Ab Initio Study of Gas Adsorption in MetalâOrganic Frameworks

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Ab Initio Study of Gas Adsorption in Metal−Organic Frameworks Modified by Lithium: The Significant Role of Li-Containing Functional Groups Chenkai Gu,† Yang Liu,‡ Jing Liu,*,† Jianbo Hu,† and Weizhou Wang§

J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF SUSSEX on 08/07/18. For personal use only.

†

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 778 Atlantic Drive, Atlanta, Georgia 30332-0100, United States § Henan Key Laboratory of Function-Oriented Porous Materials, College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471934, China S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) are promising materials for gas adsorption. Introducing metal cations, for example, lithium cations (Li+), in the framework is an effective way to alter the gas adsorption features of MOFs. In this work, Li+ carried by different functional groups was incorporated onto a benzene linker, which is one type of the most common liker used in MOF synthesis. The interactions between the Li-modified linkers and various gas molecules were studied using MP2 method. Compared to the original benzene ring, the structures and orbitals of Li-modified linkers were significantly changed toward the direction of enhancing gas adsorption. For nonpolar gas species (CH4, H2, N2, and CO2), the induced polarizations greatly enhance the interactions between gas molecules and MOF linkers. Particularly, the expanded binding energy differences of H2/N2, CH4/CO2, and N2/ CO2 will make them easier to get separated. For polar gas species (H2O, H2S, SO2, and CO), the electrostatic interactions between gas molecules and Li+ play a significant role in enhancing gas adsorption. The strong affinities between polar gases and Li-modified linkers denote that the binding sites around Li+ can be first occupied by polar molecules such as H2O and SO2 during the practical adsorption process. This can result in the reduced adsorption capacities of other gases, such as CO2.

■

cm3/cm3 at 298 K and 35 bar). However, for most MOF materials, the gas adsorption capacities are still unsatisfactory and cannot reach the U.S. Department of Energy (DOE) targets. Therefore, further studies are required for the purpose of designing and synthesizing novel MOFs with high gas adsorption performance. The interactions between gas molecules and organic linkers of MOFs play a significant role in gas adsorption and separation.25−27 Investigations of these interactions can thus provide fundamental understandings and clues to adjust the performance of materials.28,29 The incorporations of metal atoms can profoundly enhance the gas adsorption capacities of MOFs.30−33 Especially, Li atom is the most popular one because of its smallest mass. Lan et al.34 investigated the doping of a series of metal atoms in covalent organic frameworks (COFs) and showed that the excess CO2 uptakes in Li-doped COFs can be enhanced up to 8 times at 298 K and

INTRODUCTION Gas adsorption is significant in both energy production and environmental protection. The adsorbent material plays a key role in gas adsorption process. Therefore, developing effective materials for gas adsorption is a long-term goal and will be increasingly urgent in the future. So far, many kinds of porous materials have been studied, including zeolites,1−3 polymers,4−6 activated carbon,7−9 and metal−organic frameworks (MOFs).10−12 Among these materials, MOFs have been considered as potential materials used in hydrogen storage,13−15 CO2 capture,16−18 and the purification of natural gas19−21 owing to their ordered structures, tunable functionality, and large surface area. In recent years, many MOF materials have been reported for their ultrahigh gas adsorption capacities. Botas et al.22 synthesized Co21-MOF-5, whose CO2 capacity was high up to 65 wt % at 273 K and 10 bar. Wang et al.23 found that the hydrogen storage capacity of PCN-12 was 3.05 wt % at 77 K and 1 bar. Peng et al.24 reported that the methane adsorption amount in HKUST-1 is 227 cm3/cm3 at 298 K and 35 bar, which is much higher than that in traditional MOF-5 (150 © XXXX American Chemical Society

Received: April 2, 2018 Revised: June 6, 2018 Published: July 24, 2018 A

DOI: 10.1021/acs.jpcc.8b03112 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C 1 bar. Mendoza-Cortés et al.35 reported that H2 uptake performance can be enhanced by doping alkali metal atoms on the organic linkers of MOFs. Especially, compared to Na and K atoms, the Li atom shows better performance in enhancing hydrogen uptakes. So far, some methods have been applied in incorporating Li atoms to MOFs. Mavrandonakis et al.36 proposed the chemical modification of the organic linker of IRMOF-14 with −SO3Li groups for enhanced hydrogen adsorption. Hu et al.37 incorporated the Li cations to UiO66 in the form of −COOLi groups. In our previous work,38 lithium alkoxide was imported in HKUST-1 to improve the CO2 adsorption capacity. However, to the best of the authors’ knowledge, the systematical comparison of the impact of different Li-containing functional groups on gas adsorption in MOFs has not been performed. Especially, the −SLi, −PO3Li2, and −PO3HLi groups have not yet been studied as a new way to incorporate Li cations. In addition, various gas species (CH4, N2, H2, CO2, H2O, H2S, SO2, and CO) were considered for systematical comparison. In this work, Li cations were incorporated by different functional groups, including −PO3Li2, −PO3HLi, −SLi, −OLi, −SO3Li, and −COOLi. The binding mechanisms between Li+ and various gas molecules were investigated systematically. Theoretical analysis was performed on both nonpolar and polar gas molecules, including CH4, N2, H2, CO2, H2O, H2S, SO2, and CO. This work aims to reveal the changes of linker structural properties as well as gas adsorption characteristics under the influence of different Li-containing functional groups, which will be helpful to design MOFs with high gas adsorption performance.

Figure 1. Structures of organic linkers (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres).

