Article pubs.acs.org/JPCC
Mechanical, Electronic, and Adsorption Properties of Porous Aromatic Frameworks Binit Lukose, Mohammad Wahiduzzaman, Agnieszka Kuc,* and Thomas Heine School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany ABSTRACT: Diamond-like porous aromatic frameworks (PAFs) form a new set of materials that contain only carbon and hydrogen atoms within their frameworks. These structures have very low mass densities, large surface area, and high porosity. Density-functional-based calculations indicate that crystalline PAFs have mechanical stability and properties similar to those of covalent organic frameworks. Their exceptional structural properties and stability make PAFs interesting materials for hydrogen storage. Our theoretical investigations show exceptionally high hydrogen uptake at room temperature, which can reach 7 wt %, a value exceeding those of well-known metal- and covalent-organic frameworks (MOFs and COFs). For an exemplary PAF with long linkers (PAF-304), we have studied the effect of interpenetration on properties as mechanical stability and hydrogen storage.
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INTRODUCTION Porous materials have been widely investigated in the fields of materials science and technology due to their applications in many important fields, such as catalysis, gas storage and separation, template materials, molecular sensors, and so on.1−3 Designing porous materials is nowadays a simple and effective strategy following the approach of reticular chemistry,4 where predefined building blocks are used to predict and synthesize a topological organization in an extended crystal structure. The most famous and striking examples of such materials are metaland covalent-organic frameworks (MOFs and COFs).5,6 These new nanoporous materials have many advantages: high porosity and large surface areas, lowest mass densities known for crystalline materials, easy functionalization of building blocks, and good adsorption properties. Gas storage and separation by physical adsorption are very important applications of such nanoporous materials and have been major subjects of science in the last 2 decades. These applications are based on certain physical properties, namely, presence of permanent large surface area and suitable enthalpy of adsorption between the host framework and guest molecules. Attempts to produce materials with large internal surface area have been successful, and some of the more notable accomplishments include the synthesis of MOF-2107 (BET surface area of 6240 m2 g−1 and Langmuir surface area of 10 400 m2 g−1), NU-1008 (BET surface area: 6143 m2 g−1), and 3D COFs9 (BET surface area: 4210 m2 g−1 for COF-103). More recently, a new family of diamond-like porous materials emerged. So-called porous-aromatic frameworks (PAFs) have surface areas comparable to MOFs; for example, PAF-110 has a BET surface area of 5640 m2 g−1 and Langmuir surface area of 7100 m2 g−1. Because they are composed of carbon and hydrogen only, they have several advantages over frameworks containing heavy elements. MOFs with coordination bonds often suffer from low thermal and hydrothermal stability, which © 2012 American Chemical Society
might limit their applications on the industrial scale. The coordination bonds can be substituted with stronger covalent bonds, as it was realized in the case of COFs;6 however, this lowers significantly their surface areas compared with MOFs. Besides its exceptional surface area, PAF-110 has high thermal and hydrothermal stability and appears to be a good candidate for hydrogen and carbon dioxide storage applications. Experimentally obtained PAF-110 and PAF-1111 have one and four phenyl rings, respectively, in the linkers that are connected by tetragonal centers (carbon tetrahedral nodes). Lan et al. designed four PAFs (PAF-301, 302, 303, and 304) by replacing C−C bonds of diamond by phenyl, biphenyl, p-terphenyl, and quaterphenyl linkers, respectively. The hydrogen adsorption capacities of PAF-303 and 304 exceeded those of COFs.12 PAF301 and 304 correspond to the synthetically made PAF-1 and 11, respectively. In this article, we have studied structural, electronic, and adsorption properties of PAFs using the density functional based tight-binding (DFTB) method13,14 and quantized liquid density functional theory (QLDFT).15 We have found exceptionally high storage capacities at room temperature which makes PAFs very good materials for hydrogen storage devices beyond well-known MOFs and COFs. We have compared our results to the widely used classical grand canonical Monte Carlo (GCMC) calculations reported in the literature. We have also studied other properties of these materials, such as structural, energetic, electronic, and mechanical. We explored the structural variance of diamond topology by individually placing a selection of organic linkers between carbon nodes. This generally changes surface area, Received: July 6, 2012 Revised: October 8, 2012 Published: October 8, 2012 22878
dx.doi.org/10.1021/jp3067102 | J. Phys. Chem. C 2012, 116, 22878−22884
The Journal of Physical Chemistry C
Article
Figure 1. (a) Schematic diagram of the topology of PAFs: blue spheres and gray pillars represent carbon tetragonal centers and organic linkers, respectively. (b) Linkers used for the design of PAFs: (i) phenyl, (ii) biphenyl, (iii) pyrene, (iv) DPA, (v) p-terphenyl, (vi) PTCDA, and (vii) quaterphenyl.
