screen candidates and directly identify desirable candidates for synthesis.36 Therefore, it is regarded as a valuable tool to accelerate the discovery of materials for various applications including gas storage. Herein, we designed a series of functional groups (−F, −OH, −NH2, and −COOH) and metal Li-modified graphdiyne-based frameworks (denoted as GDY-R and Li@GDY-R, respectively) with a wide range of pore surface polarity and basicity to investigate their influence on the gas adsorption capacity (see Figure 1). All structures are meticulously investigated by using a multiscale simulation method, which combines density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulation to (i) screen promising adsorbent materials for CO2 capture and storage, (ii) clarify the effect of functional groups on characteristics of pore topology and

2. COMPUTATIONAL METHODS 2.1. Structure Construction. In this work, the basis unit structure was intercepted from a sheet of graphdiyne,37 as shown in Figure 1. This model containing seven primitive cells of graphdiyne captures the essential features of graphdiyne structure, e.g., flat plane carbons, benzoic rings, triangular pores, acetylenic linkages between neighboring benzoic rings, and three types of C−C bonds: C(sp)−C(sp) in the triple bonds, C(sp2)−C(sp2) in the aromatic rings, and C(sp3)− C(sp3) in the single bonds. Similar fragments of graphene (coronene, hexabenzocoronene, and circumcircumcoronene) have been manufactured and widely used for the study of CO2/CH4 adsorption.38,39 The basis unit structure was subsequently hydrogenated to form a crude three-dimensional framework (GDY-H) or functionalized by multiple functional groups (i.e., −F, −OH, −NH2, and −COOH) to constitute the modified frameworks (GDY-Rs). In practice, it is not easy to put the functional groups except carboxyl on only one side of the alkynyl groups experimentally, especially, −F and −NH2 groups. Efforts have been concentrated on the process of direct introduction of functional groups on alkynes.40,41 Importantly, Ma and co-workers have made substantial contributions to this field and developed a reliable procedure for the fluorination reaction of alkynes with electrophilic fluorinating regent N-fluorobenzenesulfonimide (NFSI) in the presence of n-butyllithium, realizing the substitution of H atoms in a series of aromatic-substituted acetylenes with −F groups.42 We proposed that the GDY-F basis unit may in principle be produced in a similar way by replacing the H atoms in GDY-H basis unit with F atoms. Even if this indiscriminately imitated method is not exactly appropriate to our model, then at least their scheme indicates an approaching direction. However, the primary purpose of the present study is to clarify the effect of various substituted functional groups with different polarity and acid−base characteristics in terminal alkynes on CO2 adsorption properties, which has not been studied thoroughly due to experimental constraints. Artfully, the computational strategy is able to drastically reduce the synthetic efforts by directly suggesting the best materials for CO2 capture and sequestration after screening candidates based on the attempts of various functional groups substitution; further experimental attention just focuses on synthesis of the most promising structure(s), neglecting other structures. All the initial structures of these hydrogenated and functionalized basis units were optimized through a computationally assisted procedure on the basis of density functional theory (DFT) geometry optimization (see Figure S1). The Perdew−Burke−Ernzerhof (PBE) functional under the generalized gradient approximation (GGA) for exchange-correlation functional was used to do the spin-unrestricted all-electron DFT calculations. The double numerical including polarization (DNP) basis set was used for all atoms. The GDY-R frameworks were constructed on the basis of the optimized structures (see Figure S2). Since the precise experimental information on the mass density of GDY-Rs is very limited, we used the density (0.542 g/ cm3) of a similar random carbon structures constructed from a collective of unmodified or OH-modified coronene-shaped graphitic

Figure 1. Functionalized graphdiyne basis unit structures and frameworks. 30003

DOI: 10.1021/acsami.7b10836 ACS Appl. Mater. Interfaces 2017, 9, 30002−30013

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ACS Applied Materials & Interfaces Table 1. Physical Characteristics of the Frameworks framework

