Hexagonal Boron Nitride with Designed Nanopores as a High

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Hexagonal Boron Nitride with Designed Nanopores as a High Efficiency Membrane for Separating Gaseous Hydrogen from Methane Yadong Zhang, Qi Shi, Yuzhen Liu, Yun-hui Wang, Zhaoshun Meng, Chuanyun Xiao, Kaiming Deng, Dewei Rao, and Ruifeng Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04918 • Publication Date (Web): 10 Aug 2015 Downloaded from http://pubs.acs.org on August 15, 2015

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The Journal of Physical Chemistry

Hexagonal

Boron

Nitride

with

Designed

Nanopores as a High Efficiency Membrane for Separating Gaseous Hydrogen from Methane Yadong Zhang,† Qi Shi,† Yuzhen Liu,† Yunhui Wang,† Zhaoshun Meng,† Chuanyun Xiao,† Kaiming Deng,† Dewei Rao,*,‡ and Ruifeng Lu*,† †

Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, P R China

‡

Institute for Advanced Materials, Jiangsu University, Zhenjiang 212013, P R China

ABSTRACT: Using first-principles calculations and molecular dynamics simulations, we theoretically explored the potential applications of hexagonal boron nitride (h-BN) for H2/CH4 separation. The h-BN with appropriate pores possesses excellent H2/CH4 selectivity (larger than 105 at room temperature). Furthermore, the adsorption energies (0.1 eV more or less) of both H2 and CH4 on the designed monolayer membranes are sufficiently low to prevent the blocking of the nanopores in a realistic separating process. Particularly, we demonstrate a highly promising membrane (h-BN with a triangular pore and a N9H9 rim) with a calculated diffusion barrier of 0.01 eV for H2 diffusion, and the simulated flux of H2 across the single layer is as large as 4.0×107 GPUs at 300 K. Additionally, the estimated permeability of H2 significantly exceeds the industrial acceptable standard for gas separation over a broad temperature range. Therefore, our results suggest that porous boron nitride nanosheets will be applicable as new membranes for gas separation.

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Compared with traditional separation technologies, membrane separation1 is more efficient and promising because of its critical advantages: its low energy consumption and it is pollution-free.2 To date, various membrane materials have been widely applied for gas separation, including hydrogen purification. It is well-known that hydrogen is an environmentally friendly energy source, and it has drawn considerable attention in the past decades. However, current production methods in industry cannot produce H2 with a sufficiently high purity.3 For example, steammethane reforming,3,4 a common method of H2 production, inevitably contains methane in the mixture during the process of catalytic decomposition of CH4.5 Therefore, it is very important to separate H2 from other less desirable gases. The traditional methods for gas separation, including pressure swing adsorption and fractional/cryogenic distillation, not only require a large amount of energy but also lead to environment problems. Although membrane separation can remedy these issues, the overall performance of the present well-known membrane materials, such as zeolite,6 silica,7 and organic polymer membranes,8,9 is still limited. To evaluate the gas separation membranes, there are two important indicators: selectivity and permeability,10 which are difficult to improve simultaneously. The ideal gas separation material should be thin and have uniform pores to permeate the desired gases. Additionally, it should be mechanically strong to sustain the pressure differentials required for permeation. Recently, graphene,10-14 its derivatives,15−18 and other one-atom-thick materials with high stability and subnanometer pores have received much attention in gas separation. Li et al.10 reported that ultrathin graphene oxide (GO) membranes with a thickness of approximately 1.8 nm have high separation selectivities (∼103) for H2 from CH4 and N2. However, the measured H2 permeance of a GO membrane is 300 GPUs (1 GPU = 3.35×10–10 mol·m–2s–1Pa–1), which is attributed to their low-density and random-distributed defects (or pores).16,19 Recently, an H2


