Article pubs.acs.org/JPCC
Cooperative Reformable Channel System with Unique Recognition of Gas Molecules in a Zeolitic Imidazolate Framework with Multilevel Flexible Ligands Benyamin Motevalli,† Huanting Wang,‡ and Jefferson Zhe Liu*,† †
Department of Mechanical and Aerospace Engineering and ‡Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia S Supporting Information *
ABSTRACT: We report a cooperative reformable channel system in a ZIF-L crystal arising from coexistence of three types of local flexible ligands. The reformable channel is able to regulate permeation of a nonspherical guest molecule, such as N2 or CO2, based on its longer molecular dimension, which is in a striking contrast to conventional molecular sieves that regulate the shorter cross-sectional dimension of the guest molecules. Our density functional theory (DFT) calculations reveal that the guest molecule induces dynamic motion of the flexible ligands, leading to the channel reformation, and then the guest molecule reorientates itself to fit in the reformed channel. Such a unique “induced fit-in” mechanism causes the gas molecule to pass through sixmembered-ring windows in the c- crystal direction of ZIF-L with its longer axis parallel to the window plane. Our experimental permeance of N2 through the ZIF-L membranes is about three times greater than that of CO2, supporting the DFT simulation predictions. the local structural flexibility is mainly achieved via either the connecting ligands or the side molecular chains purposely attached/grafted to the connecting ligands.12,13 In this Article, we report that different types of flexible ligands with different levels of local flexibility are incorporated simultaneously into a two-dimensional ZIF crystal with a leafshaped morphology (ZIF-L). Particularly the ZIF-L is composed of a “free” ligand that is incorporated in the crystal via weak hydrogen bonds and van der Waals forces, as reported in our previous work.15 Interestingly, our ab initio simulations reveal that the channel in c-crystalline direction reforms in the presence of the guests (mainly caused by the local dynamic motions of the flexible ligands) and then induces the reorientation of the guest molecule in the channel, so that it can regulate the longer dimension of the nonspherical guest molecules, such as CO2 and N2. This special recognition is distinctive from the conventional molecular sieves and the gating MOFs/ZIFs, in which the smaller dimension of the nonspherical guest molecules (called as the kinetic diameter d) dictates their transport properties, e.g., CO2 (d = 3.3 Å), usually having a higher diffusion coefficient than that of N2 (d = 3.6 Å).8 Indeed, our follow-up experiments show that the permeability of N2 through the ZIF-L membranes indeed is much higher than that of CO2, yielding an ideal selectivity of N2/CO2 of about 3, in comparison with the averaged value 0.38 of the ZIF-8 membranes, 7 supporting our simulation
1. INTRODUCTION Porous crystalline materials such as zeolites and metal organic frameworks (MOFs, including their subclass of zeolitic imidazolate frameworks (ZIFs)) have attracted tremendous research interest due to their attractive properties for a wide range of applications including absorption, separation, and catalysis.1−11 Porous crystals provide regular porous frameworks for efficient storage; meanwhile, they have the ability to distinguish guest molecules on the basis of molecular size and shape.8 In contrast, f lexible protein molecules in ion channels and enzymes give way to structural reformation of their channels and cavities, accomplishing selective recognition and capture of specific guests.12 In MOFs, various attractive forces, ranging from van der Waals force, hydrogen bond, coordination bond, to covalent bond, assemble organic ligands and metal ions into porous crystalline architectures with diverse topologies.1−6,12,13 Hence many MOFs have flexible crystal structures with “flexibility” as well as “regularity”.12 They have great potential for use as intelligent host materials with both efficient guest storage and specific recognition properties.9,12−14 There are several types of structural flexibilities observed in MOFs, including the relative motif motion of interpenetrated frameworks, structural transformation of the framework between large pore and small pore states, and local rotation and wobbling of the connecting ligands (while the framework structure remains).12 The local structural flexibility (reminiscent of the protein ion channels in cell membranes) is particularly appealing because it would give rise to intrinsically robust materials yet dynamically flexible, which will not suffer failure when the framework flexes back and forth.13 Till now, © XXXX American Chemical Society
Received: May 19, 2015 Revised: June 23, 2015
A
DOI: 10.1021/acs.jpcc.5b04761 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C predictions. The combination of a new type of structural flexibilities and the counterintuitive recognition/selectivity properties that result from this flexibility demonstrates a new way for designing soft porous PCPs for the desirable “intelligence” in guest recognition.