A basis set of 6-31++G(d,p) was chosen to optimize the structures.45 It was found that the interaction energy can decrease by more than half using the basis set superposition error (BSSE) correction. Therefore, BSSE was applied to rectify results with the counterpoise method proposed by Boys and Bernardi.46 The atomic charges were calculated using the CHELPG method47 on optimized structures, which can help to understand the electrostatic interactions between the interacting molecules. The BE can be calculated as follows

■

MODEL AND SIMULATION METHOD Initial Structure. The entire periodic MOF structure contains hundreds of atoms, which requires much more computational resources. Therefore, organic molecules with suitable size were treated as research objects. The benzene ring was a basic component of the ligand in MOFs and thus was treated as the basic structure studied in this work. Li cations were introduced to the benzene ring with different functional groups including dilithium phenyl phosphate (C6H5−PO3Li2), lithium hydrogen phenyl phosphate (C6H5−PO3HLi), lithium mercaptobenzene (C6H5−SLi), lithium phenoxide (C6H5− OLi), lithium benzene sulfinate (C6H5−SO3Li), and lithium benzoate (C6H5−COOLi). The formed various Li-modified benzene derivatives are presented in Figure 1. Vibrational frequency analysis was performed to confirm that the converged results were local minima. All possible sites, including the sites around the lithium clusters (LS) and the sites on top of the benzene ligands (BS), were considered to seek out the preferential adsorption sites. Computational Details. Density functional theory (DFT) and MP2 methods are two kinds of widely used quantum chemical methods.39 The dispersion interactions play a significant role in almost all cases involving weak physisorption. However, DFT method cannot treat dispersion interactions adequately.40 Therefore, MP2 methods were performed in all of the interaction calculations here. In terms of selective adsorption separations, both binding energy (BE)40−43 and free energy30,44 of adsorption have been employed to measure the interactions between gas molecules and absorbent materials. In this work, BE was chosen as a parameter, which has been widely used to described the interactions between gas molecules and MOF materials.

BE = E(AB) − E(A) − E(B) + E(BSSE)

(1)

In this equation, E(AB) represents the total energy of the optimized interacting system. E(A) and E(B) represent the energy of the adsorbate and substrate, respectively. E(BSSE) represents the BSSE energy from the incompleteness of the basis set or the inconsistency between the dimer and the monomer calculations. Both dispersion force and electrostatic interaction can contribute to the binding of gas molecules in MOFs. The conventional symmetry-adapted perturbation theory (SAPT) calculations were thus performed to further analyze the compositions of total interaction energies with the basis set of 6-31++G(d,p).48 The BE components contain the electrostatic energy (Ees), the exchange repulsion energy (Eexch), the effective dispersion energy including the dispersion-induced exchange energy (Edisp * = Edisp + Edisp‑exch), and the effective induction energy including the induction-induced exchange energy (E*ind = Eind + Eind‑exch). The molecular orbitals were calculated with B3LYP/6-31+ +G(d,p) method, which has been successfully employed to calculate the molecular orbitals in previous works.49,50 The geometry configurations, BEs, partial charges, and molecular orbitals were calculated and analyzed using Gaussian 09 program package.51 The SAPT calculations were carried out with Q-Chem computational suites.52 B


Article

The Journal of Physical Chemistry C

Figure 2. MP2-optimized structures and atomic charges (CHELPG) of CH4 interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

Figure 3. MP2-optimized structures and atomic charges (CHELPG) of H2 interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

■

RESULTS AND DISCUSSION

Figure S2 shows the highest occupied molecular orbitals (HOMOs) of Li-modified benzene. Compared to the benzene ring, the enhanced delocalizations of HOMO occur in Limodified benzene derivatives. Especially, the delocalization of electron densities turned into asymmetry in C6H5−OLi, C6H5−SLi, and C6H5−PO3Li2. The HOMO region shows little constraint on electrons and thus has the nature of an electron donor. When gas molecules are adsorbed at these regions, the extra electrons are obtained, leading to the enhancement of intermolecular interactions. Figure S3 shows the lowest unoccupied molecular orbitals (LUMOs) of Li-modified benzene. The symmetries of LUMO in the benzene ring were completely broken, and there was a region of LUMO around each Li+ cation. The LUMO region

Structures and Orbitals of Linkers. The optimized geometric structures of diverse linker molecules are shown in Figure 1. The Li cations were located beside electronegative atoms including O and S. To confirm the bond types, the electron localization functions (ELFs) were calculated. C6H5− COOLi was taken as an example because all of its atoms were in the same plane and thus were easily observable. Figure S1 shows the contour line map of ELF of C6H5−COOLi. Li+ shows a higher electronic localization, and there is no closed contour line between Li+ and O, which represents a typical ionic bond. In other Li-containing linker molecules, the bond types of Li+−O and Li+−S are also ionic bonds. C


Article


Figure 4. MP2-optimized structures and atomic charges (CHELPG) of N2 interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; N, blue spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

