Theoretical Simulation of CH4 Separation from H2 in CAU-17

Article pubs.acs.org/JPCC

Theoretical Simulation of CH4 Separation from H2 in CAU-17 Materials Baolei Zhou, Wenliang Li,* and Jingping Zhang* Faculty of Chemistry, Northeast Normal University, Changchun 130024, China ABSTRACT: By combining grand canonical Monte Carlo simulations and dispersion-corrected density functional theory (DFT-D3), the adsorption and separation properties for gases (including H2 and CH4) in the recently synthesized metal− organic frameworks CAU-17 were investigated. By systematically screening the separation properties of 6 binary mixtures, extremely high CH4/H2 selectivity was found for CAU-17, when comparing with several classic porous materials (including IRMOF-1, UMCM-1, Cu-BTC, COF-5, and ZIF-8). Moreover, the functionalized CAU-17 also expressed the extraordinarily high CH4/H2 selectivity. Then the origin for high CH4/H2 selectivity in CAU-17 materials was further investigated in depth. The CH4 uptakes in CAU-17 materials are higher than those in classic porous materials, while the H2 loadings are in contrary to CH4. The difference in isosteric heats of adsorption for CH4 and H2 is the key factor for high CH4/H2 selectivity. In addition, the investigation on the capacity of three channels for CH4 and H2 indicates that the rectangle channel predominantly contributes for CH4 loadings. The interaction energy from DFT-D3 calculations between the gas and channels reveals that CAU17 can selectively adsorb CH4 over H2. zeolites,5 metal−organic frameworks (MOFs),6 covalent organic frameworks (COFs),7 and zeolitic imidazolate frameworks (ZIFs)8 have already caused widespread attention. They have been widely used in a number of applications, such as sorption,9 separation,10 nonlinear optics, catalysis,11,12 biological imaging,13 drug delivery,14,15 and so forth. For example, HKUST-1 has been reported to have a CH4 uptake of about 10 mmol/g at 35 bar and 295 K.16 Nevertheless, the highest CH4 capacities for activated carbons are 4 mmol/g under the same conditions.17 Furukawa found that COFs revealed high uptakes for H2, CH4, and CO2 compared with carbon materials.18 The Yildirim group investigated the adsorption of CH4 and H2 on ZIF-8 at different temperatures (from 30 to 300 K).19 Hydrogen and methane as an alternative clean energy source have been used in vehicle application. However, hydrogen often can be gained via a methane steam reforming reaction and methane dry reforming.20 Before hydrogen can be used effectively, it is a very important process that hydrogen has to be separated from synthetic gas, which includes CH4, CO, CO2, H2, and H2O. Yang et al. investigated the separation performance between H2 and CH4, and the selectivity is about 20 in Cu-BTC and 5 in MOF-5 at low pressure.21 Tong et al. selected a suite of COFs and valued the CH4/H2, CO2/ CH4, and CO2/H2 separation properties.22 Keskin reported the adsorption and diffusion properties of two-dimensional COFs (COF-5, COF-6, and COF-10) for separation of CH4/H2

1. INTRODUCTION As one of the most concerning greenhouse gases, CO2 is usually released from heavy burning of combustible fossils.1 To change this situation, great efforts have been made to ease the burden on the environment,2 including employing clean energy, such as solar energy, geothermal energy, biomass energy, water energy, etc. Hydrogen is an alternative clean energy source with no contribution to the greenhouse effect and also possesses a high-energy yield. Nevertheless, devoting hydrogen energy into commercial application is still a huge challenge because hydrogen storage is still the most critical issue. CH4, the main ingredient for natural gas (NG), has the maximum ratio of H/C and can produce less CO2 than other hydrocarbon fuels. However, natural gas has a low volumetric energy density (VED) at room temperature.3 This physical property becomes the major impediment in its widespread application. Many methods have been proposed to enhance the storage capacity of natural gas, such as compressed natural gas (CNG), liquefied natural gas (LNG), and adsorbed natural gas (ANG). CNG technology always needs higher pressure (200 to 300 bar), which means that the methane is stored as a supercritical fluid in an oil reservoir attached to vehicles. To date, the liquefied natural gas (LNG) has been widely used for long distance transportation.4 However, a defect is the high cost of running a cryogenic system. ANG technology, which stores natural gas in lightweight carriers (for example, porous sorbent materials), is a very promising and efficient strategy at relatively low pressures (40 bar). As for now, a wide range of porous materials have been investigated. Besides the traditional activated carbons and © 2017 American Chemical Society

