Computational Design of Porous Organic Frameworks for High

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Computational Design of Porous Organic Frameworks for High-Capacity Hydrogen Storage by Incorporating Lithium Tetrazolide Moieties Yingxin Sun,† Teng Ben,‡ Lin Wang,† Shilun Qiu,*,§ and Huai Sun*,† †

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China, The Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, China, and §State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China ‡

ABSTRACT We propose to incorporate a lithium tetrazolide group into porous materials for enhancing hydrogen storage capacity. The lithium tetrazolide group is much more stable and polarized than the models made by doping aromatic groups with lithium atoms. More importantly, each of the lithium tetrazolide provides 14 binding sites for hydrogen molecules with modest interaction energies. The advantage of multiple binding sites with modest binding energies is partially demonstrated by constructing a new porous aromatics framework (PAF-4) with the lithium tetrazolide moieties and predicting its hydrogen uptake using firstprinciples GCMC simulations. The predicted hydrogen uptake reaches 4.9 wt % at 233 K and 10 MPa, which exceeds the 2010 DOE target of 4.5 wt %. SECTION Nanoparticles and Nanostructures

(Figure 1). By removing a proton, the molecule becomes aromatic and negatively charged. The aromaticity means that the group is stable, and the net negative charge implies a strong electrostatic field enhancing the interaction with hydrogen molecules. In addition, the negative charge is distributed to the five-member ring in which four nitrogen atoms are “naked” (without hydrogen attached), which provides ready access by more hydrogen molecules. The tetrazolide anion interacts with the lithium cation via ionic bonds. The calculated MP2 bond energy of Liþ with CHN4- is more than 620 kJ/mol, much greater than that obtained for the neutral Li/C6H6 complex. In addition, the charge separation is significant. Full geometry optimizations show that the Liþ can be bonded to CHN4- in five positions; among them, three are symmetrically independent, coplanar, side-coplanar, and vertical isomers. The most stable isomer for the isolated molecule is the coplanar, presumably due to high electron density of the lone pair orbitals on the nitrogen atoms. Our calculations at the RI-MP2/def2-TZVPP level of theory with BSSE corrections show that the isolated tetrazolide anion (CHN4-) is able to bind up to 10 hydrogen molecules with modest (average 6.5 kJ/mol) interaction strengths. As shown in Figure 2, the total binding energies scale up linearly, and the increment of the binding energy is nearly constant (about 6.0 kJ/mol), indicating the independence of hydrogen adsorption sites. The isolated lithium tetrazolide binds up to

T

he widespread application of hydrogen as a clean alternative to fossil fuels is limited by the lack of a convenient, cheap, and safe storage system. Much effort has been invested in studying porous organic frameworks with large surface areas and lower density.1-8 Yet, the targeted capacity of hydrogen storage for on-board use has not been reached. It has been reported that lithium doping may significantly increase the interaction energy between hydrogen and adsorbent materials.9-11 However, early reported12 success of Li doping on carbon nanotubes had been shown13 to be false due to impurities. Several experimental works demonstrated that Li-doped pristine carbon materials such as multiwall carbon nanotubes, graphite, intercalated graphite, or graphene do not show increased hydrogen uptakes.13-15 Although some computational works16,17 support the hypothetical intensification of hydrogen uptakes by Li doping on phenyl or phenyl-like moieties in MOF and COF materials, the widely used charge-transfer mechanism so far has been challenged by experimental and theoretical works.18,19 Our MP2 calculations performed on benzene and lithium clusters confirmed that the binding is weak (ca. 16 kJ/mol), and charge separation between lithium and benzene is minimal (see Table S1 of Support Information, SI). In this work, we suggest incorporating a salt group consisting of an aromatic tetrazolide anion (CHN4-) and lithium cation (Liþ) into porous materials along the direction of the doping cation Liþ via redox reactions.20-22 The tetrazolide group has been used as a part of organic linkers in MOFs.23-26 The tetrazolide anion (CHN4-) can be made by reducing tetrazole (CH2N4), a nonaromatic five-member ring compound

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Received Date: July 1, 2010 Accepted Date: August 27, 2010 Published on Web Date: September 07, 2010

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DOI: 10.1021/jz100894u |J. Phys. Chem. Lett. 2010, 1, 2753–2756