GDY-Rs

Li-doped

R

−H

−F

−OH

−NH2

−COOH

−OH

−NH2

no. of basis units atoms in the unit cell dimensions (Å3) Vtot (cm3/g) Vup(cm3/g) porosity, Φc (%) yup (%) surface area (m2/g) DM (Å)

20 2880 45.44 1.20 0.12 65.19 10.14 2675 14.71

20 2880 48.44 1.28 0.11 69.38 8.78 2885 18.21

20 3240 48.12 1.25 0.10 67.85 8.07 2773 15.59

20 3600 47.97 1.25 0.27 70.49 21.91 2890 12.55

20 3960 52.21 1.28 0.14 69.14 10.67 3139 16.16

20 3240 48.23 1.34 0.14 72.51 10.38 3109 14.38

20 3560 48.09 1.27 0.17 68.88 13.00 3024 13.00

Table 2. Lennard−Jones Parameters and Atomic Partial Charges for Adsorbates and Adsorbents DREIDING force field

gas molecule models atom

C(CH4)

H(CH4)

C(CO2)

O(CO2)

C

H

O

N

F

σ (Å) ε (K) q (e)

3.40 55.05 −0.612

2.65 7.90 0.153

2.80 27.00 0.748

3.05 79.00 −0.374

3.473 47.856

2.846 7.649

3.033 48.158

3.263 39.007

3.093 36.483

basis units for all GDY-Rs.38,39 The physical properties of the GDY-Rs models are shown in Table 1. 2.2. Simulation Details. Grand canonical Monte Carlo (GCMC) simulations were performed to calculate the adsorption isotherms of pure component of CO2 and CO2/CH4 mixtures at 298 K using the MUSIC code.43 Periodic boundary conditions were employed in all simulations which involved a total of 2 × 107 steps. The first 40% of the total steps were used for equilibration, and the remaining steps were sampled to analyze the thermodynamic properties. The Peng− Robinson equation of state was chosen to calculate the gas-phase density and experimental fugacity.44,45 CH4 was described as a five-site rigid regular tetrahedral model,46 and CO2 was modeled as a three-site rigid linear molecule. The Lennard-Jones (LJ) potential parameters of CO2 and CH4 were taken from the TraPPE model developed by Potoff, Siepmann,47 and Sun.46 The LJ + Coulomb potentials were used to describe the interactions between host and sorbate atoms, and each atom of the host or guest was treated explicitly. The LJ potential parameters of the framework atoms were taken from the DREIDING force field,48 except the Li atom, whose parameters (σ = 2.184 Å, ε = 12.58 K) were taken from universal force field (UFF).49 All the potential parameters used in the present work are provided in Table 2. All cross-interaction parameters between different types of atoms were calculated using the Lorentz−Berthelot mixing rules. The charge distribution was calculated using the B3LYP/6-31G* DFT and ChelpG methods. A cutoff radius 12.8 Å was used for the LJ interactions, and the Ewald summation technique was applied to calculate Coulomb interactions. The employed computational strategy has presented good reliability in describing the gas adsorption property of porous carbon materials,38,39,50 porous coordination network (PCN),51 COFs,52 and MOFs.53−55

