- Email: [email protected]
Highly selective 3D porous graphene membrane for organic gas separation derived from polyphenylene
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Highly selective 3D porous graphene membrane for organic gas separation derived from polyphenylene Hao Jiang a,*, Xin-Lu Cheng b,** a b
General Education Department, Sichuan Police College, Luzhou, 646000, China Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, 610065, China
highlights
graphical abstract
A CMS with elliptical cylinder channels
separates
gas
with
similar molecular size. The gas selectivity is effectively ameliorated by fine-tuning the CNT structure. The selectivity of ethylene over acetylene (methane) is about 13.8 (5.5).
article info
abstract
Article history:
In this paper, a 3D nanoporous carbon molecular sieve (CMS) membrane is proposed to
Received 27 January 2019
investigate the diffusion and separation properties of ethylene/methane and ethylene/
Received in revised form
acetylene binary mixtures permeating through the structural deformated carbon nanotube
19 July 2019
(CNT) channels. Combining the results obtained from density functional theory (DFT)
Accepted 23 July 2019
calculations and molecular dynamics (MD) simulations, we find that the organic gas
Available online 16 August 2019
permeability and selectivity can be effectively ameliorated by fine-tuning the geometric structure of CNTs gas separation channels. By virtue of the intrinsic structural character-
Keywords:
istics, this hybrid CMS configuration established elliptical cylinder channels to separate the
Carbon molecular sieve membrane
organic gas molecules with similar molecular size. Compared with channels with a circular
Gas separation
cross section, the gas selectivity for channels with an elliptical cross section is larger, and it
Structural deformation
increases with an increasing pressure. The selectivity of ethylene over acetylene (methane)
DFT calculations
increased to ~13.8 (5.5) in deformed CNTs channels, which is more than doubled over the
MD simulations
original CNT channels. This distinguished hybridization configuration may pave a promising avenue to utilize gas separation materials. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Jiang), [email protected] (X.-L. Cheng). https://doi.org/10.1016/j.ijhydene.2019.07.178 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
24268
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
Introduction Ethylene is one of the most essential feedstocks for the synthesis of many polymers and organics in industrial production [1,2]. In general, ethylene is mostly in the organic gas mixtures containing methane, acetylene, ethane and other hydrocarbon molecules. In order to obtain ethylene gas of single component, diversified approaches and materials has been researched [3e5]. Compared with conventional gas separation methods (e.g. catalytic cracking and distillation, etc.), membrane-based separation technology has demonstrated outstanding advantage in convenient operation [6e8]. Among various membrane materials implemented for selective gas separation, the performances of CMS membranes are extremely eye-catching. Although these organic gas molecules are of a similar kinetic diameter to ethylene, plenty of CMS membranes with intrinsic apertures have displayed impressive competency for effective organic gas separation. For example, Liang et al. reported the ethane/ethylene adsorption selectivity with a series of asphaltebased activated carbons (A-ACs) in which the highest selectivity for typical cracked gas mixture (ethylene: ethane ¼ 15:1) can reach up to 16.3 [9]. Salinas et al. developed a CMS membrane derived from PIM-6FDA-OH which possesses an ethylene/ethane (ethylene: ethane ¼ 1:1) selectivity of 15.6 at P ¼ 20 bar and T ¼ 308 K [10]. Kim et al. also reported propylene/propane separation selectivity through CMS membrane which can reach an 18.0 at P ¼ 2 bar and T ¼ 298 K [11]. The organic gas separation properties in these paradigms are predominantly dependent on the size of gas molecules and geometric configuration of nanochannels, where an effective aperture >4 A is required [12e15]. Nevertheless, the stability of the molecular sieve can be severely affected when the nanopores in CMS are too large or dense. More rational types, shapes and sizes of nanopores are needed to break the permeabilityselectivity trade-off for fast and precise organic gas separation [16,17]. The usage of CNT-graphene configurations as gas permeation and purification materials opened up a novel orientation to devise eximious composite membrane system for gas separation. The resultant CNTs-graphene nanohybrid networks are found to have excellent separation selectivity for many gas mixture systems due to the distinctive interfacial morphology [18e20]. This versatile 3D framework retains the hierarchical structural characters of CNTs and the intrinsic physical properties of graphene. In this configuration, the CNTs are employed as molecule-selective tubular micropore channels with well-controlled tube size and graphene networks are applied as gas isolation membranes with modest adsorption energy. The discrepancy of interactions between gas molecules and CNTs determines which species is preferably transported through the tubular channels. The highly ordered nanochannels in this robust molecular-sieving membrane provide a novel solution to the stability problem which caused by the pore density and size. Inspired by the purification of gas mixtures with CNTgraphene molecular-sieving membrane, a 3D nanoporous CMS composed of CNTs embedded in the AA stacked polyphenylene membranes is proposed in this article. Compared
with original graphene, this nanoporous graphene (NPG) membrane has many excellent intrinsic properties. First of all, because a part of C atoms are replaced by H atoms, the density of NPG dramatically reduced compared to original graphene. More importantly, this NPG membrane can emerge different tensile strain along different orientation under the same uniaxial tensile stress. Interestingly enough, the vertical orientation would shrink, when the NPG membrane is stretched in one orientation [21]. The interaction between CNTs and NPG membranes caused the orifices of CNTs to change from circular to elliptical [22]. As mentioned above, the shape of nanopore plays an important role in the gas diffusion and separation process [23]. Elliptical pore can increase the permeability of linear gas molecules and reduce the permeability of spherical gas molecules. A detailed understanding of the gas permeability and selectivity in designed CNTs-NPG membrane during gas separation process is quite necessary. The structural deformation of CNTs in CNTs-NPG membrane may exploit a novel nanomaterial to achieve the efficient gas permeation and purification. Theoretical simulations are executed to elucidate the mechanisms and properties of organic gases separation in CNTs-NPG molecular sieve. In general, different organic gas molecules have different molecular mass, structure, and interaction with solid material surface. Hence, some representative linear (ethylene and acetylene) and spherical (methane) organic gases are selected as separation targets. MD simulations are implemented to discuss the selectivity of CNTs-NPG molecular sieve to organic gas mixtures (ethylene/methane and ethylene/acetylene). DFT calculations are carried out to calculate the interaction energy between organic gas molecule and molecular sieve during the gas separation process.
