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Improving Carbon Dioxide Storage Capacity of Metal Organic Frameworks by Li Alkoxide Functionalization: A Molecular Simulation Study Jianbo Hu, Jing Liu, Yang Liu, and Xiao Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01119 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 5, 2016

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The Journal of Physical Chemistry

Improving Carbon Dioxide Storage Capacity of Metal Organic Frameworks by Li Alkoxide Functionalization: A Molecular Simulation Study Jianbo Hu, Jing Liu,* Yang Liu, Xiao Yang State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China ABSTRACT: Metal organic frameworks represent a new kind of porous solids and have shown promising potential for CO2 capture and seperation. In this work, grand canonical Monte Carlo simulations were performed to explore Li alkoxide functionalization for improving CO2 adsorption capacity in HKUST-1, MOF-143 and MOF-399. The results show that Li alkoxide functionalization remarkably improves the CO2 uptake ability in all three kinds of MOFs at 298 K, especially at low pressure range. The CO2 uptake amount in Li functionalized HKUST-1 increased more than 1,700% compared with its unfunctionalized form at 1 kPa. Furthermore, the extension of organic linkers leads to lower CO2 adsorption capacity at low pressure range due to the lower isosteric heat, but higher CO2 adsorption capacity at high pressure range resulting from the increase of total free volume. Specifically, the incorporation of Li atoms onto the frameworks induced a shift of preferential adsorption sites for CO2. The CO2 molecules were first adsorbed around the Li atoms in the three Li functionalized MOFs.

1. INTRODUCTION

1 ACS Paragon Plus Environment


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Carbon dioxide (CO2) is a major component of greenhouse gases and plays a significant role in global warming. The capture of CO2 from coal- or gas-burning power plants is an attractive route to reduce CO2 emissions into the atmosphere.1,2 Many approaches, including cryogenic distillation, amine scrubbing, membrane separation, and sorbent adsorption, have been proposed for CO2 capture.3 Compared to other methods, CO2 capture by adsorption in porous materials is superior in aspects of high energy efficiency, low capital cost, large separation capability and can easily be scaled up. Owing to their large volume, variety structures and tunable functionality, metal organic frameworks (MOFs) have been considered as an ideal platform for the development of next generation CO2 capture materials.4-7 Yaghi et.al8 investigated the CO2 adsorption abilities of nine MOFs with different topologies and found that the CO2 adsorption capacity of MOF-177 can be up to 33 mmol/g at 35 bar, which is far greater than that in any other reported porous materials for CO2 storage. Francesco et.al9 compared the CO2 adsorption performance of HKUST-1 with that of 13X zeolite, which is considered as benchmark materials. The results indicated that HKUST-1 has a noticeably higher adsorption capacity toward CO2 than 13X zeolite. Comparing to traditional experimental methods, simulations can provide more details of chemical phenomena in molecular level.10,11 Recently, numerous simulation studies have been performed for screening high CO2 capture materials.12-16 Snurr et.al12 studied the mechanism of the steps in CO2 isotherms of MOFs by molecular simulation and suggested that the attractive electrostatic interactions between CO2 molecules are responsible for the unusual shape of the adsorption isotherms studied. Babarao and Jiang13 simulated the storage of CO2 in a series of MOFs at room temperature, and reported that the organic linker plays a critical role in tuning the free volume and accessible surface area and determining CO2 uptakes at high pressures. Yang et


