Methane Uptakes in Covalent Organic Frameworks with Double

Jan 5, 2015 - The methane uptakes of six double halogen substituted covalent organic ..... Innovation Team (IRT13022) of China, and NFFTBS (J1103306)...
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Methane Uptakes in Covalent Organic Frameworks with Double Halogen Substitution Jinghao Hu, Jianfei Zhao,† and Tianying Yan* Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Metal- and Molecular-Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: The methane uptakes of six double halogen substituted covalent organic frameworks (COFs) based on COF-102 were simulated with grand canonical Monte Carlo simulation at 298 K and pressure ranges from 1 to 80 bar. The simulation shows that COF-102-1,4-2I reaches the DOE target of 180 V(STP)/V for methane delivery. The current study highlights the correlation between the structure and the adsorption property of the double halogen substituted COF-102. In COF-102-1,4-2I, the triangle arrangement of the six I atoms around the central B3O3 ring brings close contact between I atom and B3O3 ring, and thus enhances the attraction of CH4 with high CH4 density in the vicinity above and below this region, especially in particular adsorption sites. Such favorable structural arrangement, altogether with the strongest I−CH4 attraction among the halogen substituent in this study, gives the highest isosteric heat as well as the CH4 uptakes at 298 K and 35 bar in the hypobaric region. The result in this study demonstrates that double halogen substituted COF-102 is capable of increasing CH4 uptakes for practical applications.

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INTRODUCTION

Since it is difficult to attempt every substituent in practice, computer simulation11 is a powerful tool to explore the gas uptake with properly designed materials. Snurr and co-workers generated 137 953 conceivable MOFs with 102 building blocks.12 Over 300 hypothetical MOFs were predicted to have better performance for CH4 uptake than all the known materials. Moreover, their latest work13 shows that increasing the volumetric density of sites that interact strongly with CH4 along with raising the delivery temperature may be a suitable strategy to greatly improve the CH4 deliverable capacity of nanoporous materials. As for COFs, Lan et al. designed Lidoping COFs, and the CH4 uptake was almost doubled as demonstrated by the subsequent computer simulations.14 Another method to promote the uptake of COFs is ligand modification. Goddard and co-workers15 modified the moiety by replacing −H’s on meta-positions with diverse alkyl groups or by changing the phenylene group into anthracene and transethylene. They simulated the uptakes of a variety of designed COFs and found that the delivery amount of CH4 in COF-102-Ant and COF-103-Eth-trans reached the goal of DOE. Meanwhile, GCMC simulations provide complementary informations about the preferential siting of adsorbate molecules to understand the underlying mechanisms of gas

Nowadays, the combustion of fossil fuels has caused huge problems, as the main reason for the greenhouse effect.1 Natural gas, with the lowest ratio of carbon-to-hydrogen emissions among fossil fuels, is an ideal fuel to reduce the emission of carbon dioxide. However, the storage of natural gas, which consists mainly of methane (CH4), is still an important subject. According to the target by US Department of Energy (DOE), the capacity for CH4 uptake has to reach 180 V(STP)/ V at 35 bar, in which V(STP)/V refers to the volume of CH4 per volume of the adsorbent and STP refers to the standard temperature and pressure (298 K and 1.01 bar). In 1999 and 2005, Yaghi and co-workers synthesized the metal organic frameworks (MOFs)2 and the two-dimensional (2D)3 and three-dimensional (3D) 4,5 covalent organic frameworks (COFs). Both materials have porosity and crystalline structures, while COFs have lower densities because they are made of light elements such as B, C, O, H, and Si without metal elements, and thus are more favorable when counting gravimetric uptake. Many studies have been performed on the adsorption properties of MOFs and COFs of CH4 since then.6−8 Some MOFs have been confirmed experimentally to surpass the DOE target of CH4 uptake. At 35 bar and 273 K, the excess volumetric CH4 uptake is 220 V(STP)/V for PCN-149 and 190 V(STP)/V for Ni-MOF-74.10 On the other hand, none of the COFs have been reported experimentally to surpass the DOE target, to our best knowledge. © 2015 American Chemical Society