2. METHODOLOGY Our DFT calculations were carried out using the Vienna Ab initio Simulation Package (VASP 5.3.3) with the generalized gradient approximation (GGA) and the projector augmented wave (PAW) method.16−18 The cutoff energy of plane-wave basis set was 400 eV. The Monkhost−Pack k-point mesh of 2 × 2 × 1 was used for one primitive cell of ZIF-L. A similar k-point mesh density was applied for other supercells. These setups were tested to ensure the convergence of total energy within 1 meV/atom. Atomic positions were optimized with the conjugate gradient method. To understand the significantly different permeability of ZIFL in b- and c-orientation, the energy barriers for the gas molecules diffusing through the membrane were calculated. Note that due to the complex channel network of ZIF-L and its high flexibility finding the diffusion path directly from VASP was difficult. For this reason, a Genetic Algorithm code was developed in order to roughly estimate the diffusion path. Then, the points on the approximated path were used as the initial location of the gas molecules. Moreover, to ensure capturing the framework flexibility, at each of these locations, a cross-sectional parallel plane was placed along the diffusion path in b- and c-directions. Then, the gas molecule was allowed to relax in each of the cross-section planes. Accordingly, the obtained total energy profile was used to estimate the energy barrier. At last, to understand how the gases interact with their environment, a charge difference analysis was performed. To do this, the electronic charge densities were obtained for the following systems: the guest−host complex, the reformed framework without a guest molecule, and the isolated guest molecule in the same supercell box. Then, the charge redistribution was obtained by subtracting the charge densities of the guest−host complex from the summations of the charges of the reformed framework and the isolated gas molecule.
Figure 1. Crystal structure analysis of ZIF-Lthree types of flexible ligands with different levels of degrees of freedom. (a) SOD topology of ZIF-8 framework. (b) Topology of ZIF-L crystal structure that shares part of the SOD topology of ZIF-8. Both ZIF-8 and ZIF-L are formed by coordination of a zinc ion and 2-methylimidazole. ZIF-L can be seen as a variation of ZIF-8. (c) Sketch of the ZIF-L crystal structure. Three types of flexible ligands are shown. Note that the “free” ligands (FLs) block the 6-membered-ring (6-MR) window from both sides. The 6-MRs are the only passage apertures in the SOD topology framework of ZIF-8. (d) The three types of flexible ligands have different levels of flexibility (namely, different degrees of freedom).