Models and methods The construction procedure of molecular sieve used in this article is as follows: the fundamental structural unit consists of a (6, 6) armchair type CNT fixed perpendicularly over AA stacked bilayer polyphenylene membranes. The polyphenylene membranes have an interlayer distance of 20 A. The link junction between CNT and NPG membrane created six hexagons and the same number of heptagons with a negative Gaussian curvature. Whereafter, the unit is expanded into a periodic molecular-sieving model consisting four CNTs. DFT calculations are performed to compute the interaction energy between the studied organic gas molecules and NPG membrane (CNT channel) during the gas separation process. The exchange-correlation energy of interacting electrons is handled by the GGA of PBE exchange correlation functional as implemented in Dmol3 module embedded in Materials Studio (MS) software and double numerical basis set with polarization function (DNP) is applied to increase the accuracy [24e27]. The convergence threshold of SCF is set to be 1 106 a.u to control the energy and electron density [28]. To speed up SCF convergence, the thermal smearing and the size of DIIS is specified as 5 103 Ha and 6, respectively [29]. In order to obtain high quality simulation results, the structure parameters of geometric optimization are specified as 1 105 Ha,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
2 103 Ha/ A and 5 103 A for total energy, atomic forces and maximum displacement, respectively. DFT þ D scheme [30] is employed to depict the weak van der Waals (vdW) interactions [31] between organic gas molecules and molecular sieve. The interaction energy between organic gas molecules and molecular sieve (Eb) is obtained from the following expression: Eb ¼ Etotal - ECNTs-NPG - Egas, where Etotal represents the total energy of the selected molecular sieve module with one organic gas molecule, ECNTs-NPG is the energy of original molecular sieve module, and Egas is the energy of one organic gas molecule. Simultaneously, a series of MD simulations are implemented to detect the organic gas molecules diffusion and separation characteristics in this molecular-sieving configuration. All the MD simulations are actualized in the NVT ensemble controlled by the Nose-Hoover thermostat algorithm [32] by using the LAMMPS code [33]. The interactions between substances are handled using the ReaxFF potential [34]. The height of the simulation box is 10 nm and the dimension of NPG membrane is 5.4 5 nm2. To avoid the spillover of organic gas molecules, the orientations parallel to the NPG membranes are implemented with periodic boundary condition, while the orientations perpendicular to the NPG membranes are limited with reflective wall boundary conditions. A nonequilibrium system is adopted, where the simulation box is divided into two chambers (feed side and permeate side) via a bichamber setup with the CNTs-NPG model in the middle. A given number of gas molecules (120 and 480 for 2 and 8 MPa, respectively, a molar ratio of 1:1 for every binary mixture) are stacked uniformly and alternatively on the upper surface (feed side) of the molecular sieve membrane before the simulations, as shown in Fig. 1(a)-(b). Each simulation is performed 1.0 107 equilibration steps at 300 K with each step taking 0.5 fs. During MD simulations, CNTsNPG molecular sieve is modeled as fully flexible structure. To avert the vertical displacement of the CNTs-NPG molecular sieve caused by the collisions with gas particles, one “central” carbon atom in NPG membrane on the feed side is immobilized. In this way, the vibration of other atoms in molecular sieve in response to collisions with gas molecules can be retained by this fixed "central" carbon atom. The quantities of permeated molecules are monitored every 2.5 ps. To examine the stability of CNTs-NPG molecular sieve during the gas separation process, the temperature and total energy of ethylene/methane gas separation system at P ¼ 2 MPa are revealed as a function of the simulation time, as shown in Fig. 1 (c).
Results and discussion Geometric characteristics of CNTs-NPG molecular sieve and verification of force field parameters As shown in Fig. 2(a), structural deformation emerged in the CNTs-NPG molecular sieve unit owing to the interaction between CNT and NPG membrane. The CNT pore is shrunk in armchair orientation while stretched in zigzag orientation which results in the pore emerged an elliptical shape. Contrary to the CNT channel, the NPG membrane is stretched in
24269
armchair orientation and shrunk in zigzag orientation. The optimized aperture of CNT is approximately 7.61 A in armchair orientation, which can be compared with (5, 5) CNT (7.47 A). Meanwhile, the aperture in zigzag orientation is approximately 8.92 A, which can be compared with (7, 7) CNT (9.49 A). The final CNTs-NPG molecular sieve shows the bulk density of 0.758 g/cm3. The accessible surface area is approximately 1714.24 m2/g which is much larger than those of zeolites, mesoporous silica, and pillared graphene (260e1400 m2/g) [ [19,35,36]]. To validate the accuracy of the selected parameters, the diffusion and separation of CH4/N2 in pillared graphene are simulated with ReaxFF potential to compare with ref [19]. The results are exhibited in Fig. 2(b), which coincides well with those calculated by COMPASS potential. Initially, the blocking effect of CH4 on N2 is relatively weak, which results in lower gas selectivity. Thus, the results simulated by ReaxFF potential are closer to reality. Besides, considering that the ReaxFF potential contains non-bonded interactions (vdW and Coulomb interactions), hydrogen bond interactions, and bonded interactions (the energy of bond, lone-pairs, over-coordination, under-coordination, valence angle, coalition (three-body conjugation), conjugated bonds (four-body conjugation), and torsion angles) etc, it can describe the gas separation process more accurate compared with COMMPASS potential.
Energy profiles The interaction energy between gas molecules and CNT channels is scanned along the z orientation (the axis direction of CNT) to understand the penetration of organic gas molecules through the CNT channels where zero point corresponds to the middle of CNT channel (Fig. 3). In the initial state, gas molecules are placed above the CNTs-NPG membrane with z ¼ 20 A. The spherical methane molecule is placed above the channel with three H atoms near the membrane and the fourth H atom is far away from membrane [37], while the linear ethylene and acetylene molecules are placed parallel to the membrane. To have a better understanding, the electron density isosurfaces for different organic gas species are plotted in Fig. 4. The distribution of electron density is related to the interaction between gas molecules and the inner wall of CNT channel. The values of interaction energy reach the peaks at z ¼ 0 A and have a good symmetry corresponding to this point, which is caused by the vdW forces and electrostatic interactions between organic molecules and CNT channel. When gas molecules are located at z ¼ 0 point, the vdW forces and electrostatic interactions reach the maximum values. With the distance between gas molecule and z ¼ 0 point increasing, the vdW forces and electrostatic interactions gradually decrease. The final results showed that the interaction between ethylene and CNT channel is the strongest, reaching 20.25 kcal/mol; methane comes second, reaching 15.37 kcal/mol; and acetylene is the weakest, only 12.26 kcal/mol. The difference of interaction energy renders ethylene prior to methane and acetylene to pass through the molecular sieve, especially for the separation of ethylene/ acetylene mixture.
24270
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
Fig. 1 e Schematic illustration of the CNTs-NPG molecular sieve with binary mixture (a) ethylene/methane and (b) ethylene/ acetylene at P ¼ 2 MPa. Grey and green nodes represent C and H atoms, respectively. (c) The temperature and total energy fluctuations with time during the MD simulation at 300 K for ethylene/methane gas separation system. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2 e (a) Top view of the CNTs-NPG molecular sieve unit with structural deformation. C and H atoms are denoted with gray and green balls, respectively. (b) Selectivity of CH4/N2 in the pillared graphene configuration simulated by different potentials at 300 K. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Diffusive separation of ethylene/methane
Fig. 3 e Energy profiles for methane, ethylene, and acetylene passing through the CNT channel.
The research started with the diffusive separation of ethylene/ methane binary mixture to investigate the selectivity of linear and spherical organic gases in CNTs-NPG molecular sieve. Initially, the permeation of organic gas molecules in CNT channels is rather random and free, which depends on their initial positions relative to the CNTs mouth. Then, ethylene molecules tend to attract other ethylene molecules and hinder the permeation of methane molecules, when a certain number of ethylene molecules reached the vicinity of the CNT channel mouth. As time goes on, further flow in CNT channels is more preferential for ethylene molecules. In consideration of the almost identical kinetic diameter for ethylene and methane molecules (the kinetic diameters of ethylene and methane are ~3.8 and ~3.9 A, respectively), this phenomenon
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
24271
Fig. 4 e Electron density isosurfaces for (a) methane, (b) ethylene, and (c) acetylene as they go through the CNT channel.