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al14 investigated the adsorption behaviors of CO2 in nine typical MOFs and found that a suitable pore size between 1.0 and 2.0 nm is essential for a MOF to have high CO2 adsorption capacity. Thus, molecular simulation is an indispensable tool for screening the most appropriate MOF material for CO2 capture application. The interaction of the CO2 with organic linkers is a critical parameter that has a significant influence on the CO2 adsorption in MOFs.5 The adsorption energy can be enhanced by installing highly charged groups, such as polar organic substituents or exposed metal cation sites.17 One promising strategy is incorporation of metal atoms such as lithium into MOF linkers. Simulations of Li doped into some typical MOF linkers indicated that the doped Li atoms can effectively donate electron density to MOF linkers.18,19 Numerous simulation and experiment investigations demonstrated that Li doping into MOF linkers can significantly enhance the H2 adsorption capacities of MOFs,20-25 only a few studies have been performed in the area of CO2 capture.26-29 Lan et.al26 investigated the effect of doped metals (Li, Na, K, Be, Mg, Ca, Sc and Ti) into COFs on CO2 capture and the results show that Li is the best surface modifier of COFs for CO2 capture, the excess CO2 uptakes of the Li-doped COFs can be enhanced by four to eight times compared to the undoped COFs at 298 K and 1 bar. However, the effect of Li doping on CO2 adsorption capacity is still unclear. To achieve even higher CO2 adsorption capacity, it is important to further investigate the effect of Li doping on CO2 adsorption in MOFs. In this work, we investigated the effect of Li doping into MOF linkers for CO2 capture by grand canonical Monte Carlo (GCMC) simulations. The Li atoms were introduced in the structure of various different MOFs: HKUST-1, MOF-143 and MOF-399 by creating Li alkoxide groups on the organic linkers of MOFs. The CO2 adsorption capacity of the functionalized MOFs were calculated and compared with that of unfunctionalized MOFs to investigate the effect of Li



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alkoxide functionalization on CO2 adsorption. To our best knowledge, this is the first work to explore the effect of Li alkoxide functionalization on CO2 storage capacity.

2. MODEL AND SIMULATION METHOD 2.1. MOF Structure. The structures of HKUST-1,30 MOF-14331 and MOF-39931 were constructed from experimental X-ray diffraction (XRD) data30,31 as shown in Figure 1 a-c. HKUST-1 and MOF-399 have the same primitive tbo topology with Cu2 paddlewheels linked by different triangular organic linkers: HKUST-1 contains benzene-1,3,5-tricarboxylate (BTC) links, whereas MOF-399 contains benzene-1,3,5-triyl-tris (benzene-4,1-diyl)) tribenzoate (BBC) links. MOF-143 is composed by the Cu2 paddlewheels bridged by the 4,40,400-(benzene-1,3,5triyltribenzoate (BTB) links, resulting in an augmented pto net. The structural properties of these three MOFs are given in Table 1. The accessible surface area was calculated using a probe molecule with the diameter equal to the van der walls diameter of CO2 (3.3 Å). The total free volume was calculated using a probe molecules in size of 0.0 Å. Probing the material with a probe size of 0.0 Å can determine the total free volume in the unit cell that is not occupied by framework atom.32 Li-modified MOFs were constructed by adding a single Li alkoxide group to each linker, as shown in Figure 1 d-f. This method can incorporate Li cations into MOF linkers more stably, and has been demonstrated experimentally.22,23,25 The obtained functionalized MOF structures were then optimized with constant cell size using the Accelrys Forcite module.33 2.2. Force Field. The interactions of gas-adsorbent and gas-gas were described using the combination of site-site Lennard-Jones (LJ) and a Coulombic potentials:

ui j ( r ) =

∑

α ∈i β ∈j

  4ε αβ 

 σ  αβ  γ αβ 

12

   

 σ αβ − γ  αβ

   

6

 qαqβ  +  4πε 0γ αβ 

   

(1)

where ε0 = 8.8542 × 10-12 C2N-1m-2 is the permittivity of the vacuum, and σαβ, εαβ are the collision 4 ACS Paragon Plus Environment

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diameter and well depth, respectively. qα and qβ are the partial charges of the interaction atoms. In this study, CO2 was represented as a rigid linear molecule with three charged LennardJones(LJ) sites located on each atoms and with a C-O bond length of 1.16 Å, taken from the TraPPE force field developed by Potoff and Siepmann.34 The intrinsic quadrupole moment is approximately described by partial point charges centered at each LJ sites (qO = −0.35 e and qC= 0.70 e). Dreding force field,35 which has successfully been used to study the adsorption and separation of gases in various MOFs,36-38 was used to calculate the interactions between adsorbates and adsorbents. LJ parameters for Cu and Li were taken from the Universal Force Field (UFF).39 All Lennard-Jones parameters were mixed by use of Lorentz-Berthelot rules, as shown in Table 2. In all simulations, atomic partial charges for the MOFs are required as input parameters. The partial atomic charges in these MOFs were derived from DFT calculations using the CHELPG method.40 DFT calculations were performed on clusters isolated from the unit cell of each MOF in B3LYP level using Gaussian 03 package. Specifically, the LANL2DZ basis set was used for Cu atoms, while the 6-31G* basis set41 was used for the remaining atoms. For the cleaved clusters of these MOFs, the terminations are connected with the organic linkers, and the clusters