bonded to the free end of the TL, and its ring is parallel to the rings of Hmim connecting ligands (CL) in the framework, suggesting a weak van der Waals interaction. The FLs are particularly interesting because they block the 6-membered-ring (6-MR) window from both sides, which is the only passage aperture in the SOD-type frameworks (Figure 1c). What makes ZIF-L special is the existence of different ligands with different degrees of flexibility as shown in Figure 1d. The CLs can rotate around the Zn−Zn axis. The TLs can rotate around the metal− nitrogen axis and side swing around the Zn center about the bcrystal direction. The FLs possess two translation degrees of freedom in the bc plane and one rotation degree of freedom about the a-crystal axis. These flexible ligands break the channels of ZIF-L into two types of separated zero-dimension cages and cavities (Figure 2a): a larger cushion-shaped cage (9.4 Å × 7.0 Å × 5.3 Å) and a smaller cavity (3.6 Å × 2.8 Å × 2.3 Å). Along the b- and ccrystalline directions, the flexible ligands separate the cage and the cavity. Although ZIF-L (like ZIF-8) does not have a continuous channel system, it exhibits significant adsorption of CO2 and CH4,15 suggesting that the channels must be reformable to accommodate these guest molecules. 3.2. Ab Initio Simulations to Investigate How Guest Transports in ZIF-L. In our ab initio simulations, a gas molecule is dragged to transport from one cage to the other along either the b- or c-crystal direction. At each step, the center
3. RESULTS AND DISCUSSIONS 3.1. Crystal Structure Analysis. The ZIF-L was synthesized from zinc nitride hexahydrate and 2-methylimidazole (Hmim) in deionized water at room temperature, and its key synthesis parameter was the Hmim/zinc ion molar ratio (e.g., 8). A detailed synthesis process was reported in our previous work.15 The ZIF-L crystal comprises the identical building blocks of ZIF-8 Hmim and zinc nitrate but possesses a different topology. The SOD topology of ZIF-8 (Figure 1a) can be seen as a periodic alternation of two two-dimensional crystal layers (in ab plane), namely, A- and B- layer, stacking in c crystal direction. The Zn centers between adjacent layers are connected by Hmim ligands. The ZIF-L shares part of the SOD topology, but the connecting Hmim ligands between the two layers are removed and the A-layers shifted in the b-direction slightly (Figure 1b). Such a 2D network is further stabilized by the interdigitating interactions with the help of the Hmim “terminal” ligands (TL) and the Hmim “free” ligands (FL) (Figure 1c,d). Notably the TL only has one end coordinatedly bonded to zinc centers. The Hmim FL is only hydrogen B
DOI: 10.1021/acs.jpcc.5b04761 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Binding energy profiles for guest molecule transport along channels in b- and c-crystalline directions. (a) Crystal structures of ZIF-L and its noncontinuous channel system that comprises a larger cushion-shaped cage (9.4 Å × 7.0 Å × 5.3 Å) and a small cavity (3.6 Å × 2.8 Å × 2.3 Å). The flexible ligands separate the cages and the cavities. For guest molecule transport along the channels in b- or c-direction, these flexible ligands must dynamically move, yielding a reformable channel system to accommodate the guest molecules. (b,c) The binding energy (BE) profile for three gas molecules H2, CO2, and N2 diffusing along b- and c-channels, respectively, calculated using the ab initio DFT method. The difference between the highest and lowest BE represents the diffusion energy barrier.
of mass of the gas molecule is allowed to relax in the plane that is perpendicular to the channel axial direction; meanwhile, the ZIF-L crystal is fully relaxed. Figure 2b and 2c shows the binding energy (BE) profiles for three different gas molecules (H2, CO2, and N2). The BE is calculated as the total energy difference between the relaxed host−guest complex at a given position along the diffusion path and the summation of the total energy of the free guest molecule and the host ZIF-L. It represents the physical interaction strength of the host−guest complex. Figure 2b and 2c shows that the most stable adsorption site for the three types of gas molecules is inside the cage. Two CH3 heads protrude toward the center of the cage, constraining the void space and leading to the cushionshaped morphology (Figure S1, Supporting Information). Thus, the most stable adsorption sites of the relatively large CO2 or N2 are not at the exact cage center but are closer to the TLs that separate the cage and its neighboring cavity. Figure 2b and 2c also shows that the CO2 has a stronger affinity inside both cages than the N2 molecule, manifested by a more negative BE value. This is reasonable because CO2 has a stronger quadrupole moment, and it often has a higher binding energy in zeolite or MOF/ZIF crystals, such as Chabazite and ZIF-8.19,20 Figure 3 shows the charge difference analysis of the guest−host complex when the CO2 or N2 molecule is absorbed at the most favorable adsorption site inside the main cage. The yellow isosurfaces represent the electron accumulation regions, whereas the blue isosurfaces denote the electron depletion regions. Clearly the CO2 case has a much more profound charge redistribution than the N2 case, indicating a much stronger affinity with the ZIF-L framework (Figure 2). Note that the charge redistribution takes place for both CO2 and ZIF-L. This could be because CO2 has a moderate quadrupole moment that polarizes the ZIF-L framework upon its adsorption. In contrast, for the N 2 case, the charge redistribution mainly occurs surrounding the N2 molecule,
Figure 3. Charge difference analysis for (a) CO2 and (b) N2 adsorbed in the main cages of ZIF-L. The more profound charge redistribution indicates a stronger affinity of CO2 with ZIF-L.