Received: July 19, 2017 Revised: August 28, 2017 Published: August 28, 2017 20197

DOI: 10.1021/acs.jpcc.7b07108 J. Phys. Chem. C 2017, 121, 20197−20204

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The Journal of Physical Chemistry C mixtures.23 They also further investigated selectivity of 20 different nanoporous materials for CH4/H2 mixtures.24 Recently, Inge et al. synthesized a new bismuth-based MOF (CAU-17) which has a complicated structure.25 The novel pore topology from simple building units and good thermal stability motivate us to explore the adsorption and separation mechanisms and find its potential application. In this work, the adsorption and separation properties of H2, CH4 in CAU17, and its derivatives were investigated by combining classical Grand canonical Monte Carlo (GCMC) and dispersion corrected density functional theory (DFT-D3). The separation properties of the binary mixture were further screened, and the separation mechanism was discussed in detail.

Table 1. Lennard-Jones Potential Parameters for Adsorbates and Adsorbents species CH4 H2 CO2 N2

adsorbents

2. MODELS AND COMPUTATIONAL METHODS 2.1. COF and MOF Structures. In this work, we investigate a bismuth-based MOF, [Bi(BTC)(H2 O)]·2H 2O·MeOH, named CAU-17, where BTC = 1,3,5-benzenetricarboxylate. CAU-17 has a singularly abstruse structure with helical Bi−O rods linked together by BTC3− ligands.25 According to the shape of the channels, three types of channels (including triangle, rectangular, and hexagon channels) are formed in the crystal structure. To illustrate the effect of functional group on CAU-17, we designed a sequence of functionalized MOFs by replacing one H atom on benzene rings in the BTC3− ligands with other substituent groups, including −F, −Cl, −Br, −I, and −OH as shown in Figure 1. In addition, several typical porous materials, for instance, IRMOF-1, UMCM-1, Cu-BTC, COF-5, and ZIF-8, are selected to compare with CAU-17 and its derivatives.

site

ε/kb (K)

σ (Å)

CH4 H2 C O N2 C H O N B Zn Bi Cu F Cl Br I

148.00 36.70 27.0 79.0 94.95 47.90 7.66 48.158 39.007 47.806 27.718 260.67 2.518 36.486 142.571 186.202 256.66

3.73 2.958 2.80 3.05 3.549 3.47 2.85 3.033 3.263 3.58 4.045 3.89 3.114 3.093 3.519 3.519 3.697

q (e)

+0.70 −0.35

simulations by MUSIC code,35 with a cutoff of 12.0 Å. Super cells are applied to all porous frameworks, which were expanded to at least 24 Å along each dimension. During each simulation, the number of trial moves was 4 × 107, with the first 2 × 107 steps for equilibrium, and the subsequent 2 × 107 steps for data collection. GCMC simulations would give the absolute uptake Nabs, while experiments typically produce the excess uptake Nex. Thus, it is necessary to convert all absolute loadings to excess loadings to compare with the experimental data.36 It is calculated by the following equation Nex = Nabs − ρg Vg

(1)

where ρg is the fluid density of bulk gas calculated from the PR EOS,37 and vg is the free volume of adsorbent accessible to the guest molecules. It is calculated from the ideal gas law38 Figure 1. Model of CAU-17-X structure. Atom color scheme: Bi, pink; C, gray; O, red; H, white; X, yellow (X = H, F, Cl, Br, I, and OH).