saturated with –CH3 group were found to be good enough for the calculations. The

clusters used in the calculations and the resulting atomic charges are shown in Figure 2. 2.3. Grand Canonical Monte Carlo Simulation. The conventional GCMC simulation technique was employed for investigating the adsorption of CO2 in the MOFs. The simulation box representing MOF-399 contained 1 (1 × 1 × 1) unit cells, while 8 (2 × 2 × 2) unit cells were adopted for other MOFs. The framework was treated as rigid with atoms frozen at their crystallographic positions during simulations. A cutoff radius of 12.8 Å was applied to calculate



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the non-bonded interactions, and the Coulomb interactions were handled using the Ewald summation technique with tin-foil boundary condition.42 All GCMC simulations included a 1.0 × 107 cycle equilibration period followed by a 1.0 × 107 cycle production run. Each cycle is N steps where N is the number of molecules in the system (which fluctuates during a GCMC simulation). In each step, random insertion, deletion, and translation moves were sampled with equal probabilities. The input fugacities were calculated using the Peng-Robinson equation of state. All simulated adsorption data reported in this work is the excess quantity. The isosteric heat of adsorption qst was calculated from: qst = RT −

Uf f N − Uf f N

2

− N

N N

−

Usf N − Usf N

2

− N

N

(2)

N

Where R is the gas constant, N is the number of molecules adsorbed, and 〈 〉 indicates the ensemble average. The first and the second terms are the contributions from the molecular thermal energy and adsorbate - adsorbate interaction energy Uff, respectively. The third term is the contribution from the adsorbent – adsorbate interaction energy Usf.

3. RESULTS AND DISCUSSIONS 3.1. Validation of the Force Field. To confirm the reliability of the force field and the method for calculating atomic partial charges employed in this work, the adsorption isotherms of CO2 were simulated and compared with available experimental data, as shown in Figure 3. As can be seen from the figure, the simulated isotherms are in reasonable agreement with the corresponding

experimental

results

for

HKUST-1.43,44

Considering

the

experimental

uncertainties, the force fields used are applicable to the MOFs considered in this work. It should be noticed that no experimental data of CO2 adsorption in MOF-143 and MOF-399 has been reported by far, thus their simulated data cannot be verified by experimental data. However, the


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good agreement obtained between simulated and experimental data for HKUST-1 indicates that the force field and charge calculating method used in this work should also be applicable to MOF-143 and MOF-399 consisting of Cu2 paddlewheels clusters and aromatic linkers with similar chemical functionality. 3.2. CO2 Adsorption Properties in Unfunctionalized MOFs. The CO2 adsorption isotherms were calculated by GCMC simulation. Figure 4a shows the gravimetric CO2 uptake amounts in HKUST-1, MOF-143 and MOF-399 at 298 K up to 100 kPa. One can see that the CO2 uptake amount of HKUST-1 is higher than that of MOF-143 and MOF-399 at low pressure. The CO2 uptake amount in HKUST-1 is 3.16 mmol/g at 1 bar, while that in MOF-143 and MOF399 are only 0.91 and 0.94 mmol/g, respectively. This can be attributed to the higher open metal site molarity45 of HKUST-1 than that of MOF-143 and MOF-399. Figure 5 shows the changes of isosteric heat of CO2 adsorption in the three MOFs with the increase of CO2 loading amount. The isosteric heat of CO2 adsorption in HKUST-1 is 28.29 kJ/mol at initial dilution, and then it decreases as CO2 loading increase. In MOF-143 and MOF-399, the isosteric heat of CO2 adsorption at initial dilution is 12.56 and 11.05 kJ/mol, respectively, and then it keeps constant with the increase of CO2 loading. The isosteric heat of CO2 adsorption in HKUST-1 is larger than that in MOF-143 or MOF-399, which is corresponding with the CO2 adsorption amount in these three MOFs. As the pressure increasing, the CO2 adsorption isotherm of HKUST-1 and MOF-143 are saturated at 1500 kPa and 3000 kPa, respectively, whereas CO2 adsorption in MOF-399 is still unsaturated even over 5000 kPa, as shown in Figure 4b. The CO2 uptake amounts of HKUST-1, MOF-143 and MOF-399 at 5000 kPa are 14.30, 41.68 and 61.26 mmol/g, respectively. This can be attributed to the fact that the extension from BTC to BTB or BBC has significantly increased



