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Understanding the Effect of Ligands on CH Storage and CH/CH, CH/ CO Separation in Metal-Organic Frameworks With Open Cu(II) Sites 2

Yujin Ji, Lifeng Ding, Yuanyuan Cheng, Hao Zhou, Siyuan Yang, Fan Li, and Youyong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08370 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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The Journal of Physical Chemistry

Understanding the Effect of Ligands on C2H2 Storage and C2H2/CH4, C2H2/CO2 Separation in Metal-organic Frameworks with Open Cu(II) Sites Yujin Ji,a Lifeng Ding,b* Yuanyuan Cheng,c Hao Zhou,b Siyuan Yang,b Fan Li,d and Youyong Li,a* a Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215213, China b Department of Chemistry, Xi’an JiaoTong-Liverpool University, 111 Ren’ai Road, Suzhou Dushu Lake Higher Education Town, Jiangsu Province, 215123, China c School of Environmental Science & Engineering, Suzhou University of Science & Technology, Suzhou, China

d Electricity Transmission Asset Management, National Grid House, Warwick, CV34 6DA, United Kingdom Yujin Ji and Lifeng Ding contributed equally to this work Corresponding authors: Lifeng Ding ([email protected]); Youyong Li ([email protected])

ABSTRACT: Safe and efficient storage and separation of acetylene pose a significant challenge in industry. In this study, we investigated 11 open Cu(II) site (OCS) based metal-organic frameworks (MOFs) formed by various organic ligands for their C2H2 adsorption capacities and their C2H2/CO2, C2H2/CH4 separation performance using both Grand Canonical Monte Carlo (GCMC) simulations and density functional theory (DFT) calculations. Our simulations revealed that both OCSs and organic ligands of the MOFs play key roles in promoting C2H2 storage capacity and the 1 ACS Paragon Plus Environment


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separation of C2H2 over CH4 and CO2 under 2 bar. Judicious selection of organic ligands with suitable dimension and functional sites, such as methyl group, Lewis basic nitrogen site and fluorine group can facilitate C2H2 adsorption in addition to OCS, and help distinguish C2H2 from CH4 and CO2. Short ligands presented in the MOFs, such as MOF-505 which gives the highest volumetric C2H2 storage capacity under 2 bar, not only increases the density of OCSs, but also creates overlapped interaction regions for guest molecules. GCMC simulation results suggested that NOTT-106 had the 2nd highest volumetric C2H2 storage capacity because of its methyl functionalized ligands. DFT calculations however suggested that the Lewis basic nitrogen functionalized ligand of ZJU-40 might have a stronger affinity with C2H2 than that of NOTT-106, which indicated that C2H2 storage capacity in ZJU-40 should be better than that in NOTT-106. On the other hand, MOF-505, ZJU-40 and NOTT-108 showed excellent C2H2/CH4 separation performance as well as modest C2H2/CO2 separation capability.

INTRODUCTION Acetylene of high purity is a key starting material for many important chemical compounds, such as, acrylic glass, polyester and polyurethane in industry.1 The development of low pressure acetylene (C2H2) storage and separation technique is of importance to C2H2 industry because of the highly reactive nature of C2H2 which will explode above a pressure of 2 atm at room temperature. The current C2H2 storage method in acetone suffers from low C2H2 purity and high separation cost.2 Porous materials with high surface area provide an alternative solution for C2H2 storage and separation at low pressures.3,4 A recent advance in C2H2 storage technology is the development of storing C2H2 in metal-organic frameworks (MOFs) with open metal sites.5 MOF is a novel class of porous crystalline materials consisting of metal or metal oxide clusters bridged by diverse organic ligands. MOFs feature large internal surface areas and high permanent porosity.6 The presence of open metal sites and suitable pore size, both of which provide 2 ACS Paragon Plus Environment

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strong binding sites with gas molecules, allow the MOFs to be the ideal storage materials for a wide range of gas molecules.7,8 Moreover, the tuneable pore surface functionality and pore geometry of MOFs afford them good capabilities to separate gas molecules with similar size.9 Separation of C2H2/CO2 and C2H2/CH4 also has great industrial importance due to the fact that CO2 and CH4 are common impurities existing in the process of producing or transporting C2H2.10,11 Recent studies revealed that MOFs with open metal sites gave remarkable C2H2 storage capacities up to 230 cm3(STP)cm-3 as well as good C2H2/CO2 separation performance.5,12 Among the reported MOFs with open metal sites, MOFs containing OCSs have been proposed as suitable C2H2 storage/separation porous materials. The OCS-based MOFs do not only show outstanding C2H2 storage capacity. It has been demonstrated that OCS-based MOFs can adsorb and desorb densely packed C2H2 molecules with good reversibility over multiple cycles.13 Hu et al. synthesized Cu2(ebtc), which exhibited high acetylene storage of 160 cm3g-1 at 295K under 1 bar.14 Xiang et al.5 and Fischer et al.15 reported that HKUST-1 had a high C2H2 storage capacity of 201 cm3

