Article pubs.acs.org/JPCC
Hydrogen Storage in Novel Li-Doped Corrole Metal-Organic Frameworks Taxiarchis Stergiannakos,† Emmanuel Tylianakis,‡ Emmanouel Klontzas,† Pantelis N. Trikalitis,† 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 S Supporting Information *
ABSTRACT: A new metal-organic framework (MOF) has been designed based on a carboxy functionalized corrole ligand acting as a building block. The H2 storage properties of this MOF was examined by applying a multi-scale theoretical technique which combines ab initio calculations and grand canonical Monte Carlo simulations. Ab initio calculations showed that Li doping increases the interaction energy between the hydrogen molecules and the newly proposed Lidoped corrole linker, compared to the undoped one. The value of the interaction energy was found to be 3.58 kcal·mol−1 for the first hydrogen molecule. Li-doped corrole linker can host up to 10 hydrogen molecules in both the convex and the concave side. GCMC atomistic simulations verified that the proposed Li-doped material shows higher adsorption capacities than the nondoped one and this enhancement is more pronounced at low pressures. The newly proposed corrole-MOF can also find applications in the area of gas adsorption and catalysis.
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INTRODUCTION The search for alternate energy sources is now one of the most demanding needs of global society. Energy requirements worldwide are growing, along with the environmental problems caused by energy resources based on fossil fuels. Hydrogen is considered as an ideal energy carrier and is characterized by significant benefits in relation to alternative methodologies, especially for automotive applications. One of the main problems encountered with regard to the wide commercialization of hydrogen is its efficient storage in suitable solid materials. There have been many efforts by scientific groups worldwide to find materials with the desired properties for that purpose. Hydrogen can be stored in materials by either physisorption or chemisorption. In the first case, hydrogen keeps the molecular structure, while in the second it dissociates in atomic form. Materials classified as physisorbent include various nanoporous structures such as carbon-based materials, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and microporous polymers, while metal or chemical hydrides are typical chemisorbing cases. In order for these materials to qualify for hydrogen storage, capacity targets established by the U.S. Department of Energy (DOE) should be met. The system capacities targeted for 2015 are 5.5 wt % and 0.040 kg/L for gravimetric and volumetric storage, respectively.1 In fact, since these targets concern the system capacities, suitable materials must have storage capacities considerably larger than the above figures. There have been numerous experimental and computational studies for hydrogen storage in MOF materials in the past few © 2012 American Chemical Society
years. MOFs present exceptional physicochemical characteristics such as large surface areas and pore volumes but in most cases lack the existence of strong interactions of hydrogen with the framework structure. Computational studies have shown that these interactions are only a few kilojoules per mole,2 whereas in the case of frameworks with unsaturated metal centers this interaction energy can be slightly higher.3 In order to increase the interaction energy between hydrogen and the host structure, doping with alkali metal atoms has been proposed as a very promising strategy.4 There have been several successful experimental attempts in the recent past which support the possibility to incorporate Li in the internal surface of a MOF.5−9 Indeed, increased interactions between hydrogen and doped material result due to stronger electrostatic interactions of hydrogen with alkali metal cations. Lithium, as the hardest alkali metal, shows the strongest interaction with hydrogen molecules among other alkali metals, through chargeinduced dipole and quadrupole moments.10 Earlier theoretical studies on Li-doped MOF11−13 or COF14,15 materials have shown an enhancement of the interaction energy by a factor of approximately 3 compared with the undoped MOFs and an increase in the corresponding H2 gravimetric and volumetric capacities.16 The number of lithium atoms over the organic primary building units considerably affects the number of hydrogen molecules that can strongly interact with the doped Received: November 15, 2011 Revised: March 13, 2012 Published: March 15, 2012 8359
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material.17 In the case of decoration with metallic lithium, a limited number of lithium atoms must be inserted in order to prevent the formation of lithium clusters. This phenomenon is not present in the case of formation of chemical bonds which incorporate cationic lithium atoms. In the present work, we propose a new MOF structure which is based on the structure of the IRMOF family of materials. The proposed structure consists of the same inorganic secondary building unit as in IRMOF, the well-known octahedral cluster [Zn4O(−CO2)6], and a dicarboxylate functionalized corrolebased ligand as the organic primary building unit (Figure 1a). Corrole (Figure 1b) is a nonplanar, aromatic organic molecule with four nitrogen atoms in the core of the molecule and may serve as a trivalent anionic ligand. Therefore, corrole, in its anionic form, can hold up to three lithium cations through strong electrostatic interactions (Figure 1c). We carried out quantum mechanics calculations to investigate the structure of the proposed Li-doped corrole (Li-Cor) ligand and the interactions of multiple hydrogen molecules with it. Furthermore, we performed grand canonical Monte Carlo simulations in the periodic structure of Li-Cor MOF to calculate the corresponding gravimetric and volumetric capacities at 77 and 300 K for pressures up to 100 bar.