and there is a negligible charge redistribution of ZIF-L framework. This could be attributed to the zero quadruple moment of N2. This observation generally agrees with the phenomena in the classic porous zeolite adsorbents, such as Chabazite.20 C
DOI: 10.1021/acs.jpcc.5b04761 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 4. Host−guest interaction in ZIF-L: guest adaptively fitted-in reformed channels that are results of a dynamic flexible structure response to the presence of a guest. (a) Crystal structure of ZIF-L to highlight the molecular gates that are composed of different flexible ligands. There is one molecular gate (b-Gate) that separates the main and secondary cages in the b-channel. Three molecular gates exist, in addition to the known 6-MR gate in the SOD framework, between the cage and the cavity in the c-channel. (b) Demonstration of the opening of b-Gate when a CO2 molecule approaches and passes through. (c) A sequence of gate opening in the c-channel as CO2 approaches and passes through, where the c-Gate-1 FL bends, the c-Gate-2 FL shifts away and rotates, the c-Gate-3 CL rotates, and the c-Gate-3 TLs wobble, to give way to the guest molecule, namely, CO2. (d) The reformation process of the b-channel. The opening of b-Gate gives rise to the formation of a new channel protruding from the cage to the cavity until a complete connection. (e) The reformation process of the c-channel. A sequence of gate opening generates the formation of a new channel extending from the cage to the cavity. Interestingly at the moment that the c-Gage-2 opens and the c-Gate-1 just closes, the CO2 reorientates itself so that its longer molecular dimension is perpendicular to the axial direction of the diffusion path.
0.39 eV, respectively. In both channels, CO2 has a higher energy barrier than N2 (as a result of the higher BE at saddle points), indicating a slower diffusion. This is in a sharp contrast to ZIF-8. Given that ZIF-L shares identical building blocks and similar framework topology to ZIF-8, the multiple flexible ligands and thus a more reformable channel in ZIF-L must play significant roles to understand such a counterintuitive observation in ZIF-L. 3.3. Reformable Channels Leading to Adaptive Host− Guest Complexes. In the b-channel, a single gate (b-Gate) separates the cage and the cavity, which is composed of two TLs, one FL, and one CL (Figure 4a and 4b). The reformation of the channel as a result of the dynamic motion of the flexible ligands is shown in Figure 4d. As the CO2 and N2 approach the aperture, the two TLs move outward, and the FL moves
For some bridging sites in the diffusion path that connect the cage and cavity, CO2 and N2 have a positive BE value, representing the repulsive interactions between the guests and the crowded channels filled with the flexible ligands. The position where we have the most positive BE value can be seen as the spot with the narrowest channel cross-section size (termed as saddle point). It is widely accepted that a molecule with a smaller diameter should experience a weaker repulsive interaction from the aperture and thus a lower BE value at the saddle point. CO2 has a smaller kinetic diameter than N2, 3.3 Å vs 3.64 Å. In Figure 2b and c, it is surprising to see that the CO2 has more positive BE results at the saddle point than those of N2. The energy barriers in the b-channel for CO2, N2, and H2 are calculated as 0.428, 0.265, and 0.082 eV, respectively. Along the c-channel, the energy barrier results are 0.94, 0.557, and D