Table 1. Calculated Cell Parameters a (a = b = c, α = β = γ = 60°), Mass Densities (ρ), Formation Energies (EForm), Band Gaps (Δ), Bulk Moduli (B), and H2-Accessible Free Volume and Surface Area of PAFs Studied in the Present Worka
a
PAFs
a (Å)
ρ (g cm−3)
EForm (kJ mol−1)
PAF-phnl (PAF-301) PAF-biph (PAF-302) PAF-pyrn PAF-DPA PAF-ptph (PAF-303) PAF-PTCDA PAF-qtph (PAF-304)
9.7 16.7 16.6 21.0 23.7 23.6 30.8
0.85 0.32 0.42 0.19 0.16 0.24 0.10
−121 −122 −124 −122 −119 −122 −119
Δ (eV) 4.7 3.6 2.6 3.5 3.2 1.8 2.9
B (GPa)
H2 accessible free volume (%)
H2 accessible surface area (m2 g−1)
36.0 13.2 19.2 8.7 5.6 9.5 3.5
35 73 66 84 86 81 91
2398 5697 5090 7240 6735 5576 7275
(5.5) (4.0) (2.8) (3.7) (3.3) (1.9) (3.0)
In parentheses are given HOMU-LUMO gaps of the corresponding saturated linkers.
parameters have been determined by fitting first-principles calculations at the second-order Møller−Plesset (MP2) level of theory using the quadruple-ζ QZVPP basis set with correction for the basis set superposition error (BSSE). QLDFT calculations use a grid spacing of 0.5 Bohr radii and a potential cutoff of 5000 K.
mass density, and isosteric heat of adsorption, which is reflected in the adsorption isotherms.
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METHODS Using a conjugate-gradient method, we have fully optimized the crystal structures (atomic positions and unit cell parameters) of PAFs. The calculations were performed using dispersioncorrected self-consistent charge DFTB method, as implemented in the deMonNano code.16,17 Periodic boundary conditions were applied to a (3 × 3 × 3) supercell, thus representing frameworks of the crystalline solid state. Electronic density of states (DOS) have been calculated using the DFTB+ code18 with k-point sampling, where the k space was determined by reaching convergence for the total energy according to the scheme proposed by Monkhorst and Pack.19 The bulk modulus (B) of a solid at absolute zero can be calculated as B=V
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RESULTS AND DISCUSSION Design and Structure of PAFs. Following Lan et al.,12 we have designed a set of PAFs by replacing C−C bonds in the diamond lattice by a set of organic linkers: phenyl, biphenyl, pyrene, para-terphenyl, perylenetetracarboxylic acid (PTCDA), diphenylacetylene (DPA), and quaterphenyl. (See Figure 1.) We label the respective crystal structures as PAF-phnl (PAF301 in ref 12), PAF-biph (PAF-302 in ref 12), PAF-pyrn, PAFptph (PAF-303 in ref 12), PAF-PTCDA, PAF-DPA, and PAFqtph (PAF-304 in ref 12). Such a design of frameworks should result in materials with high stability due to the parent diamond-topology and pure covalent bonding of the network. The selected linkers differ in their length, width, and the number of aromatic rings. These should play an important role for hydrogen adsorption properties: aromatic systems exhibit large polarizabilities and interact with H2 molecules via London dispersion forces. Long linkers introduce high pore volume and low weight to the network, whereas wide linkers offer large internal surface area and high heat of adsorption. Hence, long linkers are of advantage for high gravimetric capacity, whereas wide linkers enhance volumetric capacity. Proper optimization of the linker size should result in a perfect candidate for hydrogen storage applications. Selected structural and mechanical properties of the investigated PAF structures are given in Table 1. Frameworks created with the above-mentioned linkers have mass densities that range from 0.85 g cm−3 (for phenyl) to 0.1 g cm−3 (for
d2E dV 2
