Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (1): 52-60

The article information

Jie Gong, Wei Li, Song Li
龚捷, 李炜, 李松
Influence of Functional Groups and Modi cation Sites of Metal-Organic Frameworks on CO2/CH4 Separation: A Monte Carlo Simulation Study
官能团及修饰位点对金属有机骨架化合物(MOFs)CO2/CH4分离性能的影响:蒙特卡洛模拟研究
Chinese Journal of Chemical Physics, 2018, 31(1): 52-60
化学物理学报, 2018, 31(1): 52-60
http://dx.doi.org/10.1063/1674-0068/31/cjcp1705108

Article history

Received on: May 26, 2017
Accepted on: July 5, 2017
Influence of Functional Groups and Modi cation Sites of Metal-Organic Frameworks on CO2/CH4 Separation: A Monte Carlo Simulation Study
Jie Gonga,b,c, Wei Lia,b,c, Song Lia,b,c     
Dated: Received on May 26, 2017; Accepted on July 5, 2017
a. State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China;
b. Shenzhen Research Institute of Huazhong University of Science and Technology, Shenzhen 518057, China;
c. Nano Interface Centre for Energy, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
*Author to whom correspondence should be addressed. Song Li, E-mail:songli@hust.edu.cn
Abstract: In order to explore the in uence of modification sites of functional groups on landfill gas (CO2/CH4) separation performance of metal-organic frameworks (MOFs), six types of organic linkers and three types of functional groups (i.e. -F, -NH2, -CH3) were used to construct 36 MOFs of pcu topology based on copper paddlewheel. Grand canonical Monte Carlo simulations were performed in this work to evaluate the separation performance of MOFs at low (vacuum swing adsorption) and high (pressure swing adsorption) pressures, respectively. Simulation results demonstrated that CO2 working capacity of the unfunctionalized MOFs generally exhibits pore-size dependence at 1 bar, which increases with the decrease in pore sizes. It was also found that -NH2 functionalized MOFs exhibit the highest CO2 uptake due to the enhanced Coulombic interactions between the polar -NH2 groups and the quadrupole moment of CO2 molecules, which is followed by -CH3 and -F functionalized ones. Moreover, positioning the functional groups -NH2 and -CH3 at sites far from the metal node (site b) exhibits more significant enhancement on CO2/CH4 separation performance compared to that adjacent to the metal node (site a).
Key words: Metal-organic frameworks    Pore-size dependence    Functional groups    Modification sites    Interaction energy    
Ⅰ. INTRODUCTION

The increasing greenhouse gas emission and global warming are drawing worldwide attention. One way to mitigate this trend is switching to CH$_4$-based energy sources that emit comparatively less CO$_2$ per unit of energy than coal-or petroleum-based fuels due to their higher hydrogen to carbon (H/C) ratio. Municipal or industrial landfill gas is a promising and potential source to obtain CH$_4$ [1], however the separation of CO$_2$ from landfill gas is essential because the presence of a large amount of CO$_2$ impurities (40% to 60%) greatly reduces the combustion efficiency and is corrosive to pipelines or cylinders [2]. Conventional technologies for CO$_2$/CH$_4$ separation include absorption, cryogenic distillation, membrane separation, and adsorption [3]. Among these, vacuum/pressure swing adsorption (VSA/PSA) is promising because of the easy control, low operation cost, and superior energy efficiency [4]. A key issue of designing a VSA/PSA system is selecting a highly selective adsorbent.