obviously not caused by the difference of molecule size, but rather by differences of interactions between organic gas molecules and CNT channels e higher in the case of ethylene (20.25 kcal/mol), which speeds up its movement to CNT channels compared to methane (15.37 kcal/mol). More ethylene molecules in the feed chamber gradually migrate to the permeate chamber via the CNT channels in CNTs-NPG molecular sieve. After 2500 ps, almost no gas penetration can be observed in all CNTs-NPG molecular sieves, which confirmed the equilibrium state of gas on both sides of the model is achieved. After the systems have reached equilibrium, quite a number of ethylene molecules diffuse through the CNT channels, while much less number of methane molecules is observed in permeate side. By monitoring the ethylene/methane amounts in the permeate side, the final time-evolution permeating amounts with different kinds of gas molecules can be counted, as shown in Fig. 5(a)-(b). To quantify the separation efficiency of organic gas mixture in the CNTs-NPG molecular sieve, the separation factor is revealed by the following formula:
Sx=y
. xper yper . ¼ xfee yfee
(1)
where xper (yper) represents the fraction of molecules of x (y) type in the permeate side and xfee (yfee) represents the fraction of molecules of x (y) type in the feed side. The final selectivity of ethylene over methane is presented in Fig. 5(c). It is larger at higher pressure condition which can be ascribed to the stronger gas intermolecular interaction force promoting ethylene to pass CNT channels more easily. The phenomenon that a higher permeation flux emerged in CNTs-NPG molecular sieve in the initial state and decayed with the elapse of simulation time fits perfectly with this conclusion. Meanwhile, the availability of the same type of molecule is also the decisive factor of ethylene separating from methane. Compared to 3.54 at 2 MPa, the selectivity of ethylene over
methane increases to 5.47 at 8 MPa. Exceptionally high gas densities can be observed at the vicinity of CNT channel mouth because of the congestion effect which leading to the gas molecules are in line waiting for permeating through the channels. In the simulation, plenty of gas molecules are adsorbed on the NPG surface. The surface coverage and the density of adsorbed gas increase with an increasing pressure [38]. The adsorbed gas concentration affects surface diffusion for adsorbed gas and gas transport in CNTs channels. The most stable absorption configurations of methane and ethylene above the NPG membrane are presented in Fig. 6(a)(b), respectively. Generally, gas molecules tend to orient parallel to the NPG membrane, and a steady structure layer of organic gas molecule forms, which is presumably attributed to the favorable interfacial interaction energy. The adsorption energies are calculated to be 7.84 [36] and 11.85 kcal/mol for methane and ethylene on NPG surface, respectively. The corresponding optimum heights are 2.43 and 2.71 A for methane and ethylene above NPG surface, respectively. The difference of gas adsorption energy leads to the selective adsorption of ethylene on NPG membrane, which can form effective barrier to methane, improving the selectivity of ethylene molecules in CNTs-NPG molecular sieve. The surface diffusion capacity of gas decreases with an increase in adsorption capacity. Methane with smaller adsorption capacity has a stronger surface diffusion capacity compared to ethylene. By observing the molecular trajectories in the process of gas penetration, we found that the adsorbed molecules on the NPG membrane can also inhibit the molecular crossing-back motions from the permeate side to feed side.
Diffusive separation of ethylene/acetylene In this section, the diffusive separation of linear organic gases (ethylene/acetylene) in CNTs-NPG molecular sieve is briefly analyzed. The separation equilibrium states of ethylene/ acetylene mixture achieved by molecular sieve at different
24272
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
Fig. 5 e The lateral view of ethylene/methane mixture (1:1, v/v) separation at (a) 2 and (b) 8 MPa after 5 ns. (c) Selectivity of ethylene over methane in the CNTs-NPG molecular sieve predicted by MD simulations at 300 K.
Fig. 6 e The electron density isosurfaces of stable adsorption configurations (top view) of (a) methane, (b) ethylene, and (c) acetylene on NPG membrane.
pressures are shown in Fig. 7(a)-(b). The variation trend of selectivity of ethylene/acetylene with pressure is similar to that of ethylene/methane binary mixture, as shown in Fig. 7(c). The selectivity of ethylene/methane in CNTs-NPG molecular sieve can reach 8.5 at 2 MPa and 13.8 at 8 MPa, respectively. The congestion effect of gas at the CNT channel mouth under high pressure obviously prolongs the time for the system to reach equilibrium. This is because much more gas molecules accumulate at the vicinity of the CNT channel mouth at high pressure. When the pressure is at 2 MPa, it takes about 1000 ps for the system to reach equilibrium. When
the pressure rises to 8 MPa, the time required for the system to reach equilibrium is prolonged to about 2500 ps. Initially, ethylene has not yet achieved a good barrier effect which results in a poor gas separation effect. With the passage of simulation time, the ethylene molecules continue to aggregate at the vicinity of the CNT channel mouth, and the blocking effect on the penetration of the acetylene molecules is enhanced, so that the selectivity of the gas mixture is improved. Subsequently, the ethylene and acetylene permeate proportionally until the final equilibrium achieved. Besides, the optimum height and corresponding adsorption
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
24273
Fig. 7 e The lateral view of ethylene/acetylene mixture separation at (a) 2 and (b) 8 MPa, respectively. (c) Selectivity of ethylene over acetylene from the binary mixture (1:1, v/v) at 300 K.
Fig. 8 e Top view of the rotation mode of organic gas molecules in undeformed and deformed CNTs. (a) methaneundeformed CNT, (b) ethylene-undeformed CNT, (c) acetylene-undeformed CNT, (d) methane-deformed CNT, (e) ethylenedeformed CNT, and (f) acetylene-deformed CNT. The gray and green spheres represent C and H atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
24274
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
energy are, respectively, 2.67 A and 10.65 kcal/mol for acetylene on NPG membrane, as shown in Fig. 6(c).
Effect of structure deformation of CNT To elucidate the effect of structural deformation of CNT on the molecular motion and selective separation of organic gas in CNTs-NPG molecular sieve, systematic simulations are carried out by using the models without CNT deformation to compare with the results provided in the previous paragraphs. During the simulations, four CNT pillars in models are fixed while ensuring that the atoms in NPG membranes are flexible. Comparing the movement of gas molecules in deformed and undeformed CNTs, dramatic differences can be found in the aspect of molecule rotation. In the process of gas separation, molecular motion is usually accompanied by rotation which severely affected by the moleculeenanopore wall collisions and the intermolecular collisions. Notably, the orientation of gas molecules inside the channels can minimize any possible steric hindrance and electrostatic repulsion from the inner wall of CNTs. When the organic gas molecules move in a CNT channel without structural deformation, the rotation modes
of various molecules are revealed in Fig. 8(a)e(c), respectively, and rotate in the orientation indicated by the red arrow. Methane and acetylene molecules rotate around any radial diameter of the CNT, as shown in Fig. 8(a) and (c), respectively. Ethylene molecule can be successfully located in the center of the channel with its C]C bond parallel to the axial orientation of the CNT and rotates around axial centerline of CNT, shown in Fig. 8(b). Nevertheless, the rotation mode of gas molecules changes obviously, when the molecules diffuse in a CNT channel with structural deformation. The rotation of gas molecule is effectively restrained in the armchair orientation, as the CNT channels are shrunk in the armchair orientation. Meanwhile, with the stretch of channels in the zigzag orientation, the rotation of gas molecule in the zigzag orientation is strengthened. All organic gas molecules rotate around the CNT radial diameter in the armchair orientation, as shown in Fig. 8(d)e(f), respectively. By comparison, the rotational mode of ethylene molecule varies most obviously. The final separation performances of ethylene/methane and ethylene/acetylene mixtures in undeformed molecular sieve are shown in Fig. 9(a)e(d), respectively. The selectivity of ethylene over methane is 1.64 at 2 MPa and 2.33 at 8 MPa,
Fig. 9 e Separation of organic gas mixtures in undeformed molecular sieve. (a)-(b) Separation schematic diagram of ethylene/methane mixture at 2 and 8 MPa, respectively. (c)-(d) Separation schematic diagram of ethylene/acetylene mixture at 2 and 8 MPa, respectively. Selectivity of (e) ethylene/methane and (f) ethylene/acetylene in undeformed molecular sieve at 300 K.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
which is merely 46.3% and 42.6%, respectively, corresponding to the same model with structural deformation as shown in Fig. 9(e). Then again, selectivity of ethylene over acetylene is 4.08 at 2 MPa and 5.41 at 8 MPa, which is 48.0% and 39.2%, respectively, corresponding to the same model with structural deformation as shown in Fig. 9(f). Obviously, the structural deformation of CNTs can simultaneously enhance the selectivity of ethylene over methane and acetylene. The different organic gas separation ratios compared with undeformed CNTs-NPG molecular sieve proved the fact that the deformation of CNTs can be, indeed, considered as an effective technical mean for improving gas separation.
