Insights into the H2/CH4 Separation Through Two-Dimensional Graphene Channels: Influence of Edge Functionalization
© Xu et al. 2015
Received: 2 December 2015
Accepted: 14 December 2015
Published: 23 December 2015
A molecular simulation technique is employed to investigate the transport of H2/CH4 mixture through the two-dimensional (2D) channel between adjacent graphene layers. Pristine graphene membrane (GM) with pore width of 0.515~0.6 nm is found to only allow H2 molecules to enter rather than CH4, forming a molecular sieve. At pore widths of 0.64~1.366 nm, both H2 and CH4 molecules could fill into the GM channel, where the permeability of methane is more preferential than that of hydrogen with the largest CH4/H2 selectivity (1.89) at 0.728 nm. The edge functionalization by –H, –F, –OH, –NH2, and –COOH groups could significantly alter gas permeability by modifying the active surface area of the pore and tuning attractive and/or repulsive interaction with molecules at the entrance of channel. At the pore width of 0.6 nm, the H2 permeability of molecular sieve is enhanced by –H, –F, and –OH groups but restrained by –NH2, especially –COOH with a passing rate of zero. At pore widths of 0.64 and 0.728 nm, both –H and –F edge-functionalized GMs show a preferential selectivity of methane over hydrogen, while the favorable transport for GM–OH is changed from H2 molecules at 0.64 nm to CH4 molecules at 0.728 nm. For GM–NH2, it exhibits an excellent hydrogen molecular sieve at 0.64 nm and then turns into a significant H2/CH4 selectivity at 0.728 nm. Meanwhile, small H2 molecules start to enter the channel of GM–COOH at the pore width up to 0.728 nm. For the largest pore width of 1.336 nm, the influence of edge functionalization becomes small, and a comparable CH4/H2 selectivity is observed for all the considered membranes.
In the past few decades, membrane separation technologies exhibit many fascinating properties including low energy consumption, facile operation, and high cost effectiveness and thus have attracted much research attention [1–3]. Among various membrane materials, graphene-based materials have the two-dimensional (2D) carbon sheets with large surface area, chemical stability, mechanical robustness, and high impermeability, and thus, they are considered as one of the most potential classes of separation membranes [4–11]. Because of the high impermeability, the perfect graphene is a good barrier layer for gases and liquids. For molecular separation, therefore, a graphene-based membrane needs to be functionalized with nanopores or nanochannels.
For the porous membranes, the selective molecular permeation could be enabled by opening and controlling the holes on the 2D graphene sheet. In the real-word, however, it is extremely difficult to fabricate a large-area monolayer graphene material with controllable and uniform high-density nanopores. Alternatively, we could prepare a separation membrane with stacked 2D graphene sheets, and the gas molecules could selectively permeate through the 2D channels by controlling the interlayer spacing. Now, the interlayer channel size could be tuned by oxidation  and intercalating different-sized cross-linking molecules [12, 13], nanoparticles , and nanowires .
The 2D channels between two stacked graphene nanosheets may allow the special molecules to pass through but reject the unwanted molecules. For example, Qiu et al. reported that the nanochannels within chemically converted graphene sheets can be controlled by hydrothermal treatment, leading to the selective passing of water and small metal nanoparticles . Nair et al. found that the submicrometer-thick graphene oxide membranes could completely reject liquids, vapors, and gases (He, Ar, H2, and N2), but water is allowed to permeate facilely . They suggested that this seemingly incompatible phenomenon is attributed to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets . Recently, Li prepared an ultrathin 1.8-nm-thick graphene oxide membrane by a facile filtration method, which exhibits the ultra-high selectivity for H2/CO2 and H2/N2 mixtures . In addition, the permeability and selectivity of graphene-based membranes to ions and molecules have also been investigated in aqueous media .