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the surface area, which is directly related to the excess CO2 uptake amount at high pressure range.13,14 However, although the surface area of MOF-399 is higher than that of MOF-143, the CO2 adsorption amount of MOF-399 is smaller than that of MOF-143 when the pressure is lower than 3500 kPa. This can be attributed to that the interactions between the CO2 molecules and the framework atoms in MOF-399 are too weak to physically adsorb CO2 molecules effectively at moderate pressure. Thus, the amount adsorbed in MOF-399 does not show evident correlation with its accessible surface area up to 3500 kPa as depicted in Figure 4b. It is worth noting that the uptake amount of CO2 in MOF-399 at 298 K and 5000 kPa exceeds those of MOF-200 and MOF-210 (54.68 mmol/g)46 at similar conditions, which are the CO2 adsorption capacity record holder by far. The volumetric capacity is critical to be considered for the practical limits associated with the tank volume required to house the adsorbent for CO2 storage. The CO2 volumetric uptake capacities have also been investigated as shown in Figure 4c. HKUST-1 has the highest CO2 uptake capacity among all three materials when the pressure is lower than 2500 kPa. This is due to its high crystal density (0.879 g/cm3) compared with other two MOFs. When the pressure is higher than 2500 kPa, the excess CO2 adsorption amount of MOF-143 is higher than that of HKUST-1 because of the higher void fraction of MOF-143 than that of HKUST-1. In comparison, MOF-399 has the lowest predicted volumetric CO2 capacity at all pressure range among the three MOFs among all materials. The low crystal density of MOF-399 (0.126 g/cm3) drastically reduces its volumetric geometric surface area (surface area per volume) (Table 1), which is related to the volumetric capacity of CO2 in MOFs. Correspondingly, MOF-399 is not saturated even over 5000 kPa due to its highest void fraction, while HKUST-1 and MOF-143 are saturated at 1500 and 3000 kPa, respectively.


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The knowledge of CO2 adsorption sites in MOFs is important in understanding the adsorption mechanism. Figure 6 shows the center of mass (COM) probability distributions of CO2 at different pressures in HKUST-1, MOF-143 and MOF-399. As shown in Figure 6 a-c, the preferential adsorption sites for CO2 are different in the three MOFs: CO2 molecules are mainly adsorbed in the small cage in HKUST-1 at low pressure, while in MOF-143 and MOF-399, the open metal sites become the preferential adsorption sites. Normally, the interaction force between CO2 and open metal site is large, which results in that the open metal sites are the preferential adsorption sites, such as the cases of MOF-143 and MOF-399 in this work. However, HKUST-1 is different from those materials because the pore size of the small cage in HKUST-1 is around 5 Å, which is accessible to CO2 molecules. Meanwhile, the small cage has a dimension and three-fold symmetry, and thus the small cage is well matched with the adsorbed CO2 molecules, leading to multiple interactions between the CO2 molecules and the surrounding framework. Therefore, the CO2 molecules are mainly adsorbed in the small cages in HKUST-1 at low pressure. With the increase of pressure, as shown in Figure 6 d-i, many CO2 molecules begin to be adsorbed in the large pores in HKUST-1, while in MOF-399 and MOF-143, the CO2 molecules start to adsorb on the linkers. 3.3. Effect of Li Alkoxide Functionalization. The effect of Li alkoxide functionalization on CO2 adsorption in MOFs was calculated by GCMC simulations. As shown in Figure 7 a, Li alkoxide functionalization remarkably improves the CO2 uptake ability in all three MOFs at 298 K and low pressure range. For instance, the CO2 uptake amount of Li alkoxide functionalized HKUST-1 is 12.05 mmol/g at 100 kPa and 298 K, which is about four times higher than that of unfunctionalized HKUST-1. For Li alkoxide functionalized MOF-143 and MOF-399, the CO2 uptake amounts at 298 K and 100 kPa are 3.01 and 2.71 mmol/g, respectively, which also