Vg =

RNmT Pmm

(2)

where T, R, and P are the temperature, gas constant, and pressure, respectively. Nm is the number of adsorbed probe molecules per molar mass (mm) of the adsorbents, which is calculated from the GCMC simulations of nonadsorbed He gas in two materials at room temperature and low pressure. The isosteric heat of adsorption (Qst) reflects the difference of the partial molar enthalpy of the adsorbate in the bulk phase and the partial molar internal energy in the adsorbed phase39 for each molecule. This is given by the following equation

2.2. Potential Models. CH4 was treated as a united atom.26 The above potential model was successfully used in previous works.27 CO2 was treated as a three-site rigid linear molecule,28 while H2 and N2 were treated as a one-site model.29,30 All the adsorbate molecules and porous frameworks were treated as rigid structures. Early studies have displayed that the Dreiding force field could give accurate predictions of gas adsorption and separation in porous materials.31 As a consequence, the Dreiding force field was chosen for the potential parameters of framework atoms apart from metal atoms after verification by comparing with experimental data. For metal atoms, such as Cu or Bi atoms, the parameters came from the UFF,32 which were not available from Dreiding. For the charge of absorbents, we used the QEq33 method based on Ewald sums. Lorentz− Berthelot mixing rules were used to describe the Lennard-Jones cross-interaction parameters between gases and sorbent, i.e., εij = (εiiεjj)1/2 and σij = (σii + σjj)/2.34 All LJ potential parameters are provided in Table 1. 2.3. GCMC Simulations. The CH4 capacities of these selected porous materials were determined by GCMC

Q st = RT −

⎛ ∂U ⎞ ⎜ ⎟ ⎝ ∂N ⎠T , V

(3)

where U is the adsorption energy in the adsorbed phase that contains the adsorbate−adsorbate and adsorbate−adsorbent interaction energies; T is the temperature; R is the universal gas constant, and N is the adsorption loading. The adsorption selectivity is used to evaluate the ability of separation in different porous materials. The adsorption selectivity for component i relative to component j is defined by the following equation 20198


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The Journal of Physical Chemistry C

Figure 2. Adsorption isotherms of CH4 in (a) IRMOF-1, (b) ZIF-8 at 300 K, (c) COF-5, (d) Cu-BTC, and (e) UMCM-1 at 298 K and adsorption isotherms of H2 in (f) IRMOF-1, (g) ZIF-8 at 300 K, (h) COF-5, (i) Cu-BTC, and (j) UMCM-1 at 77 K. (The red line is experimental data, and the black line is simulation results.)

Sads(i/j) =

xi /xj yi /yj

impurities in structure, as well as the inadequacy of the force field. To overcome the inaccuracy of the force field, ab initio calculations could be a choice for describing interactions between adsorbent and adsorbate; however, computational cost increases drastically with the increasing system size, allowing only the description of small systems (less than 100s of atoms) in GCMC simulations.48,49 In this work, CAU-17 has a large system size (about 180 atoms in a unit cell), which is far beyond our current affordance. Another method of accurate modeling is introducing an ab initio derived force field. In the process of the calculations, it is crucial to “cut” a model cluster from the periodic framework, which means some important chemical bond will be cut and results in importing a new error. In this work, CAU-17 revealed a complexity for a MOF structure (54 unique nodes and 135 edges) by topological analysis.25 Choosing a simplified model cluster for CAU-17 may introduce even larger deviations. Furthermore, the agreement between simulation results based on Dreiding force field and experimental data is satisfactory for the needs of the present work. Therefore, all GCMC simulations will be performed by using the Dreiding force field. The adsorption isotherms of H2 in typical porous materials were compared to the corresponding experimental data18,19,39,46,50 as shown in Figure 2, and good agreement can also be found according to the comparison. Therefore, the simulation method used here is credible for the current systems and encourages us to perform the following investigations on gas adsorption and separation.