(STP)g-1 and 186 cm-3(STP)g-1 at 298 K under 1 bar using experimental approach and

GCMC simulations, respectively. Hui-min et al.16 and Xu et al.17 synthesized ZJU-40 and ZJU-5 with Lewis basic nitrogen sites whose C2H2 adsorption capacity could outperform their isoreticular MOF, NOTT-101. Pang et al reported that FJI-H8 which were formed by OCS and dimethoxy-biphenyl ligands show a record-high acetylene uptake of 196 cm3(STP)cm-3 at 295 K and 1 atm.13 The understanding of the interactions between gas molecules (C2H2, CH4 and CO2) and the MOFs with OCSs and organic ligands is essential to point directions for new MOFs with enhanced C2H2 storage capacity and C2H2/CH4, C2H2/CO2 separation capabilities. Recent research efforts were focusing on investigating the interaction of C2H2, CH4 and CO2 with OCSs.5,15,18 Less attention was given to the organic

ligands

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geometry/functionality of the MOFs. In this work, we highlight the effect of ligands of 3 ACS Paragon Plus Environment


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MOFs with OCSs on their C2H2 storage capacities and C2H2/CH4, C2H2/CO2 separation performance through GCMC simulations and DFT calculations. Our theoretical investigation enriches the understanding of the effect of ligands which is beneficial to the design and synthesis of OCS based MOFs for C2H2 storage and separation. COMPUTATIONAL METHODOLOGIES MOFs structures A series of isoreticular Nbo-type MOFs which are formed by Cu paddlewheel clusters connected by tetracarboxylic ligands are investigated for C2H2 storage and C2H2/CO2, C2H2/CH4 separation. Different tetracarboxylic ligands incorporated in the MOFs contain two m-benenzedicarboxylate moieties linked with different spacers which vary the MOF species. Figure 1 gives all the ligands as well as their corresponding MOFs. All the MOFs are MOF-505 analogs which contain two types of cages: one is formed by 12 ligands linked with 6 Cu paddlewheel clusters (cage A); the other is surrounded by 6 ligands and 12 paddle wheel clusters (cage B). These two cages are stacked alternatively close to each other. Cage A whose size is relatively smaller than cage B has the OCSs pointing towards the inner center of the cage, while in cage B the open Cu sites point away from the center. To evaluate how the length of ligands affect the MOFs’ C2H2 storage and C2H2/CH4, C2H2/CO2 separation performance, the MOFs were grouped into three groups, MOF505 which has the shortest ligand forms type I MOF. Cu2(ebtc), NOTT-101, NOTT-106, NOTT108 and ZJU-40 whose ligands’ length vary from 11 to 14.5 Å are grouped as type II MOFs. NOTT-103, PCN-46, NJU-Bai12, HNUST-2 and NOTT-102 whose ligands’ lengths change from 16 to 21 Å are grouped as type III MOFs.

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Figure 1. Schematic representation of 11 ligands and their corresponding MOFs studied in this work. (Color scheme: O, red; C, green; H, white; F, brown; N, blue; Cu, pink) GCMC simulations The adsorption isotherms of pure C2H2, CO2 and CH4 as well as C2H2/CO2 and C2H2/CH4 binary mixtures in the MOFs were produced using GCMC simulations. All GCMC simulations were carried out using the RASPA package.19 Each GCMC simulation consisted of a 100,000 cycles equilibration run, followed by a 100,000 cycles production run. Every cycle had N Monte Carlo (MC) moves. The number N, which was the number of adsorbed guest molecules in the MOFs, was fluctuating during the GCMC simulations. MC moves adopted in this study included translational move, rotational move, reinsertion move, addition and deletion move. All the MC moves were tried with equal probability during the simulations. The error bars of the simulation results were usually less than 1%. All the atoms’ positions within MOFs’ framework were kept fixed during the simulations. C2H2, CO2 and CH4 molecules were treated as rigid molecules in the simulations. The geometric surface area of the MOFs was measured through Duren’s method.20 The pore size distribution histogram of the MOFs was determined through Talu et al.’s method.21 In this work, Leonard Jones (LJ) potential and coulomb interaction were used to simulate the interactions between C2H2, CO2, CH4 and the MOFs. Universal Force Field (UFF) was used for the MOFs.22 The partial charges of the MOFs’ atoms were calculated using REPEAT method.23