Metal-organic frameworks (MOFs), a new class of porous crystalline materials composed of self-assembled metallic species and organic linkers, have widespread applications, included but not limited to gas storage, gas separation, catalysis, thermal conversion, drug delivery, harmful substance storage, and biomedical imaging [5-7]. MOFs exhibit ultrahigh surface areas, controllable pore sizes, and shapes as well as "tailor-made" framework functionalities, hence giving rise to millions of diverse structures due to the various combinations of building blocks and substituents [8, 9]. As promising adsorbents, MOFs have received extensive attention to promote their selectivity and adsorption capacity for CO$_2$/CH$_4$ separation [10, 11]. Wilmer $et$ $al$. [12] implemented a large-scale computational screening over 1.3$\times$10$^5$ hypothetical MOFs, and discovered correlations between structural characteristics ($e$.$g$., pore size, surface area, and pore volume), as well as chemical characteristics ($i$.$e$., functional groups), for CO$_2$/CH$_4$ separation performance. Bae $et$ $al$. [13] studied the adsorption of CO$_2$ and CH$_4$ in a mixed-ligand metal-organic framework Zn$_2$(NDC)$_2$ (DPNI) (NDC=2, 6-naphthalenedicarboxylate, DPNI=$N$, $N$$'$-di-(4-pyridyl)-1, 4, 5, 8-naphthalene tetracarboxydiimide) using volumetric adsorption measurements and grand canonical Monte Carlo (GCMC) simulations, in which a selectivity of 30 for CO$_2$ over CH$_4$ was reported.

Functionalization has also been shown to impose profound influences on the adsorption properties of MOFs. In 2002, Yaghi's group [14] functionalized MOF-5 with the organic groups -Br, -NH$_2$, -OC$_3$H$_7$, -OC$_5$H$_{11}$, -C$_2$H$_4$, and -C$_4$H$_4$, and the pore size of frameworks can be expanded by the long molecular struts biphenyl, tetrahydropyrene, pyrene, and terphenyl. Deng $et$ $al$. [15] successfully deployed multivariate (MTV) link synthetic strategy to synthesize 18 analogues (also named MTV-MOFs) of MOF-5 with up to eight distinct functionalities in one phase. It was found that the performance of MTV-MOFs is not a simple linear combination of their constituents, and one member of the 18 derivatives exhibits significantly enhanced selectivity towards CO$_2$ over CO compared to its best same-link counterparts. McDaniel $et$ $al$. [16] reported that the absolute gas uptake normally is not merely a sum of linear contributions from its constituent functionalities but rather exhibits a synergistic enhancement due to cooperative adsorbate-linker interactions involving multiple functionalities. Efforts have also been made to introduce polar functional groups with high affinity towards CO$_2$ to boost the CO$_2$/CH$_4$ separation [17, 18]. Yang $et$ $al$. [19] explored the effects of seven functional groups on the CO$_2$/CH$_4$ separation performance of UiO-66(Zr) computationally and found that -SO$_3$H and -CO$_2$H functionalized MOFs showed the highest CO$_2$/CH$_4$ selectivity, good working capacity, and medium ranged CO$_2$ adsorption enthalpy. Walton $et$ $al$. [20] synthesized a new monomethyl-functionalized UiO-66 (UiO-66-MM), which exhibited a much higher CO$_2$/CH$_4$ selectivity due to the enhanced van der Waals interactions with CO$_2$. Couck $et$ $al$. [21] reported functionalized MIL-53(Al) with amino groups, whose CO$_2$/CH$_4$ selectivity was increased by several orders of magnitude without compensating its CO$_2$ adsorption capacity. Mu $et$ $al$. [22] found that the counterbalance between the enhanced adsorption resulted from the electron-donating functional groups and the steric hindrance effects of functionality was essential in designing high-performing MOF materials for CO$_2$/CH$_4$ separation. Gomez-Gualdron $et$ $al$. [23] found that triple bonds adjacent to the inorganic zirconium nodes provided more efficient methane packing around the nodes at high pressures. Such findings have led us to ponder over the steric effects of modification sites of functional groups. Nonetheless, to the best of our knowledge, little work has been implemented to explore the influence of modification sites or a combination of both modification sites and the type of functional group on CO$_2$/CH$_4$ separation.

In order to identify the correlation between the functional groups and their modification sites of MOFs for CO$_2$/CH$_4$ separation, 36 MOFs of the same topology assembled from one of the six types of linkers ($i$.$e$. L1, L2, L3, L4, L5, L6) (FIG. 1) with or without functional groups positioning at varying sites to metal were investigated by grand canonical Monte Carlo (GCMC) simulations. This study may provide molecular insight into effective design of promising MOFs for the CO$_2$/CH$_4$ separation.