Conclusion In summary, the diffusive separation characteristics of organic gas mixture in the CNTs-NPG molecular sieve have been intensively investigated via DFT calculations and MD simulations. The gas separation is based on the selective adsorption of ethylene and the differences of interactions between organic gas species and CNT channels. The structural deformation of CNT channels enables CNTs-NPG molecular sieve effectively to improve the selectivity of linear/ spherical and linear/linear organic gas molecules simultaneously. The selectivity of ethylene over methane and acetylene in deformed models is more than twice that of the undeformed models. This work elaborates a novel CMS material via structural modification which is a breakthrough towards the ultimate target of excellent high-selectivity molecular sieves.
Acknowledgments We thank the financial support from the National Natural Science Foundation of China (11774248 and 11474207). Meanwhile, we are grateful to the support of our calculation from Analytical & Testing Center Sichuan University, P. R. China.
references
LC, Mecerreyes D, Freire CSR, Rebelo LPN, [1] Tome Marrucho IM. Polymeric ionic liquid membranes containing IL-Agþ for ethylene/ethane separation via olefin-facilitated transport. J Mater Chem A 2014;2:5631e9. [2] Chen DL, Wang N, Xu C, Tu G, Zhu W, Krishna RA. Combined theoretical and experimental analysis on transient breakthroughs of C2H6/C2H4 in fixed beds packed with ZIF-7, micropor. Mesopor Mat 2015;208:55e65. [3] Zhang XX, et al. Separation of methane/ethylene gas mixtures using wet ZIF-8. Ind Eng Chem Res 2015;54:7890e8. [4] Hu TL, et al. Microporous metaleorganic framework with dual functionalities for highly efficient removal of acetylene from ethylene/acetylene mixtures. Nat Commun 2015;6:1e9. rrez-Sevillano JJ, [5] Lahoz-Martı´n FD, Martin-Calvo A, Gutie Calero S. Effect of light gases in the ethane/ethylene separation using zeolitic imidazolate frameworks. J Phys Chem C 2018;122:8637e46.
24275
[6] Wang Y. Experiments and modeling for recovery of hydrogen and ethylene from fluid catalytic cracking (FCC) dry gas utilizing hydrate formation. Fuel 2017;209:473e89. [7] Leo MB, Dutta A, Farooq S. Process synthesis and optimization of heat pump assisted distillation for ethyleneethane separation. Ind Eng Chem Res 2018;57:11747e56. [8] Salinas O, Ma X, Litwiller E, Pinnau I. High-performance carbon molecular sieve membranes for ethylene/ethane separation derived from an intrinsically microporous polyimide. J Membr Sci 2016;500:115e23. [9] Liang W, Zhang Y, Wang X, Wu Y, Zhou X, Xiao J, Li Y, Wang H, Li Z. Asphalt-derived high surface area activated porous carbons for the effective adsorption separation of ethane and ethylene. Chem Eng Sci 2017;162:192e202. [10] Salinas O, Ma X, Wang Y, Han Y, Pinnau I. Carbon molecular sieve membrane from a microporous spirobisindane-based polyimide precursor with enhanced ethylene/ethane mixedgas selectivity. RSC Adv 2017;7:3265e72. [11] Kim SJ, Leea PS, Chang JS, Nam SE, Park YI. Preparation of carbon molecular sieve membranes on low-cost alumina hollow fibers for use in C3H6/C3H8 separation. Separ Purif Technol 2018;194:443e50. [12] Wu K, Chen Z, Li X, Guo C, Wei M. A model for multiple transport mechanisms through nanopores of shale gas reservoirs with real gas effecteadsorption-mechanic coupling. Int J Heat Mass Transf 2016;93:408e26. [13] Wu K, Li X, Guo C, Wang C, Chen Z. A unified model for gas transfer in nanopores of shale-gas reservoirs:coupling pore diffusion and surface diffusion. SPE J 2016;21(05):1583e611. [14] Wu K, et al. Nanoconfinement effect on n-alkane flow. J Phys Chem C 2019. https://doi.org/10.1021/acs.jpcc.9b03903. [15] Jin B, Zhang X, Li F, Zhang N, Zong Z, Cao S, Chen Z Li X. Influence of nanopore density on ethylene/acetylene separation by monolayer graphene. Phys Chem Chem Phys 2019;21:6126e32. [16] Ma Z, Zhao X, Tang Q, Zhou Z. Computational prediction of experimentally possible g-C3N3 monolayer as hydrogen purification membrane. Int J Hydrogen Energy 2014;39:5037e42. [17] Chang X, Xue Q, He D, Zhu L, Li X, Tao B. 585 divacancydefective germanene as a hydrogen separation membrane: a DFT study. Int J Hydrogen Energy 2017;42:24189e96. [18] Wesołowski RP, Terzyk AP. Pillared graphene as a gas separation membrane. Phys Chem Chem Phys 2011;13:17027e9. [19] Zhou S, Lu X, Wu Z, Jin D, Guo C, Wang M, Wei S. Diffusion and separation of CH4/N2 in pillared graphene nanomaterials: a molecular dynamics investigation. Chem Phys Lett 2016;660:272e6. [20] Wesołowski RP, Terzyk AP. Dynamics of effusive and diffusive gas separation on pillared graphene. Phys Chem Chem Phys 2016;18:17018e23. [21] Jungthawan S, Reunchan P, Limpijumnong S. Theoretical study of strained porous graphene structures and their gas separation properties. Carbon 2013;54:359e64. [22] Jiang H, Cheng XL. Simulations on methane uptake in tunable pillared porous graphene hybrid architectures. J Mol Graph Model 2018;85:223e31. [23] Wu K, Chen Z, Li X. Real gas transport through nanopores of varying cross-section type and shape in shale gas reservoirs. Chem Eng J 2015;281:813e25. [24] Perdew JP, Wang Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys Rev B 1992;45:13244e9. [25] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [26] Delley B. From molecules to solids with the DMol3 approach. J Chem Phys 2000;113:7756e64.
24276
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
[27] Wu T, Xue Q, Ling C, Shan M, Liu Z, Tao Y, Li X. Fluorinemodified porous graphene as membrane for CO2/N2 separation: molecular dynamic and first-principles simulations. J Phys Chem C 2014;118:7369e76. [28] Sun Q, Li Z, Searles DJ, Chen Y, Lu G, Du A. Charge controlled switchable CO2 capture on boron nitride nanomaterials. J Am Chem Soc 2013;135:8246e53. [29] Pulay P. Improved SCF convergence acceleration. J Comput Chem 1982;3:556e60. [30] Grimme S. Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J Comput Chem 2006;27:1787e99. [31] Thonhauser T, Cooper VR, Li S, Puzder A, Hyldgaard P, Langreth DC. Van der Waals density functional: selfconsistent potential and the nature of the van der Waals bond. Phys Rev B Condens Matter Mater Phys 2007;76. 1251121-12511211. [32] Braga C, Travis KP. A configurational temperature Nose Hoover thermostat. J Chem Phys 2005;123. 134101113410115.