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COMPUTATIONAL DETAILS To study the Li-modified corrole with ab initio calculations, the cluster approximation18 was used. In that respect, the corrole ligand was separated from the proposed structure and the dangling bonds were saturated with H atoms. Second-order Moller−Plesset (MP2) perturbation theory in the resolution of identity (RI) approximation19 was applied to our calculations along with the def2-TZVPP 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. All calculations were performed with the TURBOMOLE program.20 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.21 These corrections have been proven essential since the BSSE may become critical for nonbonding interactions. The periodic structure of corrole MOF was optimized by using molecular mechanics methods. The construction of the initial crystal structures was followed by a simultaneous optimization procedure for both unit cell volume and atomic coordinates with the XTALMIN program of the TINKER package.22 For the optimization of the structure, we used an extended MM3 force field that was explicitly parametrized for IRMOF-1 on the basis of first-principles calculations.23 This force field was proven to be accurate for conformational energies of the inorganic part of the IRMOFs [Zn4O(−CO2)6]. Moreover, we implemented some extra parameters in our calculations for organic linker (corrole) in order to have a better description for the torsions based on our ab initio calculations. The optimized structure for the IRMOF is presented in Figure 1a, while structural details of the optimized structure can be found in the Supporting Information. Adsorption of hydrogen in the Li-Cor MOF was studied by employing grand canonical Monte Carlo (GCMC) simulations.
Figure 1. (a) Corrole MOF periodic structure, (b) corrole molecule, and (c) Li-doped corrole molecule.
Lennard-Jones potential was used for the description of the interactions. In order to take into account quantum effects, the Feymnan−Hibbs formalization24 was applied to the potential for every thermodynamic state. Hydrogen molecule was treated 8360
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using the united atom model, located at the center of mass of the molecule, whereas for the framework an atomistic model was used. Lennard-Jones parameters for hydrogen were taken as σHH = 0.296 nm and εHH = 34.2 K, while for the host atoms the DREIDING force field25 data were used. Lorenz−Berthelot mixing rules were used for calculation of parameters between guest and host molecules. In the case of Li (for which this force field does not contain any parameters), the potential was fitted to the ab initio binding energy in order to extract the parameters. Two different sets of Li parameters, εLiH and σLiH, were used since the binding energies at above and below the primary unit are different, as revealed from MP2 calculations. Truncation of the interaction after a cutoff of 17 Å was applied, and no long-range corrections were added. The periodic box that describes our systems was large enough to ensure that no finite-size effects exist for our frameworks. For each thermodynamic state, 10 million moves were attempted for the production stage and another 10 million for the equilibration stage. For each thermodynamic state of temperature and pressure, chemical potential was determined by using the fugacity as it was calculated by the Peng−Robinson equation of state.
Table 1. MP2 Interaction Energies for Multiple Hydrogen Molecules Interacting with Li-Cora method
no. of H2
RI-MP2 1 H2 conc. (def2TZVPP) 1 H 2 conv. 2 H2 conv. 3 H2 conv. 4 H2 conv. 5 H2 conv.
total interaction energy (BBSE-corrected values) (kcal·mol−1)
avg interaction energy per H2 (BSSE-corrected values) (kcal·mol−1)
2.68
2.68
3.58
3.58
6.69
3.35
10.23
3.41
12.88
3.22
14.96
2.99
a
conc. and conv. stand for concave side and convex side of corrole, respectively.
the T-shape geometrical configuration due to the interaction with Li. An attempt was made to add a sixth H2, but that molecule was moved away to a virtual second interaction shell around Li. The average interaction energy in the case of three, four, and five H2 was 3.41, 3.22, and 2.99 kcal·mol−1, respectively. In the concave side, the maximum number of H2 that could interact simultaneously was found also to be five. Next, H2 adsorption isotherms were calculated for Li-doped and undoped frameworks at both cryogenic and ambient temperatures for a pressure range of 1−100 bar. Figure 2 presents the total adsorption isotherms at both temperatures in gravimetric and volumetric terms. The corresponding isotherms for the excess uptake are presented in Figure S8 of the Supporting Information. As can be seen from Figure 2, both systems show good adsorption behavior especially at low temperatures. The strong binding energy in the case of the Lidoped system is reflected to the enhanced hydrogen uptake in comparison to the undoped one. At 77 K the DOE systems targets are reached at 40 and 20 bar for the undoped and doped material, respectively. It is important to note here that for the undoped framework, the pressure of 40 bar is among the lowest pressures needed for unmodified MOFs to attain DOE targets. The enhancement in H2 adsorption for the Li-doped framework compared to the undoped parent material is more pronounced at low pressures where the calculated improvement is 10-fold and 2-fold more at 300 and 77 K, respectively. At ambient temperature the observed storage capacity increase is significantly higher because under these conditions adsorption is particularly weak and the addition of physisorption-based strong interactions causes very pronounced effects. This improvement can also be seen in the snapshots taken from these simulations in Figure 3. These snapshots correspond to 77 K and 1 bar and indicate after careful inspection that the enhancement is indeed due to the presence of Li cations. The main contribution originates from the two Li atoms which provide binding energy considerably higher as already mentioned earlier in the text. Adsorption isotherms can be used as well for the calculation of isosteric heat of adsorption for these two materials. The heat of adsorption was determined to be 1.24 and 3.39 kcal/mol for the undoped and the doped material, respectively. Both of these values lie above the corresponding value for IRMOF-1 material, calculated to be 0.97 kcal/mol. The dependence of the isosteric heat of