Figure 5. MP2-optimized structures and atomic charges (CHELPG) of CO2 interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

systems. Especially, the BEs of gas molecules at LS sites are much larger than those at BS sites. Figure 2 shows the MP2-optimized structures, CHELPG charges, and BEs of CH4 interacting with linker molecules. In the benzene···CH4 system (Figure 2a), the CH4 molecule was adsorbed above the benzene with a C−H bond pointing to the center of the benzene ring. CH4 is a nonpolar molecule, and its interaction with benzene mainly stems from dispersion force with a BE of only −1.80 kJ/mol. However, the BEs are much larger at LS sites of lithium-modified systems, ranging from −11.22 to −15.27 kJ/mol (as shown in Figure 2). In these systems, all of the Li cations show significantly larger interactions with the CH4 molecule. The additional interactions mainly derive from induction force and electrostatic

has a strong affinity for electrons, showing the nature of electron acceptors. If gas molecules move to these regions, they will lose some electrons, resulting in the valent decrease of Li during the adsorption process. Interactions of Nonpolar Gas Molecules with Linkers. Interactions of CH4, H2, N2, and CO2 with linker molecules, including C6H6, C6H5−PO3Li2, C6H5−PO3HLi, C6H5−SLi, C6H5−OLi, C6H5−SO3Li, and C6H5−COOLi, were calculated systematically. All of the stable configurations of gas−linker interactions are shown in Figures 2−5. In C6H5−PO3Li2···gas systems, the optimized conformations at BS sites are the same as LS sites and thus are not shown. In general, for both BS and LS sites of Li-modified linker molecules, the BEs of gas molecules are higher than those of initial benzene···gas D


Article

The Journal of Physical Chemistry C force, as shown in Table 1. The H atoms in CH4 were located outside the line of C−Li, rather than pointing to Li+, because

Table 2. SAPT BE Decompositions (kJ/mol) of H2 Interacting with Li-Modified Linker Molecules

Table 1. SAPT BE Decompositions (kJ/mol) of CH4 Interacting with Li-Modified Linker Molecules benzene C6H5−PO3Li2 C6H5−PO3HLi C6H5−SLi C6H5−OLi C6H5−SO3Li C6H5−COOLi

site

Ees

Eexch

Eind *

Edisp *

BS LS BS LS BS LS BS LS BS LS BS LS

−4.28 −9.87 −6.16 −10.17 −5.15 −16.89 −5.21 −9.96 −6.81 −10.74 −4.82 −9.89

10.77 15.42 13.96 15.50 12.52 28.61 12.85 14.26 14.48 16.72 11.57 14.85

−0.47 −12.34 −0.78 −13.52 −0.59 −16.98 −1.13 −15.72 −1.18 −15.80 −0.27 −13.68

−7.62 −4.72 −10.59 −4.35 −10.03 −11.05 −9.48 −2.78 −10.90 −4.24 −9.34 −3.85

benzene C6H5−PO3Li2 C6H5−PO3HLi C6H5−SLi C6H5−OLi C6H5−SO3Li C6H5−COOLi

site

Ees

Eexch

E*ind

E*disp


−2.53 −6.84 −1.80 −8.29 −2.32 −10.50 −3.12 −6.93 −3.44 −8.11 −2.35 −7.24

4.56 7.90 5.73 9.97 5.40 16.16 5.21 8.44 5.92 9.90 4.83 8.54

−0.31 −5.91 −0.40 −6.77 −0.36 −7.30 −0.73 −8.33 −0.57 −7.53 −0.31 −6.93

−3.35 −1.76 −4.38 −2.34 −4.01 −6.24 −3.74 −1.09 −4.31 −2.24 −3.73 −1.64

Table 3. SAPT BE Decompositions (kJ/mol) of N2 Interacting with Li-Modified Linker Molecules

the electrostatic interaction between Li+ and H_CH4 is repulsive force due to the same electropositivity of the Li cation and H_CH4. Especially, among the linker molecules, C6H5−SLi shows the largest BE (−15.27 kJ/mol) with the CH4 molecule at the LS site, which can be attributed to the additional interaction between CH4 and benzene caused by their close locations. Both H2 and N2 are nonpolar diatomic molecules; thus, they are analyzed together for the sake of convenient comparison. The MP2-optimized structures of interacting systems for H2 and N2 are shown in Figures 3 and 4, respectively. The atomic charges calculated by the CHELPG method and the BEs are also given. In the benzene−H2 system (Figure 3a) and benzene−N2 system (Figure 4a), both H2 and N2 were adsorbed at the site above the center of benzene. Both H2 and N2 are nonpolar gas molecules; hence, the contributions of dispersion forces (H2: −3.35 kJ/mol, N2: −8.20 kJ/mol) are more than those of electrostatic forces (H2: −2.53 kJ/mol, N2: −5.34 kJ/mol) when H2 and N2 are interacting with the benzene ring. The strength of the dispersion force increases with increasing molecular mass of the interacting molecules; therefore, the dispersion interaction in benzene···N2 (−8.20 kJ/mol, Table 3) is higher than that in benzene···H2 (−3.35 kJ/mol, Table 2). In addition, the BE of benzene···N2 (−4.61 kJ/mol, Figure 4a) is higher than that of benzene···H2 (−1.74 kJ/mol, Figure 3a). After introducing Li cations, the interactions of H2 and N2 molecules with linker molecules were enhanced both at BS sites and LS sites. When interacting with Li+, dipoles were induced in both H2 and N2 molecules, which further enhanced the interactions. The induction energies at LS sites were enhanced by more than 10 times, compared to the induction energies at BS sites, as shown in Tables 2 and 3 The BEs of H2 and N2 at LS sites are improved for more than 4 times (shown in Figures 3 and 4). Here, more charge transfers were observed in the N2 molecule than in the H2 molecule because the N2 molecule has lone pair electrons and is easier to donate electrons. This results in higher BEs of N2 than those of H2 when interacting with the same linker molecules. From the perspective of linker species, both of the BE trends of H2 and N2 are C6H5−SLi > C6H5−OLi > C6H5−SO3Li > C6H5− COOLi > C6H5−PO3HLi > C6H5−PO3Li2. Figure S4 shows a plot of gas BEs versus the distances between H2/N2 and Li+.