To explore the mechanism of molecular transport, molecular dynamic (MD) simulation, as a powerful tool, has been employed. For 2D graphene channels, MD simulation showed that the water molecules cannot fill the 2D graphene capillaries with interplanar distance below 0.6 nm, whereas the capillaries with interplanar distance ranging from 0.6 to 1.0 nm could be filled by one and two layers of water molecules . Vieira-Linhares and Seaton studied the transport mechanism of H2/CH4 mixture in the 2D graphite channels by a combination of grand canonical molecular dynamics (GCMC) and dual control volume GCMC (DCV-GCMD) methods . They found the 2D channels show a sieving effect below 0.6 nm, significant selective adsorption of methane between about 0.63 and 1 nm, and poorer separation bigger than 1.0–1.2 nm . Furukawa and Nitta investigated that the permeation of pure and mixed gases (CH4 and C2H6) across carbon membranes with the pore shapes of diamond, zigzag, and straight paths by the nonequilibrium molecular dynamics simulations (NEMD) . Xu et al. performed NEMD simulations on the effect of temperature on the transport and separation of the CO2/CH4 gases through a 2D carbon nanopore . By the NEMD technique, MacElroy and Boyle studied the effect of pressures on the transport of binary H2/CH4 mixtures through a model slit carbon membrane . Recently, Jin et al. reported the flow of methane in the nanochannels of graphite layers at low and high pressures by DCV-GCMD simulations .
Theoretical investigations suggested that the nature of the functional groups at the edges of the pores plays a key role in the selective molecular separation for the porous single-layer graphene [6, 24, 25]. Nevertheless, to our best knowledge, the investigation of the influence of the functionalization at the sheet edge on the molecular permeation and separation is rather scarce for the 2D graphene channels. In this work, a molecular simulation technology is used to systematically study the effect of edge functionalization on the H2/CH4 separation through 2D graphene channels with width of 0.515~1.366 nm.
MD calculations were performed using the Discover code in Materials Studio of Accelrys Inc. . The interatomic interactions are described by the force field of a condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS) . COMPASS is a first ab initio force field, which has presented a good reliability in describing the absorbate–adsorbent interaction for the gas molecules in carbon-based materials [24, 27]. The cutoff distance for truncation of the intermolecular interactions was set to 1.28 nm, and the Ewald sum technique was used to calculate the electrostatic interaction. The Andersen thermostat method was used to control the temperature of the system at 300 K . MD simulations were carried out in the canonical (NVT) ensemble. During the simulations, the time steps were set to 1 × 107, with a fixed time step of 1 fs. Data was collected every 5 ps.
Electron density was calculated using PBE functional with the double-ξ numerical polarization (DNP) basis set, which was implemented in the DMol3 code in the Materials Studio of Accelrys Inc. [30, 31]. The tolerances of energy, gradient, and displacement convergence were 1 × 10−5 hartree, 2 × 10−2 hartree/nm, and 5 × 10−4 nm, respectively.
where x i (x j ) is the mole fraction of component i (j) entering the pore and y i (y j ) are the mole fractions of component i (j) in the gas phase.
Results and Discussion
Pristine Graphene Membranes
W = 0.515 − 0.6 nm
Number of gas molecules entering the channel of GM and corresponding CH4 selectivity
W = 0.64 − 1.366 nm
At the pore width of 0.64 nm, CH4 molecules start to enter the channel. At 10 ns, 86 CH4 molecules have diffused into the pore (Table 1). For H2, the amount of entered molecules increases to 74 at 0.64 nm. The corresponding selectivity of CH4 over H2 is found to be 1.39. At the pore width of 0.728 nm, the amounts of entered CH4 and H2 molecules increase to 145 and 77, respectively. The CH4/H2 selectivity also increases to 1.88 at 0.728 nm. However, when the pore size further increases to 0.855, 1.111, and 1.366 nm, the amount of entered CH4 molecules gradually decreases to 129, 121, and 110, respectively, although more and more H2 molecules (80, 82, and 86) diffuse into the channel. Correspondingly, the CH4/H2 selectivity decreases to 1.61, 1.47, and 1.28 for the pores of 0.855, 1.111, and 1.366 nm, respectively. Therefore, it can be seen that with the increasing pore width from 0.515 nm to 1.366 nm, the number of H2 molecules entering the pore increases gradually, whereas a pronounced peak of CH4 is found at 7.28 Å with the maximum CH4/H2 selectivity of 1.89.