DOI: 10.1021/acs.jpcc.5b04761 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Additionally the shortest intermolecular distance between the N2 and FL (∼2.54 Å) is much shorter than that between CO2 and FL (∼3.04 Å) in Figure S3 (Supporting Information). All this evidence suggests a strong chemical affinity of N2 to the FL, which should compensate the larger strain energy arising from the larger size of N2. The less positive being at the saddle point of N2 leads to a lower energy barrier and thus a better recognition over CO2. Along the c-channel, three molecular gates together with the 6-MR gate in the SOD framework separate the cage and cavity (Figure 4a and 4c). The c-Gate-1 and c-Gate-2, each composing just one FL, completely block the 6-MR gate window, and the c-Gate-3 is composed of one CL and two TLs. As the guest approaches, a sequence of channel reformations occurs: first FL (c-Gate 1) bends, followed by deformation of the 6-MR gate, then the second FL (c-gate 2) moves away from the 6-MR, and finally the CL rotates and two TLs wobble (c-gate 3), to give way to the guest molecule. The reformation of the c-channel (Figure 4e) is triggered by this sequence of gate openings (Figure 4c). To fit in the reformed new channel, the guest molecule CO2 should adaptively reorientate itself as seen in Figure 4e. Interestingly at the moment at which the gas is passing through the 6-MR, the two flexible FLs enforce a reorientation of the CO2 molecule so that it parallelly moves into the center of the 6-MR gate (Figure 5a and Figure S4, Supporting Information). A similar observation is obtained for N2 (Figure 5b) but not for the small H2. It should be noted that the initial orientation of the gas molecule in our simulation was chosen to be consistent with the general perception, i.e., the axis of the molecules perpendicular to the plane of the 6-MR. Relaxation in our ab initio simulations led to the reorientation of the CO2 or N2 molecule to the final configuration as shown in Figure 6, indicating that it is energetically favorable. Note
upward; meanwhile the CL rotates, to increase aperture size (Figure 4b). With the gate opening, a new channel (represented by the blue isosurface) forms and extrudes from the cage toward the cavity until the complete connection. Interestingly, the gas molecule adaptively reorientates itself to fit in the reformed channel as seen in Figure 4d. It is observed that the flexible ligands of the b-gate deform more to allow the passage of N2 than CO2 (Figure 4d and Figure S2, Supporting Information), which is consistent with the fact that N2 has a larger cross-section diameter. However, the resultant higher strain energy is contradictory to the calculated less positive BE result, in comparison with CO2 (Figure 2b). Figure 5a,b shows a charge difference analysis for both CO2 and N2 in the center of the b-Gate aperture. The more significant electron redistribution between N2 and the FL is observed. The Bader charge analysis indeed shows a more pronounced charge transfer taking place for the case of N2 (Figure 5c,d).
Figure 6. (a,b) The host−guest configuration at aperture centers of cchannels for CO2 and N2, respectively. The longer CO2 molecule clearly deforms the 6-MR window more than the N2 molecule.
