ARTICLE pubs.acs.org/JPCC
Experimental and Computational Study of Functionality Impact on SodaliteZeolitic Imidazolate Frameworks for CO2 Separation Hedi Amrouche,†,‡ Sonia Aguado,§ Javier Perez-Pellitero,|| Celine Chizallet,|| Flor Siperstein,‡ David Farrusseng,§ Nicolas Bats,|| and Carlos Nieto-Draghi*,† †
IFP Energies nouvelles, 1 et 4, Avenue de Bois-Preau, 92852 Rueil-Malmaison, France School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Sackville Street, Manchester M601QD, U.K. § IRCELYON. University of Lyon, CNRS. 2, Av. Albert Einstein, Villeurbanne, F-69626 France IFP Energies nouvelles, Rond-point de l'echangeur de Solaize, BP3, 69360 Solaize, France
)
‡
bS Supporting Information ABSTRACT: This study deals with the enhancement of CO2 uptake by ligand functionalization of zeolitic imidazolate framework (ZIF) materials. The ligand dipole moment could be considered as one of the main criteria for CO 2 adsorption enhancement. To verify this hypothesis, an experimental computational study was performed on an isoreticular ZIF series with sodalite (SOD) topology using published structures (ZIF-8, ZIF-90, and ZIF-Cl) as well as hypothetical structures (ZIFCOOH and ZIF-NO2) designated using DFT calculations. An analysis of structural and adsorptive properties was proposed for these materials used to separate CO2 from CH4, CO, or N2 gas. The accuracy of the calculated results was validated by comparison with our own experimental results. An exponential relationship between the ligand dipole moments and the isosteric heat of adsorption of CO2 was highlighted. Modifying the nature of the linker (dipole moment) allows a 5- to 7-fold improvement in CO2 selectivity for CO2/CH4, CO2/N2, and CO2/CO mixtures.
1. INTRODUCTION The capture of carbon dioxide gas is one of the most important challenges facing the scientific community today. In recent decades, human activity has been responsible for the release of a huge quantity of CO2 into the atmosphere. This unusually high atmospheric concentration has amplified the greenhouse effect, creating a phenomenon that climatologists call “global warming”. If this tendency is not contained, it could be responsible for serious ecologic disturbances in the near future. Energy production units are recognized to be the main source of CO2 emissions.1 Due to its stability, CO2 is usually the final product of combustion reactions between a fossil fuel (such as coal or heating oil) and oxygen. Due to increasing energy needs and the slow emergence of green energies, one of the short-term alternatives proposed to limit atmospheric CO2 emissions is CO2 capture. The N2/CO2 separation of CO2 from combustion gases appears to be the main problem. CO2 capture is also an issue in the natural gas industry, as the presence of carbon dioxide in natural gas, which is mainly composed of methane, reduces its caloric power and also induces pipeline corrosion. In this case, a gas separation process is necessary. Among the various solutions for gas purification, Pressure Swing Adsorption (PSA) is known to be one of the most efficient and affordable processes for removing CO2 from natural gas.2,3 This methodology could be extended to separate gas mixtures obtained r 2011 American Chemical Society
from combustion reactions. PSA is a three-step process: first, the gas mixture is injected into a separation column; then, during a separation step, CO2 is adsorbed by a porous adsorbent while the other gases are evacuated; and finally, the adsorbent is regenerated to restart the process. This separation process requires an adsorbent of sufficient capacity and selectivity. Although zeolite materials are recognized to be effective for gas separation, their renewability is energy intensive.4 Research for new materials is therefore strongly recommended for the successful design of adsorption separation processes that are competitive with respect to the liquid separation processes used today. The recent emergence of a new class of microporous materials, zeolitic imidazolate frameworks (ZIFs), offers promising applications for gas storage and separation.5 These materials, which consist of transition metal ions (Zn2+ or Co2+, among others) connected by imidazolate-like linkers,6 have topologies similar to zeolite materials. Moreover, by tuning the nature of the organic linker, it is possible to create a huge diversity of structures of controlled pore size and chemical functionality.79 This attractive feature of ZIFs leads to interesting adsorption (CO2, CH4, Received: March 25, 2011 Revised: June 1, 2011 Published: July 22, 2011 16425
dx.doi.org/10.1021/jp202804g | J. Phys. Chem. C 2011, 115, 16425–16432
The Journal of Physical Chemistry C
ARTICLE
samples were outgassed under vacuum (∼104 mbar) at 473 K for 12 h before the start of the measurements.
