Enhancement of CO2 Adsorption in Magnesium Alkoxide IRMOF-10

Aug 31, 2015 - Ab initio calculations and GCMC simulations were performed in order to study the CO2 adsorption from Mg modified IRMOF-10. The Mg catio...
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Enhancement of CO2 Adsorption in Magnesium Alkoxide IRMOF-10 Taxiarchis Stergiannakos,† Emmanuel Klontzas,† Emmanuel Tylianakis,‡ and George E. Froudakis*,† †

Department of Chemistry and ‡Department of Materials Science and Technology, University of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece ABSTRACT: Ab initio calculations and GCMC simulations were performed in order to study the CO2 adsorption from Mg modified IRMOF-10. The Mg cations were introduced in the linker of IRMOF-10 by creating Mg alkoxide groups. Accurate MP2 calculations on the Mg alkoxide linker showed that up to 4 CO2 molecules can simultaneously interact with the Mg atom of the alkoxide group. The average interaction energy ranged from −14.2 to −8.9 kcal·mol−1 for one to four interacting CO2. GCMC simulations were also employed using a modified LJ potential in order to predict the excess CO2 adsorption isotherms at 300 K and up to 40 bar. The predicted isotherms showed a clear enhancement of the CO2 uptake when one or two Mg alkoxide groups were introduced per linker with respect to the unmodified IRMOF-10. This enhancement was more pronounced at low pressures. At 10 bar, the volumetric capacity became 5 or 7 times larger than in the case of the unmodified IRMOF-10 by introducing 1 or 2 Mg alkoxide groups, respectively. Based on this significant enhancement, we propose the Mg functionalization as a general strategy for improving the CO2 storage capacity in MOFs and similar materials.

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INTRODUCTION The emission of greenhouse gases has been recognized as one of the main issues that has to be controlled in order to avoid further undesirable changes in the global climate. The capture of these gases has been a major challenge for the scientific community during the past years, especially for the amounts produced due to human activities. CO2 is considered as the primary greenhouse gas, accounting for 77% of the human contribution to greenhouse effect. CO2 capture and storage has attracted a lot of attention, since it is believed that the elimination of the large CO2 emissions due to the human activities will have a major impact on the decrease of the global temperature.1 A tremendous need exists to develop efficient methods to capture CO2 close to the production site. Carbon Capture and Storage (CCS)2 technology is the most indicated to achieve reduction of the CO2 emissions. Capture of CO2 in CCS scheme can be performed by three possible capture techniques, precombustion capture, postcombustion capture, and oxyfuel combustion, with the postcombustion capture to be considered as the most efficient, especially when we want to reduce the overall energy cost of the procedure. During this procedure, suitable porous solid materials must be developed with increased efficiency for CO2 capture over a mixture of gases compared to materials already used in this procedure, such as monoethanolamine solvents (MEA). During the past years, many different porous materials were studied for their ability to capture CO2 from a mixture of gases and to store CO2 in their porous networks.3 A significant category of such materials is metal−organic frameworks (MOF), which are composed of inorganic building units interconnected with properly function© XXXX American Chemical Society

alized organic linkers, in order to form a framework material with a porous network. Several families of these types of materials have been synthesized such as IRMOFs, ZIFs, MILs and UiO.3−6 In general, these types of materials present extremely high physicochemical properties such as high surface area and pore volume and high thermal and chemical stability, which make them promising materials for this application. Moreover, the properties of these materials can be tuned by changing the inorganic building unit, the organic spacer between inorganic building units, or the topology of the framework. The introduction of functional groups on the organic linkers shows the way to design novel materials with pronounced interaction with CO2. This is a very challenging task toward materials with high CO2 capacity and selectivity. In order to study the adsorption of carbon dioxide in porous materials like MOF, computational chemistry methods have significantly contributed to understanding of the phenomena which govern CO2 adsorption in these materials and to develop new materials with enhanced adsorption properties. Quantum chemistry calculations have been widely used to study the interaction of CO212−16 and other gases with MOFs.7 Such calculations are essential in understanding the CO2 adsorption in MOFs, since they can provide us with CO2 binding strengths and also reveal the nature of the interaction. CO2 can interact either with sites located on the inorganic building unit of a MOF or with the organic linker. In the first case, a lot of computational studies have been performed in several MOF, Received: June 3, 2015 Revised: August 26, 2015