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The partial charges and LJ parameters for C2H2 and CO2 were from Fischer et al.15 LJ parameters of CH4 were from the single site united atom TraPPE model.24 Lorenz-Berthelot mixing rules were used to mix LJ interaction pairs. Special attention was needed to cope with gas adsorption in MOFs involving open metal sites, because generic force fields will usually fail to describe the strong interaction of gas molecules with the open metal sites.25 In this study, specific interatomic potentials which were derived from quantum-mechanical calculations were used to deal with the interactions of C2H2, CO2 and CH4 with OCSs of the MOFs. The parameters of two interaction pairs (C2H2 with open Cu site and CO2 with open Cu site) which were produced by fitting the DFT energy surface were taken from Fischer et al.’s work.15 The parameters for the interaction of united atom of CH4 with open Cu site which was produced by Koh et al’s work26 take a Morse potential form. Adsorption based selectivity which was used to evaluate C2H2/CO2 and C2H2/CH4 separation performances in the MOFs was defined as S = (xA/xB)/(yA/yB), where xA and xB are the mole fractions of gases A and B in the adsorbed phase; and yA and yB and the mole fractions of A and B in the bulk phase. To ensure that the force fields adopted in this work could produce reasonable adsorption isotherms of C2H2, CO2 and CH4 in the MOFs, we firstly validated our simulated isotherms of C2H2, CO2 and CH4 in several MOFs against their experimental adsorption isotherms. As shown in Figures S1-S3, the simulated C2H2 adsorption isotherm in ZJU-40 and Cu2(ebtc) at 273K matched well with the experimental data. The simulated CH4 adsorption isotherm in Cu2(ebtc) at 273K and the CO2 adsorption isotherm in NJU-Bai12 and PCN-46 were in line with the experimental data. Discrepancy was observed between the simulated C2H2 and CH4 adsorption isotherm in MOF-505 and their experimental data. The possible explanations are: (1) MOF-505 synthesized in experimental work maybe imperfect or still contained some solvent in the crystals. (2) The adopted force fields which were derived using HKUST-1 may not be transferable to MOF-505. Considering the similar chemistry environment of HKUST-1 and MOF-505, we


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reckon that explanation (1) is more likely the case than explanation (2). Overall, the validation supported that the force fields used in this work could allow the simulations to produce correct adsorption isotherm and behaviors of C2H2, CO2 and CH4 in the MOFs. Quantum mechanical calculations The periodic density functional theory (DFT) calculations of the binding energies and positions of C2H2, CO2 and CH4 in the MOFs were conducted using the Vienna ab initio simulation package (VASP)2728 with the projector augmented wave (PAW) method29. The formula of exchange functional with generalized gradient approximation (GGA) proposed by Perdew, Burke and Ernzerhof (PBE)30 was chosen to optimize the ground state of geometrical structures of metal organic frameworks at the Gamma point until all the force and energy of self-consistent calculations reach to 0.02 eV/Å and 10-5 eV. Meanwhile, the non-local vdw-DF2 correction functional was chosen to account for the weak dispersion interaction between the gas molecules and the MOFs.31 The planewave energy cutoff was set to 500 eV for the plane-wave expansion of electronic wave function. The binding energies of C2H2, CO2 and CH4 adsorbed in different sites of the MOFs were calculated as ∆E = (Ecomplex – Ehost – Eguest)/N, where Ecomplex is the energy of the binding complex; Ehost is the energy of the host (MOFs or MOFs with pre-loaded gas molecules) before the binding; Eguest is the energy of the guest (gas molecules) before the binding; N is the number of guest molecules adsorbed in the MOFs. All the binding energies were obtained after geometry optimization. RESULTS AND DISCUSSIONS



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Figure 2. Simulated volumetric adsorption isotherms for 11 OCS-based MOFs at 298K: (a) C2H2 uptake; (b) CH4 uptake; (c) CO2 uptake. Lines are for eyes only. Red lines represent type I MOF. Yellow lines represent type II MOFs. Green lines represent type III MOFs. GCMC simulations of C2H2, CH4 and CO2 adsorption in the MOFs Figure 2 gives the absolute volumetric adsorption isotherms of C2H2, CH4 and CO2 in the MOFs, which were obtained from the GCMC simulations. In terms of C2H2 storage capacity under 2 bar, as shown in Figure 2 (a), Type I MOF-505 has the highest volumetric C2H2 uptake under 2 bar. Type II MOFs rank the second. Type III MOFs with the longest ligands have the lowest volumetric C2H2 uptake (Figure 2 (b) and (c)). Similar rankings for CH4 and CO2 volumetric uptake were observed for the three types of MOFs. C2H2 uptake in MOF-505 rises quicker than other MOFs under 0.8 bar and reaches a plateau above 0.8 bar. The GCMC simulation predicts that the absolute C2H2 volumetric uptake can reach 150 cm3/cm3 under 1 bar, which is comparable to the C2H2 uptake in MgMOF-74 (167 cm3(STP)cm-3).5 The reported experimental C2H2 uptake in MOF-5055 was smaller than that predicted by our GCMC simulations. The possible explanations were discussed in the force field validation part above. CO2 and CH4 volumetric uptakes in MOF-505 are almost two-fold as the rest of the MOFs. And their adsorptions are yet to be saturated up to 2 bar. Overall, the effect of the length of the ligands plays a key role in affecting C2H2, CH4 and CO2 uptake in the MOFs. As shown in Table S1, the length of the ligands is inversely proportional to the density of OCSs which provide strong