[33] Plimpton S. Fast parallel algorithms for short-range molecular dynamics. J Comput Phys 1995;117:1e19. [34] Zheng M, Li XX, Guo L. Algorithms of GPU-enabled reactive force field (ReaxFF) molecular dynamics. J Mol Graph Model 2013;41:1e11. [35] Sing KS. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl Chem 1985;57:603e19. [36] Lu XQ, Jin DL, Wei SX, Wang ZJ, An CH, Guo WY. Strategies to enhance CO2 capture and separation based on engineering absorbent materials. J Mater Chem A 2015;3:12118e32. [37] Wei S, Zhou S, Wu Z, Wang M, Wang Z, Guo W, Lu X. Mechanistic insights into porous graphene membranes for helium separation and hydrogen purification. Appl Surf Sci 2018;441:631e8. [38] Wu K, Li X, Wang C, Yu W, Chen Z. Model for surface diffusion of adsorbed gas in nanopores of shale gas reservoirs. Ind Eng Chem Res 2015;54:3225e36.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Highly selective 3D porous graphene membrane for organic gas separation derived from polyphenylene Hao Jiang a,*, Xin-Lu Cheng b,** a b
General Education Department, Sichuan Police College, Luzhou, 646000, China Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, 610065, China
highlights
graphical abstract
A CMS with elliptical cylinder channels
separates
gas
with
similar molecular size. The gas selectivity is effectively ameliorated by fine-tuning the CNT structure. The selectivity of ethylene over acetylene (methane) is about 13.8 (5.5).
article info
abstract
Article history:
In this paper, a 3D nanoporous carbon molecular sieve (CMS) membrane is proposed to
Received 27 January 2019
investigate the diffusion and separation properties of ethylene/methane and ethylene/
Received in revised form
acetylene binary mixtures permeating through the structural deformated carbon nanotube
19 July 2019
(CNT) channels. Combining the results obtained from density functional theory (DFT)
Accepted 23 July 2019
calculations and molecular dynamics (MD) simulations, we find that the organic gas
Available online 16 August 2019
permeability and selectivity can be effectively ameliorated by fine-tuning the geometric structure of CNTs gas separation channels. By virtue of the intrinsic structural character-
Keywords:
istics, this hybrid CMS configuration established elliptical cylinder channels to separate the
Carbon molecular sieve membrane
organic gas molecules with similar molecular size. Compared with channels with a circular
Gas separation
cross section, the gas selectivity for channels with an elliptical cross section is larger, and it
Structural deformation
increases with an increasing pressure. The selectivity of ethylene over acetylene (methane)
DFT calculations
increased to ~13.8 (5.5) in deformed CNTs channels, which is more than doubled over the
MD simulations
original CNT channels. This distinguished hybridization configuration may pave a promising avenue to utilize gas separation materials. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Jiang), [email protected] (X.-L. Cheng). https://doi.org/10.1016/j.ijhydene.2019.07.178 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
24268
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
Introduction Ethylene is one of the most essential feedstocks for the synthesis of many polymers and organics in industrial production [1,2]. In general, ethylene is mostly in the organic gas mixtures containing methane, acetylene, ethane and other hydrocarbon molecules. In order to obtain ethylene gas of single component, diversified approaches and materials has been researched [3e5]. Compared with conventional gas separation methods (e.g. catalytic cracking and distillation, etc.), membrane-based separation technology has demonstrated outstanding advantage in convenient operation [6e8]. Among various membrane materials implemented for selective gas separation, the performances of CMS membranes are extremely eye-catching. Although these organic gas molecules are of a similar kinetic diameter to ethylene, plenty of CMS membranes with intrinsic apertures have displayed impressive competency for effective organic gas separation. For example, Liang et al. reported the ethane/ethylene adsorption selectivity with a series of asphaltebased activated carbons (A-ACs) in which the highest selectivity for typical cracked gas mixture (ethylene: ethane ¼ 15:1) can reach up to 16.3 [9]. Salinas et al. developed a CMS membrane derived from PIM-6FDA-OH which possesses an ethylene/ethane (ethylene: ethane ¼ 1:1) selectivity of 15.6 at P ¼ 20 bar and T ¼ 308 K [10]. Kim et al. also reported propylene/propane separation selectivity through CMS membrane which can reach an 18.0 at P ¼ 2 bar and T ¼ 298 K [11]. The organic gas separation properties in these paradigms are predominantly dependent on the size of gas molecules and geometric configuration of nanochannels, where an effective aperture >4 A is required [12e15]. Nevertheless, the stability of the molecular sieve can be severely affected when the nanopores in CMS are too large or dense. More rational types, shapes and sizes of nanopores are needed to break the permeabilityselectivity trade-off for fast and precise organic gas separation [16,17]. The usage of CNT-graphene configurations as gas permeation and purification materials opened up a novel orientation to devise eximious composite membrane system for gas separation. The resultant CNTs-graphene nanohybrid networks are found to have excellent separation selectivity for many gas mixture systems due to the distinctive interfacial morphology [18e20]. This versatile 3D framework retains the hierarchical structural characters of CNTs and the intrinsic physical properties of graphene. In this configuration, the CNTs are employed as molecule-selective tubular micropore channels with well-controlled tube size and graphene networks are applied as gas isolation membranes with modest adsorption energy. The discrepancy of interactions between gas molecules and CNTs determines which species is preferably transported through the tubular channels. The highly ordered nanochannels in this robust molecular-sieving membrane provide a novel solution to the stability problem which caused by the pore density and size. Inspired by the purification of gas mixtures with CNTgraphene molecular-sieving membrane, a 3D nanoporous CMS composed of CNTs embedded in the AA stacked polyphenylene membranes is proposed in this article. Compared
with original graphene, this nanoporous graphene (NPG) membrane has many excellent intrinsic properties. First of all, because a part of C atoms are replaced by H atoms, the density of NPG dramatically reduced compared to original graphene. More importantly, this NPG membrane can emerge different tensile strain along different orientation under the same uniaxial tensile stress. Interestingly enough, the vertical orientation would shrink, when the NPG membrane is stretched in one orientation [21]. The interaction between CNTs and NPG membranes caused the orifices of CNTs to change from circular to elliptical [22]. As mentioned above, the shape of nanopore plays an important role in the gas diffusion and separation process [23]. Elliptical pore can increase the permeability of linear gas molecules and reduce the permeability of spherical gas molecules. A detailed understanding of the gas permeability and selectivity in designed CNTs-NPG membrane during gas separation process is quite necessary. The structural deformation of CNTs in CNTs-NPG membrane may exploit a novel nanomaterial to achieve the efficient gas permeation and purification. Theoretical simulations are executed to elucidate the mechanisms and properties of organic gases separation in CNTs-NPG molecular sieve. In general, different organic gas molecules have different molecular mass, structure, and interaction with solid material surface. Hence, some representative linear (ethylene and acetylene) and spherical (methane) organic gases are selected as separation targets. MD simulations are implemented to discuss the selectivity of CNTs-NPG molecular sieve to organic gas mixtures (ethylene/methane and ethylene/acetylene). DFT calculations are carried out to calculate the interaction energy between organic gas molecule and molecular sieve during the gas separation process.