Received: December 26, 2014 Revised: January 3, 2015 Published: January 5, 2015 2010

DOI: 10.1021/jp512908k J. Phys. Chem. C 2015, 119, 2010−2014

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The Journal of Physical Chemistry C absorption.16−18 Snurr et al. performed a GCMC simulation on the gas adsorption sites of argon and nitrogen in IRMOF-1, which agreed well with the experimental X-ray diffraction studies determined sites19 with only one exception and successfully explained why the site in the smaller cavities is only filled up at high loadings.20 Moreover, through analyzing the positions of adsorbate molecules during a GCMC simulation with snapshots, Mendoza-Cortés et al. pointed out that the sites which are more populated on methane adsorption are those where two edges converge in 3D-COFs.15 Tina Düren suggusted that the methane molecules prefer to be near the walls while there is still plenty of open space in the middle of the cage in IRMOF-1.7 Our previous study21 explored the effects of a variety of substituents on the basic COFs, by replacing one H atom on each benzene ring with other substituent groups on CH4 uptakes, using grand canonical Monte Carlo (GCMC) simulation.11 The results show that all these substituents are beneficial to CH4 uptake, especially the halogen groups (−Cl, −Br, and −I). The best adsorbents of CH4 delivery are COF102-I (169 V(STP)/V), COF-102-Br (168 V(STP)/V), and COF-102-Cl (165 V(STP)/V) at 298 K and 35 bar. Thus, this time we continue to study double substitution on COF-102,4 choosing the halogen atoms (−Cl, −Br, and −I) as the substituent, without changing the symmetry of the original COFs, with the purpose of achieving better CH4 adsorbents. Figure 1 shows the building units of COF-102. Although for

the optimization. The optimized coordinates as well as the cell parameters are tabulated in the Supporting Information. We employed GCMC simulation11 for our calculation of amount and isosteric heat of adsorption. The van der Waals (vdW) interactions of COF−CH4 and CH4−CH4 were simulated by Lennard-Jones potential, and the parameters of each atom of COFs were obtained from DREIDING force field.22 A singlesite model was adopted for CH4 molecule, whose LennardJones parameters were taken from previous work.23 We choose 2 × 2 × 2 supercell as our model, which was held rigid in GCMC simulations. The simulations were run for totally 4 million MC steps by using a homemade code, with the first 1 million steps for equilibrating and the following 3 million steps for sampling. The isodensity distribution is performed with single unit cell and sampled for 200 million MC steps. The temperature was set at 298 K, and the pressure was 1−80 bar with 10 bar increment. Additionally, five independent simulations were performed at 35 bar. The isosteric heat Qst was calculated from the following equation Q st =

⟨N ⟩⟨U ⟩ − ⟨NU ⟩ + RT ⟨N 2⟩ − ⟨N ⟩2

in which ⟨ ⟩ refers to the ensemble average, ⟨U⟩ is the average energy of the configurations, ⟨N⟩ is the average number of particles, R is the Boltzmann constant, and T is the temperature.

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RESULTS AND DISCUSSION Figure 2a,b shows the simulated total volumetric uptake isotherms of CH4 at 298 K and the isosteric heats of adsorption (Qst’s), respectively, for the six double halogen substituted COF-102s and the basic COF-102. It is shown in Figure 2a that the amount of absorbed CH4 grows when the pressure goes up. The higher the pressure is, the flatter the isotherms, and the uptakes approach to saturation is due to the volume effect at high pressure.21For the different substituent, the curves show that all the modified COFs perform better than the basic COF102. Apart from that, it is shown in Figure 2a that the CH4 uptake by the −I substituent is higher than that by −Br, while −Cl has the least uptake at hypobaric region below 40 bar. This is inconsistent with the Qst’s in Figure 1b, because at low and medium pressure the leading factor of the uptake is the adsorptive capabilities characterized by the higher Qst. Thus, the double halogen substituted COF-102 of stronger attraction will adsorb more CH4. On the other hand, at high pressure, since the pores approach saturation, Qst is no longer the leading factor while the volume effect becomes important. Since a substituent with heavy halogen atom occupies more space and leaves less for CH4, as shown in Table S2 in the Supporting Information, the trend of the uptake reverses with −Cl > −Br > −I at 80 bar. The inset of Figure 2a shows the CH4 delivery, calculated by subtracting the total uptake at 1 bar from those above 1 bar. Also shown is the DOE target for CH4 delivery, 180 V(STP)/V at 35 bar. For better comparison, the CH4 delivery of the six double halogen substituted COF-102s at 35 bar are reproduced in Table 1. As can be seen, though all the double halogen substituted COF-102s have higher CH4 delivery than the basic COF-102, COF-102-1,4-2I promotes the CH4 delivery by 25.1% and reaches the DOE target. Given that only two H atoms on each benzene ring are substituted by two I atoms of the building unit TBPM in Figure 1, such amplification is quite