FIG. 1 Illustration of six types of linkers and two modification sites: L1, L2, L3, L4, L5, L6. The sites in orange and green refer to modification sites that are close to site a and far from site b the metal node respectively, and sites in light blue mean that there is only one type of modification sites on the linker.
Ⅱ. METHODS A. Construction of hypothetical MOFs

As shown in FIG. 1, the six types of linkers are terephthalic acid (L1), 3, 3$'$-(1, 4-phenylene)dipropiolic acid (L2), biphenyl-4, 4-dicarboxylic acid (L3), 4, 4-stilbenedicarboxylic acid (L4), pyrene-2, 7-dicarboxylic acid (L5), and 4, 4$'$-ethyne-1, 2-diyldibenzoic acid (L6), respectively. MOFs comprised of these linkers are named as MOF 1-6 accordingly, and each MOF consists of only one type of linkers. Three types of functional groups, -F, -NH$_2$, and -CH$_3$ were chosen as representatives of functionalities with different affinities towards CO$_2$ adsorbates, and positioned to symmetrical sites of aromatic ring as highlighted in FIG. 1. According to FIG. 1, L1 and L2 only have one type of modification sites, while L3-L6 possess two types of modification sites: adjacent to the metal node (denoted as site a) and far from the metal node (denoted as site b). According to the definitions above, the MOF derivatives are named by integration of the linker type, modification sites, and functional group, $e$.$g$. MOF 1-CH$_3$ and 5b-NH$_2$. Density functional theory (DFT) structure optimization of all MOF derivatives was conducted to ensure that they are geometrically and chemically meaningful, and it is observed that introducing functional groups does not induce significant structural change.

B. Computational details

Grand canonical Monte Carlo (GCMC) simulations were conducted to obtain CO$_2$ adsorption performance of MOFs. Transferable potentials for phase equilibria (TraPPE) force field parameters [24] were used for CO$_2$ and CH$_4$ adsorbates, and Lorentz-Berthelot mixing rules were used to calculate the van der Waals interaction parameters between the atoms of MOFs and adsorbates. Universal force field (UFF) [25] and density-derived electrostatic and chemical (DDEC) [26] partial charges were employed for atoms of both framework and functional groups. The reliability of UFF force field has been validated in previous works [27, 28]. DDEC partial charges were calculated by fitting the electrostatic potential surface from the plane-wave density functional theory (DFT) calculations using the Vienna $ab$ $initio$ software package (VASP) [29-31]. The electron-ion interaction was described by the projector augmented wave (PAW) scheme with an energy cutoff of 450 eV. A 1$\times$1$\times$1 Monkhorst-Pack $k$-point mesh in the reciprocal space with spin polarization was adopted for Brillouin zone sampling.

GCMC simulations were conducted in version 1.9 of RASPA simulation code [32]. The functionalized and geometrically modified MOF derivatives were used for GCMC simulations and all atoms of MOFs were held fixed during the simulations. Ewald method was employed for describing the long-range electrostatic interactions [33]. All the interatomic interactions were modeled using the standard 12-6 Lennard-Jones (LJ) potential and Coulombic potential, both of which adopt a cutoff of 12.8 Å. Each simulation cell was replicated in all three dimensions to ensure the cell length was more than twice of the cutoff. All simulations consist of an equilibration of 5$\times$10$^4$ Monte Carlo cycles followed by another 5$\times$10$^4$ cycles to obtain the ensemble-averaged properties. Each cycle contains $N$ Monte Carlo moves including insertion, deletion, translation, rotation, and identity change of adsorbates with equal probabilities, where $N$ equals to the number of molecules in the simulation at the beginning. Monte Carlo moves including translation, rotation, reinsertion, and deletion of adsorbate molecules were performed with equal probabilities. All calculations were conducted at 298 K.

The adsorption working capacity was computed by subtracting the CO$_2$ adsorption capacity of a MOF at 0.1 bar from that at 1 bar under the vacuum-swing adsorption (VSA) conditions and subtracting the CO$_2$ adsorption capacity at 1 bar from that at 5 bar under the pressure-swing adsorption (PSA) conditions [34]. The CO$_2$/CH$_4$ selectivity of CO$_2$/CH$_4$ gas mixture with a molar ratio of 1:1 at 1 and 5 bar was calculated using the following equation, respectively.