Models and methods The construction procedure of molecular sieve used in this article is as follows: the fundamental structural unit consists of a (6, 6) armchair type CNT fixed perpendicularly over AA stacked bilayer polyphenylene membranes. The polyphenylene membranes have an interlayer distance of 20 A. The link junction between CNT and NPG membrane created six hexagons and the same number of heptagons with a negative Gaussian curvature. Whereafter, the unit is expanded into a periodic molecular-sieving model consisting four CNTs. DFT calculations are performed to compute the interaction energy between the studied organic gas molecules and NPG membrane (CNT channel) during the gas separation process. The exchange-correlation energy of interacting electrons is handled by the GGA of PBE exchange correlation functional as implemented in Dmol3 module embedded in Materials Studio (MS) software and double numerical basis set with polarization function (DNP) is applied to increase the accuracy [24e27]. The convergence threshold of SCF is set to be 1 106 a.u to control the energy and electron density [28]. To speed up SCF convergence, the thermal smearing and the size of DIIS is specified as 5 103 Ha and 6, respectively [29]. In order to obtain high quality simulation results, the structure parameters of geometric optimization are specified as 1 105 Ha,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
2 103 Ha/ A and 5 103 A for total energy, atomic forces and maximum displacement, respectively. DFT þ D scheme [30] is employed to depict the weak van der Waals (vdW) interactions [31] between organic gas molecules and molecular sieve. The interaction energy between organic gas molecules and molecular sieve (Eb) is obtained from the following expression: Eb ¼ Etotal - ECNTs-NPG - Egas, where Etotal represents the total energy of the selected molecular sieve module with one organic gas molecule, ECNTs-NPG is the energy of original molecular sieve module, and Egas is the energy of one organic gas molecule. Simultaneously, a series of MD simulations are implemented to detect the organic gas molecules diffusion and separation characteristics in this molecular-sieving configuration. All the MD simulations are actualized in the NVT ensemble controlled by the Nose-Hoover thermostat algorithm [32] by using the LAMMPS code [33]. The interactions between substances are handled using the ReaxFF potential [34]. The height of the simulation box is 10 nm and the dimension of NPG membrane is 5.4 5 nm2. To avoid the spillover of organic gas molecules, the orientations parallel to the NPG membranes are implemented with periodic boundary condition, while the orientations perpendicular to the NPG membranes are limited with reflective wall boundary conditions. A nonequilibrium system is adopted, where the simulation box is divided into two chambers (feed side and permeate side) via a bichamber setup with the CNTs-NPG model in the middle. A given number of gas molecules (120 and 480 for 2 and 8 MPa, respectively, a molar ratio of 1:1 for every binary mixture) are stacked uniformly and alternatively on the upper surface (feed side) of the molecular sieve membrane before the simulations, as shown in Fig. 1(a)-(b). Each simulation is performed 1.0 107 equilibration steps at 300 K with each step taking 0.5 fs. During MD simulations, CNTsNPG molecular sieve is modeled as fully flexible structure. To avert the vertical displacement of the CNTs-NPG molecular sieve caused by the collisions with gas particles, one “central” carbon atom in NPG membrane on the feed side is immobilized. In this way, the vibration of other atoms in molecular sieve in response to collisions with gas molecules can be retained by this fixed "central" carbon atom. The quantities of permeated molecules are monitored every 2.5 ps. To examine the stability of CNTs-NPG molecular sieve during the gas separation process, the temperature and total energy of ethylene/methane gas separation system at P ¼ 2 MPa are revealed as a function of the simulation time, as shown in Fig. 1 (c).
Results and discussion Geometric characteristics of CNTs-NPG molecular sieve and verification of force field parameters As shown in Fig. 2(a), structural deformation emerged in the CNTs-NPG molecular sieve unit owing to the interaction between CNT and NPG membrane. The CNT pore is shrunk in armchair orientation while stretched in zigzag orientation which results in the pore emerged an elliptical shape. Contrary to the CNT channel, the NPG membrane is stretched in
24269
armchair orientation and shrunk in zigzag orientation. The optimized aperture of CNT is approximately 7.61 A in armchair orientation, which can be compared with (5, 5) CNT (7.47 A). Meanwhile, the aperture in zigzag orientation is approximately 8.92 A, which can be compared with (7, 7) CNT (9.49 A). The final CNTs-NPG molecular sieve shows the bulk density of 0.758 g/cm3. The accessible surface area is approximately 1714.24 m2/g which is much larger than those of zeolites, mesoporous silica, and pillared graphene (260e1400 m2/g) [ [19,35,36]]. To validate the accuracy of the selected parameters, the diffusion and separation of CH4/N2 in pillared graphene are simulated with ReaxFF potential to compare with ref [19]. The results are exhibited in Fig. 2(b), which coincides well with those calculated by COMPASS potential. Initially, the blocking effect of CH4 on N2 is relatively weak, which results in lower gas selectivity. Thus, the results simulated by ReaxFF potential are closer to reality. Besides, considering that the ReaxFF potential contains non-bonded interactions (vdW and Coulomb interactions), hydrogen bond interactions, and bonded interactions (the energy of bond, lone-pairs, over-coordination, under-coordination, valence angle, coalition (three-body conjugation), conjugated bonds (four-body conjugation), and torsion angles) etc, it can describe the gas separation process more accurate compared with COMMPASS potential.
Energy profiles The interaction energy between gas molecules and CNT channels is scanned along the z orientation (the axis direction of CNT) to understand the penetration of organic gas molecules through the CNT channels where zero point corresponds to the middle of CNT channel (Fig. 3). In the initial state, gas molecules are placed above the CNTs-NPG membrane with z ¼ 20 A. The spherical methane molecule is placed above the channel with three H atoms near the membrane and the fourth H atom is far away from membrane [37], while the linear ethylene and acetylene molecules are placed parallel to the membrane. To have a better understanding, the electron density isosurfaces for different organic gas species are plotted in Fig. 4. The distribution of electron density is related to the interaction between gas molecules and the inner wall of CNT channel. The values of interaction energy reach the peaks at z ¼ 0 A and have a good symmetry corresponding to this point, which is caused by the vdW forces and electrostatic interactions between organic molecules and CNT channel. When gas molecules are located at z ¼ 0 point, the vdW forces and electrostatic interactions reach the maximum values. With the distance between gas molecule and z ¼ 0 point increasing, the vdW forces and electrostatic interactions gradually decrease. The final results showed that the interaction between ethylene and CNT channel is the strongest, reaching 20.25 kcal/mol; methane comes second, reaching 15.37 kcal/mol; and acetylene is the weakest, only 12.26 kcal/mol. The difference of interaction energy renders ethylene prior to methane and acetylene to pass through the molecular sieve, especially for the separation of ethylene/ acetylene mixture.
24270
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
Fig. 1 e Schematic illustration of the CNTs-NPG molecular sieve with binary mixture (a) ethylene/methane and (b) ethylene/ acetylene at P ¼ 2 MPa. Grey and green nodes represent C and H atoms, respectively. (c) The temperature and total energy fluctuations with time during the MD simulation at 300 K for ethylene/methane gas separation system. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2 e (a) Top view of the CNTs-NPG molecular sieve unit with structural deformation. C and H atoms are denoted with gray and green balls, respectively. (b) Selectivity of CH4/N2 in the pillared graphene configuration simulated by different potentials at 300 K. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Diffusive separation of ethylene/methane
Fig. 3 e Energy profiles for methane, ethylene, and acetylene passing through the CNT channel.
The research started with the diffusive separation of ethylene/ methane binary mixture to investigate the selectivity of linear and spherical organic gases in CNTs-NPG molecular sieve. Initially, the permeation of organic gas molecules in CNT channels is rather random and free, which depends on their initial positions relative to the CNTs mouth. Then, ethylene molecules tend to attract other ethylene molecules and hinder the permeation of methane molecules, when a certain number of ethylene molecules reached the vicinity of the CNT channel mouth. As time goes on, further flow in CNT channels is more preferential for ethylene molecules. In consideration of the almost identical kinetic diameter for ethylene and methane molecules (the kinetic diameters of ethylene and methane are ~3.8 and ~3.9 A, respectively), this phenomenon
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
24271
Fig. 4 e Electron density isosurfaces for (a) methane, (b) ethylene, and (c) acetylene as they go through the CNT channel.