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DOI: 10.1021/acs.jpcc.5b05294 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C including M-MOF-74, MIL materials, and HKUST-1.8,9 The above-mentioned MOFs include under-coordinated metal sites which are available to bind CO2 molecules and it has been found that they present high binding affinity toward CO2. The calculated binding energies from quantum mechanical calculations have been found to be in the range of 5 to 15 kcal/mol, depending on the structure of the inorganic building unit and the type of corresponding metal atom.10 Mg containing inorganic building units has shown the best performance in terms of binding energy with respect to other metals, such as Cu or Fe. On the other hand, the interaction of the CO2 with organic linkers is crucial, since the linkers have a dominant role in enhancing the adsorption in MOFs. Furthermore, functional groups can be used in order to tune the interaction with CO2. A few studies have been published on the interaction of CO2 with organic molecules by means of quantum chemical calculations.11−18 Some of them16−18 were devoted to give insight into the solubility of some organic molecules in supercritical CO2. The rest of these studies were targeted on examining the nature of CO2 interaction with already existing or proposed linkers for MOFs or ZIFs. In a previous study,12 we performed highquality quantum mechanical calculations, including CCSD(T) methods, to investigate the interaction of CO2 with nitrogen containing organic molecules that could act as linkers in ZIFs. We also performed a benchmark of various methods on the accuracy of the prediction of the binding energies with respect to the CCSD(T)/CBS method. Vitillo et al.15 performed MP2 calculations to study the interaction of CO2 with aliphatic and aromatic amines. Torrisi13,14 used density functional theory (DFT) to study the interaction of CO2 with a range of substituted aromatic molecules, including substitution with single or multiple atoms or functional groups. The highest binding energy was found for the interaction of CO2 with C6H5−SO3H (−3.8 kcal/mol). Classical simulations are also essential in the understanding of the adsorption of CO2 in MOFs. Molecular dynamics and Monte Carlo methods have been employed to study the diffusion properties and the storage capacity of CO2 in these materials in order to give insight on the adsorption phenomena in a larger scale. As has been mentioned in previous studies,12,15 electrostatics and dispersion forces mainly contribute to the binding energy of CO2. It has been shown12 that dispersion forces contribute almost equally in many cases depending on the system. The electrostatic contributions are based on the permanent quadrupole moment of CO2 and its ability to act as a weak Lewis acid (through the C deficient atom) or as a weak Lewis base (through the electron-rich O atoms). In this work, we performed a combination of accurate quantum chemistry calculations with large scale classical Monte Carlo simulations, to investigate the nature of the interaction of CO2 with Mg modified IRMOF-10 and predict the uptake of the functionalized MOF. Mg cations were introduced in the structure of IRMOF-10 by creating Mg alkoxide groups on the organic linkers of IRMOF-10. Suitable IRMOF-10 linkers which can by metalated with Mg have already been reported in the literature by different experimental groups.19−21 CO2 adsorption is significantly enhanced in magnesium containing MOF, as it has been shown in the case of Mg-MOF-74. By introducing the magnesium cation on the linker, more CO2 molecules will be able to simultaneously interact with magnesium cation (whereas in the case of Mg-MOF-74, only

one CO2 can interact with each Mg) with an increased binding energy. This will lead to a great enhancement of CO2 storage ability of IRMOF-10. In order to explore this possibility, we performed ab initio calculations with the MP2 methods to find the interaction energies and the corresponding equilibrium geometries of the CO2 with the Mg alkoxide IRMOF-10. Furthermore, we used QM data in order to modify existing parameters of the force field used for the GCMC simulations. To quantify the enhancement on the interaction energies of CO2 with the modified IRMOF-10, we carried out GCMC simulations to predict the excess adsorption isotherms at 300 K and pressures up to 40 bar, together with the isosteric heat of adsorption as a function of CO2 loading.

