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30
Storage and separation of CO2 and CH4 in boron imidazolate frameworks: a theoretical study from Monte Carlo simulation
B. Assfour*1,2, S. Leoni1
1 Institut für Physikalische Chemie, Technische Universität Dresden,
01062 Dresden, Germany
2Department of Chemistry, Atomic Energy Commission, P.O. Box 6091, Damascus, Syria
cscientific@aec.org.sy
PACS 87.15.ag DOI 10.17586/2220-8054-2015-6-3-320-331
In this work, the storage of pure CO2 and CH4 gases and separation of their binary mixture in new type of nanostructured materials called boron imidazolate frameworks (BIFs) have been investigated using atomistic simulation to provide information for material selection in adsorbent designs. Adsorption isotherms and adsorption selectivities were computed using grand canonical Monte Carlo (GCMC). Our results showed that BIFs exhibit significantly higher selectivities for separation of CO2 from CH4 compared to other widely studied metal organic framework (MOF) materials. Keywords: Adsorption, MOFs, BIFs, Gas Separation and Selectivity. Received: 3 March 2015 Revised: 30 March 2015
1. Introduction
The continued combustion of fossil fuel causes a rapid increase in the concentration of carbon dioxide (CO2) in earth's atmosphere, which is believed to be responsible for global warming and climate change [1]. In order to avoid CO2 from reaching the atmosphere, the capture of CO2 and the utilization of a clean energy source are prominent. Methane, the primary component of natural gas, is an appealing energy source. Compared to gasoline, methane provides much more energy because of its higher hydrogen-to-carbon ratio, and has much lower CO2 emissions [2]. However, the practical storage of methane in automobiles is still a prime challenge. According to U.S. Department of Energy (DOE) requirements, the storage capacity of material-based adsorbed methane should exceed 180 cm3 (STP). cm-3 at 298 K and 35 bar for practical on-board methane storage, where cm3 (STP).cm-3 means standard temperature and pressure equivalent volume of methane per volume of the adsorbent material. In term of energy density, this is equivalent to methane compressed at 250 bar and room temperature [3].
In parallel to the environmentally friendly fuel research, a large amount of work has been focused on the development of novel techniques for the separation, capture, and storage of CO2. The former is a key step in carbon sequestration for the prevention of global warming. Furthermore, CO2 is an impurity in natural gas, biogas and syngas [4]. Its presence will reduce the overall energy content of gas streams. Therefore, beside the storage of pure CO2 and CH4 components, it is of large importance to separate their mixtures. One promising method for separation of CO2 from its mixtures is adsorption separation by nanoporous materials. Among the potential adsorbent candidates (i.e. zeolites, activated carbon and
so on) [5,6], metal organic frameworks (MOFs) have emerged as a front-runner. MOFs are ultra-porous crystalline materials that are able to trap and store voluminous amounts of gas molecules (i.e. CH4, H2, N2 and CO2) [7]. The fact that the structure of MOFs can be tailored and chemically functionalized to selectively adsorb specific gas in a mixture gives them an enormous advantage over other nanoporous materials. Furthermore, MOFs have a versatile ability to store and release gases with fast kinetics and high reversibility over multiple cycles.
Boron imidazolate frameworks (BIFs) [8], a novel subclass of MOFs, are considered as promising materials for hydrogen storage applications [9]. Unlike the zinc imidazolate system (ZIFs), BIFs contain light chemical elements (i.e. Li and B) as framework vertices linked by different imidazolate ligands (im). The coordinatively unsaturated centers presented in BIFs have been shown to be favorable for enhancing gas uptake capacity [10]. Recently, Yang et al. have prepared novel neutral boron-imidazolate-framework (BIF-35) based on the assembly of tetradentate B(im)4 ligands and (CdBr) units, illustrating the ability to include a much wider variety of metal ions than traditional tetrahedral metal centers in order to produce new BIFs [11].