3. COMPUTATIONAL METHODS
Figure 1. Linkers of the sodalite solids studied functionalized at the 2 position.
N2, or H2)1014 and gas separation (CO2/CH4, CO2/CO, or CO2/N2) properties, which have been investigated both experimentally and computationally.1520 Despite some examples of charged ZIF frameworks,2125 the CO2 selectivities of neutral structures6 remain too low to challenge those of ionic zeolite materials.26 To design the most appropriate ZIF material from among the large variety of structures available, it is important to understand the gas adsorption mechanism of ZIFs and to identify the relevant parameters influencing the adsorption. According to recent experimental6,27 and computational28 reports, CO2 adsorption is sensitive to functionality effects. In this study, a ZIF isoreticular series based on the SOD topology is studied using molecular modeling to evaluate the impact of ligand choice on CO2, CO, CH4, and N2 adsorption properties. The previously described ZIF-829 Zn(meIm), ZIF-9030 Zn(Ica), and ZIF-Cl31 Zn(cIm) were studied; also, we predicted the behavior of two synthetic solids, ZIF-COOH Zn(carbacIm) and ZIF-NO2 Zn(nIm) (Figure 1). Note that ZIF-NO2 has already been synthesized with cobalt as the metal cation,7 but to the best of our knowledge, experimental results on its zinc counterpart have not been reported. In this work, the structure (including lattice parameters) of the different materials and, in particular, of the hypothetical ZIFCOOH and ZIF-NO2 were obtained from ab initio calculations. Such an approach has been successfully applied to similar materials.32,33 Geometric properties such as accessible surface area, pore volume, and window diameter were then analyzed for each solid. Grand Canonical Monte Carlo (GCMC) simulations and/or experimental measurements were carried out on the ZIFs to observe their adsorptive properties in the presence of pure CO2, CH4, and N2 gases. The selectivities for the CO2/CH4, CO2/N2, and CO2/CO mixtures were then computed for the solids studied. Finally, the effect of the functional group on the adsorptive properties was discussed, and a criterion was proposed for quantifying the structures’ behavior for CO2 adsorption.
2. EXPERIMENTAL METHODS 2.1. Synthesis. For the synthesis of ZIF-90, a solid mixture of 0.36 g (1.38 mmol) of Zn(NO3)2 3 4H2O and 0.193 g (2.01 mmol) of imidazole-2-carboxaldehyde is dissolved in 20 mL of DMF. Afterward, the solution is poured into a vial and heated in an oven at 373 K for 18 h. After the synthesis, the resulting powder is washed 3 times with DMF and then with EtOH. The samples are dried at 358 K for 3 h and then overnight under vacuum at room temperature. The structure of ZIF-90 was verified by XRD in comparison with the published structure.27 The ZIF-8 employed is a commercial solid from BASF. For additional details, see the Supporting Information. 2.2. Adsorption Isotherm Measurement. The CO2 adsorption/desorption isotherms at 303 K were measured by volumetric analysis on a BELSORP-HP apparatus. The different
3.1. Structure Design. The isoreticular series was designed using data from the Crystallographic Cambridge Data Centre (CCDC)34 for the ZIF-8 structure (a = b = c = 17.012 Å). The methyl functional groups were then substituted with new functional groups (HCO, COOH, NO2, and Cl) to create the new ZIFs. Next, periodic density functional theory optimizations were carried out with the VASP35,36 code to optimize the atomic positions and the lattice parameters, within the generalized gradient approximation (GGA) parametrized by Perdew and Wang, PW91,37 and the electronion interaction was described using the projector augmented wave (PAW)38 scheme with an energy cutoff of 450 eV. Due to the large size of the lattice (≈17 Å), the Brillouin zone is sampled only at the Γ point. The structure was optimized in two steps, the first one being a relaxation of the atomic positions at constant lattice parameter. The second step includes lattice optimization (atoms still being allowed to relax) until the forces on each atom are lower than 0.02 eV/Å, to obtain the lowest energy configuration for each structure. To study the impact of the lattice variation on the adsorption properties, the adsorption isotherms of fixed and modified cell parameters were compared. In this approach, we have not included dispersion corrections for our calculations, so the lattice parameters obtained should be considered as a first approximation. Electrostatic charges were evaluated by DFT for the fitting of our homemade force field (see next paragraph). In harmony with our previous