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surpass the values of their unfunctionalized forms. Figure 8 shows the changes of isosteric heat of CO2 adsorption in the Li alkoxide functionalized MOFs with the increase of CO2 loading amount. The data are also compared with that in the unfunctionalized MOFs. The isosteric heat of CO2 adsorption in Li alkoxide functionalized HKUST-1, MOF-143 and MOF-399 at initial dilution is 45.49, 32.48 and 24.42 kJ/mol, respectively, and then it decreases as CO2 loading increases. The isosteric heat of CO2 adsorption heat in Li alkoxide functionalized MOFs is higher than that in unfunctionalized forms, which explains the remarkably improved CO2 adsorption amounts in the Li alkoxide functionalized MOFs at low pressure range. The incorporation of Li atoms in the MOFs can increase the density of open metal sites, which is related to the number of open metal sites pre unit volume. Increasing the density of open metal sites increases the density of sites with high potential energy in the MOFs. Thus the isosteric heat of CO2 adsorption increases in this work. Furthermore, the trend of CO2 adsorption in Li alkoxide functionalized HKUST-1 is non-linear at low pressure due to the high binding energy, while it is linear for all other MOFs studied in this work. The saturation pressure for Li alkoxide functionalized HKUST-1 is about 500 kPa, which is much lower than that for unfunctionalized HKUST-1, as shown in Figure 7b. This can be attributed to that the incorporation of Li atom into HKUST-1 leads to a much higher binding energy of CO2, lower accessible surface area and total free volume than those of unfunctionalized forms. It means the CO2 binding strength increases but the binding sites available for CO2 decrease after Li incorporation. Thus the CO2 adsorption isotherm of Li functionalized HKUST-1 would be saturated at a lower pressure than that of unfunctionalized HKUST-1. The uptake amount of CO2 in Li alkoxide functionalized HKUST-1, MOF-143 and MOF-399 at 3000 kPa and 298 K are 15.73, 42.37 and 41.75 mmol/g, respectively, which are


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corresponded with the order of accessible surface area of MOFs. The CO2 adsorption capacity of Li alkoxide functionalized HKUST-1 and MOF-143 are similar to that of their unfunctionalized forms at 3000 kPa, the reason is that the incorporation of Li atoms cannot increase the accessible surface areas of MOFs (Table 1). Figure 9 shows the percentage contribution of Li alkoxide functionalization at different pressure. The contribution of Li alkoxide functionalization to the CO2 adsorption capacity was defined as (NLi - N)/N, where NLi and N denote the CO2 adsorption capacity in Li alkoxide functionalized MOFs and unfunctionalized MOFs, respectively. It was obvious that the effect of Li alkoxide functionalization decreased as the pressure increases. The enhancement can be as large as more than 1,700% at initial pressure, which indicated an unprecedented enhancement in CO2 adsorption capacity. Furthermore, it can be seen from Figure 9 that the contribution of Li alkoxied functionalization is more evident in HKUST-1 at initial pressure, whereas it becomes less obvious with increasing pressure. This can be attributed to the fact that the CO2 adsorption amount in HKUST-1 is higher than that of the other two MOFs. Thus the contribution from Li alkoxide functionalization in HKUST-1 decreases more quickly than that in the other two MOFs. The CO2 adsorption sites in Li funcitionalized MOFs were studied by examining the COM probability distributions. Figure 10 shows the probability distribution plots of COM of CO2 in Li alkoxide functionalized MOFs. Specifically, the COM probability distribution of CO2 adsorption in Li alkoxide functionalized HKUST-1 at pressure of 0.1 kPa, 100 kPa and 1000 kPa was examined due to its low saturation pressure (500 kPa). As shown in Figure 10, CO2 molecules are first adsorbed on the Li atoms in all three Li alkoxide functionalized MOFs, then the CO2 molecules begin to distribute in the pores with the increase of pressure. This indicates that the Li atoms are the preferential sites for CO2 adsorption in the Li alkoxede functionalized MOFs. The



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shift of preferential adsorption sites for CO2 can be attributed to that the electrostatic interactions between CO2 and the framework, which is the predominant factor that determines the preferential adsorption sites for CO2 in the MOFs. The incorporation of Li atoms changed the distribution of electrostatic potential.