In contrast to the extensive studies on other MOFs, investigations of BIFs are rather limited. Zheng and co-workers integrated metal carboxylates and boron imidazolates to prepare a novel family of BIFs called MC-BIFs [12]. Amongst these materials, MC-BIF-2H exhibits extraordinary volumetric capacity for storing CO2 (81 L/L at 273 K and ambient pressure), comparable to that previously reported for a highly porous ZIF-69 (83 L/L) [13]. Zhang et al. developed a new strategy for the design of zeolite-type MOFs. They successfully synthesized the first interrupted zeolite A (4-connected tetrahedral framework) with a 3-connected network denoted as BIF-20. The initial H2 uptake of BIF-20 is remarkably high. However, The CO2 uptake capacity (34.8 cm3/g under ambient condition) is comparable to that reported for other ZIFs [14]. Wang et al. synthesized new set of BIFs with ACO and ABW topologies. The ACO-type material shows interesting gas selectivity of CO2 over N2 and CH4 [15]. Recently, Zang and co-workers [16] reported highly porous ctn-type BIF material with high CO2 storage capacity (104.3 cm3/g at 273 K). Moreover, this material presents high selectivity for the adsorption of CO2 over CH4 at ambient conditions. Jayaramulu et al. have synthesized a new three-dimensional boron based MOF, and made use of it as a precursor to produce a borocarbonitride (BC4N). Besides having an unusual coral-like morphology, BC4N has a high BET surface area (988 m2/g) and exhibits significant CO2 and H2 uptake [17].
Complementary to experimental studies, a number of computational investigations of gas adsorption in various types of nanoporous compounds have been reported. Most of these studies investigated the chemical structure, network topology, and porosity effects on the adsorption properties of the materials [18-20].
In this paper, we extend our previous work on BIFs by using molecular modeling to examine CO2 storage and separation (relative to CH4) in series of BIFs with different topologies. The results of this work may provide useful information for unlocking the potential of these materials in storage and separation applications and help the design of new adsorbents with improved properties by studying the effect of topology on CO2 adsorption. The crystal structures of these materials are shown in Fig 1. For a description of topology symbols, see the Database of Zeolite Structures, http://www.iza-structure.org/databases/.
Fig. 1. Crystal structures of the MOFs studied in this work. The structures are not drawn to scale. B:pink, C: grey, N:blue, Li: cyan. Hydrogen atoms are omitted for clarity
2. Model
The methane and carbon dioxide adsorption and separation in BIF structures were simulated with the grand canonical Monte-Carlo (GCMC) method using the multipurpose simulation code MUSIC [21]. The code MUSIC was successfully used to simulate the adsorption of different gases, such as CH4, CO2 and H2, on a variety of nanoporous materials [22-26]. In the grand canonical ensemble, the chemical potential, temperature and volume are constant. The chemical potential was converted to fugacity using Peng-Robinson equation of state [27]. The intermolecular interactions were modeled by the Lennard-Jones pair potential between all sites to represent van der Waals interactions. A methane molecule is modeled as united atom, where potential parameters are taken from Goodbody et al. [28]. The carbon dioxide molecule is modeled using TraPPe force field [29]. In this model, the CO2 is treated as three-site liner molecule, where the C-O bond measures 1.16 A. This model can reproduce the experimental gas-phase quadrupole moment of carbon dioxide by placing partial charges on C (+0.70 e) and O (-0.35 e) atoms. The potential parameters of BIF atoms were taken from the universal force field (UFF) of Rappe et al. [30], which has been widely used to study the adsorption of different gases on nanoporous materials [23, 31-33]. Lorentz-Berthelot mixing rules were used to calculate mixed Lennard-Jones parameters. The electrostatic interactions between CO2 molecules and the BIF structure were accounted for by placing point charges on each atom. The Bader partitioning scheme was performed on electronic density with the program Dgrid [34] to calculate the charge on individual atoms. The electronic density was calculated with the all-electron, full-potential local orbital (FPLO) minimal basis method [35]. The FPLO method does not have any atomic (or muffin-tin) spheres so that the whole space is treated in a uniform manner. The atomic
charges were calculated for zni topology, and kept permanent from structure to structure. The Ewald summation technique is used to calculate the electrostatic interactions.