Similar to the pores of 0.515 and 0.6 nm, the concentration profiles suggest that most of H2 molecules, which go into the pore, could penetrate the membrane to the offside vacuum boxes at 0.64 − 1.366 nm (Fig. 2d-h). However, CH4 molecules entering the pore prefer to stay in the channel with few molecules passing into the offside vacuum box, especially the channel of 0.728 nm, indicating an optimal CH4-adsorbed pore of 0.728 nm. As a lightweight nonpolar molecule, the main interaction of H2 with graphene sheets is the weak Van der Waals terms. Therefore, H2 molecules could easily pass through the membrane without accumulation in the channel, and the H2 permeability is proportional to the width of the channel (active surface area). For CH4 molecules, the strong electrostatic interaction with graphene sheets results in the packing of methane in the channel.
For H2 molecules, the channels show a similar single and double potential energy well at the width of 0.515~0.64 and 0.728~1.366 nm, respectively (see Fig. 4a). However, all the energy wells are very shallow, which are less than 1.09 kJ mol−1. Therefore, H2 molecules are unfavorable to adsorb on the channel surfaces, and the main factor influencing H2 permeability is the active surface area (pore width) rather than the interaction energy.
Five functional groups were considered to explore the effect of edge groups on the transport properties of the channel, i.e., hydrogen (–H), fluorine (–F) hydroxyl (–OH), amine (–NH2), and carboxyl (–COOH). The pore widths of 0.6, 0.64, 0.728, and 1.336 nm are selected as models of sieving membrane, monolayer adsorbed membrane, optimum membrane, and bilayer adsorbed membrane, respectively.
W = 0.6 nm
Number of gas molecules entering the channel of edge-functionalized GM and corresponding CH4 selectivity
W = 0.64 nm
At the pore size of 0.64 nm, both H2 and CH4 molecules could enter the channel of pristine GM. When the edge is functionalized, the amount of H2 molecules entering the pore is enhanced for GM–H (79), GM–F (85), and GM–OH (87), whereas –NH3 and –COOH groups (69 and 0) play a negative role, similar to the situation at 0.6 nm. For the CH4 permeability, only –H and –F groups (90 and 105) play a positive role, while a negative influence is found for –OH, –NH2, and –COOH groups, especially –NH2 and –COOH, which still completely reject CH4 molecules to enter the channel. Therefore, we can find that after edge functionalization, the CH4/H2 separation selectivity for GM–H (1.14) and GM–F (1.23) is comparable with that for GM (1.20), but an inverse CH4/H2 selectivity (0.88) is presented for GM–OH. Furthermore, when the edges of GM are functionalized by the bigger –NH2 and –COOH groups, GM–NH2 becomes an H2 molecular sieve, while GM–COOH remains to completely prohibit the penetration of both H2 and CH4 molecules.
The profiles of interaction energy at 0.64 nm pore indicate that similar to the situation at 0.6 nm, interaction energy with H2 molecules at channel entrance is strengthened after edge functionalization by –F, –OH, and –H groups, while an energy barrier is formed at the entrance of –NH2 and –COOH functionalized membranes but lower than that at 0.6 nm (Fig. 5b). For CH4 molecules, it is interesting that an energy well is formed at the entrance of GM–F, GM–H, and GM (Fig. 5b), which may be caused by the strong electrostatic interaction between CH4 molecules and the polar edge of the membranes. Furthermore, the order of depth of energy well (GM–F > GM–H > GM) is in accordance with that of CH4 permeability. However, because of a steric effect, GM–OH, GM–NH2, and GM–COOH form a high energy barrier at the entrance to restrain the pass of CH4 molecules. Especially, the extra high energy barrier of GM–NH2 and GM–COOH results in a zero probability of CH4 penetration at 0.64 nm.