obviously not caused by the difference of molecule size, but rather by differences of interactions between organic gas molecules and CNT channels e higher in the case of ethylene (20.25 kcal/mol), which speeds up its movement to CNT channels compared to methane (15.37 kcal/mol). More ethylene molecules in the feed chamber gradually migrate to the permeate chamber via the CNT channels in CNTs-NPG molecular sieve. After 2500 ps, almost no gas penetration can be observed in all CNTs-NPG molecular sieves, which confirmed the equilibrium state of gas on both sides of the model is achieved. After the systems have reached equilibrium, quite a number of ethylene molecules diffuse through the CNT channels, while much less number of methane molecules is observed in permeate side. By monitoring the ethylene/methane amounts in the permeate side, the final time-evolution permeating amounts with different kinds of gas molecules can be counted, as shown in Fig. 5(a)-(b). To quantify the separation efficiency of organic gas mixture in the CNTs-NPG molecular sieve, the separation factor is revealed by the following formula:
Sx=y
. xper yper . ¼ xfee yfee
(1)
where xper (yper) represents the fraction of molecules of x (y) type in the permeate side and xfee (yfee) represents the fraction of molecules of x (y) type in the feed side. The final selectivity of ethylene over methane is presented in Fig. 5(c). It is larger at higher pressure condition which can be ascribed to the stronger gas intermolecular interaction force promoting ethylene to pass CNT channels more easily. The phenomenon that a higher permeation flux emerged in CNTs-NPG molecular sieve in the initial state and decayed with the elapse of simulation time fits perfectly with this conclusion. Meanwhile, the availability of the same type of molecule is also the decisive factor of ethylene separating from methane. Compared to 3.54 at 2 MPa, the selectivity of ethylene over
methane increases to 5.47 at 8 MPa. Exceptionally high gas densities can be observed at the vicinity of CNT channel mouth because of the congestion effect which leading to the gas molecules are in line waiting for permeating through the channels. In the simulation, plenty of gas molecules are adsorbed on the NPG surface. The surface coverage and the density of adsorbed gas increase with an increasing pressure [38]. The adsorbed gas concentration affects surface diffusion for adsorbed gas and gas transport in CNTs channels. The most stable absorption configurations of methane and ethylene above the NPG membrane are presented in Fig. 6(a)(b), respectively. Generally, gas molecules tend to orient parallel to the NPG membrane, and a steady structure layer of organic gas molecule forms, which is presumably attributed to the favorable interfacial interaction energy. The adsorption energies are calculated to be 7.84 [36] and 11.85 kcal/mol for methane and ethylene on NPG surface, respectively. The corresponding optimum heights are 2.43 and 2.71 A for methane and ethylene above NPG surface, respectively. The difference of gas adsorption energy leads to the selective adsorption of ethylene on NPG membrane, which can form effective barrier to methane, improving the selectivity of ethylene molecules in CNTs-NPG molecular sieve. The surface diffusion capacity of gas decreases with an increase in adsorption capacity. Methane with smaller adsorption capacity has a stronger surface diffusion capacity compared to ethylene. By observing the molecular trajectories in the process of gas penetration, we found that the adsorbed molecules on the NPG membrane can also inhibit the molecular crossing-back motions from the permeate side to feed side.
Diffusive separation of ethylene/acetylene In this section, the diffusive separation of linear organic gases (ethylene/acetylene) in CNTs-NPG molecular sieve is briefly analyzed. The separation equilibrium states of ethylene/ acetylene mixture achieved by molecular sieve at different
24272
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
Fig. 5 e The lateral view of ethylene/methane mixture (1:1, v/v) separation at (a) 2 and (b) 8 MPa after 5 ns. (c) Selectivity of ethylene over methane in the CNTs-NPG molecular sieve predicted by MD simulations at 300 K.
Fig. 6 e The electron density isosurfaces of stable adsorption configurations (top view) of (a) methane, (b) ethylene, and (c) acetylene on NPG membrane.
pressures are shown in Fig. 7(a)-(b). The variation trend of selectivity of ethylene/acetylene with pressure is similar to that of ethylene/methane binary mixture, as shown in Fig. 7(c). The selectivity of ethylene/methane in CNTs-NPG molecular sieve can reach 8.5 at 2 MPa and 13.8 at 8 MPa, respectively. The congestion effect of gas at the CNT channel mouth under high pressure obviously prolongs the time for the system to reach equilibrium. This is because much more gas molecules accumulate at the vicinity of the CNT channel mouth at high pressure. When the pressure is at 2 MPa, it takes about 1000 ps for the system to reach equilibrium. When
the pressure rises to 8 MPa, the time required for the system to reach equilibrium is prolonged to about 2500 ps. Initially, ethylene has not yet achieved a good barrier effect which results in a poor gas separation effect. With the passage of simulation time, the ethylene molecules continue to aggregate at the vicinity of the CNT channel mouth, and the blocking effect on the penetration of the acetylene molecules is enhanced, so that the selectivity of the gas mixture is improved. Subsequently, the ethylene and acetylene permeate proportionally until the final equilibrium achieved. Besides, the optimum height and corresponding adsorption
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
24273
Fig. 7 e The lateral view of ethylene/acetylene mixture separation at (a) 2 and (b) 8 MPa, respectively. (c) Selectivity of ethylene over acetylene from the binary mixture (1:1, v/v) at 300 K.
Fig. 8 e Top view of the rotation mode of organic gas molecules in undeformed and deformed CNTs. (a) methaneundeformed CNT, (b) ethylene-undeformed CNT, (c) acetylene-undeformed CNT, (d) methane-deformed CNT, (e) ethylenedeformed CNT, and (f) acetylene-deformed CNT. The gray and green spheres represent C and H atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
24274
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
energy are, respectively, 2.67 A and 10.65 kcal/mol for acetylene on NPG membrane, as shown in Fig. 6(c).
Effect of structure deformation of CNT To elucidate the effect of structural deformation of CNT on the molecular motion and selective separation of organic gas in CNTs-NPG molecular sieve, systematic simulations are carried out by using the models without CNT deformation to compare with the results provided in the previous paragraphs. During the simulations, four CNT pillars in models are fixed while ensuring that the atoms in NPG membranes are flexible. Comparing the movement of gas molecules in deformed and undeformed CNTs, dramatic differences can be found in the aspect of molecule rotation. In the process of gas separation, molecular motion is usually accompanied by rotation which severely affected by the moleculeenanopore wall collisions and the intermolecular collisions. Notably, the orientation of gas molecules inside the channels can minimize any possible steric hindrance and electrostatic repulsion from the inner wall of CNTs. When the organic gas molecules move in a CNT channel without structural deformation, the rotation modes
of various molecules are revealed in Fig. 8(a)e(c), respectively, and rotate in the orientation indicated by the red arrow. Methane and acetylene molecules rotate around any radial diameter of the CNT, as shown in Fig. 8(a) and (c), respectively. Ethylene molecule can be successfully located in the center of the channel with its C]C bond parallel to the axial orientation of the CNT and rotates around axial centerline of CNT, shown in Fig. 8(b). Nevertheless, the rotation mode of gas molecules changes obviously, when the molecules diffuse in a CNT channel with structural deformation. The rotation of gas molecule is effectively restrained in the armchair orientation, as the CNT channels are shrunk in the armchair orientation. Meanwhile, with the stretch of channels in the zigzag orientation, the rotation of gas molecule in the zigzag orientation is strengthened. All organic gas molecules rotate around the CNT radial diameter in the armchair orientation, as shown in Fig. 8(d)e(f), respectively. By comparison, the rotational mode of ethylene molecule varies most obviously. The final separation performances of ethylene/methane and ethylene/acetylene mixtures in undeformed molecular sieve are shown in Fig. 9(a)e(d), respectively. The selectivity of ethylene over methane is 1.64 at 2 MPa and 2.33 at 8 MPa,
Fig. 9 e Separation of organic gas mixtures in undeformed molecular sieve. (a)-(b) Separation schematic diagram of ethylene/methane mixture at 2 and 8 MPa, respectively. (c)-(d) Separation schematic diagram of ethylene/acetylene mixture at 2 and 8 MPa, respectively. Selectivity of (e) ethylene/methane and (f) ethylene/acetylene in undeformed molecular sieve at 300 K.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
which is merely 46.3% and 42.6%, respectively, corresponding to the same model with structural deformation as shown in Fig. 9(e). Then again, selectivity of ethylene over acetylene is 4.08 at 2 MPa and 5.41 at 8 MPa, which is 48.0% and 39.2%, respectively, corresponding to the same model with structural deformation as shown in Fig. 9(f). Obviously, the structural deformation of CNTs can simultaneously enhance the selectivity of ethylene over methane and acetylene. The different organic gas separation ratios compared with undeformed CNTs-NPG molecular sieve proved the fact that the deformation of CNTs can be, indeed, considered as an effective technical mean for improving gas separation.