Figure 1. (a) Building units of COF-102, tetra(4dihydroxyborylphenyl)methane (TBPM), and (b) the same as part a with the 4-dihydroxyborylphenyl fragment highlighted, in which R1, R2, R3, and R4 are all H atoms in the basic COF-102. For the double halogen substituents in this study, they are named as COF-102-i,j-2X when substituting the H atoms on Ri and Rj with halogen atom (X).

each substituent there are four different ways to substitute the two H atoms on benzene by two halogen atoms, preliminary attempts show that the structure of the COF-102-1,2-2X and COF-102-2,4-2X, after geometrical optimization, results in disordered framework, which may not guarantee their experimental synthesis by self-condensation of the building block in Figure 1. Moreover, their adsorption properties are worse than COF-102-1,4-2X or COF-102-1,3-2X. Thus, we only take into account COF-102-1,4-2X and COF-102-1,3-2X in this study, and there are totally six double halogen substituted COF-102s for each COF-102-1,4-2X and COF102-1,3-2X with −X denoting −Cl, −Br, and −I.

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COMPUTATIONAL METHODS In this study, we took COF-102 as basic adsorbents, and choose −Cl, −Br, and −I groups as the substitution. We substituted two type of −H in each COF with −X to create derived COFs. To obtain stable configurations for subsequent calculation, we optimized the newly designed COFs by using DREIDING force field22 with conjugate gradient algorithm, without transforming the symmetry of COFs, and all the derived frameworks still held I4̅3d space group and ctn topology after 2011