site

Ees

Eexch

Eind *

Edisp *


−5.34 −15.96 −7.33 −16.46 −6.51 −20.07 −7.68 −15.88 −7.02 −17.39 −5.83 −15.94

12.73 14.45 17.19 14.72 15.28 21.38 16.63 12.32 16.39 15.94 14.66 13.32

−0.30 −10.67 −0.51 −11.73 −0.35 −14.31 −0.81 −13.39 −0.64 −13.63 −0.31 −11.69

−8.20 −3.36 −11.16 −3.10 −9.89 −5.11 −9.92 −2.08 −10.90 −3.09 −9.49 −2.72

The results indicate a positive correlation between the increase of BEs and the decrease of H2/N2−Li distances. Figure 5 shows the minimum-energy structures, CHELPG charges, and BEs of CO2 interacting with various linker molecules. In the benzene−CO2 system (Figure 5a), the CO2 molecule was located on top of the benzene ring. This configuration is mainly controlled by the interaction between the CO2 quadrupole and the π electrons of the benzene ring, which contains both dispersion force (−10.75 kJ/mol, Table 4) and electrostatic force (−11.48 kJ/mol, Table 4). The BE in the benzene−CO2 system is −5.42 kJ/mol. When interacting with C6H5−PO3Li2 (Figure 5b), C6H5−PO3HLi (Figure 5d), Table 4. SAPT BE Decompositions (kJ/mol) of CO2 Interacting with Li-Modified Linker Molecules


E

site

Ees

Eexch

E*ind

E*disp


−11.48 −66.84 −18.60 −65.88 −5.89 −53.64 −66.47 −89.66 −19.05 −57.89 −21.51 −56.08

18.89 55.89 23.96 49.94 16.34 42.23 55.49 77.95 22.30 41.64 22.52 38.71

−1.71 −18.36 −2.42 −18.45 −1.13 −19.31 −25.12 −24.70 −2.70 −19.58 −2.19 −17.72

−10.75 −17.91 −14.48 −12.27 −11.36 −13.51 −17.25 −15.04 −13.91 −9.45 −12.46 −8.75


Article


Figure 6. MP2-optimized structures and atomic charges (CHELPG) of H2O interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

Figure 7. MP2-optimized structures and atomic charges (CHELPG) of H2S interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

C6H5−OLi (Figure 5h), C6H5−SO3Li (Figure 5j), and C6H5− COOLi (Figure 5l) at LS sites, the CO2 molecules lean against the O−Li bonds. This indicates that the CO2 molecules are mainly adsorbed by two types of simultaneous interactions: electrostatic interactions between O_CO2 and the Li cation and electrostatic interactions between C_CO2 and the O atom of linker molecules. The introduction of −SLi group significantly improved the BE to −36.97 kJ/mol at the LS site (shown in Figure 5f), which is higher than other groups. On the basis of the analysis above, C6H5−SLi shows the largest affinities to CH4, H2, N2, and CO2. Therefore, the interactions of gas molecules with C6H5−SLi were further analyzed, comparing with the gas−benzene interactions. It should be noted that the benzene−H2 BE is −1.74 kJ/mol and the benzene−CH4 BE is −1.80 kJ/mol. The BE difference is only 0.06 kJ/mol, and thus, it does not allow for selective

adsorption separation of H2/CH4. When C6H5−SLi is used instead, the BEs of linker molecules with H2 and CH4 were increased to −9.25 and −15.27 kJ/mol, respectively. The BE difference was expanded to 6.02 kJ/mol, which could be useful for effective separation of H2/CH4 mixtures by selective adsorption of CH4 in MOFs. From the practical perspective, there is a consequent enhancement in the selective adsorption of H2/CH4 caused by incorporating Li+ to MOF linkers. A similar enhancement can also be observed in other mixtures, such as H2/N2, CH4/CO2, and N2/CO2, which could be supported by the expanded BE difference. Interactions of Polar Gas Molecules with Linkers. The above analysis suggests that the interactions between linker molecules and nonpolar gas molecules are significantly enhanced by incorporating Li+. This motivates us to investigate the interactions between the Li-modified linker and polar gas F


Article


Figure 8. MP2-optimized structures and atomic charges (CHELPG) of SO2 interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

Figure 9. MP2-optimized structures and atomic charges (CHELPG) of CO interacting with (a) C6H6, (b) C6H5−PO3Li2, (c,d) C6H5−PO3HLi, (e,f) C6H5−SLi, (g,h) C6H5−OLi, (i,j) C6H5−SO3Li, and (k,l) C6H5−COOLi (H, white spheres; C, gray spheres; O, red spheres; S, yellow spheres; P, green spheres; and Li, purple spheres). BS represents the site on top of the benzene ring; LS represents the site around the Li cation.