$ {S_{{\rm{C}}{{\rm{O}}_{\rm{2}}}/{\rm{C}}{{\rm{H}}_4}}} = \frac{{{q_{{\rm{C}}{{\rm{O}}_2}}}/{p_{{\rm{C}}{{\rm{O}}_2}}}}}{{{q_{{\rm{C}}{{\rm{H}}_4}}}/{p_{{\rm{C}}{{\rm{H}}_4}}}}} $ (1)

where $q$ refers to the gas uptake of MOFs from gas mixture and $p$ is the partial pressure of the adsorbates. Helium void fraction and surface area (SA) were computed in RASPA as well. Largest cavity diameter (LCD) and pore limiting diameter (PLD) were obtained from the Zeo++ package [35].

Ⅲ. RESULTS AND DISCUSSION

The helium void fraction (HVF), surface area (SA), largest cavity diameter (LCD), and pore limiting diameter (PLD) of functionalized and unfunctionalized MOFs are provided in Table Ⅰ. It is found that the LCDs of parent structures follow the order of MOF 1$<$5$<$3$<$2$<$4$<$6, and functionalization does not bring about significant change in the SA or pore size of frameworks. Besides, functionalization at site b generally results in slightly smaller LCD, and slightly larger SA compared to site a.

Table Ⅰ Results of helium void fraction (HVF), surface area (SA), largest cavity diameter (LCD), and pore limiting diameter (PLD). LCD and PLD were calculated from the Zeo++ package [35]. Helium void fraction and SA were calculated using helium and nitrogen as the probing atom, respectively at 298 K and 1 bar in RASPA.

The calculated CO$_2$ working capacity and CO$_2$/CH$_4$ selectivity of all MOF derivatives at VSA conditions are shown in FIG. 2. It was observed that CO$_2$ uptake of the six parent MOFs decreases in the order of MOF 1$>$5$>$3$>$2$\approx$4$>$6, which is in exactly opposite trend to their variations in LCDs: MOF-1$<$5$<$3$<$2$<$4$<$6, suggesting the high CO$_2$ adsorption in small pores, consistent with previous report [36]. The density distribution map in FIG. 3 also evidenced the highest CO$_2$ adsorption in MOF-1, and the adsorbed CO$_2$ molecules were mostly adjacent to the metal node. Regarding the functionalization effects, the CO$_2$ working capacity of the functionalized MOFs exhibits the order of -NH$_2$$>$-CH$_3$$>$parent$>$-F except for 4a, where 4a-CH$_3$ and 4a-NH$_2$ exhibit similar working capacity. It was also illustrated that the CO$_2$/CH$_4$ selectivity of the vast majority of MOFs presents similar tendency to CO$_2$ working capacity except for 3a-F and 4a-F, whose selectivities are slightly higher than their parent counterparts. The increased ratio of CO$_2$/CH$_4$ uptake of 3a-F and 4a-F is possibly due to a larger decrease in CH$_4$ uptake than CO$_2$ uptake compared to their parent MOFs as shown in FIG. S1 in supplementary materials, in which there is a 13.09% reduction in CH$_4$ uptake and 9.85% reduction in CO$_2$ uptake of 3a-F in contrast to MOF-3, and a 12.42% reduction in CH$_4$ uptake and 9.95% reduction in CO$_2$ uptake of 4a-F in contrast to MOF-4. On the contrary, -F functionalization of other MOFs except MOF-3 and MOF-4 resulted in a larger decrease ($\sim$4%) in CO$_2$ uptake than CH$_4$ uptake, thus causing a reduced selectivity. However, introducing polar functionalities (such as -NH$_2$) imposes significant positive effects on the adsorption of quadrupolar CO$_2$ molecules compared to nonpolar CH$_4$ molecules, thus leading to greatly enhanced CO$_2$/CH$_4$ selectivity. It should be noted that the CO$_2$/CH$_4$ selectivity from GCMC simulation is comparable with the results of experimental works. Venna $et$ $al$. [37] reported that ZIF-8 membranes exhibited a CO$_2$/CH$_4$ selectivity ranging from 4.1 to 7.0 at 99.5 kPa. Zhang $et$ $al$. [38] also experimentally investigated a series of metal-organic materials (MOMs) whose CO$_2$/CH$_4$ selectivity ranged from 7 to 11 at 1 bar. Yang $et$ $al$. [39] studied the CO$_2$/CH$_4$ separation performance of UiO-66(Zr) by both experimental measurements and molecular simulations, and the good agreement in CO$_2$/CH$_4$ selectivity (5-7) between experimental and computational studies was observed. With regard to modification sites, functionalization of MOFs at site b by -NH$_2$ and -CH$_3$ showed higher CO$_2$ working capacity than those at site a, which is probably related to the increased surface area upon functionalization. Moreover, -F functionalized MOFs at site b exhibited larger decrease in adsorption capacity and CO$_2$/CH$_4$ selectivity than those at site a. Such tendency is well supported by the total interaction energy including host-adsorbate and adsorbate-adsorbate interaction energies shown in FIG. 4.