The simulation box, representing each BIF structure, contains 8 (2x2x2) unit cells. Adsorbents were treated as rigid with atom positions taken from Ref [36]. The periodic boundary conditions were employed in all dimensions to mimic the crystalline periodicity. Interactions beyond 13 A were neglected. Each simulation point consisted of 5x106 Monte-Carlo steps to reach equilibrium followed by an extra 5x106 steps to take the statistical average.
GCMC simulation delivers the absolute amount adsorbed (Nabs), whereas experiments give the excess amount of adsorption (Nex). In order to make the comparison, the excess amount should be converted into absolute as:
Nex = Nabs - PpVfree (1)
where ppis the density of the bulk phase [37], and Vfree is the is the available pore volume per unit cell of the sorbent estimated using a non-adsorbing species (helium) as a probe [23, 25, 26].
The accessible surface area was calculated by "rolling" a probe molecule with a diameter equal to the Lennard-Jones parameter for N2 (3.681 A) over the framework's surface as described in Ref [38]. Calculation of the isosteric heat of adsorption (Qst) at zero coverage was performed through the fluctuations over the internal energy and from fluctuations of number of particles in the system by considering a very low pressure. These calculations are reported in more detail elsewhere [39].
In adsorption-based separation process, a good indication of the ability for separation is the adsorption selectivity for different components in gas mixture. The adsorption selectivity of component i relative to component j is defined by:
x/
s= € (2)
where xi and xj are the molar fractions of component i and j in adsorbed phase and yi and yj are the molar fractions in the gas phase.
3. Results and discussion
3.1. Adsorption of pure components of CH4 and CO2
Table 1 gives the density, pore volume and accessible surface area for all five BIFs as well as the initial isosteric heats of adsorption for pure CO2 and CH4 in each material. All materials provide moderate surface area and low density, indicating that they may be promising candidates for gas storage applications [2]. The material with FAU topology has the lowest density (0.492 g/cm3) as well as the highest surface area (2773 m2/g), which is higher than that of zeolites, porous silica and lower than some MOFs [40] or COFs [25]. As is the case for most CH4 sorbents, the Qst is in the range of 12-18 kJ.mol-1 [41-46]. However, for MOFs exhibiting very high methane uptake, such as PCN-14 [47], the Qst is much larger (15-30 kJ.mol-1), which is known to be responsible for the exceptionally high methane uptake at low pressure.
In the given material, the Qst of CO2 is larger than that of CH4, indicating that CO2 is more strongly adsorbed. Consequently, BIFs show more affinity toward CO2 at lower pressures. This is expected because the CO2 molecule has a greater interaction with the adsorbent than CH4 because it contains three interaction sites plus the role of electrostatic
Table 1. Summary of calculated properties of different BIFs
Material density (g/cm3) pore volume (cm3/g) accessible surface area for N2 Initial Qst for CH4 (kJ/mol) Initial Qst for CO2 (kJ/mol)
RHO 0.566 1.11 2654 12.15 38.52
FAU 0.492 1.41 2773 14.61 36.55
DFT 0.730 0.74 1686 17.33 49.93
GIS 0.627 0.97 2625 13.11 36.11
MER 0.635 0.95 2613 12.57 37.72
interactions, which is expected to be enhanced compared to other MOFs due to the asymmetries in the metal sites. The Qst is in the range of 36-50 kJ.mol-1 higher than those for other MOFs, i.e. UiO-68(Zr) (20 kJ.mol-1) [48], MOF-5 (34 kJ.mol-1) [49], CPO-27. (38-43 kJ mol-1) [44]. For both CO2 and CH4, The Qst is the largest in DFT. This is probably due to the lower porosity.