work,39 the Electrostatic Surface Potential (ESP) atomic charges of the different solids have been estimated using cluster DFT calculations. This method provided isotherms that agree reasonably well with experiments. The ESP fitting methodology was applied by using the Jaguar40 package: the B3LYP functional combined with the pseudo potential LanL2DZ was applied for the transition metal, and the double-ζ basis set 631G** was applied for the rest of the atoms. Other methods also exist for determining the electrostatic contributions to adsorption calculations for nanoporous materials. Watanabe et al. have recently published a comparative work in which solidgas electrostatic contributions can be directly obtained from periodic DFT calculations.41 Two approaches in particular are worth mentioning: the first one uses the repeating electrostatic potential extracted atomic (REPEAT) method, in which point charges are determined without building nonperiodic clusters,42 and the second one makes direct use of the electrostatic potential energy surface (EPES) calculated from plane wave DFT.41 Although these approaches present the advantage of requiring neither cluster nor fitting procedures, they do require the refitting of the gassolid vdW intermolecular interactions. This fact is crucial if the prediction of gas selectivity between polar and nonpolar molecules is to be computed. The charge calculation procedure used in this work (by ESP fitting using cluster DFT) preserves the accuracy for gas adsorption of both polar and nonpolar molecules. 3.2. Adsorption Simulations. The simulations of gas adsorption were carried out by using Monte Carlo (MC) simulations in the Grand Canonical (GC) ensemble, with fixed fugacity f, lattice volume V, and temperature T. Fugacities were determined from previous simulation in the NPT ensemble, with N equal 16426
dx.doi.org/10.1021/jp202804g |J. Phys. Chem. C 2011, 115, 16425–16432
The Journal of Physical Chemistry C
ARTICLE
Table 1. Comparison between Structural Parameters Computed with VASP and Experimental Valuesa lattice parameter [Å] b
c
ZnZn distance [Å]
ZnImZn Angle
dp [Å]
dw [Å]
Sa [m2/g]
Vp [cm3/g]
17.0042
17.0075
17.0042
6.012
143.6
11.47
3.00
1395
0.50
17.012024
17.0120
17.0120
6.015
144.8
11.60
3.40
1445
0.52
17.1906
17.1916
17.1904
6.081
147.5
10.88
3.02
1269
0.43
17.271625
17.2716
17.2716
6.106
143.9
10.74
3.02
1351
0.48
17.0437
17.0437
17.0437
6.025
141.8
11.10
2.97
1200
0.41
16.982426
16.9824
16.9824
ZIF-NO2
17.0757
17.0791
17.0766
6.069
144.4
10.61
3.22
950
0.40
ZIF-COOH
17.2479
17.2479
17.2479
6.105
147.4
10.99
3.36
1261
0.43
ZIF ZIF-8 ZIF-90 ZIF-Cl
a
a
dp is the pore diameter and dw the six-membered ring window diameter calculated with the Material Studio package. Sa is the accessible surface area calculated using the method developed by D€uren et al.18 and Vp the pore volume.
to 100 particles, P varying from 10 to 5000 kPa, and T fixed at 303.15 K. Four gases were considered: CO2, CO, CH4, and N2. The EPM2 model by Harris and Young43 is used to model the guestguest interactions between CO2 molecules. In the case of CO, a new interatomic potential was derived from different experimental thermodynamic properties available for this gas (see Supporting Information).39 The CH4 molecules were described by means of a single LJ center, according to the model proposed by Moller et al.44 Finally, N2 was modeled using the Delhommelle model.45 It involves two LJ centers separated by a fixed distance of 1.098 Å, as well as two negative electrostatic charges located on the atomic centers, plus one positive charge on the center of mass. The system was considered rigid, and consequently, the solidsolid interactions were null. The guest host interactions were described by a transferable force field dedicated to the ZIF structures and inspired by the Universal Force Field (UFF). Its accuracy and transferability have already been extensively tested.39,46 The MC simulations were carried out using the GIBBS47 code v8.3. All simulations were performed using a 2 2 2 unit cells simulation box for which the cutoff radius was fixed to 17 Å. The Lennard-Jones tail correction was neglected, while the long-range electrostatic contributions were calculated using the Ewald methodology with ten vectors on the reciprocal space and a screening factor nR of 2.5. Finally, periodic boundary conditions were applied in all three dimensions. Technical details about the derivation of the force field and the calculation of thermodynamic properties (isosteric heat of adsorption, porosity, etc.) are available in the Supporting Information.