FIG. 2 CO$_2$ working capacity and CO$_2$/CH$_4$ selectivity of the parent and functionalized MOFs at 298 K from GCMC simulations. CO$_2$ working capacity was calculated as the difference in pure CO$_2$ uptake at 1 bar and 0.1 bar. CO$_2$/CH$_4$ selectivity was calculated from a CO$_2$/CH$_4$ mixture with the molar ratio of 1:1 at 1 bar.
FIG. 3 Density distribution map of CO$_2$ adsorbates within the parent MOFs (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 at 298 K, 1 bar. The white lines represent the supercells of MOF frameworks and the colored regions represent the scaled adsorbate densities. All the number densities of CO$_2$ adsorbates were scaled by the maximum density in MOF 1 for better comparison of the adsorption capacity of the MOFs.
FIG. 4 The interaction energy including host-adsorbate and adsorbate-adsorbate interaction energies for (a) parent, (b) -F, (c) -NH$_2$, and (d) -CH$_3$ functionalized MOFs, as well as Coulomb and van der Waals contributions of host-adsorbate interaction energy for (e) parent, (f) -F, (g) -NH$_2$, and (h) -CH$_3$ functionalized MOFs at 1 bar.

To better comprehend the observations in FIG. 2, the computed host-adsorbate and adsorbate-adsorbate interaction energy as well as Coulombic and van der Waals contributions are shown in FIG. 4. The total interaction energy including host-adsorbate and adsorbate-adsorbate interactions within parent MOFs in FIG. 4(a) is in the order of MOF 1$>$5$>$3$>$4$>$2$>$6, consistent with the CO$_2$ adsorption working capacity shown in FIG. 2. The high interaction energy for MOF-1 can be attributed to the overlapped potential well in small pores [40]. Upon functionalization, the trend in total interaction energy is generally identical, and in general -NH$_2$ functionalized MOFs (FIG. 4(d)) exhibit the strongest total interaction energy followed by -CH$_3$ (FIG. 4(c)) and -F functionalized ones (FIG. 4(b)). Coulombic interactions between frameworks and CO$_2$ molecules contributed most to the host-adsorbate interaction compared to their parent or -CH$_3$ and -F functionalized counterparts, suggesting the strong affinity between Lewis basic -NH$_2$ and Lewis acid CO$_2$ as reported in previous work [41]. Apart from -NH$_2$ functionalized MOFs, -CH$_3$ functionalized MOFs that exhibit good adsorption performance can be traced to the strong van der Waals interactions between the guest molecules and the functional groups, consistent well with the work of Walton $et$ $al$. [20]. On the contrary, -F modified MOFs show a decreased CO$_2$ adsorption in contrast to their parent counterparts due to the electronic charge withdrawing nature of fluorine functional group as evidenced by DFT calculations of Torrisi $et$ $al$. [42]. Moreover, both -NH$_2$ and -CH$_3$ functionalization at site b show a significant increase in host-adsorbate interactions compared with the functionalization at site a, hence resulting in the higher CO$_2$ uptake capacity and selectivity. This is probably because placing functionality at site a reduces the number of favorable adsorption sites near the metal node, leading to the slight increase in CO$_2$ uptake, while functionalization at site b provides additional adsorption sites within frameworks towards CO$_2$, thus significantly enhancing CO$_2$ uptake.

