Article pubs.acs.org/JPCC
From Inorganic to Organic Strategy To Design Porous Aromatic Frameworks for High-Capacity Gas Storage Lan Huang, Xiaoping Yang, and Dapeng Cao* State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *
ABSTRACT: Developing high-capacity gas storage materials is still an important issue, because it is closely related to carbon dioxide capture and hydrogen storage. This work proposes a “from inorganic to organic” strategy, that is, using tetrakis(4bromophenyl)methane (TBM) to replace silicon in zeolites, to design porous aromatic frameworks (PAF_XXXs) with extremely high pore volume and accessible surface area, because the silicon atom in the silicon-based zeolites and the TBM ligand have the same coordination manner. Through the adoption of this strategy, 115 organic PAF_XXXs based on the inorganic zeolite structures were designed. These designed PAF_XXXs have the same topology with the corresponding matrix zeolites but possess significantly higher porosity than matrix zeolites. In general, the surface area, pore volume, and pore size of PAF_XXX are in the ranges of 4600−6000 m2/g, 2.0−7.9 g/cm3, and 10−55 Å, respectively. In particular, the hydrogen uptake of PAF_RWY reaches 5.9 wt % at 100 bar and 298 K, exceeding the DOE 2015 target (5.5 wt %) for hydrogen storage. Moreover, PAF_RWY is also a promising candidate for methane storage and CO2 capture, owing to its extremely high pore volume and accessible surface area.
1. INTRODUCTION Porous materials are of scientific and technological importance because of the presence of voids of controllable dimensions at atomic, molecular, and nanometer scales, enabling them to discriminate and interact with molecules and clusters.1 They are widely used as catalysts1 and adsorbents.2,3 Basically, porous materials include inorganic and organic, two major categories. Inorganic porous materials contain zeolites, activated carbon, aluminophosphate molecular sieves, etc. Zeolites were first found by Swedish mineralogist Cronstedt in 1756 and have become one of the most important inorganic porous materials. Approximately 40 natural zeolites and hundreds of synthetic zeolites have been discovered.4,5 Each structure is defined by different framework types with three-letter codes approved by the Structure Commission of the IZA (IZA-SC).5 Because of their microporous features with uniform pore dimensions, permeability of hydrocarbon, internal acidity, and thermal stability,6 zeolites have been widely used as adsorbents, catalysts, detergents, and so on. As pointed out by Olsbye et al., the cavity and pore size of zeolite catalysts control the product selectivity of conversion methanol to hydrocarbons.7 Therefore, the topological structures of porous materials play a key role in their properties and applications. Recently, organic porous materials, such as covalent organic frameworks (COFs), 2,8−14 metal−organic frameworks (MOFs),15−20 and conjugated microporous polymers, have been designed and synthesized experimentally. COFs are a new class of high hydrothermally stable organic porous materials © XXXX American Chemical Society
and hold the advantages of designability, controllable pore size, and high surface area.21 Compared to zeolites (pore size < 20 Å), COFs often have a much broader range of pore size from several angstroms to tens of nanometers, which enables COFs with exceptional abilities in gas adsorption and separation. For example, Babarao et al. found that COF-105 and COF-108 show exceptionally high CO2 storage capacity, even surpassing the experimentally measured highest CO2 uptake in MOF177.22 Furukawa and Yaghi also concluded that 3D COFs rival the best metal−organic frameworks in their uptakes and are promising adsorbents for hydrogen, methane, and carbon dioxide.11 Most recently, Ben et al. used tetrakis(4-bromophenyl)methane (TBM) as ligand to successfully synthesize a new class of porous aromatic frameworks (PAFs) with diamond-like structure through Yamamoto-type Ullmann cross-coupling reaction.23−25 The PAF-1 shows not only high Brunauer− Emmett−Teller (BET) surface areas of 5600 m2/g and high uptake of CO2 and H2 but also high hydrothermal stability.25 Surprisingly, as a basic unit of PAFs, the TBM ligand holds a similar coordination with silicon atoms in the silicon-based zeolites, where each silicon atom connects with four oxygen atoms to form a tetrahedral subunit. Received: December 24, 2014 Revised: January 22, 2015
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DOI: 10.1021/jp5128404 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Scheme 1. Design Strategy from Inorganic Zeolites to Porous Organic PAF_XXX Materials
Inspired by the similar coordination of TBM ligands with silicon atom in zeolites, in this work, we first proposed a novel strategy to design porous organic PAF materials based on inorganic zeolite structures. We then explored the adsorption properties of the newly designed PAF for hydrogen, methane, and carbon dioxide and recommended several high-performance gas storage materials. Finally, some discussion was also addressed.