GDY-OH and 1.28 cm3/g in GDY-F and GDY-COOH. The expansion of Vtot is generally in accordance with the variation of DM, that is, the first two largest DM values (18.21 and 16.16 Å) exist in GDY-F and GDY-COOH, which is then followed by GDY-OH (15.59 Å). GDY-NH2 constitutes an exception, where a shortening of DM is observed (12.55 vs 14.71 Å) when −H is replaced by −NH2. The SA and Φc increase significantly from 2675 m2/g (GDY-H) to 3139 m2/g (GDY-COOH) and from 65.19% (GDY-H) to 70.49% (GDY-NH2), respectively. The yup of GDY-Rs is ∼10% with tiny fluctuations, and the Vup is in the range of 0.10−0.14 cm3/g with the exception of the extremely large yup and Vup (21.9% and 0.27 cm3/g) in GDYNH2. After doping Li in GDY-OH and GDY-NH2, both Vtot and SA are further increased, but distinct trends of Φc and Vup are followed for different systems, i.e., increased to 72.51% and 0.14 cm3/g for Li@GDY-OH and reduced to 68.88% and 0.17 cm3/g for Li@GDY-NH2. To further understand the pore structures, pore size distributions (PSDs) of GDY-Rs and Li@GDY-Rs are analyzed in Figure 2. We can find that all the modified GDYs are typical of microporous carbon materials with the pore sizes in 3.4− 18.2 Å, dominated by supermicropores (within 7.0−20.0 Å) with less contents of ultramicropores ( GDY-COOH > GDY-H > GDY-F. Presser et al. have suggested that at low pressures, smaller pores contribute more to the amount of adsorbed CO2.6 Further studies found that compared to wider micropores or total pore volume the portion of micropores with sizes smaller than 0.7 nm (i.e., the ultramicropores) has more pronounced influence on the CO2 adsorption properties under ambient pressure.6,58,59 For GDYRs, the ultramicropores volume (in cm3/g) decreases in the sequence of GDY-NH2 (0.27) > GDY-COOH (0.14) > GDYH (0.12) > GDY-F (0.11) > GDY-OH (0.10) (see Table 1), following generally the order of the CO2 uptake in the considered materials with the exception of GDY-OH, which has the smallest ultramicropores volume but exhibits the second largest CO2 uptake. This indicates that it is really the ultramicropores that contribute importantly to the CO2 adsorption capacity at low pressures. Combined with the CO2 uptakes at high pressures (>6 MPa), we can further find that the ultramicropores yield increasingly crucial contributions on CO2 adsorption at ambient pressure or even lower pressures, but at high pressures, it shows significant restrictive effect. Furthermore, the isosteric heats of adsorption (Qst) of CO2 in GDY-Rs were calculated. As shown in Figure 4, the calculated Qst ranging from ∼24 to ∼30 kJ/mol at 0.02 MPa is comparable with those of N-doped polypyrrole-based porous carbon (19−32 kJ/mol),60 poly-based carbon (24−32 kJ/

Figure 2. Geometric PSDs as a function of pore diameter for GDY-Rs. The red vertical lines (7 Å) represent the boundary between ultramicropores (15 Å) in GDY-OH accompanied by multiplying the composition of the supermicropores with sizes of 7−14.5 Å. The pore size of all the GDY-Rs and Li doped frameworks is below 16 Å except for a few large pores (>16 Å) in GDY-F. It is well-known that it is the micropores

Figure 3. Absolute adsorption isotherms of CO2 in GDY-Rs at 298 K and (a) pressures up to 10 MPa and (b) in the low-pressure region. 30005

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structure. The second highest Qst makes GDY-OH the second largest CO2 adsorption capacity, even though it has the smallest ultramicropores volume. However, the smallest CO2 adsorption capacity of GDY-F is due to the lowest Qst combined with the most contracted ultramicroporous structure. On the basis of the Lewis acid−base theory, both −NH2 and −OH are electrondonating groups which can be viewed as Lewis basic sites for anchoring the acidic CO2 molecules. The electron-donating ability or Lewis basicity of −NH2 group is much stronger than that of −OH and −COOH groups, thus GDY-NH2 shows much larger positive effect than GDY-OH and GDY-COOH at low pressures.63 However, −F is an electron-withdrawing group which can be viewed as Lewis acid site and thus has a negative effect on CO2 adsorption.19,63 It is evident from Figure 3a that apart from GDY-NH2 the total CO2 uptake in the functionalized GDY-Rs is obviously higher than the corresponding value in GDY-H at high pressures which indicates that appropriate functionalization can also efficiently improve the saturated CO2 adsorption capacity. From Figure 3a, it can be clearly identified the largest saturated CO2 adsorption capacity comes with modification with −COOH. The saturated CO2 adsorption capacity follows the order of GDY-COOH > GDY-F ≈ GDY-OH > GDY-H > GDY-NH2, which is in remarkable contrast with the situation observed at lower pressures. Generally, the order of the adsorption capacity of GDY-Rs at the higher pressures follows the trend of their surface area and total pore volume except GDY-NH2 (see Table 1). Thus, the CO2 uptakes in GDY-Rs are closely related to their pore properties because higher surface area can afford more abundant CO2 physical adsorption sites, while larger pore volume can make the active adsorption sites more accessible to the adsorbates. For example, the largest enhancement of CO2 uptake in GDY-COOH is mainly attributed to the well-developed micropore structure (the largest Vtot of 1.28 cm3/g, highest surface area of 3139 m2/g,