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COMPUTATIONAL DETAILS In order to study the Mg2+ modified linker with ab initio calculations, the supermolecule approach22 was used. According to this, the biphenyl ligand was separated from the proposed structure and the dangling bonds were saturated with hydrogen atoms. Second-order Møller−Plesset (MP2) perturbation theory in the Resolution of Identity (RI) approximation was applied to our calculations along with the def2-TZVPP23 basis set, together with the corresponding auxiliary basis set for the RI approximation. All structures were optimized without any symmetry constraints and the optimized minimum-energy structures were verified as stationary points on the potential energy surface by performing numerical harmonic vibrational frequency calculations. The SCF (Self-Consistent Field Hartree−Fock) convergence criteria were set at 10−8 au during calculations. All calculations were performed with the TURBOMOLE program.24 On top of this, all binding energies are corrected for the Basis Set Superposition Error (BSSE) with the CounterPoise (CP) method as proposed by Boys and Bernardi.25 The equation for the calculation of the binding energy is defined as ghost ghost BE = Ecomplex − E linker − ECO + ΔΕdeform 2

where Ecomplex is the total energy of the complex, Elinker,ghost is the energy of the Mg alkoxide linker calculated at the complex geometry in the presence of the ghost basis of CO2 molecule, ECO2,ghost is the energy of CO2 calculated at the complex geometry in the presence of the ghost basis of the Mg modified linker, and ΔEdeform is the deformation energy, defined as the difference between the isolated interacting molecules (CO2 and the Mg modified linker) in the complex geometry and in their optimized structures. These corrections have been proven essential since the BSSE may become critical for nonbonding interactions. To determine the ability of the original and the modified IRMOF-10 to adsorb carbon dioxide, we carried out Monte Carlo simulations in the Grand Canonical ensemble, i.e., keeping constant the system volume, the temperature, and the chemical potential. Since chemical potential corresponds to a given thermodynamic state of temperature and pressure, the adsorption was calculated for a pressure range up to 40 bar at 300 K, constructing in this way an isotherm. We calculated the fugacity coefficients by using the Peng−Robinson equation of state.26 For each structure, a cubic periodic box of size 68.5 × 68.5 × 68.5 Å3 was used for the unmodified and the single and double magnesium modified IRMOF-10, respectively. The periodic box dimensions were large enough to ensure that no B

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The Journal of Physical Chemistry C finite size effects will affect the results. For each point of the predicted isotherms, 107 steps were conducted to let the system equilibrate, followed by 107 production steps in order to calculate the average number of adsorbed molecules. The probability for the creation, destruction, translation, rotation, and reinsertion of a trial molecule was set to 0.20. For the description of the interactions between the host and the sorbate atoms, 6−12 Lennard-Jones (LJ)27 + Coulomb potentials were used and each atom of the host or the guest was treated explicitly. The potential used has the following formula:

where N is the number of adsorbed molecules, V is the potential energy, R is the ideal gas constant, and T is the temperature in K. Brackets represents ensemble averages.

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RESULTS AND DISCUSSION In our previous work, we proposed experimentally feasible modification steps to introduce magnesium cations to IRMOF10 organic linker.32 In the final structure, a biphenyl linker with a magnesium alkoxide group was present. The optimized geometries of the modified biphenyl linker with one and two Mg2+ can be viewed in Figure 1.

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqj σij σij Vij = 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ 4πε0rij ⎣⎝ ij ⎠