(4)

where x is the mole fraction of two components in the adsorbed phase, and y is the corresponding mole fractions in the bulk phase. 2.4. First-Principles Calculations. First-principles calculations are introduced to investigate the vdW interaction between porous materials and gas molecules with the PBE1PBE-D3 functional40 by Gaussian 09 software.41 The basis set 6-31G* is used for nonmetal atoms, while LANL2DZ42−44 is used for metal atom Bi. Basis set superposition error (BSSE) was not taken into consideration, as it was partially absorbed by the D3 short-range parametrization.45

3. RESULTS AND DISCUSSION 3.1. Verifying the Simulation Method. The uptakes of CH4 and H2 in several typical porous materials, such as IRMOF-1, ZIF-8, COF-5, Cu-BTC, and UMCM-1, are demonstrated in Figure 2 for verifying the accuracy of our simulation method. For CH4, good agreement between experimental data18,19,39 and the simulation results was observed for COF-5, IRMOF-1, and UMCM-1. However, simulation results exceed the corresponding experimental uptakes in ZIF-846 and Cu-BTC,47 and the discrepancy between them might be caused by crystal imperfections, the 20199


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The Journal of Physical Chemistry C 3.2. Comparison of Relative Selectivity. UMCM-1 demonstrated remarkable adsorption capacity for CH 4 storage51 and CO2 storage52 because of the high surface area, high free volume, and low framework density. Therefore, UMCM-1 is selected as a comparison case. The relative selectivity, SA/UMCM‑1, is defined as the ratio of the selectivity of binary mixtures in porous materials A to UMCM-1 by the following equation: SA/UMCM‑1 = SA/SUMCM‑1. Here, SA and SUMCM‑1 are the selectivity of binary mixtures in porous materials A and UMCM-1, respectively. As shown in Figure 3,

For the classic porous materials, the selectivities are kept at a relatively low level. Additionally, the functionalized CAU-17 derivatives have better performance than pristine CAU-17. CAU-17-Br shows the highest selectivity of CH4 over H2. The selectivity of CAU-17-Br changes from 416.3 to 206.6 with increasing pressure. 3.4. Adsorption Isotherms of H2 and CH4. In order to explore the reason for the high selectivity of CH4/H2 in CAU17 and its derivatives, adsorption isotherms of CH4 and H2 on selected porous materials were simulated and shown in Figure 5. CH4 uptakes in Cu-BTC are similar to the original CAU-17 and larger than the other typical materials in the range of 0−1.0 bar as depicted in Figure 5a. Additionally, amounts of CH4 uptakes in the substituted CAU-17 are larger than the original CAU-17. Therefore, original CAU-17 has good performance for CH4 adsorption, and the functionalized groups can enhance the adsorption capacity of CH4 under the same conditions. For hydrogen adsorption in Figure 5b, the situation is in sharp contrast to methane adsorption; i.e., CAU-17 and its derivatives show much lower H2 uptakes than the typical porous materials, which results in high selectivity of CH4/H2in CAU-17 and its derivatives. 3.5. Isosteric Heat of Adsorption (Qst). The Qst values of different gases were calculated to better understand gas adsorption in the porous materials. Figure 6 displays that the Qst values of CH4 are much higher than H2 in these materials. In addition, the pristine and functionalized CAU-17 materials have higher isosteric heats for CH4 in comparison with other porous materials, corresponding to stronger affinity for CH4. In Figure 6b, the trend of the isosteric heats for H2 is similar to that for CH4 in Figure 6a; however, the difference in the isosteric heats of H2 between porous materials is smaller than that of CH4 in Figure 6a. As a result, the high selectivity of CH4/H2 in pristine and functionalized CAU-17 originates from high affinity for CH4. On the other hand, the substituent groups possess a high affinity with gas molecules, which significantly improves the Qst value at the same ambient temperature. 3.6. Preferential Adsorption Sites. To understand the adsorption mechanism, preferential adsorption sites for CH4 on CAU-17 were investigated. In Figure 7a, CAU-17 possesses three shapes of 1D channels: triangle, rectangular, and hexagon channels. In order to investigate the contribution of each channel for CH4 adsorption, the divided CH4 uptakes in three channels were analyzed. Figure 7b depicts the percentage of CH4 loadings in each kind of channel over total CH4 loadings from GCMC simulations at low pressure. It is found that most of the CH4 molecules are adsorbed in the rectangular channel in CAU-17, whereas the triangle channel adsorbs the least molecules in a pressure range (0−1 bar). The order of CH4 loadings is as follows: rectangular channel > hexagon channel > triangle channel. Interestingly, with the increasing pressure, the percentage of CH4 uptakes increases slowly in the hexagon channel (with the largest pore diameter), while the one for the rectangular channel decreases accordingly. At low pressure, the fit rectangular channel plays an important role in CH4 adsorption by providing preferred adsorption sites, while at high pressure, the larger channel could provide enough free volume for additional CH4. Figure 8 displays the ensemble average from the GCMC simulations for the CH4/H2 binary mixture in CAU-17 at 298 K and six pressures. The gas molecules are adsorbed primarily in the rectangular channel at 1 kPa in Figure 8a, while a very small amount of molecules are adsorbed in the hexagon channel. This