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affinity to C2H2, CH4 and CO2. Among type II MOFs whose ligands share similar length, NOTT106 whose ligand contains methyl group substituent has the highest C2H2, CO2 and CH4 volumetric uptake under 2 bar. Figure 3 illustrates the number of C2H2, CO2 and CH4 molecules adsorbed per primitive unit cell (UC) as a function of pressure at 298K. Along with their adsorption density plots in Figure 5 and Figure S10 – S17, they reveal a molecule-level insight of how the ligands affect C2H2, CO2 and CH4 adsorption in the MOFs. The maximum C2H2 loading in one MOF-505 primitive unit cell is about 10 molecules as can be seen from Figure 3 (a). There are six OCSs in each primitive unit cell. Assuming each OCS attracts one C2H2 molecule, there should be extra four C2H2 molecules occupying non-OCS adsorption sites. As shown in C2H2 adsorption density plots in Figure 5, there are three types of adsorption sites for C2H2 in all the OCS-based MOFs. Site I is the OCS. Site II is surrounded by three Cu clusters and three ligands which form a basket pocket shaped site (we term this site as “basket pocket site”). This basket pocket also acts as a small pore window connecting small cage (Cage A) and large cage (Cage B) of the MOFs. Site III is located near the other big pore window connecting cage A and cage B. Site III is mostly influenced by the linkers’ interaction as it locates far away from the OCS. In each primitive unit cell, there are six type I sites, two type II sites and two type III sites. For a maximum C2H2 loading, each of them will attract one C2H2 molecule. In total, they contribute 10 C2H2 molecules in MOF-505. As shown in Figure 3, all the adsorption sites are saturated for C2H2 in MOF-505 by 0.8 bar. The C2H2 adsorption site I and site II in other MOFs are similarly dense as that of MOF-505. C2H2 adsorption site III in other MOFs however are less dense than that in MOF-505. And the adsorption site is also loosely distributed in the pore space. C2H2 adsorption site III in MOFs other than MOF-505 is possible to accommodate more than one C2H2 molecule due to the large pore size of cage B. It explains that C2H2 loading per unit cell in these MOFs keeps increasing under 2 bar. In NOTT-106, which has the 2nd highest C2H2 loading, shows dense adsorption



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islands in site III. The methyl substituents in the phenyl ring of NOTT-106 allow overlapped interaction potential which enhanced C2H2 adsorption affinity in site III. C2H2 loading per UC in HNUST-2 and NOTT-102 closely follow that of NOTT-106, although they have relatively low volumetric C2H2 loading because of their long ligands. The high C2H2 loading per unit cell in HNUST-2 and NOTT-102 is due to their large area planar aromatic ring structure. The linear triple bonds in NJU-Bai12 which provide less surface area than HNUST-2 and NOTT-102 has lower C2H2 adsorption capacity. It should be noted that the force field used here does not differentiate the interaction affinity of the carbon atoms from the aromatic rings or from the triple bonds. For CH4 and CO2, their adsorption loadings per UC under 2 bar are relatively smaller than that of C2H2 in all the MOFs. The overall CH4 and CO2 adsorption loading rankings per UC in the MOFs are also similar to that of C2H2. It is interesting to note that CO2 and CH4’s main adsorption sites in the MOFs are not the OCSs (site I). Instead, they both show a very strong adsorption preference in site II under 1 bar and 298K, as shown in Figure 5 and Figure S10 – S17. Fisher et al. also found that the most favorite adsorption site of CO2 in HKUST-1 is its pore window site instead of the OCS.15 The adsorption site preference of C2H2 in OCS should facilitate separation of C2H2/CH4 and C2H2/CO2 in the MOFs.


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Figure 3. Simulated adsorption isotherms (number of guest molecules per UC) for 11 OCS-based MOFs at 298K: (a) C2H2 uptake; (b) CH4 uptake; (c) CO2 uptake. Red lines represent type I MOF. Yellow lines represent type II MOFs. Green lines represent type III MOFs.

Figure 4. Isosteric heat of adsorption isotherms of C2H2 in 11 OCS-based MOFs at 298K: (a) the total isosteric heat of adsorption isotherms of C2H2 in the MOFs; (b) the host-guest interaction energy contribution to the isosteric heat of adsorption of C2H2 in the MOFs; (c) the guest-guest interaction energy contribution to the isosteric heat of adsorption of C2H2 in the MOFs. Red lines represent type I MOF. Yellow lines represent type II MOFs. Green lines represent type III MOFs.