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permeation of almost 8 orders-of-magnitude across the atomically thin and porous graphene has been obtained by Park and coworkers,18 whereas the gaseous H2/CO2 selectivity is below 10. Most recently, Peng et al.19 fabricated 1-nm-thick molecularly sieved membranes using metalorganic framework nanosheets that have an excellent H2 permeance of near 3000 GPUs with H2/CO2 selectivity that is larger than 200. From the theoretical point-of-view, the design and the search for new membrane materials with a high permeability and a high selectivity has become a hot topic in the gas separation field. A large number of computational studies for two-dimensional (2D) membrane materials have demonstrated that the rim of pores can greatly influence the gas permeation and separation properties,20-22 specifically with the introduction of heteroatoms, such as N and B. Jiang et al.20 created nanoscale pores by removing two neighboring rings from graphene, and their calculations show extremely good productivity and efficiency for H2/CH4 separation across Nsubstituted pores. From density functional theory (DFT) calculations and/or molecular dynamics (MD) simulations, other 2D carbon materials with inherent uniform pores, such as synthesized polyphenylene23 and graphydiyne,24 exhibit potential applications in versatile gas separations with (or without) physical or chemical modifications. Among them, our previous work25 has shown that B or N-doping in polyphenylene can significantly improve the selectivity for H2/X (X = CH4, CO, CO2) separations. Hexagonal boron nitride (h-BN), a monolayer material that is called “white graphene,” can be fabricated via a reaction between boric acid and urea at high temperature.26-28 It has remarkable mechanical properties, and Coleman et al. found that h-BN that is exfoliated in liquids can be used as a sieve membrane.29,30 Ideal graphene31 and C6032 can completely block the penetration of all atoms and molecules under ambient conditions. Similarly, we assume that the



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gaseous molecules cannot penetrate through the perfect 2D h-BN structure, although a noticeable proton transport through monolayer h-BN has been achieved in recent experiments.33 Interestingly, pioneering experimental investigations claimed that in h-BN the pores with a dangling N rim are preferably formed as triangle shapes.34,35 The holes in h-BN can be drilled via either electron beam punching or chemical etching techniques, as employed in graphene. Inspired by the above studies, and based on the experimentally observed pores of h-BN, we designed three triangle nanopores with different rim terminations in h-BN for H2/CH4 separation, which are named N9H9, N9H3, and B9H9 and are displayed in Figures 1a, 1b, and 1c, respectively. State-of-the-art DFT calculations determined the adsorption energies of H2 and CH4 on the h-BN surfaces and the diffusion energy barriers for these molecules passing through the pores, whereas MD simulations investigated the diffusion and separation situations under realistic conditions. All of the adsorption energies of H2 and CH4 on the three designed h-BN membranes are very low and will not block the nanopores in practical processes. The selectivities of N9H9, N9H3, and B9H9 for H2/CH4 separation at room temperature are on the order of 105, 1017, and 1052, respectively, and the permeability properties of all three of the h-BN structures reach the industrially acceptable standard for gas separation above 280 K, and the N9H9 membrane has a high permeance of 4.0 × 107 GPUs (0.013 mol·m–2s–1Pa–1) and outperforms the other two in terms of H2 permeation. We propose that suitable nanopores endow h-BN materials with a satisfying trade-off that simultaneously possesses a high permeability of H2 and a high selectivity for H2 separation from CH4. In this work, first-principles DFT calculations were performed using the Vienna Ab-initio Simulation Package36 with the generalized gradient approximation of Perdew, Burke, and Emzerhof37 for the exchange correlation potential. To study the diffusion of gas molecules


