First principles molecular dynamics simulation of a taskspecific. ionic liquid based on silver-olefin complex:

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1 First principles molecular dynamics simulation of a taskspecific ionic liquid based on silver-olefin complex: atomistic insight into separation process De-en Jiang,* and Sheng Dai, Chemical Sciences Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 jiangd@ornl.gov First principles molecular dynamics simulation of a silver-olefin complex-based ionic liquid *To whom correspondence should be addressed. jiangd@ornl.gov. Phone: (865) Fax: (865)

2 First principles molecular dynamics based on density functional theory is applied to a hypothetical ionic liquid whose cations and anions are silver-ethylene complex [Ag(C H 4 ) + ] and tetrafluoroborate [BF - 4 ], respectively. This ionic liquid represents a group of task-specific silver complex-based ionic liquids synthesized recently. Molecular dynamics simulations at two temperatures are performed for five picoseconds. Events of association, dissociation, exchange, and recombination of ethylene with silver cation are observed. A mechanism of ethylene transfer similar to the Grotthus type of proton transfer in water is identified, where a silver cation accepts one ethylene molecule and donates another to a neighboring silver cation. This mechanism may contribute to fast transport of olefins through ionic liquid membranes based on silver complexes for olefin/paraffin separation. Keywords: first principles molecular dynamics, density functional theory, ionic liquids, silver-olefin complex, ethylene

3 . Introduction Olefin/paraffin separations such as separation of ethylene from ethane are an important industrial process. Cryogenic distillation is employed commercially at large scales to fulfill the purpose. However, as a thermally driven process, cryogenic distillation is energy-intensive. Therefore, there is a strong driving force to move towards more energy-efficient processes such as membranes for olefin/paraffin separations. The apparent distinction between olefin and paraffin is the presence of the double bond in olefins. It is well known that the π-bond in olefins can form complexes with Ag + (or Cu + ) through d-π interaction. -5 Therefore, Ag + can be introduced into membranes for olefin/paraffin separation via facilitated transport of olefins. Various membranes containing Ag salts have been reported for olefin/paraffin separations. 6-6 One basic strategy employed by many is to introduce Ag salts into polymer membrane matrices. 6-8,0-6 Recently, a new strategy to synthesize novel ionic liquids whose cations consist of Ag + complexes was introduced. 7 The resulting ionic liquids were successfully used for olefin/paraffin separation via an immobilized liquid membrane process. 8 The neutral organic ligands complexed to Ag + can be olefins, amines, or amides. It has been demonstrated that Ag-olefin and Ag-amide complex-based ionic liquids show remarkable olefin/paraffin permselectivity. These task-specific ionic liquids for olefin/paraffin separations benefit from the high concentration of Ag +, which leads to high solubility of olefin and may increase the transport of olefin through the membrane at the same time. Two mechanisms of olefin transport through the ionic liquid membrane have been proposed: (a) diffusion of Ag complexes and (b) hopping of olefin along neighboring Ag + centers. As a first step toward understanding the mechanism of olefin transport through these ionic liquids, here we use first principle (namely, density functional theory) molecular dynamics simulations to examine structures and dynamics of a hypothetical and also the simplest Ag + complex-based ionic liquid, Ag(C H 4 ) + BF 4 -. We chose Ag(C H 4 ) + BF 4 - because first principles computations are timeconsuming and we are limited to systems that can capture the essence of the experimental systems but are as small as possible. As such, Ag(C H 4 ) + BF 4 - is an excellent model system. We note that 3