Figure 4. CO2 uptake and isosteric heats of adsorption in GDY-Rs at 298 K and 0.02 MPa.

mol),61 spherical nitrogen-containing microporous carbon (25−31 kJ/mol),62 and nitrogen-doped porous carbons derived from benzimidazole-linked polymers (24−35 kJ/mol).58 Qst decreases in the order of GDY-NH2 (29.96) > GDY-OH (29.76) > GDY-COOH (27.19) > GDY-H (26.71) > GDY-F (23.84), following strictly the decreasing trend of CO2 adsorption capacity. That is to say, at ultralow pressures, the CO2 uptake correlates well with its interactions with GDY-Rs which can be significantly increased by appropriate surface functionalities. Therefore, the functional groups in conjunction with the ultramicroporous structures are the determinant factors for the CO2 adsorption at ultralow pressures. The largest CO2 capacity of GDY-NH2 is a consequence of the synergistic effect of the surface functionality with the highest Qst combining with the most developed ultramicroporous

Figure 5. Stable adsorption configurations of CO2 (top view (up) and side view (down)) at the sites of hydroxyl group (a−c), triangular area formed by three acetylenic linkages (d), alkyne group (e), and phenyl ring (f) on GDY-OH surface. 30006

DOI: 10.1021/acsami.7b10836 ACS Appl. Mater. Interfaces 2017, 9, 30002−30013

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Figure 6. Snapshots of the structures of GDY-NH2 with adsorbed CO2 at various pressures: (a) 0.005 MPa; (b) 0.02 MPa; (c) 0.1 MPa; (d) 0.5 MPa; (e) 1 MPa; (f) 3 MPa; and (g) 10 MPa.

configurations of CO2 on GDY-OH surface. It is found that the adsorption energies of CO2 at hydroxyl groups are as high as 9.63−13.94 kJ/mol, indicating the hydroxyl group is a strong CO2 adsorption site. Therefore, introduction of functional groups (such as hydroxyl) has a significant influence on the CO2 adsorption, especially at low pressures. Furthermore, the CO2 adsorption energy above the alkyne group (−CC−) and triangular area formed by three acetylenic linkages are calculated to be 3.84 and 4.78 kJ/mol, respectively, which are apparently higher than that above phenyl ring (1.50 kJ/mol). This phenomenon can be attributed to the charge distributions over the modified GDY. Mulliken charges analyses from Figure S3 suggest that the C atoms in the alkyne groups possess some positive charges, while the C atoms in phenyl rings are negatively charged. Therefore, interaction of the electronegative O atoms in CO2 with the electropositive alkyne groups is attractive, while with the phenyl rings, it is repulsive. This suggests that alkyne groups are more beneficial for CO2 adsorption compared to phenyl rings. The common adsorption mechanism can be generalized as follows: The CO2 molecules first occupy the most energetically favorable functionality sites at ultralow pressures followed by filling the ultramicropores volume in the first stage of CO2 capture. At intermediate pressures, the CO2 molecules tend to