On the other hand, the CO$_2$ working capacity and CO$_2$/CH$_4$ selectivity at 5 bar of PSA conditions are shown in FIG. 5. Comparing FIG. 2 with FIG. 5, both the CO$_2$ working capacity and CO$_2$/CH$_4$ selectivity at 5 bar were higher than those at 1 bar. The density distribution maps of adsorbed CO$_2$ molecules within frameworks at 5 bar were similar to those at 1 bar (FIG. 6), and the region close to the metal node was still the favorable adsorption sites. The pores of MOF 1 were filled with CO$_2$ molecules (FIG. 6(a)). Moreover, the overall order of CO$_2$ working capacity has changed from MOF 1$>$5$>$3$>$4$\approx$2$>$6 at the low-pressure VSA condition to 5$>$1$>$3$>$4$\approx$2$>$6 at the high-pressure PSA condition, where MOF-5 exhibited the highest working capacity at PSA instead of MOF-1 at VSA. All functionalized MOFs of 1 have the lower CO$_2$ working capacity than their parent counterparts. Such an observation was mainly attributed to the smallest pores of MOF-1 (LCD=9.32 Å) among all parent MOFs, where adsorption saturation occurred at 5 bar (FIG. S2 in supplementary materials). Therefore, placing functional groups within frameworks of MOF 1 leads to the decreased CO$_2$ working capacity due to the reduced accessible space for CO$_2$ adsorption at 5 bar, which is in agreement with the findings of Babarao $et$ $al$. [43]. Nevertheless, MOFs 2-6 exhibit increased CO$_2$ working capacity upon functionalization by -NH$_2$ and -CH$_3$, which is similar to the tendency observed at 1 bar. Moreover, the adsorption performance of the functionalized MOFs also falls in the order of -NH$_2$$>$ -CH$_3$$>$parent$>$-F, similar to the observations at 1 bar of FIG. 2. The similarity in CO$_2$/CH$_4$ selectivity of 5 and 1 bar can be attributed to the almost identical impact that the pressure change imposes on CH$_4$ uptake and CO$_2$ uptake (FIG. S3 in supplementary materials). Additionally, similar to the results at 1 bar, functionalization at site b by -NH$_2$ and -CH$_3$ exhibits slightly higher CO$_2$ working capacity than that at site a, and -F functionalized MOFs displays the opposite tendency. However, the difference in adsorption performance of MOFs functionalized at site a and b at 5 bar is less significant than that at 1 bar.

FIG. 5 CO$_2$ working capacity and the CO$_2$/CH$_4$ selectivity of the parent and modified MOFs at 298 K by GCMC simulations. Working capacity was calculated with pure CO$_2$ uptake at 5 bar subtracting that at 1 bar, while CO$_2$/CH$_4$ selectivity was calculated using a mixture of CO$_2$ and CH$_4$ with the molar ratio of 1:1.
FIG. 6 Density distribution map of CO$_2$ adsorbates within (a) the parent MOF 1, (b) 2, (c) 3, (d) 4, (e) 5 and (f) 6 at 298 K, 5 bar. The white lines represent the supercells of MOF frameworks and the colored regions represent the adsorbates. All the number densities of CO$_2$ adsorbates were scaled by the maximum density in MOF 1 for better comparison of the adsorption capacity of the MOFs.

The host-adsorbate and adsorbate-adsorbate interactions of CO$_2$-MOFs at 5 bar, and Coulombic and van der Waals contributions to host-adsorbate interactions are shown in FIG. 7 to help understand CO$_2$/CH$_4$ separation performance in FIG. 5. The tendency in total interaction energy of parent MOFs in FIG. 7(a) is similar to that in FIG. 4(a), consistent with their CO$_2$ working capacity at 1 and 5 bar except for MOF 1 due to the adsorption saturation at 5 bar. As for effects of functional groups, similar to the findings at 1 bar, the strongest Coulombic interaction was observed between -NH$_2$ functionalized MOFs and CO$_2$ molecules as shown in FIG. 7(c), leading to the highest CO$_2$ working capacity at 5 bar, which is followed by -CH$_3$ and -F functionalized MOFs. Moreover, the host-adsorbate interactions for MOFs functionalized by -NH$_2$ and -CH$_3$ at site b are still slightly higher than those modified at site a, which also agrees with their CO$_2$ uptake at 5 bar. On the contrary, the host-adsorbate interaction energy of -F functionalized MOFs at site b is lower than those at site a, similar to the tendency in CO$_2$ working capacity.