DOI: 10.1021/jp512908k J. Phys. Chem. C 2015, 119, 2010−2014

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mono- to difunctionalized COF-102-I, the same as the Cl and Br substituted COF-102s. At this point, it is highly desirable if there were experimental studies on the CH4 delivery with halogen substituted COF-102s to compare with. It is notable from Figure 2 that, for the same substituent, the uptakes of COF-102-1,4-2X are always higher than those of COF-102-1,3-2X, and so is the corresponding Qst. More strikingly, the CH4 delivery of COF-102-1,4-2X at 298 K and 35 bar is consistently higher than that of COF-102-1,3-2X, as shown in Table 1, but the surface area and free pore volume of the former are consistently smaller than the latter, as shown in Table S2 in the Supporting Information. Thus, there must be correlation between the COF structure and the CH4 uptake. To investigate the different adsorption behavior by the two different double halogen substituted COF-102s, we compare the isodensity distributions of CH4 in COF-102-1,4-2I and COF-102-1,3-2I in Figure 3, in which the density of CH4 is 5 times higher than their individual average densities. The center of Figure 3 highlights a randomly selected hexatomic B3O3 ring and the six closest I atoms, which can be regarded as the building block of COF-102. The isodensity distribution is symmetric above and below each B3O3 ring, and that above the central B3O3 ring is not displayed for clear view It is shown in Figure 3 that the distribution of CH4 is highly anisotropic, regulated by the frameworks. Apart from that, the isodensity distribution of CH4 around a central B3O3 ring is nearly a triangle for COF-102-1,4-2I, while it is like a clover fan for COF-102-1,3-2I. Thus, the adsorption area in COF-102-1,42I is larger than that in COF-102-1,3-2I, in agreement with higher CH4 uptakes by COF-102-1,4-2I. Obviously, such apparent isodensity distribution is affected by the spatial arrangement of I atoms. As shown in Figure 3a, the 6 highlighted I atoms in COF-102-1,4-2I arrange to a triangle surrounding the central B3O3 ring. Consequently, one of the I atoms on each benzene in COF-102-1,4-2I is closer to the B3O3 ring than the other. The close contact between I and B3O3 leads to enhanced attraction in particular at the adsorption sites in the mutual vicinity of I and B3O3. These adsorption sites are the ones that dominate the adsorption properties due to the nonlinearity of the Boltzmann factor, and this results in higher CH4 density in the vicinity between the two groups. Thus, the attraction of CH4 is enhanced. On the contrary, the 6 highlighted I atoms in COF-102-1,3-2I arrange to a hexagon surrounding the central B3O3 ring, and none of the I atoms is close to the B3O3 ring, as shown in Figure 3b. The lack of close contact between I atom and the B3O3 ring in COF-102-1,3-2I cannot take such advantage of the enhanced attraction, and thus results in lower CH4 uptake than COF-102-1,3-2I. To explain this idea further, we performed a 400 millon steps GCMC simulation and calculated the sites with the highest densities to locate the adsorbate CH4 in COF-102-1,4-2I and COF-102-1,3-2I, which can be regarded as the adsorption sites dominating the adsorption properties. Interestingly, our simulations show that the sites found are still a nearly held I4̅3d space group, and there are totally 48 sites with the same symmetries in one COF-102 cell. Figure 4 displays the locations of these sites in a partial structure of the framework, which can be regarded as the building block of COF-102,4 as mentioned above. It can be seen in Figure 4 that although in both graphs the three adsorption sites form a regular triangle, the locations of these sites are not the same. The adsorption sites for COF-102-1,4-2I are closer to one of the I atom than the other one, while the distances between the adsorption sites

Figure 2. (a) Isotherms of total volumetric CH4 uptake at 298 K from 0 to 80 bar of the six double halogen substituted COF-102s and the basic COF-102. The inset shows isotherms of delivery volumetric CH4 uptake. The cross curve shows the DOE target. (b) Isosteric heats of adsorption at 298 K of the six double halogen substituted COF-102s and the basic COF-102.

Table 1. Simulated CH4 Delivery at 298 K and 35 bar by the Six Double Halogen Substituted COF-102a COF-102-1,4 -2X COF-102-1,4 -2I COF-102-1,4 -2Br COF-102-1,4 -2Cl

CH4 delivery V(STP)/Vb 181.1 ± 0.7 175.7 ± 0.9 171.1 ± 0.8

COF-102-1,3 -2X COF-102-1,3 -2I COF-102-1,3 -2Br COF-102-1,3 -2Cl

CH4 delivery V(STP)/Vb 166.4 ± 0.5 163.7 ± 0.9 161.8 ± 0.6

a

The simulated CH4 delivery uptake of basic COF-102 at 298 K and 35 bar is 144.8 ± 0.4 V(STP)/V, which is in reasonable agreement with the experimental result of 143.5 V(STP)/V24 at the same conditions. bError bars are represented by the standard deviation based on 5 independent GCMC simulations.

impressive. However, the increments of the CH4 delivery between COF-102-1,4-2I and monofunctionalized COF-102I21 are quite modest, only 7.1%. The restriction of the further promotions of the CH4 delivery from the mono- to difunctionalized COF-102-I is attributed to two aspects: On one hand, the additional I atoms yield higher isoteric heat and thus higher uptake at low pressure, while on the other hand, the additional I atom occupies more space and flattens the growth of uptake at medium and high pressure due to the volume effect. Consequently, the increase from the mono- to difunctionalized COF-102-I is 48.2% at 1 bar while it is only 9.9% at 35 bar. Since the CH4 delivery is defined as subtracting the total uptake at 1 bar from those above 1 bar, the high promotion at 1 bar and low promotion at 35 bar together restrict the further promotions of the CH4 delivery from the 2012

DOI: 10.1021/jp512908k J. Phys. Chem. C 2015, 119, 2010−2014

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Figure 3. Isodensity distribution of CH4 in (a) COF-102-1,4-2I and (b) COF-102-1,3-2I at 298 K and 35 bar. The sticks refer to the structure of the COFs, and the spheres in green, red, and pink denote I, O, and B atoms, respectively. Only the 6 I atoms close to a randomly selected central hexatomic B3O3 ring are highlighted with green spheres for clarity. The gray clouds denote the isodensity of CH4 with the density 5 times higher than their individual average densities.