H2O and H2S with benzene is dipole−π interactions, which contain both dispersion force and electrostatic force. However, the benzene···H2O interaction is stronger because the O atom shows stronger electronegativity than S atom and obtains more electrons, resulting in higher dipole of H2O than that of H2S. This can also be supported by the higher electrostatic energy of H2O (−14.86 kJ/mol, Table 5) than that of H2S (−13.03 kJ/ mol, Table 6). In addition, the BE of H2O (−8.47 kJ/mol, Figure 6a) is larger than that of H2S (−7.60 kJ/mol, Figure 7a). After introducing Li cations, the BEs of H2O and H2S increase sharply, with the ranges −81.90 to −68.02 kJ/mol and −51.75 to −37.70 kJ/mol at LS sites, respectively (as shown in Figures 6 and 7). Generally, the electrostatic interaction of H2O with the Li cation is stronger than that of H2S because the Li cation has more positive charges when interacting with

molecules. In this part, gas molecules including H2O, H2S, SO2, and CO were systematically analyzed. In C6H5−PO3Li2··· gas systems, the optimized conformations at BS sites are the same as LS sites and thus are not shown. Generally, the BEs of gas molecules at BS sites are much less than those at LS sites. Especially, in some cases, including Figures 6e,g, 7g, 8g, and 9g, the gas molecules are adsorbed at BS sites but they interacted with benzene rings and Li cations simultaneously. This results in the similar BEs at BS sites to those at LS sites. H2O and H2S are hydrides of same group elements (O, S) and thus are comparatively analyzed in this section. The MP2optimized structures of interacting systems for H2O and H2S are presented in Figures 6 and 7, respectively. In Figures 6a and 7a, the H2O and H2S molecules were adsorbed on top of benzene, with H_H2O and H_H2S (positive charge center) leaning to the benzene side. The major source of interaction of G


Article

The Journal of Physical Chemistry C Table 5. SAPT BE Decompositions (kJ/mol) of H2O Interacting with Li-Modified Linker Molecules benzene C6H5−PO3Li2 C6H5−PO3HLi C6H5−SLi C6H5−OLi C6H5−SO3Li C6H5−COOLi

Table 7. SAPT BE Decompositions (kJ/mol) of the SO2 Interacting with Li-Modified Linker Molecules

site

Ees

Eexch

E*ind

E*disp


−14.86 −133.95 −30.95 −131.73 −121.93 −121.93 −134.49 −91.08 −33.66 −117.60 −110.67 −88.27

14.72 86.90 24.58 81.84 73.01 72.99 81.50 32.81 27.07 62.91 58.94 34.29

−2.43 −23.89 −4.52 −23.43 −22.94 −22.93 −26.06 −16.69 −5.65 −19.47 −18.34 −15.13

−7.74 −12.89 −10.61 −11.50 −14.14 −14.13 −14.15 −2.72 −10.90 −8.88 −8.34 −3.54


Table 6. SAPT BE Decompositions (kJ/mol) of H2S Interacting with Li-Modified Linker Molecules benzene C6H5−PO3Li2 C6H5−PO3HLi C6H5−SLi C6H5−OLi C6H5−SO3Li C6H5−COOLi

site

Ees

Eexch

Eind *

Edisp *


−13.03 −67.67 −23.44 −63.76 −22.01 −77.24 −77.39 −51.43 −26.50 −64.22 −57.89 −57.89

19.93 60.17 30.28 55.07 28.64 71.28 72.25 33.54 33.40 51.40 47.31 47.33

−1.43 −20.79 −2.89 −20.11 −2.84 −23.42 −24.23 −20.00 −4.07 −19.06 −18.57 −18.57

−11.35 −10.70 −15.96 −9.38 −14.61 −18.84 −16.22 −3.13 −16.39 −9.99 −7.58 −7.59

site

Ees

Eexch

E*ind

E*disp


−23.70 −954.63 −26.75 −275.96 −27.04 −110.61 −174.83 −362.58 −36.07 −109.00 −134.55 −134.30

28.81 1291.63 35.97 331.78 36.59 104.57 192.75 473.25 62.47 81.64 129.70 129.31

−3.90 −755.05 −5.36 −99.73 −5.92 −31.96 −58.28 −164.58 −5.51 −33.13 −42.29 −42.23

−14.23 −85.23 −19.19 −38.58 −18.35 −26.29 −33.83 −45.37 −23.86 −18.59 −19.99 −19.96

molecules with Li cations are much weaker, only ranging from −12.25 to −18.38 kJ/mol at LS sites. This can be attributed to the relatively weak polarity of the CO molecule, which can be supported by its lower electrostatic energies than the other polar gas molecules studied in this work (Table 8). Table 8. SAPT BE Decompositions (kJ/mol) of the CO Interacting with Li-Modified Linker Molecules benzene C6H5−PO3Li2 C6H5−PO3HLi C6H5−SLi C6H5−OLi

H2O than H2S. In the case of the C6H5−SLi system, the BEs of H2O and H2S are the largest, up to −81.90 and −51.75 kJ/mol at LS sites, respectively (shown as Figures 6f and 7f). H2O and H2S are located over the plane of the benzene ring, with O and S atoms obliquing to Li+. Here, the directionalities are mainly attributed to electrostatic interactions between O_H2O and S_H2S atoms and Li cations. Figure 8 shows the minimum-energy structures, CHELPG charges, and BEs of SO2 interacting with linker molecules. In the systems of C6H5−PO3Li2···CO2 (Figure 8b), C6H5− PO3HLi···SO2 (Figure 8d), C6H5−OLi···SO2 (Figure 8h), C6H5−SO3Li···SO2 (Figure 8j), and C6H5−COOLi···SO2 (Figure 8l), the SO2 molecules are adsorbed by two simultaneous electrostatic interactions: electrostatic interactions between O_SO2 and the Li cation and electrostatic interactions between S_SO2 and the O atom of linker molecules. Specially, the BE in C6H5−PO3Li2···SO2 (−88.31 kJ/mol, shown in Figure 8b) is much higher than those of other SO2-adsorption systems. In this system, the SO2 molecule obtains interactions from both Li cations of −PO3Li2 group simultaneously. In particular, the dispersion force, electrostatic force, and induction force of this configuration are much higher than others, as shown in Table 7. Figure 9 shows the minimum-energy structures, CHELPG charges, and BEs of CO interacting with linker molecules. For all Li-containing systems, the CO molecule tends to be adsorbed with its oxygen end pointing to Li cations. Compared with H2S, H2O, and SO2, the interactions between CO