where V and E are the volume and energy, respectively. The calculated total energy as a function of volume was fitted to calculate the bulk modulus. Hydrogen adsorption calculations have been carried out using QLDFT within the LIE-0 approximation,15 thus including many-body interparticle interactions and quantum effects implicitly through the excess functional. The PAF-H2 nonbonded interactions were represented by the classical Morse atomic-pair potential. Force-field parameters were taken from Han et al.,20 who originally developed them for studying hydrogen adsorption in COFs that incorporate similar building blocks as PAFs. The authors adopted the approach to the shapeless particle approximation of QLDFT. These PAF-H2 22879
dx.doi.org/10.1021/jp3067102 | J. Phys. Chem. C 2012, 116, 22878−22884
The Journal of Physical Chemistry C
Article
quaterphenyl). The lowest mass-density obtained for a crystal structure was for COF-108 with ρ = 0.171 g cm−3.9 Our results show that PAF-DPA and PAF-ptph have mass densities close to the one of COF-108, whereas PAF-qtph with ρ = 0.1 g cm−3 exhibits the lowest ρ for all PAFs investigated in this study. Whereas the large cell size and the small mass density of PAF-qtph are an advantage for high gravimetric hydrogen storage capacity, other PAFs (e.g., PAF-pyrn with wide linker) would compromise gravimetric for high volumetric capacity. Because both of them are important for practical applications, a balance between them is crucial. Energetic and Mechanical Properties. We have investigated energetic stability of PAFs by calculating their formation energies. We regarded the formation of PAFs as dehydrogenation reaction between saturated linkers and CH4 molecules. For a unit cell containing n carbon nodes and m organic linkers, the formation energy (Eform) is given by
Figure 2. Calculated bulk modulus B as a function of the volume of the material. PAFs are highlighted in blue and compared with other framework materials. Red curve denotes the fitting to a 1/V function (V, volume).
Eform = Ecell + 4nE H 2 − (mE L + nECH4)
More importantly, the values of band gaps are directly related to the HOMO−LUMO gaps of the corresponding saturated linkers, which are just slightly larger. Also, more extended linkers (cf. PAF-biph and PAF-pyrn or PAF-ptph and PAFPTCDA) reduce the band gap. In general, conjugated rings reduce the band gap more effectively than C−C bond networks (cf. PAF-DPA, PAF-ptph, or PAF-qtphl). The extreme cases for this behavior are graphene, which is metallic, and diamond, which is an insulator. Overall, the band gap may be tuned by placing suitable linkers in the diamond network. Similar results have been reported for MOFs.26,27 We have calculated the electronic DOS for all PAF structures. Figure 3a shows the carbon DOS of PAFs arranged in such a way that the band gaps decrease going from the top to the bottom of the Figure. PAFs with larger band gaps have a larger population of energy states at the top of valence band and bottom of conduction band, whereas for linkers with smaller band gaps, the distribution of energy states is rather spread. To study the impact of sp3 carbon atoms in the DOS, we plotted their contributions against the total carbon DOS in the case of PAF-phnl and PAF-pyrn as examples. (See Figure 3b,c). In both cases, the sp3 carbons exhibit no energy states in the band gap and in the close vicinity of band edges. Hydrogen Adsorption Properties. One of the potential applications of PAFs is hydrogen adsorption. We have calculated the gravimetric and volumetric capacities and analyzed them to understand the contributions of the linkers on the hydrogen uptake in a different range of temperatures and pressures. H2-accessible free volume and surface area of the PAFs are given in Table 1. The free pore volume is necessary to assess the excess adsorption capacity. In our simulation, the free pore volume is defined to be that where the H2−host interaction energy is