Figure 3. Relative selectivity of binary mixtures in different materials calculated by GCMC for mixed CH4/H2 (50:50), CO2/CH4 (15:85), CO2/H2 (20:80), CO2/N2 (15:85), CH4/N2 (50:50), and H2/N2 (50:50) at 298 K and 1 bar (total pressure).

Cu-BTC exhibits high selectivity of CO2 over CH4, which is consistent with experimental data.53 Furthermore, CAU-17 demonstrates extremely high selectivity for CH4/H2 compared with the other five kinds of porous materials. Hence, we want to further explore the separation mechanism of CH4 in CAU17. 3.3. Adsorption of CH4/H2 Binary Mixtures. Figure 4 shows the selectivity for equimolar CH4/H2 mixtures at 298 K

Figure 4. Adsorption selectivity of binary mixtures of CH4/H2 at 298 K.

in low pressure. The colorful bands represent the selectivity of CH4/H2 in CAU-17 and its derivatives, while the black ones are classic porous materials. It can be seen that the selectivity of CH4/H2 in CAU-17 or its derivatives is higher than those of classic porous materials for the whole pressure range. The selectivity follows the order of CAU-17-Br > CAU-17-I > CAU17-Cl > CAU-17-OH > CAU-17-F > CAU-17 > Cu-BTC > ZIF-8 > IRMOF-1 > COF-5 > UMCM-1. The selectivities rapidly reduce in CAU-17 and its derivatives with increasing pressure, and the selectivities gradually reach stable. 20200


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Figure 5. Adsorption isotherms of (a) CH4 and (b) H2 at 298 K in metal−organic frameworks and covalent−organic materials at low pressure.

Figure 6. Isosteric heats of (a) CH4 and (b) H2.

Figure 7. (a) Structures of channels in CAU-17 and (b) the percentage of each channel for CH4 adsorption values in total adsorption values under pressure of 0−100 kPa in CAU-17.

Figure 8. Ensemble average from the GCMC simulations for CH4 and H2 adsorption in CAU-17 at 298 K under pressure of (a) 1 kPa, (b) 3 kPa, (c) 5 kPa, (d) 7 kPa, (e) 9 kPa, and (f) 10 kPa (hydrogen, red spheres; methane, green spheres).

gases tend to fill in the hexagon channel, while gases are nearly

is caused by the proper channel size, where the rectangular channel shows stronger affinity with gases than other channels. Meanwhile, the porous material provides more attractive sites for CH4 than H2, which can be proved from the dense CH4 but rare H2 in all channels. With the increasing pressure, mixture

kept saturated in the rectangular channel. 3.7. Channel−Gas Interaction. DFT calculations were performed to investigate the weak interaction between gases and the host channels.41 Due to the large size of the host 20201