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Figure 5. The simulated adsorption density plots of C2H2, CH4 and CO2 absorbed in MOF-505, NOTT-101 and NOTT-106 at 298 K and 1 bar. (Color scheme: O, red; C, green; H, white; Cu, brown) The isosteric heat of adsorption of C2H2, CH4 and CO2 as a function of pressure in Figure 4 and S9 reveals that the ranking of adsorption enthalpies for C2H2, CH4 and CO2 in the MOFs is in line with the ranking of their volumetric loading in the MOFs. Overall, the adsorption enthalpies of 12 ACS Paragon Plus Environment

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C2H2 in the MOFs vary from 22 to 30 kJ/mol. The adsorption enthalpies of CH4 is about half of the enthalpies of C2H2, ranging from 15 to 18 kJ/mol. The adsorption enthalpies of CO2 are close to that of C2H2, varying from 18 to 25 kJ/mol. All the adsorption enthalpies of C2H2, CH4 and CO2 in the MOFs remain almost constant up to 2 bar. There are quick drops of host-guest interaction energies as pressure increase, but that drops are compensated by the quick rise of the guest-guest interaction energies. (Figure 4 (b) and (c)). For MOF-505, a slight rise of adsorption enthalpies of C2H2 and CO2 as pressure increases are observed, which is due to the strong guestguest interactions in the compact pore structure of MOF-505. MOF-505 has the highest adsorption enthalpies of C2H2, CH4 and CO2 up to 2 bar. Type II MOFs follow the second. Type III MOFs have the lowest adsorption enthalpies. MOFs constructed by long ligands form large pores (Figure 1, Table S1). Guest molecules adsorbed in large pores receives less overlapped interaction than that in small pores. As shown in Figure S7, the number of C2H2 adsorbed per UC is almost inversely proportional to the pore size of cage A. It should be noted that those NOTT101 isostructural analogs (NOTT-106, NOTT-108 and ZJU-40) have higher C2H2 adsorption enthalpy than that of NOTT-101. NOTT-106 whose ligands contains methyl group substituent has the 2nd highest C2H2 adsorption enthalpy among all the MOFs. NOTT-108 with fluorine group also provides larger C2H2 adsorption enthalpy than that of NOTT-101. ZJU-40 whose ligand contains Lewis basic nitrogen site also gives better C2H2 adsorption enthalpy. The grafting or modification of phenyl ring in OCS-MOFs is a viable strategy to enhance C2H2 interaction affinity, which in turn helps to raise the adsorption capacity.



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Figure 6. C2H2/CH4 (a) and C2H2/CO2 (b) adsorption selectivity obtained from simulated adsorption isotherms of the equimolar mixture at 298K in the MOFs.

Figure 7. Isosteric heat of adsorption differences: (a) C2H2 and CH4 and (b) C2H2 and CO2 at 298K. It has been illustrated that C2H2, CO2 and CH4 have different adsorption sites and adsorption enthalpies in the MOFs, which might be potentially promising to separate C2H2 and CO2, CH4. We carried out equal molar C2H2/CO2 and C2H2/CH4 binary mixture GCMC adsorption simulation in the MOFs. As can be seen from Figure 6, OCS-based MOFs that show good C2H2/CH4 selectivities are promising to separate C2H2 and CH4, which is owning to the fact that


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CH4 can only compete the basket pocket sites (site II) of the MOFs with C2H2, while C2H2 can also be adsorbed in site I and site III. MOF-505 has the highest C2H2/CH4 selectivity of 9 up to 2 bar. Type II MOFs’ C2H2/CH4 selectivity came second, ranging from around 4 to 7. Type III MOFs’ selectivities of C2H2/CH4 ranged from 4 to 5. It can be concluded that C2H2 adsorption in MOFs with small pore size can strongly repel CH4 adsorption due to the large adsorption heat difference. It is worth noting that Cu2(ebtc) and PCN-46 rank as the MOFs with the 2nd best and the best C2H2/CH4 selectivity among type II and type III MOFs, respectively. This is because the adsorption enthalpy differences of C2H2 and CH4 in Cu2(ebtc) and PCN-46 are the highest among type II and type III MOFs, respectively, as shown in Figure 7. According to GCMC simulations, inclusion of triple bonds in OCS-based MOFs, such as Cu2(ebtc) and PCN-46, is a viable strategy to enhance their C2H2/CH4 selectivity. NOTT-106 whose C2H2 adsorption capacity is higher than NOTT-108 and Cu2(ebtc) gives a lower C2H2/CH4 selectivity than that of NOTT-108 and Cu2(ebtc). It is owning to that the adsorption enthalpy difference of C2H2 and CH4 in NOTT-108 and Cu2(ebtc) are higher than that of NOTT-106, as shown in Figure 7. Methyl group in NOTT106 should interact more strongly with CH4 than that of fluorine group in NOTT-108 and triple bonds in Cu2(ebtc). The differences of C2H2/CO2 adsorption enthalpies in the MOFs are smaller than that of C2H2/CH4. (Figure 7 (b)) Nevertheless, it is demonstrated that the MOFs can still discriminate C2H2 over CO2 through the simulated binary adsorption calculations. OCS can preferably bind C2H2 over CO2 according to adsorption density analysis discussed above. (Figure 5 and Figure 10 – S17) MOF-505 has the highest C2H2/CO2 selectivity, ranging from 3.5 to 2 up to 0.8 bar. And the selectivity drops when pressure continues to rise. It can be explained that when pressure is low, the strongest C2H2 adsorption sites (site I and site II) in MOF-505 can repel CO2 adsorption. However, when pressure increases, CO2 and C2H2’s adsorption heat difference in MOF-505 decreases as CO2 starts to populate in site II and site III, which reduces the C2H2/CO2 selectivity.



