High-Capacity Hydrogen Storage in Porous Aromatic Frameworks with

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High-Capacity Hydrogen Storage in Porous Aromatic Frameworks with Diamond-like Structure Jianhui Lan,† Dapeng Cao,*,† Wenchuan Wang,† Teng Ben,‡ and Guangshan Zhu‡ †

Division of Molecular and Materials Simulation, Key Lab for Nanomaterials, Ministry of Education of China, Beijing University of Chemical Technology, Beijing 100029, China, and ‡State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China

ABSTRACT We have used the multiscale simulation method to evaluate the hydrogen storage performance of a recently designed new class of porous materials, PAF-30X (X=1-4), with diamond-like structure. Our simulation results show that the hydrogen uptakes of PAFs mainly depend on their densities and free volumes. Among the four frameworks, PAF-304 and PAF-303 possess significantly higher gravimetric hydrogen uptakes than the recently reported covalent organic framework-102 (COF-102) and belong to the most promising candidates for hydrogen storage so far. In particular, at T = 298 K and 100 bar, the gravimetric hydrogen uptake of PAF-304 reaches 6.53 wt %, which is the highest among all of the present porous materials without modification. SECTION Energy Conversion and Storage

H

indicate that the hydrogen uptake of PAF-302 reaches 10.7 wt % at 77 K and 48 bar while the carbon dioxide uptake reaches 1300 mg/g at 298 K and 40 bar, which are all among of the highest scores of the nanoporous materials so far. Encouraged by our previous results of PAF-302, in this Letter, we attempt to explore the performance of hydrogen storage in the whole class of PAFs, including PAF-301, -303, and -304, by using the multiscale simulation method, which combines the first-principles calculations and grand canonical ensemble Monte Carlo (GCMC) simulation. Previous investigations8,13-16 have indicated that this multiscale simulation method is a powerful and versatile tool for designing adsorbents for gas adsorption and separation. In the Supporting Information of our previous paper,11 we presented the detailed structural information of PAF-30X (X = 1-3) frameworks. Here, we also present the structure parameters of PAF-304 shown in Figure 1d. For convenience, we list the structure parameters of all PAFs (PAF-30X (X = 1-4)) in Table S1 in the Supporting Information. All four frameworks belong to the cubic space group P1. With the increase of the number of phenyl rings between two neighboring tetrahedrally bonded carbon atoms, the density of PAFs decreases correspondingly. It can be found from Table S1 (Supporting Information) that PAF-301 shows the largest density of about 0.8364 g/cm3, while PAF-304 shows the smallest density of about 0.0998 g/cm3. In addition, the free volume of PAFs increases significantly with the increase of the number of phenyl rings linked between the neighboring tetrahedrally bonded carbon atoms.

ydrogen, as a viable energy carrier, may play an important role in future energy plans. One of the main challenges that constrains the widespread application of hydrogen at present is the lack of high-capacity hydrogen adsorbents. In the past few years, a lot of research interest has been focused on porous materials, which not only possess high surface area but also reversibly adsorb or desorb hydrogen under these conditions studied.1-8 Designing porous materials by topology is currently an effective strategy in the development of reticular chemistry. It simplifies the complex problem of porous structure prediction into a topological organization of molecular building blocks to some extent. A series of porous materials has been synthesized by using topology, for example, metal-organic frameworks (MOFs)9 and covalent organic frameworks (3D COFs).4,10 Most recently, a new class of porous aromatic frameworks (PAFs) with diamond-like structure was proposed in our previous work11 because diamond is an exceptionally stable crystal of carbon atoms, in which each carbon atom is tetrahedrally bonded to its four neighboring atoms by covalent bonds. Inspired by this idea, the class of PAFs, which show not only diamond-like structural stability but also high surface area, was figured out. The structures of PAFs (termed PAF-30X, X=1-4, where 3 means 3D structure and X denotes the number of phenyl rings used to replace the C-C bond), obtained by replacing the C-C bond in diamond with single or multiple phenyl rings and then performing geometry optimizations with the Forcite module of the Material Studio suite of programs,12 are given in Figure 1. Impressively, by self condensation of the tetrahedral building block, tetrakis(4-bromophenyl)methane, we have synthesized the PAF-302 (also termed as PAF-1 in our previous work), which exhibits not only a high Langmuir surface area of 7100 m2/g but also high thermal and hydrothermal stabilities.11 The measurements in the experiment

r 2010 American Chemical Society

Received Date: December 29, 2009 Accepted Date: February 24, 2010 Published on Web Date: March 01, 2010