where ε0 is the vacuum permittivity constant, rij is the interatomic distance between interacting atoms i and j, qi and qj are the corresponding partial charges for atoms i and j, and eij and σij are the LJ potential well depth and the repulsion distance between atoms i and j, respectively. Both CO2 and the IRMOF-10 were considered to be rigid and were represented by atomistic models. Carbon dioxide was treated using the TraPPE28 model, as a linear three center rigid molecule in the sense that neither the bond length nor the angle was allowed to vary during the simulations. For the electrostatic interactions between CO2 molecules and the host material, point charges equal to −0.35 e and 0.7e were placed at oxygen and carbon sites of CO2, respectively. These charges, together with CO2 bond length of 1.16 Å are proven to describe accurately its quadrupole moment. For the van der Waals interactions we used potential parameters according to TraPPE model developed by Potoff and Siepmann28 i.e., ε = 27 K and σ = 2.8 Å for carbon atom and ε = 79 K and σ = 3.05 Å for the oxygen center. For each MOF structure, the necessary potential parameters were taken from the DREIDING force field,29 except for Mg and O atoms. Lorenz-Berthelot mixing rules were used to describe the CO2−IRMOF-10 interactions. These parameters are proven to describe correctly these dispersion interactions in previous studies.30,31 Nevertheless, this is not the case for magnesium since this atom has not yet been used in earlier studies. To test the applicability of its DREIDING parameters we performed ab initio calculations at the MP2 level of theory, scanning the CO2 distance from this Mg atom and tried to reproduce the corresponding QM energies using these parameters together with electrostatic interactions. Then, we fitted the parameters of our classical potential to reproduce the quantum chemical data. The parameters revealed from this procedure were determined to be ε = 4.968 kcal/mol and σ = 2500 K for Mg and 5000 K and σ = 2.959 Å for O atom. The cutoff value for the LJ potential was set to 12.8 Å. Electrostatic interactions were also considered for the interaction of CO2 with the host material and they were treated with the Ewald summation method. The partial charges of the framework atoms were calculated with the ESP method as implemented in Turbomole24 and the resulting partial charges were slightly tuned in order to keep the molecular structure in the periodic box neutral. The values for the isosteric heat of adsorption were calculated by the simulations by using the following formula Q st = RT −

Figure 1. Optimized geometries of IRMOF-10 linker modified with (a) one and (b) two Mg alkoxide groups.

In this study, we extended our previous work with the magnesium alkoxide IRMOF-10 by studying the energetics and the geometries of the interaction of CO2 with a single Mg2+ modified linker. We also aimed to find the maximum number of CO2 molecules which could simultaneously interact with the magnesium atom of the magnesium alkoxide group. To do so, sequential addition of CO2 molecules was performed until the maximum number of CO2 molecules that give negative binding energy was attained, creating in this way a first surrounding shell of CO2 molecules around magnesium atom. Whenever we added an extra CO2, the average value of the corresponding interaction energy was calculated and the values are presented in Table 1. For the interaction of the first CO2 with the magnesium atom of the alkoxide group, three different geometrical configurations were identified (Figure 2), according to the results obtained from our MP2 calculations. In the first Table 1. Calculated MP2 Binding Energies for the Interaction of Multiple CO2 Molecules with the Magnesium Alkoxide Linkera method RI-MP2 (def2TZVPP)

⟨VN ⟩ − ⟨V ⟩⟨N ⟩ ⟨N 2⟩ − ⟨N ⟩2

a

C

number of CO2’s 1 1 1 2 3 4

CO2 a) CO2 b) CO2 c) CO2 CO2 CO2

total interaction energy

average interaction energy per CO2

−13.6 −8.3 −14.2 −27.2 −32.4 −35.8

−13.6 −8.3 −14.2 −13.6 −10.8 −8.9

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simultaneously form a first interaction sphere around the Mg atom of the group. The average interaction energy per CO2 for the maximum number of interacting CO2 was calculated to be −8.9 kcal·mol−1. The optimized structures of the complexes are shown in Figure 3 and the corresponding average binding

Figure 2. Three distinct optimized geometries that were obtained for the interaction of a single CO2 molecule with the magnesium alkoxide linker.