C6H5−SO3Li C6H5−COOLi

site

Ees

Eexch

E*ind

E*disp


−5.55 −26.22 −5.44 −22.92 −6.21 −35.54 −8.43 −24.45 −7.44 −24.95 −6.65 −22.80

12.94 23.29 12.55 11.52 13.68 32.59 15.99 10.10 13.80 12.82 14.47 10.58

−0.63 −8.27 −0.70 −11.71 −0.64 −13.87 −1.55 −13.51 −0.97 −13.78 −0.75 −11.71

−8.23 −11.82 −8.85 −2.95 −9.39 −12.09 −9.88 −2.04 −10.07 −2.98 −9.59 −2.63

From practical standpoint, during the experimental process, the strong binding site around Li+ will be first occupied by polar solvent molecules. In addition, H2O, SO2, and H2S show much larger BEs than CO2 and thus can result in competitive adsorption when capturing CO2 from industrial gas mixtures.

■

CONCLUSIONS In this work, ab initio calculations were performed to compare the role of different functional groups in incorporating Li+ for higher gas adsorption amount. From the aspect of BE, the −SLi group was proven to be the most effective one. By incorporating Li+ with various functional groups, the symmetries of HOMO and LUMO in linker molecules are profoundly changed, which can help interactions with gas molecules and enhance gas adsorption. For nonpolar gas molecules, including CH4, H2, N2, and CO2, the interactions with benzene is dominant by dispersion force. However, after incorporating Li+, the electrostatic interaction plays a more important role. The induced polarizations of gas molecules further enhance the interactions. Moreover, in the Licontaining systems, the expanded BE difference of gas mixture can make gas separation easier. For polar gas molecules, the dipole−π interaction plays a key role in gas−benzene systems. H


Article

The Journal of Physical Chemistry C However, after incorporating Li+, electrostatic forces between Li+ and gas molecules sharply enhance the interactions. Our work would be helpful for the development of new MOFs with high gas adsorption efficiency.

■

(12) Liu, Y.; Kasik, A.; Linneen, N.; Liu, J.; Lin, Y. S. Adsorption and Diffusion of Carbon Dioxide on ZIF-68. Chem. Eng. Sci. 2014, 118, 32−40. (13) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Hydrogen Storage in Metal-Organic Frameworks. Chem. Rev. 2011, 112, 782− 835. (14) Fu, J.; Liu, Y.; Tian, Y.; Wu, J. Density Functional Methods for Fast Screening of Metal-Organic Frameworks for Hydrogen Storage. J. Phys. Chem. C 2015, 119, 5374−5385. (15) Yang, Y.; Liu, J.; Zhang, B.; Liu, F. Mechanistic studies of mercury adsorption and oxidation by oxygen over spinel-type MnFe2O4. J. Hazard. Mater. 2017, 321, 154−161. (16) Yang, Q.; Xue, C.; Zhong, C.; Chen, J.-F. Molecular simulation of separation of CO2 from flue gases in Cu-BTC metal-organic framework. AIChE J. 2007, 53, 2832−2840. (17) Wang, K.; Huang, H.; Liu, D.; Wang, C.; Li, J.; Zhong, C. Covalent Triazine-Based Frameworks with Ultramicropores and High Nitrogen Contents for Highly Selective CO2 Capture. Environ. Sci. Technol. 2016, 50, 4869−4876. (18) Shan, B.; Yu, J.; Armstrong, M. R.; Wang, D.; Mu, B.; Cheng, Z.; Liu, J. A cobalt metal-organic framework with small pore size for adsorptive separation of CO2 over N2 and CH4. AIChE J. 2017, 63, 4532−4540. (19) He, Y.; Xiang, S.; Zhang, Z.; Xiong, S.; Fronczek, F. R.; Krishna, R.; O’Keeffe, M.; Chen, B. A Microporous Lanthanide-Tricarboxylate Framework with the Potential for Purification of Natural Gas. Chem. Commun. 2012, 48, 10856−10858. (20) Guo, H.-c.; Shi, F.; Ma, Z.-f.; Liu, X.-q. Molecular Simulation for Adsorption and Separation of CH4/H2 in Zeolitic Imidazolate Frameworks. J. Phys. Chem. C 2010, 114, 12158−12165. (21) Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Introduction to MetalOrganic Frameworks. Chem. Rev. 2012, 112, 673−674. (22) Botas, J. A.; Calleja, G.; Sánchez-Sánchez, M.; Orcajo, M. G. Cobalt Doping of the MOF-5 Framework and Its Effect on GasAdsorption Properties. Langmuir 2010, 26, 5300−5303. (23) Wang, X.-S.; Ma, S.; Forster, P. M.; Yuan, D.; Eckert, J.; López, J. J.; Murphy, B. J.; Parise, J. B.; Zhou, H.-C. Enhancing H2 Uptake by ″Close-Packing″ Alignment of Open Copper Sites in Metal-Organic Frameworks. Angew. Chem., Int. Ed. 2008, 47, 7263−7266. (24) Peng, Y.; Krungleviciute, V.; Eryazici, I.; Hupp, J. T.; Farha, O. K.; Yildirim, T. Methane Storage in Metal-Organic Frameworks: Current Records, Surprise Findings, and Challenges. J. Am. Chem. Soc. 2013, 135, 11887−11894. (25) Kim, K. C.; Fairen-Jimenez, D.; Snurr, R. Q. Computational Screening of Functional Groups for Capture of Toxic Industrial Chemicals in Porous Materials. Phys. Chem. Chem. Phys. 2017, 19, 31766−31772. (26) Hu, J.; Liu, Y.; Liu, J.; Gu, C. Effects of water vapor and trace gas impurities in flue gas on CO2 capture in zeolitic imidazolate frameworks: The significant role of functional groups. Fuel 2017, 200, 244−251. (27) Hu, J.; Liu, Y.; Liu, J.; Gu, C.; Wu, D. High CO2 adsorption capacities in UiO type MOFs comprising heterocyclic ligand. Micropor. Mesopor. Mater. 2018, 256, 25−31. (28) Frysali, M. G.; Klontzas, E.; Tylianakis, E.; Froudakis, G. E. Tuning the interaction strength and the adsorption of CO2 in metal organic frameworks by functionalization of the organic linkers. Micropor. Mesopor. Mater. 2016, 227, 144−151. (29) Frysali, M. G.; Klontzas, E.; Froudakis, G. E. Ab Initio Study of the Adsorption of CO2 on Functionalized Benzenes. ChemPhysChem 2014, 15, 905−911. (30) Yu, K.; Kiesling, K.; Schmidt, J. R. Trace Flue Gas Contaminants Poison Coordinatively Unsaturated Metal-Organic Frameworks: Implications for CO2 Adsorption and Separation. J. Phys. Chem. C 2012, 116, 20480−20488. (31) Kim, K. C.; Lee, C. Y.; Fairen-Jimenez, D.; Nguyen, S. B. T.; Hupp, J. T.; Snurr, R. Q. Computational Study of Propylene and Propane Binding in Metal-Organic Frameworks Containing Highly Exposed Cu+ or Ag+ Cations. J. Phys. Chem. C 2014, 118, 9086−9092.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b03112. Contour line map of ELF of C6H5−COOLi; HOMOs of linker molecules; LUMOs of linker molecules; and plot of BEs versus H2−Li+/N2−Li+ distances (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yang Liu: 0000-0003-4313-0932 Jing Liu: 0000-0001-6520-9612 Weizhou Wang: 0000-0002-4309-9077 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51676079, 21773104). REFERENCES