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through the pores, the climbing image nudged elastic band (CI-NEB) method38 was used for the minimum energy pathway calculations,39 and the van der Waals correction was considered by the DFT-D2 approach.40 The energy cutoff was set to 500 eV, and the Brillouin zone was sampled with 3×3×1 Monkhorst-Pack k-point grids.41 The convergence criteria was 10–5 eV in energy, and all the structural relaxations were accomplished until the force on each atom reduced by less than 10–3 eV/Å. The vacuum space was set to 20 Å, which was large enough to avoid the interaction of periodic images. The MD simulations for H2 and CH4 permeations were conducted by employing the LAMMPS package42 in the constant-volume and constant-temperature (NVT) ensemble at 300 K and were controlled by the Nose-Hoover thermostat method with a time step of 1 fs. The number of permeated molecules was collected every 10 ps. The cutoff distance for the Lennard-Jones and Coulombic interaction was 12 Å. The long-ranged electrostatic interaction was calculated using the PPPM method.43 For the atoms of the N9H9 and B9H9 membranes, the Mulliken charges and the Lennard-Jones parameters were obtained from the literature,44 which have been verified in many studies on gas adsorption and separation. In accordance with prior experimental observations,34,35 we first studied the adsorption of H2 and CH4 on the triangular N-rim pores without any termination of h-BN. The C-H bond of CH4 dissociated around the pores, and the H atom from CH4 passivated the nearest nitrogen of the pore, which suggests that the N dangling bonds at the pores of the h-BN were not stable. Thereafter, we considered the pore triangles of the h-BN that was fully and partially terminated with H atoms. For the perfect h-BN sheet, the optimized lattice constant of the unit cell was determined to be 2.5 Å, which agrees well with previous works.45 However, for the porous h-BN structures with the supercell shown in Figure 1, the N-H bond length was determined to be 0.996



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Å, which was shorter than the 1.137 Å of B-H. In Figures 2a, 2b, and 2c, we present the energy profiles of H2 and CH4 diffusing through the pores with N9H9, N9H3, and B9H9 rims, respectively. Table 1 summarizes the calculated adsorption energies (Ead) of H2 and CH4 on three materials and the diffusion barriers. The Ead is defined as Ead = Eh− BN + Egas − Egas / h − BN , where EhBN,

Egas, and Egas/h-BN are the total energies of the isolated gas molecule, the monolayer h-BN

membrane, and the gas-adsorbed membrane, respectively. The diffusion energy barrier (Ebarrier) is defined as Ebarrier = ETS − EAS , where ETS represents the total energy of the transition state (TS) for a gas molecule diffusing through the h-BN pore, and EAS represents the adsorption state (AS) along the diffusion pathway. Generally, the calculated Ead values of both H2 and CH4 on the three types of pores are all less than 0.1 eV except for the adsorption energy of CH4 adsorbed on the N9H9 pore. Thus, the designed porous h-BN structures are good candidates for H2/CH4 separation because they will not block the pores in a practical diffusion process. Next, we found that the calculated Ebarrier of H2 across the N9H9 pore is only 0.01 eV, which is much smaller than the calculated values of graphdiyne (0.1 eV),24 graphene with hypothetical pores (0.22 eV),20 porous graphene (PG, 0.37 eV),46 and PG-ES1 (0.13 eV),47 suggesting a high permeation of H2 diffusing through the h-BN pores. Importantly, the H2 penetration with such low diffusion barriers across the pores is easily realized with negligible energy consumption. For CH4, the calculated Ebarrier across the N9H9 pore is 0.32 eV, which is sufficient to prevent the gas permeation across the membrane under mild conditions. For N9H3 and B9H9, the corresponding barriers are 0.26 eV and 0.49 eV for H2 diffusion, respectively, whereas they are 1.28 eV and 3.65 eV for CH4 diffusion, respectively. These values indicate that CH4 molecules cannot diffuse easily through these pores compared


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with H2 molecules at room temperature. Additionally, the large difference between the Ebarrier of H2 and CH4 indicates the large selectivity for separating H2 from CH4. To understand the separation property of the designed h-BN membranes, we computed the selectivity of H2/CH4 using the Arrhenius equation:

47

SH2

/ CH

4

=

(

A H2 exp − E A

CH

4

e x p (− E

H2

CH

/ RT 4

)

/ RT

)

, where

A is the diffusion prefactor, R is the gas constant, and T is the temperature. Here, we assume that the diffusion prefactor of H2 and CH4 are identical (