configuration (Figure 2a), a coplanar orientation was observed between the O and C atoms of the CO2 and the magnesium and O atoms of the magnesium alkoxide group. The CO2 binding energy for this configuration was calculated to be −13.6 kcal·mol−1. The O atom of the CO2 is located near magnesium atom (OCO2−Mg: 2.11 Å), where the C atom of the CO2 is located near the O atom of the magnesium alkoxide group. This orientation is expected if we consider that the Mg and O atoms of the alkoxide group are positively and negatively charged, respectively. The O−C−O angle was slightly distorted by 3.7° in this configuration, which is attributed to interaction strength of the CO2 with the alkoxide group and the cooperative interaction of the O and C atoms of CO2 with Mg and O atoms of the alkoxide, respectively. In the second configuration (Figure 2b), CO2 interacted similar to the geometrical configuration presented in Figure 2a, but in this case, the O−C−O angle was greatly distorted (by 43.9°) compared to the angle of a free CO2. The binding energy was calculated to be −8.3 kcal·mol−1. Despite the large distortion of the O−C−O angle with respect to the first configuration, the binding energy was found to be somewhat lower for the second configuration. The reduced value for the binding energy of the CO2 with the linker in the second configuration can be attributed to the simultaneous change of the geometrical characteristics of the alkoxide group, which led to an energetic balance of the energy gain due to the distortion of the CO2 molecule in the complex. Considering the distortion mentioned above, the increase of the length of the nearby Mg− O bond with respect to the opposite Mg−O bond of the alkoxide and the distance between the C atom of CO2 and the O atom (1.54 Å) of the alkoxide, we deduce that CO2 is chemically absorbed. The OCO2−Mg distance also became shorter and found to be 1.94 Å. We were also able to calculate a significant charge transfer of 0.35e toward CO2 by performing a charge analysis with the Natural Bond Orbital (NBO) method as implemented in Turbomole,24 which further validated the chemical bonding of CO2 on Mg alkoxide. In the third configuration (Figure 2c), CO2 is located on top of the neighboring phenyl ring with a parallel orientation with respect to the plane of this phenyl ring, where one of the O atoms of CO2 looking toward the magnesium atom. The binding energy for the third configuration was calculated to be −14.2 kcal· mol−1. In this case, the CO2 is stabilized in this position by the cooperative interaction of the CO2 with the magnesium atom and the electron cloud of the phenyl ring. Similar values for the angle distortion for CO2 (3.5°) and for the OCO2−Mg distance (2.11 Å) were found to the first binding configuration described above. We continued with the successive addition of more CO2 molecules in the vicinity of the Mg atom of the modified linker and we found that up to four CO 2 molecules can

Figure 3. Optimized geometries for the interaction of multiple CO2 with the Mg alkoxide linker: (a) two CO2, (b) three CO2, and (c) four CO2.

energies are summarized in Table 1. The optimized geometries obtained were a combination of the three distinct configurations which were calculated for the first CO2 and were described above. When two CO2 molecules were interacting with the magnesium alkoxide group (Figure 3a), they were located on opposite sides with respect to the plane formed by the oxygen and the magnesium atoms of the alkoxide group. The distance of the closest oxygen atom of each CO2 from magnesium atom was 2.12 and 2.13 Å, respectively. The angles of both CO2 molecules are slightly distorted by 3.8°. One of them is oriented parallel to the plane of the phenyl ring, similar to the orientation that was described above for the 1 CO2 in Figure 2c. The average binding energy was calculated to be −13.6 kcal·mol−1. When three CO2 molecules interacted with the magnesium atom (Figure 3b), we found that one of the them became distorted by 41.8°, where the other two were only slightly distorted by 3.6° and 3.9°, respectively. The two undistorted CO2 molecules are located at opposite sides similar to the geometry presented in Figure 3a where the distorted CO2 molecule interacts with a geometrical configuration similar to the case presented in Figure 2b. For the CO2 with the large angle bend, the distances of the closest oxygen atom of the molecule from the magnesium and that of the carbon atom of CO2 from the closest oxygen atom of alkoxide group were 1.98 and 1.57 Å, respectively. These values are slightly larger than the corresponding values for the geometry in Figure 2b, which can be attributed to the presence of the other two CO2 molecules. For the two CO2 molecules with the smallest bend, the distances of the closest oxygen atoms of these molecules from the magnesium atom were 2.20 and 2.18 Å, respectively. They were oriented in a parallel fashion with respect to the plane of the two phenyl rings of the biphenyl linker. The binding energy per CO2 molecule was calculated to be −10.8 kcal·mol−1. in the presence of four CO2 molecules around Mg alkoxide group, we found that two of them became distorted by 41°, where the other two were only slightly distorted by 2.3°. Furthermore, the distance of the closest oxygen atom of the molecule from the magnesium and the distance of the carbon atom of CO2 from the closest oxygen atom of alkoxide group were 1.99 and 1.59 Å, respectively. These values are slightly larger than the corresponding values for the case in Figure 3b. For the undistorted CO2 molecules the equilibrium distances of the closest oxygen atoms of the molecules from the magnesium atom were 2.24 Å. The D