that this position corresponds to the highest BE point in Figure 2c. It should be pointed out that the channel reformation in ZIF-L is a dynamic process. After the passage, the flexible ligands return to their original positions, and the reformed connecting channels vanish (Figure 4d and 4e). The guest molecular parallelly fitted in the center of the 6MR causes the in-plane deformation of the 6-MR as shown in Figure 5a and 5b. The interatomic distances of the two hydrogen atoms of two CLs on opposite sides of the 6-MR increase to 6.48 Å for CO2 and 6.13 Å for N2 from the value of 5.95 Å in the guest-free ZIF-L crystal. Note that the cell volume change due to this deformation is negligible. At this position, the charge difference analysis does not show significant host−
Figure 5. Host−guest configuration and interactions at aperture centers of b-channels. (a,b) The charge difference analysis of CO2 and N2 at the aperture center of the b-channel. (c,d) The Bader analysis for the charge transfer in the host−guest complex. A more significant charge transfer for N2 implies a stronger host−guest interaction. E
DOI: 10.1021/acs.jpcc.5b04761 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
■
guest interaction differences for CO2 and N2 (Figure S5, Supporting Information). Thus, the more significant deformation of the 6-MR caused by CO2 should be the reason for the more positive BE value at the saddle point (Figure 2c). It is interesting that such a reformable channel (owing to the two flexible FL gates) can regulate the longer dimension of a nonspherical molecule, leading to the better recognition of N2 over CO2. 3.4. Comparison with Experimental Gas Permeation Properties of b- and c-Oriented ZIF-L Membranes. To justify our ab initio simulation results, the gas permeation properties of ZIF-L crystals in b- and c-directions were experimentally determined. The details are reported in ref 21. The permeance results in our experiments show that the N2 has a higher permeance than CO2 in both b- and c-oriented ZIF-L membranes, confirming our simulation predictions. The single gas permeance data yield a selectivity of N2/CO2 as 1.38 and 3.0 in b- and c-orientated membranes, respectively. This is contrary to the ZIF-8 membrane, where the averaged selectivity of N2/CO2 is about 0.38 (Table S1, Supporting Information).22−31 The better selectivity in the c-orientation membrane than that in the b-orientated membrane is overall consistent with our energy barrier results. The permeabilities of all three gases are lower in the c-orientated membrane, which is consistent with our simulation results as well.
AUTHOR INFORMATION
Corresponding Author
*Email:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge the financial support from Australian Research Council. The simulation works were carried out using Raijin provided from National Computational Infrastructure through the NCMAS scheme.
■
REFERENCES
(1) Wu, H.; Zhou, W.; Yildirim, T. Hydrogen Storage in a Prototypical Zeolitic Imidazolate Framework-8. J. Am. Chem. Soc. 2007, 129, 5314−5315. (2) D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon Dioxide Capture: Prospects for New Materials. Angew. Chem., Int. Ed. 2010, 49, 6058−6082. (3) Huang, A. S.; Wang, N. Y.; Kong, C. L.; Caro, J. OrganosilicaFunctionalized Zeolitic Imidazolate Framework ZIF-90 Membrane with High Gas-Separation Performance. Angew. Chem., Int. Ed. 2012, 51, 10551−10555. (4) Li, J. R.; Ma, Y. G.; McCarthy, M. C.; Sculley, J.; Yu, J. M.; Jeong, H. K.; Balbuena, P. B.; Zhou, H. C. Carbon Dioxide Capture-Related Gas Adsorption and Separation in Metal-Organic Frameworks. Coord. Chem. Rev. 2011, 255, 1791−1823. (5) Jiang, H. L.; Liu, B.; Akita, T.; Haruta, M.; Sakurai, H.; Xu, Q. Au@ZIF-8: CO Oxidation over Gold