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RESULTS AND DISCUSSION Quantum mechanics calculations were applied in order to determine the geometrical characteristics and the maximum number of H2 that can interact simultaneously with Li-Cor as well as the geometry and the energetics of H2 with it. The equilibrium geometry of the Li-Cor can be seen in Figure 1c. The curvature of corrole is affected upon substitution of the protons with the Li cations, leading to a change of the geometry of the ligand from planar to curved. Accordingly, two Li cations are located in the convex side of corrole and only one in the concave side. Structural details about Li-Cor can be found in the Supporting Information file. The next step was to find the energetics of the interaction of H2 with Li-Cor and the maximum number of H2 that can interact simultaneously with it. To do so, successive addition of H2 molecules was performed, until the maximum number of H2 molecules was reached. In every addition, the average value of the corresponding interaction energy was calculated. Two distinct calculations were performed to find the interaction energy of the first H2 in the convex and in the concave side of Li-Cor. MP2 calculations showed that the corresponding interaction energies are 3.58 and 2.68 kcal·mol−1, respectively (see Table 1). In both sides H2 interacted with Li by forming a T-shape geometrical configuration (Figure S2, Figure S3), which has also been observed in previous studies of Licontaining molecular systems.26 A second H2 was then added in the convex and the concave side of Li-Cor and followed by a separate calculation for each side. For the convex side, it was found that the second H2 interacted with the second Li of that side with an average interaction energy of 3.35 kcal·mol−1, forming the same T-shape configuration as the first one. In the case of the concave side, the second H2 did not interact directly with the single Li due to H2−H2 repulsions due to the limited available space around it. The energetically preferred position was located over the center of the five-member ring of corrole having an end-on geometrical configuration. Successive additions of H2 molecules were then performed in each side of the Li-Cor separately to find the maximum number of H2 that can be accommodated in each side. The maximum number of H2 was found to be five in the convex side, all having 8361
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to 10 H2 molecules simultaneously, with the first H2 molecule exhibiting an interaction energy of 3.58 kcal·mol−1. GCMC simulations predict a clear enhancement in gravimetric and volumetric capacity at 77 and 300 K, which is more pronounced at lower pressures. The effect of Li doping can be visualized from the snapshots taken at 77 K and 1 bar, which show an increment in the relative density of H2 around Li compared to the unmodified corrole. Except from hydrogen storage, we believe that corrole-MOF will have interesting adsorption properties for other gases, such as CO2 or CH4. Moreover, due to ability of the corrole ring to host different metal atoms, corrole-MOF will be an interesting candidate for catalytic processes.
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ASSOCIATED CONTENT
S Supporting Information *
Structure details of the optimized Li-Cor and Li-Cor MOF structures, optimized structures from the successive addition of multiple H2 molecules in Li-Cor as derived from ab initio calculations, excess gravimetric and volumetric isotherms of Lidoped and undoped corrole-MOF, and details about the calculation of isosteric heat of adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +30-2810545055. Fax: +30-2810545001. E-mail:
[email protected]. Notes
Figure 2. Gravimetric and volumetric adsorption isotherms at 77 K and 300 K and for a pressure range up to 100 bar. Blue symbols and lines are for Li-doped corrole, whereas red ones for the undoped. Open and close symbols represent data at 298 K and 77 K, respectively.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research has been co-financed 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 Program: Heracleitus II. Investing in knowledge society through the European Social Fund. The authors wish to acknowledge co-funding of this research by European Union- European Regional Development Fund and Greek Ministry of Εducation/EYDE-ETAK through program ESPA 2007-2013 / EPAN II / Action “SYNERGASIA” (09ΣΥΝ-42-831).
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Figure 3. Snapshots taken from the GCMC simulations for the pristine (left) and the Li-doped corrole MOF at 77 K and 1 bar.
adsorption on the hydrogen loading was also studied and is shown in Figure S9 of the Supporting Information.
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CONCLUSIONS A new MOF structure was designed that was based on the corrole molecule which acts as a primary organic building unit, while keeping the same inorganic secondary building unit as in the IRMOF family of materials. We propose this new structure as a promising material for hydrogen storage. In order to enhance the ability of this material to store hydrogen, we have doped the corrole with three Li cations by replacing an equal number of acidic hydrogen atoms. Ab initio calculations in the Li-doped corrole indicate that this corrole can interact with up 8362
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