4. CONCLUSIONS The effect of Li alkoxide functionalization on CO2 capture was investigated by grand canonical Monte Carlo simulations. The results show that Li alkoxide functionalization remarkably improves the CO2 uptake ability in all three kinds of MOFs at 298 K, especially at low pressure range. The CO2 uptake amount in Li functionalized HKUST-1 increased more than 1, 700% compared with its unfunctionalized form at 1 kPa. Furthermore, the extension of organic linkers leads to lower CO2 adsorption capacity at low pressure range due to the lower isosteric heat, but higher CO2 adsorption capacity at high pressure range resulting from the increase of total free volume. Specifically, the incorporation of Li atoms onto the frameworks induced a shift of preferential adsorption sites for CO2, the CO2 molecules were first adsorbed around the Li atoms in the three Li functionalized MOFs. ■ AUTHOR INFORMATION Corresponding author *Tel: +86 27 87545526; fax: +86 27 87545526; e-mail address: [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS


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This work was supported by National Basic Research Program of China (2014CB238904), Natural Science Foundation of Hubei Province (2015CFA046), and the Foundation of State Key Laboratory of Coal Combustion (FSKLCCB1502). ■ REFERENCES (1) Haszeldine, R. S. Carbon Capture and Storage: How Green Can Black Be? Science 2009, 325, 1647−1652. (2) Wang, M.; Liu, J.; Hu, J.; Liu, F. O2-CO2 Mixed Gas Production Using a Zr-Doped CuBased Oxygen Carrier. Ind. Eng. Chem. Res. 2015, 54, 9805−9812. (3) Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO2 Capture Technology-the U.S. Department of Energy's Carbon Sequestration Program. Int. J. Greenh. Gas. Con. 2008, 2, 9−20. (4) Li, J. R.; Ma, Y.; McCarthy, M. C.; Sculley, J.; Yu, J.; Jeong, H. K.; Balbuena, P. B.; Zhou, H. C. Carbon Dioxide Capture-Related Gas Adsorption and Separation in Metal-Organic Frameworks. Coordin. Chem. Rev. 2011, 255, 1791−1823. (5) 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. (6) Zhang, Z.; Yao, Z.-Z.; Xiang, S.; Chen, B. Perspective of Microporous Metal-Organic Frameworks for CO2 Capture and Separation. Energ. Environ. Sci. 2014, 7, 2868−2899. (7) Liu, Y.; Kasik, A.; Linneen, N.; Liu, J.; Lin, Y. Adsorption and Diffusion of Carbon Dioxide on ZIF-68. Chem. Eng. Sci. 2014, 118, 32−40.



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Structural and Limiting CO2 Adsorption Properties of the MOFs Studied in This Work Sacc

Sacc

qst0

g/cm3 cm3/g fraction

m2/g

m2/cm3

(kJ/mol)

11.130

0.88

0.818

0.72

2368

2081

28.79

27.472

20.431

0.34

2.527

0.86

5396

1834

12.56

MOF-399

68.312

43.231

0.126

7.449

0.94

6445

812

11.05

Li-HKUST-1

26.34

-

0.943

0.722

0.68

2082

1963

45.49

Li-MOF-143

27.472

-

0.354

2.403

0.85

5306

1878

32.48

Li-MOF-399

68.312

-

0.130

7.222

0.94

6376

828

24.42

Unit cells

dpore

Å

Å

HKUST-1

26.34

MOF-143

MOF

ρcrsy

Vfree

Void



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2. Lennard-Jones Parameters Used in This Work Atom types

σ (Å)

ε/kb (k)