978

DOI: 10.1021/jz900475b |J. Phys. Chem. Lett. 2010, 1, 978–981

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Table 1. van der Waals Force Field Parameters for Nonbond Interaction between H2 and PAFs Derived from the First-Principles Calculationsa parameters atom typesb c

H_A 3 3 3 H_A H_ 3 3 3 H_A

D (kcal/mol)

re (Å)

γ

0.0182 0.0124

3.5698 3.2010

10.7094 12.0027

C_R 3 3 3 H_A 0.0892 3.2400 11.6000 C_3 3 3 3 H_A 0.0620 3.2400 11.0062 a b H_A denotes H in a H2 molecule. H_ denotes H bonded to the hydrocarbon rings. C_R and C_3 denote the resonant and tetrahedralcoordinated C, respectively. c The force field parameters are derived from previous literature.2

Figure 1. Unit cells of PAFs, (a) PAF-301, (b) PAF-302, (c) PAF-303, and (d) PAF-304, derived from topology design and geometry optimization with the force field method. Here, gray and pink spheres represent carbon and hydrogen atoms, respectively, while the blue polyhedron represents the tetrahedrally bonded carbon atoms. In addition, the yellow sphere denotes the pores in 3D PAFs.

Figure 2. Comparison of predicted and experimental isotherms of H2 in PAF-302 at 77 and 150 K, respectively.

The pore size is also of great importance for judging the performance of porous materials. The centers of the largest cavity in PAF-301, PAF-302, PAF-303, and PAF-304 are 3.8, 7.4, 11.6, and 15.5 Å from the nearest H atom, respectively. Because H has a van der Waals radius of 1.2 Å, the diameters of the spherical cavities are determined to be 5.2, 12.4, 20.8, and 28.6 Å, respectively. Clearly, the pore sizes of PAF-303 and PAF-304 exceed the lower limit of 20 Å for mesoporous material. That is to say, when linking more than two hydrocarbon rings between two neighboring tetrahedrally bonded carbon atoms of diamond, the PAFs belong to mesoporous materials generally. In our multiscale simulation method, first, the interaction energies between H2 and PAFs were calculated by performing a series of single-point energy calculations with Gaussian 03 software.17 Then, the calculated discrete potential energies were fitted to a Morse potential function (see our previous work8,13-16), achieving the corresponding force field parameters. Using the fitted force field parameters as input, the GCMC simulations were implemented to evaluate the adsorption uptakes of hydrogen in these PAFs. Table 1 lists the fitted force field parameters for the interaction between H2 and PAFs. The calculation details can be referred to in our previous publications.8,13-16 Partial computational details are also presented in the Supporting Information. As mentioned above, we have measured the hydrogen isotherms of PAF-302 experimentally.11 Therefore, these data can be used to calibrate our results predicted by the multiscale method. The experimental and simulated isotherms of hydrogen adsorption in PAF-302 at 77 and 150 K are displayed in

Figure 2, from which we can see that the force field parameters derived from our first-principles calculations give a satisfying prediction to the experimental data. The good agreement of the simulation results and experimental data of hydrogen in PAF-302 allows us to further predict the hydrogen uptakes of the four PAFs. Figure 3a and b shows the simulated total and excess gravimetric H2 isotherms of the four PAFs at T = 77 K, respectively. From Table S1 (see the Supporting Information), it can be found that when replacing the C-C bond in diamond with one phenyl ring, the designed PAF-301 has a large density of about 0.8364 g/cm3 and a low free volume of about 40.92%. Therefore, the total and excess hydrogen gravimetric uptakes of PAF-301 do not exceed 5 wt % at T = 77 K, indicating that this framework is not suitable for hydrogen storage. When replacing the C-C bond in diamond with two phenyl rings, we obtained the microporous framework, PAF-302, which has been synthesized experimentally in our previous work.11 The results show that the hydrogen uptake of PAF-302 is slightly higher than the recently reported COF-10218 while slightly lower than MOF-177.5 For example, the maximum hydrogen excess uptake of PAF-302 reaches 7.00 wt % at T = 77 K, while those for COF-102 and MOF-177 are 6.84 and 7.40 wt %, respectively. The slight differences of PAF-302, COF-102, and MOF-177 in hydrogen storage are mainly attributed to their similarity in density and free volume. As listed in Table S1 (see the Supporting Information), the density and free volume of PAF-302 are 0.3150 g/cm3 and 77.60%, respectively, which are comparable to those of COF-102 (0.41 g/cm3, 71.12%)4,8 and MOF-177 (0.42 g/cm3, 68%).7,19


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Figure 3. Simulated H2 isotherms in PAFs at T = 77 K. (a) Total gravimetric isotherms. (b) Excess gravimetric isotherms.