(1) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic point Sources. ChemSusChem 2009, 2, 796−854. (2) Huang, Y.; Xiao, Y.; Huang, H.; Liu, Z.; Liu, D.; Yang, Q.; Zhong, C. Ionic liquid functionalized multi-walled carbon nanotubes/ zeolitic imidazolate framework hybrid membranes for efficient H2/ CO2 separation. Chem. Commun. 2015, 51, 17281−17284. (3) Yang, J.; Yuan, N.; Xu, M.; Liu, J.; Li, J.; Deng, S. Enhanced Mass Transfer on Hierarchical Porous Pure Silica Zeolite Used for Gas Separation. Micropor. Mesopor. Mater. 2018, 266, 56−63. (4) Yuan, D.; Lu, W.; Zhao, D.; Zhou, H.-C. Highly Stable Porous Polymer Networks with Exceptionally High Gas-Uptake Capacities. Adv. Mater. 2011, 23, 3723−3725. (5) Yang, Y.; Liu, J.; Shen, F.; Zhao, L.; Wang, Z.; Long, Y. Kinetic Study of Heterogeneous Mercury Oxidation by HCl on Fly Ash Surface in Coal-Fired Flue Gas. Combust. Flame 2016, 168, 1−9. (6) Wang, Z.; Liu, J.; Zhang, B.; Yang, Y.; Zhang, Z.; Miao, S. Mechanism of Heterogeneous Mercury Oxidation by HBr over V2O5/ TiO2 Catalyst. Environ. Sci. Technol. 2016, 50, 5398−5404. (7) Wang, G.; Tian, Y.; Jiang, J.; Wu, J. Multimodels Computation for Adsorption Capacity of Activated Carbon. Adsorpt. Sci. Technol. 2018, 36, 508−520. (8) Zhou, J.; Su, W.; Sun, Y.; Deng, S.; Wang, X. Enhanced CO2 Sorption on Ordered Mesoporous Carbon CMK-3 in the Presence of Water. J. Chem. Eng. Data 2016, 61, 1348−1352. (9) Yuan, B.; Wang, J.; Chen, Y.; Wu, X.; Luo, H.; Deng, S. Unprecedented performance of N-doped activated hydrothermal carbon towards C2H6/CH4, CO2/CH4, and CO2/H2 separation. J. Mater. Chem. A 2016, 4, 2263−2276. (10) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Carbon Dioxide Capture in Metal-Organic Frameworks. Chem. Rev. 2011, 112, 724− 781. (11) Hu, Z.; Nalaparaju, A.; Peng, Y.; Jiang, J.; Zhao, D. Modulated Hydrothermal Synthesis of UiO-66(Hf)-Type Metal-Organic Frameworks for Optimal Carbon Dioxide Separation. Inorg. Chem. 2016, 55, 1134−1141. I


Article


ment and Quantum Chemical Calculations. Chem. Phys. 2006, 320, 75−83. (50) Choi, Y. C.; Kim, W. Y.; Park, K. S.; Tarakeshwar, P.; Kim, K. S.; Kim, T.-S.; Lee, J. Y. Role of Molecular Orbitals of the Benzene in Electronic Nanodevices. J. Chem. Phys. 2005, 122, 094706. (51) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.02; Gaussian. Inc.: Wallingford, CT, 2009. (52) Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange, A. W.; Behn, A.; Deng, J.; Feng, X.; et al. Advances in Molecular Quantum Chemistry Contained in the QChem 4 Program Package. Mol. Phys. 2015, 113, 184−215.