CO2-C

2.8

27

CO2-O

3.05

79

MOF-Cu

3.114

2.546

MOF-Li

2.184

12.58

MOF-C

3.473

47.856

MOF-O

3.033

48.158

MOF-H

2.846

7.649


Page 20 of 33

Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


List of Figures Captions Figure 1. MOF structures used in this work. (a) HKUST-1, (b) MOF-143, (c) MOF-399, (d) LiHKUST-1, (e) Li-MOF-143 and (f) Li-MOF-399. (C: gray; H: white; O: red; Cu: orange; Li: purple). Figure 2. Clusters used for deriving partial charges and the atomic partial charges obtained. (a) HKUST-1, (b) MOF-143, (c) MOF-399, (d) Li-HKUST-1, (e) Li-MOF-143 and (f) LiMOF-399. (C: gray; H: white; O: red; Cu: orange; Li: purple). Figure 3. Comparison of simulated pure CO2 isotherms and experimental data in HKUST-1 up to 3000 kPa at 295 K. (Sim represents simulated data, Exp represents experimental data). Figure 4. Simulated CO2 adsorption isotherms in HKUST-1 (black, square), MOF-143 (red, circle) and MOF-399 (blue, triangle) at 298 K. (a) Low pressure gravimetric (0-100 kPa), (b) high pressure gravimetric (0-5000 kPa) and (c) high pressure volumetric. Figure 5. Variation of isosteric heat with CO2 uptake amount in HKUST-1 (black, square), MOF-143 (red, circle) and MOF-399 (blue, triangle) at 298 K. Figure 6. Probability distribution plots of COM of CO2 in HKUST-1 (left), MOF-143 (center) and MOF-399 (right) at different pressure. (a, b, c) low pressure, (d, e, f) intermediate pressure and (g, h, i) high pressure. (C: gray; H: white; O: red; Cu: orange). Figure 7. Simulated CO2 adsorption isotherms in Li alkoxide functionalized MOFs at 298 K. (a) 0-100 kPa; (b) 0-3000 kPa. Figure 8. Variation of isosteric heat with CO2 uptake amount in Li alkoxide functionalized MOFs at 298 K.



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Page 22 of 33

Figure 9. Percentage contribution of Li alkoxide functionalization to CO2 adsorption amount in HKUST-1, MOF-143 and MOF-399 at 298 K. (a) 0-400 kPa; (b) 0-3000 kPa. Figure 10. Probability distribution plots of COM of CO2 in Li alkoxide functionalized HKUST-1 (left), MOF-143 (center) and MOF-399 (right) at different pressure. (a, b, c) low pressure, (d, e, f) intermediate pressure and (g, h, i) high pressure. (C: gray; H: white; O: red; Cu: orange; Li: purple).


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) HKUST-1

(b) MOF-143

(c) MOF-399

(d) Li modified HKUST-1

(e) Li modified MOF-143

(f) Li modified MOF-399

Figure 1. MOF structures used in this work. (a) HKUST-1, (b) MOF-143, (c) MOF-399, (d) LiHKUST-1, (e) Li-MOF-143 and (f) Li-MOF-399. (C: gray; H: white; O: red; Cu: orange; Li: purple).



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Page 24 of 33

1.062

1.080

-0.061 -0.089 -0.669 0.123 -0.129 0.090 0.090 -0.198

0.101 -0.114 -0.655 -0.001

0.070 0.784 0.111 0.789

(a) HKUST-1

(b) MOF-143

1.130 0.199 0.595 0.128-0.252-0.701 -0.124 -0.109 0.111 0.126 0.092 0.062 -0.206 0.150 -0.178 0.040 0.070

1.050

0.797 -0.649 0.529 -0.336

-0.680 0.033 0.120 -0.200 0.798

0.193

(d) Li modified HKUST-1

(c) MOF-399 1.030 1.066

0.679 0.914 -0.265-0.154 -0.074 0.119 -0.6720.0810.144 -0.1570.002 0.1460.081 0.2930.010 0.073 -0.500 0.2470.232 0.092-0.065 -0.071 -0.265 0.146 0.028 -0.090

0.115 -0.054 0.100 0.100-0.054 -0.106 -0.115 0.140 -0.106-0.676 -0.195 0.033 -0.159 0.127 0.115 0.032 -0.134 -0.0350.175 0.115 0.051 0.394-0.159 0.745-0.728-0.134 0.140 0.1000.210 -0.201 -0.054 0.115 -0.106 0.798

-0.040

0.130

-0.613

(f) Li modified MOF-399

(e) Li modified MOF-143

Figure 2. Clusters used for deriving partial charges and the atomic partial charges obtained. (a) HKUST-1, (b) MOF-143, (c) MOF-399, (d) Li-HKUST-1, (e) Li-MOF-143 and (f) Li-MOF-399. (C: gray; H: white; O: red; Cu: orange; Li: purple).


Page 25 of 33

16

Uptake amount (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14 12 10 Sim(295K) Exp(Ref 42/295K) Exp(Ref 43/Sample A/295K)

8 6 4 2 0

0

500

1000

1500

2000

2500

3000

Pressure (kPa) Figure 3. Comparison of simulated pure CO2 isotherms and experimental data in HKUST-1 up to 3000 kPa at 295 K. (Sim represents simulated data, Exp represents experimental data).