W = 0.728 nm
When the pore width increases to 0.728 nm, all of the pristine and functionalized GMs (–F, –H, –OH, –NH2, and –COOH) allow H2 molecules to pass. Compared to pristine GM, the amount of H2 molecules entering the channel increases to 81~85 for GM–H, GM–F, GM–OH, and GM–NH2, except GM–COOH, where only 11 H2 molecules could enter. For CH4 molecules, permeability of GM–F, GM–H, and GM–OH is up to maximum (142~145) at 0.728 nm, similar to GM (145). However, only eight CH4 molecules enter the pore of GM–NH2, even there is still no CH4 molecule diffusing into the pore of GM–COOH. As shown in Table 2, the CH4/H2 selectivity for GM–H, GM–F, and GM–OH (1.71 ~ 1.73) is slightly less than that for GM (1.88), while a much low value (0.10) is found for GM–NH2, indicating an inversely high H2 selectivity. In addition, GM–COOH can be as a molecular sieve at pore size of 0.728 nm to allow only H2 to pass.
The interaction energy between gas molecules and the membrane was also calculated at pore size of 0.728 nm (see Fig. 5c). As shown in Fig. 5c, the H2 energy barrier at the entrance of GM–NH2 disappears, according with the comparable permeability of H2 with GM, GM–H, GM–F, and GM–OH. In addition, the height of H2 energy barrier for GM–COOH has become lower than the energetic zero, suggesting the acceptability of H2 transport. For CH4 molecules, both GM–NH2 and GM–COOH form an energy barrier at the entrance of the channel. However, the barrier for GM–NH2 is relatively low and could be overcome by methane molecules, whereas the barrier for GM–COOH is still too high to be overcome.
W = 1.366 nm
At the large pore size of 1.366 nm, the values of H2 and CH4 permeability for both pristine and functionalized membranes fluctuate in a small region of 85~92 and 107~112, respectively, suggesting the influence of edge functionalization is slight for the large pore. As discussed above (see Fig. 4), for both H2 and CH4 molecules, the interaction energy profiles (i.e., along x axis) at 1.366 nm show a double potential well across the channel with location near the surfaces of membranes. Although edge functionalization has some effects on the potential well at the channel entrance (Fig. 5d), the large active surface area may play an important role in gas transport at big 1.336 nm pore and thus results in a comparable gas permeability for all the considered membranes.
The separation of binary H2/CH4 mixture through the 2D graphene channels has been investigated via molecular simulation calculations. The results show that for the pristine GM, the membrane with a pore width of 0.515~0.6 nm can be a molecular sieve, which allows small H2 molecules to enter but forbids large CH4 to pass. Although both H2 and CH4 molecules could transport into the 0.64~1.366 nm channels, a favorable selectivity of methane over hydrogen is observed, with a maximum value of 1.89 at 0.728 nm.
The edge functionalization of GM could modify the active surface area of the pore and tune attractive and/or repulsive interaction with molecules at the entrance of channel, which has a striking influence on the transport of H2/CH4 mixture. (1) At the small pore width of 0.6 nm, the edge modification by –H, −F, and –OH groups could improve the H2 permeability of molecular sieve, but a negative effect is observed for –NH3, especially –COOH, which completely prohibits hydrogen to penetrate. (2) A preferential CH4/H2 selectivity is found for GM–H and GM–F at pore widths of both 0.64 and 0.728 nm, respectively, while the selectivity for GM–OH changes form H2 molecules at 0.64 nm to CH4 molecules at 0.728 nm. For GM–NH2, it always favors transport of H2 at pores between 0.64~0.728 nm, as reflected by an excellent hydrogen molecular sieve property at 0.64 nm and a significant H2/CH4 selectivity at 0.728 nm. With the increasing channel of GM–COOH up to 0.728 nm, gas molecules begin to enter the pore but are restricted to small H2 molecules. When the pore width further increases to 1.336 nm, the influence of edge functionalization becomes weak, resulting in a comparable CH4/H2 selectivity for all the considered membranes.
This work was supported by the National Natural Science Foundation of China (21476264), Distinguished Young Scientist Foundation of Shandong Province (JQ201215), Shandong Provincial Natural Science Foundation (ZR2015BQ009), Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (BS2012NJ015), and the Fundamental Research Funds for the Central Universities (12CX02014A and 15CX08010A).
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