4 Ag(C H 4 ) + BF - 4 does exist as a solid under a certain C H 4 pressure 9 and longer olefins such as -butene do make Ag(olefin) BF 4 more stable and easy to melt. For example, Ag(-butene) BF 4 has a melting point of ~37.5 o C, 9 which can be classified as an ionic liquid. 0 We employ first principles MD simulations for two reasons: (a) force fields targeted for Ag-olefin complexes are not available and (b) the bonding between Ag and olefin is not very strong. So we may capture olefin exchange events between Ag + centers within a time scale accessible to first principles MD simulations if we run the simulations at a relatively high temperature. As we will show below, our strategy indeed worked. The rest of the paper is organized as follows. We first explain the methods used in Sec., then show gas-phase energetics for interactions between C H 4, Ag +, and BF 4 - in Sec. 3. as a preparation for presenting and discussing the structures and dynamics of the Ag(C H 4 ) + - BF 4 ionic liquid in Sec. 3., and we conclude in Sec. 4.. Method Gas phase energetics was computed with the Gaussian 03 program at the B3LYP level of theory. The basis set used was 6-3+G(d), except that the LANLDZ effective core potential was used for Ag. The Vienna Ab Initio Simulation Package (VASP),3 was used to perform DFT-based first principles molecular dynamics simulation with planewave bases and periodic boundary conditions and within the generalized-gradient approximation (GGA) for electron exchange and correlation. 4 Projectoraugmented wave (PAW) method 5,6 was used within the frozen core approximation to describe the electron-core interaction. A kinetic energy cutoff (50 ev) was used. 4 pairs of Ag(C H 4 ) + - and BF 4 were placed within a simple cubic cell of 8.3 Å 8.3 Å 8.3 Å, giving a density of.63 g/cm 3. 7 Only the Γ-point was used for k-sampling. The deuterium isotope was used for hydrogen, so a greater time step could be used. Constant-temperature MD was run for ps with a time step of fs, and then a microcanonical MD simulation was followed for 5 ps with a time step of 0.5 fs. The microcanonical simulation was used for data analysis. Two rather high simulation temperatures (54 K and 67 K) were pursued to accelerate the MD simulations so at least one dissociation or association event happens between Ag + and C H 4 within our simulation time scale. Relative energy fluctuations for the 5 ps 4

5 microcanonical simulations were at 54 K and at 67 K; relative temperature fluctuations were 54 ± 3 K and 67 ± 7 K. 3. Results and discussion 3.. Gas-phase structure and energetics. As a preparation for the later discussion of the liquid structure and dynamics, here we first examine the gas phase structure and energetics of the three components of the ionic liquid: Ag +, C H 4, and BF - 4. Table presents the energetics of association for formation of Ag(C H 4 ) + x (x=,, and 3) and Ag + BF - 4, and one can see that the first two C H 4 molecules bind to Ag + quite strongly, while the binding strength for the third C H 4 decreases significantly, in accompany with increased Ag-C distances by ~0. Å (Figure ). Therefore, Ag(C H 4 ) + 3 may readily lose one C H 4 to become Ag(C H 4 ) +. This result has implications for C H 4 transfer in the Ag(C H 4 ) + - BF 4 ionic liquid, as we will see in Sec. 3.. We note that gas-phase Ag(C H 4 ) + x clusters have been detected by mass spectrometry recently, and our computed binding energetics agrees with the previous one. 8,9 Moreover, a single crystal containing Ag(C H 4 ) + 3 was obtained recently, 30 indicating that despite its age Ag-olefin chemistry still attracts researchers interest. The nature of bonding between Ag + and C H 4 has been analyzed extensively before Table also shows that the binding energy between Ag + and BF - 4 is at 5.5 ev, mainly due to the electrostatic interaction. In a crystal, this long-range Coulombic interaction will be modulated by interactions with other ions surrounding the pair. We can estimate the effective binding energy between Ag + and BF - 4 within a lattice. Assuming a Madelung constant of.75, we arrive at a cohesive energy of roughly 9. ev for the AgBF 4 lattice. 37 Suppose there are six nearest BF - 4 ions around Ag +, the effective binding between Ag + and one BF - 4 ion within the AgBF 4 lattice is estimated to be around.5 ev. This energy is comparable to the binding energy between Ag + and C H 4. Due to its local character, the bonding between Ag + and C H 4 is less affected by other ions. Moreover, the bond distance between Ag and F is similar to that between Ag and C (Figure ). Hence, one expects that the AgBF 4 lattice will readily absorb C H 4 molecules due to favorable reactions between Ag + and C H 4. This experiment was indeed performed, albeit 40 years ago, and Ag(C H 4 ) x BF 4 (x=,, and 3) solids were obtained. 9,38 5