Figure 4. Adsorption sites of CH4 in partial structure of (a) COF-102-1,4-2I and (b) COF-102-1,3-2I at 298 K and 35 bar by GCMC simulation. The spheres in gray, white, brown, red, and pink denote C, H, I, O, and B atoms, respectively. The yellow spheres represent the sites with the highest densities of the CH4 adsorption, and the average densities are (a) 46.427, (b) 22.686. The green dashed line highlights the distance among the adsorption sites and some atoms.

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CONCLUSION In summary, the CH4 uptakes of six double halogen substituted COF-102s were simulated with GCMC simulations at 298 K and pressure ranging from 1 to 80 bar. We found that the leading factor of CH4 adsorption is the attraction of the substituent atom in hypobaric region while the volume effect plays the leading role in the hyperbaric region. Among the six double halogen substituted COF-102s simulated in this study, the CH4 delivery uptake of COF-102-1,4-2I is 181.1 ± 0.7 V(STP)/V, which reaches the DOE target of 180 V(STP)/V. Moreover, the current study also highlights the correlation between the structure and the CH4 adsorption property of the double halogen substituted COF-102. Specifically, the triangle arrangement of the six I atoms around the central B3O3 ring brings close contact between I atom and B3O3 ring, and thus enhances the attraction of CH4 and causes higher CH4 density in the vicinity above and below this region especially in particular adsorption sites. Such a favorable structural arrangement, altogether with the strongest I−CH4 attraction among the halogen substituent in this study, gives the highest Qst as well as the CH4 uptakes at 298 K and 35 bar in the hypobaric region. Since all the double halogen substituted COF-102s in this study preserve the I4̅3d space group and ctn topology of the basic COF-102,4 we expect that experimental synthesis may be achieved with moderate efforts. At this point, we are looking

and two I atoms are nearly the same for COF-102-1,3-2I. Moreover, the adsorption sites in Figure 4a are closer to the nearest O atoms than in Figure 4b. Thus, we consider that the vdW interaction between the CH4 at the adsorption sites and COF-102-1,4-2I is stronger than that of COF-102-1,3-2I due to the closer contact between each other, which will yield a higher probabilities of methane adsorption for COF-102-1,4-2I at these sites. Actually, the average density value of a total of 48 adsorption sites calculated by GCMC simulation is 46.427 for COF-102-1,4-2I and 22.686 for COF-102-1,3-2I (densities normalized for the unit cell), showing the former with better adsorption properties than the latter at these sites, which agreed well with the views mentioned above that the close contact between I and B3O3 leads to enhanced attraction in particular adsorption sites. Other halogen atom substitutions show similar features for COF-102-1,4-2X and COF-102-1,3-2X, in which X denotes Cl or Br. The reason that COF-102-1,4-2I gives the highest CH4 uptakes may be attributed to correlation between the spatial arrangement of the substituted I atoms and the strongest I−CH4 attraction, which gives the highest Qst as well as the CH4 uptakes at 298 K and 35 bar in the hypobaric region. 2013

DOI: 10.1021/jp512908k J. Phys. Chem. C 2015, 119, 2010−2014

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forward to the experimental CH4 uptakes by the double halogen substituted COF-102s.

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ASSOCIATED CONTENT

S Supporting Information *

Computational methods, the coordinates of the optimized basic COF-102 and the six double halogen substituted COF-102s in this study, as well as their surface area per cell and free pore volume per cell. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

† Department of Chemistry, Brown University, Providence, RI 02912, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21373118, 21421001), the MOE Innovation Team (IRT13022) of China, and NFFTBS (J1103306).

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DOI: 10.1021/jp512908k J. Phys. Chem. C 2015, 119, 2010−2014