and relatively large porosity of 69.14%). More specifically, the adsorption capacity of GDY-Rs is strongly correlated to the supermicropores volume with sizes of 7−16.5 Å. Although GDY-NH2 has the second largest surface area (2890 m2/g) and total pore volume (1.25 cm3/g) as well as the largest total porosity (70.49%), the largest loss of CO2 uptake can be ascribed to the smallest supermicropores volume because of the large fraction of ultramicropores (GDY-NH2: yup, 21.91%; Vup, 0.27 cm3/g vs others: yup, 8−11%; Vup, 0.10−0.14 cm3/g; see Table 1), which is hard for multilayer adsorption of CO2 at high pressures. Although GDY-F has larger surface area (2885 vs 2773 m2/g) and total pore volume (1.28 vs 1.25 cm3/g) than GDY-OH, they have comparable CO2 capacity at high pressures because of the fraction of large pores (1.65 nm), which offsets the CO2 capacity of GDY-F. In order to provide basic insight into the CO2−adsorbent interaction, the adsorption properties of CO2 on the GDY-OH surface are investigated by using DFT. The adsorption energy (Eads) can be expressed as follows: Eads = ECO2 + Esurf − ECO2 + surf

(1)

where ECO2, Esurf, and ECO2+surf are the total energies of the CO2 molecule, adsorbent surface, and CO2−surface adsorption system, respectively. Figure 5 shows the stable adsorption 30007

DOI: 10.1021/acsami.7b10836 ACS Appl. Mater. Interfaces 2017, 9, 30002−30013

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fluorine functionality give intermediate and comparable positive effect, while the influence of amino functionality is not affirmative but negative. 3.3. Separation of CO2/CH4 Mixture by GDY-Rs. In addition to the high CO2 uptake, the question of whether GDY-Rs have high separation performance prompts us to investigate their CO 2 /CH 4 adsorption selectivity. The adsorption isotherms for the CO2/CH4 mixture (bulk composition 50:50) in GDY-Rs are shown in Figure S4. It can be seen that CO2 absorption is more favorable than CH4 adsorption in the entire pressure range (0−4.5 MPa), and the saturated CO2 absorption capacity is far above the earlierreached saturated absorption capacity of CH4. This competitive advantage is by virtue of the special properties of adsorbates: (i) the larger quadrupole moment and polarizability of CO2, which predict stronger interaction of CO2 with framework, and (ii) the exclusive effect of the adsorbates’ sizes, in which CO2 (0.33 nm) has a little smaller kinetic diameter than CH4 (0.38 nm), allowing CO2 molecules to facilely access to larger proportion of micropores.64 To evaluate the merit of GDY-Rs for gas separation, the separation performance for the equimolar CO2/CH4 mixture is quantified by the selectivity defined as Si/j = (xi/xj)(yj/yi), where xi and yi are the mole fractions of component i in the adsorbed and bulk phases, respectively. Figure 8 presents the

stuff the larger micropores through layer by layer adsorption; eventually, CO2 reaches a dynamic equilibrium involving the free volume filled at high pressures (see Figure 6). To quantitatively show the functionalization effect on the CO2 adsorption capacity, we evaluated the functionalized contribution (FC) using the following percentage ratio: FC =

NR − NH × 100% NH

(2)

where NR and NH denote the CO2 uptake of GDY-Rs and GDY-H, respectively. It is clear that the values above zero illustrate a positive impact, and the values no more than zero demonstrate a negative influence. Figure 7 shows the

Figure 7. Functionalized contribution of GDY-Rs on the CO2 adsorption at 298 K.