6 Although heating up solid Ag(C H 4 ) x BF 4 leads to dissociation, other Ag(olefin) BF 4 compounds such as Ag(-butene) BF 4 can melt at a relatively low temperature (~37.5 o C) before dissociation Structures and dynamics of Ag(C H 4 ) + BF - 4. From the 5 ps microcanonical ensemble MD simulations at 54 K and 67 K, we obtained the pair correlation functions [g(r)] for C and F around Ag. Figure displays g(r) of C around Ag. The sharp peak at.4 Å corresponds to coordination of C H 4 around Ag, as shown in Figure. The peak probability decreases with temperature because high temperature leads to more Ag(C H 4 ) + and Ag(C H 4 ) + 3 (see Figure 3) and Ag(C H 4 ) + 3 has longer Ag-C bonds. Figure 4 presents g(r) of F around Ag. One can see that two shells of F surround Ag at.5 and 4.6 Å, respectively. The second peak corresponds to mainly those F atoms connected to the first shell of F through B. The first peak probability actually increases slightly with temperature. This increase is correlated with the decrease in probability at.4 Å for C around Ag. Figure 5 looks into typical coordination environments for three selected Ag atoms at the end of the 5 ps MD simulation at 54 K. Figure 5b represents the dominant configuration where two C H 4 molecules and three F atoms surround Ag. Next we examine the association, dissociation, and exchange events regarding the complexation between C H 4 and Ag + at 54 K and 67 K. We are especially interested in exchange events because they may shed light on the mechanism by which olefin is transported through Ag + complex-based ionic liquid membranes. Figure 6 documents the events along the time line. One can see more events happen at 67 K, which is expected. To look further into the exchange events, we use the change in the number of Ag-C bonds (defined as a distance less than 3. Å) as an indicator of event occurrences. Figure 7 displays how the number of Ag-C bonds changes with time for two neighboring Ag atoms at 54 K, together with snapshots along the way. At 3000 fs, frame a shows that both Ag atoms have two C H 4 molecules complexed, while one C H 4 (x) of Ag is close to Ag and an isolated C H 4 (y) is close to Ag. Next, in two quick steps, C H 4 y is complexed to Ag, and C H 4 x is complexed to Ag. Frame b shows that C H 4 x is shared by Ag and Ag. This state is short-lived and can be considered as a transition state, because in less than 5 fs C H 4 x is dissociated away from Ag. So an exchange event 6

7 for Ag (that is, C H 4 x is exchanged for C H 4 y) is concerted with an association event for Ag (that is, C H 4 x is added). Then Ag has three C H 4 coordinated for ~000 fs (see frame c) before it dissociates away C H 4 z in two steps, leading to frame d. From frame a to frame d, the net result is that one isolated C H 4 molecule is transported by ~3 Å within 500 fs. This mechanism is quite similar to the Grotthus-type mechanism of proton transfer through water. At 67 K, we also observed an exchange event within 5 ps. Figure 8 displays the detailed process. At 000 fs, frame a shows that one of the two C H 4 molecules (x) bonded to Ag is close to Ag and an isolated C H 4 (y) is close to Ag. Then in two steps, C H 4 x is complexed to Ag, leading to frame b in which Ag and Ag share C H 4 x. What happens next is that Ag loses one Ag-C bond with C H 4 z and then Ag loses one Ag-C bond with C H 4 x, leading to frame c. Subsequently in two quick steps, C H 4 y is added to Ag, and simultaneously Ag recovers the lost Ag-C bond with C H 4 z, resulting in a state that frame d represents. After ~00 fs, C H 4 x is dissociated away from Ag, leading to frame e. The net result is that an isolated C H 4 (y) exchanges a C H 4 (x) coordinated to Ag, and the exchanged C H 4 (x) is transferred to Ag. The exchange and the transfer are also concerted. The common feature between Figures 7 and 8 is that both involve a stage where C H 4 is shared by two neighboring Ag atoms. The difference is that in Figure 7, the addition of an isolated C H 4 to Ag precedes the sharing of a C H 4 molecule and the shared state is short-lived, while in Figure 8, the addition of an isolated C H 4 happens after the sharing of a C H 4 molecule between Ag and Ag and the shared state stays relatively longer. We now discuss the implications of our findings from Figures 7 and 8 on the recently reported olefinfacilitated transport through Ag + complex-based ionic liquids. 8 The synthesized ionic liquids comprise Ag-complex cations and trifluoromethanesulfonimide (Tf N - ) anions. The cation is a complex between Ag + and neutral organic ligands which include olefins (such as -hexexe, -pentene, and isoprene), amines (such as propylamine), and amides (such as N,N-dimethylbenzamide). Liquid membranes based on Ag-olefin and Ag-amide complexes have been shown to have permselectivity as high as 795 for olefin/paraffin separations. 8 Two mechanisms of olefin transport through the membrane have been 7