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C2H2/CO2 selectivity in other MOFs varies from 1 to 2. Cu2(ebtc), NOTT-108 and PCN-46 have the 2nd, 3rd and 4th highest C2H2/CO2 selectivities, respectively. As shown in Figure 7 (b), Cu2(ebtc), NOTT-108 and PCN-46 have the leading adsorption heat differences of CO2 and C2H2. The triple bonds included in Cu2(ebtc) and PCN-46 and fluorine group included in NOTT-108 may help to enhance the C2H2/CO2 selectivity. It is worth mentioning that ZJU-40 does not show good C2H2/CO2 selectivity over its isostructural analog, NOTT-101, while in experimental study, ZJU-40 did show better C2H2/CO2 selectivity than that of NOTT-101.16 It is likely that the generic UFF force field cannot accurately address the interactions of the Lewis basic nitrogen site of ZJU-40 with C2H2 and CO2.

Figure 8. The DFT optimized binding positions of guest molecules in NOTT-108 and ZJU-40: (a) C2H2 loaded in NOTT-108; (b) CH4 loaded in NOTT-108; (c) CO2 loaded in NOTT-108; (b)


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C2H2 loaded in ZJU-40; (b) CH4 loaded in ZJU-40; (b) CO2 loaded in ZJU-40. (Color scheme: O, red; C, green; H, white; F, brown; N, blue; Cu, pink)

Figure 9. The DFT calculated C2H2 binding energies for the OCSs (blue color), the basket pocket sites (red color), and the linker sites (yellow color) of the MOFs.

Figure 10. The DFT calculated CH4 binding energies for the OCSs (blue color), the basket pocket sites (red color), and the linker sites (yellow color) of the MOFs. 17 ACS Paragon Plus Environment


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Figure 11. The DFT calculated CO2 binding energies for the OCSs (blue color), the basket pocket sites (red color), and the linker sites (yellow color) of the MOFs. DFT calculations of C2H2, CH4 and CO2 binding in the MOFs The force fields used for non-OCS atoms of the MOFs were taken from generic UFF in the GCMC simulations. The combination of generic force field, such as UFF with point charge columbic interaction has shown satisfactory applications in various gases adsorption simulations in MOFs. Nevertheless, the accuracy of the generic force fields which assign the same LJ parameters for the same atom species in different chemical environment is worth being compared with quantum mechanical calculations.4 In this work, the binding energies of C2H2, CO2 and CH4 loaded in different locations of the MOFs were also obtained using DFT calculation using VASP. The binding energies of guest molecules (C2H2, CO2 and CH4) in three adsorption sites (site I: OCS, site II: basket site, and site III: linker site) of the MOFs (Figure 9-11, Table1, and Table S2S3) were obtained in three steps. Step 1, six OCS (site I) of the primitive cells of the MOFs were saturated by manually inserting one guest molecules for each OCS. The guest molecules’ binding 18 ACS Paragon Plus Environment

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energies in site I were obtained after optimizing the guest molecules’ positions. Step 2, two guest molecules were inserted into the two basket pocket positions (site II) in the primitive cells. The binding energies of guest molecules in site II were also obtained after the optimization. Step 3, one final guest molecule was inserted close to the ligands’ position. The guest molecules’ binding energy in the site III were again obtained after the optimization. Because the potential well close to site III was not as deep as site I and II, it was rather difficult to locate the local minimum of the potential well close to the linkers’ position through one DFT optimization process. Three to four initial trial positions close to the linkers were tried. The one with highest binding energy was reported. Although the final optimized position of site III does not agree precisely with the adsorption site III produced from the adsorption density plot of GCMC simulations (as shown in Figure 8 and Figure S18 – S28), it provides insight into the binding affinities of C2H2, CO2 and CH4 with the various ligands in the MOFs at a quantum mechanical level. As can be seen in Figure 9- 10, the most preferred binding site for C2H2, CO2 and CH4 in the MOFs is the basket pocket position (site II), whose binding energies are higher than those in site I and III. The DFT binding energies of C2H2 in site II of the MOFs are close the experimental adsorption enthalpies in MOFs, such as Cu2(ebtc), ZJU-40 and NOTT-101, as listed in table 1. The optimized binding positions of C2H2 in site I of the MOFs are parallel to Cu-O bonds of the OCSs. The Cu-C (C2H2) distances are around 2.7 to 2.8 Å for all the MOFs. The binding positions of C2H2 in site II are pointing into the pocket centers. Overall, the DFT calculation shows that the magnitude of the C2H2 binding energies in site I (around 30 kJ/mol) and II (around 35 kJ/mol) are similarly strong. The binding energy at site III varies among all the MOFs because of the functional groups presented in the linkers which provide different binding affinities. MOF505's site III has the highest binding energy with C2H2 (around 33 kJ/mol) because of its compact pore structure which allows C2H2 molecule to receive interactions from multiple surface walls. The site III positions of type II MOFs (Cu2(ebtc), NOTT-101, ZJU-40, NOTT-106 and NOTT-