functionalized contribution for CO2 adsorption in GDY-Rs at 298 K as a function of pressure. It is noted that the largest variation of FC exists in GDY-NH2, showing an exponential decay trend from an extremely high value (∼120%) at 0.005 MPa to zero at ∼2.5 MPa and finally to a negative constant (ca. −3%) in the higher pressure range (>4.5 MPa). GDY-F also shows a relatively large variations in the FC but in an opposite trend, i.e., increasing sharply with the increase of pressure from an initial negative value of ca. −34% in the lower pressure range (0−3.5 MPa) and then tending to be constant (∼5%) in the higher pressure range (3.5−10 MPa). The FC curve of GDYOH and especially GDY-COOH is relatively smooth, in which the FC of GDY-OH shows a similar decreasing trend as that of GDY-NH2 at the pressures below 1.5 MPa from the initial value of ∼32% and then increases slightly from ∼1% to a constant of ∼5% at 5.0 MPa, while the curve of GDY-COOH presents a gradually rising trend from ∼31 to ∼35% in the very preliminary stage (0−0.5 MPa) then decreases tardily to an invariable value of ∼18% at 4.0 MPa. These facts demonstrate that different functionalities of GDY have different influences on the CO2 adsorption capacities, especially at the low pressures. Specifically speaking, at low pressures, the amino functionality provokes an exceptionally high improvement in CO2 adsorption, the positive effects of hydroxyl and carboxyl groups are weaker, while fluorine functionality brings about a negative influence. At high pressures, the carboxyl group has the largest positive effect on CO2 adsorption, hydroxyl and

Figure 8. Selectivity of CO2 over CH4 for the equimolar CO2/CH4 mixture in GDY-Rs at 298 K.

CO2/CH4 selectivity of GDY-Rs at 298 K as a function of pressure. We find that the selectivity curves of the OH-, NH2-, and COOH-modified GDYs follow generally similar trend as that of GDY-H, i.e., an initial sharp and small decrease followed by a milder linear increase as the pressure increases, consistent with the shape of the CO2/CH4 selectivity curves of PPN-1’s65 and IRMOF-X’s (X = 1, OH, CH3, NH2, and F).63 The initial decrease in selectivity is caused by the heterogeneity of adsorption sites in the modified GDYs, i.e., the less favorable sites are occupied by adsorbates as pressure increases; the latter increase of the selectivity curves is due to the higher saturated capacity of CO2 compared to CH4, since increasing the pressure progressively favors the component with higher adsorption capacity.65 Furthermore, it is also noticed that 30008

DOI: 10.1021/acsami.7b10836 ACS Appl. Mater. Interfaces 2017, 9, 30002−30013

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ACS Applied Materials & Interfaces