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molecules are populated in the vicinity of Liþ first and then distributed to other locations. The average values of the binding energy per H2 molecule are ∼5-6 kJ/mol. The lithium tetrazolide can be introduced as a part of the building blocks of the porous organic framework. Figure 3 shows a new material (named PAF-4) constructed based on a recently described PAF-127 by introducing two Li(þ)CHN4(-) groups to the middle phenyl ring of the building blocks. A primitive cubic cell with P1 symmetry was constructed and optimized using the universal force field (UFF)28 and charge equilibrium model (QEQ). 29 Although the coplanar is the most stable isomer for the isolated lithium tetrazolide molecule, the side-coplanar configuration becomes the most stable in the crystal based on the energy minimization. The optimized edge length is 33.8 Å, the density is 0.2631 g/cm3, the free volume is 86.2%, and the Langmuir surface area is 5524.94 m2/g. GCMC simulations were carried out to predict adsorption isotherms for the designed material based on a force field derived from the ab initio data using the same approach as reported before.30 The derived force field accurately represents the interaction energies between hydrogen molecules and the Li(þ)-CHN4(-) group. Parameters representing interactions between the hydrogen molecule and hydrocarbon groups are taken from our previous work.30 The force field parameters and validation results are given in the SI. Due to the strong binding (>620 kJ/mol), the position of the lithium cations was fixed. Figure 4 presents the simulated adsorption isotherms at 77 and 233 K and up to 10 MPa. Additional data (200 and 298 K) are given in the SI. At 77 K and 10 MPa, the gravimetric adsorption of H2 is 20.7 wt %, and the volumetric adsorption is 68.6 g/L. At T = 233 K, the maximum gravimetric uptake is 4.9 wt %, which reaches the 2010 DOE target of 4.5 wt %. The inadequate volume capacity is due to the large free volume.

14 hydrogen molecules as both the anion and cation provide binding sites. The initial binding energy is greater than 10 kJ/ mol. Two regions with different slopes can be identified, indicating that the binding sites are dependent; hydrogen

Figure 1. Formation of lithium tetrazolide. One of five possible structures of lithium tetrazolide is shown. The color codes are purple-lithium, blue-nitrogen, gray-carbon, and white-hydrogen. The five different positions are indicated by arrows; (a) two sidecoplanar, (b) one coplanar, and (c) two vertical (above and below the five-member ring).

Figure 2. Variation of the total binding energy with an increase in the number of hydrogen molecules for the CHN4- anion and the coplanar, side-coplanar, and vertical CHN4-Li isomers.

Figure 3. The building block (left) and unit cell (right) of the proposed porous aromatic framework (PAF-4) containing Li(þ)-CHN4(-) moieties. The yellow ball denotes the free volume.

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DOI: 10.1021/jz100894u |J. Phys. Chem. Lett. 2010, 1, 2753–2756

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Figure 4. Gravimetric and volumetric adsorption isotherms of H2 predicted at 77 and 233 and up to 10 MPa. The round dot indicates the DOE 2010 target for gravimetric uptake.

Figure 5. Isosteric heats of adsorption for hydrogen molecules calculated at 77 and 233 K and up to 10 MPa; the line indicates the trend.

The calculated isosteric heats (Qst) of H2 adsorption at 77 and 233 K are shown in Figure 5. The results demonstrate that the isosteric heat reaches 14.5 kJ/mol at low temperature (77 K) and low loading amount due to strong interactions between hydrogen molecules and the Li(þ)-CHN4(-) moieties. The curve drops quickly and converges to ∼3-4 kJ/mol as the pressure increases, reflecting that additional molecules are distributed in various binding sites. At 233 K, the loading amount is low, and the average isosteric heat is roughly 5 kJ/mol. The fact of multiple binding sites with modest binding energies is of great interest for hydrogen storage because it implies substantial uptake and advantage in adsorption/desorption kinetics. Using a simple Langmuir adsorption model, it can be easily demonstrated that the number of interaction sites, in addition to binding energies and surface area, is an important factor that determines the total amount of hydrogen adsorbed. To summarize, we propose for the first time incorporating a lithium tetrazolide group in porous materials for enhancing hydrogen storage capacity. The lithium tetrazolide group is much more stable and polarized than the models made by doping aromatic groups with lithium atoms. More importantly, each of the lithium tetrazolide group provides 14 binding sites for hydrogen molecules with modest interaction energies. The advantage of multiple binding sites with modest binding energies is partially demonstrated by constructing a new porous aromatics framework (PAF-4) with the lithium tetrazolide moieties and predicting its hydrogen uptake

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using GCMC simulations based on the force field derived from first-principles data. The predicted hydrogen uptake reaches 4.9 wt % at 233 K and 10 MPa, which exceeds the 2010 DOE target of 4.5 wt %, a step close to the 2015 DOE target of 5.5 wt %.

SUPPORTING INFORMATION AVAILABLE The quantum mechanics calculations, parametrization of the force field, predicted adsorption isotherms, and predicted isosteric heats. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Fax: þ86-4318516-8331. E-mai: [email protected] (S.Q.); Tel: þ86-21-5474-8987. Fax: þ86-21-5474-1297. E-mail: [email protected] (H.S.).

ACKNOWLEDGMENT The study was supported by the National

Science Foundation of China NSAF Program (No. 10676021) and the National Basic Research Program of China (No. 2007CB209701).

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