4. RESULTS AND DISCUSSION 4.1. Structure Analysis. Periodic DFT calculations were used to resolve the geometry of the structures in question. Some of the most relevant parameters obtained from these calculations are given in Table 1. The ZIF-8, ZIF-90, and ZIF-Cl lattice parameters are compared to the experimental values found in the literature.2931,34 The value obtained for ZIF-8 is within 0.1% of its experimental value, while for ZIF-90 and ZIF-Cl the observed deviation becomes more significant due to the constriction of the imidazole ligand. Indeed, according to the experimental ZIF-90 structure, the NN distance is dNN = 2.413 Å, which is too high for an imidazolate function (usually dNN = 2.228 Å, resulting from a geometric optimization performed with Jaguar40 on
the Ica ligand with the 6-31G**+ basis set and the B3LYP functional). The computed NN distance (dNN = 2.247 Å) obtained in periodic relaxation is in better agreement with the database value (dNN = 2.2133 Å) for the imidazole molecule.48 For the other solids, no published data were found for comparison. They exhibit lattice parameters ranging from 17.248 to 17.076 Å. The impact of the functional groups on the framework has been studied by measuring the average ZnImZn angle and ZnZn distance. The results obtained are also reported in Table 1. The orders of the ZnImZn angle and ZnZn distance are the following Zn Zn Zn Zn Zn Zndistance : dZn < dZn < dZn < dZn < dZn-Zn CH3 Cl NO2 HCO COOH
Zn Im Zn Im Zn Zn Im Znangle : θCl < θZn CH3 Im Zn Im Zn Im Zn < θZn < θZn < θZn NO2 HCO COOH
This classification is in agreement with the bulk order of the functional groups. The bulky functions cause an expansion of the tetrahedral building block and consequently of the lattice parameters. The variation of the window diameter, pore size, and free accessible surface area is small for the different materials studied, as shown in the topology study (Table 1). Nevertheless, we note that the porous volume of ZIF-8 is slightly greater than that of the other structures. This tendency can be explained by the chemical functional group sizes and their orientations. In particular, the orientation of the linker also depends on its nature. Indeed, functional groups are reoriented until they find their equilibrium state as illustrated in Figure 2; i.e., the functional group steric hindrance and dipole moments play a major role in this reorientation. The higher the dipole moments, the stronger the repulsion between linkers, and therefore the distance between functional groups is increased. This observation is more appreciable for the four-membered ring windows, in which the imidazolates are closer to each other (dC1_C1 = 5 Å). Nevertheless, as can be seen in Figure 2, this phenomenon is not observed for the six-membered rings, in which the distances become too significant to have repulsion between linkers (dC1_C1 = 8 Å). These assumptions can be verified by considering the window sizes obtained after a Connolly surface determination: the pore openings for the six-membered rings are similar for all solids studied (≈3.5 Å) (see Table 2). On the contrary, in the case of the 16427
dx.doi.org/10.1021/jp202804g |J. Phys. Chem. C 2011, 115, 16425–16432
The Journal of Physical Chemistry C
ARTICLE
Figure 2. Orientation of the imidazolate groups for the ZIF-NO2 around the four-membered rings.