8 proposed: (a) diffusion of Ag-olefin complexes and (b) hopping of guest olefin molecules along neighboring Ag + sites. Due to the limitation of the length and time scales accessible to our first principles DFT MD simulations, we cannot meaningfully address the diffusion of Ag + complexes within our simulations. But our simulations can shed some light on the hopping mechanism. If we loosely define hopping as the transfer of olefin from one Ag to another, Figures 7 and 8 show that hopping indeed happens via a state where the hopping olefin is shared by two neighboring Ag atoms. Beyond diffusion and hopping mechanisms, our simulations indicate that there may be a third mechanism, the Grotthus type, operating at least in the short range; namely, a chain reaction of accepting one guest olefin and donating another takes place among Ag +. Under this mechanism, the rate of olefin transport is determined by re-orientation of Ag + complexes, which is presumably faster than diffusion. For ligands whose size and bonding strength to Ag + are comparable to those of the guest olefin molecule, the Grotthus-type mechanism is highly likely to operate, at least in short ranges. Our simulations exemplify this scenario in that C H 4 acts as both the ligands and the guest molecules. For ligands which are either much bigger or binds much stronger to Ag +, the ligand/ag ratio will be an important factor. In making the Ag-complex-based ionic liquids, the ratio of ligand to Ag can usually be tuned from one to two. 8 When the ratio of ligand to Ag is two, then each Ag has only one site open to bind a guest olefin molecule and then hopping along neighboring sites would be a favorable path. If the ratio of ligand to Ag is one, then two sites on each Ag + are open for guest olefin molecules and the Grotthus-type mechanism is likely to operate. Another possibility is that the three mechanisms discussed above may work together in reality. Further experiments and simulations are needed to elucidate the dominant transport mechanism of olefins through Ag complex-based ionic liquid membranes. 4. Summary and conclusions Using DFT-based first principles method, we performed molecular dynamics simulation for an Ag complex-based ionic liquid, Ag(C H 4 ) + BF - 4. We found that although the number of C H 4 complexed to Ag is predominantly two, increased dissociation and association of Ag-olefin bonds with temperature 8

9 lead to more occurrences of coordination numbers of one and three. Gas phase association energetics indicates that binding of the third C H 4 to Ag + is favorable, even though the strength is much less than the first two. We identified exchange and transfer events of C H 4 at the temperatures examined (54 K and 67 K) within a simulation time of 5 ps. A mechanism of C H 4 transfer, similar to the Grotthus mechanism of proton transfer in water, was observed. The mechanism involves a concerted chain of reactions in which one Ag + accepts one C H 4 and donates another C H 4 to a nearby Ag + via a state of shared C H 4 between the two Ag atoms. This mechanism may contribute to the facilitated transport of olefin through Ag complex-based ionic liquid membranes that have been reported recently. 8 Acknowledgement. This work was supported by Office of Basic Energy Sciences, U.S. Department of Energy under Contract No. DE-AC05-00OR75 with UT-Battelle, LLC, and used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC0-05CH3. References and Notes [] Tropsch, H.; Mattox, W. J. J. Am. Chem. Soc. 935, 57, [] Eberz, W. F.; Welge, H. J.; Yost, D. M.; Lucas, H. J. J. Am. Chem. Soc. 937, 59, [3] Winstein, S.; Lucas, H. J. J. Am. Chem. Soc. 938, 60, [4] Hepner, F. R.; Trueblood, K. N.; Lucas, H. J. J. Am. Chem. Soc. 95, 74, [5] Trueblood, K. N.; Lucas, H. J. J. Am. Chem. Soc. 95, 74, [6] Antonio, M. R.; Tsou, D. T. Ind. Eng. Chem. Res. 993, 3, [7] Ho, W. S.; Dalrymple, D. C. J. Membr. Sci. 994, 9, 3-5. [8] Thoen, P. M.; Noble, R. D.; Koval, C. A. J. Phys. Chem. 994, 98, [9] Tsou, D. T.; Blachman, M. W.; Davis, J. C. Ind. Eng. Chem. Res. 994, 33, [0] Yamaguchi, T.; Baertsch, C.; Koval, C. A.; Noble, R. D.; Bowman, C. N. J. Membr. Sci. 996, 7,