The predicted gravimetric methane uptakes in selected BIFs at 298 K are shown in Figure 2. As expected, all materials show type I for total and excess isotherms, with profiles that depend on the material. The material with DFT topology shows the highest excess uptake almost all over the pressure range. Its saturation uptake is calculated to be 248 cm3/g at 40 bar, which is close to 252 cm3/g (290 K, 35 bar) for PCN-14, which holds the current record for methane storage [47]. Although, recently Peng et al. [50] discovered that packed HKUST-lexhibits a room-temperature volumetric methane uptake that exceeds any value reported to date. The material with FAU topology shows a slightly higher saturation gravimetric uptake (254 cm3/g) but at higher saturation pressure (70 bar). The total uptake that a material can store is more relevant to the practicability of using CH4 as a fuel. In terms of total methane uptake, the best material at 100 bar is the one with FAU topology, with 405 cm3/g followed by GIS and RHO (336 cm3/g) and DFT (306 cm3/g) respectively.
Fig. 2. Excess and total gravimetric CH4 uptake in BIFs
Concerning the volumetric uptake (Figure 3), we see that all BIFs materials do not reach the DOE target in a total volumetric uptake basis at 35 bar. However, we predict that materials with FAU and RHO topologies perform nicely, having uptakes of 200 and
190 v(STP)/v at 100 bar, respectively, suggesting that they could be suitable for practical applications of methane storage.
220 -r 200-J? 180-
¿L 1600>
ra 140 -Q.
= 120-O
tj 100E
3 80-O
> 60-m
¡2 4020-
0-| 0
Pressure[bar]
Fig. 3. Total volumetric CH4 uptake in BIFs
High and low pressure CO2 isotherms for all five BIFs were calculated and are presented in Figure 4. Unlike the CH4 isotherm, CO2 isotherms show dramatic steps, similar to those reported for CO2 in other MOFs [32, 51-53] indicating a very high affinity for CO2 gas. Compared with other porous materials [22, 54], BIFs reach their saturation uptakes at relatively low pressures (around 10 bar). The material with FAU topology shows the highest excess and total uptake (figure 4) almost over the entire pressure range. The total CO2 storage capacity for FAU at 50 bar reaches the value of 450 mg.g-1. Such value is significantly lower than those reported for other recently synthesized materials such as MOF-200 (2400 mg.g-1) and NU-100 (2315 mg.g-1). This is due to very large pore volumes provided by these materials, where the amounts of CO2 uptake are directly related to the total pore volume [54]. However, BIFs present very high CO2 capacities at low pressure range (left panel of figure 4). For instance the calculated excess capacity of FUA at 1 bar (298 mg.g-1) is almost three times higher than that reported for NU-100 (110 mg.g-1) under the same conditions [22] and more than 50% greater than that of ZIF-69 (130 mg.g-1) at 273 K, which makes this type of materials very promising candidates for CO2 capture, especially at low pressures.
3.2. Adsorption of CO2/CH4 mixture
In this section, the CO2 selectivity for CO2/CH4 mixtures with different pressures at room temperature is discussed. Figure 5 shows the adsorption isotherms for an equimolar mixture of CH4/CO2 in BIFs as a function of pressure. We find that in all BIFs, CO2 is more readilly adsorbed than CH4 at low pressures (below 10 bar). This is due to stronger interaction with structures as explained above. However, at high pressures, the number of CH4 molecules adsorbed per unit cell increases exponentially and CH4 become more favorably adsorbed.
Figure 5 (right bottom) illustrates the effect of pressure on the CO2 selectivity at room temperature for five BIFs. It shows that selectivities for CO2 are different between BIFs at low pressures with the order RHO > FAU > GIS > DFT > MER. It can be seen that selectivities decrease exponentially along with rising pressure and become close at pressures
Pressure[bar] Pressure[bar]
Fig. 4. Low (left) and high (right) excess gravimetric CO2 uptake in BIFs
higher than 1.0 bar. Therefore, we conclude that the topology of BIFs plays the main role on the CO2 selectivity rather than pressure.