A

H

2

/

A

C

H

4

=

1

).46-48 Based on the

above equation, the selectivities of H2 over CH4 at room temperature (T = 300 K) for the N9H9, N9H3, and B9H9 membranes were calculated to be 1.49×105, 1.47×1017, and 8.0×1052, respectively. These values are significantly higher than those of silica membranes (103),7 graphdiyne (104),24 zeolite membranes (103),49 and the recently developed GO membranes (∼103).10 As emphasized in the introduction, the permeability of the desired gas is also an important factor in a membrane. In this work, the permeance is calculated via the following equation, P =F/∆p, in which F is the molar flux (mol m–2 s–1) of the gas and ∆p is the difference in partial pressures (Pa) of the measured gas. The flux is defined as F =N × f, where N represents the number of gas molecules colliding with the wall and f is the probability for a molecule to diffuse through the pore at a given velocity. In the formula of

N =

p A ×

(2 π

m k BT

)

1/ 2

, p, A, m, kB, and T

denote the pressure, Avogadro constant, mass of the molecule, Boltzmann constant, and temperature,

respectively.42

The

probability

is

found

using:

f =

∫

∞ vB

f ( v ) dv

,

where f ( v) represents the Maxwell distribution and the kinetic energy at vB can surmount the barrier. We assume that the partial pressure difference, ∆p, is 1 bar across the pore and the



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incoming pressure is 3 bar. Figure 3 displays the permeance of H2 across the proposed h-BN single-layered membranes. The highlighted line represents the industrially acceptable permeance for gas separation.50 As shown in Figure 3, the h-BN pore with the N9H9 rim exhibits the best permeability of H2 over a wide temperature range, and the N9H3 rim is superior to the B9H9 rim for H2 permeation. Even for the B9H9 rim, the H2 permeance exceeds the industrially acceptable standard above 280 K. Thus, we infer that the designed h-BN membranes are workable at ambient conditions. Further MD simulations were performed to examine the performance of the N9H9 membrane. Similar to previous works,51-53 gas molecules were placed between two N9H9 membrane walls, and the box had a size of 43.30×50.01×120 Å3 with a pore density of 0.73 nm– 2

. The membrane was frozen during the simulation, and the initial conditions were 25 atm and

300 K. At the beginning of the MD simulation, single-component 74 H2 (or 74 CH4) molecules were interspersed between the membranes, as shown in Figure 4 (Figure S2). The cross terms in the force fields were evaluated by the Lorentz-Berthelot mixing rule, and the detailed parameters are listed in Table S1.54-56 The system reached an equilibrium state in 2 ns, and some selected snapshots of H2 passing through the N9H9 membranes are provided in Figure 4. Thus, we found that when more time elapses, more H2 molecules cross the pores. Figure 5 shows the number profiles of the penetrated molecules as a function of time for H2 and CH4, from which we estimated the gas permeation rate. From the inset of Figure 5, the forward H2 flux was 37 molecules when the equilibrium was approached at approximately 100 ps, corresponding to a high value of 4.0 × 107 GPUs. The MD confirmed that the porous h-BN nanosheet with the N9H9 rim is an excellent H2/CH4 separation membrane. As shown in Figure 5, no CH4 molecule can penetrate the N9H9 pore, which has also been demonstrated by LAMMPS simulations for


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the mixed gases of H2 and CH4 between the N9H9 membrane, as displayed in Figure S3 and Figure S4. In conclusion, we theoretically proposed that h-BN materials with experimentally produced triangular pores are highly efficient monolayer membranes for H2/CH4 separation. The suggested porous h-BN nanosheets can fulfill the requirements of both a high permeability of H2 and a high selectivity of H2 compared with CH4. The DFT calculations accounted for the weak dispersion, and we used accurate CI-NEB TS search methods to provide convincing results. The MD simulations unanimously established the outstanding performance for the practical H2/CH4 separation using thin h-BN materials with N9H9 pores, and the high H2 permeation is attributed to the very small adsorption energy and barrier for free H2 diffusion. Moreover, the adsorption energy of CH4 is important for preventing blockage of the pores. In view of the recent breakthroughs in molecular-sieving membrane studies, this work will be of broad interest and will guide the relevant theoretical and experimental efforts for developing more 2D membranes for gas separation.