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cavity of the hexagon channel along with the z axis. Next, we continue to investigate the energy curves along with x and y axes in the hexagon channel. The gas molecule moved along with the x/y axis (blue/magenta line) principle line of the hexagon channel from the center of the hexagon channel. Results show that the IEs are gradually increased with the decreasing distance between CH4 and the inner wall of the channel in the x/y direction within a certain range. The minimum of energy curves for CH4 is −3.35/−2.77 kcal/mol with the distance along the x/y direction being 3.94/2.95 Å. In short, three channels show three consequences. The triangle channel with the largest IEs will adsorb CH4 first, if not blocked by the extremely narrow windows. The rectangular channel contributes most for CH4/H2 selectivity, as well as adsorption capacity, in moderate condition. The hexagon channel is expected to adsorb much more gases than the other two channels in high pressure when the other two channels are fully saturated.

structure for DFT calculation, a simplified model is adopted instead of the original periodic structure. Figure 9 presents interaction energies (IEs) of CH4/H2 scanned along the different channels. The Cartesian coordinate

4. CONCLUSIONS In summary, the adsorption and separation properties of gases in CAU-17 and its derivatives have been investigated by a combination of DFT-D3 and GCMC simulations. After comparing with 5 classic porous materials (including IRMOF1, UMCM-1, Cu-BTC, COF-5, and ZIF-8), the extraordinary high CH4/H2 selectivity was found in CAU-17 and its derivatives. This prominent feature encourages us to investigate in depth. The loadings of pure CH4 are favored in CAU-17 and its derivatives compared to other porous materials, while the loadings of pure H2 are in contrary to CH4. CAU-17 and its derivatives represent remarkable CH4 separation properties compared to other porous materials, such as Cu-BTC, IRMOF1, UMCM-1, COF-5, and ZIF-8. CAU-17-Br exhibits promising gas separation properties. Investigations on gas adsorption in different channels confirm that proper pore size with higher isosteric heat results in high CH4 affinity, which is in line with DFT-D3 calculations. The IEs between the channels and gas molecules reveal that CAU-17 can selectively adsorb CH4 over H2. The triangle channel with the largest IEs but very limited free volume may not be suitable in practical application, as the narrow windows would block the gas diffusion, and extremely few gases can be adsorbed even in 1 bar. The rectangular channel contributes the most for CH4/H2 selectivity, as well as adsorption capacity, in moderate conditions. More rectangular channels with this size in porous materials are expected to present higher CH4/H2 selectivity.

Figure 9. Models and IEs of the host and the guest in the (a) triangle, (b) rectangle, and (c) hexagon channels. (d) Different direction of hexagon channel (the filled square symbols are CH4, and the open circle symbols are H2).

z of the gas molecule’s center of mass represents its position for a (a) triangle channel and (b) rectangular channel, while for the (c) hexagon channel, three directions are considered because of its large free volume. In the process of simulation, the gas molecule keeps rigid, and it passes through the center of the channel, along the principle line of the host. The IEs between the triangle channel and each single gas molecule are shown in Figure 9a. The IE varies with the position of the gas molecule in channels. The curve for the CH4 fluctuates a lot with two distinct minima; nevertheless, the curve for H2 is relatively flat. The IEs are minima with guest molecules close to the carboxylate groups of the 1,3,5benzenetricarboxylate (BTC3−) ligands. The corresponding minima are −6.06 and −10.74 kcal/mol, respectively. However, because of the narrow triangle channel with very limited free volume, very few gases could be adsorbed in this channel as shown in Figure 8. For CH4 in the triangle channel, even though it has stronger IEs with the channel, the narrow windows will hinder gas diffusion along this channel in practical applications. Figure 9b shows those two minima sites for CH4 and one minimum for H2 in the rectangular channel. The strongest IEs for CH4 and H2 are −4.91 and −1.62 kcal/mol, respectively. Although the IEs in the rectangular channel are not as strong as those in the triangle channel, there are no obvious windows blocking the gas diffusion. Because of the large cavity of the hexagon channel, three directions are considered. As depicted in Figure 9c, the relatively flat energy curves result from gases scanning in the center of the large

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-431-85099521. *E-mail: [email protected]. Phone: +86-431-85098019. ORCID

Jingping Zhang: 0000-0001-8004-3673 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573036 and 21303011), the Education Department of Jilin Province (111099108), and Jilin Provincial Research Center of Advanced Energy Materials (Northeast Normal University). 20202


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