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The Journal of Physical Chemistry C undistorted CO2 molecules had the same orientation as we had described previously for other cases. The binding energy per CO2 was calculated at −8.9 kcal·mol−1. Next we tried to insert a fifth CO2 molecule around Mg alkoxide group, but the optimization procedure moved one of the molecules away from the Mg atom with respect to the other four molecules. The space around the Mg alkoxide group became saturated with 4 CO2 and any additional CO2 will be located in a virtual second sphere around the alkoxide group. Considering all the descriptions mentioned above of the optimized structures, we found that the addition of CO2 molecules led to the gradual increase of the distances between the adsorbed CO2 molecules and the Mg atom of the alkoxide group for both distorted and nondistorted CO2 molecules. Further, we observed that there was a slight deviation of the distortion of the O−C−O angle as we increase the number of CO2 for the case of the slightly distorted CO2, where for the case of the distorted CO2 molecules there was a gradual decrease of the same angle. Finally, we found that as we added more CO2 molecules around Mg alkoxide group, the binding energy per CO2 decreased. In order to evaluate the CO2 capture ability of the Mg modified IRMOF-10, we predicted the CO2 excess adsorption isotherms by performing GCMC simulations. The simulations were performed for two different Mg alkoxide IRMOF-10. In the first case, one Mg alkoxide group was introduced in each linker of the periodic box (Figure 1a), where in the second case each of the linkers had 2 Mg alkoxide groups (Figure 1b). We obtained excess volumetric and gravimetric isotherms at 300 K and pressures up to 40 bar, which are presented in Figures 4 and 5. The results obtained from these isotherms for IRMOF10 are in very good agreement with previous findings reported by Walton33 and De Toni.34 As can be seen in Figures 4a and 5a, a type V isotherm is obtained for the IRMOF-10, which is expected for confined adsorption environments where the solid−fluid interactions are weaker than the fluid−fluid interactions. The isotherms for the singly and doubly Mg modified IRMOF-10 follow a Langmuir type I isotherm. The transition of the isotherms from type V to type I is attributed to the enhanced fluid−solid interactions that are introduced in the modified IRMOF-10 due to the presence of the Mg alkoxide groups in the linkers. The calculated interaction energy of CO2 with the magnesium alkoxide linker was −13.6 kcal·mol−1 at the RIMP2/TZVPP level of theory, where in the bare IRMOF-10 the corresponding interaction energy is typically in the range of 2−2.5 kcal·mol−1. The substitution of the linker of IRMOF-10 with the singly modified Mg alkoxide linker greatly affected the total uptake of CO2 in low pressure (up to 1 bar) and higher pressure regimes (up to 40 bar). The enhancement was more pronounced in the low pressure region, as can be seen in Figures 4b and 5b with respect to IRMOF-10. At 1 bar, the volumetric uptake for the unmodified IRMOF-10 was 7 cm3(STP)/cm3 and when we introduced a Mg alkoxide group in the linker, the total uptake reached 180 cm3(STP)/cm3. Similar to volumetric uptake, the gravimetric uptake for the unmodified IRMOF-10 and Mg alkoxide IRMOF-10 was found at 1 mmol/g and 21 mmol/g at 1 bar, respectively. At higher pressures, the volumetric uptake was still much larger for the largest part of the pressure range, but as the pressure reach 40 bar, the enhancement becomes smaller. A similar trend was observed for the gravimetric uptake at the high pressure regime. We noticed that both volumetric and gravimetric uptakes for IRMOF-10 at 40 bar can be

Figure 4. Excess volumetric isotherms for bare IRMOF-10 and singleand double- Mg alkoxide modified IRMOF-10 at 300 K and pressure ranges up to a) 40 bar and b) 1 bar.