10 [] Bai, S.; Sridhar, S.; Khan, A. A. J. Membr. Sci. 998, 47, [] Ryu, J. H.; Lee, H.; Kim, Y. J.; Kang, Y. S.; Kim, H. S. Chem.-Eur. J. 00, 7, [3] Kim, J. H.; Min, B. R.; Lee, K. B.; Won, J. G.; Kang, Y. S. Chem. Commun. 00, [4] Kim, J. H.; Kang, Y. S.; Won, J. Macromol. Res. 004,, [5] Hess, S.; Staudt-Bickel, C.; Lichtenthaler, R. N. J. Membr. Sci. 006, 75, [6] Kim, J. H.; Min, B. R.; Kim, Y. W.; Kang, S. W.; Won, J.; Kang, Y. S. Macromol. Res. 007, 5, [7] Huang, J. F.; Luo, H. M.; Dai, S. J. Electrochem. Soc. 006, 53, J9-J3. [8] Huang, J. F.; Luo, H.; Liang, C.; Jiang, D. E.; Dai, S. Ind. Eng. Chem. Res. 008, 47, [9] Quinn, H. W.; Glew, D. N. Can. J. Chem. 96, 40, 03-. [0] Welton, T. Chem. Rev. 999, 99, [] Gaussian 03, Revision D.0, M. J. Frisch, et al., Gaussian, Inc., Wallingford CT, 004. [] Kresse, G.; Furthmüller, J. Phys. Rev. B 996, 54, [3] Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 996, 6, [4] Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 996, 77, [5] Blöchl, P. E. Phys. Rev. B 994, 50, [6] Kresse, G.; Joubert, D. Phys. Rev. B 999, 59, [7] This density was chosen rather arbitrarily because of the hypothetical nature of the ionic liquid Ag(C H 4 ) + BF - 4. We considered two aspects when choosing the density. First, the synthesized Ag complex ionic liquids with the common anion Tf N - have an average density of.55 g/cm 3. 8 Because BF - 4 is much smaller than Tf N -, we expect that the density of Ag(C H 4 ) + BF - 4 will be significantly larger than.55 g/cm 3. However, to accelerate the MD simulations, we pursued quite high simulation temperatures (54 K and 67 K), which would result in volume expansion. Coupling these two factors, we decided on.63 g/cm 3. Using a higher density to examine the difference is planned for the future. Moreover, performing constant-pressure MD simulations is currently being considered (but this will need another program since the program we used in this work does not have the constant-pressure MD capability now). [8] Manard, M. J.; Kemper, P. R.; Carpenter, C. J.; Bowers, M. T. Int. J. Mass Spectrom. 005, 4, [9] Manard, M. J.; Kemper, P. R.; Bowers, M. T. Int. J. Mass Spectrom. 005, 4, [30] Krossing, I.; Reisinger, A. Angew. Chem.-Int. Edit. 003, 4,