Nanoparticles Deposited to Metal−Organic Framework. J. Am. Chem. Soc. 2009, 131, 11302− 11303. (6) Lu, G.; Hupp, J. T. Metal−Organic Frameworks as Sensors: A ZIF-8 Based Fabry−Pérot Device as a Selective Sensor for Chemical Vapors and Gases. J. Am. Chem. Soc. 2010, 132, 7832−7833. (7) Yao, J. F.; Wang, H. T. Zeolitic Imidazolate Framework Composite Membranes and Thin Films: Synthesis and Applications. Chem. Soc. Rev. 2014, 43, 4470−4493. (8) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry, and Use; Wiley: New York, 1974. (9) Shang, J.; Li, G.; Singh, R.; Gu, Q.; Nairn, K. M.; Bastow, T. J.; Medhekar, N.; Doherty, C. M.; Hill, A. J.; Liu, J. Z.; et al. Discriminative Separation of Gases by a “Molecular Trapdoor” Mechanism in Chabazite Zeolites. J. Am. Chem. Soc. 2012, 134, 19246−19253. (10) Shang, J.; Li, G.; Singh, R.; Xiao, P.; Liu, J. Z.; Webley, P. A. Potassium Chabazite: A Potential Nanocontainer for Gas Encapsulation. J. Phys. Chem. C 2010, 114, 22025−22031. (11) Shang, J.; Li, G.; Gu, Q.; Singh, R.; Xiao, P.; Liu, J. Z.; Webley, P. A. Temperature Controlled Invertible Selectivity for Adsorption of N2 and CH4 by Molecular Trapdoor Chabazites. Chem. Commun. 2014, 50, 4544−4546. (12) Horike, S.; Shimomura, S.; Kitagawa, S. Soft Porous Crystals. Nat. Chem. 2009, 1, 695−704. (13) Deng, H. X.; Olson, M. A.; Stoddart, J. F.; Yaghi, O. M. Robust Dynamics. Nat. Chem. 2010, 2, 439−443. (14) Rabone, J.; Yue, Y. F.; Chong, S. Y.; Stylianou, K. C.; Bacsa, J.; Bradshaw, D.; Darling, G. R.; Berry, N. G.; Khimyak, Y. Z.; Ganin, A. Y.; et al. An Adaptable Peptide-Based Porous Material. Science 2010, 329, 1053−1057. (15) Chen, R. Z.; Yao, J. F.; Gu, Q. F.; Smeets, S.; Baerlocher, C.; Gu, H. X.; Zhu, D. R.; Morris, W.; Yaghi, O. M.; Wang, H. T. A TwoDimensional Zeolitic Imidazolate Framework with a Cushion-Shaped Cavity for CO2 Adsorption. Chem. Commun. 2013, 49, 9500−9502. (16) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54 (16), 11169−11186.
4. CONCLUSIONS In this paper, using ab initio simulations, we reveal that the multilevel local flexible ligands in ZIF-L crystal give rise to a cooperative reformable channel system that is triggered by the guest molecules. Such a reformable channel system enables a unique guest recognition scheme; i.e., the passage of a guest is dictated by its longer molecular dimension, in a striking contrast to other conventional molecular sieving materials. The N2 is predicted to have a higher permeance than that of CO2, which is confirmed by our later experiments. This unique recognition scheme is promising to separate guest molecules with similar kinetic diameters (e.g., N2 and CH4, N2 and O2, etc.). It is worth noting that the leaf-like morphology of the ZIF-L crystal is particularly suitable for membrane applications to harness this special recognition scheme in the future. In the end, considering that ZIF-L and ZIF-8 share the identical building blocks and similar framework topology but have very different gas permeation properties, some valuable insights can be obtained. There is only one 6-MR gate separating the neighboring cages in ZIF-8, which endows relatively easy transport of a guest molecule from one cage to the next. Thus, despite the role of local wobbling of CLs in the gas transport, the selection mechanism in ZIF-8 is largely in line with the traditional molecular sieving materials. In contrast, the long and reformable channels in ZIF-L, which are caused by the dynamic motion of the multiple flexible ligands (particularly the FLs), share the same spirit of the protein channels in cell membranes, endowing the new recognition scheme. Incorporation of multilevel flexible ligands in the frameworks might serve as a general route for intelligent soft porous PCP designs.