modification models, where Li atoms are physisorbed into the pores of GDY and/or between layers of GDY as nonframework atoms. Figure 10 gives the CO2 adsorption capacities of the Li-decorated frameworks at 298 K and 0.005− 4.0 MPa. For comparison, the data of the corresponding nondoped ones are also shown in the figure. An inspection of the CO2 isotherm of Li@GDY-OH (Figure 10a) elucidates that Li doping could greatly enhance the CO2 adsorption capacity of GDY-OH in a wide pressure range (0.005−4.0 MPa); this enhancement is gradually diminished as the pressure increases so that the uptake curves of Li@GDY-OH and GDY-OH intersect at 4.0 MPa. Unexpectedly, the CO2 uptake in Li@ GDY-NH2 is only slightly enhanced in the range of 0.005−0.2 MPa, compared to the undoped situation (Figure 10b). Above that, the CO2 adsorption capacity of Li@GDY-NH2 is substantially lower than that of GDY-NH2 showing an enlarging divergence trend (Figure 10a). Note that the Lidecorated GDY-OH displays an extraordinarily high CO2 uptake of 4.83 mmol/g at room temperature and ambient pressure, which is comparable to or higher than those of the most recently reported carbon and other porous materials for capturing CO2 under identical conditions (summarized in Table S2). It is also worth noting that the cohesive energy of the Li@GDY-OH basis unit (6.21 eV/atom) is just slightly lower than that of the GDY-H basis unit (6.39 eV/atom). Therefore, the Li@GDY-OH basis unit is expected to be energetically stable enough for its formation. To explore the influence of Li doping on the CO 2 adsorption, the adsorption of CO2 on the Li@GDY-OH surface are also calculated. The CO2 adsorption configurations and energies at the sites of the Li atoms and hydroxyl groups are shown in Figure 11. From Figure 11a−f, it can be clearly identified that CO2 is most stably adsorbed at the Li atoms in Li@GDY-OH with the adsorption energies of 38.66−40.11 kJ/ mol, which are about twice those at the hydroxyl groups. Similar situation is also observed in Li@GDY-NH2 (see Figure S5). This indicates that strong CO2 affinity of Li+ ions contributes mainly to the enhancement of the gas adsorption. The strong CO2 binding energy at Li cation may mainly originate from the electrostatic interactions between the electrostatic charge at Li and the quadrupole and chargeinduced dipole of CO2,21,68 mirrored by the fact that the increment of the total interaction energy induced by Li doping is contributed mainly by Coulombic interactions at ultralow pressures (see Figures S7 and S8). The electrostatic interactions favor the tilt adsorption of CO2 via one end oxygen as shown in Figure 11. Furthermore, Li doping also strengthens the adsorption of CO2 at the hydroxyl groups; the adsorption energies are 12.33 (11.87), 17.30 (16.93), and 16.48 (16.25) kJ/mol at H1, H2, and H3 sites in Li@GDY-OH, respectively, compared to 9.63, 13.94, and 13.49 kJ/mol in the case of GDY-OH (see Figures 11 and 5). Therefore, Li atoms in Li@GDY-OH also exert an inductive effect for CO2 adsorption. Note that the inductive effect in Li@GDY-NH2 is relatively weak (see Figure S5). The influence of Li doping on CO2 uptake can thus be explained by the fact that at the ultralow pressures (0.005−0.02 MPa) the CO2 molecules bind preferentially at Li atoms strongly (see Figure S6), so both Li-doped materials show enhanced CO2 uptake capacities. With the increase of pressure, the CO2 molecules tend to adsorb either in a close vicinity of Li and/or at the amino and hydroxyl groups. As mentioned above, the different inductive effects suggest that doping of Li in GDY-

substitution with OH, NH2, and COOH can significantly enhance the CO2/CH4 selectivity over the whole pressure range where the selectivity curves of GDY-OH and GDY-NH2 are nearly parallel to GDY-H. The largest enhancement exists in GDY-NH2, and GDY-OH shows an intermediate positive effect. The influence of GDY-COOH is comparable to that of GDY-OH at lower pressures and is strengthened with the increase of pressure. The enhancement of CO2/CH4 selectivity indicates that the induced polarity of these functional groups has more positive influence on the adsorption of CO2 than CH4. Interestingly, substitution of H with F results in the decrease of CO2/CH4 selectivity, which shows a linear increase trend nearly parallel to that of GDY-H. The remarkable decrease of selectivity at the lower pressures is a result of the negative effect of the adsorption of acidic CO2 molecules at Lewis acidic fluorine sites as mentioned above, and the loss of the selectivity of CO2/CH4 in the whole pressure range may be due to the cooperation of the fluorine functionality and PSD on the CO2 and CH4 adsorption. In summary, decorating different functional groups in GDYs results in different affinities toward various components in gas mixture showing variation in adsorption selectivity, which is associated with the intrinsic characteristics of functional groups and gas molecules, such as polarity and acid−base properties.17 In addition, pore structure regulated by decoration of functional groups, which is more favorable to certain molecules over others, is also a factor for gas adsorption selectivity. 3.4. CO2 Storage and Separation Performance of Li@ GDY-Rs. The lightest metal, Li, can easily lose its valence electron to form cationic Li with a strong polarizing power, which improves the affinity of host materials toward CO2.66,67 Simulations by Lan et al. show that the excess CO2 uptakes in the lithium-doped COFs can be enhanced by 4−8 times compared to that of the undoped COFs at 298 K and 1 bar.21 Here, we investigate the influence of Li doping on the CO2 storage and separation performance of GDY-Rs. For simplification, only the doping of GDY-NH2 and GDY-OH is considered, since functionalization with −NH2 and −OH in GDY has been shown to have the positive effects on both the CO2 adsorption at lower pressures and CO2/CH4 selectivity in the whole pressure range considered. Lithium modification is adopted through introducing lithium in the form of alkoxy salt (O−Li group)23 to substitute a pair of para-sites-OH and NH2 in the basis unit of GDY-OH and GDY-NH2, respectively, named Li@GDY-OH and Li@GDY-NH2 (see Figure 9). In general, there are two types of Li doping, i.e., physical modification and chemical modification. In our model, chemical modification of Li doping was selected, where Li is covalently bound to the O atom of O−Li group, making it part of the basis unit. This structure is more stable than the physical