(32) Liu, Y.; Liu, J.; Chang, M.; Zheng, C. Theoretical studies of CO2 adsorption mechanism on linkers of metal-organic frameworks. Fuel 2012, 95, 521−527. (33) Pramudya, Y.; Mendoza-Cortes, J. L. Design Principles for High H2 Storage Using Chelation of Abundant Transition Metals in Covalent Organic Frameworks for 0−700 bar at 298 K. J. Am. Chem. Soc. 2016, 138, 15204−15213. (34) Lan, J.; Cao, D.; Wang, W.; Smit, B. Doping of Alkali, AlkalineEarth, and Transition Metals in Covalent-Organic Frameworks for Enhancing CO2 Capture by First-Principles Calculations and Molecular Simulations. ACS Nano 2010, 4, 4225−4237. (35) Mendoza-Cortés, J. L.; Han, S. S.; Goddard, W. A., III High H2 Uptake in Li-, Na-, K-Metalated Covalent Organic Frameworks and Metal Organic Frameworks at 298 K. J. Phys. Chem. A 2012, 116, 1621−1631. (36) Mavrandonakis, A.; Klontzas, E.; Tylianakis, E.; Froudakis, G. E. Enhancement of Hydrogen Adsorption in Metal−Organic Frameworks by the Incorporation of the Sulfonate Group and Li Cations. A Multiscale Computational Study. J. Am. Chem. Soc. 2009, 131, 13410−13414. (37) Hu, Z.; Khurana, M.; Seah, Y. H.; Zhang, M.; Guo, Z.; Zhao, D. Ionized Zr-MOFs for highly efficient post-combustion CO2 capture. Chem. Eng. Sci. 2015, 124, 61−69. (38) Hu, J.; Liu, J.; Liu, Y.; Yang, X. Improving Carbon Dioxide Storage Capacity of Metal Organic Frameworks by Lithium Alkoxide Functionalization: A Molecular Simulation Study. J. Phys. Chem. C 2016, 120, 10311−10319. (39) Yang, Y.; Liu, J.; Liu, F.; Wang, Z.; Miao, S. Molecular-Level Insights into Mercury Removal Mechanism by Pyrite. J. Hazard. Mater. 2018, 344, 104−112. (40) Yoon, J. W.; Chang, H.; Lee, S.-J.; Hwang, Y. K.; Hong, D.-Y.; Lee, S.-K.; Lee, J. S.; Jang, S.; Yoon, T.-U.; Kwac, K.; et al. Selective Nitrogen Capture by Porous Hybrid Materials Containing Accessible Transition Metal Ion Sites. Nat. Mater. 2017, 16, 526. (41) Parkes, M. V.; Greathouse, J. A.; Hart, D. B.; Sava Gallis, D. F.; Nenoff, T. M. Ab initio molecular dynamics determination of competitive O2 vs. N2 adsorption at open metal sites of M2(dobdc). Phys. Chem. Chem. Phys. 2016, 18, 11528−11538. (42) Rosnes, M. H.; Sheptyakov, D.; Franz, A.; Frontzek, M.; Dietzel, P. D. C.; Georgiev, P. A. On the Elusive Nature of Oxygen Binding at Coordinatively Unsaturated 3d Transition Metal Centers in Metal-Organic Frameworks. Phys. Chem. Chem. Phys. 2017, 19, 26346−26357. (43) Henley, A.; Lennox, M. J.; Easun, T. L.; Moreau, F.; Schröder, M.; Besley, E. Computational Evaluation of the Impact of Incorporated Nitrogen and Oxygen Heteroatoms on the Affinity of Polyaromatic Ligands for Carbon Dioxide and Methane in MetalOrganic Frameworks. J. Phys. Chem. C 2016, 120, 27342−27348. (44) Stoneburner, S. J.; Livermore, V.; McGreal, M. E.; Yu, D.; Vogiatzis, K. D.; Snurr, R. Q.; Gagliardi, L. Catechol-Ligated Transition Metals: A Quantum Chemical Study on a Promising System for Gas Separation. J. Phys. Chem. C 2017, 121, 10463−10469. (45) Szabo, A.; Ostlund, N. S. Modern Quantum Chemistry: Introduction to Advanced Electronic Theory; McGraw-Hill: New York, 1989. (46) Boys, S. F.; Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553−566. (47) Breneman, C. M.; Wiberg, K. B. Determining Atom-Centered Monopoles from Molecular Electrostatic Potentials. The Need for High Sampling Density in Formamide Conformational Analysis. J. Comput. Chem. 1990, 11, 361−373. (48) Jeziorski, B.; Moszynski, R.; Szalewicz, K. Perturbation Theory Approach to Intermolecular Potential Energy Surfaces of van der Waals Complexes. Chem. Rev. 1994, 94, 1887−1930. (49) Chakraborty, A.; Kar, S.; Guchhait, N. Photoinduced Intramolecular Charge Transfer (ICT) Reaction in Trans-Methyl p(dimethylamino) Cinnamate: A Combined Fluorescence MeasureJ


Ab Initio Study of Gas Adsorption in MetalâOrganic Frameworks

Recommend Documents

Ab Initio Study of Gas Adsorption in MetalâOrganic Frameworks