Conclusion In summary, the diffusive separation characteristics of organic gas mixture in the CNTs-NPG molecular sieve have been intensively investigated via DFT calculations and MD simulations. The gas separation is based on the selective adsorption of ethylene and the differences of interactions between organic gas species and CNT channels. The structural deformation of CNT channels enables CNTs-NPG molecular sieve effectively to improve the selectivity of linear/ spherical and linear/linear organic gas molecules simultaneously. The selectivity of ethylene over methane and acetylene in deformed models is more than twice that of the undeformed models. This work elaborates a novel CMS material via structural modification which is a breakthrough towards the ultimate target of excellent high-selectivity molecular sieves.
Acknowledgments We thank the financial support from the National Natural Science Foundation of China (11774248 and 11474207). Meanwhile, we are grateful to the support of our calculation from Analytical & Testing Center Sichuan University, P. R. China.
references
LC, Mecerreyes D, Freire CSR, Rebelo LPN, [1] Tome Marrucho IM. Polymeric ionic liquid membranes containing IL-Agþ for ethylene/ethane separation via olefin-facilitated transport. J Mater Chem A 2014;2:5631e9. [2] Chen DL, Wang N, Xu C, Tu G, Zhu W, Krishna RA. Combined theoretical and experimental analysis on transient breakthroughs of C2H6/C2H4 in fixed beds packed with ZIF-7, micropor. Mesopor Mat 2015;208:55e65. [3] Zhang XX, et al. Separation of methane/ethylene gas mixtures using wet ZIF-8. Ind Eng Chem Res 2015;54:7890e8. [4] Hu TL, et al. Microporous metaleorganic framework with dual functionalities for highly efficient removal of acetylene from ethylene/acetylene mixtures. Nat Commun 2015;6:1e9. rrez-Sevillano JJ, [5] Lahoz-Martı´n FD, Martin-Calvo A, Gutie Calero S. Effect of light gases in the ethane/ethylene separation using zeolitic imidazolate frameworks. J Phys Chem C 2018;122:8637e46.
24275
[6] Wang Y. Experiments and modeling for recovery of hydrogen and ethylene from fluid catalytic cracking (FCC) dry gas utilizing hydrate formation. Fuel 2017;209:473e89. [7] Leo MB, Dutta A, Farooq S. Process synthesis and optimization of heat pump assisted distillation for ethyleneethane separation. Ind Eng Chem Res 2018;57:11747e56. [8] Salinas O, Ma X, Litwiller E, Pinnau I. High-performance carbon molecular sieve membranes for ethylene/ethane separation derived from an intrinsically microporous polyimide. J Membr Sci 2016;500:115e23. [9] Liang W, Zhang Y, Wang X, Wu Y, Zhou X, Xiao J, Li Y, Wang H, Li Z. Asphalt-derived high surface area activated porous carbons for the effective adsorption separation of ethane and ethylene. Chem Eng Sci 2017;162:192e202. [10] Salinas O, Ma X, Wang Y, Han Y, Pinnau I. Carbon molecular sieve membrane from a microporous spirobisindane-based polyimide precursor with enhanced ethylene/ethane mixedgas selectivity. RSC Adv 2017;7:3265e72. [11] Kim SJ, Leea PS, Chang JS, Nam SE, Park YI. Preparation of carbon molecular sieve membranes on low-cost alumina hollow fibers for use in C3H6/C3H8 separation. Separ Purif Technol 2018;194:443e50. [12] Wu K, Chen Z, Li X, Guo C, Wei M. A model for multiple transport mechanisms through nanopores of shale gas reservoirs with real gas effecteadsorption-mechanic coupling. Int J Heat Mass Transf 2016;93:408e26. [13] Wu K, Li X, Guo C, Wang C, Chen Z. A unified model for gas transfer in nanopores of shale-gas reservoirs:coupling pore diffusion and surface diffusion. SPE J 2016;21(05):1583e611. [14] Wu K, et al. Nanoconfinement effect on n-alkane flow. J Phys Chem C 2019. https://doi.org/10.1021/acs.jpcc.9b03903. [15] Jin B, Zhang X, Li F, Zhang N, Zong Z, Cao S, Chen Z Li X. Influence of nanopore density on ethylene/acetylene separation by monolayer graphene. Phys Chem Chem Phys 2019;21:6126e32. [16] Ma Z, Zhao X, Tang Q, Zhou Z. Computational prediction of experimentally possible g-C3N3 monolayer as hydrogen purification membrane. Int J Hydrogen Energy 2014;39:5037e42. [17] Chang X, Xue Q, He D, Zhu L, Li X, Tao B. 585 divacancydefective germanene as a hydrogen separation membrane: a DFT study. Int J Hydrogen Energy 2017;42:24189e96. [18] Wesołowski RP, Terzyk AP. Pillared graphene as a gas separation membrane. Phys Chem Chem Phys 2011;13:17027e9. [19] Zhou S, Lu X, Wu Z, Jin D, Guo C, Wang M, Wei S. Diffusion and separation of CH4/N2 in pillared graphene nanomaterials: a molecular dynamics investigation. Chem Phys Lett 2016;660:272e6. [20] Wesołowski RP, Terzyk AP. Dynamics of effusive and diffusive gas separation on pillared graphene. Phys Chem Chem Phys 2016;18:17018e23. [21] Jungthawan S, Reunchan P, Limpijumnong S. Theoretical study of strained porous graphene structures and their gas separation properties. Carbon 2013;54:359e64. [22] Jiang H, Cheng XL. Simulations on methane uptake in tunable pillared porous graphene hybrid architectures. J Mol Graph Model 2018;85:223e31. [23] Wu K, Chen Z, Li X. Real gas transport through nanopores of varying cross-section type and shape in shale gas reservoirs. Chem Eng J 2015;281:813e25. [24] Perdew JP, Wang Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys Rev B 1992;45:13244e9. [25] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [26] Delley B. From molecules to solids with the DMol3 approach. J Chem Phys 2000;113:7756e64.
24276
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 2 6 7 e2 4 2 7 6
[27] Wu T, Xue Q, Ling C, Shan M, Liu Z, Tao Y, Li X. Fluorinemodified porous graphene as membrane for CO2/N2 separation: molecular dynamic and first-principles simulations. J Phys Chem C 2014;118:7369e76. [28] Sun Q, Li Z, Searles DJ, Chen Y, Lu G, Du A. Charge controlled switchable CO2 capture on boron nitride nanomaterials. J Am Chem Soc 2013;135:8246e53. [29] Pulay P. Improved SCF convergence acceleration. J Comput Chem 1982;3:556e60. [30] Grimme S. Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J Comput Chem 2006;27:1787e99. [31] Thonhauser T, Cooper VR, Li S, Puzder A, Hyldgaard P, Langreth DC. Van der Waals density functional: selfconsistent potential and the nature of the van der Waals bond. Phys Rev B Condens Matter Mater Phys 2007;76. 1251121-12511211. [32] Braga C, Travis KP. A configurational temperature Nose Hoover thermostat. J Chem Phys 2005;123. 134101113410115.
[33] Plimpton S. Fast parallel algorithms for short-range molecular dynamics. J Comput Phys 1995;117:1e19. [34] Zheng M, Li XX, Guo L. Algorithms of GPU-enabled reactive force field (ReaxFF) molecular dynamics. J Mol Graph Model 2013;41:1e11. [35] Sing KS. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl Chem 1985;57:603e19. [36] Lu XQ, Jin DL, Wei SX, Wang ZJ, An CH, Guo WY. Strategies to enhance CO2 capture and separation based on engineering absorbent materials. J Mater Chem A 2015;3:12118e32. [37] Wei S, Zhou S, Wu Z, Wang M, Wang Z, Guo W, Lu X. Mechanistic insights into porous graphene membranes for helium separation and hydrogen purification. Appl Surf Sci 2018;441:631e8. [38] Wu K, Li X, Wang C, Yu W, Chen Z. Model for surface diffusion of adsorbed gas in nanopores of shale gas reservoirs. Ind Eng Chem Res 2015;54:3225e36.