FIG. 7 Host-adsorbate and adsorbate-adsorbate interactions for (a) parent, (b) -F, (c) -NH$_2$ and (d) -CH$_3$ functionalized MOFs, as well as Coulomb and van der Waals contributions to the host-adsorbate interaction for (e) parent, (f) -F, (g) -NH$_2$, and (h) -CH$_3$ functionalized MOFs at 5 bar.

The host-adsorbate and adsorbate-adsorbate interactions of CO$_2$-MOFs at 5 bar, and Coulombic and van der Waals contributions to host-adsorbate interactions are shown in FIG. 7 to help understand CO$_2$/CH$_4$ separation performance in FIG. 5. The tendency in total interaction energy of parent MOFs in FIG. 7(a) is similar to that in FIG. 4(a), consistent with their CO$_2$ working capacity at 1 and 5 bar except for MOF 1 due to the adsorption saturation at 5 bar. As for effects of functional groups, similar to the findings at 1 bar, the strongest Coulombic interaction was observed between -NH$_2$ functionalized MOFs and CO$_2$ molecules as shown in FIG. 7(c), leading to the highest CO$_2$ working capacity at 5 bar, which is followed by -CH$_3$ and -F functionalized MOFs. Moreover, the host-adsorbate interactions for MOFs functionalized by -NH$_2$ and -CH$_3$ at site b are still slightly higher than those modified at site a, which also agrees with their CO$_2$ uptake at 5 bar. On the contrary, the host-adsorbate interaction energy of -F functionalized MOFs at site b is lower than those at site a, similar to the tendency in CO$_2$ working capacity.

The enhancement in pure CO$_2$ uptake at 1 bar (FIG. 8(a)) and 5 bar (FIG. 8(b)) as well as that of the total interaction energy at 1 bar (FIG. 8(c)) and 5 bar (FIG. 8(d)) were calculated using the following equation:

$ {\rm{Enhancement}}/\% = \frac{{{X_{{\rm{functionalized}}}} - {X_{{\rm{parent}}}}}}{{{X_{{\rm{parent}}}}}} \times 100\% $ (2)
FIG. 8 Enhancement in pure CO$_2$ uptake of MOFs at (a) 1 bar and (b) 5 bar, and enhancement in CO$_2$ total interaction energy of MOFs compared with their parent counterparts at the pressure of (c) 1 bar and (d) 5 bar respectively. Computation was conducted using pure CO$_2$ at 298 K.

where $X$ refers to the CO$_2$ uptake or the total interaction energy of parent or functionalized MOFs. Comparing the enhancement in CO$_2$ working capacity, the enhancement at 1 bar (FIG. 8(a)) is generally more significant than that at 5 bar (FIG. 8(b)). Similar tendency was observed in the enhancement in the total interaction energy of MOFs at 1 bar (FIG. 8(c)) and 5 bar (FIG. 8(d)). In general, -NH$_2$ functionalized MOFs exhibited the most significant enhancement in CO$_2$ working capacity and the interaction energy, followed by -CH$_3$ and -F functionalized MOFs, which is similar to the former observations at 1 bar (FIG. 2 and FIG. 4) and 5 bar (FIG. 5 and FIG. 7). It should be noted that the enhancement in CO$_2$ working capacity and interaction energy upon functionalization was only observed for -NH$_2$ and -CH$_3$ functionalized MOFs except MOF 3a-CH$_3$ at 1 bar and MOF 1 at 5 bar. The slightly decreased CO$_2$ working capacity of 3a-CH$_3$ at 1 bar was supported by the reduced interaction energy correspondingly in FIG. 8(c). As demonstrated previously, functionalization at site a reduced the number of favorable adsorption sites around the metal node compared with 3b-CH$_3$, leading to the slightly decreased adsorption of 3a-CH$_3$. For MOF 1-NH$_2$ and 1-CH$_3$ at 5 bar, although their total interaction energy was increased compared with their parent, the CO$_2$ working capacity was decreased, which can be attributed to the limited space for CO$_2$ adsorption. In addition, all -F functionalized MOFs exhibited decreased CO$_2$ working capacity in contrast to their unfunctionalized counterparts regardless of the increased or decreased interaction energy, consistent with the observations in FIG. 2 and FIG. 5.