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108) show C2H2 binding energies around 25 to 32 kJ/mol. The type III MOFs which have the largest pore size have the lowest C2H2 binding energies at site III (varying from 15 to 20 kJ/mol). Considering that the C2H2 binding energies in site I and II of the MOFs are all relatively strong in the MOFs, the binding energies in site III play a key role in enhancing C2H2 storage in the OCSbased MOFs. The C2H2 loading difference per unit cell of the MOFs is mainly due to the binding energy difference at site III where the chemistry environment varies. It is worth noting that C2H2 binding energies in site III of Cu2(ebtc), ZJU-40, NOTT-106 and NOTT-108 are slightly higher than that of their isostructural analog, NOTT-101. This finding provides support to our GCMC simulation and the experimental results16, both of which demonstrated that C2H2 adsorption capacities in ZJU-40, NOTT-106 and NOTT-108 were higher than that in NOTT-101. It can be concluded that the modifications of phenyl group (NOTT-101) through methyl group substitution (NOTT-106), fluorine substitution (NOTT-108) or Lewis basic nitrogen group functionalization (ZJU-40) are viable solutions to enhance C2H2 storage. And the reason that the binding energies of C2H2 with the linkers of ZJU-40 and MOF-108 are higher than that of NOTT-101 is because the fluorine functionalization (NOTT-108) or Lewis basic nitrogen group (ZJU-40) functionalization of the phenyl group (NOTT-101) will polarize the aromatic linker, which enhances the binding interaction of C2H2. As shown in the partial charge calculation of framework atoms using REPEAT method, (all the partial charges of the framework atoms are attached in the CIF files in supporting information) fluorine atoms in the linkers of NOTT-108 carry around -0.16e partial charge and nitrogen atoms in the linkers of ZJU-40 carry around 0.37e partial charge. Compared to the relative neutral phenyl group in NOTT-101, it is expected that C2H2 will have higher electrostatic interaction with the linkers of NOTT-108 and ZJU-40 than that of NOTT-101. Xin-Juan et al. also concluded that electrostatic interaction was one of the key factors in the binding strength of C2H2 in the MOFs.18 Moreover, as shown in Figure 9 and Table 1, C2H2 binding energy with methyl group substation (NOTT-106) is not higher than


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those of ZJU-40 or NOTT-108, which is opposite to the results from GCMC simulations, where NOTT-106 showed better C2H2 adsorption capacity than ZJU-40 and NOTT-108. As shown in Figure 9 and Table S2, DFT calculations overestimated the binding energies of CH4 in site II, as the values are higher than the reported experimental adsorption enthalpies. The adsorption enthalpies from GCMC simulations at a low coverage results agree quite well with the experimental results though. Wu et al.32 studied methane storage in two OCS-based MOFs (PCN11 and PCN-14) using powder neutron diffraction experiments. They pointed out that the binding energies of CH4 in cage window sites (comparable to the basket pocket site in this work) are larger than that in OCSs. The DFT results may have overestimated the binding energies of CH4 in the MOFs, but it revealed correctly that CH4 binding energies in the basket pocket sites are higher than that in the OCSs. The optimized binding positions of CH4 in site I of the MOFs lie above OCS with three hydrogen atoms of CH4 pointing towards Cu. The Cu-C (CH4) distances are around 3.0 Å. The Cu-C (CH4) distances at site II are around 5.0 to 5.5 Å. For CO2's binging energies in the MOFs, DFT calculations also suggested that site II of the MOFs provides the strongest binding energies with CO2. The DFT results in site I and II match well with the experimental and GCMC simulation results. The Cu-O (CO2) distances are around 2.5 Å.



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Figure 12. The relative DFT binding energy differences between C2H2 and CH4 for the OCSs (blue), the basket pocket sites (red), and the linker sites (yellow).

Figure 13. The relative DFT binding energy differences between C2H2 and CO2 for the OCSs (blue), the basket pocket sites (red), and the linker sites (yellow).