3.5

HKUST-1 MOF-143 MOF-399

(a)


3.0 2.5 2.0 1.5 1.0 0.5 0.0

0 10 20 30 40 50 60 70 80 90 100

Pressure (kPa) (b)


60 50 40 30 20 10 0

0

1000

2000

3000

4000

5000

Pressure (kPa)

350

(c)

300 250

3

3

Uptake amount (cm (STP)/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

200 150 100 50 0

0

1000

2000

3000

4000

5000

Pressure (kPa)

Figure 4. Simulated CO2 adsorption isotherms in HKUST-1 (black, square), MOF-143 (red, circle) and MOF-399 (blue, triangle) at 298 K. (a) Low pressure gravimetric (0-100 kPa), (b) high pressure gravimetric (0-5000 kPa) and (c) high pressure volumetric.


Page 26 of 33

Page 27 of 33

30

Isosteric heat (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27 24 21


18 15 12 9 0.0

0.2

0.4

0.6

0.8

2.5 3.0


Figure 5. Variation of isosteric heat with CO2 uptake amount in HKUST-1 (black, square), MOF-143 (red, circle) and MOF-399 (blue, triangle) at 298 K.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HKUST-1

MOF-143

Page 28 of 33

MOF-399 0.01

0.00 (a) P = 10 kPa

(b) P = 10 kPa

(c) P = 10 kPa

0.05

0.00 (d) P = 1000 kPa

(e) P = 1000 kPa

(f) P = 1000 kPa

0.1

0.00 (g) P = 3000 kPa

(h) P = 3000 kPa

(i) P = 3000 kPa

Figure 6. Probability distribution plots of COM of CO2 in HKUST-1 (left), MOF-143 (center) and MOF-399 (right) at different pressure. (a, b, c) low pressure, (d, e, f) intermediate pressure and (g, h, i) high pressure. (C: gray; H: white; O: red; Cu: orange).


Page 29 of 33

(a)


12 10

Li-HKUST-1 Li-MOF-143 Li-MOF-399


8 6 4 2 0

0

20

40

60

80

100

Pressure (kPa)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45 40 35 30 25 20 15 10 5 0

(b)

0

500

1000 1500 2000 2500 3000

Pressure (kPa)

Figure 7. Simulated CO2 adsorption isotherms in Li alkoxide functionalized MOFs at 298 K. (a) 0-100 kPa; (b) 0-3000 kPa.



45

Isosteric heat (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

40 35 30


Li-HKUST-1 Li-MOF-143 Li-MOF-399

25 20 15 10 0.0 0.5 1.0 1.5 2.0 2.5 3.0

10 12

Uptake amount (mmol/g) Uptake amount (mmol/g) Figure 8. Variation of isosteric heat with CO2 uptake amount in Li alkoxide functionalized MOFs at 298 K.


1800

(a)


1600 1400 1200 1000 800 600 400 200 0

0

100

200

300

400

Pressure (kPa)

Contribution of Li modified (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Contribution of Li modified (%)

Page 31 of 33

2000

(b)

1600 1200 800 400 200 100 0

0

500 1000 1500 2000 2500 3000

Pressure (kPa) Figure 9. Percentage contribution of Li alkoxide functionalization to CO2 adsorption amount in HKUST-1, MOF-143 and MOF-399 at 298 K. (a) 0-400 kPa; (b) 0-3000 kPa.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Li-HKUST-1

Li-MOF-143

Page 32 of 33

Li-MOF-399

0.01

0.00 (a) P = 0.1 kPa

(b) P = 10 kPa

(c) P = 10 kPa

0.05

0.00 (d) P = 100 kPa

(e) P = 1000 kPa

(f) P = 1000 kPa

0.1

0.00 (g) P = 1000 kPa

(h) P = 3000 kPa

(i) P = 3000 kPa

Figure 10. Probability distribution plots of COM of CO2 in Li alkoxide functionalized HKUST-1 (left), MOF-143 (center) and MOF-399 (right) at different pressure. (a, b, c) low pressure, (d, e, f) intermediate pressure and (g, h, i) high pressure. (C: gray; H: white; O: red; Cu: orange; Li: purple).


Page 33 of 33

Table of contents image Uptake amount (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12 10

HKUST-1 Li-HKUST-1

8 6 4 2 0

0

20

40

60

80

100

Pressure (kPa)


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