11 [3] Dewar, M. Bull. Soc. Chim. Fr. 95, 8, C79. [3] Chatt, J.; Duncanson, L. A. J. Chem. Soc. 953, [33] Ziegler, T.; Rauk, A. Inorg. Chem. 979, 8, [34] Chen, J. P.; Yang, R. T. Langmuir 995,, [35] Chen, N.; Yang, R. T. Ind. Eng. Chem. Res. 996, 35, [36] Jiang, D. E.; Sumpter, B. G.; Dai, S. Langmuir 006,, [37] Kittel, C. Introduction to Solid State Physic; Wiley: New York, 996, pp [38] Quinn, H. W.; McIntyre, J. S.; Peterson, D. J. Can. J. Chem. 965, 43,

12 Table. Gas-phase association energetics at the B3LYP level. a Association reaction b Energy (ev) Ag + + C H 4 Ag(C H 4 ) Ag(C H 4 ) + + C H 4 Ag(C H 4 ) + Ag(C H 4 ) + + C H 4 Ag(C H 4 ) 3 + Ag + + BF 4 - Ag + BF a 6-3+G(d) for H, B, C, and F, and LANLDZ for Ag. b See Figure for structures

13 Figure captions: Figure. Gas phase structures for Ag(C H 4 ) +, Ag(C H 4 ) +, Ag(C H 4 ) 3 +, and the Ag + BF 4 - pair. Bond lengths (Å) are shown for Ag-C and Ag-F. Ag, green; C, black; H, white; B, red; F, blue. The same color scheme is used in all subsequent figures. Figure. Pair correlation function of C around Ag at 54 K and 67 K. Figure 3. Distribution of coordination number of C H 4 around Ag at the end of simulations at 54 K and 67 K. Figure 4. Pair correlation function of F around Ag at 54 K and 67 K. Figure 5. Typical first coordination shells of Ag + at the end of simulation at 54 K. Figure 6. Events along the time line at 54 K and 67 K. Each vertical bar represents the onset of an event. D, dissociation; A, association; E, exchange; R, recombination. Figure 7. Change of number of Ag-C bonds for Ag and Ag versus time at 54 K, together with four snapshots. Figure 8. Change of number of Ag-C bonds for Ag and Ag versus time at 67 K, together with five snapshots. 3

14 Ag(C H 4 ) + Ag(C H 4 ) Ag(C H 4 ) 3 + Ag + BF 4 - Figure. Gas phase structures for Ag(C H 4 ) +, Ag(C H 4 ) +, Ag(C H 4 ) 3 +, and the Ag + BF 4 - pair. Bond lengths (Å) are shown for Ag-C and Ag-F. Ag, green; C, black; H, white; B, red; F, blue. The same color scheme is used in all subsequent figures. 4

15 g(r) for C around Ag r (Å) 54 K 67 K Figure. Pair correlation function of C around Ag at 54 K and 67 K. 5

16 Percentage (%) K 67 K Coordination number of C H 4 around Ag Figure 3. Distribution of coordination number of C H 4 around Ag at the end of simulations at 54 K and 67 K. 6

17 g(r) F around Ag K 67 K r (Å) Figure 4. Pair correlation function of F around Ag at 54 K and 67 K. 7

18 (a) (b) (c) Figure 5. Typical first coordination shells of Ag + at the end of simulation at 54 K. 8

19 67 K D A A R A Events D E 54 K A E D A Time (ps) Figure 6. Events along the time line at 54 K and 67 K. Each vertical bar represents the onset of an event. D, dissociation; A, association; E, exchange; R, recombination. 9

20 Number of Ag-C bonds around Ag a b Ag Ag y y x x d c y y x x z z Time (fs) Figure 7. Change of number of Ag-C bonds for Ag and Ag versus time at 54 K, together with four snapshots. 0

21 Number of Ag-C bonds around Ag Time (fs) Ag Ag a c e d b x y y x y y x y x x z z z Figure 8. Change of number of Ag-C bonds for Ag and Ag versus time at 67 K, together with five snapshots.

Experiment Section Fig. S1 Fig. S2

Experiment Section Fig. S1 Fig. S2 Electronic Supplementary Material (ESI) for ChemComm. This journal is The Royal Society of Chemistry 2018 Supplementary Materials Experiment Section The STM experiments were carried out in an ultrahigh