■
Article
ASSOCIATED CONTENT
* Supporting Information S
Details of ab initio simulations and supporting evidence for indepth analysis. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b04761. F
DOI: 10.1021/acs.jpcc.5b04761 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (17) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865− 3868. (18) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59 (3), 1758−1775. (19) Perez-Pellitero, J.; Amrouche, H.; Siperstein, F. R.; Pirngruber, G.; Nieto-Draghi, C.; Chaplais, G.; Simon-Masseron, A.; Bazer-Bachi, D.; Peralta, D.; Bats, N. Adsorption of CO2, CH4, and N2 on Zeolitic Imidazolate Frameworks: Experiments and Simulations. Chem. - Eur. J. 2010, 16, 1560−1571. (20) Shang, J.; Li, G.; Singh, R.; Xiao, P.; Danaci, D.; Liu, J. Z.; Webley, P. A. Adsorption of CO2, N2, and CH4 in Cs-Exchanged Chabazite: A Combination of van der Waals Density Functional Theory Calculations and Experiment Study. J. Chem. Phys. 2014, 140, 084705. (21) Zhong, Z.; Yao, J.; Chen, R.; Low, Z.; He, M.; Liu, J. Z.; Wang, H. Oriented Two-dimensional Zeolitic Imidazolate Framework-L Membranes and their Gas Permeation Properties. J. Mater. Chem. A 2015, DOI: 10.1039/C5TA03707G. (22) Bux, H.; Liang, F. Y.; Li, Y. S.; Cravillon, J.; Wiebcke, M.; Caro, J. Zeolitic Imidazolate Framework Membrane with Molecular Sieving Properties by Microwave-Assisted Solvothermal Synthesis. J. Am. Chem. Soc. 2009, 131, 16000−16001. (23) Shah, M.; Kwon, H. T.; Tran, V.; Sachdeva, S.; Jeong, H. K. One Step in situ Synthesis of Supported Zeolitic Imidazolate Framework ZIF-8 Membranes: Role of Sodium Formate. Microporous Mesoporous Mater. 2013, 165, 63−69. (24) Pan, Y. C.; Lai, Z. P. Sharp Separation of C2/C3 Hydrocarbon Mixtures by Zeolitic Imidazolate Framework-8 (ZIF-8) Membranes Synthesized in Aqueous Solutions. Chem. Commun. 2011, 47, 10275− 10277. (25) Tao, K.; Kong, C. L.; Chen, L. High Performance ZIF-8 Molecular Sieve Membrane on Hollow Ceramic Fiber via Crystallizing-Rubbing Seed Deposition. Chem. Eng. J. 2013, 220, 1−5. (26) Pan, Y. C.; Wang, B.; Lai, Z. P. Synthesis of Ceramic Hollow Fiber Supported Zeolitic Imidazolate Framework-8 (ZIF-8) Membranes with High Hydrogen Permeability. J. Membr. Sci. 2012, 421, 292−298. (27) Shekhah, O.; Swaidan, R.; Belmabkhout, Y.; du Plessis, M.; Jacobs, T.; Barbour, L. J.; Pinnau, I.; Eddaoudi, M. The Liquid Phase Epitaxy Approach for the Successful Construction of Ultra-Thin and Defect-Free ZIF-8 Membranes: Pure and Mixed Gas Transport Study. Chem. Commun. 2014, 50, 2089−2092. (28) Huang, K.; Dong, Z. Y.; Li, Q. Q.; Jin, W. Q. Growth of a ZIF-8 Membrane on the Inner-Surface of a Ceramic Hollow Fiber via Cycling Precursors. Chem. Commun. 2013, 49, 10326−10328. (29) McCarthy, M. C.; Varela-Guerrero, V.; Barnett, G. V.; Jeong, H. K. Synthesis of Zeolitic Imidazolate Framework Films and Membranes with Controlled Microstructures. Langmuir 2010, 26, 14636−14641. (30) Tao, K.; Cao, L. J.; Lin, Y. C.; Kong, C. L.; Chen, L. A Hollow Ceramic Fiber Supported ZIF-8 Membrane with Enhanced Gas Separation Performance Prepared by Hot Dip-Coating Seeding. J. Mater. Chem. A 2013, 1, 13046−13049. (31) Zhang, X. F.; Liu, Y. G.; Kong, L. Y.; Liu, H. O.; Qiu, J. S.; Han, W.; Weng, L. T.; Yeung, K. L.; Zhu, W. D. A Simple and Scalable Method for Preparing Low-Defect ZIF-8 Tubular Membranes. J. Mater. Chem. A 2013, 1, 10635−10638.
G
DOI: 10.1021/acs.jpcc.5b04761 J. Phys. Chem. C XXXX, XXX, XXX−XXX