Table 1. Force Field Parameters for Adsorbates and Adsorbents molecule CH4 H2 CO2 PAF_XXX
2. DESIGN STRATEGY AND COMPUTATIONAL DETAILS Scheme 1 shows the design strategy from inorganic structure to organic PAF materials. By using TBM ligand to replace the silicon atom in zeolites, we can obtain a series of porous organic PAF materials with a similar topology to the matrix zeolite. Apparently, the topological structures of PAFs are determined by the zeolites typess, so we adopted the zeolite topology type to name the newly designed materials as “PAF_XXX”. Totally, 115 new PAF_XXXs are constructed and optimized through COMPASS force field26 via forcite tools in Material Studio. In the grand canonical Monte Carlo (GCMC) simulations, we adopted the Dreiding force fields,27 which have been proved to be accurate to predict the gas adsorption in MOFs and COFs.28,29 The parameters are well-established and confirmed by previous works.29 Therefore, CH4 and H2 are modeled as a spherical united atom, whereas CO2 is modeled by a three-site model in which TraPPE force fields of Kaneko and Seaton et al.30 are adopted. The potentials were optimized to quantitatively reproduce the vapor−liquid equilibrium data of CO2. Note that, in the simulation of CO2 adsorption in PAF_XXXs, we considered only the electrostatic interactions in CO2 molecules and ignored the atomic charges of the PAF_XXXs, owing to the fact that there are no metal atoms in PAF_XXXs, which had been validated in our previous works.29 Finally, the Lorentz−Berthelot combining rules were used to calculate the cross-interaction parameters. The force field parameters for adsorbents and adsorbates are listed in Table 1. The GCMC simulations were performed to predict adsorption properties of PAF_XXXs. For CH4 and H2 represented by the spherical models, three types of moves (i.e., translation, insertion, and deletion) were attempted, wheras for CO2 an additional rotation move was also added. To avoid conversion of chemical potential into pressure, the normal move acceptance probability is transformed to relate with the component fugacity of bulk phase, which can be calculated by the Peng−Robinson equation of state (EOS).34
atom
ε/kb (K)
σ (Å)
q (e)
ref
C O C H
148.0 36.7 27.0 79.0 47.9 7.66
3.73 2.958 2.80 3.05 3.47 2.85
0 0 0.70 −0.35 0 0
31 32 33 27
The GCMC simulations give the results of absolute adsorption amount Nab, whereas the experimental adsorption isotherm is often reported in the excess adsorption amount Nex. Therefore, Nab is converted to the excess uptake (Nex) by Nex = Nab − ρbVpore, where ρb is the density of the bulk adsorbate calculated from the Peng−Robinson EOS and Vpore is the pore volume of the adsorbent accessible to the gas molecules. In this work, Vpore and pore size were calculated by the Zeo++ suite of codes.35 The accessible surface area (Sacc) is calculated with a probe radius of 1.8405 Å, which corresponds to the N2 molecule.36 The calculated results from Zeo++ are reasonable compared to previous simulation data of PAF-302. For example, the Sacc, pore size, and pore volume are 5962.8 m2/ g, 13.38 Å, and 2.636 cm3/g, respectively, whereas in our previous simulations, these values were 5735m2/g, 12.4 Å,37 and 2.34 cm3/g, respectively.29 Therefore, the same method was adopted to calculate the whole family of newly designed PAF_XXXs. All GCMC simulations were performed by the MUSIC code.38 Periodical boundary conditions were applied in all three dimensions, and the cutoff radius was set to 12.0 Å, the same as in our previous works.29 For each pressure point, 2 × 107 Monte Carlo trial moves were performed, of which the first half was used for equilibration and the second half for ensemble average. Detailed structures and parameters of these PAF_XXXs can be found in Table S1 in the Supporting Information.
3. RESULTS AND DISCUSSION Figure 1 shows the statistical analysis of pore size, accessible surface area (Sacc), density, and pore volume (Vpore) of all 115 PAF_XXXs constructed here. The pore size of these PAF_XXXs covers a range from 10 to 55 Å, and most PAF_XXX materials (56%) have pore sizes of about 20−30 Å. However, the pore size of the silicon-based zeolites with the same topology is often