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The relative binding energy differences of C2H2 at the three binding sites of the MOFs over CH4 and CO2 were calculated to gain insight of how the three sites can differentiate C2H2 from CH4 and CO2. The relative binding energy differences were obtained through (∆E(C2H2)- ∆E(CH4 or CO2))/∆E(C2H2), where ∆E(C2H2) is the binding energy of C2H2; ∆E(CH4 or CO2)) is the binding energy of CH4 or CO2. As shown in Figure 12 - 13, OCS has the largest relative binding energy difference for C2H2 over CH4 and CO2, which indicates that the main site which can preferably adsorb C2H2 over CH4 and CO2 is the OCS. This finding is in line with the results from GCMC calculations where the adsorption density plot indicates that C2H2 would be densely populated in open Cu site, while for CO2 or CH4, the main adsorption site is the basket pocket site. The second site that can distinguish C2H2 from CH4 and CO2 is the linker site where the relative binding energy difference of C2H2 is also higher than that of CH4 and CO2. It is particularly true for type I and II MOFs, where the small pore size can enhance the effect of the binding energy difference induced from the linkers. MOF-505's linker position has the largest C2H2/CO2 and C2H2/CH4 binding energy difference which makes MOF-505 the best MOF candidate to separate C2H2/CH4 and C2H2/CO2. It is important to note that ZJU-40 is the 2nd best MOF whose Lewis basic nitrogen functionalized linker can separate both C2H2/CH4 and C2H2/CO2, while GCMC simulation above suggested that NOTT-108 which has the fluorine-functionalized linker is the 2nd best MOF. Owning to the fact that the general UFF force field used for GCMC simulation does not consider the detailed chemistry environment of the linkers. It is possible that ZJU-40 should have a better capability to separate C2H2/CH4 and C2H2/CO2 than that of NOTT-108. Table 1.

Comparison of C2H2 adsorption enthalpies from experimental studies and GCMC

simulations with the DFT binding energies of C2H2 in the MOFs MOFs

Qst (exp)a

Qst (GCMC)b

DFT (site I)c

DFT (site II)d

DFT (site III)e

kJ mol-1

kJ mol-1

kJ mol-1

kJ mol-1

kJ mol-1



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MOF-505

24.7033

27.70

33.57

34.01

33.38

NJU-Bai12

-

22.90

31.44

35.26

14.51

PCN-46

-

24.06

34.81

39.13

14.32

Cu2(ebtc)

34.5033

23.24

29.89

34.65

32.05

ZJU-40

34.5034

23.88

30.82

35.82

29.20

HNUST-2

-

24.28

30.78

36.97

20.50

NOTT-101

37.1033

23.79

30.73

34.70

25.98

NOTT-102

22.0034

24.70

33.66

36.02

16.06

NOTT-103

30.8033

23.25

31.21

41.12

18.26

NOTT-106

-

25.53

31.80

34.38

23.42

NOTT-108

-

26.20

30.76

40.44

28.53

a

Experimental isosteric heats of adsorption.

b

Isosteric heats of adsorption from GCMC

simulations at a low coverage. cDFT binding energies at site I (OCS) of the MOFs. dDFT binding energies at site II (OCS) of the MOFs. eDFT binding energies at site III (OCS) of the MOFs. CONCLUSION: To summarize, C2H2 storage and separation capacity in OCS-based MOFs are affected by an interplay of the dimension and the functional groups of the ligands. Judicious selection of organic ligands with suitable dimension and functional sites, such as methyl group, Lewis basic nitrogen site and fluorine group can facilitate C2H2 adsorption in addition to OCS, and help distinguish C2H2 from CH4 and CO2. According to GCMC simulations and DFT calculations, MOF-505 appeared to be the best MOF to store C2H2 because of its compact pore size which allowed not only OCS, but also ligands to fully interact with C2H2. Nonetheless, considering that our simulation data of MOF-505 does not agree very well with the experimental data.5 Difficulty in realizing the theoretical C2H2 adsorption capacity in MOF-505 still exists. Moreover, the fast rise of C2H2 adsorption in MOF-505 at low pressure poses a challenge to the release of C2H2. Among 24 ACS Paragon Plus Environment

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the type II MOFs, NOTT-106, ZJU-40 and NOTT-108 whose ligands with methyl substituent, Lewis basic nitrogen sites and fluorine substituent provide enhanced affinity with C2H2 in addition to open Cu sites provide an alternative solution for C2H2 storage. ASSOCIATED CONTENT Supporting Information. Lennard-Jones parameters, point charges, MOFs structural details, computational methods, and additional adsorption isotherms and adsorption enthalpies are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors: Lifeng Ding ([email protected]); Youyong Li ([email protected]) ACKNOWLEDGMENT The authors acknowledge financial support from the Chinese Young Scholar National Science Foundation Grant (21403171) and the Xi’an JiaoTong-Liverpool University Research Development Fund (PGRS-13-03-08). The authors acknowledge utilization of the computational resources from the Shenzhen Cloud Computing Center. The work was also supported by the Ministry

of

Science

and

Technology

of

China

(Grants

2017YFA0204802

and

2017YFB0701601), the National Natural Science Foundation of China (Grants 51761145013, 21673149), and Collaborative Innovation Center of Suzhou Nano Science and Technology.

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TOC Graphic


CH4

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