Figure 9. Structures of lithium decorated graphdiyne basis unit. (a) Li@GDY-OH and (b) Li@GDY-NH2. 30009

DOI: 10.1021/acsami.7b10836 ACS Appl. Mater. Interfaces 2017, 9, 30002−30013

Research Article

ACS Applied Materials & Interfaces

Figure 10. Absolute adsorption isotherms of CO2 in Li-decorated frameworks at 298 K and (a) pressures up to 4.0 MPa and (b) in the low-pressure region. The absolute adsorption isotherms in the corresponding frameworks without Li doping are also shown for comparison.

Figure 11. Stable adsorption configurations of CO2 (top view (up) and side view (down)) at the sites of Li atom (a−c) and hydroxyl group (d−i) on Li@GDY-OH surface.

situation of non-Li-doped frameworks, the enhanced CO2 framework and CO2−CO2 total interactions in the Li-decorated systems (see Figures S7−S9) greatly weaken the condensability of adsorbed CO2 and thus predict the decreasing trend of CO2 uptake at high pressures even though the total available pore

OH can enhance CO2 uptake to a larger extent and in a broader pressure range than Li@GDY-NH2. Furthermore, the remarkable loss of ultramicropores volume after Li doping in GDYNH2 (0.17 vs 0.27 cm3/g) results in the lowering of CO2 adsorption capacity above 0.2 MPa. Compared with the 30010

DOI: 10.1021/acsami.7b10836 ACS Appl. Mater. Interfaces 2017, 9, 30002−30013

ACS Applied Materials & Interfaces

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volume is enlarged after being Li-decorated. This phenomenon can be reflected by the spatial-averaged equilibrium density of CO2, which is 17.07 (14.03) mmol/cm3 in Li@GDY-OH (Li@ GDY-NH2) and 18.66 (17.00) mmol/cm3 in GDY-OH (GDYNH2). The CO2/CH4 selectivity of the Li-decorated GDY-X (X = OH and NH2) in Figure 12 shows that introduction of Li atoms

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10836. Atomic partial charges, CO2 uptake values of various porous carbon materials, CO2/CH4 competitive adsorption isotherms, GDY-Rs frameworks, CO2 adsorption configurations, snapshots of the CO2 adsorption structures, and average CO2-adsorbent interaction energies (PDF)

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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 Natural Science Foundation of Shandong Province (ZR2015BQ009 and ZR2016BL12), Qingdao independent innovation program (16-5-1-88-jch), and the Fundamental Research Funds for the Central Universities (15CX05068A and 15CX08010A)

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REFERENCES

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Figure 12. Selectivity of CO2 over CH4 for the equimolar CO2/CH4 mixture in Li-decorated frameworks at 298 K.

can significantly improve the CO2 separation performance, especially at low pressures (0.005−0.1 MPa). It is worth mentioning that the CO2/CH4 selectivities are 13.0 for Li@ GDY-OH and 14.7 for Li@GDY-NH2 at 298 K and 1 atm, which are comparable to or even higher than those of mesoPOF-2 (14),69 nitrogen-doped porous carbons (12),58 PECONF-4 (12),15 dual-functionalized MOFs (10.5),70 BILP-1 (10),71 ZIF (10),72 BUT-11 (9),73 pyrene-derived benzimidazole-linked polymers (