obtained at much smaller pressure when an Mg alkoxide group per linker was introduced in IRMOF-10. The introduction of the second Mg alkoxide group in the linker further enhanced the amount of CO2 adsorbed with respect to singly modified Mg alkoxide linker, which was again more pronounced in the lower pressure regime of the volumetric and gravimetric isotherms. At 1 bar, the corresponding volumetric and gravimetric uptake for the IRMOF-10 with two Mg alkoxide groups was found at 330 cm3(STP)/cm3 and 35 mmol/g, respectively, which were much larger than both the unmodified and the modified with one Mg alkoxide group IRMOF-10. This enhancement can be attributed to the doubling of the Mg alkoxide binding sites for the CO2 in the linker, adding in this way a second strong binding site with equal binding strength per linker. In their previous work for IRMOF-10, De Toni34 concluded that the increase of the effective pore volume accessible to the sorbate molecules can be achieved by homogeneously increasing the interaction energy of the CO2 with the pore surface. A similar increase in the effective pore volume can be achieved by introducing more binding sites of specific binding strength rather than continuously increasing the binding strength, as found by the introduction of the second Mg alkoxide group. An interesting point of the predicted adsorption isotherms was found at the high pressure regime for both volumetric and gravimetric isotherms. In both cases, the difference in the total CO2 uptake was reduced with respect to lower pressures. In the case of the volumetric isotherms presented in Figure 4a, the E

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Figure 6. Isosteric heat of adsorption variation as a function of the excess gravimetric uptake for the bare, single and double Mg alkoxide IRMOF-10 at 300 K.

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CONCLUSIONS Combined ab initio calculations and GCMC simulations were carried out in order to enhance the ability of IRMOF-10 to capture carbon dioxide by introducing Mg alkoxide groups in the linkers of the structure. Ab initio calculations in the modified IRMOF-10 linker indicated that the modified linker can interact with up to four CO2 molecules per Mg alkoxide group simultaneously or to up to eight CO2 molecules when the doubly modified cation of the biphenyl linker is considered. The highest binding energy for the CO2 molecule was calculated to be −14.2 kcal·mol−1 at the RI-MP2/def2TZVPP level of theory, where in the bare IRMOF-10 the corresponding interaction energy is typically in the range of 2− 2.5 kcal·mol−1. Furthermore, GCMC simulations showed a clear enhancement of the CO2 uptake in both gravimetric and volumetric capacity at 300 K and pressures up to 40 bar for the functionalized MOF. This is more pronounced at low pressures. At 10 bar, the volumetric capacity becomes 5 or 7 times larger than the unmodified IRMOF-10 by introducing 1 or 2 Mg alkoxide groups, respectively. Based on this significant enhancement we propose Mg functionalization as a general strategy for improving the CO2 storage capacity of MOFs.

Figure 5. Excess gravimetric isotherms for bare IRMOF-10 and singleand double- Mg alkoxide IRMOF-10 at 300 K and pressure ranges up to a) 40 bar and b) 1 bar.

difference in the uptake was still quite large even at 40 bar between the Mg alkoxide IRMOF-10 and pure IRMOF-10. However, this was not the case for the gravimetric adsorption isotherms (Figure 5a), where the difference in the excess CO2 uptake between the Mg alkoxide IRMOF-10 and the IRMOF10 was predicted to be significantly reduced. Moreover, at this pressure, the uptake was found to be almost equal for single and double modified IRMOF-10. This effect can be attributed to the additional molecular weight of the structure due to introduction of the Mg alkoxide groups, which reduced the gravimetric capacity as the pressure increased. An advantage of the introduction of the metal alkoxide groups is that saturation uptake could be achieved at lower pressures than in the case of the unmodified IRMOF-10. We also calculated the isosteric heat of adsorption of CO2 in the original and the Mg modified IRMOF-10, which is presented in Figure 6 as a function of the excess gravimetric CO2 loading. As can be seen in Figure 6, the isosteric heat of adsorption calculated from the GCMC simulations was in very good agreement with the obtained MP2 binding energies for single and double Mg modified IRMOF-10. For single Mg alkoxide modified IRMOF-10, the isosteric heat of adsorption decrease as a function of the CO2 loading and become nearly constant over 8 mmol/g. In the case of the double Mg alkoxide groups per linker in IRMOF-10, the isosteric heat of adsorption deviates slightly from its initial value as CO2 uptake increases for the range of the uptake which was considered in the plot.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Tel.: +30 2810 545055, fax: +30 2810 545001. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present study was cofinanced by the European Union (European Social Fund − ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) - Research Funding Programs: Heracleitus II and Thales. Investing in knowledge society through the European Social Fund. Data Center of the University of Crete is gratefully acknowledged for providing computational resources. F

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DOI: 10.1021/acs.jpcc.5b05294 J. Phys. Chem. C XXXX, XXX, XXX−XXX