As the linker between the neighboring tetrahedrally bonded carbon atoms increases to three phenyl rings, the PAF-303 framework shows much smaller density (0.1611 g/cm3) and significantly higher free volume (87.50%) than those of PAF302, which are almost the same as those of COF-105 (0.18 g/cm3, 88.22%) and COF-108 (0.17 g/cm3, 88.84%).4,8 Our simulation results indicate that PAF-303 presents very high gravimetric hydrogen uptakes at T = 77 K. For example, the simulated total hydrogen gravimetric uptake of PAF-303 reaches 16.85 wt % at T = 77 K and p = 100 bar, while the simulated maximum excess uptake reaches 8.06 wt % (see Figure 3). These results show that PAF-303 gives almost the same hydrogen uptakes as COF-105 and COF-108.8 However, as the linker between the neighboring tetrahedrally bonded carbon atoms increases to four phenyl rings, the PAF-304 framework shows the smallest density of about 0.0998 g/cm3 and the highest free volume of about 92.10% among all of the four PAFs and the 3D COFs4,10 and MOFs.5,7 Impressively, the PAF-304 exhibits a total hydrogen gravimetric uptake of about 22.38 wt % at 77 K and p = 100 bar and a maximum excess gravimetric uptake of 8.42 wt % at 77K and 60 bar due to its extremely low density and unprecedented high free volume. These interesting numbers indicate that PAF-304 is the most promising hydrogen adsorbent to date. As mentioned above, PAF-302 has been synthesized experimentally by self-condensation of the tetrahedral building block, tetrakis(4-bromophenyl)methane. It is expected that the designed mesoporous PAF304 will be fabricated experimentally by self-condensation of the tetrahedral building block, the tetrakis(40 -bromobiphenyl4-yl)methane, in the near future. Owing to the practical application of hydrogen at room temperature, we also predicted the hydrogen uptakes of PAFs at T = 298 K. As shown in Figure 4, the total hydrogen gravimetric uptakes of PAF-301 and PAF-302 are only 0.83 and 2.21 wt % at T = 77 K and p = 100 bar, which is still a poor performance. However, when replacing the C-C covalent bonds of diamond with three and four phenyl rings, the hydrogen uptakes of the resultant PAF-303 and PAF-304 reach 4.16 and 6.53 wt % at T = 77 K and p = 100 bar, which are nearly doubled and tripled, respectively, compared to that of PAF-302. Impressively, PAF-304 exhibits the highest hydrogen uptake of about 6.53 wt % at T = 298 K and p = 100 bar among all of the present porous materials without surface modification. In summary, we have used the multiscale simulation method to evaluate the hydrogen storage performance of


Figure 4. Computed total gravimetric isotherms of H2 in the 3D PAF frameworks at T = 298 K.

recently designed porous materials PAF-30X (X = 1-4) with diamond-like structure. Moreover, our simulation results show that the hydrogen uptakes of PAFs mainly depend on their densities and free volumes. Among the four frameworks, PAF-304 and PAF-303 possess significantly higher gravimetric hydrogen uptakes than PAF-302 and belong to the most promising candidates for hydrogen storage so far. In particular, at T = 298 K and 100 bar, the gravimetric hydrogen uptake of PAF-304 reaches 6.53 wt %, which is the highest among all of the present porous materials without modification. Although PAF-302 has been prepared experimentally in our recent work, how to synthesize the PAF-303 and -304 still needs further effort in experiment. It is expected that this work can motivate the development of corresponding experiments.

SUPPORTING INFORMATION AVAILABLE

The structure parameters of PAFs and partial details of the first-principles calculations and grand canonical Monte Carlo simulations. 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. E-mail: [email protected] and [email protected]. Fax: þ8610-64427616.

ACKNOWLEDGMENT This work is supported by NSF of China (20776005, 20736002), National Basic Research Program of China

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(2007CB209706), Beijing Novel Program (2006B17), and NCET Program (NCET-06-0095) from the Ministry of Education of China.

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High-Capacity Hydrogen Storage in Porous Aromatic Frameworks with

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