Keskin studied the adsorption selectivities of ZIF type materials for equimolar CO2/ CH4 mixture [55]. Results showed that adsorption selectivities of ZIFs are better than those of IRMOFs due to their smaller pore sizes and better confinement of CO2 molecules. In contrast to BIFs, the selectivities for ZIFs increase with pressure. ZIF-3 (DFT topology) provides the best selectivity with 5 at 0.1 bar and up to 12 at 50 bar, whereas, for BIFs, RHO has the maximum selectivity with 50 at 0.1 bar down to almost 0 at 50 bar. Therefore, in sources such as flue gas, where the typical anticipated partial pressure of CO2 is about 0.1 bar [56], BIFs outperform ZIFs and related MOFs because of their suitable pore sizes. However, at high pressures MOFs are better for CO2 capture because of their larger pore sizes. This makes BIFs very promising candidates for CO2 separation at low pressures.
4. Preferential adsorption sites
To understand the adsorption mechanism for CH4 and CO2 on BIFs, preferential adsorption sites were investigated. Since the picture of adsorption sites for all BIFs is similar, we choose DFT BIF as a representative. Figure 6 shows equilibrium snapshots of adsorbed CH4/CO2 mixture in the simulation unit cell at three different pressures, 0.1, 1.0, and 10 bar. It is clear that CO2 molecules preferentially adsorb in the small pores formed by imidazole linkers (figure 7 (a)), while CH4 molecules cannot enter the small pores because their sizes are smaller than the kinetic diameter of CH4. Therefore CH4 molecules are accommodated in the large pore. With increasing pressure, the CO2 molecules also adsorb in large pores near to the polar centers in the framework because the electrostatic contribution of CO2 is larger than that of CH4 (figure 7 (b)). At higher pressures, the adsorption sites are saturated and the electrostatic interaction decreases to a negligible value. Therefore, CO2 molecules are no longer favorable and together with CH4 molecules, fill in the free space in the pores far from the charge centers. This behavior explains the large uptake of CO2 at low pressure.
Generally, in separation processes, the differences in electronic properties and size of molecules are used. However, the relatively small difference in kinetic diameters between CO2 (3.30 A) and CH4 (3.76 A) makes separation based solely on molecule size a very difficult task. Therefore, many MOFs with unsaturated metal sites were synthesized to enhance adsorption of quadrupolar CO2 over non-quadrupolar CH4 and to make the separation beneficial. Bae et al. [57] found that incorporation of Li cations into MOFs, by either chemical reduction
Fig. 5. Equimolar mixture adsorption isotherms of CH4/CÜ2in five BIFs. Adsorption selectivity of BIFs for CO2/CH4 mixture at room temperature (right bottom)
or cation exchange, significantly improved the CO2/CH4 selectivity. In case of BIFs both effects are presented, the unsaturated metal sites and optimal pore size. This may explain the large CO2 uptake at low pressure relative to other MOFs.
Fig. 6. Snapshots of mixture of CO2 and CH4 in DFT for low ((a) 0.1 bar), normal ((b) 1 bar), and intermediate ((c) 10 bar) loading. CH4 molecules are represented in green
a) b)
Fig. 7. CO2 adsorption site in the small (a) and in the large cage (b). Distances are reported in angstroms
5. Conclusions
We have used computational modeling to study the storage and separation of CO2 and CH4 in a new class of porous materials known as BIFs. Our results show that BIFs could be suitable for practical applications of methane storage, particularly from a gravimetric point of view. We have also established that BIFs are highly selective for the absorption of CO2 at low pressures as compared to other MOFs. Such high selectivities are believed to be the result of both appropriate pore sizes and unsaturated metal sites, which are known to significantly influence the CO2 uptake capacity. The results of this work will provide guidelines for the optimum choice of BIF materials to be used in gas storage and separation applications.
Acknowledgments
The author is highly grateful to Prof. Dr. I.othman and Dr. Z.ajji for giving the opportunity and help performing this work.
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