ASSOCIATED CONTENT Supporting Information. The LJ potential parameters for molecular dynamics simulations; Electron density for H2 diffusing through pores from first-principles calculations; Permeation of single-component CH4, permeation and snapshots of the H2 and CH4 mixture through the N9H9 membrane; Cartesian coordinates of pore structures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author



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*E-mail: [email protected]; [email protected]. Telephone: +86-511-88783268; +86-2584315882. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by NSF of China Grant (21373113, 21403111), Fundamental Research Funds for the Central Universities (30920140111008, 30920140132037), Jiangsu Province Science Foundation for Youths (BK2012394, BK20140526), China Postdoctoral Science Foundation funded project with Grant No. 2014M561576, and the Research Foundation for Advanced Talents of Jiangsu University with Grant No. 13JDG100. REFERENCES (1) Sedigh, M. G.; Onstot, W. J.; Xu, L.; Peng, W. L.; Tsotsis, T. T.; Sahimi, M. Experiments and Simulation of Transport and Separation of Gas Mixtures in Carbon Molecular Sieve Membranes. J. Phys. Chem. A 1998, 102, 8580−8589. (2) Kluiters, S. C. A. Status Review on Membrane Systems for Hydrogen Separation. Energy Research Centre of the Netherlands: Petten, The Netherlands, 2004. (3) Ockwig, N. W.; Nenoff, T. M. Membranes for Hydrogen Separation. Chem. Rev. 2007, 107, 4078−4110. (4) Ockwig, N. W.; Nenoff, T. M. Membranes for Hydrogen Separation (additions). Chem. Rev. 2010, 110, 2573−2573. (5) Guo, X. G.; Fang, G. Z.; Li, G.; Ma, H.; Fan, H. J.; Yu, L.; Ma, C.; Wu, X.; Deng, D. H.; Wei, M. M. et al. Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen. Science 2014, 344, 616−619. (6) Cui, Y.; Kita, H.; Okamoto, K. Preparation and Gas Separation Performance of Zeolite T Membrane. J. Mater. Chem. 2004 , 14 , 924–932. (7) Oyama, S. T.; Lee, D.; Hacarlioglu, P.; Saraf, R. F. Theory of Hydrogen Permeability in Nonporous Silica Membranes. J. Membr. Sci. 2004, 244, 45–53. (8) Ho, B. P.; Chul, H. J.; Young, M. L.; Anita, J. H.; Steven, J. P.; Stephen, T. M.; Elizabeth, V. W.; Benny, D. F.; David, J. C. Polymers with Cavities Tuned for Fast Selective Transport of Small Molecules and Ions. Science 2007, 318, 254−258. (9) Dong, J. H.; Lin, Y. S.; Kanezashi, M.; Tang, Z. Microporous Inorganic Membranes for High Temperature Hydrogen Purification. J. Appl. Phys. 2008, 104, 121301–121317.


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Table 1. Adsorption energies (Ead in eV) of the computed gas molecules on the N9H9, N9H3 and B9H9 membranes and the energy barriers (Ebarrier in eV) of the diffusion of the gases through the pores. Membrane N9H9 N9H3 B9H9

Property Ead Ebarrier Ead Ebarrier Ead Ebarrier

H2 0.081 0.014 0.069 0.258 0.053 0.487

CH4 0.125 0.322 0.098 1.280 0.090 3.636

Figure 1. Geometric structures and their lattice parameters (values in parenthesis) used in this paper: (a) N9H9-rim, (b) N9H3-rim, (C) B9H9-rim (H, white balls; B, pink balls; N, blue balls).


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Figure 2. Diffusion energy barrier profiles for H2 and CH4 interacting with the pores of (a) N9H9, (b) N9H3 and (c) B9H9. The insets correspond to the process of H2 diffusing through the pore: initial state (IS), transition state (TS), adsorption state (AS).



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Figure 3. Permeance as a function of temperature for the H2 molecules diffusing through the pores of N9H9, N9H3 and B9H9.

Figure 4. Snapshots of the H2 molecules passing through the N9H9 membrane in the 0 to 5000 ps MD simulations at 300 K. Green, blue, and pink balls represent H, N and B atoms, respectively.


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Figure 5. Single-component gas permeation through the N9H9 membrane from the MD simulations at 300 K.

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Hexagonal Boron Nitride with Designed Nanopores as a High

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