Table 2. Ligand Dipole Moment and Isosteric Heat of Adsorption at Zero Coverage of All ZIFs Reported ZIF
ligand μ [D]
qst°(CO2) [kJ/mol]
qst°(CH4) [kJ/mol]
ZIF-8
1.18
15.58
12.28
ZIF-90
3.15
23.01
11.76
ZIF-NO2 ZIF-COOH
3.33 1.85
26.67 19.87
10.77 13.87
ZIF-Cl
0.44
15.14
11.43
four-membered ring windows, we can even note the appearance of a channel for the functional groups with the highest dipole moments. If we order the four-membered rings according to aperture size (CH3 < Cl < COOH < HCO < NO2), we obtain a sequence in agreement with the dipole moments presented in Table 2. The orientation of the ligands may have some consequences on the lattice parameters and on the accessible surface area, but it is difficult to extract a clear tendency. Nevertheless, we can note that the order of the accessible surface area is inversely related to the aperture window size presented above. An interesting point for discussion is the permeability of proposed structures for CO2, CH4, N2, and CO. As remarked before, since of all the solids have similar pore window apertures (between 3 and 3.4 Å), we can then expect that the two hypothetical solids (COOH and NO2) were permeable for the gases studied here. The concept of pore window is somewhat artificial for judging the permeability of ZIF materials due to the high flexibility of the organic linkers. In fact, ZIF-7 and SIM-1 are ZIF materials with small window apertures (e3 Å) that allow CO2 and bulky hydrocarbon molecules to diffuse inside their pores.46,49,50 4.2. Pure Component Adsorption. To study the influence of the functional groups on the gas uptake capacity, excess adsorption isotherms were determined for pure CO2, CH4, and N2 at 303.15 K and pressures ranging from 10 to 5000 kPa. We compared our simulated results with experimental data when available. The simulated results show a good level of agreement with the experimental data reported in this work, as can be seen in Figure 3. Note that a similar level of agreement was also attained in a previous work in which we compared experimental and simulation results of CH4 adsorption using ZIF-8.39 It is clear
Figure 3. Simulationexperimental comparison for CO2 adsorption at 303 K on ZIF-8 and ZIF-90. The experimental data of ZIF-8 and ZIF-90 come from this work. The simulated structures were obtained using periodic DFT calculations or CCDC using ref 24 and ref 25 for ZIF-8 and ZIF-90, respectively.
from Figure 3 that ZIF-90 represents a much better material for CO2 capture than does ZIF-8. In addition, we can be confident in the isotherm predictions using our force field, which has been extended here to study other solids. The comparison between isotherms computed from the DFT structure and CCDC crystallographic data shows a very good agreement at low pressure, and a slight deviation is observed at high pressure (Figure 3). This deviation is due to small modifications of pore volumes (see Table 1). This fact may also indicate that, if pore diameters are sufficiently larger than the kinetic diameter of adsorbed molecules, the adsorption process is not strongly affected by small variations of the lattice parameter or the precise orientation of the organic linkers. It is worth mentioning that the adsorption isotherms obtained with DFT optimized structures slightly underestimate the ones obtained using CCDC structures (as well as the experimental ones). For the sake of consistency in the analysis of the results between the existing structures and the hypothetical ones (where no experimental crystallographic data are available), we have decided to use the DFT optimized structures for all solids. The CH4 adsorption isotherms calculated for the materials with different functional groups are shown in Figure 4. These results reveal that CH4 uptake is affected by the functional groups. 16428
dx.doi.org/10.1021/jp202804g |J. Phys. Chem. C 2011, 115, 16425–16432
The Journal of Physical Chemistry C Hence, the adsorption capacity could be classified as a function of the chemical group as follows: For low pressures (1000 kPa): NO2 < Cl < HCO < COOH < CH3 At low pressures, the order is representative of the gassolid vdW interaction strength.51 Taking into account the model used to describe the methane molecule, the order in adsorption uptake can be explained based on dispersiverepulsive interaction strength. At high pressures, the strongest solidgas vdW interaction is
Figure 4. Simulated CH4 isotherms of ZIFs studied, at 303.15 K.
Figure 5. Simulated CO2 isotherms of ZIF-8, ZIF-90, ZIF-NO2, ZIFCOOH, and ZIF-Cl at 303.15 K.
ARTICLE
observed for ZIF-8. Meanwhile, HCO, Cl, and COOH functional groups show moderate vdW interactions for the CH4 gas. Finally, the nitro group (NO2) is the least attractive group for CH4 sorption. At high pressures, the gas adsorption capacity is mainly affected by the accessible surface area and next by the accessible volume.51 The isotherm order therefore depends on the combination of the functional groups and the structural geometric properties. Nevertheless, it is difficult to measure this effect because the maximum storage capacity of the ZIF studied is not yet reached at 5 MPa (Figure 4). As can be seen in Figure 5 in the case of CO2 adsorption, the role of the functional groups becomes more significant. At low pressures (