Ⅳ. CONCLUSION

As an effective strategy of improving CO$_2$/CH$_4$ separation performance of MOFs, functionalization is frequently employed experimentally and theoretically. In this work, we employed GCMC simulations to study the impact of different functional groups at varying modification sites on their CO$_2$/CH$_4$ separation performance in MOFs with pcu topology and six types of linkers. CO$_2$ working capacity shows pore-size dependence, and parent MOF structures with smaller pores generally have larger CO$_2$ uptake. MOF-1 with the smallest pores exhibited the highest uptake upon functionalization at 1 bar, but a slight increment and even a decrement compared to parent MOFs at 5 bar. This observation sheds light on the importance of tailoring the pore size of MOFs at different operation conditions to balance between the gain in framework-adsorbate affinity due to potential well overlap in small pores and the loss of available adsorption sites. Moreover, the performance of different functional groups decrease in the order of -NH$_2>$-CH$_3$$>$-F, rendering guidance in the choice of functionalities in synthetic process. Placing the functional groups at site b imposed more obvious impact on CO$_2$ adsorption due to the introduction of additional adsorption sites compared to that at site a. Besides, both the influence of different functional groups and modification sites are more evident at 1 bar compared to that at 5 bar, because the host-adsorbate interactions play a more dominant role in CO$_2$ adsorption at low pressures. Our findings have shown the interplay among pore size, functional groups, modification sites and operation conditions for applications. We believe this work will guide synthetic experiments to design MOFs with improved adsorption capacity and selectivity for CO$_2$/CH$_4$ separation. However, further efforts are needed to extrapolate such findings to MOFs of other topologies.

Supplementary materials: FIG. S1 shows CH$_4$ uptake of the MOFs with the molar ratio of CO$_2$:CH$_4$=1:1, FIG. S2 shows pure CO$_2$ adsorption isotherms of MOF 1 and 5, and CO$_2$/CH$_4$ selectivities of all MOF structures with a molar ratio of CO$_2$:CH$_4$=1:1 are also provided.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.51606081) and the Basic Research Foundation of Shenzhen (No.JCYJ20160506170043770).

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官能团及修饰位点对金属有机骨架化合物(MOFs)CO2/CH4分离性能的影响:蒙特卡洛模拟研究
龚捷a,b,c, 李炜a,b,c, 李松a,b,c     
a. 华中科技大学煤燃烧国家重点实验室, 武汉 430074;
b. 深圳华中科技大学研究院, 深圳 518057;
c. 华中科技大学纳米界面输运与能源转换中心, 武汉 430074
摘要: 本文采用巨正则蒙特卡罗(GCMC)模拟,研究金属有机骨架中官能团修饰的种类及位点对垃圾填埋气(CO2/CH4)吸附分离性能的影响.选用六种不同有机配体,分别组成六种铜基pcu构型MOFs,并采用三种官能团(-F、-NH2、-CH3)对其有机配体上距金属团簇不同距离的位点进行修饰,获得36种MOFs结构,并对它们在低压真空变压吸附(VSA)和高压变压吸附(PSA)的CO2/CH4分离性能进行模拟.结果表明,在低压下,未修饰MOFs的CO2吸附量呈现出随孔径增大而减小的趋势.三种官能团中,-NH2修饰的MOFs因其强库伦相互作用而具有最优的CO2吸附性能,其次,具有强范德华相互作用的MOF-CH3,而-F具有吸电子效应,且其作为单原子官能团的弱范德华相互作用使得它的CO2吸附性能略逊于未修饰MOFs.此外,将官能团修饰于距金属团簇较远的位点(位点b)时,所造成的影响比修饰于近位点(位点a)更明显.
关键词: 金属有机骨架化合物    孔径    官能团    修饰位点    相互作用